Neil Saigal

Evaluation of serotonin 5-HT1A receptors in rodent models using [18F]mefway PET

Serotonin 5‐HT1A receptors have been investigated in various CNS disorders, including epilepsy, mood disorders, and neurodegeneration. [18F]Mefway (N‐{2‐[4‐(2′‐methoxyphenyl)piperazinyl]ethyl}‐N‐(2‐pyridyl)‐N‐(cis/trans−4′‐[18F]fluoromethylcyclohexane)‐carboxamide) has been developed as a suitable positron emission tomography (PET) imaging agent for these receptors. We have now evaluated the suitability of [18F]trans‐mefway in rat and mouse models using PET and computerized tomography (CT) imaging and corroborated with ex vivo and in vitro autoradiographic studies.

Synthesis and Evaluation of 18F-FE-PEO in Rodents: An 18F-Labeled Full Agonist for Opioid Receptor Imaging

We have investigated the opioid receptor (OR) agonist (20R)-4,5-α-epoxy-6-(2-18F-fluoroethoxy)-3-hydroxy-α,17-dimethyl-α-(2-phenyleth-1-yl)-6,14-ethenomorphinan-7-methanol (18F-FE-PEO) as a candidate OR PET ligand. This tracer is attractive because it combines 18F labeling, is suited to the slow kinetics of high-affinity ligands, and has agonist binding, which has been shown to be more sensitive to changes in OR occupation than is antagonist binding. Methods: Agonist potency and off-target binding were investigated in vitro, and autoradiographic studies on rat brain sections were used to assess binding patterns. Quantification of the tracer in vivo was investigated using small-animal PET in rats with blood sampling. Results: 18F-FE-PEO was obtained by direct nucleophilic radiofluorination and subsequent deprotection with a yield of 28% ± 15%, a specific activity of 52–224 MBq/nmol, and a radiochemical purity of more than 97% (90 min from end of bombardment). In vitro studies showed it to be a full agonist ligand, which selectively binds to OR with high affinity, although it is not selective to a single OR subtype (inhibition constant, 0.4–1.6 nM across OR subtypes). Autoradiography binding patterns were consistent with the known distribution of OR, although nondisplaceable signal typically constituted one third of the signal in OR-dense regions. Although metabolites were present in blood (∼40% of plasma radioactivity was nonparent 3 h after injection), no significant metabolite fraction was found in brain tissue, aiding PET quantification. A plasma input 2-tissue-compartment model provided good fits to the PET data, and regional distribution volumes from the latter correlated well with those from Logan plot analysis (r2 = 0.98). The cerebellum had the lowest distribution volume, but the time–activity curve data could not be adequately fitted with a 1-tissue-compartment model. Reference tissue models using the cerebellum as the reference region did not provide good fits to the data, so blood-based kinetic analysis is recommended. Conclusion: As the first 18F-labeled OR agonist ligand, 18F-FE-PEO is a useful addition to the existing OR ligand portfolio.

Comparative assessment of (18) F-Mefway as a serotonin 5-HT1A receptor PET imaging agent across species: Rodents, nonhuman primates, and humans.

We have developed 18F‐trans‐Mefway (18F‐Mefway) for positron emission tomography (PET) imaging studies of serotonin 5‐HT1A receptors which are implicated in various brain functions. Translation of imaging the 5‐HT1A receptor in animal models to humans will facilitate an understanding of the role of the receptor in human brain disorders. We report comparative brain distribution of 18F‐Mefway in normal mice, rats, monkeys, and healthy human volunteers. Mefway was found to be very selective, with subnanomolar affinity for the 5‐HT1A receptor. Affinities of >55 nM were found for all other human‐cloned receptor subtypes tested. Mefway was found to be a poor substrate (>30 μM) for the multidrug resistance 1 protein, suggesting low likelihood of brain uptake being affected by P‐glycoprotein. Cerebellum was used as a reference region in all imaging studies across all species due to the low levels of 18F‐Mefway binding. Consistent binding of 18F‐Mefway in cortical regions, hippocampus, and raphe was observed across all species. 18F‐Mefway in the human brain regions correlated with the known postmortem distribution of 5‐HT1A receptors. Quantitation of raphe was affected by the resolution of the PET scanners in rodents, whereas monkeys and humans showed a raphe to cerebellum ratio of approximately 3. 18F‐Mefway appears to be an effective 5‐HT1A receptor imaging agent in all models, including humans. 18F‐Mefway therefore may be used to quantify 5‐HT1A receptor distribution in brain regions for the study of various CNS disorders. J. Comp. Neurol. 524:1457–1471, 2016.