Tackling the Limitations of Copolymeric Small Interfering RNA Delivery Agents by a Combined Experimental-Computational Approach.

Despite the first successful applications of nonviral delivery vectors for small interfering RNA in the treatment of illnesses, such as the respiratory syncytial virus infection, the preparation of a clinically suitable, safe, and efficient delivery system still remains a challenge. In this study, we tackle the drawbacks of the existing systems by a combined experimental-computational in-depth investigation of the influence of the polymer architecture over the binding and transfection efficiency. For that purpose, a library of diblock copolymers with a molar mass of 30 kDa and a narrow dispersity (D < 1.12) was synthesized. We studied in detail the impact of an altered block size and/or composition of cationic diblock copolymers on the viability of each respective structure as a delivery agent for polynucleotides. The experimental investigation was further complemented by a computational study employing molecular simulations as well as an analytical description of systemic properties. This is the first report in which molecular dynamics simulations of RNA/cationic polymer complexes have been performed. Specifically, we developed and employed a coarse-grained model of the system at the molecular level to study the interactions between polymer chains and small interfering RNA. We were further able to confirm a threshold lengthbinding block/lengthnonbinding block ratio, which is required for efficient complexation of siRNA, and it was possible to find a correlation between the length of the cationic block and the size of the resulting polyplex. Hence, the combined insights from the experiments and the theoretical investigation resulted in a wealth of information about the properties of cationic diblock copolymers employed as RNA delivery agents, in particular regarding the molecular and mechanistic details of the interaction between the two components of a polyplex.

Chemical tags for site-specific fluorescent labeling of biomolecules.

This review focuses on the various approaches to covalently attach a chromophore to a biomolecule of interest in site-specific manner. Novel methods like inverse electron-demand Diels-Alder reaction, Pictet-Spengler ligation and enzyme tags like SNAP and Halo-tags are critically discussed and compared to established techniques like copper-free click reaction and native chemical ligation. Selected examples in which the tags have been exploited for in vitro or in vivo imaging are reviewed and evaluated.

Octreotide-Mediated Tumor-Targeted Drug Delivery via a Cleavable Doxorubicin-Peptide Conjugate.

Although recent methods for targeted drug delivery have addressed many of the existing problems of cancer therapy associated with undesirable side effects, significant challenges remain that have to be met before they find significant clinical relevance. One such area is the delicate chemical bond that is applied to connect a cytotoxic drug with targeting moieties like antibodies or peptides. Here we describe a novel platform that can be utilized for the preparation of drug-carrier conjugates in a site-specific manner, which provides excellent versatility and enables triggered release inside cancer cells. Its key feature is a cleavable doxorubicin-octreotide bioconjugate that targets overexpressed somatostatin receptors on tumor cells, where the coupling between the two components was achieved through the first cleavable disulfide-intercalating linker. The tumor targeting ability and suppression of adrenocorticotropic hormone secretion in AtT-20 cells by both octreotide and the doxorubicin hybrid were determined via a specific radioimmunoassay. Both substances reduced the hormone secretion to a similar extent, which demonstrated that the tumor homing peptide is able to interact with the relevant cell surface receptors after the attachment of the drug. Effective drug release was quickly accomplished in the presence of the physiological reducing agent glutathione. We also demonstrate the relevance of this scaffold in biological context in cytotoxicity assays with pituitary, pancreatic, and breast cancer cell lines.

The guanidinium group as a key part of water-soluble polymer carriers for siRNA complexation and protection against degradation.

Here, the preparation of a novel block copolymer consisting of a statistical copolymer N-(2-hydroxypropyl) methacrylamide-s-N-(3-aminopropyl) methacrylamide and a short terminal 3-guanidinopropyl methacrylamide block is reported. This polymer structure forms neutral but water-soluble nanosized complexes with siRNA. The siRNA block copolymer complexes are first analyzed using agarose gel electrophoresis and their size is determined with fluorescence correlation spectroscopy. The protective properties of the polymer against RNA degradation are investigated by treating the siRNA block copolymer complexes with RNase V1. Heparin competition assays confirm the efficient release of the cargo in vitro. In addition, the utilization of microscale thermophoresis is demonstrated for the determination of the binding strength between a fluorescently labeled polyanion and a polymer molecule.

Overcoming the barrier of CD8+T cells: Two types of nano-sized carriers for siRNA transport.

Bioengineering immune cells via gene therapy offers treatment opportunities for currently fatal viral infections. Also cell therapeutics offer most recently a breakthrough technology to combat cancer. These primary human cells, however, are sensitive to toxic influences, which make the utilization of optimized physical transfection techniques necessary. The otherwise commonly applied delivery agents such as LipofectamineⓇ or strongly cationic polymer structures are not only unsuitable for in vivo experiments, but are also highly toxic to immune cells. This study aimed to improve the design of polymeric carrier systems for small interfering RNA, which would allow efficient internalization into CD8+T-cells without affecting their viability and thereby removing the current limitations in the field. Here, two new carrier systems for small interfering RNA were tested. One is a cationic diblock copolymer, in which less than 10% of the monomers were modified with triphenylphosphonium cations. This moiety is lipophilic, promotes uptake and it is mostly known for its mitotropic properties. Furthermore, cationic nanohydrogel particles were synthesized in exceedingly small sizes (Rh < 14 nm). After full physicochemical characterization of the two carriers, extensive cytotoxicity studies were performed and the concentration dependent uptake into CD8+T-cells was tested in correlation to incubation time and protein content of the surrounding medium. Both carriers facilitated efficient complexation of siRNA as well as significant internalization into primary human cells in less than three hours of incubation. In addition, neither of the delivery systems reduced cell viability making them good candidates to transport siRNA into CD8+T-cells efficiently. STATEMENT OF SIGNIFICANCE: This study provides insights into the design of polymeric delivery agents as the method of choice for overcoming the limitations of cell manipulation. Until now, CD8+T-cells, which have become a treatment tool for currently fatal diseases, have not yet been made accessible for gene silencing by polymeric siRNA carrier systems. Choosing appropriate modification approaches for two chemically different polymer structures, we were, in both cases, able to achieve significant uptake in these cells even at low concentrations and without inducing cytotoxicity. These results remove current limitations and pave the way for bioengineering via gene therapy.

Overcoming drug resistance by cell-penetrating peptide-mediated delivery of a doxorubicin dimer with high DNA-binding affinity.

We describe the synthesis and characterization of a novel bioconjugate, consisting of an octaarginine cell-penetrating peptide and a highly DNA-affine doxorubicin dimer. The linkage between the two components is composed of a cleavable disulfide bond, which enables the efficient intracellular delivery of the cytotoxic payload within the reductive environment of the cytosol, mediated through glutathione. To determine the DNA-binding affinity of the dimeric drug molecule, microscale thermophoresis was applied. This is the first utilization of this method to assess the binding interactions of an anthracycline drug with nucleic acids. The cytotoxic effect of the peptide-drug conjugate, studied with drug-sensitive and doxorubicin-resistant cancer cells, demonstrates that the bioconjugate can successfully overcome drug resistance in neuroblastoma cells.