Cyclic RGD and isoDGR Integrin Ligands Containing cis-2-amino-1-cyclopentanecarboxylic (cis-β-ACPC) Scaffolds

Integrin ligands containing the tripeptide sequences Arg-Gly-Asp (RGD) and iso-Asp-Gly- Arg (isoDGR) were actively investigated as inhibitors of tumor angiogenesis and directing unit in tumor-targeting drug conjugates. Reported herein is the synthesis, of two RGD and one isoDGR cyclic peptidomimetics containing (1S,2R) and (1R,2S) cis-2-amino-1-cyclopentanecarboxylic acid (cis-β-ACPC), using a mixed solid phase/solution phase synthetic protocol. The three ligands were examined in vitro in competitive binding assays to the purified α(v)β(3) and α(5)β(1) receptors using biotinylated vitronectin (α(v)β(3)) and fibronectin (α(5)β(1)) as natural displaced ligands. The IC50 values of the ligands ranged from nanomolar (the two RGD ligands) to micromolar (the isoDGR ligand) with a pronounced selectivity for α(v)β(3) over α(5)β(1). In vitro cell adhesion assays were also performed using the human skin melanoma cell line WM115 (rich in integrin α(v)β(3)). The two RGD ligands showed IC(50) values in the same micromolar range as the reference compound (cyclo[RGDfV]), while for the isoDGR derivative an IC(50) value could not be measured for the cell adhesion assay. A conformational analysis of the free RGD and isoDGR ligands by NMR (VT-NMR and NOESY experiments) and computational studies (MC/EM and MD), followed by docking simulations performed in the α(V)β(3) integrin active site, provided a rationale for the behavior of these ligands toward the receptor.