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Share-based Compensation (Tables) | 3 Months Ended | 12 Months Ended
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Mar. 31, 2024 | Dec. 31, 2023
Share-Based Payment Arrangement [Abstract] |  |
Schedule of Estimated Weighted Average Grant Date Fair Value Per Share |  | The fair values of employee stock options granted in the year ended December 31, 2022 were estimated at the date of grant using the Black-Scholes model with the following assumptions and had the following estimated weighted average grant date fair value per share:
|  | Year Ended December 31, 2022
Expected term |  |  | 5.5 years |
Expected volatility |  |  | 107 | %
Risk-free interest rate |  |  | 3.56 | %
Expected dividend yield |  |  | 0 | %
Weighted-average grant date fair value per share |  | $ | 1.87 |
Schedule of Stock Option Activities | Stock option transactions for the three months ended March 31, 2024 under ARCA’s stock incentive plans were as follows:
|  | Options Outstanding
|  | Number of Options |  | Weighted Average Exercise Price |  | Weighted Average Remaining Contractual Term (in years)
Options outstanding – December 31, 2023 |  | 616,707 |  |  | $ | 4.76 |  | 7.35
Granted |  | 42,000 |  |  |  | 1.64 |  |
Exercised |  | — |  |  |  | — |  |
Forfeited and cancelled |  | (12,862 | ) |  |  | 17.45 |  |
Options outstanding – March 31, 2024 |  | 645,845 |  |  | $ | 4.30 |  | 6.82
Options exercisable – March 31, 2024 |  | 502,517 |  |  | $ | 4.71 |  | 6.55
Options vested and expected to vest – March 31, 2024 |  | 645,816 |  |  | $ | 4.30 |  | 6.82
| A summary of ARCA’s stock option activities for the years ended December 31, 2023 and 2022, and related information as of December 31, 2023, is as follows:
|  | Options Outstanding
|  | Number of Options |  | Weighted Average Exercise Price |  | Weighted Average Remaining Contractual Term (in years) |  | Aggregate Intrinsic Value (in thousands)
Options outstanding – December 31, 2021 |  | 904,123 |  |  | $ | 5.59 |  |  |  |  |
Granted |  | 60,000 |  |  |  | 2.31 |  |  |  |  |
Exercised |  | — |  |  |  | — |  |  |  |  |
Forfeited and cancelled |  | (259,163 | ) |  |  | 4.74 |  |  |  |  |
Options outstanding – December 31, 2022 |  | 704,960 |  |  | $ | 5.62 |  | 8.29 |  | $ | 18
Granted |  | — |  |  |  | — |  |  |  |  |
Exercised |  | — |  |  |  | — |  |  |  |  |
Forfeited and cancelled |  | (88,253 | ) |  |  | 11.68 |  |  |  |  |
Options outstanding – December 31, 2023 |  | 616,707 |  |  | $ | 4.76 |  | 7.35 |  | $ | —
Options exercisable – December 31, 2023 |  | 463,068 |  |  | $ | 5.28 |  | 7.29 |  | $ | —
Options vested and expected to vest – December 31, 2023 |  | 616,661 |  |  | $ | 4.76 |  | 7.35 |  | $ | —
Schedule of RSU Activity |  | A summary of RSU activity for the year ended December 31, 2023 is presented below:
|  | Restricted Stock Units Outstanding
|  | Number of Shares |  | Weighted Average Grant Date Fair Value
RSUs outstanding – December 31, 2021 |  | — |  |  | $ | —
Granted |  | 91,000 |  |  |  | 2.21
Vested and released |  | — |  |  |  | —
Forfeited and cancelled |  | — |  |  |  | —
RSUs outstanding – December 31, 2022 |  | 91,000 |  |  | $ | 2.21
Granted |  | — |  |  |  | —
Vested and released |  | (91,000 | ) |  |  | 2.21
Forfeited and cancelled |  | — |  |  |  | —
RSUs outstanding – December 31, 2023 |  | — |  |  | $ | —
Schedule of Non-cash, Share-based Compensation Expense | For the three months ended March 31, 2024 and 2023, ARCA recognized the following non-cash, share-based compensation expense in the statements of operations (in thousands):
|  | Three Months Ended March 31,
|  | 2024 |  | 2023
General and administrative |  | $ | 114 |  | $ | 163
Research and development |  |  | — |  |  | 41
Total |  | $ | 114 |  | $ | 204
| For the years ended December 31, 2023 and 2022, ARCA recognized the following non-cash, share-based compensation expense (in thousands):
|  | Years Ended December 31,
|  | 2023 |  | 2022
General and administrative |  | $ | 526 |  | $ | 423
Research and development |  |  | 160 |  |  | 133
Total |  | $ | 686 |  | $ | 556

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- Definition Tabular disclosure for stock option plans. Includes, but is not limited to, outstanding awards at beginning and end of year, grants, exercises, forfeitures, and weighted-average grant date fair value. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 718 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (c)(1) -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (d) -SubTopic 10 -Topic 718 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (e) -SubTopic 10 -Topic 718 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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- Definition Tabular disclosure of the significant assumptions used during the year to estimate the fair value of stock options, including, but not limited to: (a) expected term of share options and similar instruments, (b) expected volatility of the entity's shares, (c) expected dividends, (d) risk-free rate(s), and (e) discount for post-vesting restrictions. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 718 -SubTopic 10 -Subparagraph (f)(2) -Name Accounting Standards Codification -Paragraph 2 -Section 50 -Publisher FASB -URI https://asc.fasb.org/1943274/2147480429/718-10-50-2 + Details
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