Simultaneous quantification of amiloride and hydrochlorothiazide in human plasma by liquid chromatography-tandem mass spectrometry.

A selective, sensitive and precise assay based on solid phase extraction and liquid chromatography-tandem mass spectrometry (LC-MS/MS) was developed for the simultaneous determination of amiloride (AMI) and hydrochlorothiazide (HCTZ) in human plasma. Sample clean-up with 250 µL of plasma was done on Phenomenex Strata™-X extraction cartridges using their labeled internal standards (AMI-15N3 and HCTZ-13C,d2). Chromatography was performed on Hypersil Gold C (50 mm×3.0 mm, 5 µm) column using acetonitrile with 4.0 mM ammonium formate (pH 4.0, adjusted with 0.1% formic acid) (80:20, v/v) as the mobile phase. Detection was carried out on a triple quadrupole API 5500 mass spectrometer utilizing an electrospray ionization interface and operating in the positive ionization mode for AMI and negative ionization mode for HCTZ. Multiple reaction monitoring was used following the transitions at 230.6/116.0, 233.6/116.0, 296.0/204.9 and 299.0/205.9 for AMI, AMI-15N3, HCTZ and HCTZ-13C,d2, respectively. Calibration curves were linear (≥0.9997) over the concentration range of 0.050-50.0 and 0.50-500 ng/mL for AMI and HCTZ, respectively, with acceptable accuracy and precision. The signal-to-noise ratio at the limit of quantitation was ≥14 for both the analytes. The mean recovery of AMI and HCTZ from plasma was 89.0% and 98.7%, respectively. The IS-normalized matrix factors determined for matrix effect ranged from 0.971 to 1.024 for both the analytes. The validated LC-MS/MS method was successfully applied to a bioequivalence study using 5 mg AMI and 50 mg HCTZ fixed dose tablet formulation in 18 healthy Indian volunteers with good reproducibility.