Quantum dot-based theranostics

Abstract

Luminescent semiconductor nanocrystals, also known as quantum dots (QDs), have advanced the fields of molecular diagnostics and nanotherapeutics. Much of the initial progress for QDs in biology and medicine has focused on developing new biosensing formats to push the limit of detection sensitivity. Nevertheless, QDs can be more than passive bio-probes or labels for biological imaging and cellular studies. The high surface-to-volume ratio of QDs enables the construction of a "smart" multifunctional nanoplatform, where the QDs serve not only as an imaging agent but also a nanoscaffold catering for therapeutic and diagnostic (theranostic) modalities. This mini review highlights the emerging applications of functionalized QDs as fluorescence contrast agents for imaging or as nanoscale vehicles for delivery of therapeutics, with special attention paid to the promise and challenges towards QD-based theranostics.

Fig. 1

(a) Schematic representation of a multimodal QD, where the QD serves as both a diagnostic agent (imaging) and a nanoscaffold to incorporate multiple functional modalities, such as a targeting ligand (peptide, antibody, or protein) and a therapeutic agent. Upon interacting with the target cell, the cell-penetrating ligand can then be exposed, allowing the multifunctional QD to enter the cell. Stimuli-sensitive antennae may be triggered by local stimuli (pH, temperature, or enzyme), allowing subsequent intracellular release of the drug from the drug-loaded vesicle. (b) Requirements of an ideal theranostic process in the human body may include: (1) escape from the clearance of reticuloendothelial system (RES, mainly liver, spleen, and bone marrow), allowing longer blood circulation time, (2) accumulate in the pathological zone, targeting specific cell types, (3) penetrate the cell efficiently, leaving minimum damage to the cell, (4) overcome the delivery barriers, leading to efficient intracellular release, and (5) bear a diagnostic agent (imaging, optical or magnetic), allowing for real-time monitoring of the treatment, while maintaining minimum toxicity to the healthy cells.

Fig. 2

Cellular internalization of QD conjugates by chemical/biological and physical approaches. (a) Unconjugated QDs typically are uptaken via non-specific endocytosis, resulting in aggregation in the cytosol, shown as punctate fluorescence staining. (b) Delivery assisted by transfection agent (liposome, micelle or polymer) or ligand-modification on the QDs is usually more specific and efficient than non-specific endocytosis alone. (c) The electrical field in electroporation temporarily permeabilizes the cell membrane, allowing the QD conjugates to be delivered directly into the cells to achieve high gene expression. However, aggregations are still observed in the endolysosomes and cytosol for both transfection agent and electroporation assisted deliveries. (d) Microinjection or needle penetration, allows QDs to be delivered directly into the cytoplasm or even nucleus, and bypass the endosome/lysosome, thus avoiding enzymatic degradation. QD conjugates delivered through different approaches result in contrasting patterns in cell labeling. Figures reprinted with permission from Wiley-VCH Verlag GmbH & Co.

Fig. 3

Cellular delivery of QDs. (a) Monodispered QDs can be delivered by microinjection. The single nature of QDs can be validated by the blinking phenomenon. The ability to deliver controlled amounts of QDs with spatial and temporal precision is particularly useful for single-molecule studies. (b) The labeling pattern of QDs is phenotype-dependent. Unlabeled QDs are found aggregated throughout the cytoplasm in the PC3 cells, as previously observed along the endosomal pathway. Strikingly, a single clump of QDs is localized around the perinuclear region of PC3-PSMA cells. As a result, cancer phenotypes can easily be identified by the contrasting labeling pattern of QDs. Figures reprinted with permission from the American Chemistry Society and Wiley-VCH Verlag GmbH & Co.

Fig. 4

QD conjugates/complexes for intracellular gene trafficking. (a) QD-FRET system: QDs, as an energy donor, are conjugated onto the plasmid DNA (pDNA), whereas Cy5, the energy acceptor, is functionalized on the cationic polymer. The FRET-mediated Cy5 emission upon complex coacervation provides a digital indicator of the interaction between pDNA and the polymer. Consequently, the FRET signal is abrogated when DNA is released into the cytosol., (b) Two-step FRET system: the two-step energy transfer is constructed from the QD donor to the first acceptor, nuclear dye (ND, energy transfer E₁₂), as a relay donor to the second acceptor, Cy5 (energy transfer E₂₃). Similar to the one step QD-FRET system, the “OFF” signal from Cy5 (from E₂₃) signifies the DNA escape. Moreover, dual-labeled pDNA provides an additional dimension after the DNA unpacks from the complexes, allowing simultaneous detection of DNA release and degradation, during gene delivery. (c) siRNA tracking system: siRNA/QDs complexes are generated with surface modified QDs (proton-sponge coating or Amphipol80) or transfection agent (polymers or Lipofatamine23) encapsulated QDs, to trace siRNA delivery.

Fig. 5

Possible constructs of multifunctional QDs for theranostics: (a) QD–aptamer(Apt)–doxorubicin(Dox) conjugate, shortened as QD–Apt(Dox), is presented for synchronous cancer imaging and traceable drug delivery towards QD-based theranostics. The targeting modality, RNA aptamer, is functionalized onto the diagnostic modality (QDs) to probe the cancer cells. The therapeutic modality, doxorubicin (Dox), is intercalated into the aptamer. The sensing of drug loading and release relies on the bi-FRET (dual donor–quencher) design. In the drug-loading state: both QD and Dox fluorescence are turned “OFF”, since the QD fluorescence is quenched by the Dox and the Dox fluorescence is in turn quenched by the aptamer. In the drug release state: the Dox is released from the QD–Apt complex, turning both QD and Dox fluorescence back “ON”. During drug transport: the Dox fluorescence was used as a tracable dye. (Figure adapted from ref. 83). (b) Representation of an idealized nanoplatform of an “all-in-one” workstation. Multiply functionalized QDs may constitute an integrated nanoplatform, for example, able to target the tumor, transport/release the drug payload, and image the therapeutic response simultaneously. The QD–liposome (QD–L) system, although not experimentally demonstrated yet, is envisaged as a potential candidate towards QD-based therapeutics. In the current QD–L system, QDs are typically incorporated into the bilayer membrane, or functionalized onto a liposome, forming a QD–lipid vesicle. Liposomes have proved to be excellent drug and gene carriers. Integration of QD–L with other targeting ligands and therapeutic agents may achieve the goal of theranostics.