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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Mar. 01, 2019	Jun. 29, 2018
Document and Entity Information
Entity Registrant Name	RHYTHM PHARMACEUTICALS, INC.
Entity Central Index Key	0001649904
Document Type	10-K
Document Period End Date	Dec. 31, 2018
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	Yes
Entity Voluntary Filers	No
Entity Current Reporting Status	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Shell Company	false
Entity Ex Transition Period	true
Entity Public Float			$ 640.8
Entity Common Stock, Shares Outstanding		34,425,830
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Current assets:
Cash and cash equivalents	$ 49,542	$ 34,236
Short-term investments	202,519	113,846
Prepaid expenses and other current assets	6,628	2,589
Total current assets	258,689	150,671
Property, plant and equipment, net	1,120	840
Restricted cash	401	225
Total assets	260,210	151,736
Current liabilities:
Accounts payable	7,640	2,427
Deferred rent		83
Accrued expenses and other current liabilities	5,942	4,210
Total current liabilities	13,582	6,720
Long-term liabilities:
Deferred rent	372	228
Total liabilities	13,954	6,948
Commitments and contingencies
Stockholders’ equity:
Common stock, $0.001 par value: 120,000,000 shares authorized; 34,410,725 and 27,284,140 shares issued and outstanding December 31, 2018 and December 31, 2017, respectively	34	27
Additional paid-in capital	430,824	255,013
Accumulated deficit	(184,602)	(110,252)
Total stockholders’ equity	246,256	144,788
Total liabilities, convertible preferred stock and stockholders’ equity	$ 260,210	$ 151,736

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2013
CONSOLIDATED BALANCE SHEETS
Temporary equity, par value per share	$ 0.001	$ 0.001
Temporary equity, shares authorized	10,000,000	10,000,000
Temporary equity, shares issued	0	0
Temporary equity, shares outstanding	0	0
Common stock, par value per share	$ 0.001	$ 0.001	$ 0.001
Common stock, authorized	120,000,000	120,000,000
Common stock, issued	34,410,725	27,284,140	10,196,292
Common stock, outstanding	34,410,725	27,284,140

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating expenses:
Research and development	$ 50,337	$ 22,894	$ 19,594
Selling, general, and administrative	28,080	9,518	6,311
Total operating expenses	78,417	32,412	25,905
Loss from operations	(78,417)	(32,412)	(25,905)
Other income (expense):
Revaluation of Series A Investor Instrument and Series A Investor Right/Obligation		(1,863)
Interest income, net	4,353	566	33
Total other income (expense):	4,353	(1,297)	33
Net loss and comprehensive loss	(74,064)	(33,709)	(25,872)
Net loss attributable to common stockholders	$ (74,064)	$ (37,582)	$ (29,074)
Net loss attributable to common stockholders per common share, basic and diluted	$ (2.39)	$ (2.83)	$ (2.85)
Weighted average common shares outstanding, basic and diluted	31,004,047	13,267,960	10,196,292

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) - USD ($) $ in Thousands	Common stock	Preferred stock Series A convertible preferred shares	Preferred stock Series A-1 Junior Preferred Stock	Additional Paid-in Capital	Accumulated Deficit	Total
Beginning balance at Dec. 31, 2015		$ 40,000
Beginning balance (in shares) at Dec. 31, 2015		40,000,000
Ending balance at Dec. 31, 2016		$ 40,000
Ending balance (in shares) at Dec. 31, 2016		40,000,000
Beginning balance at Dec. 31, 2015	$ 10			$ 42,662	$ (50,671)	$ (7,999)
Beginning balance (in shares) at Dec. 31, 2015	10,196,292
Increase (Decrease) in Stockholders' Equity
Stock compensation expense				1,168		1,168
Net loss					(25,872)	(25,872)
Ending balance at Dec. 31, 2016	$ 10			43,830	(76,543)	(32,703)
Ending balance (in shares) at Dec. 31, 2016	10,196,292
Increase (Decrease) in Convertible Preferred Stock
Issuance of Series A Convertible Preferred Stock		$ 40,622
Settlement of Series A investor instrument		328		1,863		$ 1,863
Conversion of Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock into common stock on a 9.17 to 1 basis		$ (80,950)
Conversion of Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock into common stock on a 9.17 to 1 basis (in shares)		(80,949,999)
Ending balance (in shares) at Dec. 31, 2017						0
Increase (Decrease) in Stockholders' Equity
Stock compensation expense				2,278		$ 2,278
Issuance of common stock in connection with exercise of stock options				700		700
Issuance of common stock in connection with exercise of stock options (in shares)	152,671
Change in unrealized loss on marketable securities				(141)		(141)
Issuance of Series A Convertible Preferred Stock				(108)		(108)
Settlement of Series A investor instrument		$ 328		1,863		1,863
Exchange of common stock held by LLC entity for Series A‑1 Junior Preferred Stock	$ (8)		$ 79	(71)
Issuance of common stock on exchange of preferred stock (in shares)	(8,578,661)		78,666,209
Issuance of common stock upon completion of initial public offering, net of offering costs	$ 8			125,650		125,658
Issuance of common stock upon completion of initial public offering, net of offering costs (in shares)	8,107,500
Conversion of Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock into common stock on a 9.17 to 1 basis	$ 17		$ (79)	81,012		80,950
Conversion of Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock into common stock on a 9.17 to 1 basis (in shares)	17,406,338		(78,666,209)
Net loss					(33,709)	(33,709)
Ending balance at Dec. 31, 2017	$ 27			255,013	(110,252)	$ 144,788
Ending balance (in shares) at Dec. 31, 2017	27,284,140
Increase (Decrease) in Convertible Preferred Stock
Issuance of Series A Convertible Preferred Stock (in shares)		40,949,999				0
Ending balance (in shares) at Dec. 31, 2018						0
Increase (Decrease) in Stockholders' Equity
Stock compensation expense				6,390		$ 6,390
Shares issued for license agreement				4,448		4,448
Shares issued for license agreement (in shares)	223,544
Issuance of common stock in connection with exercise of stock options				1,808		1,808
Issuance of common stock in connection with exercise of stock options (in shares)	311,241
Change in unrealized loss on marketable securities				8		8
Issuance of common stock upon completion of initial public offering, net of offering costs	$ 7			162,871		162,878
Issuance of common stock upon completion of initial public offering, net of offering costs (in shares)	6,591,800
Net loss					(74,064)	(74,064)
Ending balance at Dec. 31, 2018	$ 34			430,824	(184,602)	$ 246,256
Ending balance (in shares) at Dec. 31, 2018	34,410,725
Increase (Decrease) in Convertible Preferred Stock
Issuance of Series A Convertible Preferred Stock (in shares)						0
Increase (Decrease) in Stockholders' Equity
Adoption of new accounting standard				$ 286	$ (286)

CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical)	12 Months Ended
	Dec. 31, 2017
CONSOLIDATED STATEMENTS OF CONVERTIBLE PREFERRED STOCK AND STOCKHOLDERS' EQUITY (DEFICIT)
Conversion ratio of Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock into common stock	9.17

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (74,064)	$ (33,709)	$ (25,872)
Adjustments to reconcile net loss to cash used in operating activities:
Non-cash research and development license expense	4,448
Stock-based compensation expense	6,390	2,278	1,168
Depreciation and amortization	442	223	144
Non-cash rent expense	61	(76)	11
Mark to market revaluation of series A investor instrument		1,863
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(6,286)	(1,889)	41
Deferred issuance costs		9	1,472
Tenant improvement allowance			376
Accounts payable, accrued expenses and other current liabilities	6,953	1,946	160
Deferred grant income			(249)
Due to related parties		(105)	(470)
Net cash used in operating activities	(62,056)	(29,460)	(23,219)
Investing activities
Purchases of short-term investments	(248,592)	(126,917)	(15,222)
Maturities of short-term investments	162,166	17,006	11,169
Purchases of property, plant and equipment	(722)	(133)	(1,057)
Net cash used in investing activities	(87,148)	(110,044)	(5,110)
Financing activities
Net proceeds from issuance of common stock	162,878	125,658
Proceeds from the exercise of stock options		40,842
Net proceeds from issuance of Series A Convertible Preferred Stock	1,808	700
Net cash provided by financing activities	164,686	167,200
Net increase in cash, cash equivalents and restricted cash	15,482	27,696	(28,329)
Cash, cash equivalents and restricted cash at beginning of period	34,461	6,765	35,094
Cash, cash equivalents and restricted cash at end of period	$ 49,943	$ 34,461	$ 6,765

Nature of Business	12 Months Ended
Nature of Business	Dec. 31, 2018
Nature of Business
Nature of Business	1. Nature of Business Rhythm Pharmaceuticals, Inc. (the “Company”), is a biopharmaceutical company focused on the development and commercialization of therapeutics for the treatment of rare genetic disorders that result in severe, life‑threatening metabolic disorders. The Company's lead product candidate is setmelanotide (“RM‑493”), which is a potent, first‑in‑class, melanocortin‑4, or MC4, receptor, agonist peptide for the treatment of rare genetic disorders of obesity caused by MC4 pathway deficiencies. MC4 pathway deficiencies result in the disruption of satiety signals and energy homeostasis in the body, which, in turn, leads to intense feelings of hunger and to obesity. The Company is currently evaluating setmelanotide for the treatment of six single gene related, or monogenic, MC4 pathway deficiencies: pro‑opiomelanocortin, or POMC, leptin receptor, or LEPR, Bardet‑Biedl syndrome, Alström syndrome, POMC heterozygous, and POMC epigenetic disorders for which there are currently no effective or approved treatments. The Company believes that the MC4 pathway is a compelling target for treating these genetic disorders because of its critical role in regulating appetite and weight by promoting satiety and weight control, and that peptide therapeutics are uniquely suited for activating this target. In March 2018 the Company acquired exclusive, worldwide rights from Takeda Pharmaceutical Company Limited (“Takeda”) to develop and commercialize T-3525770 (now “RM-853”). RM-853 is a potent, orally available ghrelin o-acyltransferase (“GOAT”) inhibitor currently in preclinical development for Prader-Willi Syndrome (“PWS”). PWS is a rare genetic disorder that results in hyperphagia and early-onset, life-threatening obesity, for which there are no approved therapeutic options. Corporate Reorganization The Company is a Delaware corporation organized in February 2013 under the name Rhythm Metabolic, Inc. Prior to the Company's organization and the Corporate Reorganization referred to below, the Company was part of Rhythm Pharmaceuticals, Inc. (the “Predecessor Company”), a Delaware corporation which was organized in November 2008 and which commenced active operations in 2010. In March 2013, the Predecessor Company underwent a corporate reorganization, (the “Corporate Reorganization”), pursuant to which all of the outstanding equity securities of the Predecessor Company were exchanged for units of Rhythm Holding Company, LLC, a newly‑organized limited liability company (the “LLC entity”). After the consummation of this exchange and as part of the Corporate Reorganization, the Predecessor Company contributed setmelanotide and the MC4 receptor agonist program to the Company and distributed to the LLC entity all of the then issued and outstanding shares of the Company's stock. The result of the Corporate Reorganization was that the Company and the Predecessor Company became wholly‑owned subsidiaries of the LLC entity and the two product candidates and related programs that were originally held by the Predecessor Company were separated, with relamorelin and the ghrelin agonist program being retained by the Predecessor Company and setmelanotide and the MC4 receptor agonist program being held by the Company. The Predecessor Company, after consummation of the Corporate Reorganization, is referred to within these Notes to Financial Statements as the Relamorelin Company and/or Motus. On October 13, 2015, the Relamorelin Company changed its name to Motus Therapeutics, Inc. (“Motus”) and the Company changed its name to Rhythm Pharmaceuticals, Inc. On December 15, 2016, Motus was sold to a large pharmaceutical company. On August 21, 2017, the LLC entity distributed to its members all of its shares of the Company (see Note 5 for further discussion). Liquidity The Company has incurred operating losses and negative cash flows from operations since inception, incurred a net loss of $74,064 $33,709 and $25,872 during the years ended December 31, 2018, 2017 and 2016, respectively, and had an accumulated deficit of $184,602 as of December 31, 2018. The Company has primarily funded these losses through the proceeds from the sales of common and preferred stock as well as capital contributions received from the Predecessor Company, the Relamorelin Company and the LLC entity. To date, the Company has no product revenue and management expects operating losses to continue for the foreseeable future. The Company has devoted substantially all of its resources to its drug development efforts, comprising research and development, manufacturing, conducting clinical trials for its product candidates, protecting its intellectual property, pre-commercialization activities and general and administrative functions relating to these operations. The future success of the Company is dependent on its ability to develop its product candidates and ultimately upon its ability to attain profitable operations. At December 31, 2018, the Company had $252,061 of cash and cash equivalents and short‑term investments on hand. In the future, the Company will be dependent on obtaining funding from third parties, such as proceeds from the issuance of debt, sale of equity and funded research and development programs, to maintain the Company's operations and meet the Company's obligations. There is no guarantee that additional equity or other financings will be available to the Company on acceptable terms, or at all. If the Company fails to obtain additional funding when needed, the Company would be forced to scale back, terminate its operations or seek to merge with or be acquired by another company. Management believes that the Company's existing cash resources will be sufficient to fund the Company's operating plan into the second half of 2020.

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The Company's consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”).

The Company has historically existed and functioned as part of the consolidated businesses of the Predecessor Company. As noted above, the Predecessor Company's setmelanotide and the MC4 receptor agonist program were transferred to the Company as part of the Corporate Reorganization on March 21, 2013. These financial statements include the results of operations of setmelanotide and the MC4 receptor agonist program from its inception. As part of the Corporate Reorganization, the Company also entered into a formal payroll services intercompany agreement with the Relamorelin Company. On November 16, 2016, the employees of the Relamorelin Company that were providing services to the Company, terminated their employment contracts with the Relamorelin Company and entered into new employment agreements with the Company. On December 15, 2016, the Relamorelin Company closed on its sale to a large pharmaceutical company. During 2016, costs have been allocated to the Company for the purposes of preparing the financial statements based on a specific identification basis or, when specific identification is not practicable, a proportional cost allocation method which allocates expenses based upon the percentage of employee time and research and development effort expended on the Company's business as compared to total employee time and research and development effort of the combined Motus and Rhythm. The proportional use basis adopted to allocate shared costs is in accordance with the guidance of SEC Staff Accounting Bulletin (“SAB”) Topic 1B, Allocation Of Expenses And Related Disclosure In Financial Statements Of Subsidiaries, Divisions Or Lesser Business Components Of Another Entity. Management has determined that the method of allocating costs to the Company is reasonable. Cost allocation was no longer required subsequent to the 2016 sale of the Relamorelin Company.

Management believes that the statements of operations include a reasonable allocation of costs and expenses incurred by the Relamorelin Company, which benefited the Company. However, such amounts may not be indicative of the actual level of costs and expenses that would have been incurred by the Company if it had operated as an independent company or of the costs and expenses expected to be incurred in the future. Management has not presented an estimate of what the expenses of the Company would have been on a standalone basis as it was not practicable to make a reasonable estimate. As such, the financial information herein may not necessarily reflect the financial position, results of operations and cash flows of the Company expected in the future or what it would have been had it been an independent company during the periods presented.

As described above, Relamorelin Company employee costs are allocated to the Company based on a proportional use method. For those employees who became employees of the Company on November 16, 2016, their full employment cost was $2,727 for the year ended December 31, 2016.

On September 22, 2017, the Company's board of directors approved a 1-for-9.17 reverse stock split of the Company's issued and outstanding shares of common stock. All share and per share amounts in the financial statements have been retrospectively adjusted for all periods presented to give effect of the reverse stock split.

On October 5, 2017, the Company filed an amended and restated certificate of incorporation with the Secretary of State of the State of Delaware to increase its authorized number of shares of common stock to 120,000,000 shares of common stock, $0.001 par value per share and 10,000,000 shares of preferred stock, $0.001 par value per share.

On October 10, 2017 the Company completed its initial public offering (“IPO”) of 8,107,500 shares of common stock at an offering price of $17.00 per share, which included the exercise in full by the underwriters of their option to purchase up to 1,057,500 additional shares of common stock. The Company received gross proceeds of approximately $137,828 or net proceeds of $125,658 after deducting underwriting discounts, commissions and estimated offering expenses. In connection with the IPO, the Company’s outstanding shares of convertible preferred stock were automatically converted into 17,406,338 shares of common stock.

On June 25, 2018 the Company completed its public offering of 6,591,800 shares of common stock at an offering price of $26.42 per share, which included the exercise in full by the underwriters of their option to purchase up to 859,800 additional shares of common stock. The Company received gross proceeds of approximately $174,155 or net proceeds of $162,878 after deducting underwriting discounts, commissions and offering expenses.

Use of Estimates

The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities and the disclosure of contingent liabilities at the date of the financial statements and the reported amounts of revenue and expenses during the reporting period. The Company bases its estimates on historical experience and other market‑specific or other relevant assumptions that it believes to be reasonable under the circumstances. This process may result in actual results differing materially from those estimated amounts used in the preparation of the financial statements if these results differ from historical experience, or other assumptions do not turn out to be substantially accurate, even if such assumptions are reasonable when made. Significant estimates relied upon in preparing these financial statements include the allocation of costs from the Relamorelin Company in accordance with SAB Topic 1B, accrued expenses, stock‑based compensation expense, the valuation allowance on the Company's deferred tax assets, and the fair value of the Series A Investor Instrument (see Note 4).

Principles of Consolidation

The consolidated financial statements include the accounts of Rhythm Pharmaceuticals, Inc. and its subsidiaries. All significant intercompany balances and transactions have been eliminated in consolidation.

Off‑Balance Sheet Risk and Concentrations of Credit Risk

Financial instruments, which potentially subject the Company to significant concentration of credit risk, consist primarily of cash and cash equivalents and short‑term investments, which are maintained at two federally insured financial institutions. The deposits held at these two institutions are in excess of federally insured limits. The Company has not experienced any losses in such accounts and management believes that the Company is not exposed to significant credit risk due to the financial position of the depository institutions in which those deposits are held. The Company has no off‑balance sheet risk, such as foreign exchange contracts, option contracts, or other foreign hedging arrangements.

Segment Information

Operating segments are defined as components of an entity about which separate discrete information is available for evaluation by the chief operating decision maker, or decision‑making group, in deciding how to allocate resources and in assessing performance. The Company views its operations and manages its business in one operating segment operating exclusively in the United States.

Cash and Cash Equivalents

The Company considers all highly liquid investments with original or remaining maturity from the date of purchase of three months or less to be cash equivalents. Cash and cash equivalents includes bank demand deposits, U.S. treasury bills and money market funds that invest primarily in U.S. government treasuries.

Short‑term Investments

Short‑term investments consist of investments with original maturities greater than 90 days, as of the date of purchase. The Company has classified its investments with maturities beyond one year as short term, based on their highly liquid nature and because such marketable securities represent the investment of cash that is available for current operations. The Company considers its investment portfolio available‑for‑sale. Accordingly, these investments are recorded at fair value, which is based on quoted market prices. Unrealized gains and losses are reported as a component of accumulated other comprehensive income (loss) in stockholders’ equity (deficit). Realized gains and losses and declines in value judged to be other than temporary are included as a component of other income (expense), net based on the specific identification method. When determining whether a decline in value is other than temporary, the Company considers various factors, including whether the Company has the intent to sell the security, and whether it is more likely than not that the Company will be required to sell the security prior to recovery of its amortized cost basis. Fair value is determined based on quoted market prices.

Restricted Cash

Restricted cash consists of security deposits in the form of letter of credits placed in a separate restricted bank accounts as required under the terms of the Company’s new lease arrangement for its corporate office in Boston, Massachusetts and the Company’s corporate travel credit cards .

Deferred Issuance Costs

Deferred issuance costs, which consist of direct incremental legal and accounting fees relating to the IPO, were capitalized and included in non-current assets. The deferred issuance costs were to be offset against IPO proceeds upon the consummation of the offering. In the event the offering was terminated, deferred issuance costs would be expensed.

The Company had capitalized $1,825 of deferred issuance costs related to a prior registration statement confidentially submitted to the Securities and Exchange Commission in 2016. In the fourth quarter of 2016, the Company wrote off these deferred issuance costs to general and administrative expenses because the offering was postponed significantly in excess of 90 days. As a result, the costs were not deemed realizable as the Company incurred similar costs in connection with its IPO in October 2017.

Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consist primarily of costs incurred in advance of services being received, including services related to clinical trial programs.


	December 31,
	2018		2017
Prepaid research and development costs	$	5,288	$	1,533
Other current assets		1,340		1,056
Prepaid expenses and other current assets	$	6,628	$	2,589

Property, Plant and Equipment

Property, Plant and Equipment consists of the following:


	Useful	December 31,
	Life	2018		2017
Leasehold improvements	*	$	1,103	$	891
Office equipment	5 years		70		70
Computers and software	3 years		411		19
Furniture and fixtures	5 years		345		227
			1,929		1,207
Less accumulated depreciation and amortization			(809)		(367)
Property, Plant and Equipment, net		$	1,120	$	840

*Shorter of asset life or lease term.

Fair Value Measurements

Fair value is the price that would be received to sell an asset or paid to transfer a liability in an orderly transaction between market participants at the measurement date. Financial assets and liabilities carried at fair value are classified and disclosed in one of the following three categories:

Level 1 — Quoted market prices in active markets for identical assets or liabilities.

Level 2 — Observable inputs other than Level 1 prices, such as quoted prices for similar assets or liabilities; quoted prices in markets that are not active; or other inputs that are observable or can be corroborated by observable market data for substantially the full term of the assets or liabilities.

Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

The Company’s cash equivalents and marketable securities at December 31, 2018 and 2017 were carried at fair value, determined according to the fair value hierarchy; see Note 4.

The carrying amounts reflected in the consolidated balance sheets for accounts payable and accrued expenses approximate their fair values due to their short-term maturities at December 31, 2018 and 2017, respectively.

2017 Series A Investor Instrument

The Company has classified its 2017 Series A Investor Instrument (See Note 4) as a liability as it is a free‑standing financial instrument. The 2017 Series A Investor Instrument was recorded at fair value upon the issuance of the Company’s series A preferred stock in January 2017, and subsequently remeasured to fair value at each reporting period. Changes in fair value of the financial instrument is recognized as a component of other income (expense), net in the statement of operations and comprehensive loss. The Company estimated the fair value of the 2017 Series A Investor Right/Obligation as the probability‑weighted present value of the expected benefit of the investment.

The Company used the Black‑Scholes option‑pricing model, which incorporates assumptions and estimates, to value the 2017 Series A Investor Call Option and assessed these assumptions and estimates on a quarterly basis as additional information impacting the assumptions was obtained. Estimates and assumptions impacting the fair value measurement include the fair value per share of the underlying series A preferred stock, the expected term of the Series A Investor Call Option, risk‑free interest rate, expected dividend yield and expected volatility of the price of the underlying preferred stock. The Company determined the fair value per share of the underlying preferred stock by taking into consideration the most recent sale of our convertible preferred stock and the investors' right to invest in a subsequent tranche. As the Company was a private company and lacked company‑specific historical and implied volatility information of its stock, it estimated its expected stock volatility based on the historical volatility of publicly traded peer companies for a term comparable to the estimated term of the Series A Investor Call Option. The risk‑free interest rate was determined by reference to the U.S. Treasury yield curve for time periods approximately equal to the estimated term of the Series A Investor Call Option. A dividend yield of zero was assumed. The fair value of the Series A Investor Instrument is determined to be the sum of the fair values of the 2017 Series A Investor Right/Obligation and the 2017 Investor Call Option.

Government Grants

The Company obtained an Orphan Products Development grant entitled “Phase 2 study of the melanocortin 4 receptor agonist RM‑493 for the treatment of Prader‑Willi syndrome” in 36 patients. The grant was awarded by the Public Health Service (“PHS”) Food and Drug Administration. The PHS grant is for a total of $999 and is effective July 2015 through June 2018 for reimbursement of expenses relating to the Phase 2 Prader‑Willi Study.

The Company recognizes government grants upon the determination that it will comply with the conditions attached to the grant arrangement and the grant will be received. Government grants are recognized in the statements of operations on a systematic basis over the periods in which the Company recognizes the related costs for which the government grant is intended to compensate. Government grants for research and development efforts are deducted in reporting the related expense in the statement of operations. Government grant income received during the year ended December 31, 2018, 2017 and 2016 of $210, zero and $642, respectively, and is included as a deduction to research and development expense in the consolidated statements of operations.

Research and Development Expenses

Costs incurred in the research and development of the Company’s products are expensed to operations as incurred. Research and development expenses consist of costs incurred in performing research and development activities, including salaries and benefits, facilities costs, overhead costs, contract services and other outside costs, both directly incurred and allocated from the Relamorelin Company. The value of goods and services received from contract research organizations or contract manufacturing organizations in the reporting period are estimated based on the level of services performed and progress in the period for which the Company has not yet received an invoice from the supplier.

Nonrefundable advance payments for goods or services to be received in the future for use in research and development activities are recorded as prepaid expenses, and expensed as the related goods are delivered or the services are performed.

Income Taxes

The Company is taxed as a C corporation for federal income tax purposes. Income taxes for the Company are recorded in accordance with FASB ASC Topic 740, Income Taxes (“ASC 740”), which provides for deferred taxes using an asset and liability approach. Income taxes have been calculated on a separate tax return basis. Certain of the Company’s activities and costs have been included in the tax returns filed by the Relamorelin Company and the LLC entity. Prior to the Corporate Reorganization, the Company’s operations were included in the tax returns filed by the Predecessor Company. The Company has filed tax returns on its own behalf since the Corporate Reorganization.

The Company accounts for income taxes under the asset and liability method, which requires the recognition of deferred tax assets and liabilities for the expected future tax consequences of events that have been included in the financial statements. Under this method, the Company determined deferred tax assets and liabilities on the basis of the differences between the financial statement and tax bases of assets and liabilities by using enacted tax rates in effect for the year in which the differences are expected to reverse. The effect of a change in tax rates on deferred tax assets and liabilities is recognized in income in the period that includes the enactment date. The Company recognized deferred tax assets to the extent that it believes that these assets are more likely than not to be realized. In making such a determination, the Company considers all available positive and negative evidence, including future reversals of existing taxable temporary differences, projected future taxable income, tax-planning strategies, and results of recent operations. If the Company determines that it would be able to realize our deferred tax assets in the future in excess of their net recorded amount, it would make an adjustment to the deferred tax asset valuation allowance, which would reduce the provision for income taxes.

The Company records uncertain tax positions in accordance with ASC 740 on the basis of a two-step process in which (1) it determines whether it is more likely than not that the tax positions will be sustained on the basis of the technical merits of the position and (2) for those tax positions that meet the more-likely-than-not recognition threshold, the Company recognizes the largest amount of tax benefit that is more than 50 percent likely to be realized upon ultimate settlement with the related tax authority.

The Company recognizes interest and penalties related to unrecognized tax benefits on the income tax expense line in the accompanying consolidated statement of operations. As of December 31, 2018, no accrued interest or penalties are included on the related tax liability line in the consolidated balance sheet.

Net Loss Per Share Attributable to Common Shareholders

Basic net loss per share attributable to common stockholders is calculated by dividing net loss attributable to common stockholders by the weighted average shares outstanding during the period, without consideration for Common Stock equivalents. Net loss attributable to common stockholders is calculated by adjusting the net loss of the Company for cumulative preferred stock dividends. During periods of income, the Company allocates participating securities a proportional share of income determined by dividing total weighted average participating securities by the sum of the total weighted average common shares and participating securities (the “two class method”). The Company's convertible preferred stock participates in any dividends declared by the Company and are therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, the Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. Diluted net loss per share attributable to common stockholders is calculated by adjusting weighted average shares outstanding for the dilutive effect of Common Stock equivalents outstanding for the period, determined using the treasury‑stock and if‑converted methods. For purposes of the diluted net loss per share attributable to common stockholders calculation, convertible preferred stock and stock options are considered to be Common Stock equivalents but have been excluded from the calculation of diluted net loss per share attributable to common stockholders, as their effect would be anti‑dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented.

The following table includes the potential common shares, presented based on amounts outstanding at each period end, that were excluded from the computation of diluted net loss per share attributable to common stockholders for the periods indicated:


	Year Ended
	December 31,
	2018		2017		2016
Stock options		2,616,530		1,832,639		1,064,394
Series A convertible preferred shares		—		—		4,362,050
Total		2,616,530		1,832,639		5,426,444

Basic and diluted earnings per share is calculated as follows:


	Year Ended
	December 31,
	2018		2017		2016
Numerator:
Net loss	$	(74,064)	$	(33,709)	$	(25,872)
Cumulative dividends on convertible preferred shares		—		(3,873)		(3,202)
Loss attributable to common shares—basic and diluted	$	(74,064)	$	(37,582)	$	(29,074)
Denominator:
Weighted-average number of common shares—basic and diluted		31,004,047		13,267,960		10,196,292
Loss per common share—basic and diluted	$	(2.39)	$	(2.83)	$	(2.85)

Patent Costs

Costs to secure and defend patents are expensed as incurred and are classified as general and administrative expenses. Patent costs were $637, $180 and $231 for the years ended December 31, 2018, 2017 and 2016, respectively.

Subsequent Events

The Company considers events or transactions that occur after the balance sheet date but prior to the issuance of the financial statements to provide additional evidence for certain estimates or to identify matters that require additional disclosure. Subsequent events have been evaluated as required.

Application of New or Revised Accounting Standards

From time to time, new accounting pronouncements are issued by the FASB and adopted by the Company as of the specified effective date. Unless otherwise discussed, the Company believes that the impact of recently issued standards that are not yet effective will not have a material impact on its financial position or results of operations upon adoption.

In April 2012, the Jump‑Start Our Business Startups Act (the “JOBS Act”) was signed into law. The JOBS Act contains provisions that, among other things, reduce certain reporting requirements for an “emerging growth company.” As an emerging growth company, the Company elected to not take advantage of the extended transition period afforded by the JOBS Act for the implementation of new or revised accounting standards, and as a result, will comply with new or revised accounting standards on the relevant dates on which adoption of such standards is required for non‑emerging growth companies.

In February 2016, the FASB issued ASU 2016‑02, Leases (Topic 842). ASU 2016‑02 which requires lessees to recognize a right-of-use asset and lease liability for most lease arrangements. The new standard is effective for annual reporting periods beginning after December 15, 2018. A modified retrospective transition approach is required to be applied to leases existing as of, or entered into after, the beginning of the earliest comparative period presented in the financial statements, with certain practical expedients available. Currently, the Company is gathering information, reviewing its portfolio of existing leases, and continuing to evaluate the potential changes to the Company’s future financial reporting and disclosures that may result from adopting this ASU. The Company plans to elect the practical expedient which will allow it to not apply the amended lease accounting guidance to comparative periods that will be presented. The Company expects that all of its lease commitments will be subject to the new standard with the cumulative effect of adoption recognized to retained earnings on January 1, 2019.

In November 2016, the FASB issued ASU No. 2016‑18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016‑18”) that changes the presentation of restricted cash and cash equivalents on the statement of cash flows. Restricted cash and restricted cash equivalents will be included with cash and cash equivalents when reconciling the beginning‑of‑period and end‑of‑period total amounts shown on the statement of cash flows. This amendment is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. The Company adopted this ASU January 1, 2018 and has applied its content to statements of cash flows for the year ended December 31, 2018, 2017 and 2016 presented herein.

In May 2017, the FASB issued ASU 2017‑09, Compensation‑Stock Compensation (Topic 718): Scope of Modification Accounting, (“ASU 2017‑09”). ASU 2017‑09 provides clarity and reduces both (1) diversity in practice and (2) cost and complexity when applying the guidance in Topic 718, to a change to the terms or conditions of a share‑based payment award. The amendments in ASU 2017‑09 should be applied prospectively to an award modified on or after the adoption date. This ASU is effective for fiscal years, and interim periods within those years, beginning after December 15, 2017. The adoption of this ASU did not have a material impact on the Company's financial position or results of operations.

In June 2018, the FASB issued ASU 2018‑07, Compensation‑Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting, (“ASU 2018-07”). ASU 2018‑07 expands the scope of Topic 718 to include share-based payment transactions for acquiring goods and services from nonemployees. An entity should apply the requirements of Topic 718 to nonemployee awards except for certain exemptions specified in the amendment. This ASU is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018, and early adoption is permitted. The Company has early adopted ASU 2018‑07 in July 2018. The guidance has been adopted using the modified-retrospective approach, which requires that unsettled equity-classified awards for which a measurement date has not been established to be measured using the adoption date fair value. The adoption of this ASU did not have a material impact on the Company's financial position or results of operations.

In August 2018, the FASB issued ASU 2018-13, Changes to the Disclosure Requirements for Fair Value Measurement (“ASU 2018-13”). ASU 2018-13 removes, modifies and adds to the disclosure requirements on fair value measurements in Topic 820. The amendments on changes in unrealized gains and losses, the range and weighted average of significant unobservable inputs used to develop Level 3 fair value measurements, and the narrative description of measurement uncertainty should be applied prospectively for only the most recent interim or annual period presented in the initial fiscal year of adoption. All other amendments should be applied retrospectively to all periods presented upon their effective date. This guidance will become effective in fiscal years beginning after December 15, 2019, including interim periods within that reporting period. Early adoption is permitted upon issuance of this updated guidance. An entity is permitted to early adopt any removed or modified disclosures upon issuance of this updated guidance and delay adoption of the additional disclosures until their effective date. The Company does not plan to early adopt this ASU, and we are currently evaluating the impact of this guidance on our consolidated financial statements.

In August 2018, the FASB issued ASU 2018-15, Customer’s Accounting for Implementation Costs in a Cloud Computing Arrangement That Is a Service Contract (“ASU 2018-15”). ASU 2018-15 helps entities evaluate the accounting for fees paid by a customer in a cloud computing arrangement (hosting arrangement) by providing guidance for determining when the arrangement includes a software license. The amendments align the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal-use software (and hosting arrangements that include an internal use software license). The accounting for the service element of a hosting arrangement that is a service contract is not affected by the amendments. This guidance will become effective in fiscal years beginning after December 15, 2019. Early adoption is permitted, including adoption in any interim period. The Company has early adopted ASU 2018‑15 in the fourth quarter of 2018 and the adoption of this ASU did not have a material impact on the Company's financial position or results of operations.

Accrued Expenses

12 Months Ended

Dec. 31, 2018

Accrued Expenses

Accrued Expenses

3. Accrued Expenses

Accrued expenses consisted of the following:


	December 31,		December 31,
	2018		2017
Research and development costs	$	2,614	$	2,771
Professional fees		858		327
Payroll related		2,410		1,094
Other		60		18
Accrued expenses	$	5,942	$	4,210

Fair Value of Financial Assets and Liability

12 Months Ended

Dec. 31, 2018

Fair Value of Financial Assets and Liability

Fair Value of Financial Assets and Liability

4. Fair Value of Financial Assets and Liability

As of December 31, 2018 and 2017, the carrying amount of cash and cash equivalents and short‑term investments was $252,061 and $148,082, respectively, which approximates fair value. Cash and cash equivalents and short‑term investments includes investments in money market funds that invest in U.S. government securities that are valued using quoted market prices. Accordingly, money market funds and government funds are categorized as Level 1 and had a total balance of $37,019 and $34,698 as of December 31, 2018 and 2017, respectively. The financial assets valued based on level 2 inputs consist of corporate debt securities and commercial paper, which consist of investments in highly-rated investment-grade corporations.

A financial liability was recognized by the Company during the year ending December 31, 2017 related to the 2017 Series A Investor Instrument. The liability was valued based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. Upon the closing of the second tranche of the 2017 Series A preferred financing in August 2017, this liability was settled.

The following tables present information about the Company's financial assets measured at fair value on a recurring basis and indicate the level of the fair value hierarchy utilized to determine such fair values:


	Fair value Measurements as of
	December 31, 2018 using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash Equivalents:
Corporate Debt Securities and Commercial Paper	$	—	$	5,976	$	—	$	5,976
Money Market Funds		37,019		—		—		37,019
Marketable Securities:
Corporate Debt Securities and Commercial Paper		—		202,519		—		202,519
Total	$	37,019	$	208,495	$	—	$	245,514


	Fair value Measurements as of
	December 31, 2017 using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash Equivalents:
Corporate Debt Securities	$	—	$	15,104	$	—	$	15,104
Money Market Funds		17,753		—		—		17,753
Marketable Securities:
Corporate Debt Securities		—		96,901		—		96,901
U.S. Treasury Securities		16,945		—		—		16,945
Total	$	34,698	$	112,005	$	—	$	146,703

Marketable Securities

The following tables summarize the Company's marketable securities:


	December 31, 2018
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Assets
Corporate Debt Securities and Commercial Paper (due within 1 year)	$	202,653	$	23	$	(157)	$	202,519
U.S. Treasury Securities (due within 1 year)		—		—		—		—
	$	202,653	$	23	$	(157)	$	202,519


	December 31, 2017
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Assets
Corporate Debt Securities (due within 1 year)	$	97,029	$	—	$	(128)	$	96,901
U.S. Treasury Securities (due within 1 year)		16,958		—		(13)		16,945
	$	113,987	$	—	$	(141)	$	113,846

Below is a roll forward of the fair value of the financial liability, the 2017 Series A Investor Instrument for the year ended December 31, 2017:


	2017 Series A Investor
	Instrument
Fair value at December 31, 2016	$	—
Fair value upon the January 2017 Initial Closing, net		328
Change in fair value		1,863
Reclassification of liability upon August 2017 Second Tranche Closing		(2,191)
Fair value at December 31, 2017	$	—

The fair value of the Series A Investor Instrument is the sum of the probability‑weighted fair value of the 2017 Investor Right/Obligation and the 2017 Series A Call Option.

The following assumptions and inputs were used in determining the fair value of the 2017 Series A Investor Call Option valued using the Black‑ Scholes option pricing model:


	August 2017 Second Tranche Closing
Series A Convertible Preferred Stock Exercise Price	$	1.00
Series A Convertible Preferred Stock Fair Value	$	1.33
Expected term		1.5 months
Expected volatility		64.0	%
Expected interest rate		0.95	%
Expected dividend yield		—

In August 2017, upon the closing of the second tranche of the series A preferred stock financing, the 2017 Series A Investor Call Option expired unexercised.

The Company estimated the fair value of the 2017 Series A Investor Right/Obligation as the probability‑weighted present value of the expected benefit of the investment. The expected benefit is the difference between the expected future value of shares issued upon the second tranche closing and the investment price for the second tranche closing. The expected future value is estimated as a weighted average of IPO and remain private scenarios, and the future value is converted to a present value assuming a closing date of August 15, 2017 and a nominal, risk‑free discount rate.

- Definition

The entire disclosure for the fair value of financial instruments (as defined), including financial assets and financial liabilities (collectively, as defined), and the measurements of those instruments as well as disclosures related to the fair value of non-financial assets and liabilities. Such disclosures about the financial instruments, assets, and liabilities would include: (1) the fair value of the required items together with their carrying amounts (as appropriate); (2) for items for which it is not practicable to estimate fair value, disclosure would include: (a) information pertinent to estimating fair value (including, carrying amount, effective interest rate, and maturity, and (b) the reasons why it is not practicable to estimate fair value; (3) significant concentrations of credit risk including: (a) information about the activity, region, or economic characteristics identifying a concentration, (b) the maximum amount of loss the entity is exposed to based on the gross fair value of the related item, (c) policy for requiring collateral or other security and information as to accessing such collateral or security, and (d) the nature and brief description of such collateral or security; (4) quantitative information about market risks and how such risks are managed; (5) for items measured on both a recurring and nonrecurring basis information regarding the inputs used to develop the fair value measurement; and (6) for items presented in the financial statement for which fair value measurement is elected: (a) information necessary to understand the reasons for the election, (b) discussion of the effect of fair value changes on earnings, (c) a description of [similar groups] items for which the election is made and the relation thereof to the balance sheet, the aggregate carrying value of items included in the balance sheet that are not eligible for the election; (7) all other required (as defined) and desired information.

+ References + Details

Preferred Stock and Common Stock	12 Months Ended
Preferred Stock and Common Stock	Dec. 31, 2018
Preferred Stock and Common Stock
Preferred and Common Stock	5. Preferred Stock and Common Stock Preferred Stock Upon the closing of the IPO in October 2017, the Series A convertible preferred stock automatically converted into shares of common stock on a 9.17‑for‑1 basis. Common Stock In March 2013, the Company issued 10,196,292 shares of common stock at a purchase price of $0.001 per share. Prior to August, 2017, the LLC entity owned all of these shares. On August 21, 2017, the LLC entity exchanged 8,578,646 of its shares of the Company's common stock for 78,666,209 shares of the Company's series A‑1 junior preferred stock and the LLC entity distributed all of its shares of the Company's series A‑1 junior preferred stock to the holders of its preferred units and the remaining 1,617,646 shares of its common stock to the holders of its common units. Following this distribution, the LLC entity no longer owns any of the Company's shares. The series A‑1 junior preferred stock is not redeemable and does not have a stated dividend or liquidation preference. These shares converted to common stock on a 9.17‑to‑1 basis upon the closing of the IPO in October 2017. In September 2017, the Company's board of directors approved a 1-for-9.17 reverse stock split of the Company's issued and outstanding shares of common stock. All shares and per share amounts in the financial statements have been retrospectively adjusted for all periods presented to give effect of the reverse stock split. On October 10, 2017 the Company completed its IPO of 8,107,500 shares of common stock at an offering price of $17.00 per share, which included the exercise in full by the underwriters of their option to purchase up to 1,057,500 additional shares of common stock. The Company received gross proceeds of approximately $137,828 or net proceeds of $125,658 after deducting underwriting discounts, commissions and estimated offering expenses. In connection with the IPO, the Company’s outstanding shares of convertible preferred stock were automatically converted into 17,406,338 shares of common stock. On April 3, 2018, in association with the Takeda license agreement, the Company issued 223,544 shares of common stock. See Note 7 for further discussion. On June 25, 2018 the Company completed its public offering of 6,591,800 shares of common stock at an offering price of $26.42 per share, which included the exercise in full by the underwriters of their option to purchase up to 859,800 additional shares of common stock. The Company received gross proceeds of approximately $174,155 or net proceeds of $162,878 after deducting underwriting discounts, commissions and offering expenses.

Stock-based Compensation

12 Months Ended

Dec. 31, 2018

Stock-based Compensation

Stock-based Compensation

6. Stock‑based Compensation

2017 Equity Incentive Plan

2017 Plan Overview

Prior to August 2015, we did not have our own equity compensation plan. In August 2015, our Board of Directors and our stockholders approved and we adopted the 2015 equity incentive plan, as amended and in effect prior to the closing of our IPO, or the 2015 Plan, which we terminated upon consummation of our IPO and replaced with the 2017 equity incentive plan, or the 2017 Plan. The 2017 Plan provides for the grant of incentive and non‑qualified stock options and restricted stock awards to employees, consultants, advisors and directors, as determined by the board of directors. As of December 31, 2018 the Company has reserved 4,793,168 shares of common stock to be issued under the Plan. The number of shares authorized under the 2017 Plan will be increased each January 1, commencing on January 1, 2017 and ending on (and including) January 1, 2027, by an amount equal to 4% of the outstanding shares of stock outstanding as of the end of the immediately preceding fiscal year. On January 1, 2018 and 2019, 1,091,366 and 1,376,429 shares, respectively, were added to the 2017 Plan. Notwithstanding the foregoing, our board of directors may act prior to January 1 for a given year to provide that there will be no such January 1 increase in the number of shares authorized under the 2017 Plan for such year, or that the increase in the number of shares authorized under the 2017 Plan for such year will be a lesser number than would otherwise occur pursuant to the preceding sentence. Shares of common stock issued upon exercise of stock options are generally issued from new shares of the Company. The Plan provides that the exercise price of incentive stock options cannot be less than 100% of the fair market value of the common stock on the date of the award for participants who own less than 10% of the total combined voting power of stock of the Company, and not less than 110% for participants who own more than 10% of the Company's voting power. Options and restricted stock granted under the Plan will vest over periods as determined by the Company's board of directors. For options granted to date, the exercise price equaled the fair value of the common stock as determined by the board of directors on the date of grant.

The Company estimates the fair value of stock‑based awards to employees and non‑employees using the Black‑Scholes option‑pricing model, which requires the input of highly subjective assumptions, including (a) the expected volatility of the underlying common stock, (b) the expected term of the award, (c) the risk‑free interest rate, and (d) expected dividends. Due to the lack of a public market for the trading of its common stock and a lack of company‑specific historical and implied volatility data, the Company based its estimate of expected volatility on the historical volatility of a group of companies in the pharmaceutical and biotechnology industries in a similar stage of development as the Company that are publicly traded. For these analyses, the Company selected companies with comparable characteristics to its own including enterprise value, risk profiles and with historical share price information sufficient to meet the expected life of the stock‑based awards. The Company computes the historical volatility data using the daily closing prices for the selected companies' shares during the equivalent period of the calculated expected term of its stock‑based awards. The Company will continue to apply this process until a sufficient amount of historical information regarding the volatility of its own stock price becomes available. The Company estimated the expected life of its employee stock options using the “simplified” method, whereby, the expected life equals the average of the vesting term and the original contractual term of the option. The risk‑free interest rates for periods within the expected life of the option are based on the U.S. Treasury yield curve in effect during the period the options were granted.

The Company was historically required to estimate forfeitures at the time of grant, and revise those estimates in subsequent periods if actual forfeitures differ from estimates. The Company used historical data to estimate pre‑vesting option forfeitures and record stock‑based compensation expense only for those awards that are expected to vest. To the extent that actual forfeitures differ from its estimates, the difference was recorded as a cumulative adjustment in the period the estimates were revised. Stock‑based compensation expense recognized in the financial statements is based on awards that are ultimately expected to vest. Upon adopting ASU 2016‑09 on January 1, 2017, the Company elected to account for forfeitures as they occur. The adoption did not have a material impact on the Company’s financial position, results of operations or cash flows.

The grant date fair value of awards subject to service‑based vesting, net of estimated forfeitures, is recognized ratably over the requisite service period, which is generally the vesting period of the respective awards. The Company's stock option awards typically vest over a service period that ranges from three to four years and includes awards with one year cliff vesting followed by ratable monthly and quarterly vesting thereafter and ratable monthly and quarterly vesting beginning on the grant date.

During the years ended December 31, 2018, 2017 and 2016, the Company granted 1,218,790, 1,112,717 and 164,229 common stock option awards to certain directors, employees and non‑employees, respectively.

Using the Black-Scholes option pricing model, the weighted average grant date fair value relating to outstanding stock options granted under the Company’s stock option plan during the year ended December 31, 2018 was $17.27.

The fair value of share options granted to employees and directors was estimated at the date of grant using the Black‑Scholes option pricing model with the following weighted‑average assumptions:


	Year ended
	December 31,
	2018		2017		2016
Risk‑free interest rate	2.73	%	1.97	%	1.39	%
Expected term (in years)	5.89		5.95		6.25
Expected volatility	62.21	%	66.18	%	74.20	%
Expected dividend yield	—		—		—

The Company has early adopted ASU 2018‑07 in July 2018. The guidance has been adopted using the modified-retrospective approach, which requires that unsettled equity-classified awards for which a measurement date has not been established to be measured using the adoption date fair value. The adoption of this ASU did not have a material impact on the Company's financial position or results of operations.

Prior to the adoption of ASU 2018-07, options granted to non-employees used an expected term of 10 years, which is the contractual term of each option. All other assumptions used to calculate the grant date fair value are generally consistent with the assumptions used for options granted to employees.

A summary of the Company's common stock option activity for the year ended December 31, 2018 is as follows:


				Weighted‑
		Weighted		Average
		Average		Remaining		Aggregate
	Number of	Exercise		Contractual		Intrinsic
	Options	Price		Term		Value
Outstanding as of December 31, 2017	1,832,639	$	7.04	8.48	$	—
Granted	1,218,790		28.45	—		—
Exercised	(316,736)		5.71	—		7,980
Cancelled	(118,163)		18.21	—		—
Outstanding as of December 31, 2018	2,616,530	$	16.67	8.58	$	29,424
Options vested and expected to vest as of December 31, 2018	2,616,530	$	16.67	8.58	$	29,424
Options exercisable at December 31, 2018	853,530	$	8.83	7.86	$	15,432

The following summarizes information about stock options at December 31, 2018 by range of exercise prices:


			Weighted
			Average		Weighted			Weighted
			Remaining		Average			Average
Range of		Number	Contractual		Exercise	Number		Exercise
Exercise Prices		Outstanding	Term		Price	Exercisable		Price
$ 4.59	$ 6.88	1,258,425	7.97	$	6.00	644,446	$	5.70
$ 7.52	$ 26.88	895,215	8.98		23.36	202,847		18.11
$ 28.00	$ 35.85	462,890	9.50		32.70	6,237		30.60
		2,616,530	8.58	$	16.67	853,530	$	8.83

The following table summarizes the classification of the Company's stock‑based compensation expenses related to stock options, restricted stock units and the employee stock purchase plan recognized in the Company's statements of operations and comprehensive loss.



	Year Ended
	December 31,
	2018		2017		2016
Research and development	$	2,793	$	927	$	507
Selling, general, and administrative		3,597		1,351		661
Total	$	6,390	$	2,278	$	1,168

Stock-based compensation expense by award type recognized during the years ended December 31, 2018, 2017 and 2016 was as follows:



	Year Ended
	December 31,
	2018		2017		2016
Stock options	$	6,241	$	2,084	$	993
Employees stock purchase plan		65		—		—
Restricted stock units		84		194		175
Total	$	6,390	$	2,278	$	1,168

During 2017, there were three awards subject to modification accounting under ASC 718-20-35-3 through 35-4. Per terms of separation with a former employee, three months of accelerated vesting was granted for the former employee’s three stock option awards. As a result, the Company recognized incremental expense for the stock option awards of $254.

As of December 31, 2018, the Company has unrecognized compensation cost of $19,897 related to non‑vested employee, non‑employee and director awards that is expected to be recognized over a weighted‑average period of 2.67 years.

LLC Incentive Plan

The Company was allocated stock compensation expense from the LLC entity's plan using the same proportional use basis for other shared costs, see Note 2. The remainder of this Note discloses the stock‑based compensation activity of the Predecessor Company and the LLC entity.

Original Plan

The Predecessor Company had one stock based compensation plan—the 2010 equity incentive plan, as amended (the “Original Plan”). The Original Plan previously provided for the grant of incentive and non‑qualified stock options and restricted stock grants to employees, consultants, advisors and directors, as determined by the board of directors of the Predecessor Company.

As a result of the Corporate Reorganization, all outstanding option grants under the Original Plan were cancelled. Each holder of a stock option that was cancelled was issued a restricted common unit of the LLC entity in its place on a one‑for‑one basis. Restricted common unit vesting agreements were contracted between the LLC entity and the restricted common unit holder granting the holder the same vesting terms as originally granted in the respective option agreement. Any unvested portion of the stock option at the Corporate Reorganization would continue to vest under those original time frames and conditions. Exercise prices were eliminated as they are not applicable to common unit instruments, and all equity incentive grants after the Corporate Reorganization were of restricted common units.

The holder of a restricted common unit is entitled to one vote per unit. After the payment of all preferential amounts to the holders of the convertible preferred units, the holder of a restricted common unit is entitled to their pro rata share of the remaining consideration, if any, based on the number of restricted common units held by the holder.

Restricted Common Units

Upon the Corporate Reorganization, all 615,685 common stock options of the Predecessor Company under the Original Plan outstanding as of March 21, 2013 were exchanged on a one‑for‑one basis for 615,685 restricted common units of the LLC entity. Vesting continued on the same schedule as originally granted per the respective option agreement.

All restricted common units granted subsequent to the Corporate Reorganization were valued at the fair value of the LLC entity's common unit on the date of grant and will be expensed over their respective service period. Forfeitures are required to be estimated at the time of grant and revised, if necessary, in subsequent periods if actual forfeitures differ from those estimates. The term “forfeitures” is distinct from “cancellations” and represents only the unvested portion of the surrendered unit. Ultimately, the actual expense recognized over the vesting period will only be for those options that vest.

A summary of the LLC entity's restricted common unit activity for the year ended December 31, 2018 is as follows:


		Weighted‑
		Average
		Grant Date
	Number of	Fair Value
	Units	Per Unit
Outstanding unvested as of December 31, 2017	23,569	$	5.06
Granted	—		—
Vested	(16,555)		5.06
Cancelled	(7,014)		5.06
Outstanding unvested as of December 31, 2018	—	$	—

2017 Employee Stock Purchase Plan

The Company’s board of directors has adopted and the Company’s stockholders have approved the 2017 Employee Stock Purchase Plan (the “2017 ESPP”), which became effective in connection with the completion of the Company’s IPO in October 2017. As of December 31, 2018 a total of 272,841 shares of common stock were reserved for issuance under this plan. In addition, The number of shares authorized under the 2017 ESPP will be increased each January 1, commencing on January 1, 2019 and ending on (and including) January 1, 2027, by an amount equal to the lesser of 1% of outstanding shares as of the end of the immediately preceding fiscal year and 682,102. On January 1, 2019, 344,107 shares were added to the 2017 ESPP. Notwithstanding the foregoing, our board of directors may act prior to January 1 of a given year to provide that there will be no such January 1 increase in the number of shares authorized under the ESPP for such year, or that the increase in the number of shares authorized under the ESPP for such year will be a lesser number than would otherwise occur pursuant to the preceding sentence. No shares were issued under this plan during the year ended December 31, 2018.

Significant Agreements	12 Months Ended
Significant Agreements	Dec. 31, 2018
Significant Agreements
Significant Agreements	7. Significant Agreements License Agreements The Predecessor Company entered into a license agreement on February 26, 2010 with Ipsen Pharma, S.A.S. (“Ipsen”) that granted full worldwide right for two programs that include the clinical candidates setmelanotide, which is in Phase 3 clinical trials, and relamorelin. As a result of the Corporate Reorganization described in Note 1, the Ipsen license was converted to separate license agreements for the setmelanotide program held by the Company and the relamorelin program held by the Relamorelin Company, respectively. Under the terms of the setmelanotide Ipsen license agreement, assuming that setmelanotide is successfully developed, receives regulatory approval and is commercialized, Ipsen may receive aggregate payments of up to $40,000 upon the achievement of certain development and commercial milestones and royalties on future product sales in the mid‑single digits. Substantially all of such aggregate payments of up to $40,000 are for milestones that may be achieved no earlier than first commercial sale of setmelanotide. In the event that the Company executes a sublicense agreement, it shall make payments to Ipsen, depending on the date of such sublicense agreement, ranging from 10% to 20% of all revenues actually received under such sublicense agreement. In July 2017, the Company made a prepayment on the first milestone event associated with this license agreement. The first milestone relates to the initiation of a Phase 3 study for setmelanotide in a pivotal multi-center human clinical trial in a large number of patients. The prepayment associated with this milestone was $1,000 and was recorded as research and development expenses during the three months ended March 31, 2018 when the milestone criteria was met in full. In January 2016, the Company entered into a license agreement with Camurus AB, or Camurus, for the use of Camurus' drug delivery technology. The contract includes a non‑refundable and non‑creditable signing fee of $500, which was paid during January 2016. The Camurus agreement also includes up to $7,750 in one‑time, non‑refundable development milestones achievable upon certain regulatory successes. The Company is also required to pay to Camurus, mid to mid‑high single digit royalties, on a product‑by‑product and country‑by‑country basis of annual net sales, until the later of (i) 10 years after the date of first commercial sale of such product in such country; or (ii) the expiration of the last to expire valid claim of all licensed patent rights in such country covering such product. The Company is also required to pay one‑time, non‑refundable, non‑creditable sales milestones upon the achievement of certain sales levels for such product that cannot be in excess of $57,000. In March 2017, the Company achieved the first milestone event associated with this license agreement. The Company completed the first manufactured batch using the Camurus drug delivery technology and filed an investigational new drug application with the FDA. The fee associated with this milestone was $250. In December 2017, the Company achieved the second milestone event associated with this license agreement. The Company completed the Phase I proof of concept study using the Camurus drug delivery technology. The fee associated with this second milestone was $1,000 and was recorded as research and development expense. In March 2018, the Company entered into a license agreement with Takeda, for the rights of a program that includes the clinical candidate RM-853, which is a GOAT inhibitor, which is currently in preclinical development for PWS. Pursuant to the license agreement the Company was required to pay a non‑refundable and non‑creditable signing fee, which the Company settled by issuing on April 3, 2018, 223,544 shares of common stock valued at $4,448. Under the terms of the license agreement, assuming that RM-853 is successfully developed, receives regulatory approval and is commercialized, the Company is also required to pay up to $70,000 in one‑time, non‑refundable development milestone payments upon the achievement of certain clinical and regulatory milestones. The Company is also required to pay up to $70,000 in one‑time, non‑refundable, non‑creditable sales milestone payments upon the achievement of certain sales levels. The Company is also required to pay to Takeda, mid to mid‑high single digit royalties (subject to certain potential reductions over time), on a product‑by‑product and country‑by‑country basis of annual net sales, of each product in such country, beginning on the first commercial sale of a product in such country, and continuing until the latest of (i) 10 years after the date of first commercial sale of such product in such country; or (ii) the expiration of the last to expire valid claim of a Takeda patents covering the composition or use of such product in such country; or (iii) the expiration of all regulatory exclusivity for such product in such country. The Company recorded the fair value of the common stock to be issued to the licensors as research and development expense, as the license does not have a future alternative use, in accordance with ASC Topic 730, Research and Development.

Commitments and Contingencies

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies

Commitments and Contingencies

8. Commitments and Contingencies

The Company’s corporate headquarters are located in Boston, Massachusetts, where it leases approximately 6,800 square feet of office space pursuant to lease agreements expiring in May 2021. This facility houses the Company’s research, clinical, regulatory, commercial and administrative personnel. In August 2018, the Company amended its existing lease agreement and the new lease term will commence in May 2019 and has a term of six years with a five-year renewal option to extend the lease. The Company will lease approximately 13,600 square feet of office space. Rent expense for the years ended December 31, 2018, 2017 and 2016 was $359, $215 and $179, respectively.

Future minimum payments under the Lease Agreement as of December 31, 2018, are as follows:


2019	$	425
2020		786
2021		802
2022		818
2023		834
Thereafter		1,353
Total	$	5,018

Related-Party Transactions	12 Months Ended
Related-Party Transactions	Dec. 31, 2018
Related-Party Transactions
Related-Party Transactions	9. Related‑Party Transactions Expenses paid directly to consultants and vendors considered to be related parties amounted to $2,005, $2,400 and $619 for the years ended December 31, 2018, 2017 and 2016, respectively. Outstanding payments due to these related parties as of December 31, 2018 and 2017 were $260 and $90, respectively and were included within accounts payable on the balance sheet. The Company shared costs with the Relamorelin Company, its affiliate, including payroll, facilities, information technology and other research and development and general and administrative overhead costs. Additionally, the Relamorelin Company had paid certain Company expenses directly on behalf of the Company. Shared costs incurred by the Relamorelin Company and Company expenses paid by the Relamorelin Company on behalf of the Company are allocated from the Relamorelin Company to the Company as described in Note 1 and Note 2. These net costs totaled $1,570 for the year ended December 31, 2016. The Relamorelin Company was sold to a large pharmaceutical company on December 15, 2016.

Income Tax

12 Months Ended

Dec. 31, 2018

Income Tax

Income Tax

10. Income Tax

In the Company's financial statements, income taxes, including deferred tax balances, have been calculated on a separate tax return basis. Certain of the Company's activities and costs have been included in the tax returns filed by the Relamorelin Company and the LLC entity. Prior to the Corporate Reorganization, the Company's operations were included in the tax returns filed by the Predecessor Company. The Company has filed tax returns on its own behalf since the Corporate Reorganization.

For the years ended December 31, 2018, 2017 and 2016 the Company did not have a current or deferred income tax expense or benefit as the entity has incurred losses since inception and has provided a full valuation allowance against its deferred tax assets.

A reconciliation of the income tax benefit at the federal statutory tax rate to the Company's effective income tax rate follows:


	As of
	December 31,
	2018		2017		2016
Statutory tax rate	21.00	%	34.00	%	34.00	%
State tax, net of federal benefit	6.90	%	4.08	%	2.63	%
Research and development credit	1.52	%	1.87	%	1.34	%
Orphan drug credit	1.95	%	2.29	%	2.15	%
Non deductible deferred issuance costs	—	%	—	%	(2.40)	%
Tax law change	—	%	(27.98)	%	—	%
Stock compensation	0.46	%	(1.84)	%	—	%
Investor instrument revaluation	—	%	(1.88)	%	—	%
Other	0.05	%	(0.07)	%	(1.32)	%
Change in valuation allowance	(31.88)	%	(10.47)	%	(36.40)	%
Effective tax rate	—	%	—	%	—	%

The principal components of the Company's deferred tax assets are as follows:


	As of
	December 31,
	2018		2017
Deferred tax assets:
Net operating loss carryforwards	$	35,776	$	18,325
Research and development credits		3,456		2,317
Orphan drug credit		4,286		2,333
Capitalized license fee		1,760		500
Other		2,401		599
Total gross deferred tax assets		47,679		24,074
Valuation allowance		(47,679)		(24,074)
Net deferred tax assets	$	—	$	—

On December 22, 2017, H.R.1, known as the Tax Cuts and Jobs Act ("The Act"), was enacted. This new law did not have a significant impact on the Company’s consolidated financial statements for the year ended December 31, 2017 because it maintains a valuation allowance on all of its net operating losses and other deferred tax assets. However, the reduction of the United States federal corporate tax rate from 35% to 21% resulted in increases to the amounts reflected in “Tax law change” in the Company’s tax reconciliation table above for the year ended December 31, 2017.

As permitted by SEC Staff Accounting Bulletin No. 118, Income Tax Accounting Implications of the Tax Cuts and Jobs Act, the Company recorded provisional estimates during the year ended December 31, 2017, and have subsequently finalized the accounting analysis based on the guidance, interpretations and data available as of December 31, 2018. No further adjustments were made upon finalization of our accounting analysis.

ASC 740 requires a valuation allowance to reduce the deferred tax assets reported if, based on the weight of available evidence, it is more likely than not that some portion or all of the deferred tax assets will not be realized. After consideration of all the evidence, both positive and negative, the Company has recorded a full valuation allowance against its deferred tax assets at December 31, 2018 and 2017, because the Company's management has determined that is it more likely than not that these assets will not be realized. The increase in the valuation allowance of $23,605 in 2018 and $3,586 in 2017 primarily relates to the net loss incurred by the Company during each period, partially offset by the federal rate reduction from 34% to 21% as a result of The Act in 2017.

As of December 31, 2018, the Company had federal and state net operating loss carryforwards of approximately $136,239 and $113,383, respectively, which are available to reduce future taxable income. The net operating loss carryforwards expire at various times beginning in 2033 for federal and state purposes. Of the federal net operating loss carryforwards at December 31, 2018, $63,073 can be carried forward indefinitely.

As of December 31, 2018, the Company had federal and state research tax credits of approximately $2,857 and $758, respectively, which may be used to offset future tax liabilities. Additionally, as of 2018, the Company had a federal orphan drug credit related to qualifying research of $4,286. These tax credit carryforwards will begin to expire at various times beginning in 2033 for federal purposes and 2028 for state purposes.

The net operating loss and tax credit carryforwards are subject to review and possible adjustment by the Internal Revenue Service and state tax authorities. Net operating loss and tax credit carryforwards may become subject to an annual limitation in the event of certain cumulative changes in the ownership interest of significant stockholders over a three‑year period in excess of 50%, as defined under Sections 382 and 383 of the Internal Revenue Code, respectively, as well as similar state provisions and other provisions within the Internal Revenue Code. This could limit the amount of tax attributes that can be utilized annually to offset future taxable income or tax liabilities. The amount of the annual limitation is determined based on the value of the Company immediately prior to the ownership change. Subsequent ownership changes may further affect the limitation in future years.

The Company has not recorded any reserves for uncertain tax positions as of December 31, 2018 and 2017. The Company has not, as yet, conducted a study of research and development credit carryforwards. This study may result in an adjustment to the Company's research and development credit carryforwards; however, until a study is completed and any adjustment is known, no amounts are being presented as an uncertain tax position. A full valuation allowance has been provided against the Company's research and development credits and, if an adjustment is required, this adjustment would be offset by an adjustment to the valuation allowance. Thus, there would be no impact to the balance sheets or statements of operations and comprehensive loss if an adjustment were required.

Interest and penalty charges, if any, related to unrecognized tax benefits will be classified as income tax expense in the accompanying statements of operations and comprehensive loss. As of December 31, 2018 and 2017, the Company had no accrued interest or penalties related to uncertain tax positions.

The Company is subject to examination by the U.S. federal, state and local income tax authorities for tax years 2013 forward. The Company is not currently under examination by the Internal Revenue Service or any other jurisdictions for any tax years.

Selected Quarterly Financial Data (unaudited)

12 Months Ended

Dec. 31, 2018

Selected Quarterly Financial Data (unaudited)

Selected Quarterly Financial Data (unaudited)

11. Selected Quarterly Financial Data (unaudited)

The following table contains selected quarterly financial information from 2018 and 2017. The Company believes that the following information reflects all normal recurring adjustments necessary for a fair statement of the information for the periods presented. The operating results for any quarter are not necessarily indicative of results for any future period.


	Three months ended
	March 31,		June 30,		September 30,		December 31,
	2018		2018		2018		2018

Total revenue	$	—	$	—	$	—	$	—
Total operating expenses		17,001		15,021		19,244		27,151
Other income (expense), net:		542		609		1,558		1,644
Net loss and comprehensive loss		(16,459)		(14,412)		(17,686)		(25,507)
Net loss attributable to common stockholders	$	(16,459)	$	(14,412)	$	(17,686)	$	(25,507)
Net loss attributable to common stockholders per common share, basic and diluted	$	(0.60)	$	(0.52)	$	(0.52)	$	(0.74)

	Three months ended
	March 31,		June 30,		September 30,		December 31,
	2017		2017		2017		2017

Total revenue	$	—	$	—	$	—	$	—
Total operating expenses		6,389		6,754		8,286		10,983
Other income (expense), net:		29		(48)		(1,730)		452
Net loss and comprehensive loss		(6,360)		(6,802)		(10,016)		(10,531)
Net loss attributable to common stockholders	$	(7,526)	$	(8,008)	$	(11,429)	$	(10,619)
Net loss attributable to common stockholders per common share, basic and diluted	$	(0.74)	$	(0.78)	$	(1.78)	$	(0.41)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Summary of Significant Accounting Policies

Basis of Presentation

Use of Estimates

Principles of Consolidation

Off-Balance Sheet Risk and Concentrations of Credit Risk

Off‑Balance Sheet Risk and Concentrations of Credit Risk

Segment Information

Cash and Cash Equivalents

Short-term Investments

Short‑term Investments

Restricted Cash

Deferred Issuance Costs

Prepaid Expenses and Other Current Assets

Prepaid expenses and other current assets consist primarily of costs incurred in advance of services being received, including services related to clinical trial programs.


	December 31,
	2018		2017
Prepaid research and development costs	$	5,288	$	1,533
Other current assets		1,340		1,056
Prepaid expenses and other current assets	$	6,628	$	2,589

Property, Plant and Equipment

Property, Plant and Equipment consists of the following:


	Useful	December 31,
	Life	2018		2017
Leasehold improvements	*	$	1,103	$	891
Office equipment	5 years		70		70
Computers and software	3 years		411		19
Furniture and fixtures	5 years		345		227
			1,929		1,207
Less accumulated depreciation and amortization			(809)		(367)
Property, Plant and Equipment, net		$	1,120	$	840

*Shorter of asset life or lease term.

Fair Value Measurements

Level 1 — Quoted market prices in active markets for identical assets or liabilities.

Level 3 — Unobservable inputs that are supported by little or no market activity and that are significant to the fair value of the assets or liabilities.

The Company’s cash equivalents and marketable securities at December 31, 2018 and 2017 were carried at fair value, determined according to the fair value hierarchy; see Note 4.

2017 Series A Investor Instrument

Government Grants

Research and Development Expenses

Income Taxes

Net Loss Per Share Attributable to Common Shareholders


	Year Ended
	December 31,
	2018		2017		2016
Stock options		2,616,530		1,832,639		1,064,394
Series A convertible preferred shares		—		—		4,362,050
Total		2,616,530		1,832,639		5,426,444

Basic and diluted earnings per share is calculated as follows:


	Year Ended
	December 31,
	2018		2017		2016
Numerator:
Net loss	$	(74,064)	$	(33,709)	$	(25,872)
Cumulative dividends on convertible preferred shares		—		(3,873)		(3,202)
Loss attributable to common shares—basic and diluted	$	(74,064)	$	(37,582)	$	(29,074)
Denominator:
Weighted-average number of common shares—basic and diluted		31,004,047		13,267,960		10,196,292
Loss per common share—basic and diluted	$	(2.39)	$	(2.83)	$	(2.85)

Patent Costs

Subsequent Events

Application of New or Revised Accounting Standards

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Summary of Significant Accounting Policies

Schedule of prepaid expenses and other current assets


	December 31,
	2018		2017
Prepaid research and development costs	$	5,288	$	1,533
Other current assets		1,340		1,056
Prepaid expenses and other current assets	$	6,628	$	2,589

Schedule of property, plant and equipment


	Useful	December 31,
	Life	2018		2017
Leasehold improvements	*	$	1,103	$	891
Office equipment	5 years		70		70
Computers and software	3 years		411		19
Furniture and fixtures	5 years		345		227
			1,929		1,207
Less accumulated depreciation and amortization			(809)		(367)
Property, Plant and Equipment, net		$	1,120	$	840

*Shorter of asset life or lease term.

Schedule of potential common shares, presented based on amounts outstanding at each period end, excluded from computation of diluted net loss per share attributable to common stockholders


	Year Ended
	December 31,
	2018		2017		2016
Stock options		2,616,530		1,832,639		1,064,394
Series A convertible preferred shares		—		—		4,362,050
Total		2,616,530		1,832,639		5,426,444

Schedule of basic and diluted earnings per share


	Year Ended
	December 31,
	2018		2017		2016
Numerator:
Net loss	$	(74,064)	$	(33,709)	$	(25,872)
Cumulative dividends on convertible preferred shares		—		(3,873)		(3,202)
Loss attributable to common shares—basic and diluted	$	(74,064)	$	(37,582)	$	(29,074)
Denominator:
Weighted-average number of common shares—basic and diluted		31,004,047		13,267,960		10,196,292
Loss per common share—basic and diluted	$	(2.39)	$	(2.83)	$	(2.85)

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2018

Accrued Expenses

Schedule of accrued expenses


	December 31,		December 31,
	2018		2017
Research and development costs	$	2,614	$	2,771
Professional fees		858		327
Payroll related		2,410		1,094
Other		60		18
Accrued expenses	$	5,942	$	4,210

Fair Value of Financial Assets and Liability (Tables)

12 Months Ended

Dec. 31, 2018

Fair Value of Financial Assets and Liability

Schedule of financial assets and liabilities measured at fair value on a recurring basis


	Fair value Measurements as of
	December 31, 2018 using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash Equivalents:
Corporate Debt Securities and Commercial Paper	$	—	$	5,976	$	—	$	5,976
Money Market Funds		37,019		—		—		37,019
Marketable Securities:
Corporate Debt Securities and Commercial Paper		—		202,519		—		202,519
Total	$	37,019	$	208,495	$	—	$	245,514


	Fair value Measurements as of
	December 31, 2017 using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash Equivalents:
Corporate Debt Securities	$	—	$	15,104	$	—	$	15,104
Money Market Funds		17,753		—		—		17,753
Marketable Securities:
Corporate Debt Securities		—		96,901		—		96,901
U.S. Treasury Securities		16,945		—		—		16,945
Total	$	34,698	$	112,005	$	—	$	146,703

Schedule of marketable securities


	December 31, 2018
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Assets
Corporate Debt Securities and Commercial Paper (due within 1 year)	$	202,653	$	23	$	(157)	$	202,519
U.S. Treasury Securities (due within 1 year)		—		—		—		—
	$	202,653	$	23	$	(157)	$	202,519


	December 31, 2017
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Assets
Corporate Debt Securities (due within 1 year)	$	97,029	$	—	$	(128)	$	96,901
U.S. Treasury Securities (due within 1 year)		16,958		—		(13)		16,945
	$	113,987	$	—	$	(141)	$	113,846

2017 Series A Investor Instrument

Fair Value of Financial Assets and Liability

Schedule of the fair value of financial liabilities


	2017 Series A Investor
	Instrument
Fair value at December 31, 2016	$	—
Fair value upon the January 2017 Initial Closing, net		328
Change in fair value		1,863
Reclassification of liability upon August 2017 Second Tranche Closing		(2,191)
Fair value at December 31, 2017	$	—

Schedule of assumptions and inputs were used in determining fair value using Black Scholes option pricing model


	August 2017 Second Tranche Closing
Series A Convertible Preferred Stock Exercise Price	$	1.00
Series A Convertible Preferred Stock Fair Value	$	1.33
Expected term		1.5 months
Expected volatility		64.0	%
Expected interest rate		0.95	%
Expected dividend yield		—

Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2018

Stock-based Compensation

Summary of common stock option activity


				Weighted‑
		Weighted		Average
		Average		Remaining		Aggregate
	Number of	Exercise		Contractual		Intrinsic
	Options	Price		Term		Value
Outstanding as of December 31, 2017	1,832,639	$	7.04	8.48	$	—
Granted	1,218,790		28.45	—		—
Exercised	(316,736)		5.71	—		7,980
Cancelled	(118,163)		18.21	—		—
Outstanding as of December 31, 2018	2,616,530	$	16.67	8.58	$	29,424
Options vested and expected to vest as of December 31, 2018	2,616,530	$	16.67	8.58	$	29,424
Options exercisable at December 31, 2018	853,530	$	8.83	7.86	$	15,432

Schedule of range of exercise prices of stock options


			Weighted
			Average		Weighted			Weighted
			Remaining		Average			Average
Range of		Number	Contractual		Exercise	Number		Exercise
Exercise Prices		Outstanding	Term		Price	Exercisable		Price
$ 4.59	$ 6.88	1,258,425	7.97	$	6.00	644,446	$	5.70
$ 7.52	$ 26.88	895,215	8.98		23.36	202,847		18.11
$ 28.00	$ 35.85	462,890	9.50		32.70	6,237		30.60
		2,616,530	8.58	$	16.67	853,530	$	8.83

Summary of stock-based compensation expense by award type



	Year Ended
	December 31,
	2018		2017		2016
Stock options	$	6,241	$	2,084	$	993
Employees stock purchase plan		65		—		—
Restricted stock units		84		194		175
Total	$	6,390	$	2,278	$	1,168

Summary of restricted common unit activity


		Weighted‑
		Average
		Grant Date
	Number of	Fair Value
	Units	Per Unit
Outstanding unvested as of December 31, 2017	23,569	$	5.06
Granted	—		—
Vested	(16,555)		5.06
Cancelled	(7,014)		5.06
Outstanding unvested as of December 31, 2018	—	$	—

2017 Equity Incentive Plan

Stock-based Compensation

Summary of stock-based compensation expense



	Year Ended
	December 31,
	2018		2017		2016
Research and development	$	2,793	$	927	$	507
Selling, general, and administrative		3,597		1,351		661
Total	$	6,390	$	2,278	$	1,168

2017 Equity Incentive Plan | Employees and directors

Stock-based Compensation

Schedule of significant assumptions used to compute the fair values of employee and director stock options awarded


	Year ended
	December 31,
	2018		2017		2016
Risk‑free interest rate	2.73	%	1.97	%	1.39	%
Expected term (in years)	5.89		5.95		6.25
Expected volatility	62.21	%	66.18	%	74.20	%
Expected dividend yield	—		—		—

Commitments and Contingencies (Tables)

12 Months Ended

Dec. 31, 2018

Commitments and Contingencies

Schedule of future minimum payments under the operating lease agreements


2019	$	425
2020		786
2021		802
2022		818
2023		834
Thereafter		1,353
Total	$	5,018

Income Tax (Tables)

12 Months Ended

Dec. 31, 2018

Income Tax

Schedule of reconciliation of the income tax benefit at the federal statutory tax rate to effective income tax rate


	As of
	December 31,
	2018		2017		2016
Statutory tax rate	21.00	%	34.00	%	34.00	%
State tax, net of federal benefit	6.90	%	4.08	%	2.63	%
Research and development credit	1.52	%	1.87	%	1.34	%
Orphan drug credit	1.95	%	2.29	%	2.15	%
Non deductible deferred issuance costs	—	%	—	%	(2.40)	%
Tax law change	—	%	(27.98)	%	—	%
Stock compensation	0.46	%	(1.84)	%	—	%
Investor instrument revaluation	—	%	(1.88)	%	—	%
Other	0.05	%	(0.07)	%	(1.32)	%
Change in valuation allowance	(31.88)	%	(10.47)	%	(36.40)	%
Effective tax rate	—	%	—	%	—	%

Schedule of principal components of deferred tax assets


	As of
	December 31,
	2018		2017
Deferred tax assets:
Net operating loss carryforwards	$	35,776	$	18,325
Research and development credits		3,456		2,317
Orphan drug credit		4,286		2,333
Capitalized license fee		1,760		500
Other		2,401		599
Total gross deferred tax assets		47,679		24,074
Valuation allowance		(47,679)		(24,074)
Net deferred tax assets	$	—	$	—

Selected Quarterly Financial Data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Selected Quarterly Financial Data (unaudited)

Schedule of Quarterly Financial Information


	Three months ended
	March 31,		June 30,		September 30,		December 31,
	2018		2018		2018		2018

Total revenue	$	—	$	—	$	—	$	—
Total operating expenses		17,001		15,021		19,244		27,151
Other income (expense), net:		542		609		1,558		1,644
Net loss and comprehensive loss		(16,459)		(14,412)		(17,686)		(25,507)
Net loss attributable to common stockholders	$	(16,459)	$	(14,412)	$	(17,686)	$	(25,507)
Net loss attributable to common stockholders per common share, basic and diluted	$	(0.60)	$	(0.52)	$	(0.52)	$	(0.74)

	Three months ended
	March 31,		June 30,		September 30,		December 31,
	2017		2017		2017		2017

Total revenue	$	—	$	—	$	—	$	—
Total operating expenses		6,389		6,754		8,286		10,983
Other income (expense), net:		29		(48)		(1,730)		452
Net loss and comprehensive loss		(6,360)		(6,802)		(10,016)		(10,531)
Net loss attributable to common stockholders	$	(7,526)	$	(8,008)	$	(11,429)	$	(10,619)
Net loss attributable to common stockholders per common share, basic and diluted	$	(0.74)	$	(0.78)	$	(1.78)	$	(0.41)

Nature of Business (Details) $ in Thousands	3 Months Ended								12 Months Ended
Nature of Business (Details) $ in Thousands	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Nature of Business
Number of product candidates \| item									2
Net loss	$ 25,507	$ 17,686	$ 14,412	$ 16,459	$ 10,531	$ 10,016	$ 6,802	$ 6,360	$ 74,064	$ 33,709	$ 25,872
Accumulated deficit	184,602				110,252				184,602	110,252
Total revenue									0	0	$ 0
Carrying amount of cash and cash equivalents and short term investments	$ 252,061				$ 148,082				$ 252,061	$ 148,082

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1A
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669619-108580

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=109239629&loc=SL4573702-111684

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 220
-SubTopic 10
-Section 45
-Paragraph 1B
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=114867106&loc=SL7669625-108580

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (c)(1)
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Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 22
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8736-108599

Reference 6: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
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-Section 55
-Paragraph 4J
-URI http://asc.fasb.org/extlink&oid=84234705&loc=SL4591551-111686

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3000-108585

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 45
-Paragraph 19
-URI http://asc.fasb.org/extlink&oid=108774443&loc=SL4569616-111683

Reference 9: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 32
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=115929826&loc=d3e8933-108599

Reference 10: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 810
-SubTopic 10
-Section 55
-Paragraph 4K
-URI http://asc.fasb.org/extlink&oid=84234705&loc=SL4591552-111686

+ Details

Summary of Significant Accounting Policies (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Jun. 25, 2018	Oct. 10, 2017	Dec. 31, 2018	Dec. 31, 2017	Oct. 05, 2017	Mar. 31, 2013
Common stock shares authorized			120,000,000	120,000,000
Common stock, par value per share			$ 0.001	$ 0.001		$ 0.001
Temporary equity, shares authorized			10,000,000	10,000,000
Temporary equity, par value per share			$ 0.001	$ 0.001
Issuance of stock (in shares)	6,591,800
Price per share	$ 26.42
Gross proceeds	$ 174,155		$ 162,878	$ 125,658
Net proceeds	$ 162,878
Common stock, outstanding			34,410,725	27,284,140
IPO
Issuance of stock (in shares)		8,107,500
Price per share		$ 17.00
Gross proceeds		$ 137,828
Net proceeds		$ 125,658
Conversion of Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock into common stock (in shares)		17,406,338
Underwriter option to purchase
Issuance of stock (in shares)	859,800
Maximum \| Underwriter option to purchase
Issuance of stock (in shares)		1,057,500
Common stock
Common stock shares authorized					120,000,000
Common stock, par value per share					$ 0.001
Preferred stock
Preferred stock, authorized					10,000,000
Preferred stock par value per share					$ 0.001

Summary of Significant Accounting Policies - Additional Information (Details) $ in Thousands		12 Months Ended
	Sep. 22, 2017	Dec. 31, 2018 USD ($) segment item	Dec. 31, 2016 USD ($)	Dec. 31, 2017 USD ($)
Summary of Significant Accounting Policies
Employment cost			$ 2,727
Stock split	0.109051
Off Balance Sheet Risk and Concentrations of Credit Risk
Number of federally insured financial institutions \| item		2
Segment Information
Number of operating segments \| segment		1
Deferred Issuance Costs
Capitalized deferred issuance costs		$ 1,825
Prepaid Expenses and Other Current Assets
Prepaid research and development costs		5,288		$ 1,533
Other current assets		1,340		1,056
Prepaid expenses and other current assets		$ 6,628		$ 2,589
Investor Instrument
Expected dividend yield (as a percent)		0.00%

Summary of Significant Accounting Policies - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment
Property, Plant and Equipment, gross	$ 1,929	$ 1,207
Less accumulated depreciation and amortization	(809)	(367)
Property, Plant and Equipment, net	1,120	840
Leasehold improvements
Property, Plant and Equipment
Property, Plant and Equipment, gross	$ 1,103	891
Office equipment
Property, Plant and Equipment
Useful Life	5 years
Property, Plant and Equipment, gross	$ 70	70
Computers and software
Property, Plant and Equipment
Useful Life	3 years
Property, Plant and Equipment, gross	$ 411	19
Furniture and fixtures
Property, Plant and Equipment
Useful Life	5 years
Property, Plant and Equipment, gross	$ 345	$ 227

Summary of Significant Accounting Policies - Government Grants (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2018 USD ($) person	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Government Grant
Expected dividend yield (as a percent)	0.00%
Number of patients \| person	36
Amount of grants awarded from the Public Health Service	$ 999
Accrued interest or penalties	0
Government Grants \| Research and development
Government Grant
Government grant income	$ 210	$ 0	$ 642

Summary of Significant Accounting Policies - Shares Excluded For EPS (Details) - shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Employees stock purchase plan
Shares excluded from the computation of diluted net loss per share
Unvested shares from share-based compensation that were anti-dilutive	2,616,530	1,832,639	1,064,394
Series A convertible preferred shares
Shares excluded from the computation of diluted net loss per share
Unvested shares from share-based compensation that were anti-dilutive			4,362,050
Series A convertible preferred shares \| Employees stock purchase plan
Shares excluded from the computation of diluted net loss per share
Unvested shares from share-based compensation that were anti-dilutive	2,616,530	1,832,639	5,426,444

Summary of Significant Accounting Policies - Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net loss	$ (25,507)	$ (17,686)	$ (14,412)	$ (16,459)	$ (10,531)	$ (10,016)	$ (6,802)	$ (6,360)	$ (74,064)	$ (33,709)	$ (25,872)
Cumulative dividends on convertible preferred shares										(3,873)	(3,202)
Loss attributable to common shares—basic and diluted	$ (25,507)	$ (17,686)	$ (14,412)	$ (16,459)	$ (10,619)	$ (11,429)	$ (8,008)	$ (7,526)	$ (74,064)	$ (37,582)	$ (29,074)
Weighted—average number of common shares—basic and diluted									31,004,047	13,267,960	10,196,292
Loss per common share—basic and diluted	$ (0.74)	$ (0.52)	$ (0.52)	$ (0.60)	$ (0.41)	$ (1.78)	$ (0.78)	$ (0.74)	$ (2.39)	$ (2.83)	$ (2.85)
Costs to secure and defend patents									$ 28,080	$ 9,518	$ 6,311
Patent Costs
Costs to secure and defend patents									$ 637	$ 180	$ 231

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

+ Details

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Accrued Expenses
Research and development costs	$ 2,614	$ 2,771
Professional fees	858	327
Payroll related	2,410	1,094
Other	60	18
Accrued expenses	$ 5,942	$ 4,210

Fair Value of Financial Assets and Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value of Financial Assets and Liability
Carrying amount of cash and cash equivalents and short term investments	$ 252,061	$ 148,082
Fair value of financial assets and liability
Marketable Securities:	202,653	113,987
Fair value of assets	202,519	113,846
Corporate Debt Securities
Fair value of financial assets and liability
Marketable Securities:	202,653	97,029
Government Funds
Fair value of financial assets and liability
Marketable Securities:		16,958
Recurring \| Estimated fair value
Fair value of financial assets and liability
Fair value of assets	245,514	146,703
Recurring \| Corporate Debt Securities \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:	5,976
Marketable Securities:	202,519	96,901
Recurring \| Money Market Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:	37,019	17,753
Recurring \| Government Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:		15,104
Marketable Securities:		16,945
Recurring \| Level 1 \| Estimated fair value
Fair value of financial assets and liability
Fair value of assets	37,019	34,698
Recurring \| Level 1 \| Money Market Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:	37,019	17,753
Recurring \| Level 1 \| Government Funds \| Estimated fair value
Fair value of financial assets and liability
Marketable Securities:		16,945
Recurring \| Level 2 \| Estimated fair value
Fair value of financial assets and liability
Fair value of assets	208,495	112,005
Recurring \| Level 2 \| Corporate Debt Securities \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:	5,976
Marketable Securities:	$ 202,519	96,901
Recurring \| Level 2 \| Government Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:		$ 15,104

Fair Value of Financial Assets and Liability - Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Fair Value of Financial Assets and Liability
Amortized Cost	$ 202,653	$ 113,987
Gross Unrealized Gains	23
Gross Unrealized Losses	(157)	(141)
Fair Value	202,519	113,846
Corporate Debt Securities
Fair Value of Financial Assets and Liability
Amortized Cost	202,653	97,029
Gross Unrealized Gains	23
Gross Unrealized Losses	(157)	(128)
Fair Value	$ 202,519	96,901
Government Funds
Fair Value of Financial Assets and Liability
Amortized Cost		16,958
Gross Unrealized Losses		(13)
Fair Value		$ 16,945

Fair Value of Financial Assets and Liability - Financial Liabilities (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Jun. 25, 2018
Assumption used to estimate the fair value of asset and liability by utilizing the Black-Scholes option pricing model
Fair Value			$ 26.42
Expected dividend yield (as a percent)	0.00%
2017 Series A Investor Instrument
Roll forward of the fair value of financial liabilities
Fair value upon initial closing, net		$ 328
Change in fair value through the date of settlement		1,863
Reclassification of liability upon August 2017 Second Tranche Closing		$ (2,191)
August 2017 Second Tranche Closing \| 2017 Series A Investor Instrument
Assumption used to estimate the fair value of asset and liability by utilizing the Black-Scholes option pricing model
Exercise Price		$ 1.00
Fair Value		$ 1.33
Expected term		1 month 15 days
Expected volatility (as a percent)		64.00%
Expected interest rate (as a percent)		0.95%

Preferred and Common Stock (Details) $ / shares in Units, $ in Thousands						12 Months Ended
	Jun. 25, 2018 USD ($) $ / shares shares	Apr. 03, 2018 shares	Oct. 10, 2017 USD ($) $ / shares shares	Sep. 22, 2017	Aug. 21, 2017 shares	Dec. 31, 2018 USD ($) $ / shares shares	Dec. 31, 2017 USD ($) $ / shares shares	Oct. 05, 2017 $ / shares	Mar. 31, 2013 $ / shares shares
Preferred and Common Stock
Common stock, issued						34,410,725	27,284,140		10,196,292
Issuance of stock (in shares)	6,591,800
Remaining shares distributed to holders of common stock					1,617,646
Temporary equity, par value per share \| $ / shares						$ 0.001	$ 0.001
Price per share \| $ / shares	$ 26.42
Gross proceeds \| $	$ 174,155					$ 162,878	$ 125,658
Net proceeds \| $	$ 162,878
Purchase price per share \| $ / shares						$ 0.001	$ 0.001		$ 0.001
Conversion ratio							9.17
Stock split				0.109051
IPO
Preferred and Common Stock
Issuance of stock (in shares)			8,107,500
Price per share \| $ / shares			$ 17.00
Gross proceeds \| $			$ 137,828
Net proceeds \| $			$ 125,658
Conversion of Series A Convertible Preferred Stock and Series A-1 Junior Preferred Stock into common stock (in shares)			17,406,338
Conversion ratio			9.17
Underwriter option to purchase
Preferred and Common Stock
Issuance of stock (in shares)	859,800
Common stock
Preferred and Common Stock
Issuance of common stock on exchange of preferred stock					8,578,646
Purchase price per share \| $ / shares								$ 0.001
Series A‑1 Junior Preferred Stock
Preferred and Common Stock
Issuance of common stock on exchange of preferred stock					78,666,209
Maximum \| Underwriter option to purchase
Preferred and Common Stock
Issuance of stock (in shares)			1,057,500
License agreement \| Takeda
Preferred and Common Stock
Issuance of stock (in shares)		223,544

Stock-based Compensation - 2017 Equity Incentive Plan (Details) $ / shares in Units, $ in Thousands		1 Months Ended	12 Months Ended
	Jan. 01, 2019 shares	Aug. 31, 2015	Dec. 31, 2018 $ / shares shares	Dec. 31, 2017 USD ($) item shares	Dec. 31, 2016 shares	Jan. 01, 2018 shares
Stock-based compensation
Expected dividend yield			0.00%
Nonemployees
Stock-based compensation
Expected term (in years)			10 years
2017 Equity Incentive Plan
Stock-based Compensation
Common stock reserved for issuance	1,376,429		4,793,168			1,091,366
Percentage increase in authorized shares under the plan			4.00%
Vesting period of stock option awards				3 months
Grant cliff vesting period		1 year
Number of common stock options granted			1,218,790	1,112,717	164,229
Weighted average grant date fair value relating to outstanding stock options \| $ / shares			$ 17.27
Number of awards subject to modification \| item				3
Incremental expenses \| $				$ 254
2017 Equity Incentive Plan \| Exercise Price Not Less Than 100% Of Fair Market Value
Stock-based Compensation
Exercise price of incentive stock options over fair market value of the common stock		100.00%
2017 Equity Incentive Plan \| Exercise Price Not Less Than 110% Of Fair Market Value
Stock-based Compensation
Exercise price of incentive stock options over fair market value of the common stock		110.00%
2017 Equity Incentive Plan \| Employees and directors
Stock-based compensation
Risk-free interest rate			2.73%	1.97%	1.39%
Expected term (in years)			5 years 10 months 21 days	5 years 11 months 12 days	6 years 3 months
Expected volatility			62.21%	66.18%	74.20%
2017 Employee Stock Purchase Plan
Stock-based Compensation
Common stock reserved for issuance			272,841
Percentage increase in authorized shares under the plan			1.00%
Number of common stock options granted			0
2017 Employee Stock Purchase Plan \| Forecast
Stock-based Compensation
Common stock reserved for issuance	682,102
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	344,107
Maximum \| 2017 Equity Incentive Plan
Stock-based Compensation
Vesting period of stock option awards		4 years
Maximum \| 2017 Equity Incentive Plan \| Exercise Price Not Less Than 110% Of Fair Market Value
Stock-based Compensation
Voting power of stock		10.00%
Minimum \| 2017 Equity Incentive Plan
Stock-based Compensation
Vesting period of stock option awards		3 years
Minimum \| 2017 Equity Incentive Plan \| Exercise Price Not Less Than 100% Of Fair Market Value
Stock-based Compensation
Voting power of stock		10.00%

Stock-based Compensation - Common stock option activity (Details) - 2017 Equity Incentive Plan - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Number of Units
Outstanding at beginning of the period	1,832,639
Granted	1,218,790	1,112,717	164,229
Exercised	(316,736)
Cancelled	(118,163)
Outstanding at end of the period	2,616,530	1,832,639
Options vested and expected to vest at end of the period	2,616,530
Options exercisable at end of the period	853,530
Weighted‑ Average Grant Date Fair Value Per Unit
Weighted average grant date fair value relating to outstanding stock options	$ 17.27
Weighted Average Exercise Price
Outstanding at beginning of the period	7.04
Granted	28.45
Exercised	5.71
Cancelled	18.21
Outstanding at end of the period	16.67	$ 7.04
Options vested and expected to vest at end of the period	16.67
Options exercisable at end of the period	$ 8.83
Weighted Average Remaining Contractual Term
Outstanding at end of the period	8 years 6 months 29 days	8 years 5 months 23 days
Options vested and expected to vest at end of the period	8 years 6 months 29 days
Options exercisable at end of the period	7 years 10 months 10 days
Aggregate Intrinsic Value
Exercised	$ 7,980
Outstanding at end of the period	29,424
Options vested and expected to vest at the end of the period	29,424
Options exercisable at the end of the period	$ 15,432

Stock-based Compensation - Range of exercise prices (Details)	12 Months Ended
	Dec. 31, 2018 $ / shares shares
Range of exercise prices
Number of stock outstanding \| shares	2,616,530
Weighted average remaining contractual term (in years)	8 years 6 months 29 days
Weighted average exercise price (in dollars per share)	$ 16.67
Number of exercisable options \| shares	853,530
Weighted average exercise price (in dollars per share)	$ 8.83
Exercise price $ 4.59 to $6.05
Range of exercise prices
Exercise Price Range, Lower Range Limit	4.59
Exercise Price Range, Upper Range Limit	$ 6.88
Number of stock outstanding \| shares	1,258,425
Weighted average remaining contractual term (in years)	7 years 11 months 19 days
Weighted average exercise price (in dollars per share)	$ 6.00
Number of exercisable options \| shares	644,446
Weighted average exercise price (in dollars per share)	$ 5.70
Exercise price $ 6.14 to $6.88
Range of exercise prices
Exercise Price Range, Lower Range Limit	7.52
Exercise Price Range, Upper Range Limit	$ 26.88
Number of stock outstanding \| shares	895,215
Weighted average remaining contractual term (in years)	8 years 11 months 23 days
Weighted average exercise price (in dollars per share)	$ 23.36
Number of exercisable options \| shares	202,847
Weighted average exercise price (in dollars per share)	$ 18.11
Exercise price $ 7.52 to $30.51
Range of exercise prices
Exercise Price Range, Lower Range Limit	28.00
Exercise Price Range, Upper Range Limit	$ 35.85
Number of stock outstanding \| shares	462,890
Weighted average remaining contractual term (in years)	9 years 6 months
Weighted average exercise price (in dollars per share)	$ 32.70
Number of exercisable options \| shares	6,237
Weighted average exercise price (in dollars per share)	$ 30.60

Stock-based Compensation - Compensation expense (Details) - 2017 Equity Incentive Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-based Compensation
Stock based compensation expense	$ 6,390	$ 2,278	$ 1,168
Unrecognized compensation expense related to non-vested portion of awards	$ 19,897
Weighted average period for recognition of unrecognized compensation expense	2 years 8 months 1 day
Research and development
Stock-based Compensation
Stock based compensation expense	$ 2,793	927	507
Selling, general and administrative
Stock-based Compensation
Stock based compensation expense	3,597	1,351	661
Stock options
Stock-based Compensation
Stock based compensation expense	6,241	2,084	993
Employees stock purchase plan
Stock-based Compensation
Stock based compensation expense	65
Restricted stock units
Stock-based Compensation
Stock based compensation expense	$ 84	$ 194	$ 175

Stock-based Compensation - LLC Incentive Plan (Details)		12 Months Ended
Stock-based Compensation - LLC Incentive Plan (Details)	Mar. 21, 2013 shares	Dec. 31, 2018 Vote item $ / shares shares	Dec. 31, 2010 item
LLC Incentive Plan
Number of Units
Outstanding unvested at beginning of the period		23,569
Vested		(16,555)
Cancelled		(7,014)
Outstanding unvested at end of the period
Weighted‑ Average Grant Date Fair Value Per Unit
Outstanding unvested at beginning of the period \| $ / shares		$ 5.06
Vested \| $ / shares		5.06
Cancelled \| $ / shares		$ 5.06
Outstanding unvested at end of the period \| $ / shares
LLC Incentive Plan \| Restricted stock units
Stock-based Compensation
Number of common stock options granted	615,685
Original plan \| Predecessor
Stock-based Compensation
Number of stock based compensation plans \| item			1
Original plan \| Restricted stock units
Stock-based Compensation
Number of units issued \| item		1
Number of votes \| Vote		1

Significant Agreements (Details) shares in Thousands			1 Months Ended			12 Months Ended
Significant Agreements (Details) shares in Thousands	Apr. 03, 2018 USD ($) shares	Feb. 26, 2010 USD ($) item	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Jan. 31, 2016 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant Agreements
Shares issued for license agreement						$ 4,448,000
Non-cash research and development license expense						$ 4,448,000
Ipsen \| Predecessor \| License agreement
Significant Agreements
Number of programs \| item		2
Milestone prepayment			$ 1,000,000
Ipsen \| Predecessor \| License agreement \| Maximum
Significant Agreements
Aggregate payment upon achievement of development and commercial milestones		$ 40,000,000
Ipsen \| Predecessor \| Sublicense agreement \| Minimum
Significant Agreements
Payment based on revenue received, as percentage		10.00%
Ipsen \| Predecessor \| Sublicense agreement \| Maximum
Significant Agreements
Payment based on revenue received, as percentage		20.00%
Camurus \| Predecessor \| License agreement
Significant Agreements
Non-refundable and non-creditable signing fee					$ 500,000
Royalty payment period					10 years
Milestone fee				$ 250,000.000			$ 1,000,000
Camurus \| Predecessor \| License agreement \| Maximum
Significant Agreements
One-time non-refundable development milestone payment					$ 7,750,000
One-time non-refundable non-creditable sales milestone payment					$ 57,000,000
Takeda
Significant Agreements
Shares issued for license agreement	$ 223,544
Shares issued for license agreement (in shares) \| shares	4,448
One-time non-refundable development milestone payment	$ 70,000,000
One-time non-refundable non-creditable sales milestone payment	$ 70,000,000
Period after date of first commercial sale (in years)	10 years

Commitments and Contingencies (Details) $ in Thousands	1 Months Ended	12 Months Ended
Commitments and Contingencies (Details) $ in Thousands	Aug. 31, 2018 ft²	Dec. 31, 2018 USD ($) ft²	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Commitments and Contingencies
Approximate operating lease rental space \| ft²	13,600	6,800
Lease term	6 years
Lease renewal term	5 years
Rent expense		$ 359	$ 215	$ 179
Future minimum payments under the operating lease agreements
2019		425
2020		786
2021		802
2022		818
2023		834
Thereafter		1,353
Total		$ 5,018

Related-Party Transactions (Details) - USD ($)	12 Months Ended
Related-Party Transactions (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Relamorelin Company and/or Motus
Related Party Transaction
Net costs			$ 1,570,000
Accounts Payable
Related Party Transaction
Outstanding payments due to consultants	$ 260,000	$ 90,000
Consultant
Related Party Transaction
Expenses paid to consultants	$ 2,005,000	$ 2,400,000.000	$ 619,000

Income Tax - Reconciliation of income tax rate (Details)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliation of the income tax benefit
Statutory tax rate	21.00%	34.00%	34.00%
State tax, net of federal benefit	6.90%	4.08%	2.63%
Research and development credit	1.52%	1.87%	1.34%
Orphan drug credit	1.95%	2.29%	2.15%
Non deductible deferred issuance costs			(2.40%)
Tax law change		(27.98%)
Stock compensation		(1.88%)
Non deductible warrant expense	0.46%	(1.84%)
Other	0.05%	(0.07%)	(1.32%)
Change in valuation allowance	(31.88%)	(10.47%)	(36.40%)

Income Tax - Deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforwards	$ 35,776	$ 18,325
Net operating loss indefinite carryforwards	63,073
Research and development credits	3,456	2,317
Orphan drug credit	4,286	2,333
Capitalized license fee	1,760	500
Other	2,401	599
Total gross deferred tax assets	47,679	24,074
Valuation allowance	$ (47,679)	$ (24,074)

Income Tax - Tax Cuts and Jobs Act (Details)	12 Months Ended
Income Tax - Tax Cuts and Jobs Act (Details)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Income Tax
U.S. corporate tax rate	21.00%	34.00%	34.00%

Income Tax - Operating loss carryforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Operating loss carryforwards
Increase decrease in valuation allowance	$ 23,605	$ 3,586
Federal net operating loss carryforwards	136,239
State net operating loss carryforwards	113,383
Federal orphan drug credit related to qualifying research	4,286
Accrued interest or penalties related to uncertain tax positions	0	$ 0
Research tax credits \| Federal
Operating loss carryforwards
Research tax credits	2,857
Research tax credits \| State
Operating loss carryforwards
Research tax credits	$ 758

Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Selected Quarterly Financial Data (unaudited)
Total revenue									$ 0	$ 0	$ 0
Total operating expenses	$ 27,151	$ 19,244	$ 15,021	$ 17,001	$ 10,983	$ 8,286	$ 6,754	$ 6,389	78,417	32,412	25,905
Other income (expense), net:	1,644	1,558	609	542	452	(1,730)	(48)	29	4,353	(1,297)	33
Net loss and comprehensive loss	(25,507)	(17,686)	(14,412)	(16,459)	(10,531)	(10,016)	(6,802)	(6,360)	(74,064)	(33,709)	(25,872)
Net loss attributable to common stockholders	$ (25,507)	$ (17,686)	$ (14,412)	$ (16,459)	$ (10,619)	$ (11,429)	$ (8,008)	$ (7,526)	$ (74,064)	$ (37,582)	$ (29,074)
Net loss attributable to common stockholders per common share, basic and diluted	$ (0.74)	$ (0.52)	$ (0.52)	$ (0.60)	$ (0.41)	$ (1.78)	$ (0.78)	$ (0.74)	$ (2.39)	$ (2.83)	$ (2.85)

- Definition

The consolidated profit or loss for the period, net of income taxes, including the portion attributable to the noncontrolling interest.

+ References

+ Details