This adverse outcome pathway details the linkage between the antagonism of the androgen receptor (AR) leading to adverse effects in the male foetus. For a more detailed explanation of this pathway, with supporting references, please refer to the project report to the OECD . The AR is involved in the mediation of various cellular processes including proliferation, differentiation and apoptosis in many tissues. The two main events regulated by AR mediated gene expression are urogenital tract differentiation during gestation and sexual changes during puberty. The AR can be activated by the binding of the endogenous androgens testosterone and its metabolite 5-alpha-dihydrotestosterone (DHT), which can activate gene expression at the transcription level. In mammals, virilisation of the external genitalia is driven by DHT while the differentiation of the Wolffian duct is driven by testosterone. Chemicals which bind to the AR may cause disruption by agonism, antagonism or by both mechanisms. Agonists will mimic the action of the endogenous androgens, whilst antagonists will block the receptor and prevent activation. Androgen receptor antagonists divide into steroid-like and non-steroidal compounds. Several classes or chemical categories are indicated by the data . These include the steroidal class (cyproterone acetate), the flutamide/ “aryl amide” class which includes bicalutamide, linuron and hydantoin analogs (such as nilutamide, vinclozin), the quinoline analog class, and the phthalimide derivatives. The best characterised class are synthetic anilides for which the model compound is flutamide.
Flutamide exhibits potent anti-androgenic activity and in animals shows dose dependent decreases in the weight of accessory sex organs at doses of 1mg/kg and above. In utero exposure to flutamide in rats has been shown to cause feminisation of external genitalia, nipple retention and alteration of androgen-dependent testicular descent in male foetuses. A number of flutamide derivatives with in vitro binding data have demonstrated in vivo activity . In vitro the relative binding affinity (RBA) to the AR can be measured using assays which compare the competitive binding versus a control compound such as DHT or a synthetic androgen (metribolone (R1881) or mibolerone). Although this assay can measure binding it cannot distinguish between agonists and antagonists . Transcriptional activation in cells transfected with human AR can be used to identify agonism or antagonism with respect to that induced by a known concentration of DHT . Short term in vivo studies may use the Hershberger assay or acute studies involving castrated rat models .