Selectivity of synthetic CXCR4 agonists

CXCR4, a widely distributed chemokine receptor, is known for its major role in cancer metastasis and AIDS pathogenesis. Pharmacotherapy with antagonistic ligands is greatly hindered since it blocks the many essential physiological roles of CXCR4, causing serious undesired effects. CXCR4 has only one known endogenous ligand, CXCL12, a 68 amino acid peptide that only binds to CXCR4 and CXCR7. To improve this situation we have discovered CXCR4 agonists from in silico homology modeling and the crystal structure of CXCR4: we hypothesized that peptide chimera of T140 (a potent inverse agonist of CXCR4) and the N‐terminal fragments of CXCL12 could act as agonists on CXCR4; such compounds proved to be the first potent synthetic CXCR4 agonists. We have shown that these synthetic peptides were able to induce chemotaxis both in vitro and in vivo, and to activate the typical Gi signaling pathway of CXCR4 in nanomolar concentrations, similarly to CXCL12. This study aims to assess the CXCR4 selectivity of these synthetic agonists. Binding assays on CXCR7 and CXCR4 were performed and showed affinities nanomolar for CXCR4 but micromolar for CXCR7, indicating up to a thousand‐fold selectivity. Functional selectivity was assessed on β‐arrestin‐2 recruitment to CXCR4 and to CXCR7. The surprising result is that our synthetic CXCR4 agonists appear to induce β‐arrestin recruitment on CXCR7 only and not on CXCR4.