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Molecular imaging provides the in vivo characterization of cellular molecular events involved in normal and pathologic
processes. With the advent of optical molecular imaging, specific molecules, proteins and genes may be tagged with a
luminescent reporter and visualized in small animals. This powerful new tool has pushed in vivo optical imaging to the
forefront as it allows for direct determination of drug
bio-distribution and uptake kinetics as well as an indicator of
biochemical activity and drug efficacy. Although optical imaging encompasses diverse techniques and makes use of
various wavelengths of light, a great deal of excitement in molecular research lies in the use of tomographic and
fluorescence techniques to image living tissues with near-infrared (NIR) light. Nonionizing, noninvasive near-infrared
optical imaging has great potential to become promising alternative for breast cancer detection. Fluorescence
spectroscopy studies of human tissue suggest that a variety of lesions show distinct fluorescence spectra compared to
those of normal tissue. It has also been shown that exogenous dyes exhibit selective uptake in neoplastic lesions and may
offer the best contrast for optical imaging. Use of exogenous agents would provide fluorescent markers, which could
serve to detect embedded tumors in the breast. In particular, the ability to monitor the fluorescent yield and lifetime may
also enable biochemical specificity if the fluorophore is sensitive to a specific metabolite, such as oxygen.
As a first step, we have synthesized and characterized one such NIR fluorescent dye conjugate, which could potentially
be used to detect estrogen receptors (ER)[2] . The conjugate was synthesized by ester formation between 17-β estradiol
and a hydrophilic derivative of indocyanine green (ICG) cyanine dye, bis-1, 1-(4-sulfobutyl) indotricarbocyanine-5-
carboxylic acid, sodium salt. The ester formed was found to have an extra binding ability with the receptor cites as
compared to ICG, which was established by the partition coefficient studies. The replacement of the sodium ion in the
ester by a larger glucosammonium ion was found to enhance the hydrophilicity and reduce the toxic effect on the cell
lines. The excitation and emission peaks for the conjugate were recorded in the NIR region as 750nm and 788nm
respectively. The ester was found nontoxic on adenocarcinoma breast cancer cell lines MCF-7/MDA-MB-231. Specific
binding and endocytosis of the estrogen-labeled conjugate was studied on the MCF-7 (ER positive) and MDA-MB-231
(ER negative). Conjugate staining of MCF-7 cells showed ~ 4-fold increase in signal intensity compared to MDA-MB-
231. Further, estrogen molecules were found to be specifically localized to the nuclear region of MCF-7 cells, whereas
MDA-MB-231 showed plasma membrane staining.
This technique offers the potential of noninvasive detection of hormone receptor status in breast cancer cells and would
help in decreasing the load of unnecessary biopsies. Here, we have reported the progress made in the development of a
novel NIR external contrast agent and the work is in progress to use this conjugate for the molecular based, diagnostic
imaging of breast cancer.
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