Water Distribution and Clustering on the Lyophilized IgG1 Surface: Insight from Molecular Dynamics Simulations

Structural Features of the Glassy State and Their Impact on the Solid-State Properties of Organic Molecules in Pharmaceutical Systems

This paper reviews the structure and properties of amorphous active pharmaceutical ingredients (APIs), including small molecules and proteins, in the glassy state (below the glass transition temperature, Tg). Amorphous materials in the neat state and formulated with excipients as miscible amorphous mixtures are included, and the role of absorbed water in affecting glass structure and stability has also been considered. We defined the term "structure" to indicate the way the various molecules in a glass interact with each other and form distinctive molecular arrangements as regions or domains of varying number of molecules, molecular packing, and density. Evidence is presented to suggest that such systems generally exist as heterogeneous structures made up of high-density domains surrounded by a lower density arrangement of molecules, termed the microstructure. It has been shown that the method of preparation and the time frame for handling and storage can give rise to variable glass structures and varying physical properties. Throughout this paper, examples are given of theoretical, computer simulation, and experimental studies which focus on the nature of intermolecular interactions, the size of heterogeneous higher density domains, and the impact of such systems on the relative physical and chemical stability of pharmaceutical systems.

Mechanistic Insights into the Adsorption of Monoclonal Antibodies at the Water/Vapor Interface

Monoclonal antibodies (mAbs) are active components of therapeutic formulations that interact with the water-vapor interface during manufacturing, storage, and administration. Surface adsorption has been demonstrated to mediate antibody aggregation, which leads to a loss of therapeutic efficacy. Controlling mAb adsorption at interfaces requires a deep understanding of the microscopic processes that lead to adsorption and identification of the protein regions that drive mAb surface activity. Here, we report all-atom molecular dynamics (MD) simulations of the adsorption behavior of a full IgG1-type antibody at the water/vapor interface. We demonstrate that small local changes in the protein structure play a crucial role in promoting adsorption. Also, interfacial adsorption triggers structural changes in the antibody, potentially contributing to the further enhancement of surface activity. Moreover, we identify key amino acid sequences that determine the adsorption of antibodies at the water-air interface and outline strategies to control the surface activity of these important therapeutic proteins.

Molecular Dynamics Simulation of an Iron(III) Binding Site on the Fc Domain of IgG1 Relevant for Visible Light-Induced Protein Fragmentation

Molecular dynamics simulations were employed to investigate the interaction between Fe(III) and an iron-binding site composed of THR259, ASP252, and GLU261 on the Fc domain of an IgG1. The goal was to provide microscopic mechanistic information for the photochemical, iron-dependent site-specific oxidative fragmentation of IgG1 at THR259 reported in our previous paper. The distance between Fe(III) and residues of interest as well as the binding pocket size was examined for both protonated and deprotonated THR259. The Fe(III) binding free energy (ΔG) was estimated by using an umbrella sampling approach. The pKa shift of the THR259 hydroxyl group caused by the presence of nearby Fe(III) was estimated based on a thermodynamic cycle. The simulation results show that Fe(III) resides inside the proposed binding pocket and profoundly changes the pocket configuration. The ΔG values indicate that the pocket possesses a strong binding affinity for Fe(III). Furthermore, Fe(III) profoundly lowers the pKa value of the THR259 hydroxyl group by 5.4 pKa units.

Adjustment of specific residual moisture levels in completely freeze-dried protein formulations by controlled spiking of small water volumes

The residual moisture (RM) level strongly impacts the stability of freeze-dried biopharmaceuticals. On the one hand, the RM should not be too high to keep the reaction potential of water molecules low and to avoid a decrease in the glass transition temperature through the plasticizing effect of water. On the other hand, overdrying has been described to negatively impact protein stability. Consequently, an optimal RM has to be established and justified for approval by authorities. Therefore, end products with different RM are analyzed over storage. The different RM levels are typically obtained by slightly varying the lyophilization process itself; but deviations from the original process are critical. Additionally, samples can be taken during lyophilization runs, e.g. at the end of primary drying or after the ramp into secondary drying. This, however, does not allow for good control over the RM. Here we present the adaption of a headspace water spiking technique using a microliter syringe to precisely increase and adjust the RM. This technique is suitable to reproducibly introduce water in the microliter scale to achieve RM of less than 1% [w/w] with high precision depending on the dry cake weight. In addition, we show that by using different fill volumes and cake densities, an equal three-dimensional water distribution throughout the whole cake can be achieved. Furthermore, potential limitations and risks in manual RM introduction are discussed.