Aiman Abrahim

Pharmacoresistance in epilepsy: A pilot PET study with the p-glycoprotein substrate R-[(11)C]verapamil

Summary:  Purpose and Methods: Regional overexpression of the multidrug transporter P‐glycoprotein (P‐gp) in epileptic brain tissue may lower target site concentrations of antiepileptic drugs and thus contribute to pharmacoresistance in epilepsy. We used the P‐gp substrate R‐[11C]verapamil and positron emission tomography (PET) to test for differences in P‐gp activity between epileptogenic and nonepileptogenic brain regions of patients with drug‐resistant unilateral temporal lobe epilepsy (n = 7). We compared R‐[11C]verapamil kinetics in homologous brain volumes of interest (VOIs) located ipsilateral and contralateral to the seizure focus. Results: Among different VOIs, radioactivity was highest in the choroid plexus. The hippocampal VOI could not be used for data analysis because it was contaminated by spill‐in of radioactivity from the adjacent choroid plexus. In several other temporal lobe regions that are known to be involved in seizure generation and propagation ipsilateral influx rate constants K1 and efflux rate constants k2 of R‐[11C]verapamil were descriptively increased as compared to the contralateral side. Parameter asymmetries were most prominent in parahippocampal and ambient gyrus (K1, range: −3.8% to +22.3%; k2, range: −2.3% to +43.9%), amygdala (K1, range: −20.6% to +31.3%; k2, range: −18.0% to +38.9%), medial anterior temporal lobe (K1, range: −8.3% to +14.5%; k2, range: −14.5% to +31.0%) and lateral anterior temporal lobe (K1, range: −20.7% to +16.8%; k2, range: −24.4% to +22.6%). In contrast to temporal lobe VOIs, asymmetries were minimal in a region presumably not involved in epileptogenesis located outside the temporal lobe (superior parietal gyrus, K1, range: −3.7% to +4.5%; k2, range: −4.2% to +5.8%). In 5 of 7 patients, ipsilateral efflux (k2) increases were more pronounced than ipsilateral influx (K1) increases, which resulted in ipsilateral reductions (10%–26%) of R‐[11C]verapamil distribution volumes (DV). However, for none of the examined brain regions, any of the differences in K1, k2 and DV between the epileptogenic and the nonepileptogenic hemisphere reached statistical significance (p > 0.05, Wilcoxon matched pairs test). Conclusions: Even though we failed to detect statistically significant differences in R‐[11C]verapamil model parameters between epileptogenic and nonepileptogenic brain regions, it cannot be excluded from our pilot data in a small sample size of patients that regionally enhanced P‐gp activity might contribute to drug resistance in some patients with temporal lobe epilepsy.

Tariquidar-Induced P-Glycoprotein Inhibition at the Rat Blood–Brain Barrier Studied with (R)-11C-Verapamil and PET

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.

A positron emission tomography microdosing study with a potential antiamyloid drug in healthy volunteers and patients with Alzheimer's disease

This work describes a microdosing study with an investigational, carbon 11‐labeled antiamyloid drug, 1,1′‐methylene‐di‐(2‐naphthol) (ST1859), and positron emission tomography (PET) in healthy volunteers (n = 3) and patients with Alzheimers disease (n = 6). The study aimed to assess the distribution and local tissue pharmacokinetics of the study drug in its target organ, the human brain. Before PET studies were performed in humans, the toxicologic characteristics of ST1859 were investigated by an extended single‐dose toxicity study according to guidelines of the Food and Drug Administration and European Medicines Agency, which are relevant for clinical trials with a single microdose. After intravenous bolus injection of 341 ± 21 MBq [11C]ST1859 (containing <11.4 nmol of unlabeled ST1859), peripheral metabolism was rapid, with less than 20% of total plasma radioactivity being in the form of unchanged parent drug at 10 minutes after administration. In both the control and patient groups, uptake of radioactivity into the brain was relatively fast (time to reach maximum concentration, 9–17 minutes) and pronounced (maximum concentration [standardized uptake value], 1.3–2.2). In both healthy volunteers and patients, there was a rather uniform distribution of radioactivity in the brain, including both amyloid‐beta‐rich and ‐poor regions, with slow washout of radioactivity (half‐life, 82–185 minutes). In conclusion, these data provide important information on the blood‐brain barrier penetration and metabolism of an investigational antiamyloid drug and suggest that the PET microdosing approach is a useful method to describe the target‐organ pharmacokinetics of radiolabeled drugs in humans.

The third-generation P-glycoprotein inhibitor tariquidar may overcome bacterial multidrug resistance by increasing intracellular drug concentration

OBJECTIVES The use of efflux pump inhibitors may be a powerful strategy to overcome transporter-mediated bacterial multidrug resistance. In the present study, we set out to investigate the potency of tariquidar, a third-generation P-glycoprotein inhibitor in clinical development, for overcoming bacterial resistance towards ciprofloxacin. METHODS Staphylococcus aureus 29213 (SA29213) and S. aureus 1199B (SA1199B), which overexpresses the multidrug transporter NorA, as well as Pseudomonas aeruginosa 27853 and Stenotrophomonas maltophilia BAA-85, which expresses SmeDEF, were exposed to ciprofloxacin in the presence and absence of tariquidar or, for comparative reasons, elacridar. Activity of both P-glycoprotein inhibitors was evaluated by determination of MICs and time-kill curves, and by quantification of uptake of ciprofloxacin into bacterial cells. RESULTS Activity of tariquidar and elacridar was comparable for S. aureus strains, and both dose-dependently increased susceptibility towards ciprofloxacin. Highest effects were observed for SA1199B, where the addition of tariquidar resulted in a 10-fold reduction of the ciprofloxacin MIC, while no effect was observed for P. aeruginosa. For S. maltophilia, elacridar but not tariquidar improved susceptibility. Uptake of [14C]ciprofloxacin and modification of susceptibility showed significant correlations (r=0.89, P<0.0001). Tariquidar had no intrinsic activity against any strain tested. CONCLUSIONS We conclude that tariquidar has potent inhibitory effect against certain bacterial efflux pumps in vitro. Their high activity at clinically achievable concentrations might yield this class of drugs promising for future applications in infectious diseases.

A combined accelerator mass spectrometry-positron emission tomography human microdose study with 14C- and 11C-labelled verapamil.

Background and ObjectiveIn microdose studies, the pharmacokinetic profile of a drug in blood after administration of a dose up to 100μg is measured with sensitive analytical techniques, such as accelerator mass spectrometry (AMS). As most drugs exert their effect in tissue rather than blood, methodology is needed for extending pharmacokinetic analysis to different tissue compartments. In the present study, we combined, for the first time, AMS analysis with positron emission tomography (PET) in order to determine the pharmacokinetic profile of the model drug verapamil in plasma and brain of humans. In order to assess pharmacokinetic dose linearity of verapamil, data were acquired and compared after administration of an intravenous microdose and after an intravenous microdose administered concomitantly with an oral therapeutic dose.MethodsSix healthy male subjects received an intravenous microdose [0.05 mg] (period 1) and an intravenous microdose administered concomitantly with an oral therapeutic dose [80 mg] of verapamil (period 2) in a randomized, crossover, two-period study design. The intravenous dose was a mixture of (R/S)-[14C] verapamil and (R)-[11C]verapamil and the oral dose was unlabelled racaemic verapamil. Brain distribution of radioactivity was measured with PET whereas plasma pharmacokinetics of (R)- and (S)-verapamil were determined with AMS. PET data were analysed by pharmacokinetic modelling to estimate the rate constants for transfer (k) of radioactivity across the blood-brain barrier.ResultsMost pharmacokinetic parameters of (R)- and (S)-verapamil as well as parameters describing exchange of radioactivity between plasma and brain (influx rate constant [K1] =0.030 ±0.003 and 0.031±0.005 mL/ mL/min and efflux rate constant [k2] = 0.099 ± 0.006 and 0.095 ± 0.008 min−1 for period 1 and 2, respectively) were not statistically different between the two periods although there was a trend for nonlinear pharmacokinetics for the (R)-enantiomer. On the other hand, all pharmacokinetic parameters (except for the terminal elimination half-life [t1/2]) differed significantly between the (R)- and (S)-enantiomers for both periods. The maximum plasma concentration (Cmax), area under the plasma concentration-time curve (AUC) from 0 to 24 hours (AUC24) and AUC from time zero to infinity (AUC∞) were higher and the total clearance (CL), volume of distribution (Vd) and volume of distribution at steady state (Vss) were lower for the (R)- than for the (S)-enantiomer.ConclusionCombining AMS and PET microdosing allows long-term pharmacokinetic data along with information on drug tissue distribution to be acquired in the same subjects thus making it a promising approach to maximize data output from a single clinical study.

Decreased blood-brain barrier P-glycoprotein function with aging

P-glycoprotein (P-gp) acts at the blood-brain barrier (BBB) as an active cell membrane efflux pump for several endogenous and exogenous compounds. The P-gp substrate (R)-[11C]verapamil (VPM) can be used to measure P-gp-mediated transport at the BBB in vivo with positron emission tomography (PET). The distribution volume (DV) of VPM has been shown to inversely reflect P-gp function in the BBB [1].

Pharmacoresistance in Epilepsy: A Pilot PET Study with the P-Glycoprotein Substrate R -[11 C]verapamil: R-[11 C]VERAPAMIL PET IN PHARMACORESISTANT EPILEPSY

Summary:  Purpose and Methods: Regional overexpression of the multidrug transporter P‐glycoprotein (P‐gp) in epileptic brain tissue may lower target site concentrations of antiepileptic drugs and thus contribute to pharmacoresistance in epilepsy. We used the P‐gp substrate R‐[11C]verapamil and positron emission tomography (PET) to test for differences in P‐gp activity between epileptogenic and nonepileptogenic brain regions of patients with drug‐resistant unilateral temporal lobe epilepsy (n = 7). We compared R‐[11C]verapamil kinetics in homologous brain volumes of interest (VOIs) located ipsilateral and contralateral to the seizure focus. Results: Among different VOIs, radioactivity was highest in the choroid plexus. The hippocampal VOI could not be used for data analysis because it was contaminated by spill‐in of radioactivity from the adjacent choroid plexus. In several other temporal lobe regions that are known to be involved in seizure generation and propagation ipsilateral influx rate constants K1 and efflux rate constants k2 of R‐[11C]verapamil were descriptively increased as compared to the contralateral side. Parameter asymmetries were most prominent in parahippocampal and ambient gyrus (K1, range: −3.8% to +22.3%; k2, range: −2.3% to +43.9%), amygdala (K1, range: −20.6% to +31.3%; k2, range: −18.0% to +38.9%), medial anterior temporal lobe (K1, range: −8.3% to +14.5%; k2, range: −14.5% to +31.0%) and lateral anterior temporal lobe (K1, range: −20.7% to +16.8%; k2, range: −24.4% to +22.6%). In contrast to temporal lobe VOIs, asymmetries were minimal in a region presumably not involved in epileptogenesis located outside the temporal lobe (superior parietal gyrus, K1, range: −3.7% to +4.5%; k2, range: −4.2% to +5.8%). In 5 of 7 patients, ipsilateral efflux (k2) increases were more pronounced than ipsilateral influx (K1) increases, which resulted in ipsilateral reductions (10%–26%) of R‐[11C]verapamil distribution volumes (DV). However, for none of the examined brain regions, any of the differences in K1, k2 and DV between the epileptogenic and the nonepileptogenic hemisphere reached statistical significance (p > 0.05, Wilcoxon matched pairs test). Conclusions: Even though we failed to detect statistically significant differences in R‐[11C]verapamil model parameters between epileptogenic and nonepileptogenic brain regions, it cannot be excluded from our pilot data in a small sample size of patients that regionally enhanced P‐gp activity might contribute to drug resistance in some patients with temporal lobe epilepsy.

Pharmacoresistance in epilepsy

Summary:  Purpose and Methods: Regional overexpression of the multidrug transporter P‐glycoprotein (P‐gp) in epileptic brain tissue may lower target site concentrations of antiepileptic drugs and thus contribute to pharmacoresistance in epilepsy. We used the P‐gp substrate R‐[11C]verapamil and positron emission tomography (PET) to test for differences in P‐gp activity between epileptogenic and nonepileptogenic brain regions of patients with drug‐resistant unilateral temporal lobe epilepsy (n = 7). We compared R‐[11C]verapamil kinetics in homologous brain volumes of interest (VOIs) located ipsilateral and contralateral to the seizure focus. Results: Among different VOIs, radioactivity was highest in the choroid plexus. The hippocampal VOI could not be used for data analysis because it was contaminated by spill‐in of radioactivity from the adjacent choroid plexus. In several other temporal lobe regions that are known to be involved in seizure generation and propagation ipsilateral influx rate constants K1 and efflux rate constants k2 of R‐[11C]verapamil were descriptively increased as compared to the contralateral side. Parameter asymmetries were most prominent in parahippocampal and ambient gyrus (K1, range: −3.8% to +22.3%; k2, range: −2.3% to +43.9%), amygdala (K1, range: −20.6% to +31.3%; k2, range: −18.0% to +38.9%), medial anterior temporal lobe (K1, range: −8.3% to +14.5%; k2, range: −14.5% to +31.0%) and lateral anterior temporal lobe (K1, range: −20.7% to +16.8%; k2, range: −24.4% to +22.6%). In contrast to temporal lobe VOIs, asymmetries were minimal in a region presumably not involved in epileptogenesis located outside the temporal lobe (superior parietal gyrus, K1, range: −3.7% to +4.5%; k2, range: −4.2% to +5.8%). In 5 of 7 patients, ipsilateral efflux (k2) increases were more pronounced than ipsilateral influx (K1) increases, which resulted in ipsilateral reductions (10%–26%) of R‐[11C]verapamil distribution volumes (DV). However, for none of the examined brain regions, any of the differences in K1, k2 and DV between the epileptogenic and the nonepileptogenic hemisphere reached statistical significance (p > 0.05, Wilcoxon matched pairs test). Conclusions: Even though we failed to detect statistically significant differences in R‐[11C]verapamil model parameters between epileptogenic and nonepileptogenic brain regions, it cannot be excluded from our pilot data in a small sample size of patients that regionally enhanced P‐gp activity might contribute to drug resistance in some patients with temporal lobe epilepsy.

In vivo dose finding of tariquidar using (R)-[11C]verapamil μPET

Address: 1Department of Radiopharmaceuticals, ARCGmbH, Seibersdorf, Austria, 2Department of Clinical Pharmacology, Medical University of Vienna, Austria, 3Department of Medical Computer Sciences, Medical University of Vienna, Austria, 4Department of Nuclear Medicine, Medical University of Vienna, Austria and 5Department of Pharmacology, Toxicology and Pharmacy, School of Veterinary Medicine Hannover, Germany

P-Glycoprotein inhibition at the blood-brain barrier visualized with (R)-[11C]verapamil μPET

Address: 1Department of Radiopharmaceuticals, ARC GmbH, Seibersdorf, Austria, 2Department of Clinical Pharmacology, Medical University of Vienna, Austria, 3Department of Medical Computer Sciences, Medical University of Vienna, Austria, 4Department of Nuclear Medicine, Medical University of Vienna, Austria and 5Department of Pharmacology, Toxicology and Pharmacy, School of Veterinary Medicine Hannover, Germany