Positron emission tomography (PET) imaging of P-glycoprotein (P-gp) in the blood-brain\nbarrier can be important in neurological diseases where P-gp is affected, such as Alzheimer�´s disease.\nRadiotracers used in the imaging studies are present at very small, nanomolar, concentration,\nwhereas in vitro assays where these tracers are characterized, are usually performed at micromolar\nconcentration, causing often discrepant in vivo and in vitro data. We had in vivo rodent PET data of\n[11C]verapamil, (R)-N-[18F]fluoroethylverapamil, (R)-O-[18F]fluoroethyl-norverapamil, [18F]MC225\nand [18F]MC224 and we included also two new molecules [18F]MC198 and [18F]KE64 in this\nstudy. To improve the predictive value of in vitro assays, we labeled all the tracers with tritium\nand performed bidirectional substrate transport assay in MDCKII-MDR1 cells at three different\nconcentrations (0.01, 1 and 50 �¼M) and also inhibition assay with P-gp inhibitors. As a comparison,\nwe used non-radioactive molecules in transport assay in Caco-2 cells at a concentration of 10 �¼M and\nin calcein-AM inhibition assay in MDCKII-MDR1 cells. All the P-gp substrates were transported\ndose-dependently. At the highest concentration (50 �¼M), P-gp was saturated in a similar way as after\ntreatment with P-gp inhibitors. Best in vivo correlation was obtained with the bidirectional transport\nassay at a concentration of 0.01 �¼M. One micromolar concentration in a transport assay or calcein-AM\nassay alone is not sufficient for correct in vivo prediction of substrate P-gp PET ligands.
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