Interference of the antihormone RU486 in the determination of testosterone and estradiol by enzyme-immunoassay

The antihormone RU486 (mifepristone, 11beta-(4-dimethylaminophenyl)-17beta-hydroxy-17alpha-(prop- 1-ynyl)-estra-4,9-dien-3-one) is currently used in many endocrinological studies and in clinical practice as a contraceptive agent. The results presented here indicate that the synthetic steroid RU486 may interfere in determinations of testosterone and estradiol when using some commercial kits. Although the cross reactivity is low (0.30% for testosterone and 0.16% for estradiol), in animal experiments RU486 doses higher than 5 mg/kg may produce false positive results in the estimation of plasma testosterone (specially in castrated male or female mice) or estradiol concentrations, even in samples obtained 48 h after the administration of this antihormone.