Design, Synthesis, and Biological Evaluation of New 2,6,7-Substituted Purine Derivatives as Toll-like Receptor 7 Agonists for Intranasal Vaccine Adjuvants.

TLR7/8 agonists are versatile immune stimulators capable of treating various diseases such as viral infections, autoimmune, and cancer. Despite the structural similarity of TLR7/8, their immune stimulation mechanisms and time-course responses significantly differ. In this study, a new series of TLR7-selective agonists was synthesized utilizing the economical building block 2,6-dichloropurine. Compound 27b showed the most potent activity on hTLR7 with an EC 50 of 17.53 nM and demonstrated high hTLR7 selectivity (224 folds against TLR8). 27b effectively stimulated the secretion of proinflammatory cytokines in mouse macrophages and enhanced intranasal vaccine efficacy against influenza A virus in vivo. Assessment of humoral and mucosal antibody titers confirmed that 27b elevates IgG and IgA levels, protecting against both homologous and heterologous influenza viral infections. These findings suggest that 27b is a promising candidate as a vaccine adjuvant to prevent viral infections or as a robust immunomodulator with prolonged activity for treating immune-suppressed diseases.