Development of Circular CpG Oligodeoxynucleotides for Prolonged Immunostimulation and Synergistic Antitumor Chemotherapy in Melanoma

Cancer immunotherapy harnesses the host immune system to treat tumors. TLR-9, a key bridge between innate and adaptive immunity, is activated by CpG oligodeoxynucleotides (ODNs). However, linear CpG ODNs suffer from rapid nuclease degradation in vivo, resulting in a short half-life and reduced immunostimulatory activity. Here, we developed covalently closed circular CpG ODNs based on FDA-approved sequences (1018 and 7909). The circular CpG ODNs exhibited superior nuclease resistance and significantly enhanced immunostimulatory activity in both human and murine immune cells. In the poorly immunogenic B16–F10 melanoma model, combining circular CpG ODNs with dacarbazine (DTIC) chemotherapy produced markedly stronger and more sustained antitumor efficacy, with enhanced tumor regression and improved survival, compared to linear CpG ODN/DTIC combinations. Our findings establish circular CpG ODNs as a promising platform for cancer immunotherapy, offering improved long-term stability and potent immunomodulatory capacity, particularly in combination with chemotherapy.