A Phase I Study of a Novel Anti-PD-1 Monoclonal Antibody

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 (R²=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<0.001). Response was better correlated with levels of PD-L1 RNA (P\=0.028) than PD-L1 protein (IHC; P\=0.39 to 0.84). RNA-seq analysis of paired baseline and on-treatment biopsies (all doses) showed increased expression of genes involved in adaptive immune response pathways, including chemokines and chemokine receptors such as CXCL9. Decreased expression of cell cycle genes such as CDKN1B was observed in on-treatment biopsies.

Conclusion:

Based on a mixed set of tumor types with possible sensitivity to anti-PD-1, high TMB, IFNG signaling, and PD-L1 were associated with response to PF-06801591, as reported for other anti-PD-1 antibodies. Albeit based on a small sample size and diverse indication profile, baseline expression of CTLA4 appears to be highly associated with response and PD-L1 RNA levels may be better associated with response than protein expression by IHC. Increased expression of genes/pathways associated with adaptive immune activation in on-treatment biopsies indicates an active, immunomodulatory mechanism with anti-PD-1 therapy. This ongoing study will evaluate predictive biomarkers in dose-expansion cohorts of PF-06801591 in non-small cell lung cancer and urothelial cancer.