Iodo-QNB cortical binding and brain perfusion: effects of a cholinergic basal forebrain lesion in the rat

This study deals with the question of whether in vivo application of [125I]iodo-quinuclidinyl-benzilate (QNB) is able to demonstrate changes in cortical muscarinic receptor density induced by a cholinergic immunolesion of the rat basal forebrain cholinergic system, and whether the potential effects on IQNB distribution in vivo are also associated with effects on regional cerebral perfusion. Immunolesioned and control animals were injected with (R,S) [125]iodo-QNB and with [99mTc]-d,l-hexamethylpropyleneamine oxime (HMPAO). The cerebral distribution of both tracers was imaged using double tracer autoradiography. Impaired cholinergic transmission was paralleled by a 10-15% increase of [125I]iodo-QNB binding in the regions of cortex and hippocampus. The local cerebral blood flow remained unchanged after cholinergic lesion.     

In vitro and in vivo characterization of 4-[125I]iododexetimide binding to muscarinic cholinergic receptors in the rat heart

4-[125I]iododexetimide binding to muscarinic cholinergic receptors (mAChR) was evaluated in the rat heart. 4-[125I]iododexetimide displayed high in vitro affinity (Kd = 14.0 nM) for rat myocardial mAChR. In vivo, there was high accumulation of 4-[125I]iododexetimide in the rat atrium and ventricle which could be blocked by approximately 60% by preinjection of atropine. In contrast, accumulation of the radiolabeled stereoisomer, 4-[125I]iodolevetimide, was 63% lower than 4-[125I]iodolevetimide and was not blocked by atropine. The blood clearance of 4-[125I]iododexetimide was rapid, providing heart-to-blood ratios of up to 14:1; however, heart-to-lung and heart-to-liver ratios were below unity. The data indicate that 4-[125I]iododexetimide binds potently to rat mAChR. However, since nonspecific binding is relatively high, it is not clear whether iododexetimide labeled with 123I will be useful in SPECT imaging studies of myocardial mAChR. Further studies in humans are indicated.     

In vivo imaging of the spinal cord cholinergic system with PET

PURPOSE: Our goal was to demonstrate the feasibility of an in vivo noninvasive method for imaging spinal cord cholinergic terminals using (+)-4-[18F]fluorobenzyltrozamicol ([18F]FBT) and PET. METHOD: In vitro and in vivo experiments in rats were conducted to demonstrate the specific binding characteristics, localization, and time course of [3H]FBT binding in the spinal cord. PET imaging was then performed on seven rhesus monkeys. RESULTS: The rat studies demonstrate high specific binding in the spinal cord with a distribution coinciding with the known distribution of cholinergic terminals. In vivo tracer concentrations in the spinal cord and basal ganglia were of the same magnitude. With use of [18F]FBT and PET in the rhesus monkey, the spinal cord was clearly visualized, with tracer concentration in the spinal cord being approximately one-fourth of that seen in the basal ganglia. CONCLUSION: This work demonstrates the feasibility of imaging cholinergic terminals in vivo in the spinal cord using [18F]FBT and PET.     

Influence of acetylcholine on binding of 4-[i]iododexetimide to muscarinic brain receptors

The distribution of nicotinic and muscarinic cholinergic receptors in the human brain in vivo has been successfully characterized using radiolabeled tracers and emission tomography. The effect of acetylcholine release into the synaptic cleft on receptor binding of these tracers has not yet been investigated. The present study examined the influence of acetylcholine on binding of 4-[¹²⁵I]iododexetimide to muscarinic cholinergic receptors of porcine brain synaptosomes in vitro. 4-Iododexetimide is a subtype-unspecific muscarinic receptor antagonist with high affinity. Acetylcholine competed with 4-[¹²⁵I]iododexetimide in a dose-dependent manner. A concentration of 500 μM acetylcholine inhibited 50% of total specific 4-[¹²⁵I]iododexetimide binding to synaptosomes when both substances were given simultaneously. An 800 μM acetylcholine solution reduced total specific 4-[¹²⁵I]iododexetimide binding by about 35%, when acetylcholine was given 60 min after incubation of synaptosomes with 4-[¹²⁵I]iododexetimide. Variations in the synaptic acetylcholine concentration might influence muscarinic cholinergic receptor imaging in vivo using 4-[¹²³I]iododexetimide. Conversely, 4-[¹²³I]iododexetimide might be an appropriate molecule to investigate alterations of acetylcholine release into the synaptic cleft in vivo using single photon emission computed tomography.     

Reproducibility of repeated measures of cholinergic terminal density using.

[18F](+)-4-fluorobenzyltrozamicol (FBT), which selectively binds to the vesicular acetylcholine transporter in the presynaptic cholinergic neuron, has previously been shown to be a useful ligand for the study of cholinergic terminal density in the basal ganglia with PET. The goal of this study was to assess the test-retest variability of [18F]FBT and PET measurements under baseline conditions in the basal ganglia. METHODS: After approval from the Animal Care and Use Committee, 6 rhesus monkeys underwent a series of 2 [18F]FBT PET scans (time between scans, 32-301 d) under isoflurane anesthesia. Each scan was initiated on the bolus injection of the radiotracer and consisted of 26 frames acquired during 180 min. Arterial blood samples were collected over the course of each scan to determine the metabolite-corrected arterial input function. Tissue time-activity curves were obtained from the scan data by drawing regions of interest over the basal ganglia and cerebellum. The distribution volume ratio for the basal ganglia was then determined for each scan by taking the ratio of the basal ganglia (specific binding) to cerebellum (nonspecific binding) distribution volume. Distribution volumes were derived using the Logan graphic analysis technique as well as a standard 3-compartment model. Additionally, the radioactivity concentration ratio was calculated as the ratio of the average [18F]FBT concentration in the basal ganglia to that in the cerebellum during the last half of the study (85-170 min). The constant K1, determined using the standard 3-compartment model, was used as an index of blood flow changes between studies. RESULTS: For all subjects, the test-retest variability was less than 15% for the distribution volume ratio and 12% for the radioactivity concentration ratio. Good agreement was found between the distribution volume ratio calculated using the graphic technique and the standard 3-compartment model. Using K1 as an index, the variability in blood flow seen in both the basal ganglia and the cerebellum was significantly reduced in their ratio. CONCLUSION: These results show the reproducibility of [18F]FBT and PET measurements in the basal ganglia.     

The potential of radioiodinated (-)-m-iodovesamicol for diagnosing cholinergic deficit dementia

We investigated changes in the brain distribution of (-)-[(125)I]-m-iodovesamicol [(-)-[(125)I]mIV] in cholinergic denervation rats produced by a unilateral lesion of the nucleus basalis magnocellularis (NBM). Dual-tracer ex vivo autoradiographic analysis using (-)-[(125)I]mIV and [(99m)Tc]HMPAO was conducted to the effect of regional cerebral perfusion on the brain distribution of (-)-[(125)I]mIV in a unilateral NBM-lesioned rat. (-)-[(125)I]mIV binding in the ipsilateral cortex to the lesion significantly reduced by 10.4 %, compared with that in the contralateral cortex, while (-)-[(125)I]mIV binding in the ipsilateral caudate putamen, hippocampus and thalamus did not change. The rate of reduction in the (-)-[(125)I]mIV binding (10.4 %) was significantly higher than that of [(99m)Tc]HMPAO accumulation (4.0%) in the ipsilateral cortex to the lesion (P < 0.01). These results suggested that radioiodinated (-)-mIV may be useful in the study of dementia characterized by degeneration of the cholinergic neurotransmitter system, such as Alzheimer's disease.     

(E)-[125I]-5-AOIBV: a SPECT radioligand for the vesicular acetylcholine transporter

The premise that, over the course of Alzheimer's disease (AD), changes in the levels of the vesicular acetylcholine transporter (VAChT) occur in parallel with changes to other cholinergic marker proteins provides the basis for the applicability of benzovesamicol derivatives as radioligands for AD studies by single photon emission computed tomography or positron emission tomography. We report the synthesis of enantiopure benzovesamicol derivatives: (R,R) or (S,S)-(E)-2-hydroxy-5-(3-iodoprop-2-en-1-oxy)-3-(4-phenylpiperidino)tetralin [(R,R)-AOIBV: K_d=0.45 nM or (S,S)-5-AOIBV: K_d=4.3 nM] and their corresponding tributyltin precursors for radioiodination. (R,R or S,S)-5-AOIBV was labeled with iodine-125 from their corresponding n-tributyltin precursors. Both compounds were obtained with radiochemical and optical purity greater than 97% and in radiochemical yields ranging 34–36%. To determine if these compounds could provide an advantage when compared to [¹²⁵I]-iodo benzovesamicol (IBVM), IBVM was also labeled and used as the reference compound in all ex vivo experiments. Ex vivo biodistribution experiments in rats revealed that [¹²⁵I]-(R,R)-5-AOIBV displayed the most suitable pharmacological profile as the radioactivity distribution corresponded well with the known VAChT brain density. Moreover, pre-injection of vesamicol prevented the uptake of [¹²⁵I]-(R,R)-5-AOIBV in striatum, cortex and hippocampus, demonstrating selectivity for the VAChT. However, even if time activity curves of [¹²⁵I]-(R,R)-5-AOIBV confirmed that this compound could be used to visualize the VAChT in vivo, at each point of the kinetic study, [¹²⁵I]-(R,R)-5-AOIBV showed a lower specific binding compared to [¹²⁵I]-IBVM. These results made [¹²⁵I]-( R,R)-5-AOIBV inferior to [¹²⁵I]-IBVM for the VAChT exploration in vivo.     

Fundamental study on nuclear medicine imaging of cholinergic innervation in the brain: changes of neurotransmitter and receptor in animal model of Alzheimer's disease

A fundamental study was performed on the nuclear medicine imaging of cholinergic innervation in the brain. In a cholinergic denervation model prepared by producing an unilateral basal forebrain lesion in the rat, which is reported to be one of animal models of Alzheimer's disease, quantitative determination of acetylcholine in parietal cortices revealed statistically significant 31% decrease on an average in the ipsilateral side relative to the contralateral side to the lesion. In vitro receptor autoradiography showed no significant differences in total, M1 and M2 muscarinic acetylcholine receptors between the ipsilateral and contralateral cortices to the lesion. Simultaneous mapping of presynaptic cholinergic innervation using 3H-2-(4-phenylpiperidino) cyclohexanol (AH5183) demonstrated significant 14% decrease of AH5183 binding on an average in the ipsilateral relative to the contralateral fronto-parieto-temporal cortices to the lesion. These results suggest that AH5183 is a promising ligand for mapping cholinergic innervation in nuclear medicine imaging.     

Effects of atropine treatment on in vitro and in vivo binding of 4-[125I]-dexetimide to central and myocardial muscarinic receptors

Upregulation of muscarinic cholinergic receptors (mAChR) after chronic atropine treatment has been described previously. The present study was designed to evaluate 4-iodine-125 dexetimide as an agent to determine changes in the number of mAChR. Rats were injected subcutaneously with atropine (500 mg/kg) either once or chronically, once daily for 10 days, and sacrificed 24 h later. In vitro binding assays with 4-[125I]-dexetimide showed significant increases in the number of mAChR in cerebra (21%) and ventricles (45%) after chronic atropine treatment but not after acute treatment. The affinity of binding to cerebral and ventricular mAChR declined after acute and chronic atropine treatment. In vivo studies were carried out involving intravenous injection of 4-[125I]-dexetimide 24 h after atropine treatment. Binding was markedly reduced in the brain and heart. Upregulation of mAChR, as seen in in vitro studies, could not be observed because of the remaining atropine. Occupancy of mAChR by atropine persisted as long as 7 days after one dose. The results of these studies indicate that 4-[125I]-dexetimide binding reflects the effects of atropine on central and peripheral muscarinic cholinergic receptors in vitro and in vivo.     

In vivo imaging of muscarinic cholinergic receptors in temporal lobe epilepsy with a new PET tracer: [76Br]4-bromodexetimide.

Muscarinic acetyl cholinergic receptors (mAChRs) may be involved in the pathophysiology of partial epilepsy. Previous experimental and imaging studies have reported medial temporal abnormalities of mAChR in patients with medial temporal lobe epilepsy (MTLE). Suitable radiotracers for mAChR are required to evaluate these disturbances in vivo using PET. Dexetimide is a specific mAChR antagonist that has been labeled recently with 76Br. This first study in humans focused on regional distribution and binding kinetics of [76Br]4-bromodexetimide (BDEX) in patients with MTLE. METHODS: Ten patients with well-lateralized MTLE had combined MRI, 18F-fluorodeoxyglucose (FDG) PET and 76Br-BDEX PET studies. Time-activity curves were generated in PET-defined regions of interest, including the medial, polar and lateral regions of the temporal lobe; the basal ganglia; the external and medial occipital cortex; and the white matter. RESULTS: The highest radioactivity concentration was observed in the basal ganglia and in the cortical regions, whereas radioactivity was lower in the white matter. On late images of PET studies, 76Br-BDEX uptake was statistically significantly decreased only in the medial temporal region ipsilateral to the seizure focus (1.37 +/-0.28, P < 0.01) as determined by FDG PET imaging, anatomic MRI and electroencephalogram correlation, compared with the contralateral medial temporal region (1.46 +/- 0.31). CONCLUSION: 76Br-BDEX concentration is reduced in the temporal lobe ipsilateral to the seizure focus in patients with MTLE. This preliminary study suggests that 76Br-BDEX is a suitable radiotracer for studies of mAChR in humans. Further studies are required to investigate the potential value of 76Br-BDEX PET in other neurological disorders with muscarinic disturbances.     

Effects of atropine treatment on in vitro and in vivo binding of 4-[125I]-dexetimide to central and myocardial muscarinic receptors

Upregulation of muscarinic cholinergic receptors (mAChR) after chronic atropine treatment has been described previously. The present study was designed to evaluate 4-iodine-125 dexetimide as an agent to determine changes in the number of mAChR. Rats were injected subcutaneously with atropine (500 mg/kg) either once or chronically, once daily for 10 days, and sacrificed 24 h later. In vitro binding assays with 4-[¹²⁵I]-dexetimide showed significant increases in the number of mAChR in cerebra (21%) and ventricles (45%) after chronic atropine treatment but not after acute treatment. The affinity of binding to cerebral and ventrical mAChR declined after acute and chronic atropine treatment. In vivo studies were carried out involving intravenous injection of 4-[¹²⁵I]-dexetimide 24 h after atropine treatment. Binding was markedly reduced in the brain and heart. Upregulation of mAChR, as seen in in vitro studies, could not be observed because of the remaining atropine. Occupancy of mAChR by atropine persisted as long as 7 days after one dose. The results of these studies indicate that 4-[¹²⁵I]-dexetimide binding reflects the effects of atropine on central and peripheral muscarinic cholinergic receptors in vitro and in vivo. Offprint requests to: U. Scheffel     

Synthesis and evaluation of [125I]I-TSA as a brain nicotinic acetylcholine receptor alpha7 subtype imaging agent

INTRODUCTION: Some in vitro investigations have suggested that the nicotinic acetylcholine receptor (nAChR) alpha7 subtype is implicated in Alzheimer's disease, schizophrenia and others. Recently, we developed (R)-3'-(5-bromothiophen-2-yl)spiro[1-azabicyclo[2.2.2]octane-3,5'-[1',3']oxazolidin]-2'-one (Br-TSA), which has a high affinity and selectivity for alpha7 nAChRs. Therefore we synthesized (R)-3'-(5-[125I]iodothiophen-2-yl)spiro[1-azabicyclo[2.2.2]octane-3,5'-[1',3']oxazolidin]-2'-one ([125I]I-TSA) and evaluated its potential for the in vivo detection of alpha7 nAChR in brain. METHODS: In vitro binding affinity of I-TSA was measured in rat brain homogenates. Radioiodination was accomplished by a Br-I exchange reaction. Biodistribution studies were undertaken in mice by tail vein injection of [(125)I]I-TSA. In vivo receptor blocking studies were carried out by treating mice with methyllycaconitine (MLA; 5 nmol/5 mul, i.c.v.) or nonradioactive I-TSA (50 micromol/kg, i.v.). RESULTS: I-TSA exhibited a high affinity and selectivity for the alpha7 nAChR (K(i) for alpha7 nAChR = 0.54 nM). Initial uptake in the brain was high (4.42 %dose/g at 5 min), and the clearance of radioactivity was relatively slow in the hippocampus (alpha7 nAChR-rich region) and was rather rapid in the cerebellum (alpha7 nAChR poor region). The hippocampus to cerebellum uptake ratio was 0.9 at 5 min postinjection, but it was increased to 1.8 at 60 min postinjection. Although the effect was not statistically significant, administration of I-TSA and MLA decreased the accumulation of radioactivity in hippocampus. CONCLUSION: Despite its high affinity and selectivity, [125I]I-TSA does not appear to be a suitable tracer for in vivo alpha7 nAChR receptor imaging studies due to its high nonspecific binding. Further structural optimization is needed.     

IMPY, a potential beta-amyloid imaging probe for detection of prion deposits in scrapie-infected mice

INTRODUCTION: A potential single-photon emission computed tomography imaging agent for labeling of A beta plaques of Alzheimer's disease, IMPY (2-(4'-dimethylaminophenyl)-6-iodo-imidazo[1,2-a]pyridine), would be effective in detection of prion amyloid deposits in transmissible spongiform encephalopathies (TSEs). METHODS: In vitro autoradiographic studies were carried out with [125 I]IMPY on brain sections from scrapie-infected mice and age-matched controls. Competition study was performed to evaluate the prion deposit binding specificity with nonradioactive IMPY. RESULTS: Binding of [125 I]IMPY was observed in infected brain sections, while on age-matched control brain sections, there was no or very low labeling. Prion deposit binding was confirmed by histoblots with prion protein-specific monoclonal antibody 2D6. In the presence of nonradioactive IMPY, the binding of [125 I]IMPY was significantly inhibited in all regions studied. CONCLUSIONS: These findings indicate that IMPY can detect the prion deposits in vitro in scrapie-infected mice. Labeled with 123 I, this ligand may be useful to quantitate prion deposit burdens in TSEs by in vivo imaging.     

PET imaging with [18F]fluoroethoxybenzovesamicol ([18F]FEOBV) following selective lesion of cholinergic pedunculopontine tegmental neurons in rat

Introduction[¹⁸F]fluoroethoxybenzovesamicol ([¹⁸F]FEOBV) is a PET radiotracer with high selectivity and specificity to the vesicular acetylcholine transporter (VAChT). It has been shown to be a sensitive in vivo measurement of changes of cholinergic innervation densities following lesion of the nucleus basalis of Meynert (NBM) in rat. The current study used [¹⁸F]FEOBV with PET imaging to detect the effect of a highly selective lesion of the pedunculopontine (PPTg) nucleus in rat.MethodsAfter bilateral and selective lesions of the PPTg cholinergic neurons, rats were scanned using [¹⁸F]FEOBV, then sacrificed, and their brain tissues collected for immunostaining and quantification of the VAChT.ResultsComparisons with control rats revealed that cholinergic losses can be detected in the brainstem, lateral thalamus, and pallidum by using both in vivo imaging methods with [¹⁸F]FEOBV, and ex vivo measurements. In the brainstem PPTg area, significant correlations were observed between in vivo and ex vivo measurements, while this was not the case in the thalamic and pallidal projection sites.ConclusionsThese findings support PET imaging with [¹⁸F]FEOBV as a reliable in vivo method for the detection of neuronal terminal losses resulting from lesion of the PPTg. Useful applications can be found in the study of neurodegenerative diseases in human, such as Parkinson’s disease, multiple system atrophy, progressive supranuclear palsy, or dementia with Lewy bodies.     

Radiosynthesis and preliminary evaluation of 5-[123/125I]iodo-3-(2(S)-azetidinylmethoxy)pyridine: a radioligand for nicotinic acetylcholine receptors

The radiochemical syntheses of 5-[125I]iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-[125I]-iodo-A-85380, [125I]1) and 5-[123I]-iodo-A-85380, [123I]1, were accomplished by radioiodination of 5-trimethylstannyl-3-((1-tert-butoxycarbonyl-2(S)-azetidinyl)metho xy)pyridine, 2, followed by acidic deprotection. Average radiochemical yields of [125I]1 and [123I]1 were 40-55%; and the average specific radioactivities were 1,700 and 7,000 mCi/mumol, respectively. Binding affinities of [125I]1 and [123I]1 in vitro (rat brain membranes) were each characterized by a Kd value of 11 pM. Preliminary in vivo assay and ex vivo autoradiography of mouse brain indicated that [125I]1 selectively labels nicotinic acetylcholine receptors (nAChRs) with very high affinity and specificity. These studies suggest that [123I]1 may be useful as a radioligand for single photon emission computed tomography (SPECT) imaging of nAChRs.     

Synthesis and biological characterization of stable and radioiodinated (+/-)-trans-2-hydroxy-3-P[4-(3-iodophenyl)piperidyl]-1,2,3,4-tetrahydronaphthalene (3'-IBVM)

The vesamicol analogue (±)-trans-2-Hydroxy-3-{4-(3-iodophenyl)piperidyl}-1,2,3,4-tetrahydronaphthalene (3′-IBVM), a potent ligand for the vesicular acetylcholine transporter (VAChT), was evaluated as a potential radiotracer for studying VAChT density in vivo. In radioligand binding experiments, 3′-IBVM displays subnanomolar affinity for VAChT and 100-fold selectivity for VAChT over σ1 and σ2 receptors. Consistent with this profile, radioiodinated (±)-3′-IBVM distributed heterogenously in the rat brain following a bolus IV injection, displaying high concentrations in the striatum and moderate to low concentrations in the cortex and cerebellum, respectively. However, co-injection of the radiotracer with the sigma ligand haloperidol resulted in significant reductions of radiotracer levels in all brain regions examined. Therefore, radioiodinated (±)-IBVM appears to bind to both VAChT and sigma receptors in vivo.     

Evaluation of radioiodinated (−)-<Emphasis Type="Italic">o</Emphasis>-iodovesamicol as a radiotracer for mapping the vesicular acetylcholine transporter

We evaluated the potencies of radioiodinated (-)-o-iodovesamicol [(-)-oIV] as a selective vesicular acetylcholine transporter (VAChT) mapping agent. (-)-[125I]oIV exhibited significant accumulation (about 2.8% of the injected dose) in rat brain. The regional brain distribution of radioactivity was similar for both (-)-[125I]olV and (-)-[3H]vesamicol. The accumulation of (-)-[125I]oIV in the brain was significant reduced by post-administration of unlabeled vesamicol (0.5 /micromol/kg(-1)) and (-)-oIV (0.5 micromol/kg(-1)). On the other hand, the post-administration of sigma ligands hardly affected the accumulation of (-)-[125I]oIV in the brain. These studies showed that (-)-[125I]oIV, as well as [3H] vesamicol, bound to VAChT with high affinity in the rat brain. Furthermore, (-)-[125I]oIV binding in the ipsilateral cortex to the lesion was significantly reduced by 17.0%, compared with that in the contralateral cortex in a unilateral NBM-lesioned rat. These results suggested that radioiodinated (-)-oIV may potentially be useful for the diagnosis of cholinergic neurodegenerative disorders.     

Cholinergic imaging in dementia spectrum disorders

The multifaceted nature of the pathology of dementia spectrum disorders has complicated their management and the development of effective treatments. This is despite the fact that they are far from uncommon, with Alzheimer’s disease (AD) alone affecting 35 million people worldwide. The cholinergic system has been found to be crucially involved in cognitive function, with cholinergic dysfunction playing a pivotal role in the pathophysiology of dementia. The use of molecular imaging such as SPECT and PET for tagging targets within the cholinergic system has shown promise for elucidating key aspects of underlying pathology in dementia spectrum disorders, including AD or parkinsonian dementias. SPECT and PET studies using selective radioligands for cholinergic markers, such as [¹¹C]MP4A and [¹¹C]PMP PET for acetylcholinesterase (AChE), [¹²³I]5IA SPECT for the α₄β₂ nicotinic acetylcholine receptor and [¹²³I]IBVM SPECT for the vesicular acetylcholine transporter, have been developed in an attempt to clarify those aspects of the diseases that remain unclear. This has led to a variety of findings, such as cortical AChE being significantly reduced in Parkinson’s disease (PD), PD with dementia (PDD) and AD, as well as correlating with certain aspects of cognitive function such as attention and working memory. Thalamic AChE is significantly reduced in progressive supranuclear palsy (PSP) and multiple system atrophy, whilst it is not affected in PD. Some of these findings have brought about suggestions for the improvement of clinical practice, such as the use of a thalamic/cortical AChE ratio to differentiate between PD and PSP, two diseases that could overlap in terms of initial clinical presentation. Here, we review the findings from molecular imaging studies that have investigated the role of the cholinergic system in dementia spectrum disorders.     

Evaluation of radioiodinated 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine as a ligand for SPECT investigations of brain nicotinic acetylcholine receptors

5-Iodo-3-(2(S)-azetidinylmethoxy)pyridine (5IA), an A-85380 analog iodinated at the 5-position of the pyridine ring, was evaluated as a radiopharmaceutical for investigating brain nicotinic acethylcholine receptors (nAChRs) by single photon emission computed tomography (SPECT). [123/125I]5IA was synthesized by the iododestannylation reaction under no-carrier-added conditions and purified by high-performance liquid chromatography (HPLC) with high radiochemical yield (50%), high radiochemical purity (> 98%), and high specific radioactivity (> 55 GBq/micromol). The binding affinity of 5IA for brain nAChRs was measured in terms of displacement of [3H]cytisine and [125I]5IA from binding sites in rat cortical membranes. The binding data revealed that the affinity of 5IA was the same as that of A-85380 and more than seven fold higher than that of (-)-nicotine, and that 5IA bound selectively to the alpha4beta2 nAChR subtype. Biodistribution studies in rats indicated that the brain uptake of [125I]51A was rapid and profound. Regional cerebral distribution studies in rats demonstrated that the accumulation of [125I]5IA was consistent with the density of high affinity nAChRs with highest uptake observed in the nAChR-rich thalamus, moderate uptake in the cortex and lowest uptake in the cerebellum. Administration of the nAChR agonists (-)-cytisine and (-)-nicotine reduced the uptake of [125I]5IA in all regions studied with most pronounced reduction in the thalamus, and resulted in similar levels of radioactivity throughout the brain. [125I]5IA binding sites were shown to be saturable with unlabeled 5IA. Behavioral studies in mice demonstrated that 5IA did not show signs of behavioral toxicity. Furthermore, SPECT studies with [123I]5IA in the common marmoset demonstrated appropriate brain uptake and regional localization for a high-affinity nAChR imaging radiopharmaceutical. These results suggested that [123I]5IA is a promising radiopharmaceutical for SPECT studies of central nAChRs in human subjects.     

In vitro and in vivo evaluation of [123I]IBZM: a potential CNS D-2 dopamine receptor imaging agent

In vitro binding characteristics of a CNS dopamine D-2 receptor imaging agent, (S)-N-[(1-ethyl-2-pyrrolidinyl)] methyl-2-hydroxy-3-iodo-6-methoxybenzamide [( 125I]IBZM), was carried out in rats. Also brain images, as well as organ biodistribution were determined in a monkey following the administration of 123I-labeled compound. The S-(-)-I[125I]IBZM showed high specific dopamine D-2 receptor binding in rat striatum (Kd = 0.426 +/- 0.082 nM, Bmax = 480 +/- 22 fmol/mg of protein). Competition of various ligands for the IBZM binding displayed the following rank order of potency: spiperone greater than S(-)IBZM much greater than R(+)IBZM greater than or equal to S(-)BZM greater than dopamine greater than ketanserin greater than SCH-23390 much greater than propranolol, norepinephrine, serotonin. In vivo planar images of a monkey injected with [123I]IBZM demonstrated a high concentration in basal ganglia of brain. The ratios of activity in the basal ganglia to cerebellum and the cortex to cerebellum in monkey brain were 4.93 and 1.44, respectively, at 120 min postinjection. These preliminary results indicate that [123I]IBZM is a potentially promising imaging agent for the investigation of dopamine D-2 receptors in humans.