CTW0404 is an experimental drug which is described as a positive allosteric modulator of the 5-HT<sub>2A</sub> receptor, with neutral or negative modulation of the related 5-HT<sub>2B</sub> and 5-HT<sub>2C</sub> subtypes. It does not produce the head-twitch response in animal studies, and reduces the head-twitch response produced by DOI, suggesting that it may be a biased allosteric modulator selective for certain signalling pathways.