Cross-Reactivity and Functionality of Approved Human Immune Checkpoint Blockers in Dogs

SIMPLE SUMMARY: Immunotherapy with immune checkpoint inhibitors (ICI) has been a major advance in the management of cancers. To increase response rates, numerous new cancer immunotherapies are under development, to be used as monotherapies or as combination with existing ICIs. Lately, companion dogs suffering from neoplastic diseases have gained recognition for the development of cancer drugs. Despite evidence for PD-1/PD-L1 blockade eliciting anti-tumor responses, there are currently no commercially available ICIs for use in dogs. Here, we explored the potential use of seven FDA-approved human ICIs in dogs. Atezolizumab is cross-reactive, blocks PD-1/PD-L1 interaction and increases cytokine production of T cells derived from healthy donors and dog cancer patients in vitro. Response to atezolizumab seems to be dependent on the composition of blood lymphocytes and tumor type. Our data provides a rationale for testing promising treatment modalities and combination therapies involving PD-1/PD-L1 blockade in dog trials to advance veterinary and human medicine. ABSTRACT: Background: Rodent cancer models have limitations in predicting efficacy, tolerability and accompanying biomarkers of ICIs in humans. Companion dogs suffering from neoplastic diseases have gained attention as a highly relevant translational disease model. Despite successful reports of PD-1/PD-L1 blockade in dogs, no compounds are available for veterinary medicine. Methods: Here, we assessed suitability of seven FDA-approved human ICIs to target CTLA-4 or PD-1/PD-L1 in dogs. Cross-reactivity and blocking potential was assessed using ELISA and flow cytometry. Functional responses were assessed on peripheral blood mononuclear cells (PBMCs) derived from healthy donors (n = 12) and cancer patient dogs (n = 27) as cytokine production after stimulation. Immune composition and target expression of healthy donors and cancer patients was assessed via flow cytometry. Results: Four candidates showed cross-reactivity and two blocked the interaction of canine PD-1 and PD-L1. Of those, only atezolizumab significantly increased cytokine production of healthy and patient derived PBMCs in vitro. Especially lymphoma patient PBMCs responded with increased cytokine production. In other types of cancer, response to atezolizumab appeared to correlate with a lower frequency of CD8 T cells. Conclusions: Cross-functionality of atezolizumab encourages reverse translational efforts using (combination) immunotherapies in companion dog tumor patients to benefit both veterinary and human medicine.