Ex vivo pembrolizumab pharmacology for personalized PD-1 inhibitor therapy reveals a critical gap between receptor occupancy and T cell functionality

PURPOSE: Targeting the programmed death (ligand)-1 (PD-1/PD-L1) axis with immune checkpoint inhibitors (ICIs), like pembrolizumab, has improved cancer survival. Unfortunately, the optimal dose remains unknown and less than 50 % of patients respond. Understanding PD-1 receptor pharmacology and developing early-response biomarkers are crucial to personalize therapy. We hypothesize that characterizing individual pre-treatment variability in immune responses to pembrolizumab will enhance PD-1 receptor pharmacology insights and improve treatment response prediction. Hence, this study evaluates the performance of a newly developed ex vivo immunopharmacological bioassay under healthy and pathological states.

METHODS: Peripheral blood mononuclear cells from healthy individuals and non-small cell lung cancer (NSCLC) patients were stimulated in vitro in the presence of pembrolizumab. PD-1 expression, interleukin-2 (IL-2) secretion, T cell differentiation, expression of activation markers and phosphorylation of T cell signalling molecules were analysed.

RESULTS: In healthy individuals, receptor saturation occurred at >0.025 μg/mL pembrolizumab. Yet IL-2 production still increased significantly beyond this concentration. NSCLC patients showed significantly higher PD-1 expression and IL-2 production than healthy individuals. Nevertheless, pembrolizumab induced IL-2 production similarly in both cohorts. In NSCLC patients, pembrolizumab inhibited differentiation towards CD4 central memory T cells. Remarkably, phosphorylation of proximal phospho-markers in response to pembrolizumab varied between NSCLC patients, potentially discriminating responders from non-responders.

CONCLUSIONS: We highlight the importance of evaluating T cell functionality alongside PD-1 receptor occupancy. We identified PD-1-induced modulation of phosphorylation of proximal signalling molecules as potential predictors for ICI treatment response in NSCLC. We recommend further development of this immunopharmacological bioassay for personalization of ICI treatment.