Longer-Term Follow-Up of Western Patients Showed Improving, Favorable Trend in Overall Survival in Global Phase III HARMONi Clinical Trial for Ivonescimab Plus Chemotherapy in 2L+ EGFRm NSCLC

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With Longer-Term Follow-Up of Western Patients, Ivonescimab Plus Chemotherapy Demonstrated Improving Global OS Trend with Nominal p-value of 0.0332 vs. Chemotherapy Alone; North American Patients’ OS HR=0.70 Consistent Median Overall Survival Observed in Western, Asian Patients in Longer-Term Follow-Up Analysis of Western Patients Presented at Presidential Symposium at WCLC 2025 Conference Call to be Held at 8:00am ET on Monday, September 8, 2025 MIAMI --(BUSINESS WIRE)-- Summit Therapeutics Inc. (NASDAQ: SMMT) ("Summit," "we," or the "Company") today announced data from the Phase III HARMONi trial featuring the novel, potential first-in-class investigational bispecific antibody, ivonescimab. The data was presented this morning as part of the Presidential Symposium at the International Association for the Study of Lung Cancer’s (IASLC) 2025 World Conference on Lung Cancer (WCLC 2025) in Barcelona, Spain . The HARMONi presentation, Ivonescimab vs Placebo Plus Chemo, Phase 3 in Patients with EGFR+ NSCLC Progressed with 3rd gen EGFR-TKI Treatment: HARMONi, evaluated ivonescimab plus platinum-doublet chemotherapy compared to placebo plus platinum-doublet chemotherapy in patients with epidermal growth factor receptor (EGFR)-mutated, locally advanced or metastatic non-squamous non-small cell lung cancer (NSCLC) who have progressed after treatment with a 3rd generation EGFR tyrosine kinase inhibitor (TKI). This is a clinical setting with a patient population where PD-1 monoclonal antibodies have previously been unsuccessful in Phase III global clinical trials in showing either a progression-free survival (PFS) or overall survival (OS) benefit, the two primary endpoints of this clinical study. The trial results were presented by Jonathan Goldman , MD, Professor of Medicine at UCLA in the Hematology/Oncology Division, UCLA Director of Clinical Trials in Thoracic Oncology, Associate Director of Early Drug Development, and Chair of University of California Lung Cancer Consortium . Clinically Meaningful Efficacy As previously disclosed, ivonescimab in combination with chemotherapy showed a positive trend in OS in the primary analysis without achieving a statistically significant benefit with a hazard ratio of 0.79 (95% CI: 0.62 – 1.01; p=0.057). The statistical analysis plan for the study required a p-value of 0.0448 in order to achieve statistical significance at the time of the primary analysis of overall survival. Median overall survival was 16.8 months for those patients administered ivonescimab plus chemotherapy vs. 14.0 months for those receiving placebo plus chemotherapy. It was noted at the time of the primary analysis that the median follow-up time for western patients was 9.2 months and less than the median overall survival at the time of the primary analysis, and these patients may continue to be followed for long-term outcomes. Primary Analysis (DCO: Apr 2025 ) Ivonescimab + Chemo (n=219) Placebo + Chemo (n=219) Median Overall Survival, ITT 16.8 mos 14.0 mos Hazard Ratio 0.79 (95% CI: 0.62 – 1.01; p=0.057) DCO = data cut-off; ITT = i ntention t o t reat population; mos = months In September 2025 , an additional analysis was performed, whereby the western patients were followed to increase their time on study (Asian patients were locked at the time of the primary analysis). In this analysis that included longer-term follow-up of western patients (median follow-up time of western patients of 13.7 months), a hazard ratio consistent with the primary analysis was observed with an improved nominal p-value (HR=0.78; 95% CI: 0.62 – 0.98; nominal p=0.0332). Median OS for this analysis remained the same in both arms from the primary analysis. Median OS in western patients receiving ivonescimab was 17.0 months compared to 14.0 months for those receiving placebo (HR=0.84); median OS in North American patients, specifically, had not yet been reached in the ivonescimab arm compared to 14.0 months in the placebo arm (HR=0.70). The hazard ratios for western patients in totality, as well as patients from the North American and European regions individually, improved from the primary OS analysis to the analysis with longer-term follow-up of western patients. Consistent benefit was observed across pre-defined subgroups. Longer-Term Follow-Up of Western Patients Analysis (DCO: Sept 2025 ) Ivonescimab + Chemo Placebo + Chemo Median Overall Survival, ITT 16.8 mos (n=219) 14.0 mos (n=219) Hazard Ratio , ITT 0.78 (95% CI: 0.62 – 0.98; nominal p=0.0332) Median Overall Survival, Western 17.0 mos (n=83) 14.0 mos (n=82) Hazard Ratio , Western 0.84 Median Overall Survival, N. America Not Reached (n=43) 14.0 mos (n=50) Hazard Ratio , N. America 0.70 Median Overall Survival, Asia 16.7 mos (n=136) 14.0 mos (n=137) Hazard Ratio , Asia 0.76 DCO = data cut-off; ITT = i ntention t o t reat population; mos = months Note: North American patients are a subset of Western patients. As previously disclosed at the prespecified primary data analysis for PFS, ivonescimab in combination with chemotherapy demonstrated a statistically significant and clinically meaningful improvement with a hazard ratio of 0.52 (95% CI: 0.41 – 0.66; p

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