Our vision for STAR-0215 is supported by preclinical data showing potent inhibition of the production of bradykinin by plasma kallikrein and a long plasma half-life that could potentially enable patients to dose less frequently. Multiple experiments have confirmed that STAR-0215 is approximately 10-fold more potent than lanadelumab, a monoclonal antibody inhibitor of plasma kallikrein commercialized under the name TAKHZYRO and an approved preventative treatment for HAE, in inhibiting bradykinin production. In cynomolgus monkey studies, lanadelumab was observed to have a half-life of approximately 10 days, which is consistent with what has been reported in U.S. Food and Drug Administration review documents and publications for lanadelumab in non-human primates. STAR-0215 was administered at the same dose as lanadelumab and the observed half-life was approximately 34 days, which is about a three to four-fold longer half-life than observed for lanadelumab. We believe this could translate to a half-life of several months for STAR-0215 in humans. If this longer half-life is demonstrated in clinical trials, it has the potential to enable dosing once every three months or longer.
We presented new preclinical data from the STAR-0215 program at the American College of Allergy, Asthma and Immunology, or ACAAI, Annual Scientific Meeting in November 2021, demonstrating the high potency of STAR-0215 for binding to and inhibition of plasma kallikrein on a different site than lanadelumab and supporting the ability of YTE technology to extend half-life. YTE modifications in STAR-0215 are designed to enable a longer duration of action. In cynomolgus monkeys dosed with STAR-0215, the YTE modifications protected STAR-0215 from antibody clearance leading to a more than three-fold increase in plasma half-life compared to an antibody without the YTE modifications. The preclinical data to date suggest that at equal doses STAR-0215 would have a significantly longer duration of action than lanadelumab and could result in STAR-0215 being an effective preventative therapy for patients with HAE due to inhibition of the pathologic activity of plasma kallikrein for an extended time period.
January 2021 Quellis Acquisition and February 2021 Financing
In January 2021, we acquired Quellis pursuant to an Agreement and Plan of Merger, or the Merger Agreement, by and among us, Cabo Merger Sub I, Inc., a Delaware corporation and our wholly owned subsidiary, or the First Merger Sub, Cabo Merger Sub II, LLC, a Delaware limited liability company and our wholly owned subsidiary, or the Second Merger Sub, and Quellis, or the Quellis Acquisition. Pursuant to the Merger Agreement, the First Merger Sub merged with and into Quellis, pursuant to which Quellis was the surviving entity and became a wholly owned subsidiary of ours, or the First Merger. Immediately following the First Merger, Quellis merged with and into the Second Merger Sub, pursuant to which the Second Merger Sub was the surviving entity, or the Second Merger and, together with the First Merger, the Merger. Under the terms of the Merger Agreement, at the closing of the Merger, we issued to the Quellis stockholders 555,444 shares of our common stock, and 50,504 shares of newly designated Series X Preferred Stock (as described below). In addition, we assumed outstanding Quellis stock options, which became options for 55,414 shares of our common stock, and assumed a warrant exercisable for Quellis common stock, which became a warrant to purchase 2,805 shares of Series X Preferred Stock at an exercise price of $341.70 per share, and a warrant to purchase 30,856 shares of our common stock at an exercise price of $2.10 per share. Upon stockholder approval of the Conversion Proposal (as defined below) on June 2, 2021, the warrant to purchase Series X Preferred Stock was converted into the right to purchase 467,500 shares of our common stock, at a per share exercise price of $2.10 per share.
In January 2021, we also entered into a Stock Purchase Agreement, or the Purchase Agreement, with certain institutional and accredited investors pursuant to which, we sold an aggregate of 35,573 shares of Series X Preferred Stock for an aggregate purchase price of $110.0 million, or the February 2021 Financing. Our stockholders approved the conversion of the Series X Preferred Stock into shares of common stock in accordance with Nasdaq Listing Rule 5635(a), or the Conversion Proposal, at our annual stockholders meeting on June 2, 2021. On the fourth business day after the approval, each share of Series X Preferred Stock automatically converted into 166.67 shares of common stock, subject to certain beneficial ownership limitations, including that a holder of Series X Preferred Stock is prohibited from converting shares of Series X Preferred Stock into shares of common stock if, as a result of such conversion, such holder, together with its affiliates, would beneficially own more than a specified percentage (as of September 30, 2021, these percentages are set at 4.99% to 9.99% and can be adjusted by the holder to a number between 4.99% and 19.99%) of the total number of shares of common stock issued and outstanding immediately after giving effect to such conversion. Shares of Series X Preferred Stock not converted automatically are thereafter subject to conversion at the option of the holder, subject to certain beneficial ownership limitations. As of September 30, 2021, 54,622 shares of Series X Preferred Stock were converted into 9,103,664 shares of common stock and 31,455 shares of Series X Preferred Stock remained outstanding. Each share of Series X Preferred Stock is convertible into 166.67 shares of common stock and therefore the number of shares of underlying common stock issuable upon conversion of the outstanding shares Series X Preferred Stock is 5,242,501. Outstanding shares of Series X Preferred Stock are subject to conversion at the option of the holder.