CXCR4 is a GPCR that by activation of its ligand CXCL12 is involved in the pathology of several diseases, examples being cancer and rheumatoid arthritis. It has also shown to play a crucial role for HIV-1 entry into T-cells and the development of AIDS. Several cyclopentapeptides (CPPs) based on the sequence D-Tyr-Arg-Arg-2-Nal-Gly of the lead compound FC131 have shown to have antagonistic activity. ...