Small Molecules as Immune Checkpoint Inhibitors in Cancer Therapeutics

Despite the large number of cancer chemotherapeutics, cancer treatment is still not very satisfactory. Immune checkpoint inhibition has emerged as a new ray of hope in the immunotherapy approach for cancer treatment. Immune checkpoint inhibitors are molecules located on the surface of immune cells that regulate unnecessary immune responses and keep autoimmune reactions in check. Immune checkpoint inhibitors (ICIs), such as anti-programmed cell death protein-1 and anti-programmed cell death ligand-1, have been employed to activate receptors on immune cells like T-cells, which can deactivate the immune checkpoint and thus reactivate them against cancer cells. However, ICI therapy has limitations, including resistance development in patients, its suitability for all patients, multiple organ disorders, and hyper-progression. Therefore, understanding the chemical structures of small molecule ICIs may aid in designing and developing novel ICIs with improved efficacy and efficiency for cancer chemotherapy. This review’s novelty lies in its summary of the U.S. Food and Drug Administration-approved drugs, repurposed drugs, candidate drugs used alone or in combination with monoclonal antibodies, and novel potential lead molecules under preclinical investigation, which may be useful for designing new chemical entities as ICIs. The review describes 10 different drugs approved by the U.S. Food and Drug Administration that have demonstrated immune checkpoint inhibition targeting the programmed cell death ligand-1/programmed cell death protein-1 signaling, CTLA-4/CD28, TIGIT/PVR, and CD47/SIRPα pathways, as well as three repurposed drugs, 11 candidate drugs, and nine drugs in combination with monoclonal antibodies that are in various phases of clinical trials.