7. Stockholders’ Equity

Equity Offering

In March 2018, the Company completed a public offering in which the Company sold 8,050,000 shares of its common stock at a price to the public of $17.00 per share. The Company received net proceeds of $128.4 million from this offering, after deducting underwriting discounts, commissions and other offering expenses.

At-The-Market Equity Offerings

On September 28, 2017, the Company entered into an at-the-market sales agreement (the “September 2017 Sales Agreement”) with Cowen and Company, LLC to sell up to $100.0 million of the Company’s common stock registered under a shelf registration statement filed with the U.S. Securities and Exchange Commission in September 2017. As of June 30, 2018, $80.0 million remained available to be sold under the terms of the September 2017 Sales Agreement. There were no shares sold under the September 2017 Sales Agreement during the six months ended June 30, 2018.

Warrants to Acquire Common Stock

In connection with the prior issuance of convertible unsecured promissory notes, the Company issued warrants to purchase shares of common stock in 2008 and 2009. The following common stock warrants were outstanding as of June 30, 2018 and December 31, 2017:


6/30/2018	12/31/2017
Number of Shares	Number of Shares	Exercise
Underlying Outstanding	Underlying Outstanding	Price		Expiration
Warrants	Warrants	Per Share		Date
-	236,632	$	0.33	July 18, 2018
-	301,986		0.33	January 16, 2019
-	15,250		0.33	January 30, 2019

During the six months ended June 30, 2018, a total of 536,564 shares of common stock were issued to stockholders upon the net exercise of 546,709 outstanding warrants. In addition, a total of 7,159 shares of common stock were issued to stockholders upon the cash exercise of outstanding warrants at a weighted average price of $0.33 per share, with total proceeds to the Company of $2,336. As of June 30, 2018, the Company no longer had any outstanding warrants to purchase shares of its capital stock.

2003 Stock Incentive Plan

The 2003 Stock Incentive Plan (the 2003 Plan) provided for the grant of incentives and nonqualified stock options and restricted stock awards. The exercise price for incentive stock options must be at least equal to the fair value of the common stock on the grant date. Unless otherwise stated in a stock option agreement, 25% of the shares subject to an option grant will vest upon the first anniversary of the vesting start date and thereafter at the rate of one forty-eighth of the option shares per month as of the first day of each month after the first anniversary. Upon termination of employment by reasons other than death, cause, or disability, any vested options shall terminate 60 days after the termination date. Stock options terminate 10 years from the date of grant. The 2003 Plan expired on May 21, 2013.

A summary of the Company’s stock option activity under the 2003 Plan for the six months ended June 30, 2018 is as follows:



				Weighted-Average
				Remaining	Aggregate
	Outstanding	Weighted-Average		Contractual Term	Intrinsic Value
	Options	Exercise Price		(Years)	(In thousands)
Outstanding as of December 31, 2017	713,211	$	1.25	2.2
Options exercised	(30,362)		1.24
Options forfeited	—		—
Outstanding, Vested and Exercisable as of June 30, 2018	682,849		1.25	1.7	$	10,163

As of June 30, 2018, outstanding options under the 2003 Plan were fully expensed and all shares underlying outstanding options were fully vested. Total intrinsic value of the options exercised during the six months ended June 30, 2018 and 2017 was $512,313 and $103,638, respectively, and total cash received for options exercised was $37,600 and $27,358 during the six months ended June 30, 2018 and 2017, respectively. The total fair value of shares underlying options which vested in the six months ended June 30, 2017 was $1,573.

2013 Equity Incentive Plan

The Company’s board of directors adopted, and its stockholders approved, its 2013 Equity Incentive Plan (the 2013 Plan) effective on January 9, 2014. The 2013 Plan provides for the grant of incentive stock options within the meaning of Section 422 of the Internal Revenue Code to the Company’s employees and its parent and subsidiary corporations’ employees, and for the grant of nonstatutory stock options, restricted stock awards, restricted stock unit awards, stock appreciation rights, performance stock awards and other forms of stock compensation to its employees, including officers, consultants and directors. The 2013 Plan also provides for the grant of performance cash awards to the Company’s employees, consultants and directors. Unless otherwise stated in a stock option agreement, 25% of the shares subject to an option grant will typically vest upon the first anniversary of the vesting start date and thereafter at the rate of one forty-eighth of the option shares per month as of the first day of each month after the first anniversary. Upon termination of employment by reasons other than death, cause, or disability, any vested options will terminate 90 days after the termination date, unless otherwise set forth in a stock option agreement. Stock options generally terminate 10 years from the date of grant.

Authorized Shares

The maximum number of shares of common stock that initially could be issued under the 2013 Plan was 1,000,000 shares, plus any shares subject to stock options or similar awards granted under the 2003 Plan that expire or terminate without having been exercised in full or are forfeited or repurchased by the Company. The number of shares of common stock reserved for issuance under the 2013 Plan automatically increases on January 1 of each year until January 1, 2023, by 3% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, or a lesser number of shares as may be determined by the Company’s board of directors. The maximum number of shares that may be issued pursuant to exercise of incentive stock options under the 2013 Plan is 20,000,000 shares. As of January 1, 2018, the number of shares of common stock that may be issued under the 2013 Plan was automatically increased by 1,030,793 shares, representing 3% of the total number of shares of common stock outstanding on December 31, 2017, increasing the number of shares of common stock available for issuance under the 2013 Plan to 3,867,994 shares.

Shares issued under the 2013 Plan may be authorized but unissued or reacquired shares of common stock. Shares subject to stock awards granted under the 2013 Plan that expire or terminate without being exercised in full, or that are paid out in cash rather than in shares, will not reduce the number of shares available for issuance under the 2013 Plan. Additionally, shares issued pursuant to stock awards under the 2013 Plan that the Company repurchases or that are forfeited, as well as shares reacquired by the Company as consideration for the exercise or purchase price of a stock award or to satisfy tax withholding obligations related to a stock award, will become available for future grant under the 2013 Plan.

A summary of the Company’s stock option activity under the 2013 Plan for the six months ended June 30, 2018 is as follows:


				Weighted-Average
ccc				Remaining	Aggregate
	Outstanding	Weighted-Average		Contractual Term	Intrinsic Value
	Options	Exercise Price		(Years)	(In thousands)
Outstanding as of December 31, 2017	2,664,163	$	7.34	7.4
Options granted	715,329		20.16
Options exercised	(54,624)		6.48
Options forfeited	(31,495)		13.10
Outstanding as of June 30, 2018	3,293,373		8.25	7.4	$	26,958
Vested or expected to vest as of June 30, 2018	3,293,373		8.25	7.4	$	26,958
Exercisable as of June 30, 2018	1,914,278		7.77	6.4	$	15,970

The weighted-average fair value of the options granted during the six months ended June 30, 2018 and 2017 was $13.52 per share and $4.72 per share, respectively, applying the Black-Scholes-Merton option pricing model utilizing the following weighted-average assumptions:


	Six Months Ended	Six Months Ended
	June 30, 2018	June 30, 2017
Expected term	6.25 years	6.25 years
Expected volatility	73.82%	75.19%
Risk-free interest rate	2.49%	2.09%
Expected dividend yield	0%	0%

As of June 30, 2018, there was $10,995,118 of total unrecognized compensation expense related to unvested options under the 2013 Plan that will be recognized over a weighted-average period of approximately 3.0 years. Total intrinsic value of the options exercised during the six months ended June 30, 2018 and 2017 was $721,337 and $82,249, respectively, and total cash received for options exercised was $353,850 and $70,204 during the six months June 30, 2018 and 2017, respectively. The total fair value of shares underlying options which vested in the six months ended June 30, 2018 and 2017 was $2,204,404 and $2,252,276, respectively.

A restricted stock unit (RSU) is a stock award that entitles the holder to receive shares of the Company’s common stock as the award vests. The fair value of each RSU is based on the closing price of the Company’s common stock on the date of grant. The Company has granted RSUs with service conditions (service RSUs) that vest in three equal annual installments provided that the employee remains employed with the Company at each installment date. As of June 30, 2018, there was $12,331 of unrecognized compensation costs related to unvested service RSUs.

The following is a summary of RSU activity under the 2013 Plan for the six months ended June 30, 2018:



			Weighted-Average
		Number of Shares	Grant Date
	Underlying RSUs			Fair Value
Unvested at December 31, 2017		9,667	$	4.61
Granted		—		—
Forfeited		—		—
Vested		4,834		4.61
Unvested at June 30, 2018		4,833		4.61

Stock-based compensation expense was classified on the statements of operations as follows for the three and six months ended June 30, 2018 and 2017:


	Three Months Ended June 30,				Six Months Ended June 30,
	2018		2017		2018		2017
Research and development expense	$	427,355	$	314,904	$	852,564	$	626,687
General and administrative expense		723,034		676,204		1,413,655		1,207,268
Total stock-based compensation expense	$	1,150,389	$	991,108	$	2,266,219	$	1,833,955