Novel Mechanism for Inhibition of BMP Receptor in Heterotopic Ossification

Objectives: Objectives: Bone morphogenetic protein (BMP) signaling can be constitutively activated by a single substitution with an acidic residue in the regulatory (GS) subdomain of the type I receptor kinases (Activin-like kinases, ALKs), leading to heterotopic bone formation in mouse models. A single residue mutation (R206H) in human ALK2, adjacent the position of the constitutively activating substitution (Q207D) in the experimental variant protein (caALK2), causes a milder form of activation that induces heterotopic bone and secondary skeleton formation in patients presenting with Fibrodysplasia Ossificans Progressiva (FOP), a rare but severely debilitating musculoskeletal disorder. Toward development of a therapeutic, an ATP-competitive inhibitor of the receptor kinase (Dorsomorphin) has been derivatized at Oxford, Harvard and Vanderbilt to enhance selectivity and potency, and proof-of-concept for the experimental drugs demonstrated in the caALK2 mouse, however off-target effects pose a challenge to translation to the clinic and eventually bedside. As a result of our homology-based structural studies, combined with in vitro comparisons of WT, mutant and variant forms of the kinase, an alternative, mechanistically novel means of inhibition by small molecules was conceptualized. To test the hypothesis, compounds were screened by in silico docking to a crystal structure followed by in vitro analyses at the wet bench.

Methods: Methods: Over 640,000 commercially available, drug-like compounds from the UCSF ZINC library were docked in silico by supercomputer, and from the top 1000 hits, ten selected for purchase and biochemical-biophysical analyses.

Results: Results: One of the ten, compound 8, produced dramatic effects that were largely unanticipated yet nonetheless highly desirable for an initial hit in development of a therapeutic.

Conclusions: Conclusions: Through the newly opened avenue, development of a safe and effective therapy for FOP patients can be pursued that is not constrained by limitations inherent to typical competitive inhibitors of protein kinases, which share highly conserved ATP binding sites.