Day One shares surge 12.96% after-hours on positive 3-year FIREFLY-1 trial data and Mersana acquisition.

Day One Biopharmaceuticals (DAWN) surged 12.96% in after-hours trading following the release of positive three-year follow-up results from its pivotal Phase 2 FIREFLY-1 trial of OJEMDA (tovorafenib), demonstrating sustained efficacy and safety in treating pediatric low-grade glioma. The data, announced ahead of the 2025 Society for Neuro-Oncology Annual Meeting, reinforced the drug’s commercial potential. Additionally, the company’s acquisition of Mersana Therapeutics, valued up to $285 million, expanded its pipeline of antibody-drug conjugate technologies, signaling strategic growth and operational synergy. These developments, emphasizing therapeutic innovation and pipeline expansion, directly aligned with the stock’s upward trajectory, overshadowing unrelated news about competitors or non-healthcare events.