Synthesis and Biological Evaluation of a New RGD-Camptothecin Conjugate Bearing a Cathepsin B-Sensitive Linker

Integrins are a large family of heterodimeric transmembrane glycoprotein receptors, composed by two non-covalently associated subunits (α and β). Integrins αVβ3 and αVβ5 have been found to be overexpressed on blood vessels in human tumors, but not on vessels in normal human tissues. For this reason, these integrins have become attractive targets for pharmacological studies mainly in the oncology area. The Gennari and Piarulli group recently developed a peptidomimetic compound containing the RGD (Arg-Gly-Asp) sequence and a diketopiperazine (DKP) scaffold as powerful αvβ3 integrin ligand (compound 1).1,2 This ligand was linked to Camptothecin through the Phe-Leu-Gly peptide sequence: a well-known cathepsin B-sensitive linker.3 The RGD-Drug conjugate 2 was examined in vitro for its ability to compete with biotinylated vitronectin for binding to the purified αVβ3 and αVβ5 receptors (Figure 1a). This tests showed that 2 can bind the αVβ3 integrin with high affinity (low nanomolar IC50 values). Compound 2 was subjected to stability assays in the presence of cathepsin B and lysosome extract, revealing that the free Camptothecin is efficiently released under these conditions. The antiproliferative activity of compound 2 has been evaluated against CEM-D9 (αVβ3 +) and CEM (αVβ3 ) cell lines. A higher activity of the RGD-drug conjugate 2 is shown towards the high αVβ3-expressing cell line CEM-D9 (Figure 1b). We gratefully acknowledge MIUR for financial support (PRIN project 2010NRREPL: Synthesis and Biomedical Applications of Tumor-Targeting Peptidomimetics).