Advanced biomaterials for the targeted delivery of immune checkpoint inhibitors to solid tumors

Immune checkpoint inhibitors (ICIs) have revolutionized the way cancer is treated by engaging the patient's own immune system to attack cancer. ICIs can also achieve favorable outcomes in patients whose cancers are unresponsive or resistant to first-line therapies. Despite these exciting prospects, ICIs are ineffective in many patients and cause immune-related adverse events (irAEs) in up to 89 % of patients. Therefore, there is a clear clinical need to reduce irAEs while maintaining or improving the therapeutic efficacy of ICIs. The local administration of ICIs through intratumoral injection or peritumoral administration has been shown to increase the potency of these therapeutics while reducing irAEs and extending survival in preclinical models. However, the rapid systemic distribution of intratumorally delivered drugs (hours) prevents this strategy from achieving even better efficacy and reduced toxicity; this is particularly problematic for ICIs due to their long biological (weeks), consequently acting at non-target sites for weeks before being cleared by the body. Engineered biomaterials have the potential to enhance local administration by improving permeation and retention, employing antibody-mediated targeting, leveraging tumor microenvironment sense-and-respond systems, or taking advantage of cell trafficking. This paper reviews the cutting-edge delivery strategies shown to improve the safety and efficacy of drugs targeting PD-1, PD-L1, and CTLA-4 and identifies the most promising strategies for clinical translation.