Purpose-Overactivity of the multidrug efflux transporter P-glycoprotein (P-gp) at the bloodbrain barrier (BBB) is believed to play an important role in resistance to central nervous system drug treatment. (R)-[ 11 C]verapamil (VPM) PET can be used to measure the function of P-gp at the BBB, but low brain uptake of VPM hampers the mapping of regional differences in cerebral P-gp function and expression. The aim of this study was to evaluate the dose-response relationship of two potent P-gp inhibitors and to investigate if increased brain uptake of VPM mediated by P-gp inhibition can be used to assess regional differences in P-gp activity.Methods-Two groups of Sprague-Dawley rats (n=12) underwent single VPM PET scans at 120 min after administration of different doses of the P-gp inhibitors tariquidar and elacridar. In an additional 6 rats, paired VPM PET scans were performed before and after administration of 3 mg/ kg tariquidar.Results-Inhibitor administration resulted in an up to 11-fold increase in VPM brain distribution volumes (DV) with ED 50 values of 3.0±0.2 and 1.2±0.1 mg/kg for tariquidar and elacridar, respectively. In paired PET scans, 3 mg/kg tariquidar resulted in regionally different enhancement of brain activity distribution, with lowest DV in cerebellum and highest DV in thalamus.Conclusion-Our data show that tariquidar and elacridar are able to increase VPM brain distribution in rat brain up to 11-fold over baseline at maximum effective doses, with elacridar being about 3 times more potent than tariquidar. Regional differences in tariquidar-induced modulation of VPM brain uptake point to regional differences in cerebral P-gp function and expression in rat brain.

Using positron emission tomography (PET) imaging we assessed, in vivo, the interaction between a microdose of (R)-[(11)C]verapamil (a P-glycoprotein (Pgp) substrate) and escalating doses of the Pgp inhibitor tariquidar (3, 4, 6, and 8 mg/kg) at the blood-brain barrier (BBB) in healthy human subjects. We compared the dose-response relationship of tariquidar in humans with data obtained in rats using a similar methodology. Tariquidar was equipotent in humans and rats in its effect of increasing (R)-[(11)C]verapamil brain uptake (expressed as whole-brain volume of distribution (V(T))), with very similar half-maximum-effect concentrations. Both in humans and in rats, brain V(T) approached plateau levels at plasma tariquidar concentrations >1,000 ng/ml. However, Pgp inhibition in humans led to only a 2.7-fold increase in brain V(T) relative to baseline scans (before administration of tariquidar) as compared with 11.0-fold in rats. The results of this translational study add to the accumulating evidence that there are marked species-dependent differences in Pgp expression and functionality at the BBB.

As P-glycoprotein (Pgp) inhibition at the blood–brain barrier (BBB) after administration of a single dose of tariquidar is transient, we performed positron emission tomography (PET) scans with the Pgp substrate (R)-[11C]verapamil in five healthy volunteers during continuous intravenous tariquidar infusion. Total distribution volume (VT) of (R)-[11C]verapamil in whole-brain gray matter increased by 273±78% relative to baseline scans without tariquidar, which was higher than previously reported VT increases. During tariquidar infusion whole-brain VT was comparable to VT in the pituitary gland, a region not protected by the BBB, which suggested that we were approaching complete Pgp inhibition at the human BBB.

The aim of this study was to develop a positron emission tomography (PET) tracer based on the dual P-glycoprotein (P-gp) breast cancer resistance protein (BCRP) inhibitor tariquidar (1) to study the interaction of 1 with P-gp and BCRP in the blood-brain barrier (BBB) in vivo. Odesmethyl-1 was synthesized and reacted with [ 11 C]methyl triflate to afford [ 11 C]-1. Small-animal PET imaging of [ 11 C]-1 was performed in naïve rats, before and after administration of unlabeled 1 (15 mg/kg, n=3) or the dual P-gp/BCRP inhibitor elacridar (5 mg/kg, n=2), as well as in wildtype, Mdr1a/b (−/−) , Bcrp1 (−/−) and Mdr1a/b (−/−) Bcrp1 (−/−) mice (n=3). In vitro autoradiography was performed with [ 11 C]-1 using brain sections of all 4 mouse types, with and without coincubation with unlabeled 1 or elacridar (1 μM). In PET experiments in rats, administration of unlabeled 1 or elacridar increased brain activity uptake by a factor of 3-4, whereas blood activity levels remained unchanged. In Mdr1a/b (−/−) , Bcrp1 (−/−) and Mdr1a/b (−/−) Bcrp1 (−/−) mice, brain-toblood ratios of activity at 25 min after tracer injection were 3.4, 1.8 and 14.5 times higher, respectively, as compared to wild-type animals. Autoradiography showed approximately 50% less [ 11 C]-1 binding in transporter knockout mice compared to wild-type mice and significant displacement by unlabeled elacridar in wild-type and Mdr1a/b (−/−) mouse brains. Our data suggest that [ 11 C]-1 interacts specifically with P-gp and BCRP in the BBB. However, further investigations are needed to assess if [ 11 C]-1 behaves in vivo as a transported or a non-transported inhibitor.