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-MSⁿ 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 CL_int 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 CL_int and CL total of halogen-substituted SARMs series.