Gold nanoparticles
Gold nanoparticles are promising molecular imaging agents; and conjugating such particles with cancer-seeking antibodies enables their direct targeting to tumors. The ability to quantitatively and noninvasively detect targeted nanoparticles in vivo could provide a promising cancer diagnostic tool.
The method
Researchers at the Engineering Faculty of Bar Ilan University in Israel have developed a tumor detection techniques based on diffusion reflectance (DR) measurements of injected gold nanorods (GNRs). The development is an inexpensive and easy-to-use method for analyzing tissue optical parameters by quantitatively measuring the in vivo concentration of GNRs which can indicate tumor size, the larger the GNR concentration the higher the EGFR amount in the cells as the EGFR concentration directly correlates with the carcinoma amount.
Technique
The DR technique involves measuring the reflected light intensity profile of an irradiated tissue at a range of source-detector distances. This profile can then be used to estimate the tissue's absorption and reduced scattering coefficients – the former of which will be influenced by the presence of highly absorbing GNRs. A higher absorption coefficient can lead to a sharper decay of the reflected light intensity profile
Even when the absorption coefficients only differed slightly, the curves exhibited distinctly different slopes. A linear relationship between the square of this slope and the absorption coefficient was observed – confirming that it's possible to extract the tissues' optical properties from the measured reflectance data.
The researchers measured the reflected light intensities from different tissue-like phantoms with a known scattering coefficient and different known absorption coefficients. The DR measurement set-up comprised a 650 nm laser diode excitation source and a 1-mm-diameter photodiode detector that was moved across the sample surface. Plots of the reflected light intensity profile reveal that a higher absorption coefficient results in a sharper slope.
Study
The researchers examine the reflected light intensity from tumor-bearing body when injected with antibody-conjugated PEG-coated GNRs and DR measurements of the tumor are performed before injection, and 15 minutes, five hours and 10 hours post-injection. Reflected light intensity profiles before the GNR injection can exhibit no negative slope implying low absorption and scattering properties. Ten hours after injection, the GNR absorption coefficient – calculated from the change in the graphs' slopes can correspond to a definite GNR concentration in the tumor.
The researchers concluded that their results prove that DR measurements can be used to calculate GNR concentration in a tumor. Such a molecular detection tool could facilitate early detection of superficial tumors, such as head-and-neck cancers, breast cancer and melanoma. The technique could also find application in image-guided therapy of such tumors.