Arg-Arg-Leu peptide-targeted near infrared imaging of experimental prostate tumors in vivo Free

5522

BACKGROUND: Tumor endothelial cells express a specific and unique cell surface signature. We have previously reported on the discovery of tumor endothelial cell (TDEC) binding peptides that possess the amino acid sequence Arg-Arg-Leu (RRL). This group has previously demonstrated by tumor ultrasonography that RRL-peptide linked microbubbles will localize to the vasculature of xenograft prostate tumors in vivo. However the tumor retention time and the whole body specificity of the peptide is unknown. In this study we link the RRL peptide to the near infrared (IR) molecules to carry out real-time whole animal imaging.

METHODS: Alexa 680 and Alexa 800 are two near-infrared dyes that emit at a wavelength minimally quenched by biologic tissue, thus permitting whole animal imaging. These molecules can be conjugated to RRL peptide. One microgram of the RRL-Alexa conjugate is injected via the intracardiac route into TRAMP (transgenic prostate cancer) mice or mice bearing subcutaneous xenografts tumors derived from TRAMP prostate tumor cells (C2 cells). A peptide containing only the glycine backbone (G7 peptide) served as the control. RESULTS: Intravenous administration of RRL-peptide-Alexa chimera to TRAMP or mice with C2 flank subcutaneous tumor results in the fluorescent visualization of tumors within the intact animal through the prolonged intra-tumor retention of the RRL-dye complex as compared to controls. The tumor could be visualized within 3 minutes and residual signal remains apparent at 2 hours. Whole animal imaging revealed the tumor and did not show an appreciable image signal at any other site or organ other than the kidneys, and IR fluorescent urine was apparent immediately upon injection of the agent. Direct time-lapse intravital microscopy of prostate tumors show real-time tumor vascular binding of the RRL peptide. Microscopy reveals deposition of the Alexa dye in tumor vasculature.

CONCLUSION: The RRL peptide homes imaging agents to prostate vasculature and is useful for the specific detection of experimental prostate tumors in vivo.