PGP and brain disorder: The P-gp radiotracers which first became available (i.e., racemic [11C]verapamil, (R)-[11C]verapamil, and [11C]N-desmethyl-loperamide) were very efficiently transported by P-gp at the BBB, resulting in very low brain uptake and a limited sensitivity to detect moderate changes in P-gp function as they are expected to occur in various brain diseases [5].