Effect of para halogen modification of S-3-(phenoxy)-2-hydroxy-2-methyl-N-(4-nitro-3-trifluoromethyl-phenyl)-propionamides on metabolism and clearance

Abstract

The purpose of this study was to better understand why para-halogen modifications of S-3-(4-halophenoxy)-2-hydroxy-2-methyl-N-(4-nitro-3-trifluoromethylphenyl) propionamide selective androgen receptor modulators (SARMs) had the opposite of expected effects on total clearance, in which electron-withdrawing groups generally protect benzene ring from hydroxylation. We determined the plasma protein binding of this series of halogen substituted SARMs and characterized the qualitative effects of B-ring halogen substitution on in vivo metabolism. In vivo metabolism of S-9, S-10, and S-11 were determined in rats using LC-MS(n) analysis. Intrinsic clearance was measured by in vitro metabolism using rat liver microsomes. Rat plasma protein binding was measured by equilibrium dialysis and drug concentrations after dialysis were analyzed by LC-MS. The major metabolic pathways of the halogen-substituted SARMs examined were very similar and included three major phase I pathways; (1) hydrolysis of the amide bond, (2) B-ring hydroxylation, and (3) A-ring nitro reduction to an aromatic amine. In plasma protein binding studies, S-1 (F, fu = 0.78 ± 0.17 %) showed the greatest unbound fraction, followed by S-9 (Cl, fu = 0.10 ± 0.04 %), S-10 (Br, fu = 0.03 ± 0.01 %), and S-11 (I, fu = 0.008 ± 0.001 %). The CLint values of S-1, S-9, S-10, and S-11 were 2.4, 2.5, 2.8, and 4.6 μL/min/mg, respectively. These findings suggest that as lipophilicity increased the free fraction was reduced thus compensating for metabolic liability and resulting in the apparent discrepancy between CLint and CL total of halogen-substituted SARMs series.