Protein engineering and formulation optimization strategies can be taken to minimize protein aggregation in the biopharmaceutical industry. Short-term stability measures such as the midpoint transition temperature (T) for global unfolding provide convenient surrogates for longer-term (e.g., 2-year) degradation kinetics, with which to optimize formulations on practical time-scales. While successful in some cases, their limitations have not been fully evaluated or understood. T values are known to correlate with chemical degradation kinetics for wild-type granulocyte colony stimulating factor (GCSF) at pH 4-5.5. However, we found previously that the T of an antibody Fab fragment only correlated with its rate of monomer loss at temperatures close to the T. Here we evaluated T, the fraction of unfolded protein (f) at temperature T, and two additional short-term stability measures, for their ability to predict the kinetics of monomer and bioactivity loss of wild-type GCSF and four variants, at 37 °C, and in a wide range of formulations. The GCSF variants introduced one to three mutations, giving a range of conformational stabilities spanning 7.8 kcal mol. We determined the extent to which the formulation rank order differs across the variants when evaluated by each of the four short-term stability measures. All correlations decreased as the difference in average T between each pair of GCSF variants increased. The rank order of formulations determined by T was the best preserved, with R-values >0.7. T-values also provided a good predictor (R = 0.73) of the aggregation rates, extending previous findings to include GCSF variant-formulation combinations. Further analysis revealed that GCSF aggregation rates at 37 °C were dependent on the fraction unfolded at 37 °C (f), but transitioned smoothly to a constant baseline rate of aggregation at f < 10. A similar function was observed previously for A33 Fab formulated by pH, ionic strength, and temperature, without excipients. For GCSF, all combinations of variants and formulations fit onto a single curve, suggesting that even single mutations destabilized by up to 4.8 kcal mol, are insufficient to change significantly the baseline rate of aggregation under native conditions. The baseline rate of aggregation for GCSF under native conditions was 66-fold higher than that for A33 Fab, highlighting that they are a specific feature of each native protein structure, likely to be dependent on local surface properties and dynamics.

Multi-domain biotherapeutic proteins present additional behavioural and analytical challenges for the optimisation of their kinetic stability by formulation. Tissue-type plasminogen activator (tPA) comprises six protein domains that exhibit a complex and pH-dependent thermal unfolding profile, due to partially independent domain unfolding. Here we have used tPA as a model for evaluating the relationships between various thermal unfolding and aggregation parameters in multi-domain proteins. We show that changes in the thermal unfolding profile of tPA were parameterised by the overall thermal mid-point transition temperature T m , and the Van't Hoff entropy for unfolding, ΔS vh , which is a measure of unfolding cooperativity. The kinetics of degradation at 45 °C, leading to aggregation, were measured as rates of monomer and activity loss. These two rates were found to be coincident at all pH. Aggregation accelerated at pH 4 due to the early unfolding of the Serine Protease N-terminal domain (SP-N), whereas at pH 5-8, the fraction unfolded at 45 °C (f 45 ) was <1%, resulting in a baseline rate of aggregation from the native ensemble.We used a Design of Experiments (DoE) approach to evaluate how formulation excipients impact and control the thermal unfolding profile for tPA, and found that the relative stability of each of the tPA domains was dependent on the formulation. Therefore, the optimisation of formulations for complex multi-domain proteins such as tPA may need to be multi-objective, with careful selection of the desired attributes that improve stability. As aggregation rates (ln v) correlated well to T m (R 2 = 0.77), ΔS vh (R 2 = 0.71), but not T agg (R 2 = 0.01), we analysed how formulation excipients and pH Page 2 of 45 ACS Paragon Plus Environment Molecular Pharmaceuticswould be able to optimise T m and ΔS vh . Formulation excipient behaviours were found to group according to their combined impact on T m and ΔS vh . The effects of each excipient were often selectively stabilising or destabilising to specific tPA domains, and changed the stability of particular domains relative to the others. The types of mechanism by which this could occur might involve specific interactions with the protein surface, or otherwise effects that are mediated via the solvent as a result of the different surface hydrophobicities and polarities of each domain.