Targeting bacterial toxins

Protein toxins constitute the main virulence factors of several species of bacteria and have proven to be attractive targets for drug development. Lead candidates that target bacterial toxins range from small molecules to polymeric binders, and act at each of the multiple steps in the process of toxin-mediated pathogenicity. Despite recent and significant advances in the field, a rationally designed drug that targets toxins has yet to reach the market. This Review presents the state of the art in bacterial toxin targeted drug development with a critical consideration of achieved breakthroughs and withstanding challenges. The discussion focuses on A-B-type protein toxins secreted by four species of bacteria, namely Clostridium difficile (toxins A and B), Vibrio cholerae (cholera toxin), enterohemorrhagic Escherichia coli (Shiga toxin), and Bacillus anthracis (anthrax toxin), which are the causative agents of diseases for which treatments need to be improved.

PEG nanocages as non-sheddable stabilizers for drug nanocrystals

Many potent drugs are difficult to administer intravenously due to poor aqueous solubility. One validated approach for addressing this issue is to process them into colloidal dispersions known as "nanocrystals" (NCs). However, NCs possess high-energy surfaces that must be stabilized with surfactants to prevent aggregation. In addition, the stabilizer provides a means of anchoring targeting moieties to the NCs for achieving deposition or uptake at specified locations. Nevertheless, a critical challenge is that the surfactant (and consequently the targeting agents) can be shed upon high dilution. This work demonstrates successful cross-linking by click chemistry of stabilizers around paclitaxel NCs to form polymeric "nanocages". Cross-linking does not cause aggregation, as evidenced by transmission electron microscopy, and the nanocages retained the particulate drug through a combination of physical entrapment and physisorption. Size measurements by dynamic light scattering showed that nanocages act as sterically stabilizing barriers to particle-particle interactions and aggregation. The nanocages were shown to be less shed from the NCs than comparable non-cross-linked stabilizers. This contribution provides crucial general tools for preparing poorly sheddable stabilizing coatings to NCs and potentially other classes of nanoparticles for which covalent attachment of the stabilizer to the particle is undesirable (e.g., a drug) or impossible (chemically inert). The presented approach also offers the possibility of more stably attaching targeting moieties to the latter by use of heterotelechelic PEG derivatives, which may favor active targeting and internalization by cells.

Treatment of calcium channel blocker-induced cardiovascular toxicity with drug scavenging liposomes

Calcium channel blocker (CCB) overdose is potentially lethal. Verapamil and diltiazem are particularly prone to acute toxicity due to their dual effect on cardiac and vascular tissues. Unfortunately, conventional decontamination measures are ineffective in accelerating blood clearance and, to date, few efforts have been made to develop antidotes. To address the issue, injectable long-circulating liposomes bearing a transmembrane pH-gradient are proposed as efficient detoxifying agents of CCB poisoning. By scavenging the drug in situ, these circulating nanocarriers can restrict its distribution in tissues and hinder its pharmacological effect. In vitro, we showed that liposomes stability in serum and their ability to sequester CCBs could be finely-tuned by modulating their internal pH, surface charge, and lipid bilayer structure. Subsequently, we verified their efficacy in reversing the cardiovascular effects of verapamil in rats implanted with telemetric pressure/biopotential transmitters. In animals orally intoxicated to verapamil, an intravenous injection of the liposomal antidote rapidly attenuated the reduction in blood pressure. Areas under diastolic, systolic, and mean pressures curves were significantly reduced by up to 60% and the time to hemodynamic recovery was shortened from 19 to only 11 h. These findings confirm the protective effect of pH-gradient liposomes against cardiovascular failure after CBB intoxication, and endorse their potential as efficient, versatile antidotes.

The copolymer P(HEMA-co-SS) binds gluten and reduces immune response in gluten-sensitized mice and human tissues

BACKGROUND & AIMS

Copolymers of hydroxyethyl methacrylate and styrene sulfonate complex with isolated gliadin (the toxic fraction of gluten) and prevent damage to the intestinal barrier in HLA-HCD4/DQ8 mice. We studied the activity toward gluten and hordein digestion and biologic effects of poly(hydroxyethyl methacrylate-co-styrene sulfonate (P(HEMA-co-SS)). We also investigated the effect of gliadin complex formation in intestinal biopsy specimens from patients with celiac disease.

METHODS

We studied the ability of P(HEMA-co-SS) to reduce digestion of wheat gluten and barley hordein into immunotoxic peptides using liquid chromatography-mass spectrometry. The biodistribution and pharmacokinetic profile of orally administered P(HEMA-co-SS) was established in rodents using tritium-labeled polymer. We assessed the capacity of P(HEMA-co-SS) to prevent the immunologic and intestinal effects induced by a gluten-food mixture in gluten-sensitized HLA-HCD4/DQ8 mice after short-term and long-term administration. We measured the effects of gliadin complex formation on cytokine release ex vivo using intestinal biopsy specimens from patients with celiac disease.

RESULTS

P(HEMA-co-SS) reduced digestion of wheat gluten and barley hordein in vitro, thereby decreasing formation of toxic peptides associated with celiac disease. After oral administration to rodents, P(HEMA-co-SS) was predominantly excreted in feces, even in the presence of low-grade mucosal inflammation and increased intestinal permeability. In gluten-sensitized mice, P(HEMA-co-SS) reduced paracellular permeability, normalized anti-gliadin immunoglobulin A in intestinal washes, and modulated the systemic immune response to gluten in a food mixture. Furthermore, incubation of P(HEMA-co-SS) with mucosal biopsy specimens from patients with celiac disease showed that secretion of tumor necrosis factor-α was reduced in the presence of partially digested gliadin.

CONCLUSIONS

The copolymer P(HEMA-co-SS) reduced digestion of wheat gluten and barley hordein and attenuated the immune response to gluten in a food mixture in rodents. It might be developed to prevent or reduce gluten-induced disorders in humans.

The journey of a drug-carrier in the body: an anatomo-physiological perspective

Recent advances in chemistry and material sciences have witnessed the emergence of an increasing number of novel and complex nanosized carriers for the delivery of drugs and imaging agents. Nevertheless, this raise in complexity does not necessarily offer more efficient systems. The lack of performance experienced by several colloidal drug carriers during the preclinical and clinical development processes can be explained by inadequate pharmacokinetic/biodistribution profiles and/or unacceptable toxicities. A comprehensive understanding of the body characteristics is necessary to predict and prevent these problems from the early stages of nanomaterial conception. In this manuscript, we review and discuss the anatomical and physiological elements which must be taken into account when designing new carriers for delivery or imaging purposes. This article gives a general overview of the main organs involved in the elimination of nanosized materials and briefly summarizes the knowledge acquired over more than 30 years of research and development in the field of drug targeting.

Interplay of chemical microenvironment and redox environment on thiol-disulfide exchange kinetics

The interplay between the chemical microenvironment surrounding disulfides and the redox environment of the media on thiol-disulfide exchange kinetics was examined by using a peptide platform. Exchange kinetics of up to 34 cysteine-containing peptides were measured in several redox buffers. The electrostatic attraction/repulsion between charged peptides and reducing agents such as glutathione was found to have a very pronounced effect on thiol-disulfide exchange kinetics (differences of ca. three orders of magnitude). Exchange kinetics could be directly correlated to peptide charge over the entire range examined. This study highlights the possibility of finely and predictably tuning thiol-disulfide exchange, and demonstrates the importance of considering both the local environment surrounding the disulfide and the nature of the major reducing species present in the environment for which their use is intended (e.g., in drug delivery systems, sensors, etc).

Development and physico-chemical characterization of a liposomal formulation of istaroxime

Istaroxime, an investigational new drug that targets defective Ca(2+) cycling without compromising cardiac efficiency, may represent a promising and safe treatment of both acute and chronic heart failure. Even though the compound demonstrated good tolerability in a phase I/II safety study, symptoms related to the gastro-intestinal tract and pain at the injection site were reported as the most frequent side effects. The aim of this study was to encapsulate istaroxime in a drug delivery system (DDS) that could minimize the pain perceived upon administration. The DDS was designed to be quickly destabilized in plasma, in order to minimize alteration of the pharmacokinetic profile of istaroxime. To meet those requirements, a balance between the encapsulation efficiency and the release rate was sought. Transmembrane pH-gradient liposomes formulated with different phosphatidylcholines were investigated as vehicles for an efficient active drug loading. Poly(ethylene glycol)-660-hydroxystearate (PEG-HS) was chosen as excipient to modulate the bilayer fluidity and the release properties of the liposomes. A fast and efficient encapsulation was obtained by modulating the drug-to-lipid ratio, the amount of PEG-HS, and the incubation temperature. High encapsulation efficiency was achieved by incubating the drug with liposomal dispersions at room temperature for 10 min. Almost complete release was obtained in physiological conditions in less than 10 min, suggesting a model formulation potentially useful for drugs presenting similar features and side effects.

siRNA nanocarriers based on methacrylic acid copolymers

Poly(ethylene glycol)-b-poly(propyl methacrylate-co-methacrylic acid) (PEG-b-P(PrMA-co-MAA) can be complexed with poly(amido amine) (PAMAM) dendrimers and nucleic acids to form pH-responsive nanosized core-shell type polyion complex micelles (PICMs). These PICMs have the ability to lose their shell and release the PAMAM/nucleic acid core under mildly acidic conditions such as those encountered in the endosomal compartment. In this work, pH-sensitive PICMs composed of PEG-b-P(PrMA-co-MAA), different PAMAMs, and siRNAs were prepared and characterized. These micelles had mean diameters ranging from 50 to 100 nm depending on the structure of the polycationic component. In order to trigger PICM uptake by receptor-mediated endocytosis, the micelles were decorated with an antibody fragment directed against the transferrin receptor (anti-CD71). The targeting ligand was stably conjugated to a semi-telechelic amino-PEG-b-P(PrMA-co-MAA) via a maleimide/activated ester bifunctional linker, yielding up to 60%-80% functionalization of the maleimide groups. The cellular uptake of the micelles was assessed on human prostate cancer cells (PC-3) via flow cytometry. Native PICMs and micelles bearing a non-specific antibody fragment were taken up to the same extent with a low efficiency, whereas anti-CD71 Fab'-decorated PICMs exhibited significantly higher uptake. The capacity of the targeted, siRNA-loaded, PICMs to downregulate the expression of the Bcl-2 anti-apoptotic oncoprotein was investigated using the appropriate unmodified or 2'-modified (2'F-RNA and 2'F-ANA) siRNA sequence. Bcl-2 mRNA and protein levels were greatly reduced when the cells were transfected with anti-CD71 decorated PICMs. Optimal silencing was achieved with the chemically modified siRNA. These data suggest that combining optimized siRNA chemistry with an effective delivery system can potentiate the activity of siRNA, thereby potentially reducing the total dose of carrier required to achieve a pharmacological effect.

Transmembrane pH-gradient liposomes to treat cardiovascular drug intoxication

Injectable scavenging nanocarriers have been proposed as detoxifying agents when there are no specific antidotes to treat pharmacological overdoses. They act by capturing the drug in situ, thereby restricting distribution in tissues. In the clinic, the only systems used for that purpose are parenteral lipid emulsions, which are relatively inefficient in terms of uptake capacity. In this study, we investigated long-circulating liposomes with a transmembrane pH gradient as treatment for diltiazem intoxication. The unique ion-trapping properties of the vesicles toward ionizable compounds were exploited to sequester the drug in the bloodstream and limit its pharmacological effect. After in vitro optimization of the formulation, the in vivo scavenging properties of the liposomes were demonstrated by examining the drug's pharmacokinetics. The reduced volume of distribution and increased area under the plasma concentration versus time curve in animals treated with liposomes indicated limited tissue distribution. The vesicles exerted a similar but more pronounced effect on deacetyl-diltiazem, the principal active metabolite of the drug. This in vivo uptake of both drug and metabolite altered the overall pharmacological outcome. In rats receiving an intravenous bolus of diltiazem, the liposomes tempered the hypotensive decline and maintained higher average blood pressure for 1 h. The detoxifying action of liposomes was even stronger when the rats received higher doses of the drug via perfusion. In conclusion, the present work provided clear evidence that liposomes with a transmembrane pH gradient are able to change the pharmacokinetics and pharmacodynamics of diltiazem and its metabolite and confirmed their potential as efficient detoxifying nanocarriers.

In vivo evaluation of pH-sensitive polymer-based immunoliposomes targeting the CD33 antigen

The purpose of this study was to evaluate in vivo a targeted pH-sensitive liposomal formulation tailored to promote the efficient intracellular delivery of 1-beta-d-arabinofuranosylcytosine (ara-C) to human myeloid leukemia cells. Specifically, pH-sensitive immunoliposomes were obtained by anchoring a copolymer of dioctadecyl, N-isopropylacrylamide and methacrylic acid in bilayers of PEGylated liposomes (LP) and by coupling the whole anti-CD33 monoclonal antibody (mAb) or its Fab' fragments. Their pharmacokinetic and biodistribution profiles were assessed in Balb/c and leukemic HL60-bearing immunodepressed (SCID) mice. In naive mice, nontargeted and pH-sensitive Fab'-LP had longer circulation times than LP with whole mAb. In SCID/HL60 (CD33(+)) mice, the pharmacokinetic and biodistribution profiles of LP and encapsulated ara-C were comparable between nontargeted and pH-sensitive Fab'-LP. In leukemic mice, only pH-insensitive, ara-C-loaded Fab' induced prolonged survival times. The apparent absence of pH-sensitive Fab'-LP effect could be related to lower exposure to ara-C in SCID mice.

pH-Responsive molecular tweezers

Molecular tweezers are dynamic devices that are able to switch from one conformation to another upon stimulation by an external trigger. In this work, we report a new water-soluble macromolecular carrier bearing a pH-responsive molecular tweezer, whose affinity for a substrate depends on the external pH. The conformational change of the switching unit was evidenced by (1)H NMR spectroscopy, and fluorescence studies conducted in aqueous media demonstrated the ability of the carrier to bind to substrates in a pH-dependent fashion.

Quantitative imaging of lymphatic function with liposomal indocyanine green

Lymphatic vessels play a major role in cancer progression and in postsurgical lymphedema, and several new therapeutic approaches targeting lymphatics are currently being developed. Thus, there is a critical need for quantitative imaging methods to measure lymphatic flow. Indocyanine green (ICG) has been used for optical imaging of the lymphatic system, but it is unstable in solution and may rapidly enter venous capillaries after local injection. We developed a novel liposomal formulation of ICG (LP-ICG), resulting in vastly improved stability in solution and an increased fluorescence signal with a shift toward longer wavelength absorption and emission. When injected intradermally to mice, LP-ICG was specifically taken up by lymphatic vessels and allowed improved visualization of deep lymph nodes. In a genetic mouse model of lymphatic dysfunction, injection of LP-ICG showed no enhancement of draining lymph nodes and slower clearance from the injection site. In mice bearing B16 luciferase-expressing melanomas expressing vascular endothelial growth factor-C (VEGF-C), sequential near-IR imaging of intradermally injected LP-ICG enabled quantification of lymphatic flow. Increased flow through draining lymph nodes was observed in mice bearing VEGF-C-expressing tumors without metastases, whereas a decreased flow pattern was seen in mice with a higher lymph node tumor burden. This new method will likely facilitate quantitative studies of lymphatic function in preclinical investigations and may also have potential for imaging of lymphedema or improved sentinel lymph detection in cancer.

Aminated linear and star-shape poly(glycerol methacrylate)s: synthesis and self-assembling properties

Over the past 10 years, polyglycerols and their structurally related analogs have received considerable attention in the biomedical field. Poly(glycidyl methacrylate) (PGMA) is a versatile polymer because its pendant epoxide groups can be opened with different functional groups to generate poly(glycerol methacrylate)s (PGOHMA) derivatives. In this work, linear and star-shape PGMAs were synthesized by atom transfer radical polymerization and then functionalized with four different amines by ring-opening addition. This resulted in the formation of polyglycerol-like polymers having both hydroxyl and amine moieties and different water-solubility. The water-insoluble polymers could form pH-sensitive nanoassemblies, while the soluble derivatives efficiently complexed a short strand polynucleotide. The aminated polyglycerol interacted more avidly with the oligonucleotide than the control poly(ethyleneimine), and high transfection efficacy could be obtained with the linear derivative. Such polymers could find practical applications for the delivery of drugs and nucleic acids.

Interaction of alpha-gliadin with polyanions: design considerations for sequestrants used in supportive treatment of celiac disease

Copolymers of sodium 4-styrene sulfonate (SS) and hydroxyethyl methacrylate (HEMA) were investigated as sequestrants of alpha-gliadin, a gluten protein, for the treatment of gluten intolerance. The interactions of alpha-gliadin with poly(SS) and poly(HEMA-co-SS) with 9 and 26 mol% SS content were studied at gastric (1.2) and intestinal (6.8) pH using circular dichroism and measurements of turbidity, dynamic light scattering and zeta potential. The interactions and their influence on alpha-gliadin secondary and aggregated structures depended mainly on the ratio of polymer negative and protein positive charges at pH 1.2, and on polymer SS content at polymer concentrations providing in excess of negative charges at either pH. Poly(SS) could not form complex particles with alpha-gliadin in a sufficient excess of negative charges. Copolymerization with HEMA enhanced the formation of complex particles. Poly(HEMA-co-SS) with intermediate SS content was found to be the most effective sequestrant for alpha-gliadin. This study provides insight into design considerations for polymer sequestrants used in the supportive treatment of celiac disease.

Tyrosine-based rivastigmine-loaded organogels in the treatment of Alzheimer's disease

Organogels can be prepared by immobilizing an organic phase into a three-dimensional network coming from the self-assembly of a low molecular weight gelator molecule. In this work, an injectable subcutaneous organogel system based on safflower oil and a modified-tyrosine organogelator was evaluated in vivo for the delivery of rivastigmine, an acetylcholinesterase (AChE) inhibitor used in the treatment of Alzheimer's disease. Different implant formulations were injected and the plasmatic drug concentration was assayed for up to 35 days. In parallel, the inhibition of AChE in different brain sections and the biocompatibility of the implants were monitored. The pharmacokinetic profiles were found to be influenced by the gel composition, injected dose and volume of the implant. The sustained delivery of rivastigmine was accompanied by a significant prolonged inhibition of AChE in the hippocampus, a brain structure involved in memory. The implant induced only a minimal to mild chronic inflammation and fibrosis, which was comparable to poly(D,L-lactide-co-glycolide) in situ-forming implants. These findings suggest that tyrosine-based organogels could represent an alternative approach to current formulations for the sustained delivery of cholinesterase inhibitors.

Serum-stable, long-circulating, pH-sensitive PEGylated liposomes

pH-sensitive liposomes have been designed to deliver active compounds, specifically to acidic intracellular organelles and to augment their cytoplasmic concentrations. These systems combine the protective effects of other liposomal formulations with specific environment-controlled drug release. They are stable at physiological pH, but abruptly discharge their contents when endocytosed into acidic compartments, allowing the drug to be released before it is exposed to the harsh environment of lysosomes. Serum-stable formulations with minimal leakage at physiological pH and rapid drug release at pH 5.0-5.5 can be easily prepared by inserting a hydrophobically modified N-isopropylacrylamide/methacrylic acid copolymer (poly(NIPAM-co-MAA)) in the lipid bilayer of sterically stabilized liposomes. The present chapter describes polymer synthesis, as well as the preparation, and characterization of large unilamelar pH-sensitive vesicles.