Background

[PubMed]

Fatty acid amide hydrolase (FAAH) is an integral membrane-bound serine hydrolase and a part of the endocannabinoid system (ECS), which comprises the cannabinoid receptors (CB1 and CB2), endogenous ligands termed endocannabinoids (anandamide and oleamide), transporters, and enzymes (1, 2). FAAH plays a key role in the hydrolysis of a number of primary and secondary fatty acid amides, controlling the levels of the neuromodulatory endocannabinoids in the ECS (3, 4). FAAH is widely expressed in many tissues, with the highest levels in the liver and brain (5). Genetic or pharmacological inactivation of FAAH in the brain leads to analgesic, anti-inflammatory, anxiolytic, and anti-depression effects in animal models (6-8).

Cyclohexylcarbamic acid 3'-carbamoylbiphenyl-3-yl ester (URB597) is an irreversible, substrate-like inhibitor of FAAH involving carbamylation of the catalytic nucleophilic Ser²⁴¹ and the O-biaryl group as the leaving group (9-11). On the basis of the structure of URB597, Wyffels et al. (12) prepared biphenyl-3-yl-4-[¹¹C]methoxyphenylcarbamate ([¹¹C]-1) for in vivo positron emission tomography (PET) imaging studies of the brain FAAH in mice. It was proposed that the carbamylation of Ser²⁴¹ would leave the [¹¹C]-methoxyanilino group bound to the FAAH for visualization of FAAH in the brain. However, the results of in vitro and ex vivo studies indicated that [¹¹C]-1 is a reversible inhibitor of FAAH, and the rapid brain washout of the tracer limits its utility as a PET agent for in vivo measurements of FAAH. Wilson et al. (13) reported the ¹¹C-radiolabeling of a close analog of URB597, 6-hydroxy-[1,1'-biphenyl]-3-yl-cyclohexylcarbamate (URB694), yielding [¹¹C-carbonyl]URB694 ([¹¹C]CURB) for PET imaging of FAAH in the brain. [¹¹C]CURB showed good brain accumulation with regional heterogeneity, irreversibility, and specific binding to FAAH in vivo in rats. Rusjan et al. (14) reported [¹¹C]CURB PET studies in human brain.

Synthesis

[PubMed]

Wilson et al. (13) synthesized [¹¹C]CURB by reaction of [¹¹C]CO₂ with cyclohexylamine for 1 min at room temperature, followed by addition of 2-phenyl-1,4-dihydroquinone. [¹¹C]CURB was purified with high-performance liquid chromatography. The specific activity of [¹¹C]CURB was 80–160 GBq/μmol (2.2–4.4 Ci/µmol) at the time of injection. The radiochemical yield, radiochemical purity, and total synthesis time were not reported. The LogD_7.4 value of [¹¹C]CURB was 2.8 ± 0.1.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Mor et al. (11) showed that unlabeled URB694 and URB597 inhibited the hydrolysis of [³H]-anandamide by FAAH with IC₅₀ values of 30.0 ± 5.8 and 7.7 ± 1.5 nM, respectively. Both compounds were irreversibly bound to FAAH.

Animal Studies

Human Studies

[PubMed]

Rusjan et al. (14) reported [¹¹C]CURB PET studies in human brain using kinetic modeling in six healthy subjects (age 19-53 yr) scanned with arterial blood sampling for 90 min. [¹¹C]CURB was 68%, 48%, and 37% intact in the plasma at 8, 20, and 90 min, respectively. High standard uptake values (SUVs) were observed in the putamen (4.3 ± 0.8), thalamus (4.2 ± 0.6), and anterior cingulate cortex (3.5 ± 0.5). Kinetic parameters were estimated regionally using a one-tissue compartment model (TCM), a 2-TCM, a 2-TCMi (with irreversible trapping), and an irreversible 3-TCM. The 2-TCMi provided the best identifiability of PET outcome measures among the models analyzed (coefficient of variation (COV) of the net influx constant K_i and the composite parameter λk₃ (λ=K₁/k₂) <5%, and COV (k₃) <10%). Binding of [¹¹C]CURB in the healthy human brain can be well identified with an irreversible two-tissue compartment kinetic model using 60 minutes of scan data.

References

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12.

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14.

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