Treatment of chemoresistant ovarian cancer with sigma2/SMAC and cisplatin.

Abstract

e16508 Background: Platinum resistance continues to be a major barrier to the successful treatment of ovarian cancer. Overexpression of the X-linked inhibitor of apoptosis proteins (XIAP) contribute to platinum resistance in ovarian cancer through inhibition of caspases and up regulation of Akt activity. Second mitochondrial-derived activators of caspases (SMAC) is an endogenous protein that binds to and reverses XIAP-mediated inhibition of caspases. In order to exploit the SMAC-mediated pro-apoptotic pathway pharmacologically, SMAC mimetics have been developed and shown to induce apoptosis in cancer cells in vitro and in vivo. Untargeted cytotoxic cancer drugs bind to both malignant and normal tissue leading to significant toxicity. We have shown previously that solid tumors upregulate the sigma-2 receptor. We have also shown that sigma-2 ligands are internalized into cancer cells and are therefore an appealing vehicle for tumor targeted therapy. The goal of this study is to test if a conjugate drug of sigma-2 ligand and a SMAC mimetic (sigma-2/SMAC) in combination with chemotherapy is capable of overcoming chemoresistance in ovarian cancer. Methods: SKOV3 and OVCAR3 ovarian cancer cell lines were treated with sigma2/SMAC (1-16μM) and/or cisplatin (.5-10μg/mL). Viability assays were used to detect cell death. Luminescence-based caspase assays were used to compare the activity of caspase-3, -7, and -9 between treatment groups to document involvement of the XIAP survival pathway. Results: We found that sigma2-SMAC is synergistic when used in combination with cisplatin. Compared to untreated cells, SKOV3 cells treated with sigma/2SMAC (4uM), cisplatin .5ug/mL, or combination therapy showed 52.6%, 117.7%, and 34.8% viability respectively (p<.05). Cisplatin and sigma2/SMAC remained synergistic at increasing doses. Similar results were observed in OVCAR3 cells. Caspase-3 and -7 increased in combination therapy 1.2-fold over Sigma/2SMAC alone (4uM) and 7-fold over cisplatin alone (.5ug/mL) in SKOV3 cells (p<.05). Conclusions: This study suggests that the sigma2/SMAC conjugate provides a targeted means for overcoming chemoresistance in ovarian cancer through inhibition of XIAP and activation of caspases.