PEG-Protein Interactions for Stable Formulations Studied by CG-MALS

Importance of the Topic

High‐concentration protein formulations often suffer from non‐specific self‐association that undermines solubility, stability and viscosity control in therapeutic products. Excipients that preferentially bind proteins can act as shields to reduce protein–protein attraction. Polyethylene glycol (PEG) is widely used as a model excipient to modulate protein interactions and improve colloidal stability in biopharmaceutical formulations.

Study Objectives and Overview

This study investigates the ability of 8 kDa PEG to influence self‐ and hetero‐association of hen egg white lysozyme at formulation‐relevant concentrations (up to 40 mg/mL) in phosphate‐buffered saline. Composition‐Gradient Multi‐Angle Static Light Scattering (CG‐MALS) is employed to quantify virial coefficients and binding affinities, providing insight into the mechanisms stabilizing or destabilizing protein monomers and oligomers.

Methodology and Instrumentation

The analysis combines classical osmotic virial coefficient theory (A2, A11) with a pseudo‐specific binding model to resolve contributions from excluded volume, repulsive forces and specific attractive interactions. Key steps include:

• Preparation of concentration gradients of lysozyme and PEG for CG‐MALS.
• Measurement of light scattering and refractive index to determine molecular weights, interaction parameters and complex stoichiometry.
• Global fitting of binding models to extract equilibrium dissociation constants (KD) for self‐ and hetero‐associations.

Instrumentation Used

• Wyatt DAWN HELEOS multi‐angle static light scattering detector
• Wyatt Calypso II composition‐gradient system with software
• Optilab T-rEX differential refractive index detector
• Wyatt Möbiuζ electrophoretic light scattering analyzer (for effective charge Z* measurements)

Key Findings and Discussion

The lysozyme–lysozyme interaction is characterized by reversible homodimer formation with KD ≈ 4.4 mM, moderated by steric repulsion (positive A2). PEG exhibits net repulsive self‐interaction consistent with excluded volume predictions.

• Cross‐virial coefficient A11 = +1.6 × 10⁻⁴ mol·mL/g² indicates net attractive PEG–lysozyme interactions, below the excluded volume reference.
• PEG does not bind appreciably to monomeric lysozyme but forms weak 2:2 complexes with dimers (KD ≈ 1 mM), suggesting both hydrophobic and electrostatic contributions.
• At PEG concentrations below ~25 mg/mL, dimer stabilization increases; higher PEG levels induce rapid hetero‐aggregation driven by multivalent binding and electrostatic attraction.

Benefits and Practical Applications

The quantitative mapping of protein–excipient interactions enables rational selection and optimization of formulation conditions. Key applications include:

• Design of stable high‐concentration antibody and protein drug formulations
• Buffer and excipient screening to minimize aggregation and viscosity
• Prediction of crowding effects and enhancement of solubility for bioprocessing

Future Trends and Potential Applications

Emerging directions leverage CG‐MALS for high‐throughput excipient screening, multi‐protein and bispecific therapeutic characterization, and dynamic monitoring of formulation stability. Integration with advanced modeling and machine learning may further accelerate formulation design.

Conclusion

CG‐MALS provides a robust, label‐free approach to dissect self‐ and hetero‐interactions in concentrated protein solutions. The study demonstrates that PEG can both stabilize lysozyme dimers and, at higher concentrations, promote aggregation. Quantitative interaction parameters guide formulation strategies to enhance colloidal stability in biopharmaceutical development.

References

• Furness, J. et al. Biomaterials, 1998, 15, 1361–1369.
• Rawat, R. et al. Biochemical and Biophysical Research Communications, 2010.