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Liabilities, Convertible Preferred Stock and Stockholders' (Deficit) Equity (Tables) | 9 Months Ended | 12 Months Ended
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Sep. 30, 2023 | Dec. 31, 2022
Share-Based Payment Arrangement [Abstract] |  |
Summary of Employee and Non-Employee Stock Option Activity |  | The following table summarizes employee and non-employee stock option activity for the year ended December 31, 2022:
|  | Shares Issuable Under Options |  |  | Weighted Average Exercise Price |  |  | Weighted Average Remaining Contractual Term |  |  | Aggregate Intrinsic Value |
|  |  |  |  |  |  |  | (in years) |  |  | (in thousands) |
Outstanding as of December 31, 2021 |  |  | 264,303 |  |  | $ | 189.00 |  |  |  | 7.90 |  |  | $ | 328 |
Granted |  |  | 235,199 |  |  |  | 45.00 |  |  |  |  |  |  |  |  |
Exercised |  |  | — |  |  |  | — |  |  |  |  |  |  |  |  |
Forfeited |  |  | (94,421 | ) |  |  | 153.50 |  |  |  |  |  |  |  |  |
Outstanding as of December 31, 2022 |  |  | 405,081 |  |  | $ | 113.75 |  |  |  | 6.72 |  |  | $ | 2 |
Options vested and expected to vest as of December 31, 2022 |  |  | 397,772 |  |  | $ | 112.50 |  |  |  | 6.78 |  |  | $ | 2 |
Options exercisable as of December 31, 2022 |  |  | 163,450 |  |  | $ | 171.00 |  |  |  | 4.93 |  |  | $ | — |
Summary of Employee Restricted Stock Activity |  | The following table summarizes employee restricted stock activity for the year ended December 31, 2022:
|  | Shares |  |  | Weighted Average Grant Date Fair Value |
Unvested restricted stock units as of December 31, 2021 |  |  | 7,600 |  |  | $ | 203.25 |
Granted |  |  | — |  |  |  | — |
Vested |  |  | — |  |  |  | — |
Forfeited |  |  | (1,940 | ) |  |  | 203.25 |
Unvested restricted stock units as of December 31, 2022 |  |  | 5,660 |  |  | $ | 203.25 |
Stock-Based Compensation Expense | Total stock-based compensation expense related to all plans was as follows (in thousands):
|  | Three Months Ended September 30, |  |  | Nine Months Ended September 30, |
|  | 2023 |  |  | 2022 |  |  | 2023 |  |  | 2022 |
Research and development (1) |  | $ | 2,965 |  |  | $ | 639 |  |  | $ | 4,136 |  |  | $ | 2,031 |
General and administrative |  |  | 1,820 |  |  |  | 926 |  |  |  | 4,269 |  |  |  | 3,653 |
Total stock-based compensation expense |  | $ | 4,785 |  |  | $ | 1,565 |  |  | $ | 8,405 |  |  | $ | 5,684 |
(1) | For the three and nine months ended September 30, 2023, $2.7 million and $2.9 million, respectively, was recognized as stock compensation expense related to the Parapyre Option Obligation. There were no such expenses for the three and nine months ended September 30, 2022.
| Total stock-based compensation expense recognized from the Company’s equity incentive plans, 2018 Plan, and the 2016 ESPP for the years ended December 31, 2022, 2021, and 2020 was as follows (in thousands):
|  | Year Ended December 31, |
|  | 2022 |  |  | 2021 |  |  | 2020 |
|  | Employees |  |  | Non- Employees |  |  | Employees |  |  | Non- Employees |  |  | Employees |  |  | Non- Employees |
Research and development |  | $ | 2,591 |  |  | $ | — |  |  | $ | 2,723 |  |  | $ | — |  |  | $ | 2,168 |  |  | $ | 36 |
General and administrative |  |  | 4,520 |  |  |  | — |  |  |  | 5,315 |  |  |  | — |  |  |  | 4,052 |  |  |  | — |
Total stock-based compensation expense |  | $ | 7,111 |  |  | $ | — |  |  | $ | 8,038 |  |  | $ | — |  |  | $ | 6,220 |  |  | $ | 36 |
Assumptions used to Estimate the Fair Value of Stock Options Granted | The following table summarizes the weighted-average Black-Scholes option pricing model assumptions used to estimate the fair value of stock options granted under the Company’s 2016 Plan, and the shares purchasable under the 2016 ESPP during the periods presented:
|  | Three Months Ended September 30, |  |  | Nine Months Ended September 30, |
|  | 2023 |  |  | 2022 |  |  | 2023 |  |  | 2022 |
2016 Plan |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 6.08 |  |  |  | 6.02 |  |  |  | 6.04 |  |  |  | 5.96 |
Expected volatility |  |  | 101 | % |  |  | 85 | % |  |  | 111 | % |  |  | 83 | %
Risk-free interest |  |  | 4.28 | % |  |  | 3.16 | % |  |  | 4.07 | % |  |  | 2.43 | %
Dividend yield |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
2016 ESPP |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 0.50 |  |  |  | 0.50 |  |  |  | 0.49 |  |  |  | 0.49 |
Expected volatility |  |  | 222 | % |  |  | 95 | % |  |  | 181 | % |  |  | 84 | %
Risk-free interest |  |  | 5.29 |  |  |  | 3.26 |  |  |  | 4.99 | % |  |  | 1.95 | %
Dividend yield |  |  | — |  |  |  | — |  |  |  | — |  |  |  | — |
| The following table summarizes the weighted-average assumptions used in calculating the fair value of the awards:
|  | Year Ended December 31, |
|  | 2022 |  |  | 2021 |  |  | 2020 |
Stock Options Granted |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 6.00 |  |  |  | 5.99 |  |  |  | 6.10 |
Expected volatility |  |  | 84 | % |  |  | 83 | % |  |  | 76 | %
Risk-free interest |  |  | 2.93 | % |  |  | 0.88 | % |  |  | 1.06 | %
Dividend yield |  |  | 0 | % |  |  | 0 | % |  |  | 0 | %
2016 ESPP |  |  |  |  |  |  |  |  |  |  |  |
Expected term (in years) |  |  | 0.49 |  |  |  | 0.50 |  |  |  | 0.50 |
Expected volatility |  |  | 84 | % |  |  | 86 | % |  |  | 76 | %
Risk-free interest |  |  | 1.95 | % |  |  | 0.08 | % |  |  | 0.75 | %
Dividend yield |  |  | 0 | % |  |  | 0 | % |  |  | 0 | %
Pre-funded Warrants for Common Stock Issued and Outstanding | As of September 30, 2023, the following pre-funded warrants for common stock were issued and outstanding:
Issue Date |  | Expiration Date |  |  | Exercise Price |  |  | Number of Warrants Outstanding |
February 8, 2019 |  |  | None |  |  | $ | 0.0025 |  |  |  | — |
April 30, 2020 |  |  | None |  |  | $ | 0.0025 |  |  |  | — |
May 20, 2022 |  |  | None |  |  | $ | 0.0025 |  |  |  | 250,000 |
Total pre-funded warrants |  |  |  |  |  |  |  |  |  |  | 250,000 |
| As of December 31, 2022, the following pre-funded warrants to purchase common stock were issued and outstanding:
Issue Date |  | Expiration Date |  |  | Exercise Price |  |  | Number of Warrants Outstanding |
February 2019 |  |  | None |  |  | $ | 0.0025 |  |  |  | 150,000 |
April 2020 |  |  | None |  |  | $ | 0.0025 |  |  |  | 474,413 |
May 2022 |  |  | None |  |  | $ | 0.0025 |  |  |  | 531,250 |
Total pre-funded warrants |  |  |  |  |  |  |  |  |  |  | 1,155,663 |
Stock Awards Granted | The following table summarizes the Company’s stock awards granted under all plans for each of the periods indicated:
|  | Three Months Ended September 30, |  |  | Nine Months Ended September 30, |
|  | 2023 |  |  | 2022 |  |  | 2023 |  |  | 2022 |
|  | Grants |  |  | Weighted Average Grant Date Fair Value |  |  | Grants |  |  | Weighted Average Grant Date Fair Value |  |  | Grants |  |  | Weighted Average Grant Date Fair Value |  |  | Grants |  |  | Weighted Average Grant Date Fair Value |
Stock options |  |  | 1,044,667 |  |  | $ | 14.50 |  |  |  | 50,806 |  |  | $ | 16.75 |  |  |  | 3,867,366 |  |  | $ | 9.65 |  |  |  | 153,686 |  |  | $ | 52.50 |

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