Characterization of the potent 5-HT(1A/B) receptor antagonist and serotonin reuptake inhibitor SB-649915: preclinical evidence for hastened onset of antidepressant/anxiolytic efficacy

Abstract

An increase in brain serotonin (5-HT) levels is thought to be a key mechanism of action responsible for generating antidepressant efficacy. It has been proven that selective serotonin reuptake inhibitors are effective antidepressants, but the delay to therapeutic onset of these agents is thought to be due to the time required for 5-HT1A, and possibly 5-HT1B, autoreceptors to desensitize. Therefore, an agent incorporating 5-HT reuptake inhibition coupled with 5-HT1A and/or 5-HT1B autoreceptor antagonism may provide a fast-acting clinical agent. The current studies review the profile of SB-649915 (6-[(1-{2-[(2-methylquinolin-5-yl)oxy]ethyl}piperidin-4-yl)methyl]-2H-1,4-benzoxazin-3(4H)-one), a novel compound with high affinity for human (h) 5-HT1A and 5-HT1B receptors (pKi values of 8.6 and 8.0, respectively) as well as the (h) 5-HT transporter (SERT) (pKi value of 9.3). SB-649915 behaved as an antagonist at both 5-HT1A and 5-HT1B receptors in vitro and in vivo, reversing 5-HT, (+)8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and SKF99101-induced functional/behavioral responses. Furthermore, it inhibited [3H]5-HT reuptake in rat cortical synaptosomes, in vitro and ex vivo. In electrophysiological studies SB-649915 had no effect on rat dorsal raphe neuronal cell firing per se, but reversed 8-OH-DPAT-induced inhibition of firing both in vitro and in vivo. In addition, in a microdialysis study, it produced an acute increase in extracellular 5-HT in forebrain structures of the rat. Finally, SB-649915 demonstrated acute anxiolytic activity in both rodent and non-human primate and reduced the latency to onset of anxiolytic behavior, compared to paroxetine, in the rat social interaction paradigm. In summary, SB-649915 is a novel, potent 5-HT1A/1B autoreceptor antagonist, and 5-HT reuptake inhibitor. This particular pharmacological profile provides a novel mechanism that could offer fast-acting antidepressant activity.

Figure 1

Effect of SB‐649915‐B on (+)8‐OH‐DPAT‐induced inhibition of cell firing in rat mid‐brain slices containing the dorsal raphe nucleus. Data are mean ± S.E.M. from three or more slices. Data published previously by Scott et al. (2006).

Figure 2

Electrically stimulated [³H]5‐HT release from guinea pig cortical slices. (A) Attenuation of exogenous 5‐HT‐induced inhibition of release (1 Hz, 3 min, 20 mA) ^† P < 0.01 vs. control; *P < 0.01 vs. 5‐HT, ANOVA with post hoc t‐test. After 30 min equilibration (t = 0), slices were stimulated at t = 12 (S1) and t = 56 (S2) min and results expressed as the S2/S1 ratio. Data are the mean of three individual experiments, each performed in duplicate. GR‐127935 is a standard 5‐HT_1B/1D receptor partial agonist. (B) Potentiation (3 Hz, 1 min, 20 mA) of [³H]5‐HT release by increasing concentrations of SB‐649915 ^† P < 0.01; *P < 0.01; ^# P= 0.07 vs. control, ANOVA with post hoc t‐test. Data published previously by Scott et al. (2006).

Figure 3

Evaluation of 5‐HT_1A receptor occupancy by SB‐649915: ex vivo occupancy measured by displacement of [³H]WAY‐100635 from rat and guinea pig cortical homogenates. Following p.o. administration of SB‐649915‐B (2 h pretreatment), animals were sacrificed and cortical tissue removed for use in [³H]WAY‐100635 binding studies (see Scott et al. 2006 for methodology). Data are expressed as mean ± S.E.M. (n = 3–4 per study group). *P < 0.05 ^## P < 0.01 vs. vehicle treated animals. ANOVA followed by Duncan's t‐test. Data published previously by Hughes et al. (2007).

Figure 4

Inhibition of [³H]‐5‐HT uptake into rat and guinea pig cortical synaptosomes by SB‐649915‐B (1, 3, and 10 mg/kg p.o.): ex vivo assessment of SSRI occupancy. Following p.o. administration of SB‐649915‐B (2 h, 15 min pretreatment), animals were sacrificed and cortical tissue removed for use in [³H]5‐HT studies (see Scott et al. 2006 for methodology). Data are expressed as mean ± S.E.M. (n = 3–4 per study group). *P < 0.05 ^## P < 0.01 vs. vehicle treated animals. ANOVA followed by Duncan's t‐test. Data published previously by Hughes et al. (2007).

Figure 5

Effect of SB‐649915‐B on (A) 8‐OH‐DPAT–induced hyperlocomotion in rat and (B) SKF99101‐induced elevation of seizure threshold in the rat maximal electroshock seizure threshold test. SB‐649915‐B was administered p.o.. Data (A) are expressed as mean ± S.E.M. over a 30‐min period (n = 8 per study group), *P < 0.05 vs. 8‐OH‐DPAT alone. ANOVA followed by Duncan's t‐test. Data (B) are expressed as current producing tonic hindlimb extensor seizure in 50% of animals (CC₅₀) ± S.E.M. values (n = 12 per treatment group). ∼P < 0.001 vs. vehicle control; *P < 0.05, ***P < 0.001 vs. SKF99101 alone group. ANOVA followed by Wilcoxon post hoc analysis. Data published previously by Hughes et al. (2007).

Figure 6

Effect of SB‐649915‐B on (+)8‐OH‐DPAT–induced inhibition of cell firing in rat dorsal raphe nucleus in vivo. SB‐649915‐B was administered p.o. Data expressed as a% pre‐drug firing rate (mean ± S.E.M., n = 6–8 per group). **P < 0.01, ***P < 0.001 vs. vehicle. ANOVA followed by Dunnet's post hoc analysis. Data published previously by Hughes et al. (2007).

Figure 7

Effect of acute administration of SB‐649915‐B on extracellular 5‐HT levels in rat frontal cortex. SB‐649915‐B was administered p.o. Data are expressed as mean% preinjection levels ± SEM (n = 9–11 per study group). Arrow denotes time of drug administration. Increases attained significance at 3 (P < 0.001) and 10 mg/kg (P < 0.0001) vs. vehicle. Two‐way ANOVA with repeated measures. Data published previously by Hughes et al. (2007).

Figure 8

Effects of SB‐649915‐B on ultrasonic vocalizations in rat pups (A) and in the human threat test (HTT) in marmosets (B). (A) SB‐649915‐B (0.1, 0.3, and 1.0 mg/kg) was administered i.p. 30 min before test. Data are expressed as mean duration of vocalization (sec) ± S.E.M. (n = 6–8 per group). SB‐649915‐B (0.3 and 1.0 mg/kg, i.p.) and fluoxetine significantly reduced vocalization compared to Vehicle. **P < 0.01 from vehicle‐treated animals (ANOVA followed by Dunnett's). (B) SB‐649915‐B (1.0, 3.0, and 10 mg/kg) was administered s.c. 360 min before test. Data are expressed as mean number of postures and jumps ± S.E.M. (n = 4 per group). SB‐649915‐B significantly reduced postures with no effect on the number of jumps. **P < 0.01compared to vehicle‐treated animals (Student's t‐test for paired group). Data published previously by Starr et al. (2007).

Figure 9

Effect of subchronic (4 and 7 days) and chronic (21 days) treatment of SB‐649915‐B in the rat high light social interaction test vs. ex vivo SERT occupancy. SB‐649915‐B was administered p.o. Social interaction scores are shown in the upper panel (solid bars) and ex vivo SERT occupancy (determined by inhibition of [³H] 5‐HT uptake) are represented in the lower (striped bar) panel. Data are presented as mean interaction time (sec) ± S.E.M. and mean% occupancy ± S.E.M. (n = 12–14). *, ** and *** denote significant change (P < 0.05, P < 0.01, or P < 0.001, respectively) from vehicle treated animals (Duncan's t‐test following ANOVA social interaction; Dunnett's following ANOVA SERT occupancy). A significant increase in interaction time was observed with SB‐649915‐B on day 7 (1.0 and 3.0 mg/kg) and 21 (1.0, 3.0 and 7.5 mg/kg) and paroxetine on day 21 with no concurrent effect on locomotion (data not shown). SB‐649915‐B significantly occupied SERT compared to vehicle. *P < 0.05, **P < 0.01 from vehicle‐treated controls. Data published previously by Starr et al. (2007).