An in vitro model for investigating the gastric mucosal retention of 14C-labelled poly(acrylic acid) dispersions

Polymers that bind from solution onto gastric mucosae can be used as a means of facilitating localised drug delivery, or act as therapeutic agents in their own right (e.g. by forming a protective layer or by inhibiting enzymes). Previous workers have used semi-quantitative methods to identify the ability of commercially available poly(acrylic acid)s to bind to gastric mucosa. In this study, the binding and retention of labelled poly(acrylic acid)s to sections of gastric mucosa from the pyloric region of pigs stomach were evaluated using 'static' and 'dynamic flow' test systems. Dispersions (3%) of 'low', 'high' and 'ultra high' (cross-linked) polymers were seen to adhere to porcine pyloric mucosa after exposure and rinsing in the 'static' system. The high molecular weight polymer showed the greatest retention in the 'dynamic' test system when washing continuously with simulated gastric acid. Changing the pH of the dispersions from 4.3 to 6.2 had little effect on polymer retention. It was concluded that polymers that were sufficiently mobile in solution to spread on, and interact with, the mucosal surface, but had a sufficiently high molecular weight to form viscous solutions and/or bioadhere to the mucosa, may be retained on the mucosal surface for the longest periods.     

Enhanced nasal retention of hydrophobically modified polyelectrolytes

Hydrophobically modified polyelectrolytes (HMP) are polymers with a high content of ionizable groups bonded to hydrophobic groups. Copolymers of poly(acrylic acid) and Pluronic surfactants constitute a special class of HMP whereby poly(propylene oxide) segments act as hydrophobes. The poly(propylene oxide) segments possess temperature-dependent aqueous solubility and the solutions of the Pluronic-poly(acrylic acid) copolymers (MW > 3,000,000) undergo a sol-gel transition when kept at body temperature. Due to the presence of the poly(acrylic acid) segments, the Pluronic-poly(acrylic acid) copolymers are bioadhesive. We have examined the hypothesis that the in-situ gelling polymer formulations of Pluronic-poly(acrylic acid) copolymers may have an enhanced retention in the nasal cavity. The effects of putative bioadhesive (Carbomer 934P) and thermogelling (Pluronic F127) polymers on nasal clearance were compared with Pluronic-poly(acrylic acid) copolymers using a rat model. The enhancement of the residence time of fluorescent labels by the Pluronic-poly(acrylic acid) copolymers was shown to be 5-8-fold that of Carbomer, and 3-6-fold that of Pluronic F127. The results unequivocally demonstrate the superior retention of the HMP that combines bioadhesive and thermogelling capabilities over either a bioadhesive polyelectrolyte or a polymer of a low molecular weight that undergoes a sol-gel transition.     

Effect of the molecular weight of water-soluble polymers on accumulation at an inflammatory site following intravenous injection

The body distribution of noncharged, water-soluble polymers, e.g. poly(vinyl alcohol) (PVA), poly(ethylene glycol) (PEG), and dextran, was studied following their intravenous injection into mice with or without carrageenan-induced inflammatory lesions at the femoral muscle. Inflammation induction did not affect their distribution profile, although polymers of high molecular weight tended to remain in the blood circulation for a longer period than those of low molecular weight. Polymers exhibited longer accumulation at the inflammatory site than at the normal site, regardless of the polymer type and the molecular weight estimated by size exclusion chromatography. Maximum accumulation was observed for the polymer having a molecular weight around 200,000, regardless of the polymer type. Pharmacokinetic studies demonstrated that the accumulation rate of polymers at the inflammatory site was higher than at the normal site and decreased with increasing molecular weight. On the other hand, the higher the molecular weight of polymers, the longer the retention time in the blood circulation. Such trends were observed for all types of polymers used, indicating that the inflammatory site accumulation of polymers injected intravenously is governed solely by the polymer molecular weight. The balance between the accumulation rate of polymers and the blood retention seemed to determine the maximum accumulation of polymers having a certain range of molecular weight.     

Development and in vitro evaluation of a mucoadhesive vaginal delivery system for nystatin

The purpose of the present study was to design and evaluate a novel vaginal delivery system for nystatin based on mucoadhesive polymers. L-Cysteine and cysteamine, respectively, were covalently attached to poly(acrylic acid), and the two different thiolated polymers were evaluated in vitro regarding their swelling behavior, mucoadhesive properties and release behavior. Tablets comprising these thiolated polymers and nystatin demonstrated a high stability in vaginal fluid simulant pH 4.2 and an increase in weight by swelling whereas control tablets comprising unmodified poly(acrylic acid) disintegrated and dissolved. The mucoadhesion time of tablets on freshly excised bovine vaginal mucosa on a rotating cylinder and the total work of adhesion of gels and tablets increased significantly due to the formation of disulfide bonds between the thiolated polymer and cysteine rich subdomaines of the mucus layer. The drug nystatin was released more slowly out of thiomer tablets and gels than out of PAA control tablets and gels. Therefore these thiolated polymers are promising delivery systems for nystatin providing a prolonged residence time and a sustained drug release in vitro under physiological relevant conditions.     

An investigation of mucus/polymer rheological synergism using synthesised and characterised poly(acrylic acid)s

A range of poly(acrylic acid)s with different average degrees of polymerisation and cross-linking densities were synthesised using a solution polymerisation process. The rheological characteristics of aqueous dispersions of these materials and those of mixtures with homogenised pigs gastric mucus were investigated using dynamic oscillatory rheology, and compared to the known mucoadhesive Carbopol 934P. From the storage moduli, the rheological synergy and relative rheological synergy were calculated, and the effects of concentration and pH on this considered. Generally, the larger the molecular weight (and degree of cross-linking), the greater the rheological synergy, with Carbopol 934P giving the most pronounced effect. Rheological synergy was seen to be concentration-dependent, and a maximum concentration to produce an optimum effect was evident. Acid pHs were seen to favour synergy, although in marked contrast to previous literature reports, the optimum mucus-polymer interaction was not observed at the half ionised value (pH = pKa) but at pH regimes that were unique to each polymer type. This could be influenced by the structural constrains imposed on potential hydrogen bonded interactions. It was concluded that synthesising poly(acrylic acid)s with better defined physicochemical properties than commercially available polymers will advance the study of the phenomenon of rheological synergy.     

Investigation of alternative compounds to poly(E-MA) as a polymeric surfactant for preparation of microcapsules by phase separation method

Various water-soluble polymers were used to examine an alternative emulsifier for poly(ethylene-alt-maleic anhydride), used in the preparation of crosslinked polyurea microcapsules. Microcapsules were successfully prepared by using the water-soluble polymers with large molecular weight alternating copolymers, namely poly(olefin-maleic anhydride), poly(olefin-maleic acid), and poly(acrylic acid). On the other hand, no microcapsule resulted from olefin-maleic acid with small molecular weight alternating copolymers. From these results, the following guidelines were obtained for the selection of polymeric surfactants suitable for crosslinked polyurea microcapsule. A polymeric surfactant must have maleic acid or a carboxyl group in order to form a crosslinked polyurea microcapsule membrane. Furthermore, to form a stronger capsule membrane it is desirable to have a maleic anhydride group. It is also important for membrane formation that the polymeric surfactant has a suitable molecular weight.     

Investigation of alternative compounds to poly(E-MA) as a polymeric surfactant for preparation of microcapsules by phase separation method

Various water-soluble polymers were used to examine an alternative emulsifier for poly(ethylene-alt-maleic anhydride), used in the preparation of crosslinked polyurea microcapsules. Microcapsules were successfully prepared by using the water-soluble polymers with large molecular weight alternating copolymers, namely poly(olefin-maleic anhydride), poly(olefin-maleic acid), and poly(acrylic acid). On the other hand, no microcapsule resulted from olefin-maleic acid with small molecular weight alternating copolymers. From these results, the following guidelines were obtained for the selection of polymeric surfactants suitable for crosslinked polyurea microcapsule. A polymeric surfactant must have maleic acid or a carboxyl group in order to form a crosslinked polyurea microcapsule membrane. Furthermore, to form a stronger capsule membrane it is desirable to have a maleic anhydride group. It is also important for membrane formation that the polymeric surfactant has a suitable molecular weight.     

Bioadhesive polymers as platforms for oral controlled drug delivery II: synthesis and evaluation of some swelling, water-insoluble bioadhesive polymers

A series of cross-linked, swellable polymers was sythesized from monomers such as acrylic acid, methacrylic acid, and others with various cross-linking agents to produce a range of polymers differing in charge densities and hydrophobicity. The densities, rate, and extent of hydration of the polymers were determined. An increase in the number of hydrophobic groups in the polymer structure reduced hydration whereas the density of the polymer was unaffected. A sensitive in vitro method for measuring adhesion of polymer to tissue from the rabbit stomach was developed. Polymers of acrylic acid loosely cross-linked (0.3%, w/w) with three different agents, divinyl glycol, 2,5-dimethyl-1,5-hexadiene, and divinylbenzene, showed the same degree of bioadhesion while poly(methacrylic acid-divinylbenzene) showed reduced bioadhesion. The small percent of cross-linking agent, irrespective of physicochemical properties, did not contribute substantially to bioadhesion, whereas the starting monomer had a large effect. The effect of pH on the bioadhesion of poly(acrylic acid-divinyl glycol) was studied at constant temperature, ionic strength, and osmolality. The polymer showed maximum adhesion at pH 5 and 6 and a minimum at pH 7. Gastrointestinal transit studies of cross-linked polymers in rats were studied. Poly(acrylic acid-divinyl glycol) and poly(methacrylic acid-divinylbenzene) were shown to have substantially longer GI transit times than the control, Amberlite 200 resin beads. The delay in transit time was due to bioadhesion of the polymer to the mucin-epithelial cell surface which was clearly observable on animal autopsy. The acrylic acid polymer showed a longer GI transit time than the methacrylic acid polymer, and this in vivo GI transit result is consistent with in vitro bioadhesion test results.     

Pluronic block copolymers and Pluronic poly(acrylic acid) microgels in oral delivery of megestrol acetate

Several Pluronic-based formulations were studied in-vitro and in a rat model with respect to the release and bioavailability of megestrol acetate (MA) after oral administration. It was demonstrated that an aqueous, micellar formulation comprising a mixture of a hydrophobic (L61) and a hydrophilic (F127) Pluronic copolymer, significantly enhanced the bioavailability of MA administered orally at relatively low doses (1-7 mg kg(-1)). Pluronic-based microgels (spherical gel particles of sub-millimetre size) were introduced as MA vehicles. The microgels comprised a cross-linked network of poly(acrylic acid) onto which the Pluronic chains were covalently attached. Microgels of Pluronic L92 and poly(acrylic acid) fabricated into tablet dosage forms exhibited dramatically lowered MA initial burst release. The MA release was pH-dependent owing to the pH sensitivity of the microgel swelling, with the drug retained by the microgel at pH 1.8 and released slowly at pH 6.8. In the rat model, a significant increase in MA bioavailability was observed when the microgel-formulated MA was administered orally at a high dose of 10 mg kg(-1), owing to the enhanced retention of the microgel. The study of the microgel passage through the gastrointestinal tract demonstrated the microgel retention characteristic of a very high molecular weight polymer and the absence of any systemic absorption of the polymer.     

Factors Influencing Gel-strengthening at the Mucoadhesive-mucus Interface

Abstract— Mechanical spectroscopy was used to examine some of the factors that may affect mucus gel strengthening: the effect of adding various concentrations of sodium chloride; mucoadhesive polymer molecular weight and its concentration; and the introduction of anionic, cationic and neutral polymers. A reduction in the storage modulus of the mucus/mucoadhesive mixture was observed with the introduction of sodium chloride. A poly(acrylic acid) with a molecular weight of 750 kDa gave the optimum mucus gel strengthening effect relative to other molecular weights. An anionic polymer was found to strengthen the mucus gel much more than a neutral or cationic polymer. It was proposed that the gel strengthening effect could be due to the formation of hydrogen bonded intermolecular complexes between the mucoadhesive and the mucus molecules. Furthermore, the complex formed is influenced by the ionic strength of the environment, and the molecular weight, nature and concentration of the mucoadhesive. In all cases the changes in the rheological properties of the mixes could be correlated directly to the strength of mucoadhesion reported in previous studies.     

Polymeric microspheres for drug delivery to the oral cavity: an in vitro evaluation of mucoadhesive potential

Polymeric microparticles were fabricated from Carbopol, polycarbophil, chitosan, or Gantrez using a "water-in-oil emulsification" solvent evaporation method. Mean particle sizes, as determined by laser diffraction, were in the range 23-38 microm. Electron microscopy revealed that all microparticles were spherical and of smooth surface morphology. In pH 7.0 phosphate buffered saline, the microspheres exhibited significantly increased swelling ratios and longer half-times of swelling than the corresponding powdered polymers. The relative merits of the potential usefulness of these microspheres as formulation tools for the enhanced retention of a therapeutic entity within the oral mucosa were evaluated by in vitro mucoadhesion tests. Tensile tests showed that all microspheres under consideration were capable of adhering to porcine esophageal mucosa, with particles prepared from the poly(acrylic acid)s exhibiting greater mucoadhesive strength than those constructed from chitosan or Gantrez. However, in elution experiments involving a challenge with artificial saliva, particles of chitosan or Gantrez were retained onto mucosal tissue for longer time periods than those assembled from the poly(acrylic acid)s.     

The effect of sofalcone on the kinetics of the generative cells and superficial epithelial cells in mouse gastric mucosa

Effects of an anti-ulcer drug, 2'-carboxymethoxy-4,4'-bis(3-methyl-2-butenyloxy) chalcone (sofalcone), on the generative cells and kinetics of superficial epithelial cells in mouse gastric mucosa, including the effect of hydrocortisone on them, were investigated by the use of 3H-thymidine autoradiography. The labeling indices and the width of the generative cell zone in the fundic mucosa were significantly decreased by administration of hydrocortisone. The decrease in the labeling indices and the width of the generative cell zone induced by administration of hydrocortisone were not inhibited by sofalcone. Hydrocortisone had no significant influence on the labeling indices of the generative cell zone in the pyloric mucosa, but decreased the width of it. The decrease in the width of the generative cell zone induced by hydrocortisone was inhibited by sofalcone. Concerning the influence on the kinetics of superficial epithelial cells, hydrocortisone inhibited the increase in the labeling indices of the superficial epithelial cells after continuous administration of 3H-thymidine in both the fundic mucosa and pyloric mucosa. In the fundic mucosa, sofalcone showed no influence on the inhibition by hydrocortisone, but in the pyloric mucosa, the effect of hydrocortisone was abolished by sofalcone. These results suggest that sofalcone inhibits the reduction of the cell production and the prolongation of the life span of superficial epithelial cells caused by hydrocortisone in the pyloric mucosa.     

Interpretation of mucoadhesive properties of polymer gel preparations using a tensile strength method.

We have developed a new tensile strength method for assessing mucoadhesive properties of polymer gels utilising freshly excised porcine nasal mucosa and a texture analyser. In conjunction with this, we propose a method for interpreting the mucoadhesive properties that is based on reasoning about the locus of the failure of a mucoadhesive joint. This involves measuring the cohesiveness of the gel and the mucus layer, respectively, and comparing these results with those obtained from a mucoadhesion measurement. Linear polymers (sodium carboxymethylcellulose, poly(acrylic acid) and sodium hyaluronate) and a cross-linked polymer (poly(acrylic acid)) were used as model polymers in this study. It was shown that the withdrawal speed of the probe should be low, about 0.1 mm s(-1), and that a contact time of 2 min was sufficient. In the mucoadhesion measurements there was no dependence of the results on the contact time in the interval 2-20 min. The tensile work appeared to be more applicable than the fracture strength for interpreting mucoadhesive properties. Furthermore, it was concluded that the interpretation procedure offers a good basis by which to assess whether the measured tensile work reflects a cohesive failure of the gel or a true interaction of the gel with the mucus layer.     

Functional polymers as therapeutic agents: Concept to market place

Biologically active synthetic polymers have received considerable scientific interest and attention in recent years for their potential as promising novel therapeutic agents to treat human diseases. Although a significant amount of research has been carried out involving polymer-linked drugs as targeted and sustained release drug delivery systems and prodrugs, examples on bioactive polymers that exhibit intrinsic therapeutic properties are relatively less. Several appealing characteristics of synthetic polymers including high molecular weight, molecular architecture, and controlled polydispersity can all be utilized to discover a new generation of therapies. For example, high molecular weight bioactive polymers can be restricted to gastrointestinal tract, where they can selectively recognize, bind, and remove target disease causing substances from the body. The appealing features of GI tract restriction and stability in biological environment render these polymeric drugs to be devoid of systemic toxicity that are generally associated with small molecule systemic drugs. The present article highlights recent developments in the rational design and synthesis of appropriate functional polymers that have resulted in a number of promising polymer based therapies and biomaterials, including some marketed products.     

Evaluation of high molecular weight poly(oxyethylene) (Polyox) polymer: studies of flow properties and release rates of furosemide and captopril from controlled-release hard gelatin capsules

The powder characteristics and the effect of the molecular weight of polymers as diluents on the release rate of furosemide and captopril from hard gelatin capsules were evaluated. The high molecular weight polymers studied were poly(oxyethylene) homopolymers (Polyox), with molecular weight ranging from 4,000,000 to 7,000,000. Powder characteristics suggested good flowability for these materials and predicted capsule fill weight uniformity. Swelling experiments showed a very high degree of swelling for these materials in both gastric and buffer solution. These polymers can sustain the release rate of both water-soluble and insoluble drugs from drug delivery systems. The low molecular weight polymers have a less pronounced sustained-release effect compared to the high molecular weight polymer material (i.e., those with 7,000,000 molecular weight). An increase in the content of polymer results in a decrease in the release rate of the drug. The solubility of the drugs clearly influenced the release rate. Release kinetics were evaluated and appeared to be influenced by the molecular weight of the polymer, the solubility of drug, and the ratio of the drug to polymer in the capsule. Bimodal release kinetics were exhibited by a number of furosemide formulations (i.e., F5 and F8).     

Evaluation of High Molecular Weight Poly(Oxyethylene) (Polyox) Polymer: Studies of Flow Properties and Release Rates of Furosemide and Captopril from Controlled-Release Hard Gelatin Capsules

The powder characteristics and the effect of the molecular weight of polymers as diluents on the release rate of furosemide and captopril from hard gelatin capsules were evaluated. The high molecular weight polymers studied were poly(oxyethylene) homopolymers (Polyox), with molecular weight ranging from 4,000,000 to 7,000,000. Powder characteristics suggested good flowability for these materials and predicted capsule fill weight uniformity. Swelling experiments showed a very high degree of swelling for these materials in both gastric and buffer solution. These polymers can sustain the release rate of both water-soluble and insoluble drugs from drug delivery systems. The low molecular weight polymers have a less pronounced sustained-release effect compared to the high molecular weight polymer material (i.e., those with 7,000,000 molecular weight). An increase in the content of polymer results in a decrease in the release rate of the drug. The solubility of the drugs clearly influenced the release rate. Release kinetics were evaluated and appeared to be influenced by the molecular weight of the polymer, the solubility of drug, and the ratio of the drug to polymer in the capsule. Bimodal release kinetics were exhibited by a number of furosemide formulations (i.e., F5 and F8).     

Polynucleotide analogues. Copolymers of vinyl bases with acrylic acid, methylacrylic acid, acrylamide, and 1-vinylpyrrolidone.

Preliminary studies are reported on the synthesis and testing of substituted vinyl polymers that are designed to have sequence specific affinity for polyribonucleic acids. Copolymers of 1-vinyluracil with acrylic acid, 2-methylacrylic acid, or 1-vinyl-2-pyrrolidone were prepared by gamma-irradiation to give the respective polymers 1,3, and 4. Similarly, 9-vinyladenine yielded copolymeric products 5 and 6 with acrylic acid or 2-methylacrylic acid. Radical initiated polymerization of 9-vinyladenine with acrylamide yielded copolymer 7. The products were characterized by elemental analysis and ultraviolet, infrared, and nuclear magnetic resonance spectroscopy. No hypochromicity could be detected on mixing polymers 1-4 with poly(adenylic acid). The acrylic acid copolymer 2 containing a high ratio of vinyluracil was a potent inhibitor of poly(adenylic acid) coded polylysine synthesis in an in vitro system. Polymers 6 and 7, containing a high proportion of vinyladenine, inhibited poly(uridylic acid) coded poly(phenylalanine) synthesis.     

Versatility of biodegradable poly(D,L-lactic-co-glycolic acid) microspheres for plasmid DNA delivery.

In this study, we have optimized different formulations of DNA encapsulated into PLGA microspheres by correlating the protocol of preparation and the molecular weight and composition of the polymer, with the main characteristics of these systems in order to design an efficient non-viral gene delivery vector. For that, we prepared poly(D,L-lactic-co-glycolic acid) (PLGA) microparticles with an optimized water-oil-water double emulsion process, by using several types of polymers (RG502, RG503, RG504, RG502H and RG752), and characterized in terms of size, zeta potential, encapsulation efficiency (EE%), morphology, DNA conformation, release kinetics, plasmid integrity and erosion. The size of the particles ranged between 0.7 and 5.7 microm depending on the protocol of formulation and the molecular mass of the polymer used. The microspheres prepared by using in their formulation polymers of high molecular weight (RG503 and RG504) were bigger in size than in the case of using a lower molecular weight polymer (RG502). The EE (%) of plasmid DNA increased with increasing the molecular mass of the polymer and by using the most hydrophilic polymer RG502H, which contains terminal acidic groups in its structure. The plasmid could be encapsulated without compromising its structural and functional integrity. Also a protective effect of PLGA on endonuclease digestion is observed. Plasmid DNA release from microspheres composed of low molecular weight or hydrophilic polymers, like RG502H, was faster than from particles containing high molecular weight or hydrophobic polymers. These PLGA microspheres could be an alternative to the viral vectors used in gene therapy, given that may be used to deliver genes and other bioactive molecules, either very rapidly or in a controlled manner.     

Effect of glycyrrhetinic acid on the cyclic nucleotide system of the rat stomach

Glycyrrhetinic acid (carbenoxolone) inhibited cyclic nucleotide phosphodiesterase (PDE) activity in a number of tissues from rat, guinea pig and dog. In the fundic mucosa, it was more active when cyclic AMP rather than cyclic GMP was used as substrate in the PDE assays. Glycyrrhetinic acid also raised the intracellular levels of cyclic AMP in rat pyloric and fundic mucosae in vivo with little effect on cyclic GMP levels. This effect, at least in the rat, was apparently mediated via PDE-inhibition, since the drug did not affect adenylyl cyclase activity from either mucosa. Studies with theophylline and papaverine, in addition to glycyrrhetinic acid, suggested that the effects of PDE inhibitors on gastric acid secretion could be related to their relative activities on cyclic AMP-PDE's or cyclic GMP-PDE's, owing to the possibly opposite effects mediated by the two cyclic nucleotides on acid secretion by the stomach.     

Thiolated polymers: self-crosslinking properties of thiolated 450 kDa poly(acrylic acid) and their influence on mucoadhesion.

This study examined the rheological and mucoadhesive properties of a self-crosslinking anionic thiolated polymer in vitro. Mediated by a carbodiimide, L-cysteine was covalently bound to poly(acrylic acid) of 450 kDa molecular mass. The resulting thiolated polymers (conjugates I and II) contained 90.5+/-15.8 and 511.6+/-52 micromol thiol groups per gram polymer, respectively (mean+/-S.D., n=3). The amount of covalently attached cysteine was therefore dependent on the concentration of carbodiimide used for the coupling reaction. Both conjugates (3%, m/v) were capable of forming inter- and/or intramolecular disulfide bonds in 100 mM phosphate buffer pH 6.8. Consequently, the apparent viscosity of conjugates I and II increased 12- and 10-fold, respectively, within 24 h of incubation at 37 degrees C. Further, rheological synergy was observed by mixing equal volumes of polymer (unmodified as well as modified) with a mucin solution. A six-fold increase in viscosity immediately after mixing could be observed for the conjugate II/mucin mixture. This clearly indicates the high interaction potential of self-crosslinking thiomers with the mucus gel layer. Mucoadhesion studies confirmed the rheological results. Tablets based on conjugate II remained attached on freshly excised porcine mucosa for about 25 times longer than the corresponding controls, which is the longest time of mucoadhesion ever found among anionic thiomers. Due to the results of the present study, self-crosslinking thiolated poly(acrylates) of 450 kDa represent very promising excipients for the development of various mucoadhesive drug delivery systems.