Total share-based payments related to stock options, restricted stock units (“RSUs”), common stock awards, and non-financing warrants were approximately $0.4 million and $0.5 million during the three months ended June 30, 2018 and 2017, respectively, and approximately $0.9 million and $1.2 million for the six months ended June 30, 2018 and 2017, respectively.

Share-based compensation expense is classified in the condensed consolidated statements of operations as follows:

Employee Stock Options

No employee stock options or RSUs were granted nor exercised during the six months ended June 30, 2018. During the six months ended June 30, 2017, the Company granted 350,000 employee stock options. The key valuation assumptions used consisted of the Company’s stock price, a risk-free rate of 0.19% and an expected volatility of approximately 110%. In addition, during the six months ended June 30, 2017, the Company extended the exercise expiration date of certain former employee stock option awards resulting in a change in incremental value of approximately $4,000, which was charged to administrative expense. The Company recognized a total of $0.3 million and $0.5 million of compensation expense related to employee stock options during the three months ended June 30, 2018 and 2017, respectively, and $0.7 million and $1.1 million during the six months ended June 30, 2018 and 2017, respectively.

Non-Employee Stock Options

During the six months ended June 30, 2018, the Company granted 10,000 non-employee stock options. The key valuation assumptions used consisted of the Company’s stock price, a risk-free rate of 1.70% and an expected volatility of 118.1%. No non-employee stock options were exercised during the six months ended June 30, 2018. There were no non-employee stock options granted or exercised during the six months ended June 30, 2017. The Company recognized approximately $26,000 and $35,000 of expense related to non-employee stock options during the three and six months ended June 30, 2018, respectively, and approximately $30,000 and $70,000 of expense during the three and six months ended June 30, 2017, respectively.

Common Stock Awards

During the three months ended June 30, 2018 and 2017, the Company granted 6,924 and 7,235 common stock awards, respectively, and 15,018 and 12,710 common stock awards during the six months ended June 30, 2018 and 2017, respectively, based on the value of the professional services provided and the average stock price during each respective quarter. As all services were rendered in each respective quarter, approximately $17,000 and $30,000 of expense related to common stock awards was recognized during the three month periods ended June 30, 2018 and 2017, respectively, and approximately $35,000 and $60,000 of expense during the six months ended June 30, 2018 and 2017, respectively. All common stock awards were authorized but not issued as of June 30, 2018.

Warrants

In connection with certain of the Company’s collaboration agreements and consulting arrangements, the Company has issued warrants to purchase shares of common stock as payment for services. As of June 30, 2018 and December 31, 2017, warrants to purchase 646,249 shares of common stock were outstanding. The fair value of these warrants was determined at each issuance date using the Black-Scholes option pricing model. The warrants are subject to re-measurement at each reporting period until the measurement date is reached. Expense is recognized on a straight-line basis over the expected service period or at the date of issuance, if there is not a service period. The Company recognized expense of approximately $59,000 and $64,000 related to collaboration and consulting warrants during the three and six months ended June 30, 2018, respectively. For the three and six months ended June 30, 2017, the Company recognized a gain of approximately $80,000 and approximately $0.1 million, respectively, related to collaboration and consulting warrants.

No collaboration or consulting service warrants were issued or exercised during the first six months of 2018 and 2017.