In October 2019, we sold 9.9 million shares of our common stock at a public offering price of $17.50 per share. We received total net cash proceeds of $162.2 million (net of $10.3 million in issuance costs) from the sale of the common stock.

In June 2014, as a component of our public offering, we sold 600,000 pre-funded warrants to purchase shares of our common stock. The pre-funded warrants have an exercise price of $0.01 per share and expire on June 30, 2021. During the six months ended June 30, 2020, warrant holders exercised 267,870 warrants, which resulted in the issuance of 267,870 shares for net cash proceeds of $2,679. As of June 30, 2020, 195,574 warrants from the June 2014 public offering remain outstanding.

The following table summarizes the stock option activity for the six months ended June 30, 2020:

	Shares (in thousands)			Weighted- average Exercise Price		Weighted- average Remaining Contractual Term (Years)
Balance at December 31, 2019		16,665		$	20.47		7.87
Granted		249		$	19.24
Exercised		(124	)	$	10.57
Expired and forfeited		(484	)	$	22.57
Balance at June 30, 2020		16,306		$	20.46		7.31

For the six months ended June 30, 2020, 124,000 shares of common stock were issued pursuant to the exercise of stock options, resulting in net proceeds of $1.3 million.

The following table summarizes stock-based compensation expense related to stock-based payment awards granted pursuant to all of our equity compensation arrangements (in thousands):

	Three Months Ended June 30,				Six Months Ended June 30,
	2020		2019		2020		2019
Research and development	$	4,681	$	4,434	$	9,968	$	9,758
General and administrative		3,304		3,714		6,735		7,394
Sales and marketing		3,129		4,558		6,385		13,456
Total stock-based compensation expense	$	11,114	$	12,706	$	23,088	$	30,608

As of June 30, 2020, there was $103.0 million of total unrecognized compensation cost related to non-vested, stock-based payment awards granted under all of our equity compensation plans and all non-plan option grants. Total unrecognized compensation cost will be adjusted for future changes in estimated forfeitures. We expect to recognize this compensation cost over a weighted-average period of 2.7 years.

We estimated the fair value of each option grant on the grant date using the Black-Scholes option pricing model with the following weighted-average assumptions:

	June 30,
	2020			2019
Risk-free interest rate		1.1	%		2.4	%
Dividend yield		0.0	%		0.0	%
Volatility		66.7	%		68.0	%
Expected life (years)		6			6

We estimated the fair value of each purchase right granted under our 1997 Employee Stock Purchase Plan, as amended, at the beginning of each new offering period using the Black-Scholes option pricing model with the following weighted-average assumptions:

	June 30,
	2020			2019
Risk-free interest rate		0.1	%		2.4	%
Dividend yield		0.0	%		0.0	%
Volatility		63.3	%		58.4	%
Expected life (months)		6			6