Target occupancy study and whole-body dosimetry with a MAGL PET ligand [11C]PF-06809247 in non-human primates

Background Monoacylglycerol lipase (MAGL) is a key serine hydrolase which terminates endocannabinoid signaling and regulates arachidonic acid driven inflammatory responses within the central nervous system. To develop [C-11]PF-06809247 into a clinically usable MAGL positron emission tomography (PET) radioligand, we assessed the occupancy of MAGL by an inhibitor in the non-human primate (NHP) brain. Additionally, we measured the whole-body distribution of [C-11]PF-06809247 in NHP and estimated human effective radiation doses. Methods Seven cynomolgus monkeys were enrolled for brain PET measurements. Two PET measurements along with arterial blood sampling were performed in each NHP: one baseline and one pretreatment condition with intravenous administration of PF-06818883, a pro-drug of a selective MAGL inhibitor (total of seven doses between 0.01 and 1.27 mg/kg). Kinetic parameters K-1, k(2) and k(3) were estimated by a two tissue compartment (2TC) model using metabolite corrected plasma radioactivity as the input function. k(4) was set as 0 according to the irreversible binding of [C-11]PF-06809247. K-i by 2TC and Patlak analysis were calculated as the influx constant. The target occupancy was calculated using K-i at baseline and pretreatment conditions. Two cynomolgus monkeys were enrolled for whole-body PET measurements. Estimates of the absorbed radiation dose in humans were calculated with OLINDA/EXM 1.1 using the adult male reference model. Results Radioactivity retention was decreased in all brain regions following pretreatment with PF-06818883. Occupancy was measured as 25.4-100.5% in a dose dependent manner. Whole-body PET showed high radioactivity uptake values in the liver, small intestine, kidney, and brain. The effective dose of [C-11]PF-06809247 was calculated as 4.3 mu Sv/MBq. Conclusions [C-11]PF-06809247 is a promising PET ligand for further studies of MAGL in the human brain.