How to improve the storage stability of aqueous polymeric film coatings

The major aim of this study was to identify an easy tool to improve the long term stability of polymeric film coatings applied from aqueous dispersions. Drug release profiles from ethylcellulose-coated theophylline pellets were monitored during 6 months open storage under ambient and stress conditions ["room temperature/ambient relative humidity (RH)" and "40 degrees C/75%RH"]. The pellets were cured for 1 or 2 d at 60 degrees C or for 1 or 2 d at 60 degrees C/75%RH (followed by 1 d at 60 degrees C for drying). Drug release was measured in 0.1 M HCl and in phosphate buffer pH 7.4. Interestingly, the addition of only small amounts of poly(vinyl alcohol)-poly(ethylene glycol) graft copolymer provided stable drug release profiles under all the investigated conditions, irrespective of the type of release medium, coating level, polymer blend ratio and curing conditions. The addition of small amounts of propylene glycol alginate resulted in unaltered drug release kinetics during open storage under ambient conditions, but decreasing theophylline release rates during open storage under stress conditions, due to further gradual polymer particle coalescence. When adding small amounts of carrageenan to the ethylcellulose coatings, essentially stable theophylline release patterns (with slight variations) were obtained. As coating conditions were not optimized for each system, further work is necessary to distinguish polymer from process effects. The observed stabilizing effects of the investigated added polymers might be attributable to their hydrophilic nature, trapping water within the coatings during film formation and, thus, facilitating polymer particle coalescence. This new concept can be used to overcome one of the major practical obstacles associated with aqueous polymeric film coatings today: storage instability.