Heat Treatment of Poloxamer-Stabilized Triglyceride Nanodispersions: Effects and Underlying Mechanism.

Lipid nanoemulsions are being investigated for the parenteral administration of poorly soluble drugs. A narrow particle size distribution in these formulations is a prerequisite for meaningful research and safe administration to patients. Autoclaving a poloxamer-stabilized trimyristin nanoemulsion resulted in moderate particle growth and a strong decrease in particle size distribution width ( Göke , K. ; Roese , E. ; Arnold , A. ; Kuntsche , J. ; Bunjes , H. Mol. Pharmaceutics 2016 , 13 , 3187 . ). In this work, the critical parameters for such a change upon autoclaving poloxamer 188-stabilized lipid nanodispersions were investigated to elucidate the underlying mechanism. Nanodispersions of triglycerides with esterified fatty acid chain lengths from C8 to C18 were treated at different temperatures and for varying durations. The influence of a decrease in poloxamer 188's cloud point was tested by adding potassium chloride to the dispersions prior to autoclaving. The influence of poloxamer 188 concentration and of the type of emulsifier was investigated. The change in particle size and particle size distribution width upon heat treatment was analyzed by dynamic or static light scattering or differential scanning calorimetry. A short esterified fatty acid chain length of the triglycerides, high temperatures, and the addition of potassium chloride were key factors for particle growth up to emulsion break up, whereas the cloud point of poloxamer 188 was irrelevant. Sodium dodecyl sulfate and sucrose laurate had negative effects on emulsion stability during autoclaving. It was concluded that the increase in particle size and the decrease in particle size distribution widths upon heat treatment resulted from heat-accelerated Ostwald ripening and not from a coalescence-based process.

Drug release studies from lipid nanoparticles in physiological media by a new DSC method.

Lipid nanoparticles are an interesting parenteral delivery system for poorly water-soluble drugs. In order to approach physiological conditions when conducting release studies from such systems the release media should preferentially contain lipophilic acceptor compartments such as lipoproteins or other colloidal lipophilic components. In practice, drug release studies under such close to physiological conditions may be complicated by the small size of lipid nanoparticles, which is in the same range as that of the potential acceptor particles. This study describes a novel differential scanning calorimetry (DSC) method for drug release measurements which works without separation of donor and acceptor particles. The technique is based on measuring the crystallization temperature of trimyristin nanoparticles by DSC. The crystallization temperature of the nanoparticles decreases proportionally with the amount of active ingredient incorporated and thus increases as a result of drug release. Liquid trimyristin nanoparticles loaded with fenofibrate, orlistat, tocopherol acetate and ubidecarenone were studied in three different release media with increasing complexity and comparability to physiological conditions: a rapeseed oil nanoemulsion, porcine serum and porcine blood. Using the new method, a correlation between release behavior and drug lipophilicity was observed: the higher the logP value of the drug, the slower the release. The extent of drug release was influenced by partition equilibrium as indicated by increased drug release in the rapeseed oil nanoemulsion compared to porcine serum and blood.

Control over Particle Size Distribution by Autoclaving Poloxamer-Stabilized Trimyristin Nanodispersions.

Lipid nanoparticles are under investigation as delivery systems for poorly water-soluble drugs. The particle size in these dispersions strongly influences important pharmaceutical properties like biodistribution and drug loading capacity; it should be below 500 nm for direct injection into the bloodstream. Consequently, small particles with a narrow particle size distribution are desired. Hitherto, there are, however, only limited possibilities for the preparation of monodisperse, pharmaceutically relevant dispersions. In this work, the effect of autoclaving at 121 °C on the particle size distribution of lipid nanoemulsions and -suspensions consisting of the pharmaceutically relevant components trimyristin and poloxamer 188 was studied. Additionally, the amount of emulsifier needed to stabilize both untreated and autoclaved particles was assessed. In our study, four dispersions of mean particle sizes from 45 to 150 nm were prepared by high-pressure melt homogenization. The particle size distribution before and after autoclaving was characterized using static and dynamic light scattering, differential scanning calorimetry, and transmission electron microscopy. Asymmetrical flow field-flow fractionation was used for particle size distribution analyses and for the determination of free poloxamer 188. Upon autoclaving, the mean particle size increased to up to 200 nm, but not proportionally to the initial size. At the same time, the particle size distribution width decreased remarkably. Heat treatment thus seems to be a promising approach to achieve the desired narrow particle size distribution of such dispersions. Related to the lipid content, suspension particles needed more emulsifier for stabilization than emulsion droplets, and smaller particles more than larger ones.

Molecular models and mutational analyses of plant specifier proteins suggest active site residues and reaction mechanism.

As components of the glucosinolate-myrosinase system, specifier proteins contribute to the diversity of chemical defenses that have evolved in plants of the Brassicales order as a protection against herbivores and pathogens. Glucosinolates are thioglucosides that are stored separately from their hydrolytic enzymes, myrosinases, in plant tissue. Upon tissue disruption, glucosinolates are hydrolyzed by myrosinases yielding instable aglucones that rearrange to form defensive isothiocyanates. In the presence of specifier proteins, other products, namely simple nitriles, epithionitriles and organic thiocyanates, can be formed instead of isothiocyanates depending on the glucosinolate side chain structure and the type of specifier protein. The biochemical role of specifier proteins is largely unresolved. We have used two thiocyanate-forming proteins and one epithiospecifier protein with different substrate/product specificities to develop molecular models that, in conjunction with mutational analyses, allow us to propose an active site and docking arrangements with glucosinolate aglucones that may explain some of the differences in specifier protein specificities. Furthermore, quantum-mechanical calculations support a reaction mechanism for benzylthiocyanate formation including a catalytic role of the TFP involved. These results may serve as a basis for further theoretical and experimental investigations of the mechanisms of glucosinolate breakdown that will also help to better understand the evolution of specifier proteins from ancestral proteins with functions outside glucosinolate metabolism.