Synthesis and preclinical evaluation of a novel, selective 111In-labelled aminoproline-RGD-peptide for non-invasive melanoma tumor imaging

In recent years, many efforts have been addressed to the development of new imaging techniques enabling early diagnosis and non-invasive monitoring of primary tumors and metastases. Among the integrin family, alpha(V)beta(3) and alpha(5)beta(1) receptors have been characterized as prototypic markers of angiogenic tumor-associated endothelial cells and their overexpression in tumor cells has been correlated with the progression of various tumor types such as melanoma. Herein, we report the synthesis, characterization and preclinical evaluation of an In-111-labelled DOTA conjugate embodying a cyclic aminoproline-RGDpeptide motif as a competent alpha(V)beta(3) integrin ligand, to be used as a radiotracer in preclinical models of human melanoma. Practical and efficient chemical and radiochemical synthetic procedures were set up, the in vitro stability and hydrophilicity of a cold c.AmpRGD)-DOTA conjugate were demonstrated, the binding affinities toward isolated alpha(V)beta(3)/alpha(5)beta(1) receptors were assayed and inhibition of cell adhesion to vitronectin and fibronectin was tested in human melanoma and endothelial progenitor cell lines. The anti-angiogenic activity of the peptide conjugates was also tested and assessed in vitro by tubulogenesis assays. The in vivo biodistribution SPECT/CT studies in healthy mice revealed high renal uptake at earlier observation times (30 min to 4 h p.i.) and complete clearance from the kidney at 48 h p.i.; the displacement experiments in human melanoma xenografts confirmed the alpha(V)beta(3)/alpha(5)beta(1) integrin specificity of tumor uptake, suggesting the In-111-labelled c.AmpRGD)-DOTA bioconjugate as a promising starting point in the search for new SPECT-imaging small-molecular probes for non-invasive visualization of tumor angiogenesis, human melanoma and other alpha(V)beta(3)/alpha(5)beta(1)-positive tumors.