Abstract

The present study characterized the in vitro androgen receptor (AR) binding affinity, in vitro and in vivo pharmacological activity, and in vivo pharmacokinetics and metabolism of acetothiolutamide, a nonsteroidal AR ligand. AR binding was determined by a competitive binding assay. In vitro AR agonist activity was examined by a cotransfection assay. Acetothiolutamide displayed high AR binding affinity (K_i = 4.9 ± 0.2 nM) and full agonist activity in the in vitro studies. Next, the androgenic, anabolic, and antiandrogenic activity of acetothiolutamide was evaluated in a castrated immature rat model. In this animal model, acetothiolutamide exhibited an overall negligible androgenic effect, but a statistically significant anabolic effect at high subcutaneous doses. Also, acetothiolutamide demonstrated a noticeable antiandrogenic effect in castrated rats supplemented with testosterone propionate. To understand the causes for the observed disparity between in vitro and in vivo activities, pharmacokinetics and metabolism of acetothiolutamide were studied in male Sprague-Dawley rats. Acetothiolutamide was rapidly cleared from rat plasma (clearance of about 45 ml/min/kg) in a concentration-independent manner after i.v. dosing. Acetothiolutamide was completely absorbed after subcutaneous administration, but not bioavailable after oral dose. In the metabolism study, the unchanged molecule and its metabolites in urine and fecal samples were detected by high-performance liquid chromatography-mass spectrometry. The structures of major metabolites were elucidated with liquid chromatography-tandem mass spectrometry. After i.v. administration, acetothiolutamide was excreted in urine and feces as unchanged drug and a variety of metabolites. Oxidation, hydrolysis, and sulfate conjugation of phase I metabolites were the major metabolic pathways of acetothiolutamide in rats. Overall, the high plasma clearance of acetothiolutamide, due to its extensive hepatic metabolism, likely contributed to its lack of androgenic activity in vivo.