We recently demonstrated that 11 C-MePPEP, a PET ligand for CB 1 receptors, has such high uptake in the human brain that it can be imaged for 210 min and that receptor density can be quantified as distribution volume (V T ) using the gold standard of compartmental modeling. However, 11 C-MePPEP had relatively poor retest and intersubject variabilities, which were likely caused by errors in the measurements of radioligand in plasma at low concentrations by 120 min. We sought to find an analog of 11 C-MePPEP that would provide more accurate plasma measurements. We evaluated several promising analogs in the monkey brain and chose the 18 F-di-deutero fluoromethoxy analog ( 18 F-FMPEP-d 2 ) to evaluate further in the human brain. Methods: 11 C-FMePPEP, 18 F-FEPEP, 18 F-FMPEP, and 18 F-FMPEP-d 2 were studied in 5 monkeys with 10 PET scans. We calculated V T using compartmental modeling with serial measurements of unchanged parent radioligand in arterial plasma and radioactivity in the brain. Nonspecific binding was determined by administering a receptorsaturating dose of rimonabant, an inverse agonist at the CB 1 receptor. Nine healthy human subjects participated in 17 PET scans using 18 F-FMPEP-d 2 , with 8 subjects having 2 PET scans to assess retest variability. To identify sources of error, we compared intersubject and retest variability of brain uptake, arterial plasma measurements, and V T . Results: 18 F-FMPEP-d 2 had high uptake in the monkey brain, with greater than 80% specific binding, and yielded less radioactivity uptake in bone than did 18 F-FMPEP. High brain uptake with 18 F-FMPEP-d 2 was also observed in humans, in whom V T was well identified within approximately 60 min. Retest variability of plasma measurements was good (16%); consequently, V T had a good retest variability (14%), intersubject variability (26%), and intraclass correlation coefficient (0.89). V T increased after 120 min, suggesting an accumulation of radiometabolites in the brain. Radioactivity accumulated in the skull throughout the entire scan but was thought to be an insignificant source of data contamination. Conclusion: Studies in monkeys facilitated our development and selection of 18 F-FMPEP-d 2 , compared with 18 F-FMPEP, as a radioligand demonstrating high brain uptake, high percentage of specific binding, and reduced uptake in bone. Retest analysis in human subjects showed that 18 F-FMPEP-d 2 has greater precision and accuracy than 11 C-MePPEP, allowing smaller sample sizes to detect a significant difference between groups.

Abstract[ 11 C]MePPEP is a high affinity, CB 1 receptor-selective, inverse agonist that has been studied in rodents and monkeys. We examined the ability of [ 11 C]MePPEP to quantify CB 1 receptors in human brain as distribution volume calculated with the "gold standard" method of compartmental modeling and compared results with the simple measure of brain uptake. A total of 17 healthy subjects participated in 26 positron emission tomography (PET) scans, with 8 having two PET scans to assess retest variability. After injection of [ 11 C]MePPEP, brain uptake of radioactivity was high (e.g., 3.6 SUV in putamen region at ~60 minutes) and washed out very slowly. A two-tissue compartment model yielded values of distribution volume (which is proportional to receptor density) that were both well identified (SE 5%) and stable between 60 and 210 minutes. The simple measure of brain uptake (average concentration of radioactivity between 40 and 80 minutes) had good retest variability (~8%) and moderate intersubject variability (16%, coefficient of variation). In contrast, distribution volume had two-fold greater retest variability (~15%) and, thus, less precision. In addition, distribution volume had three-fold greater intersubject variability (~52%). The decreased precision of distribution volume compared to brain uptake was likely due to the slow washout of radioactivity from brain and to noise in measurements of the low concentrations of [ 11 C]MePPEP in plasma. These results suggest that brain uptake can be used for within subject studies (e.g., to measure receptor occupancy by medications) but that distribution volume remains the gold standard for accurate measurements between groups.

Brain cannabinoid CB1 receptors contribute to alcohol-related behaviors in experimental animals, but their potential role in humans with alcohol dependence is poorly understood. We measured CB1 receptors in alcohol dependent patients in early and protracted abstinence, and in comparison with control subjects without alcohol use disorders, using positron emission tomography (PET) and [18F]FMPEP-d2, a radioligand for CB1 receptors. We scanned 18 male inpatients with alcohol dependence twice, within 3–7 days of admission from ongoing drinking, and after 2–4 weeks of supervised abstinence. Imaging data were compared with those from 19 age-matched healthy male control subjects. Data were also analyzed for potential influence of a common functional variation (rs2023239) in the CB1 receptor gene (CNR1) that may moderate CB1 receptor density. On the first scan, CB1 receptor binding was 20–30% lower in patients with alcohol dependence than in control subjects in all brain regions and was negatively correlated with years of alcohol abuse. After 2–4 weeks of abstinence, CB1 receptor binding remained similarly reduced in these patients. Irrespective of diagnostic status, C allele carriers at rs2023239 had higher CB1 receptor binding compared to non-carriers. Alcohol dependence is associated with a widespread reduction of cannabinoid CB1 receptor binding in the human brain and this reduction persists at least 2–4 weeks into abstinence. The correlation of reduced binding with years of alcohol abuse suggests an involvement of CB1 receptors in alcohol dependence in humans.

Electron spin lattice relaxation rates (1/T 1e ) of nitroxyl radicals for tumbling correlation times between about 0.1 and 10 ns in water:glycerol or water:sorbitol mixtures at room temperature were measured by saturation recovery at X-band (9.2 GHz), S-band (3.1 GHz), and L-band (1.9 GHz) for natural abundance 2,2,6,6-tetramethylpiperidinyl-1-oxy (tempol), tempol-d 17 , and 15 N-tempol-d 17 and for a spin-labeled derivative of ethylenediaminetetraacetic acid. Tumbling correlation times were calculated from the continuous wave EPR line shapes at X-band. The dependence of T 1e on tumbling correlation time was modeled with contributions from modulation of nitrogen nuclear hyperfine and g anisotropy, from spin rotation and from one or more thermally activated processes. At these microwave frequencies, modulation of nitrogen hyperfine anisotropy makes a substantially larger contribution than modulation of g anisotropy. Spin rotation makes a very small contribution to T 1e at the tumbling correlation times examined in these studies. Replacement of 14 N by 15 N decreases the relaxation rates as predicted for samples in which modulation of nuclear hyperfine anisotropy contributes to relaxation. Deuteration of the solvent does not affect T 1e , which indicates that the electronnuclear dipolar interaction with solvent nuclei does not make a significant contribution to T 1e of nitroxyl radicals in this motional regime. The contributions to relaxation from one or more thermally activated processes that occur at rates comparable to the microwave frequency dominate the relaxation at tumbling correlation times longer than about 2 ns at X-band and S-band. The contributions from the thermally activated process-(es) and from modulation of nitrogen nuclear hyperfine anisotropy increase as the microwave frequency is decreased, and the contribution from hyperfine anisotropy dominates at L-band.