Research progress of pH-responsive drug delivery systems in cancer immunotherapy

Cancer immunotherapy, which is an attractive strategy harnessing the host's own immune system to remove tumor cells, has been widely used in clinical practice, yet with low response rate and immune-related adverse events. Unlike traditional chemotherapy, the targets of immunotherapy exhibit high spatial heterogeneity and are distributed in different cell types or secondary organelles, resulting in off-target and on-target toxicity, which greatly reduces the efficacy and safety of treatment. Due to the altered metabolic level, tumor tissues often display a lower pH than normal tissues. In addition, the endocytosis pathway is accompanied by continuous pumping of protons. Therefore, the variation of environmental pH values could serve as an ideal stimulus for precise drug delivery and release. In recent years, pH-responsive materials (e.g., polymers, biomacromolecules, lipid nanoparticles, biofilm, inorganic nanoparticles, and metal-organic frameworks) have been widely investigated in the field of cancer immunotherapy. This paper summarizes recent strategies of pH-responsive drug delivery systems based on different types of carriers, aiming to provide some reference for the design of next generation of tumor-targeting formulations in cancer immunotherapy.