The fair value of each of the Company’s stock option grants is estimated on the date of grant using the Black-Scholes option-pricing model. The Company currently estimates its expected stock volatility based on the historical volatility of its publicly-traded peer companies and expects to continue to do so until such time as it has adequate historical data regarding the volatility of its own publicly-traded stock price. The expected term of the Company’s stock options has been determined utilizing the “simplified” method for awards that qualify as “plain vanilla” options. The expected term of stock options granted to non-employees is equal to the contractual term of the option award. The risk-free interest rate is determined by reference to the U.S. Treasury yield curve in effect at the time of grant of the award for time periods approximately equal to the expected term of the award. Expected dividend yield is based on the fact that the Company has never paid cash dividends and does not expect to pay any cash dividends in the foreseeable future. The relevant data used to determine the value of the stock option grants for the three and six months ended June 30, 2019 and 2018 were as follows:

	Three months ended				Six months ended
	June 30,				June 30,
	2019		2018		2019		2018
Risk-free interest rates	1.89 - 2.41%		2.67 - 2.93%		1.89 - 2.67%		2.67 - 2.93%
Expected dividend yield	0.00%		0.00%		0.00%		0.00%
Expected term (in years)		6.0		6.0		6.0		6.0
Expected volatility	68.6 - 68.9%		69.8 - 70.4%		68.6 - 69.5%		69.8 - 71.5%

The following table summarizes stock option activity for the six months ended June 30, 2019:

(In thousands, except per share amounts)	Shares Issuable Under Options			Weighted Average Exercise Price Per Share
Outstanding as of December 31, 2018		4,435		$	19.21
Granted		825			13.87
Exercised		—			7.15
Cancelled		(288	)		20.91
Outstanding as of June 30, 2019		4,972		$	18.23
Options vested and expected to vest at June 30, 2019		4,972		$	18.23
Options exercisable at June 30, 2019		2,911		$	18.03

The aggregate intrinsic value of options is calculated as the difference between the exercise price of the options and the fair value of the Company’s common stock for those options that had exercise prices lower than the fair value of the Company’s common stock. Options to purchase a total of approximately 118 shares of the Company’s common stock, with an aggregate intrinsic value of approximately $622, were exercised during the six months ended June 30, 2019.

At June 30, 2019 and 2018, there were options for the purchase of approximately 4,972,362 and 4,534,000 shares of the Company’s common stock outstanding, respectively, with a weighted average remaining contractual term of 7.2 years and 8.0 years, respectively, and with a weighted average exercise price of $18.23 and $19.13 per share, respectively.

The weighted average grant date fair value of options granted during the six months ended June 30, 2019 and 2018 was $8.78 and $15.35 per share, respectively.

On January 4, 2016, the Company granted 189,300 restricted stock units (“RSUs”) with performance and time-based vesting conditions to certain executives. These RSUs began vesting, and the underlying shares of common stock became deliverable, beginning when ZILRETTA was approved (the “Milestone”). The number of shares eligible for vesting varied based on the timing of achieving the Milestone. As a result of the Milestone being achieved on October 6, 2017, the number of shares of the Company’s common stock earned under these awards was 122,800, subject to ongoing employment with the Company for a period of 2 years. The 122,800 shares had an approximate value of $2.2 million as of the original grant date of which $1.6 million was recognized in the fourth quarter of 2017 upon achieving the Milestone and the remaining $0.6 million is being recognized over a period of two years.

During the six months ended June 30, 2019, the Company awarded 804,381 RSUs to employees at an average grant date fair value of $14.64 per share. The RSUs vest in four substantially equal installments on each of the first four anniversaries of the vesting commencement date, subject to the employee’s continued employment with, or services to, the Company on each vesting date. Compensation expense is recognized on a straight-line basis.

The following table summarizes the RSU activity for the six months ended June 30, 2019:

(In thousands, except per share amounts)	Number of Shares			Weighted Average Grant Date Fair Value Per Share
Nonvested balance as of December 31, 2018		252		$	22.25
Granted		804			14.64
Vested/Released		(54	)		22.96
Cancelled		(87	)		16.11
Nonvested Balance as of June 30, 2019		915		$	16.11

The Company recorded stock-based compensation expense related to stock options and RSUs and shares purchased under the Employee Stock Purchase Plan for the three and six months ended June 30, 2019 and 2018 as follows:

	For the three months ended June 30,				For the six months ended June 30,
(In thousands)	2019		2018		2019		2018
Research and development	$	1,385	$	1,137	$	2,541	$	2,287
Selling, general and administrative		2,832		2,847		5,529		5,354
Total	$	4,217	$	3,984	$	8,070	$	7,641

As of June 30, 2019, unrecognized stock-based compensation expense for stock options outstanding was approximately $23.0 million which is expected to be recognized over a weighted average period of 2.4 years. As of June 30, 2019, unrecognized stock-based compensation expense for RSUs outstanding was $12.9 million which is expected to be recognized over a period of 3.3 years.