(478c) Nanogold Particle for Enhanced Fluorescent Contrast in Fluorophore Mediated Breast Cancer Imaging

Breast cancer is the most common cancer among women, and currently, X-ray mammography is the most effective tool for detecting breast cancer. However, its ionizing radiation source and low sensitivity for younger women turns people to develop a better imaging strategy. Near-infrared (NIR) mammography, a method to localize the abnormality by optical contrast caused by the accumulated hemoglobin in tumor, can be an effective tumor detection tool. For a small sized or deeply seated breast tumor, however, enhanced optical contrast is needed, using an external contrast agent. Fluorophore is a valuable contrast agent because of its light absorbing and fluorescence emitting abilities. Indocyanine green (ICG; excitation/emission wavelengths, 780/830 nm) can be a good contrast agent, since it is one of only a few Food and Drug Administration approved fluorophores. One drawback of ICG's is its extremely low quantum yield (QY) in blood (QY= 0.012).

Nanometal particles (NMPs) hold hundreds of free electrons on surface, forming strong surface plasma. Upon photoexcitation, the surface plasma generates a surface plasmon polariton field (SPPF). This SPPF can suppress the intramolecular self-quenching of a fluorophore, resulting the artificial increase of the fluorescence QY by re-routing lone pair electrons from inside the fluororphore to outside. We have verified that the enhancement level is dependent on the metal type, particle size, the distance between a particle and a fluorophore, and the QY of a fluorophore. Because of the extremely low QY of ICG, the ICG fluorescence enhancement level by NMPs was expected to be much significant. In this study, nanogold particle (NGP) was selected because of its strong plasmon field, biocompatibility, stability, and simple surface treatment process.

ICG has no reactive groups for further functionalization, thus, Cypate, a structurally modified ICG by Achilefu et al., was used for our studies. To ensure NGP is linked with Cypate with a known distance, protein spacers at various lengths, protein A (~1 nm) and streptavidin (~3 nm), were selected for their biocompatibility and stability. 2~3 orders-of-magnitude fluorescence enhancement was exhibited for NGPs linked Cypate taking the signals by Cypate only as the control. The application of the NGPs also stabilized the fluorescence emission of Cypate with no photobleaching effect observed. For in-vitro breast tumor imaging, 30~100% more enhancement in fluorescence contrast was achieved by applying NGPs linked Cypate than that by Cypate only in the experimental tumor model.

NGPs linked Cypate is demonstrated as a potent fluorescent contrast enhancing reagent, as provides an innovative approach for seamless breast tumor detection using NIR mammography. Also, by linking breast cancer anti-receptor and anticancer drug on NGPs simultaneously with Cypate, breast cancer specific particle delivery, imaging and chemotherapy will be realized successively. Additionally, these fluorescence enhancing NGPs are effective in fluorophore mediated biosensors which have been applied for ultrasensitive diagnosis of cardiovascular diseases.

Our research was partially supported by Kentucky Science and Engineering Foundation (KSEF-148-502-03-55) for the fluorescence enhancement study by metallic nanoparticle. Sigma Xi Honor Society is also acknowledged for Bin Hong's Grants-in-Aid of Research award for the synthesis of NGPs linked Cypate.