CONDENSED CONSOLIDATED STATEMENTS OF CASH FLOWS - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2018	Dec. 31, 2017
Cash flows from operating activities:
Net loss	$ (32,703)	$ (26,512)
Adjustments to reconcile net loss to net cash used in operating activities:
Share-based compensation expense	3,679	2,409
Non-cash interest expense	376	94
Depreciation and amortization	281	308
Amortization of premiums and discounts on available-for-sale securities	(416)	0
Changes in operating assets and liabilities:
Accounts receivable	1,432	(1,436)
Prepaid expenses and other assets	51	(724)
Accounts payable	(335)	(101)
Accrued and other liabilities	791	(2,205)
Net cash used in operating activities	(26,844)	(28,167)
Cash flows from investing activities:
Purchases of short-term investments	62,462	0
Maturities of short-term investments	33,000	0
Purchases of property and equipment	(445)	(246)
Cash acquired in reverse merger	0	1,280
Proceeds from sale of property and equipment	0	0
Net cash provided by (used in) investing activities	(29,907)	1,034
Cash flows from financing activities:
Proceeds from stock purchases under employee stock purchase plan	242	110
Proceeds from the exercise of stock options	182	307
Proceeds from issuance of notes payable	0	10,000
Payments of principal on note payable	0	(4,667)
Net cash provided by financing activities	41,916	52,470
Net increase (decrease) in cash and cash equivalents	(14,835)	25,337
Cash and cash equivalents at beginning of period	47,441	22,104
Cash and cash equivalents at end of period	32,606	47,441
Supplemental disclosure of cash flow information
Cash paid for interest	489	446
Supplemental disclosure of non-cash investing and financing activities
Amortization of public offering costs	55	0
Change in unrealized loss on investments	(3)	0
Conversion of preferred stock to common stock	0	76,981
Liabilities assumed, net of non-cash assets received in reverse merger	0	1,076
Issuance of warrant classified as equity	0	127
Reclassification of preferred stock warrant (accrued liability) to common stock warrant (equity)	0	51
Accretion of redeemable convertible preferred stock to redemption value	0	5
Public Offering
Cash flows from financing activities:
Proceeds from the sale of common stock	40,782	0
Payment of issuance costs associated with the sale of common stock	(2,890)	0
Private Financing
Cash flows from financing activities:
Proceeds from the sale of common stock	1,000	40,703
Payment of issuance costs associated with the sale of common stock	(65)	(1,216)
Supplemental disclosure of non-cash investing and financing activities
Transfer of common stock issuance costs from prepaid expenses and other current assets to equity	0	331
At-the-market Offering
Cash flows from financing activities:
Proceeds from the sale of common stock	2,747	7,862
Payment of issuance costs associated with the sale of common stock	(82)	(330)
Supplemental disclosure of non-cash investing and financing activities
Transfer of common stock issuance costs from prepaid expenses and other current assets to equity	0	23
Shelf Registration
Cash flows from financing activities:
Payment of issuance costs associated with the sale of common stock	$ 0	$ (299)