SEM/EDX and confocal microscopy analysis of novel and conventional enteric-coated systems

A novel double coating enteric system (comprising an inner layer of neutralised EUDRAGIT® L 30 D-55 and organic acid, and an outer layer of standard EUDRAGIT® L 30 D-55) was developed to provide fast dissolution in proximal small intestinal conditions. The mechanisms involved in the dissolution of the double coating were investigated and compared with a conventional single layer enteric coating and an hypromellose (HPMC) sub-coated enteric system. Rates of drug release from coated prednisolone pellets were established using USP II dissolution methods (0.1 M HCl for 2 h and subsequently pH 5.5 phosphate buffer) and the coating dissolution process was illustrated using confocal laser scanning microscopy (CLSM). The distribution of sodium, as a representative ion, in the double-coating system during dissolution was determined using scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX). The double-coating system showed faster dissolution compared to the single coating and the HPMC sub-coated system in pH 5.5 buffer. The dissolution process of the double-coating was unusual; the inner coat dissolved before the outer coat and this accelerated the dissolution of the outer coat. During dissolution, sodium ions diffused from the inner coat to the outer coat. This migration of ions and the increased ionic strength and buffer capacity of the inner coat contribute to the rapid dissolution of the double-coating system.