Development of novel CXC chemokine receptor 7 (CXCR7) ligands: selectivity switch from CXCR4 antagonists with a cyclic pentapeptide scaffold. 
The CXC chemokine receptor 7 (CXCR7)/ACKR3 is a chemokine receptor that recognizes stromal cell-derived factor 1 (SDF-1)/CXCL12 and interferon-inducible T-cell α chemoattractant (I-TAC)/CXCL11.  Here, we report the development of novel CXCR7-selective ligands with a cyclic pentapeptide scaffold through an SAR study of CXC chemokine receptor 4 (CXCR4) selective antagonist FC131 [cyclo(-d-Tyr-l-Arg-l-Arg-l-Nal-Gly-), Nal = 3-(2-naphthyl)alanine].  Substitution of Gly with l-Pro switched the receptor preference of the peptides from CXCR4 to CXCR7.  The SAR study led to the identification of a potent CXCR7 ligand, FC313 [cyclo(-d-Tyr-l-Arg-l-MeArg-l-Nal-l-Pro-)], which recruits β-arrestin to CXCR7.  Investigations via receptor mutagenesis and molecular modeling experiments suggest a possible binding mode of the cyclic pentapeptide CXCR7 agonist.