Structure, dynamics, and energetics of siRNA-cationic vector complexation:a molecular dynamics study

The design and synthesis of safe and efficient nonviral vectors for gene delivery has attracted significant attention in recent years. Previous experiments have revealed that the charge density of a polycation (the carrier) plays a crucial role in complexation and the release of the gene from the complex in the cytosol. In this work, we adopt an atomistic molecular dynamics simulation approach to study the complexation of short strand duplex RNA with six cationic carrier systems of varying charge and surface topology. The simulations reveal detailed molecular-level pictures of the structures and dynamics of the RNA-polycation complexes. Estimates for the binding free energy indicate that electrostatic contributions are dominant followed by van der Waals interactions. The binding free energy between the 8(+)polymers and the RNA is found to be larger than that of the 4(+)polymers, in general agreement with previously published data. Because reliable binding free energies provide an effective index of the ability of the polycationic carrier to bind the nucleic acid and also carry implications for the process of gene release within the cytosol, these novel simulations have the potential to provide us with a much better understanding of key mechanistic aspects of gene-polycation complexation and thereby advance the rational design of nonviral gene delivery systems.

A cationic vaccine adjuvant based on a saturated quaternary ammonium lipid have different in vivo distribution kinetics and display a distinct CD4 T cell-inducing capacity compared to its unsaturated analog

Adjuvants are often composed of different constituents that can be divided into two groups based on their primary activity: the delivery system which carries and presents the vaccine antigen to antigen-presenting cells, and the immunostimulator that activates and modulates the ensuing immune response. Herein, we have investigated the importance of the delivery system and in particular its physical characteristics by comparing the delivery properties of two lipids which differ only in the degree of saturation of the acyl chains, rendering the liposomes either rigid (DDA, dimethyldioctadecylammonium) or highly fluid (DODA, dimethyldioleoylammonium) at physiological temperature. We show that these delivery systems are remarkably different in their ability to prime a Th1-directed immune response with the rigid DDA-based liposomes inducing a response more than 100 times higher compared to that obtained with the fluid DODA-based liposomes. Upon injection with a vaccine antigen, DDA-based liposomes form a vaccine depot that results in a continuous attraction of antigen-presenting cells that engulf a high amount of adjuvant and are subsequently efficiently activated as measured by an elevated expression of the co-stimulatory molecules CD40 and CD86. In contrast, the fluid DODA-based liposomes are more rapidly removed from the site of injection resulting in a lower up-regulation of co-stimulatory CD40 and CD86 molecules on adjuvant-positive antigen-presenting cells. Additionally, the vaccine antigen is readily dissociated from the DODA-based liposomes leading to a population of antigen-presenting cells that are antigen-positive but adjuvant-negative and consequently are not activated. These studies demonstrate the importance of studying in vivo characteristics of the vaccine components and furthermore show that physicochemical properties of the delivery system have a major impact on the vaccine-induced immune response. © 2012 Elsevier B.V. All rights reserved.

Mathematical modelling in batch emulsion polymerization

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Structure and dynamics of multiple cationic vectors-siRNA complexation by all-atomic molecular dynamics simulations

Understanding the molecular mechanism of gene condensation is a key component to rationalizing gene delivery phenomena, including functional properties such as the stability of the gene-vector complex and the intracellular release of the gene. In this work, we adopt an atomistic molecular dynamics simulation approach to study the complexation of short strand duplex RNA with four cationic carrier systems of varying charge and surface topology at different charge ratios. At lower charge ratios, polymers bind quite effectively to siRNA, while at high charge ratios, the complexes are saturated and there are free polymers that are unable to associate with RNA. We also observed reduced fluctuations in RNA structures when complexed with multiple polymers in solution as compared to both free siRNA in water and the single polymer complexes. These novel simulations provide a much better understanding of key mechanistic aspects of gene-polycation complexation and thereby advance progress toward rational design of nonviral gene delivery systems.

Gene delivery using cationic liposomes

The development of cationic liposomes for gene delivery has been ongoing for almost 20 years; however, despite extensive efforts to develop a successful therapeutic agent, there has been limited progress towards generating an effective pharmaceutical product. Since the introduction of N-(1-[2,3-dioley-loxy]propyl)-N,N,N-trimethylammonium chloride, an immense number of different cationic lipids have been synthesised and used to formulate cationic liposome - DNA complexes. Structural modification of the cationic lipids and the addition of components within the delivery system that can facilitate the fusion, cellular uptake and targeting of liposome - DNA complexes have all been used in a bid to enhance their transfection efficiency. Unfortunately, the overall impact of these improvements is still nominal, with the vast majority of clinical trials (∼ 85%) continuing to rely on more potent viral delivery of DNA despite their associated toxicity issues. Key characteristics of the most effective cationic liposomes for the delivery of plasmid DNA (from a consensus of the literature) is identified here and the problems of converting these attributes into an effective pharmaceutical product are outlined. © 2006 Informa UK Ltd.

Small cationic DDA:TDB liposomes as protein vaccine adjuvants obviate the need for TLR agonists in inducing cellular and humoral responses

Most subunit vaccines require adjuvants in order to induce protective immune responses to the targeted pathogen. However, many of the potent immunogenic adjuvants display unacceptable local or systemic reactogenicity. Liposomes are spherical vesicles consisting of single (unilamellar) or multiple (multilamellar) phospholipid bi-layers. The lipid membranes are interleaved with an aqueous buffer, which can be utilised to deliver hydrophilic vaccine components, such as protein antigens or ligands for immune receptors. Liposomes, in particular cationic DDA:TDB vesicles, have been shown in animal models to induce strong humoral responses to the associated antigen without increased reactogenicity, and are currently being tested in Phase I human clinical trials. We explored several modifications of DDA:TDB liposomes--including size, antigen association and addition of TLR agonists--to assess their immunogenic capacity as vaccine adjuvants, using Ovalbumin (OVA) protein as a model protein vaccine. Following triple homologous immunisation, small unilamellar vesicles (SUVs) with no TLR agonists showed a significantly higher capacity for inducing spleen CD8 IFN? responses against OVA in comparison with the larger multilamellar vesicles (MLVs). Antigen-specific antibody reponses were also higher with SUVs. Addition of the TLR3 and TLR9 agonists significantly increased the adjuvanting capacity of MLVs and OVA-encapsulating dehydration-rehydration vesicles (DRVs), but not of SUVs. Our findings lend further support to the use of liposomes as protein vaccine adjuvants. Importantly, the ability of DDA:TDB SUVs to induce potent CD8 T cell responses without the need for adding immunostimulators would avoid the potential safety risks associated with the clinical use of TLR agonists in vaccines adjuvanted with liposomes.

Liposomal cationic charge and antigen adsorption are important properties for the efficient deposition of antigen at the injection site and ability of the vaccine to induce a CMI response

With respect to liposomes as delivery vehicles and adjuvants for vaccine antigens, the role of vesicle surface charge remains disputed. In the present study we investigate the influence of liposome surface charge and antigen-liposome interaction on the antigen depot effect at the site of injection (SOI). The presence of liposome and antigen in tissue at the SOI as well as the draining lymphatic tissue was quantified to analyse the lymphatic draining of the vaccine components. Furthermore investigations detailing cytokine production and T-cell antigen specificity were undertaken to investigate the relationship between depot effect and the ability of the vaccine to induce an immune response. Our results suggest that cationic charge is an important factor for the retention of the liposomal component at the SOI, and a moderate to high (>50%) level of antigen adsorption to the cationic vesicle surface was required for efficient antigen retention in the same tissue. Furthermore, neutral liposomes expressing poor levels of antigen retention were limited in their ability to mediate long term (14 days) antigen presentation to circulating antigen specific T-cells and to induce the Th1 and Th17 arms of the immune system, as compared to antigen adsorbing cationic liposomes. The neutral liposomes did however induce the production of IL-5 at levels comparable to those induced by cationic liposomes, indicating that neutral liposomes can induce a weak Th2 response.

Increased potential of a cationic liposome-based delivery system:enhancing stability and sustained immunological activity in pre-clinical development

The combination of dimethyl dioctadecyl ammonium bromide (DDA) and the synthetic cord factor trehalose dibehenate (TDB) with Ag85B-ESAT-6 (H1 fusion protein) has been found to promote strong protective immune responses against Mycobacterium tuberculosis. The development of a vaccine formulation that is able to facilitate the requirements of sterility, stability and generation of a vaccine product with acceptable composition, shelf-life and safety profile may necessitate selected alterations in vaccine formulation. This study describes the implementation of a sterilisation protocol and the use of selected lyoprotective agents in order to fulfil these requirements. Concomitantly, close analysis of any alteration in physico-chemical characteristics and parameters of immunogenicity have been examined for this promising DDA liposome-based tuberculosis vaccine. The study addresses the extensive guidelines on parameters for non-clinical assessment, suitable for liposomal vaccines and other vaccine delivery systems issued by the World Health Organisation (WHO) and the European Medicines Agency (EMEA). Physical and chemical stability was observed following alteration in formulations to include novel cryoprotectants and radiation sterilisation. Immunogenicity was maintained following these alterations and even improved by modification with lysine as the cryoprotective agent for sterilised formulations. Taken together, these results outline the successful alteration to a liposomal vaccine, representing improved formulations by rational modification, whilst maintaining biological activity.

Facile synthesis of well-defined hydrophilic methacrylic macromonomers using ATRP and click chemistry

A range of well-defined hydrophilic methacrylic macromonomers has been synthesized by the judicious combination of atom transfer radical polymerization (ATRP) and copper-catalyzed 1,3-dipolar cycloaddition (azide-alkyne click chemistry). An azido a-functionalized ATRP initiator was used to produce well-defined homopolymers with terminal azide functionality via ATRP in protic media at 20 °C, with generally good control being achieved over both target molecular weight and final polydispersity (Mw/Mn = 1.10-1.35). Suitable methacrylic monomers include 2-aminoethyl methacrylate hydrochloride, 2-(diethylamino)ethyl methacrylate, 2-(dimethylamino)ethyl methacrylate, 2-hydroxyethyl methacrylate, 2-hydroxypropyl methacrylate, 2-(methacryloyloxy)ethyl phosphorylcholine, glycerol monomethacrylate, potassium 3-sulfopropyl methacrylate, and quaternized 2-(dimethylamino)ethyl methacrylate. These homopolymer precursors were then efficiently clicked using either propargyl methacrylate or propargyl acrylate to yield near-monodisperse (meth)acrylate-capped macromonomers with either cationic, anionic, nonionic, or zwitterionic character. Moreover, this generic route to well-defined hydrophilic macromonomers is also suitable for “one-pot” syntheses, as exemplified for 2-hydroxyethyl methacrylate and glycerol monomethacrylate-based macromonomers.