Elution behavior of insulin on high-performance size exclusion chromatography at neutral pH

The pharmacopoeia protocol for HP-SEC of insulin, using an acidic non-physiological eluent, does not represent insulin's association state in the formulation. This study aimed to evaluate insulin's elution behavior in HP-SEC in a "physiological" (aqueous, neutral pH) eluent, using on-line UV absorption and multi-angle laser light scattering detection. The effect of insulin concentration and association state in the formulation (monitored by circular dichroism) and eluent composition (zinc ion, arginine) on its elution behavior was assessed. We showed that the elution behavior of insulin in "physiological" HP-SEC is affected by both dynamic association-dissociation of insulin molecules and insulin-column interactions. Insulin molecules re-equilibrated in the HP-SEC eluent, making its elution behavior practically insensitive to the association state of insulin in the formulation. Zinc ions in the eluent promoted association of insulin to hexamers, whereas arginine overruled the effect of zinc ions and induced on-column dissociation of insulin to dimers and monomers. Combined results from "physiological" and compendial HP-SEC were shown to provide a better view of the aggregation state of heat-stressed insulin than either of the single methods. The insights obtained with this study are crucial for a proper evaluation of HP-SEC data of insulin.