Document and Entity Information - shares	9 Months Ended
Document and Entity Information - shares	Sep. 30, 2019	Nov. 01, 2019
Document and Entity Information [Abstract]
Document Type	10-Q
Document Period End Date	Sep. 30, 2019
Entity Registrant Name	Y-mAbs Therapeutics, Inc.
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Non-accelerated Filer
Entity Small Business	true
Entity Emerging Growth Company	true
Entity Ex Transition Period	true
Entity Shell Company	false
Entity Common Stock, Shares Outstanding		39,728,416
Entity Central Index Key	0001722964
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
Document Fiscal Period Focus	Q3
Amendment Flag	false

Consolidated Balance Sheets - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
CURRENT ASSETS
Cash and cash equivalents	$ 98,192	$ 147,840
Restricted cash		31
Other current assets	1,399	3,661
Total current assets	99,591	151,532
Property and equipment, net	1,689	205
Operating lease right-of-use assets	2,086
Other assets	318	187
TOTAL ASSETS	103,684	151,924
LIABILITIES
Accounts payable	7,628	5,872
Accrued liabilities	5,848	3,251
Operating lease liabilities, current portion	516
Total current liabilities	13,992	9,123
Accrued milestone and royalty payments	1,932	2,050
Operating lease liabilities, long-term portion	1,821
Other liabilities		224
TOTAL LIABILITIES	17,745	11,397
Commitments and contingencies (Note 6)
STOCKHOLDERS? EQUITY
Preferred stock, $0.0001 par value, 5,500,000 shares authorized at September 30, 2019 and December 31, 2018; none issued at September 30, 2019 and December 31, 2018
Common stock, $0.0001 par value, 100,000,000 shares authorized at September 30, 2019 and December 31, 2018; 34,593,666 and 34,193,666 shares issued at September 30, 2019, and December 31, 2018, respectively	3	3
Additional paid in capital	228,532	225,352
Accumulated other comprehensive income	131	7
Accumulated deficit	(142,727)	(84,835)
TOTAL STOCKHOLDERS' EQUITY	85,939	140,527
TOTAL LIABILITIES AND STOCKHOLDERS' EQUITY	$ 103,684	$ 151,924

Consolidated Balance Sheets (parenthetical) - $ / shares	Sep. 30, 2019	Dec. 31, 2018
Consolidated Balance Sheets
Preferred stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Preferred stock, authorized shares	5,500,000	5,500,000
Preferred stock, shares issued	0	0
Common stock, par value (in dollars per share)	$ 0.0001	$ 0.0001
Common stock, authorized shares	100,000,000	100,000,000
Common stock, shares issued	34,593,666	34,193,666

Consolidated Statements of Net Loss and Comprehensive Loss - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
OPERATING EXPENSES
Research and development	$ 19,660	$ 8,731	$ 46,665	$ 23,228
General and administrative	4,699	2,684	12,581	5,924
Total operating expenses	24,359	11,415	59,246	29,152
Loss from operations	(24,359)	(11,415)	(59,246)	(29,152)
OTHER INCOME/(EXPENSES)
Interest and other income/(expenses)	437	(11)	1,354	(62)
NET LOSS	(23,922)	(11,426)	(57,892)	(29,214)
Other comprehensive income
Foreign currency translation	134	39	124	121
COMPREHENSIVE LOSS	$ (23,788)	$ (11,387)	$ (57,768)	$ (29,093)
Net loss per share attributable to common stockholders, basic and diluted	$ (0.70)	$ (0.42)	$ (1.69)	$ (1.08)
Weighted average common shares outstanding, basic and diluted	34,371,927	27,330,579	34,253,739	26,945,432

Consolidated Statements of Changes in Stockholders' Equity - USD ($) $ in Thousands	Common Stock	Additional Paid-in Capital	Accumulated Other Comprehensive Income/(Loss)	Accumulated Deficit	Total
Balance at the beginning of period at Dec. 31, 2017	$ 3	$ 123,879	$ (169)	$ (41,561)	$ 82,152
Balance at the beginning of period (in shares) at Dec. 31, 2017	26,749,666
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		172			172
Foreign currency translation			3		3
Net loss				(7,483)	(7,483)
Balance at the end of period at Mar. 31, 2018	$ 3	124,051	(166)	(49,044)	74,844
Balance at the end of period (in shares) at Mar. 31, 2018	26,749,666
Balance at the beginning of period at Dec. 31, 2017	$ 3	123,879	(169)	(41,561)	82,152
Balance at the beginning of period (in shares) at Dec. 31, 2017	26,749,666
Increase (Decrease) in Stockholders' Equity
Net loss					(29,214)
Balance at the end of period at Sep. 30, 2018	$ 3	225,848	(48)	(70,775)	155,028
Balance at the end of period (in shares) at Sep. 30, 2018	34,193,666
Balance at the beginning of period at Dec. 31, 2017	$ 3	123,879	(169)	(41,561)	82,152
Balance at the beginning of period (in shares) at Dec. 31, 2017	26,749,666
Balance at the end of period at Dec. 31, 2018	$ 3	225,352	7	(84,835)	140,527
Balance at the end of period (in shares) at Dec. 31, 2018	34,193,666
Balance at the beginning of period at Mar. 31, 2018	$ 3	124,051	(166)	(49,044)	74,844
Balance at the beginning of period (in shares) at Mar. 31, 2018	26,749,666
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		904			904
Foreign currency translation			78		78
Net loss				(10,305)	(10,305)
Balance at the end of period at Jun. 30, 2018	$ 3	124,955	(88)	(59,349)	65,521
Balance at the end of period (in shares) at Jun. 30, 2018	26,749,666
Increase (Decrease) in Stockholders' Equity
Issuance of common stock in initial public offering, net of issuance costs		100,498			100,498
Issuance of common stock (in shares)	6,900,000
Issuance of common stock to nonemployees (in shares)	544,000
Stock-based compensation expense		395			395
Foreign currency translation			40		40
Net loss				(11,426)	(11,426)
Balance at the end of period at Sep. 30, 2018	$ 3	225,848	(48)	(70,775)	155,028
Balance at the end of period (in shares) at Sep. 30, 2018	34,193,666
Balance at the beginning of period at Dec. 31, 2018	$ 3	225,352	7	(84,835)	140,527
Balance at the beginning of period (in shares) at Dec. 31, 2018	34,193,666
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		864			864
Foreign currency translation			56		56
Net loss				(15,934)	(15,934)
Balance at the end of period at Mar. 31, 2019	$ 3	226,216	63	(100,769)	125,513
Balance at the end of period (in shares) at Mar. 31, 2019	34,193,666
Balance at the beginning of period at Dec. 31, 2018	$ 3	225,352	7	(84,835)	140,527
Balance at the beginning of period (in shares) at Dec. 31, 2018	34,193,666
Increase (Decrease) in Stockholders' Equity
Net loss					(57,892)
Balance at the end of period at Sep. 30, 2019	$ 3	228,532	131	(142,727)	85,939
Balance at the end of period (in shares) at Sep. 30, 2019	34,593,666
Balance at the beginning of period at Mar. 31, 2019	$ 3	226,216	63	(100,769)	125,513
Balance at the beginning of period (in shares) at Mar. 31, 2019	34,193,666
Increase (Decrease) in Stockholders' Equity
Stock-based compensation expense		971			971
Foreign currency translation			(66)		(66)
Net loss				(18,036)	(18,036)
Balance at the end of period at Jun. 30, 2019	$ 3	227,187	(3)	(118,805)	108,382
Balance at the end of period (in shares) at Jun. 30, 2019	34,193,666
Increase (Decrease) in Stockholders' Equity
Issuance of common stock to nonemployees (in shares)	400,000
Stock-based compensation expense		1,345			1,345
Foreign currency translation			134		134
Net loss				(23,922)	(23,922)
Balance at the end of period at Sep. 30, 2019	$ 3	$ 228,532	$ 131	$ (142,727)	$ 85,939
Balance at the end of period (in shares) at Sep. 30, 2019	34,593,666

- Definition

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

+ References + Details

Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (57,892)	$ (29,214)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	87	15
Stock-based compensation	3,180	1,471
Foreign currency transactions	123	101
Changes in assets and liabilities:
Other current assets	2,262	(1,881)
Other assets	(131)	(188)
Accounts payable	1,031	1,700
Accrued liabilities and other	2,422	344
NET CASH USED IN OPERATING ACTIVITIES	(48,918)	(27,652)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchase of property and equipment	(818)	(177)
NET CASH USED IN INVESTING ACTIVITIES	(818)	(177)
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock, net of issuance costs		101,607
Payment of offering costs for private placement		(1,002)
NET CASH PROVIDED BY FINANCING ACTIVITIES		100,605
Effect of exchange rates on cash, cash equivalents and restricted cash	57	32
NET INCREASE/(DECREASE) IN CASH, CASH EQUIVALENTS AND RESTRICTED CASH	(49,679)	72,808
Cash, cash equivalents and restricted cash at the beginning of period	147,871	90,515
Cash, cash equivalents and restricted cash at the end of period	98,192	163,323
SUPPLEMENTAL DISCLOSURE OF NON-CASH ACTIVITIES
Property and equipment purchases in accounts payable	752
Right-of-use assets obtained in exchange for lease obligations	$ 901
Common stock issuance cost in accounts payable		517
Deferred offering costs included in other assets and accounts payable and accrued liabilities and other		$ 851

ORGANIZATION AND DESCRIPTION OF BUSINESS	9 Months Ended
ORGANIZATION AND DESCRIPTION OF BUSINESS	Sep. 30, 2019
ORGANIZATION AND DESCRIPTION OF BUSINESS
ORGANIZATION AND DESCRIPTION OF BUSINESS	NOTE 1—ORGANIZATION AND DESCRIPTION OF BUSINESS Y‑mAbs Therapeutics, Inc. (“we,” “us,” “our,” the “Company,” or “Y‑mAbs”) is a late-stage clinical biopharmaceutical company focused on the development and commercialization of novel antibody-based therapeutic products for the treatment of cancer. We have entered into a worldwide license and research collaboration agreement (the “MSK License Agreement”) with Memorial Sloan‑Kettering Cancer Center (“MSK”), under which we have obtained the exclusive rights to MSK’s rights to two clinical stage antibody‑based product development programs for the treatment of neuroblastoma and other oncology indications. The MSK License Agreement also includes a protein multimerization platform technology, and an option to obtain the rights to certain chimeric antigen receptor T‑cell, or CAR‑T, technologies, as well as rights to certain next‑generation humanized, affinity matured bispecific antibodies. The Company is headquartered in New York, New York and was incorporated on April 30, 2015 under the laws of the State of Delaware.

BASIS OF PRESENTATION	9 Months Ended
BASIS OF PRESENTATION	Sep. 30, 2019
BASIS OF PRESENTATION
BASIS OF PRESENTATION	NOTE 2—BASIS OF PRESENTATION The Company has not generated any revenue and has incurred losses since inception. Operations of the Company are subject to certain risks and uncertainties, including, among others, uncertainty of drug candidate development; technological uncertainty; uncertainty regarding patents and proprietary rights; uncertainty in obtaining FDA approval in the United States and regulatory approval in other jurisdictions; marketing or sales capability or experience; uncertainty in getting adequate payer coverage and reimbursement; dependence on key personnel; compliance with government regulations and the need to obtain additional financing. The Company’s drug candidates currently under development will require significant additional research and development efforts, including extensive preclinical and clinical testing and regulatory approval, prior to commercialization. These efforts require significant amounts of additional capital, adequate personnel infrastructure and extensive compliance‑reporting capabilities. The Company’s drug candidates are in the development stage. There can be no assurance that the Company’s research and development will be successfully completed, that adequate protection for the Company’s intellectual property will be obtained, that any products developed will obtain necessary government regulatory approval or that any approved products will be commercially viable. Even if the Company’s product development efforts are successful, it is uncertain when, if ever, the Company will generate significant revenue from product sales. The Company operates in an environment of rapid change in technology and substantial competition from pharmaceutical and biotechnology companies. The Company’s financial statements have been prepared on the basis of continuity of operations, realization of assets and the satisfaction of liabilities in the ordinary course of business. The Company has experienced negative cash flows and had an accumulated deficit of $142.7 million as of September 30, 2019 and $84.8 million as of December 31, 2018. Through September 30, 2019, the Company has funded its operations through proceeds from sales of shares of its common stock, including its initial public offering (“IPO”), in September 2018. As of September 30, 2019, the Company had cash and cash equivalents of $98.2 million, and as of December 31, 2018 the Company had cash and cash equivalents of $147.8 million. As of the issuance date of the quarterly financial statements for the three and nine months ended September 30, 2019, the Company expects that its cash and cash equivalents at September 30, 2019 will be sufficient to fund its operating expenses and capital expenditure requirements through at least the next twelve months. In November 2019, the Company completed a follow-on shelf public offering and raised $134.7 million, net of issuance costs. The future viability of the Company, until such time that the Company has commercialized any of its products, is dependent on its ability to raise additional capital to finance its operations. The Company may be required to raise additional capital to fund future operations through the sale of its equity securities, incurring debt, entering into licensing or collaboration agreements with partners, grants or other sources of financing. Sufficient funds may not be available to the Company at all or on attractive terms when needed from equity or debt financing. If the Company is unable to obtain additional financing from these or other sources when needed, or to the extent needed, it may be necessary to significantly reduce its current rate of spending through delaying, scaling back, or suspending certain research and development programs and other operational programs. The accompanying unaudited consolidated financial statements reflect the accounts of the Company and its wholly‑owned subsidiary and have been prepared in accordance with accounting principles generally accepted in the United States of America (“GAAP”) for interim financial information, Accounting Standards Codification (“ASC”) Topic 270-10 and with the instructions to Form 10-Q. Accordingly, these financial statements do not include all of the information and notes required by GAAP for complete financial statements. The unaudited interim financial statements include all adjustments (consisting only of normal recurring nature) necessary in the judgment of management for a fair statement of the results for the periods presented. All intercompany balances and transactions have been eliminated. The Company has evaluated subsequent events through the date of this filing. Operating results for the three and nine-month periods ended September 30, 2019 are not necessarily indicative of the results that may be expected for the year ended December 31, 2019, any other interim periods, or any future year or period. The December 31, 2018 consolidated balance sheet data was derived from audited financial statements, but does not include all disclosures required by GAAP. You should read these unaudited interim condensed consolidated financial statements in conjunction with the consolidated financial statements and notes included in the Company’s Annual Report on Form 10-K for the year ended December 31, 2018.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	9 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	Sep. 30, 2019
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES	NOTE 3—SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES Our critical accounting policies are detailed in our Annual Report on Form 10-K for the year ended December 31, 2018. Effective January 1, 2019, the Company adopted Accounting Standards Update No. 2018-09 (“ASU 2018-09”), Codification Improvements, which clarify, correct errors in, or make minor improvements to a variety of ASC topics; ASU No. 2018‑07, Compensation—Stock Compensation (Topic 718), Improvements to Nonemployee Share‑Based Payment Accounting; Accounting Standards Update No. 2018‑02, (“ASU 2018-02”), Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income; and Accounting Standards Update No. 2016‑02 (“ASU 2016‑02”), Leases. Other than the adoption of the new accounting guidance, our critical accounting policies have not changed materially from December 31, 2018. Operating Leases As described below, the Company adopted Topic 842 as of January 1, 2019. The Company determines if an arrangement includes a lease at inception. Operating lease right-of-use assets represent the Company’s right to use an underlying asset for the lease term and operating lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. In determining the net present value of lease payments, the Company uses its estimated incremental borrowing rate based on information available at the lease commencement date. Because most of the Company’s leases do not provide an implicit rate of return, an incremental borrowing rate is used based on the information available at the commencement date in determining the present value of lease payments on an individual lease basis. The Company’s incremental borrowing rate for a lease is the estimated rate of interest it would have to pay on a collateralized basis to borrow an amount equal to the lease payments under similar terms. The Company’s leases may include options to extend or terminate the lease which are included in the lease term when it is reasonably certain that it will exercise any such options. None of the Company’s leases contain any residual value guarantees. Lease expense is recognized on a straight-line basis over the expected lease term. Related variable lease costs incurred are not material to the Company. Topic 842 also provides practical expedients and certain exemptions for an entity’s ongoing accounting post implementation. The Company currently elected the short-term lease recognition exemption for all leases that qualify. This means, for those leases that qualify, we will not recognize right-of-use assets or liabilities, and this includes not recognizing right-of-use assets or liabilities for existing short-term leases of those assets in transition. We also elected the practical expedient to not separate lease and non-lease components for all of our leases. The Company has made an accounting policy election to account for each separate lease component of a contract and its associated non-lease components as a single lease component. See the Lease Agreements section in Note 6 for the related disclosures. 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, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, the accrual for research and development expenses, the accrual of milestone and royalty payments, and the valuation of stock options. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates. Segment Information The Company is engaged solely in the discovery and development of novel antibody-based therapeutic products for the treatment of cancer. Accordingly, the Company has determined that it operates in one operating segment. Recently Issued Accounting Pronouncements - Adopted In July 2018, the FASB issued Accounting Standards Update No. 2018-09 (“ASU 2018-09”), Codification Improvements, which clarify, correct errors in, or make minor improvements to a variety of ASC topics. The changes in ASU 2018-09 are not expected to have a significant effect on current accounting practices. Some of the amendments in this update do not require transition guidance and will be effective upon this update. However, many of the updates do have transition guidance with effective dates for periods beginning after December 15, 2018. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU No. 2018‑07, Compensation—Stock Compensation (Topic 718), Improvements to Nonemployee Share‑Based Payment Accounting (“ASU 2018‑07”). ASU 2018‑07 is intended to simplify aspects of share‑based compensation issued to non‑employees by making the guidance consistent with the accounting for employee share‑based compensation. ASU 2018‑07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In February 2018, the FASB issued Accounting Standards Update No. 2018‑02, (“ASU 2018-02”), Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. ASU 2018‑02 allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. ASU 2018‑07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In February 2016, the FASB issued Accounting Standards Update No. 2016‑02 (“ASU 2016‑02”), Leases, which is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018 with early adoption permitted. Under ASU 2016‑02, lessees will be required to recognize for all leases, at the commencement date of the lease, a lease liability, which is a lessee’s obligation to make lease payments arising from a lease measured on a discounted basis, and a right‑to‑use asset, which is an asset that represents the lessee’s right to use or control the use of a specified asset for the lease term. Topic 842 was subsequently amended by ASU 2017-13, Revenue and Leases: Amendments to SEC Paragraphs Pursuant to the Staff Announcement at the July 20, 2017 EITF Meeting and Rescission of Prior SEC Staff Announcements and Observer Comments; ASU 2018-01, Land Easement Practical Expedient for Transition to Topic 842; ASU No. 2018-10, Codification Improvements to Topic 842, Leases; ASU No. 2018-11, Targeted Improvements and ASU No. 2018-20, Narrow Scope Improvements for Lessors. The Company adopted the new leasing standards using the modified retrospective transition approach as of January 1, 2019, with no restatement of prior periods or cumulative adjustment to retained earnings. An entity may choose to use either (1) its effective date or (2) the beginning of the earliest comparative period presented in the financial statements as its date of initial application. The Company used the effective date as our date of initial application. Consequently, financial information was not updated and the disclosures required under the new standard are not provided for dates and periods before January 1, 2019. The new standard also provides a number of optional practical expedients in transition. The Company elected the package of practical expedients, which permits us not to reassess under the new standard our prior conclusions about lease identification, lease classification and initial direct costs. Upon adoption of the new leasing standards, the Company recognized a lease liability of $1.8 million and a related right-of-use asset of $1.5 million with the difference being due to the elimination of previously reported deferred rent. The Company subsequently entered into two new lease agreements during the three months ended March 31, 2019, and recognized an incremental lease liability and related right-of-use asset of $0.9 million. Recently Issued Accounting Pronouncements – Not Yet Adopted In August 2018, the FASB issued Accounting Standards Update No. 2018-15 (“ASU 2018-15”), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. ASU 2018-15 clarifies certain aspects of ASU 2015-05, Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement, which was issued in April 2015. Specifically, ASU 2018-14 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal use software (and hosting arrangements that include an internal-use software license). ASU 2018-15 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years with early adoption permitted. The Company is currently evaluating the impact its adoption may have on its consolidated financial statements.

NET LOSS PER SHARE

9 Months Ended

Sep. 30, 2019

NET LOSS PER SHARE

NET LOSS PER SHARE

NOTE 4—NET LOSS PER SHARE

Basic net loss per share (“EPS”) is calculated by dividing net income or loss attributable to common stockholders by the weighted average common stock outstanding. Diluted EPS is calculated by adjusting weighted average common shares outstanding for the dilutive effect of common stock options and restricted stock units. In periods in which a net loss is recorded, no effect is given to potentially dilutive securities, since the effect would be antidilutive. Securities that could potentially dilute basic EPS in the future were not included in the computation of diluted EPS because to do so would have been antidilutive. The calculations of basic and diluted net loss per share are as follows:


	Three months ended September 30,				Nine months ended September 30,
	2019		2018		2019		2018
	(in thousands, except per share amounts)

Net loss (numerator)	$	(23,922)	$	(11,426)	$	(57,892)	$	(29,214)
Weighted-average shares (denominator)		34,372		27,331		34,254		26,945
Basic and diluted net loss per share	$	(0.70)	$	(0.42)	$	(1.69)	$	(1.08)

Potentially dilutive securities excluded from the computation of diluted earnings per share relate to stock options outstanding and unvested restricted shares and RSUs and totaled 4,124,169 shares as of September 30, 2019 and 2,799,373 shares as of September 30, 2018.

ACCRUED LIABILITIES

9 Months Ended

Sep. 30, 2019

ACCRUED LIABILITIES

ACCRUED LIABILITIES

NOTE 5—ACCRUED LIABILITIES

Accrued short‑term liabilities at September 30, 2019 and December 31, 2018 are as follows:


	September 30,		December 31,
	2019		2018
	(in thousands)

Accrued milestone and royalty payments	$	268	$	1,475
Accrued clinical costs		1,079		63
Accrued compensation and board fees		1,765		1,144
Accrued rent		—		44
Accrued manufacturing costs		2,734		—
Other		2		525
Total	$	5,848	$	3,251

LICENSE AGREEMENTS AND COMMITMENTS

9 Months Ended

Sep. 30, 2019

LICENSE AGREEMENTS AND COMMITMENTS

LICENSE AGREEMENTS AND COMMITMENTS

NOTE 6—LICENSE AGREEMENTS AND COMMITMENTS

The Company has entered into two license agreements and certain other agreements with MSK. The license agreements are further described below as the MSK License Agreement and the CD33 License Agreement. These license agreements with MSK grant the Company certain patent rights and intellectual property rights. In consideration of obtaining the patent rights and intellectual property rights, the Company agreed to make certain payments and issue shares of the Company’s common stock to MSK. Certain of the payments are contingent milestone and royalty payments, the terms of which are further described below. Amounts disclosed in the consolidated balance sheets for accrued milestone and royalty payments are inclusive of obligations under the MSK License Agreement and CD33 License Agreement, collectively.

MSK License Agreement

On August 20, 2015, we entered into the MSK License Agreement that grants us a worldwide, sublicensable license to MSK’s rights to certain patent rights and intellectual property rights related to certain know‑how to develop, make, and commercialize licensed products and to perform services for all therapeutic and diagnostic uses in the field of cancer diagnostics and cancer treatments.

The patents and patent applications covered by this agreement are directed, in part, to naxitamab, an anti GD2 antibody, and omburtamab, which is an anti B7‑H3 antibody, as well as affinity matured versions of these certain antibodies and certain single chain variable fragments (Fv) constructs, and their use for immunotherapy, targeting the treatment of neuroblastoma, diffuse intrinsic pontine glioma (DIPG), osteosarcoma and other oncology indications. Upon entering into the MSK License Agreement in 2015 and in exchange for the licenses, we paid MSK an upfront payment, issued 1,428,500 shares of our common stock to MSK and agreed to provide certain anti‑dilution rights to MSK. In addition, we are required to pay to MSK certain royalty and milestone payments. We expensed the upfront payment and the issuance of shares to MSK in 2015. We also recorded expenses related to common stock issued related to certain anti‑dilution rights held by MSK.

The MSK License Agreement requires us to pay to MSK mid-to-high single digit royalties based on annual net sales of licensed products or the performance of licensed services by us and our affiliates and sublicensees. We are obligated to pay annual minimum royalties of $80,000 over the royalty term, commencing on the fifth anniversary of the license agreement. These amounts are non‑refundable but are creditable against royalty payments otherwise due under the MSK License Agreement. Total expensed minimum royalty payments in 2016 under the MSK License Agreement were $1,200,000, upon determination that the payment of such minimum royalties was probable and the amount was estimable in 2016. As of September 30, 2019, $18,000 of the accrued minimum royalties were recorded as short-term accrued liabilities, and $1,182,000 were recorded as long‑term accrued liabilities. As of December 31, 2018, all accrued minimum royalties were recorded as long-term accrued liabilities. We are also obligated to pay MSK certain clinical, regulatory and sales‑based milestone payments under the MSK License Agreement. Certain of the clinical and regulatory milestone payments become due at the earlier of completion of the related milestone activity or the date indicated in the MSK License Agreement. Total clinical and regulatory milestones potentially due under the MSK License Agreement are $2,450,000 and $9,000,000, respectively. There are also sales‑based milestones that become due should the Company achieve certain amounts of sales of licensed products resulting from the license arrangement with MSK, with total sales‑based milestones potentially due of $20,000,000. The Company records milestones in the period in which the contingent liability is probable and the amount is reasonably estimable. In addition, to the extent we enter into sublicense arrangements, we are obligated to pay to MSK a percentage of certain payments that we receive from sublicensees of the rights licensed to us by MSK, which percentage will be based upon the achievement of certain clinical milestones. The Company has not entered into any sublicenses related to the MSK License Agreement. Failure by the Company to meet certain conditions under the arrangement could cause the related license to such licensed product to be canceled and could result in termination of the entire arrangement with MSK. In addition, the Company may terminate the MSK License Agreement with prior written notice to MSK.

No milestones were expensed in the three or nine months ended September 30, 2019 or 2018. We paid $725,000 under this arrangement in the first quarter of 2019. As of September 30, 2019, $150,000 of accrued milestone obligations were recorded in accrued short term liabilities and $300,000 was recorded within long-term liabilities. As of December 31, 2018, $875,000 of accrued milestone obligations were recorded in accrued short term liabilities and $300,000 was recorded within long-term liabilities. These milestone‑related charges were recorded as research and development expense in 2016, upon determination that payment of these clinical milestone obligations was probable after satisfying the financing requirements described herein.

Research and development is inherently uncertain and as described above, should such research and development fail, the MSK License Agreement is cancelable at the Company’s option. The Company also considered the development risk and each party’s termination rights under the agreement when considering whether any regulatory‑based milestone payments, certain of which also contain time‑based payment requirements, were probable. Given the uncertainty associated with research and development and the Company’s ability to cancel the MSK License Agreement, such regulatory‑based obligations were determined not to be probable as of September 30, 2019 and December 31, 2018 and therefore have not been accrued.

CD33 License Agreement

On November 13, 2017, we entered into an exclusive license agreement for certain MSK rights in connection with certain CD33 bispecific antibodies, which we refer to as the CD33 License Agreement. The CD33 License Agreement obligates us to pay to MSK mid to high single digit royalties based on annual net sales of licensed products or the performance of licensed services by us and our affiliates and sublicensees. We are obligated to pay annual minimum royalties of $40,000 over the royalty term, increasing to $60,000 once a patent within the licensed rights is issued, and commencing on the tenth anniversary of the CD33 License Agreement. These amounts are non‑refundable but are creditable against royalty payments otherwise due under the CD33 License Agreement. We are also obligated to pay MSK certain fees under a sponsored research agreement under the CD33 License Agreement. In addition, milestone payments become due upon achievement of the related clinical, regulatory or sales‑based milestone defined in the CD33 License Agreement. Certain of the clinical and regulatory milestone payments become due at the earlier of completion of the related milestone activity or the date indicated in the CD33 License Agreement. Total potential clinical and regulatory milestones potentially due under the CD33 License Agreement are $550,000 and $500,000, respectively. There are also sales‑based milestones that become due should the Company achieve certain amounts of sales of licensed products resulting from the CD33 License Agreement with MSK, with total sales‑based milestones potentially due of $7,500,000. Failure by the Company to meet certain conditions under the CD33 License Agreement could cause the related license to such licensed product to be canceled and could result in termination of the arrangement with MSK. In addition, the Company may terminate the CD33 License Agreement with prior written notice to MSK. The Company records milestones in the period in which the contingent liability is probable and the amount is reasonably estimable. In addition, to the extent we enter into sublicense arrangements, we are obligated to pay to MSK a percentage of certain payments that we receive from sublicensees of the rights licensed to us by MSK, which percentage will be based upon the achievement of certain clinical milestones. The Company has not entered into any sublicenses related to the CD33 License Agreement.

No milestones were expensed in the three or nine months ended September 30, 2019 or 2018. None of the clinical milestone obligations previously accrued were paid in the three or nine months ended September 30, 2019 or 2018. The total amount accrued in prior periods of $550,000 was recorded as $100,000 of accrued short‑term liabilities and $450,000 of accrued long‑term liabilities as of September 30, 2019, and $550,000 as accrued long‑term liabilities as of December 31, 2018. These milestone‑related charges were recorded as research and development expense in 2017. Research and development is inherently uncertain and as described above, should such research and development fail, the CD33 License Agreement is cancelable at the Company’s option. The Company considered risks as well as each party’s termination rights under the CD33 License Agreement when considering whether any regulatory‑based milestone payments and minimum royalty payments, certain of which also contain time‑based payment requirements, were probable. Given the uncertainty associated with research and development and the Company’s ability to cancel the CD33 License Agreement, such obligations were determined not to be probable as of September 30, 2019 and December 31, 2018 and therefore have not been accrued.

MabVax sublicense agreement

On June 27, 2018, we entered into a sublicense agreement with MabVax Therapeutics Holding, Inc., (“MabVax”) pursuant to which MabVax has sublicensed to the Company certain of MabVax’s patent rights and know-how for development and commercialization of products for the prevention or treatment of neuroblastoma by means of administering a bi-valent ganglioside vaccine, granted to MabVax pursuant to an exclusive license agreement between MabVax and MSK. Under the sublicense agreement, the Company has paid an up-front license fee of $700,000 to MabVax and paid an additional $600,000 upon the first anniversary of the sublicense agreement. The initial license fee of $700,000 was expensed and paid upon execution of the agreement and the continuation fee of $600,000 was accrued in the fourth quarter of 2018 and paid in the third quarter of 2019. The Company has agreed to become solely responsible for future amounts payable to MSK and to handle other of MabVax’ obligations applicable to the licensed indication towards MSK. This includes the obligation to pay development milestones totaling $1,400,000 and mid-single-digit royalty payments to MSK. Research and development is inherently uncertain and as described above, should such research and development fail, the MabVax sublicense agreement is cancelable at the Company’s option. The Company considered risks as well as each party’s termination rights under the MabVax sublicense agreement when considering whether any milestone payments and minimum royalty payments were probable. Given the uncertainty associated with research and development and the Company’s ability to cancel the MabVax sublicense agreement, such obligations were determined not to be probable as of September 30, 2019 and December 31, 2018 and therefore have not been accrued.

Other agreements

On November 5, 2015, we entered into a sponsored research agreement, which we refer to as the SRA, with MSK pursuant to which we agreed to pay MSK to provide research services related to the intellectual property licensed under the MSK License Agreement. The SRA was amended on September 13, 2019, and will expire five years from the date of the amendment. During the three months ended September 30, 2019 and 2018, we incurred research and development expenses of $306,000 and $297,000, respectively, under the SRA, and in the nine months ended September 30, 2019 and 2018, we incurred expenses of $918,000 and $891,000, respectively, under the SRA.

On March 20, 2016, we entered into a master data services agreement, which we refer to as the MDSA, with MSK pursuant to which we committed to make certain payments in exchange for services provided by approximately two full time employees at MSK, who are engaged in transferring clinical data, databases, regulatory files and other know‑how included in the MSK License Agreement to the Company. On October 1, 2018 the MDSA was amended to increase the resources to approximately five full time employees. During the three months ended September 30, 2019 and 2018, we incurred expenses of $205,000 and $60,000, respectively, under the MDSA, and in the nine months ended September 30, 2019 and 2018, we incurred expenses of $547,000 and $273,000, respectively, under the MDSA.

On June 21, 2017, we entered into a master clinical trial agreement, which we refer to as the CTA, with MSK pursuant to which we committed to fund certain clinical trials at MSK. Under the MSK License Agreement, the funding of clinical activities is limited to a five-year period. During the three months ended September 30, 2019 and 2018, we incurred expenses of $513,000 and $363,000, respectively, under the CTA, and in the nine months ended September 30, 2019 and 2018, we incurred expenses of $2,647,000 and $2,504,000, respectively, under the CTA.

On June 27, 2017, we entered into two separate core facility service agreements, which we refer to as the CFAs, with MSK pursuant to which we committed to obtaining certain laboratory services from MSK. During the three months ended September 30, 2019 and 2018, we incurred expenses of $186,000 and $28,000, respectively, under the CFAs, and in the nine months ended September 30, 2019 and 2018, we incurred expenses of $674,000 and $223,000, respectively, under the CFAs.

On November 13, 2017, we entered into a CD33 sponsored research agreement, which we refer to as the CD33 SRA, with MSK pursuant to which we agreed to pay MSK to provide research services over a period of two years related to the intellectual property licensed under the CD33 License Agreement. During the three months ended September 30, 2019 and 2018, we incurred expenses of $174,000 and $167,000, respectively, under the CD33 SRA, and in the nine months ended September 30, 2019 and 2018, we incurred expenses of $521,000 and $501,000, respectively, under the CD33 SRA.

Lease Agreements

In February 2019, the Company entered into a lease agreement in connection with its 4,500 square feet laboratory in New Jersey. The term of the lease is three years from the date the Company occupied the premises, with an option to extend for an additional two years which the Company expects to exercise and has included in the determination of the related lease liability. Fixed rent payable under the lease is approximately $144,000 per annum and is payable in equal monthly installments of approximately $12,000.

In January 2018, the Company entered into a lease agreement in connection with its corporate headquarters in New York, New York. The term of the lease is five years from the date the Company occupied the premises. Fixed rent payable under the lease is approximately $384,000 per annum and is payable in equal monthly installments of approximately $32,000.

Additionally, the Company entered into a three‑year lease agreement for the lease of certain office space in Denmark in February 2018, as amended in November 2018 and February 2019. Fixed rent payable under the lease is approximately $227,000 per annum and is payable in equal monthly installments of approximately $19,000.

As described above in Note 3, the Company adopted Topic 842 as of January 1, 2019. Prior period amounts have not been adjusted and continue to be reported in accordance with the Company’s historical accounting under Topic 840.

Total operating lease costs were $174,000 and $91,000 for the three months ended September 30, 2019 and 2018, respectively, and $502,000 and $181,000 for the nine months ended September 30, 2019 and 2018, respectively.

During the three months ended September 30, 2019, the expenses were recorded as $129,000 in research and development expense and $45,000 in general and administrative expense. During the three months ended September 30, 2018, the expenses were all recorded in general and administrative expense. During the nine months ended September 30, 2019, the expenses were recorded as $367,000 in research and development expense and $135,000 in general and administrative expense. During the nine months ended September 30, 2018, the expenses were all recorded in general and administrative expense.

Cash paid for amounts included in the measurement of lease liabilities for the three and nine months ended September 30, 2019 was $188,000 and $419,000, respectively, and was included in net cash used in operating activities in the Company’s Consolidated Statements of Cash Flows.

Maturities of operating lease liabilities at September 30, 2019 were as follows (in thousands):


	Operating Leases
	at September 30, 2019

Remainder of 2019	$	186
Years ending December 31,
2020		749
2021		753
2022		646
2023		539
Thereafter		77
Total lease payments		2,950
Less: Imputed interest		(613)
Total operating lease liabilities at September 30, 2019	$	2,337

Disclosures related to periods prior to the adoption of Topic 842:

Future minimum lease payments, including imputed interest, under non-cancelable operating leases at December 31, 2018 were as follows (in thousands):


	Contractual Obligations
	at December 31, 2018

2019	$	510
2020		616
2021		616
2022		616
2023		462
Thereafter		64
Total lease payments		2,884
Less: Imputed interest		—
Contractual obligations at December 31, 2018	$	2,884

Operating lease liabilities are based on the net present value of the remaining lease payments over the remaining lease term. In determining the present value of lease payments, the Company uses its estimate of its incremental borrowing rate based on the information available at the lease commencement date. As of September 30, 2019, the weighted average remaining lease term is 4.1 years and the weighted average discount rate used to determine the operating lease liability was 11.0%.

STOCKHOLDERS' EQUITY	9 Months Ended
STOCKHOLDERS' EQUITY	Sep. 30, 2019
STOCKHOLDERS' EQUITY
STOCKHOLDERS' EQUITY	NOTE 7—STOCKHOLDERS’ EQUITY Authorized Stock As of September 30, 2019 and December 31, 2018, the Company has authorized a total of 105,500,000 shares, 100,000,000 of which are common stock, par value $0.0001 per share, and 5,500,000 of which are preferred stock, par value $0.0001 per share. Common Stock Each share of common stock is entitled to one vote. Common stockholders are entitled to receive dividends, as may be declared by the board of directors, if any, subject to preferential dividend rights of the preferred stock, none of which have been issued. The Company had issued 34,593,666 shares of its common stock as of September 30, 2019 and 34,193,666 shares of its common stock as of December 31, 2018. Preferred Stock Preferred stock may be issued from time to time in one or more series with such designations, preferences and relative participating, optional or other special rights and qualifications, limitations or restrictions as approved by the Company’s Board of Directors. No preferred stock has been issued as of September 30, 2019 or December 31, 2018. Stock grant agreements with non‑employees In August 2015, we entered into certain stock grant agreements with non‑employees of the Company. We agreed to issue a total of 2,800,000 shares to two non‑employee researchers who were involved in the development of technology licensed from MSK in consideration for their prior service. These two researchers were employees of MSK. The shares are released according to a vesting schedule. A total of 560,000 shares were issued in 2015, and a total of 448,000 shares issued in each of 2016 and 2017. In 2018, a total of 448,000 shares were issued to the two non-employee researchers, and upon completion of the IPO, we issued an additional 96,000 shares pursuant to a stock grant agreement and the issuance did not result in proceeds to the Company. A total of 400,000 shares were issued in 2019 and 400,000 shares are to be issued in 2020 to one of the non-employee researchers, subject to certain conditions, such that the total grant will have been issued. The total award was expensed at its estimated fair value in 2015, as no future service was required to continue to vest in and receive the shares of common stock. In August 2016, the Company repurchased and retired a total of 83,600 shares from the two non‑employees of the Company at an amount equal to the estimated fair value of $4.38 per share. The transaction reduced the Company’s shareholders’ equity by $366,000. In April 2018, the Company granted 72,373 common stock options to one of the non‑employee researchers employed by MSK under our 2015 Equity Incentive Plan (the “2015 Plan”). The options become exercisable over a four‑year period, with the first twenty‑five percent (25%) exercisable twelve (months) from the date of grant and the remainder becoming exercisable ratably each month over the three years thereafter. The contractual term of the option award is 10 years from the date of grant. The total award was expensed at its estimated fair value in April 2018, as no future service was required by the non‑employee to continue to vest in the option grant. The shares will become immediately exercisable upon the occurrence of a change in control, as defined in the 2015 Plan as further described in Note 8, Share-Based Compensation. Issuance of common stock In August 2019, we issued 400,000 shares of our common stock. The issuance was made pursuant to a stock grant agreement and did not result in proceeds to the Company. In September 2018, we completed an initial public offering and issued 6,900,000 shares of common stock at a purchase price of $16.00 per share for an aggregate consideration of $99,507,000, net of issuance costs. Upon completion of the IPO, we also issued 96,000 shares of our common stock pursuant to a stock grant agreement and the issuance did not result in proceeds to the Company. In August 2018, we issued 448,000 shares of our common stock. The issuance was made pursuant to stock grant agreements and did not result in proceeds to the Company.

SHARE-BASED COMPENSATION

9 Months Ended

Sep. 30, 2019

SHARE-BASED COMPENSATION

SHARE-BASED COMPENSATION

NOTE 8—SHARE-BASED COMPENSATION

2015 Equity Incentive Plan

Our board of directors and stockholders have approved and adopted the 2015 Plan, which provided for the grant of incentive stock options, within the meaning of Section 422 of the Code (the Internal Revenue Code), to our employees and any parent and subsidiary corporations’ employees, and for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock and restricted stock units to our employees, directors and consultants and our parent and subsidiary corporations’ employees and consultants. A total of 4,500,000 shares of our common stock were reserved for issuance pursuant to the 2015 Plan. Options granted under the 2015 Plan vest according to the schedule specified in the grant agreements, which is generally a four year period and generally become immediately exercisable upon the occurrence of a change in control, as defined. Upon the 2018 Equity Incentive Plan (the “2018 Plan’) becoming effective in September 2018, no further grants are allowed under the 2015 Plan.

2018 Equity Incentive Plan

Our board of directors and stockholders approved and adopted the 2018 Plan, which became effective upon the Company’s initial public offering in September 2018 and which provides for the grant of incentive stock options, within the meaning of Section 422 of the Code (the Internal Revenue Code), to our employees and any parent and subsidiary corporations’ employees, and for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock and restricted stock units to our employees, directors and consultants and our parent and subsidiary corporations’ employees and consultants. A total of 5,500,000 shares of our common stock, inclusive of the awards previously granted under the 2015 Equity Incentive Plan, are reserved for issuance pursuant to the 2018 Plan. In addition, the number of shares available for issuance under the 2018 Plan will also include an annual increase on the first day of each fiscal year beginning in 2019, equal to 4% of the outstanding shares of common stock as of the last day of our immediately preceding fiscal year. The exercise price of options granted under the plans must at least be equal to the fair market value of our common stock on the date of grant. The term of an incentive stock option may not exceed 10 years, except that with respect to any participant who owns more than 10% of the voting power of all classes of our outstanding stock, the term must not exceed five years and the exercise price must equal at least 110% of the fair market value on the grant date. The administrator will determine the methods of payment of the exercise price of an option, which may include cash, shares or other property acceptable to the administrator, as well as other types of consideration permitted by applicable law. Options granted under the 2018 Plan vest according to the schedule specified in the grant agreements, which is generally a four year period and generally become immediately exercisable upon the occurrence of a change in control, as defined.

Stock Option Valuation

During the three month periods ended September 30, 2019 and 2018, stock based compensation for stock option grants were $1,324,000 and $395,000, respectively, for options granted to employees and directors. During the three months ended September 30, 2019, the expenses were recorded as $263,000 in research and development expense and $1,061,000 in general and administrative expense. During the three months ended September 30, 2018, the expenses were recorded as $67,000 in research and development expense and $328,000 in general and administrative expense.

During the nine month periods ended September 30, 2019 and 2018, stock based compensation for stock option grants were $3,124,000 and $1,471,000, respectively, for options granted to employees, non-employees and directors. During the nine months ended September 30, 2019, the expenses were recorded as $577,000 in research and development expense and $2,547,000 in general and administrative expense. During the nine months ended September 30, 2018, the expenses were recorded as $192,000 in research and development expense and $694,000 in general and administrative expense for options granted to employees and directors. During the nine months ended September 30, 2018, $585,000 was recorded in research and development expense for options granted to non-employees.

The following table summarizes common stock options issued and outstanding:


						Weighted
		Weighted		Aggregate		average
		average		intrinsic		remaining
		exercise		value		contractual
	Options	price		(in thousands)		life (years)
Outstanding and expected to vest at December 31, 2018	3,357,873	$	7.74	$	43,224	7.90
Granted	356,000	$	21.50
Outstanding and expected to vest at September 30, 2019	3,713,873	$	9.06	$	63,144	7.39
Exercisable at September 30, 2019	2,243,931	$	4.04	$	49,408	6.43

There were no stock options granted during the three months ended September 30, 2019. The weighted average fair value of stock options granted during the three months ended September 30, 2018 was $7.54.

The weighted average fair value of stock options granted during the nine month ended September 30, 2019 and 2018 was $12.34 and $6.77, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock.

As of September 30, 2019, we had $12.1 million of unrecognized compensation related to employee stock options that is expected to vest over a period of 2.63 years. As of December 31, 2018, we had $10.8 million of unrecognized compensation related to employee stock options that are expected to vest over a period of 2.82 years.

Restricted Stock Unit Activity

During the three and nine months ended September 30, 2019, stock-based compensation for restricted stock unit grants was $21,000 and $56,000, respectively. During the three months ended September 30, 2019, the expenses were recorded as $18,000 in research and development expense and $2,000 in general and administrative expense. During the nine months ended September 30, 2019, the expenses were recorded as $50,000 in research and development expense and $6,000 in general and administrative expense. There was no stock based compensation for restricted stock units during the three or nine months ended September 30, 2018.

The following table summarizes restricted stock units issued and outstanding:


		Restricted Stock Units
Outstanding and expected to vest at December 31, 2018		—
Granted		10,296
Outstanding and expected to vest at September 30, 2019		10,296

The weighted average fair value of restricted stock units granted during the three and nine months ended September 30, 2019 was $23.11. As of September 30, 2019, we had $183,000 of unrecognized compensation related to employee restricted stock units that are expected to vest over a period of 2.30 years.

RELATED PARTY TRANSACTIONS	9 Months Ended
RELATED PARTY TRANSACTIONS	Sep. 30, 2019
RELATED PARTY TRANSACTIONS
RELATED PARTY TRANSACTIONS	NOTE 9—RELATED PARTY TRANSACTIONS MSK is a shareholder of the Company. Under the MSK License Agreement, the CD33 License Agreement, CTA, CFAs, SRA and MDSA, we have expensed costs in the total amount of $1,384,000 and $914,000 in the three months ended September 30, 2019 and 2018, respectively, for milestones, minimum royalties, and research and development costs. We expensed costs in the total amounts of $5,307,000 and $4,526,000 in the nine months ended September 30, 2019 and 2018, respectively, under these agreements with MSK. Please refer to Note 6 for additional details on our agreements with MSK. As of September 30, 2019 and December 31, 2018, we had a total of $3,829,000 and $4,475,000, respectively, recorded as accounts payable and accrued liabilities related to amounts due to MSK.

INCOME TAXES	9 Months Ended
INCOME TAXES	Sep. 30, 2019
INCOME TAXES
INCOME TAXES	NOTE 10—INCOME TAXES The Company provided no current and deferred income taxes on net losses of $23.9 millon and $11.4 million for the three month periods ended September 30, 2019 and 2018, respectively, and the net losses of $57.9 million and $29.2 million for the nine month periods ended September 30, 2019 and 2018, respectively. The Company recognizes income tax benefits for tax positions determined more likely than not to be sustained upon examination, based on the technical merits of the positions. As of September 30, 2019 and December 31, 2018, the Company has determined that there were no uncertain tax positions. The Company’s tax returns for years 2018, 2017, 2016, and 2015 are open for tax examination by U.S. federal and state, and the Danish tax authorities. The valuation allowance related primarily to net U.S. deferred tax assets from operating losses, research and development tax credit carryforwards, and acquired intangibles. The Company maintains a full valuation allowance on its U.S. and foreign deferred tax assets. The assessment regarding whether a valuation allowance is required considers both positive and negative evidence when determining whether it is more‑likely‑than‑not that deferred tax assets are recoverable. In making this assessment, significant weight is given to evidence that can be objectively verified. In its evaluation, the Company considered its cumulative loss in recent years and its forecasted losses in the near‑term as significant negative evidence. Based upon review of available positive and negative evidence, the Company determined that the negative evidence outweighed the positive evidence and a full valuation allowance on its U.S. and foreign deferred tax assets will be maintained. The Company will continue to assess the realizability of its deferred tax assets and will adjust the valuation allowance as needed.

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	9 Months Ended
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES (Policies)	Sep. 30, 2019
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Operating Leases	Operating Leases As described below, the Company adopted Topic 842 as of January 1, 2019. The Company determines if an arrangement includes a lease at inception. Operating lease right-of-use assets represent the Company’s right to use an underlying asset for the lease term and operating lease liabilities represent its obligation to make lease payments arising from the lease. Operating lease right-of-use assets and liabilities are recognized at the lease commencement date based on the present value of lease payments over the lease term. In determining the net present value of lease payments, the Company uses its estimated incremental borrowing rate based on information available at the lease commencement date. Because most of the Company’s leases do not provide an implicit rate of return, an incremental borrowing rate is used based on the information available at the commencement date in determining the present value of lease payments on an individual lease basis. The Company’s incremental borrowing rate for a lease is the estimated rate of interest it would have to pay on a collateralized basis to borrow an amount equal to the lease payments under similar terms. The Company’s leases may include options to extend or terminate the lease which are included in the lease term when it is reasonably certain that it will exercise any such options. None of the Company’s leases contain any residual value guarantees. Lease expense is recognized on a straight-line basis over the expected lease term. Related variable lease costs incurred are not material to the Company. Topic 842 also provides practical expedients and certain exemptions for an entity’s ongoing accounting post implementation. The Company currently elected the short-term lease recognition exemption for all leases that qualify. This means, for those leases that qualify, we will not recognize right-of-use assets or liabilities, and this includes not recognizing right-of-use assets or liabilities for existing short-term leases of those assets in transition. We also elected the practical expedient to not separate lease and non-lease components for all of our leases. The Company has made an accounting policy election to account for each separate lease component of a contract and its associated non-lease components as a single lease component. See the Lease Agreements section in Note 6 for the related disclosures.
Use of Estimates	Use of Estimates The preparation of financial statements in conformity with GAAP requires management to make estimates and assumptions that affect the reported amounts of assets and liabilities, the disclosure of contingent assets and liabilities at the date of the financial statements and the reported amounts of expenses during the reporting period. Significant estimates and assumptions reflected in these financial statements include, but are not limited to, the accrual for research and development expenses, the accrual of milestone and royalty payments, and the valuation of stock options. Estimates are periodically reviewed in light of changes in circumstances, facts and experience. Changes in estimates are recorded in the period in which they become known. Actual results could differ from those estimates.
Segment Information	Segment Information The Company is engaged solely in the discovery and development of novel antibody-based therapeutic products for the treatment of cancer. Accordingly, the Company has determined that it operates in one operating segment.
Recently Issued Accounting Pronouncements	Recently Issued Accounting Pronouncements - Adopted In July 2018, the FASB issued Accounting Standards Update No. 2018-09 (“ASU 2018-09”), Codification Improvements, which clarify, correct errors in, or make minor improvements to a variety of ASC topics. The changes in ASU 2018-09 are not expected to have a significant effect on current accounting practices. Some of the amendments in this update do not require transition guidance and will be effective upon this update. However, many of the updates do have transition guidance with effective dates for periods beginning after December 15, 2018. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In June 2018, the FASB issued ASU No. 2018‑07, Compensation—Stock Compensation (Topic 718), Improvements to Nonemployee Share‑Based Payment Accounting (“ASU 2018‑07”). ASU 2018‑07 is intended to simplify aspects of share‑based compensation issued to non‑employees by making the guidance consistent with the accounting for employee share‑based compensation. ASU 2018‑07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In February 2018, the FASB issued Accounting Standards Update No. 2018‑02, (“ASU 2018-02”), Reclassification of Certain Tax Effects from Accumulated Other Comprehensive Income. ASU 2018‑02 allows a reclassification from accumulated other comprehensive income to retained earnings for stranded tax effects resulting from the Tax Cuts and Jobs Act. ASU 2018‑07 is required to be adopted for annual periods beginning after December 15, 2018, including interim periods within those fiscal years. Early adoption is permitted. The adoption of this standard on January 1, 2019 did not have a material impact on our consolidated financial statements and related disclosures. In February 2016, the FASB issued Accounting Standards Update No. 2016‑02 (“ASU 2016‑02”), Leases, which is effective for fiscal years, and interim periods within those years, beginning after December 15, 2018 with early adoption permitted. Under ASU 2016‑02, lessees will be required to recognize for all leases, at the commencement date of the lease, a lease liability, which is a lessee’s obligation to make lease payments arising from a lease measured on a discounted basis, and a right‑to‑use asset, which is an asset that represents the lessee’s right to use or control the use of a specified asset for the lease term. Topic 842 was subsequently amended by ASU 2017-13, Revenue and Leases: Amendments to SEC Paragraphs Pursuant to the Staff Announcement at the July 20, 2017 EITF Meeting and Rescission of Prior SEC Staff Announcements and Observer Comments; ASU 2018-01, Land Easement Practical Expedient for Transition to Topic 842; ASU No. 2018-10, Codification Improvements to Topic 842, Leases; ASU No. 2018-11, Targeted Improvements and ASU No. 2018-20, Narrow Scope Improvements for Lessors. The Company adopted the new leasing standards using the modified retrospective transition approach as of January 1, 2019, with no restatement of prior periods or cumulative adjustment to retained earnings. An entity may choose to use either (1) its effective date or (2) the beginning of the earliest comparative period presented in the financial statements as its date of initial application. The Company used the effective date as our date of initial application. Consequently, financial information was not updated and the disclosures required under the new standard are not provided for dates and periods before January 1, 2019. The new standard also provides a number of optional practical expedients in transition. The Company elected the package of practical expedients, which permits us not to reassess under the new standard our prior conclusions about lease identification, lease classification and initial direct costs. Upon adoption of the new leasing standards, the Company recognized a lease liability of $1.8 million and a related right-of-use asset of $1.5 million with the difference being due to the elimination of previously reported deferred rent. The Company subsequently entered into two new lease agreements during the three months ended March 31, 2019, and recognized an incremental lease liability and related right-of-use asset of $0.9 million. Recently Issued Accounting Pronouncements – Not Yet Adopted In August 2018, the FASB issued Accounting Standards Update No. 2018-15 (“ASU 2018-15”), Customer’s Accounting for Implementation Costs Incurred in a Cloud Computing Arrangement That Is a Service Contract. ASU 2018-15 clarifies certain aspects of ASU 2015-05, Customer’s Accounting for Fees Paid in a Cloud Computing Arrangement, which was issued in April 2015. Specifically, ASU 2018-14 aligns the requirements for capitalizing implementation costs incurred in a hosting arrangement that is a service contract with the requirements for capitalizing implementation costs incurred to develop or obtain internal use software (and hosting arrangements that include an internal-use software license). ASU 2018-15 is effective for fiscal years beginning after December 15, 2019, and interim periods within those fiscal years with early adoption permitted. The Company is currently evaluating the impact its adoption may have on its consolidated financial statements.

NET LOSS PER SHARE (Tables)

9 Months Ended

Sep. 30, 2019

NET LOSS PER SHARE

Schedule of basic and diluted net loss per share


	Three months ended September 30,				Nine months ended September 30,
	2019		2018		2019		2018
	(in thousands, except per share amounts)

Net loss (numerator)	$	(23,922)	$	(11,426)	$	(57,892)	$	(29,214)
Weighted-average shares (denominator)		34,372		27,331		34,254		26,945
Basic and diluted net loss per share	$	(0.70)	$	(0.42)	$	(1.69)	$	(1.08)

ACCRUED LIABILITIES (Tables)

9 Months Ended

Sep. 30, 2019

ACCRUED LIABILITIES

Summary of accrued short-term liabilities


	September 30,		December 31,
	2019		2018
	(in thousands)

Accrued milestone and royalty payments	$	268	$	1,475
Accrued clinical costs		1,079		63
Accrued compensation and board fees		1,765		1,144
Accrued rent		—		44
Accrued manufacturing costs		2,734		—
Other		2		525
Total	$	5,848	$	3,251

LICENSE AGREEMENTS AND COMMITMENTS (Tables)

9 Months Ended

Sep. 30, 2019

LICENSE AGREEMENTS AND COMMITMENTS

Maturities of operating lease liabilities

Maturities of operating lease liabilities at September 30, 2019 were as follows (in thousands):


	Operating Leases
	at September 30, 2019

Remainder of 2019	$	186
Years ending December 31,
2020		749
2021		753
2022		646
2023		539
Thereafter		77
Total lease payments		2,950
Less: Imputed interest		(613)
Total operating lease liabilities at September 30, 2019	$	2,337

Future minimum lease payments, Prior to adoption of Topic 842

Future minimum lease payments, including imputed interest, under non-cancelable operating leases at December 31, 2018 were as follows (in thousands):


	Contractual Obligations
	at December 31, 2018

2019	$	510
2020		616
2021		616
2022		616
2023		462
Thereafter		64
Total lease payments		2,884
Less: Imputed interest		—
Contractual obligations at December 31, 2018	$	2,884

SHARE-BASED COMPENSATION (Tables)

9 Months Ended

Sep. 30, 2019

SHARE-BASED COMPENSATION

Schedule of common stock options issued and outstanding


						Weighted
		Weighted		Aggregate		average
		average		intrinsic		remaining
		exercise		value		contractual
	Options	price		(in thousands)		life (years)
Outstanding and expected to vest at December 31, 2018	3,357,873	$	7.74	$	43,224	7.90
Granted	356,000	$	21.50
Outstanding and expected to vest at September 30, 2019	3,713,873	$	9.06	$	63,144	7.39
Exercisable at September 30, 2019	2,243,931	$	4.04	$	49,408	6.43

Schedule of restricted stock units issued and outstanding


		Restricted Stock Units
Outstanding and expected to vest at December 31, 2018		—
Granted		10,296
Outstanding and expected to vest at September 30, 2019		10,296

BASIS OF PRESENTATION (Details) - USD ($) $ in Thousands		9 Months Ended
BASIS OF PRESENTATION (Details) - USD ($) $ in Thousands	Nov. 08, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018
Issuance of stock
Accumulated deficit			$ 142,727	$ 84,835
Cash and cash equivalents			$ 98,192	$ 147,840
Proceeds from issuance of common stock, net of issuance costs		$ 101,607
Follow-on shelf public offering \| Subsequent Event
Issuance of stock
Proceeds from issuance of common stock, net of issuance costs	$ 134,700

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Leases and Segments (Details) $ in Millions	9 Months Ended
	Sep. 30, 2019 USD ($) segment
SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES
Residual value guarantees \| $	$ 0.0
Number of operating segments \| segment	1

SUMMARY OF SIGNIFICANT ACCOUNTING POLICIES - Recently issued accounting pronouncements (Details) $ in Thousands		3 Months Ended
	Jan. 01, 2019 USD ($)	Mar. 31, 2019 USD ($) lease	Sep. 30, 2019 USD ($)
New accounting pronouncements
Lease liability			$ 2,337
Operating lease right-of-use assets			$ 2,086
Number of lease agreements entered into during the period \| lease		2
Increase in lease liability upon entering into new lease agreements		$ 900
Increase in right-of-use assets upon entering into new lease agreements		$ 900
ASU 2016-02
New accounting pronouncements
Change in Accounting Principle, Accounting Standards Update, Adopted	true
Change in Accounting Principle, Accounting Standards Update, Transition Option Elected	Modified Retrospective
New Accounting Pronouncement or Change in Accounting Principle, Prior Period Not Restated	true
Lease, Practical Expedients, Package	true
ASU 2016-02 \| Adjustment
New accounting pronouncements
Lease liability	$ 1,800
Operating lease right-of-use assets	$ 1,500

NET LOSS PER SHARE - Basic and diluted (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
NET LOSS PER SHARE
Net loss (numerator)	$ (23,922)	$ (18,036)	$ (15,934)	$ (11,426)	$ (10,305)	$ (7,483)	$ (57,892)	$ (29,214)
Weighted-average shares (denominator)	34,371,927			27,330,579			34,253,739	26,945,432
Basic and diluted net loss per share (in dollars per share)	$ (0.70)			$ (0.42)			$ (1.69)	$ (1.08)

- Definition

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

+ References + Details

ACCRUED LIABILITIES (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Accrued short term liabilities
Accrued milestone and royalty payments	$ 268	$ 1,475
Accrued clinical costs	1,079	63
Accrued compensation and board fees	1,765	1,144
Accrued rent		44
Accrued manufacturing costs	2,734
Other	2	525
Total	$ 5,848	$ 3,251

LICENSE AGREEMENTS AND COMMITMENTS - MSK License Agreement (Details) - MSK License Agreement - USD ($) $ in Thousands		3 Months Ended			9 Months Ended		12 Months Ended
	Aug. 20, 2015	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2016	Dec. 31, 2018
Agreements
Issuance of common stock to strategic partner (in shares)	1,428,500
Annual minimum royalties payable from fifth anniversary	$ 80
Expensed minimum royalty payments							$ 1,200
Clinical milestones potentially due	2,450
Regulatory milestones potentially due	9,000
Sales-based milestones potentially due	$ 20,000
Clinical milestone expense		$ 0		$ 0	$ 0	$ 0
Milestone payment			$ 725
Accrued short-term liabilities
Agreements
Short-term accrued minimum royalties		18			18
Accrued milestone payments, current		150			150			$ 875
Accrued long-term liabilities
Agreements
Long-term accrued minimum royalties		1,182			1,182
Accrued milestone payments, noncurrent		$ 300			$ 300			$ 300

LICENSE AGREEMENTS AND COMMITMENTS - CD33 License Agreement (Details) - CD33 License Agreement - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018	Nov. 13, 2017
Agreements
Annual minimum royalties payable						$ 40
Annual minimum royalties payable from tenth anniversary						60
Clinical milestones potentially due						550
Regulatory milestones potentially due						500
Sales-based milestones potentially due						$ 7,500
Clinical milestone expense	$ 0	$ 0	$ 0	$ 0
Clinical milestone payment	0	$ 0	0	$ 0
Accrued milestone payments	550		550
Accrued short-term liabilities
Agreements
Accrued milestone payments, current	100		100
Accrued long-term liabilities
Agreements
Accrued milestone payments, noncurrent	$ 450		$ 450		$ 550

LICENSE AGREEMENTS AND COMMITMENTS - MabVax sublicense agreement (Details) - MabVax sublicense agreement - USD ($) $ in Thousands		1 Months Ended	3 Months Ended
	Jun. 27, 2018	Jun. 30, 2019	Sep. 30, 2019	Dec. 31, 2018
Agreements
License fee paid	$ 700	$ 600
License fee expense				$ 700
Continuation fee				$ 600
Continuation fee paid			$ 600
Development milestone payments due	$ 1,400

LICENSE AGREEMENTS AND COMMITMENTS - Other agreements (Details) $ in Thousands							3 Months Ended		9 Months Ended
	Sep. 13, 2019	Oct. 01, 2018 individual	Nov. 13, 2017	Jun. 27, 2017 agreement	Jun. 21, 2017	Mar. 20, 2016 individual	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)
Agreements
Research and development							$ 19,660	$ 8,731	$ 46,665	$ 23,228
SRA
Agreements
Research service period	5 years
Research and development							306	297	918	891
MDSA
Agreements
Research and development							205	60	547	273
CTA
Agreements
Research service period					5 years
Research and development							513	363	2,647	2,504
CFAs
Agreements
Number of service agreements \| agreement				2
Research and development							186	28	674	223
CD33-SRA
Agreements
Research service period			2 years
Research and development							$ 174	$ 167	$ 521	$ 501
Researchers, Employees of MSK \| MDSA
Agreements
Number of individuals \| individual		5				2

LICENSE AGREEMENTS AND COMMITMENTS - Lease agreements (Details) $ in Thousands	1 Months Ended
	Feb. 28, 2019 USD ($) ft²	Feb. 28, 2018	Jan. 31, 2018 USD ($)
Laboratory, New Jersey
Leases
Leased area (in square feet) \| ft²	4,500
Lease term	3 years
Option to extend	true
Option to extend, period	2 years
Fixed rent payable per annum	$ 144
Lease payable per month	12
Corporate headquarters, New York
Leases
Lease term			5 years
Fixed rent payable per annum			$ 384
Lease payable per month			$ 32
Office space, Denmark
Leases
Lease term		3 years
Fixed rent payable per annum	227
Lease payable per month	$ 19

LICENSE AGREEMENTS AND COMMITMENTS - Lease costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Leases
Total operating lease costs	$ 174		$ 502
Rental expense for operating leases		$ 91		$ 181
Cash paid for amounts included in the measurement of lease liabilities	188		419
Research and development expense
Leases
Total operating lease costs	129		367
General and administrative expense
Leases
Total operating lease costs	$ 45		$ 135

LICENSE AGREEMENTS AND COMMITMENTS - Lease maturities (Details) $ in Thousands	Sep. 30, 2019 USD ($)
Future minimum commitments under all non-cancelable operating leases
Remainder of 2019	$ 186
2020	749
2021	753
2022	646
2023	539
Thereafter	77
Total lease payments	2,950
Less: Imputed interest	(613)
Total operating lease liabilities	$ 2,337

LICENSE AGREEMENTS AND COMMITMENTS - 2018 Lease maturities (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Future minimum commitments under non-cancelable operating leases, 2018
2019		$ 510
2020		616
2021		616
2022		616
2023		462
Thereafter		64
Total lease payments		2,884
Less: Imputed interest	$ (613)
Contractual obligations		$ 2,884

LICENSE AGREEMENTS AND COMMITMENTS - Lease term and discount rate (Details)	Sep. 30, 2019
LICENSE AGREEMENTS AND COMMITMENTS
Weighted average remaining lease term	4 years 1 month 6 days
Weighted average discount rate	11.00%

STOCKHOLDERS' EQUITY - Authorized, Common and Preferred Stock (Details)	9 Months Ended	12 Months Ended
	Sep. 30, 2019 USD ($) Vote $ / shares shares	Dec. 31, 2018 USD ($) $ / shares shares
STOCKHOLDERS' EQUITY
Total shares authorized	105,500,000	105,500,000
Common stock, authorized shares	100,000,000	100,000,000
Common stock, par value (in dollars per share) \| $ / shares	$ 0.0001	$ 0.0001
Preferred stock, authorized shares	5,500,000	5,500,000
Preferred stock, par value (in dollars per share) \| $ / shares	$ 0.0001	$ 0.0001
Number of votes per common share \| Vote	1
Common stock dividends issued \| $	$ 0	$ 0
Common stock, shares issued	34,593,666	34,193,666
Preferred stock, shares issued	0	0

STOCKHOLDERS' EQUITY - Stock grant agreements with non-employees, Stock (Details) $ / shares in Units, $ in Thousands		1 Months Ended					3 Months Ended		9 Months Ended	12 Months Ended
	Aug. 15, 2015 individual	Aug. 31, 2019 shares	Sep. 30, 2018 shares	Aug. 31, 2018 shares	Aug. 31, 2016 USD ($) individual $ / shares shares	Aug. 31, 2015 individual shares	Sep. 30, 2019 shares	Sep. 30, 2018 shares	Sep. 30, 2019 individual shares	Dec. 31, 2020 individual shares	Dec. 31, 2018 individual shares	Dec. 31, 2017 shares	Dec. 31, 2016 shares	Dec. 31, 2015 shares
Nonemployees \| Researchers, Employees of MSK
Share-Based Compensation Awards
Shares agreed to be issued						2,800,000
Number of individuals \| individual	2				2	2			1		2
Shares repurchased and retired					83,600
Estimated fair value of stock repurchased and retired (in dollars per share) \| $ / shares					$ 4.38
Reduction in equity \| $					$ 366
Nonemployees \| Researchers, Employees of MSK \| Forecast
Share-Based Compensation Awards
Number of individuals \| individual										1
Common Stock
Share-Based Compensation Awards
Issuance of common stock to nonemployees (in shares)							400,000	544,000
Common Stock \| Nonemployees \| Researchers, Employees of MSK
Share-Based Compensation Awards
Issuance of common stock to nonemployees (in shares)		400,000		448,000					400,000		448,000	448,000	448,000	560,000
Common Stock \| Nonemployees \| Researchers, Employees of MSK \| Forecast
Share-Based Compensation Awards
Issuance of common stock to nonemployees (in shares)										400,000
Common Stock \| Nonemployees \| Individual with stock grant agreement with issuance at IPO
Share-Based Compensation Awards
Issuance of common stock to nonemployees (in shares)			96,000

STOCKHOLDERS' EQUITY - Stock grant agreements with non employees, Stock options (Details)	1 Months Ended	3 Months Ended	9 Months Ended
	Apr. 30, 2018 individual shares	Sep. 30, 2019 shares	Sep. 30, 2019 shares
Share-Based Compensation Awards
Granted (in shares)		0	356,000
Stock options \| Nonemployees
Share-Based Compensation Awards
Number of individuals \| individual	1
2015 Plan \| Nonemployees
Share-Based Compensation Awards
Granted (in shares)	72,373
Vesting period	4 years
Term of award	10 years
2015 Plan \| Nonemployees \| Tranche one
Share-Based Compensation Awards
Vesting period	12 months
Vesting (as a percent)	25.00%
2015 Plan \| Nonemployees \| Tranche two
Share-Based Compensation Awards
Vesting period	3 years

STOCKHOLDERS' EQUITY - Issuance of common stock, IPO (Details) $ / shares in Units, $ in Thousands	1 Months Ended	3 Months Ended	9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares shares	Sep. 30, 2018 $ / shares shares	Sep. 30, 2018 USD ($) $ / shares
Issuance of stock
Proceeds from issuance of common stock, net of issuance costs \| $			$ 101,607
IPO
Issuance of stock
Share price (in dollars per share) \| $ / shares	$ 16.00	$ 16.00	$ 16.00
Proceeds from issuance of common stock, net of issuance costs \| $	$ 99,507
Common Stock
Issuance of stock
Shares issued (in shares) \| shares		6,900,000
Common Stock \| IPO
Issuance of stock
Shares issued (in shares) \| shares	6,900,000

STOCKHOLDERS' EQUITY - Issuance of common stock, Stock grant agreements (Details) - Common Stock - shares	1 Months Ended			3 Months Ended		9 Months Ended	12 Months Ended
	Aug. 31, 2019	Sep. 30, 2018	Aug. 31, 2018	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Share-Based Compensation Awards
Issuance of common stock to nonemployees (in shares)				400,000	544,000
Nonemployees \| Individual with stock grant agreement with issuance at IPO
Share-Based Compensation Awards
Issuance of common stock to nonemployees (in shares)		96,000
Nonemployees \| Researchers, Employees of MSK
Share-Based Compensation Awards
Issuance of common stock to nonemployees (in shares)	400,000		448,000			400,000	448,000	448,000	448,000	560,000

SHARE-BASED COMPENSATION - 2015 Plan (Details) - 2015 Plan	9 Months Ended
	Sep. 30, 2019 shares
Share-Based Compensation Awards
Shares reserved for issuance pursuant to the plan	4,500,000
Shares available for grant	0
Stock options \| Employees and nonemployees owning less than 10% of voting power
Share-Based Compensation Awards
Vesting period	4 years

SHARE-BASED COMPENSATION - 2018 Plan (Details)	3 Months Ended	9 Months Ended
SHARE-BASED COMPENSATION - 2018 Plan (Details)	Sep. 30, 2019 shares	Sep. 30, 2019 shares
Share-Based Compensation Awards
Options granted (in shares)	0	356,000
2018 Plan
Share-Based Compensation Awards
Shares reserved for issuance pursuant to the plan	5,500,000	5,500,000
Annual increase on share reserve (as a percent)		4.00%
2018 Plan \| Stock options \| Employees and nonemployees owning less than 10% of voting power
Share-Based Compensation Awards
Term of award		10 years
Vesting period		4 years
2018 Plan \| Stock options \| Participants owning more than 10% of voting power
Share-Based Compensation Awards
Term of award		5 years
2018 Plan \| Stock options \| Participants owning more than 10% of voting power \| Minimum
Share-Based Compensation Awards
Ownership (as a percent)	10.00%	10.00%
Option price as percentage of fair market value of common stock on the date of grant		110.00%

SHARE-BASED COMPENSATION - Expense (Details) - Stock options - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Employees and directors
Share-Based Compensation Awards
Stock-based compensation expenses	$ 1,324	$ 395
Employees and directors \| Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses	263	67		$ 192
Employees and directors \| General and administrative expense
Share-Based Compensation Awards
Stock-based compensation expenses	$ 1,061	$ 328		694
Employees, non-employees, directors
Share-Based Compensation Awards
Stock-based compensation expenses			$ 3,124	1,471
Employees, non-employees, directors \| Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses			577
Employees, non-employees, directors \| General and administrative expense
Share-Based Compensation Awards
Stock-based compensation expenses			$ 2,547
Nonemployees \| Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses				$ 585

SHARE-BASED COMPENSATION - Stock options (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Options
Outstanding at beginning of period (in shares)			3,357,873
Granted (in shares)	0		356,000
Outstanding at end of period (in shares)	3,713,873		3,713,873		3,357,873
Exercisable at end of period (in shares)	2,243,931		2,243,931
Weighted average exercise price
Outstanding at beginning of period (in dollars per share)			$ 7.74
Granted (in dollars per share)			21.50
Outstanding at end of period (in dollars per share)	$ 9.06		9.06		$ 7.74
Exercisable at end of period (in dollars per share)	$ 4.04		$ 4.04
Aggregate intrinsic value and Weighted average remaining contractual life (years)
Outstanding (in dollars)	$ 63,144		$ 63,144		$ 43,224
Exercisable (in dollars)	$ 49,408		$ 49,408
Outstanding (in years)			7 years 4 months 21 days		7 years 10 months 24 days
Exercisable (in years)			6 years 5 months 5 days
Weighted average grant-date fair value of stock options granted (in dollars per share)		$ 7.54	$ 12.34	$ 6.77

SHARE-BASED COMPENSATION - Unrecognized compensation (Details) - Stock options - Employees - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2019	Dec. 31, 2018
Unrecognized compensation related to employee stock options
Unrecognized compensation	$ 12.1	$ 10.8
Expected vesting period	2 years 7 months 17 days	2 years 9 months 26 days

SHARE-BASED COMPENSATION - Restricted Stock Unit Activity (Details) - RSUs - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Share-Based Compensation Awards
Stock-based compensation expenses	$ 21	$ 0	$ 56	$ 0
Restricted stock units issued and outstanding
Granted (in shares)			10,296
Outstanding and expected to vest at end of period (in shares)	10,296		10,296
Weighted average fair value of units granted in period (in dollars per share)	$ 23.11		$ 23.11
Unrecognized compensation	$ 183		$ 183
Expected vesting period			2 years 3 months 18 days
Research and development expense
Share-Based Compensation Awards
Stock-based compensation expenses	18		$ 50
General and administrative expense
Share-Based Compensation Awards
Stock-based compensation expenses	$ 2		$ 6

RELATED PARTY TRANSACTIONS (Details) - MSK - MSK License Agreement, the CD33 License Agreement, CTA, CFAs, SRA and MDSA - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2018
Related party transactions
Expenses	$ 1,384	$ 914	$ 5,307	$ 4,526
Due to related parties	$ 3,829		$ 3,829		$ 4,475

INCOME TAXES - Expense (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
INCOME TAXES - Expense (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income tax expense
Current income taxes	$ 0	$ 0	$ 0	$ 0
Deferred income taxes	0	0	0	0
Losses before taxes	$ (23,900)	$ (11,400)	$ (57,900)	$ (29,200)

SUBSEQUENT EVENTS (Details) - USD ($) $ / shares in Units, $ in Thousands		9 Months Ended
	Nov. 08, 2019	Sep. 30, 2018
Subsequent Events
Proceeds from issuance of common stock, net of issuance costs		$ 101,607
Subsequent Event \| Follow-on shelf public offering
Subsequent Events
Shares issued (in shares)	5,134,750
Share price (in dollars per share)	$ 28
Proceeds from issuance of common stock, net of issuance costs	$ 134,700

OTHER BENEFITS	9 Months Ended
OTHER BENEFITS	Sep. 30, 2019
OTHER BENEFITS
OTHER BENEFITS	NOTE 11—OTHER BENEFITS On October 1, 2018, the Company adopted a defined contribution 401(k) savings plan (the 401(k) plan) covering all U.S. employees of the Company. Participants may elect to defer a percentage of their pretax or after-tax compensation to the 401(k) plan, subject to defined limitations. The plan allows for a discretionary match by the Company. The Company made no matching contributions to the plan during the three or nine months ended September 30, 2019 and 2018. The Company has established a retirement program for employees of the Company’s Danish subsidiary pursuant to which all such employees can contribute an amount at their election from their base compensation and may receive contributions from our Danish subsidiary. Contributions from our Danish subsidiary were immaterial during the three and nine months ended September 30, 2019 and 2018. In addition, health insurance benefits for our Danish employees are fully paid for by such employees. Our Danish subsidiary does not incur any costs for these health insurance benefits.

SUBSEQUENT EVENTS	9 Months Ended
SUBSEQUENT EVENTS	Sep. 30, 2019
SUBSEQUENT EVENTS
SUBSEQUENT EVENTS	NOTE 12-SUBSEQUENT EVENTS In November 2019, the Company completed a follow-on shelf public offering of 5,134,750 shares of its common stock, including underwriters overallotment option, at a public offering price of $28 per share for an aggregate consideration of $134.7 million net of issuance costs.

ORGANIZATION AND DESCRIPTION OF BUSINESS (Details)	9 Months Ended
ORGANIZATION AND DESCRIPTION OF BUSINESS (Details)	Sep. 30, 2019 agreement
ORGANIZATION AND DESCRIPTION OF BUSINESS
Number of product development programs for which entity has obtained exclusive rights	2

LICENSE AGREEMENTS AND COMMITMENTS (Details)	Sep. 30, 2019 agreement
LICENSE AGREEMENTS AND COMMITMENTS
Number of license agreements	2

OTHER BENEFITS (Details)	3 Months Ended		9 Months Ended
OTHER BENEFITS (Details)	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
OTHER BENEFITS
Employer matching contribution to defined contribution plan (as a percent)	0.00%	0.00%	0.00%	0.00%