A major pathological hallmark of Alzheimer's disease is accumulation of amyloid-β in senile plaques in the brain. Evidence is accumulating that decreased clearance of amyloid-β from the brain may lead to these elevated amyloid-β levels. One of the clearance pathways of amyloid-β is transport across the blood-brain barrier via efflux transporters. P-glycoprotein, an efflux pump highly expressed at the endothelial cells of the blood-brain barrier, has been shown to transport amyloid-β. P-glycoprotein function can be assessed in vivo using (R)-[(11)C]verapamil and positron emission tomography. The aim of this study was to assess blood-brain barrier P-glycoprotein function in patients with Alzheimer's disease compared with age-matched healthy controls using (R)-[(11)C]verapamil and positron emission tomography. In 13 patients with Alzheimer's disease (age 65 ± 7 years, Mini-Mental State Examination 23 ± 3), global (R)-[(11)C]verapamil binding potential values were increased significantly (P = 0.001) compared with 14 healthy controls (aged 62 ± 4 years, Mini-Mental State Examination 30 ± 1). Global (R)-[(11)C]verapamil binding potential values were 2.18 ± 0.25 for patients with Alzheimer's disease and 1.77 ± 0.41 for healthy controls. In patients with Alzheimer's disease, higher (R)-[(11)C]verapamil binding potential values were found for frontal, parietal, temporal and occipital cortices, and posterior and anterior cingulate. No significant differences between groups were found for medial temporal lobe and cerebellum. These data show altered kinetics of (R)-[(11)C]verapamil in Alzheimer's disease, similar to alterations seen in studies where P-glycoprotein is blocked by a pharmacological agent. As such, these data indicate that P-glycoprotein function is decreased in patients with Alzheimer's disease. This is the first direct evidence that the P-glycoprotein transporter at the blood-brain barrier is compromised in sporadic Alzheimer's disease and suggests that decreased P-glycoprotein function may be involved in the pathogenesis of Alzheimer's disease.

Tariquidar, a potent, nontoxic, third-generation P-glycoprotein (P-gp) inhibitor, is a possible reversal agent for central nervous system drug resistance. In animal studies, tariquidar has been shown to increase the delivery of P-gp substrates into the brain by severalfold. The aim of this study was to measure P-gp function at the human blood-brain barrier (BBB) after tariquidar administration using PET and the model P-gp substrate (R)-11 C-verapamil. Methods: Five healthy volunteers underwent paired (R)-11 C-verapamil PET scans and arterial blood sampling before and at 2 h 50 min after intravenous administration of tariquidar (2 mg/kg of body weight). The inhibition of P-gp on CD56-positive peripheral lymphocytes of each volunteer was determined by means of the 123 Rh efflux assay. Tariquidar concentrations in venous plasma were quantified using liquid chromatography/mass spectrometry. Results: Tariquidar administration resulted in significant increases (Wilcoxon test for paired samples) in the distribution volume (DV, 124% 6 15%) and influx rate constant (K 1 , 149% 6 36%) of (R)-11 C-verapamil across the BBB (DV, 0.65 6 0.13 and 0.80 6 0.07, P 5 0.043; K 1 , 0.034 6 0.009 and 0.049 6 0.009, P 5 0.043, before and after tariquidar administration, respectively). A strong correlation was observed between the change in brain DV after administration of tariquidar and tariquidar exposure in plasma (r 5 0.90, P 5 0.037). The mean plasma concentration of tariquidar achieved during the second PET scan (490 6 166 ng/mL) corresponded to 100% inhibition of P-gp function in peripheral lymphocytes. Conclusion: Tariquidar significantly increased brain penetration of (R)-11 C-verapamil-derived activity due to increased influx. As opposed to peripheral P-gp function, central P-gp inhibition appeared to be far from complete after the administered tariquidar dose.

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Summary: Purpose and Methods: Regional overexpression of the multidrug transporter P-glycoprotein (P-gp) in epileptic brain tissue may lower target site concentrations of antiepileptic drugs and thus contribute to pharmacoresistance in epilepsy. We used the P-gp substrate R-[11 C]verapamil and positron emission tomography (PET) to test for differences in P-gp activity between epileptogenic and nonepileptogenic brain regions of patients with drug-resistant unilateral temporal lobe epilepsy (n = 7). We compared R-[11 C]verapamil kinetics in homologous brain volumes of interest (VOIs) located ipsilateral and contralateral to the seizure focus. Results: Among different VOIs, radioactivity was highest in the choroid plexus. The hippocampal VOI could not be used for data analysis because it was contaminated by spill-in of radioactivity from the adjacent choroid plexus. In several other temporal lobe regions that are known to be involved in seizure generation and propagation ipsilateral influx rate constants K 1 and efflux rate constants k 2 of R-[11 C]verapamil were descriptively increased as compared to the contralateral side. Parameter asymmetries were most prominent in parahippocampal and ambient gyrus (K 1 , range: −3.8% to +22.3%; k 2 , range: −2.3% to +43.9%), amygdala (K 1 , range: −20.6% to +31.3%; k 2 , range: −18.0% to +38.9%), medial anterior temporal lobe (K 1 , range: −8.3% to +14.5%; k 2 , range: −14.5% to +31.0%) and lateral anterior temporal lobe (K 1 , range: −20.7% to +16.8%; k 2 , range: −24.4% to +22.6%). In contrast to temporal lobe VOIs, asymmetries were minimal in a region presumably not involved in epileptogenesis located outside the temporal lobe (superior parietal gyrus, K 1 , range: −3.7% to +4.5%; k 2 , range: −4.2% to +5.8%). In 5 of 7 patients, ipsilateral efflux (k 2 ) increases were more pronounced than ipsilateral influx (K 1 ) increases, which resulted in ipsilateral reductions (10%-26%) of R-[11 C]verapamil distribution volumes (DV). However, for none of the examined brain regions, any of the differences in K 1 , k 2 and DV between the epileptogenic and the nonepileptogenic hemisphere reached statistical significance (p > 0.05, Wilcoxon matched pairs test). Conclusions: Even though we failed to detect statistically significant differences in R-[11 C]verapamil model parameters between epileptogenic and nonepileptogenic brain regions, it cannot be excluded from our pilot data in a small sample size of patients that regionally enhanced P-gp activity might contribute to drug resistance in some patients with temporal lobe epilepsy.

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.