Absolute quantification of imipramine and its metabolites in vivo utilizing calibrators from radiolabeled in vitro incubations

Bifunctionalized isotopologs as calibrants for standard-free quantitation of drug metabolites in plasma by liquid chromatography coupled with radiometry and mass spectrometry

The reported method involves a novel workflow that eliminates the need for authentic reference standards for the quantitation of drug metabolites in biological samples using a single multi-isotopically labeled compound bearing both radio and stable isotopes. The resulting radio and stable bifunctionalized isotopolog (RADSTIL) of the parent drug is employed as a substrate for in vitro biotransformation to targeted RADSTILs of metabolites as calibrants. Inclusion of a radio label enables both radiometric and mass spectrometric detection. The addition of stable labels ensures the subsequent isotopic interference-free quantitation of unlabeled metabolites in preclinical and clinical samples. This affords a more accurate quantitation workflow compared with the current semi-quantitation method, which utilizes isotopic interfering radio isotopologs of metabolites alone as calibrants. The proof-of-concept is illustrated with (¹⁴ C,¹³ C₂ )-acetaminophen where in vitro biotransformation produced (¹⁴ C,¹³ C₂ )-sulfate and (¹⁴ C,¹³ C₂ )-glucuronide calibrants. Absolute quantitation of the acetaminophen metabolites was then achieved by liquid chromatography coupled with radiometry and mass spectrometry. Quantitative data obtained by this method fell within 82-86% of the values from conventional LC-MS/MS method.