The synthesis and in vivo evaluation of [18F]PF-9811: a novel PET ligand for imaging brain fatty acid amide hydrolase (FAAH)

Abstract

Introduction Fatty acid amide hydrolase (FAAH) is responsible for the enzymatic degradation of the fatty acid amide family of signaling lipids, including the endogenous cannabinoid (endocannabinoid) anandamide. The involvement of the endocannabinoid system in pain and other nervous system disorders has made FAAH an attractive target for drug development. Companion molecular imaging probes are needed, however, to assess FAAH inhibition in the nervous system in vivo. We report here the synthesis and in vivo evaluation of [18F]PF-9811, a novel PET ligand for non-invasive imaging of FAAH in the brain. Methods The potency and selectivity of unlabeled PF-9811 were determined by activity-based protein profiling (ABPP) both in vitro and in vivo. [18F]PF-9811 was synthesized in a 3-step, one-pot reaction sequence, followed by HPLC purification. Biological evaluation was performed by biodistribution and dynamic PET imaging studies in male rats. The specificity of [18F]PF-9811 uptake was evaluated by pre-administration of PF-04457845, a potent and selective FAAH inhibitor, 1 h prior to radiotracer injection. Results Biodistribution studies show good uptake (SUV∼0.8 at 90 min) of [18F]PF-9811 in rat brain, with significant reduction of the radiotracer in all brain regions (37%–73% at 90 min) in blocking experiments. Dynamic PET imaging experiments in rat confirmed the heterogeneous uptake of [18F]PF-9811 in brain regions with high FAAH enzymatic activity, as well as statistically significant reductions in signal following pre-administration of the blocking compound PF-04457845. Conclusions [18F]PF-9811 is a promising PET imaging agent for FAAH. Biodistribution and PET imaging experiments show that the tracer has good uptake in brain, regional heterogeneity, and specific binding as determined by blocking experiments with the highly potent and selective FAAH inhibitor, PF-04457845.

Determining target engagement in living systems

“…Here, in vivo imaging has a clear advantage in that it is non-invasive, albeit without a means to easily assess probe interactions with the broader proteome. Thus chemoproteomic and imaging approaches are complementary strategies for measuring target engagement in vivo , and these approaches can even be combined 54 .…”

11CO2 fixation: a renaissance in PET radiochemistry

“…62 Whereas a [ 18 F]fluoroethyl derivative, [ 18 F]PF-9811, was recently reported, 63 we have prepared an isotopologue by [ 11 C]CO 2 fixation, namely, [ 11 C- carbonyl ]PF-04457845 (Figure 1). The precursors for [ 11 C]CO 2 fixation are a non-nucleophilic primary aromatic amine and a cyclic secondary aliphatic amine.…”

Synthesis and Preliminary PET Imaging Studies of a FAAH Radiotracer ([11C]MPPO) Based on α-Ketoheterocyclic Scaffold

“…As shown in Figure 1, there are continued efforts in the search for suitable FAAH PET tracers, including anadamide analogs, 24 [ 11 C-methyl]URB597 analog, 25 [ 11 C-carbonyl]URB694 ([ 11 C]CURB; 1 ), 26,27 [ 18 F]DOPP ( 2 ), 28,29 [ 11 C]PF-04457845 ( 4 ), 30,31 [ 18 F]PF-9811 ( 5 ), 32 MK-3168 ( 6 ), 30 and most recently [ 11 C]MFTC ( 3 ). 33 To date, only two PET ligands have been reported in human studies, namely [ 11 C]CURB ( 1 ) 34,35 and [ 11 C]MK-3168 ( 6 ).…”