Lillian Siu

Lillian Siu, MD, FRCPC is a Senior Staff Physician in the Phase I Program and Professor of Medicine at Princess Margaret Cancer Centre and the University of Toronto, where she leads one of North America's most productive early-phase oncology programs. She is an internationally recognized leader in phase I trial design and conduct, with particular expertise in head and neck cancer and immuno-oncology. Dr. Siu was among the key investigators in the KEYNOTE-001 trial—the landmark first-in-human and dose escalation study of pembrolizumab (MK-3475) in solid tumors—contributing patient cohorts and translational analyses that established pembrolizumab's safety profile, optimal dosing, and early efficacy signals across multiple histologies. She has led or co-led phase I trials of over 60 novel agents across multiple drug classes. Her work on iRECIST (immune-modified RECIST criteria) has been foundational in establishing standardized novel response assessment tools for immunotherapy phase I and phase II trials. Dr. Siu is a member of the RECIST Working Group and has contributed extensively to the definition of response endpoints appropriate for novel agents including immunotherapies, ADCs, and bispecifics.