As of December 31, 2021, the only equity compensation plans from which the Company may currently issue new awards are the Company’s 2013 Stock Incentive Plan (as amended to date, the “2013 Plan”) and 2017 Employee Stock Purchase Plan (the “2017 ESPP”), each as more fully described below.

The Company's board of directors adopted the 2013 Plan, which was approved by the Company’s stockholders effective July 26, 2013. Amendments to the 2013 Plan were approved by the Company’s stockholders in June 2014, June 2015, June 2017 and June 2019. The 2013 Plan is intended to further align the interests of the Company and its stockholders with its employees, including its officers, non-employee directors, consultants and advisers by providing equity-based incentives. The 2013 Plan allows for the issuance of incentive stock options intended to qualify under Section 422 of the Internal Revenue Code, non-statutory stock options, stock appreciation rights, restricted stock awards, restricted stock units (“RSUs”), other stock-based awards and performance awards. The total number of shares of common stock authorized for issuance under the 2013 Plan is 5,653,057 shares of the Company’s common stock, plus such additional number of shares of common stock (up to 868,372 shares) as is equal to the number of shares of common stock subject to awards granted under the Company’s 2005 Stock Incentive Plan or 2008 Stock Incentive Plan (the “2008 Plan”), to the extent such awards expire, terminate or are otherwise surrendered, canceled, forfeited or repurchased by the Company at their original issuance price pursuant to a contractual repurchase right.

As of December 31, 2021, options to purchase a total of 4,721,038 shares of common stock and 575,703 RSUs were outstanding and up to 199,873 shares of common stock remained available for grant under the 2013 Plan.

The Company has not made any awards pursuant to other equity incentive plans, including the 2008 Plan, since the Company’s stockholders approved the 2013 Plan. As of December 31, 2021, options to purchase a total of 155,968 shares of common stock were outstanding under the 2008 Plan. In addition, as of December 31, 2021, non-statutory stock options to purchase an aggregate of 325,000 shares of common stock were outstanding that were issued outside of the 2013 Plan to certain employees in 2017, 2015 and 2014 pursuant to the Nasdaq inducement grant exception as a material component of new hires’ employment compensation.

The Company’s board of directors adopted the 2017 ESPP which was approved by the Company’s stockholders and became effective June 7, 2017. An amendment to the 2017 ESPP was approved by the Company’s stockholders in June 2019. The 2017 ESPP is intended to qualify as an "employee stock purchase plan" as defined in Section 423 of the Internal Revenue Code, and is intended to encourage our employees to become stockholders of ours, to stimulate increased interest in our affairs and success, to afford employees the opportunity to share in our earnings and growth and to promote systematic savings by them. The total number of shares of common stock authorized for issuance under the 2017 ESPP is 412,500 shares of common stock, subject to adjustment as described in the 2017 ESPP. Participation is limited to employees that would not own 5% or more of the total combined voting power or value of the stock of the Company after the grant. As of December 31, 2021, 196,225 shares remained available for issuance under the 2017 ESPP.

The 2017 ESPP provides for offerings to employees to purchase common stock with offerings beginning on dates determined by the compensation committee of the board of directors or on the first business day thereafter. Each offering begins a “plan period” during which payroll deductions are to be made and held for the purchase of common stock at the end of the plan period. The compensation committee may, at its discretion, choose a plan period of 12 months or less for subsequent offerings and/or choose a different commencement date for offerings. During each plan period participating employees may elect to have a portion of their compensation, ranging from 1% to 10% of compensation as defined by the plan, withheld and used for the purchase of common stock at the end of each plan period. The purchase price is equal to 85% of the lower of the fair market value of a share of common stock on the first trading date of each plan period or the fair market value of a share of common stock on the last trading day of the plan period, and is limited by participant to $25,000 in fair value of common stock per year as well as other quarterly plan limitations as defined by each plan.

For the years ended December 31, 2021, 2020 and 2019, the Company issued 49,117, 75,999, and 60,953, shares of common stock, in each year respectively, under the 2017 ESPP and received proceeds of $0.1 million for each year, as a result of stock purchases.

The Company recognizes non-cash compensation expense for stock-based awards under the Company’s equity incentive plans and employee stock purchases under the Company’s 2017 ESPP as follows:

Total stock-based compensation expense attributable to stock-based payments made to employees and directors and employee stock purchases included in operating expenses in the Company's statements of operations for the years ended December 31, 2021, 2020 and 2019 was as follows:


(in thousands)	2021		2020		2019
Stock-based compensation:
Research and development
Employee Stock Purchase Plan	$	28	$	88	$	36
Equity Incentive Plan		546		673		1,312
	$	574	$	761	$	1,348
General and administrative
Employee Stock Purchase Plan	$	3	$	9	$	20
Equity Incentive Plan		1,960		2,871		2,477
	$	1,963	$	2,880	$	2,497

Total stock-based compensation expense	$	2,537	$	3,641	$	3,845

Note 11. Stock-based Compensation (Continued)

During the years ended December 31, 2021, 2020 and 2019, the weighted average fair market value of stock options granted was $1.54, $1.25, and $1.64, respectively.

Assumptions Used in Determining Fair Value of Stock Options

Inherent in the Black-Scholes option-pricing model are the following assumptions:

Volatility. The Company estimates stock price volatility based on the Company’s historical stock price performance over a period of time that matches the expected term of the stock options.

Risk-free interest rate. The risk-free interest rate is based on the U.S. Treasury yield curve in effect at the time of grant commensurate with the expected term assumption.

Expected term. The expected term of stock options granted is based on an estimate of when options will be exercised or cancelled in the future.

Dividend rate. The dividend rate is based on the historical rate, which the Company anticipates will remain at zero.

Forfeitures. The Company accounts for forfeitures when they occur. Ultimately, the actual expense recognized over the vesting period will be for only those shares that vest. See Note 2.

The fair value of each option award at the date of grant was estimated using the Black-Scholes option pricing model. All options granted during the three years in the period ended December 31, 2021 were granted at exercise prices equal to the fair market value of the common stock on the dates of grant.

The following weighted average assumptions apply to the options to purchase 1,356,700, 1,215,382, and 1,279,016 shares of common stock granted to employees and directors during the years ended December 31, 2021, 2020 and 2019, respectively:


	2021		2020		2019
Average risk-free interest rate		0.4%		1.0%		2.1%
Expected dividend yield		—		—		—
Expected lives (years)		3.6		3.9		3.7
Expected volatility		94%		84%		84%
Weighted average exercise price (per share)	$	2.68	$	2.08	$	2.75

All options granted during the years ended December 31, 2021, 2020 and 2019 were granted at exercise prices equal to the fair market value of the common stock on the dates of grant.

Stock Option Activity

The following table summarizes stock option activity for the year ended December 31, 2021.


($ in thousands, except per share data)	Stock Options	Weighted-Average Exercise Price		Weighted-Average Remaining Contractual Life (in years)	Aggregate Intrinsic Value
Outstanding at December 31, 2020	4,614,323	$	9.78	6.8	$	2,949
Granted	1,356,700		2.68
Exercised	(22,500)		2.11
Forfeited	(261,426)		4.44
Expired	(485,091)		11.52
Outstanding at December 31, 2021 (1)	5,202,006	$	8.06	5.9	$	—
Exercisable at December 31, 2021	4,096,875	$	9.62	5.1	$	—

(1)

Includes both vested stock options as well as unvested stock options for which the requisite service period has not been rendered but that are expected to vest based on achievement of a service condition.

Note 11. Stock-based Compensation (Continued)

In March 2021, the Company accelerated the vesting of 1,535,578 options, which were previously granted from 2019 to 2021. The modification results in an incremental stock based compensation charge that was not significant. As of December 31, 2021, there was $1.2 million of unrecognized compensation cost related to unvested options, which the Company expects to recognize over a weighted average period of 2.1 years.

Restricted Stock Activity

The following table summarizes restricted stock activity for the year ended December 31, 2021:


	Time-based Awards			Market/Performance-based Awards
($ in thousands, except per share data)	Number of Shares	Weighted-Average Grant Date Fair Value		Number of Shares	Weighted-Average Grant Date Fair Value
Nonvested shares at December 31, 2020	354,003	$	2.27	549,318	$	1.54
Granted
Cancelled	(48,563)		2.31	(42,290)		1.54
Vested	(236,765)		2.25	—		—
Nonvested shares at December 31, 2021	68,675	$	2.30	507,028	$	1.54

Time-based Restricted Stock Units

In December 2020, the Company’s Chief Executive Officer was granted an award of 128,170 RSUs, pursuant to the 2013 Plan, in lieu of salary pursuant to a January 10, 2020 amendment to the officers’ employment agreement. The RSUs were fully vested on the grant date.

In March 2021, the Company accelerated the vesting of 137,872 unvested time-based restricted stock units which were previously granted in 2019 and 2020. The modification results in an incremental stock based compensation charge that was not significant. During the year ended December 31, 2021, the Company recognized $0.3 million of compensation expense related to these awards. As of December 31, 2021, there was $0.1 million of unrecognized compensation cost related to the Company’s time-based RSUs, which is expected to be recognized over a weighted average period of 1.6 years.

Market/Performance-based Restricted Stock Units

In July 2020, the Company granted RSUs to certain employees, including executive officers, under the 2013 Plan, with vesting that may occur upon a combination of specific performance and/or market conditions. Accordingly, the Company views these RSUs as two separate awards: (i) an award that vests if the market condition is achieved, and (ii) an award that vests whether or not the market condition is achieved, so long as the performance condition is achieved.

The Company is currently recognizing compensation expense for these awards over the estimated requisite service period of 2.36 years based on the estimated fair value when considering the market condition of the award, which was determined using a Monte Carlo simulation. During the year ended December 31, 2021, the Company recognized $0.3 million of compensation expense related to these awards. As of December 31, 2021, the remaining unrecognized compensation cost for the market-based component of these awards, which is expected to be recognized over a weighted-average period of 0.9 years, is $0.3 million. In addition, should the performance condition be achieved, the Company would recognize an additional $0.3 million of compensation expense.