In January 2013, our Board of Directors adopted our 2013 Equity Incentive Plan (“2013 Plan”), which became effective upon the closing of our IPO in May 2013. As of December 31, 2018, we are authorized to issue 18,297,465 shares of common stock under the 2013 Plan. The 2013 Plan had 5,203,731 shares of common stock available for future issuance as of December 31, 2018, subject to automatic annual increases each January 1st and will continue through January 1, 2023. The automatic annual share increase is equal to 5 % of the total number of outstanding shares of our common stock on December 31st of the preceding fiscal year, unless our Board of Directors elects to forego or reduce such increase. Further, all remaining shares available under the 2003 Equity Incentive Plan, or the 2003 Plan, were transferred to the 2013 Plan upon adoption. The 2013 Plan provides for the granting of incentive stock options, nonstatutory stock options, stock appreciation rights, restricted stock awards, restricted stock unit awards, performance stock awards, performance cash awards and other stock awards to employees, officers, directors and consultants.

In July 2017, our Board of Directors adopted an Inducement Plan (“2017 Plan”) with 1,500,000 shares authorized for issuance to new employees entering into employment with Portola in accordance with Nasdaq Listing Rule 5635(c)(5). The 2017 Plan had 1,246,421 shares of common stock available for future issuance as of December 31, 2018. In December 2018, our Board of Directors authorized an additional 1,000,000 shares under the 2017 Plan for issuance to new employees entering into employment with Portola in accordance with NASDAQ Listing Rule 5635(c)(5).

Incentive stock options may be granted with exercise prices of not less than 100% of the estimated fair value of our common stock and nonstatutory stock options may be granted with an exercise price of not less than 85% of the estimated fair value of the common stock on the date of grant. Stock options granted to a stockholder owning more than 10% of our voting stock must have an exercise price of not less than 110% of the estimated fair value of the common stock on the date of grant. Stock options are generally granted with terms of up to ten years and vest over a period of four years.

The following table summarizes stock option activity under our 2013 Plan and 2017 Plan, and related information during the year ended December 31, 2018:

	Shares
	Subject to			Weighted-
	Outstanding			Average Exercise
	Options			Price Per Share
Balance at December 31, 2017		6,514,538		$	31.36
Options granted		2,452,738			36.36
Options exercised		(699,974	)		16.92
Options canceled		(759,612	)		42.12
Balance at December 31, 2018		7,507,690		$	33.25

Additional information related to the status of stock options at December 31, 2018, is as follows (aggregate intrinsic value in thousands):

			Weighted-
			Average		Remaining
			Exercise Price		Contractual		Aggregate
	Shares		Per Share		Life		Intrinsic Value
Outstanding		7,507,690	$	33.25		6.5	$	4,833
Vested		4,149,238	$	31.05		5.0	$	4,819

The aggregate intrinsic values of stock options outstanding and vested were calculated as the difference between the exercise price of the stock options and the fair value of our common stock as of December 31, 2018. The aggregate intrinsic value of stock options exercised was $11.6 million, $39.3 million and $1.0 million for the years ended December 31, 2018, 2017 and 2016, respectively.

The weighted-average grant date fair value of employee stock options granted during the years ended December 31, 2018, 2017 and 2016 was $21.01, $24.08 and $17.15 per share, respectively. The total estimated grant date fair value of stock options vested during the years ended December 31, 2018, 2017 and 2016 was $30.6 million, $23.0 million and $20.8 million, respectively.

We recognized stock-based compensation expenses of $29.7 million, $25.5 million and $21.2 million in 2018, 2017 and 2016 respectively relating to the employee stock options. As of December 31, 2018, total unamortized employee stock-based compensation was $60.5 million, which is expected to be recognized over the remaining estimated vesting period of 2.6 years.

In May 2016, the Compensation Committee of our Board of Directors approved the commencement of granting performance stock option awards to our executive and senior officers. PSOs represent a contingent right to purchase our Common Stock upon achievement of specified conditions. The PSOs granted in May 2016 were fully vested by the fourth quarter of 2017 when regulatory approval of Andexxa was achieved and when the manufacturing goal related to our lead programs was met in 2017. A portion of PSOs granted in May 2016 were forfeited and cancelled when regulatory approval of Andexxa was not achieved by the fourth quarter of 2016.

We recognized stock-based compensation expense of $2.3 million, $0.5 million and $0.5 million in 2017, 2016 and 2015, respectively, relating to these PSOs. As of December 31, 2017, the stock-based compensation expense for these PSOs had been fully recognized. The aggregate intrinsic value of PSOs exercised for the years ended December 31, 2018, 2017 and 2016 was $0.2 million, $0.4 million and zero, respectively. The weighted-average grant date fair value of these PSOs granted during 2016 was $14.25. There was no grant of PSOs during 2017 and 2018. The following table summarizes PSO activities under our 2013 Plan and related information:

	Shares
	Subject to			Weighted-
	Outstanding			Average Exercise
	PSOs			Price Per Share
Balance at December 31, 2017		164,783		$	23.76
Options granted		—			—
Options exercised		(17,448	)		23.76
Options canceled		(4,000	)		23.76
Balance at December 31, 2018		143,335		$	23.76

Additional information related to the status of PSOs at December 31, 2018, is as follows (aggregate intrinsic value in thousands):

Weighted-

Average

Remaining

Exercise Price

Contractual

Aggregate

Per Share

Life

Intrinsic Value

Outstanding and Vested

143,335

23.76

4.0

In January 2015, the Compensation Committee of our Board of Directors approved the commencement of granting restricted stock units to our employees. RSUs are share awards that entitle the holder to receive freely tradable shares of our Common Stock upon vesting. The RSUs cannot be transferred, and until they vest, the awards are subject to forfeiture if employment terminates prior to the release of the vesting restrictions. The RSUs, generally vest in equal amounts on each of the first three year anniversaries of the grant date, provided the employee remains continuously employed with us. The fair value of the RSUs is equal to the closing price of our Common Stock on the grant date.

The following table summarizes RSU activities under our 2013 Plan and 2017 Plan and related information:

	Shares
	Subject to			Weighted-
	Outstanding			Average Grant Date
	RSUs			Fair Value Per Share
Balance at December 31, 2017		600,334		$	27.87
RSUs granted		758,156			37.62
RSUs released		(279,555	)		28.49
RSUs canceled		(99,657	)		40.13
Balance at December 31, 2018		979,278		$	34.00

Additional information related to the status of RSUs at December 31, 2018, is as follows (aggregate intrinsic value in thousands):

Remaining

Contractual

Aggregate

Life

Intrinsic Value

Outstanding

979,278

1.2

19,116

The total grant date fair value of RSUs vested during the years ended December 31, 2018, 2017 and 2016 was $8.0 million, $6.0 million and $1.7 million, respectively. The weighted-average grant date fair value of RSUs granted during the years ended December 31, 2018, 2017 and 2016 was $37.6, $27.15 and $28.01 per share respectively.

We recognized stock-based compensation expenses of $11.3 million, $8.1 million and $5.3 million in the years ended December 31, 2018, 2017 and 2016, respectively, relating to these RSUs. As of December 31, 2018, there was $21.6 million of unrecognized compensation costs related to these RSUs, which is expected to be recognized over an estimated weighted-average period of 2.0 years.

In January 2015, the Compensation Committee of our Board of Directors approved the commencement of granting performance stock units to our employees. PSUs are share awards that entitle the holder to receive freely tradable shares of our Common Stock upon achievement of specified market or performance conditions. In January 2016, the Compensation Committee of our Board of Directors approved a program to award up to 102,906 PSUs to the management team based on the achievement of certain commercial and regulatory goals related to andexanet alfa and betrixaban, respectively. In January 2017, the Compensation Committee of our Board of Directors approved a program to award up to 143,750 PSUs to the management team based on the achievement of certain regulatory goals related to andexanet alfa. In March 2018, the Compensation Committee of our Board of Directors approved a program to award up to 102,600 PSUs to the management team based on the achievement of certain regulatory and net revenue goals.

The following table summarizes PSU activities under our 2013 Plan and related information:

	Shares
	Subject to			Weighted-
	Outstanding			Average Grant Date
	PSUs			Fair Value Per Share
Balance at December 31, 2017		304,754		$	25.34
PSUs granted		102,600			32.66
PSUs released		(218,107	)		25.26
PSUs canceled		(35,744	)		27.48
Balance at December 31, 2018		153,503		$	29.85

Additional information related to the status of PSUs at December 31, 2018, is as follows (aggregate intrinsic value in thousands):

Remaining

Contractual

Aggregate

Life

Intrinsic Value

Outstanding

153,503

0.7

2,996

The total grant date fair value of PSUs vested in 2018, 2017 and 2016 was $5.7 million, $2.1 million and $0.7 million, respectively. The weighted-average grant date fair value of PSUs granted in 2018, 2017 and 2016 was $32.66, $25.54 and $33.49 per share, respectively.

We recognized stock-based compensation expenses of $2.7 million, $2.4 million and $2.5 million in the years ended December 31, 2018, 2017 and 2016, respectively, relating to these PSUs. As of December 31, 2018, there was $0.5 million of unrecognized compensation costs related to these PSUs, which is expected to be recognized over an estimated weighted-average period of 0.7 years.

The Board of Directors adopted the 2013 ESPP, effective upon the completion of the initial public offering of our common stock. As of December 31, 2018, we reserved a total of 1,818,314 shares of common stock for issuance under the 2013 ESPP. The reserve for shares available under the ESPP automatically increases on January 1st each year, beginning in 2014, by an amount equal to 2% of the total number of outstanding shares of our common stock on December 31^st of the preceding fiscal year unless the Board of Directors elects to forego or reduce such increases. Since 2015, the Board of Directors elected to completely forego the automatic share increases available under the ESPP. The ESPP had 1,505,810 shares of common stock available for future issuance as of December 31, 2018. Eligible employees may purchase common stock at 85% of the lesser of the fair market value of our Common Stock on the first or last day of the offering period.

We have granted options to purchase shares of common stock to consultants in exchange for services performed. We granted options to purchase 37,000, 50,000 and 52,000 shares with average exercise prices of $45.16, 38.14 and $24.85 per share, respectively, during the years ended December 31, 2018, 2017, and 2016, respectively. These options vest upon grant or various terms up to four years. We recognized non-employee stock compensation expense of $12.1 million (including $9.2 million from the liability-classified Lonza award), $3.9 million and less than $0.1 million during the years ended December 31, 2018, 2017 and 2016, respectively. The fair value of non-employees’ options was measured using the Black-Scholes option-pricing model reflecting the same assumptions as applied to employee options in each of the reported years, other than the expected life assumption, which is assumed to be the remaining contractual life of the option. The compensation costs of these arrangements are subject to remeasurement over the vesting terms as earned.

Stock-based compensation expense is reflected in the consolidated statements of operations as follows (in thousands):

	Year Ended December 31,
	2018			2017		2016
Research and development	$	27,694		$	19,779	$	12,905
Selling, general and administrative		28,712			23,505		17,457
Subtotal	$	56,406		$	43,284	$	30,362
Capitalized stock-based compensation costs		(1,046	)		—		—
Stock-based compensation expense included in total expenses	$	55,360		$	43,284	$	30,362

Stock-based compensation capitalized into inventory is recognized as cost of sales when the related product is sold.

The fair value of our stock options including performance stock options and purchase rights under our ESPP were determined using the Black-Scholes option valuation model. Option valuation models require the input of subjective assumptions and these assumptions can vary over time. The risk-free rate is based on U.S. Treasury zero-coupon issues with remaining terms similar to the expected terms of the awards. The expected term of employee options granted is determined using the simplified method (based on the midpoint between the vesting date and the end of the contractual term). As sufficient trading history does not yet exist for our common stock, our estimate of expected volatility is based on the weighted average volatility of other companies with similar products under development, market, size and other factors and our volatility. To date, we have not declared or paid any cash dividends and do not have any plans to do so in the future. Therefore, we used an expected dividend yield of zero.

The following table illustrates the weighted-average assumptions for the Black-Scholes option-pricing model used in determining the fair value of these awards:

	Year Ended December 31,
	2018	2017	2016
Risk-free interest rate
Stock options	2.55% - 3.03%	1.70% - 2.27%	1.01% - 2.10%
Performance stock options	–	–	1.34% - 1.50%
ESPP	1.10% - 2.28%	0.47% - 1.10%	0.26% - 0.50%
Expected term
Stock options	5.0 - 6.1 years	5.0 - 6.1 years	5.0 - 6.1 years
Performance stock options	–	–	5.4 - 6.4 years
ESPP	0.5 years	0.5 years	0.5 years
Expected volatility
Stock options	59% - 62%	60% - 65%	62% - 66%
Performance stock options	–	–	65% - 66%
ESPP	49% - 65%	61% - 80%	54% - 99%
Dividend yield
Stock options	–	–	–
Performance stock options	–	–	–
ESPP	–	–	–