Nicotine, the addictive chemical in tobacco smoke, initiates its actions in brain through nicotinic acetylcholine receptors (nAChRs). In particular, nAChRs containing ␤ 2 -subunits (␤ 2 *-nAChRs) the most prevalent subtype, mediate the reinforcing properties of nicotine. We hypothesized that abnormal numbers of ␤ 2 *-nAChRs during early abstinence contribute to the perpetuation of addiction to tobacco smoking. Using molecular imaging, specifically single-photon emission computed tomography with the nAChR agonist radiotracer ]5-IA uptake was significantly higher throughout the cerebral cortex (26 -36%) and in the striatum (27%) than in nonsmokers, suggesting higher ␤ 2 *-nAChR in recently abstinent smokers. ␤ 2 *-nAChR availability in recently abstinent smokers correlated with the days since last cigarette and the urge to smoke to relieve withdrawal symptoms but not the severity of nicotine dependence, severity of nicotine withdrawal, or the desire to smoke. Higher brain ␤ 2 *-nAChR during early abstinence indicates that, when smokers quit smoking, they do so in the face of a significant increase in the receptors normally activated by nicotine. Greater ␤ 2 *-nAChR availability during early abstinence may impact the ability of smokers to maintain abstinence.

Purpose Tau deposition is a key pathological feature of Alzheimer’s disease (AD) and other neurodegenerative disorders. The spreading of tau neurofibrillary tangles across defined brain regions corresponds to the observed level of cognitive decline in AD. Positron-emission tomography (PET) has proved to be an important tool for the detection of amyloid-beta (Aβ) aggregates in the brain, and is currently being explored for detection of pathological misfolded tau in AD and other non-AD tauopathies. Several PET tracers targeting tau deposits have been discovered and tested in humans. Limitations have been reported, especially regarding their selectivity. Methods In our screening campaign we identified pyrrolo[2,3- b :4,5- c ’]dipyridine core structures with high affinity for aggregated tau. Further characterization showed that compounds containing this moiety had significantly reduced monoamine oxidase A (MAO-A) binding compared to pyrido[4,3- b ]indole derivatives such as AV-1451. Results Here we present preclinical data of all ten fluoropyridine regioisomers attached to the pyrrolo[2,3- b :4,5- c ’]dipyridine scaffold, revealing compounds 4 and 7 with superior properties. The lead candidate [ 18 F]PI-2620 (compound 7 ) displayed high affinity for tau deposits in AD brain homogenate competition assays. Specific binding to pathological misfolded tau was further demonstrated by autoradiography on AD brain sections (Braak I-VI), Pick’s disease and progressive supranuclear palsy (PSP) pathology, whereas no specific tracer binding was detected on brain slices from non-demented donors. In addition to its high affinity binding to tau aggregates, the compound showed excellent selectivity with no off-target binding to Aβ or MAO-A/B. Good brain uptake and fast washout were observed in healthy mice and non-human primates. Conclusions Therefore, [ 18 F]PI-2620 was selected for clinical validation. Electronic supplementary material The online version of this article (10.1007/s00259-019-04397-2) contains supplementary material, which is available to authorized users.

In an effort to develop selective radioligands for in vivo imaging of neuronal nicotinic acetylcholine receptors (nAChRs), we synthesized 5-iodo-3-(2(S)-azetidinylmethoxy)pyridine (5-iodo-A-85380) and labeled it with 125 I and 123 I. Here we present the results of experiments characterizing this radioiodinated ligand in vitro. The affinity of 5-[ 125 I]iodo-A-85380 for ␣4␤2 nAChRs in rat and human brain is defined by K d values of 10 and 12 pM, respectively, similar to that of epibatidine (8 pM). In contrast to epibatidine, however, 5-iodo-A-85380 is more selective in binding to the ␣4␤2 subtype than to other nAChR subtypes. In rat adrenal glands, 5-iodo-A-85380 binds to nAChRs containing ␣3 and ␤4 subunits with 1/1000th the affinity of epibatidine, and exhibits 1/60th and 1/190th the affinity of epibatidine at ␣7 and muscle-type nAChRs, respectively. Moreover, unlike epibatidine and cytisine, 5-[ 125 I]iodo-A-85380 shows no binding in any brain regions in mice homozygous for a mutation in the ␤2 subunit of nAChRs. Binding of 5-[125 I]iodo-A-85380 in rat brain is reversible, and is characterized by high specificity and a slow rate of dissociation of the receptor-ligand complex (t 1/2 for dissociation ϳ2 h). These properties, along with other features observed previously in in vivo experiments (low toxicity, rapid penetration of the blood-brain barrier, and a high ratio of specific to nonspecific binding), suggest that this compound, labeled with 125 I or 123 I, is superior to other radioligands available for in vitro and in vivo studies of ␣4␤2 nAChRs, respectively.Nicotinic acetylcholine receptors (nAChRs) are excitatory ligand-gated cation channels that are widely distributed in mammalian organisms, appearing in the central and peripheral nervous systems, neuromuscular junctions, and adrenal glands. The nAChR channel complex is composed of five protein subunits, which form a pore that is permeable to Na ϩ , K ϩ , and Ca 2ϩ (Lindstrom, 1995;Holladay et al., 1997).To date, ␣, ␤, ␥, ␦, and ⑀ subunits have been isolated and cloned from mammalian and avian tissues, with nine varieties of ␣ and four varieties of ␤ subunits identified. The ␣1, ␤1, ␥, ␦, and ⑀ subunits form the neuromuscular junction receptor, the very first nAChR to be characterized. The other subunits (␣2-␣9 and ␤2-␤4) are found predominantly throughout the nervous system (Lindstrom, 1995;Holladay et al., 1997). This subunit diversity affords a large potential for a variety of nAChR subtypes, exhibiting distinct cationconducting properties and pharmacological heterogeneity. Based on binding properties and pharmacological sensitivity, major nAChR subtypes in mammalian brain can be categorized as ␣-bungarotoxin-sensitive (␣7) and ␣-bungarotoxin-insensitive (e.g., ␣4␤2) (Lindstrom, 1995;Holladay et al., 1997). Accordingly, 125 I-␣-bungarotoxin has been the radioligand of choice for in vitro characterization of the ␣7 subtype of nAChR, whereas tritiated agonists, such as nico-

Nicotine and other constituents of tobacco smoke elevate dopamine (DA) and serotonin (5-HT) levels in brain and may cause homeostatic adaptations in DA and 5-HT transporters. Since sex steroids alter DA and 5-HT transporter expression, the effects of smoking on DA and 5-HT transporter availability may differ between sexes. In the present study, DA and 5-HT transporter availabilities were quantitated using single photon emission computed tomography (SPECT) imaging approximately 22 h after bolus administration of [123I]beta-CIT, an analog of cocaine which labels DA and 5-HT transporters. Forty-two subjects including 21 pairs of age-, race-, and gender-matched healthy smokers and nonsmokers (12 female and 9 male pairs) were imaged. Regional uptake was assessed by the outcome measures, V3", which is the ratio of specific (i.e., ROI-cerebellar activity) to nondisplaceable (cerebellar) activity, and V3, the ratio of specific to free plasma parent. Overall, striatal and diencephalic [123I]beta-CIT uptake was not altered by smoking, whereas brainstem [123I]beta-CIT uptake was modestly higher (10%) in smokers vs. nonsmokers. When subgrouped by sex, regardless of smoking status, [123I]beta-CIT uptake was higher in the striatum (10%), diencephalon (15%), and brainstem (15%) in females vs. males. The sex*smoking interaction was not significant in the striatum, diencephalon, or brainstem, despite the observation of 20% higher brainstem [123I]beta-CIT uptake in male smokers vs. nonsmokers and less than a 5% difference between female smokers and nonsmokers. The results demonstrate higher DA and 5-HT transporter availability in females vs. males and no overall effect of smoking with the exception of a modest elevation in brainstem 5-HT transporters in male smokers. Although these findings are preliminary and need validation with a more selective 5-HT transporter radiotracer, the results suggest that brainstem 5-HT transporters may be regulated by smoking in a sex-specific manner.

The metabotropic glutamate receptor subtype 5 (mGlu5) activates calcium mobilization via binding of glutamate, the major excitatory neurotransmitter in the central nervous system. Allosteric modulation of the receptor has recently emerged as a promising alternative method of regulation to traditional regulation through orthosteric ligands. We now report three novel compounds that bind to the allosteric 2-methyl-6-(phenylethynyl)-pyridine (MPEP) site on mGlu5 but have only partial inhibition or no functional effects on the mGlu5 response. Two of these compounds, 2-(2-(3-methoxyphenyl)ethynyl)-5-methylpyridine (M-5MPEP) and 2-(2-(5-bromopyridin-3-yl)ethynyl)-5-methylpyridine (Br-5MPEPy), act as partial antagonists of mGlu5 in that they only partially inhibit the response of this receptor to glutamate. The third compound, 5-methyl-6-(phenylethynyl)-pyridine (5MPEP), acts as a neutral allosteric site ligand that binds to the MPEP site and has no effects alone. However, 5MPEP blocks the effects of both the allosteric antagonist MPEP and potentiators 3,3'-difluorobenzaldazine and 3-cyano-N-(1,3-diphenyl-1H-pyrazol-5-yl)benzamide (CDPPB). This compound also blocks depolarization effects of both MPEP and CDPPB in neurons in the subthalamic nucleus. These novel compounds provide valuable new insight into the pharmacology of allosteric sites on G protein-coupled receptors and provide valuable new tools for determining the effects of allosteric site ligands in native systems.

18 F-PI-2620 is a PET tracer with high binding affinity for aggregated tau, a key pathologic feature of Alzheimer disease (AD) and other neurodegenerative disorders. Preclinically, 18 F-PI-2620 binds to both 3-repeat and 4-repeat tau isoforms. The purpose of this firstin-humans study was to evaluate the ability of 18 F-PI-2620 to detect tau pathology in AD patients using PET imaging, as well as to assess the safety and tolerability of this new tau PET tracer. Methods: Participants with a clinical diagnosis of probable AD and healthy controls (HCs) underwent dynamic 18 F-PI-2620 PET imaging for 180 min. 18 F-PI-2620 binding was assessed visually and quantitatively using distribution volume ratios (DVR) estimated from noninvasive tracer kinetics and SUV ratio (SUVR) measured at different time points after injection, with the cerebellar cortex as the reference region. Time-activity curves and SUVR were assessed in AD and HC subjects, as well as DVR and SUVR correlations and effect size (Cohen's d) over time. Results: 18 F-PI-2620 showed peak brain uptake around 5 min after injection and fast washout from nontarget regions. In AD subjects, focal asymmetric uptake was evident in temporal and parietal lobes, precuneus, and posterior cingulate cortex. DVR and SUVR in these regions were significantly higher in AD subjects than in HCs. Very low background signal was observed in HCs. 18 F-PI-2620 administration was safe and well tolerated. SUVR time-activity curves in most regions and subjects achieved a secular equilibrium after 40 min after injection. A strong correlation (R 2. 0.93) was found between noninvasive DVR and SUVR for all imaging windows starting at more than 30 min after injection. Similar effect sizes between AD and HC groups were obtained across the different imaging windows. 18 F-PI-2620 uptake in neocortical regions significantly correlated with the degree of cognitive impairment. Conclusion: Initial clinical data obtained in AD and HC subjects demonstrated a high image quality and excellent signal-to-noise ratio of 18 F-PI-2620 PET for imaging tau deposition in AD subjects. Noninvasive quantification using DVR and SUVR for 30-min imaging windows between 30 and 90 min after injection-for example, 45-75 min-provides robust and significant discrimination between AD and HC subjects. 18 F-PI-2620 uptake in expected regions correlates strongly with neurocognitive performance.

Context β2*-nicotinic acetylcholine receptor (β2*-nAChR) availability is higher in recently abstinent smokers compared to never smokers. Variations in β2*-nAChR availability over the course of abstinence may be related to the urge to smoke, the extent of nicotine withdrawal and successful abstinence. Objective To examine changes in β2*-nAChR availability during acute and prolonged abstinence from tobacco smoking and to determine how changes in β2*-nAChR availability were related to clinical features of tobacco smoking. Design Tobacco smokers participated in up to 4 [123I]5-IA-85380 ([123I]5-IA) single photon emission computed tomography (SPECT) scans during abstinence: 1 day (n=7), 1 week (n=17), 2 weeks (n=7), 4 weeks (n=11), and 6-12 weeks (n=6). Age-matched nonsmokers participated in 1 [123I]5-IA SPECT scan. All subjects completed 1 magnetic resonance imaging study. Setting Academic imaging center. Participants Tobacco smokers (n=19) and an age-matched nonsmoker comparison group (n=20). Main Outcome Measure [123I]5-IA SPECT images were converted to distribution volume and were analyzed using regions of interest. Results Compared to nonsmokers, β2*-nAChR availability in the striatum, cortex and cerebellum of smokers was not different at one day of abstinence, significantly higher at 1week of abstinence and not different at 4 or 6-12 weeks of abstinence. In smokers, at 6-12 weeks of abstinence, β2*-nAChR availability was significantly lower in the cortex and cerebellum compared to 1 week of abstinence. Additionally, cerebellar β2*-nAChR availability at 4 weeks of abstinence was positively correlated with craving on the day of scan. Conclusions These data suggest higher β2*-nAChR availability persists up to 1 month of abstinence, and normalizes to nonsmoker levels by 6-12 weeks of abstinence from tobacco smoking. These marked and persistent changes in β2*-nAChR availability may contribute to difficulties with tobacco cessation.

Degeneration of the dopaminergic nigrostriatal system and of noradrenergic neurons in the locus coeruleus are important pathological features of Parkinson's disease. There is an urgent need to develop therapies that slow down the progression of neurodegeneration in Parkinson's disease. In the present study, we tested whether the highly specific metabotropic glutamate receptor 5 antagonist, 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine, reduces dopaminergic and noradrenergic neuronal loss in monkeys rendered parkinsonian by chronic treatment with low doses of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. Weekly intramuscular 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine injections (0.2-0.5 mg/kg body weight), in combination with daily administration of 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine or vehicle, were performed until the development of parkinsonian motor symptoms in either of the two experimental groups (1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine versus 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle). After 21 weeks of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine treatment, all 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle-treated animals displayed parkinsonian symptoms, whereas none of the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine-treated monkeys were significantly affected. These behavioural observations were consistent with in vivo positron emission tomography dopamine transporter imaging data, and with post-mortem stereological counts of midbrain dopaminergic neurons, as well as striatal intensity measurements of dopamine transporter and tyrosine hydroxylase immunoreactivity, which were all significantly higher in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine-treated animals than in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle-treated monkeys. The 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine treatment also had a significant effect on the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced loss of norepinephrine neurons in the locus coeruleus and adjoining A5 and A7 noradrenaline cell groups. In 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/vehicle-treated animals, almost 40% loss of tyrosine hydroxylase-positive norepinephrine neurons was found in locus coeruleus/A5/A7 noradrenaline cell groups, whereas the extent of neuronal loss was lower than 15% of control values in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine/3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine-treated monkeys. Our data demonstrate that chronic treatment with the metabotropic glutamate receptor 5 antagonist, 3-[(2-methyl-1,3-thiazol-4-yl) ethynyl] pyridine, significantly reduces 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine toxicity towards dopaminergic and noradrenergic cell groups in non-human primates. This suggests that the use of metabotropic glutamate receptor 5 antagonists may be a useful strategy to reduce degeneration of catecholaminergic neuro...