Use of X-ray scattering to aid the design and delivery of membrane-active drugs

Small-Angle X-ray Scattering for PEGylated Liposomal Doxorubicin Drugs: An Analytical Model Comparison Study

Liposomal delivery systems are recognized as efficient and safe platforms for chemotherapeutic agents, with doxorubicin-loaded liposomes being the most representative nanopharmaceuticals. Characterizing the structure of liposomal nanomedicines in high spatial and temporal resolution is critical to analyze and evaluate their stability and efficacy. Small-angle X-ray scattering (SAXS) is a powerful tool increasingly used to investigate liposomal delivery systems. In this study, we chose a Doxil-like PEGylated liposomal doxorubicin (PLD) as an example and characterized the liposomal drug structure using synchrotron SAXS. Classical analytical models, including the spherical-shell or flat-slab geometries with Gaussian or uniform electron density profiles, were used to model the internal structure of the liposomal membrane. A cylinder model was applied to fit the scattering from the drug crystal loaded in the liposomes. The high-resolution structures of the original drug, Caelyx, and a similar research drug prepared in our laboratory were characterized using these analytical models. The structural parameters of PLDs, including the thickness of the liposomal membrane and morphology of the drug crystal, were further compared. The results demonstrated that both spherical-shell and flat-slab geometries with Gaussian electron density distribution were suitable to elucidate the structural features of the liposomal membrane under a certain range of scattering vectors, while models with uniform electron density distribution exhibited poor fitting performance. This study highlights the technical features of SAXS, which provides structural information at the nanoscale for liposomal drugs. The demonstrated methods are reliable and easy-to-use for the structural analysis of liposomal drugs, which are helpful for a broader application of SAXS in the production and regulation of nanopharmaceuticals.

What can we learn about amphiphile-membrane interaction from model lipid membranes?

Surface-active amphiphiles find applications in a wide range of areas of industry such as agrochemicals, personal care, and pharmaceuticals. In many of these applications, interaction with cell membranes is a key factor for achieving their purpose. How do amphiphiles interact with lipid membranes? What are their bases for membrane specificity? Which biophysical properties of membranes are susceptible to modulation by amphiphilic membrane-effectors? What aspects of this interaction are important for performing their function? In our work on membrane biophysics over the years, questions like these have arisen and we now share some of our findings and discuss them in this review. This topic was approached focusing on the membrane properties and their alterations rather than on the amphiphile structure requirements for their interaction. Here, we do not aim to provide a comprehensive list of the modes of action of amphiphiles of biological interest but to help in understanding them.

Mapping Fusogenicity of Ciprofloxacin-Loaded Liposomes with Bacterial Cells

The process of liposome fusion with cellular membrane plays key role in delivering encapsulated drug molecule into the cell. This process becomes very important for molecules having low permeability as they fail to reach the site of action located inside the cell. Ciprofloxacin (CIP), a broad-spectrum BCS class IV antibiotic, has poor permeability. In the present work, CIP-loaded liposomes were prepared using solvent evaporation method and optimized by 3² factorial design approach. The optimized batch of CIP-loaded liposomes was characterized for size, entrapment efficiency, zeta potential, FTIR, and microbial susceptibility study on Staphylococcus aureus (gram-positive bacteria) and Escherichia coli (gram-negative bacteria). Confocal microscopy was used to study the fusogenicity process of CIP-loaded liposomes with bacterial cells. Additionally, the kinetics of fusogenicity process was studied using SAXS for the first time. Surprisingly, the rate of fusion of CIP-loaded liposomes with cell wall of S. aureus was twice when compared to the cell wall of E. coli. It is believed that the current work can act as a roadmap in selection of proper excipients while developing formulations which would expedite the fusogenicity and may execute pharmacological activity of poorly penetrable drug molecules at lower dose.

The efficacy of trivalent cyclic hexapeptides to induce lipid clustering in PG/PE membranes correlates with their antimicrobial activity

Various models have been proposed for the sequence of events occurring after binding of specific antimicrobial peptides to lipid membranes. The lipid clustering model arose by the finding that antimicrobial peptides can induce a segregation of certain negatively charged lipids in lipid model membranes. Anionic lipid segregation by cationic peptides is initially an effect of charge interaction where the ratio of peptide and lipid charges is thought to be the decisive parameter in the peptide induced lipid demixing. However, the sequence of events following this initial lipid clustering is more complex and can lead to deactivation of membrane proteins involved in cell division or perturbation of lipid reorganization essential for cell division. In this study we used DSC and ITC techniques to investigate the effect of binding different cyclic hexapeptides with varying antimicrobial efficacy, to phosphatidylglycerol (PG)/phosphatidylethanolamine (PE) lipid membranes and their ability to induce lipid segregation in these mixtures. We found that these cyclic hexapeptides consisting of three charged and three aromatic amino acids showed indeed different abilities to induce lipid demixing depending on their amino acid composition and their sequence. The results clearly showed that the cationic amino acids are essential for electrostatic binding but that the three hydrophobic amino acids in the peptides and their position in the sequence also contribute to binding affinity and to the extent of induction of lipid clustering. The efficacy of these different hexapeptides to induce PG clusters in PG/PE membranes was found to be correlated with their antimicrobial activity.

Global small-angle X-ray scattering data analysis for multilamellar vesicles: the evolution of the scattering density profile model

The highly successful scattering density profile (SDP) model, used to jointly analyze small-angle X-ray and neutron scattering data from unilamellar vesicles, has been adapted for use with data from fully hydrated, liquid crystalline multilamellar vesicles (MLVs). Using a genetic algorithm, this new method is capable of providing high-resolution structural information, as well as determining bilayer elastic bending fluctuations from standalone X-ray data. Structural parameters such as bilayer thickness and area per lipid were determined for a series of saturated and unsaturated lipids, as well as binary mixtures with cholesterol. The results are in good agreement with previously reported SDP data, which used both neutron and X-ray data. The inclusion of deuterated and non-deuterated MLV neutron data in the analysis improved the lipid backbone information but did not improve, within experimental error, the structural data regarding bilayer thickness and area per lipid.