Note 8 - Stock-based Compensation (Tables)

12 Months Ended

Dec. 31, 2020

Notes Tables

Share-based Payment Arrangement, Expensed and Capitalized, Amount [Table Text Block]

	Year Ended December 31,
	2020		2019		2018
Research and development	$	10,222	$	13,977	$	6,867
Selling, general and administrative		4,572		3,742		2,529
Total stock-based compensation expense	$	14,794	$	17,719	$	9,396

Share-based Payment Arrangement, Activity [Table Text Block]

							Weighted
							Average
	Awards			Options			Exercise
	Available			Outstanding			Price
Balance at December 31, 2017		468			14,452		$	6.06
Plan amendment		4,400			−			−
Restricted stock awards granted		(13	)		−			−
Stock option awards granted		(4,272	)		4,272			7.15
Stock option awards exercised		−			(1,011	)		2.92
Stock option awards cancelled		222			(222	)		7.44
Balance at December 31, 2018		805			17,491			6.49
Plan amendment		4,000			−			−
Restricted stock awards granted		(27	)		−			−
Stock option awards granted		(4,511	)		4,511			3.91
Stock option awards exercised		−			(251	)		3.75
Stock option awards cancelled		701			(701	)		6.82
Balance at December 31, 2019		968			21,050			5.96
Plan amendment		8,000			−			−
Restricted stock awards granted		(31	)		−			−
Stock option awards granted		(7,469	)		7,469			8.06
Stock option awards exercised		−			(510	)		3.56
Stock option awards cancelled		3,124			(3,124	)		6.93
Balance at December 31, 2020		4,592			24,885		$	6.52

							Weighted
							Average
	Awards			Options			Exercise
	Available			Outstanding			Price
Balance at December 31, 2019		171			1,329		$	3.60
Plan amendment		2,900			−			−
Stock option awards granted		(3,002	)		3,002			4.02
Stock option awards cancelled		160			(160	)		4.15
Balance at December 31, 2020		229			4,171		$	3.88

Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block]

	2020			2019			2018
Expected Life		5.5			5.5			5.5
Expected Volatility		84	%		81	%		82	%
Expected Dividend Yield		0.0	%		0.0	%		0.0	%
Risk-Free Interest Rate		0.4	%		1.8	%		2.7	%

Share-based Compensation Arrangement by Share-based Payment Award, Options, Vested and Expected to Vest, Outstanding and Exercisable [Table Text Block]

					Outstanding					Exercisable
						Weighted		Weighted				Weighted
						Average		Average				Average
						Remaining		Exercise				Exercise
Range				Number		Life		Price		Number		Price
$	0	to	3		2,514		6.1	$	2.19		1,414	$	1.25
	3	to	6		12,689		7.0		4.22		6,854		2.48
	6	to	9		10,852		9.2		7.88		2,005		4.89
	9	to	12		2,347		4.5		10.85		2,129		4.51
	12	to	15		559		4.1		12.30		539		6.43
	15	to	18		95		4.5		15.39		95		10.57
$	0	to	18		29,056		7.5	$	6.14		13,036	$	3.27

Share-based Payment Arrangement, Option, Activity [Table Text Block]

	Non-Vested Stock Option Awards			Weighted Average Grant-Date Fair Value
Balance December 31, 2019		11,688		$	3.77
Stock option awards granted		10,471			4.69
Stock option awards vested		(4,543	)		4.12
Stock option awards forfeited		(1,596	)		3.96
Balance December 31, 2020		16,020		$	4.25