Consolidated Statements of Cash Flows - USD ($) $ in Thousands	9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018
Cash flows from operating activities:
Net loss	$ (60,355)	$ (42,579)
Adjustments to reconcile net loss to net cash used in operating activities
Stock-based compensation expense	6,806	5,534
Non-cash interest expense	2,469	265
Change in fair value of derivative liability	(7,334)
Depreciation and amortization expense	1,856	1,704
(Gain)/loss on disposal of property and equipment	7
Changes in operating assets and liabilities:
Accounts receivable	(913)	(14)
Prepaid expenses and other current assets	(500)	382
Inventory	(678)	20
Operating lease assets	552
Accounts payable	32	(1,003)
Accrued expenses and deferred rent	543	112
Operating lease liabilities	(596)
Net cash used in operating activities	(58,111)	(35,579)
Cash flows from investing activities:
Purchases of property and equipment	(1,637)	(1,410)
Net cash used in investing activities	(1,637)	(1,410)
Cash flows from financing activities:
Proceeds from issuance of 2026 convertible notes, net of issuance costs	37,275
Proceeds from exercise of stock options	34	346
Proceeds from issuance of common stock pursuant to employee stock purchase plan	294	119
Proceeds from issuance of common stock offering, net	28,647	55,961
Repayment of notes payable		(4,114)
Net cash provided by financing activities	66,250	52,312
Net increase in cash, cash equivalents and restricted cash	6,502	15,323
Cash, cash equivalents and restricted cash at beginning of period	60,676	43,152
Cash, cash equivalents and restricted cash at end of period	67,178	58,475
Supplemental disclosure of non-cash investing and financing activities:
Additional right of use asset and related lease liability	2,044
Additions to property and equipment included in accounts payable and accrued expenses at balance sheet dates	464	198
Derivative liability in connection with issuance of 2026 convertible notes	$ 16,434
Public offering costs included in accounts payable and accrued expenses at balance sheet dates		$ 11