Abstract
Immunoglobulin G IgG and human serum albumin HSA are measured using a series of static and dynamic light scattering measurements. The results show that both SLS and DLS measurements can be used to study interactions between proteins, as well as simply their molecular weight and hydrodynamic size.
Introduction
Biopharmacueticals are a rapidly growing type of medicine where the active ingredient is a protein rather than a small molecule. The most common biopharmaceuticals are monoclonal antibodies (mAb). One of the key challenges in biopharmaceutical development is the correct formulation of the drug. The Commercial Target Profile for these biotherapeutics generally calls for high concentrations, in order to accommodate a dosage schedule consisting of a single daily user administered subcutaneous injection, as well as an expected shelf life of up to 2 years. A 'good' formulation then, would consist of the biologic at high concentration within a formulation exhibiting low viscosity and a low aggregation propensity [1]. Creating a drug formulation where these goals are met can be very difficult; therefore, advancing our understanding of the relationships and processes that result in high formulation viscosities or aggregation rates is of great importance. Beyond this, the ultimate goal is to be able to predict the sample behaviour at high concentration, based upon measured dilute solution properties early in the development cycle. By doing this, the cost and risk of formulation development of biopharmaceuticals can be significantly reduced. Two parameters that are becoming increasingly studied with respect to predicting biopharmaceutical behaviour in formulation are the second virial coefficient (A2) and the dynamic virial coefficient, or DLS interaction parameter (kD) [2, 3]. A2 is routinely measured using SLS. In a batch (cuvette-based) SLS measurement, the sample is prepared at a series of different concentrations. The light scattering intensity from each sample is measured. The scattering intensities are referenced against a light scattering standard, typically toluene. The results are used to build a Debye plot using the Zimm equation (shown below) where K is a constant, C is the sample concentration, Rθ is the Rayleigh ratio (the ratio of scattered light to incident light), A2 is the second virial coefficient, Mw is the sample molecular weight and Pθ is the angular scattering dependence. For isotropic scatterers such as proteins, Pθ is approximately equal to 1 and can be ignored.In a Debye plot, KC/Rθ is plotted as a function of sample concentration and the sample molecular weight and A2 are then calculated from the y-intercept and slope, respectively. A2 has previously been shown to have a strong correlation with sample solubility. If a sample has a negative A2, then the sample will tend to aggregate and come out of solution over time, whereas with a positive A2, a sample is more likely to stay in solution. A2 has therefore been used as a measure of sample stability; however, there is a growing recognition that this value may also have wider reaching applications related to sample viscosity [3]. The DLS interaction parameter, kD, is measured using DLS and is calculated from the concentration dependence of the measured diffusion coefficient of the sample [4], as indicated in the expression below, where Dm is the mutual (measured) diffusion coefficient, D0 is the self diffusion coefficient (the diffusion coefficient at zero concentration), and C is the sample concentration.
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