Molecular non-genetic biomarkers of effect related to methyl thiophanate cocarcinogenesis: organ- and sex-specific cytochrome P450 induction in the rat.

We used selective biochemical markers of effect to evaluate some non-genotoxic cocarcinogenic properties of methyl thiophanate (MTH) associated with cytochrome P450 (CYP) changes. Several CYP-dependent reactions were monitored in the liver, kidney and lung microsomes of male and female Sprague-Dawley rats treated (i.p.) with a single (285 or 570 mg/kg body weight) or repeated (daily 285 or 570 mg/kg body weight for three consecutive days) doses of this pesticide. No significant changes in absolute or relative liver, kidney and lung weights were observed after MTH injection. Highly specific substrates were used as probes of different isoforms, such as CYP1A1, 1A2, 2B1, 2E1 and 3A. A complex pattern of CYP induction, including organ- and sex-related differences, was observed, particularly in the liver (CYP3A, 2B1), kidney (CYP1A1, 2E1) and lung (CYP3A, 1A1). In the liver, an increase up to 29-fold in the 2B1-like activity, probed by the O-dealkylation of pentoxyresorufin, was observed at lower dose in both sexes, and the induction of CYP 1A2-mediated methoxyresorufin O-demethylase activity (up to 3.6-fold) was recorded at the higher dose in males. In the kidney, the O-deethylation of ethoxyresorufin (CYP1A1-linked) was increased up to 28.2-fold and the CYP2E1-dependent p-nitrophenol hydroxylases were enhanced up to 6.3-fold in females receiving higher multiple MTH administration. In the lung, the CYP3A-associated activity was the most induced oxidases, as exemplified by the marked increase in the O-demethylation of aminopyrine (up to 3.6-fold) in males. A weak, although significant, reduction of CYP2B1-linked oxidases was also observed in repeated treatment in the kidney (males) and lung (females). These results suggest that the induction of CYP-catalyzed drug metabolism by prolonged exposure to MTH may result in accelerated metabolism of coadministered drugs with important implications for their disposition Together with an alteration of endogenous metabolism, the adverse effects associated with CYP changes such as toxicity/cotoxicity, cocarcinogenicity and promotion may also have clinical consequences.