Targeted Tumor Cell Internalization and Imaging of Multifunctional

Aug 20, 2008 - Nano Lett. , 2008, 8 (9), pp 2851–2857. DOI: 10.1021/ .... Targeted Cellular Delivery of Quantum Dots Loaded on and in Biotinylated L...
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NANO LETTERS

Targeted Tumor Cell Internalization and Imaging of Multifunctional Quantum Dot-Conjugated Immunoliposomes in Vitro and in Vivo

2008 Vol. 8, No. 9 2851-2857

Kevin C. Weng,† Charles O. Noble,‡ Brigitte Papahadjopoulos-Sternberg,§ Fanqing F. Chen,| Daryl C. Drummond,‡ Dmitri B. Kirpotin,‡ Donghui Wang,† Yun K. Hom,† Byron Hann,† and John W. Park*,† UCSF Helen Diller Family ComprehensiVe Cancer Center, San Francisco, California 94115, Hermes Biosciences, South San Francisco, California 94080, NanoAnalytical Laboratory, San Francisco, California 94118, and Life Sciences DiVision, Lawrence Berkeley National Laboratory, Berkeley, California 94720 Received May 23, 2008; Revised Manuscript Received July 12, 2008

ABSTRACT Targeted drug delivery systems that combine imaging and therapeutic modalities in a single macromolecular construct may offer advantages in the development and application of nanomedicines. To incorporate the unique optical properties of luminescent quantum dots (QDs) into immunoliposomes for cancer diagnosis and treatment, we describe the synthesis, biophysical characterization, tumor cell-selective internalization, and anticancer drug delivery of QD-conjugated immunoliposome-based nanoparticles (QD-ILs). Pharmacokinetic properties and in vivo imaging capability of QD-ILs were also investigated. Freeze-fracture electron microscopy was used to visualize naked QDs, liposome controls, nontargeted QD-conjugated liposomes (QD-Ls), and QD-ILs. QD-ILs prepared by insertion of anti-HER2 scFv exhibited efficient receptor-mediated endocytosis in HER2-overexpressing SK-BR-3 and MCF-7/HER2 cells but not in control MCF-7 cells as analyzed by flow cytometry and confocal microscopy. In contrast, nontargeted QD-Ls showed minimal binding and uptake in these cells. Doxorubicin-loaded QD-ILs showed efficient anticancer activity, while no cytotoxicity was observed for QD-ILs without chemotherapeutic payload. In athymic mice, QD-ILs significantly prolonged circulation of QDs, exhibiting a plasma terminal half-life (t1/2) of ∼2.9 h as compared to free QDs with t1/2 < 10 min. In MCF-7/HER2 xenograft models, localization of QD-ILs at tumor sites was confirmed by in vivo fluorescence imaging.

A premise of nanomedicine is that it may be feasible to develop multifunctional constructs combining diagnostic and therapeutic capabilities, thus leading to better targeting of drugs to diseased cells. For example, it may be possible to track such constructs at the cellular and subcellular levels and to monitor their spatial and temporal interactions with tumor cells. Organic dyes have been used for optical detection of various agents including nanoparticles; however, they generally suffer from limitations such as narrow excitation/broad emission spectra, photobleaching, and biodegradation, which restrict the applicability of imaging techniques. More recently, semiconductor quantum dots * To whom correspondence should be addressed. Current address: UCSF Helen Diller Family Comprehensive Cancer Center, 1600 Divisadero St., 2nd Fl., San Francisco, CA 94115-1710. Tel: 415-502-3844. Fax: 415-3539592. E-mail: [email protected]. † UCSF Helen Diller Family Comprehensive Cancer Center. ‡ Hermes Biosciences. § NanoAnalytical Laboratory. | Lawrence Berkeley National Laboratory. 10.1021/nl801488u CCC: $40.75 Published on Web 08/20/2008

 2008 American Chemical Society

(QDs) have emerged as a promising alternative to organic dyes as fluorescent markers for in vitro and in vivo imaging; their superior brightness and photostability1-6 make them excellent candidates in the development of trackable multifunctional agents. However, as molecular probes by themselves in medical applications, QDs have a number of potential limitations such as specificity, biocompatibility, and toxicology.7 QDs have also been reported to aggregate nonselectively on the surface of cell membranes.8-10 To target them to cells of interest, QDs have been conjugated to various biocompatible entities11-13 or inserted into lipid bilayers.14 These methods sometimes involve premodification of the target cell surface or use of large quantities and multilayer couplings of QDs, which limit their practicality. As an alternative approach, we hypothesized that conjugation of QDs to immunoliposomes (ILs) would represent a feasible and novel strategy for QD-based cellular and in vivo tumor imaging, including integration of targeting, reporting,

Figure 1. Schematic showing the structure of a QD-IL nanoparticle. The liposomal core is about 100 nm in diameter as visualized by ff-EM. Derivatized CdSe/ZnS core/shell QDs are represented as a sphere with a layer of organic coating (gray) covering the outer surface of the inorganic shell (yellow) and the semiconductor core (orange). They were also characterized by ff-EM, indicating an average diameter of ∼11 nm. Surface-derivatized QDs were chemically linked to functionalized PEG-DSPE incorporated in extruded liposomes. Anti-HER2 single chain Fv fragments (scFv, arrowheads) are attached to the end of PEG chains. Nellis, D.F.; Ekstrom, D. L.; Kirpotin, D. B.; Zhu, J.; Andersson, R.; Broadt, T. L.; Ouellette, T. F.; Perkins, S. C.; Roach, J. M.; Drummond, D. C.; Hong, K.; Marks, J. D.; Park, J. W.; Giardina, S. L. Preclinical manufacture of an anti-HER2 scFv-PEG-DSPE, liposome-inserting conjugate. 1. Gram-scale production and purification. Biotechnol. Prog. 2005, 21 (1), 205–220. scFv moieties (MW ∼ 25 kDa) are not drawn to scale. QD-ILs retain an aqueous interior for loading and delivery of drugs/probes.

and drug delivery. Liposomes represent a prototypic nanoparticle system that has been fully optimized for in vivo human use as drug carriers.15 Newer liposome-based approaches include ILs in which monoclonal antibody fragments are conjugated to liposomal drugs for targeted cellspecific delivery.16 The continued evolution of ILs has led to optimized components and modular assembly strategies for specific functionalities.17 For example, ILs against HER218,19 and EGFR20,21 can efficiently deliver drugs to tumor cells and are in development for clinical testing. Our design for QD-conjugated ILs (QD-ILs) is schematically illustrated in Figure 1. To assemble the multifunctional QDIL hybrid nanoparticles with imaging, targeting, and therapeutic modalities, a 0.2 µM concentration of carboxyl CdSe/ ZnS core/shell QDs was chemically linked to 2-3 mol % amine-functionalized N-(polyethylene glycol)-1,2-distearoylsn-glycero-3-phosphoethanolamine (PEG-DSPE) incorporated in extruded liposomes through ∼2 mM zero-length cross-linker 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide (EDC). QD605 (λmax ∼ 605 nm) was used for in vitro analysis and pharmacokinetics (PK) studies. QD800 (λmax ∼ 800 nm) was used for in vivo imaging experiments for more efficient tissue penetration. Fluorescence spectrophotometry analysis of QD-conjugated liposomes (QD-Ls) indicated a mean phospholipid to QD ratio of ∼42000, corresponding to 1.7-2.2 QDs per liposome (experimental methods are available in the Supporting Information). Higher ratios of QDs/phospholipid could be achieved; however, we 2852

observed a higher extent of cross-linking and reduced internalization in vitro with such constructs (data not shown). Freeze-fracture electron microscopy (ff-EM) has been demonstrated to be a powerful technique to characterize nanoscale structures, including liposomes22 and micelles.23 Using ff-EM, we clearly visualized carboxyl QDs, both free (Figure 2a,b) and when conjugated to liposomes (Figure 2c) or HER2-targeted ILs containing doxorubicin (dox) (Figure 2d). QDs appeared as small, mostly spherical particles with shadowing behind the structure, typical for convex, hardcore particles. Diameters of the QDs investigated here ranged from 5 to 22 nm (mean, 11 nm). In contrast to QDs, liposomes and ILs displayed convex and concave fracture planes (shadow behind and shadow in front, respectively). Membrane-bound structures such as liposomes typically show fracture planes along the hydrophobic interior of bilayers as the area of weakest force interaction (hydrophobic interaction of the lipid-fatty acid tails). While both free QDs and QDs trapped in the interliposomal space showed equal diameters, QDs visible on liposomal fracture planes appeared less prominent. It is unlikely that such images depict actual QDs, which should be unable to penetrate into the hydrophobic interior of bilayers; rather, we hypothesize that these reflect imprints of surface-attached QDs showing through the semifluid, liquid-crystalline bilayers. Our ff-EM study demonstrated that conjugation of QDs resulted in some alteration of liposome size, which was also observed by dynamic light scattering measurements, showing an increase in both particle size and size distribution from 112 ( 27 to 212 ( 32 nm for extruded liposomes and QD-ILs, respectively. However, dynamic light scattering studies indicated no significant difference in size between targeted (212 ( 32 nm) and nontargeted (216 ( 37 nm) particles. Additionally, QDs were frequently observed at contact areas between liposomes/ILs. Using our methodology, the resolution limit for distinct, periodic structures in ff-EM micrographs is ∼2 nm. Therefore, the fairly long but very small and flexible PEG chains that can adopt a “mushroom” conformation were not visible. Cellular binding and uptake of free QDs and various QDnanoparticle constructs were further evaluated by flow cytometry. Target cells included human breast carcinoma cell lines with HER2 overexpression via endogenous gene amplification (SK-BR-3) or transfection (MCF-7/HER224). Parental MCF-7 was used as a negative control lacking HER2 overexpression. After 60 min of incubation at 37 °C, free carboxyl QDs bound nonspecifically to all tested cells (SKBR-3, MCF-7/HER2, and MCF-7) in a concentrationdependent manner (Figure 3). At the highest concentration of free QD tested, 10 nM, uptake was slightly higher in MCF-7 cells than in MCF-7/HER2 cells (p < 0.05, twotailed t test). In contrast, anti-HER2 QD-ILs showed specific uptake in HER2-overexpressing cells, with 16-30-fold greater uptake in SK-BR-3 and MCF-7/HER2 cells as compared with MCF-7 cells (p < 0.05 for all concentrations of QD-ILs). Furthermore, QD-ILs showed markedly greater uptake vs free QDs (50-300-fold) or vs nontargeted QDLs (900-1800-fold) in HER2-overexpressing cells at matchNano Lett., Vol. 8, No. 9, 2008

Figure 2. ff-EM micrographs of QD controls are shown. (a) QDs at 19300× and (b) QDs at 46700×. ff-EM micrographs of nanocomplexed QDs are shown. (c) QD-Ls and (d) dox-loaded QD-ILs (QD-ILs-Dox). On all electron micrographs, scale bars ) 100 nm, and the shadow direction is indicated by the arrow. Panel d revealed the cross-linked structures of QDs and liposomes, as individual and aggregated liposomes were both observed. QD-ILs-Dox also showed occasionally angular features attributable to the crystalline form of dox precipitated in the interior of liposomes.

ing concentrations. For example, in SK-BR-3 cells, uptake of 2 nM QD-ILs was ∼308-fold higher than the equivalent concentration of free QDs (p < 0.05); uptake remained ∼21fold higher than free QDs at the much higher concentration of 10 nM (p < 0.05). In non-HER2-overexpressing MCF-7 cells, QD-ILs showed equivalent and minimal uptake as nontarged QD-Ls. IL delivery of QDs corresponded to ∼2.2 × 106 QDs per SK-BR-3 or MCF-7/HER2 cell. The extremely efficient uptake of QD-ILs also allowed use of significantly reduced amounts of QD material, with the lowest test concentration of 0.5 nM still yielding significant uptake, as compared to other methods typically requiring QDs in the high nanomolar to micromolar range.11,14,25 Internalization of QD-ILs was visualized using confocal microscopy. SK-BR-3 cells were treated with free QDs, QDILs, or QD-Ls for 60 min at 37 °C (Figure 4a-g). Free carboxyl QDs were observed in close association with the cell surface (Figure 4a,b) despite cell washing and specimen processing steps. Nontargeted QD-Ls showed minimal uptake (Figure 4c). Internalization of QD-ILs was confirmed by visualizing the subcellular distribution of the conjugated QD or a separate lipid-based DiD fluorescent label, indicating that ILs had accumulated mostly in the perinuclear region, Nano Lett., Vol. 8, No. 9, 2008

consistent with endosomal routing (Figure 4d-f). Using the persistent fluorescence of QDs, we generated reconstructed three-dimensional images of cells treated by QD-ILs to map their intracellular localization (Figure 4g). This technique could be useful in studies of the spatial relations involved in intracellular trafficking of receptors. In MCF-7/HER2 cells, QD-ILs showed efficient internalization that was comparable to that observed in SK-BR-3 cells (Figure 4h); in contrast, QD-ILs showed minimal uptake in MCF-7 cells lacking HER2 overexpression (Figure 4i). To verify that internalization of QD-ILs was mediated through HER2-dependent targeting, F5-PEG-DSPE micelles were added to cell cultures for competitive blockade prior to QD-ILs treatment. Incubation with F5-PEG-DSPE micelles at 5× molar excess (as compared to the amount of scFv in QD-ILs) for 30 min prior to adding QD-ILs effectively reduced IL uptake by ∼70% in SK-BR-3 and ∼90% in MCF-7/HER2 cells, respectively. Nontargeted QDLs were also used to treat SK-BR-3, MCF-7/HER2, and MCF-7 cells to evaluate nonspecific uptake. We observed virtually no uptake nor cell surface binding for nontargeted QD-Ls (Figure 4c). 2853

Figure 3. Quantitative flow cytometry results showing fluorescence intensities (mean ( SE; n ) 6) of SK-BR-3, MCF-7/HER2 (both high HER2-overexpressing human breast cancer cell lines), and MCF-7 (low HER2-expressing human breast cancer cell line). Ten thousand cells were collected in each measurement. The columns, from left to right, represent autofluorescence, cells treated by carboxyl QDs at 2 and 10 nM, HER2-targeted QD-ILs at 0.5, 1, and 2 nM, and 2 nM nontargeted QD-Ls at 37 °C for 1 h. The fluorescence intensities were calibrated by standard fluorescent microbeads and presented in units of molecules of equivalent soluble fluorochrome (MESF) of phycoerythrin.

To install the therapeutic function, we loaded QD-Ls and QD-ILs with dox. Dox was encapsulated via the remote loading method26 either before or after covalent linking of the liposomes to QDs. While drug loading into liposomes prior to QD conjugation was highly efficient (>90% of added drug was successfully encapsulated), loading into QD-Ls was associated with ∼30% loading efficiency. Storage stability of dox encapsulated in QD-Ls and QD-ILs was evaluated, with approximately 15-30% of dox lost from QD-Ls and QD-ILs after storage in 20 mM MES buffer, pH 6, at 4 °C for 2 months. The anticancer activity of the dox-loaded nanoparticles was evaluated in HER2-overexpressing SK-BR-3 cells (Table 1). Cells (7000-8000) were cultured in 96-well sterile blackwalled assay plates (Corning, Corning, NY) and treated with various nanoparticle formulations at 37 °C for 1 h. The anticancer activity was assayed by ATP-activated luciferase assay (Promega, Madison, WI) after the cells were allowed to grow for 3 days. QD-ILs-Dox showed potent cytotoxicity against SK-BR-3 cells (IC50 ∼ 0.5 µg/mL), which was comparable to ILs-Dox without QD (IC50 ∼ 0.7 µg/mL). Nontargeted Ls-Dox with or without QDs showed ∼10-fold less cytotoxicity than their IL counterparts [p < 0.001 by one-way analysis of variance (ANOVA)]. Consistent with previous results,18 these studies confirmed that ILs but not liposomes provided intracellular dox delivery that was as efficient as the direct permeation of free dox in vitro and also indicated that this targeting effect was not compromised by QD conjugation. QD-ILs lacking dox showed minimal cytotoxicity, confirming that anticancer activity was due to drug delivery and not from the QD-IL carrier system. The 2854

slightly increased cytotoxicity of QD-conjugated nanoparticles vs their non-QD-containing counterparts (e.g., IC50 of 3.85 ( 0.24 µg/mL for QD-Ls-Dox vs 9.36 ( 0.83 µg/mL for Ls-Dox) likely resulted from some drug leakage in the presence of QDs, consistent with the stability data. It is notable that the highest concentration tested here was 5000fold lower in QD content than the concentration of CdSe/ ZnS core/shell QDs associated with normal tissue toxicity under other conditions.27 We evaluated in vivo properties of QD-ILs, including PK studies in nude mice, to address whether QD conjugation compromised the long circulation time of ILs. QD-ILs exhibited moderately prolonged circulation, with plasma terminal t1/2 of ∼2.9 h. This t1/2 was somewhat less than that for non-QD-containing ILs (t1/2 ) ∼7.7 h), possibly due to faster clearance associated with the positively charged amino groups on QD-ILs. However, t1/2 of QDs carried by ILs was greatly prolonged over that of free QDs (t1/2 < 10 min), which showed extremely rapid clearance to ∼5% of the initial plasma level by 10 min and undetectable levels after 1 h. In addition, the mice (n ) 10) were further monitored for weight change and gross toxicities for an additional 3 month period; no weight loss or obvious signs of toxicity were observed, although formal toxicology studies would be required for definitive evaluation of safety. For tumor-targeted imaging, QD-ILs were injected i.v. in nude mice bearing HER2-overexpressing MCF-7/HER2 xenografts implanted s.c. in the flank. Twenty-four hours following injection, fluorescence signals were readily detected at the tumor site as well as in mononuclear phagocytic system (MPS) organs known to mediate liposome clearance Nano Lett., Vol. 8, No. 9, 2008

Figure 4. SK-BR-3 cells (panels a-g), MCF-7/HER2 (panel h), or MCF-7 cells (panel i) were treated with free QDs or QD-conjugated nanoparticles (red fluorescence) for 60 min at 37 °C. Cells were stained by DAPI (blue fluorescence). An argon laser (488 nm) or titanium: sapphire (800 nm) was used for QD excitation. A HeNe laser (633 nm) was used for lipidic dye DiD (emission peak at 665 nm) excitation. (a and b) SK-BR-3 cells treated with free carboxyl QDs, showing nonspecific association on the cell surface (image size ) 235 µm × 235 µm, and magnification ) 40×). In panel a, QD fluorescence (red) and nucleus DAPI staining (blue) are shown. In panel b, differential interference contrast (DIC) and QD fluorescence images are combined to reveal more vividly the binding of QDs along the outline of SK-BR-3 cells. (c) SK-BR-3 cells treated with nontargeted QD-liposomes (QD-Ls), showing virtually no detectable QDs (image size ) 235 µm × 235 µm, and magnification ) 40×). (d) QD-conjugated anti-HER2 ILs (QD-ILs) internalized in SK-BR-3 cells (image size ) 235 µm × 235 µm, and magnification ) 40×). (e) Dye-incorporated (green) QD-ILs internalized in SK-BR-3 cells (image size ) 146 µm × 146 µm, and magnification ) 63×). A 0.25-0.5 mol % amount of 1,1′-dioctadecyl-3,3,3′,3′-tetramethylindodicarbocyanine perchlorate (DiD oil) was included in the lipid mixture for dual labeling. (f) Subpanels of QDs (red, top left), DiD (green, bottom left), DIC (gray scale, top right), and composite (bottom right) images show the intracellular uptake of QD-ILs, in contrast to surface binding of free QDs in panels a and b (subpanel size ) 235 µm × 235 µm, and magnification ) 40×). (g) Reconstructed 3D image of QD-ILs (red) internalized within SK-BR-3 cells (nucleus in blue). The z-stack constitutes 11 panels that span a total 30 µm elevation (image size ) 146 µm × 146 µm, and magnification ) 63×). (h) QD-ILs internalized within MCF-7/HER2 cells (image size ) 146 µm × 146 µm, and magnification ) 63×). (i) MCF-7 cells treated with QD-conjugated anti-HER2 ILs (QD-ILs), showing minimal uptake of QD-ILs (image size ) 235 µm × 235 µm, and magnification ) 40×).

(Figure 5, left panel). Tumor fluorescence reached a plateau after 24 h, with up to 18 ( 5% (mean ( SD; n ) 4) of total body fluorescence localizing to the tumor region. In separate experiments directly comparing tumor accumulation of targeted vs nontargeted nanoparticles, both versions of Nano Lett., Vol. 8, No. 9, 2008

nanoparticles showed highly efficient tumor accumulation, with 14.2 ( 3.7 and 13.7 ( 2.4% (mean ( SD; n ) 4), respectively, of total body fluorescence within the tumor at 24 h. Thus, tumor accumulation was extremely high and comparable for both targeted and nontargeted nanoparticles, 2855

Table 1. Cytotoxicity of Free Dox and Dox-Loaded Nanoparticles against HER2-Overexpressing SK-BR-3 and Nonoverexpressing MCF-7 Cancer Cells in Vitroa IC50 (µg/mL)

free Dox

Ls-Dox

ILs-Dox

QD-Ls-Dox

QD-ILs-Dox

SK-BR-3 1.30 ( 0.23 9.36 ( 0.83 0.79 ( 0.03 3.85 ( 0.24 0.45 ( 0.10 MCF-7 0.63 ( 0.03 9.55 ( 0.51 4.23 ( 0.99 3.90 ( 0.55 1.76 ( 0.25 a One hour at 37°C was used for incubation, and 3 days was allowed for regrowth after removal of nanoparticles. QD-ILs without dox were administered at equivalent QD concentrations and did not show any toxicity up to 8 nM QDs for 6 h. All values are expressed as IC50 (µg/mL) means ( standard errors with the number of replicates ) 6; p < 0.001 by one-way ANOVA. Ls-Dox, dox-loaded liposomes; ILs-Dox, dox-loaded anti-HER2 ILs; QD-Ls-Dox, dox-loaded, QD-Ls nanoparticles; and QD-ILs-Dox, dox-loaded, QD-conjugated, anti-HER2 IL nanoparticles.

Figure 5. (Left panel) In vivo fluorescence imaging of three nude mice bearing MCF-7/HER2 xenografts implanted in the lower back 30 h after i.v. injection with anti-HER2 QD-ILs. The image was acquired by using an excitation filter of 580-610 nm and an emission bandpass filter of 695-770 nm. Imaging showed that QD-ILs had localized prominently in tumors as well as in MPS organs. Units: efficiency (the fractional ratio of fluorescence emitted per incident photon). (Right panel) A 5 µm section cut from frozen tumor tissues harvested at 48 h postinjection and examined by confocal microscopy by a 63× oil immersion objective (image size, 146 µm × 146 µm). The tumor section was examined in two-color scanning mode for nuclei stained by DAPI (blue) and QD-ILs (red). For the DAPI channel, an 800 nm (titanium:sapphire) two-photon excitation and 390-465 nm emission filter was used. For QD-ILs channel, 488 (argon) or 633 nm (HeNe) for excitation and 650 nm long pass for emission were used. The distribution of QD-ILs is similar to in vitro QD-ILs uptake in SK-BR-3 cells as in Figure 4c,d, where QD-ILs likely internalized to the cytosol of MCF-7/HER2 cells.

consistent with our previous reports that long circulating liposomes and ILs localize in tumors predominantly via the enhanced permeability and retention (EPR) effect rather than via antibody-mediated targeting.21,28 Tissue sections of tumors collected 48 h postinjection were examined by confocal microscopy (Figure 5, right panel). QD-ILs showed extensive accumulation within tumor tissue and intracellularly within tumor cells; however, this technique did not allow quantitation of intracellular vs extracellular microdistribution. In summary, we have synthesized QD-ILs that selectively and efficiently internalize in HER2-overexpressing tumor cells via receptor-mediated endocytosis. Uptake of QD-ILs in HER2-overexpressing cells was markedly greater than in nonoverexpressing cells as measured by flow cytometry assay and confocal microscopy. Systemic administration of QDILs resulted in tumor localization and in vivo fluorescence imaging in a xenograft mouse model, thus demonstrating how such constructs may facilitate QD use in vivo by exploiting the biocompatible properties of liposomes. Although conjugation of QDs to liposomes or ILs increased the average diameter of the labeled liposomal nanoparticles, it effectively eliminated the nonspecific binding observed with QDs alone and enabled extended circulation time in vivo. These studies indicate that QD-ILs were able to provide, in a single nanoscale construct, tumor cell-targeting, imaging, and drug delivery functions; to our knowledge, this is the first description of a targeted lipidic nanoparticle-QD system possessing all of these capabilities. Using QD-based labeling, 2856

nanoparticle biodistribution and penetration in solid tumors, intracellular fate in target cells, and real-time dynamics of drug delivery may be investigated in more detail. This targeted delivery system may also provide a strategy for tumor cell detection and labeling applications in vivo. Acknowledgment. This work was supported by grants from the National Cancer Institute (NIH P50-CA58207-01, NIH P50-CA097257, and NIH U54-CA90788). K.C.W. is supported by the Delores R. Malone American Brain Tumor Association Fellowship Award and the National Cancer Institute Cancer Center Support Grants Pilot Program. F.F.C. is supported by DOD BCRP BC045345 grant, UCSF Prostate Cancer SPORE award (NIH Grant P50-CA89520), and CMC Institute of Nanotechnology and Nanomedicine (CMC-INN). This work was also performed under the auspices of the U.S. Department of Energy at the University of California/ Lawrence Berkeley National Laboratory under contract no. DE-AC03-76SF00098. We thank Prof. C. David James, UCSF Department of Neurological Surgery and UCSF Brain Tumor Research Center, for assistance with in vivo imaging. Supporting Information Available: Methods and threedimensional images of anti-HER2 QD-ILs internalized by SK-BR-3 cells. This material is available free of charge via the Internet at http://pubs.acs.org. Nano Lett., Vol. 8, No. 9, 2008

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