CU-CPT9b is a potent and selective TLR8 antagonist with a Kd of 21 nM. It inhibits R-848-induced NF-kappaB activation in TLR8-overexpressing HEK-Blue cells with an IC50 of 0.7 nM. This tool compound is valuable for in vitro studies investigating TLR8 signaling and its role in autoimmune, inflammatory, and antiviral research.