This is the first time that P-gp activity at the human BBB has been measured. The modest inhibition of human BBB P-gp by cyclosporine has implications for P-gp-based drug interactions at the human BBB. Our method for imaging P-gp activity can be used to identify multidrug-resistant tumors or to determine the contribution of P-gp polymorphism, inhibition, or induction to interindividual variability in drug response.

The multiple-drug resistance (MDR) transporter P-glycoprotein (P-gp) is highly expressed at the human blood-brain barrier (BBB). P-gp actively effluxes a wide variety of drugs from the central nervous system, including anticancer drugs. We have previously demonstrated P-gp activity at the human BBB using PET of 11 C-verapamil distribution into the brain in the absence and presence of the P-gp inhibitor cyclosporine-A (CsA). Here we extend the initial noncompartmental analysis of these data and apply compartmental modeling to these human verapamil imaging studies. Methods: Healthy volunteers were injected with 15 O-water to assess blood flow, followed by 11 C-verapamil to assess BBB P-gp activity. Arterial blood samples and PET images were obtained at frequent intervals for 5 and 45 min, respectively, after injection. After a 60-min infusion of CsA (intravenously, 2.5 mg/kg/h) to inhibit P-gp, a second set of water and verapamil PET studies was conducted, followed by 11 C-CO imaging to measure regional blood volume. Blood flow was estimated using dynamic 15 O-water data and a flow-dispersion model. Dynamic 11 C-verapamil data were assessed by a 2-tissuecompartment (2C) model of delivery and retention and a 1-tissuecompartment model using the first 10 min of data (1C 10 ). Results: The 2C model was able to fit the full dataset both before and during P-pg inhibition. CsA modulation of P-gp increased blood-brain transfer (K 1 ) of verapamil into the brain by 73% (range, 30%2118%; n 5 12). This increase was significantly greater than changes in blood flow (13%; range, 12%249%; n 5 12, P , 0.001). Estimates of K 1 from the 1C 10 model correlated to estimates from the 2C model (r 5 0.99, n 5 12), indicating that a short study could effectively estimate P-gp activity. Conclusion: 11 C-verapamil and compartmental analysis can estimate P-gp activity at the BBB by imaging before and during P-gp inhibition by CsA, indicated by a change in verapamil transport (K 1 ). Inhibition of P-gp unmasks verapamil trapping in brain tissue that requires a 2C model for long imaging times; however, transport can be effectively measured using a short scan time with a 1C 10 model, avoiding complications with labeled metabolites and tracer retention. The blood-brain barrier (BBB) can significantly limit drug transport into the brain in, for example, chemotherapy of brain cancer (1,2). Drug efflux at the BBB is mediated by several transport proteins, of which P-glycoprotein (P-gp) is the most important (3-5). P-gp markedly restricts BBB transport of a broad range of drugs (6,7). Several strategies have been reported to improve delivery of therapeutics to the brain through circumvention of the BBB. One such strategy is the selective inhibition of P-gp, which has been shown in human studies to increase both drug delivery to the brain and therapeutic efficacy (8) of chemotherapeutic drugs. Methods to measure P-gp activity at the human BBB are required to assess this strategy.We have recently developed a method to measure P-gp activity at the hum...

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 EC 50 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 ϳ12 M. Then, an i.v. tracer dose of [ 3 H]verapamil was administered, and 20 min after verapamil administration, the animals were sacrificed for determination of blood, plasma, and brain 3 H radioactivity by scintillation counting. The percentage increase in the brain/blood 3 H radioactivity (relative to 0 M CsA) was described by the Hill equation with E max , 1290%; EC 50 , 7.2 M; and ␥, 3.8. Previously, using [11 C]verapamil, we have shown that the human brain/blood 11 C radioactivity was increased by 79% at 2.8 M CsA blood concentration. At an equivalent CsA blood concentration, the rat brain/blood 3 H radioactivity was increased by a remarkably similar extent of 75%. This is the first time that an in vivo CsA EC 50 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.

No abstract

No abstract