Abstract
The effect of drug and polymer properties on microencapsulation of acidic drugs with enteric polymers by the surface neutralization method was studied using eight benzoic acid derivatives and three enteric polymers (carboxymethylethylcellulose (CMEC), hydroxypropylmethylcellulose acetate succinate-H (AS-H) and hydroxypropylmethylcellulose acetate succinate-L (AS-L)). Each core material containing a drug was granulated to beads from 500 to 710 μm in diameter to remove the effect of drug crystal morphology. Microcapsules (MCs) were prepared by suspending the beads in an enteric polymer solution, followed by filtration and drying. Then, the properties of the MCs produced were evaluated. The polymer content in MCs (PC) and recovery percent of the drugs in MCs were in the order AS-H>AS-L>CMEC, but the percent of recovered single-nuclear MCs was in the order CMEC>AS-L>AS-H. The PCs did not correlate with the solubilities of drugs, but showed an obvious dependency on the pH of the saturated solution of the drug. That is, as the pH decreased, the PCs increased. To estimate the PCs on the basis of drug and polymer properties, a mathematical model was established on the basis of an acid-base equilibrium equation, and the ratio of polymer acidic groups associated with hydrogen ion (FS) was calculated. A linear relationship was obtained between √((FS-αp)) values (αp : equivalency ratio of hydrogen ion to acidic groups of polymer at which the polymer begins to precipitate) and the experimental PCs. Thus, microencapsulation efficiency seemed to be predictable by using this relationship.
