TLR7 agonist18 (Compound 21a) is a selective Toll-like receptor 7 (TLR7) agonist with an EC50 of 7.8 μM. It demonstrates no cytotoxicity in an HEK293 cell line co-transfected with the hTLR7 gene and can induce the secretion of several cytokines, including IL-1β, IL-12p70, IL-8, and TNF-α. This compound is useful in research related to vaccines, asthma, allergies, and cancer treatment.