Abstract 5672: Comparative mouse pharmacokinetics of matched- and unmatched-isotype anti-PD-L1 antibodies

Abstract Immune checkpoint inhibitors (ICI) are highly effective for a minority of patients with cancer. Pre-clinical research into mechanisms of ICI resistance is reliant upon anti-PD-1 or anti-PD-L1 antibody treatment in mouse models of cancer. Common approaches utilize commercially-available rat isotype anti-PD-L1 antibodies given by intraperitoneal (IP) injection at relatively high doses (5mg/kg every 2-3 days), often for extended periods. However, there has been limited investigation into the pharmacokinetics of these antibodies with single and repeat dosing, which could inform rational dose scheduling. Non-species-matched antibodies are known to induce accelerated anti-drug immune responses, compared to species-matched antibodies. As a result, we hypothesized that matched-isotype anti-PD-L1 (based on the grafting of atezolizumab variable regions onto a mouse IgG2a scaffold) and unmatched-isotype rat IgG2b anti-PD-L1 would be associated with distinct pharmacokinetics on single and repeated dosing. We first confirmed that both matched and unmatched antibodies have nanomolar-range affinity for mouse PD-L1 by surface plasmon resonance (0.85nM and 0.37nM, respectively). Using a single IP injection in 8-week-old male C57BL/6J mice followed by repeated tail-vein plasma sampling and an indirect anti-PD-L1 ELISA, we detected a trend towards prolonged median half-life for the matched (n=17 mice) versus unmatched (n=19) antibody (91.5 versus 39.0 hours). We detected a particularly wide range of half-life measurements for matched antibody experiments (19.8 to 337.7 hours), suggesting that antibody clearance can be highly heterogeneous. Using repeated IP injection (2 doses, 14 days apart), we identified markedly accelerated antibody clearance for both antibodies resulting in comparable antibody half-lives after dose 2 (11.9 versus 12.5 hours, n=13 and n=8 mice, respectively), suggestive of a time-dependent anti-drug adaptive immune response. These findings have significant implications for in vivo antibody treatment experiments, demonstrating that repeat dosing is associated with rapid antibody clearance, irrespective of the antibody species. These data should be taken into consideration when planning longer-term antagonism of PD-1/PD-L1 signaling with antibodies in mice and strengthen the basis for short term window studies that examine the intratumoral immune biology. Furthermore, the high variability in antibody half-life suggests that antibody pharmacokinetics could confound ICI treatment response studies in vivo. Citation Format: Breanna Demestichas, Sam Kleeman, Matthew Chvasta, Tobias Janowitz. Comparative mouse pharmacokinetics of matched- and unmatched-isotype anti-PD-L1 antibodies. [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2023; Part 1 (Regular and Invited Abstracts); 2023 Apr 14-19; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2023;83(7_Suppl):Abstract nr 5672.