Abstract[ 11 C]Loperamide has been proposed for imaging P-glycoprotein (P-gp) function with positron emission tomography (PET), but its metabolism to [N-methyl-11 C]N-desmethyl-loperamide ([ 11 C] dLop; [ 11 C]3) precludes quantification. We considered that [ 11 C]3 might itself be a superior radiotracer for imaging brain P-gp function and therefore aimed to prepare [ 11 C]3 and characterize its efficacy. An amide precursor (2) was synthesized and methylated with [ 11 C]iodomethane to give [ 11 C]3. After administration of [ 11 C]3 to wild type mice, brain radioactivity uptake was very low. In P-gp (mdr-1a (−/−)) knockout mice, brain uptake of radioactivity at 30 min increased about 3.5 fold by PET measures, and over seven-fold by ex vivo measures. In knockout mice, brain radioactivity was predominantly (90%) unchanged radiotracer. In monkey PET experiments, brain radioactivity uptake was also very low, but after P-gp blockade increased more than seven-fold. [ 11 C]3 is an effective new radiotracer for imaging brain P-gp function and, in favor of future successful quantification, appears free of extensive brain-penetrant radiometabolites.

Loperamide, an opiate receptor agonist, does not cross the blood-brain barrier because it is a substrate for the permeabilityglycoprotein (P-gp) efflux pump. We evaluated 11 C-loperamide as a PET radiotracer to measure P-gp function in vivo. Methods: Monkeys were injected with 11 C-loperamide, and PET brain images were acquired for 120 min. The baseline scans were followed by scans acquired after administration of either of 2 P-gp inhibitors, (2R)-anti-5-f3-[4-(10,11-dichloromethanodibenzo-suber-5-yl)piperazin-1-yl]-2-hydroxypropoxygquinoline trihydrochloride (DCPQ) or tariquidar. Both the PET scans and ex vivo measurements were obtained in P-gp knockout and wildtype mice. Results: Pharmacologic inhibition of P-gp in monkeys dose-dependently increased brain activity, with a 3.7-fold effect at the highest DCPQ dose (8 mg/kg intravenously). This increase of brain activity was not caused peripherally, because DCPQ insignificantly changed the plasma concentration and plasma protein binding of radiotracer. Furthermore, the structurally dissimilar inhibitor, tariquidar, also increased brain uptake with potency equal to that of DCPQ. P-gp knockout mice had 3-fold higher brain activity on PET than did wild-type animals. Four radiometabolites were detected in the plasma and brains of ex vivo mice. The most lipophilic radiometabolite was found to be comobile with reference dLop on high-performance liquid chromatography. The brain concentrations of 11 C-loperamide and the putative 11 C-dLop were about 16-fold greater in P-gp knockout mice than in wild-type mice. Conclusion: Both 11 C-loperamide and its putative radiometabolite 11 C-dLop are avid P-gp substrates. 11 C-dLop may be superior to 11 C-loperamide in measuring P-gp function at the blood-brain barrier, because further demethylation of 11 CdLop will generate radiometabolites that have little entry into the brain.

Introduction [11C]Loperamide and [11C]N-desmethyl-loperamide ([11C]dLop) have been proposed as radiotracers for imaging brain P-glycoprotein (P-gp) function. A major route of [11C]loperamide metabolism is N-demethylation to [11C]dLop. We aimed to test whether inhibition of CYP3A4 with ketoconazole might reduce the metabolism of [11C]loperamide and [11C]dLop in mice, and thereby improve the quality of these radiotracers. Methods Studies were performed in wild-type and P-gp knockout (mdr–1a/b −/−) mice. During each of seven study sessions, one pair of mice, comprising one wild-type and one knockout mouse, waspretreated with ketoconazole (50 mg/kg, i.p.) while another such pair was left untreated. Mice were sacrificed at 30 min after injection of [11C]loperamide or [11C]dLop. Whole brain and plasma samples were measured for radioactivity and analyzed with radio-HPLC. Results Ketoconazole increased the plasma concentrations of [11C]loperamide and its main radiometabolite, [11C]dLop, by about two-fold in both wild-type and knockout mice, whereas the most polar radiometabolite was decreased three-fold. Furthermore, ketoconazole increased the brain concentrations of [11C]loperamide and the radiometabolite [11C]dLop by about two-fold in knockout mice, and decreased the brain concentrations of the major and most polar radiometabolite in wild-type and knockout mice by 82 and 49%, respectively. In contrast, ketoconazole had no effect on plasma and brain distribution of administered [11C]dLop and its radiometabolites in either wild-type or knockout mice, except to increase the low plasma [11C]dLop concentration. The least polar radiometabolite of [11C]dLop was identified with LC-MSn as the N-hydroxymethyl analog of [11C]dLop and this also behaved as a P-gp substrate. Conclusion In this study, ketoconazole (50 mg/kg, i.p.) proved partiallyeffective for inhibiting the N-demethylation of [11C]loperamide in mouse in vivo but had relatively smaller or no effect on [11C]dLop.