[4S06m] 17. Downstream processing - Part 1

11:20 AM - 11:40 AM JST(2:20 AM - 2:40 AM UTC)

[4DSP-04] pH-Sensitive Property of Fatty Acid Drug Molecules and its Effects on Colloidal Stability of Lipid Nanoparticles

○Nozomi Morishita Watanabe¹, Weiyu Li¹, Yukihiro Okamoto¹, Hiroshi Umakoshi¹ (1. The University of Osaka (Japan))

Keywords:

Lipid nanoparticles,Lyotropic non-lamellar liquid crystalline,Glycerol α-monooleate,2-Hydroxyoleic acid,Self-assembly system

Lyotropic non-lamellar liquid crystalline nanodispersions have emerged as sophisticated candidates for next-generation drug delivery systems (DDS). Unlike traditional lamellar liposomes, these self-assembled nanostructures—such as cubic or hexagonal phases—offer a significantly larger internal surface area and the unique ability to encapsulate both hydrophilic and hydrophobic therapeutic agents. Despite their promise, the practical application of these systems is often hindered by complexities regarding their colloidal stability. Their structural integrity is highly sensitive to external environmental stimuli, including ionic strength, temperature, and crucially, pH. Understanding the interplay between internal nanostructure and macroscopic stability is essential for developing reliable, stimuli-responsive carriers.In this study, we investigated the colloidal stability of pH-responsive self-assemblies prepared using a binary mixture of glycerol α-monooleate (MO) and 2-hydroxyoleic acid (2OHOA). The latter is a potent candidate for cancer therapy, known for its ability to regulate membrane lipid composition in malignant cells. 2OHOA features a functional carboxylic acid group that serves as a pH-responsive moiety, dictating the curvature and packing parameter of the lipid assembly. To evaluate the system’s characteristics, we employed dynamic light scattering (DLS) for particle size distribution and visual observation to detect precipitation. Furthermore, we utilized Laurdan fluorescence spectroscopy to determine the Generalized Polarization (GP) values, providing insights into the polarity and hydration levels of the internal lipid environment.Our results revealed a dramatic structural evolution across the pH spectrum. At physiological pH (around pH 7), the system primarily formed multi-lamellar or uni-lamellar vesicles. As the environment became more acidic, specifically approaching pH 3, a transition to hexosomes (inverted hexagonal phase nanodispersions) was observed. This shift is attributed to the protonation of the 2OHOA headgroups, which reduces electrostatic repulsion and alters the effective molecular geometry.The colloidal stability of these particles showed a distinct correlation with the ionization state of 2OHOA. By measuring the zeta potential across various pH levels, we calculated the "apparent pKa" of 2OHOA within the self-assembled matrix. A critical observation was made in the pH range near this pKa value: the system exhibited crystal-like precipitation. The DLS data and GP values indicated that the transition in the polarity of the inner structure and the dehydration of the lipid bilayer lead to increased intermolecular interactions between MO and 2OHOA. Specifically, as the 2OHOA molecules become partially protonated, hydrogen bonding and reduced curvature strain likely promote the formation of highly ordered, crystalline domains, which ultimately destabilize the colloidal suspension.This study demonstrates that the intermolecular properties and the resulting internal nanostructure are the primary determinants of the interaction forces acting on the particles. The transition from vesicles to hexosomes carries the risk of macroscopic instability near the pKa of the responsive lipid. By analyzing the mechanism linking self-assembly behavior to colloidal stability, we provide a fundamental basis for the rational design of functional, pH-sensitive nanocarriers.