Radiosynthesis and first evaluation in mice of [(18)F]NS14490 for molecular imaging of α7 nicotinic acetylcholine receptors

[(18)F]NS14490, a new potential radiotracer for neuroimaging of α7 nicotinic acetylcholine receptors (α7 nAChRs), was synthesized and evaluated in vitro and in vivo. Radioligand binding studies using [(3)H]methyllycaconitine and NS14490 as competitor showed a good target affinity (K(i,α7) = 2.5 nM) and a high selectivity towards other nAChRs. Radiosynthesis of [(18)F]NS14490 was performed by two different labelling procedures: a two-step synthesis using a prosthetic group, which led to 7% labelling yield, and the convenient direct nucleophilic substitution of the corresponding tosylate precursor, which resulted in 70% labelling yield. After optimisation of the isolation, purification and formulation process, biodistribution studies were performed in CD-1 mice. The brain uptake of [(18)F]NS14490 was comparably low (0.16% ID g(-1) wet weight at 5 min p.i.). The radiotracer showed a high metabolic stability in plasma and brain. Also, the target specificity was proven by pre-administration of a highly affine α7 ligand providing a rationale basis for further in vivo evaluation.

Synthesis and biological evaluation of ¹⁸F labeled fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives for sigma-1 receptor imaging

We report the synthesis and evaluation of a series of fluoro-oligo-ethoxylated 4-benzylpiperazine derivatives as potential σ(1) receptor ligands. In vitro competition binding assays showed that 1-(1,3-benzodioxol-5-ylmethyl)-4-(4-(2-fluoroethoxy)benzyl)piperazine (6) exhibits low nanomolar affinity for σ(1) receptors (K(i)=1.85 ± 1.59 nM) and high subtype selectivity (σ(2) receptor: K(i)=291 ± 111 nM; K(i)σ(2)/K(i)σ(1)=157). [(18)F]6 was prepared in 30-50% isolated radiochemical yield, with radiochemical purity of >99% by HPLC analysis after purification, via nucleophilic (18)F(-) substitution of the corresponding tosylate precursor. The logD(pH 7.4) value of [(18)F]6 was found to be 2.57 ± 0.10, which is within the range expected to give high brain uptake. Biodistribution studies in mice demonstrated relatively high concentration of radiotracers in organs known to contain σ(1) receptors, including the brain, lungs, kidneys, heart, and spleen. Administration of haloperidol 5 min prior to injection of [(18)F]6 significantly reduced the concentration of radiotracers in the above-mentioned organs. The accumulation of radiotracers in the bone was quite low suggesting that [(18)F]6 is relatively stable to in vivo defluorination. The ex vivo autoradiography in rat brain showed high accumulation of radiotracers in the brain areas known to possess high expression of σ(1) receptors. These findings suggest that [(18)F]6 is a suitable radiotracer for imaging σ(1) receptors with PET in vivo.

Cannabinoid receptor type 2 (CB2)-selective N-aryl-oxadiazolyl-propionamides: synthesis, radiolabelling, molecular modelling and biological evaluation

Background

The endocannabinoid system is involved in many physiological and pathological processes. Two receptors (cannabinoid receptor type 1 (CB1) and type 2 (CB2)) are known so far. Many unwanted psychotic side effects of inhibitors of this system can be addressed to the interaction with CB1. While CB1 is one of the most abundant neuroreceptors, CB2 is expressed in the brain only at very low levels. Thus, highly potent and selective compounds for CB2 are desired. N-aryl-((hetero)aromatic)-oxadiazolyl-propionamides represent a promising class of such selective ligands for the human CB2. Here, a library of various derivatives is studied for suitable routes for labelling with ¹⁸F. Such ¹⁸F-labelled compounds can then be employed as CB2-selective radiotracers for molecular imaging studies employing positron emission tomography (PET).

Results

By varying the N-arylamide substructure, we explored the binding pocket of the human CB2 receptor and identified 9-ethyl-9H-carbazole amide as the group with optimal size. Radioligand replacement experiments revealed that the modification of the (hetero)aromatic moiety in 3-position of the 1,2,4-oxadiazoles shows only moderate impact on affinity to CB2 but high impact on selectivity towards CB2 with respect to CB1. Further, we could show by autoradiography studies that the most promising compounds bind selectively on CB2 receptors in mouse spleen tissue. Molecular docking studies based on a novel three-dimensional structural model of the human CB2 receptor in its activated form indicate that the compounds bind with the N-arylamide substructure in the binding pocket. ¹⁸F labelling at the (hetero)aromatic moiety at the opposite site of the compounds via radiochemistry was carried out.

Conclusions

The synthesized CB2-selective compounds have high affinity towards CB2 and good selectivity against CB1. The introduction of labelling groups at the (hetero)aromatic moiety shows only moderate impact on CB2 affinity, indicating the introduction of potential labelling groups at this position as a promising approach to develop CB2-selective ligands suitable for molecular imaging with PET. The high affinity for human CB2 and selectivity against human CB1 of the herein presented compounds renders them as suitable candidates for molecular imaging studies.

Development of radioligands for the imaging of α7 nicotinic acetylcholine receptors with positron emission tomography

Molecular imaging of brain structures by highly sensitive non-invasive techniques offers unique possibilities in the understanding of physiological and pathological processes in the central nervous system. In particular, the quantitative analysis by positron emission tomography (PET) of α7 nicotinic acetylcholine receptors (α7 nAChR), which are involved in different signalling pathways in the brain, is assumed to provide important information on the relation between receptor dysfunction and the pathogeneses of neuropsychiatric brain diseases, but the applicability of this imaging approach is still hampered due to insufficient imaging agents. This paper presents the recent efforts made to develop PET radiotracers targeting α7 nAChR as well as the current state of the evaluation of the most promising radiolabelled compounds in animal models and humans.

Synthesis and biological evaluation of novel 4-benzylpiperazine ligands for sigma-1 receptor imaging

We report the synthesis and evaluation of 4-benzylpiperazine ligands (BP-CH(3), BP-F, BP-Br, BP-I, and BP-NO(2)) as potential σ(1) receptor ligands. The X-ray crystal structure of BP-Br, which crystallized with monoclinic space group P2(1)/c, has been determined. In vitro competition binding assays showed that all the five ligands exhibit low nanomolar affinity for σ(1) receptors (K(i)=0.43-0.91nM) and high subtype selectivity (σ(2) receptor: K(i)=40-61nM; K(i)σ(2)/K(i)σ(1)=52-94). [(125)I]BP-I (1-(1,3-benzodioxol-5-ylmethyl)-4-(4-iodobenzyl)piperazine) was prepared in 53±10% isolated radiochemical yield, with radiochemical purity of >99% by HPLC analysis after purification, via iododestannylation of the corresponding tributyltin precursor. The logD value of [(125)I]BP-I was found to be 2.98±0.17, which is within the range expected to give high brain uptake. Biodistribution studies in mice demonstrated relatively high concentration of radiolabeled substances in organs known to contain σ(1) receptors, including the brain, lung, kidney, heart, and spleen. Administration of haloperidol 5min prior to injection of [(125)I]BP-I significantly reduced the concentration of radioactivity in the above-mentioned organs. The accumulation of radiolabeled substance in the thyroid was quite low suggesting that [(125)I]BP-I is relatively stable to in vivo deiodination. These findings suggest that the binding of [(125)I]BP-I to σ(1) receptors in vivo is specific.

Molecular imaging of σ receptors: synthesis and evaluation of the potent σ1 selective radioligand [18F]fluspidine

PURPOSE

Neuroimaging of σ(1) receptors in the human brain has been proposed for the investigation of the pathophysiology of neurodegenerative and psychiatric diseases. However, there is a lack of suitable (18)F-labelled PET radioligands for that purpose.

METHODS

The selective σ(1) receptor ligand [(18)F]fluspidine (1'-benzyl-3-(2-[(18)F]fluoroethyl)-3H-spiro[[2]benzofuran-1,4'-piperidine]) was synthesized by nucleophilic (18)F(-) substitution of the tosyl precursor. In vitro receptor binding affinity and selectivity were assessed by radioligand competition in tissue homogenate and autoradiographic approaches. In female CD-1 mice, in vivo properties of [(18)F]fluspidine were evaluated by ex vivo brain section imaging and organ distribution of intravenously administered radiotracer. Target specificity was validated by organ distribution of [(18)F]fluspidine after treatment with 1 mg/kg i.p. of the σ receptor antagonist haloperidol or the emopamil binding protein (EBP) inhibitor tamoxifen. In vitro metabolic stability and in vivo metabolism were investigated by LC-MS(n) and radio-HPLC analysis.

RESULTS

[(18)F]Fluspidine was obtained with a radiochemical yield of 35-45%, a radiochemical purity of ≥ 99.6% and a specific activity of 150-350 GBq/μmol (n = 6) within a total synthesis time of 90-120 min. In vitro, fluspidine bound specifically and with high affinity to σ(1) receptors (K (i) = 0.59 nM). In mice, [(18)F]fluspidine rapidly accumulated in brain with uptake values of 3.9 and 4.7%ID/g and brain to blood ratios of 7 and 13 at 5 and 30 min after intravenous application of the radiotracer, respectively. By ex vivo autoradiography of brain slices, resemblance between binding site occupancy of [(18)F]fluspidine and the expression of σ(1) receptors was shown. The radiotracer uptake in the brain as well as in peripheral σ(1) receptor expressing organs was significantly inhibited by haloperidol but not by tamoxifen. Incubation with rat liver microsomes led to a fast biotransformation of fluspidine. After an incubation period of 30 min only 13% of the parent compound was left. Seven metabolites were identified by HPLC-UV and LC-MS(n) techniques. However, [(18)F]fluspidine showed a higher metabolic stability in vivo. In plasma samples ∼ 94% of parent compound remained at 30 min and ∼ 67% at 60 min post-injection. Only one major radiometabolite was detected. None of the radiometabolites crossed the blood-brain barrier.

CONCLUSION

[(18)F]Fluspidine demonstrated favourable target affinity and specificity as well as metabolic stability both in vitro and in animal experiments. The in vivo properties of [(18)F]fluspidine offer a high potential of this radiotracer for neuroimaging and quantitation of σ(1) receptors in vivo.

Evaluation of spirocyclic 3-(3-fluoropropyl)-2-benzofurans as sigma1 receptor ligands for neuroimaging with positron emission tomography

A series of various N-substituted 3-(3-fluoropropyl)-3H-spiro[[2]benzofuran-1,4'-piperidines] (7) has been synthesized. In receptor binding studies, the N-benzyl derivative 7a (WMS-1813) revealed extraordinarily high sigma(1) receptor affinity (K(i) = 1.4 nM) and excellent sigma(1)/sigma(2) selectivity (>600 fold). In vitro biotransformation of 7a with rat liver microsomes led to three main metabolites. N-Debenzylation was inhibited by introduction of an N-phenylethyl residue (7 g). The PET tracer [(18)F]7a was synthesized by nucleophilic substitution of the tosylate 13 with K[(18)F]F-K222-carbonate complex. The decay corrected radiochemical yield of [(18)F]7a was 35-48% with a radiochemical purity of >99.5% and a specific activity of 150-238 GBq/micromol. The radiotracer properties were evaluated in female CD-1 mice by organ distribution and ex vivo brain autoradiography. The radiotracer uptake in the brain was fast and sufficient, with values of approximately 4% injected dose per gram. Target specificity of [(18)F]7a was validated in blocking studies by preapplication of haloperidol, and significant reduction in the uptake of radioactivity was observed in the brain and peripheral organs expressing sigma(1) receptors.

Time-dependent alterations of cholinergic markers after experimental traumatic brain injury

Traumatic brain injury (TBI) is one of the leading causes of death and disability. Cognitive deficits are believed to be connected with impairments of the cholinergic system. The present study was conducted to evaluate the cholinergic system in a model of focal brain injury with special attention to the time course of posttraumatic events in critical brain regions. Three groups of male Sprague-Dawley rats (post-TBI survival time: 2 h, 24 h and 72 h) were subjected to sham-operation (control) or controlled cortical impact injury. Receptor densities were determined on frozen ipsilateral sagittal brain sections with [(3)H]epibatidine (nicotinic acetylcholine receptors) and [(3)H]QNB (muscarinic acetylcholine receptors). The density of the vesicular acetylcholine transporter (vAChT) was evaluated with (-)[(3)H]vesamicol. Compared to control, vAChT was lowered (up to 50%) at each time point after trauma, with reductions in olfactory tubercle, basal forebrain, motor cortex, putamen, thalamic and hypothalamic areas and the gigantocellular reticular nucleus. Time-dependent reductions of about 20% of nAChR-density in the thalamus, hypothalamus, olfactory tubercle, gigantocellular reticular nucleus and motor cortex were observed post-TBI at 24 and 72 h. The same brain regions showed reductions of mAChR at 24 and 72 h after trauma with additional decreases in the corpus callosum, basal forebrain and anterior olfactory nucleus. In conclusion, cholinergic markers showed significant time-dependent impairments after TBI. Considering the role of the cholinergic system for cognitive processes in the brain, it seems likely that these impairments contribute to clinically relevant cognitive deficits.

3-(4-(6-Fluoroalkoxy-3,4-dihydroisoquinoline-2(1H)-yl)cyclohexyl)-1H-indole-5-carbonitriles for SERT imaging: chemical synthesis, evaluation in vitro and radiofluorination

Aminocyclohexyl indoles bind with high affinity and specificity toward the serotonin transporter (SERT). Based on this structural lead, we designed fluoroalkoxydihydroisoquinoline-cyclohexyl indole carbonitriles for future application as (18)F-labeled tracers for SERT imaging by PET. Six compounds, three pairs of cis- and trans-isomer derivatives, respectively, were synthesized and evaluated in vitro. The chemistry of the new compounds, their affinity and specificity data, the general route to the phenolic precursor for labeling, and the successful (18)F-fluoroalkylation of one pair of compounds are described herein.

In vivo measurement of nicotinic acetylcholine receptors with [18F]norchloro-fluoro-homoepibatidine

Functional changes of nicotinic acetylcholine receptors (nAChR) are important during age-related neuronal degeneration. Recent studies demonstrate the applicability of the nAChR ligand 2-[(18)F]F-A-85380 for neuroimaging of patients with dementias. However, its binding kinetics demands a 7-h acquisition time limiting its practicality for clinical PET studies. Thus, the authors developed [(18)F]norchloro-fluoro-homoepibatidine ([(18)F]NCFHEB) for nAChR imaging. The kinetics of the two enantiomers of [(18)F]NCFHEB were compared with 2-[(18)F]F-A85380 in porcine brain to evaluate their potential for human neuroimaging. Twenty-four juvenile female pigs were studied with PET using [(18)F]NCFHEB. Nine animals received an additional i.v. injection (1 mg/kg) of the nAChR agonist A81418 before radiotracer administration followed by infusion (2 mg/kg/7h) thereafter. Several compartment models were applied for quantification. (-)- and (+)-[(18)F]NCFHEB showed a twofold to threefold higher brain uptake than 2-[(18)F]F-A-85380. All three radiotracers displayed spatially heterogeneous binding kinetics in regions with high, moderate, or low specific binding. The equilibrium of specific binding of (-)-[(18)F]NCFHEB was reached earlier than that of (+)-[(18)F]NCFHEB or 2-[(18)F]F-A85380. Continuous administration of the nAChR agonist A81418 inhibited the specific binding of (-)- and (+)-[(18)F]NCFHEB but not of 2-[(18)F]F-A85380. The peripheral metabolism of (+)-[(18)F]NCFHEB proceeded somewhat slower than that of the other radiotracers. Both enantiomers of [(18)F]NCFHEB are appropriate radiotracers for neuroimaging of nAChR in pigs. Their binding profile in vivo appears to be more selective than that of 2-[(18)F]F-A85380. (-)-[(18)F]NCFHEB offers a faster equilibrium of specific binding than 2-[(18)F]F-A85380.

Norchloro-fluoro-homoepibatidine (NCFHEB) - a promising radioligand for neuroimaging nicotinic acetylcholine receptors with PET

Cholinergic neurotransmission depends on the integrity of nicotinic acetylcholine receptors (nAChRs), and impairment of both is characteristic for various neurodegenerative diseases. Visualization of specific receptor subtypes by positron emission tomography (PET) has potential to assist with diagnosis of such neurodegenerative diseases and with design of suitable therapeutic approaches. The goal of our study was to evaluate in vivo the potential of (18)F-labelled (+)- and (-)-norchloro-fluoro-homoepibatidine ([(18)F]NCFHEB) in comparison to 2-[(18)F]F-A-85380 as PET tracers. In the brains of NMRI mice, highest levels of radioactivity were detected at 20 min post-injection of (+)-[(18)F]NCFHEB, (-)-[(18)F]NCFHEB, and 2-F-[(18)F]-A-85380 (7.45, 5.60, and 3.2% ID/g tissue, respectively). No marked pharmacological adverse effects were observed at 25 mug NCFHEB/kg. Uptake studies in RBE4 cells and in situ perfusion studies suggest an interaction of epibatidine and NCFHEB with the carrier-mediated choline transport at the blood-brain barrier. The data indicate that (+)- and (-)-[(18)F]NCFHEB have potential for further development as PET tracers.