CU-CPT9b is a specific TLR8 antagonist, with an IC 50 of 0.7 nM. CU-CPT9b shows high binding affinity towards TLR8 with a K d of 21 nM In Vitro CU-CPT9b is a specific TLR8 antagonist, with an IC 50 of 0.7±0.2 nM. ITC experiments have confirmed the strong binding of CU-CPT9b with a K d of 21 nM. It is shown that CU-CPT-9b binds to the inactive TLR8 dimer in a similar way to CU-CPT8m. CU-CPT9b utilizes hydrogen bonds with G351 and V520*, which are conserved among TLR8/antagonist structures. Additionally, CU-CPT9b forms water-mediated contacts with S516* and Q519*, which are not observed in TLR8/CU-CPT8m structure, suggesting that the enhanced potency of CU-CPT9b derives from the new interactions with these polar residues. The orientation of Y567* also changes to facilitate van der Waals interactions with CU-CPT9b as compared to TLR8/CU-CPT8m. MCE has not independently confirmed the accuracy of these methods. They are for reference only. Form:Solid IC50& Target:TLR8 0.7 nM (IC 50 ) TLR8 21 nM (Kd)