THE ACTIVITY OF XENOBIOTIC ENZYMES AND THE CYTOTOXICITY OF MITOXANTRONE IN MCF-7 HUMAN - BREAST-CANCER CELLS TREATED WITH INDUCING AGENTS

This study investigated the effect of inducers on the major enzymes re sponsible for metabolising the quinone antitumour agent mitoxantrone, and on its cytotoxicity in MCF 7 human breast cancer cells. Four induc ers were used: 1,2-benzanthracene (BA); phenobarbitone (PB); rifampici n (R) and dexamethasone (DEX). Of these, BA was the most effective, in creasing cytochrome P450 dependent metabolism 64-fold and DT-diaphoras e activity 1,6-fold. R did not cause an increase in any of the enzyme activities measured and, in fact, inhibited glutathione peroxidase act ivity. PB and DEX increased NADPH cytochrome c reductase activity but had no effect on either DT-diaphorase or cytochrome P450 dependent act ivities. BA potentiated the cytotoxicity of mitoxantrone in terms of l eakage of lactate dehydrogenase (LDH) activity and loss of reduced glu tathione (GSH) and protein from cultures. PB had a smaller potentiatin g effect on cytotoxicity and DEX had no effect. Studies with the enzym e inhibitors, dicoumarol (inhibits DT-diaphorase) and metyrapone (inhi bits cytochrome P450), indicate that at least two reactive species are involved in mitoxantrone cytotoxicity. One intermediate, formed by cy tochrome P450, caused LDH leakage and GSH depletion. Formation of the second intermediate. was catalysed by DT-diaphorase and this hydroquin one caused loss of intracellular protein and GSH. We propose that auto oxidation of the hydroquinone resulting in generation of reactive oxyg en species contributes to mitoxantrone cytotoxicity. Concomitant expos ure to inducing agents may alter the cytotoxicity associated with many cytotoxic drugs, not just mitoxantrone, and this is an important cons ideration as many cytotoxics have a narrow therapeutic index.