6-[18F]Fluoro-A-85380: an in vivo tracer for the nicotinic acetylcholine receptor

Single photon emission computed tomography experience with (S)-5-[(123)I]iodo-3-(2-azetidinylmethoxy)pyridine in the living human brain of smokers and nonsmokers

(S)-5-[(123)I]iodo-3-(2-azetidinylmethoxy)pyridine (5-[(123)I]IA), a novel potent radioligand for high-affinity alpha4beta2* neuronal nicotinic acetylcholine receptors (nAChRs), provides a means to evaluate the density and the distribution of nAChRs in the living human brain. We sought in healthy adult smokers and nonsmokers to (1) evaluate the safety, tolerability, and efficacy of 5-[(123)I]IA in an open nonblind trial and (2) to estimate the density and the distribution of alpha(4)beta(2)* nAChRs in the brain. Single photon emission computed tomography (SPECT) was performed for 5 h after the i.v. administration of approximately 0.001 microg/kg ( approximately 10 mCi) 5-[(123)I]IA. Blood pressure, heart rate, and neurobehavioral status were monitored before, during, and after the administration of 5-[(123)I]IA to 12 healthy adults (8 men and 4 women) (6 smokers and 6 nonsmokers) ranging in age from 19 to 46 years (mean = 28.25, standard deviation = 8.20). High plasma-nicotine level was significantly associated with low 5-[(123)I]IA binding in: (1) the caudate head, the cerebellum, the cortex, and the putamen, utilizing both the Sign and Mann-Whitney U-tests; (2) the fusiform gyrus, the hippocampus, the parahippocampus, and the pons utilizing the Mann-Whitney U-test; and (3) the thalamus utilizing the Sign test. We conclude that 5-[(123)I]IA is a safe, well-tolerated, and effective pharmacologic agent for human subjects to estimate high-affinity alpha4/beta2 nAChRs in the living human brain.

Development of radioligands with optimized imaging properties for quantification of nicotinic acetylcholine receptors by positron emission tomography

AIMS

There is an urgent need for positron emission tomography (PET) imaging of the nicotinic acetylcholine receptors (nAChR) to study the role of the nicotinic system in Alzheimer's and Parkinson's diseases, schizophrenia, drug dependence and many other disorders. Greater understanding of the underlying mechanisms of the nicotinic system could direct the development of medications to treat these disorders. Central nAChRs also contribute to a variety of brain functions, including cognition, behavior and memory.

MAIN METHODS

Currently, only two radiotracers, (S)-3-(azetidin-2-ylmethoxy)-2-[(18)F]fluoropyridine (2-[(18)F]FA) and (S)-5-(azetidin-2-ylmethoxy)-2-[(18)F]fluoropyridine (6-[(18)F]FA), are available for studying nAChRs in human brain using PET. However, the "slow" brain kinetics of these radiotracers hamper mathematical modeling and reliable measurement of kinetic parameters since it takes 4-7 h of PET scanning for the tracers to reach steady state. The imaging drawbacks of the presently available nAChR radioligands have initiated the development of radioligands with faster brain kinetics by several research groups.

KEY FINDINGS

This minireview attempts to survey the important achievements of several research groups in the discovery of PET nicotinic radioligands reached recently. Specifically, this article reviews papers published from 2006 through 2008 describing the development of fifteen new nAChR (11)C-and (18)F-ligands that show improved imaging properties over 2-[(18)F]FA.

SIGNIFICANCE

The continuous efforts of radiomedicinal chemists led to the development of several interesting PET radioligands for imaging of nAChR including [(18)F]AZAN, a potentially superior alternative to 2-[(18)F]FA.

Derivatives of (−)-7-Methyl-2-(5-(pyridinyl)pyridin-3-yl)-7-azabicyclo[2.2.1]heptane Are Potential Ligands for Positron Emission Tomography Imaging of Extrathalamic Nicotinic Acetylcholine Receptors

A series of novel racemic 7-methyl-2-(5-(pyridinyl)pyridin-3-yl)-7-azabicyclo[2.2.1]heptane derivatives with picomolar in vitro binding affinity at nicotinic acetylcholine receptors (nAChRs) were synthesized and their enantiomers were resolved by semipreparative chiral HPLC. The (-)-enantiomers showed substantially greater in vitro inhibition binding affinity than the corresponding (+)-enantiomers. The compounds with best binding affinities have been radiolabeled with positron emitting isotopes 11C and 18F as potential radioligands for positron emission tomography imaging of the nAChR. In vivo enantioselectivity of the radiolabeled (-)-7-methyl-2-(5-(pyridinyl)pyridin-3-yl)-7-azabicyclo[2.2.1]heptane derivatives was observed in biodistribution studies in rodents and baboon. One of the radiolabeled compounds, (-)-7-methyl-2-exo-[3'-(2-[18F]fluoropyridin-5-yl))-5'-pyridinyl]-7-azabicyclo[2.2.1]heptane, exhibited good properties as a first practical PET radioligand for imaging of extrathalamic nAChR in baboon brain and holds promise for further investigation for human studies.

Measurement of the alpha4beta2* nicotinic acetylcholine receptor ligand 2-[(18)F]Fluoro-A-85380 and its metabolites in human blood during PET investigation: a methodological study

2-[(18)F]fluoro-A-85380 (2-[(18)F]FA) is a new radioligand for noninvasive imaging of alpha4beta2* nicotinic acetylcholine receptors (nAChRs) by positron emission tomography (PET) in human brain. In most cases, quantification of 2-[(18)F]FA receptor binding involves measurement of free nonmetabolized radioligand concentration in blood. This requires an efficient and reliable method to separate radioactive metabolites from the parent compound. In the present study, three analytical methods, thin layer chromatography (TLC), high-performance liquid chromatography (HPLC) and solid phase extraction (SPE) have been tested. Reversed-phase TLC of deproteinized aqueous samples of plasma provides good estimates of 2-[(18)F]FA and its metabolites. However, because of the decreased radioactivity in plasma samples, this method can be used in humans over the first 2 h after radioligand injection only. Reliable quantification of the parent radioligand and its main metabolites was obtained using reversed-phase HPLC, followed by counting of eluted fractions in a well gamma counter. Three main and five minor metabolites of 2-[(18)F]FA were detected in human blood using this method. On average, the unchanged 2-[(18)F]FA fraction in plasma of healthy volunteers measured at 14, 60, 120, 240 and 420 min after radioligand injection was 87.3+/-2.2%, 74.4+/-3%, 68.8+/-5%, 62.3+/-8% and 61.0+/-8%, respectively. In patients with neurodegenerative disorders, the values corresponding to the three last time points were significantly lower. The fraction of nonmetabolized 2-[(18)F]FA in plasma determined using SPE did not differ significantly from that obtained by HPLC (+gamma counting) (n=73, r=.95). Since SPE is less time-consuming than HPLC and provides comparable results, we conclude that SPE appears to be the most suitable method for measurement of 2-[(18)F]FA parent fraction during PET investigations.

[18F]fluoropyridines: From conventional radiotracers to the labeling of macromolecules such as proteins and oligonucleotides

Molecular in vivo imaging with the high-resolution and sensitive positron emission tomography (PET) technique requires the preparation of a positron-emitting radiolabeled probe or radiotracer. For this purpose, fluorine-18 is becoming increasingly the radionuclide of choice due to its adequate physical and nuclear characteristics, and also because of the successful use in clinical oncology of 2-[18F]fluoro-2-deoxy-D-glucose ([18F]FDG), which is currently the most widely used PET-radiopharmaceutical and probably the driving force behind the growing availability and interest for this positron-emitter in radiopharmaceutical chemistry. With a few exceptions, radiofluorinations involving fluorine-18 of high specific radioactivity (e.g. > 185 GBq/micromole) had, until recently, been limited to nucleophilic substitutions in homoaromatic and aliphatic series with [18F]fluoride. Considering chemical structures showing a fluoropyridinyl moiety, nucleophilic heteroaromatic substitution at the ortho-position with no-carrier-added [l8F]fluoride, as its K[18F]F-K222 complex, appears today as a highly efficient method for the radiosynthesis of radiotracers and radiopharmaceuticals. This chapter summarizes the recent applications of this methodology and highlights its potential in the design and preparation of, often drug-based, fluorine-18-labeled probes of high specific radioactivity for PET imaging, including macromolecules of biological interest such as peptides, proteins and oligonucleotides.

Change of central cholinergic receptors following lesions of nucleus basalis magnocellularis in rats: search for an imaging index suitable for the early detection of Alzheimer's disease

Cholinergic system in the central nervous system is involved in the memory function. Thus, because the dysfunction of cholinergic system that project to the cerebral cortex from nucleus basalis of Meynert (nbM) would be implicated in the memory function deficits in Alzheimer's disease (AD), evaluating cholinergic function may be useful for the early detection of AD. In this study, because the nucleus basalis magnocellularis (NBM) in rats is equivalent to nbM in human, we investigated the change in cholinergic receptors in the frontal cortex of rats with unilateral lesion to the NBM to find an appropriate index for the early detection of AD using techniques of nuclear medicine. The right NBM was injected with ibotenic acid. [(18)F]FDG-PET images were obtained 3 days later. Some rats were sacrificed at 1 week, whereas others were subjected to a second [(18)F]FDG-PET at 4 weeks then sacrificed for membrane preparation. The prepared membranes were subjected to radioreceptor assays to measure the density of nicotinic and muscarinic acetylcholine receptors. Glucose metabolism had decreased on the damaged side compared to the control side at 3 days, but at 4 weeks, there was no difference between the sides. Nicotinic acetylcholine receptors had significantly decreased in density compared to the control side at both 1 and 4 weeks. However, muscarinic receptors were not affected. These results suggested that neuronal dysfunction in AD could be diagnosed at an early stage by imaging nicotinic acetylcholine receptors.

Synthesis and radiosynthesis of [18F]FPhEP, a novel α4β2-selective, epibatidine-based antagonist for PET imaging of nicotinic acetylcholine receptors

FPhEP (1, (+/-)-2-exo-(2'-fluoro-3'-phenyl-pyridin-5'-yl)-7-azabicyclo[2.2.1]heptane) belongs to a recently described novel series of 3'-phenyl analogues of epibatidine, which not only possess subnanomolar affinity and high selectivity for brain alpha4beta2 neuronal nicotinic acetylcholine receptors (nAChRs), but also were reported as functional antagonists of low toxicity (up to 15 mg/kg in mice). FPhEP (1, K(i) of 0.24 nM against [(3)H]epibatidine) as reference as well as the corresponding N-Boc-protected chloro- and bromo derivatives (3a,b) as precursors for labelling with fluorine-18 were synthesized in eight and nine steps, respectively, from commercially available N-Boc-pyrrole (overall yields=17% for 1, 9% for 3a and 8% for 3b). FPhEP (1) was labelled with fluorine-18 using the following two-step radiochemical process: (1) no-carrier-added nucleophilic heteroaromatic ortho-radiofluorination from the corresponding N-Boc-protected chloro- or bromo derivatives (3 a,b-1mg) and the activated K[(18)F]F-Kryptofix(222) complex in DMSO using microwave activation at 250 W for 1.5 min, followed by (2) quantitative TFA-induced removal of the N-Boc-protective group. Radiochemically pure (>99%) [(18)F]FPhEP ([(18)F]-1, 2.22-3.33 GBq, 66-137 GBq/micromol) was obtained after semi-preparative HPLC (Symmetry C18, eluent aq 0.05 M NaH(2)PO(4)/CH(3)CN, 80:20 (v:v)) in 75-80 min starting from a 18.5 GBq aliquot of a cyclotron-produced [(18)F]fluoride production batch (10-20% nondecay-corrected overall yield). In vitro binding studies on rat whole-brain membranes demonstrated a subnanomolar affinity (K(D) 660 pM) of [(18)F]FPhEP ([(18)F]-1) for nAChRs. In vitro autoradiographic studies also showed a good contrast between nAChR-rich and -poor regions with a low non-specific binding. Comparison of in vivo Positron Emission Tomography (PET) kinetics of [(18)F]FPhEP ([(18)F]-1) and [(18)F]F-A-85380 in baboons demonstrated faster brain kinetics of the former compound (with a peak uptake at 20 min post injection only). Taken together, the preliminary data obtained confirm that [(18)F]FPhEP ([(18)F]-1) has potential for in vivo imaging nAChRs in the brain with PET.

Synthesis and binding studies of epibatidine analogues as ligands for the nicotinic acetylcholine receptors

Neuronal nicotinic acetylcholine receptors (nAChRs) are transmembrane ligand-gated ion channels. Recent research demonstrated that selective nAChR ligands may have therapeutic potential in a number of CNS diseases and disorders. The alkaloid epibatidine is a highly potent non-opioid analgesic and nAChR agonist, but too toxic to be a useful ligand. To develop ligands selective for distinct nAChR subtypes and with reduced toxicity, a series of epibatidine and homoepibatidine analogues were synthesized. (+/-)-8-Methyl-3-(pyridin-3-yl)-8-azabicyclo[3,2,1]oct-2-ene, showed high affinity towards alpha4beta2 (Ki=2 nM), subtype selectivity (alpha4beta2/alpha7 affinity ratio>100) and relatively low toxicity in mice and can be labeled with 11C and 18F as positron emission tomography (PET) tracers for imaging of nAChRs.

New-generation radiotracers for nAChR and NET

Advances in radiotracer chemistry and instrumentation have merged to make positron emission tomography (PET) a powerful tool in the biomedical sciences. Positron emission tomography has found increased application in the study of drugs affecting the brain and whole body, including the measurement of drug pharmacokinetics (using a positron-emitter-labeled drug) and drug pharmacodynamics (using a labeled tracer). Thus, radiotracers are major scientific tools enabling investigations of molecular phenomena, which are at the heart of understanding human disease and developing effective treatments; however, there is evidently a bottleneck in translating basic research to clinical practice. In the meantime, the poor ability to predict the in vivo behavior of chemical compounds based on their log P's and affinities emphasizes the need for more knowledge in this area. In this article, we focus on the development and translation of radiotracers for PET studies of the nicotinic acetylcholine receptor (nAChR) and the norepinephrine transporter (NET), two molecular systems that urgently need such an important tool to better understand their functional significance in the living human brain.

Mapping brain activity following administration of a nicotinic acetylcholine receptor agonist, ABT-594, using functional magnetic resonance imaging in awake rats

Administration of ABT-594, a potent agonist for nicotinic acetylcholine receptors with selectivity for the alpha4beta2 receptor subtype, is known to modulate a diverse array of behaviors including those associated with nociception, anxiety and motor function. In this study, we sought to gain insight into the neural actions of ABT-594, in vivo, by conducting functional magnetic resonance imaging in awake and anesthetized rats. Using T(2)*-weighted gradient echo imaging and an ultrasmall superparamagnetic iron oxide contrast agent, functional imaging was conducted on a 4.7 T magnet to measure changes in relative cerebral blood volume. In awake, restrained, male Sprague-Dawley rats that were acclimated to the imaging environment, injection of ABT-594 (0.03-0.3 micromol/kg, i.v.) evoked changes to relative cerebral blood volume in several neural regions including the cingulate, somatosensory, motor, auditory, and pre-frontal cortices as well as the thalamus and the periaqueductal gray/dorsal raphe. These effects were typically bimodal with significant decreases in relative cerebral blood volume at the 0.03 micromol/kg dose and increases at the higher doses (0.1 and 0.3 micromol/kg). The decreases and increases in relative cerebral blood volume were often observed within the same region, but triggered by different doses. Both increases and decreases in relative cerebral blood volume were blocked by pretreatment with the noncompetitive nicotinic acetylcholine receptor antagonist, mecamylamine (5 micromol/kg, i.p.) in awake rats. Administration of ABT-594 (0.1 micromol/kg, i.v.) to alpha-chloralose-anesthetized rats did not significantly alter relative cerebral blood volume in any brain region suggesting an anesthetic-related interference with the effects of ABT-594. The neural regions affected by administration of ABT-594 corresponded well to the known pre-clinical behavioral profile for this compound, and demonstrate the utility of using functional magnetic resonance imaging in awake animals to study pharmacological action.

Novel pyridyl ring C5 substituted analogues of epibatidine and 3-(1-methyl-2(S)-pyrrolidinylmethoxy)pyridine (A-84543) as highly selective agents for neuronal nicotinic acetylcholine receptors containing beta2 subunits

Introduction of a hydrophobic or hydrogen-bonding alkynyl group into the C5 position of the pyridyl ring of epibatidine and A-84543 significantly increased the selectivity for neuronal nicotinic acetylcholine receptors (nAChRs) containing beta2 subunits over nAChRs containing beta4 subunits (K(i) ratio up to 92000-fold). Our data indicate that the extracellular domains of the nAChRs are sufficiently different to allow for the design of novel ligands with high affinity and selectivity for the nAChR subtypes.

Design and synthesis of a new [18F]fluoropyridine-based haloacetamide reagent for the labeling of oligonucleotides: 2-bromo-N-[3-(2-[18F]fluoropyridin-3-yloxy)propyl]acetamide

Based on the recently highlighted potential of nucleophilic heteroaromatic ortho-radiofluorinations in the preparation of fluorine-18-labeled radiotracers and radiopharmaceuticals for PET, a [(18)F]fluoropyridine-based bromoacetamide reagent has been prepared and used in prosthetic group introduction for the labeling of oligonucleotides. [(18)F]FPyBrA (2-bromo-N-[3-(2-[(18)F]fluoropyridin-3-yloxy)propyl]acetamide) was designed as a radiochemically feasible reagent, its pyridinyl moiety both carrying the radioactive halogen (fluorine-18) and allowing its efficient incorporation via a nucleophilic heteroaromatic substitution, and its 2-bromoacetamide function, ensuring the efficient alkylation of a phosphorothioate monoester group born at the 3'- or 5'-end of single-stranded oligonucleotides. [(18)F]FPyBrA (HPLC-purified) was efficiently prepared in 18-20% non-decay-corrected yield (based on starting [(18)F]fluoride) using a three-step radiochemical pathway in 80-85 min. The developed procedure involves (1) a high-yield nucleophilic heteroaromatic ortho-radiofluorination as the fluorine-18 incorporation-step (70-85% radiochemical yield) and uses [3-(3-tert-butoxycarbonylaminopropoxy)pyridin-2-yl]trimethylammonium trifluoromethanesulfonate as precursor for labeling, followed by (2) rapid and quantitative TFA-removal of the N-Boc-protective group and (3) condensation with 2-bromoacetyl bromide (45-65% radiochemical yield). Typically, 3.3-3.7 GBq (90-100 mCi) of HPLC-purified [(18)F]FPyBrA could be obtained in 80-85 min, starting from 18.5 GBq (500 mCi) of a cyclotron production batch of [(18)F]fluoride. [(18)F]FPyBrA was regioselectively conjugated with 9-mer and 18-mer single-stranded oligonucleotides, provided with a phosphorothioate monoester group at their 3'-end. Both natural phosphodiester DNAs and in vivo-stable 2'-methoxy and -fluoro-modified RNAs were used. Conjugation uses optimized, short-time reaction conditions (MeOH/0.1 M PBS pH 7.4, 15 min, 120 degrees C), both compatible with the chemical stability of the oligonucleotides (ONs) and the half-life of fluorine-18. Conjugated [(18)F]ONs were finally purified by RP-HPLC and desalted using a Sephadex NAP-10 column. The whole radiosynthetic procedure, including the preparation of the fluorine-18-labeled reagent, the conjugation with the oligonucleotide, and the HPLC purification and formulation lasted 140-160 min. [(18)F]FPyBrA represents a valuable alternative to the already reported N-(4-[(18)F]fluorobenzyl)-2-bromoacetamide for the design and development of oligonucleotide-based radiopharmaceuticals for PET.

Update on PET radiopharmaceuticals: life beyond fluorodeoxyglucose

Twenty-eight years after its inception, 2-[18F]FDG- is still the most widely used radiopharmaceutical for PET studies, but numerous more specific radiotracers have been developed and applied in neuroscience and oncology. The advances in radiotracer chemistry, especially the nucleophilic substitution reaction, have played the pivotal role in synthesizing various no-carrier-added 18F-labeled radiotracers for PET studies of various receptor systems. This article lists some of the radiotracers that are available for PET studies in neuroscience and oncology. The prospects for developing other new radiotracers for imaging other organ diseases also seem to be promising.

Positron emission tomography radiochemistry

Factors that place constraints on radio-chemists who are seeking to design and develop radiopharmaceuticals for PET imaging studies include the short half-lives of 11C and 18F, minimum radiochemical yield and specific activity requirements, and high radiation fields that are associated with multi-Curie quantities of PET radionuclides. Nevertheless, during the past 20 years, considerable progress has been made in the development and application of a variety of PET radiotracers for a range of imaging studies in human subjects. We have highlighted a few areas of radiochemistry that focused on PET radiotracers that are described in this issue. Although the number of PET radiotracers synthesized is in the hundreds [6], much work remains to develop specific and useful PET radiotracers for a host of new and exciting noninvasive imaging applications.

Radiotracers for positron emission tomography imaging

Over the past 30 years, advances in radiotracer chemistry and positron emission tomography instrumentation have merged to make positron emission tomography a powerful scientific tool in the biomedical sciences. However, despite the increasing reliance of the biomedical sciences on imaging and the new needs for functional information created by the sequencing of the human genome, the development of new radiotracers with the specificity and kinetic characteristics for quantitative analysis in vivo remains a slow process. In this article, we focus on advances in the development of the radiotracers involved in neurotransmission, amino acid transport, protein synthesis, and DNA synthesis. We conclude with a brief section on newer radiotracers that image other molecular targets and conclude with a summary of some of the scientific and infrastructure needs that would expedite the development and introduction of new radiotracers into biomedical research and the practice of medicine.

[125/123I] 5-Iodo-3-pyridyl ethers. syntheses and binding to neuronal nicotinic acetylcholine receptors

Three 3-pyridyl ether nicotinic ligands-(S)-5-Iodo-3-[(2-pyrrolidinyl)-methoxy]pyridine (5-iodo-A-85865), (S)-5-Iodo-3-[1-(methyl)-2-pyrrolidinyl-methoxy]pyridine (5-Iodo-A-84543), and (S)-5-iodo-3-[1-methyl-(2-azetidinyl)-methoxy]pyridine (5-iodo-N-Me-A-85380) were labeled with I-125/I-123, and their ability to label high-affinity brain nicotinic acetylcholine receptors (nAChRs) was evaluated. The most promising ligand, [123/125I] 5-iodo-A-85865, showed approximately 65% inhibition of radioactivity uptake in thalamus in mice pretreated with cytisine. Preliminary SPECT imaging studies with [123I] 5-iodo-A-85865 revealed a distribution profile consistent with nAChRs (thalamus > frontal cortex > cerebellum) and a more rapid pharmacokinetic profile relative to azetidinyl 3-pyridyl ether based ligands.

Radiosynthesis of 5-(2-(4-pyridinyl)vinyl)-6-chloro-3-(1-[(11)C]methyl-2-(S)-pyrrolidinylmethoxy)pyridine, a high affinity ligand for studying nicotinic acetylcholine receptors by positron emission tomography

5-(2-(4-pyridinyl)vinyl)-6-chloro-3-(1-methyl-2-(S)-pyrrolidinylmethoxy)pyridine (1b) exhibited high affinity for nicotinic acetylcholine receptors in the in vitro competition binding assays, with a K(d) value in the low picomolar range, performed at room temperature and at physiological temperature. An efficient radiochemical synthesis of 5-(2-(4-pyridinyl)vinyl)-6-chloro-3-(1-[(11)C]methyl-2-(S)-pyrrolidinylmethoxy)pyridine (1c), a potential tracer for the study of nAChR by positron emission tomography, has been developed.

PET in psychopharmacology

Emission tomography techniques and, in particular, positron emission tomography (PET) enable the in vivo study of several physiological and neurochemical variables in human subjects using methods originally developed for quantitative autoradiography. In particular, PET allows one to evaluate in human subjects: (a) the effect of specific neurochemical challenges on regional brain function at rest or under activation; (b) the activity of neurotransmitters and the regional expression of specific molecular targets during pathology including their modulation by drug treatment; (c) the kinetics of drug disposition and activity directly in the target organ. This is of primary interest in the field of biological psychiatry and in psychoactive drugs development, where it is particularly difficult to reproduce human diseases using animal models in view of the peculiarity of this field and the large heterogeneity of each psychiatric illness also inside the same clinical definition. The aim of this paper is to review the principal strategies and the main results of the use of PET in psychopharmacology.

Beta-amyloid activates the mitogen-activated protein kinase cascade via hippocampal alpha7 nicotinic acetylcholine receptors: In vitro and in vivo mechanisms related to Alzheimer's disease

Alzheimer's Disease (AD) is the most common of the senile dementias, the prevalence of which is increasing rapidly, with a projected 14 million affected worldwide by 2025. The signal transduction mechanisms that underlie the learning and memory derangements in AD are poorly understood. beta-Amyloid (Abeta) peptides are elevated in brain tissue of AD patients and are the principal component of amyloid plaques, a major criterion for postmortem diagnosis of the disease. Using acute and organotypic hippocampal slice preparations, we demonstrate that Abeta peptide 1-42 (Abeta42) couples to the mitogen-activated protein kinase (MAPK) cascade via alpha7 nicotinic acetylcholine receptors (nAChRs). In vivo elevation of Abeta, such as that exhibited in an animal model for AD, leads to the upregulation of alpha7 nAChR protein. alpha7 nAChR upregulation occurs concomitantly with the downregulation of the 42 kDa isoform of extracellular signal-regulated kinase (ERK2) MAPK in hippocampi of aged animals. The phosphorylation state of a transcriptional mediator of long-term potentiation and a downstream target of the ERK MAPK cascade, the cAMP-regulatory element binding (CREB) protein, were affected also. These findings support the model that derangement of hippocampus signal transduction cascades in AD arises as a consequence of increased Abeta burden and chronic activation of the ERK MAPK cascade in an alpha7 nAChR-dependent manner that eventually leads to the downregulation of ERK2 MAPK and decreased phosphorylation of CREB protein.

Ligands for in vivo imaging of nicotinic receptor subtypes in Alzheimer brain

The neuronal nicotinic acetylcholine receptors (nAChR) are involved in functional processes in brain including cognitive function and memory. A severe loss of the nAChRs has been detected in brain of patients with Alzheimer's disease (AD). There is a great interest to image nAChRs noninvasive for detection of receptor impairments even at a presymptomatic stage of AD as well for monitoring outcome of drug treatment. (S) [11C]Nicotine, has so far been the only nAChR ligand used in positron emission tomography (PET) studies for visualizing nAChRs in human brain. In order to develop PET/SPECT nAChRs ligands for detection of subtypes of nAChRs nicotine analogues, epibatidine and A-85380 compounds have been characterized in vitro and investigated in vivo. Epibatidine and A-85380 have been found to have higher specific signals and more favorable kinetic parameters than nicotine and its analogues. The epibatidine and A-85380 compounds can also be radiolabeled with high specific radioactivity, show affinities for the nAChRs in the pM range and readily cross the blood-brain barrier. In addition they reversibly bind to the nAChRs and show low non-specific binding and moderately fast metabolism. Due to a probably high alpha4beta2 nAChR selectivity combined with low toxicity, the A-85380 analogs presently seem to be the most promising nAChR ligand imaging of subtypes of nAChRs in human brain.

Development of ligands for in vivo imaging of cerebral nicotinic receptors

Nicotinic acetylcholine receptors (nAChRs) mediate a variety of brain functions. Findings from postmortem studies and clinical investigations have implicated them in the pathophysiology and treatment of Alzheimer's and Parkinson's diseases and other CNS disorders (e.g. Tourette's syndrome, epilepsy, nicotine dependence). Therefore, it ultimately might be useful to image nAChRs noninvasively for diagnosis, for studies on how changes in nAChRs might contribute to cerebral disorders, for development of therapies targeted at nAChRs, and to monitor the effects of such treatments. To date, only (S)-(-)-nicotine, radiolabeled with 11C, has been used for external imaging of nAChRs in human subjects. Since this radiotracer presents drawbacks, new ligands, with more favorable properties, have been synthesized and tested. Three general classes of compounds, namely, nicotine and its analogs, epibatidine and related compounds, and 3-pyridyl ether compounds, including A-85380, have been evaluated. Analogs of A-85380 appear to be the most promising candidates because of their low toxicity and high selectivity for the alpha4beta2 subtype of nAChRs.