Mass Balance Model with Donnan Equilibrium Accurately Describes Unusual pH and Excipient Profiles during Diafiltration of Monoclonal Antibodies

Abstract

There is extensive experimental data showing that the final pH and buffer composition after protein diafiltration (DF), particularly with monoclonal antibodies, can be considerably different than that in the DF buffer due to electrostatic interactions between the charged protein and the charged ions. Previous models for this behavior have focused on the final (equilibrium) partitioning and are unable to explain the complex pH and concentration profiles during the DF process. The objective of this study is to develop a new model for antibody DF based on solution of the transient mass balance equations, with the permeate concentrations of the charged species evaluated assuming Donnan equilibrium across the semipermeable membrane in combination with electroneutrality constraints. Model predictions are in excellent agreement with experimental data obtained during DF of both acidic and basic monoclonal antibodies, with the protein charge determined from independent electrophoretic mobility measurements. The model is able to predict the entire pH/histidine concentration profiles during DF, providing a framework for the development of DF processes that yield the desired antibody formulation.

Stereospecific interactions between histidine and monoclonal antibodies

“…The solid curves in Figure are the predicted pH profiles for diafiltration into the 20 mM solutions of L ‐ and D ‐histidine using the mass balance–Donnan equilibrium model developed by Baek et al (). The model involves no adjustable parameters; the only quantity that is different in the model calculations using l ‐ and d ‐histidine is the antibody charge, which was taken directly from Table .…”

“…A 1 mM phosphate buffer was used as the running buffer with the histidine quantified using an inline refractive index detector (Agilent Technologies, Palo Alto, CA). Additional details are provided by Baek, Singh, Arunkumar, Borwankar, and Zydney ().…”

Mechanistic model of pH and excipient concentration during ultrafiltration and diafiltration processes of therapeutic antibodies

“…3 Frequently, the pH and excipient concentrations in the final UF/DF pool differ significantly from those of the formulation buffer. [4][5][6][7][8] These differences complicate the design process beyond simply specifying a diafiltration buffer with the same pH and excipient concentrations as designed for the final product. Often iterative laboratory experimentation is required to To accurately model the UF/DF process, the interactions between the protein and excipients must be mathematically described.…”

“…While the electrostatic interactions and volume exclusion effects often dominate UF/DF excipient partitioning, it should be noted that additional molecular interactions, such as specific and nonspecific binding, may also influence partitioning for certain excipient types and solution conditions. 4,7,9 The electrostatic interactions in UF/DF systems have been previously modeled using two main theories: Donnan equilibrium 4,7,8 and Poisson-Boltzmann theory. 5,6 Both models account for the partitioning of charged excipients across the UF/DF membrane based on the presence of charged protein on the retentate side of the membrane.…”

“…10 Models applying Donnan equilibrium have been used to predict UF/DF retentate excipient concentrations and pH. 4,7,8 The Poisson-Boltzmann theory uses equations that describe electrostatic interactions at the microscopic scale using mean-field theory. This theory is a combination of the Poisson equation, which describes the charge density of the retentate using the electrical potential of the charged protein molecules, and the Boltzmann distribution equation, which describes the distribution of charged solute molecules as a function of the electrical potential generated by the proteins.…”