Aptamers as immune checkpoint inhibitors in cancer immunotherapy: targeting CTLA-4/B7 and PD-1/PD-L1 pathways

The increasing burden of cancer underscores the urgent need for more effective treatment strategies. Cancer immunotherapy is a revolutionary, novel approach that harnesses the immune system's natural anticancer response; however, its success is often limited by tumour immune evasion mechanisms. Given the central role of cytotoxic T-cells in anticancer immunity, efforts have been directed toward enhancing their activity. One key strategy involves targeting immune checkpoint receptors such as cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4), and programmed death 1 (PD-1), which suppress T-cell function. The blockade of these receptors using immune checkpoint inhibitors (ICIs) effectively restores T-cell activity, with FDA-approved monoclonal antibody inhibitors currently in clinical use. However, their adverse effects and inherent limitations highlight the need for alternative approaches - one of which is aptamers. As oligonucleotides, aptamers have numerous advantages, including ease of production, increased stability, small size, non-immunogenicity, and potential for conjugation with multiple molecules. Despite challenges such as vulnerability to nuclease degradation and excessive renal filtration, modifications have been explored to improve their therapeutic potential. Bispecific aptamers present an improved approach, with novel modified bispecific aptamers showing notable potential. Despite promising preclinical results, there are still currently no aptamer-based ICIs approved for clinical use. Given current advancements, aptamers as ICIs hold great promise as a next-generation cancer immunotherapy, offering a more precise, effective, and potentially safer alternative to traditional ICIs. This review suggests a novel DBCO-modified bispecific aptamer that simultaneously targets CTLA-4 and PD-L1 with improved localisation to the tumour site, which exemplifies a next-generation strategy.