Cancer remains one of the most challenging issues in modern medicine. In developed countries, its mortality has remained relatively constant for the past 35 years. Nanotechnology-based cancer treatments have received considerable attention in recent decades as they may provide unique approaches for early prediction, prevention, and diagnosis, as well as personalized therapy.1Nanoparticles composed of polymers, lipids, and inorganic materials loaded with drugs have been developed for targeted-therapy delivery systems. Additional efforts have focused on developing novel materials for the early detection of cancer, based on optically active agents such as quantum dots (QDs, semiconductor nanocrystals), gold, or magnetic (for example, iron oxide) nanoparticles.2 Advantages in using QDs and gold nanoparticles over traditional fluorophores include their enhanced optical properties, stability in biological fluids, and potential for targeting tumors.
An exciting concept in nanomedicine is the fusion of therapy with diagnostics, known as ‘theragnostics’.3 Designed to increase the efficiency and safety of treatment, the nanosystems used in theragnostics have multiple functions: diagnosis, delivery of targeted therapy, and monitoring the therapeutic response (see Figure 1). Many groups are developing multifunctional nanoparticles with the ability to detect tumor cells early, through the use of various imaging modalities, and to control the release of therapeutic agents at the designated site.4–7 Because of their combinatorial character, theragnostic nanoparticles require a suitable carrier able to encapsulate many active agents as well as accommodate appropriate surface functionalization to facilitate cellular recognition. This concept is deceptively simple: the specific optical and therapeutic properties of each integrated element must be preserved for the nanosystem to be useful.
Figure 1. Schematic illustration of theragnostic nanotechnology action. PEG: Poly(ethylene glycol).
We recently developed a theragnostic platform encapsulating hydrophilic QDs and anticancer drugs. Very little work has been done on the encapsulation of hydrophilic chemotherapeutic agents, and no theragnostic platforms containing those drugs have been reported. We first prepared cadmium sulfide (CdS) QDs using a polymeric solution—such as chitosan or poly(vinyl acetate-co-maleic acid)—as the reaction medium.8,9 This method proved facile and reliable for producing monodisperse QDs with good optical properties. Additionally, the nanoparticles required no further functionalization to achieve water dispersibility. Owing to the characteristics of the polymeric shell, they showed excellent biocompatibility.
We then sought to incorporate the hydrophilic (water-soluble) drug gemcitabine, a nucleoside promoter of apoptosis (programmed cell death), in the platform. Gemcitabine was dissolved in a solution containing chitosan and CdS QDs. Using a simple gelation procedure in a water-in-oil microemulsion, we generated particles with dimensions of 50–100nm, suitable for intravenous administration. The surface of these nanoparticles is a hydrophilic shell that is easy to functionalize with antibodies, proteins, or other ligands for targeting cancer cells. Our novel multifunctional nanosystem showed good fluorescence (see Figure 2), as well as long-term stability in body fluid. We are currently evaluating the cellular uptake, biocompatibility, and drug-release profile of this system.
Figure 2. Theragnostic nanoplatform of hydrophilic QDs and gemcitabine encapsulated in chitosan nanoparticles. (Left) Optical image of multifunctional nanoparticles. (Right) Transmission electron microscopy image of cadmium sulfide (CdS) prepared in chitosan.
Having demonstrated that hydrophilic chemotherapeutics can be encapsulated in this way, we turned our attention to hydrophobic (non-water-soluble) drugs. The challenge in this case is to produce nanoparticles that have an internal hydrophobic domain and hydrophilic shell, required for dispersibility in body fluid. We prepared a multifunctional nanoplatform containing cadmium selenide/zinc sulfide (CdSe/ZnS) QDs as the imaging agents, co-encapsulated with the anticancer drug docetaxel in a silica nanoparticle. CdSe/ZnS QDs were obtained in nonpolar solvents and capped with organic ligands, resulting in bright, monodisperse, and stable nanocrystals. Docetaxel was selected as the drug model because of its wide clinical use in the treatment of metastatic breast and lung cancer. The silica nanoparticles were synthesized in an oil-in-water microemulsion, using octadecyl triethoxysilane as the precursor. Using this organo-modified silane precursor ensured the preparation of silica nanoparticles with an oily core surrounded by a dense, hydrophilic silica shell (see Figure 2).
The oily core of the nanoparticle allowed high loading efficiency: encapsulation of many QDs and high drug concentration. The strong fluorescence emission of the QDs was preserved in the silica nanoparticles (see Figure 3), owing to the core-shell structure of the silica carrier. The semiconductor nanocrystals co-encapsulated with docetaxel showed the same maximum wavelength of emission, albeit with a slight decrease in intensity that may be due to Förster resonance, a type of energy-transfer process. To improve biocompatibility, the carrier surface was functionalized with poly(ethylene glycol) and folic acid was chemically attached as a target ligand. Nanoparticles decorated with folic acid are preferentially internalized by cancer cells, which express elevated levels of folate receptors. The cellular uptake process was observed by confocal laser scanning microscopy, and showed significant internalization of the theragnostic agent into the targeted tumor cells. We are currently investigating the toxicity of this nanosystem in cell cultures and animals. Further work will study the biodistribution and drug release of multifunctional theragnostic agents by measuring the fluorescence emission of both docetaxel and QDs.
Figure 3. Theragnostic nanotechnology consisting of cadmium selenide/zinc sulfide (CdSe/ZnS) QDs and docetaxel encapsulated in silica nanoparticles. (Left) Transmission electron microscopy image of silica nanoparticles, with a core-shell structure and oily internal droplet, in aqueous solution. (Right) Fluorescence emission spectra of multifunctional silica nanoparticles loaded with CdSe/ZnS QDs only (red) and with docetaxel (blue). a.u.: Arbitrary units.
In summary, we have explored the potential of novel nanoplatforms as theragnostic systems for the development of more effective, less toxic treatment regimens for cancer. Aside from being useful tools for studying the drug-delivery process and efficiency of chemotherapy at the cellular level, these nanosystems offer the possibility of identifying patients who do not respond to specific therapy, thus helping to move personalized medicine forward.
Ludmila Otilia Cinteza
Physical Chemistry Department
University of Bucharest
Ludmila Otilia Cinteza is an associate professor whose research interests include nanoparticles for biomedical applications and nanomaterials for the conservation and restoration of works of art.
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