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,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 and biological evaluation of S‐[11C]methylated mercaptoimidazole piperazinyl derivatives as potential radioligands for imaging 5‐HT1A receptors by positron emission tomography (PET)

The novel 2‐mercaptoimidazole derivatives, 1‐[4‐((2‐methoxyphenyl)‐1‐piperazinyl)butyl]‐2‐mercaptoimidazole ( 3 ) and methyl[4‐((2‐methoxyphenyl)‐1‐piperazinyl))butyl] (2‐mercapto‐1‐methylimidazol‐5‐yl)methanamide ( 8 ), were efficiently labelled with ¹¹C through methylation of the thioketone function with [¹¹C]methyl iodide. The resulting radioligands 1‐[4‐((2‐methoxyphenyl)‐1‐piperazinyl))butyl]‐2‐thio[¹¹C]methylimidazole ([¹¹C] 9 ) and methyl[4‐((2‐methoxyphenyl)‐1‐piperazinyl))butyl] (2‐thio[¹¹C]methyl‐1‐methylimidazol‐5‐yl)‐methanamide ([¹¹C] 10 ) were synthesized in radiochemical yields of 20–30% (decay‐corrected, related to [¹¹C]CO₂) at a specific radioactivity of 0.2–0.4 Ci/µmol within 40–45 min including HPLC‐purification. The radiochemical purity exceeded 99%. The reference compounds 9 and 10 were tested in a competitive receptor binding assay to determine their affinity toward the 5‐HT_1A receptor. Both compounds exhibit excellent sub‐nanomolar affinities (IC₅₀=0.576±0.008 nM ( 9 ); IC₅₀=0.86±0.02 nM ( 10 )) for the 5‐HT_1A receptor while displaying a high selectivity towards the 5‐HT_2A subtype of receptors (IC₅₀>480 nM). By contrast, compound 9 also shows substantial binding for the alpha1‐adrenergic receptor (IC₅₀=3.00±0.02 nM) when compared with compound 10 (IC₅₀=54.5±0.6 nM). Preliminary biodistribution studies in rats showed an initial brain uptake of 1.14±0.11 and 0.37±0.04% ID/g after 5 min, which decreased to 0.18±0.04 and 0.16±0.01% ID/g after 60 min for compounds [¹¹C] 9 and [¹¹C] 10 , respectively. For both compounds, the cerebellum and rest of the brain uptake are very similar at the different time points. Unlike [¹¹C] 9 , the radioligand [¹¹C] 10 has significant uptake and retention in the adrenal glands. Due to their washout from the brain compounds [¹¹C] 9 and [¹¹C] 10 seem not to be good candidates as radioligands for imaging 5‐HT_1A receptors by PET. Copyright © 2005 John Wiley & Sons, Ltd.     

Synthesis and in vivo evaluation of [O‐methyl‐11C] 2‐(4‐methoxyphenyl)‐N‐(4‐methylbenzyl)‐N‐(1‐methyl‐ piperidin‐4‐yl)acetamide as an imaging probe for 5‐HT2A receptors

2‐(4‐Methoxyphenyl)‐N‐(4‐methylbenzyl)‐N‐(1‐methylpiperidin‐4‐yl)acetamide (AC90179, 4 ), a highly potent and selective competitive 5‐HT_2A antagonist, was labeled by [¹¹C]‐methylation of the corresponding desmethyl analogue 5 with [¹¹C]methyl triflate. The precursor molecule 5 for radiolabeling was synthesized from p‐tolylmethylamine in three steps with 46% overall yield. [¹¹C]AC90179 was synthesized in 30 min (30 ± 5% yield, EOS) with a specific activity of 4500 ± 500 Ci/mmol and >99% chemical and radiochemical purities. Positron emission tomography studies in anesthetized baboon revealed that [¹¹C] 4 Penetrates the blood–brain barrier (BBB) with a rapid influx and efflux of the tracer in all brain regions. Due to lack of tracer retention or specific binding, [¹¹C] 4 cannot be used as PET ligand for imaging 5‐HT_2A receptors. Copyright © 2006 John Wiley & Sons, Ltd.     

Synthesis and evaluation of [O‐methyl‐11C]4‐[3‐[4‐(2‐methoxyphenyl)‐ piperazin‐1‐yl]propoxy]‐4‐aza‐tricyclo[5.2.1.02,6]dec‐8‐ene‐3,5‐dione as a 5‐HT1A receptor agonist PET ligand

4‐[3‐[4‐(2‐Methoxyphenyl)piperazin‐1‐yl]propoxy]‐4‐aza‐tricyclo[5.2.1.02,6]dec‐8‐ene‐3,5‐dione (4), a potent and selective 5‐HT_1A agonist, was labeled by ¹¹C‐methylation of the corresponding desmethyl analogue 3 with ¹¹C‐methyl triflate. The precursor molecule 3 was synthesized from commercially available endo‐N‐hydroxy‐5‐norbornene‐2,3‐dicarboximide in two steps with an overall yield of 40%. Radiosynthesis of ¹¹C‐4 was achieved in 35 min in 20±5% yield (n=6) at the end of synthesis with a specific activity of 2600±250 Ci/mmol. In vivo positron emission tomography (PET) studies in baboon revealed rapid uptake of the tracer into the brain. However, lack of specific binding indicates that ¹¹C‐4 is not useful as a 5‐HT_1A agonist PET ligand for clinical studies. Copyright © 2008 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.     

Synthesis,pharmacological evaluations,and molecular docking studies on a new 1,3,4,11b‐tetrahydro‐1H‐fluoreno[9,1‐cd]azepine framework: Rigidification of D1 receptor selective 1‐phenylbenzazepines and discovery of a new 5‐HT6 receptor scaffold

The novel 1,3,4,11b‐tetrahydro‐1H‐fluoreno[9,1‐cd]azepine framework, a structurally rigidified variant of the 1‐phenylbenzazepine template, was synthesized via direct arylation as a key reaction. Evaluation of the binding affinities of the rigidified compounds across a battery of serotonin, dopamine, and adrenergic receptors indicates that this scaffold unexpectedly has minimal affinity for D₁ and other dopamine receptors and is selective for the 5‐HT₆ receptor. The affinity of these systems at the 5‐HT₆ receptor is significantly influenced by electronic and hydrophobic interactions as well as the enhanced rigidity of the ligands. Molecular docking studies indicate that the reduced D₁ receptor affinity of the rigidified compounds may be due in part to weaker H‐bonding interactions between the oxygenated moieties on the compounds and specific receptor residues. Key receptor–ligand H‐bonding interactions, salt bridges, and π–π interactions appear to be responsible for the 5‐HT₆ receptor affinity of the compounds. Compounds 10 (6,7‐dimethoxy‐2,3,4,11b‐tetrahydro‐1H‐fluoreno[9,1‐cd]azepine) and 12 (6,7‐dimethoxy‐2‐methyl‐2,3,4,11b‐tetrahydro‐1H‐fluoreno[9,1‐cd]azepine) have been identified as structurally novel, high affinity (K_i = 5 nM), selective 5‐HT₆ receptor ligands.     

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.     

Optimization and synthesis of an 18F‐labeled dopamine D3 receptor ligand using [18F]fluorophenylazocarboxylic tert‐butylester

There is still no efficient fluorine‐18‐labeled dopamine D₃ subtype selective receptor ligand for studies with positron emission tomography. We aim at improving the D₃ selectivity and hydrophilicity of a candidate ligand by changing the substitution pattern to a 2,3‐dichlorophenylpiperazine and hydroxylation of the butyl chain. The compound [¹⁸F]3 exhibited D₃ affinity of K_i = 3.6 nM, increased subtype selectivity (K_i(D₂/D₃) = 60), and low affinity to 5‐HT_1A and α₁ receptors (K_i (5‐HT_1A/D₃) = 34; K_i (α₁/D₃) = 100). The two‐step radiosynthesis was optimized for analog [¹⁸F]4 by reducing the necessary concentration of the precursor amine (57 mM), which reacted with [¹⁸F]fluorophenylazocarboxylic tert‐butylester under basic conditions. The optimization of the base (Cs ₂CO₃, 23 mM) and the adjustment of reaction temperature led to the radiochemical yield of 63% after 5 min at 35°C. The optimized reaction conditions were transferred on to the synthesis of [¹⁸F]3 with an overall non‐decay corrected yield of 8‐12% in a specific activity of 32‐102 GBq/µmol after a total synthesis time of 30‐35 min. This provides a D ₃ radioligand candidate with improved attributes concerning selectivity and radiosynthesis for further preclinical studies.     

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,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.     

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.     

Novel Fused Pyrrole Heterocyclic Ring Systems as Structure Analogs of LE 300: Synthesis and Pharmacological Evaluation as Serotonin 5‐HT2A,Dopamine and Histamine H1 Receptor Ligands

LE 300 represents a structurally novel type of antagonists acting preferentially at the dopamine D₁/D₅ receptors and the serotonin 5‐HT_2A receptor. This compound consists of a ten‐membered central azecine ring fused to an indole ring on one side and a benzene moiety on the other side. To estimate the importance of the indole and / or phenyl moieties in this highly active benz‐indolo‐azecine, both rings were removed and replaced with a 1H‐pyrrole counterpart. Accordingly, some new analogs of LE 300 namely, pyrrolo[2,3‐g]indolizine, pyrrolo[3,2‐a]quinolizine rings and their corresponding dimethylpyrrolo[2,3‐d]azonine, and dimethylpyrrolo[2,3‐d]azecine were synthesized to be evaluated for their activity at the 5‐HT_2A and dopamine D₁, D_2L, D₄, D₅ receptors in relation to LE 300 . In addition, their activity at the H₁‐histamine receptors was also determined. The results suggested that the rigid pyrrolo[2,3‐g]indolizine 7 and pyrrolo[3,2‐a]quinolizine 8 analogs lacked biological activity in the adopted three bioassays. However, their corresponding flexible pyrrolo[2,3‐d]azonine 11 and pyrrolo[2,3‐d]azecine 12 derivatives revealed weak partial agonistic activity and weak antagonistic potency at the serotonin 5‐HT_2A and histamine H₁ receptors, respectively. Meanwhile, they showed no affinity to any of the four utilized dopamine receptors. Variation in ring size did not contribute to a significant influence on the three tested bioactivities. Removal of the hydrophobic moiety (phenyl ring) and replacement of the indole moiety with a 1H‐pyrrole counterpart led to a dramatic alteration in the profile of activity of such azecine‐type compounds.     

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.     

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.     

Radiosynthesis and preclinical evaluation of [11C]Cimbi-701 – Towards the imaging of cerebral 5-HT7 receptors

The serotonin 7 (5-HT₇) receptor is suggested to be involved in a broad variety of CNS disorders, but very few in vivo tools exist to study this important target. Molecular imaging with positron emission tomography (PET) would enable an in vivo characterization of the 5-HT₇ receptor. However, no clinical PET radiotracer exists for this receptor, and thus we aimed to develop such a tracer. In this study, we present the preclinical evaluation of [¹¹C]Cimbi-701. Cimbi-701 was synthesized in a one-step procedure starting from SB-269970. Its selectivity profile was determined using an academic screening platform (NIMH Psychoactive Drug Screening Program). Successful radiolabeling of [¹¹C]Cimbi-701 and subsequent in vivo evaluation was conducted in rats, pigs and baboon. In vivo specificity was investigated by 5-HT₇ and σ receptor blocking studies. P-gp efflux transporter dependency was investigated using elacridar. [¹¹C]Cimbi-701 could successfully be synthesized. Selectivity profiling revealed high affinity for the 5-HT₇ (K_i = 18 nM), σ-1 (K_i = 9.2 nM) and σ-2 (K_i = 1.6 nM) receptors. In rats, [¹¹C]Cimbi-701 acted as a strong P-gp substrate. After P-gp inhibition, rat brain uptake could specifically be blocked by 5-HT₇ and σ receptor ligands. In pig, high brain uptake and specific 5-HT₇ and σ-receptor binding was found for [¹¹C]Cimbi-701 without P-gp inhibition. Finally, low brain uptake was found in baboons. Both the specific σ-receptor binding and the low brain uptake of [¹¹C]Cimbi-701 displayed in baboon discouraged further translation to humans. Instead, we suggest exploration of this structural class as results indicate that selective 5-HT₇ receptor imaging might be possible when more selective non-P-gp substrates are identified.     

Synthesis and biological evaluation of a radioiodinated spiropiperidine ligand as a potential σ1 receptor imaging agent

We report the synthesis and evaluation of 1′‐(4‐[¹²⁵I]iodobenzyl)‐3H‐spiro[isobenzofuran‐1,4′‐piperidine] ([¹²⁵I]Spiro‐I) as a potential SPECT tracer for imaging of σ₁ receptors. [¹²⁵I]Spiro‐I was prepared in 55–65% isolated radiochemical yield, with radiochemical purity of >99%, via iododestannylation of the corresponding tributyltin precursor. In receptor binding studies, Spiro‐I displayed low nanomolar affinity for σ₁ receptors (σ₁: K_i=2.75±0.12 nM; σ₂: K_i=340 nM) and high subtype selectivity (σ₂/σ₁=124). Biodistribution in mice demonstrated relatively high concentration of radioactivity in organs known to contain σ₁ receptors, including the lung, kidney, heart, spleen, and brain. Administration of haloperidol 5 min prior to injection of [¹²⁵I]Spiro‐I significantly reduced the concentration of radioactivity in the above‐mentioned organs. These findings suggest that the binding of [¹²⁵I]Spiro‐I to σ₁ receptors in vivo is specific. Copyright © 2010 John Wiley & Sons, Ltd.     

Synthesis and evaluation of a 11C‐labelled angiotensin II AT2 receptor ligand

Three ¹¹C‐radiolabelled high‐affinity nonpeptide AT₂ receptor‐selective ligands were synthesized and one of these was evaluated as positron emission tomography (PET) tracer. The labelling reaction was performed via palladium(0)‐mediated aminocarbonylation of the aryl iodide substrate using [¹¹C]carbon monoxide as the labelled precursor. As an example, starting with 10.0 GBq [¹¹C]carbon monoxide, 1.10 GBq of the product N‐butoxycarbonyl‐3‐[4‐(N‐benzyl‐[¹¹C]carbamoyl)‐phenyl]‐5‐isobutylthiophene‐2‐sulphonamide [¹¹C]4d was obtained in 36% decay‐corrected radiochemical yield (from [¹¹C]carbon monoxide), 42 min from end of bombardment with a specific activity of 110 GBq·µmol^?1. The N‐isopropyl‐[¹¹C]carbamoyl‐analogue [¹¹C]4c (radiochemical purity >95%) was studied employing autoradiography, organ distribution, and small animal PET. In vitro autoradiography showed specific binding in the pancreas and kidney. Organ distribution in six rats revealed a high uptake in the liver, intestine, kidney, and adrenals. Small animal PET showed rapid and reversible uptake in the kidneys followed by accumulation in the urinary bladder suggesting fast renal excretion of the tracer. In addition, high accumulation was also seen in the liver. For future studies, more metabolically stable tracers will need to be developed. To the best of our knowledge, this is the first attempt of the use of PET imaging for the detection of expressed, fully functional AT₂ receptors in living subjects. Copyright © 2010 John Wiley & Sons, Ltd.     

Anti‐Alzheimer's multitarget‐directed ligands with serotonin 5‐HT6 antagonist,butyrylcholinesterase inhibitory,and antioxidant activity

Serotonin 5‐HT₆ receptors, butyrylcholinesterase (BuChE) and oxidative stress are related to the pathophysiology of Alzheimer's disease. Inhibition of BuChE provides symptomatic treatment of the disease and the same effect was demonstrated for 5‐HT ₆ antagonists in clinical trials. Oxidative stress is regarded as a major and primary factor contributing to the development of Alzheimer's disease; therefore, antioxidant agents may provide a disease‐modifying effect. Combining BuChE inhibition, 5‐HT ₆ antagonism, and antioxidant properties may result in multitarget‐directed ligands providing cognition‐enhancing properties with neuroprotective activity. On the basis of the screening of the library of 5‐HT ₆ antagonists against BuChE, we selected two compounds and designed their structural modifications that could lead to improved BuChE inhibitory activity. We synthesized two series of compounds and tested their affinity and functional activity at 5‐HT ₆ receptors, BuChE inhibitory activity and antioxidant properties. Compound 12 with K _i and K _b values against 5‐HT ₆ receptors of 41.8 and 74 nM, respectively, an IC ₅₀ value of 5 µM against BuChE and antioxidant properties exceeding the activity of ascorbic acid is a promising lead structure for further development of anti‐Alzheimer's agents.