Effect of comonomer ratio on hydrocortisone diffusion from sustained-release composite capsules.

The daily in vitro release of hydrocortisone from composite polymer capsules is reported here for over 120 days. Increase in vinyl acetate comonomer content of the ethylene-vinyl acetate copolymer matrix brought about an increase in the diffusion rate. Variation in the initial drug content of the capsules from 40 mg to 20 mg affects the daily drug release less significantly than the variation in copolymer ratio. The correlation between vinyl acetate comonomer content and the percent crystallinity of the copolymer matrix is suggested as one of the possible major factors in controlling diffusion rate from this drug-polymer system. The diffusion constant (D) calculated was 0.212 X 10(10) cm2/sec when the copolymer carrier has 30% vinyl acetate content and 0.430 X 10(11) cm2/sec when the copolymer carrier has 20% vinyl acetate content for capsules with 20 mg initial drug content, and 0.118 X 10(-11) cm2/sec and 0.226 X 10(-11) cm2/sec, respectively, for capsules with 40 mg initial drug content.