Chemical pathways of peptide degradation. VII. Solid state chemical instability of an aspartyl residue in a model hexapeptide.

The chemical stability of an Asp-hexapeptide (Val-Tyr-Pro-Asp-Gly-Ala) in lyophilized formulations was evaluated as a function of multiple formulation variables--specifically pH of the bulk solution, temperature, moisture content, and type of bulking agent (amorphous vs. crystalline). The disappearance of the starting Asp-hexapeptide in the solid state conformed to pseudo-first-order reversible kinetics. This type of degradation profile was accounted for by the product distribution. The factorial experimental design of this study allowed statistical analysis of the effects of individual formulation variable (main effects) as well as those of two-factor interactions on the degradation of the Asp-hexapeptide. Analysis of Variance (ANOVA) calculations of the main effects indicated that while the influence of pH of the starting solution was not statistically significant, residual moisture level, temperature, and, especially, type of bulking agent had a significant impact on the solid state chemical reactivity of the hexapeptide. Furthermore, depending on which type of excipient was used in the lyophilized formulations, residual moisture level and temperature could be important stability variables. These types of factorial experiments have proven to be useful in the rapid identification of significant formulation variables in a given system and, consequently, in optimization of formulations.