Determination of silodosin and its active glucuronide metabolite KMD-3213G in human plasma by LC-MS/MS for a bioequivalence study.

A sensitive and selective liquid chromatography-tandem mass spectrometry (LC-MS/MS) method is described for the simultaneous determination of silodosin (SLD) and its active metabolite silodosin β-d-glucuronide (KMD-3213G) in human plasma. Liquid-liquid extraction of plasma samples was carried out with ethyl acetate and methyl tert-butyl ether solvent mixture using deuterated analogs as internal standards. The extraction recoveries of SLD and KMD-3213G were in the ranges 90.8-93.4 and 87.6-89.9%, respectively. The extracts were analyzed on a Symmetry C18 (50 × 4.6 mm, 5 μm) column under gradient conditions using 10 mm ammonium formate in water and methanol-acetonitrile (40:60, v/v), within 6.0 min. For MS/MS measurements, ionization of the analytes was carried out in the positive ionization mode and the transitions monitored were m/z 496.1 → 261.2 for SLD and m/z 670.2 → 494.1 for KMD-3213G. The method showed good linearity, accuracy, precision and stability in the range 0.10-80.0 ng/mL for SLD and KMD-3213G. The IS-normalized matrix factors obtained were highly consistent, ranging from 0.962 to 1.023 for both analytes. The method was used to support a bioequivalence study of SLD and its metabolite in healthy volunteers after oral administration of 8 mg silodosin capsules.