In situ click chemistry is a target-guided synthesis approach for discovering novel lead compounds by assembling organic azides and alkynes into triazoles inside the affinity site of target biogenic molecules such as proteins. We report in situ click chemistry screening with human D-amino acid oxidase (hDAO), which led to the identification of a more potent hDAO inhibitor. The hDAO inhibitors have chemotherapeutic potential as antipsychotic agents. The new inhibitor displayed competitive inhibition of hDAO and showed significantly increased inhibitory activity against hDAO compared with that of an anchor molecule of in situ click chemistry.Key words fragment-based lead discovery; templated reaction; ligand-binding site; drug discovery; triazole formation; ligand affinity Although D-amino acids exist in a wide range of organisms, they are nonetheless often called unnatural amino acids. D-Serine is abundant in the forebrain and acts as an endogenous co-agonist of N-methyl-D-aspartate (NMDA) receptors to enhance neurotransmission. 1) D-Amino acid oxidase (DAO) was the first flavoenzyme to be identified, in 1935 by Krebs,2) and catalyzes the oxidative deamination reaction of D-amino acids. This reaction by human DAO (hDAO) in the brain primarily converts D-serine to hydroxypyruvate to regulate the concentration of D-serine. The overexpression of hDAO in the brain therefore causes neuropsychiatric disorders such as schizophrenia, a serious public health problem that affects nearly 0.8% of the global population, by reducing the amount of D-serine below the level required for adequate neuronal function.3,4) Furthermore, a protein from the human gene G72 has been identified as an interacting partner to activate hDAO, and the association of both DAO and gene G72 with schizophrenia together with activation of hDAO activity by a G72 protein product points to the involvement of the regulation of NMDA receptor. 5) Thus, hDAO inhibitors have potential to be lead compounds to the development of therapeutic agents for schizophrenia.Within the realm of click chemistry research, triazole formation between organic azide and alkyne is widely recognized. Triazole formation includes the 1,3-dipolar reaction known as the Huisgen cycloaddition, [6][7][8][9] and catalytic reactions that selectively afford the corresponding 1,4-substituted triazole or 1,5-substituted triazole using monovalent copper 10,11) or ruthenium reagents, 12,13) respectively. In situ click chemistry is a target-guided synthesis (TGS) method for the fast and efficient production of potential inhibitors against target biomolecules such as enzymes. Using this approach, complementary alkyne and azide building are assembled at binding sites and then accelerate the Huisgen 1,3-dipolar cycloaddition reaction to afford the corresponding conventional triazole, as illustrated in Fig. 1.To date, in situ click chemistry research has garnered success in inhibitor explorations where acetylcholinester-