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.
Keywords: directed evolution; human FcRn; human FcγRIIa; serum half-life; therapeutic protein.
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