CONJUGATED BRANCHED PEPTIDES AS TARGETING AGENTS FOR TUMOR IMAGING AND THERAPY

Membrane receptors for endogenous peptides like Somatostatin, Neurotensin, Bombesin and many others, are over-expressed in different human cancers and might be targeted as tumor-specific antigens. Efficiency of peptide-receptor targeting depends on the presence and concentration of receptors on neoplastic tissues. Our goal is producing peptide molecules that can be used for both a specific receptor-tracing test and for therapy or in vivo imaging, by delivering radio or chemotherapeutic moieties to the same receptors on tumor cells. We previously demonstrated that peptides synthesized in a branched form known as as Multiple Antigen Peptides (MAP) can retain biological activity and become extremely resistant to endogenous proteases and peptidases (1). An important possible development of protease-resistant MAPs, is the application to tumor therapy and in vivo imaging. Analogues of neurotensin (NT) and its functional fragment NT(8- 13) are under investigation to target a variety of human tumors overexpressing NT receptors. We demonstrated that the tetra-branched MAP form of NT and NT(8-13), fully retain their biological activity and become much more resistant to proteases (1). NT oligo-branched peptides are therefore candidates for the development of new specific tumor-targeted biomolecules. In the present work, different NT analogues were synthesised as tetra-branched MAPs, where the NT sequences behave as the tumor targeting moiety, while functional units for treatment or tracing are bound at the C-terminus of the molecule. Tetra-branched NT conjugated to fluorophores were synthesized and tested for receptor binding on different human tumor cell lines. Once verified that the conjugation of the branched peptide did not reduce its ability to bind the receptor, we tested our peptides on samples from surgical resection of colon and pancreas adenocarcinoma, in comparison with healthy tissues from the same patients. Confocal images show a general good staining by branched NT peptides in most tumor samples and a much lower staining in healthy tissue from the same patients, indicating a good selectivity of the peptide. The same tetra-branched peptide was conjugated to chemotherapic drugs and tested for cytotoxicity toward tumor cells in vitro and in animal models. Results indicate promising features of branched NT for selective tumor targeting.