Stockholders' Equity (Tables)
|
12 Months Ended |
Dec. 31, 2023 |
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| Summary of Stock Option Activity |
Stock option activity is summarized as follows:
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Number of
Outstanding
Stock Options |
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Weighted-
Average
Exercise Price |
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Weighted
Remaining Contractual Term |
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Aggregate
Intrinsic Value |
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Outstanding as of December 31, 2022 |
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2,956,170 |
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|
$ |
7.45 |
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Granted |
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1,112,885 |
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$ |
2.21 |
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Canceled/forfeited/expired |
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(112,228 |
) |
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$ |
6.42 |
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Outstanding as of December 31, 2023 |
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3,956,827 |
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$ |
6.00 |
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|
7.94 |
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|
$ |
— |
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| Summary of Assumptions Used to Determine Fair Value of Employee Stock Purchase Plan |
The assumptions used in the Black-Scholes option pricing model to determine the fair value of the ESPP offering were as follows:
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Years Ended December 31, |
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2023 |
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2022 |
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2021 |
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Expected term (in years) |
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|
0.50 |
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|
0.50 |
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|
0.50 |
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Risk-free interest rate |
|
4.54% – 5.38% |
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|
0.06% – 4.54% |
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0.04% – 0.12% |
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Expected volatility |
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40.9% – 58.5% |
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51.9% – 95.9% |
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89.7% – 108.12% |
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Expected dividend yield |
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|
0.0 |
% |
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|
0.0 |
% |
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|
0.0 |
% |
|
| Schedule of Restricted Stock Units Activity |
Occasionally, we grant restricted stock units to employees. The fair value of restricted stock is determined by the closing price of our common stock reported on the Nasdaq Capital Market on the date of grant. Restricted stock unit activity is summarized as follows:
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Number of Outstanding
Restricted Stock Units |
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Weighted-Average
Grant Date
Fair Value |
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Balance as of December 31, 2022 |
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|
121,438 |
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|
$ |
5.09 |
|
Released |
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(58,111 |
) |
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$ |
4.69 |
|
Balance as of December 31, 2023 |
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|
63,327 |
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$ |
5.45 |
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| Schedule of Allocation of Stock-Based Compensation for All Options Including Performance Options with Market Condition and Restricted Stock Units |
The allocation of stock-based compensation for all options, including performance options with market condition and restricted stock units is as follows (in thousands):
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Years Ended December 31, |
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2023 |
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2022 |
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2021 |
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Research and development |
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$ |
600 |
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$ |
528 |
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$ |
295 |
|
General and administrative |
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|
2,000 |
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|
1,626 |
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|
1,319 |
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Total stock-based compensation expense |
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$ |
2,600 |
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$ |
2,154 |
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$ |
1,614 |
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| Summary of Warrants Outstanding |
Warrants Warrants outstanding for the purchase of common stock as of December 31, 2023 were as follows:
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Number |
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Exercise Price |
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Expiration |
Outstanding |
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Per Share |
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|
Date |
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2,978 |
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$ |
50.37 |
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June 2024 |
|
2,886 |
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$ |
51.98 |
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December 2024 |
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5,864 |
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| Summary of Common Stock Reserved for Future Issuance |
Common Stock Reserved for Future Issuance Common stock reserved for future issuance was as follows:
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December 31, 2023 |
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Common stock warrants |
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|
5,864 |
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Common stock options and restricted stock units |
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|
4,020,154 |
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Shares available under the 2015 equity incentive plan |
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|
3,964,555 |
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Shares available under the 2022 inducement plan |
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|
95,367 |
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Shares available under the employee stock purchase plan |
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|
727,311 |
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8,813,251 |
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| Employee Stock Option [Member] |
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| Summary of Assumptions Used to Determine Fair Value of Employee Stock Option Grants and Performance Options with Market Condition |
The assumptions used in the Black-Scholes option pricing model to determine the fair value of the employee stock option grants were as follows:
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Years Ended December 31, |
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2023 |
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|
2022 |
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|
2021 |
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Expected term (in years) |
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5.51 – 6.09 |
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|
5.98 – 6.08 |
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5.50 – 6.08 |
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Risk-free interest rate |
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3.58% – 4.5% |
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1.7% – 4.2% |
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0.6% – 1.3% |
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Expected volatility |
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81.5% – 83.0% |
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84.2% – 86.5% |
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86.3% – 104.8% |
|
Expected dividend yield |
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|
0.0 |
% |
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|
0.0 |
% |
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|
0.0 |
% |
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