Solubility effects on drug transport through pH-sensitive, swelling-controlled release systems: Transport of theophylline and metoclopramide monohydrochloride

Poly(vinyl alcohol) and poly(acrylic acid) sequential interpenetrating network pH-sensitive microspheres for the delivery of diclofenac sodium to the intestine

Sequential interpenetrating network (IPN) of poly(vinyl alcohol) (PVA) and poly(acrylic acid) (PAA) were prepared and crosslinked with glutaraldehyde (GA) to form pH-sensitive microspheres by the water-in-oil (w/o) emulsification method. Microspheres were used to deliver a model anti-inflammatory drug, diclofenac sodium (DS), to the intestine. The formed IPN was analyzed by Fourier transform infrared spectroscopy (FTIR). Differential scanning calorimetry (DSC) and X-ray diffraction (XRD) analyses were done on the drug-loaded microspheres to confirm the polymorphism of DS. Results indicated a molecular level dispersion of DS in the IPN. Microspheres formed were spherical with the smooth surfaces as evidenced by scanning electron microscopy (SEM). Particle size and size distribution was studied using laser light diffraction particle size analyzer. Particle size analysis was also done by optical microscope for the selected microspheres; the change in diameter of the microspheres when soaked in different media at different time intervals was measured by optical microscope. Microspheres showed a pulsatile swelling behavior when the pH of the swelling media was changed. The swelling data were fitted to an empirical equation to understand the phenomenon of water transport as well as to calculate the diffusion coefficient (D). Values of D in acidic media were lower than those found in basic media. The values of D decrease with increasing crosslinking of the matrix. In-vitro release studies have been performed in 1.2 and 7.4 pH media to simulate gastric and intestinal conditions. The results indicated a dependence on the pH of the release media, extent of crosslinking and the amount of drug loading.

Effect of microenvironment pH of swellable and erodable buffered matrices on the release characteristics of diclofenac sodium

The aim of this work is to design pH-dependent swellable and erodable-buffered matrices and to study the effect of the microenvironment pH on the release pattern of diclofenac sodium. Buffered matrix tablets containing diclofenac sodium, physically mixed with hydrophilic polymer (hydroxypropyl methylcellulose [HPMC]) and pH-dependent solubility polymer (Eudragit L100-55) were prepared with different microenvironment pHs. The release of diclofenac sodium from the buffer matrices was studied in phosphate buffer solutions of pH 5.9 and 7.4. The swelling and erosion matrices containing only HPMC and Eudragit L100-55 were studied in phosphate buffer solution of pH similar to the microenvironment pHs of the matrices. Drug release from matrices was found to be linear as a function of time. Amount of drug released was found to be higher in the medium of pH 7.4 than that of pH 5.9. The rate of drug release increased with the increase of the microenvironment pH of the matrices as determined from the slope. The pattern of drug release did not change with the change of microenvironment pH. The swelling and erosion occurred simultaneously from matrices made up of HPMC and Eudragit L100-55. Both extent of swelling and erosion increased with increase of the medium pH. It was concluded from this study that changing the pH within the matrix influenced the rate of release of the drug without affecting the release pattern.

Stimulus-responsive "smart" hydrogels as novel drug delivery systems

Recently, there has been a great deal of research activity in the development of stimulus-responsive polymeric hydrogels. These hydrogels are responsive to external or internal stimuli and the response can be observed through abrupt changes in the physical nature of the network. This property can be favorable in many drug delivery applications. The external stimuli can be temperature, pH, ionic strength, ultrasonic sound, electric current, etc. A majority of the literature related to the development of stimulus-responsive drug delivery systems deals with temperature-sensitive poly(N-isopropyl acrylamide) (pNIPAAm) and its various derivatives. However, acrylic-based pH-sensitive systems with weakly acidic/basic functional groups have also been widely studied. Quite recently, glucose-sensitive hydrogels that are responsive to glucose concentration have been developed to monitor the release of insulin. The present article provides a brief introduction and recent developments in the area of stimulus-responsive hydrogels, particularly those that respond to temperature and pH, and their applications in drug delivery.

Hydrogels: from controlled release to pH-responsive drug delivery

Hydrogels are one of the upcoming classes of polymer-based controlled-release drug delivery systems. Besides exhibiting swelling-controlled drug release, hydrogels also show stimuli-responsive changes in their structural network and hence, the drug release. Because of large variations in physiological pH at various body sites in normal as well as pathological conditions, pH-responsive polymeric networks have been extensively studied. This review highlights the use of hydrogels (a class of polymeric systems) in controlled drug delivery, and their application in stimuli-responsive, especially pH-responsive, drug release.

Novel crosslinking methods to design hydrogels

Hydrogels are presently under investigation as matrices for the controlled release of bioactive molecules, in particular pharmaceutical proteins, and for the encapsulation of living cells. For these applications, it is often required that the gels degrade under physiological conditions. This means that the originally three-dimensional structure has to disintegrate preferably in harmless products to ensure a good biocompatibility of the hydrogel. In this overview, different chemical and physical crosslinking methods used for the design of biodegradable hydrogels are summarized and discussed. Chemical crosslinking is a highly versatile method to create hydrogels with good mechanical stability. However, the crosslinking agents used are often toxic compounds, which have been extracted from the gels before they can be applied. Moreover, crosslinking agents can give unwanted reactions with the bioactive substances present in the hydrogel matrix. Such adverse effects are avoided with the use of physically crosslinked gels.

Chemically modified polyacrylamide-g-guar gum-based crosslinked anionic microgels as pH-sensitive drug delivery systems: preparation and characterization

New spherically shaped cross-linked hydrogels of polyacrylamide-grafted guar gum were prepared by the emulsification method. These were selectively derivatized by saponification of the -CONH2 group to the -COOH group. The derived microgels were characterized by FTIR and elemental analyses. The derivatized microgels were responsive to pH and ionic strength of the external medium. The swelling of microgels increased when the pH of the medium changed from acidic to alkaline. Transport parameters, viz., solvent front velocity and diffusion coefficients were calculated from a measurement of the dimensional response of the microgels under variable pH conditions. The variation in pH changed the transport mechanism from Case II (in 0.1 N HCl) to non-Fickian (in pH 7.4 buffer), and these processes are relaxation-controlled. Ionic strength exerted a profound influence on the swelling of the microgels. Swelling was reversible and pulsatile with the changing environmental conditions. The pH-sensitive microgels were loaded with diltiazem hydrochloride and nifedipine (both antihypertensive drugs) and their release studies were performed in both the simulated gastric and intestinal pH conditions. The release was relatively quicker in pH 7.4 buffer than observed in 0.1 N HCl; the release followed non-Fickian transport in almost all the cases.

Evaluation of poly(2-hydroxyethyl methacrylate) gels as drug delivery systems at different pH values

Studies of dynamic and equilibrium swelling, structural characterisation and solute transport in swollen poly(2-hydroxyethyl methacrylate) gels (pHEMA) cross-linked with tripropyleneglycol diacrylate (TPGDA) were done for a wide range of TPGDA concentrations. The influence of the pH on these pHEMA properties was evaluated. In swelling studies it was found that in changing the pH from 6.5 to 12.0, a large increase in swelling occurred, from approximately 48 to 55%, for the lowest concentration of TPGDA (1 mol%), and from 40 to 80% for the highest concentration (10 mol%). Fourier transform infrared (FTIR) and differential scanning calorimetry (DSC) measurements were made after the equilibrium swelling of the gels at different pH values, to explain these results. The advantage of using these gels as controlled drug delivery systems is illustrated using salicylic acid (SA) as a model drug. The loading and the release of the SA were made at different pH values and the results obtained showed that it is possible to modulate the hydrogel performance by controlling an external factor, the pH at which the drug loading and release were performed.

Adsorption of drugs onto a poly(acrylic acid) grafted cation-exchange membrane

The influence of pH, ionic strength and the concentration of albumin in the adsorption medium as well as the charge and lipophilicity of a model drug on their adsorption onto poly(acrylic acid) grafted poly(vinylidene fluoride) (PAA-PVDF) membranes was evaluated. The PAA-PVDF membrane is a responsive porous polymer membrane that we have studied for controlled drug delivery. Sodium salicylate (anionic), flunitrazepam (neutral), primidone (neutral), desipramine (cationic) and thioridazine (cationic) were used as model drugs. The extent of drug adsorption was dependent on pH. Drug adsorption was enhanced by the dissociation of the grafted PAA chains and by a positive charge and a high lipophilicity of the drug. Increasing the ionic strength of the medium retarded the adsorption of the cationic drugs. Interestingly, the present results showing that drugs are adsorbed onto the membrane while albumin is not adsorbed onto the membrane suggest that the PAA-PVDF membrane may be suitable for separating drugs from proteinaceous substances for subsequent monitoring and evaluation.