Irindalone (; developmental code name LU-21098 or Lu 21-098) is a predominantly peripherally acting, highly potent, and selective serotonin 5-HT<sub>2</sub> receptor antagonist which was under development for the treatment of hypertension but was never marketed. It is known to act as a serotonin 5-HT<sub>2A</sub> and 5-HT<sub>2C</sub> receptor antagonist. In addition, unlike ketanserin and cinanserin, the drug blocks the serotonin receptors in the rat fundus strip, which may correspond to the serotonin 5-HT<sub>2B</sub> receptor. It shows weak affinity for the ñ<sub>1</sub>-adrenergic receptor, where it is antagonistic as well. Irindalone was first described in the scientific literature by 1988. It reached phase 2 clinical trials for hypertension prior to the discontinuation of its development in 1989. In addition to hypertension, irindalone was later studied in the early 2000s for use in combination with a selective serotonin reuptake inhibitor (SSRI) to augment serotonin levels via serotonin 5-HT<sub>2C</sub> receptor blockade and enhance antidepressant efficacy.