Dissolution of Gelatin Capsules: Evidence and Confirmation of Cross-Linking

Cross-linking is a common problem in the dissolution of gelatin capsules. Cross-linking is characterized by a bridge across the peptide backbone of the gelatin molecule which creates water insoluble membranes or pellicles during dissolution testing. The chemical covalent bonding between gelatin chains is typically triggered by catalytic amounts of aldehyde and/or from exposure to high temperature and humidity. Cross-linking gelatin capsules results in a slower release of the drug or no release at all in common dissolution media. If the gelatin capsule dissolution fails acceptance criteria due to evidence of cross-linking, the US Pharmacopeia allows the use of enzymes in the dissolution medium and requires twotier dissolution testing. Studies have shown that drugs from cross-linked capsules are available in vivo to the patient, justifying the use of enzymes in the in vitro dissolution test. The literature contains several examples supporting evidence of cross-linking. However, confirming the degree of cross-linking continues to be a practical challenge due to a lack of quantitative procedures for measuring the extent of cross-linking and variability in options for formulations, excipients, level and types of stresses, and the experience level of the analysts. This article reviews the methodologies to confirm the evidence of cross-linking such as visual observation, capsule shell switch test, and spectroscopic determination of cross-linking, with an aim to facilitate the discussion and establishment of an acceptable standard approach.