Crystal structure of the Fab fragment of burosumab, the first-in-class anti-FGF23 medicine for treating X-linked hypophosphatemia and tumor-induced osteomalacia

Burosumab is a first-in-class therapy approved to treat X-linked hypophosphatemia and tumor-induced osteomalacia, a condition associated with excessive fibroblast growth factor 23 (FGF23) production. This antibody drug directly targets the excessive FGF23 in patients and inhibits its regulation of phosphate homeostasis and vitamin D metabolism. Here, the Fab fragment of burosumab was expressed, purified, and crystallized. The crystals belonged to the space group P21, with unit cell parameters a = 49.51, b = 66.22, c = 62.05 Å, and β = 105.99°. An asymmetric unit of the crystal contains one Fab fragment with a Matthews coefficient of 1.93 Å3 Da-1. The structure was determined at a resolution of 1.45 Å, with Rwork/Rfree = 0.169/0.196. The complementarity determining regions of burosumab construct a negatively charged patch, a putative hot spot for FGF23 binding. This high-resolution structure could be used for predicting the binding mode between burosumab and FGF23.