The use of 13C solid state NMR to elucidate physico-chemical association in Eudragit RS100 microencapsulated acyl esters of salicylic acid.

A series of homogeneous Eudragit RS100 matrix microspheres containing molecularly dispersed acylated esterified homologues of salicylic acid, (acetylsalicylic acid, valerylsalicylic acid, or caprylsalicylic acid) were prepared in order to investigate the effect of encapsulation on solid-state orientation of the encapsulated molecule. Electrostatic association of the drug with the charged quaternary residues in the polymer may be responsible for the previously observed stability of acetylsalicylic acid (ASA) in aqueous swollen ASA-loaded Eudragit RS100 microspheres. Evaluation of the 13C nuclear magnetic resonance spectra for evidence of structural association of the incorporated probe molecules indicated that alteration of the microenvironment of the incorporated solutes had occurred. For instance, increasing the aliphatic character of the acyl side chain resulted in an increase in the upfield shift of the acyl bearing aromatic ring carbon, (C2), in the incorporated probe molecule as compared to the unincorporated probe molecule. Similarly, a downfield perturbation in the chemical shift of the free acid bearing aromatic ring carbon, (C1), was also observed. This microenvironment electrostatic shielding in the proximity of the ester carbonyl is attributed to an increase in the association of the probe molecule with the polymer subunits. Thereby, it is postulated that the matrix incorporated probe molecule is essentially shielded from hydrolytic attack until it is liberated into the external aqueous environment.

The influence of microencapsulation using Eudragit RS100 on the hydrolysis kinetics of acetylsalicylic acid.

Homogeneous Eudragit RS100 matrix microspheres containing molecularly dispersed acetylsalicylic acid (ASA) were prepared in order to investigate the effect of encapsulation on the decomposition rate of a hydrolytically susceptible drug. ASA-loaded microspheres of this non-eroding polymer matrix were analysed at predetermined time points following immersion of the microspheres in temperature controlled buffer systems at pH 1.2 or pH 12.1 at 30, 40 or 50 degrees C. The mass balance of the total amount of solutes (ASA and SA) initially located within the microsphere interior was equal to the sum of the amount of solutes remaining in the microsphere interior and the amount of solutes in the aqueous phase at any time during the course of the study. Each analysis involved the quantitation of four species; the drug and decomposition product, salicylic acid (SA), in both the microspheres phase and the external aqueous phase. A simple model system using first-order rate approximations for the concurrent Fickian diffusion and hydrolysis decomposition of the drug resulted in a multiexponential expression which adequately described the time-course profile of the drug. SA-loaded microspheres were used as a control under similar conditions to determine the magnitude of the contribution of microsphere phase hydrolysis of ASA to the overall rate of drug loss from the microspheres. Results indicated that microspheres phase hydrolysis of ASA was minimal. Even after 900 h of immersion in pH 12.1 buffer some ASA remained within the microsphere. It is postulated that the matrix incorporated drug is essentially shielded from hydrolytic attack until it is liberated into the external aqueous environment. Electrostatic association of the drug with the charged quaternary residues in the polymer along with the limiting availability of water within the microsphere may be responsible for the observed stability of ASA in aqueous swollen ASA-loaded Eudragit microspheres.

The effect of surfactants on the rate of decarboxylation of p-aminosalicylic acid in acidic aqueous solutions.

p-Aminosalicylic acid, which exists in four ionic forms and whose decarboxylation is well known, was used as a model drug to investigate the effects of surfactants with different charges on its stability. The greatest reduction in the rate of decarboxylation at 50 degrees C occurred when the charge on the micelles was opposite to that of the charge on the drug or when both the drug and the micelle had a neutral charge. Thus, at the highest surfactant concentration, 3 or 5% (w/v), there was a 59% reduction in the rate at pH 2.68 in the presence of the nonionic surfactant polyoxyethylene 24 monocetyl ether, 69% reduction in the rate at pH 4.88 in the presence of the cationic surfactant hexadecyl trimethylammonium bromide, and a 43% reduction at pH 1.01 in the presence of the anionic surfactant sodium cetyl sulfate. The decrease in the rate of decarboxylation was attributed to the partitioning of the drug into the micelles, which provided a phase for improved stabilization. Partition coefficients and rate constants for decarboxylation of the drug inside the micelles were calculated.

Physico-chemical evaluation of acetylsalicylic acid-Eudragit RS100 microspheres prepared using a solvent-partition method.

Controlled release homogeneous matrix microspheres containing acetylsalicylic acid (ASA) were prepared by a simple mechanical process using Eudragit RS100 as the matrix polymer. A drug-polymer solution in a binary solvent of methylene chloride/acetone (9:1) was prepared and infused at a rate of 15 microliters/min as small droplets into a flowing stream of mineral oil where partition of the solvent occurred. A series of experiments was conducted in which the polymer to drug ratio in the infusion solution was fixed at 5:1, 4:1, 3:1 or 2:1 while varying the infusion solution viscosity by altering the infusion solution total solids concentration. Results indicate that microsphere mean particle size was maintained at 200-300 microns once the infusion solution viscosity exceeded 2 cps. The physical state of the ASA incorporated into the microspheres, as confirmed by SEM and thermal analysis, was amorphous in nature until a drug loading of 24% was reached. Drug loading for each polymer to drug ratio increased in a proportional manner with respect to the initial drug concentration of the infusion solution. Dissolution release profiles were found to be biphasic and best analysed according to the semi-empirical equation of Ritger-Peppas, Mt/Mx = k2tn, for the initial phase and by the square-root model of Higuchi, Qt = k1t1/2 for the latter phase. This difference was attributed to the lack of a barrier effect to the drug diffusion process during the latter dissolution phase when the microspheres are fully hydrated.

A method to control particle size of cellulose acetate trimellitate microspheres.

A method for controlling microsphere particle size by regulating the ratio of polymer to solvent concentration and volume fraction, for an emulsion-solvent removal type of microencapsulation system, has been investigated. Viscosity of the external phase was kept constant by using light mineral oil in all experiments. Viscosity of the polymer solution, the internal phase, was modified by changing the ratio of the polymer to solvent concentration. Microspheres were obtained by adding the internal phase to the external phase and stirring the mixture for 30 min. A non-solvent was then added to the system to harden the polymer and recover the microspheres. Polymer concentration was modified by adding extra solvent to the mineral oil, just before the addition of the internal phase and also by adding extra solvent to the polymer phase. Similar, but not identical, results were obtained in both of these systems. A plot of particle size versus polymer to the solvent ratio resulted in sigmoidal curves. The term solvent means the solvent available to the polymer after mixing with mineral oil. A separate curve was obtained for each polymer concentration used in the experiments. When the internal phase volume fraction was incorporated as a variable in the plot of particle size, the three sigmoidal curves merged into a single curve, irrespective of the polymer concentration. The equation developed for controlling the particle size, as a function of polymer and solvent concentration and phase volume ratio, also was tested in systems that contained tartrazine as a model drug. Dissolution experiments were carried out and dissolution rate was correlated to particle size. Microspheres size was controlled by polymer and solvent concentration and phase volume ratio.

Effect of viscosity and interfacial tension on particle size of cellulose acetate trimellitate microspheres.

The influence of the viscosity of the internal and external phases and the interfacial tension between the two phases, in the emulsion type of microencapsulation system was investigated. The viscosity of mineral oil (external phase) was measured by a capillary viscometer and the viscosity of cellulose acetate trimellitate solution (internal phase) was measured by a Brookfield viscometer. The viscosity of the two phases were measured prior to mixing and at 5 and 60 min after mixing the phases. It was observed that the viscosity of the mineral oil phase prior to mixing had little effect on the average diameter of the microspheres, until a high concentration of light mineral oil was used. A graph of viscosity ratio of the internal phase to the external phase shows that a minimum viscosity ratio may be required before particle size increases. Results are discussed with respect to viscosity effects of mineral oil and polymer solution, as influenced by the solvent uptake by the mineral oil. The interfacial tension between the two phases was measured by pendant drop method. Interfacial tension was measured at 5 and 60 min after the two phases came in contact. The interfacial tension between the mineral oil and polymer solution ranged up to 7 dyne/cm and the particle size was not affected appreciably by the interfacial tension. Particle size and morphological analysis of the microspheres were determined using microscopy and scanning electron microscope.

Phase diagram studies for microencapsulation of pharmaceuticals using cellulose acetate trimellitate.

Phase diagrams were prepared to indicate the region of microcapsule formation for the following system: cellulose acetate trimellitate, light mineral oil, and the solvent mixture (acetone:ethanol), using chloroform as the hardening agent. The effect of sorbitan monoleate, sorbitan monolaurate, and sorbitan trioleate on the region of the phase diagram for the formation of microcapsules was investigated. The results indicate that microcapsules are readily formed when the polymer concentration is in the 0.5-1.5% range and the solvent concentration is in the 5-10% range. Aggregation of microcapsules was minimized by using lower solvent concentration. Low concentrations of sorbitan monooleate in mineral oil (less than or equal to 1%) gave products that had smoother coats and more uniform particle size. Surfactants with low hydrophile:lipophile balance produced larger regions on the phase diagram for microencapsulation compared with a surfactant with higher hydrophile:lipophile balance. A mechanism for microencapsulation is described. Tartrazine microcapsules produced using different concentrations of surfactant were tested for dissolution characteristics in both acidic and neutral conditions. Tartrazine-containing microcapsules prepared by using 3% sorbitan monooleate had the lowest release in acidic conditions. The effect of surfactant and formulation concentration on microcapsule size was studied by analyzing the particle size distribution for both blank and tartrazine-containing microcapsules. The smallest microcapsule size was obtained when the sorbitan monooleate concentration was 3%. It appears that there is an upper limit for the surfactant concentration that could be used to achieve successful microencapsulation.

Formulation parameters affecting the preparation and properties of microencapsulated ion-exchange resins containing theophylline.

A method of microencapsulating theophylline ion-exchange resins with ethylcellulose was developed to produce smooth and uniform coats which were predominantly mononucleated. This was achieved by controlling the amount of ethylcellulose and the particle size, and through the use of a protective colloid, polyisobutylene. The rate of release of theophylline was influenced by the ion-exchange resin crosslinking, the amount of ethylcellulose, and the smoothness of the coat. Mesh size and polyisobutylene did not appear to affect the rate in a regular manner. It was found that the release rate from coated resins with low crosslinking followed a logarithmic plot, indicating membrane-controlled release, whereas coated resins with high crosslinking fitted a t1/2 plot, suggesting particle diffusion control.

Stability studies of hydralazine hydrochloride in aqueous solutions.

The hydrolysis kinetics of hydralazine hydrochloride was studied at pH 1 to 12 at 35 degrees, 50 degrees, and 70 degrees C. The hydrolysis takes place by water attack on the dicationic and the cationic forms of the drug. In addition there is hydroxyl attack on the cationic and the neutral forms of the drug. The drug is not subject to attack by acetate and carbonate buffers, but its decomposition is catalyzed by HPO4 = of the H2PO4-, HPO4 = buffer system. The hydrolysis rate follows first order kinetics under nitrogen, at constant pH, temperature, and buffer concentration. The pH profile indicates that hydralazine has maximum stability near pH 3.5. The drug decomposes to phthalazine and other products. The Arrhenius activation energy was found to be 17.9 kcal/mol at pH 3.5 and the time required for 10% loss (t0.9) was estimated from the Arrhenius parameters to be 1.56 years at 25 degrees C.

On the number of Purkinje cells in the human cerebellum: unbiased estimates obtained by using the "fractionator".

Stereological estimates of the numbers of Purkinje cell nucleoli in human cerebellar cortex have been obtained from systematic random samples of tissue by using the fractionator. The estimates are unbiased by fixation, section thickness, or sampling errors and are independent of any assumptions about cell shape, size, or spatial orientation. Twelve brains from aged subjects of both sexes were examined. The average complement of nucleoli in four female brains (age range 71-93 years) amounted to 14.8 millions (with an observed coefficient of variation between subjects of 29%). For three male brains (76-91 years), the corresponding estimates were 15.7 millions (10%). No significant sex differences were found for these small samples. Five brains of unknown sex and age yielded values of 15.8 millions (18%). For the twelve brains examined, the total number of Purkinje cell nucleoli per cerebellum was found to be 15.4 millions (19%). Estimated numbers showed a significant positive correlation with cerebellar weights. The number of nucleoli in an individual cerebellum was obtained with high precision in as short a time as 4 hours.

Rate of release of organic carboxylic acids from ion-exchange resins.

Organic anions with similar properties, but different molecular weights, were bound to anion-exchange resins with different cross-linking. It was found that the capacity of the ion-exchange resin for the anions, the percentage of organic anion released, and the rate of release depends on the crosslinking of the ion-exchange resin, the molecular weight of the anion, and the moisture content of the resin. Self-diffusion coefficients for the release rates were determined.

Some factors affecting the microencapsulation of pharmaceuticals with cellulose acetate phthalate.

Phase diagrams were prepared to indicate the region of microcapsule formation for the following system: cellulose acetate phthalate, light mineral oil, acetone: 95% ethanol solvent, and sorbitan monooleate. The microencapsulation of pharmaceuticals having widely different solubility properties was carried out. Various factors affecting microencapsulation, namely trituration, order and time of addition of the pharmaceutical, core size, and the core to coat ratio, were investigated. The evidence for a mechanism of microencapsulation is also presented. The phase diagrams showed that microcapsules readily formed when the cellulose acetate phthalate concentration was in the 0.93-3.85% range and the polymer solvent concentration was in the 7-16% range. Aggregation of microcapsules was minimized at low solvent concentrations. Pharmaceuticals could be microencapsulated regardless of their solubility in the polymer solvent or hardening liquid. The size of the microencapsulated pharmaceutical increased as the core:coat ratio increased to a maximum of 1.5:1. There is an upper size limit of the pharmaceuticals which can be coated.

Preparation and evaluation of microencapsulated and coated ion-exchange resin beads containing theophylline.

Anion-exchange resin beads in the theophylline form were prepared and coated with paraffin or encapsulated with ethylcellulose by various methods; one of these products was subsequently coated with paraffin in order to achieve a wide range of release rates and patterns of release. The various products were subjected to rate studies, and it was found that the patterns of release and the rates could be controlled by the cross-linking of the resin and the coating procedures used. A medium and almost constant rate of release was obtained by using ethylcellulose microencapsulation, a coating of hard paraffin, or a combination of both. The amount of coating and the total amount of drug released was also determined.

Simple rapid method for the preparation of enteric-coated microspheres.

A method is presented for encapsulating high molecular weight biological materials such as viral antigen, concanavalin A, and other proteins with cellulose acetate phthalate. The method is simple, inexpensive, and rapid; the process takes approximately 15 min. Capsules generated by this method are produced as microspheres 1-3 mm in diameter. They are stable for at least 6 h in simulated gastric conditions, but disintegrate rapidly under simulated intestinal conditions. Encapsulation had no effect on the activity of the biological materials. The method has potentially wide application for encapsulation of drugs and other substances.

Preparation and evaluation of microencapsulated ion-exchange resin beads.

Ion-exchange resin beads in the benzoate form were coated by several microencapsulation techniques to alter and improve characteristics, especially the control of drug release, of this type of drug delivery system. The most successful techniques included polymer-polymer interaction, temperature change, and nonsolvent addition. The microencapsulated beads then were studied with respect to the release rate of the organic anion to determine the effects of microencapsulation. The release rate of the organic anion could be controlled over a wide range, depending on the encapsulating material characteristics. Factors affecting the extent and rate of release as result of microencapsulation are discussed.

Influence of wax coatings on release rate of anions from ion-exchange resin beads.

Ion-exchange resin beads were coated with various waxes to improve and control their release. The in vitro release rates of benzoate ions from the coated-resin beads were then investigated using a rotating sieve basket technique. The dramatic differences in release rates observed with the different waxes can be discussed in terms of the wax to resin ration and the solubility characteristics of the waxes. The initial release rates can be expressed in terms of a mathematical expression previously reported for the diffusion of ions in ion-exchange resins, thereby aiding in the elucidation of the effect of the waxes on release.