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The PET ligand (11)C-PBR28 (N-((2-(methoxy-(11)C)-phenyl)methyl)-N-(6-phenoxy-3-pyridinyl)acetamide) binds to the 18-kDa translocator protein (TSPO), a biomarker of glia. In clinical studies of TSPO, the ligand total distribution volume, V(T), is frequently the reported outcome measure. Since V(T) is the sum of the ligand-specific distribution volume (V(S)) and the nondisplaceable-binding distribution volume (V(ND)), differences in V(ND) across subjects and groups will have an impact on V(T). Methods: Here, we used a recently developed method for simultaneous estimation of V(ND) (SIME) to disentangle contributions from V(ND) and V(S). Data from 4 previously published (11)C-PBR28 PET studies were included: before and after a lipopolysaccharide challenge (8 subjects), in alcohol use disorder (14 patients, 15 controls), in first-episode psychosis (16 patients, 16 controls), and in Parkinson disease (16 patients, 16 controls). In each dataset, regional V(T) estimates were obtained with a standard 2-tissue-compartment model, and brain-wide V(ND) was estimated with SIME. V(S) was then calculated as V(T) − V(ND). V(ND) and V(S) were then compared across groups, within each dataset. Results: A lower V(ND) was found for individuals with alcohol-use disorder (34%, P = 0.00084) and Parkinson disease (34%, P = 0.0032) than in their corresponding controls. We found no difference in V(ND) between first-episode psychosis patients and their controls, and the administration of lipopolysaccharide did not change V(ND). Conclusion: Our findings suggest that in TSPO PET studies, nondisplaceable binding can differ between patient groups and conditions and should therefore be considered.

The effects of tobacco smoking on the immune system of the brain are not well elucidated. Although nicotine is immunosuppressive, other constituents in tobacco smoke have inflammatory effects. PET imaging of the 18-kDa translocator protein (TSPO) provides a biomarker for microglia, the primary immunocompetent cells of the brain. This work compared brain TSPO levels in 20 tobacco smokers (abstinent for at least 2 h) and 20 nonsmokers using a fully quantitative modeling approach for the first time, to our knowledge. Methods: (11)C-PBR28 (N-((2-(methoxy-(11)C)-phenyl)methyl)-N-(6-phenoxy-3-pyridinyl)acetamide) PET scans were acquired with arterial blood sampling to estimate the metabolite-corrected input function. (11)C-PBR28 volumes of distribution were estimated throughout the brain with multilinear analysis. Results: Statistical analyses revealed no evidence of significant differences in regional (11)C-PBR28 volumes of distribution between smokers and nonsmokers (whole-brain Cohen d = 0.09) despite adequate power to detect medium effect sizes. Conclusion: These findings inform previous PET studies reporting lower TSPO radiotracer concentrations in the brain (measured as SUV) for tobacco smokers than for nonsmokers by demonstrating the importance of accounting for radiotracer concentrations in plasma. These findings suggest that nonsmokers and smokers have comparable TSPO levels in the brain. Additional work with other biomarkers is needed to fully characterize the effects of tobacco smoking on the brain immune system.

In vivo estimation of β(2)-nicotinic acetylcholine receptor availability with molecular neuroimaging is complicated by competition between the endogenous neurotransmitter acetylcholine and the radioligand (123)I-3-[2(S)-2-azetidinylmethoxy]pyridine ((123)I-5-IA). We examined whether binding of (123)I-5-IA is sensitive to increases in extracellular levels of acetylcholine in humans, as suggested in nonhuman primates.Six healthy subjects (31 ± 4 y) participated in a (123)I-5-IA SPECT study. After baseline scans, physostigmine (1-1.5 mg) was administered intravenously over 60 min, and 9 additional scans were obtained.We observed a significant reduction in the total volume of distribution after physostigmine administration (29% ± 17% in the cortex, 19% ± 15% in the thalamus, 19% ± 15% in the striatum, and 36% ± 30% in the cerebellum; P0.05). This reduction reflected a combination of a region-specific 7%-16% decrease in tissue concentration of tracer and a 9% increase in plasma parent concentration.These data suggest that increases in acetylcholine compete with (123)I-5-IA for binding to β(2)-nicotinic acetylcholine receptor. Additional validation of this paradigm is warranted, but it may be used to interrogate changes in extracellular acetylcholine.

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.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.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.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.

The study of the effects of sex and hormones on brain chemistry and neurotransmission is of increasing importance as evidence emerges of sex differences in behavioral symptoms and treatment response in neuropsychiatric disorders. The nicotinic acetylcholine receptor (nAChR) system has been implicated in a variety of psychiatric disorders, including tobacco smoking, for which there is strong evidence supporting sex differences in behaviors and response to smoking cessation treatments. We examined the availability of nAChR containing the beta(2) subunit in healthy men and women and the influence of menstrual phase among women.Ten men and 19 women nonsmokers underwent one (123)I-5-IA-85380 ((123)I-5-IA) SPECT scan and one MRI scan. A subset of 9 women, aged 18-39 y, underwent a second (123)I-5-IA scan. These 9 women were scanned during the early follicular (days 4-7 in 8 subjects and day 11 in 1 subject) and mid-luteal (days 19-25) phases of their menstrual cycle. Hormone levels were measured in all women to confirm the phase of the cycle.Regional brain activity (kBq/cm(3)) was higher (39%-54%) in women than in men nonsmokers. When regional brain activity was normalized to total plasma parent to correct for individual differences in radiotracer metabolism (V(T)'), differences of 10%-16% were observed, with women greater than men. In contrast, when regional brain activity was normalized to free plasma parent (V(T)), there was less than a 4% difference by sex in regional brain beta(2)-nAChR availability. These sex differences in kBq/cm(3) and V(T)' resulted from significantly higher levels of total plasma parent, free fraction (f(1)), and free plasma parent in women than in men nonsmokers. No differences in plasma measures or brain beta(2)-nAChR availability were observed across the menstrual cycle for any outcome measure.Overall, these findings demonstrate no significant difference in brain beta(2)-nAChR availability between men and women nonsmokers or across the menstrual cycle. Importantly, these findings demonstrate sex differences in radiotracer metabolism and plasma protein binding and highlight the critical need to measure plasma radiotracer levels and f(1) in studies that include both sexes.