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Document and Entity Information - USD ($) $ in Millions	12 Months Ended
Document and Entity Information - USD ($) $ in Millions	Dec. 31, 2018	Feb. 28, 2019	Jun. 30, 2018
Document And Entity Information [Abstract]
Document Type	10-K
Amendment Flag	false
Document Period End Date	Dec. 31, 2018
Document Fiscal Year Focus	2018
Document Fiscal Period Focus	FY
Entity Registrant Name	GLYCOMIMETICS INC
Entity Central Index Key	0001253689
Current Fiscal Year End Date	--12-31
Entity Well-known Seasoned Issuer	No
Entity Current Reporting Status	Yes
Entity Voluntary Filers	No
Entity Filer Category	Accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Common Stock, Shares Outstanding		43,168,969
Entity Public Float			$ 538.3

Balance Sheets - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Current Assets:
Cash and cash equivalents	$ 209,917,595	$ 123,924,738
Prepaid expenses and other current assets	2,351,524	3,294,884
Total current assets	212,269,119	127,219,622
Property and equipment, net	957,226	1,106,899
Prepaid research and development expenses	1,560,607	204,364
Deposits	52,320	52,320
Total assets	214,839,272	128,583,205
Current liabilities:
Accounts payable	2,663,579	2,647,091
Accrued bonuses	1,727,184	1,883,051
Accrued expenses	4,273,620	3,566,607
Deferred rent	98,771	78,028
Total current liabilities	8,763,154	8,174,777
Deferred rent, net of current portion	611,623	707,003
Total liabilities	9,374,777	8,881,780
Stockholders' equity:
Preferred stock; $0.001 par value; 5,000,000 shares authorized; no shares issued and outstanding at December 31, 2018 and December 31, 2017
Common stock; $0.001 par value; 100,000,000 shares authorized, 43,160,751 shares issued and outstanding at December 31, 2018; 100,000,000 shares authorized, 34,359,799 shares issued and outstanding at December 31, 2017	43,159	34,358
Additional paid-in capital	405,972,075	271,944,173
Accumulated deficit	(200,550,739)	(152,277,106)
Total stockholders' equity	205,464,495	119,701,425
Total liabilities and stockholders' equity	$ 214,839,272	$ 128,583,205

Balance Sheets (Parenthetical) - $ / shares	Dec. 31, 2018	Dec. 31, 2017
Balance Sheets
Preferred stock, par value	$ 0.001	$ 0.001
Preferred stock, shares authorized	5,000,000	5,000,000
Preferred stock, shares issued	0	0
Preferred stock, shares outstanding	0	0
Common stock, par value	$ 0.001	$ 0.001
Common stock, shares authorized	100,000,000	100,000,000
Common stock, shares issued	43,160,751	34,359,799
Common stock, shares outstanding	43,160,751	34,359,799

Statements of Operations and Comprehensive Loss - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Statements of Operations and Comprehensive Loss
Revenue			$ 18,500
Costs and expenses:
Research and development expense	$ 40,091,773	$ 24,100,092	23,281,820
General and administrative expense	11,413,050	9,832,188	8,650,165
Total costs and expenses	51,504,823	33,932,280	31,931,985
Loss from operations	(51,504,823)	(33,932,280)	(31,913,485)
Interest income	3,231,190	651,212	103,647
Net loss and comprehensive loss	$ (48,273,633)	$ (33,281,068)	$ (31,809,838)
Basic and diluted net loss per common share (in dollars per share)	$ (1.18)	$ (1.13)	$ (1.50)
Basic and diluted weighted average number of common shares (in shares)	41,044,621	29,395,756	21,256,312

Statements of Stockholders' Equity - USD ($)	Common Stock	Additional Paid-In Capital	Accumulated Deficit	Total
Balance at Dec. 31, 2015	$ 19,049	$ 127,619,078	$ (87,166,473)	$ 40,471,654
Balance, shares at Dec. 31, 2015	19,050,204
Issuance of common stock, net of issuance costs	$ 4,146	23,597,809		23,601,955
Issuance of common stock, net of issuance costs, shares	4,145,584
Exercise of options and warrants	$ 54	72,539		72,593
Exercise of options and warrants, shares	54,235
Stock-based compensation		2,964,767		2,964,767
Net loss			(31,809,838)	(31,809,838)
Balance at Dec. 31, 2016	$ 23,249	154,254,193	(118,976,311)	35,301,131
Balance, shares at Dec. 31, 2016	23,250,023
Issuance of common stock, net of issuance costs	$ 11,038	113,536,146		113,547,184
Issuance of common stock, net of issuance costs, shares	11,038,647
Exercise of options	$ 71	373,305		373,376
Exercise of options, shares	71,129
Stock-based compensation		3,760,802		3,760,802
Net loss			(33,281,068)	(33,281,068)
Balance at Dec. 31, 2017	$ 34,358	271,944,173	(152,277,106)	119,701,425
Balance, shares at Dec. 31, 2017	34,359,799
Cumulative adjustment upon implementation of ASU No. 2016-09 for stock-based compensation forfeitures \| ASU 2016-09		19,727	(19,727)
Issuance of common stock, net of issuance costs	$ 8,050	128,417,030		128,425,080
Issuance of common stock, net of issuance costs, shares	8,050,000
Exercise of options and warrants	$ 751	1,023,774		1,024,525
Exercise of options and warrants, shares	750,952
Stock-based compensation		4,587,098		4,587,098
Net loss			(48,273,633)	(48,273,633)
Balance at Dec. 31, 2018	$ 43,159	$ 405,972,075	$ (200,550,739)	$ 205,464,495
Balance, shares at Dec. 31, 2018	43,160,751

Statements of Cash Flows - USD ($)	12 Months Ended
Statements of Cash Flows - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Operating activities
Net loss	$ (48,273,633)	$ (33,281,068)	$ (31,809,838)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation	275,123	263,541	190,540
Loss on disposal of property and equipment	168
Stock-based compensation expense	4,587,098	3,760,802	2,964,767
Changes in assets and liabilities
Prepaid expenses and other current assets	943,360	(2,816,381)	(36,693)
Prepaid research and development expenses	(1,356,243)	555,167	(63,000)
Deposits			(52,320)
Accounts payable	16,488	1,061,880	1,000,975
Accrued expenses and bonuses	551,146	724,797	(2,504,179)
Deferred rent	(74,637)	(37,099)	578,483
Net cash used in operating activities	(43,331,130)	(29,768,361)	(29,731,265)
Investing activities
Purchases of property and equipment	(125,618)	(294,107)	(704,202)
Net cash used in investing activities	(125,618)	(294,107)	(704,202)
Financing activities
Proceeds from issuance of common stock, net of issuance costs	128,425,080	113,572,189	23,601,955
Proceeds from exercise of stock options and warrants	1,024,525	373,376	72,593
Net cash provided by financing activities	129,449,605	113,945,565	23,674,548
Net change in cash and cash equivalents	85,992,857	83,883,097	(6,760,919)
Cash and cash equivalents, beginning of period	123,924,738	40,041,641	46,802,560
Cash and cash equivalents, end of period	$ 209,917,595	123,924,738	40,041,641
Non-cash investing activities and financing activities
Property acquisition costs included in accounts payable and accrued expenses		20,000	$ 21,292
Issuance costs associated with financing included in accounts payable and accrued expenses		$ 25,005

Description of the Business	12 Months Ended
Description of the Business	Dec. 31, 2018
Description of the Business
Description of the Business	GLYCOMIMETICS, INC. Notes to Financial Statements 1. Description of the Business GlycoMimetics, Inc. (the Company), a Delaware corporation headquartered in Rockville, Maryland, was incorporated in 2003. The company is a clinical stage biotechnology company focused on the discovery and development of novel glycomimetic drugs to address unmet medical needs resulting from diseases in which carbohydrate biology plays a key role. Glycomimetics are molecules that mimic the structure of carbohydrates involved in important biological processes. Using its expertise in carbohydrate chemistry and knowledge of carbohydrate biology, the Company is developing a pipeline of proprietary glycomimetics that inhibit disease-related functions of carbohydrates, such as the roles they play in inflammation, cancer and infection. The Company’s executive personnel have devoted substantially all of their time to date to the planning and organization of the Company, the process of hiring scientists, initiating research and development programs and securing adequate capital for anticipated growth and operations. The Company has not commercialized any of its drug candidates and planned commercial operations have not commenced. The Company has incurred significant losses in the development of its drug candidates. The Company has not generated revenues from product sales. As a result, the Company has consistently reported negative cash flows from operating activities and net losses, had an accumulated deficit of $200,550,739 at December 31, 2018 and expects to continue incurring losses for the foreseeable future.

Significant Accounting Policies

12 Months Ended

Dec. 31, 2018

Summary of Significant Accounting Policies

Basis of Presentation and Significant Accounting Policies [Text Block]

2. Summary of Significant Accounting Policies

Basis of Accounting

The accompanying financial statements were prepared based on the accrual method of accounting in accordance with U.S. generally accepted accounting principles (GAAP).

Segment Information

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

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 disclosure of assets and liabilities at the date of the financial statements and the reported amounts of revenues and expenses during the reporting periods. Although actual results could differ from those estimates, management does not believe that such differences would be material.

Cash and Cash Equivalents

Cash and cash equivalents consist of investment in money market funds with commercial banks and financial institutions. The Company considers all investments in highly liquid financial instruments with an original maturity of three months or less at the date of purchase to be cash equivalents. Cash equivalents are stated at amortized cost, plus accrued interest, which approximates fair value.

Fair Value Measurements

The Company’s financial instruments include cash and cash equivalents. The fair values of the financial instruments approximated their carrying values at December 31, 2018 and 2017, due to their short-term maturities. The Company accounts for recurring and nonrecurring fair value measurements in accordance with ASC 820, Fair Value Measurements. ASC 820 defines fair value, establishes a fair value hierarchy for assets and liabilities measured at fair value, and requires expanded disclosures about fair value measurements. The ASC hierarchy ranks the quality of reliability of inputs, or assumptions, used in the determination of fair value, and requires assets and liabilities carried at fair value to be classified and disclosed in one of the following three categories:

Level 1—Fair value is determined by using unadjusted quoted prices that are available in active markets for identical assets and liabilities.

Level 2—Fair value is determined by using inputs, other than Level 1 quoted prices, that are directly and indirectly observable. Inputs can include quoted prices for similar assets and liabilities in active markets or quoted prices for identical assets and liabilities in inactive markets. Related inputs can also include those used in valuation or other pricing models that can be corroborated by observable market data.

Level 3—Fair value is determined by inputs that are unobservable and not corroborated by market data. Use of these inputs involves significant and subjective judgments to be made by a reporting entity. In instances where the determination of the fair value measurement is based on inputs from different levels of fair value hierarchy, the fair value measurement will fall within the lowest level input that is significant to the fair value measurement in its entirety.

The Company periodically evaluates financial assets and liabilities subject to fair value measurements to determine the appropriate level at which to classify them each reporting period. This determination requires the Company to make subjective judgments as to the significance of inputs used in determining fair value and where such inputs lie within the ASC 820 hierarchy.

The Company had no assets or liabilities that were measured using quoted prices for similar assets and liabilities or significant unobservable inputs (Level 2 and Level 3 assets and liabilities, respectively) as of December 31, 2018 and 2017. The carrying value of cash held in money market funds of approximately $207.9 million and $121.9 million as of December 31, 2018 and 2017, respectively, is included in cash and cash equivalents and approximates market values based on quoted market prices (Level 1 inputs).

Concentration of Credit Risk

Credit risk represents the risk that the Company would incur a loss if counterparties failed to perform pursuant to the terms of their agreements. Financial instruments that potentially expose the Company to concentrations of credit risk consist primarily of cash and cash equivalents. Cash and cash equivalents consist of investment in money market funds with major financial institutions in the United States. These deposits and funds may be redeemed upon demand and, therefore, bear minimal risk. The Company does not anticipate any losses on such balances.

Property and Equipment

Property and equipment are recorded at cost and depreciated on a straight-line basis over estimated useful lives ranging from three to seven years. Upon retirement or disposition of assets, the costs and 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 are charged to operations as incurred; major replacements that extend the useful life are capitalized. Depreciation and amortization are computed using the straight-line method over the following estimated useful lives:


		ESTIMATED USEFUL LIVES
Furniture and fixtures		7 years
Laboratory equipment		5 years
Office equipment		5 years
Computer equipment		5 years
Computer software		3 years
Leasehold improvements		Shorter of lease term or useful life

Impairment of Long-Lived Assets

The Company periodically assesses the recoverability of the carrying value of its long-lived assets in accordance with the provisions of ASC 360, Property, Plant, and Equipment. ASC 360 requires that long-lived assets and certain identifiable intangible assets be reviewed for impairment whenever events or changes in circumstances indicate that the carrying amount of an asset may not be recoverable. Recoverability of the long-lived asset is measured by a comparison of the carrying amount of the asset to future undiscounted net cash flows expected to be generated by the asset. If the carrying value exceeds the sum of undiscounted cash flows, the Company then determines the fair value of the underlying asset. Any impairment to be recognized is measured by the amount by which the carrying amount of the assets exceeds the estimated fair value of the assets. Assets to be disposed of are reported at the lower of the carrying amount or fair value, less costs to sell. As of December 31, 2018 and 2017, the Company determined that there were no impaired assets and had no assets held for sale.

Revenue Recognition

Effective January 1, 2018, the Company adopted Accounting Standards Codification, or ASC, Topic 606, Revenue from Contracts with Customers (Topic 606), using the full retrospective transition method. Under this method, the Company would have been required to revise its financial statements, if applicable, for the years ended December 31, 2017 and 2016, and applicable interim periods within those years, as if Topic 606 had been effective for those periods. However, Topic 606 did not have any impact on the Company’s revenue recognition upon adoption. This standard applies to all contracts with customers, except for contracts that are within the scope of other standards, such as leases, insurance, collaboration arrangements and financial instruments. Under Topic 606, an entity recognizes revenue when its customer obtains control of promised goods or services in an amount that reflects the consideration which the entity expects to receive in exchange for those goods and services. To determine revenue recognition for arrangements that an entity determines are within the scope of Topic 606, the entity performs the following five steps: (i) identify the contract(s) with the customer(s); (ii) identify the performance obligations in the contract; (iii) determine the transaction price; (iv) allocate the transaction price to the performance obligations in the contract; and (v) recognize revenue when (or as) the entity satisfies a performance obligation. The Company only applies the five-step model to contracts when it is probable that the entity will collect the consideration it is entitled to in exchange for the goods and services it transfers to the customer. At contract inception, the Company assesses the goods or services promised within each contract that falls under the scope of Topic 606, determines those that are performance obligations and assesses whether each promised good or service is distinct. The Company then recognizes as revenue the amount of the transaction price that is allocated to the respective performance obligation when (or as) the performance obligation is satisfied.

The Company enters into licensing agreements which are within the scope of Topic 606, under which it licenses certain of its product candidates’ rights to third parties. The terms of these arrangements typically include payment of one or more of the following: non-refundable, up-front license fees; development, regulatory and commercial milestone payments; and royalties on net sales of the licensed product, if and when earned. In determining the appropriate amount of revenue to be recognized as it fulfills its obligation under each of its agreements, the Company performs the five steps under ASC 606 as described above. As part of the accounting for these arrangements, the Company must develop assumptions that require judgment to determine the stand-alone selling price, which may include forecasted revenues, development timelines, reimbursement of personnel costs, discount rates and probabilities of technical and regulatory success.

Licensing of Intellectual Property: If the license to the Company’s intellectual property is determined to be distinct from the other performance obligations identified in the arrangement, the Company recognizes revenue from non-refundable, up-front fees allocated to the license when the license is transferred to the licensee and the licensee is able to use and benefit from the license. For licenses that are bundled with other promises, the Company utilizes judgment to assess the nature of the combined performance obligation to determine whether the combined performance obligation is satisfied over time or at a point in time and, if over time, the appropriate method of measuring progress for purposes of recognizing revenue from non-refundable, up-front fees. The Company evaluates the measure of progress each reporting period, and, if necessary, adjusts the measure of performance and related revenue recognition.

Milestone Payments: At the inception of each arrangement that includes development milestone payments, the Company evaluates whether the milestones are considered probable of being reached and estimates the amount to be included in the transaction price using the most likely amount method. If it is probable that a significant revenue reversal will not occur, the associated milestone value is included in the transaction price. Milestone payments that are not within the control of the Company or the licensee, such as regulatory approvals, are not considered probable of being achieved until those approvals are received. The transaction price is then allocated to each performance obligation on a relative stand-alone selling price basis, for which the Company recognizes revenue as or when the performance obligations under the contract are satisfied. At the end of each subsequent reporting period, the Company re-evaluates the probability of achievement of such development milestones and any related constraint and, if necessary, adjusts its estimate of the overall transaction price. Any such adjustments are recorded on a cumulative catch-up basis, which would affect license, collaboration and other revenues and earnings in their period of adjustment.

Royalties: For arrangements that include sales-based royalties, including milestone payments based on the level of sales, and for which the license is deemed to be the predominant item to which royalties relate, the Company recognizes revenue at the later of (i) when the related sales occur or (ii) when the performance obligation to which some or all of the royalty has been allocated has been satisfied (or partially satisfied). To date, the Company has not recognized any royalty revenue from its license agreements.

The Company has entered into a license agreement (the Pfizer Agreement) with Pfizer Inc. (Pfizer) in October 2011. The Pfizer Agreement calls for a non-refundable up-front payment and milestone payments upon achieving significant milestone events. The Pfizer Agreement also contemplates royalty payments on future sales of an approved product. There are no performance, cancellation, termination or refund provisions in the Pfizer Agreement that contain material financial consequences to the Company. For a complete discussion of the Company’s accounting for the Pfizer Agreement, see Note 10, “Research and License Agreements.”

Accrued Liabilities

The Company is required to estimate accrued liabilities as part of the process of preparing its financial statements. The estimation of accrued liabilities involves identifying services that have been performed on the Company’s behalf, and then estimating the level of service performed and the associated cost incurred for such services as of each balance sheet date. Accrued liabilities include professional service fees, such as for lawyers and accountants, contract service fees, such as those under contracts with clinical monitors, data management organizations and investigators in conjunction with clinical trials, and fees to contract manufacturers in conjunction with the production of clinical materials. Pursuant to the Company’s assessment of the services that have been performed, the Company recognizes these expenses as the services are provided. Such assessments include: (i) an evaluation by the project manager of the work that has been completed during the period; (ii) measurement of progress prepared internally and/or provided by the third-party service provider; (iii) analyses of data that justify the progress; and (iv) the Company’s judgment.

Research and Development Costs

Except for payments made in advance of services, research and development costs are expensed as incurred. For payments made in advance, the Company recognizes research and development expense as the services are rendered. Research and development costs primarily consist of salaries and related expenses for personnel, laboratory supplies and raw materials, sponsored research, depreciation of laboratory facilities and leasehold improvements, and utilities costs related to research space. Other research and development expenses include fees paid to consultants and outside service providers including clinical research organizations and clinical manufacturing organizations.

Stock-Based Compensation

Stock-based payments are accounted for in accordance with the provisions of ASC 718, Compensation—Stock Compensation. The fair value of stock-based payments is estimated, on the date of grant, using the Black-Scholes-Merton model. The resulting fair value is recognized ratably over the requisite service period, which is generally the vesting period of the option.

The Company has elected to use the Black-Scholes-Merton option pricing model to value any options granted. The Company will reconsider use of the Black-Scholes-Merton model if additional information becomes available in the future that indicates another model would be more appropriate or if grants issued in future periods have characteristics that prevent their value from being reasonably estimated using this model.

A discussion of management’s methodology for developing some of the assumptions used in the valuation model follows:

Expected Dividend Yield—The Company has never declared or paid dividends and has no plans to do so in the foreseeable future.

Expected Volatility—Volatility is a measure of the amount by which a financial variable such as share price has fluctuated (historical volatility) or is expected to fluctuate (expected volatility) during a period. Prior to the Company’s initial public offering, there was not a market for the Company’s shares. The Company utilizes the historical volatilities of a peer group (e.g., several public entities of similar size, complexity, and stage of development), along with the Company’s historical volatility since its initial public offering to determine its expected volatility.

Risk-Free Interest Rate—This is the U.S. Treasury rate for the week of each option grant during the year, having a term that most closely resembles the expected life of the option.

Expected Term—This is a period of time that the options granted are expected to remain unexercised. Options granted have a maximum term of 10 years. The Company estimates the expected life of the option term to be 6.25 years. The Company uses a simplified method to calculate the average expected term.

Expected Forfeiture Rate—Effective with the adoption of Accounting Standards Update (ASU) No. 2016-09 on January 1, 2017, Compensation – Stock Compensation (Topic 718): Improvements to Employee Share-Based Payment Accounting, the Company has elected to account for forfeitures as they occur. The forfeiture rate is the estimated percentage of options granted that is expected to be forfeited or canceled on an annual basis before becoming fully vested.

Income Taxes

The Company accounts for income taxes using the asset and liability method in accordance with ASC 740, Income Taxes. Deferred income taxes are recognized for the tax consequences in future years of differences between the tax bases of assets and liabilities and the financial reporting amounts at each year-end based on enacted tax laws and statutory tax rates applicable to the periods in which the differences are expected to affect taxable income. A valuation allowance is established when necessary to reduce deferred tax assets to the amount expected to be realized.

The Company accounts for uncertain tax positions pursuant to ASC 740. Financial statement recognition of a tax position taken or expected to be taken in a tax return is determined based on a more-likely-than-not threshold of that tax position being sustained. If the tax position meets this threshold, the benefit to be recognized is measured as the tax benefit having the highest likelihood of being realized upon ultimate settlement with the taxing authority. The Company recognizes interest accrued related to unrecognized tax benefits and penalties in the provision for income taxes.

Comprehensive Loss

Comprehensive loss comprises net loss and other changes in equity that are excluded from net loss. For the years ended December 31, 2018, 2017 and 2016, the Company’s net loss was equal to comprehensive loss and, accordingly, no additional disclosure is presented.

Adopted Accounting Standards

In May 2014, the FASB issued Topic 606, Revenue from Contracts with Customers, which amended the guidance for accounting for revenue from contracts with customers. Topic 606 superseded the revenue recognition requirements in ASC Topic 605, Revenue Recognition. The Company adopted this new standard on January 1, 2018 using the full retrospective transition method. The Company evaluated the Pfizer Agreement to determine the impact of the new revenue standard on the up-front and milestone payments within the Pfizer Agreement and determined that the transition to the new revenue standard had no material impact on the prior financial statements presented. There were no financial statement line items affected by the transition. For further discussion on the adoption of this standard, see “Revenue Recognition” above and Note 10, “Research and License Agreements.”

In January 2017, the FASB issued ASU No. 2017-01, Business Combinations (Topic 805), Clarifying the Definition of a Business. The guidance changed the definition of a business to assist entities with evaluating when a set of transferred assets and activities is a business. The new guidance requires an entity to evaluate if substantially all of the fair value of the gross assets acquired is concentrated in a single identifiable asset or a group of similar identifiable assets; if so, the set of transferred assets and activities is not a business. The Company adopted this ASU as of January 1, 2018. The adoption of this ASU had no impact on the Company’s financial statements for the year ended December 31, 2018.

In May 2017, the FASB issued ASU No. 2017-09, Compensation – Stock Compensation (Topic 718), Scope of Modification Accounting, which clarifies when a change to the terms or conditions of a share-based payment award must be accounted for as a modification. The new guidance requires modification accounting if the fair value, vesting conditions or classification of the award is not the same immediately before and after a change to the terms and conditions of the award. The Company adopted this ASU on a prospective basis as of January 1, 2018. The adoption of this ASU had no impact on the Company’s financial statements for the year ended December 31, 2018.

Accounting Standards Not Yet Adopted

In August 2018, the U.S. Securities and Exchange Commission (SEC) adopted the final rule under SEC Release No. 33-10532, Disclosure Update and Simplification. This final rule amends certain disclosure requirements that are redundant, duplicative, overlapping, outdated or superseded. In addition, the amendments expand the disclosure requirements on the analysis of stockholders' equity for interim financial statements. Under the amendments, an analysis of changes in each caption of stockholders' equity presented in the balance sheet must be provided in a note or separate statement. The analysis should present a reconciliation of the beginning balance to the ending balance of each period for which a statement of comprehensive income is required to be filed. This final rule is effective for the Company for all filings made on or after November 5, 2018. The SEC staff clarified that the first presentation of the changes in shareholders' equity may be included in the first Form 10-Q for the quarter that begins after the effective date of the amendments. The adoption of the final rule did not have a material impact on the Company’s consolidated financial statements. The Company will change its presentation of stockholder’s equity in the first quarter of 2019.

In February 2016, the FASB issued ASU No. 2016-02, Leases (Topic 842), which supersedes existing guidance on accounting for leases in Leases (Topic 840) and generally requires all leases, including operating leases, to be recognized in the statement of financial position as right-of-use assets and lease liabilities by lessees. The provisions of ASU No. 2016-02 are to be applied using a modified retrospective approach and are effective for reporting periods beginning after December 15, 2018; early adoption is permitted. The Company has elected the transition option provided under ASU No. 2018-11, which will not require adjustments to comparative periods nor require modified disclosures in those comparative periods. In addition, the Company elected the transition package of practical expedients permitted within the new standard, which among other things, allows the carryforward of the historical lease classification. Adoption of the standard will result in the recognition of right-of-use assets and related lease liabilities for operating leases of approximately $3.6 million and $4.3 million, respectively, as of January 1, 2019. The difference between these amounts will be comprised of adjustments related to unamortized balances of deferred rent, lease incentives, and prepaid rent existing as of the effective date. The adoption of the standard is not expected to materially affect net earnings.

In June 2018, the FASB issued ASU No. 2018-07, Compensation – Stock Compensation (Topic 718): Improvements to Nonemployee Share-based Payment Accounting, to simplify the accounting for share-based payments to nonemployees by aligning it with the accounting for share-based payments to employees, with certain exceptions. The provisions of ASU 2018-07 are effective for reporting periods beginning after December 15, 2018, including interim periods within that fiscal year. Upon transition, the Company will be required to measure these nonemployee awards at fair value as of the adoption date. The Company adopted this ASU as of January 1, 2019. The adoption of this ASU is expected to have an immaterial impact on the Company’s financial statements for the year ending December 31, 2019.

In November 2018, the FASB issued ASU No. 2018-18, Collaborative Arrangements (Topic 808): Clarifying the Interaction between Topic 808 and Topic 606. The amendment clarifies that certain transactions between collaborative arrangement participants should be accounted for as revenue under Topic 606 when the collaborative arrangement participant is a customer in the context of a unit of account. In those situations, all the guidance in Topic 606 should be applied, including recognition, measurement, presentation, and disclosure requirements. The amendment also adds unit-of-account guidance in Topic 808 to align with the guidance in Topic 606 (that is, a distinct good or service) when an entity is assessing whether the collaborative arrangement or a part of the arrangement is within the scope of Topic 606. Lastly, the amendment requires that in a transaction with a collaborative arrangement participant that is not directly related to sales to third parties, presenting the transaction together with revenue recognized under Topic 606 is precluded if the collaborative arrangement participant is not a customer. For public business entities, the amendments are effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years. The Company is currently evaluating these clarifications in the accounting and presentation for its collaborative arrangements within the scope of Topic 808.

With the exception of the new standards discussed above, there have been no new accounting pronouncements that have significance, or potential significance, to the Company’s financial statements.

Net Loss Per Share of Common Stock

12 Months Ended

Dec. 31, 2018

Net Loss Per Share of Common Stock

Net Loss Per Share of Common Stock

3. Net Loss Per Share of Common Stock

Basic net loss per common share is determined by dividing net loss by the weighted-average number of common shares outstanding during the period, without consideration of common stock equivalents. Diluted net income per share is computed by dividing net income by the weighted-average number of common stock equivalents outstanding for the period. The treasury stock method was used to determine the dilutive effect of the Company’s stock option grants, restricted stock units and warrants.

The following table sets forth the computation of basic and diluted earnings per share for the years ended December 31, 2018, 2017 and 2016:



	2018		2017		2016
Net loss	$	(48,273,633)	$	(33,281,068)	$	(31,809,838)
Basic and diluted net loss per common share	$	(1.18)	$	(1.13)	$	(1.50)
Basic and diluted weighted average common shares outstanding		41,044,621		29,395,756		21,256,312

The following potentially dilutive securities outstanding at December 31, 2018, 2017 and 2016 have been excluded from the computation of diluted weighted average shares outstanding, as they would be anti-dilutive:



	2018	2017	2016
Warrants	—	553,868	553,868
Stock options and restricted stock units	3,937,167	3,399,124	2,817,674

Prepaid Expenses and Other Current Assets

12 Months Ended

Dec. 31, 2018

Prepaid Expenses and Other Current Assets

Prepaid Expenses and Other Current Assets

4. Prepaid Expenses and Other Current Assets

The following is a summary of the Company’s prepaid expenses and other current assets at December 31:



	2018		2017
Prepaid research and development expenses	$	1,608,768	$	2,941,196
Other prepaid expenses		329,634		251,733
Other receivables		413,122		101,955
Prepaid expenses and other current assets	$	2,351,524	$	3,294,884

Property and Equipment

12 Months Ended

Dec. 31, 2018

Property and Equipment

Property and Equipment

5. Property and Equipment

Property and equipment, net consisted of the following at December 31:


	2018		2017
Furniture and fixtures	$	334,300	$	314,024
Laboratory equipment		1,389,036		1,325,667
Office equipment		11,085		11,085
Computer equipment		233,282		192,430
Leasehold improvements		573,165		573,165
Property and equipment		2,540,868		2,416,371
Less accumulated depreciation		(1,583,642)		(1,309,472)
Property and equipment, net	$	957,226	$	1,106,899

Depreciation of property and equipment totaled $275,123, $263,541 and $190,540 for the years ended December 31, 2018, 2017 and 2016, respectively.

Accrued Expenses

12 Months Ended

Dec. 31, 2018

Accrued Expenses

Accrued Expenses

6. Accrued Expenses

The following is a summary of the Company’s accrued expenses at December 31:



	2018		2017
Accrued research and development expenses	$	3,483,741	$	2,702,445
Accrued consulting and other professional fees		140,397		227,811
Accrued employee benefits		385,789		331,930
Other accrued expenses		263,693		304,421
Accrued expenses	$	4,273,620	$	3,566,607

Operating Leases

12 Months Ended

Dec. 31, 2018

Operating Leases

Operating Leases

7. Operating Leases

The Company leases office and research space in Rockville, Maryland under an operating lease with a term through October 31, 2023 (as amended to date, the Lease) that is subject to annual rent increases. The Company has the right to sublease or assign all or a portion of the premises, subject to the conditions set forth in the Lease. The Lease may be terminated early by either the landlord or the Company in certain circumstances. In connection with the Lease, the Company received rent abatement as a lease incentive. The annual rent increases and rent abatement have been recognized as deferred rent that is being adjusted on a straight-line basis over the term of the Lease.

In March 2016, the Company amended the Lease (the Lease Amendment) to lease additional space beginning on June 1, 2016. In addition to the other terms of the Lease, the Lease Amendment provided for a tenant improvement allowance reflected in the Company’s 2016 financial statements as an increase in capitalized leasehold improvements as incurred and an increase in deferred rent. In May 2016, the Company also paid a security deposit of $52,320 to be held until the expiration or termination of the Company’s obligations under the Lease, which comprises the deposits balance on the balance sheets. The term of the Lease Amendment for the additional space continues through October 31, 2023, the same date as for the premises originally leased under the Lease, subject to the Company’s renewal option set forth in the Lease. The Company also has a one-time option to terminate the Lease effective as of October 31, 2020.

Deferred rent related to the Lease was $710,394 and $785,031 at December 31, 2018 and December 31, 2017, respectively. Total rent expense under the Company’s leases was $896,758, $890,170 and $784,739 for the years ended December 31, 2018, 2017 and 2016, respectively.

The following table presents the future minimum lease payments as of December 31, 2018 under the Lease:


YEAR	AMOUNT
2019	$	989,502
2020		1,014,239
2021		1,039,595
2022		1,065,585
2023		906,669
After 2023		—
Total	$	5,015,590

Stockholders' Equity

12 Months Ended

Dec. 31, 2018

Stockholders' Equity

Disclosure of stockholders' equity and share-based compensation

8. Stockholders’ Equity

Common Stock

At-The-Market Equity Offerings

On March 1, 2016, the Company entered into an at-the-market sales agreement with Cowen and Company, LLC to sell the Company’s securities under a shelf registration statement filed in March 2015. As of December 31, 2016, the Company had issued and sold 668,791 shares of common stock under the at-the-market sales agreement. The shares were sold at a weighted average price per share of $6.336, for aggregate net proceeds of $3.9 million, after deducting commissions and offering expenses. During the period from January 1, 2017 through May 23, 2017, the Company issued and sold an additional 1,388,647 shares of common stock under the at-the-market sales agreement. The shares were sold at a weighted average price per share of $5.55, for aggregate net proceeds of $7.4 million, after deducting commissions and offering expenses. The at-the market sales agreement was terminated on May 23, 2017.

On September 28, 2017, the Company entered into a new at-the-market sales agreement with Cowen and Company, LLC to sell the Company’s securities under a shelf registration statement filed in September 2017. As of December 31, 2017, the Company had issued and sold 1,600,000 shares of common stock under the at-the-market sales agreement. The shares were sold at a weighted average price per share of $12.50, for aggregate net proceeds of $19.3 million, after deducting commissions and offering expenses. As of December 31, 2018, $80.0 million remained available to be sold under the terms of the September 2017 at-the-market sales agreement. There were no shares sold under the September 2017 Sales Agreement during the year ended December 31, 2018.

Public Offerings of Common Stock

In June 2016, the Company completed a public offering in which the Company sold 3,476,793 shares of its common stock at a price to the public of $6.10 per share. The Company received net proceeds of $19.7 million from this offering, after deducting underwriting discounts, commissions and other offering expenses.

In May 2017, the Company completed a public offering in which the Company sold 8,050,000 shares of its common stock at a price to the public of $11.50 per share. The Company received net proceeds of $86.8 million from this offering, after deducting underwriting discounts, commissions and other offering expenses.

In March 2018, the Company completed a public offering in which the Company sold 8,050,000 shares of its common stock at a price to the public of $17.00 per share. The Company received net proceeds of $128.4 million from this offering, after deducting underwriting discounts, commissions and other offering expenses.

Warrants to Acquire Company Stock

In connection with the prior issuance of convertible unsecured promissory notes, the Company issued warrants to purchase shares of common stock in 2008 and 2009. As of December 31, 2017 and 2016, warrants to purchase an aggregate of 553,868 shares were outstanding, each with an exercise price of $0.33 per share. During the year ended December 31, 2018, all of the outstanding warrants were exercised; a total of 536,564 shares of common stock were issued to stockholders upon the net exercise of 546,709 outstanding warrants, and 7,159 shares of common stock were issued to stockholders upon the cash exercise of outstanding warrants, for total proceeds to the Company of $2,336. The Company no longer has any outstanding warrants to purchase shares of its capital stock. For the year ended December 31, 2017, no warrants were exercised or expired. For the year ended December 31, 2016, a total of 23,275 warrants were exercised at a weighted average exercise price of $0.33 per share and a total of 1,544 warrants expired.

2003 Stock Incentive Plan

The 2003 Stock Incentive Plan (the 2003 Plan) provided for the grant of incentives and nonqualified stock options and restricted stock awards. The exercise price for incentive stock options must be at least equal to the fair value of the common stock on the grant date. Unless otherwise stated in a stock option agreement, 25% of the shares subject to an option grant will vest upon the first anniversary of the vesting start date and thereafter at the rate of one forty-eighth of the option shares per month as of the first day of each month after the first anniversary. Upon termination of employment by reasons other than death, cause, or disability, any vested options shall terminate 60 days after the termination date. Stock options terminate 10 years from the date of grant. The 2003 Plan expired on May 21, 2013.

A summary of the Company’s stock option activity under the 2003 Plan for the year ended December 31, 2018 is as follows:



				Weighted-Average
				Remaining	Aggregate
	Outstanding	Weighted-Average		Contractual Term	Intrinsic Value
	Options	Exercise Price		(Years)	(In thousands)
Outstanding as of December 31, 2017	713,211	$	1.25	2.2
Options exercised	(46,131)		1.29
Options forfeited	—		—
Outstanding, Vested and Exercisable as of December 31, 2018	667,080		1.24	1.2	$	5,488

During 2018, 2017 and 2016 the Company issued 46,131, 16,608 and 28,368 shares of common stock, respectively, in conjunction with exercises of stock options granted under the 2003 Plan. The Company received cash proceeds from the exercise of these stock options of $59,659, $27,357 and $44,771 during 2018, 2017 and 2016, respectively. Total intrinsic value of the options exercised during the years ended December 31, 2018, 2017 and 2016 was $716,920, $103,638 and $97,707, respectively.

As of December 31, 2018, the options under the 2003 Plan were fully expensed and all shares outstanding in the plan were fully vested as of December 31, 2017. The total fair value of shares vested in the years ended December 31, 2017 and 2016, was $1,573 and $16,024, respectively. There were no options granted from this plan in 2018, 2017 or 2016.

2013 Equity Incentive Plan

The Company’s board of directors adopted, and its stockholders approved, its 2013 Equity Incentive Plan (the 2013 Plan) effective on January 9, 2014. The 2013 Plan provides for the grant of incentive stock options within the meaning of Section 422 of the Internal Revenue Code (the Code), to the Company’s employees and its parent and subsidiary corporations’ employees, and for the grant of nonstatutory stock options, restricted stock awards, restricted stock unit awards, stock appreciation rights, performance stock awards and other forms of stock compensation to its employees, including officers, consultants and directors. The 2013 Plan also provides for the grant of performance cash awards to the Company’s employees, consultants and directors. Unless otherwise stated in a stock option agreement, 25% of the shares subject to an option grant will typically vest upon the first anniversary of the vesting start date and thereafter at the rate of one forty-eighth of the option shares per month as of the first day of each month after the first anniversary. Upon termination of employment by reasons other than death, cause, or disability, any vested options shall terminate 90 days after the termination date, unless otherwise set forth in a stock option agreement. Stock options generally terminate 10 years from the date of grant.

Authorized Shares

The maximum number of shares of common stock that may be issued under the 2013 Plan was 1,000,000 shares, plus any shares subject to stock options or similar awards granted under the 2003 Plan that expire or terminate without having been exercised in full or are forfeited to or repurchased by the Company. The number of shares of common stock reserved for issuance under the 2013 Plan will automatically increase on January 1 of each year, beginning on January 1, 2015 and ending on January 1, 2023, by 3% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares as may be determined by the Company’s board of directors. The maximum number of shares that may be issued pursuant to exercise of incentive stock options under the 2013 Plan is 20,000,000. As of January 1, 2019, the number of shares of common stock that may be issued under the 2013 Plan was automatically increased by 1,294,822 shares, representing 3% of the total number of shares of common stock outstanding on January 1, 2019, increasing the number of shares of common stock available for issuance under the 2013 Plan to 5,162,816 shares.

Shares issued under the 2013 Plan may be authorized but unissued or reacquired shares of common stock. Shares subject to stock awards granted under the 2013 Plan that expire or terminate without being exercised in full, or that are paid out in cash rather than in shares, will not reduce the number of shares available for issuance under the 2013 Plan. Additionally, shares issued pursuant to stock awards under the 2013 Plan that the Company repurchases or that are forfeited, as well as shares reacquired by the Company as consideration for the exercise or purchase price of a stock award or to satisfy tax withholding obligations related to a stock award, will become available for future grant under the 2013 Plan.

Stock Options

A summary of the Company’s stock option activity under the 2013 Plan for the year ended December 31, 2018 is as follows:


				WEIGHTED-
		WEIGHTED-		AVERAGE	AGGREGATE
		AVERAGE		REMAINING	INTRINSIC
	OUTSTANDING	EXERCISE		CONTRACTUAL	VALUE (IN
	OPTIONS	PRICE		TERM(YEARS)	THOUSANDS)
Outstanding as of December 31, 2017	2,664,163	$	7.34	7.4
Options granted	845,479		19.22
Options exercised	(144,182)		6.68
Options forfeited	(100,206)		13.89
Outstanding as of December 31, 2018	3,265,254		8.39	7.1	$	7,489
Vested or expected to vest as of December 31, 2018	3,265,254		8.39	7.1	$	7,489
Exercisable as of December 31, 2018	2,020,307		7.66	6.1	$	4,539

The weighted-average fair value of the options granted during the years ended December 31, 2018, 2017 and 2016 was $12.90, $4.76 and $3.39 per share, respectively, applying the Black-Scholes-Merton option pricing model utilizing the following weighted-average assumptions:


	2018	2017	2016
Expected term	6.25 years	6.25 years	6.25 years
Expected volatility	73.75%	75.20%	68.98%
Risk-free interest rate	2.55%	2.08%	1.70%
Expected dividend yield	0%	0%	0%

As of December 31, 2018, there was $9,129,271 of total unrecognized compensation expense related to unvested options that will be recognized over a weighted-average period of approximately 2.7 years. The total fair value of shares vested in the years ended December 31, 2018, 2017 and 2016 was $3,003,632, $3,506,568 and $3,053,086, respectively. During the years ended December 31, 2018, 2017 and 2016, the Company received cash of $962,530, $346,019 and $27,825, respectively, and issued 144,182, 54,521 and 3,500 shares of common stock, respectively, in conjunction with exercises of stock options granted under the 2013 Plan. The intrinsic value of the options exercised for the years ended December 31, 2018, 2017 and 2016 was $1,344,026, $385,701 and $2,275, respectively.

Restricted Stock Units (RSUs)

An RSU is a stock award that entitles the holder to receive shares of the Company’s common stock as the award vests. The fair value of each RSU is based on the closing price of the Company’s stock on the date of grant. The Company has granted RSUs with service conditions (service RSUs) that vest in three equal annual installments provided that the employee remains employed with the Company. As of December 31, 2018, $1,099 of unrecognized compensation costs related to unvested service.

The following is a summary of RSU activity for the 2013 Plan for the year ended December 31, 2018:



			Weighted-Average
		Number of Shares	Grant Date
	Underlying RSUs			Fair Value
Unvested at December 31, 2017		9,667	$	4.61
Granted		—		—
Forfeited		—		—
Vested		4,834		4.61
Unvested at December 31, 2018		4,833		4.61

Total stock-based compensation expense associated with stock options and RSUs was classified as follows on the statement of operations for the years ended December 31:



	2018		2017		2016
Research and development expense	$	1,709,390	$	1,280,909	$	1,033,005
General and administrative expense		2,877,708		2,479,893		1,931,762
Total stock-based compensation expense	$	4,587,098	$	3,760,802	$	2,964,767

Income Taxes

12 Months Ended

Dec. 31, 2018

Income Taxes

Income Taxes

9. Income Taxes

The components of the gross deferred tax asset and related valuation allowance at December 31 were as follows:


	2018		2017
Deferred tax assets:
Net operating loss carryforward	$	41,687,577	$	30,813,509
Capitalized start-up costs		1,501,895		1,695,688
Patent amortization		119,888		135,358
Research and orphan drug credits		28,123,082		21,293,509
Deferred rent		51,400		71,938
Deferred compensation		3,293,221		2,274,885
Other		95,564		69,906
Total gross deferred tax assets		74,872,627		56,354,793
Valuation allowance		(74,872,627)		(56,354,793)
Deferred tax assets		—		—
Deferred tax liabilities:
Total deferred tax liabilities		—		—
Net deferred tax assets	$	—	$	—

Based on the Company’s operating history and management’s expectation regarding future profitability, management believes the realization of the Company’s deferred tax assets does not meet the more-likely-than-not criteria under ASC 740, Income Taxes. Accordingly, a full valuation allowance has been established as of December 31, 2018 and 2017.

On December 22, 2017, the Tax Cuts and Jobs Act (the “TCJA”) was enacted into law. The TCJA contained several key tax provisions including the reduction of the corporate income tax rate from 35% to 21% effective January 1, 2018, as well as a variety of other changes, including the limitation of the tax deductibility of interest expense, acceleration of expensing of certain business assets and reductions in the amount of executive pay that can qualify as a tax deduction. ASC Topic 740 required the Company to recognize the effect of the tax law changes in the period of enactment. The Company re-measured certain of its U.S. deferred tax assets and liabilities, based on the rates at which they are expected to reverse in the future. As of December 31, 2017, the tax benefit recorded related to the re-measurement of the deferred tax balance was $15.2 million, which was offset by the related valuation allowance. The Company did not adjust the provisional amount estimated at December 31, 2017 upon its final analysis during the year ended December 31, 2018 of the income tax effects due to the enactment of the TCJA.

As of December 31, 2018, the Company had $151.5 million of U.S. Federal and state net operating losses, $8.5 million of research and development tax credits and $19.6 million of orphan drug tax credits available to carry forward. A portion of the net operating loss carryforwards will begin to expire in 2026, the research and development tax credits in 2023 and the orphan drug tax credit in 2033.

The Company’s tax attributes, including net operating losses and credits, are subject to any ownership changes as defined under Internal Revenue Code Sections 382 and 383. A change in ownership could affect the Company’s ability to utilize its net operating losses and credits. As of December 31, 2018, the Company does not believe that an ownership change has occurred. Any future ownership changes may cause a limitation on the Company’s ability to utilize existing tax attributes.

The Company files income tax returns in the U.S. federal jurisdiction and in the State of Maryland. The Company’s federal income tax returns for tax years 2003 and after remain subject to examination by the U.S. Internal Revenue Service. The Company’s Maryland income tax returns for the tax years 2006 and thereafter remain subject to examination by the Comptroller of Maryland. In addition, all of the net operating losses, research and development tax credit and orphan drug credit carryforwards that may be used in future years are still subject to adjustment.

The Company did not have unrecognized tax benefits as of December 31, 2018 and 2017, and does not anticipate this to change significantly over the next 12 months. The Company will recognize interest and penalties accrued on any unrecognized tax benefits as a component of income tax expense. Reconciliations between the statutory federal income tax rate and the effective income tax rate of income tax expense is as follows as of December 31:


	2018		2017		2016
U.S. Federal statutory tax rate	21.0	%	34.0	%	34.0	%
State taxes	5.7		4.4		4.6
Research credit	0.8		1.3		1.0
Orphan drug credit	10.4		11.5		9.0
Other	0.6		0.3		—
Stock-based compensation	(0.1)		(0.7)		(0.7)
Change in valuation allowance	(38.4)		(5.1)		(47.9)
Effective change due to corporate tax rate reduction	—		(45.7)		—
Provision for income taxes	—	%	—	%	—	%

Research and License Agreements	12 Months Ended
Research and License Agreements	Dec. 31, 2018
Research And License Agreements [Abstract]
Research and License Agreement	10. Research and License Agreements The Company has entered into a research services agreement (the Research Agreement) with the University of Basel (the University) for biological evaluation of selectin antagonists. Certain patents covering the rivipansel compound remain subject to provisions of the Research Agreement. Under the terms of the Research Agreement, the Company will owe a 10% payment to the University for all future milestone and royalty payments received from Pfizer with respect to rivipansel. There were no payments recorded for the years ended December 31, 2018, 2017 or 2016. The Company and Pfizer have entered into the Pfizer Agreement in October 2011, which provides Pfizer an exclusive worldwide license to rivipansel for vaso-occlusive crisis associated with sickle cell disease and for other diseases for which the drug candidate may be developed. The Company was responsible for completion of a Phase 2 clinical trial, after which Pfizer assumed all further development and commercialization responsibilities. Upon execution of the Pfizer Agreement, the Company received an up-front payment of $22.5 million. The Pfizer Agreement also provides for potential milestone payments of up to $115.0 million upon the achievement of specified development milestones, including the dosing of the first patients in Phase 3 clinical trials for up to two indications and the first commercial sale of a licensed product in the United States and selected European countries for up to two indications; potential milestone payments of up to $70.0 million upon the achievement of specified regulatory milestones, including the acceptance of the Company’s filings for regulatory approval by regulatory authorities in the United States and Europe for up to two indications; and potential milestone payments of up to $135.0 million upon the achievement of specified levels of annual net sales of licensed products. Pfizer has the right to terminate the Pfizer Agreement by giving prior written notice. The Company received a $15.0 million non-refundable milestone payment in May 2014 and an additional non-refundable milestone payment of $20.0 million in June 2015. The Company has not recognized any revenue under the Pfizer Agreement subsequent to the year ended December 31, 2015. The Company assessed this arrangement in accordance with Topic 606, and concluded that the contract counterparty, Pfizer, is a customer. The Company identified the following performance obligations under the contract: (1) an exclusive worldwide license to rivipansel for vaso-occlusive crisis associated with sickle cell disease and for other diseases for which the drug candidate may be developed; and (2) research and development (R&D) services to develop the rivipansel compound for commercial use related to the Phase 2 clinical trial and delivery of data to Pfizer. In addition to the rivipansel license and R&D services, management also considered whether the Company’s participation in a Joint Steering Committee (JSC) constituted a promise. The JSC was formed solely for communication purposes between Pfizer and the Company relating to Pfizer’s progress in further developing rivipansel for commercial use. The Company’s involvement in the JSC is limited to attending the JSC meetings on a semi-annual basis to receive progress updates from Pfizer; Pfizer is responsible for calling and organizing the meetings. Given the minimal level of involvement by the Company, participation in the JSC is not considered a significant aspect of the arrangement and the related costs, such as employee time, are not material. Therefore, management views the Company’s participation in JSC as administrative only and did not further evaluate its participation in the JSC in identifying the performance obligations in the Pfizer Agreement. Under the Pfizer Agreement, in order to evaluate the appropriate transaction price, the Company determined that the up-front amount constituted the entirety of the consideration to be included in the transaction price and to be allocated to the performance obligations based on the Company’s best estimate of their relative stand-alone selling prices. The transaction price of the up-front fee was equal to the $22.5 million received. The fixed up-front consideration is recognized under ASC 606 based on when control of the combined performance obligation is transferred to the customer, which corresponds with the service period (through March 2013). None of the clinical or regulatory milestones have been included in the transaction price, as all milestone amounts were fully constrained. Event-driven milestones are a form of variable consideration as the payments are variable based on the occurrence of future events. As part of its evaluation of the constraint, the Company considered numerous factors, including that receipt of the milestones is outside the control of the Company and is contingent upon success in future clinical trials and the licensee’s efforts. Recognition of event-driven milestones should be recognized when the variable consideration is no longer constrained. There are no changes in accounting necessary for the $15.0 million milestone payment recognized in May 2014 or the $20.0 million milestone payment recognized in June 2015 as a result of Pfizer dosing the first patient in the Phase 3 clinical trial of rivipansel. Future event-driven milestones will be recognized when the constraint no longer applies. Any consideration related to sales-based milestones (including royalties) will be recognized when the related sales occur as they were determined to relate predominantly to the license granted to Pfizer and therefore have also been excluded from the transaction price. The Company will re-evaluate the transaction price in each reporting period and as uncertain events are resolved or other changes in circumstances occur. In evaluating the Pfizer Agreement, the Company considered that there were no significant financing components identified, no non-cash consideration was paid by Pfizer and no consideration was paid by the Company to Pfizer as part of the arrangement.

Employee Benefit Plan	12 Months Ended
Employee Benefit Plan	Dec. 31, 2018
Defined Contribution Plan [Abstract]
Compensation and Employee Benefit Plans, Other than Share-based Compensation [Text Block]	11. Employee Benefit Plan The Company has a defined contribution plan under the Internal Revenue Code Section 401(k). This plan covers substantially all employees who meet minimum age and service requirements and allows participants to defer a portion of their annual compensation on a pre-tax basis. For the years ended December 31, 2018, 2017 and 2016, the Company made a discretionary match of 50% up to the first 3% of employee contributions. All matching contributions have been paid by the Company. The Company’s matching contributions vest in full at the employee’s third anniversary of employment and all employer contributions thereafter vest immediately. The total Company matching contributions were approximately $94,000, $88,000 and $79,000 for the years ended December 31, 2018, 2017 and 2016, respectively.

Quarterly Financial Information (Unaudited)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information (Unaudited)

Quarterly Financial Information (Unaudited)

12. Quarterly Financial Information (Unaudited)

Summarized quarterly financial information for each of the years ended December 31, 2018 and 2017 are as follows:


	Quarter Ended
	December 31,		September 30,		June 30,		March 31,
	2018		2018		2018		2018
Revenue	$	—	$	—	$	—	$	—
Net loss	$	(13,906,915)	$	(11,575,111)	$	(11,278,763)	$	(11,512,844)
Loss per share—basic and diluted	$	(0.32)	$	(0.27)	$	(0.26)	$	(0.33)


	Quarter Ended
	December 31,		September 30,		June 30,		March 31,
	2017		2017		2017		2017
Revenue	$	—	$	—	$	—	$	—
Net loss	$	(9,257,858)	$	(7,950,101)	$	(8,141,796)	$	(7,931,313)
Loss per share—basic and diluted	$	(0.27)	$	(0.24)	$	(0.30)	$	(0.34)

Summary of Significant Accounting Policies (Policies)

12 Months Ended

Dec. 31, 2018

Summary of Significant Accounting Policies

Basis of Accounting

The accompanying financial statements were prepared based on the accrual method of accounting in accordance with U.S. generally accepted accounting principles (GAAP).

Segment Information

Use of Estimates

Cash and Cash Equivalents

Fair Value Measurements

Level 1—Fair value is determined by using unadjusted quoted prices that are available in active markets for identical assets and liabilities.

Concentration of Credit Risk

Property and Equipment


		ESTIMATED USEFUL LIVES
Furniture and fixtures		7 years
Laboratory equipment		5 years
Office equipment		5 years
Computer equipment		5 years
Computer software		3 years
Leasehold improvements		Shorter of lease term or useful life

Impairment of Long-Lived Assets

Revenue Recognition

Accrued Liabilities

Research and Development Costs

Stock-Based Compensation

A discussion of management’s methodology for developing some of the assumptions used in the valuation model follows:

Expected Dividend Yield—The Company has never declared or paid dividends and has no plans to do so in the foreseeable future.

Risk-Free Interest Rate—This is the U.S. Treasury rate for the week of each option grant during the year, having a term that most closely resembles the expected life of the option.

Income Taxes

Comprehensive Loss

Adopted Accounting Standards

Accounting Standards Not Yet Adopted

With the exception of the new standards discussed above, there have been no new accounting pronouncements that have significance, or potential significance, to the Company’s financial statements.

Summary of Significant Accounting Policies (Tables)

12 Months Ended

Dec. 31, 2018

Summary of Significant Accounting Policies

Schedule Of Property Plant And Equipment Useful Life


		ESTIMATED USEFUL LIVES
Furniture and fixtures		7 years
Laboratory equipment		5 years
Office equipment		5 years
Computer equipment		5 years
Computer software		3 years
Leasehold improvements		Shorter of lease term or useful life

Net Loss Per Share of Common Stock (Tables)

12 Months Ended

Dec. 31, 2018

Net Loss Per Share of Common Stock

Computation of basic and diluted net loss per common share

The following table sets forth the computation of basic and diluted earnings per share for the years ended December 31, 2018, 2017 and 2016:



	2018		2017		2016
Net loss	$	(48,273,633)	$	(33,281,068)	$	(31,809,838)
Basic and diluted net loss per common share	$	(1.18)	$	(1.13)	$	(1.50)
Basic and diluted weighted average common shares outstanding		41,044,621		29,395,756		21,256,312

Potentially dilutive securities outstanding



	2018	2017	2016
Warrants	—	553,868	553,868
Stock options and restricted stock units	3,937,167	3,399,124	2,817,674

Prepaid Expenses and Other Current Assets (Tables)

12 Months Ended

Dec. 31, 2018

Prepaid Expenses and Other Current Assets

Summary of Prepaid Expenses and Other Current Assets

The following is a summary of the Company’s prepaid expenses and other current assets at December 31:



	2018		2017
Prepaid research and development expenses	$	1,608,768	$	2,941,196
Other prepaid expenses		329,634		251,733
Other receivables		413,122		101,955
Prepaid expenses and other current assets	$	2,351,524	$	3,294,884

Property and Equipment (Tables)

12 Months Ended

Dec. 31, 2018

Property and Equipment

Summary of Property and Equipment, Net

Property and equipment, net consisted of the following at December 31:


	2018		2017
Furniture and fixtures	$	334,300	$	314,024
Laboratory equipment		1,389,036		1,325,667
Office equipment		11,085		11,085
Computer equipment		233,282		192,430
Leasehold improvements		573,165		573,165
Property and equipment		2,540,868		2,416,371
Less accumulated depreciation		(1,583,642)		(1,309,472)
Property and equipment, net	$	957,226	$	1,106,899

Accrued Expenses (Tables)

12 Months Ended

Dec. 31, 2018

Accrued Expenses

Summary of Accrued Expenses

The following is a summary of the Company’s accrued expenses at December 31:



	2018		2017
Accrued research and development expenses	$	3,483,741	$	2,702,445
Accrued consulting and other professional fees		140,397		227,811
Accrued employee benefits		385,789		331,930
Other accrued expenses		263,693		304,421
Accrued expenses	$	4,273,620	$	3,566,607

Operating Leases (Tables)

12 Months Ended

Dec. 31, 2018

Operating Leases

Schedule of Future Minimum Lease Payments

The following table presents the future minimum lease payments as of December 31, 2018 under the Lease:


YEAR	AMOUNT
2019	$	989,502
2020		1,014,239
2021		1,039,595
2022		1,065,585
2023		906,669
After 2023		—
Total	$	5,015,590

Stockholders Equity (Tables)

12 Months Ended

Dec. 31, 2018

Stock-Based Compensation Expense



	2018		2017		2016
Research and development expense	$	1,709,390	$	1,280,909	$	1,033,005
General and administrative expense		2,877,708		2,479,893		1,931,762
Total stock-based compensation expense	$	4,587,098	$	3,760,802	$	2,964,767

2003 Stock Incentive Plan

Company's Stock Option Activity



				Weighted-Average
				Remaining	Aggregate
	Outstanding	Weighted-Average		Contractual Term	Intrinsic Value
	Options	Exercise Price		(Years)	(In thousands)
Outstanding as of December 31, 2017	713,211	$	1.25	2.2
Options exercised	(46,131)		1.29
Options forfeited	—		—
Outstanding, Vested and Exercisable as of December 31, 2018	667,080		1.24	1.2	$	5,488

2013 Equity Incentive Plan

Company's Stock Option Activity


				WEIGHTED-
		WEIGHTED-		AVERAGE	AGGREGATE
		AVERAGE		REMAINING	INTRINSIC
	OUTSTANDING	EXERCISE		CONTRACTUAL	VALUE (IN
	OPTIONS	PRICE		TERM(YEARS)	THOUSANDS)
Outstanding as of December 31, 2017	2,664,163	$	7.34	7.4
Options granted	845,479		19.22
Options exercised	(144,182)		6.68
Options forfeited	(100,206)		13.89
Outstanding as of December 31, 2018	3,265,254		8.39	7.1	$	7,489
Vested or expected to vest as of December 31, 2018	3,265,254		8.39	7.1	$	7,489
Exercisable as of December 31, 2018	2,020,307		7.66	6.1	$	4,539

Weighted-Average Fair Value of Options Granted


	2018	2017	2016
Expected term	6.25 years	6.25 years	6.25 years
Expected volatility	73.75%	75.20%	68.98%
Risk-free interest rate	2.55%	2.08%	1.70%
Expected dividend yield	0%	0%	0%

Summary of RSU Activity

The following is a summary of RSU activity for the 2013 Plan for the year ended December 31, 2018:



			Weighted-Average
		Number of Shares	Grant Date
	Underlying RSUs			Fair Value
Unvested at December 31, 2017		9,667	$	4.61
Granted		—		—
Forfeited		—		—
Vested		4,834		4.61
Unvested at December 31, 2018		4,833		4.61

Income Taxes (Tables)

12 Months Ended

Dec. 31, 2018

Income Taxes

Schedule of Gross Deferred Tax Asset and Related Valuation Allowance


	2018		2017
Deferred tax assets:
Net operating loss carryforward	$	41,687,577	$	30,813,509
Capitalized start-up costs		1,501,895		1,695,688
Patent amortization		119,888		135,358
Research and orphan drug credits		28,123,082		21,293,509
Deferred rent		51,400		71,938
Deferred compensation		3,293,221		2,274,885
Other		95,564		69,906
Total gross deferred tax assets		74,872,627		56,354,793
Valuation allowance		(74,872,627)		(56,354,793)
Deferred tax assets		—		—
Deferred tax liabilities:
Total deferred tax liabilities		—		—
Net deferred tax assets	$	—	$	—

Schedule of Reconciliations Between Statutory Federal Income Tax Rate and Effective Income Tax Rate of Income Tax Expense


	2018		2017		2016
U.S. Federal statutory tax rate	21.0	%	34.0	%	34.0	%
State taxes	5.7		4.4		4.6
Research credit	0.8		1.3		1.0
Orphan drug credit	10.4		11.5		9.0
Other	0.6		0.3		—
Stock-based compensation	(0.1)		(0.7)		(0.7)
Change in valuation allowance	(38.4)		(5.1)		(47.9)
Effective change due to corporate tax rate reduction	—		(45.7)		—
Provision for income taxes	—	%	—	%	—	%

Quarterly Financial Information (Unaudited) (Tables)

12 Months Ended

Dec. 31, 2018

Quarterly Financial Information (Unaudited)

Summary of Quarterly Financial Information


	Quarter Ended
	December 31,		September 30,		June 30,		March 31,
	2018		2018		2018		2018
Revenue	$	—	$	—	$	—	$	—
Net loss	$	(13,906,915)	$	(11,575,111)	$	(11,278,763)	$	(11,512,844)
Loss per share—basic and diluted	$	(0.32)	$	(0.27)	$	(0.26)	$	(0.33)


	Quarter Ended
	December 31,		September 30,		June 30,		March 31,
	2017		2017		2017		2017
Revenue	$	—	$	—	$	—	$	—
Net loss	$	(9,257,858)	$	(7,950,101)	$	(8,141,796)	$	(7,931,313)
Loss per share—basic and diluted	$	(0.27)	$	(0.24)	$	(0.30)	$	(0.34)

Description of the Business (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Description of the Business
Accumulated deficit	$ (200,550,739)	$ (152,277,106)

Significant Accounting Policies (Detail)	12 Months Ended
Significant Accounting Policies (Detail)	Dec. 31, 2018 USD ($) segment item	Dec. 31, 2017 USD ($) item	Dec. 31, 2016
Fair Value Measurements
Number Of Operating Segments \| segment	1
Assets measured at fair value levels 2 or 3	$ 0	$ 0
Liabilities measured at fair value levels 2 or 3	0	0
Carrying value of cash held in money market fund	$ 207,900,000	$ 121,900,000
Impairment of Long-Lived Assets
Number of long-lived assets determined to be impaired \| item	0	0
Assets held for sale	$ 0	$ 0
Share-based Compensation
Expected term	6 years 3 months	6 years 3 months	6 years 3 months
Maximum
Share-based Compensation
Expected term	10 years

Summary of Significant Accounting Policies - Property and Equipment (Detail)	12 Months Ended
	Dec. 31, 2018
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Property and Equipment Estimated useful lives	P7Y
Laboratory Equipment
Property, Plant and Equipment [Line Items]
Property and Equipment Estimated useful lives	P5Y
Office Equipment
Property, Plant and Equipment [Line Items]
Property and Equipment Estimated useful lives	P5Y
Computer Equipment
Property, Plant and Equipment [Line Items]
Property and Equipment Estimated useful lives	P5Y
Minimum
Property, Plant and Equipment [Line Items]
Property and Equipment Estimated useful lives	P3Y
Maximum
Property, Plant and Equipment [Line Items]
Property and Equipment Estimated useful lives	P7Y

Significant Accounting Policies - Recently Issued Accounting Standards (Detail) - USD ($)		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2018
ASU 2014-09
New accounting pronouncement, modified retrospective method
Cumulative impact of adoption on prior financial statements		$ 0
ASU 2017-01
New accounting pronouncement, modified retrospective method
Impact of adoption on financial statements		0
ASU 2017-09
New accounting pronouncement, modified retrospective method
Impact of adoption on financial statements		$ 0
Restatement Adjustment Forecast \| ASU 2016-02
New accounting pronouncement, modified retrospective method
Transition package of practical expedients	true
Operating Lease, Right-of-Use Asset	$ 3,600,000
Operating Lease, Liability	$ 4,300,000

Net (Loss) Income Per Share of Common Stock (Detail) - USD ($)	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Net (Loss) Income Per Common Share
Net loss	$ (13,906,915)	$ (11,575,111)	$ (11,278,763)	$ (11,512,844)	$ (9,257,858)	$ (7,950,101)	$ (8,141,796)	$ (7,931,313)	$ (48,273,633)	$ (33,281,068)	$ (31,809,838)
Basic and diluted net loss per common share (in dollars per share)	$ (0.32)	$ (0.27)	$ (0.26)	$ (0.33)	$ (0.27)	$ (0.24)	$ (0.30)	$ (0.34)	$ (1.18)	$ (1.13)	$ (1.50)
Basic and diluted weighted average common shares outstanding									41,044,621	29,395,756	21,256,312
Warrants [Member]
Anti-dilutive securities excluded
Anti-dilutive securities										553,868	553,868
Stock Options and Restricted Stock Units [Member]
Anti-dilutive securities excluded
Anti-dilutive securities									3,937,167	3,399,124	2,817,674

- Definition

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

+ References + Details

Prepaid Expenses and Other Current Assets - Summary of Prepaid Expenses and Other Current Assets (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Prepaid Expenses and Other Current Assets
Prepaid research and development expenses	$ 1,608,768	$ 2,941,196
Other prepaid expenses	329,634	251,733
Other receivables	413,122	101,955
Prepaid expenses and other current assets	$ 2,351,524	$ 3,294,884

Property and Equipment (Detail) - USD ($)	12 Months Ended
Property and Equipment (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Property, Plant and Equipment [Line Items]
Property and equipment	$ 2,540,868	$ 2,416,371
Less accumulated depreciation	(1,583,642)	(1,309,472)
Property and equipment, net	957,226	1,106,899
Depreciation of property and equipment	275,123	263,541	$ 190,540
Furniture and Fixtures
Property, Plant and Equipment [Line Items]
Property and equipment	334,300	314,024
Laboratory Equipment
Property, Plant and Equipment [Line Items]
Property and equipment	1,389,036	1,325,667
Office Equipment
Property, Plant and Equipment [Line Items]
Property and equipment	11,085	11,085
Computer Equipment
Property, Plant and Equipment [Line Items]
Property and equipment	233,282	192,430
Leasehold Improvements
Property, Plant and Equipment [Line Items]
Property and equipment	$ 573,165	$ 573,165

Accrued Expenses - Summary of Accrued Expenses (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Accrued Expenses
Accrued research and development expenses	$ 3,483,741	$ 2,702,445
Accrued consulting and other professional fees	140,397	227,811
Accrued employee benefits	385,789	331,930
Other accrued expenses	263,693	304,421
Accrued expenses	$ 4,273,620	$ 3,566,607

Operating Leases (Details) - USD ($)	12 Months Ended
Operating Leases (Details) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	May 31, 2016
Operating leases, rent expense	$ 896,758	$ 890,170	$ 784,739
Security deposit paid	52,320	52,320
Operating Leases, Future Minimum Payments Due, Fiscal Year Maturity [Abstract]
2019	989,502
2020	1,014,239
2021	1,039,595
2022	1,065,585
2023	906,669
Total	5,015,590
Rockville
Deferred Rent Credit	$ 710,394	$ 785,031
Security deposit paid				$ 52,320

Stockholders' Equity - Equity Offerings (Detail) - USD ($) $ / shares in Units, $ in Millions	1 Months Ended			3 Months Ended	5 Months Ended	10 Months Ended	12 Months Ended
	Mar. 31, 2018	May 31, 2017	Jun. 30, 2016	Dec. 31, 2017	May 23, 2017	Dec. 31, 2016	Dec. 31, 2018
September 2017 At-The-Market
Equity offerings
Issuance of common stock, net of issuance costs, shares				1,600,000
Weighted average price per share issued				$ 12.50
Aggregate net proceeds from stock sale				$ 19.3
Common stock available for sale under agreement							$ 80.0
Securities sold during period							0
March 2016 At-The-Market
Equity offerings
Issuance of common stock, net of issuance costs, shares					1,388,647	668,791
Weighted average price per share issued					$ 5.55	$ 6.336
Aggregate net proceeds from stock sale					$ 7.4	$ 3.9
Public Equity Offering
Equity offerings
Issuance of common stock, net of issuance costs, shares	8,050,000	8,050,000	3,476,793
Weighted average price per share issued	$ 17.00	$ 11.50	$ 6.10
Aggregate net proceeds from stock sale	$ 128.4	$ 86.8	$ 19.7

Stockholders' Equity - Common Stock Warrants (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Class of Warrant or Right [Line Items]
Number of shares underlying outstanding warrants		553,868	553,868
Exercise price per share		$ 0.33	$ 0.33
Number of warrants exercised	546,709	0	23,275
Warrants Exercised Weighted Average Exercise Price		$ 0.33
Proceeds from cash exercise of warrants	$ 2,336
Number of warrants expired	0		1,544
Common Stock
Class of Warrant or Right [Line Items]
Shares issued upon net exercise of outstanding warrants	536,564
Shares issued upon cash exercise of warrants	7,159

Stockholders' Equity - Incentive Plans (Detail) - USD ($)		12 Months Ended
	Jan. 01, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
2003 Stock Incentive Plan
Incentive plans
Period from date of termination that any vested options shall expire		60 days
Expiration period		10 years
Intrinsic value of options exercised		$ 716,920	$ 103,638	$ 97,707
Cash proceeds from exercise of stock options		$ 59,659	$ 27,357	$ 44,771
Options exercised		46,131	16,608	28,368
Fair value of shares vested			$ 1,573	$ 16,024
2003 Stock Incentive Plan \| Upon first anniversary of start date
Incentive plans
Percent of shares subject to option grant that will vest		25.00%
2003 Stock Incentive Plan \| Each month after the first anniversary
Incentive plans
Percent of shares subject to option grant that will vest		2.083%
2013 Equity Incentive Plan
Incentive plans
Period from date of termination that any vested options shall expire		90 days
Expiration period		10 years
Unrecognized compensation expense related to unvested options		$ 9,129,271
Period for unrecognized compensation expense related to unvested options yet has not been recognized		2 years 8 months 12 days
Intrinsic value of options exercised		$ 1,344,026	385,701	2,275
Cash proceeds from exercise of stock options		$ 962,530	$ 346,019	$ 27,825
Options exercised		144,182	54,521	3,500
Fair value of shares vested		$ 3,003,632	$ 3,506,568	$ 3,053,086
2013 Equity Incentive Plan \| Upon first anniversary of start date
Incentive plans
Percent of shares subject to option grant that will vest		25.00%
2013 Equity Incentive Plan \| Each month after the first anniversary
Incentive plans
Percent of shares subject to option grant that will vest		2.083%
Common Stock
Incentive plans
Options exercised			71,129
Common Stock \| 2013 Equity Incentive Plan
Authorized shares
Common stock authorized under 2013 plan	5,162,816
Automatic increase in number of shares reserved for issuance as a percentage of the total common stock outstanding at the end of the prior year	3.00%	3.00%
Maximum number of shares that may be issued pursuant to exercise of incentive stock		20,000,000
Increase in number of shares of common stock	1,294,822
Common Stock \| 2013 Equity Incentive Plan \| Maximum
Authorized shares
Common stock authorized under 2013 plan		1,000,000

Stockholders' Equity - Company's Stock Option Activity (Detail) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
2003 Stock Incentive Plan
OUTSTANDING OPTIONS
Outstanding at beginning of period	713,211
Options granted	0	0	0
Options exercised	(46,131)	(16,608)	(28,368)
Outstanding at end of period	667,080	713,211
WEIGHTED-AVERAGE EXERCISE PRICE
Outstanding at beginning of period	$ 1.25
Options exercised	1.29
Outstanding at end of period	$ 1.24	$ 1.25
VESTED AND EXPECTED TO VEST
Outstanding options	667,080
Aggregate Intrinsic Value (in thousands)	$ 5,488
ADDITIONAL DISCLOSURES
Weighted-average exercise price exercisable at end of period	$ 1.24
Weighted-average contractual term (years) exercisable at end of period	1 year 2 months 12 days
Aggregate intrinsic value (in thousands) exercisable at end of period	$ 5,488
Weighted-average contractual term outstanding	1 year 2 months 12 days	2 years 2 months 12 days
Aggregate intrinsic value (in thousands) outstanding at end of period	$ 5,488
2013 Equity Incentive Plan
OUTSTANDING OPTIONS
Outstanding at beginning of period	2,664,163
Options granted	845,479
Options exercised	(144,182)	(54,521)	(3,500)
Options forfeited	(100,206)
Outstanding at end of period	3,265,254	2,664,163
WEIGHTED-AVERAGE EXERCISE PRICE
Outstanding at beginning of period	$ 7.34
Options granted	19.22
Options exercised	6.68
Options forfeited	13.89
Outstanding at end of period	$ 8.39	$ 7.34
VESTED AND EXPECTED TO VEST
Outstanding options	3,265,254
Weighted-Average Exercise Price	$ 8.39
Weighted-Average Remaining Contractual Term	7 years 1 month 6 days
Aggregate Intrinsic Value (in thousands)	$ 7,489
ADDITIONAL DISCLOSURES
Outstanding options exercisable at end of period	2,020,307
Weighted-average exercise price exercisable at end of period	$ 7.66
Weighted-average contractual term (years) exercisable at end of period	6 years 1 month 6 days
Aggregate intrinsic value (in thousands) exercisable at end of period	$ 4,539
Weighted-average contractual term outstanding	7 years 1 month 6 days	7 years 4 months 24 days
Aggregate intrinsic value (in thousands) outstanding at end of period	$ 7,489

Stockholders' Equity - Weighted-Average assumptions (Detail) - $ / shares	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Weighted-average assumptions
Expected term	6 years 3 months	6 years 3 months	6 years 3 months
Expected volatility	73.75%	75.20%	68.98%
Risk-free interest rate	2.55%	2.08%	1.70%
Expected dividend yield	0.00%	0.00%	0.00%
2013 Equity Incentive Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Weighted-average fair value of the options granted	$ 12.90	$ 4.76	$ 3.39

Stockholders' Equity - Summary of RSU Activity (Detail) - 2013 Equity Incentive Plan	12 Months Ended
	Dec. 31, 2018 USD ($) item $ / shares shares
NUMBER OF SHARES
Unvested at beginning of period \| shares	9,667
Vested \| shares	4,834
Unvested at end of period \| shares	4,833
WEIGHTED-AVERAGE GRANT DATE FAIR VALUE
Unvested at beginning of period \| $ / shares	$ 4.61
Vested \| $ / shares	4.61
Unvested at end of period \| $ / shares	$ 4.61
RSU [Member]
Restricted Stock Units information
Unrecognized compensation expense related to unvested service RSUs \| $	$ 1,099
Number of vesting installments \| item	3

Stockholders' Equity - Stock-Based Compensation Expense (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Stock-based compensation expense
Total stock-based compensation expense	$ 4,587,098	$ 3,760,802	$ 2,964,767
Research and Development
Stock-based compensation expense
Total stock-based compensation expense	1,709,390	1,280,909	1,033,005
General and Administrative Expense.
Stock-based compensation expense
Total stock-based compensation expense	$ 2,877,708	$ 2,479,893	$ 1,931,762

Income Taxes - Schedule of Gross Deferred Tax Asset and Related Valuation Allowance (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Deferred tax assets:
Net operating loss carryforward	$ 41,687,577	$ 30,813,509
Capitalized start-up costs	1,501,895	1,695,688
Patent amortization	119,888	135,358
Research and orphan drug credits	28,123,082	21,293,509
Deferred rent	51,400	71,938
Deferred compensation	3,293,221	2,274,885
Other	95,564	69,906
Total gross deferred tax assets	74,872,627	56,354,793
Valuation allowance	(74,872,627)	(56,354,793)
Deferred tax assets
Deferred tax liabilities:
Total deferred tax liabilities
Net deferred tax assets

Income Taxes - Tax Cuts and Jobs Act (Details) - USD ($) $ in Millions			12 Months Ended
	Jan. 01, 2018	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Effect of Tax Cuts and Jobs Act of 2017 Accounting Incomplete Provisional [Abstract]
U.S. Federal statutory tax rate	21.00%	35.00%	21.00%	34.00%	34.00%
Tax benefit related to remeasurement			$ (15.2)
Valuation allowance offset to remeasurement tax benefit			$ 15.2

Income Taxes - NOLs and carryforwards (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017
Unrecognized tax benefits	$ 0	$ 0
Research tax credit carryforward
Tax credits available to carry forward	8,500,000
Orphan drug credit carryforward
Tax credits available to carry forward	19,600,000
U.S. Federal And State
Net operating losses	$ 151,500,000

Income Taxes - Reconciliation Between Statutory Federal Income Tax Rate and Effective Income Tax Rate (Detail)			12 Months Ended
	Jan. 01, 2018	Dec. 22, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Reconciliations between the statutory federal income tax rate and the effective income tax rate
U.S. Federal statutory tax rate	21.00%	35.00%	21.00%	34.00%	34.00%
State taxes			5.70%	4.40%	4.60%
Research credit			0.80%	1.30%	1.00%
Orphan drug credit			10.40%	11.50%	9.00%
Other			0.60%	0.30%
Stock-based compensation			(0.10%)	(0.70%)	(0.70%)
Change in valuation allowance			(38.40%)	(5.10%)	(47.90%)
Effective change due to corporate tax rate reduction				(45.70%)

Research and License Agreements - Additional Information (Detail)	1 Months Ended		12 Months Ended
	Jun. 30, 2015 USD ($)	May 31, 2014 USD ($)	Dec. 31, 2018 USD ($) item	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)
Research and License Agreements
Revenue	$ 20,000,000	$ 15,000,000			$ 18,500
License Agreement \| Pfizer
Research and License Agreements
Up-front payment received			$ 22,500,000
Transaction price			22,500,000
Non-cash consideration received			$ 0
Research Services Agreement \| University Of Basel
Research and License Agreements
Royalty payment due against license fees received (as a percent)			10.00%
Consideration paid			$ 0	$ 0	$ 0
Achievement Of Specified Development Milestone \| License Agreement \| Pfizer \| Maximum
Research and License Agreements
Potential milestone payments			115,000,000
Achievement Of Specified Regulatory Milestone \| License Agreement \| Pfizer \| Maximum
Research and License Agreements
Potential milestone payments			70,000,000
Achievement Of Specified Levels Of Annual Net Sales Of Licensed Products \| License Agreement \| Pfizer \| Maximum
Research and License Agreements
Potential milestone payments			$ 135,000,000
Achievement Of First Commercial Sale Of Licensed Product \| License Agreement \| Pfizer
Research and License Agreements
Number of indications \| item			2
Achievement Of Dosing First Patient In Clinical Trial \| License Agreement \| Pfizer
Research and License Agreements
Non-refundable milestone payment received	$ 20,000,000	$ 15,000,000
Achievement Of Dosing First Patient In Clinical Trial \| License Agreement \| Pfizer \| Maximum
Research and License Agreements
Number of indications \| item			2

Employee Benefit Plan (Detail) - USD ($)	12 Months Ended
Employee Benefit Plan (Detail) - USD ($)	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Defined Contribution Plan Disclosure [Line Items]
Company matching contribution percentage	50.00%	50.00%	50.00%
Employer match contribution	$ 94,000	$ 88,000	$ 79,000
Maximum
Defined Contribution Plan Disclosure [Line Items]
Percent of employee contributions the company contributes a matching contribution	3.00%	3.00%	3.00%

Quarterly Financial Information (Unaudited) (Detail) - USD ($)	1 Months Ended		3 Months Ended								12 Months Ended
	Jun. 30, 2015	May 31, 2014	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information (Unaudited)
Revenue	$ 20,000,000	$ 15,000,000											$ 18,500
Net (loss) income			$ (13,906,915)	$ (11,575,111)	$ (11,278,763)	$ (11,512,844)	$ (9,257,858)	$ (7,950,101)	$ (8,141,796)	$ (7,931,313)	$ (48,273,633)	$ (33,281,068)	$ (31,809,838)
Loss per share - basic and diluted			$ (0.32)	$ (0.27)	$ (0.26)	$ (0.33)	$ (0.27)	$ (0.24)	$ (0.30)	$ (0.34)	$ (1.18)	$ (1.13)	$ (1.50)

- Definition

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

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