Abstract 4332: Ultra-low background optical sentinel lymph node imaging using two unique nano-particles
Authors: Kosaka Nobuyuki , Mitsunaga Makoto , Ogawa Mikako , Choyke Peter L. , Kobayashi Hisataka 
Published: February 2011

Abstract
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Optical imaging has the potential to become a robust tool in intraoperative sentinel lymph node mapping because of its portability, high spatial resolution, and absence of radiation. Most preclinical and clinical studies have used conventional fluorescent agents, which need excitation light to emit fluorescence. However, excitation light also induces autofluorescence (background signal) from intrinsic fluorophores in the normal tissue, lowering the target-to-background ratio. We developed two new methods for optical sentinel lymph node mapping to eliminate background signals using two unique nano-particles; an upconverting nano-particle (UCNP) and a self-illuminating bioluminescence resonance energy transfer quantum-dot (BRET-QD) nano-particle. UCNPs are unique nano-sized particles that emit light at shorter wavelengths with excitation in the near infrared (NIR). Since fluorophores typically emit light at longer wavelengths than the excitation light, there is no autofluorescence. Moreover, by changing the composition of the doping metals, UCNPs can be designed to emit at specific wavelengths. When UCNPs were injected into the chin of anesthetized mice, and excited at 950-nm, superficial neck lymph nodes were successfully depicted with no background signals in the visible-range (peak 550 nm) or NIR-range (peak 800 nm) in all mice. Furthermore, with serial injection of the visible and NIR-range UCNPs, simultaneous lymphatic imaging was achieved in two colors. The BRET-QD is self-illuminating and employs the substrate, coelenterazine, which reacts with luciferase conjugated on the BRET-QD. The bioluminescent light in blue efficiently excites the NIR QD, and emits photons in the NIR, therefore, there is no autofluorophore excitation. Moreover, by changing the QDs within BRET-QD, it is possible to alter their emission wavelength. BRET-QD655, (QD655 in its core), was injected at different sites (chin, ear, forepaws, and hind paws) of anesthetized mice. After BRET-QD655 injection, coelenterazine was intravenously injected and imaging was performed without excitation light. In all mice, sentinel lymph nodes in each lymphatic basin were clearly visualized with ultra-low background signals, and lasted at least 30 min after coelenterazine injections. In conclusion, we demonstrate that optical imaging using two unique nano-particles were able to depict sentinel lymph nodes with minimal autofluorescence. This enables the direct, real time sentinel lymph node mapping without extensive post processing of the images. These two novel nano-particles have the potential to be robust tools for sentinel lymph node detection.
Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 4332.