Synthesis of closo- and nido-carborane derivatives of the KRGD peptide

Peptides of the RGD family are of significant interest as vectors for targeted delivery of various therapeutic and diagnostic groups to tumor cells. Their application can be especially useful in the implementation of boron neutron capture therapy (BNCT) of malignant tumors. We have developed a method for obtaining derivatives of the lysine-arginine-glycine-aspartic acid (KRGD) peptide containing two closo- or nido-carborane fragments linked to a lysine residue. It has been shown that to obtain bis(closo-carboranyl) KRGD peptide with free functional groups, it is preferable to use protecting groups that can be removed under mild acidic conditions. Deboronation of the peptide containing closo-carborane residues made it possible to obtain a bis(nido-carboranyl) tetrapeptide containing 20 wt.% boron and having high solubility in water (up to 5 mg/mL). In vitro experiments demonstrated the low cytotoxicity of the KRGD peptide containing two nido-carborane residues (CC50 > 100 mu mol/L). The developed synthetic approach to KRGD derivatives containing 18-20 boron atoms per molecule opens the way to potential boron delivery agents for BNCT.