Preclinical validation of TGFβ inhibitors as a novel therapeutic strategy for post-traumatic heterotopic ossification

Heterotopic ossification (HO) is characterized by the abnormal growth of ectopic bone in non-skeletal soft tissues through a fibrotic pathway and is a frequent complication in a wide variety of musculoskeletal injuries. We have previously demonstrated that TGFβ levels are elevated in the soft tissues following extremity injuries. Since TGFβ mediates the initial inflammatory and wound-healing response in the traumatized muscle bed, we hypothesized that targeted inhibition of the TGFβ pathway may be able to abrogate the unbalanced fibrotic phenotype and bone-forming response observed in post-traumatic HO. Primary mesenchymal progenitor cells (MPCs) harvested from debrided traumatized human muscle tissue were used in this study. After treatment with TGFβ inhibitors (SB431542, Galunisertib/LY2157299, Halofuginone, and SIS3) cell proliferation/survival, fibrotic formation, osteogenic induction, gene expression, and phosphorylation of SMAD2/3 were assessed. In vivo studies were performed with a Sprague-Dawley rat blast model treated with the TGFβ inhibitors. The treatment effects on the rat tissues were investigated by radiographs, histology, and gene expression analyses. Primary MPCs treated with TGFβ had a significant increase in the number of fibrotic nodules compared to the control, while TGFβ inhibitors that directly block the TGFβ extracellular receptor had the greatest effect on reducing the number of fibrotic nodules and significantly reducing the expression of fibrotic genes. In vivo studies demonstrated a trend towards a lower extent of HO formation by radiographic analysis up to 4 months after injury when animals were treated with the TGFβ inhibitors SB431542, Halofuginone and SIS3. Altogether, our results suggest that targeted inhibition of the TGFβ pathway may be a useful therapeutic strategy for post-traumatic HO patients.