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Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2017	Nov. 10, 2017
Document and Entity Information
Entity Registrant Name	RHYTHM PHARMACEUTICALS, INC.
Entity Central Index Key	0001649904
Document Type	10-Q
Document Period End Date	Sep. 30, 2017
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Entity Current Reporting Status	No
Entity Filer Category	Non-accelerated Filer
Entity Common Stock, Shares Outstanding		27,284,140
Document Fiscal Year Focus	2017
Document Fiscal Period Focus	Q3

Condensed Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Current assets:
Cash and cash equivalents	$ 27,914	$ 6,540
Short-term investments	2,501	3,997
Prepaid expenses and other current assets	1,621	638
Total current assets	32,036	11,175
Property, plant and equipment, net	845	930
Deferred issuance costs	1,698	9
Restricted cash	225	225
Total assets	34,804	12,339
Current liabilities:
Accounts payable	2,271	1,895
Due to related party		105
Deferred rent	81	76
Accrued expenses and other current liabilities	3,108	2,655
Total current liabilities	5,460	4,731
Long-term liabilities:
Deferred rent	250	311
Total liabilities	5,710	5,042
Commitments and contingencies
Preferred stock:
Series A Convertible Preferred Stock, $1.00 par value: 80,950,000 shares authorized; 80,949,999 shares issued and outstanding at September 30, 2017 and 40,000,000 shares issued and outstanding at December 31, 2016; (aggregate liquidation preference of $88,864 and $44,129 at September 30, 2017 and December 31, 2016 respectively)	80,950	40,000
Stockholders’ equity (deficit):
Common stock, $0.001 par value: 29,919,979 shares authorized; 1,770,302 and 10,196,292 shares issued and outstanding and September 30, 2017 and December 31, 2016, respectively	2	10
Series A-1 Convertible Junior Preferred Stock, $0.001 par value, 78,666,209 shares authorized; 78,666,209 and no shares issued and outstanding at September 30, 2017 and December 31, 2016, respectively	79
Additional paid-in capital	47,784	43,830
Accumulated deficit	(99,721)	(76,543)
Total stockholders’ equity (deficit)	(51,856)	(32,703)
Total liabilities, convertible preferred stock and stockholders’ equity (deficit)	$ 34,804	$ 12,339

Condensed Balance Sheets (Parenthetical) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Condensed Balance Sheets
Temporary equity, par value per share	$ 1.00	$ 1.00
Temporary equity, shares authorized	80,950,000	80,950,000
Temporary equity, shares issued	80,949,999	40,000,000
Temporary equity, shares outstanding	80,949,999	40,000,000
Temporary equity, liquidation preference	$ 88,864	$ 44,129
Common stock, par value per share	$ 0.001	$ 0.001
Common stock, authorized	29,919,979	29,919,979
Common stock, issued	1,770,302	10,196,292
Common stock, outstanding	1,770,302	10,196,292
Preferred stock par value per share	$ 0.001	$ 0.001
Preferred stock, authorized	78,666,209	78,666,209
Preferred stock, issued	78,666,209	0
Preferred stock, outstanding	78,666,209	0

Condensed Statements of Operations and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016
Operating expenses:
Research and development	$ 5,971	$ 5,419	$ 16,241	$ 13,963
General and administrative	2,315	982	5,188	3,567
Total operating expenses	8,286	6,401	21,429	17,530
Loss from operations	(8,286)	(6,401)	(21,429)	(17,530)
Other income (expense):
Revaluation of Series A Investor Instrument	(1,781)		(1,863)
Interest income, net	51	10	114	24
Total other income (expense):	(1,730)	10	(1,749)	24
Net loss and comprehensive loss	(10,016)	(6,391)	(23,178)	(17,506)
Net loss attributable to common stockholders	$ (11,429)	$ (7,191)	$ (26,963)	$ (19,902)
Net loss attributable to common stockholders per common share, basic and diluted	$ (1.78)	$ (0.71)	$ (3.02)	$ (1.95)
Weighted average common shares outstanding, basic and diluted	6,404,254	10,196,292	8,918,389	10,196,292

Condensed Statements of Convertible Preferred Stock and Stockholders’ Equity (Deficit) - USD ($) $ in Thousands	Common stock	Preferred stock Series A Convertible Preferred Stock	Preferred stock Series A-1 Junior Preferred Stock	Additional Paid-in Capital	Accumulated Deficit	Total
Beginning balance at Dec. 31, 2015		$ 40,000
Beginning balance (in shares) at Dec. 31, 2015		40,000,000
Ending balance at Dec. 31, 2016		$ 40,000				$ 40,000
Ending balance (in shares) at Dec. 31, 2016		40,000,000				40,000,000
Beginning balance at Dec. 31, 2015	$ 10			$ 42,662	$ (50,671)	$ (7,999)
Beginning balance (in shares) at Dec. 31, 2015	10,196,292
Increase (Decrease) in Stockholders' Equity
Stock compensation expense				1,168		1,168
Net loss					(25,872)	(25,872)
Ending balance at Dec. 31, 2016	$ 10			43,830	(76,543)	(32,703)
Ending balance (in shares) at Dec. 31, 2016	10,196,292
Increase (Decrease) in Convertible Preferred Stock
Issuance of Series A Convertible Preferred Stock		$ 40,622
Issuance of Series A Convertible Preferred Stock (in shares)		40,949,999
Settlement of Series A investor instrument		$ 328		1,863		1,863
Ending balance at Sep. 30, 2017		$ 80,950				$ 80,950
Ending balance (in shares) at Sep. 30, 2017		80,949,999				80,949,999
Increase (Decrease) in Stockholders' Equity
Stock compensation expense				1,569		$ 1,569
Issuance of common stock in connection with exercise of stock options				700		700
Issuance of common stock in connection with exercise of stock options (in shares)	152,671
Change in unrealized loss on marketable securities				1		1
Net loss					(23,178)	(23,178)
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)
Exchange of common stock for Series A‑1 Junior Preferred Stock (in shares)	(8,578,661)		78,666,209
Ending balance at Sep. 30, 2017	$ 2		$ 79	$ 47,784	$ (99,721)	$ (51,856)
Ending balance (in shares) at Sep. 30, 2017	1,770,302		78,666,209

Condensed Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
Condensed Statements of Cash Flows - USD ($) $ in Thousands	Sep. 30, 2017	Sep. 30, 2016
Operating activities
Net loss	$ (23,178)	$ (17,506)
Adjustments to reconcile net loss to cash used in operating activities:
Stock-based compensation expense	1,569	859
Depreciation and amortization	163	89
Non-cash rent expense	(56)	29
Mark to market revaluation of Series A Investor Instrument	1,863
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	(972)	(272)
Deferred issuance costs	(1,689)	(348)
Tenant improvement allowance		376
Accounts payable, accrued expenses and other current liabilities	830	(335)
Deferred grant income		(71)
Due to related parties	(105)	139
Net cash used in operating activities	(21,575)	(17,040)
Investing activities
Purchases of short-term investments	(13,021)	(11,211)
Maturities of short-term investments	14,506	7,054
Purchases of property, plant and equipment	(78)	(1,057)
Net cash provided by (used in) investing activities	1,407	(5,214)
Financing activities
Net proceeds from issuance of Series A Convertible Preferred Stock	40,842
Proceeds from the exercise of stock options	700
Net cash provided by financing activities	41,542
Net increase (decrease) in cash and cash equivalents	21,374	(22,254)
Cash and cash equivalents at beginning of period	6,540	34,869
Cash and cash equivalents at end of period	$ 27,914	$ 12,615

Summary of Significant Accounting Policies

9 Months Ended

Sep. 30, 2017

Summary of Significant Accounting Policies

Summary of Significant Accounting Policies

2. Summary of Significant Accounting Policies

Basis of Presentation

The Company's unaudited condensed financial statements have been prepared in conformity with accounting principles generally accepted in the United States (“GAAP”) and the applicable rules and regulations of the Securities and Exchange Commission (“SEC”) regarding interim financial reporting. 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”). As permitted under these rules, certain footnotes or other financial information that are normally required by GAAP have been condensed or omitted.

The accompanying interim balance sheet as of September 30, 2017, the statements of operations and comprehensive loss for the three and nine months ended September 30, 2017 and 2016, the statement of cash flows for the nine months ended September 30, 2017 and 2016, and the statement of convertible preferred stock and stockholders' equity (deficit) for the nine months ended September 30, 2017 and the related footnote disclosures are unaudited. In management's opinion, the unaudited interim financial statements have been prepared on the same basis as the audited financial statements and include all adjustments, which include all normal recurring adjustments necessary for the fair presentation of the interim financial statements. The results for the nine months ended September 30, 2017 are not necessarily indicative of the results expected for the full fiscal year.

The Company has historically existed and functioned as part of the consolidated businesses of the Predecessor Company. As noted above, the Predecessor Company's setmelanotide and MC4R agonist programs were transferred to the Company as part of the Corporate Reorganization on March 21, 2013. These financial statements include the results of operations of setmelanotide and the MC4R agonist program from its inception. As part of the Corporate Reorganization, the Company also entered into a formal payroll services intercompany agreement with the Relamorelin Company. On November 16, 2016, the employees of the Relamorelin Company that were providing services to the Company, terminated their employment contracts with the Relamorelin Company and entered into new employment agreements with the Company. On December 15, 2016, the Relamorelin Company closed its sale to a large pharmaceutical company. At September 30, 2017, the Company had seventeen employees directly employed by the Company. During 2016, costs have been allocated to the Company for the purposes of preparing the financial statements based on a specific identification basis or, when specific identification is not practicable, a proportional cost allocation method which allocates expenses based upon the percentage of employee time and research and development effort expended on the Company's business as compared to total employee time and research and development effort of the combined Motus and Rhythm. The proportional use basis adopted to allocate shared costs is in accordance with the guidance of SEC Staff Accounting Bulletin (“SAB”) Topic 1B, Allocation Of Expenses And Related Disclosure In Financial Statements Of Subsidiaries, Divisions Or Lesser Business Components Of Another Entity. Management has determined that the method of allocating costs to the Company is reasonable. Cost allocation was no longer required subsequent to the 2016 sale of the Relamorelin Company.

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

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

On September 22, 2017, the Company's board of directors approved a 1-for-9.17 reverse stock split of the Company's issued and outstanding shares of common stock. All share and per share amounts in the financial statements have been retrospectively adjusted for all periods presented to give effect of the reverse stock split. On September 29, 2017 the Company filed a third amended and restated certificate of incorporation with the Secretary of State of the State of Delaware to reflect the reverse stock split and authorize 29,919,979 of its shares of common stock, $0.001 par value per share and 159,616,209 shares of preferred stock, $0.001 par value per share.

Use of Estimates

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

Off‑Balance Sheet Risk and Concentrations of Credit Risk

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

Segment Information

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

2017 Series A Investor Instrument

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

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

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 to 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 is therefore considered to be participating securities. Participating securities have the effect of diluting both basic and diluted earnings per share during periods of income. During periods of loss, the Company allocates no loss to participating securities because they have no contractual obligation to share in the losses of the Company. Diluted net loss per share attributable to common stockholders is calculated by adjusting weighted average shares outstanding for the dilutive effect of common stock equivalents outstanding for the period, determined using the treasury‑stock and if‑converted methods. For purposes of the diluted net loss per share attributable to common stockholders calculation, convertible preferred stock and stock options are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share attributable to common stockholders, as their effect would be anti‑dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented.

Basic and diluted earnings per share is calculated as follows:


	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2017		2016		2017		2016
Numerator:
Net loss	$	(10,016)	$	(6,391)	$	(23,178)	$	(17,506)
Cumulative dividends on convertible preferred shares		(1,413)		(800)		(3,785)		(2,396)
Loss attributable to common shares—basic and diluted	$	(11,429)	$	(7,191)	$	(26,963)	$	(19,902)
Denominator:
Weighted-average number of common shares—basic and diluted		6,404,254		10,196,292		8,918,389		10,196,292
Loss per common share—basic and diluted	$	(1.78)	$	(0.71)	$	(3.02)	$	(1.95)

Application of New or Revised Accounting Standards

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

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

In February 2016, the FASB issued ASU 2016‑02, Leases (Topic 842). ASU 2016‑02 requires lessees to recognize lease assets and lease liabilities for those leases classified as operating leases under previous GAAP. A lessee should recognize in the statement of financial position a liability to make lease payments (the lease liability) and a right‑of‑use asset representing its right to use the underlying asset for the lease term. For leases with a term of 12 months or less, a lessee is permitted to make an accounting policy election by class of underlying asset not to recognize lease assets and lease liabilities. The recognition, measurement, and presentation of expenses and cash flows arising from a lease by a lessee have not significantly changed from previous GAAP. There continues to be a differentiation between finance leases and operating leases. ASU 2016‑02 is effective for fiscal years beginning after December 15, 2018, and interim periods within those fiscal years, and early adoption is permitted. The Company is currently in the process of evaluating the impact of adoption of ASU 2016‑02 on its financial position and results of operations.

In March 2016, the FASB issued ASU 2016‑09, Improvements to Employee Share‑Based Payment Accounting (Topic 718) that changes the accounting for certain aspects of share‑based payments to employees. The guidance requires the recognition of the income tax effects of awards in the income statement when the awards vest or are settled, thus eliminating additional paid in capital pools. The guidance also allows for the employer to repurchase more of an employee's shares for tax withholding purposes without triggering liability accounting. In addition, the guidance allows for a policy election to account for forfeitures as they occur rather than on an estimated basis. The guidance is effective for annual periods beginning after December 15, 2016, and interim periods within those annual periods with early adoption permitted. Accordingly, the standard is effective for the Company on January 1, 2018. The Company adopted the standard as of January 1, 2017. The adoption did not have a material impact on the Company's financial position, results of operations or cash flows.

In November 2016, the FASB issued ASU 2016‑18, Statement of Cash Flows (Topic 230): Restricted Cash that changes the presentation of restricted cash and cash equivalents on the statement of cash flows. Restricted cash and restricted cash equivalents will be included with cash and cash equivalents when reconciling the beginning‑of‑period and end‑of‑period total amounts shown on the statement of cash flows. This amendment is effective for fiscal years beginning after December 15, 2017, including interim periods within those fiscal years. Early adoption is permitted, including adoption in an interim period. The Company adopted the standard as of January 1, 2017. The adoption did not have a material impact on the Company’s statements of cash flows.

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

In July 2017, the FASB issued ASU 2017‑11, Earnings Per Share (Topic 260), Distinguishing Liabilities from Equity (Topic 480) and Derivatives and Hedging (Topic 815): I. Accounting for Certain Financial Instruments with Down Round Features; II. Replacement of the Indefinite Deferral for Mandatorily Redeemable Financial Instruments of Certain Nonpublic Entities and Certain Mandatorily Redeemable Noncontrolling Interests with a Scope Exception. Part I of this update addresses the complexity of accounting for certain financial instruments with down round features. Down round features are features of certain equity‑linked instruments (or embedded features) that result in the strike price being reduced on the basis of the pricing of future equity offerings. Current accounting guidance creates cost and complexity for entities that issue financial instruments (such as warrants and convertible instruments) with down round features that require fair value measurement of the entire instrument or conversion option. Part II of this update addresses the difficulty of navigating Topic 480, Distinguishing Liabilities from Equity, because of the existence of extensive pending content in the FASB Accounting Standards Codification. This pending content is the result of the indefinite deferral of accounting requirements about mandatorily redeemable financial instruments of certain nonpublic entities and certain mandatorily redeemable noncontrolling interests. The amendments in Part II of this update do not have an accounting effect. ASU 2017-11 is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018. The Company is currently assessing the potential impact of adopting ASU 2017‑11 on its financial statements and related disclosures.

Preferred Stock	9 Months Ended
Preferred Stock	Sep. 30, 2017
Preferred Stock
Preferred Stock	4. Preferred Stock In January 2017, pursuant to the Series A preferred stock purchase agreement, by and among the Company and certain purchasers, and as part of an initial tranche closing, the Company issued 20,475,001 shares of Series A convertible preferred stock, par value $0.001 per share, at a purchase price of $1.00 per share, resulting in net proceeds of $20,377 to the Company (the “January 2017 Initial Tranche Closing”). The Series A preferred stock purchase agreement provided for the delayed issuance by the Company of up to an additional 20,474,998 shares of Series A convertible preferred stock as part of a second tranche closing at a purchase price of $1.00 per share (the “2017 Series A Investor Right/Obligation”). The second tranche is contingent upon: (1) the Company's cash, cash equivalents and short‑term investments balance, net of accounts payable and accrued liabilities, falling below $5.0 million and (2) the Company's satisfaction of contractual and customary representations and warranties. Unless otherwise mutually agreed upon in writing, the rights and obligations underlying the second tranche (if not previously executed) will terminate on the first to occur of the following dates: (1) the date (the “Roadshow Acceleration Date”) on which the Company files with the U.S. Securities and Exchange Commission, or SEC, the last pre‑effective amendment to the registration statement prior to the start of the Company's roadshow in connection with the Intial Public Offering, or IPO, provided, that such termination shall be contingent upon the consummation of the IPO pursuant to the same registration statement that was on file with the SEC on the Roadshow Acceleration Date, without withdrawal thereof or filing of a subsequent registration statement in replacement thereof; and (2) the date of the consummation of a Deemed Liquidation Event (as defined below). To the extent the closing of the second tranche has not already taken place, the investors in the first tranche also have a call right on the shares underlying the second tranche whereby such shares can be purchased for the same price as the second tranche (the “2017 Series A Investor Call Option”). The 2017 Series A Investor Call Option terminates upon the Roadshow Acceleration Date. The 2017 Series A Investor Right/Obligation and the 2017 Series A Investor Call Option have been evaluated and determined to be a free standing instrument, the 2017 Series A Investor Instrument. The 2017 Series A Investor Instrument is being accounted for as a liability (see Note 2). In August 2017, the Series A Investors waived the $5.0 million cash balance requirement of the Series A Investor Right/Obligation and closed the second tranche of the series A preferred stock financing. The Company issued 20,474,998 shares of Series A convertible preferred stock, par value $0.001 per share, at a purchase price of $1.00 per share, resulting in gross proceeds of $20,475 to the Company. The 2017 Series A Investor Call Option expired unexercised at that time. Upon the closing of an initial public offering with a minimum price per share and gross proceeds of at least $1.00 and $50.0 million, respectively, the Series A convertible preferred stock will automatically convert into shares of common stock on a 9.17‑for‑1 basis. The holders of the Series A convertible preferred stock have the following rights and preferences: Voting Rights The holders of Series A convertible preferred stock are entitled to vote, together with the holders of common stock, on all matters submitted to stockholders for a vote. Each preferred stockholder is entitled to the number of votes equal to the number of shares of common stock into which each preferred share is convertible at the time of such vote. In addition, pursuant to the Company's charter, the holders of record of the outstanding shares of Series A convertible preferred stock are entitled to elect one director to serve as the Series A preferred director on the board of directors of the Company. Dividends The holders of Series A convertible preferred stock are entitled to receive dividends in preference to any dividend on common stock at the rate of 8.0% per year of the original issue price. Dividends shall accrue annually, whether or not declared, and shall be cumulative. The Company may not declare, pay or set aside any dividends on shares of any other class or series of capital stock of the Company unless the holders of Series A convertible preferred stock then outstanding shall first receive, or simultaneously receive, dividends on each outstanding share of Series A convertible preferred stock. Through September 30, 2017 and December 31, 2016, no dividends had been declared or paid by the Company. Accrued dividends, whether or not declared, shall also be payable upon any liquidation event. At September 30, 2017 and December 31, 2016, cumulative preference dividends amounted to $7,914, or $0.10 per share and $4,129, or $0.10 per share, respectively. Liquidation In the event of any liquidation, dissolution or winding‑up of the Company or a Deemed Liquidation Event (as defined below), the holders of Series A convertible preferred stock then outstanding shall be entitled to be paid out of the assets of the Company available for distribution to stockholders, and before any payment shall be made to holders of common stock, an amount per share equal to greater of (i) the original issue price per share, plus any accrued but unpaid dividends thereon, whether or not declared, plus any declared but unpaid dividends thereon, if any, or (ii) such amount per share as would have been payable had all shares of Series A convertible preferred stock been converted to common stock prior to such liquidation. If upon such event, the assets of the Company available for distribution are insufficient to permit payment in full to the holders of Series A convertible preferred stock, the proceeds will be ratably distributed among the holders of Series A convertible preferred stock in proportion to the respective amounts that they would have received if they were paid in full. After payments have been made in full to the holders of Series A convertible preferred stock, the remaining assets of the Company available for distribution will be distributed among the holders of Series A convertible preferred stock, the holders of the Series A-1 convertible junior preferred stock, and the holders of common stock as if the shares of Series A convertible preferred stock and Series A-1 convertible junior preferred stock were converted to common stock immediately prior to the liquidation event. A merger, acquisition, sale of voting control or other transaction of the Company in which the stockholders of the Company do not own a majority of the outstanding shares of the surviving company shall be considered a Deemed Liquidation Event. A sale, exclusive license, transfer or other disposition of all or substantially all of the assets of the Company shall also be considered a Deemed Liquidation Event. Each share of Series A convertible preferred stock may be redeemed at the option of the holder upon the occurrence of a deemed liquidation event. As of September 30, 2017 and December 31, 2016, the liquidation preference of the outstanding shares of Series A convertible preferred stock was approximately $88,864 and $44,129, respectively. Conversion Each share of Series A convertible preferred stock is convertible into common stock at the option of the stockholder at any time after the date of issuance. In addition, each share of Series A convertible preferred stock will be automatically converted into shares of common stock, at the applicable conversion ratio then in effect, upon the earlier of (i) a qualified public offering with gross proceeds of at least $50,000 and a price of not less than $1.00 per share, subject to appropriate adjustment for any stock dividend, stock split, combination or other similar recapitalization, and (ii) the date specified by vote or written consent of the holders of at least two‑thirds of the then outstanding shares of series A preferred stock. The shares of Series A convertible preferred stock will be converted to common stock, at par value, with the remainder recorded to additional paid‑in capital. The conversion ratio of the Series A convertible preferred stock is determined by dividing the original issue price per share by the conversion price of $9.17 per share, subject to appropriate adjustment in the event of any stock dividend, stock split, combination or recapitalization affecting the Series A convertible preferred stock. As of September 30, 2017 and December 31, 2016, the outstanding shares of Series A convertible preferred stock were convertible into 8,827,692 and 4,362,050 shares of common stock, respectively.

Fair Value of Financial Assets and Liability

9 Months Ended

Sep. 30, 2017

Fair Value of Financial Assets and Liability

Fair Value of Financial Assets and Liability

5. Fair Value of Financial Assets and Liability

As of September 30, 2017 and December 31, 2016, the carrying amount of cash and cash equivalents and short‑term investments was $30,415 and $10,537, respectively, which approximates fair value. Cash and cash equivalents and short‑term investments includes investments in money market funds that invest in U.S. government securities that are valued using quoted market prices. Accordingly, money market funds and government funds are categorized as Level 1 and had a total balance of $28,722 and $7,984 as of September 30, 2017 and December 31, 2016, respectively.

A financial liability was recognized by the Company during the nine months ending September 30, 2017 related to the 2017 Series A Investor Instrument that was exercised in August 2017. The liability was valued based on significant inputs not observable in the market, which represents a Level 3 measurement within the fair value hierarchy. For the year ended December 31, 2016, the Company had no financial liabilities outstanding measured at fair value.

The following tables present information about the Company's financial assets and liabilities 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
	September 30, 2017 using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash Equivalents:
Government Funds	$	—	$	—	$	—	$	—
Money Market Funds		26,221		—		—		26,221
Marketable Securities:
Government Funds		2,501		—		—		2,501
Total	$	28,722	$	—	$	—	$	28,722
Liabilities:
2017 Series A Investor Instrument	$	—	$	—	$	—	$	—
Total	$	—	$	—	$	—	$	—


	Fair value Measurements as of
	December 31, 2016 using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash Equivalents:
Government Funds	$	2,000	$	—	$	—	$	2,000
Money Market Funds		1,987		—		—		1,987
Marketable Securities:
Government Funds		3,997		—		—		3,997
Total	$	7,984	$	—	$	—	$	7,984

Marketable Securities

The following tables summarize the Company's marketable securities:


	September 30, 2017
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Assets
Government Funds (due within 1 year)	$	2,502	$	—	$	(1)	$	2,501
	$	2,502	$	—	$	(1)	$	2,501


	December 31, 2016
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Assets
Government Funds (due within 1 year)	$	3,997	$	—	$	—	$	3,997
	$	3,997	$	—	$	—	$	3,997

Below is a roll forward of the fair value of the 2017 Series A Investor Instrument for the nine months ended September 30, 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 through the date of settlement		1,863
Reclassification of liability upon August 2017 Second Tranche Closing		(2,191)
Fair value at September 30, 2017	$	—

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

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


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

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

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

- Definition

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

+ References + Details

Summary of Significant Accounting Policies (Policies)

9 Months Ended

Sep. 30, 2017

Summary of Significant Accounting Policies

Basis of Presentation

Use of Estimates

Off-Balance Sheet Risk and Concentrations of Credit Risk

Off‑Balance Sheet Risk and Concentrations of Credit Risk

Segment Information

2017 Series A Investor Instrument

Net Loss Per Share Attributable to Common Shareholders

Basic and diluted earnings per share is calculated as follows:


	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2017		2016		2017		2016
Numerator:
Net loss	$	(10,016)	$	(6,391)	$	(23,178)	$	(17,506)
Cumulative dividends on convertible preferred shares		(1,413)		(800)		(3,785)		(2,396)
Loss attributable to common shares—basic and diluted	$	(11,429)	$	(7,191)	$	(26,963)	$	(19,902)
Denominator:
Weighted-average number of common shares—basic and diluted		6,404,254		10,196,292		8,918,389		10,196,292
Loss per common share—basic and diluted	$	(1.78)	$	(0.71)	$	(3.02)	$	(1.95)

Application of New or Revised Accounting Standards

Summary of Significant Accounting Policies (Tables)

9 Months Ended

Sep. 30, 2017

Summary of Significant Accounting Policies

Schedule of basic and diluted earnings per share


	Three Months Ended				Nine Months Ended
	September 30,				September 30,
	2017		2016		2017		2016
Numerator:
Net loss	$	(10,016)	$	(6,391)	$	(23,178)	$	(17,506)
Cumulative dividends on convertible preferred shares		(1,413)		(800)		(3,785)		(2,396)
Loss attributable to common shares—basic and diluted	$	(11,429)	$	(7,191)	$	(26,963)	$	(19,902)
Denominator:
Weighted-average number of common shares—basic and diluted		6,404,254		10,196,292		8,918,389		10,196,292
Loss per common share—basic and diluted	$	(1.78)	$	(0.71)	$	(3.02)	$	(1.95)

Fair Value of Financial Assets and Liability (Tables)

9 Months Ended

Sep. 30, 2017

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
	September 30, 2017 using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash Equivalents:
Government Funds	$	—	$	—	$	—	$	—
Money Market Funds		26,221		—		—		26,221
Marketable Securities:
Government Funds		2,501		—		—		2,501
Total	$	28,722	$	—	$	—	$	28,722
Liabilities:
2017 Series A Investor Instrument	$	—	$	—	$	—	$	—
Total	$	—	$	—	$	—	$	—


	Fair value Measurements as of
	December 31, 2016 using:
	Level 1		Level 2		Level 3		Total
Assets:
Cash Equivalents:
Government Funds	$	2,000	$	—	$	—	$	2,000
Money Market Funds		1,987		—		—		1,987
Marketable Securities:
Government Funds		3,997		—		—		3,997
Total	$	7,984	$	—	$	—	$	7,984

Schedule of marketable securities


	September 30, 2017
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Assets
Government Funds (due within 1 year)	$	2,502	$	—	$	(1)	$	2,501
	$	2,502	$	—	$	(1)	$	2,501


	December 31, 2016
			Gross		Gross
	Amortized		Unrealized		Unrealized		Fair
	Cost		Gains		Losses		Value
Assets
Government Funds (due within 1 year)	$	3,997	$	—	$	—	$	3,997
	$	3,997	$	—	$	—	$	3,997

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 through the date of settlement		1,863
Reclassification of liability upon August 2017 Second Tranche Closing		(2,191)
Fair value at September 30, 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		—

Summary of Significant Accounting Policies (Details) $ / shares in Units, $ in Thousands		9 Months Ended	12 Months Ended
	Sep. 22, 2017	Sep. 30, 2017 employee segment item $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares	Sep. 29, 2017 $ / shares shares	Mar. 31, 2013 $ / shares
Summary of Significant Accounting Policies
Number of employees directly employed by the company \| employee		17
Employment cost \| $			$ 2,727
Forward stock split	0.109051
Common stock, authorized \| shares		29,919,979	29,919,979	29,919,979
Common stock, par value per share \| $ / shares		$ 0.001	$ 0.001	$ 0.001	$ 0.001
Preferred stock, authorized \| shares		78,666,209	78,666,209	159,616,209
Preferred stock par value per share \| $ / shares		$ 0.001	$ 0.001	$ 0.001
Off Balance Sheet Risk and Concentrations of Credit Risk
Number of federally insured financial institutions \| item		2
Segment Information
Number of operating segments \| segment		1

Preferred Stock (Details)		1 Months Ended		9 Months Ended	12 Months Ended
Preferred Stock (Details)	Sep. 22, 2017	Aug. 31, 2017 USD ($) $ / shares shares	Jan. 31, 2017 USD ($) $ / shares shares	Sep. 30, 2017 USD ($) director $ / shares shares	Dec. 31, 2016 USD ($) $ / shares shares
Preferred Stock
Temporary equity, par value per share \| $ / shares				$ 1.00	$ 1.00
Stockholders' Equity Note, Stock Split, Conversion Ratio	0.109051
Temporary equity, liquidation preference				$ 88,864,000	$ 44,129,000
Number of outstanding shares held by holder at the time of voting				0.67
Conversion price \| $ / shares				$ 9.17
Number of common stock on conversion \| shares				8,827,692	4,362,050
IPO
Preferred Stock
Minimum price per share \| $ / shares		$ 1.00
Gross proceeds		$ 50,000,000
Number of shares issued on conversion \| shares		9.17
Series A Convertible Preferred Stock
Preferred Stock
Number of directors \| director				1
Preferred dividend (as a percent)				8.00%
Dividends declared and paid				$ 0	$ 0
Cumulative preference dividends				$ 7,914,000	$ 4,129,000
Cumulative preference dividends (per share) \| $ / shares				$ 0.10	$ 0.10
Temporary equity, liquidation preference				$ 88,864,000	$ 44,129,000
January 2017 initial tranche closing
Preferred Stock
Issuance of Series A Convertible Preferred Stock (in shares) \| shares			20,475,001
Temporary equity, par value per share \| $ / shares			$ 0.001
Temporary Equity, Purchase price per share			$ 1.00
Issuance of Series A Convertible Preferred Stock			$ 20,377,000
Additional shares \| shares			20,474,998
Cash, cash equivalents and short-term investments net of accounts payable and accrued liabilities			$ 5,000,000
August 2017 second tranche closing
Preferred Stock
Issuance of Series A Convertible Preferred Stock (in shares) \| shares		20,474,998
Temporary equity, par value per share \| $ / shares		$ 0.001
Temporary Equity, Purchase price per share		$ 1.00
Issuance of Series A Convertible Preferred Stock		20,475,000
Cash balance requirement waived		$ 5,000,000

Fair Value of Financial Assets and Liability (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value of Financial Assets and Liability
Carrying amount of cash and cash equivalents and short term investments	$ 30,415	$ 10,537
Fair value of financial assets and liability
Short-term investments	2,501	3,997
Fair value of financial liabilities		0
Government Funds
Fair value of financial assets and liability
Short-term investments	2,501	3,997
Level 1
Fair value of financial assets and liability
Fair value of assets	28,722	7,984
Recurring \| Estimated fair value
Fair value of financial assets and liability
Fair value of assets	28,722	7,984
Recurring \| Money Market Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:	26,221	1,987
Recurring \| Government Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:		2,000
Short-term investments	2,501	3,997
Recurring \| Level 1 \| Estimated fair value
Fair value of financial assets and liability
Fair value of assets	28,722	7,984
Recurring \| Level 1 \| Money Market Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:	26,221	1,987
Recurring \| Level 1 \| Government Funds \| Estimated fair value
Fair value of financial assets and liability
Cash Equivalents:		2,000
Short-term investments	$ 2,501	$ 3,997

Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands	Oct. 10, 2017	Oct. 05, 2017	Sep. 30, 2017	Sep. 29, 2017	Aug. 31, 2017	Aug. 21, 2017	Dec. 31, 2016	Mar. 31, 2013
Subsequent events
Common stock shares authorized			29,919,979	29,919,979			29,919,979
Common stock, par value per share			$ 0.001	$ 0.001			$ 0.001	$ 0.001
Preferred stock, authorized			78,666,209	159,616,209			78,666,209
Preferred stock par value per share			$ 0.001	$ 0.001			$ 0.001
Common stock, outstanding			1,770,302				10,196,292
IPO
Subsequent events
Minimum price per share					$ 1.00
Subsequent event
Subsequent events
Common stock, outstanding	27,284,140
Subsequent event \| IPO
Subsequent events
Issuance of stock (in shares)	8,107,500
Minimum price per share	$ 17.00
Gross proceeds	$ 137,828
Net proceeds	$ 125,780
Conversion of Series A Convertible Preferred Stock and Series A‑1 Junior Preferred Stock into common stock on a one-to-one basis (in shares)	17,406,338
Subsequent event \| Underwriter option to purchase
Subsequent events
Issuance of stock (in shares)	1,057,500
Common stock \| Subsequent event
Subsequent events
Common stock shares authorized		120,000,000
Common stock, par value per share		$ 0.001
Common stock \| Maximum
Subsequent events
Minimum price per share						$ 1.00
Preferred stock \| Subsequent event
Subsequent events
Preferred stock, authorized		10,000,000
Preferred stock par value per share		$ 0.001
2017 Stock Incentive Plan \| Subsequent event
Subsequent events
Stock Issued During Period, Percentage of Prior Year Outstanding	4.00%
2017 Stock Incentive Plan \| Maximum \| Subsequent event
Subsequent events
Issuance of stock (in shares)	4,018,538

Nature of Business	9 Months Ended
Nature of Business	Sep. 30, 2017
Nature of Business
Nature of Business	1. Nature of Business Rhythm Pharmaceuticals, Inc. (the “Company”), is a biopharmaceutical company focused on the development and commercialization of peptide therapeutics for the treatment of genetic deficiencies that result in life‑threatening metabolic disorders. The Company's lead product candidate is setmelanotide (RM‑493), which is a potent, first‑in‑class, melanocortin‑4 receptor, or MC4R, agonist for the treatment of rare genetic disorders of obesity caused by MC4 pathway deficiencies. The Company is currently evaluating setmelanotide for the treatment of six genetic disorders of obesity: pro‑opiomelanocortin, or POMC, leptin receptor, or LepR, Bardet‑Biedl syndrome, Alström syndrome, POMC heterozygous, and POMC epigenetic disorders. Corporate Reorganization The Company is a Delaware corporation organized in February 2013 under the name Rhythm Metabolic, Inc. Prior to the Company's organization and the Corporate Reorganization referred to below, the Company was part of Rhythm Pharmaceuticals, Inc. (the “Predecessor Company”), a Delaware corporation which was organized in November 2008 and which commenced active operations in 2010. In March 2013, the Predecessor Company underwent a corporate reorganization, (the “Corporate Reorganization”), pursuant to which all of the outstanding equity securities of the Predecessor Company were exchanged for units of Rhythm Holding Company, LLC, a newly‑organized limited liability company (the “LLC entity”). After the consummation of this exchange and as part of the Corporate Reorganization, the Predecessor Company contributed setmelanotide and the MC4R agonist program to the Company and distributed to the LLC entity all of the then issued and outstanding shares of the Company's stock. The result of the Corporate Reorganization was that the Company and the Predecessor Company became wholly‑owned subsidiaries of the LLC entity and the two product candidates and related programs that were originally held by the Predecessor Company were separated, with relamorelin and the ghrelin agonist program being retained by the Predecessor Company and setmelanotide and the MC4R agonist program being held by the Company. The Predecessor Company, after consummation of the Corporate Reorganization, is referred to within these Notes to Financial Statements as the Relamorelin Company and/or Motus. On October 13, 2015, the Relamorelin Company changed its name to Motus Therapeutics, Inc. (“Motus”) and the Company changed its name to Rhythm Pharmaceuticals, Inc. On December 15, 2016, Motus was sold to a large pharmaceutical company. On August 21, 2017, the LLC entity distributed to its members all of its shares of the Company (see Note 6 for further discussion). Following this distribution, the LLC entity no longer owns any of the Company’s shares. Liquidity The Company has incurred losses since inception and negative cash flows from operating activities. As of September 30, 2017, the Company had an accumulated deficit of $99,721. The Company anticipates that it will continue to incur significant operating losses for the next several years as it continues to develop setmelanotide. The Company had cash and short-term investments of $30,415 as of September 30, 2017. In addition, the Company received additional funding in connection with its initial public offering subsequent to quarter-end (see Note 10, “Subsequent Events”). The net proceeds from this offering were approximately $125.8 million after deducting underwriting discounts and commissions and offering expenses. In the future, the Company will be dependent on obtaining funding from third parties, such as proceeds from the issuance of debt, sale of equity, and funded research and development programs, to maintain the Company's operations and meet the Company's obligations. There is no guarantee that additional equity or other financings will be available to the Company on acceptable terms, or at all. If the Company fails to obtain additional funding when needed, the Company would be forced to scale back, terminate its operations or seek to merge with or be acquired by another company. Management believes that the Company's existing cash resources will be sufficient to fund the Company's operating plan into the first half of 2019.

Common Stock	9 Months Ended
Common Stock	Sep. 30, 2017
Common Stock
Common Stock	6. Common Stock In March 2013, the Company issued 10,196,292 shares of common stock at a purchase price of $0.001 per share. As of December 31, 2016, the LLC entity owned all of these shares. On August 21, 2017, the LLC entity exchanged 8,578,646 of its shares of the Company's common stock for 78,666,209 shares of the Company's series A‑1 junior preferred stock and the LLC entity distributed all of its shares of the Company's series A‑1 junior preferred stock to the holders of its preferred units and the remaining 1,617,646 shares of its common stock to the holders of its common units. Following this distribution, the LLC entity no longer owns any of the Company's shares. The series A‑1 junior preferred stock is not redeemable and does not have a stated dividend or liquidation preference. These shares will convert to common stock on a 9.17‑to‑1 basis upon the earlier of (i) a qualified public offering with gross proceeds of at least $50,000 and a price of not less than $1.00 per share, subject to appropriate adjustment for any stock dividend, stock split, combination or other similar recapitalization, and (ii) the date specified by vote or written consent of the holders of the requisite holders of series A preferred stock and series A‑1 junior preferred stock. 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.

Significant Agreements	9 Months Ended
Significant Agreements	Sep. 30, 2017
Significant Agreements
Significant Agreements	7. Significant Agreements License Agreements The Predecessor Company entered into a license agreement on February 26, 2010 with Ipsen Pharma, S.A.S. (“Ipsen”) that granted full worldwide right for two programs that include the clinical candidates setmelanotide and relamorelin. As a result of the Corporate Reorganization described in Note 1, the Ipsen license was converted to separate license agreements for the setmelanotide program held by the Company and the relamorelin program held by the Relamorelin Company, respectively. Under the terms of the setmelanotide Ipsen license agreement, assuming that setmelanotide is successfully developed, receives regulatory approval and is commercialized, Ipsen may receive aggregate payments of up to $40,000 upon the achievement of certain development and commercial milestones and royalties on future product sales in the mid‑single digits. Substantially all of such aggregate payments of up to $40,000 are for milestones that may be achieved no earlier than first commercial sale of setmelanotide. In the event that the Company executes a sublicense agreement, it shall make payments to Ipsen, depending on the date of such sublicense agreement, ranging from 10% to 20% of all revenues actually received under such sublicense agreement. In July 2017, the Company made a prepayment on the first milestone event associated with this license agreement. The first milestone relates to the initiation of a Phase 3 study for setmelanotide in a pivotal multi-center human clinical trial in a large number of patients. The prepayment associated with this milestone was $1,000. In January 2016, the Company entered into a licensing agreement with Camurus AB, or Camurus, for the use of Camurus’ drug delivery technology. The contract includes a non‑refundable and non‑creditable signing fee of $500, which was paid during January 2016. The Camurus Agreement also includes up to $7,750 in one‑time, non‑refundable development milestones achievable upon certain regulatory successes. The Company is also required to pay to Camurus royalties, mid to mid‑high single digit, 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 and 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.

Related-Party Transactions	9 Months Ended
Related-Party Transactions	Sep. 30, 2017
Related-Party Transactions
Related-Party Transactions	8. Related‑Party Transactions The Company shared costs with the Relamorelin Company, its affiliate, including payroll, facilities, information technology and other research and development and general and administrative overhead costs. Additionally, the Relamorelin Company had paid certain Company expenses directly on behalf of the Company. Shared costs incurred by the Relamorelin Company and Company expenses paid by the Relamorelin Company on behalf of the Company are allocated from the Relamorelin Company to the Company as described in Note 1 and Note 2. The Relamorelin Company was sold to a large pharmaceutical company on December 15, 2016. The LLC entity made payments on behalf of the Company totaling $105 related to allocated 2016 employee bonuses. Those costs are recorded as a payable due to the LLC entity from the Company at December 31, 2016 on the balance sheet. Expenses paid directly by the Company to consultants considered to be related parties amounted to $294, $880, $184 and $386 for the three and nine months ended September 30, 2017 and 2016, respectively. Outstanding payments due to these related parties as of September 30, 2017 and December 31, 2016 were $130 and $50, respectively, and were included within accounts payable on the balance sheet. Expenses paid by the Relamorelin Company to these related parties amounted to zero, zero, $173 and $622 for the three and nine months ended September 30, 2017 and 2016, respectively. Employees of certain holders of series A and series B convertible preferred units of the LLC entity, have been retained as consultants supporting development activities of the Company and the Relamorelin Company for which the holders are paid cash compensation pursuant to consulting arrangements. Compensation payments related to these consultants totaled $33, $77, $11 and $48 for the three and nine months ended September 30, 2017 and 2016, respectively.

Income Taxes	9 Months Ended
Income Taxes	Sep. 30, 2017
Income Taxes
Income Taxes	9. Income Taxes In the Company's financial statements, income taxes, including deferred tax balances, have been calculated on a separate tax return basis. Certain of the Company's activities and costs have been included in the tax returns filed by the Relamorelin Company and the LLC entity. Prior to the Corporate Reorganization, the Company's operations were included in the tax returns filed by the Predecessor Company. The Company has filed tax returns on its own behalf since the Corporate Reorganization. For the year ended December 31, 2016, the Company did not have a current or deferred income tax expense or benefit as the entity has incurred losses since inception and has provided a full valuation allowance against its deferred tax assets.

Nature of Business (Details) $ in Thousands		9 Months Ended
Nature of Business (Details) $ in Thousands	Oct. 10, 2017 USD ($)	Sep. 30, 2017 USD ($) item	Dec. 31, 2016 USD ($)
Subsidiary sale of stock
Number of product candidates \| item		2
Accumulated deficit		$ 99,721	$ 76,543
Carrying amount of cash and cash equivalents and short term investments		$ 30,415	$ 10,537
IPO \| Subsequent event
Subsidiary sale of stock
Net proceeds	$ 125,780

Accrued Expenses (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Accrued Expenses
Research and development costs	$ 2,005	$ 2,049
Professional fees	371	182
Payroll related	625	344
Other	107	80
Accrued expenses	$ 3,108	$ 2,655

Fair Value of Financial Assets and Liability - Marketable Securities (Details) - USD ($) $ in Thousands	Sep. 30, 2017	Dec. 31, 2016
Fair Value of Financial Assets and Liability
Amortized Cost	$ 2,502	$ 3,997
Gross Unrealized Losses	(1)
Fair Value	2,501	3,997
Government Funds
Fair Value of Financial Assets and Liability
Amortized Cost	2,502	3,997
Gross Unrealized Losses	(1)
Fair Value	$ 2,501	$ 3,997

Fair Value of Financial Assets and Liability - Financial Liabilities (Details) - 2017 Series A Investor Instrument $ / shares in Units, $ in Thousands	9 Months Ended
	Sep. 30, 2017 USD ($) $ / shares
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)
Assumption used to estimate the fair value of asset and liability by utilizing the Black-Scholes option pricing model
Exercise Price \| $ / shares	$ 1.00
Fair Value \| $ / shares	$ 1.33
Expected term	1 month 15 days
Expected volatility (as a percent)	64.00%
Expected interest rate (as a percent)	0.95%

Significant Agreements (Details) - Predecessor - License agreement $ in Thousands		1 Months Ended
	Feb. 26, 2010 USD ($) item	Jul. 31, 2017 USD ($)	Mar. 31, 2017 USD ($)	Jan. 31, 2016 USD ($)
Ipsen
Significant Agreements
Number of programs \| item	2
Aggregate payment upon achievement of development and commercial milestones	$ 40,000
Milestone prepayment		$ 1,000
Ipsen \| Minimum
Significant Agreements
Payment based on revenue received, as percentage	10.00%
Ipsen \| Maximum
Significant Agreements
Payment based on revenue received, as percentage	20.00%
Camurus
Significant Agreements
Non-refundable and non-creditable signing fee				$ 500
One-time non-refundable development milestone payment				$ 7,750
Royalty payment period				10 years
One-time non-refundable non-creditable sales milestone payment				$ 57
Milestone fee			$ 250

Income Taxes (Details) $ in Thousands	12 Months Ended
Income Taxes (Details) $ in Thousands	Dec. 31, 2016 USD ($)
Income Taxes
Current income tax expense or benefit	$ 0
Deferred income tax expense or benefit	$ 0