Selection of tumor-targeting agents on freshly excised human breast tumors using a phage display library.

The selective delivery of therapeutic agents to tumor site without harming rest of the body is a major challenge in clinical oncology today. Phage display approach has been used for searching ligands for cell-surface macromolecules on cancer cells so that they can be employed as drug targeting agents. Although basic protocols for biopanning cells are available, they are not as such suitable for screening highly complex and diverse target as whole tumor. Present study is an attempt to select peptide ligands specific to whole tumors. The cells from freshly collected human breast tumors were biopanned with phage displayed disulfide-constrained random heptapeptide library, following subtraction on normal human breast cells. Comparative analysis of amino acid frequencies in tumor-selected peptides and in random peptides from unselected library showed that selection was not random. The binding assessment of tumor-selected clones, using highly sensitive chemiluminescence ELISA, demonstrated that 47-75% of selected clones, depending on a tumor, bound to tumor cells they were panned on. Furthermore, several clones bound exclusively or preferentially to tumor cells in comparison to normal breast cells. It was interesting to note that insert sequences of tumor-binding clones from different tumors shared significant motifs. It shows the possibility of identifying ligands that may bind to tumor-specific targets common in certain tumors. The results of this investigation on seven human breast tumors demonstrated that, using procedures developed in the present study, whole tumors can be panned successfully with phage displayed library and tumor-binding ligands can be identified rapidly in high throughput manner. This is an important enabling step in identifying lead molecules for developing novel, specific, and effective agents that can be used for the diagnosis and treatment of cancer.