Active Targeting
Conjugation of antibodies, peptides, or other ligands onto the nanoparticle surface produces active targeting agents, which can selectively accumulate on specific cells or tissues. This innovative imaging approach expands the role of CT beyond its present structural imaging capabilities, endowing it with functional and molecular-based imaging capabilities as well.
Cancer
Molecularly targeted nanoparticles reach tumor tissues through the EPR effect (as in passive targeting). However, the active targeting has additive values; the nanoparticles home selectively onto specific tumors and remain at the tumor site for an extended time, thereby increasing the local accumulation of the nanoparticles in sites of interest.
Specific targeting could be achieved through the conjugation of nanoparticles to a variety of ligands, including antibodies, peptides, aptamers or small molecules that possess high affinity toward unique molecular signatures found in diseases such as cancer. Hainfeld et al. demonstrated molecular imaging of cancer with actively targeted CT contrast agents.[68] They showed that GNPs can enhance the visibility of mm-sized human breast tumors in mice, and that active tumor targeting (with anti-Her2 antibodies) is 1.6-fold more efficient than passive targeting. They also showed that the specific uptake of the targeted GNPs in the tumor's periphery was 22-fold higher than in surrounding muscle. In another study, Chanda et al. reported enhanced CT attenuation of bombesin functionalized GNPs that selectively targeted cancer receptor sites that are overexpressed in prostate, breast and small-cell lung carcinoma.[69] In our own research, we recently demonstrated, in vitro[66] and in vivo,[67] that the CT number of molecularly targeted head and neck cancer is over five-times higher than the corresponding CT number of an identical but untargeted tumor, and that active tumor targeting is more efficient and specific than passive targeting (Figure 6).[70,71] This specific interaction between antigen and antibody or receptor and its ligand was shown to be an effective strategy to improve the amount and residence time of contrast agents in tumors, as well as to provide specific molecular knowledge regarding the findings.
Figure 6.
In vivo x-ray computed tomography volume-rendered images of (A) a mouse before GNP injection, (B) a mouse 6 h postinjection of nonspecific IgG GNP as a passive targeting experiment, and (C) a mouse 6 h postinjection of anti-EGFR coated GNP that are specifically targeted the SCC head and neck tumor.
The anti-EGFR targeted GNP show clear contrast enhancement of the tumor ([C], arrow), which was undetectable without the GNP contrast agents ([A], arrow). Computed tomography (CT) numbers represent the average Hounsfield unit of the whole tumor area. All scans were performed using a clinical CT at 80 kVp, 500 mAs, collimation 0.625 x 64 mm and 0.521 pitch size (A 64-detector CT scanner, LightSpeed VCT, GE Medical Systems).
Submitted to the International Journal of Nanomedicine.71
Atherosclerosis
Rupture-prone plaques, which are rich in macrophages, have been specifically targeted by gold high-density lipoprotein nanoparticles (Au-HDL).[72,73] The targeted particles induced CT contrast enhancement specifically in macrophage-rich, rupture-prone plaques, while no significant enhancement was observed for stable plaques that are not rich in macrophages. In addition, the same group proposed a new concept of multicolor spectral CT, in which incident x-rays are divided into six different energy bins that can be used for multicolor imaging. This imaging method enabled the discrimination between Au-HDL, iodine-based contrast material, and calcium phosphate in the phantoms and in a mouse model. Accumulations of Au-HDL were detected in the aortas of the mice, while the iodine-based contrast agent and the calcium-rich tissue could also be detected and thus facilitated visualization of the vasculature and bones (skeleton), respectively, during a single scanning examination (Figure 7).[72,73] In another study multicolor spectral CT imaging was applied to visualize intravascular pathologic epitopes with fibrin-targeted bismuth nanoparticles in rabbit models of atherosclerosis.[74] Performing multicolor CT could enable a simultaneous tracking of two differently labeled cell populations and a study of their interactions or the interaction of cells with specific receptors, while each retains its own unique label.
Figure 7.
Multicolor spectral CT images.
(A–C) Spectral computed tomography images of thorax and abdomen in apoE–KO mouse injected 24 h earlier with Au-HDL. (D & E) Spectral CT images near bifurcation of aorta in apoE–KO mouse injected with Au-HDL and an iodinated emulsion contrast agent (Fenestra VC) for vascular imaging.
Reproduced with permission from [72].
These studies demonstrated that molecularly targeted particles yield the potential to significantly improve CT contrast and specificity through increasing the local accumulation of nanoparticles in sites of interest. However, since the active targeting approach is based on the existence and the degree of overexpression of specific tissue biomarkers, it is applicable only under particular biological conditions. Table 3 summarizes recent studies regarding passive and active in vivo CT contrast agents.
Nanomedicine. 2012;7(2):257-259. © 2012 Future Medicine Ltd.
