Abstract 3673: Preclinical toxicology and pharmacology for the 4-1BB/HER2 bispecific PRS-343: A first-in-class costimulatory T cell engager

Background. 4-1BB (CD137) is a key costimulatory immunoreceptor and a highly promising therapeutic target in cancer. To overcome toxicity and efficacy limitations of current 4-1BB targeting antibodies, we have developed PRS-343, a 4-1BB/HER2 bispecific based on Anticalin® technology. We have previously reported on the generation and characterization of PRS-343 with regard to preclinical proof-of-concept and basic drug-like properties. 1 Here, we describe the preclinical dataset supporting initiation of a first-in-patient trial. Methods and Results. The pharmacology of PRS-343 was investigated by ex vivo assays based on mixed culture of human PBMC and tumor cell lines. We find that 4-1BB costimulated T cells prominently increase production of IL-2, GM-CSF, TNF-α and IFN-γ. Using a set of immortal cancer cell lines spanning a range of HER2 surface copy numbers, we identified a copy number threshold above which PRS-343 reliably elicited T cell costimulation with a high potency and an EC50 in the subnanomolar range. PRS-343 was well tolerated in a repeat-dose study in cynomolgus monkeys, with no overt toxicity and no significant drug-related toxicological findings. Pharmacokinetic assessment confirmed dose-proportional exposure of the animals during the course of the study. In a mouse model of human PBMC-induced xenograft-vs-host disease (xGvHD), PRS-343 did not show an acceleration of xGvHD development, in contrast to a 4-1BB targeting benchmark. Again utilizing ex vivo assays, we found no PRS-343 induced T cell costimulation in a panel of primary cells, showing that physiological levels of HER2 are insufficient for activation. In a cytokine release assay, proinflammatory cytokine induction by PRS-343 in the absence of a primary TCR stimulation was negligible. Conclusion. The ex vivo experiments described indicate that HER2 expression level is expected to be a reliable marker for patient stratification for PRS-343. The toxicology assessment of PRS-343 indicates that the benign toxicity profile of trastuzumab is retained in PRS-343 with regard to HER2 targeting, and that PRS-343 is expected to elicit its costimulatory effects strictly on T cells also receiving a primary TCR signal and strictly localized to HER2-positive tumors. This is in agreement with in vivo mouse model data showing PRS-343 leads to tumor-localized CD8+ T cell expansion, 1 and supports the potential of PRS-343 as an efficacious yet well tolerable 4-1BB costimulating agent. The reported data is the basis for the trial design of a first in patient Phase 1 trial with PRS-343. 1 Cancer Immunol Res 2016;4(11 Suppl):Abstract nr B016. Citation Format: Marlon J. Hinner, Rachida-Siham Bel Aiba, Thomas Jaquin, Sven Berger, Manuela Durr, Corinna Schlosser, Andrea Allersdorfer, Christine Rothe, Louis A. Matis, Shane A. Olwill. Preclinical toxicology and pharmacology for the 4-1BB/HER2 bispecific PRS-343: A first-in-class costimulatory T cell engager [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2017; 2017 Apr 1-5; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2017;77(13 Suppl):Abstract nr 3673. doi:10.1158/1538-7445.AM2017-3673