Our Team


Percival Barretto-Ko

President & Chief Executive Officer

Percival is the President & Chief Executive Officer of Plexium, the premier next-generation Targeted Protein Degradation company.

Percival joined Plexium in 2021 with extensive biopharmaceutical experience across various therapeutic areas, functions, and geographies. Prior to Plexium, Percival was the Chief Business Officer of Astellas Pharma Inc., where he spearheaded the execution of the company’s Corporate Strategic Plan and its efforts to enhance internal and external innovation. Percival’s earlier roles at Astellas included President of Astellas Pharma US, SVP International, and SVP Corporate Strategy, Business Development & Government Affairs. Before joining Astellas, Percival held increasingly senior commercial positions with the Roche Group in the United States and the United Kingdom. Percival earned a BS in Biological Sciences and Chemistry with Honors from Cornell University and an MBA from Yale University. In addition, Percival served as a Sloan Fellow for Innovation and Global Leadership at the MIT Sloan School of Management, where he was awarded an MS degree in Management. Percival is a board member of Americares, a health-focused relief and development organization that serves people affected by poverty or disaster with life-changing medicine and health programs.

Stephen Mullennix

Chief Financial Officer
Stephen Mullennix

Mr. Mullennix joins Plexium as Chief Financial Officer in May 2022.

Mr. Mullennix joins Plexium from ProSciento, Inc. where he served as Chief Financial Officer and played a critical role in advancing clinical methodologies for drugs and devices in the metabolic disease arena. Prior to ProSciento, Mr. Mullennix was Chief Financial Officer and Chief Operating Officer of UroGen Pharma, Ltd (NYSE:URGN). During his tenure at UroGen, he helped the company build its management team and corporate infrastructure that successfully launched its first FDA-approved drug for urothelial cancer and progressed a pipeline of Uro-Oncology drug candidates. Before UroGen, Mr. Mullennix was Chief Financial Officer and Senior Vice President of Operations at SolarReserve, LLC, a solar energy technology company, where he supported the company’s global expansion and oversaw multiple corporate and project financings. Prior to SolarReserve, he served as a Managing Director at US Renewables Group, a private equity investment fund with more than $750 million of assets under management. Mr. Mullennix has been directly involved in $3.3 billion of public, private, equity, debt and M&A transactions. He earned his MBA from the Anderson School at University of California Los Angeles and BA in Economic Geography from Dartmouth College.

Simon Bailey, Ph.D., MBA

Executive Vice President, Drug Discovery

The Head of Drug Discovery at Plexium, Simon oversees the company’s efforts to identify novel protein degrader drugs that address significant unmet medical needs.

Simon joined Plexium in 2019, bringing more than 25 years of experience in medicinal chemistry and small molecule drug discovery leadership. From 1995-2015, Simon held positions of increasing responsibility at Pfizer, including leading the Diabetes and Oncology Medicinal Chemistry Departments. Subsequently, Simon was the Head of Research and Global Pipeline Leader for Intercept Pharmaceuticals. There, he led the company’s programs from inception to Phase 2 in the area of fibrotic liver disease. Simon holds a Ph.D. and was the Hibbert Prize winner in synthetic organic chemistry from the University of Manchester. He completed postdoctoral studies under Prof. Leo Paquette at the Ohio State University and obtained an MBA from the University of California, San Diego. Simon is a 2021 recipient of the American Chemical Society’s Hero of Chemistry award for his role in the discovery of Lorbrena™.

Kevin Eastwood

Executive Vice President, Corporate Business Development

Kevin is the Executive Vice President of Corporate Development for Plexium.

He leads corporate strategy and is responsible for establishing external partnerships, managing strategic alliances, and supporting corporate financing initiatives. With over 30 years of industry experience, Kevin joined Plexium after working with large biotechnology companies, such as Agouron Pharmaceuticals and Gilead, and venture-backed start-ups, such as Achillion Pharmaceuticals and eFFECTOR Therapeutics. Prior to joining Plexium, Kevin was the Vice President of Corporate Development for the California Institute for Biomedical Research. He began his business development career at Agouron Pharmaceuticals and has significant experience across strategic partnerships, in-licensing, out-licensing, and M&A. Kevin graduated from Missouri State University with a degree in biology.

Suzanne Zoumaras

Executive Vice President, Head of Human Resources

Ms. Zoumaras joined Plexium as Executive Vice President, Head of Human Resources in September 2022.

Prior to joining Plexium, Ms. Zoumaras was a Co-Founder and Managing Partner of Human Capital Resource Partners, a human capital and advisory services consulting firm. Before that, she served as Executive Vice President & Chief Human Capital Officer at Arena Pharmaceuticals, Executive Vice President and Chief Human Resource Officer at Teradata, and as Senior Vice President of Global Human Resources at Atmel Corporation (acquired by Microchip). She is also an adjunct professor in the Fowler School of Management at San Diego State University where she teaches Total Rewards. Ms. Zoumaras received her BS in Finance from San Diego State University and an HR Certificate, Human Resources from the University of California, San Diego.

Daniel Sipes

Senior Vice President, Head of Technology

Mr. Sipes leads the Engineering, Software, Informatics, and Compound Management groups.

Before joining Plexium, Daniel served as SVP, Strategy and Operations, Site Head San Diego at Strateos (formerly Transcriptic), where he was responsible for building and operating remotely controlled cloud labs for automated chemistry and biology. Before Strateos, he was Director, Advanced Automation Technologies at the Novartis Institutes for BioMedical Research (NIBR) San Diego location (formerly GNF). While at NIBR, Daniel developed and implemented new technologies for drug discovery, assay development, high throughput screening, automated cell culture, and protein production. Before that, he held positions focused on high throughput screening and compound management at Kalypsys, Inc. and Ligand Pharmaceuticals. Daniel started his career in the Hybridoma Laboratory at Genentech. He received his BS in Molecular Biology from California State University, Sacramento, and an MS, in Immunology from the University of California, Davis.

Peggy Thompson, Ph.D.

Vice President, Biology

Peggy is the Vice President of Biology, leading the targeted protein degradation drug discovery efforts at Plexium.

Peggy joined Plexium in 2020 with more than 20 years of experience in drug discovery and development in the biotechnology industry. Prior to Plexium, she was a co-founder and Executive Director of Biology at eFFECTOR Therapeutics, where she established their technology platform for targeting dysregulated mRNA translation for supporting oncology drug development. She also led their eIF4A program (Zotatifin, Phase II) from concept to clinical development in solid tumor malignancies. Before that, Peggy served as the Director of Biology at Anadys Pharmaceuticals, Inc., where she held positions of increasing responsibility, including leading the drug discovery biology efforts and supporting the development of Setrobuvir until the acquisition by Roche Pharmaceuticals. Prior to Anadys, Peggy held positions at Biopraxis and the National Institutes of Health. She was an NIH postdoctoral fellow in Biophysics at the University of California, Berkeley, and completed her Ph.D. in Chemistry from the University of California, San Diego. She received her B.S. with Highest Distinction in Biochemistry from Washington State University.

Kevin Freeman-Cook, Ph.D.

 VP, Head of Medicinal and Computational Chemistry

Kevin is the Vice President of Medicinal and Computational Chemistry, leading the design and synthesis of targeted protein degraders at Plexium.

Kevin joined Plexium in 2022 with extensive pharmaceutical industry experience. Before joining Plexium, Kevin worked for twenty-two years at Pfizer, where he held positions of increasing responsibility, including Research Fellow. Kevin led numerous medicinal chemistry teams and drug discovery programs during this time. Kevin spent ten years in Groton, CT working in the therapeutic areas of allergy and respiratory disease, inflammation, antibacterials, oncology, and diabetes. In 2009, Kevin moved to Pfizer’s La Jolla, CA site, where he focused on medicinal chemistry design in oncology. Over these years, Kevin worked in and led teams that have nominated five compounds for clinical development across multiple therapeutic areas. Dr. Freeman-Cook received his BA in chemistry from Carleton College in 1994 and a Ph.D. in organic chemistry from the University of Colorado Boulder in 1999.

Board of Directors

Mike Grey

Executive Chairman

Percival Barretto-Ko

President & CEO, Plexium

Cariad Chester

Partner, TCG X

Ian Peikon

Partner, Lux Capital

Rob Hopfner

Managing Partner, Pivotal Bioventure Partners

Gorjan Hrustanovic

Managing Director at BVF Partners L.P.

Eric Shiozaki

Partner DCVC

Brian Wong, M.D., Ph.D.

President and CEO of RAPT Therapeutics