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 $0.7 million during the three months ended March 31, 2018 and 2017, respectively.

Share-based payments are 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 three months ended March 31, 2018. During the three months ended March 31, 2017, the Company granted 175,000 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 110%. In addition, during the three months ended March 31, 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 $0.4 million and $0.6 million of compensation expense related to employee stock options during the three months ended March 31, 2018 and 2017, respectively. No RSUs were granted nor exercised during the three months ended March 31, 2017.

Non-Employee Stock Options

During the three months ended March 31, 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 1.7% and an expected volatility of 118.1%. No non-employee stock options were exercised during the three months ended March 31, 2018 or 2017. The Company recognized approximately $9,000 and $44,000 of expense related to non-employee stock options during the three months ended March 31, 2018 and 2017, respectively.

Common Stock Awards

During the three months ended March 31, 2018 and 2017, the Company granted 8,094 and 5,475 common stock awards, 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 $31,000 of expense related to common stock awards was recognized during each of the three-month periods ended March 31, 2018 and 2017, respectively. All common stock awards were authorized but not issued as of March 31, 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 March 31, 2018 and December 31, 2017, warrants to purchase 646,249 shares of common stock were outstanding. The fair value of these warrants were 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 approximately $5,000 of expense on revaluation of warrants for the three months ended March 31, 2018. During the three months ended March 31, 2017, the Company recognized an approximate gain of $15,000 as a result of a reduction in estimated fair value of warrants. No warrants were granted or exercised in connection with collaboration agreements and consulting services during the three months ended March 31, 2018 and 2017.