Natural Blockers of PD-1/PD-L1 Interaction for the Immunotherapy of Triple-Negative Breast Cancer-Brain Metastasis

Simple Summary Aggressive types of breast cancer spread to the brain and can form a new tumor there. The treatment of such a tumor is more difficult because there is a membrane around the brain that limits the entrance of drugs. Currently, chemotherapy is the most well-known treatment for these patients, but it cannot pass through that membrane and such patients often die within two years. Here, we looked at some of the drug candidates that are extracted from plants and traditional herbal medicines and these candidates can activate the immune system to kill cancer. We reviewed whether these molecules could pass the brain membrane to activate the immune system inside the brain to kill cancer there. The limited treatment options for triple-negative breast cancer with brain metastasis (TNBC-BM) have left the door of further drug development for these patients wide open. Although immunotherapy via monoclonal antibodies has shown some promising results in several cancers including TNBC, it cannot be considered the most effective treatment for brain metastasis. This is due to the protective role of the blood-brain barrier (BBB) which limits the entrance of most drugs, especially the bulky ones such as antibodies, to the brain. For a drug to traverse the BBB via passive diffusion, various physicochemical properties should be considered. Since natural medicine has been a key inspiration for the development of the majority of current medicines, in this paper, we review several naturally-derived molecules which have the potential for immunotherapy via blocking the interaction of programmed cell death protein-1 (PD-1) and its ligand, PD-L1. The mechanism of action, physicochemical properties and pharmacokinetics of these molecules and their theoretical potential to be used for the treatment of TNBC-BM are discussed.