Rhythm Pharmaceuticals (RYTM) 10-K 1 Mar 21 2020 FY Annual report Financial data

Document and Entity Information

Document and Entity Information - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2020	Feb. 19, 2021	Jun. 30, 2020
Document and Entity Information
Document Type	10-K
Document Annual Report	true
Document Transition Report	false
Document Period End Date	Dec. 31, 2020
Entity File Number	001-38223
Entity Registrant Name	RHYTHM PHARMACEUTICALS, INC.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	46-2159271
Entity Address, Address Line One	222 Berkeley Street
Entity Address, Address Line Two	12th Floor
Entity Address, City or Town	Boston
Entity Address, State or Province	MA
Entity Address, Postal Zip Code	02116
City Area Code	857
Local Phone Number	264-4280
Title of 12(b) Security	Common Stock, $0.001 par value per share
Trading Symbol	RYTM
Security Exchange Name	NASDAQ
Entity Well-known Seasoned Issuer	Yes
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
ICFR Auditor Attestation Flag	true
Entity Shell Company	false
Entity Public Float			$ 828.1
Entity Common Stock, Shares Outstanding		50,181,164
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	FY
Entity Central Index Key	0001649904
Amendment Flag	false

CONSOLIDATED BALANCE SHEETS

CONSOLIDATED BALANCE SHEETS - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Current assets:
Cash and cash equivalents	$ 100,854	$ 62,294
Short-term Investments	71,938	230,165
Prepaid expenses and other current assets	8,876	9,945
Total current assets	181,668	302,404
Property and equipment, net	3,195	3,671
Right-of-use asset	1,807	2,045
Restricted cash	403	403
Total assets	187,073	308,523
Current liabilities:
Accounts payable	4,900	10,415
Accrued expenses and other current liabilities	12,559	13,530
Lease liability	535	472
Total current liabilities	17,994	24,417
Long-term liabilities:
Lease liability	2,551	3,086
Total liabilities	20,545	27,503
Commitments and contingencies (Notes 5 and 9)
Stockholders' equity:
Common stock, $0.001 par value: 120,000,000 shares authorized; 44,235,903 and 43,996,753 shares issued and outstanding at December 31, 2020 and December 31, 2019, respectively	44	44
Additional paid-in capital	625,762	606,307
Accumulated other comprehensive income	49
Accumulated deficit	(459,327)	(325,331)
Total stockholders' equity	166,528	281,020
Total liabilities and stockholders' equity	$ 187,073	$ 308,523

CONSOLIDATED BALANCE SHEETS (Pa

CONSOLIDATED BALANCE SHEETS (Parenthetical) - $ / shares	Dec. 31, 2020	Dec. 31, 2019
CONSOLIDATED BALANCE SHEETS
Preferred stock par value per share	$ 0.001	$ 0.001
Preferred stock, authorized	10,000,000	10,000,000
Preferred stock, issued	0	0
Common stock, par value per share	$ 0.001	$ 0.001
Common stock, authorized	120,000,000	120,000,000
Common stock, issued	44,235,903	43,996,753
Common stock, outstanding	44,235,903	43,996,753

CONSOLIDATED STATEMENTS OF OPER

CONSOLIDATED STATEMENTS OF OPERATIONS AND COMPREHENSIVE LOSS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating expenses:
Research and development	$ 90,450	$ 109,450	$ 50,337
Selling, general, and administrative	46,125	36,550	28,080
Total operating expenses	136,575	146,000	78,417
Loss from operations	(136,575)	(146,000)	(78,417)
Other income (expense):
Interest income, net	2,579	5,271	4,353
Total other income, net	2,579	5,271	4,353
Net loss	$ (133,996)	$ (140,729)	$ (74,064)
Net loss per share, basic and diluted	$ (3.04)	$ (3.86)	$ (2.39)
Weighted-average common shares outstanding, basic and diluted	44,127,220	36,422,450	31,004,047
Other comprehensive loss:
Net loss	$ (133,996)	$ (140,729)	$ (74,064)
Unrealized gain on marketable securities	49	144
Comprehensive loss	$ (133,947)	$ (140,585)	$ (74,064)

CONSOLIDATED STATEMENTS OF STOC

CONSOLIDATED STATEMENTS OF STOCKHOLDERS' EQUITY - USD ($) $ in Thousands	Common stock	Additional Paid-in CapitalCumulative adjustment effect in period of adoption	Additional Paid-in Capital	Accumulated Other Comprehensive Income	Accumulated DeficitCumulative adjustment effect in period of adoption	Accumulated Deficit	Total
Beginning balance at Dec. 31, 2017	$ 27		$ 255,013			$ (110,252)	$ 144,788
Beginning balance (in shares) at Dec. 31, 2017	27,284,140
Increase (Decrease) in Stockholders' Equity
Stock compensation expense			6,390				6,390
Shares issued for license agreement			4,448				4,448
Shares issued for license agreement (in shares)	223,544
Issuance of common stock in connection with exercise of stock options			(1,808)				(1,808)
Issuance of common stock in connection with exercise of stock options (in shares)	311,241
Unrealized gain on marketable securities			8				8
Issuance of common stock upon completion of public offering, net of offering costs	$ 7		162,871
Issuance of common stock upon completion of public offering, net of offering costs (in shares)	6,591,800
Issuance of Series A Convertible Preferred Stock							(162,878)
Net loss						(74,064)	(74,064)
Ending balance at Dec. 31, 2018	$ 34	$ 286	430,824		$ (286)	(184,602)	246,256
Ending balance (in shares) at Dec. 31, 2018	34,410,725
Increase (Decrease) in Stockholders' Equity
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 and vesting of restricted stock units	$ 1		1,563				1,564
Issuance of common stock in connection with exercise of stock options and vesting of restricted stock units (in shares)	235,833
Unrealized gain on marketable securities			144				144
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
Net loss						(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
Increase (Decrease) in Stockholders' Equity
Stock compensation expense			17,455				17,455
Issuance of common stock in connection with ESPP			522				522
Issuance of common stock in connection with ESPP (in shares)	30,052
Issuance of common stock in connection with exercise of stock options and vesting of restricted stock units			1,478				1,478
Issuance of common stock in connection with exercise of stock options and vesting of restricted stock units (in shares)	209,098
Unrealized gain on marketable securities				$ 49			49
Net loss						(133,996)	(133,996)
Ending balance at Dec. 31, 2020	$ 44		$ 625,762	$ 49		$ (459,327)	$ 166,528
Ending balance (in shares) at Dec. 31, 2020	44,235,903

CONSOLIDATED STATEMENTS OF CASH

CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Operating activities
Net loss	$ (133,996)	$ (140,729)	$ (74,064)
Adjustments to reconcile net loss to cash used in operating activities:
Non-cash research and development license expense			4,448
Stock-based compensation expense	17,455	11,875	6,390
Depreciation and amortization	690	834	442
Deferred rent	(234)	203	61
Changes in operating assets and liabilities:
Prepaid expenses and other current assets	551	(6,378)	(6,286)
Tenant improvement allowance		938
Accounts payable, accrued expenses and other current liabilities	(6,446)	10,507	6,953
Net cash used in operating activities	(121,980)	(122,750)	(62,056)
Investing activities
Purchases of short-term investments	(86,869)	(295,825)	(248,592)
Maturities of short-term investments	245,614	271,240	162,166
Purchases of property and equipment	(214)	(3,385)	(722)
Net cash provided by (used in) investing activities	158,531	(27,970)	(87,148)
Financing activities
Net proceeds from issuance of common stock		161,352	162,878
Proceeds from the exercise of stock options	1,487	1,564	1,808
Proceeds from issuance of common stock from ESPP	522	558
Net cash provided by financing activities	2,009	163,474	164,686
Net increase in cash, cash equivalents and restricted cash	38,560	12,754	15,482
Cash, cash equivalents and restricted cash at beginning of period	62,697	49,943	34,461
Cash, cash equivalents and restricted cash at end of period	$ 101,257	$ 62,697	$ 49,943

Nature of Business

Nature of Business	12 Months Ended
	Dec. 31, 2020
Nature of Business
Nature of Business	1. Nature of Business Rhythm Pharmaceuticals, Inc. (the “Company” or “We”), is a commercial-stage biopharmaceutical company focused on changing the paradigm for the treatment of rare genetic diseases of obesity, which are characterized by early-onset, severe obesity and an insatiable hunger or hyperphagia. Our lead product candidate is IMCIVREE (setmelanotide), a potent melanocortin-4 receptor, or MC4R, agonist for the treatment of rare genetic diseases of obesity. We believe IMCIVREE, for which we have exclusive worldwide rights, has the potential to restore dysfunctional MC4R signaling due to impaired MC4R pathway function. 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. IMCIVREE has been approved by the U.S. Food and Drug Administration, or FDA, for chronic weight management in adult and pediatric patients six years of age and older with obesity due to proopiomelanocortin, or POMC, proprotein convertase subtilisin/kexin type 1, or PCSK1, or leptin receptor, or LEPR, deficiency confirmed by genetic testing. We expect IMCIVREE to be commercially available in the first quarter of 2021. 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. The Company’s continued development efforts are focused on obesity related to several single gene-related, or monogenic, MC4R pathway deficiencies: 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. The Company believes 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. The Company is subject to risks and uncertainties common to late-stage companies in the biotechnology industry, including but not limited to, risks associated with the commercialization of approved products, 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. Commercialization of approved products will require significant resources and in order to market IMCIVREE, the Company must continue to build its sales, marketing, managerial and other non-technical capabilities or make arrangements with third parties to perform these services. 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 though the Company has an approved product, and even if the Company’s further product development efforts are successful, it is uncertain when, if ever, the Company will realize revenue from product sales. Liquidity The Company has incurred operating losses and negative cash flows from operations since inception. As of December 31, 2020, the Company had an accumulated deficit of $459,327. 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 former parent company, Rhythm Holdings LLC. 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 of 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, 2020, the Company had $172,792 of cash and cash equivalents and short-term investments on hand. Subsequent to year end, the Company received additional funding in connection with the sale of a Rare Pediatric Disease Priority Review Voucher, or PRV, and a public offering (see Note 12, “Subsequent Events”). The net proceeds from the sale of the PRV Transfer and this offering, or the February 2021 public offering, were approximately $260,050 after deducting underwriting discounts and commissions and estimated 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 operations through at least the next twelve months from the filing of this Annual Report on Form 10-K with the SEC.

Summary of Significant Accounti

Summary of Significant Accounting Policies	12 Months Ended
	Dec. 31, 2020
Summary of Significant Accounting Policies
Summary of Significant Accounting Policies	2. Summary of Significant Accounting Policies Basis of Presentation The Company's consolidated financial statements have been prepared in conformity with accounting principles generally accepted in the United States (“GAAP”). Any reference in these notes to applicable guidance is meant to refer to the authoritative United States generally accepted accounting principles as found in the Accounting Standards Codification (“ASC”) and Accounting Standards Updates (“ASU”) of the Financial Accounting Standards Board (“FASB”). 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 accruals related to research and development expenses, assumptions used to record 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 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 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. To the extent the amortized cost basis of the available-for-sale debt securities exceeds the fair value, management assesses the debt securities for credit loss; however, management considers the risk of credit loss to be minimized by the Company's policy of investing in financial instruments issued by highly-rated financial institutions. When assessing the risk of credit loss, management considers factors such as the severity and the reason of the decline in value (i.e., any changes to the rating of the security by a rating agency or other adverse conditions specifically related to the security) and management's intended holding period and time horizon for selling. During the years ended December 31, 2020, 2019, and 2018, the Company did not recognize any credit losses related to its available-for-sale debt securities. Further, as of December 31, 2020 and 2019, the Company did not record an allowance for credit losses related to its available-for-sale debt securities. Restricted Cash Restricted cash consists of security deposits in the form of letters of credit 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, ​ 2020 2019 Prepaid research and development costs ​ $ 5,828 ​ $ 6,438 Other current assets ​ 3,048 ​ 3,507 Prepaid expenses and other current assets ​ $ 8,876 ​ $ 9,945 ​ Property and Equipment Property and Equipment consists of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Useful ​ December 31, ​ Life 2020 2019 Leasehold improvements * ​ $ 2,705 ​ $ 2,705 Office equipment 5 years ​ 70 ​ 70 Computers and software 3 years ​ 625 ​ 411 Furniture, fixtures and equipment 5 years ​ 1,237 ​ 1,237 ​ ​ 4,637 ​ 4,423 Less accumulated depreciation and amortization ​ (1,442) ​ (752) Property and equipment, net ​ $ 3,195 ​ $ 3,671 * Depreciation and amortization expense for the years ended December 31, 2020, 2019 and 2018 was $690, $834 and $442, 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, 2020 and 2019 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, 2020 and 2019, 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, 2020 or 2019. 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 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 determines 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 recognizes 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, 2020, no accrued interest or penalties are included on the related tax liability line in the consolidated balance sheet. Net Loss Per Share Basic net loss per share is computed by dividing net loss by the weighted average number of common shares outstanding during the period, without consideration of potential dilutive securities. Diluted net loss per share is computed by adjusting the weighted average shares outstanding for the potential dilutive effects of common stock equivalents outstanding during the period calculated in accordance with the treasury stock method. For purposes of the diluted net loss per share calculation, stock options and restricted stock units are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented. The following table includes the potential common shares, presented based on amounts outstanding at each period end, that were excluded from the computation of diluted net loss per share due to their anti-dilutive effect, for the periods indicated: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended ​ ​ ​ December 31, ​ ​ 2020 2019 ​ 2018 Stock options ​ ​ 5,199,235 ​ ​ 3,428,497 ​ ​ 2,616,530 ​ Restricted stock units ​ 176,537 ​ — ​ — ​ Potential common shares ​ ​ 5,375,772 ​ ​ 3,428,497 ​ ​ 2,616,530 ​ ​ Comprehensive Income (Loss) Comprehensive income (loss) represents the net change in stockholders’ 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 $524, $472 and $637 for the years ended December 31, 2020, 2019 and 2018, 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. 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 June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses-Measurement of Credit Losses on Financial Instruments In December 2019, the FASB issued ASU 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes

Accrued Expenses

Accrued Expenses	12 Months Ended
	Dec. 31, 2020
Accrued Expenses
Accrued Expenses	3. Accrued Expenses Accrued expenses consisted of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ 2020 2019 Research and development costs ​ $ 5,815 ​ $ 8,059 Professional fees ​ 648 ​ 1,439 Payroll related ​ 5,916 ​ 3,655 Other ​ 180 ​ 377 Accrued expenses ​ $ 12,559 ​ $ 13,530 ​

Fair Value of Financial Assets

Fair Value of Financial Assets	12 Months Ended
	Dec. 31, 2020
Fair Value of Financial Assets
Fair Value of Financial Assets	4. Fair Value of Financial Assets As of December 31, 2020 and 2019, the carrying amount of cash and cash equivalents and short-term investments was $172,792 and $292,459, respectively, which approximates fair value. Cash and cash equivalents and short-term investments includes investments in U.S. treasury securities and 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. 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, 2020 using: ​ Level 1 Level 2 Level 3 Total Assets: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash Equivalents: ​ ​ ​ ​ Corporate Debt Securities and Commercial Paper ​ $ — ​ $ 36,242 ​ $ — ​ $ 36,242 U.S. Treasury Securities and Money Market Funds ​ ​ 63,182 ​ ​ — ​ ​ — ​ ​ 63,182 Marketable Securities: ​ ​ ​ ​ Corporate Debt Securities and Commercial Paper ​ ​ — ​ ​ 71,938 ​ ​ — ​ ​ 71,938 Total ​ $ 63,182 ​ $ 108,180 ​ $ — ​ $ 171,362 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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 ​ Marketable Securities The following tables summarize the Company's marketable securities: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 ​ ​ ​ ​ ​ Gross ​ Gross ​ ​ ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair ​ Cost Gains Losses Value Assets ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Corporate debt securities and commercial paper (due within 1 year) ​ $ 71,895 ​ $ 43 ​ $ — ​ $ 71,938 ​ ​ $ 71,895 ​ $ 43 ​ $ — ​ $ 71,938 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2019 ​ ​ ​ ​ ​ Gross ​ Gross ​ ​ ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair ​ Cost Gains Losses Value Assets ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Corporate debt securities and commercial paper (due within 1 year) ​ $ 230,155 ​ $ 54 ​ $ (44) ​ $ 230,165 ​ ​ $ 230,155 ​ $ 54 ​ $ (44) ​ $ 230,165 ​

Right of Use Asset and Lease Li

Right of Use Asset and Lease Liability	12 Months Ended
	Dec. 31, 2020
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 4.6 years. The Company measured the lease liability associated with the office lease using a discount rate of 10% at inception. 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, 2020, 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 right of use 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 statements of cash flows and had no impact on the consolidated statements of operations. 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. The Company’s lease agreement commenced May 2019 and has a term of six years with a five-year renewal option to extend Supplemental cash flow information related to the Company’s lease for the year ended December 31, 2020, includes cash payments of $786 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, 2020, all of which is under a non-cancellable operating lease: ​ ​ ​ ​ ​ ​ Operating Lease 2021 ​ $ 802 2022 ​ 818 2023 ​ 834 2024 ​ ​ 851 2025 ​ ​ 502 Thereafter ​ — Total operating lease payments ​ ​ 3,807 Less: imputed interest ​ ​ 721 Total operating lease liability ​ $ 3,086 ​

Common Stock

Common Stock	12 Months Ended
	Dec. 31, 2020
Common Stock
Common Stock	6. Common Stock 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 a 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.

Stock-based Compensation

Stock-based Compensation	12 Months Ended
	Dec. 31, 2020
Stock-based Compensation
Stock-based Compensation	7. Stock-based Compensation 2017 Equity Incentive Plan The 2017 Plan provides for the grant of incentive and non-qualified stock options, stock appreciation rights, performance units, restricted stock, restricted stock units and stock grants to employees, consultants, advisors and directors of us or our affiliates, as determined by the board of directors. The number of shares authorized under the 2017 Plan will be increased each January 1, commencing on January 1, 2018 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, 2021, 2020 and 2019, 1,769,436, 1,759,870 and 1,376,429 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 2017 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. Awards granted under the 2017 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. As of December 31, 2020, an aggregate of 7,484,536 awards. In addition, a total of 5,375,772 shares of common stock reserved for issuance were subject to currently outstanding stock options and restricted stock units granted under the Plan. The Company estimates the fair value of stock option awards to employees and non-employees using the Black-Scholes option-pricing model, which requires the input of 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 has 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. During 2020, the Company began to estimate its volatility by using a blend of its stock price history for the length of time it has market data for its stock and the historical volatility of similar public companies for the expected term of each grant. 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. We have elected to account for forfeitures as they occur. The grant date fair value of awards subject to service-based vesting 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 one one year During the years ended December 31, 2020, 2019 and 2018, the Company granted 2,546,075, 1,445,200 and 1,218,790 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 years ended December 31, 2020, 2019 and 2018 was $13.25, $17.19 and $17.27, respectively. The total intrinsic value of stock options exercised during the years ended December 31, 2020, 2019 and 2018 was $2,661, $3,844 and $7,980, respectively. The fair value of stock 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, ​ ​ 2020 ​ 2019 ​ 2018 Risk‑free interest rate ​ 0.76 % ​ 2.40 % ​ 2.73 % Expected term (in years) ​ 6.08 ​ ​ 6.07 ​ ​ 5.89 ​ Expected volatility ​ 70.67 % ​ 66.03 % ​ 62.21 % Expected dividend yield ​ — ​ ​ — ​ ​ — ​ ​ The Company early adopted ASU 2018-07, Improvements to Nonemployee Share-Based Payment Accounting (Topic 718), 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 A summary of the Company's stock option activity for the year ended December 31, 2020 is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted ‑ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ Average ​ ​ ​ ​ ​ ​ ​ ​ ​ Average ​ ​ Remaining ​ ​ Aggregate ​ ​ ​ ​ Number of ​ Exercise ​ ​ Contractual ​ ​ Intrinsic ​ ​ ​ ​ Options ​ Price ​ ​ Term ​ ​ Value Outstanding as of December 31, 2019 3,428,497 ​ $ 21.17 ​ 7.94 ​ $ — Granted ​ ​ 2,546,075 ​ 21.22 ​ — ​ — Exercised ​ ​ (184,098) ​ 8.07 ​ — ​ 2,661 Cancelled ​ ​ (591,239) ​ 24.34 ​ — ​ — Outstanding as of December 31, 2020 5,199,235 ​ $ 21.30 ​ ​ 7.57 ​ $ 45,233 Options vested and expected to vest as of December 31, 2020 5,199,235 ​ $ 21.30 ​ ​ 7.57 ​ $ 45,233 Options exercisable at December 31, 2020 2,157,915 ​ $ 18.95 ​ ​ 5.47 ​ $ 24,218 ​ A summary of the Company's restricted stock unit activity for the year ended December 31, 2020 is as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ ​ ​ ​ ​ Average ​ ​ ​ ​ Number of ​ Grant Date ​ ​ ​ ​ RSU's ​ Fair Value Unvested as of December 31, 2019 — ​ $ — Granted ​ ​ 209,912 ​ 19.77 Vested ​ ​ (25,000) ​ 22.30 Cancelled ​ ​ (8,375) ​ 17.87 Unvested as of December 31, 2020 176,537 ​ $ 19.50 ​ As of December 31, 2020, the aggregate intrinsic value of non-vested RSUs was $5,248. 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, ​ 2020 ​ 2019 ​ 2018 Research and development ​ $ 6,055 ​ $ 5,163 ​ $ 2,793 Selling, general, and administrative ​ 11,400 ​ 6,712 ​ 3,597 Total ​ $ 17,455 ​ $ 11,875 ​ $ 6,390 ​ Stock-based compensation expense by award type recognized during the years ended December 31, 2020, 2019 and 2018 was as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended ​ ​ December 31, ​ 2020 ​ 2019 ​ 2018 Stock options ​ $ 15,915 ​ $ 11,667 ​ $ 6,241 Employees stock purchase plan ​ ​ 180 ​ ​ 208 ​ ​ 65 Restricted stock units ​ 1,360 ​ — ​ 84 Total ​ $ 17,455 ​ $ 11,875 ​ $ 6,390 ​ During 2020 and 2019, there were certain awards subject to modification accounting. Per terms of separation with a former employee, the employee’s stock option awards were amended to provide for accelerated vesting and extended time to exercise vested options. As a result, the Company recognized incremental expense for the stock option awards of $2,880 and $56, respectively. As of December 31, 2020, the Company has unrecognized compensation cost of $39,111 related to non-vested employee, non-employee and director stock option awards that is expected to be recognized over a weighted-average period of 2.83 years. The Company has unrecognized compensation cost of $2,640 related to non-vested employee restricted stock unit awards that is expected to be recognized over a weighted-average period of 2.52 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, 2020, a total of 1,000,993 shares of common stock were reserved for issuance under the 2017 ESPP. 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 2017 ESPP for such year, or that the increase in the number of shares authorized under the 2017 ESPP for such year will be a lesser number than would otherwise occur pursuant to the preceding sentence. The board of directors elected not to increase the pool on January 1, 2021. During the year ended December 31, 2020, 30,052 shares were issued under this plan.

Significant Agreements

Significant Agreements	12 Months Ended
	Dec. 31, 2020
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. The Company has recorded milestone expenses related to this license agreement of $3,000 and $1,000 during the years ended December 31, 2020 and 2018, respectively. The expenses were recorded as research and development expenses when the milestone criterias were met in full. No milestone expenses were recorded during the year ended December 31, 2019. 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 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 Research and Development.

Commitments and Contingencies

Commitments and Contingencies	12 Months Ended
	Dec. 31, 2020
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, 2020, 2019 and 2018 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, 2020, 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 $9,000 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.

Related-Party Transactions

Related-Party Transactions	12 Months Ended
	Dec. 31, 2020
Related-Party Transactions
Related-Party Transactions	10. Related-Party Transactions Expenses paid directly to consultants and vendors considered to be related parties amounted to $3,221, $2,489 and $2,005 for the years ended December 31, 2020, 2019 and 2018, respectively. Outstanding payments due to these related parties as of December 31, 2020 and 2019 were $187 and $264, respectively and were included within accounts payable on the balance sheet.

Income Tax

Income Tax	12 Months Ended
	Dec. 31, 2020
Income Tax
Income Tax	11. Income Tax For the years ended December 31, 2020, 2019 and 2018 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, ​ 2020 2019 2018 Statutory tax rate 21.00 % 21.00 % 21.00 % State tax, net of federal benefit 6.32 % 6.75 % 6.90 % Research and development credit 1.46 % 2.49 % 1.52 % Orphan drug credit 2.40 % 1.85 % 1.95 % Tax law change — % — % — % Stock compensation (0.53) % (0.10) % 0.46 % Investor instrument revaluation — % — % — % Other (0.30) % 0.20 % 0.05 % Change in valuation allowance (30.35) % (32.19) % (31.88) % Effective tax rate — % — % — % ​ The principal components of the Company's deferred tax assets and liabilities are as follows: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of ​ ​ December 31, ​ 2020 2019 Deferred tax assets: ​ ​ Net operating loss carryforwards ​ $ 102,367 ​ $ 71,524 Research and development credits ​ 10,347 ​ 7,876 Orphan drug credit ​ 10,110 ​ 6,889 Capitalized license fee ​ 2,492 ​ 1,734 Stock-based compensation ​ ​ 6,621 ​ ​ 3,628 Accrued expenses and other ​ 2,267 ​ 2,006 Total deferred tax assets ​ 134,204 ​ 93,657 Valuation allowance ​ ​ (133,596) ​ ​ (92,943) Net deferred tax assets ​ ​ 608 ​ ​ 714 Deferred tax liabilities: ​ ​ ​ ​ ​ ​ Operating lease right-of-use asset and other ​ ​ (608) ​ ​ (714) Total deferred tax liabilities ​ $ (608) ​ $ (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, 2020 and 2019, 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 $40,653 in 2020 and $45,264 in 2019 primarily relates to the net loss incurred by the Company during each period. As of December 31, 2020, the Company had federal and state net operating loss carryforwards of approximately $382,314 and $351,187, 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, 2020, $309,147 can be carried forward indefinitely. As of December 31, 2020, the Company had federal and state research tax credits of approximately $8,115 and $2,826, respectively, which may be used to offset future tax liabilities. Additionally, as of 2020, the Company had a federal orphan drug credit related to qualifying research of $10,110. 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, 2020 and 2019. 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. In March 2020, the Coronavirus Aid, Relief, and Economic Security Act, the CARES Act, was signed into law. The CARES Act includes provisions relating to several aspects of corporate income taxes. The CARES Act did not have a significant impact on the Company’s provision for income taxes. 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, 2020 and 2019, 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. ​

Subsequent Events

Subsequent Events	12 Months Ended
	Dec. 31, 2020
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 with the above notes to these consolidated financial statements and below. On January 5, 2021, the Company entered into a definitive agreement to sell its PRV, for $100,000. The PRV was granted to the Company by the U.S. FDA with the approval of IMCIVREE for chronic weight management in adult and pediatric patients 6 years of age and older with obesity due to POMC, PCSK1 or LEPR deficiency confirmed by genetic testing. The sale closed on February 17, 2021. On February 9, 2021 the Company completed a public offering of 5,750,000 shares of common stock at an offering price of $30.00 per share, which included the exercise in full by the underwriters of their option to purchase up to 750,000 additional shares of common stock. The Company received approximately $161,550 in net proceeds after deducting underwriting discounts, commissions and estimated offering expenses. The financial statements as of December 31, 2020, including share and per share amounts, do not include the effects of the PRV sale or the February public offering.

Summary of Significant Accoun_2

Summary of Significant Accounting Policies (Policies)	12 Months Ended
	Dec. 31, 2020
Summary of Significant Accounting Policies
Basis of Presentation	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	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 accruals related to research and development expenses, assumptions used to record 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	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	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	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. To the extent the amortized cost basis of the available-for-sale debt securities exceeds the fair value, management assesses the debt securities for credit loss; however, management considers the risk of credit loss to be minimized by the Company's policy of investing in financial instruments issued by highly-rated financial institutions. When assessing the risk of credit loss, management considers factors such as the severity and the reason of the decline in value (i.e., any changes to the rating of the security by a rating agency or other adverse conditions specifically related to the security) and management's intended holding period and time horizon for selling. During the years ended December 31, 2020, 2019, and 2018, the Company did not recognize any credit losses related to its available-for-sale debt securities. Further, as of December 31, 2020 and 2019, the Company did not record an allowance for credit losses related to its available-for-sale debt securities.
Restricted Cash	Restricted Cash Restricted cash consists of security deposits in the form of letters of credit 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 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, ​ 2020 2019 Prepaid research and development costs ​ $ 5,828 ​ $ 6,438 Other current assets ​ 3,048 ​ 3,507 Prepaid expenses and other current assets ​ $ 8,876 ​ $ 9,945 ​ Property and Equipment Property and Equipment consists of the following: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Useful ​ December 31, ​ Life 2020 2019 Leasehold improvements * ​ $ 2,705 ​ $ 2,705 Office equipment 5 years ​ 70 ​ 70 Computers and software 3 years ​ 625 ​ 411 Furniture, fixtures and equipment 5 years ​ 1,237 ​ 1,237 ​ ​ 4,637 ​ 4,423 Less accumulated depreciation and amortization ​ (1,442) ​ (752) Property and equipment, net ​ $ 3,195 ​ $ 3,671 * Depreciation and amortization expense for the years ended December 31, 2020, 2019 and 2018 was $690, $834 and $442, 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.
Property and Equipment	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 2019 Prepaid research and development costs ​ $ 5,828 ​ $ 6,438 Other current assets ​ 3,048 ​ 3,507 Prepaid expenses and other current assets ​ $ 8,876 ​ $ 9,945
Fair Value Measurements	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, 2020 and 2019 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, 2020 and 2019, respectively.
Government Grants	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, 2020 or 2019.
Research and Development Expenses	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	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 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 determines 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 recognizes 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, 2020, no accrued interest or penalties are included on the related tax liability line in the consolidated balance sheet.
Net Loss Per Share	Net Loss Per Share Basic net loss per share is computed by dividing net loss by the weighted average number of common shares outstanding during the period, without consideration of potential dilutive securities. Diluted net loss per share is computed by adjusting the weighted average shares outstanding for the potential dilutive effects of common stock equivalents outstanding during the period calculated in accordance with the treasury stock method. For purposes of the diluted net loss per share calculation, stock options and restricted stock units are considered to be common stock equivalents but have been excluded from the calculation of diluted net loss per share, as their effect would be anti-dilutive for all periods presented. Therefore, basic and diluted net loss per share were the same for all periods presented. The following table includes the potential common shares, presented based on amounts outstanding at each period end, that were excluded from the computation of diluted net loss per share due to their anti-dilutive effect, for the periods indicated: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended ​ ​ ​ December 31, ​ ​ 2020 2019 ​ 2018 Stock options ​ ​ 5,199,235 ​ ​ 3,428,497 ​ ​ 2,616,530 ​ Restricted stock units ​ 176,537 ​ — ​ — ​ Potential common shares ​ ​ 5,375,772 ​ ​ 3,428,497 ​ ​ 2,616,530 ​
Comprehensive Income (Loss)	Comprehensive Income (Loss) Comprehensive income (loss) represents the net change in stockholders’ 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	Patent Costs Costs to secure and defend patents are expensed as incurred and are classified as general and administrative expenses. Patent costs were $524, $472 and $637 for the years ended December 31, 2020, 2019 and 2018, respectively.
Subsequent Events	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	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. 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 June 2016, the FASB issued ASU 2016-13, Financial Instruments-Credit Losses-Measurement of Credit Losses on Financial Instruments In December 2019, the FASB issued ASU 2019-12, Income Taxes-Simplifying the Accounting for Income Taxes

Summary of Significant Accoun_3

Summary of Significant Accounting Policies (Tables)	12 Months Ended
	Dec. 31, 2020
Summary of Significant Accounting Policies
Schedule of prepaid expenses and other current assets	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ 2020 2019 Prepaid research and development costs ​ $ 5,828 ​ $ 6,438 Other current assets ​ 3,048 ​ 3,507 Prepaid expenses and other current assets ​ $ 8,876 ​ $ 9,945 ​
Schedule of property, plant and equipment	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Useful ​ December 31, ​ Life 2020 2019 Leasehold improvements * ​ $ 2,705 ​ $ 2,705 Office equipment 5 years ​ 70 ​ 70 Computers and software 3 years ​ 625 ​ 411 Furniture, fixtures and equipment 5 years ​ 1,237 ​ 1,237 ​ ​ 4,637 ​ 4,423 Less accumulated depreciation and amortization ​ (1,442) ​ (752) Property and equipment, net ​ $ 3,195 ​ $ 3,671 *
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, ​ ​ 2020 2019 ​ 2018 Stock options ​ ​ 5,199,235 ​ ​ 3,428,497 ​ ​ 2,616,530 ​ Restricted stock units ​ 176,537 ​ — ​ — ​ Potential common shares ​ ​ 5,375,772 ​ ​ 3,428,497 ​ ​ 2,616,530 ​

Accrued Expenses (Tables)

Accrued Expenses (Tables)	12 Months Ended
	Dec. 31, 2020
Accrued Expenses
Schedule of accrued expenses	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, ​ December 31, ​ 2020 2019 Research and development costs ​ $ 5,815 ​ $ 8,059 Professional fees ​ 648 ​ 1,439 Payroll related ​ 5,916 ​ 3,655 Other ​ 180 ​ 377 Accrued expenses ​ $ 12,559 ​ $ 13,530

Fair Value of Financial Assets

Fair Value of Financial Assets (Tables)	12 Months Ended
	Dec. 31, 2020
Fair Value of Financial Assets
Schedule of financial assets and liabilities measured at fair value on a recurring basis	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Fair value Measurements as of ​ ​ December 31, 2020 using: ​ Level 1 Level 2 Level 3 Total Assets: ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Cash Equivalents: ​ ​ ​ ​ Corporate Debt Securities and Commercial Paper ​ $ — ​ $ 36,242 ​ $ — ​ $ 36,242 U.S. Treasury Securities and Money Market Funds ​ ​ 63,182 ​ ​ — ​ ​ — ​ ​ 63,182 Marketable Securities: ​ ​ ​ ​ Corporate Debt Securities and Commercial Paper ​ ​ — ​ ​ 71,938 ​ ​ — ​ ​ 71,938 Total ​ $ 63,182 ​ $ 108,180 ​ $ — ​ $ 171,362 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ 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
Schedule of marketable securities	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2020 ​ ​ ​ ​ ​ Gross ​ Gross ​ ​ ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair ​ Cost Gains Losses Value Assets ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Corporate debt securities and commercial paper (due within 1 year) ​ $ 71,895 ​ $ 43 ​ $ — ​ $ 71,938 ​ ​ $ 71,895 ​ $ 43 ​ $ — ​ $ 71,938 ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ December 31, 2019 ​ ​ ​ ​ ​ Gross ​ Gross ​ ​ ​ ​ ​ Amortized ​ Unrealized ​ Unrealized ​ Fair ​ Cost Gains Losses Value Assets ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Corporate debt securities and commercial paper (due within 1 year) ​ $ 230,155 ​ $ 54 ​ $ (44) ​ $ 230,165 ​ ​ $ 230,155 ​ $ 54 ​ $ (44) ​ $ 230,165

Right of Use Asset and Lease _2

Right of Use Asset and Lease Liability (Tables)	12 Months Ended
	Dec. 31, 2020
Right Of Use Asset and Lease Liability
Schedule of operating lease maturities	​ ​ ​ ​ ​ ​ Operating Lease 2021 ​ $ 802 2022 ​ 818 2023 ​ 834 2024 ​ ​ 851 2025 ​ ​ 502 Thereafter ​ — Total operating lease payments ​ ​ 3,807 Less: imputed interest ​ ​ 721 Total operating lease liability ​ $ 3,086

Stock-based Compensation (Table

Stock-based Compensation (Tables) - 2017 Equity Incentive Plan	12 Months Ended
	Dec. 31, 2020
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, 2019 3,428,497 ​ $ 21.17 ​ 7.94 ​ $ — Granted ​ ​ 2,546,075 ​ 21.22 ​ — ​ — Exercised ​ ​ (184,098) ​ 8.07 ​ — ​ 2,661 Cancelled ​ ​ (591,239) ​ 24.34 ​ — ​ — Outstanding as of December 31, 2020 5,199,235 ​ $ 21.30 ​ ​ 7.57 ​ $ 45,233 Options vested and expected to vest as of December 31, 2020 5,199,235 ​ $ 21.30 ​ ​ 7.57 ​ $ 45,233 Options exercisable at December 31, 2020 2,157,915 ​ $ 18.95 ​ ​ 5.47 ​ $ 24,218
Summary of restricted stock unit activity	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Weighted- ​ ​ ​ ​ ​ ​ Average ​ ​ ​ ​ Number of ​ Grant Date ​ ​ ​ ​ RSU's ​ Fair Value Unvested as of December 31, 2019 — ​ $ — Granted ​ ​ 209,912 ​ 19.77 Vested ​ ​ (25,000) ​ 22.30 Cancelled ​ ​ (8,375) ​ 17.87 Unvested as of December 31, 2020 176,537 ​ $ 19.50
Summary of stock-based compensation expense	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended ​ ​ December 31, ​ 2020 ​ 2019 ​ 2018 Research and development ​ $ 6,055 ​ $ 5,163 ​ $ 2,793 Selling, general, and administrative ​ 11,400 ​ 6,712 ​ 3,597 Total ​ $ 17,455 ​ $ 11,875 ​ $ 6,390
Summary of stock-based compensation expense by award type	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year Ended ​ ​ December 31, ​ 2020 ​ 2019 ​ 2018 Stock options ​ $ 15,915 ​ $ 11,667 ​ $ 6,241 Employees stock purchase plan ​ ​ 180 ​ ​ 208 ​ ​ 65 Restricted stock units ​ 1,360 ​ — ​ 84 Total ​ $ 17,455 ​ $ 11,875 ​ $ 6,390
Employees
Stock-based Compensation
Schedule of significant assumptions used to compute the fair values of employee and director stock options awarded	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ ​ Year ended ​ ​ December 31, ​ ​ 2020 ​ 2019 ​ 2018 Risk‑free interest rate ​ 0.76 % ​ 2.40 % ​ 2.73 % Expected term (in years) ​ 6.08 ​ ​ 6.07 ​ ​ 5.89 ​ Expected volatility ​ 70.67 % ​ 66.03 % ​ 62.21 % Expected dividend yield ​ — ​ ​ — ​ ​ — ​

Income Tax (Tables)

Income Tax (Tables)	12 Months Ended
	Dec. 31, 2020
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, ​ 2020 2019 2018 Statutory tax rate 21.00 % 21.00 % 21.00 % State tax, net of federal benefit 6.32 % 6.75 % 6.90 % Research and development credit 1.46 % 2.49 % 1.52 % Orphan drug credit 2.40 % 1.85 % 1.95 % Tax law change — % — % — % Stock compensation (0.53) % (0.10) % 0.46 % Investor instrument revaluation — % — % — % Other (0.30) % 0.20 % 0.05 % Change in valuation allowance (30.35) % (32.19) % (31.88) % Effective tax rate — % — % — %
Schedule of principal components of deferred tax assets	​ ​ ​ ​ ​ ​ ​ ​ ​ ​ As of ​ ​ December 31, ​ 2020 2019 Deferred tax assets: ​ ​ Net operating loss carryforwards ​ $ 102,367 ​ $ 71,524 Research and development credits ​ 10,347 ​ 7,876 Orphan drug credit ​ 10,110 ​ 6,889 Capitalized license fee ​ 2,492 ​ 1,734 Stock-based compensation ​ ​ 6,621 ​ ​ 3,628 Accrued expenses and other ​ 2,267 ​ 2,006 Total deferred tax assets ​ 134,204 ​ 93,657 Valuation allowance ​ ​ (133,596) ​ ​ (92,943) Net deferred tax assets ​ ​ 608 ​ ​ 714 Deferred tax liabilities: ​ ​ ​ ​ ​ ​ Operating lease right-of-use asset and other ​ ​ (608) ​ ​ (714) Total deferred tax liabilities ​ $ (608) ​ $ (714)

Nature of Business (Details)

Nature of Business (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	Feb. 28, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Nature of Business
Accumulated deficit		$ (459,327)	$ (325,331)
Revenue		0	0	$ 0
Carrying amount of cash and cash equivalents and short term investments		$ 172,792	$ 292,459
Net proceeds from sale	$ 260,050

Summary of Significant Accoun_4

Summary of Significant Accounting Policies - Additional Information (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)Institutionitem	Dec. 31, 2019USD ($)
Off Balance Sheet Risk and Concentrations of Credit Risk
Number of federally insured financial institutions | Institution	2
Segment Information
Number of operating segments | item	1
Prepaid Expenses and Other Current Assets
Prepaid research and development costs	$ 5,828	$ 6,438
Other current assets	$ 3,048	$ 3,507

Summary of Significant Accoun_5

Summary of Significant Accounting Policies - Property, Plant and Equipment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Property, Plant and Equipment
Property, Plant and Equipment, gross	$ 4,637	$ 4,423
Less accumulated depreciation and amortization	(1,442)	(752)
Property, Plant and Equipment, net	3,195	3,671
Depreciation and amortization	690	834	$ 442
Leasehold improvements
Property, Plant and Equipment
Property, Plant and Equipment, gross	$ 2,705	2,705
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	$ 625	411
Furniture, fixtures and equipment
Property, Plant and Equipment
Useful Life	5 years
Property, Plant and Equipment, gross	$ 1,237	$ 1,237

Summary of Significant Accoun_6

Summary of Significant Accounting Policies - Government Grants (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)person	Dec. 31, 2019USD ($)	Dec. 31, 2018USD ($)
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	$ 0	$ 210

Summary of Significant Accoun_7

Summary of Significant Accounting Policies - Shares Excluded For EPS (Details) - shares	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Shares excluded from the computation of diluted net loss per share
Unvested shares from share-based compensation that were anti-dilutive	5,375,772	3,428,497	2,616,530
Stock options
Shares excluded from the computation of diluted net loss per share
Unvested shares from share-based compensation that were anti-dilutive	5,199,235	3,428,497	2,616,530
Restricted stock units
Shares excluded from the computation of diluted net loss per share
Unvested shares from share-based compensation that were anti-dilutive	176,537

Summary of Significant Accoun_8

Summary of Significant Accounting Policies - Net Loss Per Share and Patent Costs (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Net loss-basic and diluted	$ (133,996)	$ (140,729)	$ (74,064)
Weighted-average number of common shares-basic and diluted	44,127,220	36,422,450	31,004,047
Loss per common share-basic and diluted	$ (3.04)	$ (3.86)	$ (2.39)
Costs to secure and defend patents	$ 46,125	$ 36,550	$ 28,080
Patent Costs
Costs to secure and defend patents	$ 524	$ 472	$ 637

Summary of Significant Accoun_9

Summary of Significant Accounting Policies - Application of New or Revised Accounting Standards (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Summary of Significant Accounting Policies
Right-of-use asset	$ 1,807	$ 2,045	$ 3,265
Operating Lease, Liability	$ 3,086		$ 3,636

Accrued Expenses (Details)

Accrued Expenses (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Accrued Expenses
Research and development costs	$ 5,815	$ 8,059
Professional fees	648	1,439
Payroll related	5,916	3,655
Other	180	377
Accrued expenses	$ 12,559	$ 13,530

Fair Value of Financial Asset_2

Fair Value of Financial Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value of Financial Assets and Liabilities
Carrying amount of cash and cash equivalents and short term investments	$ 172,792	$ 292,459
Fair value of financial assets and liabilities
Marketable Securities	71,938	230,165
Corporate Debt Securities and Commercial Paper
Fair value of financial assets and liabilities
Marketable Securities	71,938	230,165
Recurring | Estimated fair value
Fair value of financial assets and liabilities
Fair value of assets	171,362	292,064
Recurring | Corporate Debt Securities and Commercial Paper | Estimated fair value
Fair value of financial assets and liabilities
Cash Equivalents	36,242	8,885
Marketable Securities		230,165
Recurring | Money Market Funds | Estimated fair value
Fair value of financial assets and liabilities
Cash Equivalents	63,182	53,014
Recurring | Level 1 | Estimated fair value
Fair value of financial assets and liabilities
Fair value of assets	63,182	53,014
Recurring | Level 1 | Money Market Funds | Estimated fair value
Fair value of financial assets and liabilities
Cash Equivalents	63,182	53,014
Recurring | Level 2 | Estimated fair value
Fair value of financial assets and liabilities
Fair value of assets	108,180	239,050
Recurring | Level 2 | Corporate Debt Securities and Commercial Paper | Estimated fair value
Fair value of financial assets and liabilities
Cash Equivalents	$ 36,242	8,885
Marketable Securities		$ 230,165

Fair Value of Financial Asset_3

Fair Value of Financial Assets - Marketable Securities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Fair Value of Financial Assets and Liabilities
Amortized Cost	$ 71,895	$ 230,155
Gross Unrealized Gains	43	54
Gross Unrealized Losses		(44)
Fair Value	71,938	230,165
Corporate Debt Securities and Commercial Paper
Fair Value of Financial Assets and Liabilities
Amortized Cost	71,895	230,155
Gross Unrealized Gains	43	54
Gross Unrealized Losses		(44)
Fair Value	71,938	230,165
Recurring | Estimated fair value | Corporate Debt Securities and Commercial Paper
Fair Value of Financial Assets and Liabilities
Amortized Cost	71,938
Fair Value		230,165
Recurring | Level 2 | Estimated fair value | Corporate Debt Securities and Commercial Paper
Fair Value of Financial Assets and Liabilities
Amortized Cost	$ 71,938
Fair Value		$ 230,165

Fair Value of Financial Asset_4

Fair Value of Financial Assets - Financial Liabilities (Details) - $ / shares	Oct. 18, 2019	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

Right of Use Asset and Lease _3

Right of Use Asset and Lease Liability (Details) - USD ($) $ in Thousands	1 Months Ended	12 Months Ended
	May 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Right Of Use Asset and Lease Liability
Remaining term of operating lease (in years)		4 years 7 months 6 days
Operating lease discount rate		10.00%
Transition relief package				true
Right-of-use asset		$ 1,807	$ 2,045	$ 3,265
Operating Lease, Liability		3,086		$ 3,636
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		551	$ 629	$ 359
Cash payments in the measurement of operating lease liability		$ 786

Right of Use Asset and Lease _4

Right of Use Asset and Lease Liability - Operating Lease Maturities (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2018
Lessee, Operating Lease, Liability, Payment, Due [Abstract]
2021	$ 802
2022	818
2023	834
2024	851
2025	502
Total operating lease payments	3,807
Less: imputed interest	721
Total operating lease liability	$ 3,086	$ 3,636

Common Stock (Details)

Common Stock (Details) - USD ($) $ / shares in Units, $ in Thousands	Oct. 18, 2019	Jun. 25, 2018	Apr. 03, 2018	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2021	Jan. 01, 2020	Jan. 01, 2019
Common Stock
Common stock, authorized				120,000,000	120,000,000
Price per share	$ 18.50	$ 26.42
Issuance of stock (in shares)	9,324,324	6,591,800
Net proceeds from issuance of common stock					$ 161,352	$ 162,878
Net proceeds--------do not use	$ 161,352	$ 162,878
Underwriter option to purchase
Common Stock
Issuance of stock (in shares)	1,216,216	859,800
License agreement | Takeda
Common Stock
Issuance of stock (in shares)			223,544
2017 Employee Stock Purchase Plan
Common Stock
Common stock reserved for issuance				1,000,993
Number of common stock options granted				30,052
2017 Equity Incentive Plan
Common Stock
Common stock reserved for issuance				5,375,772			1,769,436	1,759,870	1,376,429
Shares available for future grant				2,108,764
Number of common stock options granted				2,546,075	1,445,200	1,218,790

Stock-based Compensation - 2017

Stock-based Compensation - 2017 Equity Incentive Plan (Details) - USD ($) $ / shares in Units, $ in Thousands	Jan. 01, 2020	Jan. 01, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Jan. 01, 2021
Stock-based Compensation
Stock options total intrinsic value of stock options					$ 7,980
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	5,375,772			1,769,436
Percentage increase in authorized shares under the plan			4.00%
Stock options total intrinsic value of stock options			$ 2,661	$ 3,844
Authorized shares			7,484,536
Shares available for future grant			2,108,764
Common stock reserved for issuance	1,759,870	1,376,429	5,375,772			1,769,436
Grant cliff vesting period			1 year
Number of common stock options granted			2,546,075	1,445,200	1,218,790
Weighted average grant date fair value relating to outstanding stock options			$ 13.25	$ 17.19	$ 17.27
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
Stock-based compensation
Risk-free interest rate			0.76%	2.40%	2.73%
Expected term (in years)			6 years 29 days	6 years 25 days	5 years 10 months 20 days
Expected volatility			70.67%	66.03%	62.21%
2017 Employee Stock Purchase Plan
Stock-based Compensation
Common stock reserved for issuance			1,000,993
Number of additional shares added	439,968	344,107
Percentage increase in authorized shares under the plan			1.00%
Common stock reserved for issuance			1,000,993
Number of common stock options granted			30,052
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			1 year
Minimum | 2017 Equity Incentive Plan | Exercise Price Not Less Than 100% Of Fair Market Value
Stock-based Compensation
Voting power of stock			10.00%

Stock-based Compensation - Comm

Stock-based Compensation - Common stock option activity (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Aggregate Intrinsic Value
Stock options total intrinsic value of stock options			$ 7,980
2017 Equity Incentive Plan
Number of Units
Outstanding at beginning of the period	3,428,497
Granted	2,546,075	1,445,200	1,218,790
Exercised	(184,098)
Cancelled	(591,239)
Outstanding at end of the period	5,199,235	3,428,497
Options vested and expected to vest at end of the period	5,199,235
Options exercisable at end of the period	2,157,915
Weighted Average Grant Date Fair Value Per Unit
Weighted average grant date fair value relating to outstanding stock options	$ 13.25	$ 17.19	$ 17.27
Weighted Average Exercise Price
Outstanding at beginning of the period	21.17
Granted	21.22
Exercised	8.07
Cancelled	24.34
Outstanding at end of the period	21.30	$ 21.17
Options vested and expected to vest at end of the period	21.30
Options exercisable at end of the period	$ 18.95
Weighted Average Remaining Contractual Term
Outstanding at end of the period	7 years 6 months 25 days	7 years 11 months 8 days
Options vested and expected to vest at end of the period	7 years 6 months 25 days
Options exercisable at end of the period	5 years 5 months 19 days
Aggregate Intrinsic Value
Stock options total intrinsic value of stock options	$ 2,661	$ 3,844
Outstanding at end of the period	45,233
Options vested and expected to vest at the end of the period	45,233
Options exercisable at the end of the period	$ 24,218

Stock-based Compensation - Rest

Stock-based Compensation - Restricted Stock Unit activity (Details) - 2017 Equity Incentive Plan $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)$ / sharesshares
Number of Units
Granted | shares	209,912
Vested | shares	(25,000)
Cancelled | shares	(8,375)
Outstanding unvested at end of the period | shares	176,537
Weighted Average Grant Date Fair Value
Granted | $ / shares	$ 19.77
Vested | $ / shares	22.30
Cancelled | $ / shares	17.87
Outstanding unvested at end of the period | $ / shares	$ 19.50
Aggregate intrinsic value | $	$ 5,248

Stock-based Compensation - Comp

Stock-based Compensation - Compensation expense (Details) - 2017 Equity Incentive Plan - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Stock-based Compensation
Stock based compensation expense	$ 17,455	$ 11,875	$ 6,390
Incremental expenses	2,880	56
Research and development
Stock-based Compensation
Stock based compensation expense	6,055	5,163	2,793
Selling, general and administrative
Stock-based Compensation
Stock based compensation expense	11,400	6,712	3,597
Nonemployees
Stock-based Compensation
Unrecognized compensation expense related to non-vested portion of awards	$ 39,111
Weighted average period for recognition of unrecognized compensation expense	2 years 9 months 29 days
Employees
Stock-based Compensation
Unrecognized compensation expense related to non-vested portion of awards	$ 2,640
Weighted average period for recognition of unrecognized compensation expense	2 years 6 months 7 days
Stock options
Stock-based Compensation
Stock based compensation expense	$ 15,915	11,667	6,241
Stock options
Stock-based Compensation
Stock based compensation expense	180	$ 208	65
Restricted stock units
Stock-based Compensation
Stock based compensation expense	$ 1,360		$ 84

Significant Agreements (Details

Significant Agreements (Details) - USD ($) $ in Thousands	Apr. 03, 2018	Jan. 31, 2016	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Feb. 26, 2010
Significant Agreements
Shares issued for license agreement					$ 4,448
Non-cash research and development license expense					4,448
Ipsen
Significant Agreements
Milestone expenses			$ 3,000	$ 0	$ 1,000
Ipsen | License agreement | Maximum
Significant Agreements
Aggregate payment upon achievement of development and commercial milestones						$ 40,000
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
Royalty payment period		10 years
Camurus | License agreement | Maximum
Significant Agreements
One-time non-refundable development milestone payment		$ 7,750
One-time non-refundable non-creditable sales milestone payment		$ 57,000
Takeda
Significant Agreements
Shares issued for license agreement	$ 4,448
Shares issued for license agreement (in shares)	223,544
One-time non-refundable development milestone payment	$ 70,000
One-time non-refundable non-creditable sales milestone payment	$ 70,000
Period after date of first commercial sale (in years)	10 years

Commitments and Contingencies (

Commitments and Contingencies (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2020USD ($)
Commitments and Contingencies
Potential Milestone Payment	$ 9,000

Related-Party Transactions (Det

Related-Party Transactions (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accounts Payable
Related Party Transaction
Outstanding payments due to consultants	$ 187	$ 264
Consultant
Related Party Transaction
Net costs	$ 3,221	$ 2,489	$ 2,005

Income Tax - Reconciliation of

Income Tax - Reconciliation of income tax rate (Details)	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Reconciliation of the income tax benefit
Statutory tax rate	21.00%	21.00%	21.00%
State tax, net of federal benefit	6.32%	6.75%	6.90%
Research and development credit	1.46%	2.49%	1.52%
Orphan drug credit	2.40%	1.85%	1.95%
Stock compensation	(0.53%)	(0.10%)	0.46%
Other	(0.30%)	0.20%	0.05%
Change in valuation allowance	(30.35%)	(32.19%)	(31.88%)

Income Tax - Deferred tax asset

Income Tax - Deferred tax assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Deferred tax assets:
Net operating loss carryforwards	$ 102,367	$ 71,524
Research and development credits	10,347	7,876
Orphan drug credit	10,110	6,889
Capitalized license fee	2,492	1,734
Stock-based compensation	6,621	3,628
Other	2,267	2,006
Gross deferred tax assets	134,204	93,657
Valuation allowance	(133,596)	(92,943)
Net deferred tax assets	608	714
Deferred Tax Liabilities:
Operating lease right-of-use asset and other	(608)	(714)
Total deferred tax liabilities:	$ (608)	$ (714)

Income Tax - Operating loss car

Income Tax - Operating loss carryforward (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Operating loss carryforwards
Increase decrease in valuation allowance	$ 40,653	$ 45,264
Federal net operating loss carryforwards	382,314
State net operating loss carryforwards	351,187
Federal orphan drug credit related to qualifying research	10,110
Accrued interest or penalties related to uncertain tax positions	0	$ 0
Net operating loss indefinite carryforwards	309,147
Research tax credits | Federal
Operating loss carryforwards
Research tax credits	8,115
Research tax credits | State
Operating loss carryforwards
Research tax credits	$ 2,826

Subsequent Events (Details)

Subsequent Events (Details) - USD ($) $ / shares in Units, $ in Thousands	Feb. 09, 2021	Oct. 18, 2019	Jun. 25, 2018	Dec. 31, 2019	Dec. 31, 2018	Jan. 05, 2021
Subsequent events
Issuance of stock (in shares)		9,324,324	6,591,800
Price per share		$ 18.50	$ 26.42
Net proceeds from issuance of common stock				$ 161,352	$ 162,878
Underwriter option to purchase
Subsequent events
Issuance of stock (in shares)		1,216,216	859,800
Subsequent Events
Subsequent events
Definitive agreement price of Rare Pediatric Disease Priority Review Voucher ("PRV")						$ 100,000
Issuance of stock (in shares)	5,750,000
Price per share	$ 30
Net proceeds from issuance of common stock	$ 161,550
Subsequent Events | Underwriter option to purchase
Subsequent events
Issuance of stock (in shares)	750,000