ACTIVATION OF THIO-TEPA CYTOTOXICITY TOWARD HUMAN BREAST-CANCER CELLS BY HEPATIC CYTOCHROME-P450

The anti-tumor alkylating agent thio-TEPA is metabolized by hepatic cytochrome P450 to yield an oxo derivative. TEPA, plus one or more reactive metabolites. The contribution of this metabolism to the parent drug's cytotoxicity was evaluated in cell culture using the human breast carcinoma cell line MCF-7 as a model. Incubation of thio-TEPA in the presence of NADPH + an Aroclor 1254-induced rat liver 9000. x - supernatant fraction (S9 fraction) resulted in a dramatic increase in drug cytotoxicity, as measured using a clonogenic assay for cell survival. This increase in cytotoxicity was not evident when thio-TEPA was incubated with an uninduced rat liver homogenate, indicating that one or more Aroclor-inducible liver enzymes contribute to drug activation in this system. Metyrapone, a selective inhibitor of the Aroclor-inducible cytochrome P450 2B1, completely blocked liver S9-dependent cytotoxicity, as did an inhibitory antibody directed against P450 2B1, demonstrating that P450 2B1 is the S9 catalyst of thio-TEPA activation. Although TEPA is the major thio-TEPA metabolite formed by P450 2B 1, neither TEPA nor a TEPA metabolite is the cytotoxic species generated by the liver S9 fraction, as evidenced by the moderate cytotoxicity of TEPA in this cellular system, by the failure of S9 to activate TEPA appreciably, and by the lack of synergism between TEPA and thio-TEPA. While glutathione had little or no effect on the cytotoxicity of thio-TEPA or TEPA, low levels of extracellular glutathione (0.25-0.5 mM) fully blocked S9-dependent thio-TEPA activation, suggesting a cell surface site of action of the S9-generated cytotoxic metabolites. These metabolites may also act intracellulary, since depletion of intracellular glutathione by treatment with buthionine sulfoximine enhanced the cytotoxicity of thio-TEPA to a V79 cell transformant that stably expresses P450 2B1, but not to the parental V79 line or to MCF-7 cells. These studies establish a role for cytochrome P450 2B1 in the activation of thio-TEPA to cytotoxic metabolites that are distinct from TEPA, and furthermore suggest a mechanism of action that includes the cell surface as a novel target of this cancer chemotherapeutic agent.