Immunotherapies have revolutionized the field of cancer therapeutics, yet a substantial subset of patients fail to respond. Recent efforts have focused on identifying targets that could elicit or augment anti-tumor immune responses. One such novel target is STING or stimulator of interferon (IFN) genes, an endoplasmic protein that induces the production of pro-inflammatory cytokines such as type I IFNs. Since the discovery of STING, numerous natural and synthetic STING agonists have been tested in both pre-clinical and clinical settings in different tumors. However, the structural instability of first-generation agonists prompted the development of more stable and potent compounds. This review will highlight the latest pharmacologic classes of STING agonists, novel approaches for tumor-targeted drug delivery, and challenges in the clinical targeting of the STING pathway.

The interrogation of intrinsic and adaptive resistance to cancer immunotherapy has identified lack of antigen presentation and type I interferon signaling as biomarkers of non-T-cell-inflamed tumors and clinical progression. A myriad of pre-clinical studies have implicated the cGAS/stimulator of interferon genes (STING) pathway, a cytosolic DNA-sensing pathway that drives activation of type I interferons and other inflammatory cytokines, in the host immune response against tumors. The STING pathway is also increasingly understood to have other anti-tumor functions such as modulation of the vasculature and augmentation of adaptive immunity via the support of tertiary lymphoid structure development. Many natural and synthetic STING agonists have entered clinical development with the first generation of intra-tumor delivered cyclic dinucleotides demonstrating safety but only modest systemic activity. The development of more potent and selective STING agonists as well as novel delivery systems that would allow for sustained inflammation in the tumor microenvironment could potentially augment response rates to current immunotherapy approaches and overcome acquired resistance. In this review, we will focus on the latest developments in STING-targeted therapies and provide an update on the clinical development and application of STING agonists administered alone, or in combination with immune checkpoint blockade or other approaches.