7. Warrants

The following is a summary of activity in the number of warrants outstanding to purchase the Company’s common stock for the three months ended March 31, 2021 (in thousands):

Warrants Classified as Liabilities

Liability-classified warrants consist of warrants issued by Biozone in connection with equity financings in October 2013 and January 2014, which were assumed by the Company in connection with its merger with Biozone in January 2014. Warrants accounted for as liabilities have the potential to be settled in cash or are not indexed to the Company’s own stock.

The estimated fair value of outstanding warrants accounted for as liabilities is determined at each balance sheet date. Any decrease or increase in the estimated fair value of the warrant liability since the most recent balance sheet date is recorded in the condensed consolidated statement of operations as changes in fair value of derivative liabilities.

The fair value of the warrants classified as liabilities is estimated using the Black-Scholes option-pricing model with the following inputs as of March 31, 2021:

The fair value of the warrants classified as liabilities is estimated using the Black-Scholes option-pricing model with the following inputs as of December 31, 2020:

The Company estimates volatility using a blend of its own historical stock price volatility as well as that of market comparable entities since the Company’s common stock has limited trading history and limited observable volatility of its own. The expected life assumption is based on the remaining contractual terms of the warrants. The risk-free rate is based on the zero-coupon rates in effect at the balance sheet date. The dividend yield used in the pricing model is zero, because the Company has no present intention to pay cash dividends.