Targeting transforming growth factor beta signaling in metastatic osteosarcoma

Osteosarcoma is a rare type of bone cancer, and half of the cases affect children and adolescents younger than 20 years of age. Despite intensive efforts to improve both chemotherapeutics and surgical management, the clinical outcome for metastatic osteosarcoma remains poor. Transforming growth factor β (TGF-β) is one of the most abundant growth factors in bones. The TGF-β signaling pathway has complex and contradictory roles in the pathogenesis of human cancers. TGF-β is primarily a tumor suppressor that inhibits proliferation and induces apoptosis of premalignant epithelial cells. In the later stages of cancer progression, however, TGF-β functions as a metastasis promoter by promoting tumor growth, inducing epithelial-mesenchymal transition (EMT), blocking antitumor immune responses, increasing tumor-associated fibrosis, and enhancing angiogenesis. In contrast with the dual effects of TGF-β on carcinoma (epithelial origin) progression, TGF-β seems to mainly have a pro-tumoral effect on sarcomas including osteosarcoma (mesenchymal origin). Many drugs that target TGF-β signaling have been developed: neutralizing antibodies that prevent TGF-β binding to receptor complexes; ligand trap employing recombinant Fc-fusion proteins containing the soluble ectodomain of either type II (TβRII) or the type III receptor ((TβRIII), preventing TGF-β from binding to its receptors; antisense nucleotides that reduce TGF-β expression at the transcriptional/translational level; small molecule inhibitors of serine/threonine kinases of the type I receptor (TβRI) preventing downstream signaling; and vaccines that contain cell lines transfected with TβRII antisense genes, or target furin convertase, resulting in reduced TGF-β signaling. TGF-β antagonists have been shown to have effects on osteosarcoma in vitro and in vivo. One of the small molecule TβRI inhibitors, Vactosertib, is currently undergoing a phase 1/2 clinical trial to evaluate its effect on osteosarcoma. Several phase 1/2/3 clinical trials have shown TGF-β antagonists are safe and well tolerated. For instance, Luspatercept, a TGF-β ligand trap, has been approved by the FDA for the treatment of anemia associated with myeloid dysplastic syndrome (MDS) with ring sideroblasts/mutated SF3B1 with acceptable safety. Clinical trials evaluating the long-term safety of Luspatercept are in process.