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.