Moisture sorption profiles and tensile strength of tableted phenobarbitone formulations

Hot-melt coating: water sorption behavior of excipient films

Hot-melt coating allows encapsulation of water-labile, drug-laden substrates to form a barrier that resists moisture ingress. To understand the interaction of water with excipients that can form moisture-protective coatings, sorption behavior of films of lipidic (glyceryl behenate) and polymeric (polyvinyl alcohol) coating excipients was investigated. A simple and rapid method using a new, fully automated instrumental technique to investigate the sorption/desorption behavior of excipient films is reported. Further, the influence of temperature and film thickness on the sorption behavior of films is examined. Both excipient films displayed sorption isotherms that were classified as type III and demonstrated hysteresis during desorption. The sorption data for both films did not follow the Langmuir model, and the BET model could only be used restrictively. The GAB model fitted the sorption data at all conditions and over the entire range of water activity studied. The ability of the Young and Nelson model to explain the hysteresis behavior, from analytical and mechanistic perspectives, is evaluated. Temperature and film thickness were found to profoundly influence the nature of moisture interaction and distribution of moisture in the excipient films. An Arrhenius-type relationship was observed between equilibrium moisture content of excipient films and temperature at constant water activity.

The Effect of Hygroscopic Formulation Ingredients on the Sorption Characteristics of Tablets

In order to examine the effect of hygroscopic ingredients on the sorption characteristics of tablets, three hygroscopic additives - citric acid anhydrous (CAA), sorbitol (SI) and maltodextrin (MA) - were added at concentrations of 10% and 20% to a standard tablet granulate formulation prepared with three different initial moisture contents. The additives chosen were intended to be representative of a range of active ingredients with varying hygroscopicity characteristics.The granulate/additive mixtures, together with the corresponding additive-free mixtures, were then tabletted, and the sorption isotherms of the resulting tablets were determined. The sorption-related changes in hardness, thickness, diameter and disintegration time were also investigated.Examination of the sorption isotherms showed that the position of the "ansor-ption point" - the point where the isotherm crosses the x-axis and thus the point at which the tablets start to adsorb water - was much more dependent on the initial moisture content of the tablets than on the presence of a hygroscopic additive. The presence of a hygroscopic additive had little or no effect.The additives did not begin to have any marked effects on the sorption isotherms of the finished tablets until the relative humidity level reached 62%. Above 62%, however, the differences in the hygroscopicity characteristics of the individual additives had a direct impact on the sorption profiles of the tablets.As increasing amounts of moisture were adsorbed, tablet hardness fell whilst tablet thickness and diameter increased. The increases in thickness were in all cases greater than the increases in diameter. These findings applied to all tablets irrespective of their initial moisture content.The sorption-related changes in disintegration time did not exhibit any consistent pattern and have therefore not been interpreted.A comparison of the areas under the adsorption isotherms yielded the following results:• The three additives adsorbed differently. Their adsorption, however, was always greater than that of any of the tablet variants.• The areas under the adsorption isotherms of the additive-containing tablets were in all cases greater than the areas under the adsorption isotherms of the corresponding additive-free tablets. However, the differences were not always directly related to the concentration of the additive or to the area under the adsorption isotherm of that additive.The relevance of these findings for routine pharmaceutical practice is discussed.