Quantum Dots for Cancer Diagnosis and Therapy: Biological and Clinical Perspectives

Hua Zhang; Douglas Yee; Chun Wang

Disclosures

Nanomedicine. 2008;3(1):83-91. 

In This Article

QD Imaging for Cancer Diagnosis and Treatment

Recent developments in QD technology have already made a remarkable impact on cancer imaging. Here, three applications of QDs in cancer imaging are discussed from biological and clinical perspectives. Current achievements, challenges and future directions of QD development are described and also summarized in Table 1 .

Sentinel lymph nodes (SLNs) are the first lymph node or group of nodes to receive metastasizing cancer cells from a primary tumor. SLN biopsy is a procedure to enable the identification and removal of the SLNs. If cancer is not found in the SLNs, then further lymph-node sampling can be avoided. Currently, SLN biopsy is in routine clinical use for staging and prognosis of breast cancer and melanoma.[34] In other types of cancer, SLN is a well accepted concept, yet SLN biopsy has not been validated widely. Success in SLN biopsy requires precise nodal mapping. Current mapping techniques include using peritumoral injection of radioisotopes, such as Technetium-99m-colloidal albumin, the isosulfan blue dye or the combination of the two agents. The identification rate of these techniques in breast cancer is over 90%, with a false-negative rate of less than 10%.[34]

Recent studies have shown that the type II QDs, which emit fluorescence at the NIR region, might be used in SLN mapping. Kim et al. coated the type II QDs with polydentate phosphine, which renders them soluble and stable in serum. After intradermal injection, these QDs localized to the nearest SLN rapidly, both in mice and pigs.[13] Several reports followed, showing that QDs can be used for SLN mapping in various types of cancer.[35,36,37,38,39] The advantage of QDs over blue dye is that SLN stained with extremely bright QDs can be visible through the depth of tissue, even before the incision is made. Furthermore, QDs injected into tumors drain to the SLNs very quickly,[40] which suggests that, with the guidance of QDs, it should be possible to remove the primary tumor and the SLN in a single surgery.

Because current techniques in SLN mapping in breast cancer and melanoma have very high success rates, utilization of QDs in these cancers might not add much to currently available techniques. However, type II QDs might have great impact on SLN mapping in cancers that involve visceral organs, such as gastrointestinal, lung and prostate, which have more complex lymphatic-draining systems.[37,38,39,41] For example, in esophageal cancer, the draining regional lymph nodes vary depending on the area of esophagus involved. Furthermore, the lymph nodes of the thoracic esophagus might be pigmented, which renders blue dye less valuable in such situations.[41] Under a single excitation source, multiple QDs with different emission wavelengths may be applied to identify and sort the complicated lymphatic system. In a proof-of-principle experiment, Kobayashi et al. have successfully mapped five different lymphatic basins in the upper body of the mice, using five QDs with similar sizes but different emission spectra.[20]

When regular cells transform into cancer cells, their protein-expression profiles change dramatically. Certain proteins, called 'cancer cell-specific antigens', are expressed on the surface of cancer cells. For example, prostate-specific membrane antigen (PSMA) is expressed highly on the surface of prostate carcinoma[42] and MUC1 (CD227) is overexpressed in 90% of human breast cancers.[43] These highly expressed antigens might not be involved in the proliferation and metastasis of cancer cells but could still be used as distinct targets for cancer immunotherapy. For cancer imaging, these antigens might also serve as targets or markers for diagnosis. In principle, recognition of the primary tumor and identification of regional or distant metastases could be accomplished by QDs conjugated with the antibodies specific for these antigens. Nie's group has conducted some pioneering work using antibody-conjugated QDs to target prostate tumor. A PSMA-specific antibody was conjugated with Cd/Se QDs and injected intravenously into nude mice bearing prostate tumor xenografts. After 2 h of circulations, these conjugated QDs were localized to the tumor.[16] By contrast, the PEG-modified QDs, when injected at a concentration 15-times higher than PSMA antibody-conjugated QDs, reached the tumor only after 24 h circulation, a much longer time interval, presumably by diffusion. This passive diffusion is owing to the enhanced permeability and retention (EPR) of QDs at the tumor site because tumors have leaky blood vessels and lack an effective lymphatic-drainage system.[44] Their work demonstrated very nicely the feasibility of antibody-mediated specific tumor targeting in vivo and the ability of achieving long-circulation in vivo by proper surface modification of QDs with polymers.

Current research has focused on using QDs to target primary tumor with success in proof-of-principle experiments in small animal models. To achieve maximal impact on cancer diagnosis in the clinical setting, research efforts should move toward identifying and localizing small tumors that are not detectable currently by conventional imaging methods, both at the primary and metastatic sites. Because any injected fluid drains normally into SLNs, it is extremely challenging to detect micro-metastasized cancer cells in the lymph nodes by QD imaging. What is more practical clinically is to use conjugated QDs to identify micro-metastasis in distant organs beyond the lymphatic system. This possibility is further enhanced by the ability of fluorescent multiphoton in vivo microscopy with single-cell resolution.[45,46]

In addition to tumor antigens, many transmembrane-receptor tyrosine kinases are also selectively and highly expressed on the surface of certain types of cancer cells. In contrast to tumor-specific antigens, these tyrosine kinases have crucial roles in the proliferation, motility and metastasis of cancer cells. Therefore, they serve as molecular targets in targeted cancer therapy. Several antibodies and small-molecule inhibitors of certain receptor tyrosine kinases, such as the EGF receptor (EGFR) family members, including EGFR and HER-2, are used clinically in treating cancers that overexpress these targets. In addition, inhibitors against other tyrosine kinase receptors, such as the type I IGF receptor (IGF1R), are being tested in clinical trials.[47] Furthermore, some nonkinase cell-surface receptors, such as integrins, which mediate cell migration through interactions with the extracelluar matrix, are also attractive therapeutic targets.

Given the availability of drugs directed against cell-surface proteins, how to determine accurately the clinical benefit of target inhibition for any individual patient is a major challenge and requires a simple, direct way to identify the level of target on cancer cells. Because cancer is a heterogeneous and extremely complex disease, even in the same type of cancer, the levels of the same receptor tyrosine kinases vary greatly. For example, HER-2 is only expressed highly in 20-30% of breast cancer patients.[48] Furthermore, receptor levels can change over time, even in the same patient at different disease stages and after metastasis. For instance, HER-2 levels have been reported to change in a subset of distant metastases from breast carcinomas.[49] For primary tumor, surgical resection enables evaluation of such targets; if a high level of target expression is found, the patient will be selected for treatment. For metastatic tumors, however, it is more difficult clinically to determine the level of receptor expression when an invasive biopsy is not possible.

Therefore, the first promising application of QDs is to identify molecular target expression in primary and metastatic cancers. This could be achieved by conjugating QDs with a ligand or an antibody specific for the target, then administrating conjugated QDs in vivo for active tumor targeting. In fact, conjugation of QDs with HER-2 antibody was among the first few attempts to address this issue.[17] These QDs bind specifically to HER-2-positive breast cancer cells but not to other breast cancer cells in vitro. However, it is not clear whether conjugated QDs can reach and bind to the specific receptors on tumor cells, after intravenous delivery in vivo. In a recent study, RGD (Arg-Gly-Asp) peptide-conjugated QDs that recognize αvβ3 integrin specifically were used to target U87MG human glioblastoma tumors in vivo in a mouse xenograft model.[19] After intravenous injection of RGD-conjugated QDs through the tail vein, QD fluorescence was observed in the tumor as quickly as 20 min later and reached maximum intensity after 6 h. It is noteworthy that the QD fluorescence was retained in the tumor vasculature and did not appear to extravasate into tumor cells, despite the fact that RGD-conjugated QDs bound specifically to U87MG cells and tumor tissue in vitro, whereas the bare QDs did not. Similar phenomena were observed when other peptide-conjugated QDs (F3 or LyP-1) were injected intravenously: they were localized to tumor vasculature but not tumor cells.[50,51] Interestingly, using a high-speed confocal microscope with a high-sensitivity camera, Tada et al. tracked the delivery process of trastuzumab (an HER-2 monoclonal antibody)-conjugated type II QDs successfully to extravasation and binding to xenograft breast tumor cells in vivo in nude mice.[42] However, their report tracks single QDs at the microscopic level and the information on the percentage of total QD population that might extravasate into surrounding tissues was not available.[52]

One common mechanism of action of therapeutic antibodies against receptor tyrosine kinases is targeted downregulation with subsequent inhibition of receptor function.[47,53] Therefore, the level of receptors after antibody therapy is an excellent biodynamic marker for monitoring the therapeutic efficacy. Future development of QDs should address the ability of these probes to measure quantitatively the level of the receptor post-therapy.

Furthermore, QDs attached covalently to therapeutic agents can serve as a tracker to evaluate the delivery effectiveness and the pharmacodynamics of the agents in real time. Stroh et al. used QD-tagged silica microspheres in conjunction with multiphoton intravital microscopy to guide the design of particulate drug-delivery vehicles.[46] QDs were also conjugated with HER-2 siRNA to track the delivery of therapeutic siNRA and to monitor the effectiveness of siRNA-mediated downregulation of the receptor in breast cancer cell lines.[54]

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