Myocarditis in Cynomolgus Monkeys Following Treatment with Immune Checkpoint Inhibitors.

Purpose: Immune checkpoint inhibitors (ICI) targeting PD1, PDL1, or CTLA4 are associated with immunerelated adverse events (irAE) in multiple organ systems including myocarditis. The pathogenesis and early diagnostic markers for ICI-induced myocarditis are poorly understood, and there is currently a lack of laboratory animal model to enhance our understanding. We aimed to develop such a model using cynomolgus monkeys. Experimental Design: Chinese-origin cynomolgus monkeys were dosed intravenously with vehicle or nivolumab 20 mg/kg plus ipilimumab 15 mg/kg once weekly and euthanized on day 29. Results: Multiple organ toxicities were observed in cynomolgus monkeys, and were characterized by loose feces, lymphadenopathy, and mononuclear cell infiltrations of varying severity in heart, colon, kidneys, liver, salivary glands, and endocrine organs. Increased proliferation of CD4(+) and CD8(+) T lymphocytes as well as an increase in activated T cells and central memory T cells in the blood, spleen, and lymph nodes, were observed. Transcriptomic analysis suggested increased migration and activation of T cells and increased phagocytosis and antigen presentation in the heart. Mononuclear cell infiltration in myocardium was comprised primarily of T cells, with lower numbers of macrophages and occasional B cells, and was associated with minimal cardiomyocyte degeneration as well as increases in cardiac troponin-I and NT-proBNP. Morphologically, cardiac lesions in our monkey model are similar to the reported ICI myocarditis in humans. Conclusions: We have developed a monkey model characterized by multiple organ toxicities including myocarditis. This model may provide insight into the immune mechanisms and facilitate biomarker identification for ICI-associated irAEs.