WAY-100135 is a serotonergic drug of the phenylpiperazine family which is used in scientific research. It acts as potent 5-HT<sub>1A</sub> receptor antagonist, and was originally believed to be highly selective, but further studies have demonstrated that it also acts as a partial agonist of the 5-HT<sub>1D</sub> receptor (pK<sub>i</sub> = 7.58; virtually the same affinity for 5-HT<sub>1A</sub>), and to a much lesser extent, of the 5-HT<sub>1B</sub> receptor (pK<sub>i</sub> = 5.82). These findings may have prompted the development of the related compound WAY-100635, another purportedly selective and even more potent 5-HT<sub>1A</sub> antagonist, which was synthesized shortly thereafter. However, WAY-100635 turned out to be non-selective as well, having been shown to act additionally as a potent D<sub>4</sub> receptor agonist later on.