Imaging serotonin transporters with 123I-ADAM brain SPECT in healthy non-human primates

BACKGROUND: Serotonin transporters (SERTs) play a major role in modulating serotonergic neuronal function and are the target of many antidepressant drugs used in neuro-psychiatric disorders. To gain more information on the temporal distribution of SERTs, 2-([2-([dimethylamino]methyl)phenoxyl]thio)-5-[I]iodophenylamine (I-ADAM) single photon emission computed tomography (SPECT) was utilized in an in vivo imaging study using non-human primates. METHODS: Two female monkeys (Macaca cyclopis) were studied. Eight brain SPECT imaging examinations, each 30 min in duration, were obtained after injection of 185 MBq of I-ADAM. Images were obtained using a dual-head gamma camera equipped with ultra-high resolution fan-beam collimators. In addition to visual inspection, the radio-uptake and specific uptake ratios (SURs) of midbrain (MB), thalamus (TH), striatum (ST), temporal and frontal cortices and the whole brain in reference to the corresponding magnetic resonance image at the eight time points were measured. The SUR of MB, using cerebellum (CB) as the reference tissue, was calculated as (MB - CB)/CB, in mean counts/pixel. The SURs of the other brain regions were similarly measured. RESULTS: There was relatively high uptake of I-ADAM in the MB and TH, moderate uptake in ST, lower uptake in the cerebral cortex, and almost no uptake in the CB. The image of MB could be easily identified at the first 30 min time point. It appeared that the SURs of MB, TH and ST reached equilibrium around 210 min after injection. No adverse reactions of the primates were found during and after imaging. Brain distribution of I-ADAM in the primate appeared consistent with the known distribution of SERTs. CONCLUSION: In conjunction with a high SUR in MB, TH and ST, we speculate that I-ADAM may be a potential radioligand for SPECT studies of serotonin transporters in humans.     

123I-ADAM binding to serotonin transporters in patients with major depression and healthy controls: a preliminary study.

The serotonergic system may play an important role in the pathophysiology of major depressive disorder (MDD). Few imaging studies have examined serotonin transporter (SERT) binding in patients with MDD. We hypothesized that SERT binding activity may be altered in patients with MDD. This study compared SERT binding in patients with MDD with that in healthy controls. METHODS: We studied SERT activity in 7 patients (22-50 y old) with moderate to severe MDD and 6 healthy controls (24-56 y old) using (123)I-labeled 2-((2-((dimethylamino)methyl) phenyl)thio)-5-iodophenylamine (ADAM) and SPECT brain imaging. Subjects underwent SPECT 4 h after intravenous administration of 185 MBq (5 mCi) of (123)I-ADAM. Images were reconstructed in the axial plane, and region-of-interest demarcations were placed on the midbrain, medial temporal region, and basal ganglia region. RESULTS: (123)I-ADAM binding to SERT in the midbrain was significantly lower (P = 0.01) in MDD patients (1.81 +/- 0.07) than in controls (1.95 +/- 0.13). Age-adjusted (123)I-ADAM binding in the midbrain correlated significantly with scores on the Hamilton Depression Rating Scale (r = 0.82; P = 0.02). A significant negative correlation was observed between (123)I-ADAM SERT binding in the midbrain and age in the healthy control group (r = 0.98; P = 0.0002). SERT binding in the basal ganglia or medial temporal regions of interest did not significantly differ between groups. CONCLUSION: The findings from this preliminary study suggest the possibility of decreased SERT binding in the midbrain region of patients with MDD, with the degree of decrease correlating with the severity of depressive symptoms. There also appears to be an age-related decline in midbrain (123)I-ADAM SERT binding in healthy subjects.     

Quantification of serotonin transporters in nonhuman primates using [(123)I]ADAM and SPECT.

We reported recently a highly selective radioligand, 2-([2-([dimethylamino]methyl)phenyl]thio)-5-[(123)I]iodophenylamine (ADAM), for SPECT imaging of serotonin transporters (SERT). In this article we describe the kinetic modeling of [(123)I]ADAM and its ability to quantitatively and reproducibly measure the concentrations of SERT in the nonhuman primate brain. We also investigate simplified models of tracer behavior that do not require invasive arterial blood sampling. METHODS: Three female baboons each underwent 3 [(123)I]ADAM SPECT studies. The studies consisted of a dynamic sequence of seventy-two 5-min scans after injection of 330 +/- 50 MBq (mean +/- SD) [(123)I]ADAM. Rapid arterial blood samples were obtained and corrected for the presence of labeled metabolites. Dynamic imaging and metabolite-corrected plasma data were analyzed using graphic analysis to give the distribution volumes (DVs) of different brain regions. DV ratios (DVRs) of target to cerebellum were derived and compared against a kinetic reference tissue model and simple target-to-background ratio. RESULTS: Averaged over all 9 scans, the mean DV in the midbrain was 4.86 +/- 1.06 mL/mL and the mean DV in the cerebellum was 2.25 +/- 0.48 mL/mL. The mean test-retest repeatability of the midbrain DV was 14.5%. The reference tissue model gave a mean midbrain DVR of 2.01 +/- 0.17 and correlated strongly with the DVR calculated from the full kinetic model (correlation coefficient [R(2)] = 0.94; P < 0.001), but with much improved repeatability (test-retest, 5.4%; intersubject variability, 5.2%). Similarly, the simple ratio method gave strong correlations with the full kinetic model (R(2) = 0.89; P < 0.001) and a test-retest of 7.6%. CONCLUSION: Accurate, repeatable quantification of SERT in the nonhuman primate brain is possible using kinetic modeling of dynamic [(123)I]ADAM SPECT scans. Simplified models, which do not require arterial blood sampling, gave accurate results that correlated strongly with the full kinetic model. The test-retest reliability of the simplified reference region models was excellent. Quantification of SERT is possible using full kinetic modeling and also with simpler reference region methods.     

Characterization of the binding sites for 123I-ADAM and the relationship to the serotonin transporter in rat and mouse brains using quantitative autoradiography.

Imaging of serotonin transporter (SERT) in the central nervous system may provide an important tool to evaluate some psychiatric disorders. Recently, a novel (123)I-labeled radiotracer, 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine ((123)I-ADAM), has been developed that exhibited a high selectivity for SERT. The aim of this study was to characterize the biodistribution and specificity of (123)I-ADAM to SERT using quantitative autoradiography in both control and neurotoxin-treated animals. METHODS: (123)I-ADAM (74 MBq) was injected intravenously into the mice to access its biodistribution in the brain via quantitative autoradiography. Further, rats with serotonin depleted by intraperitoneal injection of p-chloroamphetamine (PCA) were used to evaluate the specificity of (123)I-ADAM to SERT. The levels of biogenic amines were then measured and correlated with quantitative (123)I-ADAM labeling in control and PCA-treated rat brains. RESULTS: The autoradiographic results showed that (123)I-ADAM accumulated in SERT-rich brain areas after systemic injection, including the globus pallidus, thalamus, hypothalamus, substantia nigra, interpeduncular nucleus, amygdala, and raphe nucleus. The dorsal raphe nucleus had the highest initial uptake with a peak specific binding ratio (i.e., [target - cerebellum]/cerebellum) at 120 min after injection. (123)I-ADAM uptake was dramatically decreased in the hippocampus, thalamus, amygdala, geniculate nuclei, hypothalamus, raphe nucleus, and substantia nigra in PCA-lesioned rats. The decrement in radioactivity was more prominent at higher dosages of PCA and was in parallel with the changes in amounts of serotonin and 5-hydroxyindoleacetic acid in the prefrontal cortex. CONCLUSION: This study demonstrates that regional distribution of (123)I-ADAM radioactivity is similar to the SERT localization in both rat and mouse brains. We also validated that destruction on central serotonergic neurons after PCA treatment inhibits the uptake of (123)I-ADAM in serotonin-rich brain regions. High specific binding to SERT in vivo makes (123)I-ADAM an appropriate radiotracer for solitary studies of serotonin functions in living humans.     

Imaging the availability of serotonin transporter in rat brain with 123I-ADAM and small-animal SPECT

BACKGROUND: Imaging serotonin transporters during antidepressant treatment in small animals is a useful tool for preclinical study during drug development. In this work, we aimed to demonstrate the feasibility of using 123I-ADAM and small-animal SPECT to monitor serotonin transporter availabilities in rat brains prior to and after administration of a selective serotonin re-uptake inhibitor. METHODS: Male Sprague-Dawley rats with and without administration of citalopram (4 mg x kg body weight) were examined in this study. During the process rat brains were scanned using a double-headed microSPECT system equipped with pinhole collimators. SPECT tomographic images and X-ray computed tomography (CT) were acquired after introducing 123I-ADAM via the tail vein. The 123I-ADAM specific binding was assessed by SPECT/CT fused image to draw regions of interest in the midbrain and cerebellum. Ex-vivo autoradiography was carried out as a parallel investigation to validate the SPECT technique. RESULTS: SPECT images displayed specific binding ratio in midbrain to be 0.91+/-0.30 averaged from three rats. Drug occupancies (95.47+/-1.56)% were shown after administration of citalopram in a dosage of 4 mg x kg. CONCLUSION: This study demonstrated that the serotonin transporter availability during antidepressant treatment in small animals can be assessed semi-quantitatively by using 123I-ADAM and SPECT.     

Characterization of the SPECT 5-HT2A receptor ligand 123I-R91150 in healthy volunteers: Part 1--pseudoequilibrium interval and quantification methods.

With the aim of characterizing radioiodinated 4-amino-N-1-[3-(4-fluorophenoxy)propyl]-4-methyl-4-piperidinyl]5-iodo-2-methoxybenzamide ((123)I-R91150) as a SPECT ligand for subtype 2A of the 5-hydroxytryptamine receptor (5-HT(2A)), tracer kinetic compartmental analyses were compared with the tissue ratio method (TR). The pseudoequilibrium interval after a single bolus injection was identified, and a reference database of specific uptake ratio (SUR) values was obtained. Within-scan and between-subject variability was also assessed. METHODS: Nineteen healthy men (mean age +/- SD, 24.4 +/- 3.3 y) were included and separated into 2 groups. Dynamic scans with venous blood sampling from 0 to 470 min after a single bolus injection of (123)I-R91150 was completed for 7 of the 9 subjects included in group A, and in one of them compartmental modeling was performed with an arterial blood input function using 1-tissue-compartment (1TC) and 2-tissue-compartment (2TC) models. Binding potential (BP) using the simplified reference tissue model (SRTM) (BP(SRTM)) and SUR values using TR over time were also calculated. The 10 remaining subjects (group B) underwent a single scan at pseudoequilibrium with the aim of improving the precision of mean normal SUR estimates. Regions of interest in cortical regions and basal ganglia for specific uptake, and in cerebellum for nonspecific uptake, were manually drawn on each subject's MR images and translated to the corresponding SPECT slices after coregistration. RESULTS: The 1TC model correlated well with the 2TC model (BP(2TC) = 1.04.BP(1TC) - 0.01, R(2) = 0.98), and both methods correlated with BP(SRTM) and SUR with little bias (BP(1TC) = 1.10 BP(SRTM) + 0.03, R(2) = 0.98; BP(2TC) = 1.15 BP(SRTM) + 0.01, R(2) = 0.98; BP(SRTM) = 0.99 SUR(mean) + 0.01, R(2) = 0.98). SUR values stabilized from 180 min after injection in most cortical regions, ranging from 0.51 +/- 0.10 in the orbitofrontal region to 0.27 +/- 0.09 in the parietal region. Within-scan and between-subject variability among regions ranged from 10% to 14.8%, and from 18.3% to 35.4%, respectively. CONCLUSION: (123)I-R91150 distribution agrees with autoradiography results, showing highly specific binding in cortical regions. The correlations found among 1TC, 2TC, SRTM, and TR outcome measurements support the use of TR for quantification of 5-HT(2A) receptor binding with (123)I-R91150 SPECT and a simple protocol avoiding arterial blood sampling and serial scanning over time.     

Biodistribution and imaging with (123)I-ADAM: a serotonin transporter imaging agent.

2-((2-((Dimethylamino)methyl)phenyl)thio)-5-(123)I-iodophenylamine ((123)I-ADAM) is a new radiopharmaceutical that selectively binds the central nervous system serotonin transporters. The purpose of this study was to measure its whole-body biokinetics and estimate its radiation dosimetry in healthy human volunteers. The study was conducted within a regulatory framework that required its pharmacologic safety to be assessed simultaneously. METHODS: The sample included 7 subjects ranging in age from 22 to 54 y old. An average of 12.7 whole-body scans were acquired sequentially on a dual-head camera for up to 50 h after the intravenous administration of 185 MBq (5 mCi) (123)I-ADAM. The fraction of the administered dose in 13 regions of interest (ROIs) was quantified from the attenuation-corrected geometric mean counts in conjugate views. Multiexponential functions were iteratively fit to each time-activity curve using a nonlinear, least-squares regression algorithm. These curves were numerically integrated to yield source organ residence times. Gender-specific radiation doses were then estimated with the MIRD technique. SPECT brain scans obtained 3 h after injection were evaluated using an ROI analysis to determine the range of values for the region to cerebellum. RESULTS: There were no pharmacologic effects of the radiotracer on any of the subjects, including no change in heart rate, blood pressure, or laboratory results. Early planar images showed differentially increased activity in the lungs. SPECT images demonstrated that the radiopharmaceutical localized in the midbrain in a distribution that is consistent with selective transporter binding. The dose-limiting organ in both men and women was the distal colon, which received an average of 0.12 mGy/MBq (0.43 rad/mCi) (range, 0.098-0.15 mGy/MBq). The effective dose equivalent and effective dose for (123)I-ADAM were 0.037 +/- 0.003 mSv/MBq and 0.036 +/- 0.003 mSv/MBq, respectively. The mean adult male value of effective dose for (123)I-ADAM is similar in magnitude to that of (111)In-diethylenetriaminepentaacetic acid (0.035 mGy/MBq), half that of (111)In-pentetreotide (0.81 mGy/MBq), and approximately twice that of (123)I-inosine 5'-monophosphate (0.018 mGy/MBq). The differences in results between this study and a previous publication are most likely due to several factors, the most prominent being this dataset used attenuation correction of the scintigraphic data. Region-to-cerebellum ratios for the brain SPECT scans were 1.95 +/- 0.13 for the midbrain, 1.27 +/- 0.10 for the medial temporal regions, and 1.11 +/- 0.07 for the striatum. CONCLUSION: (123)I-ADAM may be a safe and effective radiotracer for imaging serotonin transporters in the brain and the body.     

Measuring SSRI occupancy of SERT using the novel tracer [123I]ADAM: a SPECT validation study

Purpose Serotonergic brain regions play a crucial role in the modulation of emotion, and serotonergic dysfunction may contribute to several neurological disorders. [¹²³I]ADAM is a novel SPECT tracer which binds with high affinity to serotonin transporters (SERT). The objective of this study was to compare different methods for the quantification of tracer binding and to develop a simplified single-scan protocol for this tracer, as well as to investigate its potential for characterisation of the transporter occupancy versus plasma concentration curve of a selective serotonin re-uptake inhibitor (SSRI).Methods Dynamic SPECT scans were performed on 16 healthy volunteers after administration of 150 MBq [¹²³I]ADAM. Data were acquired from the time of injection until 5.5 h after injection in 30- or 45-min sessions. Each subject was scanned twice: with and without pre-treatment with the SSRI citalopram in various dosage regimens. The plasma concentration of citalopram (C_p) was determined from venous samples. Images were reconstructed by filtered back-projection with scatter and attenuation correction. Tracer binding was quantified for midbrain, striatum and thalamus using cerebellum as a reference region. Quantification was done by kinetic modelling, graphical analysis and multi-linear regression, as well as by the ratio method, with binding potential (BP₂) as the outcome measure. The SERT occupancy by citalopram was determined relative to the baseline scan for each subject, and the occupancy versus C_p curve was fitted with the E_max model.Results The highest binding of [¹²³I]ADAM was in midbrain (mean baseline BP₂±SD=1.31±0.29), with lower binding in thalamus (0.79±0.16) and striatum (0.66±0.13). There was good agreement between BP₂ values obtained by different quantification methods. Using the ratio method, the best agreement with kinetic modelling was obtained with data from the time interval [200,260] min after injection. The fitting of the midbrain occupancy curve yielded a maximum occupancy of 84% and a plasma concentration required to reach 50% of the maximum of 2.5 ng/ml, with a goodness-of-fit variability of 13% (SD).Conclusion Binding of [¹²³I]ADAM to SERT in midbrain can be quantified with a single scan starting 200 min after injection. However, the variability of estimated occupancy values may be too high for critical assessment of occupancy of SERT by SSRI.     

Characterization of 4-[18F]-ADAM as an imaging agent for SERT in non-human primate brain using PET: a dynamic study

IntroductionSerotonin transporter (SERT) has been associated with many psychiatric diseases. This study investigated the biodistribution of a serotonin transporter imaging agent, N,N-dimethyl-2-(2-amino-4-¹⁸F-fluorophenylthio)benzylamine (4-[¹⁸F]-ADAM), in nonhuman primate brain using positron emission tomography (PET).MethodsSix and four Macaca cyclopis monkeys were used to determine the transit time (i.e., time necessary to reach biodistribution equilibrium) and the reproducibility of 4-[¹⁸F]-ADAM biodistribution in the brain, respectively. The sensitivity and specificity of 4-[¹⁸F]-ADAM binding to SERT were evaluated in one monkey challenged with different doses of fluoxetine and one monkey treated with 3,4-methylendioxymethamphetamine (MDMA). Dynamic PET imaging was performed for 3 h after 4-[¹⁸F]-ADAM intravenous bolus injection. The specific uptake ratios (SURs) in the midbrain (MB), thalamus (TH), striatum (ST) and frontal cortex (FC) were calculated.ResultsThe distribution of 4-[¹⁸F]-ADAM reached equilibrium 120–150 min after injection. The mean SURs were 2.49±0.13 in MB, 1.59±0.17 in TH, 1.35±0.06 in ST and 0.34±0.03 in FC, and the minimum variability was shown 120–150 min after 4-[¹⁸F]-ADAM injection. Using SURs and intraclass coefficient of correlation, the test/retest variability was under 8% and above 0.8, respectively, in SERT-rich areas. Challenge with fluoxetin (0.75–2 mg) dose-dependently inhibited the SURs in various brain regions. 4-[¹⁸F]-ADAM binding was markedly reduced in the brain of an MDMA-treated monkey compared to that in brains of normal controls.Conclusion4-[¹⁸F]-ADAM appears to be a highly selective radioligand for imaging SERT in monkey brain.     

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

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.     

Single-photon emission tomography imaging of serotonin transporters in the nonhuman primate brain with [(123)I]ODAM.

We have described previously a selective serotonin transporter (SERT) radioligand, [(123)I]IDAM. We now report a similarly potent, but more stable IDAM derivative, 5-iodo-2-[2-[(dimethylamino)methyl]phenoxy]benzyl alcohol ([(123)I]ODAM). The imaging characteristics of this radioligand were studied and compared against [(123)I]IDAM. Dynamic sequences of single-photon emission tomography (SPET) scans were obtained on three female baboons after injection of 375 MBq of [(123)I]ODAM. Displacing doses (1 mg/kg) of the selective SERT ligand (+)McN5652 were administered 120 min after injection of [(123)I]ODAM. Total integrated brain uptake of [(123)I]ODAM was about 30% higher than [(123)I]IDAM. After 60-120 min, the regional distribution of tracer within the brain reflected the characteristic distribution of SERT. Peak specific binding in the midbrain occurred 120 min after injection, with an equilibrium midbrain to cerebellar ratio of 1. 50+/-0.08, which was slightly lower than the value for [(123)I]IDAM (1.80+/- 0.13). Both the binding kinetics and the metabolism of [(123)I]ODAM were slower than those of [(123)I]IDAM. Following injection of a competing SERT ligand, (+)McN5652, the tracer exhibited washout from areas with high concentrations of SERT, with a dissociation kinetic rate constant k(off)=0.0085+/-0.0028 min(-1) in the midbrain. Similar studies using nisoxetine and methylphenidate showed no displacement, consistent with its low binding affinity to norepinephrine and dopamine transporters, respectively. These results suggest that [(123)I]ODAM is suitable for selective SPET imaging of SERT in the primate brain, with higher uptake and slower kinetics and metabolism than [(123)I]IDAM, but also a slightly lower selectivity for SERT.     

123I-5-IA-85380 SPECT measurement of nicotinic acetylcholine receptors in human brain by the constant infusion paradigm: feasibility and reproducibility.

(123)I-5-IA-85380 ((123)I-5-IA; [(123)I]-5-iodo-3-[2(S)-azetidinylmethoxy]pyridine) is a promising SPECT radiotracer for imaging beta(2)-containing nicotinic acetylcholine receptors (beta(2)-nAChRs) in brain. Beta(2)-nAChRs are the initial site of action of nicotine and are implicated in various neuropsychiatric disorders. The feasibility and reproducibility of the bolus-plus-constant-infusion paradigm for equilibrium modeling of (123)I-5-IA using SPECT in healthy nonsmokers was studied. METHODS: Ten healthy nonsmokers (mean age +/- SD, 43.7 +/- 9.9 y) underwent two (123)I-5-IA SPECT scans within 4 wk. (123)I-5-IA was administered as a bolus (125.8 +/- 14.6 MBq) plus constant infusion (18.1 +/- 1.5 MBq/h). SPECT acquisitions (30 min) and venous blood sampling were performed every 60 min throughout the infusion (10-14 h). The test-retest variability and reliability of plasma activity (kBq/mL), the regional brain activity reflected by units of kBq/mL and %ID/mL (injected dose/mL brain tissue), and the equilibrium outcome measures V(T)' (ratio of total uptake to total plasma parent concentration) and V(T) (ratio of total uptake to free plasma parent concentration) were evaluated in 4 brain areas, including thalamus, striatum, cortex, and cerebellum. RESULTS: Linear regression analysis revealed that time-activity curves for both plasma and brain (123)I-5-IA activity stabilized by 5 h, with an average change of [2.5%/h between 6 and 8 h of infusion, permitting equilibrium modeling. The plasma free fraction (f(1)), total parent, and clearance demonstrated good test-retest variability (mean, 10.9%-12.5%), whereas the variability of free parent was greater (mean, 24.3%). Regional brain activity (kBq/mL) demonstrated good test-retest variability (11.1%-16.4%) that improved when corrected for infusion rate (mean, 8.2%-9.9%) or for injected dose (mean, 9.5%-13.3%). V(T)' demonstrated better test-retest variability (mean, 7.0%-8.9%) than V(T) (mean, 12.9%-14.6%). Reliability assessed by the intraclass correlation coefficient (ICC) was superior for kBq/mL (ICC = 0.83-0.90) and %ID/mL (ICC = 0.93-0.96) compared with V(T)' (ICC = 0.30-0.64) and V(T) (ICC = 0.28-0.60). The lower reliability of V(T) was attributed to the poor reliability of the free fraction (ICC = 0.35) and free parent (ICC = 0.68). CONCLUSION: These results support the feasibility and reproducibility of equilibrium imaging with (123)I-5-IA for measurement of beta(2)-nAChRs in human brain.     

2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine (ADAM): an improved serotonin transporter ligand

Serotonin transporters (SERT) are target-sites for commonly used antidepressants, such as fluoxetine, paroxetine, sertraline, and so on. Imaging of these sites in the living human brain may provide an important tool to evaluate the mechanisms of action as well as to monitor the treatment of depressed patients. Synthesis and characterization of an improved SERT imaging agent, ADAM (2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine)(7) was achieved. The new compound, ADAM(7), displayed an extremely potent binding affinity toward SERT ( K(i)=0.013 nM, in membrane preparations of LLC-PK(1)-cloned cell lines expressing the specific monoamine transporter). ADAM(7) also showed more than 1,000-fold selectivity for SERT over norepinephrine transporter (NET) and dopamine transporter (DAT) ( K(i)=699 and 840 nM, for NET and DAT, respectively). The radiolabeled compound [(125)I]ADAM(7) showed an excellent brain uptake in rats (1.41% dose at 2 min post intravenous [IV] injection), and consistently displayed the highest uptake (between 60-240 min post IV injection) in hypothalamus, a region with the highest density of SERT. The specific uptake of [(125)I]ADAM(7) in the hypothalamus exhibited the highest target-to-nontarget ratio ([hypothalamus - cerebellum]/cerebellum was 3.97 at 120 min post IV injection). The preliminary imaging study of [(123)I]ADAM in the brain of a baboon by single photon emission computed tomography (SPECT) at 180-240 min post IV injection indicated a specific uptake in midbrain region rich in SERT. These data suggest that the new ligand [(123)I]ADAM(7) may be useful for SPECT imaging of SERT binding sites in the human brain.     

Imaging serotonin and dopamine transporters with 123I-beta-CIT SPECT: binding kinetics and effects of normal aging.

[123I]beta-carbomethoxy-3-beta-(4-iodophenyl)-tropane (CIT) is a useful ligand for dopamine transporters (DATs) and serotonin transporters (5-HTTs). Previous SPECT studies have shown a state of sustained equilibrium in the striatum on day 2 after injection that allows quantification of striatal DATs using a simple ratio of specific-to-nondisplaceable binding. The aim of this study was to investigate the kinetics of [123I]beta-CIT uptake in the thalamus, hypothalamus, and midbrain, areas known to contain 5-HTTs in high densities. METHODS: SPECT with a triple-head camera was performed on 16 healthy volunteers (13 women, 3 men; mean age [+/-SD], 32 +/- 11 y) after intravenous bolus injection of 130 +/- 20 MBq (3.5 +/- 0.5 mCi) [123I]beta-CIT. Two individuals were scanned 1, 2, 4, 7, 10, 13, 16, and 24 h after injection, and the remaining 14 were scanned 4, 7, 10, 20, and 24 h after injection. Values from 19 previously examined healthy volunteers (8 women, 11 men; mean age, 52 +/- 20 y) were included in the analysis to study the age dependency of beta-CIT binding in striatal and 5-HTT-rich brain areas in a larger control sample. RESULTS: Peak uptake 4 h after injection, followed by stable uptake until 10 h and a slow decrease until 24 h, was observed in the thalamus-hypothalamus region. Activity in the midbrain-pons region peaked 2 h after injection. Because of a concomitant slow but steady decline of uptake in reference regions starting 4 h after injection, a higher stability of binding ratios for 5-HTT-rich brain areas was observed on day 2, suggesting that a state of transient equilibrium is reached between 20 and 24 h but that conditions are only close to transient equilibrium between 4 and 10 h after injection for 5-HTT-rich brain areas. In addition to an age-related decline of striatal [123I]beta-CIT binding of 6.6% per decade, a significant age-associated decrease of beta-CIT binding of 3-4% per decade was found in 5-HTT-rich brain areas. The decline of beta-CIT binding in these regions may be explained, at least in part, by a loss of monoamine transporters with age but may also be related to age-associated morphologic changes. CONCLUSION: [123I]beta-CIT appears to be a suitable ligand for imaging serotonin transporters with SPECT. However, careful age matching is warranted for [123I]beta-CIT SPECT studies of 5-HTT changes in patients with neuropsychiatric disorders.     

Serotonin transporters in the midbrain of Parkinson's disease patients: a study with 123I-beta-CIT SPECT.

In Parkinson's disease (PD), both neuropathologic and biochemical studies suggest that serotonin (5-hydroxytryptamine [5-HT]) neurons are affected by the disease process. The integrity of 5-HT transporters was assessed in PD patients with SPECT using 2beta-carbomethoxy-3beta-(4-(123)I-iodophenyl)tropane ((123)I-beta-CIT), which binds with high affinity to both dopamine (DA) and 5-HT transporters. METHODS: Forty-five PD patients at relatively early stages (mean Hoehn-Yahr stage, 2.0 +/- 0.7; range, 1-3) and 7 age-matched healthy control subjects had 15 scans over a 24-h period after injection of (123)I-beta-CIT using a 3-head SPECT system. In the midbrain, the 5-HT transporter parameter k(3)/k(4) was estimated by 3 noninvasive methods: pseudoequilibrium ratio (R(PE)) method, area ratio (R(A)) method, and a modified graphic method that derives the ratio of ligand distribution volumes (R(V)). Striatal V(3)", the DA transporter parameter that is equivalent to k(3)/k(4), was measured using the images acquired at 24 h after tracer injection. All measures were derived using the cerebellum as the reference region. RESULTS: In control subjects, the (123)I-beta-CIT activity in the midbrain reached a peak at 91 +/- 21 min after injection and then washed out at a slow rate (1.1%/h +/- 0.5%/h). The peak specific uptake in the midbrain occurred at 315 +/- 46 min. In PD patients, the temporal patterns of the midbrain and cerebellar activity were not significantly different from those in control subjects. None of midbrain R(PE), R(A), and R(V) was significantly different between control subjects and PD patients, whereas striatal V(3)" was bilaterally reduced in all patients, being 32% lower than that of the control subjects (P = 0.002). In PD patients, none of the midbrain outcome measures was significantly correlated with either striatal V(3)" or motor or nonmotor symptom ratings, including the Hoehn-Yahr stage and the Unified Parkinson's Disease Rating Scale scores. When the studies of 7 PD patients with depression were analyzed separately, none of the midbrain outcome measures in these patients either was different significantly from control values or correlated with the Hamilton Depression Rating Scale score. CONCLUSION: These results suggest that DA and 5-HT transporters are differentially affected in PD, and 5-HT transporters in the midbrain region may not be affected in relatively early stages of PD. Alternatively, 5-HT transporters in the remaining neurons may be upregulated, thus raising the midbrain 5-HT transporter density to almost normal levels.     

Single-photon emission tomography imaging of serotonin transporters in the non-human primate brain with the selective radioligand [(123)I]IDAM.

A new radioligand, 5-iodo-2-[[2-2-[(dimethylamino)methyl]phenyl]thio]benzyl alcohol ([(123)I]IDAM), has been developed for selective single-photon emission tomography (SPET) imaging of SERT. In vitro binding studies suggest a high selectivity of IDAM for SERT (K(i)=0.097 nM), with considerably lower affinities for norepinephrine and dopamine transporters (NET K(i)= 234 nM and DAT K(i)>10 microM, respectively). In this study the biodistribution of SERT in the baboon brain was investigated in vivo using [(123)I]IDAM and SPET imaging. Dynamic sequences of SPET scans were performed on three female baboons (Papio anubis) after injection of 555 MBq of [(123)I]IDAM. Displacing doses (1 mg/kg) of the selective SERT ligand (+)McN5652 were administered 90-120 min after injection of [(123)I]IDAM. Similar studies were performed using a NET inhibitor, nisoxetine, and a DAT blocker, methylphenidate. After 60-120 min, the regional distribution of tracer within the brain reflected the characteristic distribution of SERT, with the highest uptake in the midbrain area (hypothalamus, raphe nucleus, substantia nigra), and the lowest uptake in the cerebellum (an area presumed free of SERT). Peak specific binding in the midbrain occurred at 120 min, with a ratio to the cerebellum of 1.80+/-0.13. At 30 min, 85% of the radioactivity in the blood was metabolite. Following injection of a competing SERT ligand, (+)McN5652, the tracer exhibited rapid washout from areas with high concentrations of SERT (dissociation rate constant in the midbrain, averaged over three baboons, k(off)=0. 025+/-0.002 min(-1)), while the cerebellar activity distribution was undisturbed (washout rate 0.0059+/- 0.0003 min(-1)). Calculation of tracer washout rate pixel-by-pixel enabled the generation of parametric images of the dissociation rate constant. Similar studies using nisoxetine and methylphenidate had no effect on the distribution of [(123)I]IDAM in the brain. These results suggest that [(123)I]IDAM is suitable for selective SPET imaging of SERT in the primate brain, with high contrast, favorable kinetics, and negligible binding to either NET or DAT.     

Synthesis and pharmacological characterization of a new PET ligand for the serotonin transporter: [11C]5-bromo-2-[2-(dimethylaminomethylphenylsulfanyl)]phenylamine ([11C]DAPA)

A new PET radioligand for the serotonin transporter (SERT), [11C]-5-bromo-2-[2-(dimethylaminomethylphenylsulfanyl)]phenylamine ([11C]DAPA (10), was synthesized and evaluated in vivo in rats and baboons. [11C]DAPA (10) was prepared from its monomethylamino precursor 8 by reaction with high specific activity [11C]methyl iodide. Radiochemical yield was 24 +/- 5% based on [11C]methyl iodide at end of bombardment (EOB, n = 10) and specific activity was 1,553 +/- 939 Ci/mmol at end of synthesis (EOS, n = 10). Binding assays indicated that [11C]DAPA displays high affinity (Ki 1.49 +/- 0.28 nM for hSERT) and good selectivity for the SERT in vitro. Biodistribution studies in rats indicated that [11C]DAPA enters into the brain readily and localizes in brain regions known to contain high concentrations of SERT, such as the thalamus, hypothalamus, frontal cortex and striatum. Moreover, such binding in SERT-rich regions of the brain are blocked by pretreatment with either the selective serotonin reuptake inhibitor (SSRI) citalopram and by the cold compound itself, demonstrating that [11C]DAPA binding in the rat brain is saturable and specific to SERT. Imaging experiments in baboons indicated that [11C]DAPA binding is consistent with the known distribution of SERT in the baboon brain, with highest levels of radioactivity detected in the midbrain and thalamus, intermediate levels in the hippocampus and striatum, and lower levels in the cortical regions. Pretreatment of the baboon with citalopram 10 min before radioactivity injection blocked the binding of [11C]DAPA in all brain regions that contain SERT. Kinetic analysis revealed that, in all brain regions examined, [11C]DAPA specific to nonspecific distribution volume ratios (V(3)") are higher than [11C](+)-McN 5652 and similar to [11C]DASB. In summary, [11C]DAPA appears to be a promising radioligand suitable for the visualization of SERT in vivo using PET.     

Crossover study of (99m)Tc-TRODAT-1 SPECT and (18)F-FDOPA PET in Parkinson's disease patients.

99mTc-TRODAT-1 ([2-[[2-[[[3-(4-chlorophenyl)-8-methyl-8-azabicyclo[3,2,1]oct-2-yl]methyl](2-mercaptoethyl)amino]ethyl]amino]ethanethiolato(3-)-N2,N2',S2,S2']oxo-[1R-(exo-exo)]) is a potential agent for dopamine transporter (DAT) SPECT, whereas 6-(18)F-fluoro-L-dopa ((18)F-FDOPA) PET has been used for the quantitative assessment of presynaptic nigrostriatal dopaminergic function. The current study investigated the relationship between the 2 imaging modalities in evaluating patients with Parkinson's disease (PD). METHODS: Twenty patients in whom PD was diagnosed by generally accepted criteria were recruited. In addition to visual inspection, specific uptake ratios (SURs) of (99m)Tc-TRODAT-1 in the striatum and putamen were measured bilaterally. For PET, all patients received 100 mg of carbidopa 90 min before (18)F-FDOPA (300 MBq) injection. Images were acquired between 120 and 150 min after injection, using a whole-body PET scanner with settings identical to those for the SPECT studies. The SURs for PET were calculated similarly to those for SPECT. Individual SURs of the striatum or putamen from SPECT were correlated with the corresponding PET values using linear regression. RESULTS: A consistent image pattern between SPECT and PET was achieved by visual inspection except for 3 cases. In 1 case (a patient with Hoehn and Yahr Scale I PD), the SPECT images were more compatible with the patient's clinical findings whereas PET showed nearly normal uptake. In the other 2 cases (both patients with Hoehn and Yahr Scale II PD), PET correlated better with the clinical findings. The caudate and putamen nuclei were more discernable on PET. An acceptable correlation of SUR, however, was found between SPECT and PET in both the striatum and the putamen (P < 0.01 for both). CONCLUSION: The comparability of (99m)Tc-TRODAT-1 SPECT and (18)F-FDOPA PET suggests that (99m)Tc-TRODAT-1 SPECT may provide a reliable alternative to (18)F-FDOPA PET in the evaluation of clinical PD patients.     

Single-photon emission tomography imaging of serotonin transporters in the nonhuman primate brain with [123I]ODAM

We have described previously a selective serotonin transporter (SERT) radioligand, [¹²³I]IDAM. We now report a similarly potent, but more stable IDAM derivative, 5-iodo-2-[2-[(dimethylamino)methyl]phenoxy]benzyl alcohol ([¹²³I]ODAM). The imaging characteristics of this radioligand were studied and compared against [¹²³I]IDAM. Dynamic sequences of single-photon emission tomography (SPET) scans were obtained on three female baboons after injection of 375 MBq of [¹²³I]ODAM. Displacing doses (1 mg/kg) of the selective SERT ligand (+)McN5652 were administered 120 min after injection of [¹²³I]ODAM. Total integrated brain uptake of [¹²³I]ODAM was about 30% higher than [¹²³I]IDAM. After 60–120 min, the regional distribution of tracer within the brain reflected the characteristic distribution of SERT. Peak specific binding in the midbrain occurred 120 min after injection, with an equilibrium midbrain to cerebellar ratio of 1.50±0.08, which was slightly lower than the value for [¹²³I]IDAM (1.80± 0.13). Both the binding kinetics and the metabolism of [¹²³I]ODAM were slower than those of [¹²³I]IDAM. Following injection of a competing SERT ligand, (+)McN5652, the tracer exhibited washout from areas with high concentrations of SERT, with a dissociation kinetic rate constant k _off=0.0085±0.0028 min^–1 in the midbrain. Similar studies using nisoxetine and methylphenidate showed no displacement, consistent with its low binding affinity to norepinephrine and dopamine transporters, respectively. These results suggest that [¹²³I]ODAM is suitable for selective SPET imaging of SERT in the primate brain, with higher uptake and slower kinetics and metabolism than [¹²³I]IDAM, but also a slightly lower selectivity for SERT. Received 1 May and in revised form 31 May 1999     

Phase 1 clinical study of 123I-FP-CIT, a new radioligand for evaluating dopamine transporter by SPECT (II): Tracer kinetics in the brain

The kinetics of 123I-FP-CIT in the brain for healthy subjects were studied. Twelve dynamic SPECT data sets (0- to 6-hr after an intravenous injection) from a Phase 1 clinical trial of 123I-FP-CIT were analyzed. Tracer concentrations in the striatum, midbrain, cerebellum and cerebral cortex were measured on the SPECT images co-registered with the corresponding MR images. High tracer accumulation was observed in the striatum, which peaked at 60 min post-injection, followed by slow elimination (3%/hr). The kinetics were similar both in the cerebellum and in the cerebral cortex, which peaked at 15 min post-injection, followed by rapid elimination. Tracer accumulation in the midbrain was higher than in the cerebellum and cerebral cortex. The striatal specific/nonspecific binding ratio ((striatal-occipital)/occipital concentration ratio) was stable at 3-hr post-injection and later at a value of 3, suggesting that the specific binding of 123I-FP-CIT could be evaluated from a single SPECT image at 3- to 6-hr post-injection. The specific/nonspecific binding ratio at 4-hr post-injection showed a negative correlation with aging (r = -0.70, p = 0.01), with a decrease rate of 11%/decade (95% confidence interval: 3%-19%/decade).