An ALK2 inhibitor, BLU-782, prevents heterotopic ossification in a mouse model of fibrodysplasia ossificans progressiva

Abstract

Fibrodysplasia ossificans progressiva (FOP) is a rare genetic disease driven by gain-of-function variants in activin receptor–like kinase 2 (ALK2), the most common variant being ALK2 ^R206H . In FOP, ALK2 variants display increased and dysregulated signaling through the bone morphogenetic protein (BMP) pathway resulting in progressive and permanent replacement of skeletal muscle and connective tissues with heterotopic bone, ultimately leading to severe debilitation and premature death. Here, we describe the discovery of BLU-782 (IPN60130), a small-molecule ALK2 ^R206H inhibitor developed for the treatment of FOP. A small-molecule library was screened in a biochemical ALK2 binding assay to identify potent ALK2 binding compounds. Iterative rounds of structure-guided drug design were used to optimize compounds for ALK2 ^R206H binding, ALK2 selectivity, and other desirable pharmacokinetic properties. BLU-782 preferentially bound to ALK2 ^R206H with high affinity, inhibiting signaling from ALK2 ^R206H and other rare FOP variants in cells in vitro without affecting signaling of closely related homologs ALK1, ALK3, and ALK6. In vivo efficacy of BLU-782 was demonstrated using a conditional knock-in ALK2 ^R206H mouse model, where prophylactic oral dosing reduced edema and prevented cartilage and heterotopic ossification (HO) in both muscle and bone injury models. BLU-782 treatment preserved the normal muscle-healing response in ALK2 ^R206H mice. Delayed dosing revealed a short 2-day window after injury when BLU-782 treatment prevented HO in ALK2 ^R206H mice, but dosing delays of 4 days or longer abrogated HO prevention. Together, these data suggest that BLU-782 may be a candidate for prevention of HO in FOP.

Abstract

BLU-782 blocks heterotopic ossification in preclinical models of fibrodysplasia ossificans progressiva.

Editor’s summary

Fibrodysplasia ossificans progressiva results in ectopic bone formation through heterotopic ossification (HO), which causes fused joints and early death. Gain-of-function variants in the gene encoding the activin receptor–like kinase 2 (ALK2) protein cause the disorder. Davis et al. screened a library of small-molecule kinase inhibitors and developed a compound, BLU-782, to inhibit ALK2 and prevent HO in a mouse model expressing the most common disease-causing variant of ALK2 in humans. Muscle pinch or bone break caused HO in the mice, and prophylactic treatment with BLU-782 prevented HO formation in the injured tissue. BLU-782 treatment was required starting 2 days after injury and no later than 4 days after injury to prevent HO in these mice. This study characterizes a promising potential therapy to prevent HO in fibrodysplasia ossificans progressiva. —Brandon Berry