PET analysis of alcohol interaction with the brain disposition of [11C]flumazenil

PET measurement of "GABA shift" in the rat brain: A preclinical application of bolus plus constant infusion paradigm of [18F]flumazenil

INTRODUCTION

We measured the tiagabine-induced enhancement of the GABA_A receptor's affinity for benzodiazepine ligands ("GABA shift") using [¹⁸F]flumazenil (FMZ) PET with preclinical application of bolus plus constant infusion (B/I). Differences in quantified results of [¹⁸F]FMZ binding were compared to that of [¹⁸F]FMZ PET with single bolus injection (SB).

MATERIALS AND METHODS

Sprague-Dawley rats underwent [¹⁸F]FMZ PET scans with B/I, which consisted of baseline and "GABA shift" sessions in a scan, or scans with SB one week apart. Tiagabine (10mg/kg) was intravenously injected after the baseline session. [¹⁸F]FMZ binding potentials (BP_ND) were calculated using an equilibrium ratio method and a modeling method for B/I and SB, respectively. Regional brain BP_ND changes (%) before and after the tiagabine treatment were also calculated.

RESULTS

In PET studies with B/I (K_bol=20min), [¹⁸F]FMZ distribution in the various cortical and subcortical regions rapidly reached equilibrium. After the tiagabine treatment, [¹⁸F]FMZ BP_ND were substantially increased across the regions of interest (the frontal cortex, hippocampus, thalamus, and striatum), ranging from 3% to 7% BP_ND change (B/I) and 6-14% BP_ND change (SB), respectively. In PET studies with SB, a statistically significant increase of [¹⁸F]FMZ BP_ND was found only in the striatum, due to the greater inter-individual variance compared to those with B/I.

CONCLUSIONS

Data demonstrated that an [¹⁸F]FMZ PET study with B/I (K_bol=20min) is both reliable and sensitive for the assessment of altered GABA_A receptor function induced by tiagabine treatment in the rat brain. These results may help to improve the efficiency of the development of new GABA-targeting drugs in the preclinical stage using [¹⁸F]FMZ PET.

In vivo characterization and dynamic receptor occupancy imaging of TPA023B, an alpha 2/alpha 3/alpha 5 subtype selective gamma-aminobutyric acid-a partial agonist

BACKGROUND

A novel, high-affinity (.7-2.0 nmol) compound that selectively activates the alpha2, alpha 3, and alpha 5 (but not alpha1) gamma-aminobutyric acid-A (GABA(A)) receptor subtypes, TPA023B (2',6-difluoro-5'-[3-(1-hydroxy-1-methylethyl) imidazo[1,2-b][1,2,4]triazin-7-yl][1,1'-biphenyl]-2-carbonitrile) was pharmacologically characterized and studied by means of positron emission tomography (PET) to determine dynamic occupancies of the benzodiazepine binding site of human brain GABA(A) receptors after a single oral dose.

METHODS

Four healthy male volunteers were studied in a double-blind, randomized placebo-controlled study of which three were given a single dose of 1.5 mg TPA023B and the fourth received placebo. The time course of GABA(A) receptor occupancy was determined with multiple dynamic [(11)C]flumazenil PET studies at pre-dose baseline and 5 and 24 hours after dose. Arterial sampling and full kinetic modeling with a two-compartment model was used to calculate parametric maps of receptor availability (distribution volume V(T)) and of occupancy.

RESULTS

The GABA(A) receptor occupancy as determined from [(11)C]flumazenil V(T) values in all brain regions was reduced homogeneously, on average by 52.5 +/- 1.2% after 5 hours and 46.4 +/- 6.0% after 24 hours. No serious adverse events were encountered in humans.

CONCLUSIONS

Single oral doses of 1.5 mg of TPA023B correspond to average receptor occupancies in neocortical regions of 52% and 46% after 5 and 24 hours, respectively. Provided suitable ligands and quantification methods are available for the appropriate target, quantitative PET offers a unique tool for dynamic in vivo measurement of relevant on-site receptor occupancy.

Neuroimaging and mechanisms of drug abuse: interface of molecular imaging and molecular genetics

Whereas ligand studies can inform the end-products of dysregulation of genetic expression, reporter gene imaging can provide the means to understand the genetic origin of these end-products. As with radioligand studies, in vivo direct measurement of gene expression will allow genetic processes to be monitored over time in the same subject, use of a subject as his/her own control in intervention studies (i.e., measurement before and after an intervention), and monitoring the spatial distribution of molecular events in the whole brain. Furthermore, reporter gene imaging, by advancing knowledge of the biologic mechanisms of disease states, has important clinical implications, particularly in the development and monitoring of treatments. We expect PET to play a prominent role in the elucidation of substance abuse mechanisms and contribute significantly to the development of innovative treatment strategies.

Drug interactions at GABA(A) receptors

Neurotransmitter receptor systems have been the focus of intensive pharmacological research for more than 20 years for basic and applied scientific reasons, but only recently has there been a better understanding of their key features. One of these systems includes the type A receptor for the gamma-aminobutyric acid (GABA), which forms an integral anion channel from a pentameric subunit assembly and mediates most of the fast inhibitory neurotransmission in the adult vertebrate central nervous system. Up to now, depending on the definition, 16-19 mammalian subunits have been cloned and localized on different genes. Their assembly into proteins in a poorly defined stoichiometry forms the basis of functional and pharmacological GABA(A) receptor diversity, i.e. the receptor subtypes. The latter has been well documented in autoradiographic studies using ligands that label some of the receptors' various binding sites, corroborated by recombinant expression studies using the same tools. Significantly less heterogeneity has been found at the physiological level in native receptors, where the subunit combinations have been difficult to dissect. This review focuses on the characteristics, use and usefulness of various ligands and their binding sites to probe GABA(A) receptor properties and to gain insight into the biological function from fish to man and into evolutionary conserved GABA(A) receptor heterogeneity. We also summarize the properties of the novel mouse models created for the study of various brain functions and review the state-of-the-art imaging of brain GABA(A) receptors in various human neuropsychiatric conditions. The data indicate that the present ligands are only partly satisfactory tools and further ligands with subtype-selective properties are needed for imaging purposes and for confirming the behavioral and functional results of the studies presently carried out in gene-targeted mice with other species, including man.

Metabolite analysis of [11C]flumazenil in human plasma: assessment as the standardized value for quantitative PET studies

Analysis of carbon-11 labeled metabolites in plasma was carried out during positron emission tomography (PET) studies with a central benzodiazepine receptor ligand [11C]flumazenil ([11C]FMZ) in 24 human subjects (14-76 y.o.) including five normal volunteers and 19 patients with neurological disorders. Arterial plasma samples were obtained at 3, 5, 10, 15, 20, 30 and 60 min after i.v. injection of the tracer, and were analyzed by high-performance liquid chromatography. The rate of plasma [11C]FMZ degradation was associated with a large individual variation, but no significant difference was found in the degradation of [11C]FMZ either between male and female, young and old, or between normal subjects and patient groups. When the mean fraction of unchanged [11C]FMZ at each time point was used instead of individually measured metabolite data for the arterial input function, as much as a 30% error occurred in the distribution volume of the [11C]FMZ binding in the brain. These results indicate that the mean percentage of unchanged [11C]FMZ fraction in subjects cannot be used as the standardized value, and that the analysis of metabolites in plasma is necessary to determine the exact arterial input function for quantitative PET measurement.

Central benzodiazepine receptor imaging and quantitation with single photon emission computerised tomography: SPECT

This review discusses the current use of single photon emission computerised tomography (SPECT) for central benzodiazepine receptor imaging and quantitation. The general principles underlying SPECT imaging and receptor quantitation methods such as the kinetic, pseudo-equilibrium and steady-state (tracer infusion and bolus) approaches are described. The advantages and practical drawbacks of these techniques are highlighted.

PET-determination of benzodiazepine receptor binding in studies on alcoholism

Positron Emission Tomography (PET) and the radioligand [11C]flumazenil were used to examine benzodiazepine (BZ) receptor binding in the human brain. In a first study of healthy males acute ingestion of alcohol did not alter total radioactivity uptake or specific [11C]flumazenil binding in the neocortex or cerebellum. In a second study [11C]flumazenil binding was determined in 5 healthy male controls and 5 chronic alcohol dependent men using a saturation procedure with two PET experiments. Mean values for BZ-receptor density and affinity were similar in the two groups but the Bmax variance for the alcohol dependents was significantly larger (p < 0.05) for all regions. The present studies do not support the view that alcohol affects central BZ receptor binding in man.

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.

PET analysis of [11C]flumazenil binding to benzodiazepine receptors in chronic alcohol-dependent men and healthy controls

Benzodiazepine (BZ) receptor binding in the brain was determined in five chronic alcohol-dependent men and in five healthy male control subjects using [11C]flumazenil (Ro 15-1788) and positron emission tomography (PET). Magnetic resonance imaging was used to evaluate brain anatomy and pathology, and to define regions of interest in the brain. [11C]Flumazenil binding was quantified by applying a saturation procedure with two PET experiments, the first with high (400-3400 Ci/mmol) and the second with low (approximately 1 Ci/mmol) specific radioactivity. Radioactivity in the pons was taken as an estimate of free and nonspecifically bound radioligand. Equilibrium was defined to prevail when the derivative of specific binding (dCb(t)/dt) was equal to zero. The values were used in a Scatchard analysis for determination of the maximum density of binding sites (Bmax) and the equilibrium binding constant (Kd). The mean values of Bmax and Kd were about the same in the two groups, but the Bmax variance for the alcoholics was significantly greater for all regions of interest as compared with the healthy volunteers. The results may indicate that chronic alcohol consumption has multiple effects on the BZ receptor complex.