The coupling of specific
antibodies to imaging agents often improves imaging specificity. However, free
amine groups designed for the coupling can cause nonspecific binding of the imaging agents. We report here development of a nanocarrier, MnMEIO-silane-NH2-mPEG nanoparticles (NPs), consisting of a
manganese-doped iron oxide nanoparticle core (MnMEIO), a copolymer shell of
silane and
amine-functionalized
poly(ethylene glycol) (silane-EA-
mPEG). The key feature in MnMEIO-silane-NH2-mPEG is the flexible PEG, which masks the non-conjugated reactive
amine groups (-NH2 ↔ -NH3(+)) and reduces nonspecific binding of MnMEIO-silane-NH2-mPEG to cells. The
amine groups on MnMEIO-silane-NH2-mPEG were conjugated with the
fluorescent dye, Cy777 or
antibodies [
Erbitux (Erb)] to form a MR-optical imaging
contrast agent (MnMEIO-silane-NH2-(Erb)-mPEG) for EGFR-expressing
tumors. Confocal microscopic and flow cytometric analyses showed that MnMEIO-silane-NH2-(Erb)-mPEG displayed low nonspecific binding. Moreover, TEM images showed that MnMEIO-silane-NH2-(Erb)-mPEG were endocytosed by EGFR-expressing cells. In line with their EGFR expression levels, A431, PC-3, and Colo-205
tumors treated with MnMEIO-silane-NH2-(Erb)-mPEG NPs showed -97.1%, -49.7%, and -2.8% contrast enhancement, respectively, in in vitro T2-weighted MR imaging. In vivo T2-weighted MR imaging and optical images showed that MnMEIO-silane-NH2-(Erb)-mPEG could specifically and effectively target to EGFR-expressing
tumors in nude mice; the relative contrast enhancements were 7.94 (at 2 h) and 7.59 (at 24 h) fold higher in A431
tumors as compared to the EGFR-negative Colo-205
tumors. On the contrary, MnMEIO-silane-NH2-(Erb) NPs showed only 1.44 (at 2 h) and 1.52 (at 24 h) fold higher in EGFR-positive
tumors as compared to the EGFR-negative
tumors. Finally,
antibodies can be readily changed to allow imaging of other
tumors bearing different
antigens. These data indicate that masking surface charges on
contrast agents is a useful strategy to improve imaging efficacy.