Increased permeability-glycoprotein inhibition at the human blood-brain barrier can be safely achieved by performing PET during peak plasma concentrations of tariquidar

Abstract

The permeability-glycoprotein (P-gp) efflux transporter is densely expressed at the blood-brain barrier, and its resultant spare capacity requires substantial blockade to increase the uptake of avid substrates, blunting the ability of investigators to measure clinically meaningful alterations in P-gp function. This study, conducted in humans, examined 2 P-gp inhibitors (tariquidar, a known inhibitor, and disulfiram, a putative inhibitor) and 2 routes of administration (intravenous and oral) to maximally increase brain uptake of the avid and selective P-gp substrate (11)C-N-desmethyl-loperamide (dLop) while avoiding side effects associated with high doses of tariquidar.

Methods: Forty-two (11)C-dLop PET scans were obtained from 37 healthy volunteers. PET was performed with (11)C-dLop under the following 5 conditions: injected under baseline conditions without P-gp inhibition, injected 1 h after intravenous tariquidar infusion, injected during intravenous tariquidar infusion, injected after oral tariquidar, and injected after disulfiram. (11)C-dLop uptake was quantified with kinetic modeling using metabolite-corrected arterial input function or by measuring the area under the time-activity curve in the brain from 10 to 30 min.

Results: Neither oral tariquidar nor oral disulfiram increased brain uptake of (11)C-dLop. Injecting (11)C-dLop during tariquidar infusion, when plasma tariquidar concentrations reach their peak, resulted in a brain uptake of the radioligand approximately 5-fold greater than baseline. Brain uptake was similar with 2 and 4 mg of intravenous tariquidar per kilogram; however, the lower dose was better tolerated. Injecting (11)C-dLop after tariquidar infusion also increased brain uptake, though higher doses (up to 6 mg/kg) were required. Brain uptake of (11)C-dLop increased fairly linearly with increasing plasma tariquidar concentrations, but we are uncertain whether maximal uptake was achieved.

Conclusion: We sought to increase the dynamic range of P-gp function measured after blockade. Performing (11)C-dLop PET during peak plasma concentrations of tariquidar, achieved with concurrent administration of intravenous tariquidar, resulted in greater P-gp inhibition at the human blood-brain barrier than delayed administration and allowed the use of a lower, more tolerable dose of tariquidar. On the basis of prior monkey studies, we suspect that plasma concentrations of tariquidar did not fully block P-gp; however, higher doses of tariquidar would likely be associated with unacceptable side effects.

Trial registration: ClinicalTrials.gov NCT00605254.

Keywords: N-desmethyl-loperamide; P-glycoprotein (P-gp); positron emission tomography (PET); tariquidar.

© 2015 by the Society of Nuclear Medicine and Molecular Imaging, Inc.

Figures

FIGURE 1

Composite neocortex time-activity curves showing brain uptake of radioactivity after 11C-dLop injection. 11C-dLop was injected at baseline (●), and during intravenous infusion of tariquidar at 2 (○) and 4 mg/kg (▲). Error bars denote SD.

FIGURE 2

Representative images from 11C-dLop PET scans. Images are summed from 10 – 30 minutes post-injection. Radioligand was injected either at baseline (A) or during intravenous infusion of tariquidar at doses of 2 mg/kg (B) and 4 mg/kg (C).

FIGURE 3

Composite neocortex time-activity curves showing brain uptake of radioactivity after 11C-dLop injection at baseline (●) and after 1500 mg oral tariquidar (○). Error bars denote SD.

FIGURE 4

Composite neocortex time-activity curves showing brain uptake of radioactivity after injection of 11C-dLop at baseline (●) and after either a one-time dose of 500 mg of disulfiram (○) or 2.5 g of disulfiram over 4 days (▲). Error bars denote SD.

FIGURE 5

Dose-response curve for tarquidar and uptake of 11C-dLop in composite neocortex.