Supramolecular nanoreactors for intracellular singlet-oxygen sensitization.

An amphiphilic polymer with multiple decyl and oligo(ethylene glycol) chains attached to a common poly(methacrylate) backbone assembles into nanoscaled particles in aqueous environments. Hydrophobic anthracene and borondipyrromethene (BODIPY) chromophores can be co-encapsulated within the self-assembling nanoparticles and transported across hydrophilic media. The reversible character of the noncovalent bonds, holding the supramolecular containers together, permits the exchange of their components with fast kinetics in aqueous solution. Incubation of cervical cancer (HeLA) cells with a mixture of two sets of nanoparticles, pre-loaded independently with anthracene or BODIPY chromophores, results in guest scrambling first and then transport of co-entrapped species to the intracellular space. Alternatively, incubation of cells with the two sets of nanocarriers in consecutive steps permits the sequential transport of the anthracene and BODIPY chromophores across the plasma membrane and only then allows their co-encapsulation within the same supramolecular containers. Both mechanisms position the two sets of chromophores with complementary spectral overlap in close proximity to enable the efficient transfer of energy intracellularly from the anthracene donors to the BODIPY acceptors. In the presence of iodine substituents on the BODIPY platform, intersystem crossing follows energy transfer. The resulting triplet state can transfer energy further to molecular oxygen with the concomitant production of singlet oxygen to induce cell mortality. Furthermore, the donor can be excited with two near-infrared photons simultaneously to permit the photoinduced generation of singlet oxygen intracellularly under illumination conditions compatible with applications in vivo. Thus, these supramolecular strategies to control the excitation dynamics of multichromophoric assemblies in the intracellular environment can evolve into valuable protocols for photodynamic therapy.

Carbon quantum dot-NO photoreleaser nanohybrids for two-photon phototherapy of hypoxic tumors.

We report a conjugate between carbon quantum dots and a NO photoreleaser able to photogenerate the anticancer NO radical via an energy transfer mechanism. This nanohybrid proved toxic to cancer cells in vitro and significantly reduced tumor volume in mice bearing human xenograft BxPC-3 pancreatic tumors upon two-photon excitation with the highly biocompatible 800 nm light.

A new three-component formulation for the efficient whitening of teeth (Carbamide Plus).

OBJECTIVES: This study aimed to develop and characterise a new three-component dental whitening formulation which is as effective as the currently used carbamide peroxide but at significantly lower hydrogen peroxide concentrations. MATERIALS AND METHODS: The new formulation (Carbamide Plus) was prepared containing hydrogen peroxide, urea, and sodium tripolyphosphate and compared directly with carbamide peroxide (containing just hydrogen peroxide and urea). To evaluate the clinical effectiveness of 5% Carbamide Plus, a randomised double-blind placebo-controlled clinical trial was conducted comparing the tooth colour of 33 patients using L*a*b* scores at baseline and after a 2-week whitening treatment. The behaviour of the three components in solution was determined by (1)H and (31)P NMR spectroscopy and pH dilution experiments. RESULTS: This clinical trial revealed that 5% whitening gels containing Carbamide Plus were as effective as those containing 10% carbamide peroxide. (1)H and (31)P NMR spectroscopy revealed strong intermolecular interactions between hydrogen peroxide and both urea and sodium tripolyphosphate (STPP) with little apparent interaction between urea and STPP. CONCLUSIONS: In this manuscript, we postulate that this increased whitening efficiency is due to a marked increase in local pH upon dilution which destabilises the hydrogen peroxide and expedites the whitening process. We postulate Carbamide Plus to be a three-component adduct with two molecules of carbamide peroxide binding to a central STPP unit with no direct interaction between STPP and urea. There were no statistically significant differences between Carbamide Plus and 10% carbamide peroxide in tooth-whitening achieved at 2 weeks. These results were recorded following 2 weeks of 2-h daily wear of at-home trays. CLINICAL RELEVANCE: Carbamide Plus offers the potential of using significantly lower levels of hydrogen peroxide concentration to achieve similar dental whitening effects.

Polymeric microbubbles as delivery vehicles for sensitizers in sonodynamic therapy.

Microbubbles (MBs) have recently emerged as promising delivery vehicles for sensitizer drugs in sonodynamic therapy (SDT). The ability to selectively destroy the MB and activate the sensitizer using an external ultrasound trigger could provide a minimally invasive and highly targeted therapy. While lipid MBs have been approved for use as contrast agents in diagnostic ultrasound, the attachment of sensitizer drugs to their surface results in a significant reduction in particle stability. In this Article, we prepare both lipid and polymer (PLGA) MBs with rose bengal attached to their surface and demonstrate that PLGA MB conjugates are significantly more stable than their lipid counterparts. In addition, the improved stability offered by the PLGA shell does not hinder their selective destruction using therapeutically acceptable ultrasound intensities. Furthermore, we demonstrate that treatment of ectopic human tumors (BxPC-3) in mice with the PLGA MB-rose bengal conjugate and ultrasound reduced tumor volume by 34% 4 days after treatment while tumors treated with the conjugate alone increased in volume by 48% over the same time period. Therefore, PLGA MBs may offer a more stable alternative to lipid MBs for the site specific delivery of sensitizers in SDT.

Intracellular guest exchange between dynamic supramolecular hosts.

Decyl and oligo(ethylene glycol) chains were appended to the same poly(methacrylate) backbone to generate an amphiphilic polymer with a ratio between hydrophobic and hydrophilic segments of 2.5. At concentrations greater than 10 µg mL(-1) in neutral buffer, multiple copies of this particular macromolecule assemble into nanoparticles with a hydrodynamic diameter of 15 nm. In the process of assembling, these nanoparticles can capture anthracene donors and borondipyrromethene acceptors within their hydrophobic interior and permit the transfer of excitation energy with an efficiency of 95%. Energy transfer is observed also if nanocarriers containing exclusively the donors are mixed with nanoparticles preloaded separately with the acceptors in aqueous media. The two sets of supramolecular assemblies exchange their guests with fast kinetics upon mixing to co-localize complementary chromophores within the same nanostructured container and enable energy transfer. After guest exchange, the nanoparticles can cross the membrane of cervical cancer cells and bring the co-entrapped donors and acceptors within the intracellular environment. Alternatively, intracellular energy transfer is also established after sequential cell incubation with nanoparticles containing the donors first and then with nanocarriers preloaded with the acceptors or vice versa. Under these conditions, the nanoparticles exchange their cargo only after internalization and allow energy transfer exclusively within the cell interior. Thus, the dynamic character of such supramolecular containers offers the opportunity to transport independently complementary species inside cells and permit their interaction only within the intracellular space.

Extending the tissue penetration capability of conventional photosensitisers: a carbon quantum dot-protoporphyrin IX conjugate for use in two-photon excited photodynamic therapy.

A carbon quantum dot (CQD)-protoporphyrin (IX) sensitisier conjugate was designed to exploit the large two-photon absorption cross section of CQDs and enable the indirect excitation of the sensitiser with 800 nm irradiation via FRET.

Highly luminescent biocompatible Carbon Quantum Dots by encapsulation with an amphiphilic polymer.

Highly luminescent, water-soluble and biocompatible Carbon Quantum Dots (aqCQDs) were prepared by encapsulating the parent hydrophobic CQDs in an amphiphilic polymer. The resulting aqCQDs were non-toxic to living cells, and were found to cross the cell membrane and localise primarily in the cytosol.

Microbubble-sonosensitiser conjugates as therapeutics in sonodynamic therapy.

A Rose Bengal sonosensitiser has been covalently attached to a lipid microbubble and the resulting conjugate shown to produce higher levels of singlet oxygen, enhanced cytotoxicity in a cancer cell line and a greater reduction in tumour growth than the sonosensitiser alone.

The effects of ultrasound and light on indocyanine-green-treated tumour cells and tissues.

Photodynamic therapy (PDT) is emerging as a treatment modality for the management of neoplastic disease. Despite considerable clinical success, its application for the treatment of deep-seated lesions is constrained by the inability of visible light to penetrate deeply into tissues. An emerging alternative approach exploits the fact that many photosensitisers respond to ultrasound, eliciting cytotoxic effects on target cells and tissues; this has become known as sonodynamic therapy (SDT). The objectives of this study were 1) to determine whether the IR-absorbing dye, indocyanine green (ICG), can be employed as a sonosensitiser and 2) to determine whether ultrasound can be used to enhance ICG-mediated PDT. Exposing ICG-treated mouse fibrosarcoma cells to ultrasound at an energy density of 30 J cm(-2) decreased cell viability by 65 %. Prior exposure of ICG-treated cells to light (λ 830 nm) and subsequent treatment with ultrasound led to a 90 % decrease in cell viability. In combination treatments a synergistic effect was observed at lower doses of ultrasound. Microscopic examination of cell populations treated with light or ultrasound demonstrated the production of intracellular reactive oxygen species (ROS). Using a mouse tumour model, treatment with light, ultrasound, or a combination thereof led to respective decreases in tumour growth of 42, 67, and 98 % at day 27 post-treatment. These results could provide a means of circumventing light-penetration issues that currently challenge the widespread use of PDT in the treatment of cancer.

Photoinduced enhancement in the luminescence of hydrophilic quantum dots coated with photocleavable ligands.

In search of strategies to photoactivate the luminescence of semiconductor quantum dots, we devised a synthetic approach to attach photocleavable 2-nitrobenzyl groups to CdSe-ZnS core-shell quantum dots coated with hydrophilic polymeric ligands. The emission intensity of the resulting nanostructured constructs increases by more than 60% with the photolysis of the 2-nitrobenzyl appendages. Indeed, the photoinduced separation of the organic chromophores from the inorganic nanoparticles suppresses an electron-transfer pathway from the latter to the former and is mostly responsible for the luminescence enhancement. However, the thiol groups anchoring the polymeric envelope to the ZnS shell also contribute to the photoinduced emission increase. Presumably, their photooxidation eliminates defects on the nanoparticle surface and promotes the radiative deactivation of the excited quantum dots. This effect is fully reversible but its magnitude is only a fraction of the change caused by the photocleavage of the 2-nitrobenzyl groups. In addition, these particular quantum dots can cross the membrane of model cells and their luminescence increases by ~80% after the intracellular photocleavage of the 2-nitrobenzyl quenchers. Thus, photoswitchable luminescent constructs with biocompatible character can be assembled combining the established photochemistry of the 2-nitrobenzyl photocage with the outstanding photophysical properties of semiconductor quantum dots and the hydrophilic character of appropriate polymeric ligands.

Enhanced ROS production and cell death through combined photo- and sono-activation of conventional photosensitising drugs.

A combination of light and ultrasound activation of two conventional photosensitising drugs, methylene blue and rose bengal, was shown to generate higher levels of reactive oxygen species (ROS) and lower LD50 values than either light or ultrasound activation alone.

Supramolecular strategies to construct biocompatible and photoswitchable fluorescent assemblies.

We designed and synthesized an amphiphilic copolymer with pendant hydrophobic decyl and hydrophilic poly(ethylene glycol) chains along a common poly(methacrylate) backbone. This macromolecular construct captures hydrophobic boron dipyrromethene fluorophores and hydrophobic spiropyran photochromes and transfers mixtures of both components in aqueous environments. Within the resulting hydrophilic supramolecular assemblies, the spiropyran components retain their photochemical properties and switch reversibly to the corresponding merocyanine isomers upon ultraviolet illumination. Their photoinduced transformations activate intermolecular electron and energy transfer pathways, which culminate in the quenching of the boron dipyrromethene fluorescence. As a result, the emission intensity of these supramolecular constructs can be modulated in aqueous environments under optical control. Furthermore, the macromolecular envelope around the fluorescent and photochromic components can cross the membrane of Chinese hamster ovarian cells and transport its cargo unaffected into the cytosol. Indeed, the fluorescence of these supramolecular constructs can be modulated also intracellularly by operating the photochromic component with optical inputs. In addition, cytotoxicity tests demonstrate that these supramolecular assemblies and the illumination conditions required for their operation have essentially no influence on cell viability. Thus, supramolecular events can be invoked to construct fluorescent and photoswitchable systems from separate components, while imposing aqueous solubility and biocompatibility on the resulting assemblies. In principle, this simple protocol can evolve into a general strategy to deliver and operate intracellularly functional molecular components under optical control.

Hydrophilic CdSe-ZnS core-shell quantum dots with reactive functional groups on their surface.

We synthesized macromolecular ligands for CdSe-ZnS core-shell quantum dots incorporating multiple thiol groups, poly(ethylene glycol) chains, and either carboxylic acids or primary amines along a common poly(methacrylate) backbone. The thiol groups encourage the adsorption of these macromolecular constructs on the ZnS shell of the nanoparticles, and the poly(ethylene glycol) chains impose hydrophilic character on the resulting assemblies. Indeed, the coated quantum dots are readily soluble in water and are stable under these conditions for months over a broad pH range (4.0-12.0) and even in the presence of large salt concentrations. In addition, these nanoparticles have relatively small hydrodynamic diameters (17-30 nm) and good quantum yields (0.3-0.4). Furthermore, the pendant carboxylic acids or primary amines of the macromolecular ligands can be exploited to modify the quantum dots after the adsorption of the polymers on their surface. For example, boron dipyrromethene dyes can be connected to the hydrophilic quantum dots on the basis of amide bond formation to encourage the transfer of energy from the luminescent CdSe core to the organic dyes. Our hydrophilic nanoparticles can also cross the membrane of Chinese hamster ovarian cells and accumulate in the cytosol with limited nuclear localization. Moreover, the internalized quantum dots are not cytotoxic and have essentially no influence on cell viability. Thus, our strategy for the preparation of biocompatible quantum dots can evolve into the development of valuable luminescent probes with nanoscaled dimensions and optimal photophysical properties for a diversity of biomedical applications.

Biocompatible CdSe-ZnS core-shell quantum dots coated with hydrophilic polythiols.

We designed four polymeric ligands for semiconductor quantum dots and synthesized these macromolecular constructs in four steps, starting from commercial precursors. These ligands have a poly(methacrylate) backbone with pendant thiol groups and poly(ethylene glycol) chains. The thiol groups anchor these ligands on the surface of preformed CdSe-ZnS core-shell quantum dots, and the poly(ethylene glycol) chains impose hydrophilic character on the resulting assemblies. Indeed, three of the four sets of quantum dots are soluble in aqueous environments and are stable under these conditions for days over a wide pH range (5.0-9.0). Furthermore, the polymeric coatings wrapped around the inorganic nanoparticles preserve the photophysical properties of the CdSe core and ensure relatively compact dimensions. Specifically, the luminescence quantum yield is ca. 0.4 and the hydrodynamic diameter ranges from 15 to 29 nm with the nature of the polymeric ligand. Model studies with human umbilical vein endothelial cells demonstrated that these hydrophilic quantum dots cross the cell membrane and localize either in the cytosol or in the nucleus. The length of the poly(ethylene glycol) chains appears to guide the intracellular localization of these luminescent probes. In addition, these studies indicated that these particular nanoparticles are not cytotoxic. In fact, their cellular internalization has essentially no influence on cell growth. In summary, we developed novel polymeric ligands able to impose hydrophilic character and biocompatibility on CdSe-ZnS core-shell nanoparticles. Thus, our results can lead to a new family of valuable luminescent probes for cellular imaging, based on the unique photophysical properties of semiconductor quantum dots.

A potential new prodrug for the treatment of cystinosis: design, synthesis and in-vitro evaluation.

Nephropathic cystinosis is a rare autosomal recessive disease characterised by raised lysosomal levels of cystine in the cells of most organs. The disorder is treated by regular administration of the aminothiol, cysteamine, an odiferous and unpleasant tasting compound that along with its metabolites is excreted in breath and sweat, leading to poor patient compliance. In an attempt to improve patient compliance a series of novel prodrugs has been designed and evaluated as a potential new treatment for nephropathic cystinosis. The first of the prodrugs tested, 3a, was found to decrease the levels of intracellular cystine in cystinotic fibroblasts. This is the first report of a potential new therapeutic treatment for nephropathic cystinosis since the advent of cysteamine bitartrate.

Anion sensing with luminescent quantum dots--a modular approach based on the Photoinduced Electron Transfer (PET) mechanism.

A CdSe-ZnS quantum dot (QD) has been surface functionalised with 1-(2-mercapto-ethyl)-3-phenyl-thiourea in the fluorophore-spacer-receptor format typical of Photoinduced Electron Transfer (PET) based organic dye sensors. The resulting QD conjugate was tested for selectivity toward the tetrabutylammonium salts of fluoride, chloride, bromide, hydrogen sulfate and acetate. Addition of fluoride, chloride and acetate ions resulted in an approximate 90% quenching of the original fluorescence intensity, while bromide and hydrogen sulfate had almost no effect. The observed quench was attributed to an increase in the reduction potential of the receptor upon anion binding resulting in an increase in PET from the excited QD to the receptor and a concomitant reduction in fluorescence intensity. The selectivity and sensitivity were comparable to an analogous organic dye based sensor where a similar receptor was bound to an anthracene fluorophore. Thus a modular approach is evident where a receptor used in an organic dye based sensor can be adapted and successfully used with QD's.