Investigating the effects of chelating agents, surfactants and magnesium cations on the size of LPS aggregates in formulations causing low endotoxin recovery in limulus amebocyte lysate assays

To ensure safety, pharmaceuticals are rigorously tested for lipopolysaccharide (LPS) contamination, as this can trigger severe immune reactions in patients. Low Endotoxin Recovery (LER), describing the masking of spiked LPS controls in Limulus Amebocyte Lysate (LAL) assays, has been associated with the presence of chelating agents and surfactants in pharmaceutical formulations. The addition of excipients, such as Mg²⁺, have shown the ability to mitigate the effects of LER, however, inconsistencies in various studies regarding the influence of the excipients on LPS aggregate characteristics and LER occurrence hinder a clear understanding of the mechanisms underlying LER. In this study, dynamic light scattering (DLS) and small-angle X-ray scattering (SAXS) were employed to systematically assess the impact of chelating agents, surfactants, and divalent cations on the size and shape of LPS aggregates across various formulations. Our results indicate that surfactant-only formulations generally reduce LPS aggregate size, whereas chelating agent-only formulations do not. Notably, the smallest aggregates were observed when both chelating agents and surfactants were present, with the extent of size reduction being specific to the particular excipients used. Additionally, Mg²⁺ generally inhibited the excipients' capacity to decrease aggregate size, most effectively in phosphate-containing samples. Despite these variations in size, the overall aggregate shape remained largely unchanged in all formulations. These findings suggest that LPS aggregate size or shape does not distinguish formulations causing LER; instead, factors such as the characteristics of the LPS aggregate surface in different formulations should be explored in the future.