Development and characterization of a promising fluorine-18 labelled radiopharmaceutical for in vivo imaging of fatty acid amide hydrolase

Abstract

Fatty acid amide hydrolase (FAAH), the enzyme responsible for terminating signaling by the endocannabinoid anandamide, plays an important role in the endocannabinoid system, and FAAH inhibitors are attractive drugs for pain, addiction, and neurological disorders. The synthesis, radiosynthesis, and evaluation, in vitro and ex vivo in rat, of an 18F-radiotracer designed to image FAAH using positron emission tomography (PET) is described. Fluorine-18 labelled 3-(4,5-dihydrooxazol-2-yl)phenyl (5-fluoropentyl)carbamate, [18F]5, was synthesized at high specific activity in a one-pot three step reaction using a commercial module with a radiochemical yield of 17–22% (from [18F]fluoride). In vitro assay using rat brain homogenates showed that 5 inhibited FAAH in a time-dependent manner, with an IC50 value of 0.82 nM after a preincubation of 60 min. Ex vivo biodistribution studies and ex vivo autoradiography in rat brain demonstrated that [18F]5 had high brain penetration with standard uptake values of up to 4.6 and had a regional distribution which correlated with reported regional FAAH enzyme activity. Specificity of binding to FAAH with [18F]5 was high (>90%) as demonstrated by pharmacological challenges with potent and selective FAAH inhibitors and was irreversible as demonstrated by radioactivity measurements on homogenized brain tissue extracts. We infer from these results that [18F]5 is a highly promising candidate radiotracer with which to image FAAH in human subjects using PET and clinical studies are proceeding.

11CO bonds made easily for positron emission tomography radiopharmaceuticals

“…It was found that a linear N -alkyl substituent leads to increased potency over cyclic isomers and that the presence of a dihydrooxazole confers faster and greater brain uptake with lower nonspecific binding. These insights lead directly to the design of two fluorine-18 labeled FAAH radiotracers: [ 18 F]DOPP 75,76 and [ 18 F]FCHC. 77 Both show greater brain uptake in vivo and sensitivity to FAAH activity in vitro than [ 11 C]CURB.…”

A Double Whammy: Targeting Both Fatty Acid Amide Hydrolase (FAAH) and Cyclooxygenase (COX) To Treat Pain and Inflammation

“…The compound was administered by intraperitoneal injection in rats 30 min prior to the administration of [ 18 F]-3-(4,5-dihydrooxazol-2-yl)phenyl (5-fluoropentyl)carbamate ( 18 F-DOPP) an irreversible and brain penetrant 18 F-labelled FAAH inhibitor. [63] It was postulated that pretreatment with 3 would prevent the interaction of 18 F-DOPP with FAAH. [64] However, administration of 3 at two different dosages had no effect on brain labeling with 18 F-DOPP, which is suggestive of limited penetration.…”

Radiosynthesis and ex vivo evaluation of [11C-carbonyl]carbamate- and urea-based monoacylglycerol lipase inhibitors

“…To our knowledge, two of those radiotracers have been evaluated in humans, the irreversible radioligand [ 11 C]CURB, which has been validated in modeling studies [18], and [ 11 C]MK-3168, a reversible radioligand prepared at Merck [20]. While clinical translation is planned for two additional FAAH radiotracers [17,19], it is noteworthy that there have been no reported attempts to prepare MAGL radiotracer for imaging studies.…”