Drug migration into soft gelatin capsule shells and its effect on in-vitro availability

Analysis of the shells and contents of soft gelatin capsules containing acetomenaphthone, ephedrine, 4-hydroxyenzoic acid or phenobarbitone, dissolved in isopropyl myristate, revealed that the percentage of solute taken up by the shells increased with increasing aqueous solubility of the substrate. Thus no acetomenaphthone, which has a negligible aqueous solubility, was found in the shells, in comparison with 92% of the 4-hydroxybenzoic acid, which has a significant solubility in water. Uptake was not influenced by the solubility in isopropyl myristate. The effect of the oily solvent was studied using blends of 1-octanol and isopropyl myristate in which either 4-hydroxybenzoic acid or phenobarbitone were dissolved. Solute release shows that both release and migration can be predicted from a knowledge of the aqueous solubility of the solute and its partition coefficient between water and the non-polar solvent. Samples of capsules containing 4-hydroxybenzoic acid in isopropyl myristate were withdrawn at various stages of the manufacturing process, and the distributions between shell and contents noted. Most of the transfer took place while the capsules were being dried in rotating basket driers, and at this point 67% of the acid had migrated. This increased to 92% during tray drying, and remained so for at least 6 months after manufacture.