<p dir="ltr">Over the last century, cancer immunotherapy has become an attractive field due to the popularity of checkpoint blockades and adoptive cell therapy. Though these new frontier therapeutics are effective for certain populations, they’ve had either adverse effects on others or are non-efficacious when used to treat “cold tumors”. Hence, newer strategies are needed to sensitize cold tumors into immune-responsive “hot tumors”, which synergize with checkpoint blockades. The cyclic GMP-AMP synthase-Stimulator of INterferon Genes (cGAS-STING) pathway has been identified as a pathway that can initiate T cell infiltration and turn cold tumors into hot tumors. Therefore, STING agonists have been identified as potential remedies that could help bend the curve to increase the survival rate of cancer patients if combined with anti-PD1 and anti-CTLA4 therapies.</p><p dir="ltr">2’3’-cGAMP is a master regulator of the innate immune system and is produced by cGAS upon cancer deregulation as well as bacterial and viral infection. Although 2’3’-cGAMP is a nanomolar affinity binder to STING and has vast immunostimulatory potential, it is plagued by several limitations that prevent its use in vivo. Most medicinal chemists have focused on making phosphorothioate derivatives which circumvents 2’3’-cGAMP’s limitations, but synthesizing these analogs presents a synthetic challenge. Also, these derivatives are commonly administered via intratumoral injection, which is not an attractive mode of delivery. This dissertation tries to address some of these challenges and provide a newer platform to develop CDN-based STING agonists.</p><p dir="ltr">We describe a novel class of phosphorothioester-linked cyclic dinucleotides (endo-S-CDNs) as excellent STING agonists. Showing through structural-activity relationship (SAR) which groups are tolerated or detrimental for STING binding and cellular activity. Also, determining that these 5’-phosphorothioester-linked CDNs are resistant to cleavage by clinically relevant phosphodiesterases (PDEs). Finally, we discuss how this novel class of CDNs is suitable for subcutaneous dosing to clear tumors in different mouse models.</p>