Our 1999 Equity Incentive Plan, as amended (the “1999 EIP”) authorized awards of incentive stock options within the meaning of Section 422 of the Internal Revenue Code (the “Code”), non-qualified stock options, non-vested (restricted) stock, and unrestricted stock for up to 2.0 million shares of common stock (subject to adjustment for stock splits and similar capital changes and exclusive of options exchanged at the consummation of mergers) to employees and, in the case of non-qualified stock options, non-vested (restricted) stock, and unrestricted stock, to consultants and directors as defined in the 1999 EIP. The plan terminated on November 15, 2009.

On March 12, 2009, our Board of Directors adopted, and on June 10, 2009, our stockholders approved, our 2009 Equity Incentive Plan (the “2009 EIP”). The 2009 EIP provides for the grant of incentive stock options intended to qualify under Section 422 of the Code, nonstatutory stock options, restricted stock, unrestricted stock and other equity-based awards, such as stock appreciation rights, phantom stock awards, and restricted stock units, for up to 29.2 million shares of our common stock (subject to adjustment in the event of stock splits and other similar events). As of December 31, 2019, no shares remain available for issuance under the 2009 EIP.

On April 10, 2019, our Board of Directors adopted, and on June 19, 2019, our stockholders approved, our 2019 Equity Incentive Plan (the “2019 EIP”). The 2019 EIP provides for the grant of incentive stock options intended to qualify under Section 422 of the Code, nonstatutory stock options, restricted stock, unrestricted stock and other equity-based awards, such as stock appreciation rights, phantom stock awards, and restricted stock units, which we refer to collectively as Awards, for up to 40.2 million shares of our common stock (subject to adjustment in the event of stock splits and other similar events).

The Board of Directors appointed the Compensation Committee to administer the 1999 EIP, the 2009 EIP and the 2019 EIP. No awards will be granted under the 2019 EIP after June 19, 2029.

On March 12, 2009, our Board of Directors adopted, and on June 10, 2009, our stockholders approved, the 2009 Employee Stock Purchase Plan (the “2009 ESPP”) to provide eligible employees the opportunity to acquire our common stock in a program designed to comply with Section 423 of the Code. There were 166,666 shares of common stock reserved for issuance under the 2009 ESPP. Rights to purchase common stock under the 2009 ESPP were granted at the discretion of the Compensation Committee, which determined the frequency and duration of individual offerings under the plan and the dates when stock may have been purchased. Eligible employees participated voluntarily and may have withdrawn from any offering at any time before the stock is purchased. Participation terminated automatically upon termination of employment. The purchase price per share of common stock in an offering was 85% of the lesser of its fair value at the beginning of the offering period or on the applicable exercise date and may have been paid through payroll deductions, periodic lump sum payments, the delivery of our common stock, or a combination thereof. Unless otherwise permitted by the Board of Directors, no participant may have acquired more than 3,333 shares of stock in any offering period. No participant was allowed to purchase shares under the 2009 ESPP if such employee would own or would have been deemed to own stock possessing 5% or more of the total combined voting power or value of the Company. The 2009 ESPP plan terminated on June 10, 2019.

In the second quarter of 2019, our Board of Directors adopted the 2019 Employee Stock Purchase Plan (the “2019 ESPP”) to provide eligible employees the opportunity to acquire our common stock in a program designed to comply with Section 423 of the Code. There are 500,000 shares reserved for issuance under the 2019 ESPP. The 2019 ESPP is subject to approval by our stockholders.

Our Director’s Deferred Compensation Plan, as amended, permits each outside director to defer all, or a portion of, their cash compensation until their service as a director ends or until a specified date into a cash account or a stock account. There are 425,000 shares of our common stock reserved for issuance under this plan. As of December 31, 2019, 72,081 shares had been issued. Amounts deferred to a cash account will earn interest at the rate paid on one-year Treasury bills with interest added to the account annually. Amounts deferred to a stock account will be converted on a quarterly basis into a number of units representing shares of our common stock equal to the amount of compensation which the participant has elected to defer to the stock account divided by the applicable price for our common stock. The applicable price for our common stock has been defined as the average of the closing price of our common stock for all trading days during the calendar quarter preceding the conversion date as reported by The Nasdaq Capital Market. Pursuant to this plan, a total of 412,435 units, each representing a share of our common stock at a weighted average common stock price of $4.52, had been credited to participants’ stock accounts as of December 31, 2019. The compensation charges for this plan were immaterial for all periods presented.

On November 4, 2015, our Board of Directors adopted and approved our 2015 Inducement Equity Plan (the “2015 IEP”) in compliance with and in reliance on NASDAQ Listing Rule 5635(c)(4), which exempts inducement grants from the general requirement of the NASDAQ Listing Rules that equity-based compensation plans and arrangements be approved by stockholders. There are 1,500,000 shares of our common stock reserved for issuance under the 2015 IEP.

We primarily use the Black-Scholes option pricing model to value options granted to employees and non-employees, as well as options granted to members of our Board of Directors. All stock option grants have 10-year terms and generally vest ratably over a 3 or 4-year period.

The fair value of each option granted during the periods was estimated on the date of grant using the following weighted average assumptions:

	2019			2018			2017
Expected volatility		64	%		64	%		65	%
Expected term in years		5			6			4
Risk-free interest rate		1.8	%		2.8	%		1.7	%
Dividend yield		0	%		0	%		0	%

Expected volatility is based exclusively on historical volatility data of our common stock. The expected term of stock options granted is based on historical data and other factors and represents the period of time that stock options are expected to be outstanding prior to exercise. The risk-free interest rate is based on U.S. Treasury strips with maturities that match the expected term on the date of grant.

	Options			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term (in years)		Aggregate Intrinsic Value
Outstanding at December 31, 2018		18,613,822		$	4.15
Granted		11,579,119			3.00
Exercised		(466,940	)		3.15
Forfeited		(1,186,465	)		3.20
Expired		(1,375,389	)		5.08
Outstanding at December 31, 2019		27,164,147			3.67		7.65	$	19,782,605
Vested or expected to vest at December 31, 2019		27,164,147			3.67		7.65	$	19,782,605
Exercisable at December 31, 2019		11,785,971		$	4.33		5.83	$	3,677,098

The weighted average grant-date fair values of options granted during the years ended December 31, 2019, 2018, and 2017, was $1.77, $1.23, and $1.97, respectively.

The aggregate intrinsic value in the table above represents the difference between our closing stock price on the last trading day of fiscal 2019 and the exercise price, multiplied by the number of in-the-money options that would have been received by the option holders had all option holders exercised their options on December 31, 2019 (the intrinsic value is considered to be zero if the exercise price is greater than the closing stock price). This amount changes based on the fair market value of our stock. The total intrinsic value of options exercised during the years ended December 31, 2019, 2018, and 2017, determined on the dates of exercise, was $385,000, $399,000, and $132,000, respectively.

During 2019, 2018, and 2017, all options were granted with exercise prices equal to the market value of the underlying shares of common stock on the grant date other than certain awards dated March 2, 2018, August 6, 2018 and December 31, 2018. In March 2018, our Board of Directors approved certain awards subject to forfeiture in the event stockholder approval was not obtained to increase the shares available under our 2009 EIP. This approval was obtained in June 2018. Accordingly, these awards have a grant date of June 2018, with an exercise price as of the date the Board of Director's approved the awards in March 2018. In August 2018, our Board of Directors approved certain awards. However, the awards were not communicated until October 2018. Accordingly, these awards have a grant date of October 2018 with an exercise price as of the date the Board of Director's approved the awards in August 2018. In December 2018, our Board of Directors approved certain awards subject to forfeiture in the event stockholder approval was not obtained for our 2019 EIP. This approval was obtained in June 2019. Accordingly, these awards have a grant date of June 2019, with an exercise price as of the date the Board of Director's approved the awards in December 2018. In August 2018, our Board of Directors approved certain awards. However, the awards were not communicated until October 2018. Accordingly, these awards have a grant date of October 2018 with an exercise price as of the date the Board of Director's approved the awards in August 2018.

As of December 31, 2019, there was $20.3 million of unrecognized share-based compensation expense related to stock options granted to employees, consultants and directors for which, if all milestones are achieved, will be recognized over a weighted average period of 2.3 years.

Certain employees and consultants have been granted non-vested stock. The fair value of non-vested market-based awards is calculated based on a Monte Carlo simulation as of the date of issuance. The fair value of other non-vested stock is calculated based on the closing sale price of our common stock on the date of issuance.

	Nonvested Shares			Weighted Average Grant Date Fair Value
Outstanding at December 31, 2018		2,213,967		$	3.20
Granted		737,682			2.98
Vested		(129,675	)		2.76
Forfeited		(702,163	)		4.14
Outstanding at December 31, 2019		2,119,811		$	2.84

As of December 31, 2019, there was $3.9 million of unrecognized share-based compensation expense related to these non-vested shares for which, if all milestones are achieved, will be recognized over a period of 1.7 years. The total intrinsic value of shares vested during the years ended December 31, 2019, 2018, and 2017, was $357,000, $242,000, and $3.8 million, respectively.

Cash received from option exercises and purchases under our 2009 ESPP for the years ended December 31, 2019, 2018, and 2017, was $1.6 million, $1.2 million, and $1.0 million, respectively. We issue new shares upon option exercises, purchases under our 2009 ESPP and 2019 ESPP, vesting of non-vested stock and under the Director’s Deferred Compensation Plan. During the years ended December 31, 2019, 2018, and 2017, 84,703 shares, 140,313 shares, and 121,183 shares, were issued under the 2009 ESPP, respectively. During the years ended December 31, 2019, 2018, and 2017, 129,675 shares, 53,050 shares, and 1.1 million shares, respectively, were issued as a result of the vesting of non-vested stock.

The impact on our results of operations from share-based compensation for the years ended December 31, 2019, 2018, and 2017, was as follows (in thousands).

	Year Ended
	2019		2018		2017
Research and development	$	3,873	$	3,498	$	6,159
General and administrative		6,019		4,127		6,270
Total share-based compensation expense	$	9,892	$	7,625	$	12,429