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Chief Medical Officer
Dr. Arbet-Engels joined X4 in August 2023 as Chief Medical Officer. He brings to X4 more than 25 years of experience in strategic global drug discovery, development, and commercialization across a broad range of therapeutic areas including gastroenterology, cardiovascular-metabolism, neurology, oncology, and rare and orphan diseases.
Dr. Arbet-Engels previously served as Chief Medical Officer at several life science companies, including Neurogastrx, Millendo Therapeutics, and Poxel Pharmaceuticals, where he provided research and development leadership, advancing multiple candidates from pre-clinical through to late-stage clinical trials and assisting with both financial and strategic partnering transactions. Prior to these roles, he held a variety of senior medical and clinical positions at Biogen, Boehringer-Ingelheim Pharmaceuticals, Hoffmann-La Roche, Merck Research Laboratories, Aventis Pharmaceuticals, and Ligand Pharmaceuticals, where he led clinical development and registration, launch, and lifecycle management efforts for several new medicines, including the blockbuster therapies LANTUS® and JARDIANCE®.
Dr. Arbet-Engels has also served in educational roles at The Salk Institute for Biological Studies, University of Paris VI, and the Assistance Publique, Hospitals of Paris, and currently serves as a Member of the Board of Tutors in Biochemical Sciences at Harvard University. He received his MD and PhD in internal medicine/endocrinology-metabolism from the University of Paris, France, and completed his MBA at Rutgers University.
Chief Commercial Officer
Mr. Baldry joined X4 in November 2022 as Chief Commercial Officer, bringing more than 30 years of international life science commercial experience to the role, with particular expertise in building high- performing commercial teams, developing effective global strategies, and launching new therapeutics into both specialty and rare disease markets.
Mr. Baldry previously served in the roles of Chief Commercial Officer at Freeline Therapeutics Holdings, Chief Commercial Officer at Wave Life Sciences, and Vice President, Global Marketing and then Senior Vice President, Global Marketing & Commercial Operations at Amicus Therapeutics. In these roles, he was responsible for building and executing on global commercial strategies to support successful launches of innovative medicines in orphan diseases, including the first oral chaperone therapy for the treatment of Fabry disease.
Throughout his earlier career, Mr. Baldry held multiple leadership positions at Biogen Inc., including Vice President, Public Affairs and Vice President, New Product Commercialization, and at the Human Genetic Therapies division of Shire Pharmaceuticals, where he served as Head of Global Strategic Marketing and Head of Marketing, Market Access, and Public Affairs, Europe, Middle East, and Africa. Mr. Baldry received a BSc in Genetics from York University (U.K.) and an MBA from Concordia University (Canada).
Interim Chief Medical Officer
Dr. Stewart has served as a member of the X4 Board of Directors since March 2019 and is currently serving as interim Chief Medical Officer of the company. He previously served as Chief Medical Officer of Rhythm Pharmaceuticals, Inc., where he led the successful NDA submission and approval of ImcivreeTM (setmelanotide), an innovative treatment for rare causes of obesity, and as Head of R&D for Novelion Therapeutics Inc., where he oversaw global medical affairs for Juxtapid® and Myalept®, two marketed products for rare metabolic diseases. Dr. Stewart has served as a member of the X4 Board of Directors since March 2019 and is currently serving as interim Chief Medical Officer of the company.
He previously served as Chief Medical Officer of Rhythm Pharmaceuticals, Inc., where he led the successful NDA submission and approval of ImcivreeTM (setmelanotide), an innovative treatment for rare causes of obesity, and as Head of R&D for Novelion Therapeutics Inc., where he oversaw global medical affairs for Juxtapid® and Myalept®, two marketed products for rare metabolic diseases. Prior to that, Dr. Stewart was Chief Medical Officer at GlaxoSmithKline (GSK), with global responsibility for patient well-being across the vaccine, pharmaceutical, and consumer business units.
During his 18-year tenure at GSK, Dr. Stewart also held multiple research and development leadership roles, including Clinical Head of the Biopharma Unit and Therapy Area Head for metabolic and cardiovascular diseases. Prior to his tenure with GSK, he worked as a consultant physician and honorary senior lecturer at the Diabetes Center in Newcastle upon Tyne in the U.K. Dr. Stewart holds an MD from Southampton Medical School and is a Fellow of the Royal College of Physicians.
Chief Operating Officer
Dr. DiBiase has served as X4’s Chief Operating Officer since September 2021, having joined the company in June 2017 and previously serving in a variety of senior management roles across Technical Operations, Quality, and Program Management. Prior to joining X4, she was Vice President, Technical Operations and Program Management at Epirus Biopharmaceuticals, a small biosimilar company working to expand access to biosimilar products across the globe. Other previous roles have included Vice President, Product Operations, Specialty Care Business Unit for Pfizer Global Supply, and multiple positions of increasing responsibility throughout her 16-year tenure at Biogen that included Senior Director, CMC Management, Program Executive for Avonex®, and Associate Director, Pharmaceutical Science and Technology. Dr. DiBiase received a B.Pharm. from the University of London, a Ph.D. from the University of Rhode Island, and she completed the Executive Leadership Program at Bentley University’s McCallum Graduate School of Business.
Chief Scientific Officer
Dr. Art Taveras joined the X4 team as Chief Scientific Officer in October 2020. He brings more than 30 years of experience leading small molecule research and development programs focused on the treatment of cancer, dysregulated immune disorders, neurodegeneration and metabolic diseases. His research and leadership have led to the development of multiple IND candidates and dozens of clinical trials. He joins X4 from CoMET Therapeutics, where he served as Chief Scientific Officer leading the development of treatments for Inborn Errors of Metabolism (IEM) diseases and metabolic disorders.
Previously, he was the Founder and Chief Scientific Officer at Transform Therapeutics, a company aiming to discover CXCR2 antagonists for the treatment of cancer and served as President and Chief Scientific Officer at ShangPharma ChemPartner where he supported discovery and clinical portfolios of hundreds of pharmaceutical and biotech companies worldwide. Prior to that, Dr. Taveras was Vice President of Small Molecule Drug Discovery and CMC Development at Biogen Idec, where he led the discovery and development of neurology, immunology and oncology drug candidates at its multiple sites in the U.S. and abroad. He also was Vice President of drug discovery and development at Alantos Pharmaceuticals, which was acquired by Amgen in 2007, where he spearheaded the company’s discovery research and built its diabetes and matrix metalloproteinase franchise in the U.S. and Germany.
Dr. Taveras began his career with Schering-Plough Corporation and held multiple roles of increasing responsibility in the oncology and immunology drug discovery and research sector over a 14-year period, including leading the discovery of Navarixin®, a CXCR2 antagonist currently in clinical trials in combination with pembrolizumab for the treatment of cancer. He holds 130 issued patents and patent applications and has authored more than 90 scientific publications. Dr. Taveras received his B.S., M.S. and Ph.D. from Rensselaer Polytechnic Institute. Additionally, he attended the college of advancing studies at Boston College.
CEO & President
Dr. Ragan is a Founder of X4 and has been the company’s President and Chief Executive Officer and a member of the Board of Directors since launching the company in July 2014. Prior to founding X4, Dr. Ragan consulted as Chief Business Officer at Lysosomal Therapeutics, a private biopharmaceutical company. Prior to that, Dr. Ragan held leadership roles in corporate development and operations at Genzyme Corporation, a Sanofi company, where she led strategic partnering efforts for Genzyme’s Rare Disease business and headed the supply chain planning for Genzyme’s flagship commercial products. She also previously held business roles at Hydra Biosciences, Oscient Corporation, and Celera Corporation. Since 2019, Dr. Ragan has served as a member of the Board of Directors of Camp4 Therapeutics, a private biotechnology company. Dr. Ragan received a B.S. from Tufts University, a Ph.D. from Massachusetts Institute of Technology, and completed post-doctoral studies at Harvard Medical School.
Chief Financial Officer
Mr. Mostafa has served as X4’s Chief Financial Officer since September 2018. Prior to joining X4, Mr. Mostafa served as Chief Financial Officer of Abpro Corporation, a biotechnology company focused on antibody therapeutics. Prior to that, Mr. Mostafa held a number of investment-banking positions, including Managing Director in the healthcare investment banking group at Cantor Fitzgerald, Senior Banker in the healthcare investment banking group at Needham & Company, Vice President in the investment banking group at CRT Capital Group, and portfolio management associate in the global stock selection group at AQR Capital. He began his career as an analyst in the healthcare investment banking group at Salomon Smith Barney. Mr. Mostafa holds a A.B. in Economics from Brown University.