(706b) Cationic Multifluorescent Quantum Dot Liposomes for Cancer Cell Imaging

Quantum dot (QD) nanocrystals provide highly efficient and photostable spectral emission for medical imaging and diagnostics.^1-3 However, QDs have also exhibited cytotoxicity^4,5 and are natively hydrophobic, as a result of the synthesis process, which requires that they be surface modified with biocompatible hydrophilic coatings to be dispersed in aqueous phases. An alternative strategy that has been recently explored is to encapsulate QDs within liposomes.^6-8 A potential advantage of this strategy includes incorporating multiple QDs and delivering them to specific cell types using liposome-targeting techniques.

In this work, we describe the design of a liposomal system that can deliver multiple QDs as imaging agents to carcinoma cells. Cationic and zwitterionic multifluorescent quantum dot liposomes (QD-Ls) have been prepared with both green hydrophobic (3 nm diameter; 545 nm emission) and red hydrophilic (11 nm diameter; 620 nm emission) CdSe/ZnS core/shell quantum dots by reverse phase evaporation. Cationic liposomes were composed of dipalmitoylphosphatidylcholine (DPPC)/dipalmitoylethylphosphocholine (ethyl-DPPC)/cholesterol (6:2:2 molar basis) and zwitterionic liposomes were composed of DPPC or DPPC/cholesterol (8:2). The hydrophobic QDs were embedded within the lipid bilayer and the hydrophilic QDs were encapsulated in the aqueous core during liposome assembly at lipid to QD molar ratios of 10,000:1. QD incorporation was confirmed by fluorescence and confocal microscopy, as colocalized green and red QDs produced yellow. Incorporation did not affect QD photoactivity or damage bilayer or liposome structure, as determined from differential scanning calorimetry. Cell uptake was examined in human hepatocellular carcinoma cells (HuH-7) and showed that cationic QD-Ls were stable in vitro, exhibited high uptake, and collected near the nuclear membrane. In contrast, zwitterionic QD-Ls aggregated and exhibited low uptake. We are currently examining cancer cell targeting strategies using headgroup-modified lipids and the effect of serum proteins on QD-L stability and uptake. Given that liposomes are established and versatile platforms for creating cell-targeting therapeutic agents, we envision that multifluorescent QD-Ls may offer advanced techniques for imaging hydrophobic and hydrophilic domains simultaneously. If coupled with an encapsulated drug, QD-Ls could be multifunctional and provide detection and drug delivery in a single assembly.

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