The antiestrogenic activity of a series of alkyl-substituted polychlorinated dibenzofurans (PCDFs) were determined in the immature female Sprague-Dawley rat uterus. The compounds utilized in this study contain two, three, or four lateral substituents and include: 6methyl-1,3,8-triCDF, 6-ethyl-1,3,8-triCDF, 6-n-propyl-1,3,8-triCDF, 6-i-propyl-1,3,8-triCDF, 6-t-butyl-1,3,8-triCDF, 8-methyl-1,3,6-triCDF (two lateral substituents); 6-methyl-2,3,8-triCDF, 6-methyl-2,3,4,8-tetraCDF, 8-methyl-1,3,7-triCDF, and 8-methyl-1,2,4,7-tetraCDF (three lateral substituents); 8-methyl-2,3,7-triCDF, 8-methyl-2,3,4,7-tetraCDF (four lateral substituents). Two additional compounds, 8-methyl-2,3,7-trichlorodibenzo-p-dioxin and 8-methyl-2,3,7-tribromodibenzo-p-dioxin (four lateral substituents), were also utilized. All of the alkyl-substituted compounds inhibited estrogen-induced uterine wet weight increase and cytosolic and nuclear progesterone and estrogen receptor binding. The effects of structure on the antiestrogenic potencies were determined using 6-i-propyl-1,3,8-triCDF, 6-methyl-2,3,4,8-tetraCDF, and 8-methyl-2,3,4,7-tetraCDF as representative congeners containing two, three, and four lateral substituents, respectively. The ED50 values for antiestrogenicity were similar for the three compounds; however, the ED50 values for induction of hepatic CYP1A1-dependent activity were 73,600 (estimated), 8.52, and 5.31 μmol/kg for 6-i-propyl-1,3,8-triCDF, 6-methyl-2,3,4,8-tetraCDF, and 8-methyl-2,3,4,7-tetraCDF, respectively. Since CYP1A1 can be used as a surrogate for toxic potency in the rat then high ED50 (induction)/ED50 (antiestrogenicity) ratios would be indicative of low toxicity and high antiestrogenic potency. The ratio was 13,990 to 17,100 for 6-i-propyl-1,3,8-triCDF, whereas the corresponding value for the compounds with three or four lateral substituents varied from 0.64 to 3.34. The results suggests that the 1,3,6,8-substituted alkyl PCDFs are useful structural models for developing new aryl hydrocarbon receptor-mediated antiestrogens for future clinical use as antiestrogens.
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