Purpose: Nanoparticles are a promising technology to improve cancer diagnosis and therapy as compared to conventional chemotherapy and diagnostic methods. As a result, there are increasing preclinical research efforts to develop new nanomedicines and further improve their delivery and efficacy. We already know that nanoparticle formulation and development involves thorough characterization of properties such as size and size distribution, charge surface functionalization, pH sensitivity, etc. In addition, nanoparticles, due to the high curvature, are very surface active. Studying how nanoparticle physicochemical properties affect its surface properties may be helpful in understanding how they are transported in the body towards their target cells. In this work, we developed a method to study how physicochemical attributes affect surface properties of liposomes. We hypothesize that the physicochemical properties like particle average size, size distribution and PEGylation will have a measurable effect on their surface properties. This work also establishes a building block in the long-term goal of obtaining insight into the translation of nanoparticles from animal (preclinical) to human (clinical) stages in the drug development process.
Two liposome formulations of different particle size and consisting of phosphatidyl choline (PC) were formulated in-house using the thin film hydration method followed by extrusion for size reduction. Research-grade liposomes that mimic the clinical product Doxil (DoxomTM, Liposomics) were also used. Average particle size and polydispersity index (PDI) of the liposomal suspensions was obtained using dynamic light scattering (Mobius-122, Wyatt Technology). A sustom-made apparatus, 3D printed in-house (Figure 1), combined with Fisherbrand Microhematocrit capillary tubes was used to measure the surface tension of liposomal suspensions using the capillary-rise method. The method was validated using water surface tension measurements obtained prior to each experiment as reference. Liposomal suspensions at different lipid concentrations (0.1-4mg/ml) were measured in order to study the effect of concentration on surface tension. All measurements were performed in triplicates. Mean and standard error of surface tension measurements for all the formulations was plotted against different lipid concentrations.
The average particle sizes and PDI for the two PC formulations were 113nm, PDI 0.05 and 132nm, PDI 0.07, respectively. The surface tension measurements against different concentration values for the PC formulations and the research grade (PEGylated liposomes with encapsulated doxorubicin) are presented in Figures 2 and 3, respectively.
It was observed that minor differences in liposome size (difference of 20nm in average size and 0.02 in PDI) had a significant and measurable effect on the surface tension of the suspensions, with higher values observed for the larger liposomes. These formulations also showed a critical micelle concentration-type effect (CMC) with a dip in surface tension at certain (and different) lipid concentrations; however, unlike the typical CMC curves observed in small molecular weight surfactants, the dip was delayed in PC liposomes. This effect was not observed in the PEGylated liposomes, that acted more like traditional surfactants.
The physicochemical properties of liposomes (size, size distribution and PEGylation) were shown to have a significant and measurable effect on their surface properties. Future studies will include studies on the effect on surface tension from other physicochemical properties of liposomal suspensions such as surface charge, composition, and more thorough size distribution variations to mimic typical batch-to-batch variability.