The absorption spectra of the DMS data was used to identify aggregation in the acetate formulation and, with the CD spectra, to assess the long-term stability for both formulations. The aggregation-onset temperature of the antibody in the acetate formulation was found to decrease significantly with sample age, whereas, no tendency to aggregate was found for the lactate formulation irrespective of the age of the sample.
The results suggest that despite the significantly lower Tm of its first transition which, if taken in isolation might suggest the contrary, the antibody is more stable in the lactate than in the acetate formulation.
The data presented in this study support the assertion that Tm on its own may not be a particularly good indicator of stability and that structural changes and resistance to aggregation, once unfolding has occurred, are likely to be better indicators. The complementary nature of spectroscopic and calorimetric methods has also been demonstrated to show the importance of referring to more than one analytical method to get a complete understanding of protein behavior in different formulations.