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The capacity of α-mangostin (α-MG) and β-mangostin (β-MG) from mangosteen pericarp on P-glycoprotein (Pgp) in silico, in vitro, and ex vivo was investigated in this study. Screening with the ADMET Predictor™ program predicted the two compounds to be both a Pgp inhibitor and Pgp substrate. The compounds tended to interact with Pgp and inhibit Pgp ATPase activity. Additionally, bidirectional transport on Caco-2 cell monolayers demonstrated a significantly lower efflux ratio than that of the control (α-(44.68) and β-(46.08) MG versus the control (66.26); p < 0.05) indicating an inhibitory effect on Pgp activity. Test compounds additionally revealed a downregulation of MDR1 mRNA expression. Moreover, an ex vivo absorptive transport in everted mouse ileum confirmed the previous results that α-MG had a Pgp affinity inhibitor, leading to an increase in absorption of the Pgp substrate in the serosal side. In conclusion, α- and β-MG have the capability to inhibit Pgp and they also alter Pgp expression, which makes them possible candidates for reducing multidrug resistance. Additionally, they influence the bioavailability and transport of Pgp substrate drugs.

The transcellular transport and metabolism of eight green tea catechins (GTCs) were studied in Caco-2 monolayers, with the aim of investigating the effect of cis–trans isomerism on the membrane permeability and biotransformation of GTCs. The results showed that the catechin stereochemistry significantly affects the efflux transport rather than the absorption transport in the Caco-2 monolayers. The trans catechins showed a better transcellular permeability than their corresponding cis (epi) catechins in the efflux transport, as the efflux amount of trans catechins were all significantly higher than that of the cis (epi) catechins at each concentration and each time point tested. Moreover, the relative contents of the (+)-catechin (C)-O-sulfate, (+)-gallocatechin (GC)-O-sulfate, (−)-catechin gallate (CG)-O-sulfate, and (−)-gallocatechin gallate (GCG)-O-sulfate in the efflux transport were 2.67, 16.08, 50.48, and 31.54 times higher than that of the (−)-epicatechin (EC)-O-sulfate, (−)-epigallocatechin (EGC)-O-sulfate, (−)-epicatechin gallate (ECG)-O-sulfate, and (−)-epigallocatechin gallate (EGCG)-O-sulfate, respectively. It indicated that more metabolites were observed after the transcellular efflux of trans catechins. Furthermore, after two hours of incubation, the GTCs could significantly increase the expression of multidrug resistance-associated protein 2 (MRP2) and breast cancer-resistance protein (BCRP), and decrease the expression of P-glycoprotein in the Caco-2 cells. The regulation of GTCs on P-glycoprotein, MRP2, and BCRP could also be significantly influenced by the chemical and dimensional structure. In a conclusion, catechin stereochemistry significantly affects the transport and metabolism of GTCs when refluxed in the Caco-2 monolayers.

Positron emission tomography (PET) imaging of P-glycoprotein (P-gp) in the blood-brain barrier can be important in neurological diseases where P-gp is affected, such as Alzheimer´s disease. Radiotracers used in the imaging studies are present at very small, nanomolar, concentration, whereas in vitro assays where these tracers are characterized, are usually performed at micromolar concentration, causing often discrepant in vivo and in vitro data. We had in vivo rodent PET data of [11C]verapamil, (R)-N-[18F]fluoroethylverapamil, (R)-O-[18F]fluoroethyl-norverapamil, [18F]MC225 and [18F]MC224 and we included also two new molecules [18F]MC198 and [18F]KE64 in this study. To improve the predictive value of in vitro assays, we labeled all the tracers with tritium and performed bidirectional substrate transport assay in MDCKII-MDR1 cells at three different concentrations (0.01, 1 and 50 µM) and also inhibition assay with P-gp inhibitors. As a comparison, we used non-radioactive molecules in transport assay in Caco-2 cells at a concentration of 10 µM and in calcein-AM inhibition assay in MDCKII-MDR1 cells. All the P-gp substrates were transported dose-dependently. At the highest concentration (50 µM), P-gp was saturated in a similar way as after treatment with P-gp inhibitors. Best in vivo correlation was obtained with the bidirectional transport assay at a concentration of 0.01 µM. One micromolar concentration in a transport assay or calcein-AM assay alone is not sufficient for correct in vivo prediction of substrate P-gp PET ligands.