Influence of drug partition coefficient and pH value of sink solution on permeation from porous thick-walled ethyl cellulose microcapsules

Solvent and plasticizer influences on ethylcellulose-microcapsules

Variations in microencapsulation processes give rise to different products and it seems there are no firm rules. It is thus difficult to know what kind of product will be obtained before the research is carried out. Changes in temperature, rate, time and type of stirring can cause great modifications in the system, most of which are responsible for variations in standard techniques. In our study, we investigate the solvent influence on ethylcellulose (EC) microcapsule formation. We have selected four different solvents: ethanol as an aqueous solvent and acetone, chloroform and toluene as organic solvents. Diclofenac sodium (DFNa), a non-steroidal anti-inflammatory agent, has been used as an encapsulated substance as it is inactivated in the gastric juices. This polymer and microencapsulation process was selected after an exhaustive study with different polymers and processes. Once the solvent influence was determined, ethylphthalate was incorporated in one type of microcapsule in order to study the influence of this plasticizer on drug release by the modification of film-permeability.

Microspheres of human serum albumin with barbiturates: effect of drug partition coefficient on preparation and drug release

Human serum albumin microspheres were prepared with a series of barbiturates by the thermal denaturation method. The barbiturates were of similar general physicochemical properties but different partition coefficients. The total drug content of microspheres was dependent on the partition coefficient, and an increase in the partition coefficient caused a decrease in the drug content of finished microspheres due to drug migration to the outer organic phase during preparation. The ensemble drug release from microspheres was followed by the paddle method and by potentiometric titration with a pH-stat. Nonlinear regression analysis showed the best fit for the spherical Higuchi equation, especially first-order kinetics (biphasic). The drug partition coefficient affected the release in such a way that drugs with higher partition coefficients were released faster and to a greater extent than those with lower coefficients.

A new concept for interpretation of first-order release from albumin microspheres

A release mechanism from a microsphere that consisted of a water-swellable polymer and a uniformly dispersed, relatively small number of very slightly soluble large-core particles was considered. When the microsphere is dipped in release media the polymer swells instantly, and every core particle dissolves in the release media in the space between the core particles and the swollen polymer to make a saturated solution. It is assumed that Fickian diffusion of dissolved core substance between all spaces filled with saturated solution and outer sink occurs independent of each other, and release from one entire microsphere is the sum of diffusion from all spaces in the microsphere. Then, derived theoretical release kinetics is found to be first-order, and the derived first-order release rate constant is expressed as a function of the following parameters: radius of core particle, radius of the microsphere, solubility of core substance to media solution, density of the core particle, and permeability constant of core substance in swollen polymer. When rate constants were measured from release tests, varying each parameter, the relation between constants and each parameter follows the function. The permeability constant, which was calculated applying the function on measured rate constants and other known parameters, was in good agreement with the permeability constant measured from permeation study of planar membrane prepared in similar conditions to when preparing microspheres. These results are thought to show the validity of the mechanism and function proposed.

Aliphatic polyesters and cellulose-based polymers for controlled release applications

The general technologies for fabricating drug delivery systems are briefly discussed in the present review. Furthermore, two types of commonly used polymers, biodegradable aliphatic polyesters for parenteral administration and cellulose-based polymers for oral uses, are presented by means of selected examples of their properties and applications to control drug release.

Direct measurements on individual microcapsule dissolution as a tool for determination of release mechanism

A method is described for simultaneous microscopic observation of individual microcapsule core material dissolution together with quantitative measurement of the individual kinetics of release of the contents. These may be conductimetric in the case of ionized materials or spectrophotometric otherwise. This enables correlation of changes in core surface area during dissolution with kinetics. Surprisingly, both ethyl cellulose- and polymethacrylate-coated cores of potassium dichromate crystals, used as a model, showed localized internal dissolution universally, providing evidence of the exit of the salt via pores in the membrane, in spite of the kinetics being invariably zero order, as expected for individual microcapsules. The advantages of the method are presented.

Dissolution studies of microencapsulated 4-sulphonamidophenoxyacetic acid: effect of preparative variables on dissolution

Ethylcellulose-walled microcapsules of 4-sulphonamidophenoxyacetic acid were prepared and their in vitro dissolution characteristics were investigated. Different release kinetics must be applied according to the respective average particle size and wall content values of the microcapsule fractions. The 'dissolution model' appears to fit better with small thin-walled microcapsules whereas, for larger thicker-walled microcapsules, Higuchi-type kinetics seem to describe the release of the major part of the drug more adequately.

Preparation and release kinetics of microencapsulated cisplatin with ethylcellulose

Several kinds of microcapsules containing cisplatin were prepared by the ethylcellulose coacervation process in an attempt to administer cisplatin with chemoembolization. Microcapsules made either with mechanical stirring or with sonication showed similar properties; the chemical structure of cisplatin was not affected by the micro-encapsulation process. The release kinetics of cisplatin from ethylcellulose-walled microcapsules followed different patterns, according to the wall thickness. In each case, the release kinetics did not depend on the stirring rate of the surrounding medium. Only microcapsules with a cisplatin release ratio from 80 to 100% within 24 h were selected for later clinical use.

Preparation and biopharmaceutical evaluation of microcapsules of amoxicillin

Two methods of microencapsulation of amoxicillin, an orally administered antibiotic, were studied. One is based on dispersion of gelatin-amoxicillin mixture in liquid paraffin followed by drying and hardening with formalin-isopropanol treatment; the other is based on dispersion of ethylcellulose-amoxicillin mixture in purified water containing sodium lauryl benzene sulphonate. The microcapsules were recovered as discrete, free-flowing fine granules with a particle diameter of about 250-1000 microns. Dissolution of amoxicillin from ethylcellulose microcapsules was suppressed considerably with a zero-order dissolution pattern in solutions of various pH. Gastric-emptying-controlled rabbits were used for the in vivo evaluation of gelatin and ethylcellulose microcapsules. The ethylcellulose microcapsule containing 25 per cent amoxicillin showed a significantly sustained release pattern of amoxicillin. To establish a suitable design and for the evaluation of the sustained release microcapsules, a nomogram was made using pharmacokinetic parameters obtained after administration of a conventional formulation. It is advantageous for the preparation of sustained release microcapsules to chose pharmaceuticals having over about 2 as the ratio of the elimination rate constant, k10, to the release/absorption rate constant, kr, in the rabbit.