SB-649915, a novel, potent 5-HT_1A and 5-HT_1B autoreceptor antagonist and 5-HT re-uptake inhibitor in native tissue

An increase in brain 5-HT levels is thought to be the key mechanism of action which results in an antidepressant response. It has been proven that selective serotonin re-uptake inhibitors are effective antidepressants but the delay to therapeutic onset of these agents is thought to be due to the time required for 5-HT_1A, and possibly 5-HT_1B, autoreceptor desensitisation. Therefore an agent incorporating 5-HT re-uptake inhibition coupled with 5-HT_1A and/or 5-HT_1B autoreceptor antagonism may provide a fast acting clinical agent. The current studies describe the in vitro 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 which has high affinity for human recombinant 5-HT_1A, 5-HT_1B and 5-HT_1D receptors (pK_i values of 8.6, 8.0, 8.8, respectively) and the human recombinant 5-HT transporter (pK_i value of 9.3). SB-649915 also displays high affinity for rat, guinea pig, mouse and marmoset native tissue 5-HT_1A, 5-HT_1B and 5-HT_1D receptors and rat native tissue 5-HT transporters (pK_i values ≥ 7.5). In functional [³⁵S]GTPγS binding studies, SB-649915 (up to 1 μM) does not display intrinsic activity in HEK293 cells expressing human recombinant 5-HT_1A receptors but acts as a partial agonist at human recombinant 5-HT_1B and 5-HT_1D receptors with intrinsic activity values of 0.3 and 0.7, respectively, as compared to the full agonist 5-HT. From Schild analysis, SB-649915 caused a concentration-dependent, rightward shift of 5-HT-induced stimulation of basal [³⁵S]GTPγS binding in cells expressing human recombinant 5-HT_1A or 5-HT_1B receptors to yield pA₂ values of 9.0 and 7.9, respectively. In electrophysiological studies in rat dorsal raphe nucleus, SB-649915 did not affect the cell firing rate up to 1 μM but attenuated (+)8-hydroxy-2-(di-n-propylamino) tetralin-induced inhibition of cell firing with an apparent pK_b value of 9.5. SB-649915 (1 μM) significantly attenuated exogenous 5-HT-induced inhibition of electrically-stimulated [³H]5-HT release from guinea pig cortex. In studies designed to enhance endogenous 5-HT levels, and therefore increase tone at 5-HT_1B autoreceptors, SB-649915 significantly potentiated [³H]5-HT release at 100 and 1000 nM. In LLCPK cells expressing human recombinant 5-HT transporters and in rat cortical synaptosomes, SB-649915 inhibited [³H]5-HT re-uptake with pIC₅₀ values of 7.9 and 9.7, respectively. In summary, SB-649915 is a novel, potent 5-HT_1A/1B autoreceptor antagonist and 5-HT re-uptake inhibitor in native tissue systems and represents a novel mechanism that could offer fast acting antidepressant action.