Current landscape of RNA chemistry and delivery in cancer immunotherapy

Small interfering RNAs (siRNAs) are programmable nucleic acids that play key roles in chemical biology and can selectively silence disease-associated genes through RNA interference (RNAi). These programmable nucleic acids have emerged as a powerful class of medicines and chemical biology tools that can rewire tumor-immune signaling, target immunosuppressive genes, stimulate immune responses, and boost the immune system against immune-mediated diseases. Recent success in the rapid synthesis and applications of siRNA highlights the potential of this technology to address previously "undruggable" targets across a range of genetic, metabolic, and oncologic diseases. Despite the potential of these siRNA-based therapies, including those used in cancer immunotherapy, challenges such as off-target effects during delivery, chemical degradation of siRNA in the body, and immunogenicity limit their efficacy. This review provides a comprehensive overview of the chemical biology and chemical modifications inherent to the design of robust siRNA therapies; the nucleic acid structure-function relationships that dictate the cellular mechanisms underlying siRNA-mediated gene silencing and efficacy; and the current clinical landscape and safety of approved siRNA therapeutics for immunotherapy. We examine the growing role of computationally guided design strategies and emerging machine-learning-based methods in optimizing siRNA chemical design, and outline how recent advances in siRNA chemical modification are expected to improve targeted gene modulation in the clinic. Additionally, we examine the role of delivery systems in enhancing siRNA potency, with an emphasis on tumor-targeted and tissue-specific approaches, as well as emerging combination therapies integrating siRNA with chemotherapy, immune checkpoint blockade, siRNA and mRNA co-delivery, and prodrug activation.