18F-ZW-104: A New Radioligand for Imaging Neuronal Nicotinic Acetylcholine Receptors—In Vitro Binding Properties and PET Studies in Baboons

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.

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 evaluation of 3-¹²³I-iodo-5-[2-(S)-3-pyrrolinylmethoxy]-pyridine (niodene) as a potential nicotinic α4β2 receptor imaging agent

Nicotinic acetylcholine receptors (nAChRs) are downregulated in disease conditions such as Alzheimer's and substance abuse. Presently, (123)I-5-IA-85380 is used in human studies and requires over 6h of scanning time, thus increases patient discomfort. We have designed and synthesized 3-iodo-5-[2-(S)-3-pyrrolinylmethoxy]pyridine (niodene) with the aim to have faster binding kinetics compared to (123)I-5-IA-85380, which may reduce scanning time and help in imaging studies. Binding affinity K(i) of niodene for rat brain α4β2 receptors in brain homogenate assays using (3)H-cytisine was 0.27 nM. Niodene, 10nM displaced >95% of (18)F-nifene bound to α4β2 receptors in rat brain slices. By using the iododestannylation method, (123)I-niodene was obtained in high radiochemical purity (>95%) but with low radiochemical yield (<5%) and low specific activity (∼100 Ci/mmol). Autoradiograms show (123)I-niodene localized in the thalamus and cortex, which was displaced by nicotine (thalamus to cerebellum ratio=4; cortex to cerebellum ratio=1.6). Methods of radioiodination need to be further evaluated in order to obtain (123)I-niodene in higher radiochemical yields and higher specific activity of this potentially useful new SPECT imaging agent.

PET imaging of α4β2* nicotinic acetylcholine receptors: quantitative analysis of 18F-nifene kinetics in the nonhuman primate

The PET radioligand 2-fluoro-3-[2-((S)-3-pyrrolinyl)methoxy]pyridine ((18)F-nifene) is an α4β2* nicotinic acetylcholine receptor (nAChR) agonist developed to provide accelerated in vivo equilibrium compared with existing α4β2* radioligands. The goal of this work was to analyze the in vivo kinetic properties of (18)F-nifene with both kinetic modeling and graphical analysis techniques.

METHODS

Dynamic PET experiments were performed on 4 rhesus monkeys (female; age range, 9-13 y) using a small-animal PET scanner. Studies began with a high-specific-activity (18)F-nifene injection, followed by a coinjection of (18)F-nifene and unlabeled nifene at 60 min. Sampling of arterial blood with metabolite analysis was performed throughout the experiment to provide a parent radioligand input function. In vivo kinetics were characterized with both a 1-tissue-compartment model (1TCM) and a 2-tissue-compartment model, Logan graphical methods (both with and without blood sampling), and the multilinear reference tissue model. Total distribution volumes and nondisplaceable binding potentials (BP(ND)) were used to compare regional binding of (18)F-nifene. Regions examined include the anteroventral thalamus, lateral geniculate body, frontal cortex, subiculum, and cerebellum.

RESULTS

The rapid uptake and binding of (18)F-nifene in nAChR-rich regions of the brain was appropriately modeled using the 1TCM. No evidence for specific binding of (18)F-nifene in the cerebellum was detected on the basis of the coinjection studies, suggesting the suitability of the cerebellum as a reference region. Total distribution volumes in the cerebellum were 6.91 ± 0.61 mL/cm(3). BP(ND) values calculated with the 1TCM were 1.60 ± 0.17, 1.35 ± 0.16, 0.26 ± 0.08, and 0.30 ± 0.07 in the anteroventral thalamus, lateral geniculate body, frontal cortex, and subiculum, respectively. For all brain regions, there was a less than 0.04 absolute difference in the average BP(ND) values calculated with each of the 1TCM, multilinear reference tissue model, and Logan methods.

CONCLUSION

The fast kinetic properties and specific regional binding of (18)F-nifene promote extension of the radioligand into preclinical animal models and human subjects.

Discrepancies in the P-glycoprotein-mediated transport of (18)F-MPPF: a pharmacokinetic study in mice and non-human primates

PURPOSE

Several in vivo studies have found that the 5-HT(1A) PET radioligand (18)F-MPPF is a substrate of rodent P-glycoprotein (P-gp). However, in vitro assays suggest that MPPF is not a substrate of human P-gp. We have now tested the influence of inhibiting P-gp on the brain kinetics of (18)F-MPPF in mice and non-human primates.

METHODS

We measured the peripheral kinetics (arterial input function, metabolism, free fraction in plasma (f(P))) during (18)F-MPPF brain PET scanning in baboons with or without cyclosporine A (CsA) infusion. We measured (3)H-MPPF transport at the mouse BBB using in situ brain perfusion in P-gp/Bcrp deficient mice and after inhibiting P-gp with PSC833.

RESULTS

There was an unexpected 1.9-fold increase in brain area under the curve in CsA-treated baboons (n = 4), with no change in radiometabolite-corrected arterial input. However, total volume of distribution corrected for f(P) (V(T)/f(P)) remained unchanged. In situ brain perfusion showed that P-gp restricted the permeability of the mouse BBB to (3)H-MPPF while Bcrp did not.

CONCLUSION

These and previous in vitro results suggest that P-gp may not influence the permeability of human BBB to (18)F-MPPF. However, CsA treatment increased (18)F-MPPF free fraction, which is responsible for a misleading, P-gp unrelated enhanced brain uptake.

Effect of secondhand smoke on occupancy of nicotinic acetylcholine receptors in brain

CONTEXT

Despite progress in tobacco control, secondhand smoke (SHS) exposure remains prevalent worldwide and is implicated in the initiation and maintenance of cigarette smoking.

OBJECTIVE

To determine whether moderate SHS exposure results in brain α(4)β(2)* nicotinic acetylcholine receptor (nAChR) occupancy.

DESIGN, SETTING, AND PARTICIPANTS

Positron emission tomography scanning and the radiotracer 2-[18F]fluoro-3-(2(S)azetidinylmethoxy) pyridine (also known as 2-[(18)F]fluoro-A-85380, or 2-FA) were used to determine α(4)β(2)* nAChR occupancy from SHS exposure in 24 young adult participants (11 moderately dependent cigarette smokers and 13 nonsmokers). Participants underwent two bolus-plus-continuous-infusion 2-FA positron emission tomography scanning sessions during which they sat in the passenger's seat of a car for 1 hour and either were exposed to moderate SHS or had no SHS exposure. The study took place at an academic positron emission tomography center. Main Outcome Measure  Changes induced by SHS in 2-FA specific binding volume of distribution as a measure of α(4)β(2)* nAChR occupancy.

RESULTS

An overall multivariate analysis of variance using specific binding volume of distribution values revealed a significant main effect of condition (SHS vs control) (F(1,22) = 42.5, P < .001) but no between-group (smoker vs nonsmoker) effect. Exposure to SHS led to a mean 19% occupancy of brain α(4)β(2)* nAChRs (1-sample t test, 2-tailed, P < .001). Smokers had both a mean 23% increase in craving with SHS exposure and a correlation between thalamic α(4)β(2)* nAChR occupancy and craving alleviation with subsequent cigarette smoking (Spearman ρ = -0.74, P = .01).

CONCLUSIONS

Nicotine from SHS exposure results in substantial brain α(4)β(2)* nAChR occupancy in smokers and nonsmokers. Study findings suggest that such exposure delivers a priming dose of nicotine to the brain that contributes to continued cigarette use in smokers. This study has implications for both biological research into the link between SHS exposure and cigarette use and public policy regarding the need to limit SHS exposure in cars and other enclosed spaces.