Cyproterone acetate: a genotoxic carcinogen?

Cyproterone acetate, a widely used synthetic progestagen with antiandrogenic activity, is known for years to produce liver tumours in rats, with a higher incidence in females. This effect was attributed to a rodent-specific tumour promoting mode of action based on the detection of a strong hepato-mitogenic activity of cyproterone acetate. However, more recent studies have demonstrated that cyproterone acetate is sex-specifically activated to (a) DNA-damaging intermediate(s) in the liver of female rats which result in the formation of DNA adducts, induction of DNA repair, and increased levels of micronuclei and gene mutations. Consistent with a sex-specific genotoxicity, cyproterone acetate showed a tumour-initiating potential in a liver foci assay with female rats but not with male rats. Most important, cyproterone acetate was found to induce formation of DNA adducts in primary cultures of human hepatocytes indicating that human liver cells have the capacity to activate cyproterone acetate to genotoxic intermediates. However, the overall assessment of the preclinical data presented in this review suggests that induction of liver tumours in female rats most probably depends on both, genotoxic and mitogenic effects which would suggest a non-linear mode of action with regard to tumour formation. With the exception of DNA adduct formation all other adverse effects induced by cyproterone acetate in rat liver, including gene mutations and liver tumours, can be detected at very high dose levels only. Hence, a cancer risk estimate based on a simple linear extrapolation from high dose to low exposure conditions of recommended clinical use would be questionable. Human data from pharmacoepidemiological studies that specifically addressed the question of possible liver cancer risk in patients treated with cyproterone acetate do in principle support this interpretation. In agreement with these considerations the regulatory authorities of the European Union came to the common conclusion that a possible cancer risk associated with the clinical use of cyproterone acetate, if any, appears to be low and the risk-benefit ratios for the currently authorised indications remain favourable.