Norepinephrine transporter occupancy by nortriptyline in patients with depression: a positron emission tomography study with (S,S)-[¹8F]FMeNER-D2.

Norepinephrine transporter (NET) plays important roles in the treatment of various neuropsychiatric disorders, such as depression and attention deficit hyperactivity disorder (ADHD). Nortriptyline is a NET-selective tricyclic antidepressant (TCAs) that has been widely used for the treatment of depression. Previous positron emission tomography (PET) studies have reported over 80% serotonin transporter occupancy with clinical doses of selective serotonin reuptake inhibitors (SSRIs), but there has been no report of NET occupancy in patients treated with relatively NET-selective antidepressants. In the present study, we used PET and (S,S)-[18¹8F]FMeNER-D2 to investigate NET occupancies in the thalamus in 10 patients with major depressive disorder taking various doses of nortriptyline, who were considered to be responders to the treatment. Reference data for the calculation of occupancy were derived from age-matched healthy controls. The result showed approximately 50-70% NET occupancies in the brain as a result of the administration of 75-200 mg/d of nortriptyline. The estimated effective dose (ED50) and concentration (EC50) required to induce 50% occupancy was 65.9 mg/d and 79.8 ng/ml, respectively. Furthermore, as the minimum therapeutic level of plasma nortriptyline for the treatment of depression has been reported to be 70 ng/ml, our data indicate that this plasma nortriptyline concentration corresponds to approximately 50% NET occupancy measured with PET, suggesting that more than 50% of central NET occupancy would be appropriate for the nortriptyline treatment of patients with depression.

A randomized placebo controlled trial demonstrates the effect of dl-methylephedrine on brain functions is weaker than that of pseudoephedrine.

Intellectual drug doping in athletics by using stimulants that affect central nervous system functions has been diversified. Stimulants are regulated by the World Anti-Doping Agency according to their levels of urinary concentration. Positron emission tomography could evaluate how stimulants affect central nervous system functions. We aimed to evaluate the effect of stimulants on brain function by examining the difference in brain dopamine transporter occupancy by PET after administration of dl-methylephedrine or pseudoephedrine at the clinical maximum daily dose. Four PET scans without and with drug administration (placebo, dl-methylephedrine 150 mg and pseudoephedrine 240 mg) were performed. The concentrations of dl-methylephedrine and pseudoephedrine in plasma and urine were measured. DAT occupancies in the striatum with placebo, dl-methylephedrine and pseudoephedrine were calculated by PET images. The urinary concentration of dl-methylephedrine (12.7 µg/mL) exceeded the prohibited concentration (10 µg/mL), but the DAT occupancy with dl-methylephedrine (6.1%) did not differ (p = 0.92) from that with placebo (6.2%). By contrast, although the urinary concentration of pseudoephedrine (144.8 µg/mL) was below the prohibited concentration (150 µg/mL), DAT occupancy with pseudoephedrine was 18.4%, which was higher than that with placebo (p = 0.009). At the maximum clinical dose, dl-methylephedrine was shown to have weaker effects on brain function than pseudoephedrine.

Occupancy of serotonin and norepinephrine transporter by milnacipran in patients with major depressive disorder: a positron emission tomography study with [(11)C]DASB and (S,S)-[(18)F]FMeNER-D(2).

Antidepressants used for treatment of depression exert their efficacy by blocking reuptake at serotonin transporters (5-HTT) and/or norepinephrine transporters (NET). Recent studies suggest that serotonin and norepinephrine reuptake inhibitors that block both 5-HTT and NET have better tolerability than tricyclic antidepressants and may have higher efficacy compared to selective serotonin reuptake inhibitors. Previous positron emission tomography (PET) studies have reported >80% 5-HTT occupancy with clinical doses of antidepressants, but there has been no report of NET occupancy in patients treated with antidepressants. In the present study, we investigated both 5-HTT and NET occupancies by PET using radioligands [(11)C]DASB and (S,S)-[(18)F]FMeNER-D(2), in six patients, each with major depressive disorder (MDD), using various doses of milnacipran. Our data show that mean 5-HTT occupancy in the thalamus was 33.0% at 50 mg, 38.6% at 100 mg, 60.0% at 150 mg and 61.5% at 200 mg. Mean NET occupancy in the thalamus was 25.3% at 25 mg, 40.0% at 100 mg, 47.3% at 125 mg and 49.9% at 200 mg. Estimated ED(50) was 122.5 mg with the dose for 5-HTT and 149.9 mg for NET. Both 5-HTT and NET occupancies were observed in a dose-dependent manner. Both 5-HTT and NET occupancies were about 40% by milnacipran at 100 mg, the dose most commonly administered to MDD patients.

Effects of menopause on brain structural changes in schizophrenia.

AIM: The aim of this study was to investigate the influences of menopause on brain morphological changes in schizophrenia using magnetic resonance imaging (MRI). METHODS: Forty female schizophrenia patients, 20 premenopausal and 20 postmenopausal, and 50 female controls underwent cerebral MRI. Optimized voxel-based morphometry was performed with Statistical Parametric Mapping version 5. RESULTS: Compared with controls, regional gray matter reductions in schizophrenia patients were observed in the insula, superior temporal gyrus, anterior cingulate, parahippocampal gyrus, and thalamus. Direct comparison between the patient groups showed that the gray matter of postmenopausal patients was significantly smaller when compared with premenopausal patients in the left middle frontal gyrus, and no region had significantly lower gray matter volume in premenopausal patients relative to postmenopausal patients. Significant negative correlation between gray matter volume and the interval after menopause was found in the right superior frontal gyrus in the postmenopause patient group. CONCLUSION: Differential morphological alterations between postmenopausal and premenopausal schizophrenia patients were observed, suggesting that the female hormone plays a protective role against schizophrenia.

Depression as a Prodromal Symptom of Neurodegenerative Diseases.

Neurodegenerative diseases can manifest as psychiatric symptoms in the prodromal phase, before the onset of core symptoms such as neurological, motor, and cognitive symptoms. Positron emission tomography (PET) has made it possible to detect the pathology of some neurodegenerative diseases in vivo. Many studies have indicated that depression is a preclinical symptom of neurodegenerative diseases. Approximately 10% of non-demented participants with depression developed Alzheimer's disease (AD) during the follow-up period. The prevalence of depression/dysphoria was 42.9% in the preclinical stage of dementia with Lewy bodies. Depression was present in 33.3% of patients with preclinical behavioral-variant frontotemporal lobar degeneration. Approximately 10% of patients had a history of depression at the time of diagnosis with Parkinson's disease. PET studies have revealed the pathology of neurodegenerative diseases in some cases of geriatric depression. Increased brain amyloid-beta deposition in late-onset depression is a possible reflection of prodromal AD. The severity of depression was significantly associated with greater inferior temporal tau and marginally associated with greater entorhinal cortex tau, and depression was associated with significantly greater mean cortical tau deposition. Thus, the presence of depression as a preclinical/prodromal symptom of neurodegenerative diseases has been demonstrated by epidemiological, pathological, and biomarker studies. A growing body of evidence from PET studies indicates that some cases of geriatric depression have pathologies of degenerative neurological disease. In the future, it is expected that PET will be utilized as an imaging biomarker for diagnosis of psychiatric disorders and development of new therapeutic agents.

Effect of DL-Methylephedrine on Dopamine Transporter Using Positron Emission Tomography With [18F]FE-PE2I.

Rationale: Since ephedrine has a dopamine transporter (DAT) inhibitory effect similar to amphetamine, dl-methylephedrine, a derivative of ephedrine, is considered to have the characteristics of a central nervous system stimulant due to the DAT inhibitory effect. For example, the World Anti-Doping Agency categorizes dl-methylephedrine as a stimulant in the prohibited list for competitions. Assuming to have the same effect as ephedrine, the urinary concentration of dl-methylephedrine is regulated below 10 µg/mL, as is ephedrine. However, the extent to which dl-methylephedrine affects brain function is not yet fully understood. Objectives: The purpose of this study was to evaluate DAT occupancy by a single oral administration of a daily dose of dl-methylephedrine using positron emission tomography (PET) with [18F]FE-PE2I to characterize its stimulatory effect on the central nervous system. Methods: Nine healthy male volunteers were enrolled in the study. The experiments were designed as a placebo-controlled randomized double-blind crossover comparative study. After the first PET scan in a drug-free state, the second and third PET scans were performed with randomized dosing at 60 mg of dl-methylephedrine or placebo. The plasma and urine concentrations of dl-methylephedrine were measured just before and after the PET scans, respectively. Results: Mean urine and plasma concentrations of dl-methylephedrine were 13.9 µg/mL and 215.2 ng/mL, respectively. Mean DAT occupancy in the caudate was 4.4% for dl-methylephedrine and 1.2% for placebo. Mean DAT occupancy in the putamen was 3.6% for dl-methylephedrine and 0.5% for placebo. There was no significant difference of DAT occupancies between the groups. Conclusion: In this study, the urinary concentration of dl-methylephedrine (13.9 µg/mL) was higher than the prohibited reference value (10.0 µg/mL), and there was no significant difference in DAT occupancy between dl-methylephedrine and placebo. These findings suggest that a clinical daily dose of dl-methylephedrine may exceed the doping regulation value according to urine concentration; however, it was considered that at least the central excitatory effect mediated by DAT inhibition was not observed at the daily dose of dl-methylephedrine.

A positron emission tomography study of the serotonin1B receptor effect of electroconvulsive therapy for severe major depressive episodes.

BACKGROUND: Electroconvulsive therapy (ECT) is an effective treatment for depressive disorders, although its molecular mechanism of action is unknown. The serotonin 1B (5-HT1B) receptor is a potential target for treatment of depression and low 5-HT1B receptor binding in limbic regions has been reported in previous positron emission tomography (PET) studies of depression. METHODS: The objective of this longitudinal PET study was to examine the effect of ECT for depression on 5-HT1B receptor binding. Fifteen hospitalized patients with major depressive episodes were examined with PET and the 5-HT1B receptor selective radioligand [11C]AZ10419369, before and after ECT. Fifteen controls matched for age and sex were examined. Limbic regions with previously reported low 5-HT1B receptor binding in depression and a dorsal brain stem region were selected. RESULTS: Thirteen patients completed the study according to protocol. Eleven out of thirteen patients responded to ECT. 5-HT1B receptor binding in hippocampus increased with 30 % after ECT (p=0.021). Using linear mixed effects modelling, we observed increases in 5-HT1B receptor binding following ECT with a moderate to large effect size, which did not differ significantly between regions. In an exploratory analysis, strong correlations between changes in 5-HT1B receptor binding and agitation scores on the Hamilton Depression Rating Scale after ECT were observed. LIMITATIONS: Albeit representative of a PET study, the sample size is still small and there are potential confounding effects of medication. CONCLUSIONS: Increased 5-HT1B receptor binding was observed following ECT for depression, corresponding to previous findings of increased 5-HT1B receptor binding in hippocampus after rapid acting ketamine for treatment resistant depression.

The Repeatability of Values Measured Using the Spot Vision Screener in Healthy Children and Children with Refractive Errors.

Purpose: To determine the repeatability of refractive values measured using the Spot vision screener in healthy children and children with refractive errors. Methods: This cross-sectional study included 170 eyes of 85 healthy children (normal group), and 50 eyes of 25 children wearing spectacles for refractive errors (spectacles group). The spherical equivalent value, cylindrical value, and inter-ocular differences in the spherical equivalent values were analyzed. The repeatability of the refractive values measured using the Spot vision screener was determined using intra-class correlation coefficients (ICCs) and 95% limits of agreement. Results: In the normal group, ICC values for the spherical equivalent value, cylindrical value, and amount of anisometropia were 0.77, 0.80, and 0.64, respectively. In the spectacles group with spectacles they were 0.89, 0.67, and 0.78, respectively. In the spectacles group without spectacles they were 0.96, 0.78, and 0.97, respectively. The 95% limits of agreement between consecutive measurements for the spherical equivalent value, cylindrical value, and amount of anisometropia were within ±1.00 D in the normal group and the spectacles group with spectacles, and over ±1.00 D in the spectacles group without spectacles. Conclusions: The Spot vision screener can ensure better than moderate repeatability with and without spectacles.

Potential for imaging the high-affinity state of the 5-HT1B receptor: a comparison of three PET radioligands with differing intrinsic activity.

BACKGROUND: Over the last decade, a few radioligands have been developed for PET imaging of brain 5-HT1B receptors. The 5-HT1B receptor is a G-protein-coupled receptor (GPCR) that exists in two different agonist affinity states. An agonist ligand is expected to be more sensitive towards competition from another agonist, such as endogenous 5-HT, than an antagonist ligand. It is of interest to know whether the intrinsic activity of a PET radioligand for the 5-HT1B receptor impacts on its ability to detect changes in endogenous synaptic 5-HT density. Three high-affinity 11C-labeled 5-HT1B PET radioligands with differing intrinsic activity were applied to PET measurements in cynomolgus monkey to evaluate their sensitivity to be displaced within the brain by endogenous 5-HT. For these experiments, fenfluramine was pre-administered at two different doses (1.0 and 5.0 mg/kg, i.v.) to induce synaptic 5-HT release. RESULTS: A dose-dependent response to fenfluramine was detected for all three radioligands. At the highest dose of fenfluramine (5.0 mg/kg, i.v.), reductions in specific binding in the occipital cortex increased with radioligand agonist efficacy, reaching 61% for [11C]3. The most antagonistic radioligand showed the lowest reduction in specific binding. CONCLUSIONS: Three 5-HT1B PET radioligands were identified with differing intrinsic activity that could be used in imaging high- and low-affinity states of 5-HT1B receptors using PET. From this limited study, radioligand sensitivity to endogenous 5-HT appears to depend on agonist efficacy. More extensive studies are required to substantiate this suggestion.

Development of a 18F-labeled PET radioligand for imaging 5-HT1B receptors: [18F]AZ10419096.

INTRODUCTION: In the last decade PET has been useful in studying and understanding the 5-HT1B receptor. [11C]AZ10419369 and [11C]P943 have been applied as radioligands in these studies. Both use carbon-11 (t1/2 = 20.4 min) as radionuclide, which limits the application to PET centres that have an on-site cyclotron and radiochemistry facilities. In this paper we report the synthesis and initial evaluation of the first fluorine-18 PET radioligand to image 5-HT1B receptors in brain, [18F]AZ10419096. MATERIALS AND METHODS: A boronate-precursor for [18F]AZ10419096 was synthesized from an intermediate provided by AstraZeneca and was labeled with fluorine 18 using Cu-mediated radio-fluorination. [18F]AZ10419096 was used in PET baseline and pretreatment measurements in nonhuman primates. PET data were analyzed using SRTM using the cerebellum as reference region. Blood samples for radio-metabolite analysis were collected during PET measurements. RESULTS: Radio-fluorination gave [18F]AZ10419096 in sufficient amounts and molar activity and with high radiochemical purity to be applied in PET measurements. In a baseline PET measurement [18F]AZ10419096 showed a high brain uptake and regional distribution consistent with reported 5-HT1B receptor densities. In a pretreatment PET measurement, AR-A000002 (2.0 mg/kg) blocked the binding of [18F]AZ10419096 to 5-HT1B receptors in occipital cortex by 80%, thereby demonstrating high specific binding. CONCLUSION: [18F]AZ10419096 is the first fluorine-18 PET radioligand for imaging 5-HT1B receptors in vivo with high specific binding and binding potential. [18F]AZ10419096 is a candidate for further development for use in clinical PET studies.

Relation between dopamine synthesis capacity and cell-level structure in human striatum: a multi-modal study with positron emission tomography and diffusion tensor imaging.

Positron emission tomography (PET) study has shown that dopamine synthesis capacity varied among healthy individuals. This interindividual difference might be due to a difference in the cell-level structure of presynaptic dopaminergic neurons, i.e., cellular density and/or number. In this study, the relations between the dopamine synthesis capacity measured by PET and the parameter estimates in diffusion tensor imaging (DTI) in striatal subregions were investigated in healthy human subjects. DTI and PET studies with carbon-11 labeled L-DOPA were performed in ten healthy subjects. Age-related changes in the above parameters were also considered. Fractional anisotropy showed a significant positive correlation with age in the posterior caudate. There was significant negative correlation between dopamine synthesis capacity and mean diffusivity in the posterior caudate and putamen. Assuming that mean diffusivity reflects the density of wide-spreading axonal terminals in the striatum, the result suggests that dopamine synthesis may be related to the density of dopaminergic neuronal fibers. It is evident that PET/DTI combined measurements can contribute to investigations of the pathophysiology of neuropsychiatric diseases involving malfunction of dopaminergic neurons.

Reproducibility of PET measurement for presynaptic dopaminergic functions using L-[ß-(11)C]DOPA and [(18)F]FE-PE2I in humans.

OBJECTIVE: Recent PET studies have indicated altered presynaptic function and relation with psychotic symptoms in patients with schizophrenia. The L-[ß-(11)C]DOPA uptake rate reflects the dopamine synthesis capacity (kref), whereas the nondisplaceable binding potential (BPND) of [(18)F]FE-PE2I reflects the dopamine transporter availability. Although the kref values of L-[ß-(11)C]DOPA and the BPND of [(18)F]FE-PE2I can be potential markers for evaluating the severity of positive symptoms, test-retest reproducibility has not been confirmed. The purpose of this study was to investigate the test-retest reproducibility of kref values of L-[ß-(11)C]DOPA and that of BPND of [(18)F]FE-PE2I in the striatum and midbrain in healthy humans. MATERIALS AND METHODS: Twelve healthy male volunteers underwent two PET studies on separate days. Each PET study comprised two PET scans, one with L-[ß-(11)C]DOPA and the other with [(18)F]FE-PE2I. Volumes of interest were defined for the caudate, putamen, midbrain, and thalamus. Test-retest reproducibility was assessed in terms of intrasubject variability (absolute variability) and reliability [intraclass correlation coefficient (ICC)]. RESULTS: The absolute variability values of kref and BPND were 4.8-25.7% on average for the caudate, putamen, midbrain, and thalamus. The ICC values of the kref values of L-[ß-(11)C]DOPA were 0.78, 0.71, 0.77, and 0.77 for the caudate, putamen, midbrain, and thalamus, respectively. The ICC values of the BPND of [(18)F]FE-PE2I were 0.83, 0.88, 0.71, and 0.70 for the caudate, putamen, midbrain, and thalamus, respectively. CONCLUSION: We found good test-retest reproducibility for the kref values of L-[ß-(11)C]DOPA and that for the BPND of [(18)F]FE-PE2I in the striatum and midbrain, indicating the reliability of clinical investigation using PET with L-[ß-(11)C]DOPA and [(18)F]FE-PE2I.

Quantitative Analysis of Amyloid Deposition in Alzheimer Disease Using PET and the Radiotracer ¹¹C-AZD2184.

UNLABELLED: Characteristic neuropathologic changes in Alzheimer disease (AD) are amyloid-ß deposits and neurofibrillary tangles. Recently, a new radioligand for amyloid senile plaques, (11)C-labeled 5-(6-{[tert-butyl(dimethyl)silyl]oxy}-1,3-benzothiazol-2-yl)pyridin-2-amine ((11)C-AZD2184), was developed, and it was reported to show rapid brain uptake followed by rapid washout. In this study, (11)C-AZD2184 binding in control subjects and AD patients was examined in more detail by compartment model analysis using a metabolite-corrected arterial input function. The accuracy of simplified quantitative methods using a reference brain region was also evaluated. METHODS: After intravenous bolus injection of (11)C-AZD2184, a dynamic PET scan was obtained for 90 min in 6 control subjects and 8 AD patients. To obtain the arterial input function, arterial blood sampling and high-performance liquid chromatography analysis were performed. RESULTS: Time-activity curves in all brain regions could be described using the standard 2-tissue-compartment model. The total distribution volume ratios to reference region (DVR) in cerebral cortical regions were significantly higher in AD patients than in control subjects. Although there was no conspicuous accumulation of radioactivity in white matter as compared with other amyloid radioligands, DVR values in the centrum semiovale were more than 1 for both control subjects and AD patients, suggesting binding to myelin. The standardized uptake value ratio calculated from integrated time-activity curves in brain regions and the reference region was statistically in good agreement with DVR. CONCLUSION: Although the white matter binding of (11)C-AZD2184 may have some effect on cortical measurement, it can be concluded that the kinetic behavior of (11)C-AZD2184 is suitable for quantitative analysis. The standardized uptake value ratio can be used as a validated measure of (11)C-AZD2184 binding in clinical examinations without arterial input function.

Effects of dopamine D2 receptor partial agonist antipsychotic aripiprazole on dopamine synthesis in human brain measured by PET with L-[ß-11C]DOPA.

Dopamine D(2) receptor partial agonist antipsychotic drugs can modulate dopaminergic neurotransmission as functional agonists or functional antagonists. The effects of antipsychotics on presynaptic dopaminergic functions, such as dopamine synthesis capacity, might also be related to their therapeutic efficacy. Positron emission tomography (PET) was used to examine the effects of the partial agonist antipsychotic drug aripiprazole on presynaptic dopamine synthesis in relation to dopamine D(2) receptor occupancy and the resulting changes in dopamine synthesis capacity in healthy men. On separate days, PET studies with [(11)C]raclopride and L-[ß-(11)C]DOPA were performed under resting condition and with single doses of aripiprazole given orally. Occupancy of dopamine D(2) receptors corresponded to the doses of aripiprazole, but the changes in dopamine synthesis capacity were not significant, nor was the relation between dopamine D(2) receptor occupancy and these changes. A significant negative correlation was observed between baseline dopamine synthesis capacity and changes in dopamine synthesis capacity by aripiprazole, indicating that this antipsychotic appears to stabilize dopamine synthesis capacity. The therapeutic effects of aripiprazole in schizophrenia might be related to such stabilizing effects on dopaminergic neurotransmission responsivity.

Quantification of dopamine transporter in human brain using PET with 18F-FE-PE2I.

UNLABELLED: (18)F-(E)-N-(3-iodoprop-2E-enyl)-2ß-carbofluoroethoxy-3ß-(4-methylphenyl)nortropane ((18)F-FE-PE2I) is a new PET radioligand with a high affinity and selectivity for the dopamine transporter (DAT). In nonhuman primates, (18)F-FE-PE2I showed faster kinetics and less production of radiometabolites that could potentially permeate the blood-brain barrier than did (11)C-PE2I. The aims of this study were to examine the quantification of DAT using (18)F-FE-PE2I and to assess the effect of radiometabolites of (18)F-FE-PE2I on the quantification in healthy humans. METHODS: A 90-min dynamic PET scan was obtained for 10 healthy men after intravenous injection of (18)F-FE-PE2I. Kinetic compartment model analysis with a metabolite-corrected arterial input function was performed. The effect of radiometabolites on the quantification was evaluated by time-stability analyses. The simplified reference tissue model (SRTM) method with the cerebellum as a reference region was evaluated as a noninvasive method of quantification. RESULTS: After the injection of (18)F-FE-PE2I, the whole-brain radioactivity showed a high peak (∼3-5 standardized uptake value) and fast washout. The radioactive uptake of (18)F-FE-PE2I in the brain was according to the relative density of the DAT (striatum > midbrain > thalamus). The cerebellum showed the lowest uptake. Tissue time-activity curves were well described by the 2-tissue-compartment model (TCM), as compared with the 1-TCM, for all subjects in all regions. Time stability analysis showed stable estimation of total distribution volume with 60-min or longer scan durations, indicating the small effect of radiometabolites. Binding potentials in the striatum and midbrain were well estimated by the SRTM method, with modest intersubject variability. Although the SRTM method yielded a slight underestimation and overestimation in regions with high and low DAT densities, respectively, binding potentials by the SRTM method were well correlated to the estimates by the indirect kinetic method with 2-TCM. CONCLUSION: (18)F-FE-PE2I is a promising PET radioligand for quantifying DAT. The binding potentials could be reliably estimated in both the striatum and midbrain using both the indirect kinetic and SRTM methods with a scan duration of 60 min. Although radiometabolites of (18)F-FE-PE2I in plasma possibly introduced some effects on the radioactivity in the brain, the effects on estimated binding potential were likely to be small.

Striatal and extrastriatal dopamine D2 receptor occupancy by the partial agonist antipsychotic drug aripiprazole in the human brain: a positron emission tomography study with [¹¹C]raclopride and [¹¹C]FLB457.

RATIONALE: Second-generation antipsychotics demonstrate clinical efficacy with fewer extrapyramidal side effects compared with first-generation antipsychotics. One of the proposed explanations is the hypothesis of preferential extrastriatal dopamine D2 receptor occupancy (limbic selectivity) by antipsychotics. In the present study, we focused on aripiprazole, which has a unique pharmacological profile with partial agonism at dopamine D2 receptors and the minimal risk of extrapyramidal side effects. Previous positron emission tomography (PET) studies using high-affinity radioligands for dopamine D2 receptors have reported inconsistent results regarding regional differences of dopamine D2 receptor occupancy by aripiprazole. OBJECTIVE: To test the hypothesis of preferential binding to extrastriatal dopamine D2 receptors by aripiprazole, we investigated its regional dopamine D2 receptor occupancies in healthy young subjects. MATERIALS AND METHODS: Using PET and two radioligands with different affinities for dopamine D2 receptors, [¹¹C]raclopride and [¹¹C]FLB457, striatal and extrastriatal dopamine D2 receptor bindings at baseline and after oral administration of 6 mg aripiprazole were measured in 11 male healthy subjects. RESULTS: Our data showed that dopamine D2 receptor occupancies in the striatum measured with [¹¹C]raclopride were 70.1% and 74.1%, with the corresponding values for the extrastriatal regions measured with [¹¹C]FLB457 ranging from 46.6% to 58.4%. CONCLUSIONS: In the present study, preferential extrastriatal dopamine D2 receptor occupancy by aripiprazole was not observed. Our data suggest partial agonism at dopamine D2 receptors is the most likely explanation for the minimal risk of extrapyramidal side effects in the treatment by aripiprazole.

Central nervous system drug evaluation using positron emission tomography.

In conventional pharmacological research in the field of mental disorders, pharmacological effect and dose have been estimated by ethological approach and in vitro data of affinity to the site of action. In addition, the frequency of administration has been estimated from drug kinetics in blood. However, there is a problem regarding an objective index of drug effects in the living body. Furthermore, the possibility that the concentration of drug in blood does not necessarily reflect the drug kinetics in target organs has been pointed out. Positron emission tomography (PET) techniques have made progress for more than 20 years, and made it possible to measure the distribution and kinetics of small molecule components in living brain. In this article, we focused on rational drug dosing using receptor occupancy and proof-of-concept of drugs in the drug development process using PET.