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 ICOS and CD28 T cell costimulatory pathways designed for therapeutic application in inflammatory diseases. CD28 and ICOS bind CD80/CD86 and ICOS ligand (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 ICOS and CD28 with 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 CD28 pathway 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 choriomeningitis virus (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 xenogeneic T 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 t