RS-127445, also known as MT-500, is a drug which acts as a potent and selective antagonist at the serotonin 5-HT<sub>2B</sub> receptor, with around 1,000-fold selectivity over the closely related 5-HT<sub>2A</sub> and 5-HT<sub>2C</sub> receptors. The role of the 5-HT<sub>2B</sub> receptor in the body is still poorly understood, and RS-127445 has been a useful tool in unravelling the function of the various systems in which this receptor is expressed. The drug was under development for potential use as a pharmaceutical drug by Roche and reached phase 1 clinical trials but was discontinued for unknown reasons.
RS-127445 has been found to diminish the head-twitch response, a behavioral proxy of psychedelic effects, induced by the psychedelic drug LSD in rats but not in mice. It also blocked the persistent antidepressant-like effects produced by LSD in rats.