text: Hydroxyflutamide and its analogues, bicalutamide and nilutamide, share an anilide ring structure. The structures can be seen in figure 7, where the anilide ring is coloured red. These three compounds require an electron-deficient aromatic ring for efficient AR binding. Replacing the anilide with an alkene gives weakly active compounds, which can be attributed to the lack of intramolecular hydrogen binding or to poor hydrogen-bond donor capability. Various combinations of electron-withdrawing substitutions in the aniline ring of these drugs have not shown higher binding to the AR receptor, compared to compounds which have a chloro or trifluoromethyl group at the meta position (R1) and either a cyano or nitrogen group at the para position (R2).
  For hydroxyflutamide, a group of compounds that differed in the aromatic ring did not bind to the AR. This suggests that the bisubstitution in the hydroxyflutamide ring is essential for high AR binding affinity. It has also been demonstrated that hydroxyflutamide requires the strong hydrogen bond donor ability of the tertiary hydroxyl group and fixed conformers involved in intramolecular hydrogen binding, to bind effectively to AR.
  For bicalutamide, the antiandrogenic activities of sulfide and sulfone substitutions of the X-linkage were tested in vitro. The sulfides showed in most cases at least 2-fold higher binding affinity than corresponding sulfones. However, this relationship was reversed when the R3 group was NHSOCH, where the binding affinity of sulfone was 3-fold higher than that of sulfide. These results indicate that substituents of the B-ring largely determine the effect of the X-linkage in AR binding. Researchers have proposed that the tertiary hydroxyl group is involved in direct interaction with AR because when an acetyl group is introduced to that hydroxyl moiety, the receptor binding affinity greatly decreases.
  Nilutamide has very low affinity for AR when tested on castrated rat prostate. Modifications such as replacing the N3 atom with oxygen has little effect on affinity of the compound for prostate AR. By replacing the oxygen atom with a sulfur atom at the C2 position of the imidazole ring and adding butylalcohol to the N3 atom, the receptor binding and biological activity of the compound increases 100 times that of NSAAs. Also the compound does not bind to other steroid receptors. If a methyl group is changed for the butylalcohol group, the compound shows 3 and 10 times more antiandrogenic activity in vivo than bicalutamide and nilutamide, respectively.