Research progress of histidine-based tumor-targeting drug delivery systems

As a basic amino acid, histidine has a pK_a close to the acidity of the tumor microenvironment, thus the charge and solubility of histidine are able to vary as the pH changes. Under a neutral environment, histidine is not charged and exhibits hydrophobic properties, while it can be protonated and becomes hydrophilic when exposed to mildly acidic pH, such as tumor microenvironment. Therefore, histidine is widely used in the design of drug delivery systems to target the mildly acidic pH of tumor microenvironment. This article reviews the recent progresses of histidine-based tumor-targeting drug delivery systems, and summarizes the principles on promoting internalization and tuning drug release by taking advantage of histidine. Finally, we point out the common issues on histidine application and illustrate its future prospects.