Synthesis and evaluation of N-(4-[11C]methoxyphenethyl) linoleoylamide as a FAAH PET tracer

Objectives Fatty acid amide hydrolase (FAAH) is an enzyme responsible for terminating endocannabinoid signaling. It may play an important role in several neuropsychiatric disorders. At present, no radioligand is available for mapping FAAH in vivo. We have synthesized and evaluated N-(4-[¹¹C]methoxyphenethyl)linoleoylamide (1) as a potential metabolic trapping tracer for in vivo study of brain FAAH.

Methods Cold compound 1 and desmethyl-1 were synthesized in a 2 steps reaction. Labeling of 1 was done by heating 3 µmol desmethyl-1 with [¹¹C]MeI in 250 µl DMF at 55°C for 10 min in the presence of 1 µl TBAH. 1 was purified by RP-HPLC. Interaction with recombinant FAAH, and the biodistribution and metabolism of 1 in wild type (WT) and FAAH knock-out (KO) mice were studied.

Results 'Cold'-1 displayed in vitro interaction with FAAH as a substrate. Desmethyl-1 was synthesized with 77% yield and alkylated with [¹¹C]MeI (RCY 47%) to provide 1. 1 demonstrated brain uptake (1.44% ID/g at 1 min in WT and 0.643%ID/g at 1min in KO mice) with high blood activity at all time points. Metabolite studies showed rapid metabolisation of 1 both in blood (49.5, 7.9, 8.1% intact product at 1, 10, 30 min p.i.) and brain (47.6, 6.9, 3.0% at 1, 10, 30 min p.i.) in WT. Slower metabolisation was detected in KO in blood (89.1, 36.3, 17.7% intact product at 1, 10, 30 min p.i.) and brain (80.9, 55.1, 31.0% at 1, 10, 30 min p.i.).

Conclusions 1 interacts with FAAH as a substrate, was successfully labeled with ¹¹C and demonstrates moderate brain uptake. The metabolite profile should be further evaluated. Together these data suggest that 1 can serve as an entry point to the preparation of FAAH imaging agents.