Utility of small-animal positron emission tomographic imaging of rats for preclinical development of drugs acting on the serotonin transporter

Abstract

Visualization of neurotransmission components in living small animals using positron emission tomography (PET) has the potential of contributing to the preclinical development of neuroactive drugs, although it is yet to be examined whether quantitative animal PET data on candidate compounds can be extrapolated to humans. Here, we investigated the comparability of the occupancies of serotonin transporter (5-HTT) by therapeutic agents in rat PET studies with our predetermined data from ex- vivo animal experiments and clinical PET scans. Rats were treated with varying doses of fluvoxamine and a newly developed compound, (2S)-1-[4-(3,4-dichlorophenyl) piperidin-1-yl]-3-[2-(5-methyl-1,3,4-oxadiazol-2-yl)benzo[b]furan-4-yloxy]propan-2-ol monohydrochloride (Wf-516), and underwent PET scans with [11C]3-amino-4-(2-dimethylaminomethyl-phenylsulfanyl)-benzonitrile ([11C]DASB), a selective radioligand for in-vivo quantification of 5-HTT. PET images indicated a reduction of [11C]DASB binding to 5-HTT as a function of the doses and/or plasma concentrations of fluvoxamine and Wf-516. The doses of these drugs at half-maximal effect (15.2 mg/kg and 3.1 mg/kg, respectively), determined that using binding potentials for [11C]DASB, were comparable to those estimated by our previous ex-vivo measurements in rats (4.5 mg/kg and 1.1 mg/kg, respectively), as there was only a 3-fold difference between these results. Moreover, the plasma concentration of fluvoxamine needed for 50% occupancy of central 5-HTT (6.1 ng/ml) was almost equivalent to the value determined in human PET studies (4.6 ng/ml). These findings support the view that the conjunctive use of small-animal PET and [11C]DASB facilitates a quantitative comparison of in-development drugs targeting 5-HTT with established inhibitors and a predictive estimation of their plasma concentrations exerting therapeutic effects in humans.

Presynaptic Selectivity of a Ligand for Serotonin 1A Receptors Revealed by In Vivo PET Assays of Rat Brain

“…ED 50 estimates in the hippocampus and raphe nucleus were 5.6 mg/kg and 2.4 mg/kg, respectively, and EC 50 measures in these regions were 319 ng/mL and 125 ng/mL, respectively. The occupancy by 3 mg/kg Wf-516, corresponding to ED 50 for its blockade of 5-HTTs as reported in our previous study [24], differed by approximately 100% between the hippocampus and raphe nucleus, while 3 mg/kg pindolol, close to its ED 50 value in the raphe nucleus, resulted in an approximately 50% difference in the occupancy between these two regions, suggesting a prominent regional selectivity of Wf-516 in comparison with pindolol. Additionally, administration of 30 mg/kg fluvoxamine prior to pindolol treatment, which is capable of inhibiting more than 80% of 5-HTTs according to our previous investigation [24], had no effect on 5-HT 1A receptor occupancies by pindolol (Table 1).…”

“…The occupancy by 3 mg/kg Wf-516, corresponding to ED 50 for its blockade of 5-HTTs as reported in our previous study [24], differed by approximately 100% between the hippocampus and raphe nucleus, while 3 mg/kg pindolol, close to its ED 50 value in the raphe nucleus, resulted in an approximately 50% difference in the occupancy between these two regions, suggesting a prominent regional selectivity of Wf-516 in comparison with pindolol. Additionally, administration of 30 mg/kg fluvoxamine prior to pindolol treatment, which is capable of inhibiting more than 80% of 5-HTTs according to our previous investigation [24], had no effect on 5-HT 1A receptor occupancies by pindolol (Table 1). This result implies that binding of 5-HT 1A receptor ligands was not overtly affected by a concurrent blockade of 5-HTT and consequent increase of synaptic 5-HT, supporting the view that the characteristics of the interaction between Wf-516 and 5-HT 1A receptors do not stem from its dual action on 5-HT 1A receptor and 5-HTT.…”

“…Moreover, a recent in vivo electrophysiological study using rats has indicated that Wf-516 at low and medium doses was an antagonist for presynaptic but not postsynaptic 5-HT 1A receptors [23]. Although our previous PET study of rats demonstrated in vivo binding of Wf-516 to central 5-HTTs in a dose-dependent manner [24], the pharmacological mechanisms involved in the presynaptic/postsynaptic selectivity of binding of Wf-516 to 5-HT 1A receptors in living brains had still remained to be clarified using neuroimaging assays.…”

“…Male Wistar rats at 8 weeks of age were purchased from CLEA Japan Inc. (Tokyo, Japan). Prior to the PET measurements, neuroanatomical template magnetic resonance images (MRI) of the rat brains were generated as described elsewhere [24].…”

In Vivo Measurement of the Affinity and Density of Metabotropic Glutamate Receptor Subtype 1 in Rat Brain Using 18F-FITM in Small-Animal PET

“…In addition to its diagnostic applications, PET has been used to assess new therapeutic agents for targeting neuropsychiatric and neurodegenerative disorders in the brain (19). Tracking the dynamic biodistribution of radiolabeled compounds on maps of living animal brains has been made possible by the development of high-resolution PET scanners (20,21).…”