As of March 31, 2019, the aggregate number of shares of common stock that may be issued pursuant to stock awards under the 2015 Plan was 18,500,000 shares. The 2015 Plan also provides for the grant of nonstatutory stock options, or NSOs, incentive stock options, or ISOs, stock appreciation rights, restricted stock awards and restricted stock unit awards. At March 31, 2019, a total of 13,337,602 options were outstanding, of which 4,680,540 were exercisable, 36,455 restricted stock unit awards were outstanding, and 3,854,948 shares were available for future grant under the 2015 Plan.

On January 1, 2019, 500,000 shares of common stock were added to the 2017 Employee Stock Purchase, or 2017 ESPP. In accordance with the provisions of our 2017 ESPP, the number of shares of our common stock reserved for issuance under the 2017 ESPP automatically increased on January 1 of each calendar year. As of March 31, 2019, the aggregate number of shares of common stock reserved under the 2017 ESPP was 1,500,000 and we had 1,450,709 shares available for future issuance under the 2017 ESPP. During the three months ended March 31, 2019, 17,740 shares were issued under our 2017 ESPP. At March 31, 2019, accrued liabilities included $0.3 million of 2017 ESPP contributions for which the related shares will be issued on July 1, 2019.

The following table summarizes stock-based compensation expense for the three months ended March 31, 2019 and 2018 (in thousands):

	Three Months Ended March 31,
	2019		2018
Cost of sales - product	$	118	$	—
Research and development expenses		3,921		2,314
Selling, general and administrative expenses		14,521		4,070
Total	$	18,560	$	6,384

As of March 31, 2019, total unrecognized estimated non-cash stock-based compensation expense related to non-vested stock options was $3.1 million.