Ultrafast mass spectrometry based bioanalytical method for digoxin supporting an in vitro P-glycoprotein (P-gp) inhibition screen

The evaluation of interactions between drug candidates and transporters such as P‐glycoprotein (P‐gp) has gained considerable interest in drug discovery and development. Inhibition of P‐gp can be assessed by performing bi‐directional permeability studies with in vitro P‐gp‐expressing cellular model systems such as Caco‐2 (human colon carcinoma) cells, using digoxin as a substrate probe. Existing methodologies include either assaying 3H‐digoxin with liquid scintillation counting (LSC) detection or assaying non‐labeled digoxin with liquid chromatography–tandem mass spectrometric (LC‐MS/MS) analysis at a speed of several minutes per sample. However, it is not feasible to achieve a throughput high enough using these approaches to sustain an early liability screen that generates more than a thousand samples on a daily basis. To address this challenge, we developed an ultrafast (9 s per sample) bioanalytical method for digoxin analysis using RapidFire™, an on‐line solid‐phase extraction (SPE) system, with MS/MS detection. A stable isotope labeled analog, d3‐digoxin, was used as internal standard to minimize potential ionization matrix effect during the RF‐MS/MS analysis. The RF‐MS/MS method was more than 16 times faster than the LC‐MS/MS method but demonstrated similar sensitivity, selectivity, reproducibility, linearity and robustness. P‐gp inhibition results of multiple validation compounds obtained with this RF‐MS/MS method were in agreement with those generated by both the LC‐MS/MS method and the 3H‐radiolabel assay. This method has been successfully deployed to assess P‐gp inhibition potential as an important early liability screen for drug–transporter interaction. Copyright © 2011 John Wiley & Sons, Ltd.