In vivo optical imaging to enhance the detection of
cancer during endoscopy or surgery requires a targeted
fluorescent probe with high emission efficiency and high signal-to-background ratio. One strategy to accurately detect
cancers is to have the fluorophore internalize within the
cancer cells permitting nonbound fluorophores to be washed away or absorbed. The choice of fluorophores for this task must be carefully considered. For depth of penetration, near-infrared probes are ordinarily preferred but suffer from relatively low quantum efficiency. Although
green fluorescent protein has been widely used to image
tumors on internal organs in mice, green
fluorescent probes are better suited for imaging the superficial tissues because of the short penetration distance of green light in tissue and the highly efficient production of signal. While the fluorescence properties of green fluorophores are well-known in vitro, less attention has been paid to their fluorescence once they are internalized within cells. In this study, the emission efficiency after cellular internalization of four common green fluorophores conjugated to
avidin (Av-
fluorescein, Av-Oregon green, Av-
BODIPY-FL, and Av-
rhodamine green) were compared after each conjugate was incubated with SHIN3
ovarian cancer cells. Using the
lectin binding receptor system, the
avidin-fluorophore conjugates were endocytosed, and their fluorescence was evaluated with fluorescence microscopy and flow cytometry. While
fluorescein demonstrated the highest signal outside the cell, among the four fluorophores, internalized Av-
rhodamine green emitted the most light from SHIN3
ovarian cancer cells both in vitro and in vivo. The internalized Av-
rhodamine green complex appeared to localize to the endoplasmic vesicles. Thus, among the four common green
fluorescent dyes,
rhodamine green is the brightest green fluorescence probe after cellular internalization. This information could have implications for the design of
tumor-targeted
fluorescent probes that rely on cellular internalization for
cancer detection.