Total share-based expense related to stock options, restricted stock units (“RSUs”), common stock awards, and non-financing warrants were approximately $0.2 million and $0.4 million during the three months ended June 30, 2019 and 2018, respectively, and approximately $0.5 million and $0.9 million for the six months ended June 30, 2019 and 2018, 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, 2019 and 2018, respectively. The Company recognized a total of $0.2 million and $0.4 million of compensation expense related to employee stock options during the three months ended June 30, 2019 and 2018, respectively, and $0.5 million and $0.8 million during the six months ended June 30, 2019 and 2018, respectively.

Non-Employee Stock Options

The Company did not grant any non-employee stock options during the six months ended June 30, 2019. During the six months ended June 30, 2018, the Company granted 834 non-employee stock options. There were no non-employee stock options exercised during the six months ended June 30, 2019 and 2018, respectively. The Company did not recognize any expense related to non-employee stock options during the three and six months ended June 30, 2019, respectively, as all options were fully vested in 2018. The Company recognized approximately $26,000 and $35,000 of expense during the three and six months ended June 30, 2018, respectively.

Common Stock Awards

During the three months ended June 30, 2019 and 2018, the Company granted 1,188 and 598 common stock awards, respectively, and 1,873 and 1,273 common stock awards during the six months ended June 30, 2019 and 2018, 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 $15,000 and $17,000 of expense related to common stock awards was recognized during the three month periods ended June 30, 2019 and 2018, respectively, and approximately $32,000 and $35,000 of expense during the six months ended June 30, 2019 and 2018, respectively. Other than 7,836 shares of Common Stock issued in June 2019, which represented common stock awards authorized but not issued as of March 31, 2019, all common stock awards were authorized but not issued as of June 30, 2019.

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, 2019 and December 31, 2018, warrants to purchase 44,944 shares of common stock were outstanding, respectively. 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. These warrants have an average weighted exercise price of $124.90 and expiration dates ranging from December 2019 through May 2021. No warrants were granted or exercised in connection with collaboration or consulting services during the three and six months ended June 30, 2019 and 2018, respectively.