Human Fc gamma receptor IIIA blockade inhibits platelet destruction in a humanized-murine model of ITP

Fc gamma receptor (FcγR) IIIA is an important receptor for IgG and is involved in immune defense mechanisms as well as tissue destruction in some autoimmune diseases including immune thrombocytopenia (ITP). FcγRIIIA on macrophages can trigger phagocytosis of IgG-sensitized platelets and prior pilot studies observed blockade of FcγRIIIA increased platelet counts in ITP patients. Unfortunately, while blockade of FcγRIIIA in ITP patients increased platelet counts, its engagement by the blocking antibody drove serious adverse inflammatory reactions. These adverse events were postulated to originate from the antibody’s Fc and/or bivalent nature. Blockade of human FcγRIIIA in vivo with a monovalent construct lacking an active Fc region has not yet been achieved. To effectively block FcγRIIIA in vivo, we developed a high affinity monovalent single chain variable fragment (scFv) that can bind and block human FcγRIIIA. This scFv (17C02) was expressed in three formats: a monovalent fusion protein with albumin, a one-armed human IgG1 antibody, and a standard bivalent mouse (IgG2a) antibody. Both monovalent formats were effective in preventing phagocytosis of ITP-serum-sensitized human platelets. In vivo studies using FcγR-humanized mice demonstrated that both monovalent therapeutics were also able to increase platelet counts. The monovalent albumin fusion protein did not have adverse event activity as assessed by changes in body temperature while the one-armed antibody induced some changes in body temperature even though the Fc region function was impaired by the LALA mutation. These data demonstrate that monovalent blockade of human FcγRIIIA in vivo can potentially be a therapeutic strategy for patients with ITP.