Cytotoxic and porphyrinogenic effects of diphenyl ethers in cultured rat hepatocytes: Chlornitrofen (CNP), CNP-amino, chlomethoxyfen and bifenox

We studied the cytotoxic and porphyrinogenic effects of four diphenyl ether s (DPEs), chlornitrofen (CNP), CNP-amino, chlomethoxyfen and bifenox, in ra t hepatocytes cultured on Matrigel. Cytotoxicity was determined as a decrea se in viability measured by the release of lactate dehydrogenase. Of the DP Es examined, CNP-amino was the most cytotoxic, with an LC50 value of 0.36 m M (95% confidence interval, 0.33-0.40 mM). CNP also reduced the viability i n a concentration-dependent manner at the concentrations of 0.50 mM or abov e. In contrast, no concentration-dependent decrease in viability was observ ed in the chlomethoxyfen- and bifenox-treated hepatocytes at the concentrat ions up to 1.0 mM. To identify the enzyme involved in the metabolic activat ion of CNP-amino, inhibition studies were carried out using SKF 525-A (0.05 0 mM) and methimazole (1.0 mM). SKF 525-A, a cytochrome P450 inhibitor, qui ckened the onset of cell killing by CNP-amino, while methimazole, an inhibi tor of flavin-containing monooxygenase (FMO), partially suppressed the cyto toxicity of CNP-amino. These results suggest that FMO plays an important ro le in the cytotoxicity induced by CNP-amino, while cytochrome P450 particip ates in the detoxification, possibly via the ring-hydroxylation. The maximu m porphyrin accumulation was observed at 0.13 mM for chlomethoxyfen (18-fol d) and at 0.25 mM for CNP and bifenox (17- and 21-fold, respectively). In c ontrast to these DPEs, the porphyrinogenic effect of CNP-amino was weak, wi th the maximum accumulation at 0.13 mM (at least fivefold). The predominant species was protoporphyrin IX in all of the DPE-treated cultures. These re sults suggest that all of the DPEs examined, possibly including CNP-amino, inhibit protoporphyrinogen oxidase, resulting in the accumulation of protop orphyrin IX. (C) 1998 Elsevier Science Ltd. All,rights reserved.