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Summary of Significant Accounting Policies - Application of New or Revised Accounting Standards (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019
Summary of Significant Accounting Policies
Right-of-use asset	$ 2,045	$ 3,265
Operating Lease, Liability	$ 3,558	$ 3,636

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Current assets:
Cash and cash equivalents	$ 62,294	$ 49,542
Short-term investments	230,165	202,519
Prepaid expenses and other current assets	9,945	6,628
Total current assets	302,404	258,689
Property and equipment, net	3,671	1,120
Right-of-use asset	2,045
Restricted cash	403	401
Total assets	308,523	260,210
Current liabilities:
Accounts payable	10,415	7,640
Accrued expenses and other current liabilities	13,530	5,942
Lease liability	472
Total current liabilities	24,417	13,582
Long-term liabilities:
Lease liability	3,086
Deferred rent		372
Total liabilities	27,503	13,954
Commitments and contingencies (Note 5 and 9)
Stockholders’ equity:
Common stock, $0.001 par value: 120,000,000 shares authorized; 43,996,753 and 34,410,725 shares issued and outstanding December 31, 2019 and 2018, respectively	44	34
Additional paid-in capital	606,307	430,824
Accumulated deficit	(325,331)	(184,602)
Total stockholders’ equity	281,020	246,256
Total liabilities and stockholders’ equity	$ 308,523	$ 260,210

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, 2016		$ 40,000
Beginning balance (in shares) at Dec. 31, 2016		40,000,000
Increase (Decrease) in Convertible Preferred Stock
Settlement of Series A investor instrument		$ 328		$ 1,863		$ 1,863
Beginning balance at Dec. 31, 2016	$ 10			43,830	$ (76,543)	(32,703)
Beginning balance (in shares) at Dec. 31, 2016	10,196,292
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
Issuance of common stock upon completion of public offering, net of offering costs	$ 8			125,650		125,658
Issuance of common stock upon completion of public offering, net of offering costs (in shares)	8,107,500
Change in unrealized gain (loss) on marketable securities				(141)		(141)
Issuance of Series A Convertible Preferred Stock		40,622
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
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	$ (80,950)	$ (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	(80,949,999)	(78,666,209)
Loss attributable to common shares—basic and diluted					(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 Stockholders' Equity
Issuance of Series A Convertible Preferred Stock (in shares)		40,949,999
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
Issuance of common stock upon completion of public offering, net of offering costs	$ 7			162,871		162,878
Issuance of common stock upon completion of public offering, net of offering costs (in shares)	6,591,800
Change in unrealized gain (loss) on marketable securities				8		8
Loss attributable to common shares—basic and diluted					(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 Stockholders' Equity
Adoption of new accounting standard				286	(286)
Stock compensation expense				11,875		11,875
Issuance of common stock in connection with ESPP				558		558
Issuance of common stock in connection with ESPP (in shares)	25,871
Issuance of common stock in connection with exercise of stock options	$ 1			1,563		1,564
Issuance of common stock in connection with exercise of stock options (in shares)	235,833
Issuance of common stock upon completion of public offering, net of offering costs	$ 9			161,343		161,352
Issuance of common stock upon completion of public offering, net of offering costs (in shares)	9,324,324
Change in unrealized gain (loss) on marketable securities				144		144
Loss attributable to common shares—basic and diluted					(140,729)	(140,729)
Ending balance at Dec. 31, 2019	$ 44			$ 606,307	$ (325,331)	$ 281,020
Ending balance (in shares) at Dec. 31, 2019	43,996,753

Income Tax - Operating loss carryforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
Operating loss carryforwards
Increase decrease in valuation allowance	$ 45,264	$ 23,605
Federal net operating loss carryforwards	268,009
State net operating loss carryforwards	241,314
Federal orphan drug credit related to qualifying research	6,889
Accrued interest or penalties related to uncertain tax positions	0	$ 0
Net operating loss indefinite carryforwards	194,843
Research tax credits \| Federal
Operating loss carryforwards
Research tax credits	6,357
Research tax credits \| State
Operating loss carryforwards
Research tax credits	$ 1,922

Nature of Business	12 Months Ended
Nature of Business	Dec. 31, 2019
Nature of Business
Nature of Business	1. Nature of Business Rhythm Pharmaceuticals, Inc. (the “Company” or “We”), is a late-stage biopharmaceutical company focused on the development and commercialization of therapeutics for the treatment of rare genetic disorders which are characterized by severe, early-onset obesity and an unrelenting hunger or hyperphagia. The Company’s lead product candidate is setmelanotide, a potent melanocortin‑4 receptor, or MC4R, agonist for the treatment of rare genetic disorders of obesity. We believe setmelanotide, for which we have exclusive worldwide rights, has the potential to serve as replacement therapy for the treatment of MC4R pathway deficiencies. MC4R 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’s development efforts are initially focused on obesity related to several single gene‑related, or monogenic, MC4R pathway deficiencies: pro‑opiomelanocortin, or POMC, deficiency obesity; leptin receptor, or LEPR, deficiency obesity; Bardet‑Biedl syndrome, or BBS; Alström syndrome; POMC or LEPR heterozygous deficiency obesity; steroid receptor coactivator 1, or SRC1, deficiency obesity; SH2B adapter protein 1, or SH2B1, deficiency obesity; MC4R deficiency obesity and Smith‑Magenis syndrome, as well as additional disorders as part of investigator-initiated protocols. Currently, there are no effective or approved treatments for these MC4R pathway‑related disorders. We believe that the MC4R 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. The Company is subject to risks and uncertainties common to early-stage companies in the biotechnology industry, including but not limited to, risks associated with completing preclinical studies and clinical trials, receiving regulatory approvals for product candidates, development by competitors of new biopharmaceutical products, dependence on key personnel, protection of proprietary technology, compliance with government regulations and the ability to secure additional capital to fund operations. Product candidates currently under development will require significant additional research and development efforts, including preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel and infrastructure and extensive compliance-reporting capabilities. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will realize revenue from product sales. Corporate Reorganization The Company is a Delaware corporation organized in February 2013 under the name Rhythm Metabolic, Inc., and as of October 2015, under the name Rhythm Pharmaceuticals, Inc. Prior to its organization and a corporate reorganization, the Company was part of Rhythm Pharmaceuticals, Inc., a Delaware corporation which was organized in November 2008 and which commenced active operations in 2010. We refer to this corporation as the Predecessor Company. Liquidity The Company has incurred operating losses and negative cash flows from operations since inception. As of December 31, 2019, the Company had an accumulated deficit of $325,331. 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 and the former parent company, Rhythm Holdings LLC, or 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, 2019, the Company had $292,459 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 operations through at least the next twelve months from the filing of this Annual Report on Form 10-K with the SEC.

Related-Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Accounts Payable
Related Party Transaction
Outstanding payments due to consultants	$ 264	$ 260
Consultant
Related Party Transaction
Net costs	$ 2,489	$ 2,005	$ 2,400

Significant Agreements	12 Months Ended
Significant Agreements	Dec. 31, 2019
Significant Agreements
Significant Agreements	8. Significant Agreements License Agreements Pursuant to a license agreement with Ipsen Pharma, S.A.S., or Ipsen, the Company has an exclusive, sublicensable, worldwide license to certain patents and other intellectual property rights to research, develop, and commercialize compounds that were discovered or researched by Ipsen in the course of conducting its MC4R program or that otherwise were covered by the licensed patents. 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. 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. The fees associated with these milestones were recorded as research and development expenses during the year ended December 31, 2017 when the milestones criteria were met in full. 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.

Subsequent Events (Details) - Subsequent Events - Separation Agreement - Dr. Gottesdiener $ in Thousands	Jan. 06, 2020 USD ($) item
Subsequent events
Severance Costs	$ 531
Consulting Fees Paid	$ 10
Number of Hours of Service \| item	10
Employees stock purchase plan
Subsequent events
Incremental compensation charge	$ 2,968
Maximum \| Employees stock purchase plan
Subsequent events
Share-based Compensation Arrangement by Share-based Payment Award, Post-termination Exercise Period	2 years
Minimum \| Employees stock purchase plan
Subsequent events
Share-based Compensation Arrangement by Share-based Payment Award, Post-termination Exercise Period	90 days

Fair Value of Financial Assets and Liability (Tables)

12 Months Ended

Dec. 31, 2019

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, 2019 using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash Equivalents:
Corporate Debt Securities and Commercial Paper	$	—	$	8,885	$	—	$	8,885
Money Market Funds		53,014		—		—		53,014
Marketable Securities:
Corporate Debt Securities and Commercial Paper		—		230,165		—		230,165
Total	$	53,014	$	239,050	$	—	$	292,064


	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

Schedule of marketable securities


	December 31, 2019
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Assets
Corporate debt securities and commercial paper	$	230,155	$	54	$	(44)	$	230,165
	$	230,155	$	54	$	(44)	$	230,165


	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
	$	202,653	$	23	$	(157)	$	202,519

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 - Common stock option activity (Details) - 2017 Equity Incentive Plan - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Number of Units
Outstanding at beginning of the period	2,616,530
Granted	1,445,200	1,218,790	1,112,717
Exercised	(235,833)
Cancelled	(397,400)
Outstanding at end of the period	3,428,497	2,616,530
Options vested and expected to vest at end of the period	3,428,497
Options exercisable at end of the period	1,446,311
Weighted‑ Average Grant Date Fair Value Per Unit
Weighted average grant date fair value relating to outstanding stock options	$ 17.19	$ 17.27	$ 4.98
Weighted Average Exercise Price
Outstanding at beginning of the period	16.67
Granted	27.98
Exercised	6.63
Cancelled	24.85
Outstanding at end of the period	21.17	$ 16.67
Options vested and expected to vest at end of the period	21.17
Options exercisable at end of the period	$ 16.03
Weighted Average Remaining Contractual Term
Outstanding at end of the period	7 years 11 months 9 days	8 years 6 months 29 days
Options vested and expected to vest at end of the period	7 years 11 months 9 days
Options exercisable at end of the period	7 years 1 month 10 days
Aggregate Intrinsic Value
Stock options total intrinsic value of stock options	$ 3,844	$ 7,980	$ 588
Outstanding at end of the period	18,643
Options vested and expected to vest at the end of the period	18,643
Options exercisable at the end of the period	$ 13,872

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2019

Accrued Expenses

Schedule of accrued expenses


	December 31,		December 31,
	2019		2018
Research and development costs	$	8,059	$	2,614
Professional fees		1,439		858
Payroll related		3,655		2,410
Other		377		60
Accrued expenses	$	13,530	$	5,942

Income Tax - Reconciliation of income tax rate (Details)	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Reconciliation of the income tax benefit
Statutory tax rate	21.00%	21.00%	34.00%
State tax, net of federal benefit	6.75%	6.90%	4.08%
Research and development credit	2.49%	1.52%	1.87%
Orphan drug credit	1.85%	1.95%	2.29%
Tax law change			(27.98)
Stock compensation	(0.10%)	0.46%	(1.84%)
Investor instrument revaluation			(1.88%)
Other	0.20%	0.05%	(0.07%)
Change in valuation allowance	(32.19%)	(31.88%)	(10.47%)

Significant Agreements (Details) - USD ($)		1 Months Ended		3 Months Ended	12 Months Ended
Significant Agreements (Details) - USD ($)	Apr. 03, 2018	Mar. 31, 2017	Jan. 31, 2016	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2017	Feb. 26, 2010
Significant Agreements
Shares issued for license agreement					$ 4,448,000
Non-cash research and development license expense					$ 4,448,000
Ipsen \| License agreement
Significant Agreements
Milestone prepayment				$ 1,000,000
Ipsen \| License agreement \| Maximum
Significant Agreements
Aggregate payment upon achievement of development and commercial milestones							$ 40,000,000
Ipsen \| Sublicense agreement \| Minimum
Significant Agreements
Payment based on revenue received, as percentage							10.00%
Ipsen \| Sublicense agreement \| Maximum
Significant Agreements
Payment based on revenue received, as percentage							20.00%
Camurus \| 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 \| 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	$ 4,448,000
Shares issued for license agreement (in shares)	223,544
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

Income Tax

12 Months Ended

Dec. 31, 2019

Income Tax

Income Tax

11. 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 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, 2019, 2018 and 2017 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 is as follows:


	As of
	December 31,
	2019		2018		2017
Statutory tax rate	21.00	%	21.00	%	34.00	%
State tax, net of federal benefit	6.75	%	6.90	%	4.08	%
Research and development credit	2.49	%	1.52	%	1.87	%
Orphan drug credit	1.85	%	1.95	%	2.29	%
Tax law change	—	%	—	%	(27.98)	%
Stock compensation	(0.10)	%	0.46	%	(1.84)	%
Investor instrument revaluation	—	%	—	%	(1.88)	%
Other	0.20	%	0.05	%	(0.07)	%
Change in valuation allowance	(32.19)	%	(31.88)	%	(10.47)	%
Effective tax rate	—	%	—	%	—	%

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


	As of
	December 31,
	2019		2018
Deferred tax assets:
Net operating loss carryforwards	$	71,524	$	35,776
Research and development credits		7,876		3,456
Orphan drug credit		6,889		4,286
Capitalized license fee		1,734		1,760
Stock-based compensation		3,628		1,517
Other		2,006		884
Total deferred tax assets		93,657		47,679
Valuation allowance		(92,943)		(47,679)
Net deferred tax assets		714		—
Deferred tax liabilities:
Operating lease right-of-use asset and other		(714)		—
Total deferred tax liabilities	$	(714)	$	—

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, 2019 and 2018, 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 $45,264 in 2019 and $23,605 in 2018 primarily relates to the net loss incurred by the Company during each period.

As of December 31, 2019, the Company had federal and state net operating loss carryforwards of approximately $268,009 and $241,314, 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, 2019, $194,843 can be carried forward indefinitely.

As of December 31, 2019, the Company had federal and state research tax credits of approximately $6,357 and $1,922, respectively, which may be used to offset future tax liabilities. Additionally, as of 2019, the Company had a federal orphan drug credit related to qualifying research of $6,889. 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, 2019 and 2018. 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, 2019 and 2018, 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.

Commitments and Contingencies (Details) $ in Thousands	12 Months Ended
Commitments and Contingencies (Details) $ in Thousands	Dec. 31, 2019 USD ($)
Commitments and Contingencies
Potential Milestone Payment	$ 8,000
Regulatory Milestone Payments	3,000
Commercial Milestone Payments	$ 5,000

Preferred Stock and Common Stock	12 Months Ended
Preferred Stock and Common Stock	Dec. 31, 2019
Preferred Stock and Common Stock
Preferred Stock and Common Stock	6. 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 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 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 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 net proceeds of $125,658 after deducting underwriting discounts, commissions and 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 8 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 net proceeds of $162,878 after deducting underwriting discounts, commissions and offering expenses. On October 18, 2019 the Company completed a public offering of 9,324,324 shares of common stock at an offering price of $18.50 per share, which included the exercise in full by the underwriters of their option to purchase up to 1,216,216 additional shares of common stock. The Company received net proceeds of $161,352 after deducting underwriting discounts, commissions and offering expenses. As of December 31, 2019, an aggregate of 3,096,344 shares of common stock were reserved for future issuance under our stock plans, including outstanding stock options to purchase 2,505,267 shares of common stock and 591,077 shares available for future grant under our 2017 Employee Stock Purchase Plan.

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2019

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”).

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 research and development accrued expenses, stock‑based compensation expense and the valuation allowance on the Company's deferred tax assets. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ materially from those estimates.

Principles of Consolidation

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

Off-Balance Sheet Risk and Concentrations of Credit Risk

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 considers its chief executive officer, or CEO, as its chief operating decision maker. The Company and the CEO view the Company’s 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

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. 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 separate restricted bank accounts as required under the terms of the Company’s lease arrangement for its corporate office in Boston, Massachusetts and the Company’s corporate travel credit cards.

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. Prepaid expenses and other current assets consists of the following:


	December 31,
	2019		2018
Prepaid research and development costs	$	6,438	$	5,288
Other current assets		3,507		1,340
Prepaid expenses and other current assets	$	9,945	$	6,628

Property, Plant and Equipment

Property and Equipment

Property and Equipment consists of the following:


	Useful	December 31,
	Life	2019		2018
Leasehold improvements	*	$	2,705	$	1,103
Office equipment	5 years		70		70
Computers and software	3 years		411		411
Furniture, fixtures and equipment	5 years		1,237		345
			4,423		1,929
Less accumulated depreciation and amortization			(752)		(809)
Property and Equipment, net		$	3,671	$	1,120

*Shorter of asset life or lease term.

Depreciation and amortization expense for the years ended December 31, 2019, 2018 and 2017 was $834, $442 and $223, respectively.

Property and equipment are recorded at cost. Depreciation and amortization is calculated using the straight-line method over the estimated useful lives of the assets. Upon disposal, retirement or sale, the cost of assets disposed of and the related accumulated depreciation are removed from the accounts and any resulting gain or loss is included in the results of operations. Expenditures for repairs and maintenance that do not improve or extend the lives of the respective assets are charged to expense as incurred.

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, 2019 and 2018 were carried at fair value, determined according to the fair value hierarchy. See Note 4 for further discussion.

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, 2019 and 2018, respectively.

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, or 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 of $210 is included as a deduction to research and development expense in the consolidated statements of operations and comprehensive loss. No grant income was received during the years ended December 31, 2019 or 2017.

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. The value of goods and services received from contract research organizations, or CROs, or contract manufacturing organizations, or CMOs, 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. When evaluating the adequacy of the accrued liabilities, the Company analyzes progress of the studies or clinical trials, including the phase or completion of events, invoices received and contracted costs. Significant judgments and estimates are made in determining the accrued balances at the end of any reporting period. Actual results could differ from the Company’s estimates. The Company’s historical accrual estimates have not been materially different from the actual costs.

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 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 its 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% 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, 2019, 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 more dilutive of the treasury‑stock and if‑converted methods. For purposes of the diluted net loss per share attributable to common stockholders calculation, 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 share equivalents, 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,
	2019		2018		2017
Stock options		3,428,497		2,616,530		1,832,639

Basic and diluted earnings per share is calculated as follows:


	Year Ended
	December 31,
	2019		2018		2017
Numerator:
Net loss	$	(140,729)	$	(74,064)	$	(33,709)
Cumulative dividends on convertible preferred shares		—		—		(3,873)
Loss attributable to common shares—basic and diluted	$	(140,729)	$	(74,064)	$	(37,582)
Denominator:
Weighted-average number of common shares—basic and diluted		36,422,450		31,004,047		13,267,960
Loss per common share—basic and diluted	$	(3.86)	$	(2.39)	$	(2.83)

Comprehensive Income (Loss)

Comprehensive income (loss) represents the net change in shareholders’ equity during a period from sources other than transactions with shareholders. As reflected in the accompanying consolidated statements of operations and comprehensive loss, our comprehensive loss is comprised of net losses and unrealized gains and losses on marketable debt securities. These changes in equity are reflected net of tax.

Patent Costs

Costs to secure and defend patents are expensed as incurred and are classified as general and administrative expenses. Patent costs were $472, $637 and $180 for the years ended December 31, 2019, 2018 and 2017, 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 June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses-Measurement of Credit Losses on Financial Instruments (“ASU 2016-13”). ASU 2016-13 requires the measurement and recognition of expected credit losses for financial assets held at amortized cost. ASU 2016-13 replaces the existing incurred loss impairment model with an expected loss methodology, which will result in more timely recognition of credit losses. ASU 2016-13 is effective for annual and interim reporting periods beginning after December 15, 2019. The Company does not anticipate this standard to have a significant impact on the consolidated financial statements.

In November 2016, the FASB issued ASU 2016‑18, Statement of Cash Flows (Topic 230): Restricted Cash (“ASU 2016-18”). ASU 2016-18 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. The Company adopted this ASU on January 1, 2018 and has applied its content to statements of cash flows for the years ended December 31, 2019, 2018 and 2017 presented herein.

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 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 2018-15, 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.

Effective January 1, 2019 the Company adopted FASB ASU 2016‑02, Leases (Topic 842) (“ASU 2016‑02”). ASU 2016‑02 requires lessees to recognize a right-of-use (“ROU”) asset and lease liability for most lease arrangements. The new standard is effective for annual reporting periods beginning after December 15, 2018. The original guidance required application on a modified retrospective basis with the earliest period presented. In August 2018, the FASB issued ASU 2018-11, Targeted Improvements to ASC 842, which included an option to not restate comparative periods in transition and elect to use the effective date of ASC 842, as the date of initial application of transition, which the Company has elected. In addition, the Company elected the package of practical expedients permitted under the transition guidance within the new standard which allowed us to carry forward the historical lease classification. As a result of the adoption of ASC 842 on January 1, 2019, the Company recorded both an operating lease right-of-use asset of $3,265 and a lease liability of $3,636. Additional information and disclosures required by this new standard are contained in Note 5, Right Of Use Asset and Lease Liability.

In December 2019, the FASB issued ASU 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes (“ASU 2019-12”). ASU 2019-12 eliminates certain exceptions related to the approach for intraperiod tax allocation, the methodology for calculating income taxes in an interim period and the recognition of deferred tax liabilities for outside basis differences. The new guidance also simplifies aspects of the accounting for franchise taxes and enacted changes in tax laws or rates and clarifies the accounting for transactions that result in a step-up in the tax basis of goodwill. The standard is effective for annual periods beginning after December 15, 2020 and interim periods within, with early adoption permitted. Adoption of the standard requires certain changes to be made prospectively, with some changes to be made retrospectively. We are currently assessing the impact of this standard on our financial condition and results of operations.

Stock-based Compensation

12 Months Ended

Dec. 31, 2019

Stock-based Compensation

Stock-based Compensation

7. Stock‑based Compensation

2017 Equity Incentive Plan

The Company has a 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, 2019 the Company has reserved 5,933,764 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, 2020, 2019 and 2018, 1,759,870, 1,376,429 and 1,091,366 shares, respectively, were added to the 2017 Plan. Notwithstanding the foregoing, the 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. 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, 2019, 2018 and 2017, the Company granted 1,445,200, 1,218,790, and 1,112,717 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, 2019, 2018 and 2017 was $17.19, $17.27 and $4.98, respectively. Total intrinsic value of stock options exercised during the years ended December 31, 2019, 2018 and 2017 was $3,844, $7,980 and $588, respectively.

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,
	2019		2018		2017
Risk‑free interest rate	2.40	%	2.73	%	1.97	%
Expected term (in years)	6.07		5.89		5.95
Expected volatility	66.03	%	62.21	%	66.18	%
Expected dividend yield	—		—		—

The Company early adopted ASU 2018‑07 in July 2018. The guidance was adopted using the modified-retrospective approach, which requires that unsettled equity-classified awards for which a measurement date has not been established 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, 2019 is as follows:


				Weighted‑
		Weighted-		Average
		Average		Remaining		Aggregate
	Number of	Exercise		Contractual		Intrinsic
	Options	Price		Term		Value
Outstanding as of December 31, 2018	2,616,530	$	16.67	8.58	$	—
Granted	1,445,200		27.98	—		—
Exercised	(235,833)		6.63	—		3,844
Cancelled	(397,400)		24.85	—		—
Outstanding as of December 31, 2019	3,428,497	$	21.17	7.94	$	18,643
Options vested and expected to vest as of December 31, 2019	3,428,497	$	21.17	7.94	$	18,643
Options exercisable at December 31, 2019	1,446,311	$	16.03	7.11	$	13,872

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 consolidated statements of operations and comprehensive loss.



	Year Ended
	December 31,
	2019		2018		2017
Research and development	$	5,163	$	2,793	$	927
Selling, general, and administrative		6,712		3,597		1,351
Total	$	11,875	$	6,390	$	2,278

Stock-based compensation expense by award type recognized during the years ended December 31, 2019, 2018 and 2017 was as follows:



	Year Ended
	December 31,
	2019		2018		2017
Stock options	$	11,667	$	6,241	$	2,084
Employees stock purchase plan		208		65		—
Restricted stock units		—		84		194
Total	$	11,875	$	6,390	$	2,278

During 2019 and 2017, there were certain awards subject to modification accounting under ASC 718-20-35-3 through 35-4. Per terms of separation with a former employee, accelerated vesting and time to exercise vested options were granted for the former employee’s stock option awards. As a result, the Company recognized incremental expense for the stock option awards of $56 and $254, respectively.

As of December 31, 2019, the Company has unrecognized compensation cost of $27,885 related to non‑vested employee, non‑employee and director awards that is expected to be recognized over a weighted‑average period of 2.48 years.

2017 Employee Stock Purchase Plan

The Company has a 2017 Employee Stock Purchase Plan, or the 2017 ESPP, which became effective in connection with the completion of the Company’s IPO in October 2017. As of December 31, 2019, a total of 591,077 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. On January 1, 2020 and 2019, 439,968 and 344,107 shares, respectively, were added to the 2017 ESPP. Notwithstanding the foregoing, the 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. During the year ended December 31, 2019, 25,871 shares were issued under this plan.

Selected Quarterly Financial Data (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Selected Quarterly Financial Data (unaudited)
Revenue									$ 0	$ 0	$ 0
Total operating expenses	$ 34,224	$ 37,107	$ 44,149	$ 30,520	$ 27,151	$ 19,244	$ 15,021	$ 17,001	146,000	78,417	32,412
Other income (expense), net:	1,268	1,104	1,353	1,546	1,644	1,558	609	542	5,271	4,353	(1,297)
Net loss	(32,956)	(36,003)	(42,796)	(28,974)	(25,507)	(17,686)	(14,412)	(16,459)	(140,729)	(74,064)	(33,709)
Net loss attributable to common stockholders	$ (32,956)	$ (36,003)	$ (42,796)	$ (28,974)	$ (25,507)	$ (17,686)	$ (14,412)	$ (16,459)	$ (140,729)	$ (74,064)	$ (37,582)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.78)	$ (1.04)	$ (1.24)	$ (0.84)	$ (0.74)	$ (0.52)	$ (0.52)	$ (0.60)	$ (3.86)	$ (2.39)	$ (2.83)

- Definition

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

+ References + Details

Fair Value of Financial Assets and Liability - Financial Liabilities (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Oct. 18, 2018	Jun. 25, 2018
Assumption used to estimate the fair value of asset and liability by utilizing the Black-Scholes option pricing model
Fair Value		$ 18.50	$ 26.42
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%
Expected dividend yield (as a percent)	0.00%

Summary of Significant Accounting Policies - Net Loss per Share (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Loss attributable to common shares—basic and diluted									$ (140,729)	$ (74,064)	$ (33,709)
Cumulative dividends on convertible preferred shares											(3,873)
Loss attributable to common shares—basic and diluted	$ (32,956)	$ (36,003)	$ (42,796)	$ (28,974)	$ (25,507)	$ (17,686)	$ (14,412)	$ (16,459)	$ (140,729)	$ (74,064)	$ (37,582)
Weighted—average number of common shares—basic and diluted									36,422,450	31,004,047	13,267,960
Loss per common share—basic and diluted	$ (0.78)	$ (1.04)	$ (1.24)	$ (0.84)	$ (0.74)	$ (0.52)	$ (0.52)	$ (0.60)	$ (3.86)	$ (2.39)	$ (2.83)
Costs to secure and defend patents									$ 36,550	$ 28,080	$ 9,518
Patent Costs
Costs to secure and defend patents									$ 472	$ 637	$ 180

- 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=118930883&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=118930883&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)
-URI http://asc.fasb.org/extlink&oid=109239629&loc=SL4573702-111684

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
-SubTopic 10
-Section 55
-Paragraph 4J
-URI http://asc.fasb.org/extlink&oid=116874947&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=118932676&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=116874947&loc=SL4591552-111686

+ Details

Right of Use Asset and Lease Liability - Operating Lease Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Jan. 01, 2019
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2020	$ 786
2021	802
2022	818
2023	834
2024	851
Thereafter	502
Total operating lease payments	4,593
Less: imputed interest	1,035
Total operating lease liability	$ 3,558	$ 3,636

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2019	Feb. 28, 2020	Jun. 28, 2019
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, 2019
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 Interactive Data Current	Yes
Entity Filer Category	Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Public Float			$ 569.4
Entity Common Stock, Shares Outstanding		44,058,909
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	FY

Summary of Significant Accounting Policies - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Property, Plant and Equipment
Property, Plant and Equipment, gross	$ 4,423	$ 1,929
Less accumulated depreciation and amortization	(752)	(809)
Property, Plant and Equipment, net	3,671	1,120
Depreciation and amortization	834	442	$ 223
Leasehold improvements
Property, Plant and Equipment
Property, Plant and Equipment, gross	$ 2,705	1,103
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	411
Furniture, fixtures and equipment
Property, Plant and Equipment
Useful Life	5 years
Property, Plant and Equipment, gross	$ 1,237	$ 345

Selected Quarterly Financial Data (unaudited)

12 Months Ended

Dec. 31, 2019

Selected Quarterly Financial Data (unaudited)

Selected Quarterly Financial Data (unaudited)

13. Selected Quarterly Financial Data (unaudited)

The following table contains selected quarterly financial information from 2019 and 2018. 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,
	2019		2019		2019		2019

Total revenue	$	—	$	—	$	—	$	—
Total operating expenses		30,520		44,149		37,107		34,224
Other income (expense), net:		1,546		1,353		1,104		1,268
Net loss		(28,974)		(42,796)		(36,003)		(32,956)
Net loss attributable to common stockholders	$	(28,974)	$	(42,796)	$	(36,003)	$	(32,956)
Net loss attributable to common stockholders per common share, basic and diluted	$	(0.84)	$	(1.24)	$	(1.04)	$	(0.78)

	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		(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)

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2019	Dec. 31, 2018
CONSOLIDATED BALANCE SHEETS
Common stock, par value per share	$ 0.001	$ 0.001
Common stock, authorized	120,000,000	120,000,000
Common stock, issued	43,996,753	34,410,725
Common stock, outstanding	43,996,753	34,410,725
Preferred stock par value per share	$ 0.001	$ 0.001
Preferred stock, authorized	10,000,000	10,000,000
Preferred stock, issued	0	0

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating activities
Net loss	$ (140,729)	$ (74,064)	$ (33,709)
Adjustments to reconcile net loss to cash used in operating activities:
Non-cash research and development license expense		4,448
Stock-based compensation expense	11,875	6,390	2,278
Depreciation and amortization	834	442	223
Non-cash rent expense	203	61	(76)
Mark to market revaluation of series A investor instrument			1,863
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(6,378)	(6,286)	(1,889)
Deferred issuance costs			9
Tenant improvement allowance	938
Due to related party			(105)
Accounts payable, accrued expenses and other current liabilities	10,507	6,953	1,946
Net cash used in operating activities	(122,750)	(62,056)	(29,460)
Investing activities
Purchases of short-term investments	(295,825)	(248,592)	(126,917)
Maturities of short-term investments	271,240	162,166	17,006
Purchases of property and equipment	(3,385)	(722)	(133)
Net cash used in investing activities	(27,970)	(87,148)	(110,044)
Financing activities
Net proceeds from issuance of common stock	161,352	162,878	125,658
Proceeds from the exercise of stock options	1,564	1,808	700
Proceeds from issuance of common stock from ESPP	558
Net proceeds from issuance of Series A Convertible Preferred Stock			40,842
Net cash provided by financing activities	163,474	164,686	167,200
Net increase in cash, cash equivalents and restricted cash	12,754	15,482	27,696
Cash, cash equivalents and restricted cash at beginning of year	49,943	34,461	6,765
Cash, cash equivalents and restricted cash at end of year	$ 62,697	$ 49,943	$ 34,461

Fair Value of Financial Assets and Liability - Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value of Financial Assets and Liability
Amortized Cost	$ 230,155	$ 202,653
Gross Unrealized Gains	54	23
Gross Unrealized Losses	(44)	(157)
Fair Value	230,165	202,519
Corporate Debt Securities and Commercial Paper
Fair Value of Financial Assets and Liability
Amortized Cost	230,155	202,653
Gross Unrealized Gains	54	23
Gross Unrealized Losses	(44)	(157)
Fair Value	230,165	202,519
Estimated fair value \| Recurring \| Corporate Debt Securities and Commercial Paper
Fair Value of Financial Assets and Liability
Amortized Cost	230,165
Fair Value		202,519
Level 2 \| Estimated fair value \| Recurring \| Corporate Debt Securities and Commercial Paper
Fair Value of Financial Assets and Liability
Amortized Cost	$ 230,165
Fair Value		$ 202,519

Right of Use Asset and Lease Liability (Tables)

12 Months Ended

Dec. 31, 2019

Right Of Use Asset and Lease Liability

Schedule of operating lease maturities


	Operating Lease
2020	$	786
2021		802
2022		818
2023		834
2024		851
Thereafter		502
Total operating lease payments		4,593
Less: imputed interest		1,035
Total operating lease liability	$	3,558

Summary of Significant Accounting Policies - Shares Excluded For EPS (Details) - shares	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Summary of Significant Accounting Policies
Unvested shares from share-based compensation that were anti-dilutive	3,428,497	2,616,530	1,832,639

Related-Party Transactions	12 Months Ended
Related-Party Transactions	Dec. 31, 2019
Related-Party Transactions
Related-Party Transactions	10. Related‑Party Transactions Expenses paid directly to consultants and vendors considered to be related parties amounted to $2,489, $2,005, and $2,400 for the years ended December 31, 2019, 2018 and 2017, respectively. Outstanding payments due to these related parties as of December 31, 2019 and 2018 were $264 and $260, respectively and were included within accounts payable on the balance sheet.

Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2019

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, 2018	2,616,530	$	16.67	8.58	$	—
Granted	1,445,200		27.98	—		—
Exercised	(235,833)		6.63	—		3,844
Cancelled	(397,400)		24.85	—		—
Outstanding as of December 31, 2019	3,428,497	$	21.17	7.94	$	18,643
Options vested and expected to vest as of December 31, 2019	3,428,497	$	21.17	7.94	$	18,643
Options exercisable at December 31, 2019	1,446,311	$	16.03	7.11	$	13,872

Summary of stock-based compensation expense by award type



	Year Ended
	December 31,
	2019		2018		2017
Stock options	$	11,667	$	6,241	$	2,084
Employees stock purchase plan		208		65		—
Restricted stock units		—		84		194
Total	$	11,875	$	6,390	$	2,278

2017 Equity Incentive Plan

Stock-based Compensation

Summary of stock-based compensation expense



	Year Ended
	December 31,
	2019		2018		2017
Research and development	$	5,163	$	2,793	$	927
Selling, general, and administrative		6,712		3,597		1,351
Total	$	11,875	$	6,390	$	2,278

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,
	2019		2018		2017
Risk‑free interest rate	2.40	%	2.73	%	1.97	%
Expected term (in years)	6.07		5.89		5.95
Expected volatility	66.03	%	62.21	%	66.18	%
Expected dividend yield	—		—		—

Commitments and Contingencies	12 Months Ended
Commitments and Contingencies	Dec. 31, 2019
Commitments and Contingencies
Commitments and Contingencies	9. Commitments and Contingencies Legal Proceedings The Company, from time to time, may be party to litigation arising in the ordinary course of business. The Company was not subject to any material legal proceedings during the years ended December 31, 2019, 2018 and 2017 and to the best of its knowledge, no material legal proceedings are currently pending or threatened. Other The Company is party to various agreements, principally relating to licensed technology, that require future payments relating to milestones that may be met in subsequent periods, or royalties on future sales of specified products. See Note 8 for discussion of these arrangements. Additionally, the Company is party to various contracts with CROs and CMOs that generally provide for termination on notice, with the exact amounts in the event of termination to be based on the timing of the termination and the terms of the agreement. Based on the Company’s current development plans as of December 31, 2019, potential payments due to third parties during the next 12 months from the filing of this Annual Report on Form 10-K are estimated be approximately $8,000, including $3,000 in regulatory milestones and $5,000 in commercial milestones, in connection with our license agreements. These milestones generally become due and payable upon achievement of such milestones or sales. When the achievement of these milestones or sales have not occurred, such contingencies are not recorded in the Company’s consolidated financial statements.

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2019

Summary of Significant Accounting Policies

Schedule of prepaid expenses and other current assets


	December 31,
	2019		2018
Prepaid research and development costs	$	6,438	$	5,288
Other current assets		3,507		1,340
Prepaid expenses and other current assets	$	9,945	$	6,628

Schedule of property, plant and equipment


	Useful	December 31,
	Life	2019		2018
Leasehold improvements	*	$	2,705	$	1,103
Office equipment	5 years		70		70
Computers and software	3 years		411		411
Furniture, fixtures and equipment	5 years		1,237		345
			4,423		1,929
Less accumulated depreciation and amortization			(752)		(809)
Property and Equipment, net		$	3,671	$	1,120

*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,
	2019		2018		2017
Stock options		3,428,497		2,616,530		1,832,639

Schedule of basic and diluted earnings per share

Basic and diluted earnings per share is calculated as follows:


	Year Ended
	December 31,
	2019		2018		2017
Numerator:
Net loss	$	(140,729)	$	(74,064)	$	(33,709)
Cumulative dividends on convertible preferred shares		—		—		(3,873)
Loss attributable to common shares—basic and diluted	$	(140,729)	$	(74,064)	$	(37,582)
Denominator:
Weighted-average number of common shares—basic and diluted		36,422,450		31,004,047		13,267,960
Loss per common share—basic and diluted	$	(3.86)	$	(2.39)	$	(2.83)

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Operating expenses:
Research and development	$ 109,450	$ 50,337	$ 22,894
Selling, general, and administrative	36,550	28,080	9,518
Total operating expenses	146,000	78,417	32,412
Loss from operations	(146,000)	(78,417)	(32,412)
Other income (expense):
Revaluation of Series A Investor Instrument and Series A Investor Right/Obligation			(1,863)
Interest income, net	5,271	4,353	566
Total other income, net	5,271	4,353	(1,297)
Net loss	(140,729)	(74,064)	(33,709)
Net Income (Loss), Including Portion Attributable to Noncontrolling Interest	(140,729)	(74,064)	(33,709)
Net loss attributable to common stockholders	$ (140,729)	$ (74,064)	$ (37,582)
Net loss per share attributable to common stockholders, basic and diluted	$ (3.86)	$ (2.39)	$ (2.83)
Weighted-average common shares outstanding, basic and diluted	36,422,450	31,004,047	13,267,960
Other comprehensive loss:
Net loss	$ (140,729)	$ (74,064)	$ (33,709)
Unrealized gain on marketable securities	144	8	(141)
Comprehensive loss	$ (140,585)	$ (74,064)	$ (33,709)

Subsequent Events	12 Months Ended
Subsequent Events	Dec. 31, 2019
Subsequent Events
Subsequent Events	12. 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. The Company has evaluated all subsequent events and determined that there are no material recognized or unrecognized subsequent events requiring disclosure, other than as disclosed below. On January 6, 2020, the Company announced that Keith Gottesdiener, M.D., the Company’s Chief Executive Officer and President, intends to step down from his roles with the Company following submission of the Company’s NDA filing with the FDA. Dr. Gottesdiener is expected to remain in his roles until the earlier of (i) the planned NDA filing, (ii) the start date of a new Chief Executive Officer of the Company, and (iii) March 31, 2020 (the Termination Date). Dr. Gottesdiener will also step down as a director of the Company on such date. Thereafter, Dr. Gottesdiener is expected to serve as a consultant to the Company until December 31, 2020. In connection with the above announcement, the Company and Dr. Gottesdiener entered into a separation agreement which entitles Dr. Gottesdiener to certain severance payments and benefits as set forth therein (the Separation Agreement). In accordance with the terms of the Separation Agreement, the Company will pay Dr. Gottesdiener a severance amount equal to $531, equal to 12 months of base salary, to be paid in installments in accordance with the Company's normal payroll practices following the Termination Date. Dr. Gottesdiener will also receive an additional monthly payment for 12 months after his Termination Date which he may use to cover a portion of certain healthcare expenses. In addition, Dr. Gottesdiener will provide consulting services to the Company, and will be paid $10 per month for up to 10 hours of services, subject to the terms and conditions of the consulting agreement through December 31, 2020. In connection with the Separation Agreement, the Company modified certain equity awards held by Dr. Gottesdiener. The modification included the continuation of vesting of stock options through the end of the consulting agreement and an extension of the post-termination exercise period for vested options from 90 days to up to two years. In connection with this modification, the Company will record an incremental compensation charge of $2,968 during the three months ended March 31, 2020.

Selected Quarterly Financial Data (unaudited) (Tables)

12 Months Ended

Dec. 31, 2019

Selected Quarterly Financial Data (unaudited)

Schedule of Quarterly Financial Information


	Three months ended
	March 31,		June 30,		September 30,		December 31,
	2019		2019		2019		2019

Total revenue	$	—	$	—	$	—	$	—
Total operating expenses		30,520		44,149		37,107		34,224
Other income (expense), net:		1,546		1,353		1,104		1,268
Net loss		(28,974)		(42,796)		(36,003)		(32,956)
Net loss attributable to common stockholders	$	(28,974)	$	(42,796)	$	(36,003)	$	(32,956)
Net loss attributable to common stockholders per common share, basic and diluted	$	(0.84)	$	(1.24)	$	(1.04)	$	(0.78)

	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		(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)

Summary of Significant Accounting Policies

12 Months Ended

Dec. 31, 2019

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

Use of Estimates

Principles of Consolidation

Off‑Balance Sheet Risk and Concentrations of Credit Risk

Segment Information

Cash and Cash Equivalents

Short‑term Investments

Restricted Cash

Prepaid Expenses and Other Current Assets


	December 31,
	2019		2018
Prepaid research and development costs	$	6,438	$	5,288
Other current assets		3,507		1,340
Prepaid expenses and other current assets	$	9,945	$	6,628

Property and Equipment

Property and Equipment consists of the following:


	Useful	December 31,
	Life	2019		2018
Leasehold improvements	*	$	2,705	$	1,103
Office equipment	5 years		70		70
Computers and software	3 years		411		411
Furniture, fixtures and equipment	5 years		1,237		345
			4,423		1,929
Less accumulated depreciation and amortization			(752)		(809)
Property and Equipment, net		$	3,671	$	1,120

*Shorter of asset life or lease term.

Depreciation and amortization expense for the years ended December 31, 2019, 2018 and 2017 was $834, $442 and $223, respectively.

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.

Government Grants

Research and Development Expenses

Income Taxes

Net Loss Per Share Attributable to Common Shareholders


	Year Ended
	December 31,
	2019		2018		2017
Stock options		3,428,497		2,616,530		1,832,639

Basic and diluted earnings per share is calculated as follows:


	Year Ended
	December 31,
	2019		2018		2017
Numerator:
Net loss	$	(140,729)	$	(74,064)	$	(33,709)
Cumulative dividends on convertible preferred shares		—		—		(3,873)
Loss attributable to common shares—basic and diluted	$	(140,729)	$	(74,064)	$	(37,582)
Denominator:
Weighted-average number of common shares—basic and diluted		36,422,450		31,004,047		13,267,960
Loss per common share—basic and diluted	$	(3.86)	$	(2.39)	$	(2.83)

Comprehensive Income (Loss)

Patent Costs

Subsequent Events

Application of New or Revised Accounting Standards

Fair Value of Financial Assets and Liability

12 Months Ended

Dec. 31, 2019

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, 2019 and 2018, the carrying amount of cash and cash equivalents and short‑term investments was $292,459 and $252,061, respectively, which approximates fair value. Cash and cash equivalents 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 $53,014 and $37,019 as of December 31, 2019 and 2018, 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.

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, 2019 using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash Equivalents:
Corporate Debt Securities and Commercial Paper	$	—	$	8,885	$	—	$	8,885
Money Market Funds		53,014		—		—		53,014
Marketable Securities:
Corporate Debt Securities and Commercial Paper		—		230,165		—		230,165
Total	$	53,014	$	239,050	$	—	$	292,064


	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

Marketable Securities

The following tables summarize the Company's marketable securities:


	December 31, 2019
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Assets
Corporate debt securities and commercial paper	$	230,155	$	54	$	(44)	$	230,165
	$	230,155	$	54	$	(44)	$	230,165


	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
	$	202,653	$	23	$	(157)	$	202,519

2017 Series A Investor Instrument

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. The Company had classified its 2017 Series A Investor Instrument as a liability as it was 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 expected benefit was 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 was estimated as a weighted-average of IPO and remain private scenarios, and the future value was converted to a present value assuming a closing date of August 15, 2017 and a nominal, risk‑free discount rate.

The Company used the Black‑Scholes option‑pricing model to value the 2017 Series A Investor Call Option. The fair value of the Series A Investor Instrument is determined to be the sum of the probability‑weighted fair values of the 2017 Series A Investor Right/Obligation and the 2017 Investor Call Option.

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 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.

- 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

Stock-based Compensation - Compensation expense (Details) - 2017 Equity Incentive Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Stock-based Compensation
Stock based compensation expense	$ 11,875	$ 6,390	$ 2,278
Unrecognized compensation expense related to non-vested portion of awards	$ 27,885
Weighted average period for recognition of unrecognized compensation expense	2 years 5 months 23 days
Research and development
Stock-based Compensation
Stock based compensation expense	$ 5,163	2,793	927
Selling, general and administrative
Stock-based Compensation
Stock based compensation expense	6,712	3,597	1,351
Stock options
Stock-based Compensation
Stock based compensation expense	11,667	6,241	2,084
Employees stock purchase plan
Stock-based Compensation
Stock based compensation expense	$ 208	65
Restricted stock units
Stock-based Compensation
Stock based compensation expense		$ 84	$ 194

Nature of Business (Details) - USD ($) $ in Thousands	12 Months Ended
Nature of Business (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Nature of Business
Accumulated deficit	$ (325,331)	$ (184,602)
Revenue	0	0	$ 0
Carrying amount of cash and cash equivalents and short term investments	$ 292,459	$ 252,061

Income Tax (Tables)

12 Months Ended

Dec. 31, 2019

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,
	2019		2018		2017
Statutory tax rate	21.00	%	21.00	%	34.00	%
State tax, net of federal benefit	6.75	%	6.90	%	4.08	%
Research and development credit	2.49	%	1.52	%	1.87	%
Orphan drug credit	1.85	%	1.95	%	2.29	%
Tax law change	—	%	—	%	(27.98)	%
Stock compensation	(0.10)	%	0.46	%	(1.84)	%
Investor instrument revaluation	—	%	—	%	(1.88)	%
Other	0.20	%	0.05	%	(0.07)	%
Change in valuation allowance	(32.19)	%	(31.88)	%	(10.47)	%
Effective tax rate	—	%	—	%	—	%

Schedule of principal components of deferred tax assets


	As of
	December 31,
	2019		2018
Deferred tax assets:
Net operating loss carryforwards	$	71,524	$	35,776
Research and development credits		7,876		3,456
Orphan drug credit		6,889		4,286
Capitalized license fee		1,734		1,760
Stock-based compensation		3,628		1,517
Other		2,006		884
Total deferred tax assets		93,657		47,679
Valuation allowance		(92,943)		(47,679)
Net deferred tax assets		714		—
Deferred tax liabilities:
Operating lease right-of-use asset and other		(714)		—
Total deferred tax liabilities	$	(714)	$	—

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

Right of Use Asset and Lease Liability (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Aug. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2019
Right Of Use Asset and Lease Liability
Remaining term of operating lease (in years)		5 years 6 months
Operating lease discount rate		10.00%
Lease term (in years)	6 years
Lease renewal term (in years)	5 years	5 years
Option to extend	true
Rent expense, or operating lease costs		$ 629	$ 359	$ 215
Cash payments in the measurement of operating lease liability		425
Right-of-use asset		2,045			$ 3,265
Operating Lease, Liability		$ 3,558			$ 3,636

Summary of Significant Accounting Policies - Government Grants (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) person	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Government Grant
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	$ 0	$ 210	$ 0

Preferred Stock and Common Stock (Details) $ / shares in Units, $ in Thousands							12 Months Ended
	Oct. 18, 2018 USD ($) $ / shares shares	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, 2017	Dec. 31, 2019 $ / shares shares	Dec. 31, 2018 $ / shares shares	Oct. 05, 2017 $ / shares shares
Preferred and Common Stock
Common Stock, Shares Authorized								120,000,000	120,000,000	120,000,000
Common Stock, Par or Stated Value Per Share \| $ / shares								$ 0.001	$ 0.001	$ 0.001
Preferred Stock, Shares Authorized								10,000,000	10,000,000	10,000,000
Preferred Stock, Par or Stated Value Per Share \| $ / shares								$ 0.001	$ 0.001	$ 0.001
Common Stock, Capital Shares Reserved for Future Issuance								3,096,344
Share-based Compensation Arrangement by Share-based Payment Award, Number of Shares Available for Grant								591,077
Issuance of stock (in shares)	9,324,324	6,591,800
Remaining shares distributed to holders of common stock						1,617,646
Price per share \| $ / shares	$ 18.50	$ 26.42
Net proceeds \| $	$ 161,352	$ 162,878
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
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)	1,216,216	859,800
Common stock
Preferred and Common Stock
Issuance of common stock on exchange of preferred stock						8,578,646
Series A‑1 Junior Preferred Stock
Preferred and Common Stock
Issuance of common stock on exchange of preferred stock						78,666,209
Equity Option [Member]
Preferred and Common Stock
Common Stock, Capital Shares Reserved for Future Issuance								2,505,267
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

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Accrued Expenses
Research and development costs	$ 8,059	$ 2,614
Professional fees	1,439	858
Payroll related	3,655	2,410
Other	377	60
Accrued expenses	$ 13,530	$ 5,942

Income Tax - Deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Deferred tax assets:
Net operating loss carryforwards	$ 71,524	$ 35,776
Research and development credits	7,876	3,456
Orphan drug credit	6,889	4,286
Capitalized license fee	1,734	1,760
Stock-based compensation	3,628	1,517
Other	2,006	884
Gross deferred tax assets	93,657	47,679
Valuation allowance	(92,943)	$ (47,679)
Net deferred tax assets	714
Deferred Tax Liabilities:
Operating lease right-of-use asset and other	(714)
Total deferred tax liabilities:	$ (714)

Accrued Expenses

12 Months Ended

Dec. 31, 2019

Accrued Expenses

Accrued Expenses

3. Accrued Expenses

Accrued expenses consisted of the following:


	December 31,		December 31,
	2019		2018
Research and development costs	$	8,059	$	2,614
Professional fees		1,439		858
Payroll related		3,655		2,410
Other		377		60
Accrued expenses	$	13,530	$	5,942

Fair Value of Financial Assets and Liability (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Fair Value of Financial Assets and Liability
Carrying amount of cash and cash equivalents and short term investments	$ 292,459	$ 252,061
Fair value of financial assets and liability
Marketable Securities	230,165	202,519
Fair value of assets	230,165	202,519
Corporate Debt Securities and Commercial Paper
Fair value of financial assets and liability
Marketable Securities	230,165	202,519
Level 1 \| Money Market Funds and Government Funds in Aggregate
Fair value of financial assets and liability
Fair value of assets	53,014	37,019
Recurring \| Estimated fair value
Fair value of financial assets and liability
Fair value of assets	292,064	245,514
Recurring \| Corporate Debt Securities and Commercial Paper \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents	8,885	5,976
Marketable Securities		202,519
Recurring \| Money Market Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents	53,014	37,019
Recurring \| Level 1 \| Estimated fair value
Fair value of financial assets and liability
Fair value of assets	53,014	37,019
Recurring \| Level 1 \| Money Market Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents	53,014	37,019
Recurring \| Level 2 \| Estimated fair value
Fair value of financial assets and liability
Fair value of assets	239,050	208,495
Recurring \| Level 2 \| Corporate Debt Securities and Commercial Paper \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents	$ 8,885	5,976
Marketable Securities		$ 202,519

Summary of Significant Accounting Policies - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 USD ($) item	Dec. 31, 2018 USD ($)
Off Balance Sheet Risk and Concentrations of Credit Risk
Number of federally insured financial institutions \| item	2
Segment Information
Number of operating segments \| item	1
Prepaid Expenses and Other Current Assets
Prepaid research and development costs	$ 6,438	$ 5,288
Other current assets	3,507	1,340
Prepaid Expense and Other Assets, Current	$ 9,945	$ 6,628

Right of Use Asset and Lease Liability

12 Months Ended

Dec. 31, 2019

Right Of Use Asset and Lease Liability

Right Of Use Asset and Lease Liability

5. Right Of Use Asset and Lease Liability

The Company has a material operating lease for its head office facility and other immaterial operating leases for certain equipment. The Company’s office lease has a remaining lease term of 5.5 years. The Company has measured the lease liability associated with the office lease using a discount rate of 10%. The Company estimated the incremental borrowing rate for the leased asset based on a range of comparable interest rates the Company would incur to borrow an amount equal to the lease payments on a collateralized basis over a similar term in a similar economic environment. As of December 31, 2019, the Company has not entered into any lease arrangements classified as a finance lease.

Under ASC 842, the Company determines, at the inception of the contract, whether the contract is or contains a lease based on whether the contract provides the Company the right to control the use of a physically distinct asset or substantially all of the capacity of an asset. Leases with an initial noncancelable term of twelve months or less that do not include an option to purchase the underlying asset that the Company is reasonably certain to exercise are classified as short-term leases. The Company has elected as an accounting policy to exclude from the consolidated balance sheets a ROU asset and lease liability for short-term leases.

Upon adoption of ASC 842, the Company elected the transition relief package, permitted within the standard, pursuant to which the Company did not reassess the classification of existing leases, whether any expired or existing contracts contain a lease, and whether existing leases have any initial direct costs. The Company also elected the practical expedient of not separating lease components from non-lease components for all leases. There was no cumulative-effective adjustment to the opening balance of retained earnings. The Company reviews all material contracts for embedded leases to determine if they have a right-of-use asset.

The Company recognizes rent expense on a straight-line basis over the lease period. The depreciable life of assets and leasehold improvement are limited by the expected lease term, unless there is a transfer of title or purchase option reasonably certain of exercise.

As a result of the adoption of ASC 842 on January 1, 2019, the Company recorded both an operating lease right-of-use asset of $3,265 and a lease liability of $3,636. The standard did not materially impact the consolidated statement of cash flows and had no impact on the consolidated income statement.

The Company’s office lease includes both lease and non-lease components. Non-lease components relate to real estate taxes, insurance, operating expenses and common area maintenance, which are usually billed at actual amounts incurred proportionate to the Company’s rented square feet of the building. These non-lease components are expensed by the Company as they are incurred and are not included in the measurement of the lease liability.

The Company’s corporate headquarters is located in Boston, Massachusetts. 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 commenced May 2019 and has a term of six years with a five-year renewal option to extend the lease. As of January 1, 2019, the Company has not included the five-year renewal option to extend the lease in its measurement of the ROU asset or lease liability. Rent expense, or operating lease costs, for the years ended December 31, 2019, 2018 and 2017 were $629, $359 and $215, respectively.

Supplemental cash flow information related to the Company’s lease for the year ended December 31, 2019, includes cash payments of $425 used in the measurement of its operating lease liability.

The following table presents the maturities of the Company’s operating lease liability related to office space as of December 31, 2019, all of which is under a non-cancellable operating lease:


	Operating Lease
2020	$	786
2021		802
2022		818
2023		834
2024		851
Thereafter		502
Total operating lease payments		4,593
Less: imputed interest		1,035
Total operating lease liability	$	3,558

Stock-based Compensation - 2017 Equity Incentive Plan (Details) - USD ($) $ / shares in Units, $ in Thousands			12 Months Ended
	Jan. 01, 2020	Jan. 01, 2019	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Jan. 01, 2018
Stock-based Compensation
Common stock reserved for issuance			3,096,344
Nonemployees
Stock-based compensation
Expected term (in years)			10 years
2017 Equity Incentive Plan
Stock-based Compensation
Common stock reserved for issuance	1,759,870	1,376,429	5,933,764			1,091,366
Percentage increase in authorized shares under the plan			4.00%
Stock options total intrinsic value of stock options			$ 3,844	$ 7,980	$ 588
Grant cliff vesting period			1 year
Number of common stock options granted			1,445,200	1,218,790	1,112,717
Weighted average grant date fair value relating to outstanding stock options			$ 17.19	$ 17.27	$ 4.98
Incremental expenses			$ 56		$ 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			2.40%	2.73%	1.97%
Expected term (in years)			6 years 26 days	5 years 10 months 21 days	5 years 11 months 12 days
Expected volatility			66.03%	62.21%	66.18%
2017 Employee Stock Purchase Plan
Stock-based Compensation
Common stock reserved for issuance			591,077,000
Share-based Compensation Arrangement by Share-based Payment Award, Number of Additional Shares Authorized	439,968	344,107
Percentage increase in authorized shares under the plan			1.00%
Number of common stock options granted				25,871
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%