AbstractBackground and PurposeWhereas biased agonism on the 5‐HT_2A receptor has been ascribed to hallucinogenic properties of psychedelics, no information about biased inverse agonism on this receptor is available. In schizophrenia, increased 5‐HT_2A receptor constitutive activity has been suggested, highlighting the therapeutic relevance of inverse agonism. This study characterized the modulation of G protein activity promoted by different drugs, commonly considered as 5‐HT_2A receptor antagonists, in post‐mortem human brain cortex.Experimental ApproachModulation of [³⁵S]GTPγS binding to different subtypes of Gα proteins exerted by different 5‐HT_2A receptor drugs was determined by scintillation proximity assays in brain from human, WT and 5‐HT_2A receptor KO mice.Key ResultsMDL‐11,939 was the only drug having no effect on the basal activity of 5‐HT_2A receptor. Altanserin and pimavanserin decreased basal activation of G_i1, but not G_q/11 proteins. This effect was blocked by MDL‐11,939 and absent in 5‐HT_2A receptor KO mice. Volinanserin showed 5‐HT_2A receptor‐mediated inverse agonism both on G_i1 and G_q/11 proteins. Ketanserin exhibited 5‐HT_2A receptor partial agonism exclusively on G_q/11 proteins. On the other hand, eplivanserin and nelotanserin displayed inverse agonism on G_q/11 and/or G_i1 proteins, which was insensitive to MDL‐11,939 and was present in KO mice suggesting a role for another receptor.Conclusion and ImplicationsThe results reveal the existence of constitutively active 5‐HT_2A receptors in human pre‐frontal cortex and demonstrate different pharmacological profiles of various 5‐HT_2A receptor drugs previously considered antagonists. These findings indicate that altanserin and pimavanserin possess biased inverse agonist profile towards 5‐HT_2A receptor activation of G_i1 proteins.