TLR8 agonist 9 (Compound II-77) is an agonist for Toll-like receptor 8 ( TLR8 ) with EC 50 of 0.25-1 μM. TLR8 agonist 9 is stable in human and murine plasma, induces secretion of cytokines TNFα, with EC 50 <1 μM. TLR8 agonist 9 exhibits antitumor activity in MC38-HER2 xenograft mouse model with a tumor growth inhibition (TGI) rate of 97%.