Rapid assessment of P-glycoprotein inhibition and induction in vitro

Purpose. Using rhodamine123 (RH123) cell exclusion, 17 clinically used compounds were screened for their inhibitory effect on P-glycoprotein (P-gp), which was compared with the drugs' inhibitory activity against CYP3A4. The same assay was used to study induction of P-gp activity. Methods. P-gp inhibition was assessed using RH123 accumulation into LS180V cells as well as Rh123 transport across Caco-2 monolayers. Inhibition of CYP3A4 was determined in human liver microsomes using triazolam-4-hydroxylation. Induction of P-gp expression and activity was measured using western blot analysis and RH123 accumulation into LS180V cells, respectively. Results. The observed inhibition of RH123 cell exclusion ranged from little or no effect ( digoxin, indinavir, fexofenadine) up to a nearly 10-fold increase in RH123 accumulation ( ivermectin, terfenadine). No correlation between P-gp and CYP3A4 inhibition was observed. The rank order in P-gp inhibitory potency for terfenadine, verapamil, ritonavir, and indomethacin was identical in both LS180V and Caco-2 models. Ritonavir and St. John's wort extract showed a concentration-dependent P-gp induction, with good correlation between western blot analysis and RH123 accumulation. Conclusions. The RH123 accumulation assay in LS180V cells can be used as a valuable screening tool to study both inhibition and induction of P-gp activity and expression. This assay has the potential to predict P-gp-mediated alterations in intestinal absorption of drugs.