|
Note 12 - Stock Options and Employee Stock Purchase Plan (Tables)
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12 Months Ended |
|
Dec. 31, 2014
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| Disclosure of Compensation Related Costs, Share-based Payments [Abstract] |
|
| Schedule of Employee Service Share-based Compensation, Allocation of Recognized Period Costs [Table Text Block] |
|
|
|
Years Ended December 31,
|
|
|
|
|
2012
|
|
|
2013
|
|
|
2014
|
|
|
Research and development
|
|
$
|
511,860
|
|
|
$
|
524,495
|
|
|
$
|
1,604,215
|
|
|
General and administrative
|
|
|
531,129
|
|
|
|
528,668
|
|
|
|
1,409,068
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Total stock-based compensation expense
|
|
$
|
1,042,989
|
|
|
$
|
1,053,163
|
|
|
$
|
3,013,283
|
|
|
| Schedule of Share-based Payment Award, Stock Options, Valuation Assumptions [Table Text Block] |
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|
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2012
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|
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2013
|
|
|
2014
|
|
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Risk-free interest rate
|
|
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1.20
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%
|
|
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2.12
|
%
|
|
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2.26
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%
|
|
Expected dividend yield
|
|
|
0
|
%
|
|
|
0
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%
|
|
|
0
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%
|
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Expected option term (in years)
|
|
|
7
|
|
|
|
7
|
|
|
|
7
|
|
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Expected volatility
|
|
|
94
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%
|
|
|
94
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%
|
|
|
96
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%
|
|
| Schedule of Share-based Compensation, Stock Options, Activity [Table Text Block] |
|
|
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Number of
Shares
|
|
|
Weighted
Average Exercise
Price
|
|
|
Weighted
Average
Contractual
Term
(in years)
|
|
|
Aggregate
Intrinsic
Value
|
|
|
Outstanding as of December 31, 2013
|
|
|
1,957,069
|
|
|
$
|
5.48
|
|
|
|
|
|
|
|
|
Granted
|
|
|
967,903
|
|
|
$
|
6.70
|
|
|
|
|
|
|
|
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Exercised
|
|
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(3,050
|
)
|
|
$
|
4.20
|
|
|
|
|
|
|
|
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Cancelled
|
|
|
(74,825
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)
|
|
$
|
4.94
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
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Outstanding as of December 31, 2014
|
|
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2,847,097
|
|
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$
|
5.89
|
|
|
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8.65
|
|
|
$
|
11,796,921
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
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Exercisable as of December 31, 2014
|
|
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897,294
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|
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$
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5.04
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|
|
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7.54
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|
|
$
|
4,509,931
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
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Vested and expected to vest as of December 31, 2014
|
|
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2,716,363
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|
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$
|
5.86
|
|
|
|
8.62
|
|
|
$
|
11,255,225
|
|
|
| Schedule of Share-based Compensation, Shares Authorized under Stock Option Plans, by Exercise Price Range [Table Text Block] |
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Exercise Price or Range of Exercise Price
|
|
Options Outstanding
|
|
|
Weighted Average
Contractual Life
(Years)
|
|
|
Options Exercisable
|
|
|
|
$4.20
|
|
|
|
553,434
|
|
|
|
6.64
|
|
|
|
521,718
|
|
|
|
$5.82
|
|
|
|
1,219,689
|
|
|
|
8.85
|
|
|
|
333,785
|
|
|
$5.98
|
to
|
$6.62
|
|
|
869,525
|
|
|
|
9.50
|
|
|
|
—
|
|
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$7.32
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to
|
$11.09
|
|
|
203,172
|
|
|
|
9.10
|
|
|
|
40,514
|
|
|
$35.30
|
to
|
$36.66
|
|
|
1,271
|
|
|
|
6.70
|
|
|
|
1,271
|
|
|
$135.78
|
to
|
$4,615.92
|
|
|
6
|
|
|
|
0.83
|
|
|
|
6
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
2,847,097
|
|
|
|
8.65
|
|
|
|
897,294
|
|
|