ASTX-295

- Medical oncologist and accomplished clinical leader brings deep development and industry experience to Mosaic’s Executive Leadership team - Vince will lead research and development for Mosaic’s drug combination programs, including the build-out of the Company’s early pipeline CAMBRIDGE, England--(BUSINESS WIRE)--Mosaic Therapeutics, Ltd, (‘Mosaic’, or ‘the Company’) a clinical-stage oncology therapeutics company developing next-generation, Synergistic Precision combination therapies, today announced the appointment of Dr Vince O’Neill as Head of Research and Development (R&D) to support its next phase of growth. “We are delighted to welcome Vince to the Mosaic leadership team at this pivotal stage of the company’s evolution,” said Thomas Fuchs, CEO, Mosaic Therapeutics. “His deep expertise in targeted oncology drug development and diagnostics will be instrumental as we advance our pipeline to bring transformative therapies to patients in need.” Vince is a board-certified medical oncologist and highly experienced biotechnology executive, bringing over two decades of therapeutic and diagnostic R&D experience within industry to Mosaic Therapeutics, with a track record spanning from IND through to NDA/BLA/MAA approval. Most recently he served at BioXcel Therapeutics as EVP, Head of Product Development and Chief Medical Officer, having previously held roles including Chief R&D Officer at OnkosXcel Therapeutics, and CMO roles at MIRNA Therapeutics and Exosome Diagnostics. Additionally, he has held various senior clinical development and medical leadership positions at Sanofi-Aventis, GSK, Genentech and Roche, playing an instrumental role in the development and commercialisation of multiple high-impact oncology therapeutics, including Avastin, Venclexta, Tarceva, Merkinist, and Tafinlar. In early-stage biotech, he has also secured successful public ($420M) and private ($65M series B) financing raises as part of executive leadership teams. He obtained his BSc, MD and MBChB in Medicine from the University of Glasgow. Vince O’Neill, Head of R&D, Mosaic Therapeutics, commented: “ I look forward to working closely with the Board and the team to advance our programs into the clinic and for the opportunity to develop novel combination therapies for cancer patients, utilising innovative biomarker strategies to maximise patient benefit.” About Mosaic Therapeutics Mosaic Therapeutics is an oncology therapeutics company building the category leader in Synergistic Precision Oncology. The company’s mission is to tackle cancers with substantial unmet need and few targeted treatment options, by applying its advanced experimental and computational platform to identify oncology combinations that have synergistic activity in biomarker-defined patient populations. Born out of pioneering research at the Wellcome Sanger Institute, in collaboration with the Netherlands Cancer Institute, Mosaic has active programs across a variety of oncology indications and disease biology pathways. The Company’s pipeline is anchored by ASTX295, an MDM2 antagonist, and ASTX029, an ERK1/2 inhibitor, both in-licensed from Astex Pharmaceuticals in April 2025. Mosaic’s first clinical combination study is expected to commence in early 2027. LinkedIn: Website: Contacts Thomas Fuchs CEO, Mosaic Therapeutics E: info@mosaic-tx.com

First preclinical data on the Company’s lead program MOS101: a combination of ASTX295, a next-generation, clinical-stage MDM2 antagonist, and olaparib, an FDA-approved PARP inhibitor, in biomarker-selected solid tumors Additional data showcasing Mosaic’s Discovery Platform, PRIME, will be presented CAMBRIDGE, England--(BUSINESS WIRE)--Mosaic Therapeutics, Ltd, (‘Mosaic’, or ‘the Company’) a clinical-stage oncology therapeutics company building the category leader in Synergistic Precision Oncology, today announced it will hold a poster presentation at the American Association for Cancer Research (AACR) Annual Meeting 2026, taking place April 17-22 in San Diego, CA. Poster Presentation Details: Title: "Combination of the MDM2 antagonist ASTX295 and olaparib as a novel treatment option for BRCA2 mutant, TP53 wild-type solid tumors” Abstract Number: 3053 Date and Time: Monday, April 20, 2026, 2:00PM – 5:00PM PST Session Category: Novel Targets and Pathways Session: PO.ET09.09 Location: Poster Section 15 For more information and to view the Company’s abstract, visit the AACR Annual Meeting website . About ASTX295 ASTX295 is a potent, highly selective, potentially best-in-class MDM2 antagonist that has completed a phase I study (NCT03975387) in over 100 patients with advanced solid tumors. Rationally designed to exhibit a short half-life, ASTX295 has demonstrated a compelling safety pro minimal hematological toxicities, enabling use as a combination therapy. MDM2 is a clinically validated target for cancer therapy, frequently overexpressed across multiple tumor types. As the primary negative regulator of the p53 tumor suppressor protein, MDM2 inhibition leads to p53 stabilization and re-activation, inducing cell cycle arrest and apoptosis. Mosaic Therapeutics announced the in-licensing of ASTX295, alongside ASTX029 (an ERK1/2 inhibitor), from Astex Pharmaceuticals in April 2025, and holds full development and commercialization rights. About MOS101 Plans for a Phase 1b/2a study evaluating the combination of ASTX295 with olaparib (an approved PARP inhibitor) for patients with biomarker-selected solid tumors are being finalized. About Mosaic Therapeutics Mosaic Therapeutics is an oncology therapeutics company building the category leader in Synergistic Precision Oncology. The company’s mission is to tackle cancers with substantial unmet need and few targeted treatment options, by reinventing the traditional approach to discovering and developing novel combination medicines. Born out of pioneering research at the Wellcome Sanger Institute, in collaboration with the Netherlands Cancer Institute, and anchored around a proprietary platform that has seen 20 years of fine-tuning and development, Mosaic has active programs across a variety of oncology indications and disease biology pathways. Mosaic is applying its advanced experimental and computational platform to identify oncology combinations that have synergistic activity in biomarker-defined patient populations. The Company’s pipeline is anchored by ASTX295, an MDM2 antagonist, and ASTX029, an ERK1/2 inhibitor, both in-licensed from Astex Pharmaceuticals in April 2025. Mosaic’s first clinical combination study is expected to commence in early 2027. LinkedIn: Website: Contacts Thomas Fuchs CEO, Mosaic Therapeutics E: info@mosaic-tx.com