Modeling analysis of [11C]flumazenil kinetics studied by PET: application to a critical study of the equilibrium approaches

Brain PET imaging using 11C-flumazenil and 11C-buprenorphine does not support the hypothesis of a mutual interaction between buprenorphine and benzodiazepines at the neuroreceptor level

Among opioids, buprenorphine presents a favorable safety profile with a limited risk of respiratory depression. However, fatalities have been reported when buprenorphine is combined to a benzodiazepine. Potentiation of buprenorphine interaction with opioid receptors (ORs) with benzodiazepines, and/or vice versa, is hypothesized to explain this drug-drug interaction (DDI). The mutual DDI between buprenorphine and benzodiazepines was investigated at the neuroreceptor level in nonhuman primates (n = 4 individuals) using brain PET imaging and kinetic modelling. The binding potential (BPND) of benzodiazepine receptor (BzR) was assessed using 11C-flumazenil PET imaging before and after administration of buprenorphine (0.2 mg, i.v.). Moreover, the brain kinetics and receptor binding of buprenorphine were investigated in the same individuals using 11C-buprenorphine PET imaging before and after administration of diazepam (10 mg, i.v.). Outcome parameters were compared using a two-way ANOVA. Buprenorphine did not impact the plasma nor brain kinetics of 11C-flumazenil. 11C-flumazenil BPND was unchanged following buprenorphine exposure, in any brain region (p > 0.05). Similarly, diazepam did not impact the plasma or brain kinetics of 11C-buprenorphine. 11C-buprenorphine volume of distribution (VT) was unchanged following diazepam exposure, in any brain region (p > 0.05). To conclude, our PET imaging findings do not support a neuropharmacokinetic or neuroreceptor-related mechanism of the buprenorphine/benzodiazepine interaction.

Drug characteristics derived from kinetic modeling: combined 11C-UCB-J human PET imaging with levetiracetam and brivaracetam occupancy of SV2A

BACKGROUND

Antiepileptic drugs, levetiracetam (LEV) and brivaracetam (BRV), bind to synaptic vesicle glycoprotein 2A (SV2A). In their anti-seizure activity, speed of brain entry may be an important factor. BRV showed faster entry into the human and non-human primate brain, based on more rapid displacement of SV2A tracer ¹¹C-UCB-J. To extract additional information from previous human studies, we developed a nonlinear model that accounted for drug entry into the brain and binding to SV2A using brain ¹¹C-UCB-J positron emission tomography (PET) data and the time-varying plasma drug concentration, to assess the kinetic parameter K₁ (brain entry rate) of the drugs.

METHOD

Displacement (LEV or BRV p.i. 60 min post-tracer injection) and post-dose scans were conducted in five healthy subjects. Blood samples were collected for measurement of drug concentration and the tracer arterial input function. Fitting of nonlinear differential equations was applied simultaneously to time-activity curves (TACs) from displacement and post-dose scans to estimate 5 parameters: K₁ (drug), K₁(¹¹C-UCB-J, displacement), K₁(¹¹C-UCB-J, post-dose), free fraction of ¹¹C-UCB-J in brain (f_ND(¹¹C-UCB-J)), and distribution volume of ¹¹C-UCB-J (V_T(UCB-J)). Other parameters (K_D(drug), K_D(¹¹C-UCB-J), f_P(drug), f_P(¹¹C-UCB-J, displacement), f_P(¹¹C-UCB-J, post-dose), f_ND(drug), k_off(drug), k_off(¹¹C-UCB-J)) were fixed to literature or measured values.

RESULTS

The proposed model described well the TACs in all subjects; however, estimates of drug K₁ were unstable in comparison with ¹¹C-UCB-J K₁ estimation. To provide a conservative estimate of the relative speed of brain entry for BRV vs. LEV, we determined a lower bound on the ratio BRV K₁/LEV K₁, by finding the lowest BRV K₁ or highest LEV K₁ that were statistically consistent with the data. Specifically, we used the F test to compare the residual sum of squares with fixed BRV K₁ to that with floating BRV K₁ to obtain the lowest possible BRV K₁; the same analysis was performed to find the highest LEV K₁. The lower bound of the ratio BRV K₁/LEV K₁ was ~ 7.

CONCLUSIONS

Under appropriate conditions, this advanced nonlinear model can directly estimate entry rates of drugs into tissue by analysis of PET TACs. Using a conservative statistical cutoff, BRV enters the brain at least sevenfold faster than LEV.

Comparison of high and low molar activity TSPO tracer [18F]F-DPA in a mouse model of Alzheimer's disease

[¹⁸F]F-DPA, a novel translocator protein 18 kDa (TSPO)-specific radioligand for imaging neuroinflammation, has to date been synthesized with low to moderate molar activities (A_m's). In certain cases, low A_m can skew the estimation of specific binding. The high proportion of the non-radioactive component can reduce the apparent-specific binding by competitively binding to receptors. We developed a nucleophilic synthesis of [¹⁸F]F-DPA resulting in high A_m (990 ± 150 GBq/µmol) and performed in vivo comparison with low A_m (9.0 ± 2.9 GBq/µmol) [¹⁸F]F-DPA in the same APP/PS1-21 and wild-type mice (injected masses: 0.34 ± 0.13 µg/kg and 38 ± 15 µg/kg, respectively). The high level of microgliosis in the APP/PS1-21 mouse model enables good differentiation between diseased and healthy animals and serves better to distinguish the effect of differing A_m on specific binding. The differing injected masses affect the washout profile and shape of the time-activity curves. Ratios of standardized uptake values obtained with high and low A_m [¹⁸F]F-DPA demonstrate that there is a 1.5-fold higher uptake of radioactivity in the brains of APP/PS1-21 animals when imaging is carried out with high A_m [¹⁸F]F-DPA. The differences between APP/PS1-21 and wild-type animals showed higher significance when high A_m was used.

In vivo variation in same-day estimates of metabotropic glutamate receptor subtype 5 binding using [11C]ABP688 and [18F]FPEB

Positron emission tomography tracers [11C]ABP688 and [18F]FPEB target the metabotropic glutamate receptor subtype 5 providing quantification of the brain glutamatergic system in vivo. Previous [11C]ABP688 positron emission tomography human test-retest studies indicate that, when performed on the same day, significant binding increases are observed; however, little deviation is reported when scans are >7 days apart. Due to the small cohorts examined previously (eight and five males, respectively), we aimed to replicate the same-day test-retest studies in a larger cohort including both males and females. Results confirmed large within-subject binding differences (ranging from -23% to 108%), suggesting that measurements are greatly affected by study design. We further investigated whether this phenomenon was specific to [11C]ABP688. Using [18F]FPEB and methodology that accounts for residual radioactivity from the test scan, four subjects were scanned twice on the same day. In these subjects, binding estimates increased between 5% and 39% between scans. Consistent with [11C]ABP688, mean absolute test-retest variability was previously reported as <12% when scans were >21 days apart. This replication study and pilot extension to [18F]FPEB suggest that observed within-day binding variation may be due to characteristics of mGluR5; for example, diurnal variation in mGluR5 may affect measurement of this receptor.

Generalized paired-agent kinetic model for in vivo quantification of cancer cell-surface receptors under receptor saturation conditions

New precision medicine drugs oftentimes act through binding to specific cell-surface cancer receptors, and thus their efficacy is highly dependent on the availability of those receptors and the receptor concentration per cell. Paired-agent molecular imaging can provide quantitative information on receptor status in vivo, especially in tumor tissue; however, to date, published approaches to paired-agent quantitative imaging require that only 'trace' levels of imaging agent exist compared to receptor concentration. This strict requirement may limit applicability, particularly in drug binding studies, which seek to report on a biological effect in response to saturating receptors with a drug moiety. To extend the regime over which paired-agent imaging may be used, this work presents a generalized simplified reference tissue model (GSRTM) for paired-agent imaging developed to approximate receptor concentration in both non-receptor-saturated and receptor-saturated conditions. Extensive simulation studies show that tumor receptor concentration estimates recovered using the GSRTM are more accurate in receptor-saturation conditions than the standard simple reference tissue model (SRTM) (% error (mean  ±  sd): GSRTM 0  ±  1 and SRTM 50  ±  1) and match the SRTM accuracy in non-saturated conditions (% error (mean  ±  sd): GSRTM 5  ±  5 and SRTM 0  ±  5). To further test the approach, GSRTM-estimated receptor concentration was compared to SRTM-estimated values extracted from tumor xenograft in vivo mouse model data. The GSRTM estimates were observed to deviate from the SRTM in tumors with low receptor saturation (which are likely in a saturated regime). Finally, a general 'rule-of-thumb' algorithm is presented to estimate the expected level of receptor saturation that would be achieved in a given tissue provided dose and pharmacokinetic information about the drug or imaging agent being used, and physiological information about the tissue. These studies suggest that the GSRTM is necessary when receptor saturation exceeds 20% and highlight the potential for GSRTM to accurately measure receptor concentrations under saturation conditions, such as might be required during high dose drug studies, or for imaging applications where high concentrations of imaging agent are required to optimize signal-to-noise conditions. This model can also be applied to PET and SPECT imaging studies that tend to suffer from noisier data, but require one less parameter to fit if images are converted to imaging agent concentration (quantitative PET/SPECT).

CNS Assessment Using Functional Neuro-PET Imaging

Disorders of the brain manifest in a variety of manners ranging from feeling or thought abnormalities to total paralysis. Until recently, most imaging methods of the brain have been limited to anatomic considerations, with little information about actual function of the brain except that deduced from clinical examination and physical and cognitive assessment testing. With the advent of positron emission tomography (PET) and enhanced computer and scintigraphic image detection systems, there is keen interest in applying this imaging technique to better understand brain physiology and pathophysiology. Potential applications of PET in CNS assessment is expanding avenues for improved diagnosis and staging of disease, as well as monitoring therapeutic interventions. A general review of the radiopharmaceuticals used for neuro-PET imaging, as well as their application in situations such as cerebrovascular disease, brain activation studies, various movement disorders and dementias, depression, epilepsy, obsessive-compulsive disorder, schizophrenia, and neuropharmacology (including cerebral receptor studies) will be presented.

Investigation of Proposed Activity of Clarithromycin at GABAA Receptors Using [(11)C]Flumazenil PET

Clarithromycin is a potential treatment for hypersomnia acting through proposed negative allosteric modulation of GABAA receptors. We were interested whether this therapeutic benefit might extend to Parkinson's disease (PD) patients because GABAergic neurotransmission is implicated in postural control. Prior to initiating clinical studies in PD patients, we wished to better understand clarithromycin's mechanism of action. In this work we investigated whether the proposed activity of clarithromycin at the GABAA receptor is associated with the benzodiazepine binding site using in vivo [(11)C]flumazenil positron emission tomography (PET) in primates and ex vivo [(3)H]flumazenil autoradiography in rat brain. While the studies demonstrate that clarithromycin does not change the K d of FMZ, nor does it competitively displace FMZ, there is preliminary evidence from the primate PET imaging studies that clarithromycin delays dissociation and washout of flumazenil from the primate brain in a dose-dependent fashion. These findings would be consistent with the proposed GABAA allosteric modulator function of clarithromycin. While the results are only preliminary, further investigation of the interaction of clarithromycin with GABA receptors and/or GABAergic medications is warranted, and therapeutic applications of clarithromycin alone or in combination with flumazenil, to treat hyper-GABAergic status in PD at minimally effective doses, should also be pursued.

Simultaneous scanning of two mice in a small-animal PET scanner: a simulation-based assessment of the signal degradation

In PET imaging, research groups have recently proposed different experimental set ups allowing multiple animals to be simultaneously imaged in a scanner in order to reduce the costs and increase the throughput. In those studies, the technical feasibility was demonstrated and the signal degradation caused by additional mice in the FOV characterized, however, the impact of the signal degradation on the outcome of a PET study has not yet been studied. Here we thoroughly investigated, using Monte Carlo simulated [18F]FDG and [11C]Raclopride PET studies, different experimental designs for whole-body and brain acquisitions of two mice and assessed the actual impact on the detection of biological variations as compared to a single-mouse setting. First, we extended the validation of the PET-SORTEO Monte Carlo simulation platform for the simultaneous simulation of two animals. Then, we designed [18F]FDG and [11C]Raclopride input mouse models for the simulation of realistic whole-body and brain PET studies. Simulated studies allowed us to accurately estimate the differences in detection between single- and dual-mode acquisition settings that are purely the result of having two animals in the FOV. Validation results showed that PET-SORTEO accurately reproduced the spatial resolution and noise degradations that were observed with actual dual phantom experiments. The simulated [18F]FDG whole-body study showed that the resolution loss due to the off-center positioning of the mice was the biggest contributing factor in signal degradation at the pixel level and a minimal inter-animal distance as well as the use of reconstruction methods with resolution modeling should be preferred. Dual mode acquisition did not have a major impact on ROI-based analysis except in situations where uptake values in organs from the same subject were compared. The simulated [11C]Raclopride study however showed that dual-mice imaging strongly reduced the sensitivity to variations when mice were positioned side-by-side while no sensitivity reduction was observed when they were facing each other. This is the first study showing the impact of different experimental designs for whole-body and brain acquisitions of two mice on the quality of the results using Monte Carlo simulated [18F]FDG and [11C]Raclopride PET studies.

A data driven method for estimation of B(avail) and appK(D) using a single injection protocol with [¹¹C]raclopride in the mouse

PURPOSE

The partial saturation approach (PSA) is a simple, single injection experimental protocol that will estimate both B(avail) and appK(D) without the use of blood sampling. This makes it ideal for use in longitudinal studies of neurodegenerative diseases in the rodent. The aim of this study was to increase the range and applicability of the PSA by developing a data driven strategy for determining reliable regional estimates of receptor density (B(avail)) and in vivo affinity (1/appK(D)), and validate the strategy using a simulation model.

METHODS

The data driven method uses a time window guided by the dynamic equilibrium state of the system as opposed to using a static time window. To test the method, simulations of partial saturation experiments were generated and validated against experimental data. The experimental conditions simulated included a range of receptor occupancy levels and three different B(avail) and appK(D) values to mimic diseases states. Also the effect of using a reference region and typical PET noise on the stability and accuracy of the estimates was investigated.

RESULTS

The investigations showed that the parameter estimates in a simulated healthy mouse, using the data driven method were within 10±30% of the simulated input for the range of occupancy levels simulated. Throughout all experimental conditions simulated, the accuracy and robustness of the estimates using the data driven method were much improved upon the typical method of using a static time window, especially at low receptor occupancy levels. Introducing a reference region caused a bias of approximately 10% over the range of occupancy levels.

CONCLUSIONS

Based on extensive simulated experimental conditions, it was shown the data driven method provides accurate and precise estimates of B(avail) and appK(D) for a broader range of conditions compared to the original method.

In vivo measurement of hippocampal GABAA/cBZR density with [18F]-flumazenil PET for the study of disease progression in an animal model of temporal lobe epilepsy

Purpose

Imbalance of inhibitory GABAergic neurotransmission has been proposed to play a role in the pathogenesis of temporal lobe epilepsy (TLE). This study aimed to investigate whether [¹⁸F]-flumazenil ([¹⁸F]-FMZ) PET could be used to non-invasively characterise GABA_A/central benzodiazepine receptor (GABA_A/cBZR) density and affinity in vivo in the post-kainic acid status epilepticus (SE) model of TLE.

Methods

Dynamic [¹⁸F]-FMZ -PET scans using a multi-injection protocol were acquired in four male wistar rats for validation of the partial saturation model (PSM). SE was induced in eight male Wistar rats (10 weeks of age) by i.p. injection of kainic acid (7.5–25 mg/kg), while control rats (n = 7) received saline injections. Five weeks post-SE, an anatomic MRI scan was acquired and the following week an [¹⁸F]-FMZ PET scan (3.6–4.6 nmol). The PET data was co-registered to the MRI and regions of interest drawn on the MRI for selected structures. A PSM was used to derive receptor density and apparent affinity from the [¹⁸F]-FMZ PET data.

Key Findings

The PSM was found to adequately model [¹⁸F]-FMZ binding in vivo. There was a significant decrease in hippocampal receptor density in the SE group (p<0.01), accompanied by an increase in apparent affinity (p<0.05) compared to controls. No change in cortical receptor binding was observed. Hippocampal volume reduction and cell loss was only seen in a subset of animals. Histological assessment of hippocampal cell loss was significantly correlated with hippocampal volume measured by MRI (p<0.05), but did not correlate with [¹⁸F]-FMZ binding.

Significance

Alterations to hippocampal GABA_A/cBZR density and affinity in the post-kainic acid SE model of TLE are detectable in vivo with [¹⁸F]-FMZ PET and a PSM. These changes are independent from hippocampal cell and volume loss. [¹⁸F]-FMZ PET is useful for investigating the role that changes GABA_A/cBZR density and binding affinity play in the pathogenesis of TLE.

18F-flumazenil: a γ-aminobutyric acid A-specific PET radiotracer for the localization of drug-resistant temporal lobe epilepsy

Studies report that (11)C-flumazenil (FMZ) PET more specifically localizes the epileptogenic zone in patients with medically refractory focal epilepsy than (18)F-FDG PET. However, practical aspects of (11)C use limit clinical application. We report a phase I/IIa study assessing the clinical use of (18)F-FMZ PET for the localization of the epileptogenic zone in patients with drug-resistant temporal lobe epilepsy (TLE). Receptor binding was quantified using kinetic modeling that did not require arterial sampling.

METHODS

Dynamic (18)F-FMZ PET and static interictal (18)F-FDG PET scans were compared in healthy controls (n = 17 for (18)F-FMZ and n = 20 for (18)F-FDG) and TLE patients with mesial temporal sclerosis on MR imaging (MTS, n = 12) and with normal MR imaging (NL TLE, n = 19). Masked visual assessment of images was undertaken. Parametric images of (18)F-FMZ binding potential (BPND) were generated using the simplified reference tissue model. Region-of-interest analysis on coregistered MR images and statistical parametric mapping were used to quantify (18)F-FMZ BPND and (18)F-FDG uptake in the temporal lobe.

RESULTS

The visual assessment of static standardized uptake value images showed (18)F-FMZ PET to have high specificity (16/17 [94%]) and moderate sensitivity (21/31 [68%]) for the localization of the epileptogenic zone, with a more restricted abnormality than (18)F-FDG PET. However, the (18)F-FMZ standardized uptake value images were falsely localizing in 3 of 31 patients (10%). Region-of-interest analysis demonstrated reductions in ipsilateral hippocampal (18)F-FMZ BPND in patients with either MTS or NL TLE, compared with controls subjects. Ipsilateral hippocampal (18)F-FMZ BPND was independent of both hippocampal volume and (18)F-FDG uptake, whereas ipsilateral hippocampal volume was correlated with (18)F-FDG uptake (r(2) = 0.69, P < 0.0001). Statistical parametric mapping analysis demonstrated decreased uptake in 14 of 31 (45%) cases with (18)F-FMZ PET and 18 of 29 (62%) with (18)F-FDG PET. Cluster size was significantly smaller on (18)F-FMZ than (18)F-FDG images (37 vs. 160 voxels, P < 0.01).

CONCLUSION

(18)F-FMZ PET has potential as a clinical tool for the localization of the epileptogenic zone in the presurgical evaluation of drug-resistant TLE, providing information complementary to (18)F-FDG PET, with a more restricted region of abnormality.

PET imaging of brain inflammation during early epileptogenesis in a rat model of temporal lobe epilepsy

BACKGROUND

Recently, inflammatory cascades have been suggested as a target for epilepsy therapy. Positron emission tomography (PET) imaging offers the unique possibility to evaluate brain inflammation longitudinally in a non-invasive translational manner. This study investigated brain inflammation during early epileptogenesis in the post-kainic acid-induced status epilepticus (KASE) model with post-mortem histology and in vivo with [18F]-PBR111 PET.

METHODS

Status epilepticus (SE) was induced (N = 13) by low-dose injections of KA, while controls (N = 9) received saline. Translocator protein (TSPO) expression and microglia activation were assessed with [125I]-CLINDE autoradiography and OX-42 immunohistochemistry, respectively, 7 days post-SE. In a subgroup of rats, [18F]-PBR111 PET imaging with metabolite-corrected input function was performed before post-mortem evaluation. [18F]-PBR111 volume of distribution (Vt) in volume of interests (VOIs) was quantified by means of kinetic modelling and a VOI/metabolite-corrected plasma activity ratio.

RESULTS

Animals with substantial SE showed huge overexpression of TSPO in vitro in relevant brain regions such as the hippocampus and amygdala (P < 0.001), while animals with mild symptoms displayed a smaller increase in TSPO in amygdala only (P < 0.001). TSPO expression was associated with OX-42 signal but without obvious cell loss. Similar in vivo [18F]-PBR111 increases in Vt and the simplified ratio were found in key regions such as the hippocampus (P < 0.05) and amygdala (P < 0.01).

CONCLUSION

Both post-mortem and in vivo methods substantiate that the brain regions important in seizure generation display significant brain inflammation during epileptogenesis in the KASE model. This work enables future longitudinal investigation of the role of brain inflammation during epileptogenesis and evaluation of anti-inflammatory treatments.

Validation of in vitro cell-based human blood-brain barrier model using clinical positron emission tomography radioligands to predict in vivo human brain penetration

We have evaluated a novel in vitro cell-based human blood-brain barrier (BBB) model that could predict in vivo human brain penetration for compounds with different BBB permeabilities using the clinical positron emission tomography (PET) data. Comparison studies were also performed to demonstrate that the in vitro cell-based human BBB model resulted in better predictivity over the traditional permeability model in discovery organizations, Caco-2 cells. We evaluated the in vivo BBB permeability of [(18)F] and [(11)C]-compounds in humans by PET imaging. The in vivo plasma-brain exchange parameters used for comparison were determined in humans by PET using a kinetic analysis of the radiotracer binding. For each radiotracer, the parameters were determined by fitting the brain kinetics of the radiotracer using a two-tissue compartment model of the ligand-receptor interaction. Bidirectional transport studies with the same compounds as in in vivo studies were carried out using the in vitro cell-based human BBB model as well as Caco-2 cells. The in vitro cell-based human BBB model has important features of the BBB in vivo and is suitable for discriminating between CNS and non-CNS marketed drugs. A very good correlation (r(2) = 0.90; P < 0.001) was demonstrated between in vitro BBB permeability and in vivo permeability coefficient. In contrast, a poor correlation (r(2) = 0.17) was obtained between Caco-2 data and in vivo human brain penetration. This study highlights the potential of this in vitro cell-based human BBB model in drug discovery and shows that it can be an extremely effective screening tool for CNS programs.

Positron emission tomography imaging demonstrates correlation between behavioral recovery and correction of dopamine neurotransmission after gene therapy

In vivo gene transfer using viral vectors is an emerging therapy for neurodegenerative diseases with a clinical impact recently demonstrated in Parkinson's disease patients. Recombinant adeno-associated viral (rAAV) vectors, in particular, provide an excellent tool for long-term expression of therapeutic genes in the brain. Here we used the [(11)C]raclopride [(S)-(-)-3,5-dichloro-N-((1-ethyl-2-pyrrolidinyl)methyl)-2-hydroxy-6-methoxybenzamide] micro-positron emission tomography (PET) technique to demonstrate that delivery of the tyrosine hydroxylase (TH) and GTP cyclohydrolase 1 (GCH1) enzymes using an rAAV5 vector normalizes the increased [(11)C]raclopride binding in hemiparkinsonian rats. Importantly, we show in vivo by microPET imaging and postmortem by classical binding assays performed in the very same animals that the changes in [(11)C]raclopride after viral vector-based enzyme replacement therapy is attributable to a decrease in the affinity of the tracer binding to the D(2) receptors, providing evidence for reconstitution of a functional pool of endogenous dopamine in the striatum. Moreover, the extent of the normalization in this non-invasive imaging measure was highly correlated with the functional recovery in motor behavior. The PET imaging protocol used in this study is fully adaptable to humans and thus can serve as an in vivo imaging technique to follow TH + GCH1 gene therapy in PD patients and provide an additional objective measure to a potential clinical trial using rAAV vectors to deliver l-3,4-dihydroxyphenylanaline in the brain.

Quantification of cerebral nicotinic acetylcholine receptors by PET using 2-[18F]fluoro-A-85380 and the multiinjection approach

The multiinjection approach was used to study in vivo interactions between alpha4beta2(*) nicotinic acetylcholine receptors and 2-[(18)F]fluoro-A-85380 in baboons. The ligand kinetics was modeled by the usual nonlinear compartment model composed of three compartments (arterial plasma, free and specifically bound ligand in tissue). Arterial blood samples were collected to generate a metabolite-corrected plasma input function. The experimental protocol, which consisted of three injections of labeled or unlabeled ligand, was aiming at identifying all parameters in one experiment. Various parameters, including B'(max) (the binding sites density) and K(d)V(R) (the apparent in vivo affinity of 2-[(18)F]fluoro-A-85380) could then be estimated in thalamus and in several receptor-poor regions. B'(max) estimate was 3.0+/-0.3 pmol/mL in thalamus, and ranged from 0.25 to 1.58 pmol/mL in extrathalamic regions. Although K(d)V(R) could be precisely estimated, the association and dissociation rate constants k(on)/V(R) and k(off) could not be identified separately. A second protocol was then used to estimate k(off) more precisely in the thalamus. Having estimated all model parameters, we performed simulations of 2-[(18)F]fluoro-A-85380 kinetics to test equilibrium hypotheses underlying simplified approaches. These showed that a pseudo-equilibrium is quickly reached between the free and bound compartments, a favorable situation to apply Logan graphical analysis. In contrast, the pseudo-equilibrium between the plasma and free compartments is only reached after several hours. The ratio of radioligand concentration in these two compartments then overestimates the true equilibrium value, an unfavorable situation to estimate distribution volumes from late images after a bolus injection.

Multiinjection approach for D2 receptor binding quantification in living rats using [11C]raclopride and the beta-microprobe: crossvalidation with in vitro binding data

The purpose of this study was to quantify D2 receptors density and affinity in living rats using [11C]raclopride and to validate the multiinjection modelling approach. To this aim, we used an intracerebral beta+-sensitive probe as a highly sensitive system to quantify the radioligand activity using a single three-injection experimental paradigm. The study was divided into three main parts: (i) [11C]raclopride catabolism evaluation without and with cimetidine pretreatment (cytochrome P450 inhibitor); (ii) quantification of kinetics parameters in the striatum, enthorinal cortex, and cerebellum of living rats using a three-compartment model with an arterial input function; (iii) correlation study of in vivo and in vitro binding density and affinity values in the same striatal tissues. (i) raclopride catabolism was very reproducible between individuals; cimetidine pre-treatment resulted in a 30% reduction of raclopride metabolites. (ii) D2 striatal B'max and KdVr estimates obtained by compartmental modelling were 19.87+/-6.45 and 6.2+/-3.3 nmol/L, respectively. Cerebellum is the best candidate as a reference region with no specific binding detectable in vivo. (iii) When comparing density (Bmax/B'max) and affinity (Kd/KdVr) values in vivo and in vitro for each striatum, a high strict correlation was found (r2=0.90 and 0.72, for density and affinity, respectively). These results validate the multi-injection modelling approach coupled to beta-microprobe acquisitions as a mean to provide accurate and separate estimates of dopamine D2-receptor density and affinity, in the living rodent striatum.

Acute inhibition of cardiac monoamine oxidase A after tobacco smoke inhalation: validation study of [11C]befloxatone in rats followed by a positron emission tomography application in baboons

The in vivo characteristics of [11C]befloxatone were assessed in myocardium of rats and monkeys. A complete multicompartmental model was developed to quantify monkey cardiac monoamine oxidase A (MAO-A) binding sites using positron emission tomography (PET) and was applied to assess the acute effects of inhalation of tobacco smoke. Unknown compounds contained in tobacco smoke inhibit brain MAO. In vitro, befloxatone inhibits selectively, competitively, and reversibly MAO-A in human tissues. [11C]Befloxatone (1.85 MBq) was i.v. injected into rats. Animals were sacrificed, dissected, and samples were assessed for radioactivity. Another group of rats was pretreated with clorgyline (10 mg/kg i.v.). Monkeys were injected with [11C]befloxatone (222-370 MBq), and the chest was imaged with PET for 2 h. Presaturation and displacement experiments were performed using unlabeled befloxatone. For quantification of myocardial binding sites (Bmax), [11C]befloxatone was first injected as a tracer dose (2.7-9.3 nmol) and 20 min later injected as a mixture of labeled and unlabeled befloxatone (labeled, 10.3-41.9 nmol; unlabeled, 407-765 nmol). In rodents, cardiac uptake was high (3.39 +/- 0.5% injected dose/g tissue) and strongly inhibited (80%) by clorgyline. In monkeys, administration of unlabeled befloxatone displaced 85% of cardiac radioactivity. Bmax was found to be 208 +/- 13 pmol ml(-1) tissue. Inhalation of tobacco smoke decreased Bmax: 150 +/- 6.2 pmol ml(-1), whereas nicotine did not. [11C]Befloxatone allows a good visualization of the heart. Cardiac MAO-A Bmax was quantified and a clear effect of acute inhalation of tobacco smoke was evidenced. Therefore, a single cigarette can interfere with the cardiac turnover of catecholamines.

Imaging technologies in drug development: Anxiety and depression

Imaging technologies provide a unique access to the human brain in vivo. The use of imaging in anxiety and depression drug development has the potential to shorten and reduce the cost of the drug development process. Principles and assumptions inherent in diverse imaging technologies need be kept in mind to ensure data obtained are not misleading. A consideration of questions commonly encountered in drug development suggests specific imaging methodologies to be used to explore these.:

Parametric images of the extrastriatal D2 receptor density obtained using a high-affinity ligand (FLB 457) and a double-saturation method

The potential of positron emission tomography for the quantitative estimation of receptor concentration in extrastriatal regions has been limited in the past because of the low density of the D2 receptor sites in these regions and the insufficient affinity of the most widely used radioligands for dopamine receptors. The new method described in this paper permits the estimate of the D2 receptor concentration in the extrastriatal regions using a two-injection protocol and FLB 457, a ligand with a high affinity (20 pmol/L in vitro ) with D2 dopamine receptors. This approach is not valid for the striatal regions because some hypotheses cannot be verified (because of the high receptor concentration in these regions). The experimental protocol includes two injections with ligand doses designed to significantly occupy the extrastriatal receptor sites (approximately 90%), while leaving less than 60% of the receptor sites occupied by the ligand in the striatal regions. The results obtained using this double-saturation method are in line with the concentration estimates previously obtained using the multiinjection approach. The receptor concentration is 2.9 +/- 0.5 pmol/mL in the thalamus, 1.0 +/- 0.2 pmol/mL in the temporal cortex, and 0.35 +/- 0.13 pmol/mL in the occipital cortex. This study provides new arguments supporting the presence of a small receptor-site concentration in the cerebellum, estimated at 0.35 +/- 0.16 pmol/mL The simplicity of the calculation used to estimate the receptor concentration lends itself easily to parametric imaging. The receptor concentration is estimated pixel by pixel, without filtering. This method permits estimation of the extrastriatal D2 receptor concentration using an experimental protocol that can easily be used in patient studies (i.e., single experiment, no blood sampling, short experiment duration).

Dual-[11C]tracer single-acquisition positron emission tomography studies

The ability to study multiple physiologic processes of the brain simultaneously within the same subject would provide a new means to explore the interactions between neurotransmitter systems in vivo. Currently, examination of two distinct neuropharmacologic measures with positron emission tomography (PET) necessitates performing two separate scans spaced in time to allow for radionuclide decay. The authors present results from a dual-tracer PET study protocol using a single dynamic-scan acquisition where the injections of two tracers are offset by several minutes. Kinetic analysis is used to estimate neuropharmacologic parameters for both tracers simultaneously using a combined compartmental model configuration. This approach results in a large reduction in total study time of nearly 2 hours for carbon-11-labeled tracers. As multiple neuropharmacologic measures are obtained at nearly the same time, interventional protocols involving a pair of dual-tracer scans become feasible in a single PET session. Both computer simulations and actual human PET studies were performed using combinations of three different tracers: [11C]flumazenil, N-[11C]methylpiperidinyl propionate, and [ 11 C]dihydrotetrabenazine. Computer simulations of tracer-injection separations of 10 to 30 minutes showed the feasibility of the approach for separations down to 15 to 20 minutes or less. Dual-tracer PET studies were performed in 32 healthy volunteers using injection separations of 10, 15, or 20 minutes. Model parameter estimates for each tracer were similar to those obtained from previously performed single-injection studies. Voxel-by-voxel parametric images were of good quality for injections spaced by 20 minutes and were nearly as good for 15-minute separations, but were degraded noticeably for some model parameters when injections were spaced by only 10 minutes. The authors conclude that dual-tracer single-scan PET is feasible, yields accurate estimates of multiple neuropharmacologic measures, and can be implemented with a number of different radiotracer pairs.

Biodistribution, binding specificity and metabolism of [18F]fluoroethylflumazenil in rodents

Pre-clinical studies were carried out in order to characterize in rodents the biodistribution, the binding specificity and the metabolism of [18F]Fluoroethylflumazenil ([18F]FEF), a potential candidate for in vivo imaging of the benzodiazepine receptors. In vivo competition with flumazenil indicates that [18F]FEF binds specifically to the benzodiazepine receptor in the brain. The accumulation of [18F]FEF was significantly lower than using [3H]Flumazenil. The rather low accumulation in the brain is due to a rapid metabolism of [18F]FEF in hydrophylic metabolites which cannot cross the blood brain barrier, and are rapidly eliminated in the urine. Inhibition of the metabolism by acetaminophen (chemically induced hepatitis) led to a significant increase of the radioactivity found in the circulating blood and in the brain, while these results were not observed using classical inhibitors of the cytochrome CYP450, cimetidine and ketoconazole.

Anatomic and biochemical correlates of the dopamine transporter ligand 11C-PE2I in normal and parkinsonian primates: comparison with 6-[18F]fluoro-L-dopa

Positron emission tomography (PET) coupled to 6-[18F]Fluoro-L-Dopa (18F-Dopa) remains the gold standard for assessing dysfunctionality concerning the dopaminergic nigrostriatal pathway in Parkinson's disease and related disorders. The use of ligands of the dopamine transporters (DAT) is an attractive alternative target; consequently, the current aim was to validate one of them, 11C-PE2I, using a multiinjection modeling approach allowing accurate quantitation of DAT densities in the striatum. Experiments were performed in three controls, three MPTP-treated (parkinsonian) baboons, and one reserpine-treated baboon. 11C-PE2I B'max values obtained with this approach were compared with 18F-Dopa input rate constant values (Ki), in vitro Bmax binding of 125I-PE2I, and the number of dopaminergic neurons in the substantia nigra estimated postmortem by stereology. In the caudate nucleus and putamen, control values for 11C-PE2I B'max were 673 and 658 pmol/mL, respectively, whereas it was strongly reduced in the MPTP-treated (B'max = 26 and 36 pmol/mL) and reserpine-treated animals (B'max = 338 and 483 pmol/mL). In vivo 11C-PE2I B'max values correlated with 18F-Dopa Ki values and in vitro 125I-PE2I Bmax values in the striatum and with the number of nigral dopaminergic neurons. Altogether, these data support the use of 11C-PE2I for monitoring striatal dopaminergic disorders and the effect of potential neuroprotective strategies.

Absolute quantification by positron emission tomography of the endogenous ligand

The results of several recent papers have shown a significant influence of the endogenous neurotransmitters on the exogenous ligand kinetics measured by positron emission tomography. For example, several groups found that the percentage of D2 receptor sites occupied by the endogenous dopamine ranged from 25% to 40% at basal level. An obvious consequence of this significant occupancy is that the ligand-receptor model parameters, usually estimated by a model that does not take into account the endogenous ligand (EL) kinetics, can be significantly biased. In the current work, the authors studied the biases obtained by using the multiinjection approach. The results showed that in the classical ligand-receptor model, the receptor concentration is correctly estimated and that only the apparent affinity is biased by not taking the EL into account. At present, all absolute quantifications of the EL have been obtained through pharmacologic manipulation of the endogenous transmitter concentration, which is often too invasive a method to be used in patients. A theoretical reasoning showed that a noninvasive approach is necessarily based on both the apparent affinity measurement and on a multiregion approach. The correlation between the receptor concentration and the apparent affinity, previously observed with some ligands, verifies these two conditions; thus, the authors suggest that this correlation could be the result of the EL effect. To test this assumption experimentally, the effect of reserpine-induced dopamine depletion on the interactions between the D2 receptor sites and the FLB 457 is studied. With untreated baboons, the apparent FLB 457 affinity was smaller in the receptor-rich regions (striatum) than in the receptor-poor regions. This discrepancy disappeared after dopamine depletion, strongly suggesting that this affinity difference was related to the EL effect. Therefore, the purpose of the current study was to test the ability to quantify the EL based on the observed correlation between the receptor concentration and the apparent affinity. This approach offers a method for estimating the percentage of receptor sites occupied by the EL and, if its affinity is known, the free EL concentration. From the data obtained using FLB 457 with baboons, the authors found that approximately 53% of the D2 receptor sites are occupied by dopamine in the striatum and that the free dopamine concentration is approximately 120 nmol/L at basal level. This approach is transferable to patients, because the experimental data are obtained without pharmacologically induced modification of the EL.

Similarity and robustness of PET and SPECT binding parameters for benzodiazepine receptors

The single photon emission computed tomography (SPECT) radiotracer [123I]iomazenil is used to assess benzodiazepine receptor binding parameters. These measurements are relative indices of benzodiazepine receptor concentration (B'max). To evaluate the ability of such indices in accurately accessing the B'max the authors compared them with absolute values of B'max, measured using positron emission tomography (PET). The authors performed SPECT, PET, and magnetic resonance imaging (MRI) studies on a group composed of seven subjects. For SPECT studies, the authors administered a single injection of [123I]iomazenil and estimated the total and specific distribution volumes (DV(T SPECT), DV(S SPECT)) and the binding potential (BP) using unconstrained (BP(SPECT)) and constrained (BP(C SPECT)) compartmental models. For PET studies, the authors used a multiinjection approach with [11C]flumazenil and unlabeled flumazenil to estimate absolute values of receptor concentration, B'max, and some other binding parameters. The authors studied the correlation of different binding parameters with B'max. To study the robustness of the binding parameter measurements at the pixel level, the authors applied a wavelet-based filter to improve signal-to-noise ratio of time-concentration curves, and the calculated kinetic parameters were used to build up parametric images. For PET data, the B'max and the DV(PET) were highly correlated (r = 0.988). This confirms that it is possible to use the DV(PET) to access benzodiazepine receptor density. For SPECT data, the correlation between DV(SPECT) estimated using a two- and three-compartment model was also high (r = 0.999). The DV(T SPECT) and BP(C SPECT) parameters estimated with a constrained three-compartment model or the DV(T''SPECT) parameter estimated with a two-compartment model were also highly correlated to the B'max parameter estimated with PET. Finally, the robustness of the binding parameters allowed the authors to build pixel-by-pixel parametric images using SPECT data.

Wavelet analysis of dynamic PET data: application to the parametric imaging of benzodiazepine receptor concentration

Receptor density and ligand affinity can be assessed using positron emission tomography (PET). Biological parameters (B(max)('), k(1), k(2), k(on)/V(R), k(off)) are estimated using a compartmental model and a multi-injection protocol. Parametric imaging of the ligand-receptor model has been shown to be of special interest to study certain brain disorders. However, the low signal-to-noise ratio in kinetic curves at the pixel level hampers an adequate estimation of model parameters during the optimization procedure. For this reason, mapping requires a spatial filter, resulting in a loss of resolution. Filtering the kinetic curves in the frequency domain using the Fourier transform is not appropriate, because of difficulties in choosing a correct and efficient cutoff frequency. A wavelet-based filter is more appropriate to such tracer kinetics. The purpose of this study is to build up parametric images at the pixel level while conserving the original spatial resolution, using wavelet-based filtering. Data from [(11)C]flumazenil studies, mapping the benzodiazepine receptor density, were used. An invertible discrete wavelet transform was used to calculate the time-frequency signals of the time-concentration PET curves on a pixel-by-pixel basis. Kinetic curves observed from large regions of interest in high and low receptor-density regions were used to calibrate the threshold of wavelet coefficients. The shrunken wavelet coefficients were then transformed back to the original domain in order to obtain the filtered PET signal. Maps of all binding parameters were obtained at the pixel level with acceptable coefficients of variation of less than 30% for the B(max)(') parameter in most of the gray matter. A strong correlation between model parameter estimates using the usual regions of interest and parametric imaging was observed for all model parameters (r = 0.949 for the parameter B(max)(')). We conclude that wavelet-based filters are useful for building binding parameter maps without loss of the original spatial resolution of the PET scanner. The use of the wavelet-based filtering method can be extended far beyond the multi-injection protocol. It is likely to be also effective for other dynamic PET studies.

Quantitation of extrastriatal D2 receptors using a very high-affinity ligand (FLB 457) and the multi-injection approach

The multi-injection approach has been used to study in baboon the in vivo interactions between the D2 receptor sites and FLB 457, a ligand with a very high affinity for these receptors. The model structure was composed of four compartments (plasma, free ligand, and specifically and unspecifically bound ligands) and seven parameters (including the D2 receptor site density). The arterial plasma concentration, after correction for metabolites, was used as the input function. The experimental protocol, which consisted of three injections of labeled and/or unlabeled ligand, allowed the evaluation of all model parameters from a single positron emission tomography experiment. In particular, the concentration of receptor sites available for binding (B'max) and the apparent in vivo FLB 457 affinity were estimated in seven brain regions, including the cerebellum and several cortex regions, in which these parameters are estimated in vivo for the first time (B'max is estimated to be 4.0+/-1.3 pmol/mL in the thalamus and from 0.32 to 1.90 pmol/mL in the cortex). A low receptor density was found in the cerebellum (B'max = 0.39+/-0.17 pmol/mL), whereas the cerebellum is usually used as a reference region assumed to be devoid of D2 receptor sites. In spite of this very small concentration (1% of the striatal concentration), and because of the high affinity of the ligand, we demonstrated that after a tracer injection, most of the PET-measured radioactivity in the cerebellum results from the labeled ligand bound to receptor sites. The estimation of all the model parameters allowed simulations that led to a precise knowledge of the FLB 457 kinetics in all brain regions and gave the possibility of testing the equilibrium hypotheses and estimating the biases introduced by the usual simplified approaches.

Relationships between trait and state anxiety and the central benzodiazepine receptor: a PET study

The central benzodiazepine receptor (cBZr) has long been implicated in anxiety disorders on the basis of: (i) the well-known anxiolytic and anxiogenic properties of cBZr agonists and inverse agonists, respectively; (ii) a possibly reduced sensitivity to benzodiazepines in anxious subjects; and (iii) a putative endogenous ligand. Thus, two main hypothesis have been advanced, namely changes in the concentration or properties of the latter, and changes in the GABAA complex conformation, which contains the cBZr. Neither postmortem studies nor appropriate animal models are available to investigate these ideas. We have used positron emission tomography (PET) to measure both the density and affinity of the cBZr in multiple brain regions in unmedicated patients and age- and sex-matched healthy volunteers, and have looked for differences between groups as well as correlations between cBZr parameters and state and trait anxiety scores. We studied 10 unmedicated patients (sex ratio 1 : 1; mean age: 39 years), prospectively recruited using DSM III-R criteria, and 10 age- and gender-matched healthy unmedicated volunteers. Thanks to a PET procedure using two successive administrations of 11C-flumazenil (at high and low specific radioactivity) and previously validated by us, we estimated the Bmax, Kd and bound : free (B/F) ratios in 11 neocortical areas and in the cerebellum. Before and after the PET session, anxiety scores from Spielberger's and Covi's scales were obtained. There was no statistically significant difference in Bmax, Kd or B/F-values between the two groups for any region. Across the two groups, there were only a few marginally significant anxiety-score-PET correlations, suggesting chance findings. This is the first fully quantitative study to report on the relationships between cBZr parameters and anxiety. Using two independent approaches (i.e. group comparison and across-group correlations), we found no evidence for a link between anxiety trait or state and the cBZr in neocortex or cerebellum in this sample. These findings, if confirmed by studies on larger samples, have implications for the pharmacotherapy of anxiety disorders, and will need to be considered when designing new neurobiological models of anxiety.

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.

Parametric images of benzodiazepine receptor concentration using a partial-saturation injection

The in vivo quantification of the benzodiazepine receptor concentration in human brain using positron emission tomography (PET) and 11C-flumazenil (11C-FMZ), is usually based on a three-compartment model and on PET curves measured in a small number of large regions of interest; however, it should be interesting to estimate the receptor concentration for each pixel and to build quantified images of the receptor concentration. The main advantage is to allow screening of the receptor site localization and visual observation of the possible abnormalities. Up to now, all the methods described include complex experimental protocols, difficult to use in routine examinations. In this paper, we propose the partial-saturation approach to obtain parametric images of benzodiazepine receptor concentration and FMZ affinity. It consists of a single FMZ injection with a low specific activity, followed by Scatchard analysis. Like other parametric imaging methods, this partial-saturation approach can lead to a small percentage (< 1%) of unrealistic values in receptor-poor regions; however, it is the only method that allows receptor concentration and affinity images to be obtained from a single-injection 40-min experiment without blood sampling. We also propose a second method in which the receptor concentration map is directly deduced from the PET image acquired 5 to 10 min after a partial-saturation injection. This method assumes a known and constant FMZ affinity value but requires only very simple corrections of this PET image. It is robust (negative values are never found) and quite simple to use in routine examination of patients (no blood sampling, single injection, only 10-min experiment).

In vivo modulation of benzodiazepine receptor function after inhibition of endogenous gamma-aminobutyyric acid synthesis

The influence of decreased endogenous gamma-aminobutyric acid (GABA) concentration on benzodiazepine receptor function was studied in the brain of living baboons. Positron emission tomography and the radiotracer [11C]flumazenil combined with electroencephalography were used to determine the pharmacological properties of two bezodiazepine receptors agonists, diazepam and bretazenil, in baboons pre-treated or not with DL-allylglycine (an inhibitor of GABA synthesis). Our results show that, in vivo, DL-allylglycine reduces the affinity of benzodiazepine receptors for their agonists without altering the intrinsic capability of agonists to allosterically modulate GABAergic transmission.

Error analysis on parameter estimates in the ligand-receptor model: application to parameter imaging using PET data

Positron emission tomography and compartmental models allow the in vivo analysis of radioligand binding to receptor sites in the human brain. Benzodiazepine receptor binding was studied using a three-compartmental model and [11C]flumazenil. Four and five parameters were estimated from a single kinetic curve obtained with a multi-injection protocol, and parametric maps of receptor density and of the individual kinetic parameters were created with four-pixel sampling of the experimental images. The coefficient of variation on each estimated model parameter was calculated using the diagonal elements of the covariance matrix. However, these estimates are valid only under some statistical hypotheses which are not always verified with PET data. Thus, in order to verify the validity of the coefficient of variation of each parameter calculated with the covariance matrix, these results have been compared with the more rigorous statistical results provided by a Monte Carlo simulation. The study showed a negligible difference between the results obtained by the two methods for a low noise level in time-concentration curves encountered using large ROIs. However, this bias becomes less negligible when the noise level is high and some estimations of the coefficients of variation were unacceptable (> 100%) with the five-parameter model. Such difficulties did not occur with the four-parameter model which led to parametric images with good quality and acceptable estimates of coefficients of variation (less than 20% in about 75% of the ROIs).

Comparison of two compartmental models for describing receptor ligand kinetics and receptor availability in multiple injection PET studies

The goal of research with receptor ligands and PET is the characterization of an in vivo system that measures rates of association and dissociation of a ligand-receptor complex and the density of available binding sites. It has been suggested that multiple injection studies of radioactive ligand are more likely to identify model parameters than are single injection studies. Typically, at least one of the late injections is at a low specific activity (SA), so that part of the positron emission tomography (PET) curve reflects ligand dissociation. Low SA injections and the attendant reductions in receptor availability, however, may violate tracer kinetic assumptions, namely, tracer may no longer be in steady state with the total (labeled and unlabeled) ligand. Tissue response becomes critically dependent on the dose of total ligand, and an accurate description of the cold ligand in the tissue is needed to properly model the system. Two alternative models have been applied to the receptor modeling problem, which reduces to describing the time-varying number of available receptor sites. The first (Huang et al., 1989) contains only compartments for the hot ligand, 'hot only' (HO), but indirectly accounts for the action of cold ligand at receptor sites via SA. The second stipulates separate compartments for the hot and cold ligands, 'hot and cold' (HC), thus explicitly calculating available number of receptors. We examined these models and contrasted their abilities to predict PET activity, receptor availability, and SA in each tissue compartment. For multiple injection studies, the models consistently predicted different PET activities--especially following the third injection. Only for very high rate constants were the models identical for multiple injections. In one case, simulated PET curves were quite similar, but discrepancies appeared in predictions of receptor availability. The HO model predicted nonphysiological changes in the availability of receptor sites and introduced errors of 30-60% into estimates of B'max for test data. We, therefore, strongly recommend the use of the HC model for all analyses of multiple injection PET studies.

The uptake of Mn-DPDP by hepatocytes is not mediated by the facilitated transport of pyridoxine

Manganese-dipyridoxal diphosphate (Mn-DPDP) is a liver-selective contrast agent selectively taken up by the hepatocytes. Because of the analogy of structure with pyridoxine (vitamin B6), it was previously suggested that this compound can be selectively taken up by the facilitated transport of vitamers B6. To understand the uptake mechanism, an in vivo binding study was performed based on a competition between 54Mn-DPDP and pyridoxine on the one hand, and Mn-DPDP and [3H]pyridoxine on the other. We found that the [3H]pyridoxine levels in the liver were not significantly different 5 min after intravenous administration of several doses of Mn-DPDP (5 nmol/kg to 50 mumol/kg): 5.0 +/- 0.3% of the injected dose/g tissue. The content of 54Mn (administered as 54Mn-DPDP) in the liver was not affected by a saturation dose of pyridoxine (1 mmol/kg) and was found to be constant (+/- 10% of the injected dose/g tissue) for 60 min. These experiments showed that the uptake of Mn-DPDP is not mediated by the transporter of pyridoxine.

Quantification of benzodiazepine receptors in human brain using PET, [11C]flumazenil, and a single-experiment protocol

A kinetic method using a multiinjection protocol, positron emission tomography (PET), and [11C]flumazenil as a specific ligand was used to study in vivo the flumazenil-benzodiazepine receptor interactions in the human brain. The model structure is composed of three compartments (plasma, free, and bound ligand) and five parameters (including the benzodiazepine receptor concentration). The arterial plasma concentration, after correction for metabolites, was used as the input function. The experimental protocol, which consisted of three injections of labeled and/or unlabeled ligand, allowed the evaluation of the five model parameters in various brain regions from a single experiment. In particular, the concentration of receptor sites available for binding (B'max) and the equilibrium dissociation constant (KDVR, VR being the volume of reaction) were estimated in five brain regions, including the pons, in which these parameters are identified for the first time (B'max = 4.7 +/- 1.7 pmol/ml and KDVR = 4.4 +/- 1.3 pmol/ml). Due to the large range of measured receptor concentrations, a linear correlation between B'max and KDVR was pointed out (r = 0.88, p < 0.0005) and was interpreted as a linear relationship between B'max and VR, the parameter KD being assumed constant. This result and its concordance with the published data are discussed. Simulation of the usual two-experiment Scatchard analysis, using the pons as a reference region, showed that the bias on the receptor concentration estimates introduced by this method is significant (from 20 to 40%) but can be corrected using an estimate of the receptor concentration in the pons. Furthermore, we propose a new experimental protocol, based on a Scatchard analysis of the PET data obtained with a partial-saturation experiment. This single-injection protocol is entirely noninvasive, and thus the estimation of the benzodiazepine receptor concentration and of the flumazenil affinity is now possible in human patients using a single 1-h experiment without blood sampling.

A new human (psycho)pharmacology tool: the multiple organs coincidences counter (MOCC)

We describe a novel instrument which is capable of measuring the uptake of radioligand in human organs in vivo with the administration of very small doses of positron-emitting radioligands. This technique readily detects the displacement or reduced uptake of radioligand when a competitive agonist or antagonist is administered. This system provides no tomographic information, but the small radioactive doses involved mean that investigations can be repeated at regular intervals and that female volunteers can also participate. We administered [(11) C]flumazenil, [(11)C]diprenorphine, [(11)C]meta -hydroxyephedrine (MHED) and [(11)C]RTI 55 to healthy male volunteers and performed control, pre-loading and displacement experiments. These demonstrate the feasibility of using this technique to investigate benzodiazepine and opiate receptor occupancy, as well as occupancy at dopamine, noradrenaline and serotonin (5-HT) re-uptake sites. This method is likely to be useful in pharmacokinetic/pharmacodynamic experiments, in drug development and discovery and in the development of novel imaging radioligands.

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.

Benzodiazepine receptor quantification in vivo in humans using [11C]flumazenil and PET: application of the steady-state principle

Carbon-11-labeled flumazenil combined with positron emission tomography (PET) was used to measure the concentration (Bmax) of the benzodiazepine (Bz) receptor in the brain and its equilibrium dissociation constant (KD) for flumazenil in five normal subjects. The steady-state approach was used injecting the tracer as a bolus of high specific activity. In each subject two studies were carried out. The first study was performed at essentially zero receptor occupancy, the tracer alone study. The second study was performed at a steady-state receptor occupancy of about 50%, achieved by a prolonged constant infusion of nonlabeled ("cold") flumazenil starting 2h before the bolus tracer injection and continuing until the end of scanning period. In this second study the free concentration of unmetabolized flumazenil in plasma water was measured in multiple blood samples. The observed tissue and plasma tracer curves, calibrated in the same units of radioactivity per millimeter, were analyzed in two ways: (a) by the noncompartmental (stochastic) approach making no assumptions regarding number of compartments in the tissue, and (b) by the single-compartment approach assuming rapid exchange (mixing) of tracer between all tissue compartments. The noncompartmental and the compartmental analyses gave essentially the same values for the distribution volume of the tracer, the parameter used for quantitation of the Bz receptor. As the compartmental approach could be applied to a shorter observation period (60 min instead of 120 min) it was preferred. The five subjects had a mean KD value of 12 nM/L of water and Bmax values of the grey matter ranging from 39 +/- 11 in thalamus to 120 +/- 14 nM/L of brain in occipital cortex. Most previous studies have been based on the pseudoequilibrium approach using the brain stem as a receptor-free reference region. This yields practically the same KD but lower Bmax values than the steady-state approach presented here.

In vivo high intrinsic efficacy of triazolam: a positron emission tomography study in nonhuman primates

The triazolobenzodiazepine triazolam is a central-type benzodiazepine receptor (BZR) ligand that is widely prescribed as a hypnotic agent. Triazolam produces its effects through potentiation of gamma-aminobutyric acid-mediated neurotransmission. Findings reported from in vitro binding studies showed some discrepancies concerning the pharmacological characteristics of triazolam. The present study aims to characterize in vivo the biochemical properties of triazolam, i.e., cerebral pharmacokinetics, interaction with BZR, potency, and intrinsic efficacy. Triazolam was studied in living nonhuman primates using positron emission tomography. Two different studies were carried out: (a) a direct study using [11C]triazolam and (b) an indirect competition study using the radiolabeled BZR antagonist 1C]flumazenil. Results showed that, in the brain in vivo, triazolam binds specifically and competitively to the BZR. Its rapid cerebral kinetics is consistent with a hypnotic profile (maximal binding after 23 min, elimination half-life of 202 min). Triazolam is very potent in displacing [11C]flumazenil (ID50 = 28 +/- 6 micrograms/kg). Hill analysis of the displacement curve does not show obvious binding-site heterogeneity. Triazolam is 20 times more potent in displacing [11C]flumazenil and 50 times more potent in inhibiting pentylenetetrazol-induced paroxysmal activity than the full benzodiazepine agonist diazepam. Interestingly, the simultaneous use of positron emission tomography and EEG recording allowed us to show that triazolam-positive intrinsic efficacy is slightly higher (20%) than that of diazepam. An attractive hypothesis proposes that the severity of side effects of BZR ligands is proportional to their intrinsic efficacy. Therefore, our study shows that triazolam side effects, as for other benzodiazepines, may be related to its high intrinsic efficacy in vivo.