Agomelatine and S 21517 have moderately high affinities for 5-HT2C receptors (Ki = 0.21 μM and 0.13 μM, respectively). The metabolite 3-Hydroxy agomelatine (S 21540) has a 10-fold lower affinity (Ki = 1.8 μM). Agomelatine, S 21517 and 3-Hydroxy agomelatine (10-10 M) are antagonists at 5-HT2C receptors, with a rank order of efficacy as follows: S 21517>Agomelatine>3-Hydroxy agomelatine.