cAMP signaling in brain is decreased in unmedicated depressed patients and increased by treatment with a selective serotonin reuptake inhibitor.

Basic studies exploring the importance of the cyclic adenosine monophosphate (cAMP) cascade in major depressive disorder (MDD) have noted that the cAMP cascade is downregulated in MDD and upregulated by antidepressant treatment. We investigated cAMP cascade activity by using 11C-(R)-rolipram to image phosphodiesterase-4 (PDE4) in unmedicated MDD patients and after ~8 weeks of treatment with a selective serotonin reuptake inhibitor (SSRI). 11C-(R)-rolipram positron emission tomographic (PET) scans were performed in 44 unmedicated patients during a major depressive episode and 35 healthy controls. Twenty-three of the 44 patients had a follow-up 11C-(R)-rolipram PET scan ~8 weeks after treatment with an SSRI. Patients were moderately depressed (Montgomery-Åsberg Depression Rating Scale=30±6) and about half were treatment naïve. 11C-(R)-rolipram binding was measured using arterial sampling to correct for individual differences in radioligand metabolism. We found in unmedicated MDD patients widespread, ~20% reductions in 11C-(R)-rolipram binding compared with controls (P=0.001). SSRI treatment significantly increased rolipram binding (12%, P<0.001), with significantly greater increases observed in older patients (P<0.001). Rolipram binding did not correlate with severity of baseline symptoms, and increased rolipram binding during treatment did not correlate with symptom improvement. In brief, consistent with the results of basic studies, PDE4 was decreased in unmedicated MDD patients and increased after SSRI treatment. The lack of correlation between PDE4 binding and depressive symptoms could reflect the heterogeneity of the disease and/or the heterogeneity of the target, given that PDE4 has four subtypes. These results suggest that PDE4 inhibitors, which increase cAMP cascade activity, may have antidepressant effects.

Reduced cannabinoid CB1 receptor binding in alcohol dependence measured with positron emission tomography.

Brain cannabinoid CB1 receptors contribute to alcohol-related behaviors in experimental animals, but their potential role in humans with alcohol dependence is poorly understood. We measured CB1 receptors in alcohol dependent patients in early and protracted abstinence, and in comparison with control subjects without alcohol use disorders, using positron emission tomography and [(18)F]FMPEP-d2, a radioligand for CB1 receptors. We scanned 18 male in-patients with alcohol dependence twice, within 3-7 days of admission from ongoing drinking, and after 2-4 weeks of supervised abstinence. Imaging data were compared with those from 19 age-matched healthy male control subjects. Data were also analyzed for potential influence of a common functional variation (rs2023239) in the CB1 receptor gene (CNR1) that may moderate CB1 receptor density. On the first scan, CB1 receptor binding was 20-30% lower in patients with alcohol dependence than in control subjects in all brain regions and was negatively correlated with years of alcohol abuse. After 2-4 weeks of abstinence, CB1 receptor binding remained similarly reduced in these patients. Irrespective of the diagnostic status, C allele carriers at rs2023239 had higher CB1 receptor binding compared with non-carriers. Alcohol dependence is associated with a widespread reduction of cannabinoid CB1 receptor binding in the human brain and this reduction persists at least 2-4 weeks into abstinence. The correlation of reduced binding with years of alcohol abuse suggests an involvement of CB1 receptors in alcohol dependence in humans.

Reversible and regionally selective downregulation of brain cannabinoid CB1 receptors in chronic daily cannabis smokers.

Chronic cannabis (marijuana, hashish) smoking can result in dependence. Rodent studies show reversible downregulation of brain cannabinoid CB(1) (cannabinoid receptor type 1) receptors after chronic exposure to cannabis. However, whether downregulation occurs in humans who chronically smoke cannabis is unknown. Here we show, using positron emission tomography imaging, reversible and regionally selective downregulation of brain cannabinoid CB(1) receptors in human subjects who chronically smoke cannabis. Downregulation correlated with years of cannabis smoking and was selective to cortical brain regions. After ∼4 weeks of continuously monitored abstinence from cannabis on a secure research unit, CB(1) receptor density returned to normal levels. This is the first direct demonstration of cortical cannabinoid CB(1) receptor downregulation as a neuroadaptation that may promote cannabis dependence in human brain.

Scintigraphic detection of metastatic melanoma using indium 111/DTPA conjugated anti-gp240 antibody (ZME-018).

We evaluated the toxicity, pharmacokinetics, and localization of a monoclonal IgG2 alpha murine anti-human melanoma (gp240) antibody (ZME-018) that recognizes a tumor-associated cell surface glycoprotein of 240,000 molecular weight present in most melanomas. The antibody was conjugated with DTPA (diethylenetriamine pentaacetic acid) and labeled by chelation of 111In. One mg of antibody labeled with 5 mCi of 111In was infused, together with 0 to 40 mg of "cold" carrier ZME-018. The blood clearance, urinary excretion, and in vivo localization were determined in 26 patients. Scintigraphic images were obtained at 24 hours and 72 hours in all patients. Mild toxicity occurred in one patient. The half-time clearance of labeled monoclonal murine antibody (MoAb) from the blood increased from 16.1 hours at an antibody dose of 1 mg to 35.9 hours at 40 mg. Males showed faster clearance from the blood than did females or a single castrated male, perhaps due to selective concentration of antibody in the testes. Nonspecific uptake in liver, spleen, bone marrow, and intestine was seen in all patients. The percentage of known metastatic foci detected increased with the total dosage of antibody, from 23% at doses less than or equal to 5 mg, to 65%, 87% and 78% for 10, 20, and 40 mg, respectively. We conclude that at doses of greater than or equal to 10 mg, ZME-018 is a safe and potentially useful agent for the scintigraphic detection of metastatic malignant melanoma.

Graphical, kinetic, and equilibrium analyses of in vivo [123I] beta-CIT binding to dopamine transporters in healthy human subjects.

The in vivo kinetics of the dopamine (DA) transporter probe 123I-labeled 2 beta-carboxymethoxy-3 beta-(4-iodophenyl) tropane ([123I] beta-CIT) in striatum was investigated with single-photon emission computerized tomography (SPECT) in five healthy human subjects. The aim of this study was to derive an adequate measure of the DA transporter density that would not be affected by regional cerebral blood flow or peripheral clearance of the tracer. SPECT data were acquired on the day of injection (day 1) from 0 to 7 h and on the following day (day 2) from 19 to 25 h. Arterial sampling on day 1 was used to measure the input function. Graphical, kinetic, and equilibrium analyses were evaluated. Graphical analysis of day 1 data, with the assumption of negligible dissociation of the tracer-receptor complex (k4 = 0), was found to be blood flow-dependent. A three-compartment kinetic analysis of day 1 data were performed using a three (k4 = 0)- and a four (k4 > 0)-parameter model. The three-parameter model estimated the konBmax product at 0.886 +/- 0.087 min-1. The four-parameter model gave a binding potential (BP) of 476 ml g-1, a value consistent with in vitro measurements. The stability of the regional uptake on day 2 allowed direct measurement of the specific to nonspecific equilibrium partition coefficient (V3" = k3/k4 = 6.66 +/- 1.54). Results of day 1 kinetic analysis and day 2 equilibrium analysis were well correlated among subjects. Simulations indicated that the error associated with the day 2 equilibrium analysis was acceptable for plasma tracer terminal half-lives > 10 h. We propose the equilibrium analysis on day 2 as the method of choice for clinical studies since it does not require multiple scans or the measurement of the arterial plasma tracer concentrations.

SPECT quantification of [123I]iomazenil binding to benzodiazepine receptors in nonhuman primates: I. Kinetic modeling of single bolus experiments.

The aim of this work was to study the feasibility and reproducibility of in vivo measurement of benzodiazepine receptors with single photon emission computerized tomography (SPECT) in the baboon brain. Arterial and brain regional activities were measured for 420 min in three baboons after single bolus injection of the benzodiazepine antagonist [123I]iomazenil. Data were fit to a three-compartment model to derive the regional binding potential (BP), which corresponds to the product of the receptor density, (Bmax) and affinity (1/KD). Regional BP values (from 114 in striatum to 241 in occipital) were in good agreement with values predicted from in vitro studies. Constraining the regional volume of distribution of the nondisplaceable compartment to the value measured during tracer constant infusion experiments in baboons (Laruelle et al., 1993) improved the identifiability of the rate constants. Each experiment was repeated to investigate the reproducibility of the measurement. The regional average reproducibility was 10 +/- 5%, expressed as coefficient of variation (CV). Results of equilibrium analysis at peak uptake were in good agreement with results of kinetic analysis. Empirical counts ratio methods were found to be poorly sensitive to benzodiazepine receptor density. These studies suggest the feasibility of quantitative measurement of benzodiazepine receptors by kinetic analysis of SPECT data and the inadequacy of empirical methods of analysis, such as counts ratios, to evaluate differences in receptor density.

SPECT quantification of [123I]iomazenil binding to benzodiazepine receptors in nonhuman primates: II. Equilibrium analysis of constant infusion experiments and correlation with in vitro parameters.

In vivo benzodiazepine receptor equilibrium dissociation constant, KD, and maximum number of binding sites, Bmax, were measured by single photon emission computerized tomography (SPECT) in three baboons. Animals were injected with a bolus followed by a constant i.v. infusion of the high affinity benzodiazepine ligand [123I]iomazenil. Plasma steady-state concentration and receptor-ligand equilibrium were reached within 2 and 3 h, respectively, and were sustained for the duration (4-9 h) of the experiments (n = 15). At the end of the experiments, a receptor saturating dose of flumazenil (0.2 mg/kg) was injected to measure nondisplaceable activity. Experiments were carried out at various levels of specific activity, and Scatchard analysis was performed for derivation of the KD (0.59 +/- 0.09 nM) and Bmax (from 126 nM in the occipital region to 68 nM in the striatum). Two animals were killed and [125I]iomazenil Bmax and KD were measured at 22 and 37 degrees C on occipital homogenate membranes. In vitro values of Bmax (114 +/- 33 nM) and 37 degrees C KD (0.66 +/- 0.16 nM) were in good agreement with in vivo values measured by SPECT. This study demonstrates that SPECT can be used to quantify central neuroreceptors density and affinity.

Age-related decline in central serotonin transporter availability with [(123)I]beta-CIT SPECT.

Postmortem studies have provided limited and conflicting data regarding aging effects on the central serotonin transporter (SERT). The present study investigated the effect of age on SERT availability in the human brainstem and diencephalon with single photon emission computed tomography (SPECT) using the ligand [(123)I]2 beta-carbomethoxy-3 beta-(4-iodophenyl)tropane ([(123)I]beta-CIT). Healthy control subjects (n = 126) who ranged in age from 18 to 88 were injected with 6.0 +/- 0.8 (mean +/- SD) mCi [(123)I]beta-CIT and imaged 23.1 +/- 1.9 h later under equilibrium conditions. A ratio of specific to nondisplaceable brain uptake (i.e. , V(3)" = [brainstem-diencephalon -occipital]/occipital), a measure proportional to the binding potential (B(max)/K(D)), was derived. SERT availability (V(3)") showed a significant inverse correlation with age (r = -0.40, P < 0.0001). Linear regression analysis revealed that V(3)" declined by 29.5% over the age range 18 to 88, or approximately 4.2% per decade. These results demonstrate reductions in the availability of central SERT binding sites with age in living human subjects.

Brain SPECT imaging of amphetamine-induced dopamine release in euthymic bipolar disorder patients.

OBJECTIVE: Increased dopaminergic neurotransmission has been implicated in the pathophysiology of bipolar disorder. However, it remains unclear whether the abnormality is due to increased dopamine release or enhanced postsynaptic receptor sensitivity. In this study, dopamine receptor imaging combined with a pharmacological challenge of amphetamine was used to assess both pre- and postsynaptic aspects of dopamine neurotransmission in euthymic bipolar disorder patients. METHOD: Thirteen patients with bipolar disorder (seven medication free and six receiving mood stabilizer therapy) who had been euthymic for more than 4 weeks and 13 age- and gender-matched healthy comparison subjects were included in the study. Single photon emission computed tomography scans were obtained with the striatal dopamine (D(2)/D(3)) receptor radiotracer iodobenzamide ([(123)I]IBZM) before and after an intravenous amphetamine challenge (0.3 mg/kg). Reduction in striatal [(123)I]IBZM binding potential from the first scan to the second scan was used as an indirect measure of the amount of dopamine released. Behavioral response to amphetamine was measured with the Brief Psychiatric Rating Scale, Young Mania Rating Scale, and visual analogue scales. RESULTS: Bipolar patients and healthy subjects did not differ in terms of mood state or striatal D(2) receptor binding at baseline. Amphetamine challenge led to a significantly greater behavioral response in bipolar patients than in healthy subjects. However, there was no significant difference between the two groups in the amphetamine-induced decrease in striatal [(123)I]IBZM binding. CONCLUSIONS: In a group of euthymic patients with bipolar disorder, this study did not find evidence for increased striatal dopamine release. Instead, these data are consistent with enhanced postsynaptic dopamine responsivity in patients with bipolar disorder.

Elevated central serotonin transporter binding availability in acutely abstinent cocaine-dependent patients.

OBJECTIVE: Recent work has underscored the role of serotonergic neurotransmission in chronic neural adaptations to cocaine dependence. The authors tested for evidence of serotonergic dysfunction during acute abstinence from cocaine, a period of high risk for relapse in cocaine dependence. METHOD: Binding availability of dopamine transporters and serotonin transporters was measured in 15 cocaine-dependent subjects during acute abstinence and in 37 healthy comparison subjects by using [(123)I]beta-CIT and single photon emission computed tomography. RESULTS: Significant increases in diencephalic and brainstem serotonin transporter binding (16.7% and 31.6%, respectively) were observed in cocaine-dependent subjects. Brainstem serotonin transporter binding was significantly inversely correlated with age across diagnostic groups. CONCLUSIONS: These findings provide further evidence of serotonergic dysfunction during acute abstinence from chronic cocaine use. Age-related decline in brainstem serotonin transporter binding may underlie the poor response to selective serotonin reuptake inhibitor antidepressants seen in some elderly depressed patients.

Prediction of dopamine transporter binding availability by genotype: a preliminary report.

OBJECTIVE: Evidence of a relationship between genotype and binding availability was assessed for the dopamine and serotonin transporter genes. METHOD: The authors assessed dopamine transporter genotype at the SLC6A3 3' variable number of tandem repeats (VNTR) polymorphism and serotonin transporter genotype at the SLC6A4 promotor VNTR polymorphism in 30 healthy subjects who also underwent single photon emission computed tomography with [(123)I]beta-CIT. RESULTS: Subjects homozygous for the 10-repeat allele at the SLC6A3 locus demonstrated significantly lower dopamine transporter binding than carriers of the nine-repeat allele. There was no effect of SLC6A4 genotype upon serotonin transporter binding. CONCLUSIONS: These findings suggest that genetic variation at the SLC6A3 3' VNTR polymorphism may modify dopamine transporter function.

"Central" and "peripheral" benzodiazepine receptors: opposite changes in human epileptogenic tissue.

We measured the density of two benzodiazepine (BZ) receptor subtypes in neurosurgically obtained hippocampal tissue from the seizure focus of patients with temporal lobe epilepsy (TLE) showing mesial temporal sclerosis, the most common pathologic finding in TLE. We performed quantitative in vitro receptor autoradiography with [125I]Ro 16-0154, a probe for the central-type BZ receptor and with [3H]PK 11195, a probe for the peripheral-type BZ receptor. In comparison with autopsy and neurosurgical control groups, patients with mesial temporal sclerosis had regionally selective decreased central-type and increased peripheral-type BZ receptors. These changes paralleled regional losses of neurons and proliferation of glia. Decreases of the inhibitory central-type BZ receptor may be a component of the enhanced excitability of the seizure focus and also may allow localization of the focus by in vivo neuroreceptor imaging. Single photon emission computed tomography (SPECT) imaging of two TLE patients with [123I]Ro 16-0154 suggests that this technique may provide a more sensitive means of localizing the seizure focus than current imaging methods relying on changes in blood flow or glucose metabolism.

[123I] beta-CIT/SPECT imaging demonstrates bilateral loss of dopamine transporters in hemi-Parkinson's disease.

We have used in vivo single-photon emission computed tomography (SPECT) of the dopamine transporter with 2 beta-carboxymethoxy-3 beta-(4-iodophenyl)tropane ([123I] beta-CIT) to investigate striatal dopamine transporter loss in patients with early Parkinson's disease (PD). Striatal uptake of ([123I] beta-CIT was compared in eight early-PD patients with exclusively hemi-parkinsonism and eight age- and sex-matched healthy subjects. [123I] beta-CIT striatal uptake was reduced by approximately 53% contralateral and by 38% ipsilateral to the clinically symptomatic side in the hemi-PD patients, compared with the mean striatal uptake in age- and sex-matched healthy subjects. The relative reduction in [123I] beta-CIT uptake in the hemi-PD patients was greater in the putamen than in the caudate. These data demonstrate that SPECT imaging of the dopamine transporter with [123I] beta-CIT can identify patients with PD at the onset of motor symptoms and suggest that this technique also may be useful in identifying individuals with developing dopaminergic pathology before onset of motor symptoms.

Imaging D2 receptor occupancy by endogenous dopamine in humans.

The impact of endogenous dopamine on in vivo measurement of D2 receptors in humans was evaluated with single photon emission computerized tomography (SPECT) by comparing the binding potential (BP) of the selective D2 radiotracer [123I]IBZM before and after acute dopamine depletion. Dopamine depletion was achieved by administration of the tyrosine hydroxylase inhibitor alpha-methyl-para-tyrosine (AMPT), given orally at a dose of 1 g every six hours for two days. AMPT increased [123I]IBZM BP by 28 +/- 16% (+/- SD, n = 9). Experiments in rodents suggested that this effect was due to removal of endogenous dopamine rather than D2 receptor upregulation. Synaptic dopamine concentration was estimated as 45 +/- 25 nM, in agreement with values reported in rodents. The amplitude and the variability of the AMPT effect suggested that competition by endogenous dopamine introduces a significant error in measurement of D2 receptors in vivo with positron emission tomography (PET) or SPECT. However, these results also imply that D2 receptor imaging coupled with acute dopamine depletion might provide estimates of synaptic dopamine concentration in the living human brain.

Significance of nonuniform attenuation correction in quantitative brain SPECT imaging.

UNLABELLED: The purposes of this study were to develop a method for nonuniform attenuation correction of 123I emission brain images based on transmission imaging with a longer-lived isotope (i.e., 57Co) and to evaluate the relative improvement in quantitative SPECT images achieved with nonuniform attenuation correction. METHODS: Emission and transmission SPECT scans were acquired on three different sets of studies: a heterogeneous brain phantom filled with 1231 to simulate the distribution of dopamine transporters labeled with 2beta-carbomethoxy-3beta-(4-123I-iodophenyl)tropane (123I-beta-CIT); nine healthy human control subjects who underwent transmission scanning using two separate line sources (57Co and 123I); and a set of eight patients with Parkinson's disease and five healthy control subjects who received both emission and transmission scans after injection of 123I-beta-CIT. Attenuation maps were reconstructed using a Bayesian transmission reconstruction algorithm, and attenuation correction was performed using Chang's postprocessing method. The spatial distribution of errors within the brain was obtained from attenuation correction factors computed from uniform and nonuniform attenuation maps and was visualized on a pixel-by-pixel basis as an error image. RESULTS: For the heterogeneous brain phantom, the uniform attenuation correction had errors of 2%-6.5% for regions corresponding to striatum and background, whereas nonuniform attenuation correction was within 1%. Analysis of 123I transmission images of the nine healthy human control subjects showed differences between uniform and nonuniform attenuation correction to be in the range of 6.4%-16.0% for brain regions of interest (ROIs). The human control subjects who received transmission scans only were used to generate a curvilinear function to convert 57Co attenuation values into those for 123I, based on a pixel-by-pixel comparison of two coregistered transmission images for each subject. These values were applied to the group of patients and healthy control subjects who received transmission 57Co scans and emission 123I scans after injection of 123I-beta-CIT. In comparison to nonuniform attenuation correction as the gold standard, uniform attenuation with the ellipse drawn around the transmission image caused an approximately 5% error, whereas placement of the ellipse around the emission image caused a 15% error. CONCLUSION: Nonuniform attenuation correction allowed a moderate improvement in the measurement of absolute activity in individual brain ROIs. When images were analyzed as target-to-background activity ratios, as is commonly performed with 123I-beta-CIT, these outcome measures showed only small differences when Parkinson's disease patients and healthy control subjects were compared using nonuniform, uniform or even no attenuation correction.

Graphical analysis and simplified quantification of striatal and extrastriatal dopamine D2 receptor binding with [123I]epidepride SPECT.

UNLABELLED: The purpose of this study was to extend the graphical analysis of reversible tracer binding to account for labeled lipophilic metabolites (metabolites) in quantifying [123I]epidepride binding to striatal and extrastriatal D2 receptors and, additionally, to evaluate the feasibility of simplified analysis to measure the specific volume of distribution (V3') using single-sample blood data because the tissue ratio (RT) may be a less reliable measure of D2 binding in the presence of metabolites. METHODS: Multilinear regression analysis (MLRA) and graphical analysis (GA) using plasma parent (P) plus metabolite (M) activities as input and time activities of receptor-free (RF, cerebellum) and receptor-containing regions (RR, striatum and temporal cortex) derived V3' = (alpha(RR)(P) - alpha(RF)(P)), V3' = (1 + delta) (alpha(RR) - alpha(RF)) and RT = V3'/(V2P' + deltaV2M'), where alpha is a regression coefficient, delta is the equilibrium area ratio of M and P, and (V2P'/V2M') are the corresponding nondisplaceable distribution volumes. V3' by simplified analysis (SA) was calculated from RT determined without blood data and (V2P' + deltaV2M') with single-blood sample data. The accuracy of these three V3' values was assessed relative to the metabolite-accounted kinetic analysis (KA) for [123I]epidepride SPECT studies of 11 healthy volunteers, in which each participant had 27 scans and 30 plasma samples drawn during the 14 h after injection. RESULTS: All three V3' values (mL/g) significantly correlated with those by KA (r > or = 0.90) (striatum/temporal cortex: MLRA, 77.8 +/- 36.6/2.35 +/- 1.16; GA, 98.8 +/- 34.2/4.61 +/- 1.77; SA, 83.9 +/- 24.8/4.26 +/- 1.74; KA, 107.6 +/- 34.4/5.61 +/- 1.84). However, the correlation between RT and V3' was only moderate (r < or = 0.65) because of significant intersubject variability (23%) in (V2P' + deltaV2M'). CONCLUSION: The graphical analysis can be extended to account for metabolites in measuring D2 binding with [123I]epidepride SPECT for both high and low D2 density regions. Additionally, simplified V3' measurements with single blood sampling are feasible and may be a practical alternative to the tissue ratio RT because RT suffers as a measure of D2 binding from significant intersubject variability in the metabolite-contributed distribution volume of the nondisplaceable compartment.

Comparison of gallium-67 versus indium-111 monoclonal antibody (96.5, ZME-018) in detection of human melanoma in athymic mice.

We compared the biodistribution of two radiolabeled, whole, tumor selective monoclonal antibodies [( 111In]96.5, [111In]ZME-018) to 67Ga in nude mice bearing a human melanoma known to express p97 antigen. Localization of gallium was determined 48 hr following i.v. injection. Localization of the radiolabeled antibodies was determined at 3 days and 7 days following i.v. injection. All agents showed more or less similar absolute tumor uptake which varied between 22% and 36% of the injected dose per gram of tumor. Only the tumor uptake of [111In]96.5 antibody at 7 days was significantly lower than the 67Ga uptake at 48 hr. However, uptake in normal tissues was generally higher for both antibodies at 3 and 7 days than for 67Ga uptake at 48 hr. Therefore, the tumor-to-blood ratio for 67Ga was tenfold higher than that for either antibody, the tumor-to-muscle ratio was twofold higher. Bone was the only organ in which the tumor-to-organ ratio was consistently higher with radiolabeled antibody than with 67Ga. The tumor-to-liver and tumor-to-intestine ratios were comparable. Localization of the two tumor selective antibodies was greater than a nonspecific "control antibody" [( 111In]CEA) and change in specific activity from 0.17 mCi/mg to 3.3 mCi/mg did not influence localization. From these animal data it may be anticipated that tumor imaging with [111In]96.5 or [111In]ZME-018 will not be superior to imaging with 67Ga for detection of melanoma.

Test-retest reproducibility of extrastriatal dopamine D2 receptor imaging with [123I]epidepride SPECT in humans.

UNLABELLED: This study evaluated the test-retest reproducibility of D2 receptor quantification in the thalamus and temporal cortex using [123I]epidepride SPECT. METHODS: Ten healthy volunteers (4 men, 6 women; age range, 19-46 y) underwent 2 SPECT studies (interval, 2-26 d) using a bolus-plus-constant-infusion paradigm (bolus-to-infusion ratio = 6 h; infusion time = 9 h). Plasma clearance (in liters per hour) and free fraction (f1) of the parent tracer were measured. Radioactivity (in becquerels per gram) in the thalamus, temporal cortex, and cerebellum were normalized to the infusion rate (in becquerels per hour). Normalized striatal radioactivity was also measured to assess reproducibility in regions with a high density of receptors and better counting statistics. The outcome measures obtained were V3 (receptor density [Bmax]/equilibrium dissociation constant [KD]), V3' (f1 x Bmax/KD), and RT (specific-to-nondisplaceable tissue ratio). RESULTS: Test-retest variability and reliability (intraclass correlation coefficient) were 10.8% and 0.88, respectively, for plasma clearance and 15.3% and 0.77, respectively, for f1. The test-retest variability of brain-specific (target minus nondisplaceable) radioactivity was higher in the thalamus and temporal cortex than in the striatum, although reliability was comparable. Among the outcome measures, V3' showed better test-retest variability and reliability in the thalamus (13.3% and 0.75, respectively) and temporal cortex (13.4% and 0.86, respectively). CONCLUSION: Brain radioactivity was the main source of variability for quantification of extrastriatal D2 receptors with [123I]epidepride. The reproducibility of outcome measures in extrastriatal regions was good. However, because receptor density was lower in extrastriatal regions than in the striatum, the counting statistics in these regions were low and reproducibility was affected by the higher test-retest variability of brain-specific radioactivity. Compared with V3 and V3', RT showed less test-retest variability in the thalamus and temporal cortex but lower reliability. Moreover, measurement of RT may be affected by the presence of potential lipophilic metabolites entering the brain.

Measurement of alpha4beta2 nicotinic acetylcholine receptors with [123I]5-I-A-85380 SPECT.

UNLABELLED: Nicotinic acetylcholine receptors (nAChRs) play an important role in tobacco dependence and a potential therapeutic role in neuropsychiatric disorders such as Alzheimer's disease. [123I]5-iodo-3-[2(S)-2-azetidinylmethoxy]pyridine (5-I-A-85380) is a new SPECT tracer that labels alpha4beta2 nAChRs. The purpose of this study was to assess the usefulness of this tracer to measure regional nAChR binding in baboon brain using both a bolus/kinetic paradigm and also a bolus plus constant infusion/equilibrium paradigm. METHODS: A pair of bolus/kinetic and bolus plus constant infusion/equilibrium studies was performed in each of 3 isoflurane-anesthetized baboons. Bolus studies were performed by intravenous injection of 191-226 MBq [123I]5-I-A-85380 and image acquisition for 289-367 min. The data were analyzed with 1- and 2-tissue compartment models. Bolus plus constant infusion/equilibrium studies were performed by a bolus injection (74-132 MBq) followed by a 468- to 495-min infusion with a bolus/infusion ratio (B/I) of 4.8-5.0 h. The distribution volumes in the thalamus were measured in these 2 paradigms. To study whether the cerebellum was appropriate as a receptor-poor region, displacement studies were done in 2 baboons using the B/I paradigm with subcutaneous injection of (-)-cytisine (0.8 and 1.0 mg/kg). RESULTS: The kinetics of this tracer was best described by the 1-tissue compartment model. The 2-compartment model showed poor identifiability of rate constants. The total (specific plus nondisplaceable compartments) distribution volumes (V(T)') agreed between bolus and B/I paradigms (average percentage difference in V(T)', 16.8%). (-)-Cytisine (0.8 and 1.0 mg/kg) displaced 70% and 72% of the radioactivity in the thalamus and 36% and 55% in the cerebellum, respectively, indicating that the latter was not appropriate as a receptor-poor region. CONCLUSION: These results show the feasibility of quantifying alpha4beta2 nAChRs using [123I]5-I-A-85380 and support the use of V(T)' as an appropriate outcome measure.

SPECT imaging of the benzodiazepine receptor: feasibility of in vivo potency measurements from stepwise displacement curves.

Iodine-123-labeled Ro 16-0154 is a high affinity, reversibly binding radiotracer for the benzodiazepine (BZ) receptor. Brain uptake of this radioligand was relatively stable and showed high ratios of specific-to-nonspecific uptake, with greater than 90% displaced by intravenous administration of BZ receptor agents. Repeated injections of increasing doses of each of five BZ drugs (Ro 16-0154, Ro 15-1788, clonazepam, alprazolam, and diazepam) yielded stepwise displacement curves, which were analyzed to measure the in vivo potencies of these agents. The relatively long half-life of 123I and the stable biologic uptake of the radiotracer allowed such potency estimations in just one experiment following a single injection of radioligand. The in vivo potencies of these five agents were highly correlated with their affinities for the BZ receptor determined with in vitro homogenate binding. A single crystal probe provided potency measurements virtually identical to simultaneously performed SPECT imaging studies. In conclusion, stepwise displacement by agents administered following the injection of the radioligand 123I-Ro 16-0154 provided a reliable means of measuring the in vivo potencies of BZ receptor agents. This in vivo determination may predict the clinical potency of BZ drugs than in vitro homogenate estimations, because the in vivo measure provides the summed effects of receptor affinity, plasma protein binding, penetration of the blood-brain barrier, and metabolism of the displacing agent.