Compound Library Screening for Synergistic Drug Combinations: mTOR Inhibitor and Proteasome Inhibitor Effective Against Osteosarcoma Cells

The development of new drugs is urgently needed for new treatment strategies that can improve the prognosis of osteosarcoma (OS). In this study, we attempted to identify combinations of new molecular-targeted agents for OS.A library containing 324 compounds was used. For the first screening, MG-63 OS cells were treated with each of the compounds and cell viability was measured. After the best candidate compound was decided, the compound was included in a second screening. The combination of most effective compounds was decided. The antiproliferative effect of the combination was examined and the cell signaling mechanism was evaluated by western blot analysis. 143B OS-bearing mice were used for in vivo antitumor testing.In the first screening, bortezomib was chosen as the effective drug. In the second screening with bortezomib, everolimus was chosen. This combination showed a synergistic inhibitory effect on cell proliferation when compared to monotherapy with each of these drugs alone. Compared to monotherapy, the combination therapy enhanced the levels of cleaved poly (ADP-ribose) polymerase, caspase-3, caspase-8 and caspase-9, phospho-c-Jun N-terminal kinase, and P38. In contrast, the levels of c-MYC proto-oncogene bHLH transcription factor, survivin, and phospho-cyclin D1 were reduced. The combination effectively induced apoptosis and interfered with cell cycle progression. In the in vivo analysis, the combination therapy significantly inhibited tumor growth.The combination of everolimus and bortezomib demonstrated a synergistic effect against OS both in vitro and in vivo. These results indicate that this combination may be useful as a novel therapeutic strategy for OS.