Although quinidine has been recommended as a probe substrate for the P-gp inhibition assay using Caco-2 cell monolayer, it has not been studied widely in the in vitro system. In the present investigation, in vitro permeability studies using Caco-2 cell monolayer were carried out in order to optimize and validate quinidine as a P-gp probe substrate. In bi-directional Caco-2 assay across different passages, a good efflux ratio of more than ten was consistently obtained at 100 nM donor concentration of quinidine. Quinidine was found to have a good mass balance in the Caco-2 system. The inhibitory potencies of known P-gp inhibitors viz verapamil, ketoconazole, tacrolimus and cyclosporine A, determined over a wide concentration range, showed low apparent IC(50) values. Overall, quinidine was found to be a good probe substrate for routine use to assess the in vitro inhibitory potency of NCEs on Pgp-mediated transport.

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