Phosphatidylinositol 3-kinase and mechanistic target of rapamycin dualinhibitor, VDC597, as a therapeutic agent for canine osteosarcoma.

Canine osteosarcoma (OS) presents a significant clinical challenge in veterinary oncology. Due to the similarities in aggressive biologic behavior, mutation status, and gene expression profiles, the canine patient also provides a spontaneous animal model for OS in humans. Advancements in the treatment of OS have been slow to progress. The phosphatidylinositol 3-kinase (PI3K), AKT serine/threonine kinase (AKT), and mechanistic target of rapamycin (mTOR) signal transduction pathway is implicated in canine and human OS, and presents a potentially valuable therapeutic target. The present study investigated PI3K-AKT-mTOR signaling activity in canine OS cells and the in vitro and in vivo efficacy of a PI3K/mTOR dual inhibitor alone and in combination with cytotoxic chemotherapy drugs for treatment of canine OS. The results of this study demonstrate reduced signal transduction; increased cell death; reduced cell proliferation, migration, invasion, and vascular endothelial growth factor production in vitro; as well as reduced tumor growth and greater survival times with inhibition of PI3K-AKT-mTOR signaling in a xenograft mouse model. We also examined patient-derived tumors for immunoreactivity of forkhead box O1, a downstream target of AKT activation. Unlike the xenograft tumors that were treated with a PI3K/mTOR inhibitor, a correlation between tumor biologic behavior and forkhead box O1 immunoreactivity was not present in the patient-derived tumor sections. These findings indicate both the potential benefits of PI3K/mTOR dual inhibitors in chemotherapeutic protocols and the need for further study of patient-derived tumors to better understand the extent of PI3K-AKT-mTOR activation for the application of such targeted inhibitors. SIGNIFICANCE STATEMENT: The phosphatidylinositol 3-kinase, AKT serine/threonine kinase, and mechanistic target of rapamycin pathway is frequently dysregulated in canine osteosarcoma. VDC597, a dual inhibitor of pathway activation, can potentially improve outcomes of canine osteosarcoma. In vitro, VDC597 inhibited cellular viability, migration, and invasion, vascular endothelial growth factor production, and phosphorylation of signaling proteins, while promoting cell death. In mice treated with VDC597, tumor growth slowed and survival times increased. Effects were found both with VDC597 alone and in combination with other chemotherapy drugs.