In vitro and in vivo structure-activity relationships of novel androgen receptor ligands with multiple substituents in the B-ring

We recently reported two nonsteroidal androgen receptor (AR) ligands that demonstrate tissue-selective pharmacological activity, identifying these S-3-(phenoxy)-2-hydroxy-2methyl-N-(4-nitro-3-trifluoromethyl-phenyl)-propionamide analogs as the first members of a new class of drugs known as selective androgen receptor modulators. The purpose of these studies was to explore additional structure-activity relationships of selective androgen receptor modulators to enhance their AR binding affinity, AR-mediated transcriptional activation, and in vivo pharmacological activity. The AR binding affinity (Ki) of 29 novel synthetic AR ligands was determined by a radioligand competitive binding assay and ranged from 1.0-51 nM. Compounds with electron-withdrawing substituents at the para- and meta-positions of the B-ring demonstrated the highest AR binding affinity. The AR-mediated transcriptional activation was determined using a cotransfection assay in CV-1 cells. Most compounds with two substituents in the B-ring maintained or improved their functional activity in vitro. However, compounds with three halogen substituents exhibited significant regioselectivity. Fifteen compounds were selected to examine their pharmacological activity in castrated rats. In vivo pharmacological activity and selectivity were significantly changed by structural modification in the B-ring. Compounds with halogen groups at the para- and meta-positions of the B-ring displayed the highest pharmacological activity. Incorporating substituents at the ortho-position of the B-ring resulted in poor pharmacological activity. In vitro and in vivo agonist activities were partially correlated. In conclusion, novel selective androgen receptor modulators with improved in vivo pharmacological activity can be designed and synthesized based on the structure-activity relationship identified in these studies.