Heike Bunjes

Physicochemical characterization of lipid nanoparticles and evaluation of their drug loading capacity and sustained release potential

Abstract Drug carrier systems based on lipid nanosuspensions prepared by melt emulsification present a number of severe stability problems such as a high gelation tendency, considerable particle growth and drug expulsion. Destabilization of the emulsified lipidic carriers is related to recrystallization of the lipids. The choice of stabilizers for colloidal lipid suspensions is, therefore, restricted. Systematic surface modifications are thus limited. In addition, the drug payload of crystalline nanosuspension particles is generally low. Improved stability and loading capacities were found for amorphous lipid nanoparticles which present the characteristic signals of supercooled melts in high resolution 1 H-NMR. The NMR data indicate that such liquid but viscous carriers can, however, not immobilize the incorporated drug molecules to the same extent as a solid matrix. Sustained release over days or weeks as in slowly biodegraded solid matrices thus seems difficult to achieve with a supercooled melt. Attempts to combine the advantages of the solid crystalline lipids and the amorphous nature of the supercooled melts by generating solid but amorphous lipid suspension particles with a satisfactory long-term stability by a variation of the lipid matrix material have hitherto not been successful. Even a satisfactory stabilization of the α-modification using complex lipid mixtures to improve the loading capacity or to slow down the drug expulsion process could not be achieved. The rates of the polymorphic transitions were much higher in the colloidal lipid dispersions than in the bulk for the hard fats under investigation. Despite the fact that the properties of the lipids are superimposed with colloidal properties, significant differences between monoacid triglycerides and complex lipids were, however, found.

Crystallization tendency and polymorphic transitions in triglyceride nanoparticles

The ability of tristearin, tripalmitin, trimyristin and trilaurin to form solid lipid nanoparticles after melt-homogenization is investigated by DSC and X-ray diffraction. Upon storage at common temperatures after preparation solid nanoparticles are formed in tristearin and tripalmitin dispersions. In contrast to literature reports, colloidal dispersions of trilaurin do not form solid particles under those conditions. They should, therefore, be regarded as emulsions of supercooled melts rather than as nanosuspensions. Trimyristin nanoparticles which can be obtained in solid or liquid form have a larger incorporation capacity for the lipophilic model drug menadione in the liquid than in the solid state. The kinetics of polymorphic transitions after crystallization of triglyceride nanoparticles are slower for longer-chain than for shorter-chain triglycerides. Addition of tristearin raises the crystallization temperature of colloidally dispersed trimyristin and trilaurin facilitating solidification during production. The structure and melting behavior of the resulting mixed nanoparticles are more complex than those of nanoparticles prepared from the simple triglycerides. Depending on the mixing ratio, the time-course of polymorphic transitions after crystallization may also be altered significantly. The melting enthalpy of the mixed nanoparticle dispersions is usually not significantly different from that of dispersions of the simple triglycerides.

Lipid nanoparticles for the delivery of poorly water‐soluble drugs

Objectives  This review discusses important aspects of lipid nanoparticles such as colloidal lipid emulsions and, in particular, solid lipid nanoparticles as carrier systems for poorly water‐soluble drugs, with a main focus on the parenteral and peroral use of these carriers.

Cryogenic transmission electron microscopy (cryo-TEM) for studying the morphology of colloidal drug delivery systems.

Cryogenic transmission electron microscopy (cryo-TEM) has evolved into an indispensable tool for the characterization of colloidal drug delivery systems. It can be applied to study the size, shape and internal structure of nanoparticulate carrier systems as well as the overall colloidal composition of the corresponding dispersions. This review gives a short overview over the instrumentation used in cryo-TEM experiments and over the sample preparation procedure. Selected examples of cryo-TEM studies on colloidal drug carrier systems, including liposomes, colloidal lipid emulsions, solid lipid nanoparticles, thermotropic and lyotropic liquid crystalline nanoparticles, polymer-based colloids and delivery systems for nucleic acids, are presented in order to illustrate the wealth of information that can be obtained by this technique.

Do nanoparticles prepared from lipids solid at room temperature always possess a solid lipid matrix

Abstract Nanoparticles prepared by melt-emulsification from lipids solid at room temperature do not always represent suspension particles. Dispersed trilaurin forms metastable supercooled melts at room and refrigerator temperature. Such systems can be considered as o/w emulsions but not as dispersions of solid lipid nanoparticles.

Incorporation of the Model Drug Ubidecarenone into Solid Lipid Nanoparticles

AbstractPurpose. The impact of drug incorporation on melt-homogenized tripalmitin nanoparticles is investigated with ubidecarenone as a model drug. The dispersions are studied with respect to their drug loading capacity, localization and physical state of the drug as well as to potential changes of the nanoparticle properties due to interactions between drug and triglyceride matrix. Methods. The investigations were carried out using photon correlation spectroscopy, differential scanning calorimetry, synchrotron radiation X-ray diffraction, ultracentrifugation, and cryo- and freeze-fracture transmission electron microscopy. Results. Ubidecarenone can be incorporated into the dispersions in concentrations higher than 50% of the dispersed phase. The drug is associated with the nanoparticles such that small drug amounts are bound tightly to the carrier matrix while excess drug adheres as a liquid phase to the crystalline particles. Drug incorporation lowers the crystallization and melting temperature of the particle matrix and accelerates the transition of the triglyceride into the stable β-polymorph after crystallization. Conclusions. Drug incorporation may significantly alter important physicochemical parameters of solid lipid nanoparticles. Slow release of ubidecarenone may only be possible for the fraction of drug which is tightly bound to the matrix while the liquid fraction should be rapidly released.

Supercooled Smectic Nanoparticles: A Potential Novel Carrier System for Poorly Water Soluble Drugs

AbstractPurpose. The possibility of preparing nanoparticles in the supercooled thermotropic liquid crystalline state from cholesterol esters with saturated acyl chains as well as the incorporation of model drugs into the dispersions was investigated using cholesteryl myristate (CM) as a model cholesterol ester. Methods. Nanoparticles were prepared by high-pressure melt homogenization or solvent evaporation using phospholipids, phospholipid/bile salt, or polyvinyl alcohol as emulsifiers. The physicochemical state and phase behavior of the particles was characterized by particle size measurements (photon correlation spectroscopy, laser diffraction with polarization intensity differential scattering), differential scanning calorimetry, X-ray diffraction, and electron and polarizing light microscopy. The viscosity of the isotropic and liquid crystalline phases of CM in the bulk was investigated in dependence on temperature and shear rate by rotational viscometry. Results. CM nanoparticles can be obtained in the smectic phase and retained in this state for at least 12 months when stored at 23°C in optimized systems. The recrystallization tendency of CM in the dispersions strongly depends on the stabilizer system and the particle size. Stable drug-loaded smectic nanoparticles were obtained after incorporation of 10% (related to CM) ibuprofen, miconazole, etomidate, and 1% progesterone. Conclusions. Due to their liquid crystalline state, colloidal smectic nanoparticles offer interesting possibilities as carrier system for lipophilic drugs. CM nanoparticles are suitable model systems for studying the crystallization behavior and investigating the influence of various parameters for the development of smectic nanoparticles resistant against recrystallization upon storage.

Encapsulation of proteins in hydrogel carrier systems for controlled drug delivery influence of network structure and drug size on release rate

Novel hydrogels based on hydroxyethyl starch modified with polyethylene glycol methacrylate (HES-P(EG)₆MA) were developed as delivery system for the controlled release of proteins. Since the drug release behavior is supposed to be related to the pore structure of the hydrogel network the pore sizes were determined by cryo-SEM, which is a mild technique for imaging on a nanometer scale. The results showed a decreasing pore size and an increase in pore homogeneity with increasing polymer concentration. Furthermore, the mesh sizes of the hydrogels were calculated based on swelling data. Pore and mesh size were significantly different which indicates that both structures are present in the hydrogel. The resulting structural model was correlated with release data for bulk hydrogel cylinders loaded with FITC-dextran and hydrogel microspheres loaded with FITC-IgG and FITC-dextran of different molecular size. The initial release depended much on the relation between hydrodynamic diameter and pore size while the long term release of the incorporated substances was predominantly controlled by degradation of the network of the much smaller meshes.

Poly(vinyl alcohol) as emulsifier stabilizes solid triglyceride drug carrier nanoparticles in the alpha-modification.

Colloidal dispersions of solid lipids are under intensive investigation as drug delivery systems. In the present study, poly(vinyl alcohol) (PVA) was tested as an alternative stabilizer for triglyceride nanoparticles. The dispersions contained 10% triglyceride (trimyristin or tristearin) and 5% PVA and were prepared by high pressure melt homogenization. The nanoparticle dispersions were investigated for their thermal behavior and storage stability with special regard to the polymorphic transitions of the triglyceride matrix, including effects of storage temperature and the incorporation of model drugs (diazepam, ubidecarenone) using photon correlation spectroscopy, differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. The release of the model drug diazepam from a selected nanoparticle dispersion was investigated with differential pulse polarography. Triglyceride nanoparticles prepared with PVA displayed an unusually high stability of the metastable alpha-modification depending on the type of triglyceride and the storage conditions. In tristearin nanoparticles, the alpha-polymorph was stable for at least 9 months at refrigerator temperature and the particles exhibited a spherical shape in electron microscopic investigations. Moreover, the alpha-form in PVA-stabilized tristearin nanoparticles seemed to be highly disordered, as it did not lead to a pronounced small-angle X-ray reflection. Storage at higher temperatures led to a transformation of the particles into the beta-modification, which usually was accompanied by an increase in particle size. Incorporation of the two model drugs did not change the crystal modification of the particle matrix to a large extent. After dilution into a large volume of release medium, a large fraction of the model drug diazepam was released immediately but there was no further release over several hours. The high stability of PVA-stabilized tristearin nanoparticles with regard to particle size and alpha-modification makes them suitable as a model for investigations on the influence of the polymorphic form (e.g., in comparison with nanoparticles in the more stable beta-modification) on pharmaceutically important parameters such as drug load and drug release.

Flow cytometry as a new approach to investigate drug transfer between lipid particles.

Lipid nanoparticles and liposomal carrier systems are of growing interest for intravenous drug delivery due to their biocompatibility and targetability. It is, however, difficult to investigate their release behavior for lipophilic drugs under physiological conditions. This study describes a novel flow cytometric method studying drug transfer from such carrier systems to particles simulating physiological receptor sites. For this purpose, liquid and solid trimyristin nanoparticles or soybean phospholipid liposomes were loaded with the lipophilic fluorescent substances Nile red, temoporfin, and DiI. The transfer of these model drugs to large emulsion droplets was examined by flow cytometry. Transfer of DiI to differently sized acceptor emulsions was also monitored by separating donor and acceptor particles using ultracentrifugation. Flow cytometry revealed a completion of transfer within a few minutes for Nile red and temoporfin at considerable amounts of transferred dye. In contrast, the highly lipophilic DiI transferred over a period of weeks only for a small percentage of the dye. Ultracentrifugation results confirmed this for DiI and indicated a dependence of transfer characteristics on the acceptor surface area. Nile red transfer into a bulk oil phase as alternative acceptor system was also very slow. Flow cytometry seems to be well suited to study the intrinsic transfer of fluorescent lipophilic substances, as no kinetic hindrances like dialysis bags nor separation steps are required. Additional detailed experiments will, however, be necessary to elucidate the prevalent transfer mechanisms completely.