Highly reactive metabolites that are generated when a drug is metabolized by enzymes such as cytochrome P450 (CYP) in vivo are called reactive metabolites. Since reactive metabolites form covalent bonds with biomacromolecules including proteins and irreversibly inhibit their function, they may exhibit serious toxicity. In recent drug discovery research, it is required to avoid metabolism-related toxicity and reduce safety risks by conducting a risk assessment of reactive metabolites in an early stage of drug development. CysGlu-Dan and Dap-Dan are fluorescent trapping agents developed by Professor Tomoyuki Ohe and his team at the Faculty of Pharmacy, Keio University. They can detect and quantify a wide range of reactive metabolites.