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Stock-Based Compensation (Tables) | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2020 | Dec. 31, 2019
Share-based Payment Arrangement [Abstract] |  |
Summary of Stock-Based Compensation Expense | Total stock-based compensation expense is recognized for stock options granted to employees and non-employees and has been reported in the Company’s condensed consolidated condensed statements of operations and comprehensive loss as follows:
|  | Three Months Ended March 31, |
|  | 2020 |  |  | 2019 |
Research and development |  | $ | 400 |  |  | $ | 277 |
General and administrative |  |  | 575 |  |  |  | 386 |
Total stock-based compensation expense |  | $ | 975 |  |  | $ | 663 |
| Total stock-based compensation expense is recognized for stock-based awards granted to employees and non-employees and has been reported in the Company’s consolidated statements of operations and comprehensive loss as follows:
|  | Year Ended December 31, |
|  | 2019 |  |  | 2018 |  |  | 2017 |
|  | (In thousands) |
Research and development |  | $ | 1,542 |  |  | $ | 1,236 |  |  | $ | 246 |
General and administrative |  |  | 2,152 |  |  |  | 1,557 |  |  |  | 224 |
Total stock-based compensation expense |  | $ | 3,694 |  |  | $ | 2,793 |  |  | $ | 470 |
Summary of Stock Option Activity | The following table summarizes stock option activity under the Plans:
|  | Shares Available for Grant |  |  | Number of Options Outstanding |  |  | Weighted- Average Exercise Price per Option |  |  | Weighted- Average Remaining Contract Term |  |  | Aggregate Intrinsic Value |
|  |  |  |  |  |  |  |  |  |  |  |  |  | (In thousands) |
Outstanding, December 31, 2019 |  |  | 215,043 |  |  |  | 2,562,800 |  |  | $ | 7.85 |  |  |  | 8.84 |  |  |  |  |
Shares reserved for issuance |  |  | 1,457,789 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options cancelled |  |  | 260,365 |  |  |  | (260,365 | ) |  |  | 8.57 |  |  |  |  |  |  |  |  |
Restricted stock units cancelled |  |  | 74,053 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Outstanding, March 31, 2020 |  |  | 2,007,250 |  |  |  | 2,302,435 |  |  |  | 7.76 |  |  |  | 8.55 |  |  | $ | 21 |
Exercisable, March 31, 2020 |  |  |  |  |  |  | 577,853 |  |  |  | 10.75 |  |  |  | 7.17 |  |  |  | 12 |
Vested and expected to vest, March 31, 2020 |  |  |  |  |  |  | 2,302,435 |  |  |  | 7.76 |  |  |  | 8.55 |  |  |  | 21 |
| The following table summarizes stock option activity under the Plan:
|  | Shares Available for Grant |  |  | Number of Options Outstanding |  |  | Weighted- Average Exercise Price per Option ($) |  |  | Weighted- Average Remaining Contract Term (Years) |  |  | Aggregate Intrinsic Value ($) |
|  | (In thousands) |
Outstanding, December 31, 2018 |  |  | 1,350,582 |  |  |  | 1,122,677 |  |  |  | 11.63 |  |  |  | 9.22 |  |  |  |  |
Shares reserved for issuance |  |  | 1,122,213 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options granted |  |  | (1,896,527 | ) |  |  | 1,896,527 |  |  |  | 6.28 |  |  |  |  |  |  |  |  |
Restricted stock units granted |  |  | (813,335 | ) |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Options exercised |  |  | — |  |  |  | (7,029 | ) |  |  | 0.79 |  |  |  |  |  |  |  |  |
Options forfeited |  |  | 449,375 |  |  |  | (449,375 | ) |  |  | 10.77 |  |  |  |  |  |  |  |  |
Outstanding, December 31, 2019 |  |  | 212,308 |  |  |  | 2,562,800 |  |  |  | 7.85 |  |  |  | 8.84 |  |  |  | 200 |
Exercisable, December 31, 2019 |  |  |  |  |  |  | 461,150 |  |  |  | 11.40 |  |  |  | 7.06 |  |  |  | 37 |
Vested and expected to vest, December 31, 2019 |  |  |  |  |  |  | 2,562,800 |  |  |  | 7.85 |  |  |  | 8.84 |  |  |  | 200 |
Schedule of Assumptions to Estimate Fair Value of Stock Options for Employees and Non-Employees Using Black-Scholes Option Pricing Model | The fair value of stock options for employees and non-employees was estimated using a Black-Scholes option pricing model with the following assumptions:
|  | Three Months Ended March 31,
|  | 2020 |  | 2019
Employees: |  |  |  |
Fair value of common stock |  | N/A |  | $8.53 - $8.90
Expected term (in years) |  | N/A |  | 6.1
Expected volatility |  | N/A |  | 93.7% - 94.8%
Risk-free interest rate |  | N/A |  | 2.5% - 2.6%
Expected dividend yield |  | N/A |  | 0.0%
Non-employees: |  |  |  |
Fair value of common stock |  | $0.96 - $1.07 |  | $6.82 - $8.61
Expected term (in years) |  | 7.2 - 9.0 |  | 8.2 - 10.0
Expected volatility |  | 99.6% - 101.7% |  | 91.3% - 94.9%
Risk-free interest rate |  | 0.6% - 0.9% |  | 2.4% - 2.6%
Expected dividend yield |  | 0.0% |  | 0.0%
| The fair value of stock options for employees and non-employees was estimated using a Black-Scholes option pricing model with the following assumptions:
|  | Year Ended December 31, |
|  | 2019 |  |  | 2018 |  |  | 2017 |
Employees: |  |  |  |  |  |  |  |  |  |  |  |
Fair value of common stock |  | $ | 1.27 - $10.66 |  |  | $ | 8.57 - $15.45 |  |  | $ | 0.79 - $9.33 |
Expected term (in years) |  |  | 5.5 - 6.1 |  |  |  | 5.8 - 6.2 |  |  |  | 5.9 - 6.2 |
Expected volatility |  |  | 92.0% - 104.9 | % |  |  | 75.9% - 90.6 | % |  |  | 74.4% - 74.5 | %
Risk-free interest rate |  |  | 1.4% - 2.6 | % |  |  | 2.4% - 3.1 | % |  |  | 1.9% - 2.2 | %
Expected dividend yield |  |  | 0.0 | % |  |  | 0.0 | % |  |  | 0.0 | %
Non-employees: |  |  |  |  |  |  |  |  |  |  |  |
Fair value of common stock |  | $ | 1.23 - $10.26 |  |  | $ | 8.62 - $15.45 |  |  | $ | 0.79 - $10.28 |
Expected term (in years) |  |  | 7.4 - 10.0 |  |  |  | 8.4 - 10.0 |  |  |  | 9.4 - 10.0 |
Expected volatility |  |  | 89.7% - 99.5 | % |  |  | 78.0% - 91.2 | % |  |  | 74.6% - 77.0 | %
Risk-free interest rate |  |  | 1.7% - 2.8 | % |  |  | 2.7% - 3.1 | % |  |  | 2.3% - 2.4 | %
Expected dividend yield |  |  | 0.0 | % |  |  | 0.0 | % |  |  | 0.0 | %
Summary of Restricted Stock Activity |  | The summary of restricted stock activity and related information follows:
|  | Number of Restricted Shares Outstanding |
Unvested shares—December 31, 2018 |  |  | 1,000 |
Vested |  |  | (1,000 | )
Unvested shares—December 31, 2019 |  |  | — |
Summary of Restricted Stock Units Activity | The summary of restricted stock unit activity and related information follows:
|  | Number of Restricted Stock Units Outstanding |
Unvested shares—December 31, 2019 |  |  | 828,935 |
Vested, net of shares withheld for taxes |  |  | (1,019 | )
Cancelled |  |  | (74,053 | )
Unvested shares—March 31, 2020 |  |  | 753,863 |
| The summary of restricted stock unit activity and related information follows:
|  | Number of Restricted Stock Units Outstanding |
Unvested shares—December 31, 2018 |  |  | 24,960 |
Granted |  |  | 813,335 |
Vested |  |  | (9,360 | )
Unvested shares—December 31, 2019 |  |  | 828,935 |

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