In vivo labeling of central benzodiazepine receptors with the partial inverse agonist [3H]Ro 15-4513

Test-retest reproducibility of quantitative binding measures of [11C]Ro15-4513, a PET ligand for GABAA receptors containing alpha5 subunits

INTRODUCTION

Alteration of γ-aminobutyric acid "A" (GABAA) receptor-mediated neurotransmission has been associated with various neurological and psychiatric disorders. [11C]Ro15-4513 is a PET ligand with high affinity for α5-subunit-containing GABAA receptors, which are highly expressed in limbic regions of the human brain (Sur et al., 1998). We quantified the test-retest reproducibility of measures of [11C]Ro15-4513 binding derived from six different quantification methods (12 variants).

METHODS

Five healthy males (median age 40 years, range 38-49 years) had a 90-min PET scan on two occasions (median interval 12 days, range 11-30 days), after injection of a median dose of 441 MegaBequerels of [11C]Ro15-4513. Metabolite-corrected arterial plasma input functions (parent plasma input functions, ppIFs) were generated for all scans. We quantified regional binding using six methods (12 variants), some of which were region-based (applied to the average time-activity curve within a region) and others were voxel-based: 1) Models requiring arterial ppIFs - regional reversible compartmental models with one and two tissue compartments (2kbv and 4kbv); 2) Regional and voxelwise Logan's graphical analyses (Logan et al., 1990), which required arterial ppIFs; 3) Model-free regional and voxelwise (exponential) spectral analyses (SA; (Cunningham and Jones, 1993)), which also required arterial ppIFs; 4) methods not requiring arterial ppIFs - voxelwise standardised uptake values (Kenney et al., 1941), and regional and voxelwise simplified reference tissue models (SRTM/SRTM2) using brainstem or alternatively cerebellum as pseudo-reference regions (Lammertsma and Hume, 1996; Gunn et al., 1997). To compare the variants, we sampled the mean values of the outcome parameters within six bilateral, non-reference grey matter regions-of-interest. Reliability was quantified in terms of median absolute percentage test-retest differences (MA-TDs; preferentially low) and between-subject coefficient of variation (BS-CV, preferentially high), both compounded by the intraclass correlation coefficient (ICC). These measures were compared between variants, with particular interest in the hippocampus.

RESULTS

Two of the six methods (5/12 variants) yielded reproducible data (i.e. MA-TD <10%): regional SRTMs and voxelwise SRTM2s, both using either the brainstem or the cerebellum; and voxelwise SA. However, the SRTMs using the brainstem yielded a lower median BS-CV (7% for regional, 7% voxelwise) than the other variants (8-11%), resulting in lower ICCs. The median ICCs across six regions were 0.89 (interquartile range 0.75-0.90) for voxelwise SA, 0.71 (0.64-0.84) for regional SRTM-cerebellum and 0.83 (0.70-0.86) for voxelwise SRTM-cerebellum. The ICCs for the hippocampus were 0.89 for voxelwise SA, 0.95 for regional SRTM-cerebellum and 0.93 for voxelwise SRTM-cerebellum.

CONCLUSION

Quantification of [11C]Ro15-4513 binding shows very good to excellent reproducibility with SRTM and with voxelwise SA which, however, requires an arterial ppIF. Quantification in the α5 subunit-rich hippocampus is particularly reliable. The very low expression of the α5 in the cerebellum (Fritschy and Mohler, 1995; Veronese et al., 2016) and the substantial α1 subunit density in this region may hamper the application of reference tissue methods.

Changes in in vivo [(3)H]-Ro15-4513 binding induced by forced swimming in mice

Mice were forced to swim for 5 min in water at a temperature of 12 degrees C (cold water swim stress) or 32 degrees C (warm water swim stress), and stress-induced analgesia (SIA) was measured using the tail-flick test. The cold water swim stress induced non-opioid SIA as well as hypothermia, whereas the warm water swim stress caused opioid SIA. The in vivo binding of [(3)H]-Ro15-4513 was measured in the stressed mice and compared with that in control mice. The specific binding of [(3)H]-Ro15-4513 in the cerebral cortex, hippocampus, and cerebellum was significantly altered by forced swimming in cold water. Apparent association and dissociation rate of [(3)H]-Ro15-4513 binding were decreased, and the change in the dissociation rate was most pronounced in the hippocampus. In contrast, no significant alterations were observed in in vitro binding. The hypothermia induced by the cold water swim stress seems to be the main reason for alterations in the specific binding of [(3)H]-Ro15-4513. The kinetics of a saturable amount of [(3)H]-Ro15-4513 in the blood and brain were also measured. The relative ratio of the radioactivity concentration in the brain to that in the blood was significantly decreased by forced swimming in cold water, indicating that the cold water swim stress induced changes in the nonspecific binding of [(3)H]-Ro15-4513 in the brain. These results together with previous reports suggested that non-opioid SIA induced by the cold water swim stress might be related to alterations in the rates of general ligand-receptor interactions including GABA(A)/benzodiazepine system. Changes in the nonspecific binding might be also involved in non-opioid SIA.

Selective labelling of diazepam-insensitive GABAA receptors in vivo using [3H]Ro 15-4513

Classical benzodiazepines (BZs), such as diazepam, bind to GABAA receptors containing alpha1, alpha2, alpha3 or alpha5 subunits that are therefore described as diazepam-sensitive (DS) receptors. However, the corresponding binding site of GABAA receptors containing either an alpha4 or alpha6 subunit do not bind the classical BZs and are therefore diazepam-insensitive (DIS) receptors; a difference attributable to a single amino acid (histidine in alpha1, alpha2, alpha3 and alpha5 subunits and arginine in alpha4 and alpha6). Unlike classical BZs, the imidazobenzodiazepines Ro 15-4513 and bretazenil bind to both DS and DIS populations of GABAA receptors. In the present study, an in vivo assay was developed using lorazepam to fully occupy DS receptors such that [3H]Ro 15-4513 was then only able to bind to DIS receptors. When dosed i.v., [3H]Ro 15-4513 rapidly entered and was cleared from the brain, with approximately 70% of brain radioactivity being membrane-bound. Essentially all membrane binding to DS+DIS receptors could be displaced by unlabelled Ro 15-4513 or bretazenil, with respective ID50 values of 0.35 and 1.2 mg kg(-1). A dose of 30 mg kg(-1) lorazepam was used to block all DS receptors in a [3H]Ro 15-1788 in vivo binding assay. When predosed in a [3H]Ro 15-4513 binding assay, lorazepam blocked [3H]Ro 15-4513 binding to DS receptors, with the remaining binding to DIS receptors accounting for 5 and 23% of the total (DS plus DIS) receptors in the forebrain and cerebellum, respectively. The in vivo binding of [3H]Ro 15-4513 to DIS receptors in the presence of lorazepam was confirmed using alpha1H101R knock-in mice, in which alpha1-containing GABAA receptors are rendered diazepam insensitive by mutation of the histidine that confers diazepam sensitivity to arginine. In these mice, and in the presence of lorazepam, there was an increase of in vivo [3H]Ro 15-4513 binding in the forebrain and cerebellum from 4 and 15% to 36 and 59% of the total (i.e. DS plus DIS) [3H]Ro 15-4513 binding observed in the absence of lorazepam.

PET imaging of ischemic neuronal death in the hippocampus of living monkeys

The aim of the present study was to visualize postischemic hippocampal neuronal death in the living monkey brain, using a high-resolution positron emission tomography (PET) and novel radioligands. In preceding papers, we reported on postischemic hippocampal neuronal death in a model of Japanese monkeys (Macaca fuscata) undergoing a 20-min complete whole-brain ischemia. Using the same model here, we investigated the in vivo bindings of two radiotracers, [11C]Ro15-4513 (a type II benzodiazepine receptor ligand) and [11C](+)3-MPB (a muscarinic cholinergic receptor ligand), in the hippocampus on day 7 after ischemia, as compared to the normal hippocampus. A significant decrease in the in vivo binding of [11C]Ro154513 and [11C(+)3-MPB was observed in the postischemic monkey hippocampus on day 7 after ischemia compared to controls. Light and electron microscopic analyses of postischemic CA1 neurons showed typical features of coagulation necrosis, as associated with a marked reduction of postsynaptic densities and presynaptic vesicles. These results suggest that semiquantification of hippocampal neuronal death is possible in the living primate brain using PET, and that the same procedures can be applied for evaluating neuronal cell loss in patients with ischemic injuries and/or dementia.

Imaging the GABA-benzodiazepine receptor subtype containing the alpha5-subunit in vivo with [11C]Ro15 4513 positron emission tomography

There is evidence of marked variation in the brain distribution of specific subtypes of the GABA-benzodiazepine receptor and that particular subtypes mediate different functions. The alpha5-containing subtype is highly expressed in the hippocampus, and selective alpha5 inverse agonists (which decrease tonic GABA inhibition) are being developed as potential memory-enhancing agents. Evidence for such receptor localization and specialization in humans in vivo is lacking because the widely used probes for imaging the GABA-benzodiazepine receptors, [11C]flumazenil and [123I]iomazenil, appear to reflect binding to the alpha1 subtype, based on its distribution and affinity of flumazenil for this subtype. The authors characterized for positron emission tomography (PET) a radioligand from Ro15 4513, the binding of which has a marked limbic distribution in the rat and human brain in vivo. Competition studies in vivo in the rat revealed that radiolabeled Ro15 4513 uptake was reduced to nonspecific levels only by drugs that have affinity for the alpha5 subtype (flunitrazepam, RY80, Ro15 4513, L655,708), but not by the alpha1 selective agonist, zolpidem. Quantification of [11C]Ro15 4513 PET was performed in humans using a metabolite-corrected plasma input function. [11C]Ro15 4513 uptake was relatively greater in limbic areas compared with [11C]flumazenil, but lower in the occipital cortex and cerebellum. The authors conclude that [11C]Ro15 4513 PET labels in vivo the GABA-benzodiazepine receptor containing the alpha5 subtype in limbic structures and can be used to further explore the functional role of this subtype in humans.

Imaging of the super high affinity binding sites for [3H]Ro15-4513 in rat hippocampus: comparison between in vitro and in vivo binding

The super high affinity binding sites for [3H]Ro15-4513, a partial inverse agonist of central benzodiazepine receptors, were analyzed in rat hippocampus both in vivo and in vitro. An ultra high sensitive method of autoradiography with an imaging plate system was employed for quantitative analysis of [3H]Ro15-4513 binding. In in vitro binding, the super high affinity binding sites in the hippocampus were observed when the [3H]Ro15-4513 concentration was below 0.5 nM. In vivo, the super high affinity binding sites were only found when the injected dose of Ro15-4513 was below 3.6 microg/kg and almost disappeared when the dose was increased to 10 microg/kg. These results both in vivo and in vitro indicate that there is a significant discrepancy between actual free ligand concentration in vivo and in vitro, and that concentrations in intact brain may be much lower than previously thought.

No age-related changes in human benzodiazepine receptor binding measured by PET with [11C]Ro 15-4513

The effects of age on the binding of [11C]Ro 15-4513, a partial inverse agonist of the central benzodiazepine receptor, were studied. Sixteen healthy male volunteers (21-78 years old) participated. Regional radioactivity in the brain was followed for 45 min by positron emission tomography after a bolus injection of [11C]Ro 15-4513. Similar tracer kinetics were observed in both young and old subjects. For the quantification of receptor binding in vivo, a compartment model, in which radioactivity in the pons was used as an input function, was applied. There were no significant changes in the binding potentials with age (P > 0.1) in ten brain regions. These observations delineate an interesting difference between central benzodiazepine (BZ) receptors and other neurotransmitter receptors in the human brain measured by PET that have been shown to have a reduction with age.

Radioligands for PET studies of central benzodiazepine receptors and PK (peripheral benzodiazepine) binding sites--current status

The status of the radiochemical development and biological evaluation of radioligands for PET studies of central benzodiazepine (BZ) receptors and the so-called peripheral benzodiazepine binding sites, here discriminated and referred to as PK binding sites, is reviewed against current pharmacological knowledge, indicating those agents with present value and those with future potential. Practical recommendations are given for the preparation of two useful radioligands for PET studies, [N-methyl-11C]flumazenil for central BZ receptors, and [N-methyl-11C]PK 11195 for PK binding sites. Quality assurance and plasma metabolite analysis are also reviewed for these radioligands and practical recommendations are given on methodology for their performance.

In vivo binding of [11C]Ro15-4513 in human brain measured with PET

Ro15-4513, an azide derivative of benzodiazepine antagonist flumazenil (Ro15-1788), and Ro15-1788 were labelled with carbon 11. Sequential PET scans following injection of [11C]Ro15-4513 or [11C]Ro15-1788 into normal male healthy volunteers were measured, and kinetic analysis using pons as a reference region was performed. [11C]Ro15-4513 was highly accumulated in frontal cortex, temporal cortex, hippocampus and relatively lower accumulation in occipital cortex, whereas almost homogeneous distribution of Ro15-1788 throughout cortex area was seen. The kinetic analysis revealed that such differences of regional distribution in brain between two labelled ligands were mainly due to the regional difference of the dissociation rate constants in vivo (k4). [11C]Ro15-4513 may be a useful tool for the in vivo study of benzodiazepine receptors in human brain.

[11C]Ro 15-4513, a ligand for visualization of benzodiazepine receptor binding. Preparation, autoradiography and positron emission tomography

Ro 15-4513, a partial inverse agonist at the benzodiazepine (BZ) receptor site was labelled with 11C and used for in vitro autoradiography on human post mortem brain sections and for positron emission tomography (PET) on Cynomolgus monkeys. The total radiochemical yield of [11C]Ro 15-4513 was 30-40% with an overall synthesis time of 40 min. The specific radioactivity was about 1000 Ci/mmol at end of synthesis. In vitro autoradiography showed that [11C]Ro 15-4513 bound specifically predominantly in the neocortex of the human brain. Specific binding was also demonstrated in the basal ganglia and the cerebellar cortex. Flumazenil (Ro 15-1788) and clonazepam inhibited the binding in cerebral regions, but a significant proportion in the cerebellum was not inhibited by these agents. This proportion may represent alpha 6-containing BZ receptors. PET examination of [11C]Ro 15-4513 binding in Cynomolgus monkeys demonstrated high uptake of radioactivity in neocortex. The uptake of radioactivity was markedly displaced by high doses of Ro 15-4513 or clonazepam. [11C]Ro 15-4513 should be a useful ligand to examine BZ receptor characteristics in the living human brain by PET.

In vivo bidirectional modulatory effect of benzodiazepine receptor ligands on GABAergic transmission evaluated by positron emission tomography in non-human primates

The central type benzodiazepine receptor (BDZr), an allosteric modulatory site of the GABAA receptor-anion channel, has been shown in vitro to respond to drugs with positive efficacy (agonists), zero efficacy (competitive antagonists) and drugs with negative efficacy (inverse agonists). However, this general concept of the function of BDZr drugs has rarely been assessed in intact living brain. We report here in on a non-invasive in vivo assessment of the intrinsic efficacies of BDZr drugs in the brain of non-human primates. We have performed an in vivo simultaneous determination of fractional BDZr occupancy and the resulting pharmacological efficacies of the full agonist diazepam, the partial agonist bretazenil, the antagonist flumazenil (Ro15-1788), the partial inverse agonist Ro15-4513 and the full inverse agonist methyl beta-carboline-3-carboxylate (beta-CCM). Positron emission tomography (PET) was used to estimate fractional BDZr occupancy measured as the in vivo displacement in the brain of the positron emitter radioligand, [11C]flumazenil. Simultaneously, the proconvulsant or anticonvulsant efficacies of the BDZr drugs were measured as their abilities to facilitate or counteract the central effects of an infusion of pentylenetetrazol, a non-competitive GABA antagonist acting on the picrotoxin site of the receptor complex. This was measured using electroencephalographic recording (EEG). Our results show that, in vivo, the fractional receptor occupancy by a given drug is perfectly correlated with its resulting graded pharmacological effects, as predicted from the competitive drug receptor interaction theory. Furthermore, the slope of the relationship between fractional receptor occupancies and the resulting pharmacological effects (an index of intrinsic efficacy) strictly depends on the BDZr ligand considered. Diazepam displayed a strong positive intrinsic efficacy, and, in contrast, beta-CCM a marked negative one. Between these two extremes, the partially active drugs bretazenil and Ro15-4513, which required a large fractional receptor occupancy to produce significant anti- or proconvulsant effects, respectively, displayed only a weak intrinsic efficacy. Flumazenil did not produce any significant pharmacological effect. We observed that the in vivo intrinsic efficacies of diazepam, flumazenil and beta-CCM correlate with their intrinsic efficacies as measured by their modulatory effects on the GABA-dependent membrane chloride conductance in vitro. Thus, the intrinsic efficacies measured using PET and EEG are likely to reflect the different in vivo abilities of BDZr drugs to induce or stabilize the GABAA-benzodiazepine chloride channel in a given conformation.(ABSTRACT TRUNCATED AT 400 WORDS)

Acute effects of ethanol on regional brain glucose metabolism and transport

To evaluate the effects of ethanol in the human brain, we tested six normal subjects and six alcoholics using positron emission tomography and 2-deoxy-2-[18F]-fluoro-D-glucose (FDG) under baseline conditions and 24 hours later after ethanol administration (1 g/kg). Ethanol inhibited cortical and cerebellar glucose metabolism with relative sparing of the basal ganglia and corpus callosum. This inhibition was more pronounced in the alcoholics than in the controls. Measurement of the constants for glucose transport and utilization showed that decreased glucose metabolism was due to a reduction in glucose phosphorylation and not to a change of glucose transport into the tissue. The pattern of regional metabolic inhibition by alcohol paralleled the distribution of benzodiazepine receptors in the human brain.

Non-invasive methods to study drug disposition: positron emission tomography. Detection and quantification of brain receptors in man

Positron emission tomography (PET) has been used mostly for the study of brain blood flow and metabolism in normal controls and in a variety of neurological and psychiatric conditions. With the appropriate radiotracers, PET also allows non-invasive imaging and quantification of a growing list of neuroreceptors, the target of most psychotropic drugs. For example, 11C-carfentanil and 11C-diprenorphine, two potent opiate ligands, have been used to label opiate receptors in vivo in man. Methods have been developed to quantify receptor studies with PET in terms of receptor density and affinity. PET is a unique tool that now allows measurement of receptor occupancy in vivo and could be used by the pharmacologist to optimize drug treatment.