Discovery of targeting peptides for selective therapy of medullary thyroid carcinoma

Adenovirus efficiently infects a broad range of target cells, thereby preventing selective gene transfer. Moreover, several cell types and tissues including primary tumors are refractory to adenoviral infection, mainly because of low expression levels of coxsackie-adenovirus receptor (CAR). Thus, identification of cancer-selective ligands which yield gene transfer to neoplastic cells by minimizing transduction of normal cells is a key issue for successful cancer therapy.

We initially analyzed adenoviral receptor expression in human medullary thyroid carcinoma (MTC) cells. MTC cell-specific peptides were isolated by biopanning a phage display peptide library on cultured cancer cells and on tumors in vivo and further characterized.

We found significant differences in CAR and alphav-integrin protein levels between MTC-derived TT cells in vitro and established xenograft tumors in mice, indicating a lack of alphav-integrin expression on growing tumors. MTC-specific candidates were identified by performing three rounds of subtraction. Selected phages showed up to 22-fold higher binding efficiency for TT cells when compared with wild-type M13 phage or other human cell lines and tumor tissue in vivo. Homing to TT cells of the best binding phage was clearly blocked in the presence of specific peptide, whereas no phage competition was observed with an unspecific peptide. The best binding peptide mediated efficient internalization of the phage. Importantly, specific binding and internalization was also mediated by the identified peptide within the adenoviral context.

Our results indicate that the identified ligand should be suitable to improve selectivity of adenoviral gene transfer to medullary thyroid tumors in vivo.