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.