Leadership Team


Lewis T. “Rusty” Williams, MD, PhD

Co-founder, Chairman and CEO

Prior to Walking Fish, Dr. Williams founded and served as a director of Five Prime Therapeutics, Inc., a public biotechnology company, from January 2002 to January 2020, and served as its President and Chief Executive Officer from April 2011 to December 2017. Previously, Dr. Williams spent seven years at Chiron Corporation (“Chiron”) a biopharmaceutical company, now known as Novartis Vaccines and Diagnostics, Inc., most recently as its Chief Scientific Officer. He also served on Chiron’s board of directors from 1999 to 2001. Prior to joining Chiron, Dr. Williams was a professor of medicine at the University of California, San Francisco, and served as Director of the University’s Cardiovascular Research Institution and Daiichi Research Center. Dr. Williams also has served on the faculties of Harvard Medical School and Massachusetts General Hospital and co-founded COR Therapeutics, Inc., a biotechnology company focused on cardiovascular disease. He is a member of the National Academy of Sciences and a fellow of the American Academy of Arts and Sciences. Dr. Williams was previously a member of the board of directors of COR Therapeutics, Inc., and Beckman Coulter, Inc., each of which was a public company during his service as a director. He currently serves on the Boards of Protagonist Therapeutics, Neoleukin Therapeutics and Arcellx Therapeutics. Dr. Williams has also served as a venture partner of Quan Capital, LLP, a healthcare-focused venture capital firm, since October 2018. Dr. Williams received a B.S. from Rice University and an M.D. and a Ph.D. from Duke University.


Mark Selby, PhD

Co-founder and VP, Immunology

Mark received his Ph.D. at UCSF having studied Nerve Growth Factor in the laboratory of Dr. William J. Rutter.  Thereafter he completed postdoctoral studies with B. Matija Peterlin at UCSF, working on the role of TAT in modulating transcription from the HIV LTR.  Mark then joined Chiron as a scientist in the Vaccines division.  There he worked on HCV with Dr. Michael Houghton and DNA delivery platforms with Dr. Jeffrey Ulmer.  In 2001 Mark left to work with Dr. Alan Korman at Medarex, a company with a platform that could generate fully human antibodies from immunized transgenic mice.  Medarex was acquired by BMS in 2009. Mark left BMS in December of 2018 as a Senior Director and joined Walking Fish Therapeutics in 2019. 

Mark is a co-inventor of the anti-PD-1 antibody Nivolumab (Opdivo) which is approved in multiple oncology indications.  Mark directed much of the preclinical work that led to the identification of the clinical candidate and derived supporting data from mouse tumor models.  Mark is also a co-inventor of the combination of Nivolumab and Ipilimumab, which is approved and continues to be explored in multiple indications.  Mark is also co-inventor of the anti-LAG-3 antibody, Relatlimab as well as PD-L1, GITR and TIM-3 antibodies.  He contributed to other programs including TIGIT, CD137, ICOS, an ADCC enhanced CTLA-4 antibody, and many others.  He is a co-inventor on 49 patents, many of which are issued.  Mark co-authored 67 scientific publications.


Tom Brennan, PhD

Co-founder and VP, Preclinical Development

Tom served as the head of pharmacology at FivePrime Therapeutics Inc. from 2005 to 2018.  At FivePrime he led preclinical pharmacology validation efforts for dozens of programs leading to 5 IND’s and championed the start of the FPT155 (CD80-Fc) program which entered into a phase 1 clinical trial in 2018. During his tenure at FivePrime he also led and managed the in vivo target discovery program which led to the identification of multiple leads for metabolic, fibrotic, and oncology indications. Prior to FivePrime Tom was CEO and cofounder of a privately held biotech company focused on murine models to identify new targets for CNS disorders, and was a founding scientist at Deltagen Inc.  Tom received his PhD in the laboratory of Eric Olson where he studied myogenin and the regulation of muscle cell fate at the University of Texas Health Science Center in Houston.


Seenu Kothakota, PhD

Co-founder and VP Strategy

Dr. Kothakota was recently a leader in the Innovation Group at FivePrime Therapeutics where he worked for 15 years and had an active role in developing the discovery platform.   He played a key role in using this platform for discovery of new therapeutic proteins for cancer, diabetes and regenerative medicine.  He formerly worked at Protein Pathways and as a scientist at Chiron.  Dr. Kothakota completed his postdoctoral training with Professor Lewis Williams at University of California San Francisco, his doctoral work at University of Massachusetts with Professor Maurille Fournier and an MBA at Anderson School of Business, UCLA.