7. Stock-based Compensation
We have an incentive plan that authorized an aggregate of 20,035,000 shares of common stock for awards to employees, directors and consultants. The equity incentive plan authorizes the issuance of equity-based compensation in the form of stock options, stock appreciation rights, restricted stock, restricted stock units, performance shares and units, and other stock-based awards. In the three-month period ended June 30, 2018, we granted 2,078,340 stock options and 787,968 restricted stock units to our employees and directors primarily pursuant to our annual incentive programs. As of June 30, 2018, a total of 4,671,812 shares (including 243,940 shares related to an expired incentive plan) of common stock have been issued under our equity incentive plans.
As of June 30, 2018, a total of 3,696,351 shares were available for issuance under our equity incentive plan, and stock-based awards to purchase 12,846,533 shares (including 935,756 shares related to an expired incentive plan) of common stock were outstanding. For the three-month periods ended June 30, 2018 and 2017, stock-based compensation expense was approximately $0.8 million and $0.7 million, respectively. At June 30, 2018, total unrecognized estimated compensation cost related to unvested stock-based awards was approximately $10.4 million, which is expected to be recognized by the end of 2022 using the straight-line method.
8. Stockholders’ Equity
Equity Issuance—Healios
In March 2018, Healios purchased 12,000,000 shares of our common stock (the “Shares”) for $21.1 million, or approximately $1.76 per share, and the Healios Warrant to purchase up to an additional 20,000,000 Warrant Shares of common stock. In connection with the issuance of the Shares, we and Healios entered into an Investor Rights Agreement, which governs certain rights of Healios and us relating to Healios’ ownership of our common stock, including the Shares and the Warrant Shares. The Investor Rights Agreement provides for customary standstill and voting obligations, transfer restrictions and registration rights for Healios. Additionally, we agree to provide notice to Healios of certain equity issuances and to allow Healios to participate in certain issuances in order maintain its proportionate ownership of our common stock as of the time of such issuance. We further agreed under the Investor Rights Agreement that during such time as Healios beneficially owns more than 5.0% but less than 15.0% of our outstanding common stock, our Board of Directors (the “Board”) will nominate a Healios nominee suitable to us to become a member of the Board, and during such time as Healios beneficially owns 15.0% or more of our outstanding common stock, our Board will nominate two suitable Healios nominees to become members of the Board, at each annual election of directors. Healios nominated an individual to the Board, who was elected at the 2018 annual stockholders’ meeting. As a result of Healios’ investment, Healios became a related party, and the transactions with Healios are separately identified within these financial statements as they are related party transactions.
The value of the Healios Warrant was considered as an element of compensation in the transaction price of the Healios expansion, as discussed in Note 6. The Healios Warrant originally did not become effective until the CEA became effective in June 2018 and the first payment was made under the expansion. Upon such effectiveness, the Healios Warrant became exercisable with respect to 4,000,000 Warrant Shares and the remaining 16,000,000 Warrant Shares will become exercisable if Healios agrees to execute the option for a license in China in September 2018. Other important Healios Warrant terms include expiration in September 2020 (subject to a potential extension), fixed and floating exercise price mechanisms, and an exercise cap triggered at Healios’ ownership of 19.9% of our common stock. The Healios Warrant may be terminated by us under certain conditions.
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