Transferrin receptor binding valency determines the circulating half-life and weight-loss efficacy of Fc-FGF21 in TfR-humanized mice

Fibroblast growth factor 21 (FGF21) is a promising metabolic hormone with potent anti-obesity effects. It acts in both the central nervous system and peripheral tissues to exert its metabolic benefits. Because its receptors are expressed at low levels in the brain, we hypothesized that enhanced transport across the blood-brain barrier (BBB) would improve its efficacy. To achieve this, FGF21 was fused to Fc(TV35), an engineered Fc fragment that binds the human transferrin receptor (hTfR), to generate Fc(TV35)-FGF21. Fc(TV35)-FGF21 does not bind to mouse TfR, so it produced weight-loss effects in wild-type diet-induced obese mice comparable to those of Fc-FGF21. However, it failed to induce weight loss in homozygous hTfR-humanized mice because of a markedly shortened circulating half-life. Converting Fc(TV35)-FGF21 from a homodimer to a bispecific Fc format with a single hTfR-binding arm partially restored the circulating half-life and weight-loss efficacy. Reducing mouse hTfR zygosity from homozygous to heterozygous did not substantially improve the engineered protein's half-life or weight-loss effect. Taken together, our findings suggest that sustained systemic exposure is a major determinant of FGF21's weight-loss efficacy and that hTfR engagement can shorten its systemic half-life and compromise its therapeutic activity. These results highlight an important design limitation of TfR-based brain vehicles, especially for delivering metabolic hormones with low-abundance CNS receptor targets. As a result, further optimization of shuttle formats to minimize systemic clearance while crossing the BBB is needed for metabolic hormones.