Use of Radioiodinated Peptide Arg-Arg-Leu Targeted to Neovasculari- zation as well as Tumor Cells in Molecular Tumor Imaging.

OBJECTIVE: To explore a tumor peptide imaging agent Arginine-Arginine-Leucine (Tyr-Cys-Gly-Gly-Arg-Arg- Leu-Gly-Gly-Cys, tripeptide RRL [tRRL]) that targeted to tumor cells and tumor-derived endothelial cells (TDECs) and primarily investigate the possible relationship between tRRL and vascular endothelial growth factor receptor 2 (VEGFR-2).
METHODS: The tRRL sequence motif was identified as a tumor molecular marker specifically binding to TDECs. Tyrosine was conjugated to the amino terminal of RRL (Cys-Gly-Gly-Arg-Arg-Leu-Gly-Gly-Cys) for labeling with radionuclide iodine-131 ((131)I-tRRL). The uptake ability and molecular binding of tRRL to tumor cells and angiogenic endothelium were studied using flow cytometry and radioactivity counter in vitro. Whether VEGFR-2 is the binging site of tRRL was investigated. Biodistribution and single-photon emission computed tomography (SPECT) imaging of (131)I-tRRL were used to evaluate the effectiveness of this new imaging agent to visualize varied tumor xenografts in nude mice.
RESULTS: In vitro cellular uptake experiments revealed that tRRL could not only adhere to tumor angiogenic endothelial cells but also largely accumulate in malignant tumor cells. VEGFR-2, which is highly expressed on TDECs, was probably not the solely binding ligand for tRRL targeted to tumor angiogenic endothelium. (131)I-tRRL mainly accumulated in tumors in vivo, not other organs at 24 h after injection. SPECT imaging with (131)I-tRRL clearly visualized tumors in nude mice, especially at 24 h.
CONCLUSION: Radioiodinated tRRL offers a noninvasive nuclear imaging method for functional molecular imaging of tumors targeted to neovascularization, and may be a promising candidate for tumor radioimmunotherapeutic carrier.