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Share-based Compensation (Tables)

3 Months Ended

12 Months Ended

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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Tabular disclosure of allocation of amount expensed and capitalized for award under share-based payment arrangement to statement of income or comprehensive income and statement of financial position. Includes, but is not limited to, corresponding line item in financial statement.

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Tabular disclosure of the changes in outstanding nonvested restricted stock units.

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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.

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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.

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