Note 10. Stock-Based Compensation

As of March 31, 2020, 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 (as amended to date, the “2017 ESPP”), each as more fully described below.

Equity Incentive and Employee Stock Purchase Plans

2013 Stock Incentive Plan

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, 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 March 31, 2020, options to purchase a total of 3,977,830 shares of common stock and 376,210 restricted stock units were outstanding and up to 1,571,751 shares of common stock remained available for grant under the 2013 Plan.

Note 10. Stock-Based Compensation (Continued)

Other Awards and Inducement Grants

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 March 31, 2020, options to purchase a total of 333,512 shares of common stock were outstanding under the 2008 Plan. In addition, as of March 31, 2020, non-statutory stock options to purchase an aggregate of 393,750 shares of common stock were outstanding. These options were issued outside of the 2013 Plan to certain newly-hired employees in 2017, 2015 and 2014 pursuant to the Nasdaq inducement grant exception as a material component of such new hires’ employment compensation.

2017 Employee Stock Purchase Plan

The 2017 ESPP is intended to qualify as an "employee stock purchase plan" as defined in Section 423 of the Internal Revenue Code. 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. As of March 31, 2020, 302,493 shares remained available for issuance under the 2017 ESPP.

For the three months ended March 31, 2020 and 2019, the Company issued 18,848 and 11,096 shares of common stock, respectively, under the 2017 ESPP and received proceeds of less than $0.1 million during each period, as a result of employee stock purchases.

Accounting for Stock-based Compensation

The Company recognizes non-cash compensation expense for stock-based awards under the Company’s equity incentive plans over an award’s requisite service period, or vesting period, using the straight-line attribution method, based on their grant date fair value determined using the Black-Scholes option-pricing model. The Company also recognizes non-cash compensation for stock purchases made under the 2017 ESPP. The fair value of the discounted purchases made under the Company’s 2017 ESPP is calculated using the Black-Scholes option-pricing model. The fair value of the look-back provision plus the 15% discount is recognized as compensation expense over each plan period.

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 condensed statements of operations for the three months ended March 31, 2020 and 2019 were as follows:


	Three Months Ended
	March 31,
(in thousands)	2020		2019
Stock-based compensation:
Research and development
Employee Stock Purchase Plans	$	11	$	6
Equity Incentive Plans		193		330
	$	204	$	336
General and administrative
Employee Stock Purchase Plans	$	1	$	6
Equity Incentive Plans		545		674
	$	546	$	680
Total stock-based compensation expense	$	750	$	1,016

During the three months ended March 31, 2020 and 2019, the weighted average fair market value of stock options granted was $1.07 and $1.83, respectively.

Note 10. Stock-Based Compensation (Continued)

The following weighted average assumptions apply to the options to purchase 643,629 and 480,502 shares of common stock granted to employees and directors during the three months ended March 31, 2020 and 2019, respectively:


	Three Months Ended
	March 31,
	2020		2019
Average risk-free interest rate		1.6%		2.4%
Expected dividend yield		—		—
Expected lives (years)		3.8		3.6
Expected volatility		82.7%		82.0%
Weighted average exercise price (per share)	$	1.80	$	3.14

All options granted during the three months ended March 31, 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 three months ended March 31, 2020:


($ 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, 2019	4,220,417	$	13.08	6.6	$
Granted	643,629		1.80
Exercised	—		—
Forfeited	(38,874)		5.67
Expired	(120,080)		19.73
Outstanding at March 31, 2020 (1)	4,705,092	$	11.43	7.0	$	—
Exercisable at March 31, 2020	2,373,804	$	18.16	4.9	$	—

(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.

The fair value of options that vested during the three months ended March 31, 2020 was $0.8 million. As of March 31, 2020, there was $5.1 million of unrecognized compensation cost related to unvested options, which the Company expects to recognize over a weighted average period of 2.5 years.

Restricted Stock Activity

The following table summarizes restricted stock activity for the three months ended March 31, 2020:


($ in thousands, except per share data)	Number of Shares	Weighted-Average Grant Date Fair Value
Nonvested shares at December 31, 2019	193,625	$	3.14
Granted	237,675		1.79
Cancelled	(6,688)		2.34
Vested	(48,402)		3.14
Nonvested shares at March 31, 2020	376,210	$	2.30

As of March 31, 2020, there was $0.8 million of unrecognized compensation expense related to the restricted stock units, which is expected to be recognized over a weighted-average period of 3.3 years.