A PET study comparing receptor occupancy by five selective cannabinoid 1 receptor antagonists in non-human primates

PET molecular imaging for pathophysiological visualization in Alzheimer's disease

Alzheimer's disease (AD) is the most common dementia worldwide. The exact etiology of AD is unclear as yet, and no effective treatments are currently available, making AD a tremendous burden posed on the whole society. As AD is a multifaceted and heterogeneous disease, and most biomarkers are dynamic in the course of AD, a range of biomarkers should be established to evaluate the severity and prognosis. Positron emission tomography (PET) offers a great opportunity to visualize AD from diverse perspectives by using radiolabeled agents involved in various pathophysiological processes; PET imaging technique helps to explore the pathomechanisms of AD comprehensively and find out the most appropriate biomarker in each AD phase, leading to a better evaluation of the disease. In this review, we discuss the application of PET in the course of AD and summarized radiolabeled compounds with favorable imaging characteristics.

Central Nervous System Trial Failures: Using the Fragile X Syndrome-mGluR5 Drug Target to Highlight the Complexities of Translating Preclinical Discoveries Into Human Trials

PURPOSE/BACKGROUND

Drug trials of the central nervous system(CNS) have been plagued with uninformative failures, often because of the difficulties of knowing definitively whether dosing achieved was sufficient to modulate the intended CNS target at adequate concentrations to produce pharmacodynamic or dose-related changes in readouts of brain function. Key design elements can be introduced into early-stage trials to get at this issue.

METHODS/PROCEDURES

This commentary builds on a review of earlier clinical studies in Fragile X syndrome to explore the extent to which the chain of evidence is in place to allow for interpretation of the results as ruling in or out the utility of modulating one or another molecular target to treat this disorder. Recent and current biomarker studies in Fragile X syndrome occurring subsequent to the clinical studies are reviewed to see if they might address any chain of evidence gaps.

FINDINGS/RESULTS

Despite the strong preclinical basis for targeting molecular mechanisms, the lack of efficacy seen in clinical studies remains uninterpretable, with regard to ruling in or out the utility of targeting the mechanism in a clinical population, given the absence of studies, which address whether doses of administered drug impacted the targeted brain mechanism.

IMPLICATIONS/CONCLUSIONS

The value of pursuing clinical studies of compounds targeted to novel mechanisms in the absence of clinical pharmacological evidence of some anticipated mediating pharmacokinetic/pharmacodynamic signals is questionable. One or more biomarkers of a drug effect on brain function are needed to establish dose dependent CNS effects that allow one to interpret clinical results as ruling in or out a mechanism and providing a firm basis for continuing or not, as well as informing dose selection in any clinical efficacy trials. Initiatives to address this general need in pediatric psychopharmacology are highlighted.

Radiolabeling with [11C]HCN for Positron emission tomography

Hydrogen cyanide (HCN) is a versatile synthon for generating carbon‑carbon and carbon-heteroatom bonds. Unlike other one-carbon synthons (i.e., CO, CO2), HCN can function as a nucleophile (as in potassium cyanide, KCN) and an electrophile (as in cyanogen bromide, (CN)Br). The incorporation of the CN motif into organic molecules generates nitriles, hydantoins and (thio)cyanates, which can be converted to carboxylic acids, aldehydes, amides and amines. Such versatile chemistry is particularly attractive in PET radiochemistry where diverse bioactive small molecules incorporating carbon-11 in different positions need to be produced. The first examples of making [11C]HCN for radiolabeling date back to the 1960s. During the ensuing decades, [11C]cyanide labeling was popular for producing biologically important molecules including 11C-labeled α-amino acids, sugars and neurotransmitters. [11C]cyanation is now reemerging in many PET centers due to its versatility for making novel tracers. Here, we summarize the chemistry of [11C]HCN, review the methods to make [11C]HCN past and present, describe methods for labeling different types of molecules with [11C]HCN, and provide an overview of the reactions available to convert nitriles into other functional groups. Finally, we discuss some of the challenges and opportunities in [11C]HCN labeling such as developing more robust methods to produce [11C]HCN and developing rapid and selective methods to convert nitriles into other functional groups in complex molecules.

Peripherally Selective CB1 Receptor Antagonist Improves Symptoms of Metabolic Syndrome in Mice

Metabolic syndrome (MetS) is a complex disorder that stems from the additive effects of multiple underlying causes such as obesity, insulin resistance, and chronic low-grade inflammation. The endocannabinoid system plays a central role in appetite regulation, energy balance, lipid metabolism, insulin sensitivity, and β-cell function. The type 1 cannabinoid receptor (CB1R) antagonist SR141716A (rimonabant) showed promising antiobesity effects, but its use was discontinued due to adverse psychiatric events in some users. These adverse effects are due to antagonism of CB1R in the central nervous system (CNS). As such, CNS-sparing CB1R antagonists are presently being developed for various indications. In this study, we report that a recently described compound, 3-{1-[8-(2-chlorophenyl)-9-(4-chlorophenyl)-9H-purin-6-yl]piperidin-4-yl}-1-[6-(difluoromethoxy)pyridin-3-yl]urea (RTI1092769), a pyrazole based weak inverse agonist/antagonist of CB1 with very limited brain exposure, improves MetS related complications. Treatment with RTI1092769 inhibited weight gain and improved glucose utilization in obese mice maintained on a high fat diet. Hepatic triglyceride content and steatosis significantly improved with treatment. These phenotypes were supported by improvement in several biomarkers associated with nonalcoholic fatty liver disease (NAFLD) and nonalcoholic steatohepatitis (NASH). These results reinforce the idea that CB1 antagonists with limited brain exposure should be pursued for MetS and other important indications.

Positron Emission Tomography Imaging of the Endocannabinoid System: Opportunities and Challenges in Radiotracer Development

The endocannabinoid system (ECS) is involved in a wide range of biological functions and comprises cannabinoid receptors and enzymes responsible for endocannabinoid synthesis and degradation. Over the past 2 decades, significant advances toward developing drugs and positron emission tomography (PET) tracers targeting different components of the ECS have been made. Herein, we summarized the recent development of PET tracers for imaging cannabinoid receptors 1 (CB1R) and 2 (CB2R) as well as the key enzymes monoacylglycerol lipase (MAGL) and fatty acid amide hydrolase (FAAH), particularly focusing on PET neuroimaging applications. State-of-the-art PET tracers for the ECS will be reviewed including their chemical design, pharmacological properties, radiolabeling, as well as preclinical and human PET imaging. In addition, this review addresses the current challenges for ECS PET biomarker development and highlights the important role of PET ligands to study disease pathophysiology as well as to facilitate drug discovery.

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.

Targeting Peripheral CB1 Receptors Reduces Ethanol Intake via a Gut-Brain Axis

Endocannabinoids acting on the cannabinoid-1 receptor (CB₁R) or ghrelin acting on its receptor (GHS-R_1A) both promote alcohol-seeking behavior, but an interaction between the two signaling systems has not been explored. Here, we report that the peripheral CB₁R inverse agonist JD5037 reduces ethanol drinking in wild-type mice but not in mice lacking CB₁R, ghrelin peptide or GHS-R_1A. JD5037 treatment of alcohol-drinking mice inhibits the formation of biologically active octanoyl-ghrelin without affecting its inactive precursor desacyl-ghrelin. In ghrelin-producing stomach cells, JD5037 reduced the level of the substrate octanoyl-carnitine generated from palmitoyl-carnitine by increasing fatty acid β-oxidation. Blocking gastric vagal afferents abrogated the ability of either CB₁R or GHS-R_1A blockade to reduce ethanol drinking. We conclude that blocking CB₁R in ghrelin-producing cells reduces alcohol drinking by inhibiting the formation of active ghrelin and its signaling via gastric vagal afferents. Thus, peripheral CB₁R blockade may have therapeutic potential in the treatment of alcoholism.

Analysis of PK11195 concentrations in rodent whole blood and tissue samples by rapid and reproducible chromatographic method to support target-occupancy PET studies

In Positron Emission Tomography (PET) research, it is important to assess not only pharmacokinetics of a radiotracer in vivo, but also of the drugs used in blocking/displacement PET studies. Typically, pharmacokinetic/pharmacodynamic (PK/PD) analyses of drugs used in rodent PET studies are based on population average pharmacokinetic profiles of the drugs due to limited blood volume withdrawal while simultaneously maintaining physiological homeostasis. This likely results in bias of PET data quantification, including unknown bias of target occupancy (TO) measurements. This study aimed to develop a High Performance Liquid Chromatography (HPLC) method for PK/PD quantification of drugs used in preclinical rodent PET research, specifically the translocator 18 kDa protein (TSPO) selective drug, PK11195, that used sub-millilitre blood volumes. The lowest detection limit for the proposed HPLC method ranged between 7.5 and 10 ng/mL depending on the method used to calculate the limit of detection, and the measured average relative standard deviation for intermediate precision was equal to 17.2%. Most importantly, we were able to demonstrate a significant difference between calculated PK11195 concentrations at 0.5, 1, 2, 3, 5, 15 and 30 min post-administration and individually measured whole blood levels (significance level range from p < 0.05 to p < 0.001; one-way ANOVA, Dunnet's post hoc test, p < 0.05). The HPLC method developed here uses sub-millilitre sample volumes to reproducibly assess PK/PD of PK11195 in rodent blood. This study highlights the importance of individually measured PK/PD drug concentrations when quantifying the TO from blocking/displacement rodent PET experiments.

Relationship between receptor occupancy and the antinociceptive effect of mu opioid receptor agonists in male rats

The analgesic mechanisms of mu opioid receptor (MOR) agonists, including receptor occupancy at the site of action, are not completely understood. The aims of the present study were to evaluate: (i) receptor occupancy in the rat brain after administration of MOR agonists; (ii) the relationship between occupancy and the antinociceptive effect. Morphine (2 or 4 mg/kg) or oxycodone (1 or 3 mg/kg) was subcutaneously administered to rats. The antinociceptive effect of these drugs was measured by the hot-plate test. MOR occupancy in the thalamus was assessed by conducting an ex vivo receptor binding assay using [³H] [D-Ala², N-MePhe⁴, Gly-ol]-enkephalin, followed by autoradiographic analysis. Both drugs produced antinociception in a dose-dependent manner, and these effects disappeared after the time point at which the maximal effect was elicited. Thalamic MOR occupancy was observed in a dose-dependent manner at the time point at which maximal antinociception was elicited, and relatively low occupancy was observed when the antinociceptive effect was decreasing. Good correlation between thalamic MOR occupancy and the antinociceptive effect was observed. These findings provide direct evidence for the receptor occupancy of MOR agonists at the site of action and its relationship with the analgesic effect.

[carbonyl-11C]4-Fluoro-N-methyl-N-(4-(6-(methylamino)pyrimidin-4-yl)thiazol-2-yl)benzamide ([11C]FIMX) is an effective radioligand for PET imaging of metabotropic glutamate receptor 1 (mGluR1) in monkey brain

INTRODUCTION

Metabotropic glutamate subtype receptor 1 (mGluR1) is implicated in several neuropsychiatric disorders and is a target for drug development. [(18)F]FIMX ([(18)F]4-fluoro--N-methyl-N--(4-(6-(methylamino)pyrimidin-4-yl)thiazol-2-yl)benzamide) is an effective radioligand for imaging brain mGluR1 with PET. A similarly effective radioligand with a shorter half-life would usefully allow PET studies of mGluR1 at baseline and after pharmacological or other challenge on the same day. Here we describe the preparation of [(11)C]FIMX for evaluation in monkey with PET.

METHODS

[(11)C]FIMX was prepared via Pd-promoted carbonylation of 1-fluoro-4-iodobenzene with [(11)C]carbon monoxide, aminolysis of the [(11)C]acyl-palladium complex with the requisite Boc-protected amine, and deprotection with HCl in THF. PET scans of [(11)C]FIMX injected into a monkey were performed at baseline and after preblock of mGluR1 with measurement of the arterial input function.

RESULTS

The radiosynthesis required 42 min and gave [(11)C]FIMX in about 5% overall decay-corrected radiochemical yield and with a specific activity of about 100 GBq/μmol. PET in rhesus monkey at baseline showed that radioactivity peaked high in receptor-rich cerebellum and much lower in receptor-poor occipital cortex. Radioactivity in cerebellum declined to 32% of peak at 85 min. VT at baseline appeared stable in all brain regions after 60 min. Under mGluR1 pre-blocked condition, radioactivity uptake in all regions declined more rapidly to a low level. Receptor pre-block reduced VT from 13.0 to 1.5 in cerebellum and from 2.9 to 1.4 in occipital cortex.

CONCLUSION

[(11)C]FIMX is an effective radioligand for imaging mGluR1 in monkey with PET.