TCS-OX2-29 is an orexin antagonist. It was the first non-peptide antagonist developed that is selective for the orexin receptor subtype OX<sub>2</sub>, with an IC<sub>50</sub> of 40nM and selectivity of around 250x for OX<sub>2</sub> over OX<sub>1</sub> receptors. Orexin antagonists are expected to be useful for the treatment of insomnia, with subtype-selective antagonists such as TCS-OX2-29 potentially offering more specificity of action compared to non-selective orexin antagonists like almorexant.