Abstract CT046: Predictive biomarkers and pharmacodynamic changes in tumor RNA expression in a phase I study of anti PD-1 monoclonal antibody PF-06801591

Introduction: PF-06801591 is a humanized IgG4 monoclonal antibody that binds to programmed cell death (PD-1) receptor, blocking its interaction with PD-1 ligands. In a phase I study (NCT02573259; dose-escalation) of 40 patients (pts) with multiple solid tumor types treated with PF-06891591 intravenously (IV) or subcutaneously (SC), 7 pts showed partial response. PF-06801591 was well tolerated at all doses. Biopsy samples were evaluated for predictive biomarkers and pharmacodynamic effects. Methods: Biopsy samples at baseline and on-treatment (IV:day 29; SC:day 36) were collected from 4 dose cohorts (IV:1, 3 and 10 mg/kg; SC:300 mg). Whole-exome sequencing (WES) and RNA-seq of biopsy tissue were used to estimate tumor mutation burden (TMB) and gene expression. Regression analysis was used to identify TMB/genes/pathways potentially associated with response in baseline tissue. Differential analysis of baseline and on-treatment biopsies identified genes/pathways potentially upregulated by PF-06801591. PD-L1 (clone SP-263; Ventana) expression was evaluated in tumor biopsies by immunohistochemistry (IHC) and pathologist scoring. Results: In a combined WES analysis of samples from baseline biopsies (all doses, n=24 pts), higher TMB was significantly associated with improved objective response (R^2=0.215, P=0.013). From RNA-seq analysis, genes positively associated with response were involved in interferon-gamma (IFNG) and PD-1 signaling, and cell cycle; CTLA4 was among the genes most significantly associated with response (P