Molecular imaging and quantitative measurement of epidermal growth factor receptor expression in live cancer cells using immunolabeled gold nanoparticles.

The goal of this study was to assess whether immunolabeled nanoparticle biomarkers are comparable to fluorescent marker imaging in measuring epidermal growth factor receptor (EGFR) expression. EGFR expression was quantified using both imaging methods in four cell lines: A431 human epidermoid carcinoma cells, which are known to have high EGFR expression; two cell lines with lower EGFR expression (270-GBM human glioblastoma xenograft cells and H2224 human glioblastoma xenograft cells); and MDA-MB-453 breast carcinoma cells, which do not express EGFR. To enhance contrast of the nanoparticle biomarkers, a darkfield microspectroscopy system was used that includes a custom epi-illumination light train. Nanoparticle-bound cells were clearly distinguished from control cells not bound to nanoparticles in that they showed a significant increase in detected intensity under darkfield illumination due to nanoparticle scattering. The average nanoparticle-scattering intensity for A431 cells was 41.5 counts per cell compared with 24.7 for 270-GBM cells, 8.77 for H2224 cells, and 0.44 for MDA-MB-453 cells. The average fluorescence intensity for A431 cells was 35.3 counts per cell compared with 28.7 for 270-GBM cells, 5.91 for H2224 cells, and 2.07 for MDA-MB-453 cells. A plot of fluorescence intensity versus nanoparticle-scattering intensity for all four cell lines showed that the data agree with a linear relationship given by the following equation: NP = 1.0691 x FL - 0.3873, where NP is the nanoparticle-scattering intensity and FL is the fluorescence intensity. The covariance of the data with the trend line was R(2) = 0.9409. The average peak wavelength of nanoparticle scattering was 570.93 nm for A431 cells, 565.26 nm for 270-GBM cells, and 562.70 nm for H2224 cells (with no clear peaks observed for MDA-MB-453 cells). This spectral trend shows that nanoparticle scattering may reveal additional information about their nanoenvironment via refractive index sensitivity. Immunolabeled nanoparticles can quantify receptor expression with performance comparable to fluorescence markers and show promise to better characterize receptor expression via their refractive index sensitivity.