Impact of mono- and poly-ester fractions on polysorbate quantitation using mixed-mode HPLC-CAD/ELSD and the fluorescence micelle assay

Determination of excipient content in drug formulation is an important aspect of pharmaceutical formulation development and for analytical testing of the formulation. In this study, the influence of polysorbate subspecies, in particular mono- and poly-esters, for determining polysorbate (PS) content were investigated by comparing three of the most widely used PS quantitation approaches, the Fluorescence Micelle Assay (FMA) and Mixed-Mode High Performance Liquid Chromatography coupled with Charged Aerosol Detection (MM-CAD) or Evaporative Light Scattering Detection (MM-ELSD). FMA and MM-CAD were employed to investigate the quantitation behavior of PS20 and PS80 subspecies and corresponding degradation products in placebo formulations using forced degradation conditions at 40°C for up to 12 weeks. While both methods allowed accurate and comparable quantification of neat PS at the beginning of stress studies, pronounced differences in content determination between the methods were observed at later time points, which were attributable to substantial differences in the contribution of individual mono- and poly-esters to the overall quantitation results. It was particularly surprising to find that the main component of PS20, polyoxyethylene sorbitan monolaurate, did not show a signal at the studied concentration using FMA. Moreover, the degradation of polysorbate poly-esters, was reflected much stronger in FMA than MM-CAD results. Additional experiments employing chemical oxidation and base hydrolysis to degrade PS20, quantified by FMA and MM-ELSD, also show preferential reduction in certain subspecies depending on the degradation pathway involved. For PS20 degraded by chemical oxidation, quantitation results were lower for FMA than MM-ELSD, while the opposite trend was observed with base hydrolysis.