Aptamers targeting immune checkpoints for tumor immunotherapy: a comprehensive review

Tumor immunotherapy has transformed the cancer treatment paradigm by leveraging the host immune system to identify and eradicate tumor cells in the body. Immune checkpoint inhibitors (ICIs) targeting programmed cell death protein 1 (PD-1), programmed death ligand 1 (PD-L1), and cytotoxic T-lymphocyte-associated antigen 4 (CTLA-4) have achieved significant clinical success. However, major limitations, such as therapeutic resistance, immune-mediated toxicities, and high treatment costs, necessitate the development of alternative and more efficient strategies. Aptamers, short-chain single-stranded nucleic acid ligands with high binding specificity and affinity, have emerged as compelling candidates for cancer therapy due to their superior tissue penetration, reduced immunogenicity, and ease of chemical modification compared to antibody therapies. This review provides an inclusive overview of aptamer-based approaches for targeting immune checkpoints, with a specific emphasis on PD-1/PD-L1 and CTLA-4. In addition, we highlight recent advancements in the engineering of bispecific and multifunctional aptamers, their role in overcoming immune resistance, and their potential to improve therapeutic performance. We also discuss innovative approaches to enhance aptamer stability, bioavailability, and tumor-specific delivery through chemical tailoring and nanoparticle conjugation. Although most aptamer-based checkpoint inhibitors remain in preclinical stages, early phase clinical investigations (primarily with C-X-C motif chemokine ligand 12 (CXCL12)-targeting Spiegelmer NOX-A12 in combination settings, as well as earlier programs such as AS1411 targeting nucleolin) have demonstrated effective inhibition of immune checkpoint signaling, reactivation of T-cell function, and synergistic effects when combined with existing immunotherapies. Preclinical and early phase clinical investigations have demonstrated that aptamers can effectively inhibit immune checkpoint signaling, reactivate T-cell function, and potentiate synergistic effects when combined with existing immunotherapies. By critically evaluating current progress and identifying key translational challenges, this review provides strategic insights into the future development of aptamer-based immunotherapeutic platforms, ultimately guiding the advancement of more precise, cost-effective, and personalized cancer treatment modalities.