Engineered human FcγRIIa fusion: A novel strategy to extend serum half‐life of therapeutic proteins

The immunoglobulin G (IgG) molecule has a long circulating serum half‐life (~3 weeks) through pH‐ dependent FcRn binding‐mediated recycling. To hijack the intracellular trafficking and recycling mechanism of IgG as a way to extend serum persistence of non‐antibody therapeutic proteins, we have evolved the ectodomain of a low‐affinity human FcγRIIa for enhanced binding to the lower hinge and upper CH2 region of IgG, which is very far from the FcRn binding site (CH2–CH3 interface). High‐throughput library screening enabled isolation of an FcγRIIa variant (2A45.1) with 32‐fold increased binding affinity to human IgG1 Fc (equilibrium dissociation constant: 9.04 × 10−7 M for wild type FcγRIIa and 2.82 × 10−8 M for 2A45.1) and significantly improved affinity to mouse serum IgG compared to wild type human FcγRIIa. The in vivo pharmacokinetic profile of PD‐L1 fused with engineered FcγRIIa (PD‐L1–2A45.1) was compared with that of PD‐L1 fused with wild type FcγRIIa (PD‐L1–wild type FcγRIIa) and human PD‐L1 in mice. PD‐L1–2A45.1 showed 11.7‐ and 9.7‐fold prolonged circulating half‐life (t1/2) compared to PD‐L1 when administered intravenously and intraperitoneally, respectively. In addition, the AUCinf of PD‐L1–2A45.1 was two‐fold higher compared to that of PD‐L1–wild type FcγRIIa. These results demonstrate that engineered FcγRIIa fusion offers a novel and successful strategy for prolonging serum half‐life of therapeutic proteins. To prolong serum half‐lives of therapeutic proteins, a novel platform strategy that hijack FcRn‐mediated recycling mechanism of serum IgG was developed. For efficient association of a target protein to a human serum IgG molecule, we evolved the ectodomain of human FcγRIIa and isolated a human FcγRIIa variant (2A45.1) with 32‐fold enhanced IgG1 binding affinity. Pharmacokinetic analysis of engineered FcγRIIa variant fused model protein (PD‐L1‐2A45.1) exhibited remarkably improved circulating half‐life in mice.