Injection site

Figure 4.8. In vivo cancer imaging and targeting with bioconjugated quantum dots. By applying a spectral unmixing algorithm, it is possible to subtract background autofluorescence and visualize visible-range emitting QD aggregation at target sites. Also illustrated is evidence that actively targeted QD probes (QD-PSMA) reach the implanted tumor more efficiently than passive targeting (QD-PEG) and no targeting (QD-COOH). [Reprinted from Gao et al. (2004) in Nature Biotechnology.]

that can be detected, considerable work still needs to be done. The quality of NIR QDs needs to be improved so that they are bright, stable, and tunable. In fact, Kim et al. have predicted that the two optimum spectral windows for in vivo QD imaging are 700-900 nm and 1200-1600 nm. The ultimate goal of this technology is its application for clinical use in human subjects. In order to realize this, toxicity issues need to be addressed. As noted earlier, QDs are typically composed of CdSe, of which Cd2+ ions are especially toxic to living cells (Derfus et al., 2004a). The experiments discussed in this section so far indicate that there are no significant short-term toxic effects of QDs if they are stable and protected with a ZnS layer. However, detailed and long-term studies need to be performed on how QDs can affect the body upon degradation.

Magnetic Nanoparticles. Magnetic nanoparticles include paramagnetic gadolinium-DTPA molecules, magnetodendrimers, and superparamagnetic iron oxide nanoparticles (SPION). We shall focus on superparamagnetic nanoparticles and their applications in molecular and cellular imaging in vivo. There are several advantages of SPION that make it a good candidate for in vivo imaging. First, iron oxide is nontoxic in low dosage since it can be metabolized, and iron is essential for normal cellular growth (Arbab et al., 2003). Second,

SPION have been developed as an MRI negative contrast agent for liver imaging and are thus familiar to radiologists. Third, SPION with the appropriate coating have been shown to circulate in the blood for 24-36 hours (Weissleder et al., 2000).

In an effort to incorporate SPION into cells and track them in vivo, Lewin et al. (2000) designed a multifunctional SPION by using both a delivery peptide (Tat) and a fluorophore (FITC). Compared to untagged particles, SPION-Tat particles uptake was improved by nearly 100-fold in human CD4+ lymphocytes (Lewin et al., 2000). Similar improvements were also obtained for human CD34+ progenitor cells, mouse neural progenitor cells, and mouse splenocytes. A major finding was that upon introduction of these SPION-loaded cells into mice, MRI detected homing of these cells to the bone marrow.

Application of SPION to imaging of tumors and angiogenesis also holds great promise. In particular, labeling deep tissue tumor to determine their physical parameters such as tumor mass, volume, degree of neovascularization, and stage of metastasis can be accomplished with SPION. An even more exciting prospect is the ability to deliver SPION conjugated to anti-angiogenic therapeutic agents. In fact, targeting of integrins using Gd-based paramagnetic liposomes has been shown (Sipkins et al., 1998). In a separate study, TGF-ß receptors in the endothelium have been targeted using radioisotope-based particles (Bredow et al., 2000). Studies have also been conducted on imaging gene expression using SPION. For instance, tumor cells expressing an engineered transferrin receptor (ETR) and those not expressing the receptor were implanted onto opposite flanks of a mouse. SPION conjugated to transferrin ligand was introduced into the mouse, and MR imaging was performed in vivo after 24 hours (Weissleder et al., 2000). The results showed that the cells expressing the receptor (ETR+) had internalized SPION but not the ETR-negative cells, as determined by the negative contrast.

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