IT1t is a potent CXCR4 antagonist; inhibits CXCL12/CXCR4 interaction with an IC50 of 2.1 nM. CXCL12/CXCR4|2.1 nM (IC50)|HIV-1 (X4)|14.2 nM (IC50, in MT-4 cells)|HIV-1 (X4)|19 nM (IC50, in PBMCs) The CXCR4 is involved in chemotaxis and serves as a coreceptor for T-tropic HIV-1 viral entry and in cancer metastasis. IT1t is a small, drug-like, isothiourea derivative. IT1t shows very potent and dose-dependent inhibition of the CXCL12/CXCR4 interaction with an IC50 of 2.1 nM. This calcium flux is also inhibited by IT1t with an IC50 of 23.1. Strong electron density is observed for IT1t in the binding cavity of both subunits of the CXCR4 homodimer. In dimers of CXCR4 bound to IT1t, the monomers interact only at the extracellular side of helices V and VI, leaving at least a 4 Å gap between the intracellular regions, which is presumably filled by lipids. The IT1t compound and CVX15 peptide have both been characterized as competitive inhibitors of CXCL12, and many of the receptor-ligand contacts in the co-crystal structures presented are important for CXCL12 binding, including the acidic Asp187, Glu2887.39 and Asp972.63. The binding site of IT1t may point to the major anchor region for this domain.