Under the original merger agreement, each ACELYRIN share was to be exchanged for 0.4274 shares of Alumis common stock, which would have left Alumis stockholders owning approximately 55% and ACELYRIN stockholders approximately 45% of the combined company on a fully diluted basis. On April 20, 2025, the parties amended the merger agreement to increase the exchange ratio to 0.4814 Alumis shares per ACELYRIN share, after ACELYRIN disclosed that it had received a competing, unsolicited acquisition proposal from Tang Capital Partners (the "Tang Proposal") consisting of $3.00 per share in cash plus a contingent value right tied to 80% of net proceeds from any future out-license or disposition of ACELYRIN's development programs, according to the joint proxy statement/prospectus on Form S-4. Following the amendment, Alumis stockholders were expected to own approximately 52% and ACELYRIN stockholders approximately 48% of the combined company on a fully diluted basis. The S-4 states that the amended exchange ratio represented a premium of approximately 67% relative to the unaffected market-based exchange ratio of ACELYRIN common stock on February 6, 2025. No cash consideration, dollar-denominated enterprise value, or EV/EBITDA-type multiple has been disclosed for the transaction, consistent with its structure as a stock-for-stock exchange between two clinical-stage, pre-revenue biopharmaceutical companies.
The companies disclosed a pro forma cash position of approximately $737 million as of December 31, 2024 (Alumis approximately $289 million and ACELYRIN approximately $448 million on a preliminary basis), which they said would provide runway into 2027. No cost synergy dollar figure or phase-in timeline was disclosed; the S-4/joint proxy statement cites only qualitative "cost synergies and operating efficiencies resulting from the elimination of redundant general and administrative costs" as a consideration for the ACELYRIN board, without quantifying the amount.
Per the joint proxy statement/prospectus, the Alumis Special Committee retained Morgan Stanley & Co. LLC, which rendered a fairness opinion dated April 20, 2025 concluding the amended exchange ratio was fair, from a financial point of view, to Alumis. ACELYRIN retained Guggenheim Securities, LLC, which rendered a fairness opinion, dated February 6, 2025, as to the fairness of the original exchange ratio to ACELYRIN stockholders. The joint proxy/S-4 also discloses that ACELYRIN, assisted by Guggenheim Securities, contacted 27 strategic parties to assess a potential transaction, and that a separate process previously run by Aquilo Partners had contacted 32 parties regarding a partnership for ACELYRIN's izokibep program; neither process produced an actionable alternative to the Alumis transaction other than the Tang Proposal, which the ACELYRIN board ultimately did not pursue in lieu of the amended Alumis merger.
Morgan Stanley & Co. LLC served as financial advisor to Alumis, and Cooley LLP served as its legal counsel. Guggenheim Securities, LLC served as financial advisor to ACELYRIN. ACELYRIN's legal representation included both Fenwick & West LLP, described in the S-4 as ACELYRIN's "recently engaged corporate counsel," and Paul Hastings LLP, identified in the S-4 as "legal counsel to ACELYRIN." Fenwick & West also delivered a tax opinion that the merger would more likely than not qualify as a tax-free "reorganization" under the Internal Revenue Code.
The original merger agreement, filed as Exhibit 2.1 to the February 6, 2025 Form 8-K, provided for a $10.0 million termination fee payable by either party under specified circumstances, including termination to accept a superior proposal, and an outside date of July 7, 2025. Alumis stockholders approved the related share-issuance proposal at a special meeting held May 13, 2025, with approximately 88.98% of outstanding shares represented and 41,861,422 votes in favor versus 159,077 against, according to the Form 8-K reporting the vote (Item 5.07). The merger closed on May 21, 2025: ACELYRIN merged with Alumis' wholly owned merger subsidiary and became a wholly owned subsidiary of Alumis, with each ACELYRIN share converted into the right to receive 0.4814 shares of Alumis common stock, per the closing Form 8-K (Item 2.01). ACELYRIN stock options with an exercise price of $18.00 or less were converted into Alumis options at the exchange ratio; options with an exercise price above $18.00 were cancelled without consideration. Outstanding ACELYRIN RSUs and PSUs (with PSUs deemed earned at 100% of target) were converted into Alumis RSUs at the exchange ratio. In connection with the closing, Alumis' board was expanded and Lynn Tetrault was appointed as a new independent director; Henry Gosebruch, who had also been expected to join the board, did not do so after being named CEO of Galapagos. The combined company operates under the Alumis name, led by Alumis' pre-existing executive team, with its headquarters remaining in South San Francisco, California. As permitted under Item 9.01(a)(3) of Form 8-K, audited financial statements of the acquired business and pro forma financial information were to be filed by amendment within 71 days of the closing 8-K's required filing date; this research did not locate or confirm the filing of that amendment.