6-[18F]fluoro-A-85380, a novel radioligand for in vivo imaging of central nicotinic acetylcholine receptors

Developing Low Molecular Weight PET and SPECT Imaging Agents

Imaging agents for positron emission tomography (PET) and single-photon emission computerized tomography (SPECT) have shown their utility in many situations, answering clinical questions related to drug development and medical considerations. The discovery and development of imaging agents follow a well-understood process, with variations related to available starting points and to the envisaged imaging application. This article describes the general development path leading from the expression of an imaging need and project initiation to a clinically usable imaging agent. The definition of the project rationale, the design and optimization of early leads, and the assessment of the imaging potential of an imaging agent candidate are followed by preclinical and clinical development activities that differ from those required for therapeutic agents. These include radiolabeling with a positron emitter and first-in-human clinical studies, to rapidly evaluate the ability of a new imaging agent to address the questions it was designed to answer.

Evaluation of an Image-Derived Input Function for Kinetic Modeling of Nicotinic Acetylcholine Receptor-Binding PET Ligands in Mice

Positron emission tomography (PET) radioligands that bind with high-affinity to α4β2-type nicotinic receptors (α4β2Rs) allow for in vivo investigations of the mechanisms underlying nicotine addiction and smoking cessation. Here, we investigate the use of an image-derived arterial input function and the cerebellum for kinetic analysis of radioligand binding in mice. Two radioligands were explored: 2-[18F]FA85380 (2-FA), displaying similar pKa and binding affinity to the smoking cessation drug varenicline (Chantix), and [18F]Nifene, displaying similar pKa and binding affinity to nicotine. Time-activity curves of the left ventricle of the heart displayed similar distribution across wild type mice, mice lacking the β2-subunit for ligand binding, and acute nicotine-treated mice, whereas reference tissue binding displayed high variation between groups. Binding potential estimated from a two-tissue compartment model fit of the data with the image-derived input function were higher than estimates from reference tissue-based estimations. Rate constants of radioligand dissociation were very slow for 2-FA and very fast for Nifene. We conclude that using an image-derived input function for kinetic modeling of nicotinic PET ligands provides suitable results compared to reference tissue-based methods and that the chemical properties of 2-FA and Nifene are suitable to study receptor response to nicotine addiction and smoking cessation therapies.

A Molecular Toolbox of Positron Emission Tomography Tracers for General Anesthesia Mechanism Research

With appropriate radiotracers, positron emission tomography (PET) allows direct or indirect monitoring of the spatial and temporal distribution of anesthetics, neurotransmitters, and biomarkers, making it an indispensable tool for studying the general anesthesia mechanism. In this Perspective, PET tracers that have been recruited in general anesthesia research are introduced in the following order: 1) ¹¹C/¹⁸F-labeled anesthetics, i.e., PET tracers made from inhaled and intravenous anesthetics; 2) PET tracers targeting anesthesia-related receptors, e.g., neurotransmitters and voltage-gated ion channels; and 3) PET tracers for studying anesthesia-related neurophysiological effects and neurotoxicity. The radiosynthesis, pharmacodynamics, and pharmacokinetics of the above PET tracers are mainly discussed to provide a practical molecular toolbox for radiochemists, anesthesiologists, and those who are interested in general anesthesia.

Trapping of Nicotinic Acetylcholine Receptor Ligands Assayed by In Vitro Cellular Studies and In Vivo PET Imaging

A question relevant to nicotine addiction is how nicotine and other nicotinic receptor membrane-permeant ligands, such as the anti-smoking drug varenicline (Chantix), distribute in brain. Ligands, like varenicline, with high pKa and high affinity for α4β2-type nicotinic receptors (α4β2Rs) are trapped in intracellular acidic vesicles containing α4β2Rs in vitro Nicotine, with lower pKa and α4β2R affinity, is not trapped. Here, we extend our results by imaging nicotinic PET ligands in vivo in male and female mouse brain and identifying the trapping brain organelle in vitro as Golgi satellites (GSats). Two PET 18F-labeled imaging ligands were chosen: [18F]2-FA85380 (2-FA) with varenicline-like pKa and affinity and [18F]Nifene with nicotine-like pKa and affinity. [18F]2-FA PET-imaging kinetics were very slow consistent with 2-FA trapping in α4β2R-containing GSats. In contrast, [18F]Nifene kinetics were rapid, consistent with its binding to α4β2Rs but no trapping. Specific [18F]2-FA and [18F]Nifene signals were eliminated in β2 subunit knock-out (KO) mice or by acute nicotine (AN) injections demonstrating binding to sites on β2-containing receptors. Chloroquine (CQ), which dissipates GSat pH gradients, reduced [18F]2-FA distributions while having little effect on [18F]Nifene distributions in vivo consistent with only [18F]2-FA trapping in GSats. These results are further supported by in vitro findings where dissipation of GSat pH gradients blocks 2-FA trapping in GSats without affecting Nifene. By combining in vitro and in vivo imaging, we mapped both the brain-wide and subcellular distributions of weak-base nicotinic receptor ligands. We conclude that ligands, such as varenicline, are trapped in neurons in α4β2R-containing GSats, which results in very slow release long after nicotine is gone after smoking.SIGNIFICANCE STATEMENT Mechanisms of nicotine addiction remain poorly understood. An earlier study using in vitro methods found that the anti-smoking nicotinic ligand, varenicline (Chantix) was trapped in α4β2R-containing acidic vesicles. Using a fluorescent-labeled high-affinity nicotinic ligand, this study provided evidence that these intracellular acidic vesicles were α4β2R-containing Golgi satellites (GSats). In vivo PET imaging with F-18-labeled nicotinic ligands provided additional evidence that differences in PET ligand trapping in acidic vesicles were the cause of differences in PET ligand kinetics and subcellular distributions. These findings combining in vitro and in vivo imaging revealed new mechanistic insights into the kinetics of weak base PET imaging ligands and the subcellular mechanisms underlying nicotine addiction.

PET Imaging of the Human Nicotinic Cholinergic Pathway in Atherosclerosis

During the past years, non-neuronal vascular nicotinic acetylcholine receptors (nAChRs) increasingly have gained interest in cardiovascular research, as they are known to mediate the deleterious effects of nicotine and nitrosamines, components of tobacco smoke, on the vasculature. Because smoking is a major risk factor for the development of atherosclerosis, it is obvious that understanding the pathophysiologic role of nAChRs in the atherosclerotic disease process, as well as in the development of new diagnostic and therapeutic nAChR-related options, has become more important. Accordingly, we briefly summarize the pathophysiologic role of vascular nAChRs in the atherosclerotic disease process. We also provide an overview of currently available nAChR positron emission tomography (PET) tracers and their performance in the noninvasive imaging of vascular nAChRs, as well as potential nAChR PET tracers that might be an option for vascular nAChR PET imaging in the future.

Selective syntheses of no-carrier-added 2- and 3-[18F]fluorohalopyridines through the radiofluorination of halopyridinyl(4'-methoxyphenyl)iodonium tosylates

No-carrier-added 2- and 3-[(18)F]fluorohalopyridines were readily synthesized as potentially useful labeling synthons for prospective PET radiotracers through the selective radiofluorination of halopyridinyl(4'-methoxyphenyl)iodonium tosylates, themselves conveniently prepared in a single pot from iodohalopyridines.

Quantification of smoking-induced occupancy of beta2-nicotinic acetylcholine receptors: estimation of nondisplaceable binding

5-(123)I-iodo-85380 ((123)I-5-IA) is used to quantitate high-affinity nicotinic acetylcholine receptors (beta(2)-nAChRs) on human SPECT scans. The primary outcome measure is V(T)/f(P), the ratio at equilibrium between total tissue concentration (free, nonspecifically bound, and specifically bound) and the free plasma concentration. Nondisplaceable uptake (free plus nonspecific) of (123)I-5-IA has not been measured in human subjects. Nicotine has high affinity for beta(2)*-nAChRs (nAChRs containing the beta(2)* subunit, for which * represents other subunits that may also be part of the receptor) and displaces specifically bound (123)I-5-IA. In this study, we measured nicotine occupancy and nondisplaceable binding in healthy smokers after they had smoked to satiety.

METHODS

Eleven nicotine-dependent smokers (mean age +/- SD, 35.6 +/- 14.4 y) completed the study. One subject was excluded from subsequent analyses because of abnormal blood nicotine levels. Subjects abstained from tobacco smoke for 5.3 +/- 0.9 d and participated in a 15- to 17-h SPECT scanning day. (123)I-5-IA was administered by bolus plus constant infusion, with a total injected dose of 361 +/- 20 MBq. At approximately 6 h after the start of the infusion, three 30-min SPECT scans and a 15-min transmission-emission scan were acquired to obtain baseline beta(2)*-nAChR availability. Subjects then smoked to satiety (2.4 +/- 0.7 cigarettes), and arterial (first 40 min) and venous (until study completion) plasma nicotine and cotinine levels were collected. About 1 h after subjects had smoked to satiety, up to six 30-min SPECT scans were acquired. V(T)/f(P) data, computed from the tissue and plasma radioactivity measurements from the presmoking baseline and postsmoking scans, were analyzed using the Lassen plot method.

RESULTS

Receptor occupancy after subjects had smoked to satiety was 67% +/- 9% (range, 55%-80%). Nondisplaceable uptake was estimated as 19.4 +/- 5.8 mL x cm(-3) (range, 15-28 mL x cm(-3)). Thus, in the thalamus, where mean V(T)/f(P) is 93 mL x cm(-3), nondisplaceable binding represents approximately 20% of the total binding.

CONCLUSION

These results are in agreement with previous findings and suggest that when satiating doses of nicotine are administered to smokers, imaging of receptor availability can yield valuable data, such as quantifiable measures of nondisplaceable binding.

Synthesis and biological evaluation of novel carbon-11 labeled pyridyl ethers: candidate ligands for in vivo imaging of alpha4beta2 nicotinic acetylcholine receptors (alpha4beta2-nAChRs) in the brain with positron emission tomography

The most abundant subtype of cerebral nicotinic acetylcholine receptors (nAChR), alpha4beta2, plays a critical role in various brain functions and pathological states. Imaging agents suitable for visualization and quantification of alpha4beta2 nAChRs by positron emission tomography (PET) would present unique opportunities to define the function and pharmacology of the nAChRs in the living human brain. In this study, we report the synthesis, nAChR binding affinity, and pharmacological properties of several novel 3-pyridyl ether compounds. Most of these derivatives displayed a high affinity to the nAChR and a high subtype selectivity for alpha4beta2-nAChR. Three of these novel nAChR ligands were radiolabeled with the positron-emitting isotope (11)C and evaluated in animal studies as potential PET radiotracers for imaging of cerebral nAChRs with improved brain kinetics.

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.

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.

Quantification of cerebral nicotinic acetylcholine receptors by PET using 2-[18F]fluoro-A-85380 and the multiinjection approach

The multiinjection approach was used to study in vivo interactions between alpha4beta2(*) nicotinic acetylcholine receptors and 2-[(18)F]fluoro-A-85380 in baboons. The ligand kinetics was modeled by the usual nonlinear compartment model composed of three compartments (arterial plasma, free and specifically bound ligand in tissue). Arterial blood samples were collected to generate a metabolite-corrected plasma input function. The experimental protocol, which consisted of three injections of labeled or unlabeled ligand, was aiming at identifying all parameters in one experiment. Various parameters, including B'(max) (the binding sites density) and K(d)V(R) (the apparent in vivo affinity of 2-[(18)F]fluoro-A-85380) could then be estimated in thalamus and in several receptor-poor regions. B'(max) estimate was 3.0+/-0.3 pmol/mL in thalamus, and ranged from 0.25 to 1.58 pmol/mL in extrathalamic regions. Although K(d)V(R) could be precisely estimated, the association and dissociation rate constants k(on)/V(R) and k(off) could not be identified separately. A second protocol was then used to estimate k(off) more precisely in the thalamus. Having estimated all model parameters, we performed simulations of 2-[(18)F]fluoro-A-85380 kinetics to test equilibrium hypotheses underlying simplified approaches. These showed that a pseudo-equilibrium is quickly reached between the free and bound compartments, a favorable situation to apply Logan graphical analysis. In contrast, the pseudo-equilibrium between the plasma and free compartments is only reached after several hours. The ratio of radioligand concentration in these two compartments then overestimates the true equilibrium value, an unfavorable situation to estimate distribution volumes from late images after a bolus injection.

[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.

A-85380: a pharmacological probe for the preclinical and clinical investigation of the alphabeta neuronal nicotinic acetylcholine receptor

A-85380 [3-(2(s)-azetidinylmethoxy) pyridine] is a neuronal nicotinic acetylcholine receptor (nAChR) agonist that has been a useful tool in the investigation of the function of nAChRs in both preclinical and clinical studies. Amongst nAChR subtypes, A-85380 shows selectivity for the alpha(4)beta(2) vs. the alpha(7) or alpha(1)beta(1)deltagamma nAChRs. In functional in vitro cation flux assays, A-85380 is a potent and full agonist. A-85380 has a broad-spectrum analgesic profile with efficacy in acute, persistent, and neuropathic pain models. As demonstrated using selective nAChR antagonists or alpha(4) antisense, the alpha(4)beta(2) nAChR mediates the analgesic effects of A-85380. Interestingly, the site of action depends upon the type of pain as antinociception is mediated by descending inhibition into the spinal cord whereas anti-allodynia in neuropathic pain is mediated at both central and peripheral sites. Radiolabelled forms of A-85380 have been developed and shown to be safe for use in vivo in humans. In clinical studies using positron and photon emission tomography, marked decreases in alpha(4)beta(2) nAChRs have been seen in patients with Parkinson's and Alzheimer's disease. Although not developed as a therapeutic agent, A-85380 has proven to be an important component in the development of novel nAChR ligands for the treatment of pain and other disorders.

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.

Chemical fate of the nicotinic acetylcholinergic radiotracer [123I]5-IA-85380 in baboon brain and plasma

The fate of the nicotinic acetylcholinergic receptor radiotracer [123I]5-IA-85380 ([123I]5-IA) was studied in baboon by analyzing the chemical composition of brain tissue and plasma after intravenous administration of the tracer. Acetonitrile denaturation and high-performance liquid chromatography (HPLC) analysis showed predominantly unchanged (91-98%) parent tracer in all brain tissues examined, compared to significant metabolism (23% parent) in the plasma at 90 min postinjection, and control tissue recovery of 95-98%. [123I]5-IA was distributed to the thalamus with a standardized uptake value of 9.2 (0.04% dose/g) or a concentration 5.8 times higher than that of the cerebellum. The HPLC behavior of a synthesized sample of one hypothesized metabolite, 5-iodo-3-pyridinol (5-IP), was consistent with plasma radiometabolite fraction. Since only parent radiotracer compound was found in brain tissue, these results add confidence that information derived from single photon emission computed tomography images of 123I activity in the brain after [123I]5-IA administration can be interpreted as distribution of an intact radiotracer.

Autoradiography of 2-[18F]F-A-85380 on nicotinic acetylcholine receptors in the porcine brain in vitro

Noninvasive molecular imaging of subtypes of neuronal nicotinic acetylcholine receptors (nAChRs) will provide information on the role of these receptors in neurodegenerative diseases. The binding of the positron emission tomography ligand 2-[18F]F-A-85380 to nAChRs was investigated in the porcine brain by quantitative autoradiography in vitro. The high-affinity binding of 2-[18F]F-A-85380 to each of the investigated 12 brain areas was saturable and apparently monophasic (e.g., apparent KD value of 1.72 nM in the thalamus). The highest density of specific binding sites was observed in the thalamus (1,158 fmol/mg protein), and the lowest density was measured in the cerebellar gray matter (11 fmol/mg protein). An attempt to assess nAChR subtype specificity of 2-[18F]F-A-85380 was made by competitive autoradiography. Binding of 2-[18F]F-A-85380 coincubated with 2-F-A-85380, epibatidine, cytisine, or methyllycaconitine, respectively, indicated a specificity of 2-[18F]F-A-85380 to beta2-containing nAChRs in the porcine brain. The autoradiographic data confirmed the suitability of swine as a model for the evaluation of radioligands designed for imaging of nAChR subtypes in the living 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.

In vitro characterization of 6-[18F]fluoro-A-85380, a high-affinity ligand for alpha4beta2* nicotinic acetylcholine receptors

Nicotinic acetylcholine receptors are involved in tobacco dependence and several other neuropathologies (e.g., Alzheimer's disease, Parkinson's disease), as well as in attention, learning, and memory. Performing in vivo imaging of these receptors in humans holds great promise for understanding their role in these conditions. Recently, three radiohalogenated analogs of 3-(2(S)-azetidinylmethoxy)pyridine (A- 85380) were used successfully for the in vivo visualization of alpha4beta2* nicotinic receptors in the human brain with PET/SPECT. Herein, we present the results of the in vitro characterization of one of these radioligands, 6-[18F]fluoro-3-(2(S)-azetidinylmethoxy)-pyridine (6-[18F]fluoro-A-85380), which is a fluoro-analog of the potent nonopioid analgesic ABT-594. In human postmortem cortical tissue, 6-[18F]fluoro-A-85380 reversibly binds with high affinity to a single population of sites (Kd = 59 pM at 37 degrees C, Bmax = 0.7 pmol/g tissue). The binding is fully reversible and is characterized at 37 degrees C by T(1/2assoc) = 2.2 min (at a ligand concentration of 39 pM) and by T(1/2dissoc) = 3.6 min. 6-Fluoro-A-85380 exhibits clear selectivity for alpha4beta2* over the other major mammalian nicotinic receptor subtypes: alpha7, alpha3beta4, and muscle-type. These results suggest that 6-[18F]fluoro-A-85380 is a promising radioligand for in vivo imaging of brain alpha4beta2* nicotinic receptors.

Radiotracer development in psychiatry

Over the last 20 years a number of radiotracers that target various neurotransmitter systems have been developed for use in imaging studies in psychiatry, but there are many more targets still to be investigated. The development of a radiotracer for clinical positron emission tomography (PET) or single photon emission computed tomography (SPECT) neuroimaging studies can be a complex and lengthy process with few imaging agents successfully progressing into clinical human studies. One of the most challenging aspects in the procedure is the development of a rapid and simple radiosynthesis protocol to obtain the potential radiotracer with adequate specific activity, isolated radiochemical yield and radiochemical purity for human imaging. Once a candidate has been radiolabelled, full characterization of the radiotracer is required before it can be used in clinical human studies. Pre-clinical studies include investigation into the binding distribution, pharmacokinetics, metabolism, toxicology and dosimetry of a radiotracer. There are many points during the development procedure where a potential radiotracer can be rejected. Due to interspecies differences the development of a radiotracer can either go too far with an unsuccessful candidate or can potentially lead to rejection of a candidate too soon. It is only when the radiotracer has been used in humans can we be certain that a radiotracer is a useful imaging agent for clinical research studies. The development of new technologies, such as micro-PET or SPECT can only improve our ability to predict the success of a radiotracer.

Synthesis of a [2-Pyridinyl-18F]-labelled fluoro derivative of (−)-Cytisine as a candidate radioligand for brain nicotinic α4β2 receptor imaging with PET

In recent years, there has been considerable effort to design and synthesize radiotracers suitable for use in Positron Emission Tomography (PET) imaging of the alpha4beta2 neuronal nicotinic acetylcholine receptor (nAChR) subtype. A new fluoropyridinyl derivative of (-)-cytisine (1), namely (-)-9-(2-fluoropyridinyl)cytisine (3, K(i) values of 24 and 3462 nM for the alpha4beta2 and alpha7 nAChRs subtypes, respectively) has been synthesized in four chemical steps from (-)-cytisine and labelled with fluorine-18 (T(1/2): 119.8 min) using an efficient two-step radiochemical process [(a). nucleophilic heteroaromatic ortho-radiofluorination using the corresponding N-Boc-protected nitro-derivative, (b). TFA removal of the Boc protective group]. Typically, 20-45 mCi (0.74-1.67 GBq) of (-)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3, 2-3 Ci/micromol or 74-111 GBq/micromol) were easily obtained in 70-75 min starting from a 100 mCi (3.7 GBq) aliquot of a cyclotron-produced [18F]fluoride production batch (20-45% non decay-corrected yield based on the starting [18F]fluoride). The in vivo pharmacological profile of (-)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3) was evaluated in rats with biodistribution studies and brain radioactivity monitoring using intracerebral radiosensitive beta-microprobes. The observed in vivo distribution of the radiotracer in brain was rather uniform, and did not match with the known regional densities of nAChRs. It was also significantly different from that of the parent compound (-)-[3H]cytisine. Moreover, competition studies with (-)-nicotine (5 mg/kg, 5 min before the radiotracer injection) did not reduce brain uptake of the radiotracer. These experiments clearly indicate that (-)-9-(2-[18F]fluoropyridinyl)cytisine ([18F]-3) does not have the required properties for imaging nAChRs using PET.

Influence of acetylcholine levels on the binding of a SPECT nicotinic acetylcholine receptor ligand [123I]5-I-A-85380

Although in vitro theory indicates that ligand binding is sensitive to competition with neurotransmitters, only some imaging ligands have shown such competition in vivo. The purpose of this study was to determine whether increases in acetylcholine (ACh) levels induced by an acetylcholinesterase inhibitor, physostigmine, inhibit in vivo binding of [(123)I]5-iodo-3-(2(S)-2-azetidinyl-methoxy) pyridine (5-I-A-85380), a single photon emission computed tomography ligand for the high-affinity type nicotinic ACh receptor (nAChR). Baboons were used for seven studies with a bolus plus constant infusion equilibrium paradigm. After achieving equilibrium at 5 h, physostigmine (0.02 (n = 1), 0.067 (n = 3), and 0.2 (n = 3) mg/kg) was administered intravenously and data were acquired for up to 8 h. To confirm equilibrium conditions, [(123)I]5-I-A-85380 plasma levels were measured in four studies, including all studies with 0.2 mg/kg physostigmine. Prior to physostigmine administration, thalamic activities were stable, with changes of 1.1%/h or less, except in one study with a gradual increase of 4.2%/h. Thalamic activities were decreased by 15% in one study with 0.067 mg/kg and 14-17% in all studies with 0.2 mg/kg physostigmine administration (P = 0.009). In these studies with 0.2 mg/kg physostigmine administration, [(123)I]5-I-A-85380 plasma levels showed a transient or a sustained increase after physostigmine administration that would have increased thalamic activities. These results suggest that elevated ACh levels induced by physostigmine can effectively compete in vivo with [(123)I]5-I-A-85380 binding at nAChRs. However, decreased thalamic activities could have been caused by other mechanisms, including internalization of the receptor with an associated decreased affinity for radioligand.

Imaging methods for evaluating brain function in man

Imaging of brain function and neurotransmission is an important bridge between basic and clinical research. Regional cerebral energy metabolism and blood flow are normally coupled to regional cerebral function. Positron tomography (PET) studies of cerebral glucose metabolism and blood flow, single photon tomography (SPECT) and MRI studies of cerebral perfusion, have been used to image cerebral development and aging in man. The sensitivity, temporal resolution, spatial resolution and lack of radiation have led to the widespread utilization of blood oxygen level dependent (BOLD) and MRI perfusion techniques. PET and SPECT methods for studying cerebral neurotransmission include studies of dopaminergic, serotonergic, cholinergic, opiate and GABAergic neurotransmission in man. Studies of cerebral neurotransmission in man have helped to delineate the mechanisms of action of antipsychotic and antidepressant drugs, the diagnosis and progression of Parkinson's disease, and to evaluate neuroprotective drugs. The strengths, limitations, and application of these modalities are reviewed. The application of these methods to cerebral development and aging are briefly discussed.

Synthesis and evaluation of a novel series of 2-chloro-5-((1-methyl-2-(S)-pyrrolidinyl)methoxy)-3-(2-(4-pyridinyl)vinyl)pyridine analogues as potential positron emission tomography imaging agents for nicotinic acetylcholine receptors

Reportedly, 2-[(18)F]fluoro-A-85380, 1, a promising radiotracer for imaging the nicotinic acetylcholine receptor (nAChR) by positron emission tomography (PET) in humans, exhibits slow penetration through the blood-brain barrier (BBB) due to its low lipophilicity. A ligand for nAChRs with greater lipophilicity than that of 1 would be potentially more favorable for PET imaging of nAChR due to its faster penetration through the BBB. Herein, a novel series of compounds has been developed based on the high affinity ligand for nAChRs, 2-chloro-5-((1-methyl-2-(S)-pyrrolidinyl)methoxy)-3-(2-(4-pyridinyl)vinyl)pyridine, 3b. The in vitro binding affinities for the new series were found to be in the range of K(i) = 9-331 pM. A molecular modeling study showed differences in the comformational profiles and the electronic properties of these compounds, which provides further insight into the structure-activity relationships at nAChR. Lipophilicities of the compounds 3b-6b have been found to be substantially higher than that of 1. As a result, compounds 3b-6b might exhibit a faster penetration through the BBB than the less lipophilic 1. The N-methyl derivatives 3b and 6b demonstrated very high affinities at nAChRs (K(i) = 28 and 23 pM, respectively) and will be targets for development of (11)CH(3)-labeled derivatives as radiotracers for PET imaging of nAChRs.

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.

Behavioral and neural consequences of prenatal exposure to nicotine

OBJECTIVE

To review evidence for the neurodevelopmental effects of in utero exposure to nicotine. Concerns about long-term cognitive and behavioral effects of prenatal exposure to nicotine arise from reports of increased rates of disruptive behavioral disorders in children whose mothers smoked during pregnancy. The relatively high rate of tobacco smoking among pregnant women (25% of all pregnancies in the U.S.) underlines the seriousness of these concerns.

METHOD

This review examines the largest and most recent epidemiological and clinical studies that investigated the association of prenatal nicotine exposure with health, behavioral, and cognitive problems. Because of the numerous potential confounding variables in human research, findings from animal studies, in which environmental factors are strictly controlled, are also discussed. Finally, neural and molecular mechanisms that are likely to underlie neurodevelopmental disruptions produced by prenatal nicotine exposure are outlined.

RESULTS

A dose-response relationship between maternal smoking rates and low birth weight (potentially associated with lower cognitive ability) and spontaneous abortion is consistently found, whereas long-term developmental and behavioral effects in the offspring are still controversial, perhaps because of the difficulty of separating them from other genetic and environmental factors. Despite the wide variability of experimental paradigms used in animal studies, common physical and behavioral effects of prenatal exposure to nicotine have been observed, including low birth weight, enhanced locomotor activity, and cognitive impairment. Finally, disturbances in neuronal pathfinding, abnormalities in cell proliferation and differentiation, and disruptions in the development of the cholinergic and catecholaminergic systems all have been reported in molecular animal studies of in utero exposure to nicotine.

CONCLUSIONS

Prenatal exposure to nicotine may lead to dysregulation in neurodevelopment and can indicate higher risk for psychiatric problems, including substance abuse. Knowledge of prenatal exposure to nicotine should prompt child psychiatrists to closely monitor at-risk patients.

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.