Characterization of a Novel M4 PAM PET Radioligand [11C]PF06885190 in Nonhuman Primates (NHP)

Muscarinic acetylcholine receptors (mAChR), including M4, draw attention as therapeutic targets for several neurodegenerative diseases including Alzheimer’s disease (AD). PET imaging of M4 positive allosteric modulator (PAM) allows qualification of the distribution as well as the expression of this receptor under physiological conditions and thereby helps to assess the receptor occupancy (RO) of a drug candidate. In this study, our aims were (a) to synthesize a novel M4 PAM PET radioligand [11C]PF06885190 (b) to evaluate the brain distribution of [(11)C]PF06885190 in nonhuman primates (NHP) and (c) to analyze its radiometabolites in the blood plasma of NHP. Radiolabeling of [(11)C]PF06885190 was accomplished via N-methylation of the precursor. Six PET measurements were performed using two male cynomolgus monkeys, where three PET measurements were at baseline, two after pretreatment with a selective M4 PAM compound CVL-231 and one after pretreatment with donepezil. The total volume of distribution (V(T)) of [(11)C]PF06885190 was examined using Logan graphical analysis with arterial input function. Radiometabolites were analyzed in monkey blood plasma using gradient HPLC system. Radiolabeling of [(11)C]PF06885190 was successfully accomplished and the radioligand was found to be stable in the formulation, with radiochemical purity exceeding 99% 1 h after the end of the synthesis. [(11)C]PF06885190 was characterized in the cynomolgus monkey brain where a moderate brain uptake was found at the baseline condition. However, it showed fast wash-out as it dropped to half of the peak at around 10 min. Change of V(T) from baseline was around −10% after pretreatment with a M4 PAM, CVL-231. Radiometabolite studies showed relatively fast metabolism. Although sufficient brain uptake of [(11)C]PF06885190 was observed, these data suggest that [(11)C]PF06885190 might have too low specific binding in the NHP brain to be further applied in PET imaging.