The cell death response to γ-radiation in MCF-7 cells is enhanced by a neuroleptic drug, pimozide

Neuroleptic drugs that bind sigma sites were tested for their ability to inhibit growth and radiosensitize MCF-7 human breast cancer cells. Inhibition of growth by ∼ 50% occurred in cells exposed to pimozide (0.6 μM), haloperidol (10 μM), and the sigma ligand DTG (1,3-di(2-tolyl)guanidine, 20 μM), but no growth inhibition occurred in cells exposed to clozapine, a neuroleptic drug lacking sigma binding activity, or dextromethorphan, a selective sigma 1 binding ligand. Pimozide (2.5 μM), but not haloperidol (3.6 μM), enhanced the sensitivity of MCF-7 cells to γ radiation in clonogenic survival assays. Pimozide significantly decreased MCF-7 clonogenic survival following a 5 or 8 Gy dose of γ radiation, and the dose of radiation required for 1% survival (survival enhancement ratio, SER) was decreased by a factor of 2. Exposure of normal WI-38 human embryonic lung cells to pimozide did not increase their sensitivity to γ radiation. Pimozide (2.5 μM) activated early apoptotic changes in MCF-7 cells that were detected by the uptake of Hoechst 33342 dye, and 10 μM pimozide activated a complete apoptotic pathway resulting in the death of > 90% of the cells within 24 hours. MCF-7 cells exposed to γ radiation alone (8 Gy) showed giant cell formation, mitotic arrest, and a limited degree of apoptosis and necrosis. Within 50 hours of treatment with a combination of radiation and pimozide, cell numbers were sharply reduced compared with cultures exposed to either radiation or pimozide alone. We conclude that pimozide augmented the sensitivity of MCF-7 cells to radiation-induced cell killing through a mechanism not shared by haloperidol, but suggest that concentration of pimozide in MCF-7 cells as a result of an enrichment of sigma 2 sites might target the radiosensitization.