Brain kinetics of the new selective serotonin transporter tracer [123I]ADAM in healthy young adults

Amygdala substructure volumes and serotonin transporter in first-episode, drug- naïve major depressive disorder: A pilot study

INTRODUCTION

Amygdala and serotonergic system abnormalities have been documented in major depressive disorder (MDD). However, most studies have been conducted on recurrent MDD, and only a few have assessed their interaction. This study aimed to concurrently examine both the amygdala and serotonergic systems and their clinical relevance in first-episode, drug-naïve MDD.

METHODS

This study included 27 patients with first-episode, drug-naïve MDD and 27 age- and gender-matched healthy controls (HCs). The amygdala substructure volumes were performed with Freesurfer from a 1.5 T magnetic resonance image. Serotonin transporter (SERT) availability was detected by single-photon emission computed tomography with ¹²³I-ADAM. The Benjamini-Hochberg method was applied to adjust for multiple comparisons.

RESULTS

No significant difference was found in the amygdala substructure volume and SERT availability between the two groups, respectively. Within MDD patients, the right medial, cortical nucleus, and centromedial volumes were positively associated with caudate SERT availability, respectively. Moreover, the right lateral nucleus volume in the amygdala was positively correlated with depression severity. However, these significances did not survive correction for multiple testing.

CONCLUSIONS

There were no significant abnormalities in the amygdala substructure volumes and SERT availability in patients with first-episode, drug-naïve MDD. We did not observe an association between amygdala substructure volume and serotonergic dysregulation and their correlations with depression severity in patients with MDD. A larger sample size is warranted to elucidate the actual correlation.

Extrastriatal binding of [¹²³I]FP-CIT in the thalamus and pons: gender and age dependencies assessed in a European multicentre database of healthy controls

PURPOSE

Apart from binding to the dopamine transporter (DAT), [(123)I]FP-CIT shows moderate affinity for the serotonin transporter (SERT), allowing imaging of both monoamine transporters in a single imaging session in different brain areas. The aim of this study was to systematically evaluate extrastriatal binding (predominantly due to SERT) and its age and gender dependencies in a large cohort of healthy controls.

METHODS

SPECT data from 103 healthy controls with well-defined criteria of normality acquired at 13 different imaging centres were analysed for extrastriatal binding using volumes of interest analysis for the thalamus and the pons. Data were examined for gender and age effects as well as for potential influence of striatal DAT radiotracer binding.

RESULTS

Thalamic binding was significantly higher than pons binding. Partial correlations showed an influence of putaminal DAT binding on measured binding in the thalamus but not on the pons. Data showed high interindividual variation in extrastriatal binding. Significant gender effects with 31 % higher binding in women than in men were observed in the thalamus, but not in the pons. An age dependency with a decline per decade (±standard error) of 8.2 ± 1.3 % for the thalamus and 6.8 ± 2.9 % for the pons was shown.

CONCLUSION

The potential to evaluate extrastriatal predominant SERT binding in addition to the striatal DAT in a single imaging session was shown using a large database of [(123)I]FP-CIT scans in healthy controls. For both the thalamus and the pons, an age-related decline in radiotracer binding was observed. Gender effects were demonstrated for binding in the thalamus only. As a potential clinical application, the data could be used as a reference to estimate SERT occupancy in addition to nigrostriatal integrity when using [(123)I]FP-CIT for DAT imaging in patients treated with selective serotonin reuptake inhibitors.

SERT-to-DAT ratios in early Parkinson's disease do not correlate with the development of dyskinesias

Background

Although the treatment of Parkinson’s disease (PD) is very effective, in the course of the disease, 40% to 60% of patients develop dyskinesias. The pathophysiology of dyskinesias is still unclear. Results of preclinical research suggest that uptake and uncontrolled release of dopamine by serotonergic neurons is an important factor. Based on this model, we hypothesized that dyskinesias will develop predominantly in PD patients with a relatively preserved serotonergic system.

Methods

Between 1995 and 1998, 50 patients with early-stage untreated PD, diagnosed according to clinical criteria, and reduced striatal [¹²³I]β-carboxymethyoxy-3-beta-(4-iodophenyl) tropane (CIT) single-photon emission computed tomography (SPECT) binding were recruited. To test our hypothesis, we retrospectively assessed baseline [¹²³I]β-CIT SPECT scans for striatal dopamine transporter (DAT) and midbrain serotonin transporter (SERT) availability as well as the SERT-to-DAT ratios. We compared these data between patients that developed dyskinesias and patients that did not develop dyskinesias during a mean follow-up of 14.2 years.

Results

Approximately half of the PD patients developed dyskinesias. No differences in baseline [¹²³I]β-CIT DAT availability, SERT availability, or SERT-to-DAT ratios were found between the dyskinetic and non-dyskinetic group. The development of dyskinesias was most strongly associated with the age of onset (P = 0.002).

Conclusions

SERT-to-DAT ratios in early-stage untreated PD do not correlate with the future development of dyskinesias. However, our study does not exclude the possibility that SERT-to-DAT ratios increase with disease progression in patients that develop dyskinesias because of a slower rate of degeneration of the serotonergic system.

Effects of [123I]ADAM, a serotonin transporter radiopharmaceutical, on pregnant Sprague-Dawley rats

Serotonin transport abnormalities are implicated in neuropsychiatric disorders. [(123)I]ADAM ([(123)I]-2-([2-({dimethylamino}methyl)phenyl]thio)-5-iodophenylamine) is a novel radiotracer that targets serotonin transporters. We assessed the toxicity of [(123)I]ADAM (18.5 MBq) administered in early- and late-phases (8 and 14 day postfertilization, respectively) of pregnancy. The mortality, clinical status, and gross necropsy were measured in pregnant rats, and the fertility index was measured in rat offspring (weight, clinical observations). We found no dosing-related clinical signs. In conclusion, [(123)I]ADAM was not toxic in an animal pregnancy model.

Assessing the optimal time point for the measurement of extrastriatal serotonin transporter binding with 123I-FP-CIT SPECT in healthy, male subjects

(123)I-N-ω-fluoropropyl-2β-carboxymethoxy-3β-(4-iodophenyl)nortropane ((123)I-FP-CIT) is commonly used to assess the dopamine transporter in the striatum. However, recent studies suggest that this tracer may be used also to assess binding to monoamine transporters in the midbrain or diencephalon, which may reflect predominantly serotonin transporter (SERT) binding. However, it is still unclear at what time point after injection SPECT should be performed for optimal assessment of SERT with(123)I-FP-CIT. Therefore, we examined the time course of extrastriatal (123)I-FP-CIT binding.

METHODS

Nineteen healthy, male subjects were included, and SPECT images were acquired up to 3 h after (123)I-FP-CIT injection. Region-of-interest analysis was performed, and specific-to-nonspecific binding ratios were calculated.

RESULTS

Specific-to-nonspecific (123)I-FP-CIT binding ratios in the midbrain and diencephalon were significantly higher 2 h after injection than 1 h after injection and remained stable between 2 and 3 h after injection.

CONCLUSION

The optimal time frame for assessing (123)I-FP-CIT binding to extrastriatal SERT is between 2 and 3 h after injection of the tracer.

Development of acute and subacute toxicity with the serotonin transporter radiopharmaceutical, ADAM

It is predicted that depression will become the most common neurological disease in the new millennium. Its incidence is currently about 3% of diseases worldwide. Serotonin is an essential neurotransmitter for the central and peripheral nervous systems and plays a crucial role in neuropsychiatric disorders. (123)I-labeled ADAM was developed to facilitate an early diagnosis of serotonin transporter (SERT) abnormalities in the brain. Many studies have confirmed that the binding of this radiotracer to SERTs is associated with depression. The aim of this study was to evaluate the acute and subacute toxicity of ADAM and to determine its no observed adverse effect level (NOAEL) by administering it via intravenous injection to Sprague-Dawley rats for 14 consecutive days. None of the animals died, and no treatment-related clinical signs were observed. Urinalysis, hematology, and clinical chemistry analysis revealed that daily administration of ADAM (2-2-dimethylaminomethylphenylthio-5-iodophenylamine) for 2 weeks had no toxicological effects. It is concluded that ADAM exerts no adverse toxic effects on this animal model. The NOAEL was 155 microg/kg/day.

The SPECT tracer [123I]ADAM binds selectively to serotonin transporters: a double-blind, placebo-controlled study in healthy young men

Purpose

The tracer ¹²³I-2-([2-({dimethylamino}methyl)phenyl]thio)-5-iodophenylamine ([¹²³I]ADAM) has been developed to image serotonin transporters (SERTs) with SPECT. Preclinical studies have shown that [¹²³I]ADAM binds selectively to SERTs. Moreover, initial human studies have shown that [¹²³I]ADAM binding could be blocked by selective serotonin reuptake inhibitors (SSRIs). However, in humans it has not been proven that [¹²³I]ADAM binds selectively to SERTs.

Methods

We examined the in vivo availability of SERTs in 12 healthy young volunteers 5 h after bolus injection of [¹²³I]ADAM. To evaluate the selectivity of binding, four participants were pretreated (double-blinded design) with placebo, four with paroxetine (20 mg) and four with the dopamine/norepinephrine blocker methylphenidate (20 mg). SPECT studies were performed on a brain-dedicated system (Neurofocus), and the SPECT images were coregistered with individual MR scans of the brain. ADAM binding in SERT-rich brain areas and cerebellar cortex (representing non-specific binding) was assessed by drawing regions of interest (ROIs) on the individual MR images. Specific to non-specific ratios were used as the outcome measure.

Results

We found that specific to non-specific ratios were statistically significantly lower in paroxetine-pretreated participants than in placebo- or methylphenidate-pretreated participants. No such difference was found between groups pretreated with placebo or methylphenidate.

Conclusion

Our preliminary findings suggest that [¹²³I]ADAM binds selectively to SERTs in human brain.

Differences in the dynamics of serotonin reuptake transporter occupancy may explain superior clinical efficacy of escitalopram versus citalopram

Escitalopram the S-enantiomer of the racemate citalopram, is clinically more effective than citalopram in the treatment of major depressive disorder. However, the precise mechanism by which escitalopram achieves superiority over citalopram is yet to be determined. It has been hypothesized that the therapeutically inactive R-enantiomer competes with the serotonin-enhancing S-enantiomer at a low-affinity allosteric site on serotonin reuptake transporters (SERTs), and reduces the effectiveness of the S-enantiomer at the primary, high-affinity serotonin-binding site. This study summarizes the results of two recent single-photon emission computerized tomography studies measuring SERT occupancy in citalopram-treated and escitalopram-treated healthy volunteers, after a single dose and multiple doses (i.e. under steady-state conditions). The single-dose study showed no attenuating effect of R-citalopram. After multiple dosing, however, SERT occupancy was significantly reduced in the presence of R-citalopram. Under steady-state conditions, R-enantiomer concentrations were greater than for the S-enantiomer because of slower clearance of R-citalopram. A pooled analysis suggests that build-up of the R-enantiomer after repeated citalopram dosing may lead to increased inhibition of S-enantiomer occupancy of SERT. This review adds to the growing body of evidence regarding differences in the dynamics of SERT occupancy, that is, molecular mechanisms underlying the often-observed superior clinical efficacy of escitalopram compared with citalopram in major depressive disorder.

Evaluation of the Serotonin Transporter Ligand 123I-ADAM for SPECT Studies on Humans

Imaging serotonin transporters in the living human brain is important in several fields, such as normal psychophysiology, mood disorders, eating disorders, and neurodegenerative disorders. The aim of this study was to compare different kinetic and semiquantitative methods for assessing serotonin transporters using (123)I-labeled 2-((2-((dimethylamino)methyl)phenyl)thio)-5-iodophenylamine (ADAM) in humans: an arterial plasma input model, simplified and Logan reference tissue models, and standardized uptake value ratios.

METHODS

Nine subjects were scanned with dynamic (123)I-ADAM SPECT (mean age, 31 y; range, 24-43 y), and metabolite-corrected arterial input was measured. Tissue reference models (simplified reference tissue model, Logan reference tissue model, and ratio method) were validated against the outcome of a 1-tissue-compartment model, and performance with decreasing scan length was evaluated. The specificity of (123)I-ADAM binding was investigated in a blocking experiment.

RESULTS

Binding estimates from the simplified reference tissue and Logan reference tissue models correlated tightly with full kinetic modeling when based on a 240- or 360-min dynamic acquisition (r = 0.99); however, there were slight underestimations (3%-5%), especially in high-binding regions. Application of the ratio method to data from 200 to 240 min overestimated specific binding (on average, by 10% +/- 28%) and correlated only moderately with estimates from the 1-tissue-compartment model (r = 0.94). With an acquisition time of 0-120 min, the Logan model still yielded an acceptable outcome when a fixed clearance rate constant (k2') from the cerebellum was applied. Intravenously injected citalopram was not associated with a decrease in cerebellar binding. A lipophilic metabolite that did not seem to bind specifically to serotonin transporter was seen in 2 of 7 subjects.

CONCLUSION

Serotonin transporter binding with (123)I-ADAM SPECT can be assessed with the Logan model based on a 120-min acquisition when a constant k2' is applied. This model, because it allows for more accurate and less biased binding estimates and thus reduces the required sample size, is advantageous over the ratio method used in clinical studies so far. A single blocking experiment supported the use of the cerebellum as a reference region.

Synthesis and in vivo evaluation of halogenated N,N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine derivatives as PET serotonin transporter ligands

N, N-dimethyl-2-(2'-amino-4'-hydroxymethylphenylthio)benzylamine (38), substituted on ring A, was reported to display high binding affinity and selectivity to the human brain serotonin transporter (SERT). In an attempt to explore the potential of compounds substituted on ring B of the phenylthiophenyl core structure, three derivatives of 38 were synthesized: N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-fluorobenzylamine (35), N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-bromobenzylamine (36), and N, N-dimethyl-2-(2'-amino-4'-hydroxymethyl-phenylthio)-5-iodobenzylamine (37). The in vitro binding studies in cells transfected with human SERT, norepinephrine transporter (NET), and dopamine transporter (DAT) showed that 35, 36, and 37 exhibited high SERT affinity with K is (SERT) = 1.26, 0.29, and 0.31 nM (vs [(3)H]citalopram), respectively. [(11)C]-(35), [(11)C]-(36), and [(11)C]-( 37) were prepared by methylation of their monomethyl precursors 16, 17, and 18, with [(11)C]iodomethane in 28, 11, and 14% radiochemical yields, respectively. The microPET images of [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) showed high uptake in the monkey brain regions rich in SERT with peak midbrain to cerebellum ratios of 3.41, 3.24, and 3.00 at 85 min post-injection, respectively. In vivo bindings of [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) were shown to be specific to the SERT as displacement with citalopram (a potent SERT ligand) reduced radioactivity in SERT-rich regions to the cerebellum level. These results suggest that [(11)C]-(35), [(11)C]-(36), and [(11)C]-(37) could be potential agents for mapping human SERT by PET and radiolabeling 37 with iodine-123, which could afford the first SPECT SERT imaging agent exhibiting fast kinetics.

5-Chloro-2-(2'-((dimethylamino)methyl)-4'-iodophenylthio)benzenamine: a new serotonin transporter ligand

Two novel ligands with 4' substitution on the Phenyl Ring B of biphenylthiol, 5-chloro-2-(2'-((dimethylamino)methyl)-4'-iodophenylthio)benzenamine (7) and 2-(2'-((dimethylamino)methyl)-4'-methoxyphenylthio)-5-iodobenzenamine (8), were prepared and tested as potential serotonin transporter (SERT) imaging agents. The new ligands displayed extremely high binding affinities to SERT (K(i)=0.22+/-0.09 and 0.11+/-0.04 nM, respectively), with very low binding affinities to dopamine and norepinephrine transporters (K(i)>1000 nM). The corresponding [(125)I]7 and [(125)I]8 were successfully prepared from tri-n-butyltin derivatives. They showed good brain uptakes and prolonged retention after intravenous injection in rats (brain uptake was 1.77% and 0.98% dose/g for [(125)I]7, and 0.92% and 0.29% dose/g for [(125)I]8, at 2 and 120 min, respectively). Significantly, [(125)I]7 showed excellent uptake and prolonged retention in the hypothalamus, where SERT concentration was highest. The hypothalamus/cerebellum (HY/CB) ratios (target/background ratios) were 4.24, 7.10, 8.24 and 12.6 at 2, 4, 6 and 12 h, respectively. The HY/CB ratios for [(125)I]8 were 3.97, 5.57 and 5.06 at 1, 2 and 4 h, respectively. Adding the 4'-iodo group to the Phenyl Ring B of Compound (7) appeared to reduce the rate of clearance from the brain, and kinetics favored uptake and retention in the hypothalamus. The localization of [(125)I]7 in the hypothalamus region in the rat brain could be blocked by pretreatment with (+)McN5652, escitalopram and ADAM (2), which are all selective SERT ligands (at 2 mg/kg iv, 5 min pretreatment). Ex vivo autoradiograms of rat brain sections (at 4 h after intravenous injection of [(125)I]7) showed intense labeling in regions of the brain known to have high SERT density. The excellent selective uptake and retention in the hypothalamus region suggest that [(123)I]7 is a potential lead compound for developing new imaging agents targeting SERT-binding sites with single-photon emission computed tomography.