Verapamil P-glycoprotein transport across the rat blood-brain barrier: Cyclosporine, a concentration inhibition analysis, and comparison with human data

To predict the magnitude of P-glycoprotein (P-gp)-based drug interactions at the human blood-brain barrier (BBB), rodent studies are routinely conducted where P-gp is chemically inhibited. For such studies to be predictive of interactions at the human BBB, the plasma concentration of the P-gp inhibitor must be comparable with that observed in the clinic. Therefore, we determined the in vivo EC₅₀ of P-gp inhibition at the rat BBB using verapamil as a model P-gp substrate and cyclosporine A (CsA) as the model P-gp inhibitor. Under isoflurane anesthesia, male Sprague-Dawley rats were administered i.v. CsA to achieve pseudo steady-state CsA blood concentrations ranging from 0 to ∼12 μM. Then, an i.v. tracer dose of [³H]verapamil was administered, and 20 min after verapamil administration, the animals were sacrificed for determination of blood, plasma, and brain ³H radioactivity by scintillation counting. The percentage increase in the brain/blood ³H radioactivity (relative to 0 μM CsA) was described by the Hill equation with E_max, 1290%; EC₅₀, 7.2 μM; and γ, 3.8. Previously, using [¹¹C]verapamil, we have shown that the human brain/blood ¹¹C radioactivity was increased by 79% at 2.8 μM CsA blood concentration. At an equivalent CsA blood concentration, the rat brain/blood ³H radioactivity was increased by a remarkably similar extent of 75%. This is the first time that an in vivo CsA EC⁵⁰ of P-gp inhibition at the rat BBB has been determined and the magnitude of such inhibition was compared between the rat and the human BBB at the same blood CsA concentration.