Effects of ketoconazole on the biodistribution and metabolism of [11C]loperamide and [11C]N-desmethyl-loperamide in wild-type and P-gp knockout mice.

Introduction: [(11)C]Loperamide and [(11)C]N-desmethyl-loperamide ([(11)C]dLop) have been proposed as radiotracers for imaging brain P-glycoprotein (P-gp) function. A major route of [(11)C]loperamide metabolism is N-demethylation to [(11)C]dLop. We aimed to test whether inhibition of CYP3A4 with ketoconazole might reduce the metabolism of [(11)C]loperamide and [(11)C]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, was pretreated with ketoconazole (50 mg/kg, ip), while another such pair was left untreated. Mice were sacrificed at 30 min after injection of [(11)C]loperamide or [(11)C]dLop. Whole brain and plasma samples were measured for radioactivity and analyzed with radio-high-performance liquid chromatography.

Results: Ketoconazole increased the plasma concentrations of [(11)C]loperamide and its main radiometabolite, [(11)C]dLop, by about twofold in both wild-type and knockout mice, whereas the most polar radiometabolite was decreased threefold. Furthermore, ketoconazole increased the brain concentrations of [(11)C]loperamide and the radiometabolite [(11)C]dLop by about twofold 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 [(11)C]dLop and its radiometabolites in either wild-type or knockout mice, except to increase the low plasma [(11)C]dLop concentration. The least polar radiometabolite of [(11)C]dLop was identified with LC-MS(n) as the N-hydroxymethyl analog of [(11)C]dLop and this also behaved as a P-gp substrate.

Conclusion: In this study, ketoconazole (50 mg/kg, ip) proved partially effective for inhibiting the N-demethylation of [(11)C]loperamide in mouse in vivo but had relatively smaller or no effect on [(11)C]dLop.