Proof of mechanism studies of a novel serotonin transporter blocker using [¹¹C]DASB micro-PET and in vivo microdialysis studies

Objectives DA-8031 is a novel compound that potentially blocks serotonin transporters (SERTs). As proof of mechanism studies for this investigational drug, we measured SERT occupancy by DA-8031 and DA-8031-induced changes in extracellular serotonin levels in the rat brain using [¹¹C]DASB micro-PET and in vivo microdialysis, respectively.

Methods A series of four dynamic [¹¹C]DASB micro-PET scans were performed 1 wk apart for each of four rats 30 min after oral pretreatment with graded doses of DA-8031 (vehicle only, 10, 30 and 100 mg/kg). SERT occupancy by the doses of DA-8031 in the raphe nucleus was determined using binding potentials (BP_ND) for [¹¹C]DASB calculated by the multilinear reference tissue model. In another set of experiment, extracellular serotonin levels were monitored in the dorsal raphe nucleus of freely moving rats (n = 4 for each dose) following a single oral administration of DA-8031 (10-100 mg/kg) using in vivo microdialysis.

Results PET data indicated a reduction of [¹¹C]DASB binding to SERTs in the raphe nucleus as a function of the dose of DA-8031. SERT occupancy by the doses of DA-8031 (10-100 mg/kg) ranged between 31% and 84%. The DA-8031 dose-SERT occupancy relationship was well predicted by a sigmoid E_max model and the ED₅₀ estimate for SERT occupancy was 13.5 mg/kg, which was comparable to the ED₅₀ for behavioral modification in preclinical tests (10.0 mg/kg). In vivo microdialysis showed that DA-8031 produced a dose-dependent increase in extracellular serotonin levels in the dorsal raphe nucleus (33-81% increase for doses of 10-100 mg/kg).

Conclusions These preclinical data provide proof of mechanism for DA-8031 as a serotonin transporter blocker, warranting further clinical trials. They also offer insight into optimal drug dosing exerting therapeutic effects and minimizing adverse effects in humans.