Synthesis of novel WAY 100635 derivatives containing a norbornene group and radiofluorination of [18F]AH1.MZ as a serotonin 5‐HT1A receptor antagonist for molecular imaging

5‐HT_1A receptors are involved in a variety of psychiatric disorders and in vivo molecular imaging of the 5‐HT_1A status represents an important approach to analyze and treat these disorders. We report herein the synthesis of three new fluoroethylated 5‐HT_1A ligands (AH1.MZ, AH2.MZ and AH3.MZ) as arylpiperazine derivatives containing a norbornene group. AH1.MZ (K_i= 4.2 nM) and AH2.MZ (K_i=30 nM) showed reasonable in vitro affinities to the 5‐HT_1A receptor, whereas AH3.MZ appeared to be non‐affine toward the 5‐HT_1A receptor. The receptor profile of AH1.MZ and AH2.MZ showed selectivity within the 5‐HT system. ¹⁸F‐labelling via [¹⁸F]FETos to [¹⁸F]AH1.MZ was carried out in radiochemical yields of >70%. The final formulation of injectable solutions including [¹⁸F]FETos synthon synthesis, radiosynthesis and semi‐preparative high‐performance liquid chromatography (HPLC) separation took no longer than 130 min and provided [¹⁸F]AH1.MZ with a purity of >98% as indicated by analytical HPLC analyses. Copyright © 2009 John Wiley & Sons, Ltd.     

Synthesis and evaluation of a 18F‐labeled 4‐phenylpiperidine‐4‐carbonitrile radioligand for σ1 receptor imaging

We report the design and synthesis of several 4‐phenylpiperidine‐4‐carbonitrile derivatives as σ₁ receptor ligands. In vitro radioligand competition binding assays showed that all the ligands exhibited low nanomolar affinity for σ₁ receptors (K_i(σ₁) = 1.22–2.14 nM) and extremely high subtype selectivity (K_i(σ₂) = 830–1710 nM; K_i(σ₂)/K_i(σ₁) = 680–887). [¹⁸F]9 was prepared in 42–46% isolated radiochemical yield, with a radiochemical purity of >99% by HPLC analysis after purification, via nucleophilic ¹⁸F^‐ substitution of the corresponding tosylate precursor. Biodistribution studies in mice demonstrated high initial brain uptakes and high brain‐to‐blood ratios. Administration of SA4503 or haloperidol 5 min prior to injection of [¹⁸F]9 significantly reduced the accumulation of radiotracers in organs known to contain σ₁ receptors. Two radioactive metabolites were observed in the brain at 30 min after radiotracer injection. [¹⁸F]9 may serve as a lead compound to develop suitable radiotracers for σ₁ receptor imaging with positron emission tomography.     

Synthesis,radiofluorination and first evaluation of (±)‐[18F]MDL 100907 as serotonin 5‐HT2A receptor antagonist for PET

In some psychiatric disorders 5‐HT_2A receptors play an important role. In order to investigate those in vivo there is an increasing interest in obtaining a metabolically stable, subtype selective and high affinity radioligand for receptor binding studies using positron emission tomography (PET). Combining the excellent in vivo properties of [¹¹C]MDL 100907 for PET imaging of 5‐HT_2A receptors and the more suitable half‐life of fluorine‐18, MDL 100907 was radiofluorinated in four steps using 1‐(2‐bromoethyl)‐4‐[¹⁸F]fluorobenzene as a secondary labelling precursor. The complex reaction required an overall reaction time of 140 min and (±)‐[¹⁸F]MDL 100907 was obtained with a specific activity of at least 30 GBq/µmol (EOS) and an overall radiochemical yield of 1–2%. In order to verify its binding to 5‐HT_2A receptors, in vitro rat brain autoradiography was conducted showing the typical distribution of 5‐HT_2A receptors and a very low non‐specific binding of about 6% in frontal cortex, using ketanserin or spiperone for blocking. Thus, [¹⁸F]MDL 100907 appears to be a promising new 5‐HT_2A PET ligand. Copyright © 2008 John Wiley & Sons, Ltd.     

Synthesis of [18F]3‐[1‐(3‐fluoropropyl)‐(S)‐pyrrolidin‐2‐ylmethoxy]pyridine ([18F]NicFP): a potential α4β2 nicotinic acetylcholine receptor radioligand for PET

Nicotinic acetylcholine receptors are widely distributed throughout the human brain and are believed to play a role in several neurological and psychiatric disorders. In order to identify an effective PET radioligand for in vivo assessment of the α4β2 subtype of nicotinic receptor, we synthesized [¹⁸F]3‐[1‐(3‐fluoropropyl)‐(S)‐pyrrolidin‐2‐ylmethoxy]pyridine (NicFP). The in vitro K_D of NicFP was determined to be 1.1 nM, and the log P value obtained by HPLC analysis of the unlabelled standard was found to be 2.2. The radiosynthesis of [¹⁸F]NicFP was carried out by a nucleophilic substitution reaction of anhydrous [¹⁸F]fluoride and the corresponding mesylate precursor. After purification by HPLC, the radiochemical yield was determined to be 11.3±2.1% and the specific activity was 0.47±0.18 Ci/μmol (EOS, n = 3). The time of synthesis and purification was 99±2 min. The final product was prepared as a sterile saline solution suitable for in vivo use. Copyright © 2003 John Wiley & Sons, Ltd.     

18F‐Labeled benzylpiperazine derivatives as highly selective ligands for imaging σ1 receptor with positron emission tomography

We report the design, synthesis, and evaluation of a new series of benzylpiperazine derivatives as selective σ₁ receptor ligands. All seven ligands possessed low nanomolar affinity for σ₁ receptors (K_i(σ₁) = 0.31‐4.19 nM) and high subtype selectivity (K_i(σ₂)/K_i(σ₁) = 50‐2448). The fluoroethoxy analogues also exhibited high selectivity toward the vesicular acetylcholine transporter (K_i(VAChT)/K_i(σ₁) = 99‐18252). The corresponding radiotracers [¹⁸F] 13 , [¹⁸F] 14 , and [¹⁸F] 16 with high selectivity (K_i(σ₂)/K_i(σ₁) > 100, K_i(VAChT)/K_i(σ₁) > 1000) were prepared in 42% to 55% radiochemical yields (corrected for decay), greater than 99% radiochemical purity (RCP), and molar activity of about 120 GBq/μmol at the end of synthesis (EOS). All three radiotracers showed high initial brain uptake in mouse (8.37‐11.48% ID/g at 2 min), which was not affected by pretreatment with cyclosporine A, suggesting that they are not substrates for permeability‐glycoprotein (P‐gp). Pretreatment with SA4503 or haloperidol resulted in significantly reduced brain uptake (35%‐62% decrease at 30 min). In particular, [¹⁸F] 16 displayed high brain‐to‐blood ratios and high in vivo metabolic stability. Although it may not be an optimal neuroimaging agent because of its slow kinetics in the mouse brain, [¹⁸F] 16 can serve as a lead compound for further structural modifications to explore new potential radiotracers for σ₁ receptors.     

Syntheses and radiofluorination of two derivatives of 5‐cyano‐indole as selective ligands for the dopamine subtype‐4 receptor

Two fluoroethoxy substituted derivatives, namely 2‐[4‐(2‐(2‐fluoroethoxy)phenyl)‐piperazin‐1‐ylmethyl]indole‐5‐carbonitrile ( 5a ) and 2‐[4‐(4‐(2‐fluoroethoxy)‐phenyl)piperazin‐1‐ylmethyl]indole‐5‐carbonitrile ( 5b ) were synthesized as analogs of the selective D₄ receptor ligand 2‐[4‐(4‐fluorophenyl)piperazin‐1‐ylmethyl]indole‐5‐carbonitrile (FAUC 316). In vitro characterization using CHO‐cells expressing different dopamine receptor subtypes gave K_i values of 2.1 ( 5a ) and 9.9 nM ( 5b ) for the dopamine D₄ subtype and displayed a 420‐fold D₄‐selectivity over D₂ receptors for 5b . The para‐fluoroethoxy substituted candidate 5b revealed substantially reduced α₁ and serotoninergic binding affinities in comparison to the ortho‐fluoroethoxy substituted compound. In order to provide potential positron emission tomography (PET) imaging probes for the dopamine D₄ receptor, ¹⁸F‐labelling conditions using [¹⁸F]fluoroethyl tosylate were optimized and led to radiochemical yields of 81 ± 5% ( [¹⁸F]5a ) and 47 ± 4% ( [¹⁸F]5b ) (n = 3, decay‐corrected and referred to labelling agent), respectively. Thus, ¹⁸F‐fluoroethylation favourably at the para position of the phenylpiperazine moiety of the 5‐cyano‐indole framework proved to be tolerated by D₄ receptors and could also be applied to alternative scaffolds in order to develop D₄ radioligand candidates for PET with improved D₄ receptor affinity and selectivity. Copyright © 2005 John Wiley & Sons, Ltd.     

Antinociceptive Synergism of MD‐354 and Clonidine. Part II. The α2‐Adrenoceptor Component

Abstract: Previously, we reported that antinociceptive synergism of a 5‐HT₃/α₂‐adrenoceptor ligand MD‐354 (m‐chlorophenylguanidine) and clonidine combination occurs, in part, through a 5‐HT₃ receptor antagonist mechanism. In the present investigation, a possible role for α₂‐adrenoceptors was examined. Mechanistic studies using yohimbine (a subtype non‐selective α₂‐adrenoceptor antagonist), BRL 44408 (a preferential α_2A‐adrenoceptor antagonist) and imiloxan (a preferential α_2B/C‐adrenoceptor antagonist) on the antinociceptive actions of a MD‐354/clonidine combination were conducted. Subcutaneous pre‐treatment with all three antagonists inhibited the antinociceptive synergism of MD‐354 and clonidine in the mouse tail‐flick assay in a dose‐dependent manner (AD₅₀ = 0.33, 2.1, and 0.17 mg/kg, respectively). Enhancement of clonidine antinociception by MD‐354 did not potentiate clonidine’s locomotor suppressant activity in a mouse locomotor assay. When [ethyl‐³H]RS‐79948‐197 was used as radioligand, MD‐354 displayed almost equal affinity to α_2A‐ and α_2B‐adrenoceptors (K_i = 110 and 220 nM) and showed lower affinity at α_2C‐adrenoceptors (K_i = 4,700 nM). MD‐354 had no subtype‐selectivity for the α₂‐adrenoceptor subtypes as an antagonist in functional [³⁵S]GTPγS binding assays. MD‐354 was a weak partial agonist at α_2A‐adrenoceptors. Overall, in addition to the 5‐HT₃ receptor component, the present investigation found MD‐354 to be a weak partial α_2A‐adrenoceptor agonist that enhances clonidine’s thermal antinociceptive actions through an α₂‐adrenoceptor‐mediated mechanism without augmenting sedation.     

A novel method for the synthesis of carbon‐14‐labeled N‐[3‐(1‐methyl‐4‐piperidinyl)‐1H‐pyrrolo[3,2‐b]pyridin‐5‐yl]propanamide and its use in quantitative whole‐body autoradiography studies

Sumitriptan ( 1 ), a non‐selective 5‐HT_1B/1D agonist is an effective therapeutic agent for the acute treatment of migraine, but it is contraindicated for use in patients with known heart disease. The first Selective Serotonin One F Receptor Agonist (SSOFRA), 5‐(4′‐fluorobenz‐amido)‐3‐(N‐methyl‐piperidin‐4‐yl)‐1H‐indole ( 2 ) was demonstrated to be clinically useful in the treatment of migraine. Although 2 exhibited high affinity for the 5‐HT_1F receptor as well as high selectivity for the 5‐HT_1F receptor relative to 5‐HT_1B and 5‐HT_1D receptors, it demonstrated appreciable affinity for the 5‐HT_1A receptor. Subsequently, a program was launched to discover SSOFRA's with improved selectivity over other 5‐HT₁ receptor subtypes. As a result of these efforts, N‐[3‐(1‐methyl‐4‐piperidinyl)‐1H‐pyrrolo[3,2‐b]pyridin‐5‐yl]propanamide ( 3 ) was found to possess greater than 100‐fold selectivity over 5‐HT_1A, 5‐HT_1B and 5‐HT_1D receptors. Pursuant to a potential clinical investigation of 3 , its carbon‐14‐labeled isotopomer has been prepared by a circuitous route from unlabeled 3 and used in quantitative whole‐body autoradiography studies in rats. The results of these efforts are reported herein. Copyright © 2005 John Wiley & Sons, Ltd.     

Structure‐Based Identification of Aporphines with Selective 5‐HT2A Receptor‐Binding Activity

Selective blockade of the serotonin 5‐HT_2A receptor is a useful therapeutic approach for a number of disorders, including schizophrenia, insomnia and ischaemic heart disease. A series of aporphines were docked into a homology model of the rat 5‐HT_2A receptor using AutoDock. Selected compounds with high in silico binding affinities were screened in vitro using radioligand‐binding assays against rat serotonin (5‐HT_1A and 5‐HT_2A) and dopamine (D1 and D2) receptors. (R)‐Roemerine and (±)‐nuciferine were found to have high affinity for the 5‐HT_2A receptor (K_i = 62 and 139 nm , respectively), with (R)‐roemerine showing 20‐ to 400‐fold selectivity for the 5‐HT_2A receptor over the 5‐HT_1A, D1 and D2 receptors. Investigation into the ligand–receptor interactions suggested that the selectivity of (R)‐roemerine is due to it having stronger H‐bonding and dipole–dipole interactions with several of the key residues in the 5‐HT_2A receptor‐binding site.     

Alternative methods for labeling the 5‐HT1A receptor agonist, 1‐[2‐(4‐fluorobenzoylamino)ethyl]‐4‐(7‐methoxynaphthyl)piperazine (S14506), with carbon‐11 or fluorine‐18

1‐[2‐(4‐Fluorobenzoylamino)ethyl]‐4‐(7‐methoxynaphthyl)piperazine (S14506) is one of the most potent and selective agonists at 5‐HT_1A receptors. For the purpose of prospective 5‐HT_1A receptor imaging with positron emission tomography and the investigation of radioligand metabolic pathways, S14506 was labeled with a positron emitter, either carbon‐11 (t_1/2=20.4 min) or fluorine‐18 (t_1/2=109.7 min), at different positions. Thus, [O‐methyl‐¹¹C]S14506 was obtained in a radiosynthesis time of 35 min by treating O‐desmethyl‐S14506 with [¹¹C]iodomethane and tetrabutylammonium hydroxide in N,N–dimethylformamide. The overall decay‐corrected radiochemical yield (RCY) of [O‐methyl‐¹¹C]S14506 ranged between 6 and 24% and the specific activity (SA) between 1343 and 3101 Ci/mmol (mean 2390; n=30). [carbonyl‐¹¹C]S14506 was synthesized through a microwave‐enhanced direct coupling of in situ generated [¹¹C]organocarboxymagnesium bromide with amine precursor. RCYs ranged from 10 to 18%. [¹⁸F]S14506 was prepared via nucleophilic aromatic fluoridation of the 4‐nitro analog in 14–35% RCY and with SA ranging from 1063 to 2302 Ci/mmol (mean 1617; n=14) in a radiosynthesis time of 115 min. Heating the radiofluoridation mixture for 5 min at 180°C in a single mode microwave cavity gave similar RCY and SA to heating for 30 min in an oil bath at the same temperature. Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis,labelling and first evaluation of [18F]R91150 as a serotonin 5‐HT2A receptor antagonist for PET

In psychiatric disorders such as anxiety, depression and schizophrenia, 5‐HT_2A receptors play an important role. In order to investigate them in vivo there is an increasing interest in selective and high‐affinity radioligands for receptor binding studies using positron emission tomography (PET). Since available radioligands have disadvantages, R91150, which is a selective and high‐affinity ligand for 5‐HT_2A receptors, was labelled with fluorine‐18. This was accomplished in six steps via 4‐[¹⁸F]fluorophenol and 1‐(3‐bromopropoxy)‐4‐[¹⁸F]fluorobenzene within 190 min starting from no‐carrier‐added [¹⁸F]fluoride. The overall radiochemical yield was 3.8±2% and the specific activity was at least 335 GBq/µmol at the end of the synthesis. First ex vivo studies in mice proved the uptake of [¹⁸F]R91150 in the brain. Radiometabolite studies revealed no radiometabolites in the brain, whereas in the plasma at least two could be detected 30 min p.i. Further preclinical studies are encouraged to evaluate the potential of this new 5‐HT_2A ligand as a radiotracer for PET. Copyright © 2008 John Wiley & Sons, Ltd.     

Automated radiosynthesis of [18F]SPA‐RQ for imaging human brain NK1 receptors with PET

[¹⁸F]SPA‐RQ is an effective radioligand for imaging brain neurokinin type‐1 (NK₁) receptors in clinical research and drug discovery with positron emission tomography. For the automated regular production of [¹⁸F]SPA‐RQ for clinical use in the USA under an IND we chose to use a modified commercial synthesis module (TRACERlab FX_F‐N; GE Medical Systems) with an auxiliary custom‐made robotic cooling–heating reactor, after evaluating several alternative radiosynthesis conditions. The automated radiosynthesis and its quality control are described here. [¹⁸F]SPA‐RQ was regularly obtained within 150 min from the start of radiosynthesis in high radiochemical purity (>99%) and chemical purity and with an overall decay‐corrected radiochemical yield of 15±2% (mean±S.D.; n=10) from cyclotron‐produced [¹⁸F]fluoride ion. The specific radioactivity of [¹⁸F]SPA‐RQ at the end of synthesis ranged from 644 to 2140 mCi/µmol (23.8–79.2 GBq/µmol). Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis,radiofluorination, and preliminary evaluation of the potential 5‐HT2A receptor agonists [18F]Cimbi‐92 and [18F]Cimbi‐150

An agonist PET tracer is of key interest for the imaging of the 5‐HT_2A receptor, as exemplified by the previously reported success of [¹¹C]Cimbi‐36. Fluorine‐18 holds several advantages over carbon‐11, making it the radionuclide of choice for clinical purposes. In this respect, an ¹⁸F‐labelled agonist 5‐HT_2A receptor (5‐HT_2AR) tracer is highly sought after. Herein, we report a 2‐step, 1‐pot labelling methodology of 2 tracer candidates. Both ligands display high in vitro affinities for the 5‐HT_2AR. The compounds were synthesised from easily accessible labelling precursors, and radiolabelled in acceptable radiochemical yields, sufficient for in vivo studies in domestic pigs. PET images partially conformed to the expected brain distribution of the 5‐HT_2AR; a notable exception however being significant uptake in the striatum and thalamus. Additionally, a within‐scan displacement challenge with a 5‐HT_2AR antagonist was unsuccessful, indicating that the tracers cannot be considered optimal for neuroimaging of the 5‐HT_2AR.     

Synthesis and preliminary in vivo evaluation of 4‐[18F]fluoro‐N‐{2‐[4‐(6‐trifluoromethylpyridin‐2‐yl)piperazin‐1‐yl]ethyl}benzamide,a potential PET radioligand for the 5‐HT1A receptor

4‐Fluoro‐N‐{2‐[4‐(6‐trifluoromethylpyridin‐2‐yl)piperazin‐1‐yl]ethyl}benzamide is a full 5‐HT_1A agonist with high affinity (pK_i=9.3), selectivity and a c log P of 3.045. The corresponding PET radioligand 4‐[¹⁸F]fluoro‐N‐{2‐[4‐(6‐trifluoromethylpyridin‐2‐yl)piperazin‐1‐yl]ethyl}benzamide was synthesized by nucleophilic aromatic substitution on the nitro precursor. The fluorinating agent K[¹⁸F]F/Kryptofix 2.2.2 was both dried (9 min, 700 W) and incorporated in the precursor (5 min, 700 W) using a commercially available microwave oven. In a total synthesis time of 60 min, an overall radiochemical yield of 18% (SD=5, n=7, EOS) was obtained. Radiochemical purity was always higher than 99% and specific activity always higher than 81.4 GBq/µmol (2.2 Ci/µmol). Initial brain uptake in mice was 2.19% ID (5.47% ID/g, 2 min) but decreased rapidly (0.17% ID, 0.45% ID/g (60 min)). During the first 20 min p.i., radioactivity concentration of the brain was significantly higher than that of blood demonstrating good brain entry of the tracer. Copyright © 2004 John Wiley & Sons, Ltd.     

The synthetic cathinone psychostimulant α‐PPP antagonizes serotonin 5‐HT2A receptors: In vitro and in vivo evidence

Synthetic cathinones (SCs) are β‐keto analogs of amphetamines. Like amphetamines, SCs target monoamine transporters; however, unusual neuropsychiatric symptoms have been associated with abuse of some SCs, suggesting SCs might possess additional pharmacological properties. We performed radioligand competition binding assays to assess the affinities of nine SCs at human 5‐HT_2A receptors (5‐HT_2AR) and muscarinic M₁ receptors (M₁R) transiently expressed in HEK293 cells. None of the SCs exhibited affinity at M₁R (minimal displacement of [~K_d] [³H]scopolamine up to 10 μM). However, two SCs, α‐pyrrolidinopropiophenone (α‐PPP) and 4‐methyl‐α‐PPP, had low μM K_i values at 5‐HT_2AR. In 5‐HT_2AR–phosphoinositide hydrolysis assays, α‐PPP and 4‐methyl‐α‐PPP displayed inverse agonist activity. We further assessed the 5‐HT_2AR functional activity of α‐PPP, and observed it competitively antagonized 5‐HT_2AR signaling stimulated by the 5‐HT₂R agonist (±)‐2,5‐dimethoxy‐4‐iodoamphetamine (DOI; K_b = 851 nM). To assess in vivo 5‐HT_2AR activity, we examined the effects of α‐PPP on the DOI‐elicited head‐twitch response (HTR) in mice. α‐PPP dose‐dependently blocked the HTR with maximal suppression at 10 mg/kg (P < 0.0001), which is a moderate dose used in studies investigating psychostimulant properties of α‐PPP. To corroborate a 5‐HT_2AR mechanism, we also tested 3,4‐methylenedioxy‐α‐PPP (MDPPP) and 3‐bromomethcathinone (3‐BMC), SCs that we observed had 5‐HT_2AR K_is > 10 μM. Neither MDPPP nor 3‐BMC, at 10 mg/kg doses, attenuated the DOI HTR. Our results suggest α‐PPP has antagonist interactions at 5‐HT_2AR in vitro that may translate at physiologically‐relevant doses in vivo. Considering 5‐HT_2AR antagonism has been shown to mitigate effects of psychostimulants, this property may contribute to α‐PPPs unpopularity compared to other monoamine transporter inhibitors.     

Evaluation of [18F]2FP3 in pigs and non‐human primates

So far, no suitable 5‐HT₇R radioligand exists for clinical positron emission tomography (PET) imaging. [¹⁸F]2FP3 was first tested in vivo in cats, and the results were promising for further evaluations. Here, we evaluate the radioligand in pigs and non‐human primates (NHPs). Furthermore, we investigate species differences in 5‐HT₇R binding with [³H]SB‐269970 autoradiography in post‐mortem pig, NHP, and human brain tissue. Specific binding of [¹⁸F]2FP3 was investigated by intravenous administration of the 5‐HT₇R specific antagonist SB‐269970. [³H]SB‐269970 autoradiography was performed as previously described. [¹⁸F]2FP3 was synthesized in an overall yield of 35% to 45%. High brain uptake of the tracer was found in both pigs and NHPs; however, pretreatment with SB‐269970 only resulted in decreased binding of 20% in the thalamus, a 5‐HT₇R–rich region. Autoradiography on post‐mortem pig, NHP, and human tissues revealed that specific binding of [³H]SB‐269970 was comparable in the thalamus of pig and NHP. Despite the high uptake of [¹⁸F]2FP3 in both species, the binding could only be blocked to a limited degree with the 5‐HT₇R antagonists. We speculate that the affinity of the radioligand is too low for imaging the 5‐HT₇Rs in vivo and that part of the PET signal arises from targets other than the 5‐HT₇R.     

Automated cGMP‐compliant radiosynthesis of [18F]‐(E)‐PSS232 for brain PET imaging of metabotropic glutamate receptor subtype 5

(E)‐3‐(Pyridin‐2‐yl ethynyl)cyclohex‐2‐enone O‐(3‐(2‐[¹⁸F]‐fluoroethoxy)propyl) oxime ([¹⁸F]‐(E)‐PSS232, [¹⁸F] 2a ) is a recently developed radiotracer that can be used to visualize metabotropic glutamate receptor subtype 5 (mGlu₅) in vivo. The mGlu₅ has become an attractive therapeutic and diagnostic target owing to its role in many neuropsychiatric disorders. Several carbon‐11‐labeled and fluorine‐18‐labeled radiotracers have been developed to measure mGlu₅ receptor occupancy in the human brain. The radiotracer [¹⁸F] 2a , which is used as an analogue for [¹¹C]ABP688 ([¹¹C] 1 ) and has a longer physical half‐life, is a selective radiotracer that exhibits high binding affinity for mGlu₅. Herein, we report the fully automated radiosynthesis of [¹⁸F] 2a using a commercial GE TRACERlab? FX‐_FN synthesizer for routine production and distribution to nearby satellite clinics. Nucleophilic substitution of the corresponding mesylate precursor with cyclotron‐produced [¹⁸F]fluoride ion at 100°C in dimethyl sulfoxide (DMSO), followed by high‐performance liquid chromatography (HPLC) purification and formulation, readily provided [¹⁸F] 2a with a radiochemical yield of 40 ± 2% (decay corrected, n = 5) at the end of synthesis. Radiochemical purity for the [¹⁸F]‐(E)‐conformer was greater than 95%. Molar activity was determined to be 63.6 ± 9.6 GBq/μmol (n = 5), and the overall synthesis time was 70 minutes.     

Synthesis and preliminary PET study of the 5‐HT7 receptor antagonist [11C]DR4446

DR4446 (1‐methyl‐2a‐[4‐(4,5,6,7‐tetrahydrothieno[3,2‐c]pyridin‐5‐yl)butyl]‐2a,3,4,5‐tetrahydro‐1H‐benz[cd]indole‐2‐one) is a potent 5‐HT₇ receptor antagonist (K_i=9.7 nM) with a high selectivity over other 5‐HT family receptors (K_i for 5‐HT_1A: 770 nM; for other 5‐HT receptors: >1000 nM). As a positron emission tomography (PET) tracer for the 5‐HT₇ receptor, [¹¹C]DR4446 was synthesized at high radiochemical purity ( >98%) with specific activity of 73–120 GBq/μmol at the end of synthesis by the alkylation of the desmethyl precursor (1) with [¹¹C]CH₃I in the presence of NaH. A PET study in monkey demonstrated that [¹¹C]DR4446 had good permeability into the brain, and had a specific binding component in the brain regions including the thalamus, possibly an area in the 5‐HT₇ receptors. Metabolite analysis showed that [¹¹C]DR4446 was relatively stable and low percentages of two radio‐labeled metabolites were detected in the plasma of monkey using HPLC. Copyright © 2002 John Wiley & Sons, Ltd.     

Ortho‐[18F]Fluoronitrobenzenes by no‐carrier‐added nucleophilic aromatic substitution with K[18F]F–K222—A comparative study

The scope of the nucleophilic aromatic ortho‐fluorinations from the corresponding ortho‐halonitrobenzene precursors (halo‐to‐fluoro substitutions) with no‐carrier‐added [¹⁸F]fluoride ion as its activated K[¹⁸F]F–K₂₂₂ complex has been evaluated via the radiosynthesis of ortho‐[¹⁸F]fluoronitrobenzene, chosen as a model reaction. The parameters studied include the influence of the leaving group in the ortho position of the phenyl ring (–Cl, –Br, –l), the quantity of precursor used, the type of activation (conventional heating or microwave irradiations), the solvent, the temperature and the reaction time. The iodo‐precursor was completely unreactive and the bromo‐precursor gave only low incorporation (<10%) in the optimal conditions used (conventional heating at 145°C or microwave activation, 100 W for 120 s). Only the chloro‐precursor was found reactive in the conditions described above and up to 70% yield was observed for the formation of ortho‐[¹⁸F]fluoronitrobenzene ([¹⁸F]‐ 1 ). In all the experiments, the unwanted ortho‐[¹⁸F]fluoro‐halobenzenes, potentially resulting from the nitro‐to‐fluoro substitution, could not be detected. These results will be applied for the radiosynthesis of 5‐[¹⁸F]fluoro‐6‐nitroquipazine, a potent radioligand for the imaging of the serotonin transporter with PET. Copyright © 2002 John Wiley & Sons, Ltd.     

Synthesis,docking studies,and pharmacological evaluation of 5HT2C ligands containing the N′‐cyanoisonicotinamidine or N′‐cyanopicolinamidine nucleus

N′‐Cyanoisonicotinamidine and N′‐cyanopicolinamidine derivatives, linked to an arylpiperazine moiety, were prepared and their affinities to the 5‐HT_1A, 5‐HT_2A, and 5‐HT_2C receptors were evaluated. Several of the newly synthesized compounds, tested by binding studies, showed nanomolar affinity at the 5‐HT_1A and 5‐HT_2C receptors and moderate or no affinity for other relevant receptors (D₁, D₂, α₁, and α₂). Compound 8e (K_i = 21.4 nM) was the most affine for the 5‐HT_2C receptor, showing, at the same time, a high selectivity with respect to the other receptors analyzed. Compounds 4a and 4c , instead, showed an interesting mixed 5‐HT_1A/5‐HT_2C activity with K_i values of 21.3/11.5 and 23.2/6.48 nM, respectively. The compounds with better affinity and selectivity binding profiles toward 5‐HT_2C ( 4a , 4c , 8b , and 8e ) were selected for further in vivo assays to determine their functional activity. Finally, to rationalize the obtained results, molecular docking studies were performed. The results of the pharmacological studies showed that compounds 4a , 8b , and 8e exerted antidepressant‐like effects and 4a and 8e revealed also significant anxiolytic properties. Among the developed derivatives, the most promising compound seems to be 4a , which displayed antipsychotic‐, antidepressant‐ and anxiolytic‐like properties. No side effects, like catalepsy, motor‐impairment or ethanol‐potentiating effects, were observed after the injection of the tested compounds.