[18F]MK-9470 PET measurement of cannabinoid CB1 receptor availability in chronic cannabis users.

Δ(9) -Tetrahydrocannabinol, the main psychoactive component of cannabis, exerts its central effects through activation of the cerebral type 1 cannabinoid (CB1 ) receptor. Pre-clinical studies have provided evidence that chronic cannabis exposure is linked to decreased CB1 receptor expression and this is thought to be a component underlying drug tolerance and dependence. In this study, we make first use of the selective high-affinity positron emission tomography (PET) ligand [(18) F]MK-9470 to obtain in vivo measurements of cerebral CB1 receptor availability in 10 chronic cannabis users (age = 26.0 ± 4.1 years). Each patient underwent [(18) F]MK-9470 PET within the first week following the last cannabis consumption. A population of 10 age-matched healthy subjects (age = 23.0 ± 2.9 years) was used as control group. Parametric modified standardized uptake value images, reflecting CB1 receptor availability, were calculated. Statistical parametric mapping and volume-of-interest (VOI) analyses of CB1 receptor availability were performed. Compared with controls, cannabis users showed a global decrease in CB1 receptor availability (-11.7 percent). VOI-based analysis demonstrated that the CB1 receptor decrease was significant in the temporal lobe (-12.7 percent), anterior (-12.6 percent) and posterior cingulate cortex (-13.5 percent) and nucleus accumbens (-11.2 percent). Voxel-based analysis confirmed this decrease and regional pattern in CB1 receptor availability in cannabis users. These findings revealed that chronic cannabis use may alter specific regional CB1 receptor expression through neuroadaptive changes in CB1 receptor availability, opening the way for the examination of specific CB1 -cannabis addiction interactions which may predict future cannabis-related treatment outcome.

Automated one-step radiosynthesis of the CB1 receptor imaging agent [(18) F]MK-9470.

The fluorine-18-labeled positron emission tomography (PET) radiotracer [(18) F]MK-9470 is a selective, high affinity inverse agonist that has been used to image the cannabinoid receptor type 1 in human brain in healthy and disease states. This report describes a simplified, one-step [(18) F]radiofluorination approach using a GE TRACERlab FXFN module for the routine production of this tracer. The one-step synthesis, by [(18) F]fluoride displacement of a primary tosylate precursor, gives a six-fold increase in yield over the previous two-step method employing O-alkylation of a phenol precursor with 1,2-[(18) F]fluorobromoethane. The average radiochemical yield of [(18) F]MK-9470 using the one-step method was 30.3 ± 11.7% (n = 12), with specific activity in excess of 6 Ci/µmol and radiochemical purity of 97.2 ± 1.5% (n = 12), in less than 60 min. This simplified, high yielding, automated process was validated for routine GMP production of [(18) F]MK-9470 for clinical studies.

Quantification of the Cannabinoid Type 1 Receptor Availability in the Mouse Brain.

Introduction: The endocannabinoid system is involved in several diseases such as addictive disorders, schizophrenia, post-traumatic stress disorder, and eating disorders. As often mice are used as the preferred animal model in translational research, in particular when using genetically modified mice, this study aimed to provide a systematic analysis of in vivo cannabinoid type 1 (CB1) receptor ligand-binding capacity using positron emission tomography (PET) using the ligand [18F]MK-9470. We then compared the PET results with literature data from immunohistochemistry (IHC) to review the consistency between ex vivo protein expression and in vivo ligand binding. Methods: Six male C57BL/6J (6-9 weeks) mice were examined with the CB1 receptor ligand [18F]MK-9470 and small animal PET. Different brain regions were evaluated using the parameter %ID/ml. The PET results of the [18F]MK-9470 accumulation in the mouse brain were compared with immunohistochemical literature data. Results: The ligand [18F]MK-9470 was taken up into the mouse brain within 5 min after injection and exhibited slow kinetics. It accumulated highly in most parts of the brain. PET and IHC classifications were consistent for most parts of the telencephalon, while brain regions of the diencephalon, mesencephalon, and rhombencephalon were rated higher with PET than IHC. Conclusions: This preclinical [18F]MK-9470 study demonstrated the radioligand's applicability for imaging the region-specific CB1 receptor availability in the healthy adult mouse brain and thus offers the potential to study CB1 receptor availability in pathological conditions.

Pharmacokinetics of the cannabinoid receptor ligand [18 F]MK-9470 in the rat brain - Evaluation of models using microPET.

PURPOSE: The positron emission tomography ligand [18 F]MK-9470 is an inverse agonist that binds reversibly and with high affinity to the cannabinoid type 1 receptor. Due to its slow brain kinetics, care is required in the definition of its dissociation rates from the receptor. The goal of this study was to investigate pharmacokinetic analysis methods using an arterial input function. METHODS: Five Sprague-Dawley rats received injections of 13 to 25 MBq of [18 F]MK-9470 and were scanned over a period of 90 min. Arterial blood samples were collected throughout the scan. Data were analyzed using four different compartmental models: a reversible one-tissue model, reversible two tissue models with and without parameter constraints and an irreversible two-tissue model. The outcome values were goodness of fit measures (Akaike information criterion; standard error), pharmacokinetic modeling parameters (volume of distribution; irreversible uptake constant) and intersubject variability. RESULTS: Goodness of fit measures indicated that the experimental data are more adequately described by a two-tissue model than a one-tissue model. Differences in mean Akaike information criterion values between all two-tissue models were < 5%. Mean standard errors of model parameters were lowest for the irreversible model (range: 1% to 6%). The irreversible model delivered plausible results for all animals that were less variable compared to results of the other two-tissue models. CONCLUSIONS: A reversible two-tissue model may not deliver stable results for all animals and regions within a 90-min microPET study protocol. Stable parameters for all animals and regions are obtained when an irreversible model is used. If the acquisition time of the experiment is limited, an irreversible model provides a consistent distribution of composite outcome parameters, suggesting its suitability for use in future studies.

Whole-body biodistribution of the cannabinoid type 1 receptor ligand [18F]MK-9470 in the rat.

The endocannabinoid system participates in many processes in the body, including memory, reward, pain, motor activity, food intake, energy metabolism, and gastrointestinal functions. [18F]MK-9470 is a positron emission tomography (PET) ligand that binds with high affinity and selectivity to the cannabinoid type 1 receptor. In order to fully characterize ligand behavior, tracer uptake measured using in vivo microPET was compared with results from ex vivo tissue dissection. Twelve male Sprague-Dawley rats were divided into three subgroups and scanned over time periods of 10min, 30min and 90min using PET. Afterwards, a number of the animals' organs were dissected. Uptake of radioactivity was expressed in terms of %ID/ml and %ID/(g tissue). For comparison of in vivo and ex vivo methods, Bland-Altman plots were computed. The highest uptake of [18F]MK-9470 was found in the liver and small intestine; the brain showed less uptake, while low and unspecific binding was observed in tissue of the heart, lung, kidney and bone. In the brain, normalized uptake of [18F]MK-9470 was on average 0.25%ID/ml (range: 0.16 to 0.28%ID/ml). Bland-Altman plots revealed the best agreement between methods for the 90min acquisition protocols. High hepatic accumulation and metabolism of [18F]MK-9470 occur with mainly enteral excretion, which may vary considerably over time - a finding which may be of relevance in metabolite determination in quantitative brain studies. Comparisons between in vivo and ex vivo methods showed that whole-body distribution of [18F]MK-9470 using positron emission tomography is a preferable alternative to ex vivo biodistribution, and requires a significantly smaller number of animals.

Prenatal poly I:C age-dependently alters cannabinoid type 1 receptors in offspring: a longitudinal small animal PET study using [(18)F]MK-9470.

Evidence suggests that there is a link between the endocannabinoid system (ECS) and neuropsychiatric illnesses, including schizophrenia. Whilst the ECS has been shown to be involved in immune system regulation in various ways, it is known that infections during pregnancy can modulate the immune system of the mother and increase the risk for schizophrenia in offspring. In animal studies, maternal immune activation following administration of viral or bacterial mimics has been shown to reproduce many key structural, behavioural, and pharmacological abnormalities in offspring that resemble schizophrenia. In the present study, we used Positron Emission Tomography (PET) and [(18)F]MK-9470, a selective high-affinity inverse agonist radioligand for cannabinoid type 1 receptors (CB1R), to longitudinally assess CB1R expression in the progeny of female rats exposed to the viral mimic polyriboinosinic-polyribocytidilic acid (poly I:C) (4mg/kg i.v.) or vehicle at gestational day 15 (GD 15). PET scans were performed in offspring at postnatal days (PND) 32-42 (adolescence) and in the same animals again at PNDs 75-79 (adulthood). Sixteen regions of interest were assessed, encompassing the whole rat brain. At adolescence, offspring exposed prenatally to poly I:C had significantly lower CB1R relative Standard Uptake Values (rSUV) compared to controls in the globus pallidus (p=0.046). In adulthood, however, poly I:C exposed offspring had higher levels of CB1R rSUV in sensory cortex (p=0.034) and hypothalamus (p=0.032) compared to controls. Our results suggest that prenatal poly I:C leads to long term alterations in the integrity of the ECS that are age and region-specific. The increased CB1R expression in adulthood following poly I:C mirrors the increased CB1R observed in patients with schizophrenia in post-mortem and in vivo PET studies.