MICROSOMAL AND PEROXIDASE ACTIVATION OF 4-HYDROXY-TAMOXIFEN TO FORM DNA-ADDUCTS - COMPARISON WITH DNA-ADDUCTS FORMED IN SPRAGUE-DAWLEY RATSTREATED WITH TAMOXIFEN

Using rat liver microsomal preparations and peroxidase enzymes, we hav e investigated the formation of DNA adducts by the antiestrogen compou nd tamoxifen (TAM) and its metabolite 4-hydrooxy-tammoxifen (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 P-32-postlabeling. The DNA adduct pro duced by microsomal activation of 4-OH-TAM and TAM was the same. With cumene hydroperoxide (CuOOH) as the cofactor for the microsomal activa tion 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 sh owed that they were distinct. However one of the DNA adducts (a) produ ced by microsomal activation of 4-OH-TAM using CuOOH was the same as a dduct a produced by microsomal activation of 4-OH-TAM with NADPH. Acti vation of 4-OH-TAM with horseradish peroxidase resulted in the formati on 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 iden tical 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 an imals was similar to that produced by microsomal activation of TAM usi ng 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 eit her TAM or 4-OH-TAM using NADPH was also present as one of the adducts (1) formed in vivo following TAM treatment. These studies demonstrat e that 4-OH-TAM can be activated to form DNA adducts and that it contr ibutes to the formation of DNA adducts in the liver of rats treated wi th TAM.