5. Shared-Based Payments

Stock Option Plan

The Company’s 2000 Stock Option and Incentive Plan (“2000 Plan”) allowed for the grant of awards in respect of an aggregate of 130,725 shares, which was increased to 150,297 shares, of the Company’s common stock in the form of incentive stock options, non-qualified stock options, stock appreciation rights, restricted stock and restricted stock units and other performance awards. As of December 31, 2013, under the 2000 Plan, there were options to purchase an aggregate of 23,657 shares of common stock outstanding at a weighted average exercise price of $0.83 per share. The 2000 Plan has expired, and no further awards may be issued under the plan. Any shares of common stock subject to awards under the 2000 Plan that expire, terminate, or are otherwise surrendered, canceled, forfeited or repurchased without having been fully exercised, or resulting in any common stock being issued, will become available for issuance under the 2013 Stock Incentive Plan (“2013 Plan”) up to a specified number of shares.

Effective February 2003, the Company implemented the 2003 Equity Incentive Plan (“2003 Plan”), and it was amended and approved by the Company’s stockholders in 2005. The 2003 Plan originally allowed for the grant of awards in respect of an aggregate of 2,051,644 shares of the Company’s common stock. Between 2006 and 2010 the maximum number of shares of common stock authorized to be issued by the Company under the 2003 Plan was increased by 1,739,116 shares to 3,790,760. During the year ended December 31, 2012, the maximum number of shares of common stock authorized to be issued by the Company under the 2003 Plan was increased by 545,970 shares to 4,336,730. Stock options granted under the 2003 Plan may be either incentive stock options as defined by the Internal Revenue Code (“IRC”), or non-qualified stock options.

As of December 31, 2013, under the 2003 Plan, there were options to purchase an aggregate of 2,559,011 shares of common stock outstanding at a weighted average exercise price of $1.52 per share. Upon the completion of the IPO, the 2003 Plan was terminated, and no further awards may be issued under the plan. Any shares of common stock subject to awards under the 2003 Plan that expire, terminate, or are otherwise surrendered, canceled, forfeited or repurchased without having been fully exercised, or resulting in any common stock being issued, will become available for issuance under the 2013 Plan, up to a specified number of shares.

In October 2013, the Company implemented the 2013 Plan. The 2013 Plan provides for the grant of stock options and other stock-based awards, as well as cash-based performance awards. The aggregate number of shares of common stock initially available for issuance pursuant to awards under the 2013 Plan is 1,960,168 shares. The number of shares of common stock reserved for issuance will automatically increase on January 1 of each year from January 1, 2014 through and including January 1, 2023, by the lesser of (a) 1,960,168 shares, (b) 4.0% of the total number of shares of common stock outstanding on December 31 of the preceding calendar year, or (c) the number of shares of common stock determined by the Board of Directors. All of the shares available for issuance under the 2013 Plan are eligible for issuance pursuant to the exercise of incentive stock options. If an option expires or terminates for any reason without having been fully exercised, if any shares of restricted stock are forfeited, or if any award terminates, expires or is settled without all or a portion of the shares of common stock covered by the award being issued, such shares are available for the grant of additional awards. However, any shares that are withheld (or delivered) to pay withholding taxes or to pay the exercise price of an option are not available for the grant of additional awards. As of December 31, 2013, under the 2013 Plan, there were options to purchase an aggregate of 618,290 shares of common stock outstanding at a weighted average exercise price of $19.06 per share.

Stock Option Exchange

On March 16, 2011 (“Exchange Date”), the Company modified certain outstanding options with exercise prices of $1.88 and $4.69 (“Original Options”). These Original Options were canceled and replaced with options having an exercise price of $0.94 (“Replacement Options”), reflecting the current fair market value of the Company’s common stock on the Exchange Date. Original Options submitted for exchange were replaced on a one-for-one basis with Replacement Options. Additionally, the Replacement Options retain all terms and conditions of the Original Options except for the reduction to the exercise price as described above.

Total compensation associated with the Replacement Options consisted of the grant-date fair value of the Original Options for which the requisite service period is expected to be rendered (or has already been rendered) at the Exchange Date, plus the incremental cost associated with the modification of terms. The incremental compensation expense was measured as the excess of the fair value of the Replacement Options over the fair value of the Original Options re-measured as of the Exchange Date. A total of 1,921,894 Original Options were exchanged for Replacement Options.

The following stock-based compensation amounts were recognized for the periods indicated:


	Year Ended December 31,
	2013		2012		2011
Research and development	$	507,142	$	471,809	$	1,018,935
General and administrative		355,020		366,586		1,328,504

Total stock-based compensation expense	$	862,162	$	838,395	$	2,347,439

Employee Stock Options

The fair value of each option award is estimated on the date of grant using the Black-Scholes option-pricing model using the assumptions in the following table:


	Year Ended December 31,
	2013	2012	2011
Expected dividend yield..	0%	0%	0%
Expected volatility	53% - 67%	51%	62%
Risk-free interest rate	1.24% - 2.19%	1.18%	1.35%
Expected term.	7 years	7 years	7 years
Expected forfeiture rate..	5.06% - 5.57%	5.57%	5.58%

Expected Dividend Yield—The Company has never declared or paid dividends and has no plans to do so in the foreseeable future.

Expected Volatility—Volatility is a measure of the amount by which a financial variable such as a share price has fluctuated (historical volatility) or is expected to fluctuate (expected volatility) during a period. As the Company does not yet have sufficient history of its own volatility, the Company has identified several public entities of similar size, complexity and stage of development and calculates historical volatility using the volatility of these companies.

Risk-Free Interest Rate—This is the U.S. Treasury rate for the week of each option grant during the year, having a term that most closely resembles the expected life of the option.

Expected Term—This is the period of time that the options granted are expected to remain unexercised. Options granted have a maximum term of ten years. The Company estimates the expected life of the option term to be seven years. The Company uses a simplified method to calculate the average expected term.

Expected Forfeiture Rate—The forfeiture rate is the estimated percentage of options granted that is expected to be forfeited or canceled on an annual basis before becoming fully vested. The Company estimates the forfeiture rate based on turnover data with further consideration given to the class of the employees to whom the options were granted.

Equity instruments issued to non-employees are accounted for under the provisions of ASC 505-50, Equity Based Payments to Non-Employees. Accordingly, the estimated fair value of the equity instrument is recorded on the earlier of the performance commitment date or the date the services required are completed.

The following table summarizes stock option activity for 2013:


	Shares			Weighted- Average Exercise Price		Weighted-Average Remaining Contractual Term (Years)		Aggregate Intrinsic Value (in thousands)
Outstanding, December 31, 2012		3,249,541		$	0.94		7.3
Granted		1,307,101			10.70
Exercised		(1,268,049	)		0.85
Forfeited or expired		(87,635	)		1.11

Outstanding, December 31, 2013		3,200,958			4.90		6.9	$	67,532

December 31, 2013:
Exercisable		1,688,251			0.94		4.8		42,311
Vested and expected to vest		3,022,948			4.64		6.8		64,574

During 2013, 2012 and 2011 the Company issued 1,268,049, 863,176 and 49,883 shares of common stock, respectively, in conjunction with stock option exercises. The Company received cash proceeds from the exercise of these stock options of approximately $1.1 million, $46,826 and $53,225 during 2013, 2012 and 2011, respectively.

The weighted-average grant-date fair value of options granted during 2013, 2012 and 2011 was $6.91, $0.94 and $0.94 per share, respectively. The total intrinsic value of options exercised during 2013, 2012 and 2011 was approximately $5.4 million, $271,929 and $102,418, respectively. The total fair value of stock options which vested during 2013, 2012 and 2011 was $487,603, $879,024 and $1,108,107, respectively. As of December 31, 2013, the total unrecognized compensation expense related to non-vested stock options, net of related forfeiture estimates, was $7.6 million, which the Company expects to recognize over a weighted-average period of approximately four years.