Antje Vollrath

Fluorescence imaging of cancer tissue based on metal-free polymeric nanoparticles - a review

The utilization of fluorescent nanoparticles (FNPs), which consist of organic fluorophores embedded into a polymer matrix, seems to be a promising concept for in vivo cancer imaging showing good biocompatibility, biodegradability, and low toxicity of the agents. Polymeric nanoparticles as fluorescent nanocarriers can be systematically designed with regard to the requested task, i.e., specific accumulation in the tumor tissue. Versatile organic fluorophores can be entrapped into polymers with fine-tuned properties, which were synthesized via polymerization techniques. Moreover, the formulation of the nanoparticles can be adjusted, and passive as well as active targeting strategies can be employed. Despite their evident benefits, fluorescent polymeric nanoparticles are still not in clinical application for cancer detection due to a still existing lack of understanding of their in vivo interactions as well as their reproducible production. This review focuses on cancer imaging based on organic dyes and metal-free polymeric fluorescent nanoparticles highlighting recent interesting reports about their design and application as well as their limitations.

A Fluorescent Thermometer Based on a Pyrene-Labeled Thermoresponsive Polymer

Thermoresponsive polymers that undergo a solubility transition by variation of the temperature are important materials for the development of ‘smart’ materials. In this contribution we exploit the solubility phase transition of poly(methoxy diethylene glycol methacrylate), which is accompanied by a transition from hydrophilic to hydrophobic, for the development of a fluorescent thermometer. To translate the polymer phase transition into a fluorescent response, the polymer was functionalized with pyrene resulting in a change of the emission based on the microenvironment. This approach led to a soluble polymeric fluorescent thermometer with a temperature range from 11 °C to 21 °C. The polymer phase transition that occurs during sensing is studied in detail by dynamic light scattering.

Preparation, Cellular Internalization, and Biocompatibility of Highly Fluorescent PMMA Nanoparticles

Methacrylate monomers were functionalized with a 4-hydroxythiazole chromophore and copolymerized with methyl methacrylate via RAFT. Nanoparticles of 120 and 500 nm in size were prepared without using stabilizers/surfactants. For comparative studies, preparative ultracentrifugation was applied for the separation into small and large particle fractions. All suspensions were characterized by DLS, AUC, and SEM and tested regarding their stability during centrifugation and re-suspension, autoclavation, and incubation in cell culture media. In vitro studies with mouse fibroblast cell line and differently sized NP showed a particle uptake into cells. Biocompatibility, non-toxicity, and hemocompatibility were demonstrated using a XTT assay, a live/dead staining, and an erythrocyte aggregation and hemolysis assay.

A toolbox of differently sized and labeled PMMA nanoparticles for cellular uptake investigations

The cellular internalization of defined PMMA nanoparticles was investigated. For this purpose, the biocompatible copolymer p(MMA-stat-MAA)0.91:0.09 was synthesized by RAFT polymerization and labeled with three different fluorescent dyes (λEx = 493, 557, and 653 nm). Nanoparticles were formulated from the differently labeled copolymers into samples with relatively narrow size distribution (diameter d 300 nm) under appropriate conditions of nanoprecipitation and were subsequently characterized by DLS and SEM. Mixtures of the differently sized nanoparticle samples were applied for internalization studies using monolayer cultured HeLa cells. The localization of the nanoparticles was detected after certain time points up to 24 h by CLSM, using LysoTracker as a marker for late endosomes and lysosomes. In investigations by flow cytometry, a fast uptake of medium sized nanoparticles was found, whereas the large and small nanoparticles exhibited a slower internalization. However, small and medium sized nanoparticles were detected in the late endosomes/lysosomes, whereas the large nanoparticles exhibit little co-localization with LysoTracker. Moreover, it could be shown by using different inhibitors for clathrin-dependent (chlorpromazine), caveolin-dependent (filipin III) endocytosis and macropinocytosis (EIPA) that nanoparticles with d 300 nm were internalized via macropinocytosis.

Dextran-graft-linear poly(ethylene imine)s for gene delivery: Importance of the linking strategy

Low molar mass linear poly(ethylene imine)s (lPEI) were grafted onto dextran via different synthesis routes aiming at the elucidation of structure-property relationships of dextran-graft-linear poly(ethylene imine) (dex-g-lPEI) conjugates for gene delivery applications. Beside the molar mass of well-defined lPEIs and the linker unit, also the amount of lPEI in the polymeric vectors was varied. The synthesized dextran modifications were characterized regarding their chemical structure and showed enhanced complexation and stabilization of DNA against enzymatic degradation. The transfection efficiency of dex-g-lPEIs was increased compared to unmodified lPEI and revealed a dependency of the used linking strategy. All complexes of DNA and dex-g-lPEIs were found to be nontoxic, but the synthesis route showed a strong influence on the aggregation of red blood cells. In conclusion, the linking strategy of lPEI to dextran has a significant impact on the physicochemical characteristics of DNA/polymer complexes, the biocompatibility as well as the transfection efficiency.

3rd generation poly(ethylene imine)s for gene delivery

Cationic polymers play a crucial role within the field of gene delivery offering the possibility to circumvent (biological) barriers in an elegant way. However, polymers are accompanied either by a high cytotoxicity or low efficiency. In this study, a series of high molar mass poly(2-oxazoline)-based copolymers was synthesized introducing 2-ethyl-2-oxazoline, ethylene imine, and primary amine bearing monomer units representing a new generation of poly(ethylene imine) (PEI). The potential of these modified PEIs as non-viral gene delivery agents was assessed and compared to linear PEI by studying the cytotoxicity, the polyplex characteristics, the transfection efficiency, and the cellular uptake using plasmid DNA (pDNA) as well as small interfering RNA (siRNA). High transfection efficiencies, even in serum containing media, were achieved using pDNA without revealing any cytotoxic effects on the cell viability at concentrations up to 1 mg mL-1. The delivery potential for siRNA was further investigated showing the importance of polymer composition for different genetic materials. To elucidate the origins for this superior performance, super-resolution and electron microscopy of transfected cells were used, identifying the endosomal release of the polymers as well as a reduced protein interaction as the main difference to PEI-based transfection processes. In this respect, the investigated copolymers represent remarkable alternatives as non-viral gene delivery agents.

Labeled Nanoparticles Based on Pharmaceutical EUDRAGIT® S 100 Polymers

The pharmaceutically important polymer P(MAA-r-MMA)(1:2) (EUDRAGIT(®) S100) was investigated concerning its behavior to form nanoparticles via nanoprecipitation. The particles obtained were characterized regarding their size, shape, and characteristics using DLS, SEM, and AUC. Furthermore, the P(MAA-r-MMA)(1:2) copolymer was modified with different markers in order to achieve polymer-based nanocarrier systems, which are detectable and may be useful for controlled drug delivery devices to monitor the drug pathways. The particles were labeled by physical entrapment as well as by covalent attachment of various markers, e.g., radicals, fluorescent-, and near-infrared dyes, to the polymer. Physical entrapment of radicals into the polymeric units was performed by co-nanoprecipitation of P(MAA-r-MMA)(1:2) and a radical marker. By means of covalent binding of the markers to the polymer, a stable and more defined labeling of the particles was also performed, leading only to a low degree of modification of the pharmaceutical polymer. After nanoprecipitation, the resulting labeled particles were characterized by SEM and DLS, whereas their biocompatibility was proven by in vitro studies. In order to ensure the possibility of detection of the particles inside the body for drug delivery-, sensor-, and imaging applications, the polymeric carriers were also investigated by electron spin resonance, fluorescence, as well as near-infrared spectroscopy.