Review of immune checkpoint inhibitors in immuno-oncology

Tumor cells predominantly express self-antigens and overcoming self-tolerance is the primary challenge to effective immunotherapy. Tumors also express ligands for co-inhibitory molecules on immune cells, in order to suppress anti-tumor immunity. Over a decade ago, the first antibodies generated to block the co-inhibitory molecule CTLA-4 was tested in patients with metastatic melanoma. Results from this landmark trial have informed not only the current landscape of checkpoint blockade but also the way in which immunotherapy trial outcomes are determined. Antibodies targeting PD-1 and its ligand, PD-L1, soon followed and use of these checkpoint inhibitors (ICIs) have expanded exponentially. ICI treatment has shown long-lasting clinical benefit in several tumor types and patients refractory to other treatments can often respond to ICI therapy. On the other hand, in some tumor types, the response to ICI is short-lived and tumors eventually recur. Current clinical trials are focused on enhancing anti-tumor effects through combinations of multiple ICIs with agents which cause tumor death, particularly in solid tumors, in order to enhance antigen presentation. It is also important to define which patients will respond to therapy with ICIs as over half of all patients suffer from immune-related adverse events (irAE), some of which are severe and long-lasting.