Crosslinking studies in gelatin capsules treated with formaldehyde and in capsules exposed to elevated temperature and humidity.

Incomplete in vitro capsule shell dissolution and subsequent drug release problems have recently received attention. A modified USP dissolution method was used to follow capsule shell dissolution, and a 2,4,6-trinitrobenzenesulfonic acid (TNBS) assay was used to follow loss of epsilon-amino groups to study this shell dissolution problem postulated to be due to gelatin crosslinking. The dissolution problems were simulated using hard gelatin capsule (HGC) shells previously treated with formaldehyde to crosslink the gelatin. These methods were also used to study the effect of uncrosslinked HGC stored under stressed conditions (37 degrees C and 81% RH) with or without the presence of soft gelatin capsule shells (SGC). A 120 ppm formaldehyde treatment reduced gelatin shell dissolution to 8% within 45 min in water at 37 degrees C. A 200 ppm treatment reduced gelatin epsilon-amino groups to 83% of the original uncrosslinked value. The results also support earlier reports of non-amino group crosslinking by formaldehyde in gelatin. Under stressed conditions, HGC stored alone showed little change over 21 weeks. However, by 12 to 14 weeks, the HGC exposed to SGC showed a 23% decrease in shell dissolution and an 8% decrease in the number of epsilon-amino groups. These effects on the stressed HGC are ascribed to a volatile agent from SGC shells, most likely formaldehyde, that crosslinked nearby HGC shells. This report also includes a summary of the literature on agents that reduce gelatin and capsule shell dissolution and the possible mechanisms of this not-so-simple problem. Copyright 2001 Wiley-Liss, Inc. and the American Pharmaceutical Association J Pharm Sci 90: 79-88, 2001