Thiol-disulfide exchange in peptides derived from human growth hormone during lyophilization and storage in the solid state.

Lyophilization (freeze-drying) is frequently used to stabilize protein therapeutics. However, covalent modifications such as thiol-disulfide exchange and disulfide scrambling can occur even in the solid state. The effects of lyophilization and storage of lyophilized powders on the mechanism and kinetics of thiol-disulfide exchange have not been elucidated and are explored here. Reaction kinetics was monitored in peptides corresponding to tryptic fragments of human growth hormone (T20 + T20-T21 or T20 + cT20-T21) during different stages of lyophilization and during storage of the lyophilized powders at 22°C and ambient RH. The concentrations of reactants and products were determined using RP-HPLC and product identity confirmed using liquid chromatography-mass spectrometry. Loss of native disulfide was observed for the reaction of T20 with both linear (T20-T21) and cyclic (cT20-T21) peptides during the primary drying step; however, the native disulfides were regenerated during secondary drying with no further change till the end of lyophilization. Deviations from Arrhenius parameters predicted from solution studies and the absence of buffer effects during lyophilization suggest that factors such as temperature, initial peptide concentration, buffer type, and concentration do not influence thiol-disulfide exchange during lyophilization. Results from a "cold finger" method used to study peptide adsorption to ice indicate that there is no preferential adsorption to the ice surface and that its presence may not influence disulfide reactivity during primary drying. Overall, reaction rates and product distribution differ for the reaction of T20 with T20-T21 or cT20-T21 in the solid state and aqueous solution, whereas the mechanism of thiol-disulfide remains unchanged. Increased reactivity of the cyclic peptide in the solid state suggests that peptide cyclization does not offer protection against lyophilization and that damage induced by a process stress further affects storage stability at 22°C and ambient RH.