In April 2020, the Company issued and sold 15,442,303 shares of common stock at a public offering price of $4.75 per share and pre-funded warrants to purchase up to 13,610,328 shares of common stock at a public offering price of $4.7499 per warrant in an underwritten public offering pursuant to a shelf registration statement on Form S-3. This includes the full exercise by the underwriters of their option to purchase up to 3,789,473 additional shares of common stock. The net proceeds to the Company from this public offering were approximately $129.1 million, after deducting underwriting discounts and commissions of $8.2 million and offering costs of $0.7 million.

The public offering price for the pre-funded warrants was equal to the public offering price of the common stock, less the $0.0001 per share exercise price of each warrant. These warrants were recorded as a component of stockholders’ equity within additional paid-in capital and have no expiration date. Per the terms of the warrant agreement, the outstanding warrants to purchase shares of common stock may not be exercised if the holder’s ownership of the Company’s common stock would exceed 4.99% (“Maximum Ownership Percentage”). By written notice to the Company, the holders may increase or decrease the Maximum Ownership Percentage up to 19.99%. The revised Maximum Ownership Percentage would be effective 60 days after the notice is received by the Company.

As of June 30, 2020, the following pre-funded warrants for common stock were issued and outstanding:

Issue Date	Expiration Date	Exercise Price		Number of Warrants Outstanding
February 8, 2019	None	$	0.0001		4,000,000
April 30, 2020	None	$	0.0001		13,610,328
Total pre-funded warrants					17,610,328

The 2016 Equity Incentive Plan (“2016 Plan”) provides for an automatic increase in the number of shares reserved for issuance thereunder on January 1 of each year for the remaining term of the plan (through 2028) equal to (a) 4.0% of the number of issued and outstanding shares of common stock on December 31 of the immediately preceding year, or (b) a lesser amount as approved by the Company’s board of directors each year. As a result of this provision, on January 1, 2020 and January 1, 2019, an additional 1,163,377 and 965,603 shares, respectively, became available for issuance under the 2016 Plan.

As of June 30, 2020, the 2016 Plan had 1,341,292 shares available for future issuance.

During the three months ended June 30, 2020 and 2019, the Company issued an aggregate of 239,400 and 159,600 options to purchase common stock, respectively, under the Company’s equity incentive plans for an aggregate fair value of $1.2 million and $0.7 million, respectively.

During the six months ended June 30, 2020 and 2019, the Company issued an aggregate of 1,444,400 and 1,201,900 options to purchase common stock, respectively, under the Company’s equity incentive plans for an aggregate fair value of $7.6 million and $6.9 million, respectively.

There were no shares issued and sold under the Company’s 2016 Employee Stock Purchase Plan (“2016 ESPP”) during the three months ended June 30, 2020 and 2019. The Company issued and sold 25,928 shares for aggregate cash proceeds of $0.2 million during the six months ended June 30, 2020 and 20,132 shares for aggregate cash proceeds of $0.1 million during the six months ended June 30, 2019.

Total stock-based compensation expense related to the Company’s equity incentive plans and 2016 ESPP was as follows (in thousands):

	Three Months Ended				Six Months Ended
	June 30,				June 30,
	2020		2019		2020		2019
Research and development	$	665	$	464	$	1,197	$	866
General and administrative		935		781		1,751		1,460
Total stock-based compensation expense	$	1,600	$	1,245	$	2,948	$	2,326

The following table summarizes the weighted-average Black-Scholes option pricing model assumptions used to estimate the fair value of stock options granted under the Company’s equity incentive plans, and the shares purchasable under the 2016 ESPP during the periods presented:

	Three Months Ended						Six Months Ended
	June 30,						June 30,
	2020			2019			2020			2019
Equity Incentive Plans
Expected term (in years)		5.50			5.70			5.94			5.99
Expected volatility		77	%		78	%		75	%		81	%
Risk-free interest		0.46	%		2.05	%		1.29	%		2.50	%
Dividend yield		0	%		0	%		0	%		0	%

2016 ESPP
Expected term (in years)		—			—			0.50			0.49
Expected volatility		—			—			53	%		66	%
Risk-free interest		—			—			1.49	%		2.55	%
Dividend yield		—			—			0	%		0	%