DNA methyl-adduct dosimetry and O6-alkylguanine-DNA alkyl transferase activity determinations in rat mammary carcinogenesis by procarbazine and N-methylnitrosourea

Abstract

The metabolism of the carcinogenic antitumor drug procarbazine (PCZ) is complex with the ultimate production, among other metabolites, of a methyldiazonium ion which is also the ultimate carcinogenic species of the DNA-methylating N-nitroso compounds including N-methylnitrosourea (MNU). This suggests a similar mechanism of carcinogenic action. Following a single oral dose of [¹⁴C]PCZ (50 mg/rat) to 50 day old female Sprague-Dawley rats under the reported conditions of mammary gland carcinogenicity, the DNA adducts 7-methylguanine (7-meG) and O⁶-methylguanine (O⁶-meG) were determined in target (mammary gland) and non-target organs. The degree of DNA methylation was similar in all the organs considered. ]In the mammary gland, lung, spleen, small intestine and stomach the O⁶-meG/7-meG ratio was close to 0.11. At a lower dose of PCZ (26 mg/rat), the levels of 7-meG in the tissues were 40–60% of those produced by the higher dose. Eighty percent of the rats given the higher dose versus 37% of those given the lower dose developed mammary tumors after 20 weeks. With the higher dose of MNU (50mg/kg body wt) DNA methylation was more or less uniform in all the organs including the mammary gland, with slightly greater yields in the liver. At a lower MNU dose (25 mg/kg) the levels of 7-meG were 40–48% of those produced by the higher dose. Fifty seven percent of the rats given the higher dose versus 21% of the animals given the lower dose developed mammary gland tumors after 20 weeks. On a mol/kg body wt basis, PCZ was ∼5-times less active than MNU in the production of 7-meG in mammary gland but only ∼2-times less active than MNU in the production of mammary gland tumors. The O⁶-alkylguanine-DNA alkyltransferase (AGT) levels in the liver, kidney, spleen and lung of PCZ or MNU treated rats were ∼9–28% (expressed relative to protein content) and 10–33% (expressed relative to homogenate DNA content) of those in the corresponding organs of the saline-treated controls. However, the AGT levels of the mammary gland and brain were in the range of 45–61% (expressed relative to protein content) and 39–54% (expressed relative to homogenate DNA content) of those of the saline-treated controls. Also the mammary gland of the 50 day old female rats has the lowest AGT activity (expressed relative to DNA content). These data of the 50 day old rat may explain the extradinary susceptability of the mammary gland to the carcinogenic action of PCZ and MNU.