Acquisition of Mersana Therapeutics N O V E M B E R 2 0 2 5 Day One's Mission Inspired by the urgent needs of children, Day One creatively and intentionally develops new medicines for people of all ages with life- threatening diseases Q&A Session All Participants 2 Confidential Information. For internal use only. Forward Looking Statements This communication contains forward-looking statements. Forward-looking statements are generally identified by the words "expects", "anticipates", "believes", "intends", ...

Core Viewpoint - Jasper Therapeutics faced a significant setback due to issues with a batch of its investigational antibody drug, briquilimab, which interfered with the clinical trial results for chronic spontaneous urticaria (CSU), leading to a 55% drop in its stock price [1][2]. Group 1: Drug Batch Issue - A problematic batch of briquilimab affected the BEACON Ib/IIa clinical trial results, which involved approximately 40 patients in a randomized, double-blind, placebo-controlled study [1]. - Data from 13 patients indicated that those using the problematic batch showed "lower than expected" improvement in tryptase levels, a key pharmacodynamic outcome for the trial [1]. - The primary efficacy measure, urticaria activity score, showed "no significant impact" for the 10 patients using the affected batch [1]. Group 2: Market Reaction and Financial Impact - Following the announcement, Jasper's stock plummeted by 55%, prompting the company to consider cost-cutting measures, including potential strategic restructuring [2]. - As of March 31, Jasper had $48.8 million in cash and cash equivalents, which may be impacted by the current market environment and investor sentiment [2]. - Analysts from BMO Capital Markets suggested that the batch issue would make it harder for Jasper to attract investors in the current market climate [2]. Group 3: Future Plans and Adjustments - Jasper is investigating the specific reasons behind the batch issue and plans to provide new medication to the affected patients from an earlier cohort that showed efficacy [2]. - The company intends to add 10 to 12 new patients to the affected dosage groups to ensure data robustness, with results expected in Q4 [2]. - The initiation of the IIb clinical trial for briquilimab in CSU has been delayed to mid-2026, and the company will no longer pursue the drug for asthma indications [2][3].