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

Release behaviour of single pellets and internal fine 3D structural features co-define the in vitro drug release profile

Multi-pellet formulations are advantageous for the controlled release of drugs over single-unit dosage forms. To understand the diffusion controlled drug release mechanism, the pellet structure and drug release from a single pellet (not at dose level) were studied using synchrotron radiation X-ray computed microtomography (SR-μCT) and a sensitive LC/MS/MS method. The purpose of this article is to introduce a powerful, non-invasive and quantitative technique for studying individual pellet microstructures and to investigate the relationship between the microstructure and drug release from single pellets. The data from the single pellet dissolution measurements demonstrated that the release profile of capsules containing approximately 1,000 pellets per unit dose was the summation of the release profiles of the individual pellets. The release profiles of single tamsulosin hydrochloride (TSH) pellets formed three groups when a cluster analysis was performed, and the dissolution rate of the individual pellets correlated well with the combined effects of the drug loading, volume and surface area of the pellets (R(2) = 0.9429). In addition, the void microstructures within the pellet were critical during drug release. Therefore, SR-μCT is a powerful tool for quantitatively elucidating the three-dimensional microstructure of the individual pellets; because the microstructure controls drug release, it is an important parameter in the quality control of multi-pellet formulations.

Ammonio Polymethacrylate-Coated Diltiazem: Drug Release from Single Pellets, Media Dependence, and Swelling Behavior

Drug release from single pellets was measured on an easily assembled flow-through system. Despite heterogeneity between pellets, the sum of the individual results resembled drug release from an ensemble. A typical pellet displayed a long lag followed by rapid release. Heterogeneity appeared to result from substrate properties rather than coating uniformity. Swelling behavior in acid and buffer was measured by dynamic image analysis and related to drug release. Drug release was sensitive to dissolution temperature but swelling was not. A description of the drug release process was proposed.

Visualization of the release behaviour of microcapsules

This paper describes an experimental technique that visualizes the release behaviour that microcapsules loaded initially with a halide salt experience when immersed in aqueous media. The technique, based on the principle of silver halide photography, involves observing the effect that contact with a dilute aqueous silver nitrate solution has on individual microcapsules. Rapid precipitation of an insoluble silver halide salt on a capsule surface provides a permanent record of the location(s) on a capsule surface at which halide ion release occurs. Results obtained by examining the behaviour of individual capsules selected from one KCl and three NaCl capsule samples produced by fluidized bed coating illustrate the wide variety of halide ion release behaviour exhibited by capsules from different capsule samples as well as capsules from the same sample. Scanning electron microscopy showed that halide ion release from all capsules studied does not occur uniformly over the capsule surface, but is limited to specific points on the capsule surface. Whereas most capsules from two NaCl samples broke open spontaneously after a short immersion time, thereby producing a large plume of AgCl precipitate, capsules from one NaCl and the KCl sample remained intact even after prolonged immersion. In these latter cases, a AgCl bolus typically grows on the surface of a capsule as immersion time increased.

XEDS-mapping for explaining release patterns from single pellets

A common way to formulate controlled-release (CR) pharmaceuticals is to coat pellets of active substance with a polymer film, decrease the size of the pellets and distribute them as multiple-unit dosages in capsules. To increase the understanding of the release mechanism, the pellet shape and surface structure of pellets, before and after release in microtitre plates, have been studied by scanning electron microscope and X-ray energy-dispersive spectrometry. By performing these studies we associate release profiles during the first few hours to the microscopic structure. Pellets were divided into three classes (spherical pellets, dumbbell shaped pellets and twin-pellets) according to pellet form. Cases of burst release occurred for all three shape classes due to "open-window-defects" at the surface. Areas of thinner polymer film in the neck-region of dumbbell shaped pellets broaden the range of intermediate release rates for this pellet shape. The surface of twin pellets and dumbbell shaped pellets showed more defects, which increases the release rates in comparison to spherical pellets. All pellets with high release rates revealed ruptures in the polymer film, whereas only small cracks could be traced for pellets with slow release rates. The information gained is necessary for the development of future formulations and mathematical modelling of release patterns. The pharmaceutical used as model was remoxipride coated with a polymer film of ethyl cellulose and 10 wt.% triethyl citrate.

Gradation of microcapsule wall porosity by deposition of polymer mixtures (Eudragit RL and Eudragit RS). Phase separation of polymer mixtures and effects of external media and conditions on release

With the aim of increasing flexibility in controlling release from microcapsules, mixtures of wall polymers varying in porosity were investigated by phase separation. Eudragit RL and RS (polymethylmethacrylate linear backbone polymers) mixtures differing in polar substituent content and porosity were used as the wall material and were deposited using a non-solvent addition method. Release rates increased with polar group content of the mixtures, using theophylline, potassium dichromate or sodium chloride as model core materials. Theophylline release rate had the same relationship to polar group content as found earlier for urea permeation of cast mixed-polymer films. Release was generally accelerated in these systems when the external medium contained sodium lauryl sulphate as a wetting agent but not consistently, decreasing unexpectedly for RL-theophylline microcapsules. Localized dissolution of core substance was visible microscopically during release from single microcapsules. The release rate was sensitive to agitation intensity only at low wall to core ratios. Temperature change revealed only a single release mechanism for sodium chloride by Arrhenius equation treatment. Buffer ions penetrated coatings readily, changing theophylline release rates and providing clear evidence of diffusion via a pore-capillary mechanism.

Effect of polybutadiene on the encapsulation efficiency of ethyl cellulose microcapsules of sulphadiazine

The coacervation-inducing effect of polybutadiene (PBD), a liquid telomer of butadiene, in microencapsulation of sulphadiazine was investigated, using phase separation coacervation of ethyl cellulose, by solvent alteration, in heavy mineral oil medium. The effects of protective colloids and core-wall ratio on encapsulation efficiency were studied. Efficacy of PBD was studied compared to polyisobutylene (PIB). Experimental results revealed that PBD ensured better encapsulation efficiency compared to PIB.

A contribution to the evaluation of microcapsules by light microscopy

Light microscopy has been used for the evaluation of the internal and external structure of dry microcapsules. The method involves surface and penetrative staining with various dyes after which the microcapsules were embedded in suitable optically translucent material. Using this method the core material, its shape and position within the microcapsules either in total or as subunits of the core are clearly distinguishable from the wall material. The surface characteristics of the microcapsules can be observed with either light or fluorescent microscopy after staining with a fluorescent dye. Furthermore, it is a relatively simple and inexpensive method by comparison with the scanning electron microscopy. The natural character of microcapsules, without any artificial structures, has been maintained. It could serve as a routine auxiliary method for complex evaluation or control of the microencapsulation process and its optimization.

Papaverine hydrochloride release from ethyl cellulose-walled microcapsules

The mechanism of papaverine hydrochloride release from ethyl cellulose-walled microcapsules in both simulated gastric and intestinal fluid is discussed. The microcapsules were prepared by coacervation using different core: wall ratios. The rupture of the thin-walled microcapsules after release in simulated gastric fluid was shown and attributed to the internal osmotic pressure, supporting a mechanism for drug dissolution. The internal osmotic pressure produced only a few small holes in the thin-walled microcapsules after release in simulated intestinal fluid. No rupture of the thick-walled microcapsules after release in either medium was shown. Therefore these release data fitted diffusion-type kinetics. It is suggested that the internal osmotic pressure developed after penetration of the medium is affected by the ratio between the core dissolution rate and the drug diffusion rate through the wall.

Variation of population release kinetics in polydisperse multiparticulate systems (microcapsules, microspheres, droplets, cells) with heterogeneity of one, two or three parameters in the population of individuals

Release kinetics of active substances from ensembles of microparticles such as microcapsules, cells, droplets and liposomes constituted of individual entities releasing their contents at constant rates may follow zero order, first order, sigmoid or biphasic equations. The release equation observed depends upon the statistical distribution of release-determining parameters among the population. Typical cases are presented in terms of the distribution of two parameters, payload (m infinity) and time for complete payload release (t infinity) which also define the release rate constant (k). Heterogeneity of the two parameters generally leads to first order ensemble behaviour, whereas heterogeneity of one parameter only may lead to different ensemble release equations, viz. zero order (m infinity heterogeneous) or first order (t infinity heterogeneous). Biphasic distribution can lead to an apparent 'burst' effect, with apparent change of kinetics. The presence of a third heterogeneous parameter, lag time, yields a sigmoid ensemble release curve. These conclusions are demonstrated by simulations or experimentally, and are also valid for linearized curves of microparticles following matrix kinetics.