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

Competition, Selectivity and Efficacy of Analogs of A-84543 for Nicotinic Acetylcholine Receptors with Repositioning of Pyridine Nitrogen

Nicotinic acetylcholine receptors (nAChRs) play a crucial role in a number of clinically relevant mental and neurological pathways, as well as autonomic and immune functions. The development of subtype-selective ligands for nAChRs therefore is potentially useful for targeted therapeutic management of conditions where nAChRs are involved. We tested if selectivity for a particular nAChR subtype can be achieved through small structural modifications of a lead compound containing the nicotinic pharmacophore by changing the distance between the electronegative elements. For this purpose, analogs of A-84543 were designed, synthesized and characterized as potentially new nAChR subtype-selective ligands. Compounds were tested for their binding properties in rat cerebral cortical tissue homogenates, and subtype-selectivity was determined using stably transfected HEK cells expressing different nAChR subtypes. All compounds synthesized were found to competitively displace [(3)H]-epibatidine ([(3)H]EB) from the nAChR binding site. Of all the analogues, H-11MNH showed highest affinity for nAChRs compared to a ~ fivefold to tenfold lower affinity of A-84543. All other compounds had affinities >10,000 nM. Both A-84543 and H-11MNH have highest affinity for α2β2 and α4β2 nAChRs and show moderate affinity for β4- and α7-containing receptors. H-11MNH was found to be a full agonist with high potency at α3β4, while A-84543 is a partial agonist with low potency. Based on their unique pharmacological binding properties we suggest that A-84543 and its desmethylpyrrolidine analog can be useful as pharmacological ligands for studying nAChRs if selective pharmacological and/or genetic tools are used to mask the function of other receptors subtypes.

PET imaging of high-affinity α4β2 nicotinic acetylcholine receptors in humans with 18F-AZAN, a radioligand with optimal brain kinetics

We evaluated (-)-2-(6-[(18)F]fluoro-2,3'-bipyridin-5'-yl)-7-methyl-7-aza-bicyclo[2.2.1]heptane ((18)F-AZAN), a novel radiotracer that binds to α4β2 nicotinic acetylcholine receptors (α4β2-nAChRs) and shows high specific binding and rapid and reversible kinetics in the baboon and human brain.

METHODS

We tested safety tolerability and test-retest reliability (n = 5) and proposed initial quantification of (18)F-AZAN receptors in 3 healthy human subjects who had nicotine exposure and 9 who did not. We also present a receptor blocking study in a nicotine subject dosed with the α4β2-nAChR-selective partial agonist varenicline.

RESULTS

Radiation dosimetry PET/CT experiments indicated that most human organs received doses between 0.008 and 0.015 mSv/MBq, with an effective dose of approximately 0.014 mSv/MBq. The tracer rapidly entered the brain, and the peak was reached before 20 min, even for thalamus. Ninety-minute scans were sufficient for (18)F-AZAN to obtain the ratio at equilibrium of specifically bound radioligand to nondisplaceable radioligand in tissue (BPND) using plasma reference graphical analysis, which showed excellent reproducibility of BPND (test-retest variability < 10%) in the nAChR-rich brain regions. Regional plasma reference graphical analysis BP(ND) values exceeded 2 in the midbrain tegmental nuclei, lateral geniculate body, and thalamus for nonsmokers (n = 9) but were less than 1 in the nAChR-poor brain regions. There was a dramatic reduction of (18)F-AZAN brain uptake in smokers and varenicline-treated subjects.

CONCLUSION

(18)F-AZAN is a highly specific, safe, and effective PET radioligand for human subjects that requires only 90 min of PET scanning to estimate high-affinity α4β2-nAChR in the living human brain.

Nicotinic α4β2 receptor imaging agents. Part IV. Synthesis and biological evaluation of 3-(2-(S)-3,4-dehydropyrrolinyl methoxy)-5-(3'-¹⁸F-fluoropropyl)pyridine (¹⁸F-Nifrolene) using PET

Imaging agents for nicotinic α4β2 receptors in the brain have been under way for studying various CNS disorders. Previous studies from our laboratories have reported the successful development of agonist, ¹⁸F-nifene. In attempts to develop potential antagonists, ¹⁸F-nifrolidine and ¹⁸F-nifzetidine were previously reported. Further optimization of these fluoropropyl derivatives has now been carried out resulting in 3-(2-(S)-3,4-dehydropyrrolinylmethoxy)-5-(3'-Fluoropropyl)pyridine (nifrolene) as a new high affinity agent for nicotinic α4β2 receptors. Nifrolene in rat brain homogenate assays--labeled with ³H-cytisine--exhibited a binding affinity of 0.36 nM. The fluorine-18 analog, ¹⁸F-nifrolene, was synthesized in approximately 10%-20% yield and specific activity was estimated to be >2000 Ci/mmol. Rat brain slices indicated selective binding to anterior thalamic nuclei, thalamus, subiculum, striata, cortex and other regions consistent with α4β2 receptor distribution. This selective binding was displaced >90% by 300 μM nicotine. Thalamus to cerebellum ratio (>10) was the highest for ¹⁸F-nifrolene with several other regions showing selective binding. In vivo rat PET studies exhibited rapid uptake of ¹⁸F-nifrolene in the brain with specific retention in the thalamus and other brain regions while clearing out from the cerebellum. Thalamus to cerebellum ratio value in the rat was >4. Administration of nicotine caused a rapid decline in the thalamic ¹⁸F-nifrolene suggesting reversible binding to nicotinic receptors. PET imaging studies of ¹⁸F-nifrolene in anesthetized rhesus monkey revealed highest binding in the thalamus followed by regions of the lateral cingulated and temporal cortex. Cerebellum showed the least binding. Thalamus to cerebellum ratio in the monkey brain was >3 at 120 min. These ratios of ¹⁸F-nifrolene are higher than measured for ¹⁸F-nifrolidine and ¹⁸F-nifzetidine. ¹⁸F-Nifrolene thus shows promise as a new PET imaging agent for α4β2 nAChR.

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.

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.

Synthesis and In-Vivo Evaluation of [11C]p-PVP-MEMA as a PET Radioligand for Imaging Nicotinic Receptors

Within the class of (4-pyridinyl)vinylpyridines developed by Abbott laboratories as potent neuronal nicotinic acetylcholine receptor ligands, p-PVP-MEMA ({(R)-2-[6-chloro-5-((E)-2-pyridin-4-ylvinyl)pyridin-3-yloxy]-1-methylethyl}methylamine) is the lead compound of a novel series that do not display the traditional nicotinic-like pyrrole-ring but still possessing high subnanomolar affinity (Ki 0.077 nm—displacement of [3H](–)cytisine from whole rat brain synaptic membranes). In the present study, p-PVP-MEMA and its nor-derivative ({(R)-2-[6-chloro-5-((E)-2-pyridin-4-ylvinyl)pyridin-3-yloxy]-1-methylethyl}methylamine) as precursor for labelling with the short-lived positron-emitter carbon-11 (T1/2 20.4 min) were synthesized in 10 chemical steps from 2-hydroxy-5-nitropyridine and Boc-d-alanine. N-Alkylation of nor-p-PVP-MEMA with [11C]methyl iodide afforded [11C]p-PVP-MEMA (>98% radiochemically pure, specific activity of 86.4 GBq μmol–1) in 2% (non-decay corrected and non-optimized) radiochemical yield, in 34 min (including HPLC purification and formulation). Preliminary positron emission tomography (PET) results obtained in a Papio hamadryas baboon showed that [11C]p-PVP-MEMA is not a suitable PET-radioligand.

[18F]FPhEP and [18F]F2PhEP, two new epibatidine-based radioligands: evaluation for imaging nicotinic acetylcholine receptors in baboon brain

The radioligand 2-[(18)F]fluoro-A-85380 has been developed for imaging alpha(4)beta(2) nAChRs with PET. However, it has slow kinetics and a large fraction of bound activity is nondisplaceable. In an attempt to address these problems, two epibatidine-based alpha(4)beta(2) nicotinic antagonists, coded FPhEP and F(2)PhEP, were evaluated in vivo in baboons. They were radiolabeled with fluorine-18 from the corresponding N-Boc-protected bromo-derivatives and the no-carrier-added K[(18)F]F-Kryptofix(222) complex. Radiochemically pure [(18)F]FPhEP or [(18)F]F(2)PhEP was obtained in 80 min in amounts of 1.11-2.22 GBq (111-185 GBq/micromol). After injection of 215 MBq of [(18)F]FPhEP or [(18)F]F(2)PhEP, dynamic PET data were acquired. Thalamic radioactivity peaked at 20 min (4.9% +/- 0.2% ID/100 mL tissue) for [(18)F]FPhEP. For [(18)F]F(2)PhEP, the peak was at 45 min (3.3% +/- 0.1% ID/100 mL tissue). Regional distribution of both radiotracers was in accordance with the known distribution of nAChRs. In presaturation experiments, nicotine, cytosine, or FPhEP reduced brain radioactivity of [(18)F]FPhEP. In a displacement experiment with nicotine only a small amount of [(18)F]F(2)PhEP was dislodged. In spite of a moderate to high in vitro affinity, both ligands do not fulfill the widely adopted criteria for a PET radioligand.

QSAR study of a large set of 3-pyridyl ethers as ligands of the α4β2 nicotinic acetylcholine receptor

Extensive 3D-QSAR studies were performed on 158 diverse analogues of 3-pyridyl ethers, which are excellent ligands of alpha4beta2 neuronal nicotinic acetylcholine receptor (NnAChR). Comparative molecular field analysis (CoMFA) and comparative molecular similarity indices analysis (CoMSIA) techniques were used to relate the binding affinities with the ligand structures. Two QSAR models were obtained using CoMFA and CoMSIA techniques. The two QSAR models were proved to be statistically significant and have high predictive power. The best CoMFA model yielded the cross-validated q(2)=0.605 and the non-cross-validated r(2)=0.862. The derived model indicated the importance of steric (85.9%) as well as electrostatic (14.1%) contributions. The CoMFA model demonstrated the steric field as the major descriptor of the ligand binding. The best CoMSIA model gave q(2)=0.723 and r(2)=0.685. This model showed that steric (30.3%) and H-bond interaction (61.8%) properties played major roles in ligand binding process. The squares of correlation coefficient for external test set of 28 molecules were 0.723 and 0.685 for the CoMFA model and the CoMSIA model, respectively. The two models were further graphically interpreted in terms of field contribution maps. SAR studies were also performed on different series of compounds in order to get a more reasonable understanding of the interactions between the ligands and the receptor. With the results, we have also presumed some assistant elements as supplements to the traditional pharmacophoric elements. A crude vision of ligand localization in the ligand-binding pocket of the receptor was also obtained, which would favor for the docking study of this kind of ligands.

6-Chloro-3-(((1-[11C]methyl)-2-(S)-pyrrolidinyl)methoxy)-5-(2-fluoropyridin-4-yl)pyridine ([11C]JHU85270), a potent ligand for nicotinic acetylcholine receptor imaging by positron emission tomography

6-Chloro-3-((1-methyl)-2-(S)-pyrrolidinyl)methoxy)-5-(2-fluoropyridin-4-yl)pyridine (JHU85270), a novel high-affinity ligand for the alpha4beta2 nicotine acetylcholine receptor (nAChR) (K(i)=86, 115 pM; K(i)(JHU85270)/K(i)(epibatidine)=1.7) with a log D(7.4)=1.6 was synthesized in 56% overall yield. [(11)C]JHU85270 was synthesized from [(11)C]-methyl iodide and the corresponding normethyl precursor. The average time of radiosynthesis, purification, and formulation was 37 min from the end of bombardment. The average radiochemical yield of [(11)C]JHU85270 was 37%+/-3% (non-decay corrected). The average specific radioactivity was 398+/-165 GBq/micromol (10750+/-4468 mCi/micromol) and the radiochemical purity was greater than 99%.

Nicotinic Agonists, Antagonists, and Modulators From Natural Sources

1. Acetylcholine receptors were initially defined as nicotinic or muscarinic, based on selective activation by two natural products, nicotine and muscarine. Several further nicotinic agonists have been discovered from natural sources, including cytisine, anatoxin, ferruginine, anabaseine, epibatidine, and epiquinamide. These have provided lead structures for the design of a wide range of synthetic agents. 2. Natural sources have also provided competitive nicotinic antagonists, such as the Erythrina alkaloids, the tubocurarines, and methyllycaconitine. Noncompetitive antagonists, such as the histrionicotoxins, various izidines, decahydroquinolines, spiropyrrolizidine oximes, pseudophrynamines, ibogaine, strychnine, cocaine, and sparteine have come from natural sources. Finally, galanthamine, codeine, and ivermectin represent positive modulators of nicotinic function, derived from natural sources. 3. Clearly, research on acetylcholine receptors and functions has been dependent on key natural products and the synthetic agents that they inspired.

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.

CoMFA, CoMSIA, and Molecular Hologram QSAR Studies of Novel Neuronal nAChRs Ligands-Open Ring Analogues of 3-Pyridyl Ether

3-Pyridyl ethers are excellent nAChRs ligands, which show high subtype selectivity and binding affinity to alpha4beta2 nAChR. Although the quantitative structure-activity relationship (QSAR) of nAChRs ligands has been widely investigated using various classes of compounds, the open ring analogues of 3-pyridyl ethers have been less involved in these studies due to the greater flexibility of this kind of molecule. In this study, two three-dimensional QSAR techniques and one two-dimensional QSAR technique were used to correlate the molecular structure with the biological activity of 64 analogues of 3-pyridyl ethers. Three different QSAR models were established. Their performances in the QSAR studies of open ring analogues of 3-pyridyl ethers were evaluated by the statistical values in the corresponding models. All models exhibited satisfactory predictive power. Of these models, the HQSAR behaved optimally in terms of the statistical values with q2=0.845, r2=0.897. Finally, graphic interpretation of three different models provided coincident information about the interaction of the ligand-receptor complex and supplied useful guidelines for the synthesis of novel, potent ligands.

Epibatidine: impact on nicotinic receptor research

Epibatidine is a natural product that was isolated and identified by Daly and coworkers in 1992. Since that time, it has had a profound influence on the investigation of alpha4beta2 nicotinic cholinergic (nACh) receptor pharmacophore models, and has inspired the development of novel agents with therapeutic potential in CNS disorders. Apart from acetylcholine and nicotine, probably no other agent has had as much recent impact on nACh research as has epibatidine.