Tariquidar-induced P-glycoprotein inhibition at the rat blood-brain barrier studied with (R)-11C-verapamil and PET

Abstract

The multidrug efflux transporter P-glycoprotein (P-gp) is expressed in high concentrations at the blood-brain barrier (BBB) and is believed to be implicated in resistance to central nervous system drugs. We used small-animal PET and (R)-11C-verapamil together with tariquidar, a new-generation P-gp modulator, to study the functional activity of P-gp at the BBB of rats. To enable a comparison with human PET data, we performed kinetic modeling to estimate the rate constants of radiotracer transport across the rat BBB.

Methods: A group of 7 Wistar Unilever rats underwent paired (R)-11C-verapamil PET scans at an interval of 3 h: 1 baseline scan and 1 scan after intravenous injection of tariquidar (15 mg/kg, n = 5) or vehicle (n = 2).

Results: After tariquidar administration, the distribution volume (DV) of (R)-11C-verapamil was 12-fold higher than baseline (3.68 +/- 0.81 vs. 0.30 +/- 0.08; P = 0.0007, paired t test), whereas the DVs were essentially the same when only vehicle was administered. The increase in DV could be attributed mainly to an increased influx rate constant (K1) of (R)-11C-verapamil into the brain, which was about 8-fold higher after tariquidar. A dose-response assessment with tariquidar provided an estimated half-maximum effect dose of 8.4 +/- 9.5 mg/kg.

Conclusion: Our data demonstrate that (R)-11C-verapamil PET combined with tariquidar administration is a promising approach to measure P-gp function at the BBB.

FIGURE 1

Diagram of study set-up. After (R)-¹¹C-verapamil PET scan 1, tariquidar (15 mg/kg, n=5) or vehicle (n=2) was administered i.v. followed by scan 2, which measured the remainder of circulating activity from scan 1. At 3 h after scan 1, (R)-¹¹C-verapamil PET scan 3 was performed.

FIGURE 2

TACs in rat brain for (R)-¹¹C-verapamil PET scans recorded (A) before (open squares, n=2, scan 1) and after (filled squares, n=2, scan 3) administration of vehicle; (B) before (open squares, n=5, scan 1) and after (filled squares, n=5, scan 3) administration of tariquidar (15 mg/kg). Activity concentration is expressed as mean %ID/g (± standard deviation, SD for n=5).

FIGURE 3

Transversal and sagittal PET summation images (0-60 min) recorded before (upper row, scan 1) and after (lower row, scan 3) administration of tariquidar (15 mg/kg).

FIGURE 4

Total activity concentrations in whole blood before (open squares, n=5, scan 1) and after (filled squares, n=5, scan 3) administration of tariquidar (15 mg/kg). Activity concentration is expressed as mean %ID/g (± SD).

FIGURE 5

Brain TACs for PET scan 2 recorded from time 0-90 min following administration of vehicle (open squares, n=2) or 15 mg/kg of tariquidar (filled squares, n=5). PET scan 2 measured the remainder of circulating activity from scan 1. Activity concentration is expressed as mean %ID/g (± SD for n=5).

FIGURE 6

TACs and fits obtained from the 2T4K model in the brain of one rat: (A) before (scan 1) and (B) after administration of tariquidar (15 mg/kg, scan 3). Open circles: TAC in plasma measured; open diamonds: TAC in brain VOI measured; solid squares: TAC in brain VOI model. Note that the y-axis is in a logarithmic scale.

FIGURE 7

Brain TACs following administration of different tariquidar doses: 0 mg/kg (open squares, n=5), 1 mg/kg (filled squares, n=1), 3 mg/kg (open triangles, n=1), 5 mg/kg (filled triangles, n=1), 7.5 mg/kg (open circles, n=1) and 15 mg/kg (filled circles, n=5).