Metabolism of equilin sulfate in the dog

The metabolism of equilin sulfate was determined in female dogs receiving 2.5 mg/kg of [ 3H]equilin sulfate alone or in a preparation that contained all the components that are present in the conjugated equine estrogen product Premarin ®. The pharmacokinetic parameters of total radioactivity indicated that the drug is rapidly absorbed and it has a moderate half-life in plasma. The total radioactivity in plasma following administration of [ 3H]equilin sulfate as part of a mixture of conjugated equine estrogens had significantly lower peak concentration ( C max), a lower area under the curve (AUC), a longer terminal half-life ( t 1 2 ) and a longer mean residence time (MRT) than when [ 3H]equilin sulfate was given alone, indicating that the other components in the conjugated equine estrogen preparation altered the pharmacokinetics of equilin sulfate. An average of 26.7 ± 4.4% of the administered radioactive dose was excreted in urine of dogs receiving [ 3H]equilin sulfate. Again, a significantly lower percentage (21.4 ± 6.3%, P = 0.023) was eliminated in urine of dogs receiving [ 3H]equilin sulfate in the conjugated equine estrogen preparation, indicating that the absorption of equilin sulfate was perhaps altered by the other components in the conjugated equine estrogen preparation. Metabolite profiles of plasma and urine were similar. Equilin, equilenin, 17β-dihydroequilenin, 17β-dihydroequilin, 17α-dihydroequilenin and 17α-dihydroequilin were present in both matrices. 17β-Dihydroequilin and equilin were the two major chromatographic peaks in plasma samples. 17β-Dihydroequilenin and 17β-dihydroequilin were the major metabolites in urine. In conclusion, following oral administration of [ 3H]equilin sulfate to dogs, the radioactivity is rapidly absorbed. The disposition of equilin sulfate is altered by the other components that are present in the conjugated equine estrogen preparation Premarin ®. The reduction of the 17-keto group and aromatization of ring-B are the major metabolic pathways of equilin in the dog.