Material Event (Form 8-K)

On April 8, 2026, IDEAYA Biosciences, Inc. (the "Company") entered into a clinical collaboration agreement with AstraZeneca plc ("AstraZeneca") to evaluate the efficacy and safety of IDE849, the Company's investigational, potential first-in-classDLL3 TOP1 antibody-drug conjugate, in combination with AstraZeneca's Imfinzi^® (durvalumab), a programmed death-ligand 1 (PD-L1)inhibitor, in extensive-stage small cell lung cancer ("SCLC"). The Company will sponsor the clinical combination study and AstraZeneca will supply Imfinzi^®.

The Company is advancing a multi-site global Phase 1 clinical trial for IDE849 in DLL3 upregulated solid tumor indications, including SCLC, neuroendocrine carcinomas ("NECs"), neuroendocrine tumors ("NETs"), and melanoma. The study will be enrolling patients globally, including in North America, Europe, Australia, South America, and Asia.

DLL3 has been reported to be upregulated in multiple solid tumor types, including in SCLC, NECs, NETs, non-smallcell lung cancer, and melanoma, among others. DLL3 has limited extracellular expression in normal tissues, making it a promising potential therapeutic target in these solid tumors, for which there remains significant unmet medical need.