Synergism of heat shock protein and histone deacetylase inhibitors in synovial sarcoma

Synovial sarcoma is a cancer of young adults and is fatal in about half of cases. This malignancy carries an SYT-SSX fusion oncoprotein that is not targeted by existing drugs. Recently, we demonstrated that histone deacetylase inhibitors and the heat shock protein 90 inhibitor 17AAG inhibit synovial sarcoma in preclinical models. In this thesis we tested combinations of 17AAG with the histone deacetylase inhibitor MS-275 for synergism by performing proliferation and apoptosis assays on synovial sarcomas. Synergism was assessed by the median-effect principle of Chou and Talalay. The combination was found to be highly synergistic at multiple time points in both tested cell lines (combination indeces as low as 0.11) suggesting that reduced dose combination therapies may be effective against this disease. We next investigated the mechanism of synergism. While our work has shown that histone deacetylase inhibitors induce synovial sarcoma cell death, others have shown in related systems that these agents activate the NF- κB survival pathway, potentially limiting their therapeutic effect. As 17AAG can inhibit upstream activators of NF- κB, we proposed that 17AAG may exert its synergistic effect with histone deacetylase inhibitors by abrogating activation of NF- κB. This hypothesis was confirmed in cells exposed to the drugs alone or in combination by quantitating total cellular levels of the NF- κB inhibitor IκB, nuclear levels of the NF- κB subunit RelA, and NF- κB mediated transcription. In each of these assays, adding 17AAG reversed the NF- κB activating effects of MS-275. Additionally we demonstrated that the NF- κB inhibitor BAY-11-7085 also synergizes with MS-275. Overall our data suggests that the NF- κB pathway plays a role in the synergistic activity of 17AAG and MS-275. These findings contribute to the preclinical development of an optimal systemic therapy for synovial sarcoma.Medicine, Faculty ofPathology and Laboratory Medicine, Department ofGraduat