Bile Acid Sequestrants Based on Natural and Synthetic Gels.

Bile acid sequestrants (BASs) are non-systemic therapeutic agents used for the management of hypercholesterolemia. They are generally safe and not associated with serious systemic adverse effects. Usually, BASs are cationic polymeric gels that have the ability to bind bile salts in the small intestine and eliminate them by excretion of the non-absorbable polymer-bile salt complex. This review gives a general presentation of bile acids and the characteristics and mechanisms of action of BASs. The chemical structures and methods of synthesis are shown for commercial BASs of first- (cholestyramine, colextran, and colestipol) and second-generation (colesevelam and colestilan) and potential BASs. The latter are based on either synthetic polymers such as poly((meth)acrylates/acrylamides), poly(alkylamines), poly(allylamines) and vinyl benzyl amino polymers or biopolymers, such as cellulose, dextran, pullulan, methylan, and poly(cyclodextrins). A separate section is dedicated to molecular imprinting polymers (MIPs) because of their great selectivity and affinity for the template molecules used in the imprinting technique. Focus is given to the understanding of the relationships between the chemical structure of these cross-linked polymers and their potential to bind bile salts. The synthetic pathways used in obtaining BASs and their in vitro and in vivo hypolipidemic activities are also introduced.

Role of Hydrophobic Associations in Self-Healing Hydrogels Based on Amphiphilic Polysaccharides.

Self-healing hydrogels have the ability to recover their original properties after the action of an external stress, due to presence in their structure of reversible chemical or physical cross-links. The physical cross-links lead to supramolecular hydrogels stabilized by hydrogen bonds, hydrophobic associations, electrostatic interactions, or host-guest interactions. Hydrophobic associations of amphiphilic polymers can provide self-healing hydrogels with good mechanical properties, and can also add more functionalities to these hydrogels by creating hydrophobic microdomains inside the hydrogels. This review highlights the main general advantages brought by hydrophobic associations in the design of self-healing hydrogels, with a focus on hydrogels based on biocompatible and biodegradable amphiphilic polysaccharides.

New Quaternary Ammonium Derivatives Based on Citrus Pectin.

New citrus pectin derivatives carrying pendant N,N-dimethyl-N-alkyl-N-(2-hydroxy propyl) ammonium chloride groups were achieved via polysaccharide derivatization with a mixture of N,N-dimethyl-N-alkyl amine (alkyl = ethyl, butyl, benzyl, octyl, dodecyl) and epichlorohydrin in aqueous solution. The structural characteristics of the polymers were examined via elemental analysis, conductometric titration, Fourier Transform Infrared spectroscopy (FTIR) and 1D (1H and 13C) nuclear magnetic resonance (NMR). Capillary viscosity measurements allowed for the study of viscometric behavior as well as the determination of viscosity-average molar mass for pristine polysaccharide and intrinsic viscosity ([η]) values for pectin and its derivatives. Dynamic light scattering measurements (DLS) showed that pectin-based polymers formed aggregates in aqueous solution with a unimodal distribution. Critical aggregation concentration (cac) for the hydrophobic pectin derivatives were determined using fluorescence spectroscopy. Atom force microscopy (AFM) images allowed for the investigation of the morphology of polymeric populations obtained in aqueous solution, consisting of flocs and aggregates for crude pectin and its hydrophilic derivatives and well-organized aggregates for lipophilic pectin derivatives. Antimicrobial activity, examined using the disc diffusion method, proved that all polymers were active against Staphylococcus aureus bacterium and Candida albicans yeast.

Bile salts adsorption on dextran-based hydrogels.

Dextran-based gels bearing two types of pendant N, N-dimethyl-N-alkyl-N-(2-hydroxypropyl) ammonium chloride groups with different alkyl chain length substituents (C2 and C12/C16, respectively) at the quaternary nitrogen were synthesized and structural characteristics of the compounds were studied by elemental analysis, potentiometric titration, FTIR and NMR spectroscopy. The morphology and size of polymeric microspheres were examined by SEM and their swelling behavior in water was also investigated. The hydrogels were evaluated as sorbents for sodium cholate (NaCA) and sodium deoxycholate (NaDCA) in water and 10 mM NaCl solutions. Different isotherm models (nearest-neighbor-interaction, Langmuir, Freundlich, Dubinin-Raduskevich, Sips and Hill) were used to elucidate the adsorption mechanism and established the characteristics of the most efficient polymeric sorbent. The maximum adsorption capacity of the gels was highly controlled by gel hydrophobicity which enhanced gel-bile salt affinity but decreased binding cooperativity. Swelling porosity, ionic strength and ligand lipophilicity were other factors that also affected the adsorption process. The hydrogel having 25 mol% pendant dodecyl groups retained the maximum amount of bile salts (1051 mg NaCA/g and 1138 mg NaDCA/g). All hydrophobically modified hydrogels revealed a better affinity and strength of binding compared to commercial Cholestyramine®.

Self-assembly of dextran - b - deoxycholic acid polyester copolymers: Copolymer composition and self-assembly procedure tune the aggregate size and morphology.

Self-assembly potential of new amphiphilic block copolymers containing dextran (Mn 4500, 8000, 15,000) and a semi-rigid deoxycholic acid-oligoethyleneglycol polyester (Mn 2500-8800, 2 or 4 ethyleneglycol units), was evaluated as a function of copolymer composition and self-assembly procedure, using dynamic light scattering and transmission electron microscopy. Addition of copolymer solution to water provided small star-like micelles (∼ 100 nm), while addition of water to copolymer solution led to various morphologies and sizes (60-600 nm), depending on polymer composition. Worm-like micelles were obtained from a copolymer containing dextran with Mn 4500 and 66 wt% polyester, and vesicles were formed by copolymers prepared from dextran with Mn 8000 and containing 46 wt% polyester. Presence of a longer oligoethyleneglycol decreased the size of micelles and vesicles due to an enhanced flexibility of the polyester hydrophobic block. The results allow the selection of the most appropriate parameters to obtain the desired aggregate characteristics.

Novel amphiphilic dextran esters with antimicrobial activity.

New amphiphilic dextran esters were obtained by polysaccharide functionalization with different substituted 1,2,3-triazoles-4-carboxylic acid via in situ activation with N, N'-carbonyldiimidazole. Nitrogen-containing heterocyclic derivatives were achieved by copper(I)-catalyzed cycloaddition reaction between organic azides and ethyl propiolate. Structural characteristics of the compounds were studied by elemental analysis, Fourier transform infrared and nuclear magnetic resonance spectroscopy (1H and 13C-NMR). Thermogravimetric analysis, differential scanning calorimetry and wide-angle X-ray diffraction were used for esters characterization. Properties of polymeric self-associates, formed in aqueous solution, were studied by dynamic light scattering and transmission electron microscopy. The critical aggregation concentration values for dextran esters, determined by fluorescence spectroscopy, were in the range of 4.1-9.5 mg/dL. Antimicrobial activity, investigated for some of the polymers by disc-diffusion method, pointed out that polysaccharide esters were active.

Block copolymers containing dextran and deoxycholic acid polyesters. Synthesis, self-assembly and hydrophobic drug encapsulation.

New biocompatible amphiphilic block copolymers were prepared using two natural compounds as starting materials, a polysaccharide (dextran) and a bile acid (deoxycholic acid). The copolymers were synthesized by dipolar 1,3-cycloaddition reaction between dextran with azide end groups and deoxycholic acid - oligo(ethylene glycol)s polyester with propargyl end groups. Different copolymer composition were obtained by variation of molecular weights of dextran (Mn 4.5, 8, 15 kDa) and polyester (Mn 2-6 kDa), as well as the length of oligo(ethylene glycol) (2-4 ethylenglycol units) used for polyester synthesis. These copolymers can for micelle like aggregates in aqueous medium with nanometric size (50-600 nm) and spherical form, as assessed by light scattering, atomic force microscopy and transmission electron microscopy. Encapsulation of the hydrophobic drug curcumin in micelles could increase 68,181 times its water solubility, and curcumin release from micelles was slow and with reduced burst effect.

Influence of dextran hydrogel characteristics on adsorption capacity for anionic dyes.

A series of cationic amphiphilic dextran hydrogels with pendent N, N-dimethyl-N-alkyl-N-(2-hydroxypropyl) ammonium chloride pendent groups was obtained with various molar ratios hydrophilic (alkyl = C2)/hydrophobic groups (alkyl = C12 or C16), total content in amino groups (50-68 mol%) and water swelling capacity (3-15 g water/g dry gel). Adsorption capacity for anionic dyes (Methyl Orange and Rose Bengal) was studied as a function of hydrogel characteristics. Adsorption kinetics was mainly influenced by swelling porosity and dye molecular weight. Equilibrium data showed that the maximum sorption capacity was strongly dependent on hydrophobic segment content, which enhanced the affinity dye-gel (increased Langmuir constant KL) and contribution of ion-exchange to adsorption mechanism, and significantly decreased the maximum uptake of Rose Bengal. The hydrophilic highly swollen hydrogel had the best adsorption capacity for Rose Bengal (1700 mg/g) at 25 °C, and more hydrophobic gels could adsorb 830-900 mg Methyl Orange/g, irrespective of the medium temperature.

Dextran derivatives application as flocculants.

A large variety of hydrophilic and amphiphilic polyelectrolytes based on dextran have been previously tested as coagulating and flocculating agents in model wastewater treatment and the results are presented in this review article. The dextran derivatives are either cationic, bearing (i) pendent quaternary ammonium groups, N-alkyl-N,N-dimethyl-N-(2-hydroxypropyl)ammonium chloride and (ii)1-(2-hydroxypropyl)-3-methyl imidazolium chloride, randomly distributed along the polymer backbone or anionic, (i) dextran monobasic phosphate with HP(O)(OH) groups and (ii) dextran derivatives with carboxylic groups namely, dextran-g-poly(acrylamide-co-sodium acrylates). This paper will give an overview of the main results obtained in the separation of suspensions and emulsions containing either inorganic (clay, titanium dioxide, zirconium silicate (kreutzonit), zinc oxide and ferric oxide (hematite)) or organic (pesticides: Fastac 10EC, Decis, Dithane M45) contaminants. The investigations considered the influence of some polyelectrolyte characteristics (the charge density, the length of the alkyl substituent, the molecular mass), polyelectrolyte dose and initial concentration, as well as the properties of the model suspensions/emulsions (contaminants concentration, dispersion medium composition and pH) on the separation process. Turbidity/absorbance, zeta potential and particle dimensions measurements allowed to determine the polycation dose where the maximum separation efficiency was obtained, the flocculation mechanism and the floc size and distribution for each peculiar system.

Antimicrobial activity of chemically modified dextran derivatives.

Cationic amphiphilic dextran derivatives with a long alkyl group attached to the reductive end of the polysaccharide chain and quaternary ammonium groups attached as pendent groups to the main dextran backbone were synthesized and tested for their antimicrobial properties against several bacteria and fungi strains. Dependence of antimicrobial activity on both polymer chemical composition (dextran molar mass, length of end alkyl group and chemical structure of ammonium groups) and type of microbes was highlighted by disc-diffusion method (diameter of inhibition zone) and broth microdilution method (minimum inhibitory concentrations). Polymers had antimicrobial activity for all strains studied, except for Pseudomonas aeruginosa ATCC 27853. The best activity against Staphylococcus aureus (Minimun Inhibitory Concentration 60µg/mL) was provided by polymers obtained from dextran with lower molecular mass (Mn=4500), C12H25 or C18H37 end groups, and N,N-dimethyl-N-benzylammonium pendent groups.

Cationic gemini surfactant as a dual linker for a cholic acid-modified polysaccharide in aqueous solution: thermodynamics of interaction and phase behavior.

Understanding the thermodynamics of formation of biocompatible aggregates is a key factor in the bottom up approach to the development of novel types of drug carriers and their structural tuning using small amphiphilic molecules. We chose an anionic amphiphilic and biocompatible polymer that consists of a dextran and grafted cholic acid pendants, randomly distributed along the dextran backbone, with a degree of substitution (DS) of 15 mol% (designated Dex-15CACOONa). The thermodynamics of interaction and phase behavior of mixtures of this polyelectrolyte and a cationic gemini surfactant hexanediyl-α,ω-bis(dodecyldimethylammonium bromide) (C12C6C12Br2) or its monomer surfactant dodecyltrimethylammonium bromide (DTAB) in aqueous solution were characterized by isothermal titration calorimetry (ITC) and turbidity, together with cryogenic transmission electron microscopy (Cryo-TEM). The various critical concentrations and the enthalpy changes of the corresponding phase transitions for the oppositely charged system were obtained from the plots of the observed enthalpy change (ΔHobs) and turbidity measurements as a function of gemini concentration. The morphologies of the aggregates in various phases were observed by Cryo-TEM. Altogether these results suggest the critical role of gemini as a dual linker. At the concentrations where the crosslink between the pendant aggregates happens, the free gemini concentration is proximately zero and the aggregate retains its negative charge. The analysis of various factors involved in the interaction allowed a rationalization of the driving forces for mixed aggregate formation, which will contribute to a subsequent rational design of drug delivery systems based on this polymer/surfactant system.

Dextran based Polymeric Micelles as Carriers for Delivery of Hydrophobic Drugs.

BACKGROUND: The improvement of drugs bioavailability, especially of the hydrophobic ones, by using various nanoparticles is a very exciting field of the modern research. OBJECTIVE: The applicability of nano-sized shell crosslinked micelles based on dextran as supports for controlled release of several hydrophobic drugs (nystatin, rifampicin, resveratrol, and curcumin) was investigated by in vitro drug loading/release experiments. METHODS: The synthesized crosslinked micelles were loaded with drugs of various hydrophobicities and their retention/release behavior was followed by dialysis procedure. RESULTS: Crosslinked micelles obtained from dextran with octadecyl end groups, with or without N-(2- hydroxypropyl)-N,N-dimethyl-N-benzylammonium chloride groups attached to the main dextran chains, could retain the drugs in amounts which increased with increasing drug hydrophobicity (water insolubility), as follows: 30-60 mg rifampicin/g, 70-100 mg nystatin/g, 120-144 mg resveratrol/g and 146-260 mg curcumin/g. The rate of drug release from the loaded micelles was also dependent on the drug hydrophobicity and was always slower than the free drug recovery. Antioxidant activity of curcumin and resveratrol released from the loaded micelles was preserved. CONCLUSION: The results highlighted the potential of the new nano-sized micelles as carriers for prolonged and controlled delivery of various hydrophobic drugs.

Ionic dextran derivatives for removal of Fastac 10 EC from its aqueous emulsions.

Separation studies of Fastac 10 EC from model emulsions by cationic polysaccharides were followed by UV-vis spectroscopy and zeta potential measurements. Floc size measurements at the optimum polycation dose were also carried out using laser diffraction technology. The investigated polyelectrolytes contained various amounts of quaternary ammonium salt groups, N-ethyl(octyl)-N,N-dimethyl-2-hydroxypropyl ammonium chloride, attached to a dextran backbone. The effects of polycation dose, its charge density, emulsion pH and pesticide concentration on the flocculation performance were studied. The pesticide removal results (UV-vis spectroscopy) show that the optimum polycation dose decreased with increasing polymer charge density and the emulsion pH and increased with increasing pesticide concentration. The zeta potential values close to zero at the optimum polymer dose point to contribution, mainly, from charge neutralization mechanism for the flocculation process. The flocs size increase with increasing pesticide concentration was also observed.

New shell crosslinked micelles from dextran with hydrophobic end groups and their interaction with bioactive molecules.

Micelles formed in aqueous solution by dextran with hydrophobic (alkyl) end-groups were stabilized through divinyl sulfone crosslinking of the dextran shell. The efficacy of the crosslinking reaction was influenced by the divinyl sulfone amount, the pH and micelle concentration. Crosslinked micelles with a moderate crosslinking degree were further functionalized by attachment of 10 and 17 moles% N-(2-hydroxypropyl)-N,N-dimethyl-N-benzylammonium chloride groups along the dextran chain. The size and shape of both crosslinked micelles and their cationic derivatives were analyzed by DLS and TEM. The prepared micelles were able to bind anionic diclofenac (60-370 mg/g), hydrophobic anionic indometacin (70-120 mg/g), and hydrophobic alpha-tocopherol (170-220 mg/g) or ergocalciferol (90-110 mg/g) by hydrophobic or/and electrostatic forces. The release experiments and the antioxidant activity of bound alpha-tocopherol highlighted the potential of the new nano-sized micelles mainly as carriers for prolonged and controlled delivery of hydrophobic drugs.

Micelle-like association of polysaccharides with hydrophobic end groups.

New dextran derivatives with hydrophobic end groups were synthesized by reductive amination of dextran chain ends, followed by chemical modification of the dextran main chain by attachment of cationic groups and/or by crosslinking. Properties of the aggregates formed by hydrophobic association of the end groups were studied by fluorescence, dynamic light scattering, atomic force microscopy and transmission electron microscopy and depended on the length of the dextran chain (6, 10, 25 kDa) and the hydrophobicity of the end group (alkyl, dialkyl, bile acid). All neutral derivatives were able to form micelle-like aggregates above a critical aggregation concentration (0.008-0.159 g/dL). Polarity of the micelle hydrophobic core was close to or lower than that of neutral low molecular surfactants (polarity parameter I1/I3≈0.8-1.13), aggregation number was 20-30 and hydrodynamic radius 20-30 nm. Attachment of cationic groups to the dextran main chain increased critical aggregation concentration and core polarity, but cationic polymeric surfactants with good association ability could be obtained by an appropriate choice of the content and hydrophobicity of the cationic groups. Cross-linking of the micelle shell with divinylsulfone increased micelle stability to dilution.

Cationic amphiphilic dextran hydrogels with potential biomedical applications.

Dextran microparticles were chemically modified for the attachment of quaternary ammonium groups carrying substituents with different hydrophobicity, in order to obtain amphiphilic cationic hydrogels with different hydrophilic/lipophilic balance. These hydrogels retain various amounts of dyes: Rose Bengal, Brilliant Blue and Vitamin B12, used as models for hydrophobic, amphiphilic and hydrophilic drugs, as a function of their hydrophilic/hydrophobic properties. Bovine serum albumin (BSA) retention by hydrogels occurs in higher amounts at pH 6.9, and is influenced by electrostatic, hydrophobic forces and the swelling of the supports. Tetanus anatoxin is retained by the supports through electrostatic and/or hydrophobic forces, in amounts varying between 110 and 200 mg/g. Both proteins are gradually released, through increasing of the eluent ionic strengths. Alpha-tocopherol is retained by the hydrogels preponderantly through hydrophobic forces, in amounts varying between 130 and 300 mg/g. Measurement of the scavenging effect proved the antioxidant properties of the included drug. Based on the obtained results, one can appreciate the potential of the synthesized cationic hydrogels as supports for biomolecules or as vaccine adjuvants.

Critical role of the degree of substitution in the interaction of biocompatible cholic acid-modified dextrans with phosphatidylcholine liposomes.

The interaction between biocompatible cholic acid-modified dextrans with different pendent cholic acid groups' content and phosphatidylcholine liposomes was studied by a variety of techniques including isothermal titration calorimetry (ITC), differential scanning calorimetry (DSC), turbidity measurements, microscopy imaging (transmission electron microscopy (TEM), and cryo-scanning electron microscopy (cryo-SEM)). The variation of the interaction enthalpy with polymer concentration, as obtained by ITC, highlighted the formation of different aggregates. Complete phase modification, from vesicles covered with a few polymer chains to vesicle disintegration, was observed by turbidity measurements. DSC showed the effect of polymer addition to the liposome gel to liquid-crystalline phase transition, and microscopy images gave information about the size and morphology of the aggregates. The composition, structure, and morphology of polymer/liposome aggregates were found to be strongly influenced by the cholic acid content in the polymer (degree of substitution, DS). Along with a rather monotonous change in the polymer/liposome system's properties with increasing DS, a discontinuity in behavior could also be observed at DS = 4 mol %. For DS ≤ 4 mol %, the polymer/liposome interaction takes place mainly between individual components, and liposome disintegration occurs in a narrow concentration range, whereas for DS > 4 mol % extended physical networks are formed, which last over a wide concentration range. A mechanism of interaction, as a function of DS, is proposed and discussed in detail.

Separation of TiO2 particles from water and water/methanol mixtures by cationic dextran derivatives.

Cationic polysaccharides with N-alkyl-N,N-dimethyl-N-(2-hydroxypropyl)ammonium chloride pendent groups attached to a dextran backbone were used to flocculate titanium dioxide (TiO2) particles suspended in water as well in water/methanol mixtures (90:10 and 50:50 v/v %). The investigations were performed with respect to the polycation dose, the suspension medium composition and the length of alkyl substituent at the quaternary nitrogen (alkyl=ethyl, octyl, dodecyl). The supernatant residual turbidity values were much lower for TiO2 particles dispersed in solvent mixtures than in water. This finding was explained by the solvent mixtures effect on the charges of both the particle surface and polyelectrolyte chains. The alkyl substituent length did not affect in a dramatic way the separation of TiO2 particles. The supernatant zeta potential dependence on the polycation dose when water/methanol mixtures were used as dispersion medium indicated a charge patch mechanism for the flocculation of TiO2 particles, which was supported by particle aggregates size measurements.

Intrinsic viscosities of polyelectrolytes in the absence and in the presence of extra salt: Consequences of the stepwise conversion of dextran into a polycation.

Viscosities of dilute polymer solutions were measured in capillary viscometers for samples varying in their fraction f of charged units from 0.00 to 0.90. The dependence of the logarithm of the relative viscosity on polymer concentration c is in all cases reproduced quantitatively by three characteristic parameters: [η], the intrinsic viscosity; B, a viscometric interaction parameter (related to the Huggins constant); [η], a parameter required only for polyelectrolytes at low concentrations of extra salt. In pure water [η] increases more than 80 times as the fraction f rises from zero to 0.90 and [η] starts from zero and goes up to ≈71mL/g. Upon the addition of NaCl [η] decreases by at least one order of magnitude (depending on the value of f). The observed dependences of log [η] on logcsalt can be modeled quantitatively by Boltzmann sigmoids.

Cationic polyelectrolytes as drug delivery vectors: calorimetric and fluorescence study of Rutin partitioning.

The interaction between hydrophobically modified cationic polysaccharides based on dextran and a flavonoid drug (Rutin) was studied by isothermal titration calorimetry (ITC) and fluorescence spectroscopy, in order to assess the factors responsible for the interaction and characterize its energetics, as well as for evaluating their encapsulation capacity, for possible applicability of these polymers as drug delivery vectors. To address the importance of the hydrophobic pendant groups in the solution behavior of these polymer/drug systems, we also studied the interaction of Rutin with a cationic surfactant, cetyltrimethylammonium chloride (CTAC). The interaction enthalpies and drug binding constants for D40R30/Rutin systems were derived from ITC through a simple binding model. The binding constants were independently derived from fluorescence results, with fair agreement between the parameters obtained from both methods. By changing the Rutin concentration, we were able to get evidence for a solubility enhancement induced by the presence of the polymers, a promising effect regarding its use to improve bioavailability.