The 2015 Equity Incentive Award Plan (the “2015 Plan”) was adopted by Mirna in 2015 and remains active after the Merger, now functioning as the primary equity plan for the Company. The 2015 Plan provides for the granting of a variety of stock‑based compensation awards, including stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, deferred stock awards, dividend equivalent awards, stock payment awards, performance awards and other stock‑based awards. Pursuant to the evergreen provision of the 2015 Plan, which allows for an annual increase in the number of shares of common stock available for issuance, the Company added 1,270,073 shares to the 2015 Plan on January 1, 2019.

The 2017 Stock Incentive Plan (the “2017 Plan”) was adopted by Private Synlogic in 2017 at the time of the 2017 Reorganization and assumed by the Company during the Merger. The 2017 Plan provides for the grant of incentive stock options, non-qualified stock options, restricted and unrestricted stock awards and other stock-based awards.

As of March 31, 2019, there were 753,164 shares available for future grant under the Company’s two active equity incentive plans, the 2017 Plan and the 2015 Plan.

For a full description of the Company’s equity plans, refer to Note 12, Equity-based Compensation and Equity Incentive Plans in the Company’s 2018 Annual Report.

The following table summarizes stock option activity during the three months ended March 31, 2019 under the 2015 Plan and the 2017 Plan.

	Stock options outstanding
						Weighted
				Weighted		average		Aggregate
				average		remaining		Intrinsic
	Number of			exercise		contractual		value^(a)
	options			price		term (in years)		(in thousands)
Outstanding at December 31, 2018		1,739,884		$	11.92		9.0	$	—
Granted		1,163,980			8.54
Exercised		—			—
Forfeited		(156,821	)		13.84
Outstanding at March 31, 2019		2,747,043			10.38		9.2	$	2,030

Vested or expected to vest at March 31, 2019		2,747,043			10.38		9.2		2,030

Exercisable at March 31, 2019		558,274			13.10		8.5	$	—

(a)

The aggregate intrinsic value is calculated as the difference between the exercise price of the options and the fair market value of the underlying common stock for the options that were in the money at March 31, 2019 and December 31, 2018.

As of March 31, 2019, there was $12.7 million of unrecognized share-based compensation related to unvested stock option grants which is expected to be recognized over a weighted average period of 3.1 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur.

The following table shows restricted stock activity during the three months ended March 31, 2019:

	Restricted stock awards
				Grant date
	Number of			fair value
	shares			(per share)
Unvested at December 31, 2018		118,679		$	13.54
Granted		—			—
Vested		(18,124	)		13.53
Forfeited		(12,836	)		13.53
Unvested at March 31, 2019		87,719		$	13.55

As of March 31, 2019, there was approximately $0.1 million of unrecognized share-based compensation related to restricted stock awards granted, which is expected to be recognized over a weighted average period of 1.4 years. The total unrecognized share-based compensation cost will be adjusted for actual forfeitures as they occur.

The Company recorded total equity-based compensation expense of $1.0 million during the three months ended March 31, 2019, and $0.9 million during the three months ended March 31, 2018.

The following table summarizes equity‑based compensation expense within the Company’s consolidated statements of operations and comprehensive loss for the three months ended March 31, 2019 and 2018 (in thousands):

The following table summarizes equity‑based compensation expense by type of award for the three months ended March 31, 2019 and 2018 (in thousands):