Translational Evaluation of PET Radiotracers

National Institute Of Mental Health

Investigators

Linked publications, trials & patents

Abstract

1) Development of [11C]ZTP-1 for imaging of phosphodiesterase 4B (PDE4B) For several years, the MIB has been investigating phosphodiesterase type 4 subtype B (PDE4B), an enzyme that selectively hydrolyzes cyclic adenosine monophosphate (cAMP) to enact numerous downstream signaling events. PDE4B is widely expressed in the brain and is implicated in several neuropsychiatric disorders. Moreover, PDE4B inhibition shows anti-inflammatory and antidepressant-like effects in animal studies. [18F]PF-06445974 was developed to image human brain PDE4B using PET, thereby providing a tool for pathophysiologic studies and drug development. However, a radioligand labeled with shorter-lived [11C] would be a crucial alternative for studies that require more than one administration into the same imaging participant on a single day. Towards this end, we identified 8-Cyclopropyl-10-(3,5-difluoro-4-(methoxy)phenyl)-7,8-dihydropyrido[2',3':4,5]pyrrolo[1,2-a]pyrazin-9(6H)-1 (ZTP-1) as possessing many favorable properties for development as an [11C]-labeled PET radioligand, including high PDE4B inhibitory potency, moderate computed lipophilicity, and a methoxy group as a potential labeling site. [11C]ZTP-1 was readily obtained by 11C methylation of a synthesized O-desmethyl precursor. PET imaging of rat and rhesus monkey brains was performed with [11C]ZTP-1 at baseline and after administration of PDE4B- and PDE4D-selective inhibitors. Radiometabolite profiles for [11C]ZTP-1 were also determined ex vivo in rat plasma and brains. We found that [11C]ZTP-1 was obtained in a high activity yield and with high molar activity. Rat and monkey PET imaging showed high whole-brain radioactivity uptake with subsequent gradual washout. Challenge experiments verified a high and PDE4B-selective PET signal in rat and monkey brains. Ex vivo rat brain uptake of [11C]ZTP-1 showed less than 1% radiometabolite contamination at 30 minutes. Total distribution volume measures in monkey brains quickly reached stability. Collectively, [11C]ZTP-1 appears to be a promising, shorter-lived alternative to [18F]PF-06445974 for quantifying brain PDE4B in rodents and nonhuman primates with PET—particularly because it did not generate radiometabolites that accumulate in monkey brain—and warrants further investigation in humans. 2) PET imaging of cannabinoid type-2 receptors (CB2Rs) Cannabinoid receptors are part of the endocannabinoid system and play a crucial role in regulating various physiological processes. Given that microglial activation and neuroinflammation are upstream mechanisms underlying the pathogenesis of neurodegenerative diseases like Alzheimer’s disease, Parkinson’s disease, amyotrophic lateral sclerosis, and Huntington’s disease, CB2Rs have potential as drug or early detection targets for these disorders. In particular, CB2Rs have recently been considered as potential therapeutic targets in diseases with few or no currently approved therapies, such as neuropathic pain and neurodegenerative conditions (including Alzheimer’s disease). The lack of currently approved treatments for such disorders underscores the importance of early detection to maximize the effectiveness of available symptomatic treatments, with the goal of improving patient and caregiver quality of life and contributing to broader public health efforts. Positron emission tomography (PET) imaging of CB2Rs may aid in such detection efforts. To date, however, no effective, selective, and high-affinity PET radioligands for CB2Rs have been developed. PET imaging of cannabinoid type-2 receptors (CB2Rs) in the healthy brain remains challenging due to low receptor density and the unavailability of radiotracers with high affinity and selectivity. Because some carbon-deuterium bonds are less susceptible than carbon-proton bonds to enzymatic cleavage, deuteration of [18F]JHU94620 was pursued to potentially slow its metabolism. This international collaborative study: 1) evaluated the sensitivity of a heavily deuterated version of the agonist [18F]JHU94620 ([18F]JHU94620-d8) to detect brain CB2Rs in healthy Sprague-Dawley rats and monkeys and in a rat model of inflammation; 2) assessed the metabolic stability of [18F]JHU94620 and [18F]JHU94620-d8 in whole blood, plasma, and brain of control (FVB) mice and in the whole blood and plasma of monkeys; and 3) investigated the efflux transporter substrate liability of [18F]JHU94620-d8. Broadly, we observed species differences in the metabolic stability of [18F]JHU94620 and its deuterated analog. Deuteration of [18F]JHU94620 did not significantly affect its uptake in the brains of FVB mice, Sprague Dawley rats, or monkeys, nor did it affect metabolic stability, except in FVB mice. Whereas defluorination was observed mainly for [18F]JHU94620 in mice, no or low differences were observed between [18F]JHU94620 and [18F]JHU94620-d8 in rats and monkeys. Moreover, [18F]JHU94620-d8 had insufficient sensitivity to detect neuroinflammation in rats after unilateral injection of the inflammogen lipopolysaccharide (LPS). [18F]JHU94620-d8 was also found to be a moderate substrate for efflux transporters in monkeys but not in mice. Taken together, the findings suggest that the sensitivity of [18F]JHU94620-d8 was inadequate to detect the low density of CB2Rs that are in the agonist-preferring state in the brain. The results suggest that a radiolabeled antagonist, which does not yet exist, would be more likely than an agonist to measure the low density of CB2Rs, providing a much needed future direction for this important avenue of exploration.