Fluorescent dyes are widely used for near-infrared imaging but many applications of these dyes are limited by disadvantageous properties in aqueous solution that include concentration-dependent aggregation, poor aqueous stability in vitro and low quantum yield. For example, a particularly useful and FDA approved dye, indocyanine green (ICG), is known to strongly bind to nonspecific plasma proteins, leading to rapid elimination from the body, having a half-life of only 3-4 min. Other limiting factors displayed by ICG include: rapid circulation kinetics; lack of target specificity; and changes in optical properties due to influences such as concentration, solvent, pH, and temperature. To overcome some of these shortcomings the inclusion of the fluorescent dyes into micellar and nanoparticulate systems have been examined.
Attempts to encapsulate ICG into silica and polymer matrices have been met with only partial success. Much of this appears to stem from ICG's combined amphiphilic character and strong hydrophilicity. It contains both lipophilic groups and hydrophilic groups that promote its distribution at interfaces and its interaction with the surfactants that are often necessitated in the particles synthesis and largely limits its incorporation to the interior of nanoparticles. ICG displays a critical micelle concentration of about 0.32 mg/mL in H2O and readily partitions into aqueous environments, and, therefore, ICG encapsulation in particulate matrices suffers from significant leaching.
Nevertheless, encapsulated ICG and other fluorescent dyes remain attractive for bio-imaging techniques that non-invasively measure biological functions, evaluate cellular and molecular events, and reveal the inner mechanisms of a body. Fluorescent dye comprising nanoparticles are useful for in vitro fluorescence microscopy and flow cytometry. Additionally, fluorescent dye comprising nanoparticles are potentially valuable for photoacoustic tomography (PAT), an emerging non-invasive in vivo imaging modality that uses a non-ionizing optical (pulsed laser) source to generate contrast. A PAT signal is detected as an acoustic signal whose scattering is 2-3 orders of magnitude weaker than optical scattering in biological tissues, a primary limitation of optical imaging.
Additionally, diagnosis often necessitates the use of more than one imaging technique to integrate the strengths of multiple techniques and overcome the limitations of an individual technique to improve diagnostics, preclinical research and therapeutic monitoring. Examples of PAT complementary techniques include magnetic resonance imaging (MRI), positron emission tomography (PET), X-ray tomography, luminescence (optical imaging), and ultrasound. Typically, analysis by different techniques requires different contrast agents. Furthermore, using multiple bio-imaging techniques requires significantly greater time and expense, and can impose diagnostic complications. If the fluorescent dye comprising nanoparticles include one or more additional contrast agents, multiple bio-imaging techniques could be carried out rapidly or simultaneously. Multi-modal contrast bio-imaging agents are potentially important tools for developing and benchmarking experimental imaging technologies by carrying out parallel experiments using developing and proven techniques.
To these ends, effective and stable fluorescent dye comprising nanoparticles and methods for their preparation are needed. Such novel nanoparticles could be employed for multiple biological applications, including imaging, even multiple bio-imaging techniques, and therapeutics.
Embodiments of the invention are directed to fluorescent core-shell nanoparticle wherein a core comprising a water soluble fluorescent dye is encapsulated in a silica shell. The dye is ion-paired with a cationic polymer and/or with a multivalent cation as a precipitated non-soluble matrix. In an exemplary embodiment of the subject invention, a FDA approved fluorescent dye, indocyanine green (ICG), is used. In one embodiment, the cationic polymer is chitosan treated by tripolyphosphate. In another embodiment, the multivalent cation is Ba2+ and the dye is distributed in precipitated BaSO4. The novel core-shell nanoparticles can be monodispersed with sizes less than 100 nm.
Embodiments of the invention are directed to methods of making the novel fluorescent core-shell nanoparticle. This is done by using a water-in-oil microemulsion directed synthesis. In one embodiment, the preparation steps comprise: providing core within the water phase of a water-in-oil microemulsion where the core comprises a polymer having cationic sites, such as protonated chitosan, and/or an insoluble salt of a multivalent cation, such as a Ba2+ salt with a fluorescent dye having a plurality of anionic sites, such as ICG, and coating the core with a metal oxide layer, for example a silica layer, by condensation of a precursor, for example, ammonium carbonate catalyzed condensation of silanes.
Advantageously, fluorescent core-shell nanoparticles according to embodiments of the invention display good photostability. The synthetic methods used for the novel core-shell nanoparticle allow a multistep architecture on the nanoparticle, where, for example, the use of barium sulfate enables CT or X-ray contrast as well as near infrared fluorescence traceability and/or the inclusion of other contrast agents for robust multimodal bioimaging.
Embodiments of the invention are directed to fluorescent core-shell nanoparticles containing ionically bound ICG or other fluorescent dyes where the dye has at least one anionic site and is included within a core bound within an insoluble difunctional or multifunctional metal salt or ionically bound to a biocompatible polymer having a plurality of cationic sites and crosslinked into an insoluble polymer matrix core, and where the core is encapsulated in a metal oxide shell. Other fluorescent dyes that can be use in place of or in addition to ICG include, but are not limited to, Evans blue, bromothymol blue, and rose Bengal. For purposes of the invention, the core is a material that is formed in a first step and the shell is a material that is formed in a second step, and although in many embodiments of the invention the shell material will have limited penetration into the core material, in some embodiments of the invention, the shell material can penetrate deeply into or extending throughout the core material, yet the core and shell materials remain separate material phases. A simplified schematic representation of the particle design is shown in
Some embodiments of the invention are directed to a method of preparing the novel fluorescent core-shell nanoparticles. The method involves formation of a core by a water-in-oil microemulsion directed synthesis. The oil can be any water immiscible liquid, for example a hydrocarbon such as hexane, cyclohexane, heptane, or iso-octane. The size of the nanoparticle cores formed by this novel microemulsion method can be tuned from as little as 5 to 150 nm by controlling the molar ratio of water to surfactant and the concentrations of the reagents. The confined surfactant stabilized aqueous micelles of the microemulsion allow for the preparation of nanoparticles that have a very narrow size distribution, nearly monodispersed nanoparticles having a maximum polydispersity index (volume average particle size/number average particle size) of 1.2.
In one embodiment of the method, a water-in-oil microemulsion is generated where the micelles include the soluble fluorescent dye salt and solubilized chitosan. In other embodiments of the invention, the chitosan can be replaced with other polymers containing primary amino groups, for example, polyethylenimines (PEI) or polylysine, and can be a linear polymer, a branched polymer, a hyperbranched polymer or a dendrimers. The chitosan, or other polymer, can be dissolved in a dilute acetic acid solution and mixed with ICG, generally, but not necessarily, as a disodium salt dissolved in water and mixed with a polyanionic precipitant, for example the polyacid tripolyphosphate, where the precipitant forms ammonium cations on the chitosan which form precipitating ionic cross-links and binds the ICG. A silica shell is subsequently formed about the chitosan containing core by hydrolysis and condensation of a tetraalkoxysilane, such as tetramethoxysilane or tetraethoxysilane, at the interface of the aqueous micelle containing the chitosan ICG precipitate. Other silanes that can be combined with the tetraalkoxysilane include, but are not limited to 3-mercaptopropyltrimethoxysilane, 2-methoxy(polyethyleneoxy) propyltrimethoxysilane, and N-(Trimethoxysilyl-propyl)ethyldiaminetriacetic acid trisodium salt. An aminopropyltrialkoxysilane can be included in the silane mixture to promote encapsulation of ICG and the formation of the silica shell about the chitosan ICG precipitate core and to generate sites on the nanoparticles to which moieties are attached to modify the particles for cell targeting, promotion of particle suspension, or additionally provide signals for alternate imaging techniques, such as MRI, X-ray or PAT for multimodal imaging. Metal speckles can also be deposited on the silica shell.
In another embodiment of the invention, the ICG is combined with an insoluble multivalent cation salt where, for example, a soluble barium salt and ICG are present in the micelle of a water-in-oil microemulsion, and subsequently combined with an aqueous sodium sulfate solution present in the water-in-oil microemulsion, to precipitate a Ba-ICG/BaSO4 salt within the micelle. The barium sulfate, or other multivalent cation salt, permits formation of BaSO4-ICG/silica core-shell nanoparticles that display CT or X-ray contrast as well as MR fluorescence traceability. The ionic interaction between a single Ba2+ cation and the sulfate groups of ICG is illustrated in
The nanoparticle cores within the micelles are coated with a silica shell to form the core-shell nanoparticle having an encapsulated dye core. Traditional sol-gel silica nanoparticle formation that one might envision to coat the core within the micelles of a microemulsion is catalyzed by NH4OH. However it has been found that this traditional method can not be applied to the preparation of the novel core-shell nanoparticles according to embodiments of the invention because NH4OH causes the degradation of ICG with lose of fluorescence properties during synthesis. The degradation can not be prevented by simply using a diluted NH4OH solution. It has been discovered that by using NH4CO3, rather than NH4OH, the hydrolysis and condensation of the alkoxysilanes occurs without dye degradation. For example, approximately 24 hours after introduction of the NH4CO3 catalyst, silica shells are formed on BaSO4-ICG or Chitosan-ICG cores to yield the desired novel core-shell fluorescent nanoparticles.
The formation of silica nanoparticles by a sol-gel process involves two steps where hydrolysis of the precursor is followed by condensation to the nanoparticle. Using ammonium carbonate to catalyze generation of silica nanoparticles allows a high level of control over the condensation step. The use of ammonium carbonate appears to modulate the rate of silica particle formation and can affect the extent of condensation. The extent of condensation affects the mechanical and chemical stability of the nanoparticles. Hence, the nanoparticle can be formed in a manner that can be broken down (degraded) into smaller silica fragments. The particles can be effectively biodegradable, which provides significant advantageous for nanoparticles used for biological applications, such as carriers for diagnostic contrast agents, drug delivery vehicles, and other applications that employ nanoparticulates. The breakdown of the nanoparticle can be promoted by a biological environment's pH, temperature, ionic strength t, or other factors. In contrast, ammonium hydroxide catalyzed silica particle formation largely results in non-biodegradable silica particles.
In some embodiments of the invention, aminoalkysilanes, for example 3-aminopropyltrialkoxysilanes, can be included with the core material or with the tetraalkoxysilanes to enhance the ICG encapsulation efficiency. Inclusion of the amine sites in the silica matrix additionally allows for inclusion of groups for bioconjugation and targeting capability. Also, the aminoalkyl groups of the silica matrix in the shell's surface can be modified with polyethyleneglycol (PEG) or other oligomers or polymers with a strong affinity for water in some embodiments of the invention such that opsonization is prevented, allowing increased circulation times of the particles upon introduction to an organism. PEG modification can be carried out by the reaction of an N-hydroxysuccinimide ester (NHS) terminated PEG, or other reactive terminated PEG polymers, with the aminoalkyl containing silica shell.
To overcome issues associated with carrier particle inhomogeneity and allow for the facile obtainment of tunable monodispersed particle sizes of less than 100 nm, a water-in-oil microemulsion mediated synthesis strategy is carried out by modification of the process disclosed in Sharma et al., Chemistry of Materials, 2008, 20(19), 6087-94; Santra et al., Technology in Cancer Research & Treatment, 2004, 4(6), 593-602; Santra et al., Food and Bioproducts Processing, 2005, 83(C2), 136-40; Santra et al., Journal of Nanoscience and Nanotechnology, 2005, 5(6), 899-904; Santra et al., Chemical Communications, 2004, 24, 2810-1, all references incorporated herein by reference. For example, encapsulation of the surface active dye ICG in a microemulsion can be carried out as follows. Chitosan and/or a Ba2+ salt are dissolved in the aqueous micelles of the microemulsion, followed by addition of an ICG comprising solution such that the ICG partitions into the micelle. Subsequently a precipitant, tripolyphosphate for chitosan and/or sodium sulfate for Ba2+ salt, is added to cause precipitation within the micelle, entrapping ICG. Alternately, precipitation can be carried from a homogeneous aqueous solution that is subsequently used to form a microemulsion. Although many microemulsion systems can be used, encapsulation of the dyes occurs effectively in a reverse sodium bis(2-ethylhexyl)sulfosuccinate (AOT) microemulsion system, and does not occur as effectively in a common Triton X-100 (TX-100) microemulsion system. The precipitate containing micelles are then coated with silica or another metal oxide layer to encapsulate the dye. Again, a simplified schematic representation of a nanoparticle according to an embodiment of the invention is shown in
In embodiments of the invention, the novel core-shell nanoparticles containing ICG are fluorescent and are useful for imaging by fluorescence microscopy in vitro and quantitative cellular uptake by flow cytometry. For example, the nanoparticles are found to be non-toxic to cancer cells in vitro and can be taken up by cancer cells such as the breast cancer cells (BT474), as shown in the fluorescence microscopy image in
Photoacoustic tomography (PAT) is an emerging powerful non-ionizing deep tissue imaging technology that offers benefits of both high optical contrast and high ultrasound resolution. PAT can image with high contrast and good spatial resolution. In PAT, NIR pulsed laser light is used to generate ultrasound waves in target structures that are detected and reconstructed for image generation. This will allow non-invasive quantization of nanoparticle contrast agent concentration inside tumors. It has been demonstrated in preliminary experiments that ICG containing nanoparticles are an excellent in vitro and in vivo photoacoustic contrast agent (
The encapsulation of ICG inside of a solid silica core significantly enhances the dyes capacity for long term imaging.
The nanoparticle synthesis can be extended to the formation of multimodal nanoparticles that can be simultaneously imaged by fluorescence and, for example, magnetic resonance imaging (MRI), in the manner disclosed in Sharma, et al., “Multimodal Nanoparticles for Non-Invasive Bio-Imaging” International Application No. PCT/US08/074,630; filed Aug. 28, 2008, and incorporated herein by reference.
The ICG core-shell nanoparticles can be use for in vivo imaging as shown in
In another embodiment of the invention, ICG core-shell nanoparticles are used therapeutically, for example, for photodynamic therapy (PDT). PDT employing ICG core-shell nanoparticles and a laser, for example a diode laser with a wavelength of 805 nm, can be used to treat: Barrett's esophagus; early esophageal cancer (adenocarcinoma or squamous cell carcinoma); obstructing esophageal cancer; persistent or recurrent esophageal cancer; gastric cancer; lung cancer; and/or macular degeneration.
All patents, patent applications, provisional applications, and publications referred to or cited herein are incorporated by reference in their entirety, including all figures and tables, to the extent they are not inconsistent with the explicit teachings of this specification.
The present application claims the benefit of U.S. Provisional Application Ser. No. 61/309,261, filed Mar. 1, 2010, the disclosure of which is hereby incorporated by reference in its entirety, including any figures, tables, or drawings.
The subject invention was made with government support under the National Science Foundation, Contract No. EEC0506560. The government has certain rights to this invention.
Filing Document | Filing Date | Country | Kind | 371c Date |
---|---|---|---|---|
PCT/US2011/026038 | 2/24/2011 | WO | 00 | 1/17/2013 |
Number | Date | Country | |
---|---|---|---|
61309261 | Mar 2010 | US |