Peptides are gradually emerging as middle-sized therapeutic agents in addressing unmet medical needs. Compared to small-molecule drugs, peptides can target complex biological processes more precisely while generally less complex and more a cost-effective than large biological drugs like antibodies. Over 100 FDA-approved peptide drugs have been on the market since the first peptide hormone insulin was developed in 1923, among which about 40 drugs for treating a wide range of diseases (e.g., cancer, cardiovascular, and metabolic diseases) contain at least one tryptophan (Trp) residue, which is a key amino acid.
