Imaging of Cyclosporine Inhibition of P-Glycoprotein Activity Using ¹¹C-Verapamil in the Brain: Studies of Healthy Humans

Abstract

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...

Quantitative assessment of dynamic PET imaging data in cancer imaging

“…To relate flux to measurable quantities, we rely upon the methods described by Sokoloff for deoxyglucose, and for the moment, ignore the small contribution of de-phoshorylation (k 4 ). Under Steady state conditions for the native substance, the exchangeable compartment content, Qe, is constant, and thus (5) Re-arranging terms yields (6) For the native substance, TdR, the blood concentration is constant. Referring to the constant native compound concentration as [TdR] and substituting for CpFLT, the flux equation now becomes (7) The fraction term describes what is often termed the flux constant, Ki, which is a macro parameter (a combination of rate parameters), that appear in the flux term as a rate constant describing the overall flow of the tracer from the blood, through the metabolic pathway and into the retained compartment Qm: (8) In the analysis of FLT model parameter error, sensitivity and accuracy, we find that the parameters that are the most well defined are transport of the tracer, K 1 , and overall metabolic flux, Ki, and are also of the most biological interest as well.…”

Clinical Applications of Simultaneous PET/MR Imaging Using (R)-[11C]-Verapamil with Cyclosporin A: Preliminary Results on a Surrogate Marker of Drug-Resistant Epilepsy

“…23 However, because (R)-[ 11 C]-verapamil is a low-extraction radiotracer, its uptake is considered insensitive to changes in cerebral blood flow. 10 In addition, Muzi et al 8 reported that the CS modulation of Pgp increased the blood-brain transfer of (R)- [ 11 C]-verapamil into the brain by 73%, and this increase was significantly greater than changes in blood flow by 13%.…”

“…8 Participants underwent a 60-minute PET scan after an intravenous injection of 370 MBq of (R)-[ 11 C]-verapamil radiotracer (range, 333-407 MBq). Simultaneous acquisitions of 3D dynamic PET images and T1-weighted MR images were performed by using a Biograph mMR (Siemens, Erlangen, Germany).…”

“…Several recent clinical trials have evaluated Pgp expression in humans via positron-emission tomography by using the Pgp substrate (R)- [ 11 C]-verapamil and Pgp inhibitors, such as tariquidar or cyclosporin A (CS). 8,9 Several investigators have performed (R)-[ 11 C]-verapamil PET in patients with epilepsy to compare the expression of Pgp between patients with DRE and healthy controls, by using the percentage changes in verapamil influx or the efflux rate before and after Pgp inhibitor injection. However, to our knowledge, there has been no study for MRI-negative neocortical epilepsies, and previous methods were applied only to medial temporal lobe epilepsy (TLE).…”

Molecular Imaging of Membrane Transporters’ Activity in Cancer: a Picture is Worth a Thousand Tubes

“…The microconstants can also be used to calculate macroparameters such as 'distribution volume' (DV) of the probe. The compartmental approach has been applied to data of therefore, almost all of the effect attributed to P-gp is the decrease in K 1 (block the entry of the radioligand to the intracellular compartment) (27,37). Importantly, because blood flow influences the initial transfer of many probes into tissues, transport (K1) may be normalized by flow.…”