Condensed Balance Sheets (Current Period Unaudited) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Assets
Cash and cash equivalents	$ 13,438	$ 18,409
Marketable securities		3,260
Prepaid expenses and other current assets	1,873	2,052
Total current assets	15,311	23,721
Property and equipment, net	1,688	1,753
Operating lease right-of-use assets	1,785
Restricted cash	528	710
Other long-term assets	555	562
Total assets	19,867	26,746
Liabilities and stockholders' deficit
Accounts payable and accrued expenses	8,178	8,342
Current portion of convertible notes	6,370	3,333
Current portion of capital lease obligation		214
Current portion of operating lease liabilities	395
Other current liabilities	229	115
Total current liabilities	15,172	12,004
Convertible notes, less current portion, net	75,373	78,105
Derivative and warrant liability	1,665	2,118
Capital lease obligation, less current portion		396
Operating lease liabilities, less current portion	2,142
Other long-term liabilities	351	689
Total liabilities	94,703	93,312
Commitments and contingencies (Note D)
Stockholders’ deficit:
Common stock, $0.0001 par value, 250,000,000 shares authorized, 27,976,823 shares issued and outstanding as of March 31, 2019 (unaudited); 26,455,352 shares issued and outstanding as of December 31, 2018	3	3
Additional paid-in capital	158,644	154,623
Accumulated deficit	(233,483)	(221,192)
Total stockholders' deficit	(74,836)	(66,566)
Total liabilities and stockholders' deficit	19,867	26,746
Series A Convertible Preferred Stock [Member]
Stockholders’ deficit:
Preferred stock
Undesignated Preferred Stock [Member]
Stockholders’ deficit:
Preferred stock