Initial evaluation of123|-5-|-R91150, a selective 5-HT2A ligand for single-photon emission tomography, in healthy human subjects

The mapping of 5-HT₂ receptors in the brain using functional imaging techniques has been limited by a relative lack of selective radioligands. Iodine-123 labelled 4-amino-N-[1-[3-(4-fluorophenoxy)propyl]-4-methyl-4-piperidinyl]-5-iodo-2-methoxybenzamide (¹²³I-5-I-R91150 or¹²³I-R93274) is a new ligand for single-photon emission tomography (SPET), with high affinity and selectivity for 5-HT_2A receptors. This study reports on preliminary¹²³I-5-I-R91150 SPET, wholebody and blood distribution findings in five healthy human volunteers. Maximal brain uptake was approximately 2% of total body counts at 180 min post injection (p.i.). Dynamic SPET sequences were acquired with the brain-dedicated, single-slice multi-detector system SEM-810 over 200 min p.i. Early peak uptake (at 5 min p.i.) was seen in the cerebellum, a region free from 5HT_2A receptors. In contrast, radioligand binding in the frontal cortex increased steadily over time, up to a peak at approximately 100–120 min p.i. Frontal cortex-cerebellum activity ratios reached values of 1.4, and remained stable from approximately 100 min p.i. onwards. Multi-slice SPET sequences showed a pattern of regional variation of binding compatible with the autoradiographic data on the distribution of 5-HT_2A receptors in (cerebral cortex>striatum>cerebellum). These findings suggest that¹²³I-5-I-R91150 may be used for the imaging of 5-HT_2A receptors in the living human brain with SPET.     

Biodistribution and displacement studies of the selective 5-HT2A receptor antagonist 123I-5-I-R91150 in the normal dog

There is increasing interest in mapping receptors in vivo by using functional imaging modalities such as single photon emission tomography (SPET) and positron emission tomography (PET). Since SPET is a more accessible functional imaging modality than PET and, overall, it is more economical, radioligands suitable for this technique are in greater demand. Recently, 123I-5-I-R91150, a radioligand with high selectivity and affinity for 5-HT(2A) receptors in the brain, was introduced for SPET. This study reports on the whole-body distribution and brain uptake of the selective 123I-5-I-R91150 ligand in four normal dogs. The frontal to cerebellar ratio of uptake in time was determined in three dogs. Time-activity curve of venous blood was determined in one dog. Maximal global brain uptake was found at 10-60 min post-injection. Higher brain uptake was noted in the frontal cortical areas compared to the cerebellum. The frontal-cerebellar ratio reached the highest values at 90-180 min. Reversibility and pharmacological selectivity of ligand binding was demonstrated through displacement and blocking studies with the 5-HT(2A) receptor antagonist ketanserin. This study demonstrates that the specific 5-HT(2A) iodinated ligand can be used for imaging and semi-quantification of the 5-HT(2A) receptors in the canine brain in vivo by using SPET.     

Decreased frontal serotonin 5-HT2a receptor binding index in deliberate self-harm patients

Studies of serotonin metabolites in body fluids in attempted suicide patients and of post-mortem brain tissue of suicide victims have demonstrated the involvement of the serotonergic neurotransmission system in the pathogenesis of suicidal behaviour. Recently developed neuroimaging techniques offer the unique possibility of investigating in vivo the functional characteristics of this system. In this study the 5-HT_2a receptor population of patients who had recently attempted suicide was studied by means of the highly specific radio-iodinated 5-HT_2a receptor antagonist 4-amino-N-[1-[3-(4-fluorophenoxy)propyl]-4-methyl-4-piperidinyl]-5-iodo-2-methoxybenzamide or ¹²³I-5-I-R91150. Nine patients who had recently (1-7 days) attempted suicide and 12 age-matched healthy controls received an intravenous injection of 185 MBq ¹²³I-5-I-R91150 and were scanned with high-resolution brain single-photon emission tomography (SPET). Stereotactic realigned images were analysed semi-quantitatively using predefined volumes of interest. Serotonin binding capacity was expressed as the ratio of specific to non-specific activity. The cerebellum was used as a measure of non-specific activity. An age-dependent 5-HT_2a binding index was found, in agreement with previous literature. Deliberate self-harm patients had a significantly reduced mean frontal binding index after correction for age (P=0.002) when compared with controls. The reduction was more pronounced among deliberate self-injury patients (DSI) (P<0.001) than among deliberate self-poisoning patients (DSP). Frontal binding index was significantly lower in DSI patients than in DSP suicide attempters (P<0.001). It is concluded that brain SPET of the 5-HT_2a serotonin receptor system in attempted suicide patients who are free of drugs influencing the serotonergic system shows in vivo evidence of a decreased frontal binding index of the 5-HT_2a receptor, indicating a decrease in the number and/or in the binding affinity of 5-HT_2a receptors.     

The effect of citalopram hydrobromide on 5-HT2A receptors in the impulsive–aggressive dog, as measured with 123I-5-I-R91150 SPECT

Purpose Involvement of the serotonergic system in impulsive aggression has been demonstrated in both human and animal studies. The purpose of the present study was to investigate the effect of citalopram hydrobromide (a selective serotonin re-uptake inhibitor) on the 5-HT_2A receptor and brain perfusion in impulsive–aggressive dogs by means of single-photon emission computed tomography.Methods The binding index of the radioligand ¹²³I-5-I-R91150 was measured before and after treatment with citalopram hydrobromide in nine impulsive–aggressive dogs. Regional perfusion was measured with ^99mTc-ethyl cysteinate dimer (ECD). Behaviour was assessed before treatment and again after 6 weeks of treatment.Results A correlation was found between decreased binding and behavioural improvement in eight out of nine dogs. The 5-HT_2A receptor binding index was significantly reduced after citalopram hydrobromide treatment in all cortical regions but not in the subcortical area. None of the dogs displayed alterations in perfusion on the post-treatment scans.Conclusion This study supports previous findings regarding the involvement of the serotonergic system in impulsive aggression in dogs in general. More specifically, the effect of treatment on the 5-HT_2A receptor binding index could be demonstrated and the decreased binding index correlated with behavioural improvement.     

Decreased frontal serotonin 5-HT 2a receptor binding index in deliberate self-harm patients

Studies of serotonin metabolites in body fluids in attempted suicide patients and of post-mortem brain tissue of suicide victims have demonstrated the involvement of the serotonergic neurotransmission system in the pathogenesis of suicidal behaviour. Recently developed neuroimaging techniques offer the unique possibility of investigating in vivo the functional characteristics of this system. In this study the 5-HT2a receptor population of patients who had recently attempted suicide was studied by means of the highly specific radio-iodinated 5-HT2a receptor antagonist 4-amino-N-[1-[3-(4-fluorophenoxy) propyl]-4-methyl-4-piperidinyl]-5-iodo-2-methoxybenzamide or 123I-5-I-R91150. Nine patients who had recently (1-7 days) attempted suicide and 12 age-matched healthy controls received an intravenous injection of 185 MBq 123I-5-I-R91150 and were scanned with high-resolution brain single-photon emission tomography (SPET). Stereotactic realigned images were analysed semi-quantitatively using predefined volumes of interest. Serotonin binding capacity was expressed as the ratio of specific to non-specific activity. The cerebellum was used as a measure of non-specific activity. An age-dependent 5-HT2a binding index was found, in agreement with previous literature. Deliberate self-harm patients had a significantly reduced mean frontal binding index after correction for age (P=0.002) when compared with controls. The reduction was more pronounced among deliberate self-injury patients (DSI) (P<0.001) than among deliberate self-poisoning patients (DSP). Frontal binding index was significantly lower in DSI patients than in DSP suicide attempters (P<0.001). It is concluded that brain SPET of the 5-HT2a serotonin receptor system in attempted suicide patients who are free of drugs influencing the serotonergic system shows in vivo evidence of a decreased frontal binding index of the 5-HT2a receptor, indicating a decrease in the number and/or in the binding affinity of 5-HT2a receptors.     

Decreased 5-HT2a receptor binding in patients with anorexia nervosa.

Indirect estimations of brain neurotransmitters in patients with anorexia nervosa (AN) and low weight have demonstrated a reduction in brain serotonin (5-HT) turnover in general and led to hypotheses about dysfunction in the 5-HT(2a) receptor system. It was our aim to investigate the central 5-HT(2a) receptor binding index using SPECT brain imaging. METHODS: The 5-HT(2a) receptors of low-weight patients with AN were studied by means of the highly specific radioiodinated 5-HT(2a) receptor antagonist 4-amino-N-[1-[3-(4-fluorophenoxy)propyl]-4-methyl-4-piperidinyl]-5-iodo-2-methoxybenzamide or (123)I-5-I-R91150. Fifteen patients with clinical diagnoses of AN and 11 age-matched healthy volunteers received intravenous injections of 185 MBq (123)I-5-I-R91150 and were scanned with high-resolution brain SPECT. RESULTS: Compared with healthy volunteers, patients with AN had a significantly reduced 5-HT(2a) binding index in the left frontal cortex, the left and right parietal cortex, and the left and right occipital cortex. A significant left-right asymmetry was noted in the frontal cortex (left < right). CONCLUSION: These results are in accordance with diminished metabolic and perfusion of frontal and parietal cortices reported in recent neuroimaging studies and imply localized disturbed serotonergic function. The data are discussed in the light of possible confounding factors related to the low-weight AN status. A regional cortical reduction in 5-HT(2a) binding index is not likely to be caused by a general reduction in serotonergic function due to the possible confounding factors. Suggestions for further research are given.     

Radioiodinated SB 207710 as a radioligand in vivo: imaging of brain 5-HT4 receptors with SPET

Single-photon emission tomography (SPET) and positron emission tomography (PET), when coupled to suitable radioligands, are uniquely powerful for investigating the status of neurotransmitter receptors in vivo. The serotonin subtype-4 (5-HT₄) receptor has discrete and very similar distributions in rodent and primate brain. This receptor population may play a role in normal cognition and memory and is perhaps perturbed in some neuropsychiatric disorders. SB 207710 [(1-butyl-4-piperidinylmethyl)-8-amino-7-iodo-1,4-benzodioxan-5-carboxylate] is a selective high-affinity antagonist at 5-HT₄ receptors. We explored radioiodinated SB 207710 as a possible radioligand for imaging 5-HT₄ receptors in vivo. Rats were injected intravenously with iodine-125 labelled SB 207710, euthanised at known times and dissected to establish radioactivity content in brain tissues. Radioactivity entered brain but cleared rapidly and to a high extent from blood and plasma. Between 45 and 75 min after injection, the ratios of radioactivity concentration in each of 12 selected brain tissues to that in receptor-poor cerebellum correlated with previous measures of 5-HT₄ receptor density distribution in vitro. The highest ratio was about 3.4 in striatum. SB 207710 was labelled with iodine-123 by an iododestannylation procedure. A cynomolgus monkey was injected intravenously with [¹²³I]SB 207710 and examined by SPET. Maximal whole brain uptake of radioactivity was 2.3% of the injected dose at 18 min after radioligand injection. Brain images acquired between 9 and 90 min showed high radioactivity uptake in 5-HT₄ receptor-rich regions, such as striatum, and low uptake in receptor-poor cerebellum. At 169 min the ratio of radioactivity concentration in striatum to that in cerebellum was 4.0. In a second SPET experiment, the cynomolgus monkey was pretreated with a selective 5-HT₄ receptor antagonist, SB 204070, at 20 min before [¹²³I]SB 207710 injection. Radioactivity in all brain regions was reduced almost to the level in cerebellum by 176 min after radioligand injection. These findings show that [¹²³I]SB 207710 is an effective radioligand for imaging brain 5-HT₄ receptors in vivo.For preliminary accounts of this work, see Pike VW et al., J Nucl Med 1998; 39 (Suppl):185; Eur J Nucl Med 1999; 26:991.     

Oxotechnetium TcO[SN(R)S][S] complexes as potential 5-HT1A receptor imaging agents

A series of ^99mTcO[SN(R)S][S] complexes carrying the 1-(2-methoxyphenyl)piperazine moiety on the tridentate ligand [SN(R)S] was synthesized. For structural characterization and for in vitro binding assays the analogous oxorhenium complexes were prepared. As demonstrated by appropriate competition binding tests in rat hippocampal preparations, all oxorhenium analogues showed affinity for the 5-HT_1A receptor binding sites with IC₅₀ values at the nanomolar range (IC₅₀= 5.8–103 nM). All ^99mTcO[SN(R)S]/[S] complexes showed significant brain uptake in rats at 2 min p.i. (0.24–1.31% ID). However, a clear correlation between distribution of radioactivity in the brain and distribution of 5-HT_1A receptors could not be established.     

In vivo imaging of GABAA receptors using sequential whole-volume iodine-123 iomazenil single-photon emission tomography

Using a brain-dedicated triple-headed single-photon emission tomography (SPET) system, a sequential whole-volume imaging protocol has been devised to evaluate the regional distribution of iodine-123 iomazenil binding to GABA_A receptors in the entire brain. The protocol was piloted in eight normal volunteers (seven males and one female; mean age, 24.8±3.9 years). The patterns obtained were largely compatible with the known distribution of GABA_A receptors in the brain as reported in autoradiographic studies, with cerebral cortical regions, particularly the occipital and frontal cortices, displaying the highest¹²³I-iomazenil uptake. Measures of time to peak uptake and tracer washout rates presented with the same pattern of regional variation, with later times to peak and slower washout rates in cortical regions compared to other brain areas. Semiquantitative analysis of the data using white matter/ventricle regions as reference demonstrated a plateau of specific¹²³I-iomazenil binding in neocortical and cerebellar regions from 60–75 min onwards. These data demonstrate the feasibility of sequential, dynamic whole-volume¹²³I-iomazenil SPET imaging. The protocol may be particularly useful in the investigation of neuropsychiatric conditions which are likely to involve more than one focus of GABA abnormalities, such as anxiety disorders and schizophrenia.     

Radiosynthesis and in vivo evaluation of a series of substituted 11C-phenethylamines as 5-HT2A agonist PET tracers

Purpose

Positron emission tomography (PET) imaging of serotonin 2A (5-HT_2A) receptors with agonist tracers holds promise for the selective labelling of 5-HT_2A receptors in their high-affinity state. We have previously validated [¹¹C]Cimbi-5 and found that it is a 5-HT_2A receptor agonist PET tracer. In an attempt to further optimize the target-to-background binding ratio, we modified the chemical structure of the phenethylamine backbone and carbon-11 labelling site of [¹¹C]Cimbi-5 in different ways. Here, we present the in vivo validation of nine novel 5-HT_2A receptor agonist PET tracers in the pig brain.

Methods

Each radiotracer was injected intravenously into anaesthetized Danish Landrace pigs, and the pigs were subsequently scanned for 90?min in a high-resolution research tomography scanner. To evaluate 5-HT_2A receptor binding, cortical nondisplaceable binding potentials (BP_ND) were calculated using the simplified reference tissue model with the cerebellum as a reference region.

Results

After intravenous injection, all compounds entered the brain and distributed preferentially into the cortical areas, in accordance with the known 5-HT_2A receptor distribution. The largest target-to-background binding ratio was found for [¹¹C]Cimbi-36 which also had a high brain uptake compared to its analogues. The cortical binding of [¹¹C]Cimbi-36 was decreased by pretreatment with ketanserin, supporting 5-HT_2A receptor selectivity in vivo. [¹¹C]Cimbi-82 and [¹¹C]Cimbi-21 showed lower cortical BP_ND, while [¹¹C]Cimbi-27, [¹¹C]Cimbi-29, [¹¹C]Cimbi-31 and [¹¹C]Cimbi-88 gave rise to cortical BP_ND similar to that of [¹¹C]Cimbi-5.

Conclusion

[¹¹C]Cimbi-36 is currently the most promising candidate for investigation of 5-HT_2A receptor agonist binding in the living human brain with PET.     

Synthesis,radiolabeling and bioevaluation of a novel arylpiperazine derivative containing triazole as a 5-HT1A receptor imaging agents

IntroductionIt has been recognized that serotonin plays a main role in various pathological conditions such as anxiety, depression, aggressiveness, schizophrenia, suicidal behavior, panic and autism. 1-(2-Methoxyphenyl) piperazine pharmacophore, a fragment of the true 5-HT_1A antagonist WAY100635, is found in numerous selective 5-HT_1A imaging agents. In this paper, we have reported the synthesis of a novel derivative of 1-(2-methoxyphenyl) piperazine that is labeled with ^99mTc (CO)₃ via click chemistry.MethodsThe bidentate alkyne, propargylglycine was reacted with phenyl piperazine triazole derivative in the presence of a catalytic amount of Cu (I) to form tridentate ligand. The ligand was radiolabeled with the precursor [^99mTc] [(H₂O)₃ (CO)₃]⁺ and characterized by HPLC. The bioevaluation of radio labeled ligand was carried out in rats.ResultsTriazole complex was labeled by ^99mTc-tricarbonyl and its radiochemical yield was more than > 95% which was determined by HPLC. In vivo stability studies in human serum albumin show a 93% ratio of complex after a 24 h period. The calculated partition coefficient (logP) was 0.34 ± 0.02. Receptor binding assays indicated about 70% specific binding of radioligand to 5-HT_1A receptors. Biodistribution studies have shown brain hippocampus uptake of 0.40 ± 0.08 %ID/g at 30 min post injection.ConclusionsResults indicate that this ^99mTc-tricabonyl-arylpiperazine derivative has specific binding to 5-HT_1A receptors and presented suitable characters for its use as a CNS imaging agent.     

In vivo evaluation in rodents of [I]-3-I-CO as a potential SPECT tracer for the serotonin 5-HT2A receptor

Introduction[¹²³I]-(4-fluorophenyl)[1-(3-iodophenethyl)piperidin-4-yl]methanone ([¹²³I]-3-I-CO) is a potential single photon emission computed tomography tracer with high affinity for the serotonin 5-HT_2A receptor (K_i=0.51 nM) and good selectivity over other receptor (sub)types. To determine the potential of the radioligand as a 5-HT_2A tracer, regional brain biodistribution and displacement studies will be performed. The influence of P-glycoprotein blocking on the brain uptake of the radioligand will also be investigated.MethodsA regional brain biodistribution study and a displacement study with ketanserin were performed with [¹²³I]-3-I-CO. Also, the influence of cyclosporin A (50 mg/kg) on the brain distribution of the radioligand was investigated. For the displacement study, ketanserin (1 mg/kg) was administered 30 min after injection of [¹²³I]-3-I-CO.ResultsThe initial brain uptake of [¹²³I]-3-I-CO was quite high, but a rapid wash-out of radioactivity was observed. Cortex-to-cerebellum binding index ratios were low (1.1 – 1.7), indicating considerable aspecific binding and a low specific ‘signal’ of the radioligand. Tracer uptake was reduced to the levels in cerebellum (a 60% reduction) after ketanserin displacement. Administration of cyclosporin A resulted in a doubling of the brain radioactivity concentration.ConclusionsAlthough [¹²³I]-3-I-CO showed adequate brain uptake and could be displaced by ketanserin, high aspecific binding to brain tissue was responsible for very low cortex-to-cerebellum binding index ratios, possibly limiting the potential of the radioligand as a serotonin 5-HT_2A receptor tracer. We also demonstrated that [¹²³I]-3-I-CO is probably a weak substrate for the P-glycoprotein efflux transporter.     

Initial human studies with single-photon emission tomography using iodine-123 labelled 3-(5-cyclopropyl-1,2,4-oxadiazo-3-yl)-7-iodo-5,6-dihydro-5-methyl-6-oxo-4H-imidazo[1,5-a][1,4]-benzodiazepine (NNC 13-8241)

The iodine-123 labelled ligand 3-(5-cyclopropyl-1,2,4-oxadiazo-3-yl)-7-iodo-5,6-dihydro-5-methyl-6oxo-4H-imidazo[1,5-a][1,4]-benzodiazepine ([¹²³I]NNC 13-8241) was evaluated as a probe for in vivo imaging of benzodiazepine receptor sites in the human brain. Four healthy volunteers were imaged with a high-resolution single-photon emission tomography (SPET) scanner. The metabolism of [¹²³I]NNC 13-8241 in plasma was slow. The total brain uptake was about 1.5-fold higher than that of [¹²³I]iomazenil. The specific binding in the cortical areas was high and less intense in the thalamus. The most intense uptake was seen in the occipital cortex. The peak cortical uptake of [¹²³I]NNC 13-8241 was observed 6–10 h after the injection of tracer. The radiation burden to the patient was moderate, being 2.5·10^–2 mSv/MBq (effective dose equivalent). A slow metabolism together with favourable kinetics indicates that [¹²³I]NNC 13-8241 is a specific and promising SPET ligand for imaging benzodiazepine receptor sites in the living human brain.     

Synthesis and autoradiographic evaluation of a novel high-affinity Tc-99m ligand for the 5-HT2A receptor

The synthesis and in vitro autoradiography of a novel Tc-99m ligand with subnanomolar affinity to the 5-HT_2A receptor is reported. The complex combines the 4-(4-fluoro)-benzoyl piperidine portion derived from the 5-HT_2A receptor antagonist ketanserin with a neutral oxotechnetium(V) chelate in form of a mixed ligand “3+1” unit containing the SNS/S donor set. The analogous rhenium compound has been synthesized as a surrogate for the Tc-99m complex for use in receptor binding assays and for complete structural characterization. In competition experiments the Tc-99 complex as well as its Re analogue display subnanomolar affinity toward the 5-HT_2A receptor (K_i 0.44 nM for Tc, 0.25 nM for Re). The subnanomolar 5-HT_2A receptor binding of the Re complex was confirmed by functional in vitro antagonism of contractile effects evoked by 5-HT in rat arterial tissue. Re 1 inhibited 5-HT-induced, 5-HT_2A receptor-mediated contractions of isolated rat tail artery in a competitive fashion and possessed nanomolar affinity ( pA₂=9.08, pA₂ representing the negative decadic logarithm of the Re 1/5-HT_2A-receptor dissociation constant [mol/L]). Like ketanserin, Re 1 displayed moderate affinity for adrenergic α_1D ( pA₂=8.23) and histamine H₁ receptors ( pA₂=8.00), and was >600-fold up to 10,700-fold less active at several neurotransmitter receptor subtypes. In vitro autoradiographic studies clearly indicate the accumulation of the Tc-99m compound in 5-HT_2A-receptor-rich areas of the brain. This enrichment can be blocked by 5-HT_2A receptor antagonists such as mianserin and ketanserin and is therefore specific.     

[carbonyl-11C]Desmethyl-WAY-100635 (DWAY) is a potent and selective radioligand for central 5-HT1A receptors in vitro and in vivo

 [carbonyl-¹¹C]Desmethyl-WAY-100635 (DWAY) is possibly a low-level metabolite appearing in plasma after intravenous administration of [carbonyl-¹¹C]WAY-100635 to human subjects for positron emission tomographic (PET) imaging of brain 5-HT_1A receptors. In this study we set out to assess the ability of DWAY to enter brain in vivo and to elucidate its possible interaction with 5-HT_1A receptors. Desmethyl-WAY-100635 was labelled efficiently with carbon-11 (t _1/2 = 20.4 min) in high specific radioactivity by reaction of its descyclohexanecarbonyl analogue with [carbonyl-¹¹C]cyclohexanecarbonyl chloride. The product was separated in high radiochemical purity by high-performance liquid chromatography (HPLC) and formulated for intravenous injection. Rats were injected intravenously with DWAY, sacrificed at known times and dissected to establish radioactivity content in brain tissues. At 60 min after injection, the ratios of radioactivity concentration in each brain region to that in cerebellum correlated with previous in vitro and in vivo measures of 5-HT_1A receptor density. The highest ratio was about 22 in hippocampus. Radioactivity cleared rapidly from plasma; HPLC analysis revealed that DWAY represented 55% of the radioactivity in plasma at 5 min and 33% at 30 min. Only polar radioactive metabolites were detected. Subsequently, a cynomolgus monkey was injected intravenously with DWAY and examined by PET. Maximal whole brain uptake of radioactivity was 5.7% of the administered dose at 5 min after injection. The image acquired between 9 and 90 min showed high radioactivity uptake in brain regions rich in 5-HT_1A receptors (e.g. frontal cortex and neocortex), moderate uptake in raphe nuclei and low uptake in cerebellum. A transient equilibrium was achieved in cortical regions at about 60 min, when the ratio of radioactivity concentration in frontal cortex to that in cerebellum reached 6. The corresponding ratio for raphe nuclei was about 3. Radioactive metabolites appeared rapidly in plasma, but these were all more polar than DWAY, which represented 52% of the radioactivity in plasma at 4 min and 20% at 55 min. In a second PET experiment, in which a cynomolgus monkey was pretreated with the selective 5-HT_1A receptor antagonist, WAY-100635, at 25 min before DWAY injection, radioactivity in all brain regions was reduced to that in cerebellum. Autoradiography of post mortem human brain cryosections after incubation with DWAY successfully delineated 5-HT_1A receptor distribution. Receptor-specific binding was eliminated in the presence of the selective 5-HT_1A receptor agonist, 8-OH-DPAT [(±)-8-hydroxy-2-dipropylaminotetralin]. These findings show that: (a) intravenously administered DWAY is well able to penetrate brain in rat and monkey, (b) DWAY is a highly effective radioligand for brain 5-HT_1A receptors in rat and monkey in vivo and for human brain in vitro, and (c) the metabolism and kinetics of DWAY appear favourable to successful biomathematical modelling of acquired PET data. Thus, DWAY warrants further evaluation as a radioligand for PET studies of 5-HT_1A receptors in human brain. Received 1 October and in revised form 12 December 1997     

Comparison of iodine-123 epidepride and iodine-123 IBZM for dopamine D2 receptor imaging in clinically non-functioning pituitary macroadenomas and macroprolactinomas

We compared pituitary iodine-123 epide- pride single-photon emission tomography (SPET) and ¹²³I-IBZM SPET for the in vivo imaging of dopamine D₂ receptors in 15 patients with clinically non-functioning pituitary adenomas. Four patients with dopamine agonist-sensitive macroprolactinomas were studied as positive controls. The uptake of radioactivity in the pituitary was established using a visual scoring system and an uptake index calculated by dividing the average count rates in the pituitary area by the average count rates in the cerebellum. All four macroprolactinomas showed specific binding of ¹²³I-epidepride, but only one showed specific binding of ¹²³I-IBZM. Specific binding of ¹²³I-epidepride was demonstrated in 9 of the 15 clinically non-functioning pituitary adenomas (60%), but specific binding of ¹²³I-IBZM was shown in only 6 of these 15 cases (40%). The uptake of ¹²³I-epidepride in the pituitary region was consistently higher than that of ¹²³I-IBZM. None of the patients who showed absence of uptake of ¹²³I-epidepride in the pituitary area showed uptake of ¹²³I-IBZM in this area. In conclusion: ¹²³I-epidepride SPET is superior to ¹²³I-IBZM SPET for the visualization of dopamine receptor-positive pituitary adenomas. Therefore, ¹²³I-epidepride should replace ¹²³I-IBZM for future D₂ receptor SPET studies of pituitary adenomas. ¹²³I-epidepride SPET potentially might serve to predict the response of clinically non-functioning pituitary adenomas to dopamine agonist therapy. Received 11 July and in revised form 25 September 1998     

Synthesis and in vivo evaluation of a novel 5-HT1A receptor agonist radioligand [O-methyl- 11C]2-(4-(4-(2-methoxyphenyl)piperazin-1-yl)butyl)-4-methyl-1,2,4-triazine-3,5(2H,4H)dione in nonhuman primates

Purpose Serotonin_1A (5-HT_1A) receptors exist in high- and low-affinity states, and agonist ligands bind preferentially to the high-affinity state of the receptor and provide a measure of functional 5-HT_1A receptors. Although the antagonist tracers are established PET ligands in clinical studies, a successful 5-HT_1A receptor agonist radiotracer in living brain has not been reported. [¹¹C]MPT, our first-generation agonist radiotracer, shows in vivo specificity in baboons; however, its utility is limited owing to slow washout and immeasurable plasma free fraction. Hence we performed structure-activity relationship studies of MPT to optimize a radiotracer that will permit valid quantification of 5-HT_1A receptor binding. We now report the synthesis and evaluation of [¹¹C]MMP as an agonist PET tracer for 5-HT_1A receptors in baboons. Methods In vitro binding assays were performed in bovine hippocampal membranes and membranes of CHO cells expressing 5-HT_1A receptors. [¹¹C] labeling of MMP was performed by reacting desmethyl-MMP with [¹¹C]CH₃OTf. In vivo studies were performed in baboons, and blocking studies were conducted by pretreatment with 5-HT_1A receptor ligands WAY-100635 and (±)-8-OH-DPAT. Results MMP is a selective 5-HT_1A receptor agonist (K _i 0.15 nM). Radiosynthesis of [¹¹C]MMP was achieved in 30 ± 5% (n = 15) yield at EOS with a specific activity of 2,600 ± 500 Ci/mmol (n = 12). PET studies in baboons demonstrated specific binding of [¹¹C]MMP to 5-HT_1A receptor-enriched brain regions, as confirmed by blockade with WAY-100635 and (±)-8-OH-DPAT. Conclusion We identified [¹¹C]MMP as an optimal agonist PET tracer that shows quantifiable, specific binding in vivo to 5-HT_1A receptors in baboons.     

[18F]F15599, a novel 5-HT1A receptor agonist, as a radioligand for PET neuroimaging

Purpose  

The serotonin-1A (5-HT_1A) receptor is implicated in the pathophysiology of major neuropsychiatric disorders. Thus, the functional imaging of 5-HT_1A receptors by positron emission tomography (PET) may contribute to the understanding of its role in those pathologies and their therapeutics. These receptors exist in high- and low-affinity states and it is proposed that agonists bind preferentially to the high-affinity state of the receptor and therefore could provide a measure of the functional 5-HT_1A receptors. Since all clinical PET 5-HT_1A radiopharmaceuticals are antagonists, it is of great interest to develop a ¹⁸F labelled agonist.     

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

A new radioligand, 5-iodo-2-[[2–2-[(dimethylamino)methyl]phenyl]thio]benzyl alcohol ([¹²³I]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 μM, respectively). In this study the biodistribution of SERT in the baboon brain was investigated in vivo using [¹²³I]IDAM and SPET imaging. Dynamic sequences of SPET scans were performed on three female baboons (Papio anubis) after injection of 555 MBq of [¹²³I]IDAM. Displacing doses (1 mg/kg) of the selective SERT ligand (+)McN5652 were administered 90–120 min after injection of [¹²³I]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 [¹²³I]IDAM in the brain. These results suggest that [¹²³I]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. Received: 1 February and in revised form 2 March 1999     

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