Near-infrared optical imaging in glioblastoma xenograft with ligand-targeting alpha 3 integrin.

PURPOSE: Patients with glioblastoma usually have a very poor prognosis. Even with a combination of radiotherapy plus temozolomide, the median survival of these patients is only 14.6 months. New treatment approaches to this cancer are needed. Our purpose is to develop new cell surface-binding ligands for glioblastoma cells and use them as targeted imaging and therapeutic agents for this deadly disease.
METHODS: One-bead one-compound combinatorial cyclic peptide libraries were screened with live human glioblastoma U-87MG cells. The binding affinity and targeting specificity of peptides identified were tested with in vitro experiments on cells and in vivo and ex vivo experiments on U-87MG xenograft mouse model.
RESULTS: A cyclic peptide, LXY1, was identified and shown to be binding to the alpha 3 integrin of U-87MG cells with moderately high affinity (K (d) = 0.5 +/- 0.1 microM) and high specificity. Biotinylated LXY1, when complexed with streptavidin-Cy5.5 (SA-Cy5.5) conjugate, targeted both subcutaneous and orthotopic U-87MG xenograft implants in nude mice. The in vivo targeting specificity was further verified by strong inhibition of tumor uptake of LXY1-biotin-SA-Cy5.5 complex when intravenously injecting the animals with anti-alpha 3 integrin antibody or excess unlabeled LXY1 prior to administrating the imaging probe. The smaller univalent LXY1-Cy5.5 conjugate (2,279 Da) was found to have a faster accumulation in the U-87MG tumor and shorter retention time compared with the larger tetravalent LXY1-biotin-SA-Cy5.5 complex (approximately 64 kDa).
CONCLUSIONS: Collectively, the data reveals that LXY1 has the potential to be developed into an effective imaging and therapeutic targeting agent for human glioblastoma.