SB-649915, a novel, potent 5-HT sub(1A) and 5-HT sub(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 sub(1A), and possibly 5-HT sub(1B), autoreceptor desensitisation. Therefore an agent incorporating 5-HT re-uptake inhibition coupled with 5- HT sub(1A) and/or 5-HT sub(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 sub(1A), 5-HT sub(1B) and 5-HT sub(1D) receptors (pK sub(i) values of 8.6, 8.0, 8.8, respectively) and the human recombinant 5-HT transporter (pK sub(i) value of 9.3). SB-649915 also displays high affinity for rat, guinea pig, mouse and marmoset native tissue 5-HT sub(1A), 5-HT sub(1B) and 5- HT sub(1D) receptors and rat native tissue 5-HT transporters (pK sub(i) values >= 7.5). In functional [ super(35)S]GTP gamma S binding studies, SB-649915 (up to 1 mu M) does not display intrinsic activity in HEK293 cells expressing human recombinant 5-HT sub(1A) receptors but acts as a partial agonist at human recombinant 5- HT sub(1B) and 5-HT sub(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 [ super(35)S]GTP gamma S binding in cells expressing human recombinant 5-HT sub(1A) or 5-HT sub(1B) receptors to yield pA sub(2) 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 mu M but attenuated (+)8-hydroxy-2-(di-n-propylamino) tetralin-induced inhibition of cell firing with an apparent pK sub(b) value of 9.5. SB-649915 (1 mu M) significantly attenuated exogenous 5-HT-induced inhibition of electrically- stimulated [ super(3)H]5-HT release from guinea pig cortex. In studies designed to enhance endogenous 5-HT levels, and therefore increase tone at 5-HT sub(1B) autoreceptors, SB-649915 significantly potentiated [ super(3)H]5-HT release at 100 and 1000 nM. In LLCPK cells expressing human recombinant 5-HT transporters an