Cyclophilin Inhibitor Rencofilstat Combined with Proteasome Inhibitor Ixazomib Increases Proteotoxic Cell Death in Advanced Prostate Cancer Cells with Minimal Effects on Non-Cancer Cells

Background/Objective: Proteotoxic stress induced by inhibitors of the ubiquitin–proteasome system has been successful in multiple myeloma but not in solid cancers such as prostate cancer. Our objective is to find a combination with proteasome inhibitors that increases apoptotic cell death in all types of prostate cancer without harming non-cancer cells. Methods: The effectiveness of rencofilstat, a pan-cyclophilin inhibitor, combined with the ixazomib proteasome inhibitor, was investigated in multiple prostate cancer and non-cancer cells. Inducible knockdown of stress response XBP1s and cyclophilins A/B and inducible expression of XBP1s and cyclophilin B were developed in prostate cancer to determine functional roles. Results: Rencofilstat + ixazomib increased apoptotic cell death in prostate cancer but not in non-cancer cells. We investigated the effects on XBP1s and PERK, important unfolded protein response factors required for cells to survive proteotoxic stress. The results revealed that XBP1s had a pro-survival role early, but maintenance at later times of rencofilstat + ixazomib treatment resulted in cell death. In addition, decreased PERK and phospho-eIF2α likely maintained protein synthesis to further enhance proteotoxic stress. In contrast, rencofilstat + ixazomib did not alter XBP1s or PERK in non-cancer cells. Additional genetic experiments showed that the RCF targets cyclophilins A, B, and D had protective effects. Rencofilstat increased extracellular secretion of cyclophilin B, but rencofilstat + ixazomib reduced glycosylation and, likely, the biological function of CD147 (CypB receptor) and decreased downstream ERK signaling. Conclusions: Rencofilstat + ixazomib may be a new strategy for increasing proteotoxic stress and apoptotic cell death in advanced prostate cancer cells with less toxic side effects.