Microsomal and peroxidase activation of 4-hydroxy-tamoxifen to form DNA adducts: comparison with DNA adducts formed in Sprague-Dawley rats treated with tamoxifen.

Using rat liver microsomal preparations and peroxidase enzymes, we have investigated the formation of DNA adducts by the antiestrogen compound tamoxifen (TAM) and its metabolite 4-hydroxy-tamoxifen (4-OH-TAM). When reduced nicotinamide-adenine dinucleotide phosphate (NADPH) was used as a cofactor in microsomal activation of either 4-OH-TAM or TAM, one DNA adduct and relative DNA adduct levels of 4.6 and 3.1 x 10(-8), respectively were detected by 32P-postlabeling. The DNA adduct produced by microsomal activation of 4-OH-TAM and TAM was the same. With cumene hydroperoxide (CuOOH) as the cofactor for the microsomal activation of either 4-OH-TAM or TAM, three to six DNA adducts were produced; the relative adduct levels were 8.0 and 20.6 x 10(-8), respectively. Comparison of the DNA adduct patterns produced by 4-OH-TAM and TAM showed that they were distinct. However one of the DNA adducts (a) produced by microsomal activation of 4-OH-TAM using CuOOH was the same as adduct a produced by microsomal activation of 4-OH-TAM with NADPH. Activation of 4-OH-TAM with horseradish peroxidase resulted in the formation of a single DNA adduct and a relative adduct level of 20.7 x 10(-8). Rechromatography analysis of this DNA adduct showed that it was identical to that produced by microsomal activation of 4-OH-TAM with NADPH and one of the adducts produced using CuOOH as the cofactor. Ten DNA adducts and a relative adduct level of 15.3 x 10(-8) were detected in the liver of female Sprague-Dawley rats treated daily with 20 mg/kg of TAM for 7 days. The DNA adduct pattern in the liver of the treated animals was similar to that produced by microsomal activation of TAM using CuOOH as the co-factor. The principal DNA adduct (no. 6) formed in the livers of rats treated with TAM was the same as the principal DNA adduct formed following microsomal activation of TAM using CuOOH as a cofactor. The DNA adduct formed following microsomal activation of either TAM or 4-OH-TAM using NADPH was also present as one of the adducts (1) formed in vivo following TAM treatment. These studies demonstrate that 4-OH-TAM can be activated to form DNA adducts and that it contributes to the formation of DNA adducts in the liver of rats treated with TAM.