On July 1, 2015, in connection with the closing of the IPO, the Company effected its Restated Certificate of Incorporation, which authorizes the Company to issue 200,000,000 shares of common stock, $0.001 par value per share.

On June 18, 2019, the Company completed an underwritten public offering, in which the Company sold 26,666,667 shares of its common stock at a price to the public of $2.25 per share. The aggregate net proceeds received by the Company from the offering were approximately $55,976, after deducting underwriting discounts and commissions and offering expenses payable by the Company. In addition, the Company granted the underwriters a 30-day option to purchase up to an additional 2,666,666 shares of common stock at the public offering price, less underwriting discounts and commissions.

On June 21, 2019, the Company sold an additional 2,151,911 shares of its common stock at a price to the public of $2.25 per share. The aggregate net proceeds received by the Company were approximately $4,551, after deducting underwriting discounts and commissions and offering expenses payable by the Company.

On November 27, 2019, the Company entered into a Sales Agreement (the “Sales Agreement”) with Cowen and Company, LLC (“Cowen”) to sell shares of the Company’s common stock, par value $0.001 per share (the “Common Stock”), with aggregate gross sales proceeds of up to $25,000, from time to time, through an “at the market” equity offering program under which Cowen will act as sales agent. As of December 31, 2019, The Company has sold 128,400 shares under the Sales Agreement.

The Company’s 2012 Stock Incentive Plan, as amended, (the “2012 Plan”) provided for the Company to sell or issue common stock or restricted common stock, or to grant incentive stock options or nonqualified stock options for the purchase of common stock, to employees, members of the board of directors and consultants of the Company. The 2012 Plan is administered by the board of directors, or at the discretion of the board of directors, by a committee of the board of directors. The exercise prices, vesting and other restrictions are determined at the discretion of the board of directors, or their committee if so delegated, except that the exercise price per share of stock options may not be less than 100% of the fair market value of the share of common stock on the date of grant and the term of stock option may not be greater than ten years. The Company generally granted stock-based awards with service conditions only (“service-based” awards).

Stock options granted under the 2012 Plan generally vest over four years and expire after ten years, although options have been granted with vesting terms less than four years. As of December 31, 2019, there were no shares available for future grant under the 2012 Plan.

On June 16, 2015, the Company’s stockholders approved the 2015 Incentive Award Plan (the “2015 Plan”), which became effective on June 25, 2015. The 2015 Plan provides for the grant of incentive stock options, nonqualified stock options, stock appreciation rights, restricted stock awards, restricted stock units and other stock-based awards. The number of shares initially reserved for issuance under the 2015 Plan was the sum of (i) 2,200,000 shares of common stock and (ii) the number of shares subject to awards outstanding under the 2012 Plan that expire, terminate or are otherwise surrendered, cancelled, forfeited or repurchased by the Company on or after the effective date of the 2015 Plan. In addition, the number of shares of common stock that may be issued under the 2015 Plan is subject to increase on the first day of each calendar year, beginning in 2016 and ending in 2025, equal to the lesser of (i) 4% of the number of shares of the Company’s common stock outstanding on the last day of the preceding applicable calendar year and (ii) an amount determined by the Company’s board of directors.

Stock options granted under the 2015 Plan generally vest over four years and expire after ten years, although options have been granted with vesting terms less than four years. As of December 31, 2019, there were 2,154,460 shares available for future grant under the 2015 Plan.

On June 16, 2015, the Company’s stockholders approved the 2015 Employee Stock Purchase Plan (the “ESPP”), which became effective on June 25, 2015. A total of 365,000 shares of common stock were reserved for issuance under the ESPP. In addition, the number of shares of common stock that may be issued under the ESPP automatically increase on the first day of each calendar year, beginning in 2016 and ending in 2025, by an amount equal to the lesser of (i) 400,000 shares, (ii) 1% of the number of shares of the Company’s common stock outstanding on the last day of the applicable preceding calendar year and (iii) an amount determined by the Company’s board of directors. Offering periods under the ESPP will commence when determined by the plan administrator. As of December 31, 2019, there were 108,346 shares issued under the ESPP and 1,847,474 shares were reserved and available for issuance under the ESPP.

The ESPP provides that eligible employees may contribute up to 15% of their eligible earnings toward the semi-annual purchase of the Company's common stock. The ESPP is qualified under Section 423 of the Internal Revenue Code. The employee's purchase price is derived from a formula based on the closing price of the common stock on the first day of the offering period versus the closing price on the date of purchase (or, if not a trading day, on the immediately preceding trading day). The offering period under the ESPP has a duration of six months, and the purchase price with respect to each offering period beginning on or after such date is, until otherwise amended, equal to 85% of the lesser of (i) the fair market value of the Company's common stock at the commencement of the applicable six-month offering period or (ii) the fair market value of the Company's common stock on the purchase date. The Company estimates the fair value of common stock under the ESPP using a Black-Scholes valuation model. The fair value was estimated on the date of grant using the Black-Scholes option valuation model and the straight-line attribution approach with the following weighted-average assumptions: risk-free interest rate (1.73%); expected term (0.5 years); expected volatility (75.9%); and an expected dividend yield (0%). The Company recorded $109 and $145 of stock-based compensation expense under the ESPP for the twelve months ended December 31, 2019 and December 31, 2018, respectively. There was no stock-based compensation expense under the ESPP for the twelve months ended December 31, 2017.

The assumptions that the Company used to determine the fair value of the stock options granted to employees and directors were as follows, presented on a weighted average basis:

	Year Ended December 31,
	2019			2018			2017
Risk-free interest rate		2.64	%		2.39	%		2.20	%
Expected term (in years)		6.0			6.0			6.0
Expected volatility		88.4	%		76.0	%		80.9	%
Expected dividend yield		0	%		0	%		0	%

The following table summarizes the Company’s stock option activity for the twelve months ended December 31, 2019:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Remaining Contractual Term		Aggregate Intrinsic Value
						(in years)
Outstanding as of December 31, 2018		7,561,719		$	12.26		7.23	$	4,958
Granted		2,456,850			5.79
Exercised		(90,125	)		1.61
Forfeited		(1,617,761	)		12.82
Outstanding as of December 31, 2019		8,310,683		$	10.36		7.01	$	3,427
Options vested and expected to vest as of December 31, 2019		8,310,683		$	10.36		7.01	$	3,427
Options exercisable as of December 31, 2019		4,815,001		$	12.47		5.78	$	3,296

The weighted average grant-date fair value of stock options granted during the years ended December 31, 2019, 2018 and 2017 was $4.13, $6.53, and $7.07 per share, respectively. The total intrinsic value of stock options exercised during the years ended December 31, 2019, 2018, and 2017 was $244, $1,801, and $1,912, respectively.

The aggregate intrinsic value of stock options is calculated as the difference between the exercise price of the stock options and the fair value of the Company’s common stock for those stock options that had exercise prices lower than the fair value of the Company’s common stock.

During the year ended December 31, 2019, the Company granted performance-based stock options to employees for the purchase of an aggregate of 1.1 million shares of common stock with a grant date fair value of $4.58 per share. These stock options are exercisable only upon achievement of specified performance targets. As of December 31, 2019, none of these options were exercisable because none of the specified performance targets had been achieved. Because achievement of the specified performance targets was not deemed probable as of December 31, 2019, the Company did not record any expense for these stock options from the dates of issuance through December 31, 2019.

The Company has granted restricted stock units with time-based vesting conditions. The table below summarizes the Company’s restricted stock activity for the twelve months ended December 31, 2019:

	Number of Shares			Weighted Average Grant Date Fair Value
Unvested restricted stock units as of December 31, 2018		226,900		$	9.64
Granted		20,000		$	2.69
Forfeited		(22,500	)	$	8.77
Vested		(94,400	)	$	9.46
Unvested restricted stock units as of December 31, 2019		130,000		$	8.86

The aggregate intrinsic value of restricted stock units that vested during the years ended December 31, 2019, 2018 and 2017 was $517, $1,206, and $445, respectively.

The Company recorded stock-based compensation expense related to stock options and restricted stock units in the following expense categories of its consolidated statements of operations and comprehensive loss:

	Year Ended December 31,
	2019		2018		2017
Research and development expenses	$	4,613	$	8,388	$	8,115
General and administrative expenses		3,731		8,253		9,247
	$	8,344	$	16,641	$	17,362

As of December 31, 2019, the Company had an aggregate of $9,387 of unrecognized stock-based compensation cost, which is expected to be recognized over a weighted average period of 2.30 years.