8.	Stock Based Compensation

Stock Option Valuation

Effective July 1, 2018, the Company adopted ASU No. 2018-07, Compensation—Stock Compensation (Topic 718): Improvements to Nonemployee Share-Based Payment Accounting (“ASU 2018-07”). The table below for the nine months ended September 30, 2019 is post adoption of ASU 2018-07. The weighted average assumptions that the Company used in the Black-Scholes pricing model to determine the fair value of the stock options granted to employees, non-employees and directors for the nine months ended September 30, 2019 and the stock options granted to employees, non-employees and directors for the nine months ended September 30, 2018 were as follows:

	Nine Months Ended September 30,
	2019			2018

Risk-free interest rate		2.40	%		2.61	%
Expected term (in years)		5.79			5.78
Expected volatility		75.13	%		88.30	%
Expected dividend yield		0.00	%		0.00	%
Exercise price	$	14.42		$	17.76
Fair value of common stock	$	14.42		$	17.76

The Company recognizes compensation expense for only the portion of awards that are expected to vest.

The following table summarizes stock option activity for the nine months ended September 30, 2019 under the Second Amended and Restated 2014 Stock Option and Incentive Plan:

	Number of Shares			Weighted Average Exercise Price		Weighted Average Contractual Term (Years)		Aggregate Intrinsic Value

Outstanding as of December 31, 2018		8,615,997		$	4.48		7.51	$	94,474
Granted		1,397,616			14.42		9.58
Exercised		(106,294	)		1.72				1,110
Cancelled		(298,782	)		7.49
Outstanding as of September 30, 2019		9,608,537		$	5.85		7.30	$	88,855

Options vested and exercisable as of September 30, 2019		7,143,825		$	2.87		6.72	$	84,535
Options unvested as of September 30, 2019		2,464,712		$	14.48		9.06

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. The intrinsic value of options exercised and exercisable as of September 30, 2019 and 2018 was $84.5 million and $117.2 million respectively.

The weighted average grant-date fair value per share of stock options granted during the nine months ended September 30, 2019 and 2018 was $9.50 and $13.11, respectively.

The total fair value of options vested during the nine months ended September 30, 2019 and 2018 was $49.4 million and $49.5 million, respectively.

Stock-Based Compensation

	Three Months Ended September 30,				Nine Months Ended September 30,
	2019		2018		2019		2018

Research and development	$	893	$	2,223	$	4,711	$	5,075
General and administrative		1,801		389		5,301		5,705
Total stock based compensation expense	$	2,694	$	2,612	$	10,012	$	10,780

As of September 30, 2019, the Company had an aggregate of $20.9 million of unrecognized stock-based compensation cost, which is expected to be recognized over the weighted average period of 1.79 years.