Gheorghe Fundueanu

Preparation and characterization of pH- and temperature-sensitive pullulan microspheres for controlled release of drugs

Most part of pH- and temperature-sensitive microspheres used for the controlled delivery of drugs are not biodegradable. Therefore, the aim of this work is to prepare pH- and temperature-sensitive microspheres from biodegradable and biocompatible natural polymers. Pullulan microspheres were prepared by suspension cross-linking with epichlorohydrin of an aqueous solution of the polymer. In order to confer them temperature sensitivity, poly(N-isopropylacrylamide-co-acrylamide) was grafted onto pullulan microspheres. Then, the pH-sensitive units (-COOH) were introduced by reaction between the remaining -OH groups of the pullulan with succinic anhydride. The grafted pullulan microspheres are more hydrophilic than pullulan microspheres, their swelling degree as well as water regain increase significantly. The thermo-sensitivity of the carboxylated microspheres depends to the number and the ionization form (-COOH/-COO(-)) of carboxylic groups. At a low exchange capacity (0.35 meq/g), microspheres are thermo-sensitive both in the protonated and deprotonated form of -COOH groups. At a higher exchange capacity (2.25 meq/g), microspheres are almost unswellable in the protonated form and swell extensively in the ionized form (up to 28 times than their dried form) loosing in a great extent the thermo-sensitive properties. In isotonic phosphate buffer pH=7.4, both thermo-sensitive and pH/thermo-sensitive microspheres possess a phase transition temperature close to that of the human body temperature. Loading and release profiles of lysozyme, taken as a molecular model system, were investigated.

Removal of anionic dyes from aqueous solutions by an ion-exchanger based on pullulan microspheres.

Pullulan-graft-poly(3-acrylamidopropyl trimethylammonium chloride) (P-g-pAPTAC) microspheres were prepared by suspension cross-linking of the pullulan previously grafted with cationic moieties. Adsorption of Azocarmine B by the P-g-pAPTAC microspheres was used as a model to demonstrate the removal of anionic dyes from aqueous solutions. Batch adsorption studies concerning the effect of the contact time, pH, initial dye concentration, temperature, grafting, and the nature of sulfonated anionic dyes on the adsorption kinetics were investigated. Adsorption was shown to be independent of pH. The experimental data best fitted to the pseudo-second order model which provided values of the rate constant k(2) of 1.4×10(-4) g mg(-1) min(-1) for 100 mg L(-1) solution and of 3.7×10(-4) g mg(-1) min(-1) for 500 mg L(-1) solution. From the Langmuir isotherm linear equation, the maximum adsorption capacity determined was 113.63 mg of Azocarmine B per gram of adsorbent; the negative value of the free energy change indicated the spontaneous nature of the adsorption process.

Poly(N-isopropylacrylamide-co-acrylamide) cross-linked thermoresponsive microspheres obtained from preformed polymers: Influence of the physico-chemical characteristics of drugs on their release profiles.

Poly(N-isopropylacrylamide-co-acrylamide) copolymer was synthesized as an interesting thermoresponsive material possessing a phase transition temperature of around 36 degrees C in phosphate buffer, pH 7.4 (PB); the concentration was 10%, w/v. The copolymer maintains a sharp phase transition at a relatively high percentage of acrylamide. The lower critical solution temperature (LCST) of the copolymer is influenced by the concentration of copolymer solution in PB. The copolymer was transformed in thermoresponsive microspheres by chemical cross-linking of amide groups with glutaraldehyde. The key factors for the successful preparation of microspheres are the use of a concentrated polymer solution, a temperature (38 degrees C) that is high enough but lower than LCST, and a long reaction time (48h). The microspheres were characterized by optical and scanning electron microscopy, swelling/deswelling kinetics, swelling degree, and PB retention at different temperatures. Finally, the influence of hydrophilicity/hydrophobicity and the molecular weight of the drugs (propranolol, lidocaine, vitamin B(12)) on their release profile from thermoresponsive microspheres were examined. Above LCST the hydrogel matrix is in the dehydrated state and hydrophobic interactions between the hydrophobic drugs and the polymer occur, modulating the release rate of the drugs. For hydrophilic drugs, the release rate is modulated mainly by the steric interaction between the drug molecule and the matrix.

Preparation and characterization of starch/cyclodextrin bioadhesive microspheres as platform for nasal administration of Gabexate Mesylate (Foy®) in allergic rhinitis treatment

Bioadhesive and biodegradable microspheres were obtained by chemical cross-linking with epichlorohydrin of an alkaline solution of a mixture of starch and alpha-, beta-, or gamma-cyclodextrin (CyD). Microspheres were characterized by scanning electron microscopy, swelling degree, and water retention. The percentage of the effective CyD in microspheres was estimated by measuring the amount of iodine and typical organic compounds (TOCs) retained in the hydrophobic cavity of CyD. Gabexate Mesylate (trade name Foy); GM), an antiallergic drug, was included in microspheres by soaking in an aqueous solution containing the drug, followed by solvent evaporation or lyophilization. UV, IR, and DSC data indicated that despite the fact that GM is a hydrophilic drug, its hydrophobic moiety close to the benzene ring is able to penetrate the CyD cavity and to form stable inclusion complexes. Values of the association equilibrium constant for GM binding to CyD, obtained by UV differential spectroscopy, indicated that the affinity of the drug for alpha- and gamma-CyD is higher than that for beta-CyD. In vitro, GM was gradually released during 1h. Even if the release rate of the drug is relatively fast, the microspheres might actually provide the best platform since the material adheres to the nasal mucosa which was proved by adhesion tests. The GM integrity was checked by comparing its anti-trypsin activity before and after release.

Novel biodegradable flocculanting agents based on pullulan.

New copolymers with different grafted cationic chain content and length were synthesized by graft-polymerization of (3-acrylamidopropyl)-trimethylammonium chloride onto pullulan in aqueous solution, using potassium persulfate as an initiator. Their flocculation efficiency was studied in a clay suspension as a function of the grafted chains content and length as well as of some flocculation parameters: the polycation dose, the settling time, the parent solution concentration of polycation. All the polysaccharide samples showed flocculation ability: the higher the grafted cationic chain content and length the lower the amount of pullulan derivative was required for reaching the maximum clarity degree (of about 100%). This finding together with the negative value of the zeta potential and floc size measurements, at the optimum polymer dose, point to contributions from both neutralization and bridging mechanisms for flocculation process. The residual turbidity values also varied with the settling time and the parent solution concentration of polymer.

Pullulan–cyclodextrin microspheres: A chromatographic approach for the evaluation of the drug–cyclodextrin interactions and the determination of the drug release profiles

Pullulan microspheres containing cyclodextrin (CyD) were obtained by chemical crosslinking with epichlorohydrin of an alkaline solution of pullulan (Pul) and alpha-, beta- or gamma-CyD. The amount of alpha-, beta- and gamma-CyD in microspheres was 120, 156, and 138 micromol/g, respectively, as determined from the percentage of iodine incorporated in the hydrophobic cavity of CyDs. Microspheres were packed in a glass column and the liquid chromatographic behaviour by isocratic elution of different drugs or typical organic compounds (TOC), taken as model drugs, was investigated. The increase of the retention volume (V(R)) of each compound, depending on the interaction(s) between CyDs cavity and the considered molecule, is characterized by a broadening of the peaks. The interaction coefficient K, corresponding to the ratio between the V(R) value of each tested molecule on Pul-alpha-, Pul-beta- and Pul-gamma-CyD active stationary phase and the V(R) value of benzoic acid on St/maltodextrin neutral stationary phase, was determined. According to K values, the accurate prediction can be done on the potential drugs to be conditioned in suitable CyD cavity. Values of K allow to anticipate the release profiles of drugs considered.

Poly(vinyl alcohol) microspheres with pH- and thermosensitive properties as temperature-controlled drug delivery.

One of the most important inconveniences of the pH- and temperature-sensitive hydrogels is the loss of thermosensitivity when relatively large amounts of a pH-sensitive monomer are co-polymerized with N-isopropylacrylamide (NIPAAm). In order to overcome this drawback, we propose here a method to prepare thermosensitive poly(vinyl alcohol) (PVA) microspheres with a higher content of carboxylic groups that preserve thermosensitive properties. Moreover, PVA possesses excellent mechanical properties, biocompatibility and non-toxicity. PVA microspheres were obtained by suspension cross-linking of an acidified aqueous solution of the polymer with glutaraldehyde. Poly(N-isopropylacrylamide-co-N-hydroxymethyl acrylamide) (poly(NIPAAm-co-HMAAm)), designed to have a lower critical solution temperature (LCST) corresponding to that of the human body, was grafted onto PVA microspheres in order to confer them with thermosensitivity. Then, the pH-sensitive functional groups (COOH) were introduced by reaction between the un-grafted OH groups of PVA and succinic anhydride. The pH- and temperature-sensitive PVA microspheres display a sharp volume transition under physiological conditions around the LCST of the linear polymer. The microspheres possess good drug-loading capacity without losing their thermosensitive properties. Under simulated physiological conditions, the release of drugs is controlled by temperature.

Poly(N-isopropylacrylamide-co-hydroxyethylacrylamide) thermosensitive microspheres: the size of microgels dictates the pulsatile release mechanism.

Poly(N-isopropylacrylamide-co-N-hydroxyethylacrylamide) (poly(NIPAAm-co-HEAAm)) was prepared as a new thermosensitive copolymer possessing a sharp phase transition around the human body temperature. The effect of the copolymer concentration on the lower critical solution temperature (LCST) was determined under physiological conditions by cloud point (CP) and differential scanning calorimetric (DSC) methods. Then, thermosensitive microspheres were prepared from preformed copolymers by chemical cross-linking of hydroxyl groups with glutaraldehyde at a temperature situated slightly below LCST of the copolymer solution. The volume phase transition temperature (VPTT) of corresponding cross-linked microspheres was determined from swelling degree-temperature curve. The microspheres were loaded with model drug indomethacin by the solvent evaporation method. The DSC analysis proved that the drug is molecularly dispersed in the polymer network. Finally, the influence of the microsphere size on drug release was investigated. It was established that microspheres with the diameter ranging between 5 and 60 μm release the drug with almost the same rate below (in the swollen state) and above the VPTT (in the collapsed state). On the contrary, microspheres with the diameter ranging between 125 and 220 μm release a significantly higher amount of indomethacin below than above the VPTT. This different behavior is enough to assure a pulsatile release mechanism when the temperature changes cyclically below and above the VPTT. However, both small and large microspheres release a large amount of the drug during the collapsing process.

Fast-responsive porous thermoresponsive microspheres for controlled delivery of macromolecules

Porous thermoresponsive microspheres with a homogeneous dimension and distribution of the pores are synthesized by an original method. Poly(N-isopropylacrylamide-co-acrylamide) (poly(NIPAAm-co-AAm)) copolymer was obtained as a thermoresponsive material with a lower critical solution temperature (LCST) under physiologic-like conditions (i.e., at 37 degrees C and pH 7.4, 50mM phosphate buffer). Semitelechelic oligomers of NIPAAm (ONIPAAm) were also synthesized in the presence of 3-mercaptopropionic acid (MPA) (chain transfer molecule) which acts as a pore-forming agent. Poly(NIPAAm-co-AAm) and ONIPAAm were solubilized in acidified aqueous solution, dispersed in a mineral oil, and transformed in stable microspheres by crosslinking the amide group with glutaraldehyde at temperatures below and above the LCST of the oligomers, and always below the LCST of the polymer. Microspheres obtained at temperatures below the LCST of ONIPAAm are characterized by a homogeneous porous structure with a narrow distribution of the pore size. These microspheres are characterized by a very rapid response rate when the temperature changes below and above the body temperature. The higher is the amount of the porogen in the polymer solution, the larger is the pore size and faster is the response rate. The porous microspheres with suitable pore size are a conveyable matrix for loading and temperature-controlled release of the high molecular weight model drug blue dextran (BD).

Phosphorylated curdlan microgels. Preparation, characterization, and in vitro drug release studies.

Curdlan derivative with anionic phosphate groups was used for the first time to obtain hydrogel microspheres. The chemical cross-linking of the phosphorylated curdlan was performed with epichlorohydrin using the water-in-oil inverse emulsion technique. The optical and scanning electron microscopies were used to analyze the morphology of the microgels, whereas the FTIR spectroscopy was used to investigate their chemical structure. The main characteristics such as the swelling degree, the exchange capacity, and the thermal resistance were also studied. These new anionic microgels could be used as potential carriers for controlled release of opposite charged drugs retained through electrostatic forces. Diphenhydramine, a cationic model drug, was used to investigate the loading and the release processes in various pH media simulating physiological fluids. Several mathematical models were applied to evaluate the drug transport processes and to calculate the drug diffusion coefficients. The synthesized microspheres presented an excellent biocompatibility.