Elzasonan (CP-448,187) is a selective 5-HT<sub>1B</sub> and 5-HT<sub>1D</sub> receptor antagonist that was under development by Pfizer for the treatment of depression but was discontinued, possibly due to poor efficacy. By preferentially blocking 5-HT<sub>1B</sub> and 5-HT<sub>1D</sub> autoreceptors, elzasonan is thought to enhance serotonergic innervations originating from the raphe nucleus, thereby improving signaling to limbic regions like the hippocampus and prefrontal cortex and ultimately resulting in antidepressant effects.