Jeffery Batis

β2-Nicotinic Acetylcholine Receptor Availability During Acute and Prolonged Abstinence From Tobacco Smoking

CONTEXT Available levels of nicotinic acetylcholine receptors containing the beta(2) subunit (beta(2)*-nAChR) are higher in recently abstinent tobacco smokers compared with participants who never smoked. Variations in beta(2)*-nAChR availability during 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 beta(2)*-nAChR availability during acute and prolonged abstinence from tobacco smoking and to determine how changes in beta(2)*-nAChR availability were related to clinical features of tobacco smoking. DESIGN Tobacco smokers participated in up to 4 iodide 123-labeled 5-iodo-A-85380 ([(123)I]5-IA) single-photon emission computed tomography (SPECT) scans during abstinence at 1 day (n = 7) and 1 (n = 17), 2 (n = 7), 4 (n = 11), and 6 to 12 (n = 6) weeks. Age-matched nonsmokers participated in a single [(123)I]5-IA SPECT scan. All participants 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 The [(123)I]5-IA SPECT images were converted to distribution volume and were analyzed using regions of interest. RESULTS Compared with nonsmokers, beta(2)*-nAChR availability in the striatum, cortex, and cerebellum of smokers was not different at 1 day of abstinence, was significantly higher at 1 week of abstinence, and was not different at 4 or at 6 to 12 weeks of abstinence. In smokers, beta(2)*-nAChR availability was significantly lower in the cortex and cerebellum at 6 to 12 weeks compared with 1 week of abstinence. In addition, cerebellar beta(2)*-nAChR availability at 4 weeks of abstinence was positively correlated with craving on the day of the SPECT scan. CONCLUSIONS These data suggest that higher beta(2)*-nAChR availability persists up to 1 month of abstinence and normalizes to nonsmoker levels by 6 to 12 weeks of abstinence from tobacco smoking. These marked and persistent changes in beta(2)*-nAChR availability may contribute to difficulties with tobacco cessation.

Quantification of Smoking-Induced Occupancy of β2-Nicotinic Acetylcholine Receptors: Estimation of Nondisplaceable Binding

5-123I-iodo-85380 (123I-5-IA) is used to quantitate high-affinity nicotinic acetylcholine receptors (β2-nAChRs) on human SPECT scans. The primary outcome measure is VT/fP, 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 123I-5-IA has not been measured in human subjects. Nicotine has high affinity for β2*-nAChRs (nAChRs containing the β2* subunit, for which * represents other subunits that may also be part of the receptor) and displaces specifically bound 123I-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. 123I-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 β2*-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. VT/fP 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·cm−3 (range, 15–28 mL·cm−3). Thus, in the thalamus, where mean VT/fP is 93 mL·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.

GABAA-benzodiazepine receptor availability in smokers and nonsmokers: Relationship to subsyndromal anxiety and depression†

Many smokers experience subsyndromal anxiety symptoms while smoking and during acute abstinence, which may contribute to relapse. We hypothesized that cortical gamma aminobutyric acidA‐benzodiazepine receptor (GABAA‐BZR) availability in smokers and nonsmokers might be related to the expression of subsyndromal anxiety, depressive, and pain symptoms. Cortical GABAA‐BZRs were imaged in 15 smokers (8 men and 7 women), and 15 healthy age and sex‐matched nonsmokers, and 4 abstinent tobacco smokers (3 men; 1 woman) using [123I]iomazenil and single photon emission computed tomography (SPECT). Anxiety and depressive symptoms were measured using the Spielbergers State‐Trait Anxiety Index (STAI) and the Center for Epidemiology Scale for Depressive Symptoms (CES‐D). The cold pressor task was administered to assess pain tolerance and sensitivity. The relationship between cortical GABAA‐BZR availability, smoking status, and subsyndromal depression and anxiety symptoms, as well as pain tolerance and sensitivity, were evaluated. Surprisingly, there were no statistically significant differences in overall GABAA‐BZR availability between smokers and nonsmokers or between active and abstinent smokers; however, cortical GABAA‐BZR availability negatively correlated with subsyndromal state anxiety symptoms in nonsmokers but not in smokers. In nonsmokers, the correlation was seen across many brain areas with state anxiety [parietal (r = −0.47, P = 0.03), frontal (r = −0.46, P = 0.03), anterior cingulate (r = −0.47, P = 0.04), temporal (r = −0.47, P = 0.03), occipital (r = −0.43, P = 0.05) cortices, and cerebellum (r = −0.46, P = 0.04)], trait anxiety [parietal (r = −0.72, P = 0.02), frontal (r = −0.72, P = 0.02), and occipital (r = −0.65, P = 0.04) cortices] and depressive symptoms [parietal (r = −0.68; P = 0.02), frontal (r = −0.65; P = 0.03), anterior cingulate (r = −0.61; P = 0.04), and temporal (r = −0.66; P = 0.02) cortices]. The finding that a similar relationship between GABAA‐BZR availability and anxiety symptoms was not observed in smokers suggests that there is a difference in GABAA‐BZR function, but not number, in smokers. Thus, while subsyndromal anxiety and depressive symptoms in nonsmokers may be determined in part by GABAA‐BZR availability, smoking disrupts this relationship. Aberrant regulation of GABAA‐BZR function in vulnerable smokers may explain why some smokers experience subsyndromal anxiety and depression. Synapse 63:1089–1099, 2009.

Brain β2*-nicotinic acetylcholine receptor occupancy after use of a nicotine inhaler

The Nicotrol® (Pfizer, USA) nicotine inhaler reduces craving by mimicking the behavioural component of cigarettes and delivering controlled doses of nicotine, which binds to the beta-2 subunit-containing nicotinic acetylcholine receptors (β2*-nAChRs). Previous studies examined β2*-nAChR occupancy after administration of regular and low-nicotine cigarettes. Here, we measured occupancy of β2*-nAChRs after administration of nicotine via inhaler, and the relationship between occupancy and changes in craving for tobacco smoking and withdrawal symptoms. Tobacco smokers participated in [123I]5-IA-85380 SPECT studies with either a nicotine inhaler (n=9) or tobacco cigarette (n=4) challenge. [123I]5-IA was administered as a bolus plus constant infusion. After equilibrium was achieved, three 30-min baseline scans were collected, and subjects either used the nicotine inhaler or a regular cigarette, and up to six additional scans were obtained. Receptor occupancy was determined based on the Lassen plot method. Craving for tobacco smoking and withdrawal symptoms were evaluated pre- and post-challenge. Use of the nicotine inhaler produced an average 55.9±6.4% occupancy of β2*-nAChRs 2-5 h post-challenge, whereas use of a cigarette produced significantly higher receptor occupancy (F=10.6, p=0.009) with an average 67.6±14.1% occupancy 1.5-5 h post-challenge. There was a significant decrease in withdrawal symptoms post-nicotine inhaler use (F=6.13, p=0.04). These results demonstrate significant differences in occupancy of β2*-nAChRs by nicotine after use of the inhaler vs. a cigarette and confirm the ability of the nicotine inhaler to relieve withdrawal symptoms.

Kinetic Modeling, Test–Retest, and Dosimetry of 123I-MNI-420 in Humans

In vivo imaging of adenosine 2A receptors (A2A) in the brain has attracted significant interest from the scientific community, because studies have shown that dysregulation of these receptors is implicated in a variety of neurodegenerative and psychiatric disorders, including Parkinson and Huntington diseases. This work aimed to describe the kinetic properties, test–retest results, and dosimetry estimates of 123I-MNI-420, a SPECT radiotracer for the in vivo imaging of A2A in the brain. Methods: Nine healthy human subjects were enrolled in this study; 7 completed 123I-MNI-420 brain SPECT studies, and 2 participated in whole-body planar imaging evaluating 123I-MNI-420 biodistribution and dosimetry. For 3 of the brain SPECT studies, arterial blood was collected for invasive modeling. Noninvasive models were also explored, including Logan graphical analysis and simplified reference tissue models. Test–retest reliability was assessed in 4 subjects. To evaluate radiotracer biodistribution and dosimetry, serial whole-body images were acquired immediately after injection and at selected time points after injection. Urine samples were collected over a period of 21 h to calculate urinary excretion. Results: 123I-MNI-420 rapidly entered the human brain and displayed uptake consistent with known A2A densities. At pseudoequilibrium (reached at 90 min after radiotracer injection), stable target-to-cerebellum ratios of around 1.4–2.0 were determined. Binding potentials around 0.8–1.2 were estimated using different kinetic models and the cerebellum as the reference region. Average test–retest variability in the striatum was 4.8%, 3.5%, and 6.5% for the simplified reference tissue model, Logan graphical analysis, and standardized uptake value ratio methods, respectively. The estimated radiation effective dose determined from whole-body studies was 0.036 mSv/MBq. Conclusion: The data indicate that 123I-MNI-420 is a useful SPECT radiotracer for imaging A2A in the brain and has radiation doses that would allow for multiple scans in the same research subject each year. The availability of 123I-MNI-420 offers the possibility of investigating A2A activity in specific conditions and evaluating drug occupancy for A2A candidate therapeutics.

In vivo evaluation of [123I]MNI-420: A novel single photon emission computed tomography radiotracer for imaging of adenosine 2A receptors in brain

INTRODUCTION Dysregulation of adenosine 2A (A2A) receptor function in brain has been implicated in multiple psychiatric and neurodegenerative disorders, including schizophrenia and Parkinsons disease, making the development of an imaging agent to study A2A receptors in both healthy brain and disease states desirable. In this study, [(123)I]MNI-420 was evaluated as a potential single photon emission computed tomography (SPECT) radiotracer for imaging A2A receptors in brain. METHODS Two adult male monkeys (Macaca fascicularis) and three adult female baboons (Papio anubis) were anesthetized and imaged on Neurofocus SPECT cameras. Baboons underwent baseline and displacement studies using varying doses of caffeine (2.0-20mg/kg). Baseline and pre-blocking experiments with multiple doses of preladenant (0.01-1.2mg/kg), a highly selective A2A antagonist, were performed in cynomolgus monkeys. RESULTS Following bolus intravenous (i.v.) injection, [(123)I]MNI-420 rapidly entered the non-human primate brain. The regional brain accumulation of [(123)I]MNI-420 matched the known distribution of A2A receptors in brain (highest in the striatum). Striatum to cerebellum ratios and binding potentials of around 3.0-3.5 and 2.0-2.5, respectively, were measured in monkey and baboon brain. A dose-dependent occupancy was observed following i.v. injection of caffeine at pseudo-equilibrium conditions during displacement experiments. Pre-treatment with preladenant blocked specific binding in A2A rich regions in a dose-dependent fashion. CONCLUSIONS The data indicate that [(123)I]MNI-420 holds promise as a SPECT radiotracer for imaging A2A receptors in brain and further evaluation is warranted, in order to determine its utility as a SPECT radiotracer for imaging of A2A in brain.

Kinetic modeling and occupancy measures of the norepinephrine transporters in baboons using single photon emission computed tomography with 123I-INER

This study aims to investigate the pharmacokinetics of a recently developed radiotracer for imaging of the norepinephrine transporter (NET) in baboon brain, 123I‐INER, using single photon emission computed tomography (SPECT). In addition, it also aims to determine NET occupancy by atomoxetine and reboxetine, two selective norepinephrine reuptake inhibitors, using 123I‐INER in baboons. Baseline and preblocking studies with a high dose of atomoxetine (0.85 mg/kg) were conducted in three baboons using SPECT with 123I‐INER administered as a bolus. Kinetic modeling analysis was investigated for different models, namely invasive and reference tissue models. Bolus plus constant infusion experiments with displacement at equilibrium using six different doses of atomoxetine (0.03–0.85 mg/kg) and four different doses of reboxetine (0.5–3.0 mg/kg) were carried out in several baboons to obtain occupancy measurements as a function of dose for the two NET selective drugs. Results showed that reference tissue models can be used to estimate binding potential values and occupancy measures of 123I‐INER in different brain regions. In addition, the apparent volume of distribution was estimated by dividing concentration in tissue by the concentration in blood at 3 hours postinjection. After administration of atomoxetine or reboxetine, a dose‐dependent occupancy was observed in brain regions known to contain high densities of NET. In conclusion, pharmacokinetic properties of 123I‐INER were successfully described, and obtained results may be used to simplify future data acquisition and image processing. Dose‐dependent NET occupancy for two selective norepinephrine reuptake inhibitors was successfully measured in vivo in baboon brain using SPECT and 123I‐INER. Synapse, 2013.

Iodine‐123 labeled reboxetine analogues for imaging of noradrenaline transporter in brain using single photon emission computed tomography

Preliminary investigation of the radioiodinated (S,S)‐reboxetine analogue, 123I‐INER, in baboons showed this tracer to have promise for imaging the noradrenaline transporter (NAT) using single photon emission computed tomography (SPECT). More recently, the radioiodinated (R,S)‐stereoisomer of 123I‐INER, 123I‐NKJ64, has been synthesized and preliminary evaluation in rats has been reported. This article reports the brain distribution and pharmacokinetic properties of 123I‐NKJ64 in baboons and compares results with 123I‐INER data in the same species. SPECT studies were conducted in two ovariectomized adult female baboons using two different protocols: (1) bolus of 123I‐INER or 123I‐NKJ64; and (2) bolus plus constant infusion of 123I‐NKJ64 with reboxetine (2.0 mg/kg) administration at equilibrium. Following bolus injection, both radiotracers rapidly and avidly entered the baboon brain. The regional brain accumulation of 123I‐NKJ64 did not match the known distribution of NAT in baboon brain, contrasting with previous results obtained in rats. Conversely, the regional distribution of 123I‐INER was consistent with known distribution of NAT in baboon brain. No displacement of 123I‐NKJ64 was observed following administration of reboxetine. This contrasts with previous data obtained for 123I‐INER, where 60% of specific binding was displaced by a lower dose of reboxetine. These data suggest that 123I‐NKJ64 may lack affinity and selectivity for NAT in baboon brain and 123I‐INER is the most promising iodinated reboxetine analogue developed to date for in vivo imaging of NAT in brain using SPECT. This study highlights the importance of species differences during radiotracer development and the stereochemical configuration of analogues of reboxetine in vivo. Synapse 66:923–930, 2012.

Test/retest reproducibility of iodine-123-MNI308 in nonhuman primates

Methods: Six stump-tailed macaques (age: 17.3±6.3; mean±s.d.) were scanned on 2 separate days (time between scans: 91±37 days) with [I]MNI308 (injected activity: 92.8±32.7 MBq). Serial, dynamic SPECT scans were acquired for 3 h immediately following injection of [I]MNI308. Volumes-of-interest (VOIs) were drawn on an average image from the first SPECT scan day, and VOIs included whole brain, cerebellum, brainstem, thalamus, and frontal, temporal, parietal and occipital cortex. Retest scans were co-registered to their respective initial scans. VOI placement and scan coregistrations were conducted using commercially available software. Histopathological evidence suggested that the cerebellumwas void of amyloid plaque in stump tails, and was suitable for use as a reference region. SUVrs were calculated as the ratio of uptake in target VOIs/uptake in cerebellum. For each VOI, the average ratio for all scans from 90 to 150 min post injectionwas calculated. This post-injection interval represents a period of secular equilibriumwith unchanging SUVr values. This mean SUVr from each scan (test and retest) for a given animal was calculated, and the quotient (difference/mean), expressed as a percent, were calculated for each VOI.