Receptor occupancy and neo-epitope specific T-cell repertoires in patients with solid tissue tumors following anti-PD-1 therapy

Background: In patients undergoing chemotherapy and/or immune therapy, endogenous T cells within the tumor micro-environment (TME) and in the periphery are subject to dynamic changes. Particularly, exhausted T cells in the TME and peripheral blood mononuclear cells (PBMCs) isolated from whole blood express the inhibitory receptor, Programmed Cell Death Receptor 1 (PD-1). A small but significant number of these T cells can recognize neo-epitopes (neoE) arising from patient specific mutations. We determined receptor occupancy (RO) in patients treated with monoclonal anti-PD-1 antibody AB122, which is currently in clinical development. Methods: Multi-color flow cytometry was used to determine RO post-AB122 administration using a directly-conjugated competitive antibody to AB122. In addition, we determined RO values using the established method for nivolumab using a biotinylated anti-human IgG4 to detect bound anti-PD-1. The assays were also optimized to determine RO in whole blood from patients with solid tumors treated with AB122. Receptor occupancy using both methods were compared. Further, whole exome sequencing from archival formalin-fixed, paraffin-embedded (FFPE) tissue was performed and T cells that recognized patient-specific neoE resulting from tumor-specific mutations were identified in patient blood. Results: In the dose-escalation study for AB122 monotherapy arm, a variety of dosing regimens have been evaluated, including Q2W, Q3W and Q4W. The number of doses received ranged from 1 to 20. Data from patients in the 80 and 240 mg Q2W cohorts showed that AB122 serum concentrations ≥1.5 μg/mL (equivalent to 10 nM) are associated with full receptor occupancy. The RO associated with AB122 dosing is maintained when dosed in combination with the potent A2aR/A2bR dual adenosine receptor antagonist AB928 in an ongoing clinical trial. Conclusions: Full RO is observed with both cohorts of AB122 monotherapy (80 and 240 mg Q2W) and in the 240 mg Q2W combination with AB928. Study of peripheral target engagement and expansion of neoE-specific T cell populations in a longitudinal study of solid tumor patients treated with AB122 could provide insight towards designing successful combination immune therapy regimens in future clinical trials with AB122. Citation Format: Devika Ashok, Songming Peng, Benjamin Yuen, Matt Walters, Stephen W. Young, Lisa Seitz. Receptor occupancy and neo-epitope specific T-cell repertoires in patients with solid tissue tumors following anti-PD-1 therapy [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2019; 2019 Mar 29-Apr 3; Atlanta, GA. Philadelphia (PA): AACR; Cancer Res 2019;79(13 Suppl):Abstract nr 4100.