Use of an intravenous microdose of 14C-labeled drug and accelerator mass spectrometry to measure absolute oral bioavailability in dogs; cross-comparison of assay methods by accelerator mass spectrometry and liquid chromatography-tandem mass spectrometry.

A technique utilizing simultaneous intravenous microdosing of (14)C-labeled drug with oral dosing of non-labeled drug for measurement of absolute bioavailability was evaluated using R-142086 in male dogs. Plasma concentrations of R-142086 were measured by liquid chromatography-tandem mass spectrometry (LC-MS/MS) and those of (14)C-R-142086 were measured by accelerator mass spectrometry (AMS). The absence of metabolites in the plasma and urine was confirmed by a single radioactive peak of the parent compound in the chromatogram after intravenous microdosing of (14)C-R-142086 (1.5 microg/kg). Although plasma concentrations of R-142086 determined by LC-MS/MS were approximately 20% higher than those of (14)C-R-142086 as determined by AMS, there was excellent correlation (r=0.994) between both concentrations after intravenous dosing of (14)C-R-142086 (0.3 mg/kg). The oral bioavailability of R-142086 at 1 mg/kg obtained by simultaneous intravenous microdosing of (14)C-R-142086 was 16.1%, this being slightly higher than the value (12.5%) obtained by separate intravenous dosing of R-142086 (0.3 mg/kg). In conclusion, on utilizing simultaneous intravenous microdosing of (14)C-labeled drug in conjunction with AMS analysis, absolute bioavailability could be approximately measured in dogs, but without total accuracy. Bioavailability in humans may possibly be approximately measured at an earlier stage and at a lower cost.