Consolidated Statements of Cash Flows - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019
Cash flows from operating activities:
Net loss	$ (21,512)	$ (17,124)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	1,665	1,942
Non-cash interest expense	1,048	409
Amortization of operating lease asset		187
Change in fair value of derivative liability	3,404	(3,223)
Depreciation and amortization expense	734	589
Changes in operating assets and liabilities:
Accounts receivable	(881)	(75)
Prepaid expenses and other current assets	(234)	(667)
Inventory	(153)	(48)
Operating lease assets	188
Accounts payable	(236)	761
Accrued expenses and deferred rent	(2,617)	(1,891)
Operating lease liabilities	(260)	(246)
Net cash used in operating activities	(18,854)	(19,386)
Cash flows from investing activities:
Purchases of property and equipment	(249)	(725)
Net cash used in investing activities	(249)	(725)
Cash flows from financing activities:
Proceeds from issuance of 2026 convertible notes, net of issuance costs		37,345
Proceeds from exercise of stock options	128	1
Proceeds from issuance of common stock upon public offering, net	12,690	4,954
Net cash provided by financing activities	12,818	42,300
Net (decrease) increase in cash, cash equivalents and restricted cash	(6,285)	22,189
Cash, cash equivalents and restricted cash at beginning of period	56,201	60,676
Cash, cash equivalents and restricted cash at end of period	49,916	82,865
Supplemental disclosure of cash flow information:
Cash paid for interest	585	475
Supplemental disclosure of non-cash investing and financing activities:
Additions to property and equipment included in accounts payable and accrued expenses at balance sheet dates	$ 21	176
Derivative liability in connection with issuance of 2026 convertible notes		$ 14,685