A novel noninvasive method for assessing glutathione-conjugate efflux systems in the brain (original) (raw)
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Molecular Imaging and Biology, 2013
Purpose-Our goal was to use Positron Emission Tomography (PET) to analyze the movement of radiolabeled agents in tissue to enable direct measurement of drug delivery to the brain. Procedures-A variety of 11C-and 18F-labeled compounds were delivered directly to an agarose phantom or rat striatum. Concentration profiles were extracted for analysis and fitted to diffusion models. Results-Diffusion coefficients ranged from 0.075±0.0026 mm 2 /min ([ 18 F]Fluoride, 18 Da) to 0.0016±0.0018 mm 2 /min ([ 18 F]NPB4-avidin, 68 kDa) and matched well with predictions based on molecular weight (R 2 =0.965). The tortuosity of the brain extracellular space was estimated to be 1.56, with the tissue clearance halftime of each tracer in the brain varying from 19 to 41 minutes. Conclusions-PET is an effective modality to directly quantify the movement of locally delivered drugs or drug-carriers. This continuous, non-invasive assessment of delivery will aid the design of better drug delivery methods.
Journal of Nuclear Medicine, 2009
P-glycoprotein (P-gp) is a membrane-bound efflux pump that limits the distribution of drugs to several organs of the body. At the blood-brain barrier, P-gp blocks the entry of both loperamide and its metabolite, N-desmethyl-loperamide (N-dLop), and thereby prevents central opiate effects. Animal studies have shown that 11 C-dLop, compared with 11 C-loperamide, is an especially promising radiotracer because it generates negligible radiometabolites that enter the brain. The purposes of this study were to determine whether 11 C-dLop is a substrate for P-gp at the blood-brain barrier in humans and to measure the distribution of radioactivity in the entire body to estimate radiation exposure. Methods: Brain PET scans were acquired in 4 healthy subjects for 90 min and included concurrent measurements of the plasma concentration of unchanged radiotracer. Time-activity data from the whole brain were quantified using a 1-tissuecompartment model to estimate the rate of entry (K 1 ) of radiotracer into the brain. Whole-body PET scans were acquired in 8 healthy subjects for 120 min. Results: For brain imaging, after the injection of 11 C-dLop the concentration of radioactivity in the brain was low (standardized uptake value, ;15%) and stable after approximately 20 min. In contrast, uptake of radioactivity in the pituitary was about 50-fold higher than that in the brain. The plasma concentration of 11 C-dLop declined rapidly, but the percentage composition of plasma was unusually stable, with the parent radiotracer constituting 85% of total radioactivity after approximately 5 min. The rate of brain entry was low (K 1 5 0.009 6 0.002 mLÁcm 23 Ámin 21 ; n 5 4). For whole-body imaging, as a measure of radiation exposure to the entire body the effective dose of 11 C-dLop was 7.8 6 0.6 mSv/MBq (n 5 8). Conclusion: The low brain uptake of radioactivity is consistent with 11 C-dLop being a substrate for P-gp in humans and confirms that this radiotracer generates negligible quantities of brain-penetrant radiometabolites. In addition, the low rate of K 1 is consistent with P-gp rapidly effluxing substrates while they transit through the lipid bilayer. The radiation exposure of 11 C-dLop is similar to that of many other 11 C-radiotracers. Thus, 11 C-dLop is a promising radiotracer to study the function of P-gp at the blood-brain barrier, at which impaired function would allow increased uptake into the brain.
Drug Metabolism and Disposition, 2009
Species differences occur in the brain concentrations of drugs, but the reasons for these differences are not yet apparent. This study was designed to compare brain uptake of three radiolabeled P-glycoprotein (P-gp) substrates across species using positron emission tomography. Brain concentrations and brain-to-plasma ratios were compared; [ 11 C]verapamil in rats, guinea pigs, and monkeys; [ 11 C](S)-(2-methoxy-5-(5-trifluoromethyltetrazol-1-yl)phenylmethylamino)-2(S)-phenylpiperidine (GR205171) in rats, guinea pigs, monkeys, and humans; and [ 18 F]altanserin in rats, minipigs, and humans. The fraction of the unbound radioligand in plasma was studied along with its metabolism. The effect of P-gp inhibition was investigated by administering cyclosporin A (CsA). Pronounced species differences were found in the brain and brainto-plasma concentrations of [ 11 C]verapamil, [ 11 C]GR205171, and [ 18 F]altanserin with higher brain distribution in humans, monkeys, and minipigs than in rats and guinea pigs. For example, the brainto-plasma ratio of [ 11 C]GR205171 was almost 9-fold higher in humans compared with rats. The species differences were still present after P-gp inhibition, although the increase in brain concentrations after P-gp inhibition was somewhat greater in rats than in the other species. Differences in plasma protein binding and metabolism did not explain the species-related differences. The findings are important for interpretation of brain drug delivery when extrapolating preclinical data to humans. Compounds found to be P-gp substrates in rodents are likely to also be substrates in higher species, but sufficient blood-brain barrier permeability may be retained in humans to allow the compound to act at intracerebral targets.
Nuclear medicine and biology, 1995
beta-CIT-FP [N-(3-fluoropropyl)-2 beta-carbomethoxy-3 beta-(4-iodophenyl)nortropane] is a cocaine analogue with a high affinity for the dopamine transporter. [O-methyl-11C]beta-CIT-FP ([11C]beta-CIT-FP) was prepared by O-alkylation of the free acid with [11C]methyl iodide. The total radiochemical yield of [11C]beta-CIT-FP was 50 to 60% with an overall synthesis time of 30 min. The radiochemical purity was > 99%, and the specific radioactivity at time of injection was about 37 GBq/mumol (1000 Ci/mmol). Autoradiographic examination of [11C]beta-CIT-FP binding in human brain postmortem demonstrated specific binding in the caudate nucleus and putamen. Positron emission tomography (PET) examination of [11C]beta-CIT-FP in a Cynomolgus monkey demonstrated accumulation in the striatum with a striatum-to-cerebellum ratio of about 8 after 60 min. Equilibrium in the striatum was attained within 70 to 90 min. The radioactivity ratios of thalamus/cerebellum and neocortex/cerebellum were about...
Blocking of efflux transporters in rats improves translational validation of brain radioligands
EJNMMI Research
Background Positron emission tomography (PET) is a molecular imaging technique that can be used to investigate the in vivo pharmacology of drugs. Initial preclinical evaluation of PET tracers is often conducted in rodents due to the accessibility of disease models as well as economic considerations. Compared to larger species, rodents display a higher expression and/or activity of efflux transporters such as the P-glycoprotein (P-gp). Low brain uptake could, therefore, be species-specific and uptake in rodents not be predictive for that in humans. We hypothesized that a better prediction from rodent data could be achieved when a tracer is evaluated under P-gp inhibition. Consequently, we compared the performance of eight neuroreceptor tracers in rats with and without P-gp inhibition including a specific binding blockade. This data set was then used to predict the binding of these eight tracers in pigs. Methods PET tracers targeting serotonin 5-HT2A receptors ([18F]MH.MZ, [18F]Altans...
Assessment of [11C]-L-Methionine Transport into the Human Brain
Journal of Cerebral Blood Flow & Metabolism, 1988
Neutral amino acid transport into human brain was measured using a dual-probe positron detection system or positron emission tomography (PET). Rate constants (ml/minlcc) for brain accumulation of [lIC]L methionine measured with the dual detector ranged from 0.012 to 0.078 (average 0.031) under baseline conditions and from 0.010 to 0.017 (average 0.014) after administra tion of nonradioactive L-phenylalanine (100 mg/kg). The net rate of brain accumulation of L-methionine ranged from 0.42 to 2.89 (average 1.28) nmollmin/cc, and de creased by 27.5-91.2% (average 53.9%) after L-phenylal anine. PET-estimated accumulation rates (mllmin/cc) of
2013
A specific positron emission tomography (PET) radiotracer for the glycine transporter type 1 (GlyT1) would constitute an imaging biomarker to investigate the distribution of GlyT1 in normal individuals and those with neuropsychiatric disorders. In addition it could demonstrate the ability of a novel drug to reach its target in the brain and enable receptor occupancy studies, thus facilitating drug development. In this article we describe the evaluation in non-human primates of two candidate PET radiotracers ([ 11 C]RO5013852 and [ 11 C] RO5013853) previously characterized in the rat. Both radiotracers showed acceptable uptake in the baboon brain and heterogeneous distribution consistent with that reported for GlyT1. In vivo blockade studies with two specific glycine reuptake inhibitors (GRIs), RO5013853 and bitopertin (RG1678, reduced uptake of both tracers to homogenous levels across brain regions and demonstrated specificity of the signal. [ 11 C] RO5013853 showed a larger specific signal and slightly higher brain uptake and was therefore selected for further characterization. Quantitative compartmental analysis of PET data showed that the 2-tissue compartment model with 5 parameters was the most appropriate to describe the kinetics of [ 11 C]RO5013853. Two additional methods were used: a) the Logan graphical analysis using plasma input and, b) a linear parametric imaging approach with the 2-tissue compartmental model. These produced V T estimates of comparable magnitude, namely, pons, thalamus and cerebellum > caudate, putamen and cortical regions. High resolution autoradiography with tritiated RO5013853 was used to confirm the binding pattern observed by PET. In vivo metabolism studies in the baboon demonstrated the formation of a single, radiolabeled metabolite more polar than the parent compound. Finally, [ 11 C]RO5013853 was used to quantify the degree of cerebral GlyT1 occupancy observed in the baboon following oral administration of bitopertin, a selective GRI presently in Phase III clinical trial. Plasma concentrations of approximately 150-300 ng/mL were estimated to produce 50% GlyT1 occupancy in the thalamus, the cerebellum and the pons.
In Vivo Evaluation of (11)C-labeled Three Radioligands for Glycine Transporter 1 in the Mouse Brain
Clinical psychopharmacology and neuroscience : the official scientific journal of the Korean College of Neuropsychopharmacology, 2012
Glycine transporter 1 (GlyT-1) is one of the most attractive therapeutic targets for schizophrenia. There is great interest in developing radioligands for in vivo imaging of GlyT-1 in the brain using positron emission tomography. Here, we report the properties of three novel non-sarcosine-based radioligands [(11)C]CHIBA-3007, [(11)C]CHIBA-3009, and [(11)C]CHIBA-3011, for GlyT-1 imaging in the mouse brain in vivo. The three radioligands were synthesized by N-[(11)C] methylation of the corresponding desmethyl precursor. A pharmacological characterization of these radioligands for in vivo imaging of GlyT-1 in the brain was conducted using male ddY mice. [(11)C]CHIBA-3009 and [(11)C]CHIBA-3011 were scarcely incorporated into the brain, whereas [(11)C]CHIBA-3007 showed slight but considerable brain uptake. Regional brain uptake of [(11)C]CHIBA-3007 (medulla oblongata>cerebellum>cortex) was similar to the distribution of the GlyT-1 protein. However, pretreatment with CHIBA-3007 (1 m...
Evidence for carrier-mediated transport of glutathione across the blood-brain barrier in the rat
Journal of Clinical Investigation, 1990
Information on the origin of brain glutathione and the possibility of its transport from blood to brain is limited. We found a substantial uptake of f3Slabeled glutathione by the rat brain using the carotid artery injection technique. The brain uptake index of glutathione with and without an irreversible gammaglutamyl transpeptidase inhibitor, acivicin, was similar. No sign t differences in the regional uptake of labeled giutathione were found in rats pretreated with acivicin. The brain uptake index of tracer glutathione was similar to that of cysteine tracer and was lower than that of phenylalanine. The transport of oxidized glutathione (glutathione disulfide) across the blood-brain barrier was not significantly different from that of sucrose, an impermeable marker. Brain radioactivity 15 s after carotid artery injection of labeled glutathione to rats pretreated with acivicin was predominantly in the form of giutathione. The in vivo giutathione uptake was saturable with an apparent K., of 5.84 mM. Amino acids, amino acid analogues, and other compounds Icysteine, phenylalanine, glutathione disulfide, gamma-glutamylglutamate, gamma-glutamyl p-nitroanilide, 2-aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH)] did not affect glutathione transport. Our data suggest that glutathione is transported across the blood-brain barrier by a saturable and specific mechanism. (J. Clin.- Invest. 1990.