Therapeutic Candidate Alpn-101, a Dual ICOS/CD28 Antagonist, Potently Suppresses Human/NSG Mouse Xenograft Graft Vs. Host Disease (GvHD) in a Dose Ranging Study and Reduces Disease Activity in a Mouse Model of Hemophagocytic Lymphohistiocytosis (HLH)

Background/Purpose: ALPN-101 is a potent dual inhibitor of the ICOSand CD28T cell costimulatory pathways designed for therapeutic application in inflammatory diseases. CD28and ICOSbind CD80/ CD86and ICOSligand (ICOSL), respectively, and play critical roles in T cell activation and adaptive immunity. ALPN-101 has previously been demonstrated to have potent efficacy - superior to wild type ICOSL-Fc - in models of graft versus host disease (GvHD), a disease reflecting immune-mediated attack of recipient tissue by donor T cells. Here, we examined the efficacy of a single dose of ALPN-101 or repeat dosing with different dose levels in GvHD. We also explored the potential therapeutic benefit of ALPN-101 in another T cell-driven inflammatory disease, hemophagocytic lymphohistiocytosis(HLH), a spectrum of disorders of the immune system characterized by the excessive production of cytokines by activated T cells and macrophages accumulating in organs such as the liver, spleen, bone marrow, and brain, which mediate significant tissue damage. Methods: ALPN-101 was generated using our proprietary variant Ig domain (vIgD™) platform and is an effector-function negative Fc-fusion protein with an engineered variant Ig ICOSL domain capable of binding both ICOSand CD28with high affinity. ALPN-101 blocks the interaction of these T cell costimulatory molecules with their respective receptors, downregulating T cell activation. The dose ranging GvHD study was executed with ALPN-101 (3x weekly/4 weeks, 20 ug - 500 ug) treatment of NSGTM mice engrafted with human peripheral blood mononuclear cells (PBMC) in comparison to belatacept, a CTLA-4-Fc fusion protein CD28pathway inhibitor. Mice were monitored daily for clinical signs of GvHD. In a model of primary (inherited) HLH in which perforin-deficient (Prf1(-∕-)) mice are infected with lymphocytic choriomeningitisvirus (LCMV), we evaluated both prophylactic (days 0, 3, and 6 post LCMV infection) and delayed (days 3, 5, and 7) treatment with ALPN-101 (400ug/dose). Results: ALPN-101 significantly attenuated T cell activation in the human PBMC-NSG GvHD model at a single 100ug dose and at all multiple doses tested, protecting mice from the effects of xenogeneicT cell activation in vivo. Treated animals exhibited enhanced survival and reduced disease scores compared to control mice treated with saline or belatacept. Flow cytometric analyses of blood collected at 1-2 weeks post cell transfer demonstrated ALPN-101 reduced both the number and activation state of the transferred human CD4+ and CD8+ T cells. In the HLH model, ALPN-101 lessened several of the clinical and laboratory manifestations of HLH, including organomegaly, anemia, CD8+ T cell expansion, and liver inflammation. Conclusion: ALPN-101 is a potent T cell inhibitor capable, even with a single dose, of preventing T cell activation, such as that observed in the huPBMC-NSGTM GvHD and the LCMV-induced HLH models, and thus is a promising novel therapeutic candidate for GvHD and other inflammatory diseases. Preclinical development is underway to support clinical studies of this potentially first-in-class dual ICOSand CD28inhibitor. Disclosures Dillon:Alpine Immune Sciences: Employment, Equity Ownership. Lewis:Alpine Immune Sciences: Employment, Equity Ownership. Swanson:Alpine Immune Sciences: Employment, Equity Ownership. Evans:Alpine Immune Sciences: Employment, Equity Ownership. Levin:Alpine Immune Sciences: Employment, Equity Ownership. Rixon:Alpine Immune Sciences: Employment, Equity Ownership. Peng:Alpine Immune Sciences: Employment, Equity Ownership. Nichols:Incyte: Research Funding; Alpine Immune Sciences: Research Funding. Swiderek:Alpine Immune Sciences: Employment, Equity Ownership.