Condensed Consolidated Statements of Cash Flows (Unaudited) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
CASH FLOWS FROM OPERATING ACTIVITIES
Net loss	$ (18,728)	$ (17,167)
Adjustments to reconcile net loss to net cash used in operating activities:
Depreciation and amortization	154	100
Purchase net discount on marketable securities	19	243
Net accretion of discount on marketable securities	(48)	(275)
Stock-based compensation	1,348	1,081
Non-cash operating lease expense	152	69
Changes in operating assets and liabilities:
Prepaid expenses and other assets	(2,696)	(928)
Accounts payable	262	2,289
Operating lease liabilities	(92)	(114)
Accrued and other liabilities	(2,065)	(1,105)
Net cash used in operating activities	(21,694)	(15,807)
CASH FLOWS FROM INVESTING ACTIVITIES
Purchases of property and equipment	(1,389)	(118)
Purchases of marketable securities	(4,000)	(19,750)
Proceeds from maturities of marketable securities	20,142	33,804
Net cash provided by investing activities	14,753	13,936
CASH FLOWS FROM FINANCING ACTIVITIES
Proceeds from issuance of common stock and pre-funded warrants in public offering, net of offering costs		64,662
Proceeds from employee stock plan purchases and stock option exercises	289	399
Principal payments on finance lease obligation	(7)	(6)
Net cash provided by financing activities	282	65,055
NET (DECREASE) INCREASE IN CASH, CASH EQUIVALENTS, AND RESTRICTED CASH	(6,659)	63,184
CASH, CASH EQUIVALENTS, AND RESTRICTED CASH
Beginning of period	20,753	22,461
End of period	14,094	85,645
Supplemental Disclosure of Non-Cash Investing and Financing Information:
Leased assets obtained in exchange for lease obligations	137	$ 776
Unpaid amounts related to purchase of property and equipment	$ 1,057