Improvement in the disintegration of shellac-coated soft gelatin capsules in simulated intestinal fluid

Shellac is a natural enteric polymer, which results in good gastric resistance; however, it often dissolves too slowly in intestinal fluids. The objective of this study was to improve the disintegration of shellac-coated soft gelatin capsules in simulated intestinal fluids (phosphate buffer pH 6.8) through the addition of pore-formers, such as organic acids and hydrophilic polymers, while retaining gastric resistance. The mechanical properties (% elongation at rupture, puncture strength at break and modulus at puncture), media uptake and weight loss of shellac films were determined upon exposure in 0.1 N HCl and/or phosphate buffer pH 6.8. Organic acids (e.g., sorbic acid) acted as plasticizers, they reduced the glass transition temperature of ethanol-cast shellac films. The addition of additives effectively decreased the disintegration times in phosphate buffer pH 6.8, while the behavior in 0.1 N HCl remained unchanged. In addition, the hardness and disintegration of shellac-coated soft gelatin capsules were monitored through the whole disintegration experiments. The best disintegration was achieved with sorbic acid as pore-former. Sorbic acid remained in the shellac coating at low pH, but leached in pH 6.8 buffer, thus resulting in good gastric resistance and rapid disintegration in simulated intestinal fluids. The disintegration time of ethanolic shellac-coated soft gelatin capsules decreased with increasing amount of pore-former. The slow disintegration of aqueous shellac-coated soft gelatin capsules could be also improved by the addition of hydrophilic polymers, such as hydroxypropyl methylcellulose (HPMC). However, higher HPMC concentrations were required when compared to sorbic acid.