Dissolution Characteristics of Capsule Shells and Drug Release from Commercial Tetracycline-Hcl Capsules

Abstract

The dissolution characteristics of the gelatin shells of four brands of tetracycline-HCl capsules were examined by measuring the shell rupture time (t_r) in a modified version of two-blade stirrer apparatus under various stirrer depth, ionic strength, and pH conditions. The dissolution rate of tetracycline-HCl from these capsules was also determined using the U.S.P. XIX dissolution apparatus. While no significant effect of stirrer depth on t_r was found, increasing the basket-stirrer distance from the bottom of the flask from 0.2 to 2 cm was found to increase the dissolution rate of tetracycline-HCl from capsules significantly (p < 0.001). As the ionic strength was increased, the dissolution rates of both gelatin shell and tetracycline-HCl content were increased, however, increasing the ionic strength from 0.6 to 1.5 failed to produce any further increase in t_r. The pH of the dissolution fluid significantly (p < 0.01) influenced the dissolution rate of the capsule shell and t_r was longest at pH = 4. A linear, inverse relationship between pH and tetracycline-HCl dissolution rate constant (k_s) was obtained. While a good correlation between t_r and k_s was obtained under certain conditions, capsule shell and tetracycline-HCl content showed different dissolution behaviour under other conditions. It is expected therefore, that under the latter conditions capsule shells had their maximum effect on drug release from the capsules studied.