Michael Türk

Micronization of pharmaceutical substances by the Rapid Expansion of Supercritical Solutions (RESS): a promising method to improve bioavailability of poorly soluble pharmaceutical agents

A multitude of pharmaceutical substances are often insoluble or only slightly soluble in aqueous media and the application of oral or injectable drugs is often limited by its low bioavailability. A promising method to improve the bioavailability of pharmaceutical agents is the Rapid Expansion of Supercritical Solutions (RESS). The RESS-process enables the micronization of thermally labile materials and the formation of particles of less than 500 nm in diameter. Our current research is aimed towards an improved understanding of the relationship between process parameters and particle characteristics and to explore new areas of application for nanoscale particles. Therefore, experimental investigations and numerical simulations were performed. Measurements were carried out for Benzoic acid, the pharmaceuticals Griseofulvin and -Sitosterol with the solvents CO2 (Carbon dioxide) and CHF3 (Trifluoromethane). These experiments led to particle sizes in the range of 200–500 nm depending on solvent and pre and postexpansion conditions. RESS-modelling is focused on the flow through the nozzle, the supersonic freejet, the Mach shock and particle growth in the expansion unit. From these calculations follows that particles are formed as small as 2–8 nm in the supersonic freejet. Hence, the conditions inside the expansion chamber are one key factor to control particle size. Furthermore, experiments show that the RESS processing of Griseofulvin leads to a significantly better dissolution rate of the drug resulting in an improved bioavailability. Moreover, stable suspensions of nanoscale particles of -Sitosterol were produced by the rapid expansion of a supercritical mixture through a capillary nozzle into aqueous solutions. The particle sizes of -Sitosterol in the aqueous solution were smaller or equal to those produced by RESS into air without the surfactant solution.

Influence of thermodynamic behaviour and solute properties on homogeneous nucleation in supercritical solutions

Abstract The knowledge about the thermodynamic behaviour of dilute supercritical solutions is one of the basics for modelling processes, such as the formation of small particles by rapid expansion of supercritical solutions (RESS). RESS allows the production of particles less than 1 μm and RESS experiments show that particle size depends on solvent, solute and preexpansion conditions. However, an understanding of the underlying physical phenomena of the relationship between the process conditions and the mechanism of particle formation during RESS is still at an early stage. Because of that, there is a need to model the RESS process to get a better understanding of the influencing parameters. The calculations show a steep increase at the beginning of the freejet reaching maximum theoretical supersaturations of ≈10 8 and for an interfacial tension of 0.02 N m −1 maximum nucleation rates of about 10 +26 (cm −3 s −1 ). In the present paper, the influence of the solubility of various solutes in supercritical fluids and of the surface tension group ( σ  ·  v S 2/3 / k  ·  T ) of the diverse solutes on attainable nucleation rates under typical RESS operation conditions is investigated. The calculations show that the nucleation rate is a sensitive function of the solubility and of the unknown surface tension group. Furthermore, it is shown that the classical nucleation theory is not able to describe the trend in particle size resulting from RESS experiments in a sufficient manner. Also, the present calculations show that it is not possible to investigate homogeneous nucleation and coagulation separately and that there is an enormous need for more reliable information about the solute properties.

Phase equilibria of organic solid solutes and supercritical fluids with respect to the RESS process

In order to overcome the lack of experimental phase equilibrium data for a deeper understanding of particle formation processes using supercritical fluids, investigations have been carried out in two static equilibrium apparatuses. The first apparatus is designed as a view cell with an inner volume of 1310 cm3 placed in an air-thermostatted chamber, which allows a visual determination of the SLG-line following the first melting point method. The second equilibrium cell of approximately half the inner volume is placed in a water thermostat, which is more efficient for solubility measurements. The SLG-lines of naphthalene with carbon dioxide, trifluoromethane, and a mixture of these two solvents were experimentally determined and additional solubility measurements have been performed. The experimental results are described by means of cubic equations of state, which give, from an engineering point of view, an adequate thermodynamic model for the representation of solubility and SLG-lines.

Cellular uptake of platinum nanoparticles in human colon carcinoma cells and their impact on cellular redox systems and DNA integrity.

Supercritical fluid reactive deposition was used for the deposition of highly dispersed platinum nanoparticles with controllable metal content and particle size distribution on beta-cyclodextrin. The average particle size and size distribution were steered by the precursor reduction conditions, resulting in particle preparations <20, <100, and >100 nm as characterized by transmission electron microscopy and scanning electron microscopy (SEM). These particle preparations of different size distributions were used to address the question as to whether metallic platinum particles are able to invade cells of the gastrointestinal tract as exemplified for the human colon carcinoma cell line HT29 and thus affect the cellular redox status and DNA integrity. Combined focused ion beam and SEM demonstrated that platinum nanoparticles were taken up into HT29 cells in their particulate form. The chemical composition of the particles within the cells was confirmed by energy-dispersive X-ray spectroscopy. The potential influence of platinum nanoparticles on cellular redoxsystems was determined in the DCF assay, on the translocation of Nrf-2 and by monitoring the intracellular glutathione (GSH) levels. The impact on DNA integrity was investigated by single cell gel electrophoresis (comet assay) including the formation of sites sensitive to formamidopyrimidine-DNA-glycosylase. Platinum nanoparticles were found to decrease the cellular GSH level and to impair DNA integrity with a maximal effect at 1 ng/cm(2). These effects were correlated with the particle size in an inverse manner and were enhanced with increasing incubation time but appeared not to be based on the formation of reactive oxygen species.

Hydrodynamic and aerosol modelling of the rapid expansion of supercritical solutions (RESS-process)

Abstract The rapid expansion of a supercritical solution (RESS-process) is modelled numerically to better understand the mechanisms of particle formation and growth. A model is presented which includes the expansion process inside the capillary nozzle as well as the post-expansion process with particle growth inside the expansion chamber. Results are shown for the solid benzoic acid, griseofulvin and β-sitosterol in combination with the solvent carbon dioxide and trifluoromethane. The comparison with experimental results shows good agreement in general trends but does not yet match exactly with the measured mean particle sizes. Overall, a powerful tool for the optimisation of the RESS-process has been created which contributes to the understanding of the RESS-process.

Stabilized nanoparticles of phytosterol by rapid expansion from supercritical solution into aqueous solution.

The basic objective of this work was to form stable suspensions of submicron particles of phytosterol, a water-insoluble drug, by rapid expansion of supercritical solution into aqueous solution (RESSAS). A supercritical phytosterol/CO2 mixture was expanded into an aqueous surfactant solution. In these experiments 4 different surfactants were used to impede growth and agglomeration of the submicron particles resulting from collisions in the free jet. The concentration of the drug in the aqueous surfactant solution was determined by high-performance liquid chromatography, while the size of the stabilized particles was measured by dynamic light scattering. Submicron phytosterol particles (<500 nm) were stabilized and in most cases a bimodal particle size distribution was obtained. Depending on surfactant and concentration of the surfactant solution, suspensions with drug concentrations up to 17 g/dm3 could be achieved, which is 2 orders of magnitude higher than the equilibrium solubility of phytosterol. Long-term stability studies indicate modest particle growth over 12 months. Thus, the results demonstrate that RESSAS can be a promising process for stabilizing submicron particles in aqueous solutions.

Theoretical and experimental investigations of the micronization of organic solids by rapid expansion of supercritical solutions

Abstract The RESS-Process (Rapid Expansion of Supercritical Solutions) is an innovative and promising technology to produce small particles and offers interesting applications for difficult-to-comminute species such as certain pharmaceuticals. Our RESS-experiments are carried out with an apparatus suitable for temperatures up to 600 K and pressures up to 60 MPa. Carbon dioxide (CO2) and trifluoromethane (CHF3) have been used as supercritical solvents and cholesterol and benzoic acid as solutes. Besides the experimental investigations, the RESS-Process is modelled numerically considering the three parts capillary inlet–capillary–microscale free jet. The evolution of fluid pressure and temperature along the expansion path are used to calculate the supersaturation of the real mixtures CO2/cholesterol, CO2/benzoic acid and CHF3/benzoic acid and the respective nucleation rates. The results of these calculations are theoretical nucleation rates of about 1026 cm−3 s−1 at residence times of 10−7 s in the free jet.

Micronization of Pharmaceutical Substances by Rapid Expansion of Supercritical Solutions (RESS): Experiments and Modeling

An increasing number of newly developed pharmaceutical substances are poorly soluble in both aqueous and organic media. Thus, the application of oral or injectable drugs is often limited by its low bioavailability. An alternative and promising method to improve the bioavailability of pharmaceutical agents is the production of nanoscale particles by the rapid expansion of supercritical solutions (RESS). Our research is aimed towards an improved understanding of the underlying physical phenomena of the relationship between the process conditions and the particle characteristics. Therefore, experimental investigations and numerical simulations were performed. RESS experiments with the pharmaceutical substances β-sitosterol, griseofulvin, and ibuprofen led to particle sizes in the range of 240±80 nm. In addition, as one step towards intravenous application of poorly soluble drugs, β-sitosterol was used to produce aqueous suspensions of a water-insoluble drug with a particle size smaller than or equal to those produced by RESS into air. RESS modeling is focused on the flow through the nozzle, the supersonic free jet, the mach shock, and particle growth in the expansion unit. The comparison with experimental results shows a good agreement in the general trends but does not match exactly the measured mean particle sizes.

Drug loading into β-cyclodextrin granules using a supercritical fluid process for improved drug dissolution

To improve dissolution properties of drugs, a supercritical fluid (SCF) technique was used to load these drugs into a solid carrier. In this study, granules based on beta-cyclodextrin (betaCD) were applied as a carrier for poor water-soluble drug and loaded with a model drug (ibuprofen) using two different procedures: controlled particle deposition (CPD), SCF process and solution immersion (SI) as a conventional method for comparison. Using the CPD technique, 17.42+/-2.06wt.% (n=3) ibuprofen was loaded into betaCD-granules, in contrast to only 3.8+/-0.15wt.% (n=3) in the SI-product. The drug loading was confirmed as well by reduction of the BET surface area for the CPD-product (1.134+/-0.07m(2)/g) compared to the unloaded-granules (1.533+/-0.031m(2)/g). Such a reduction was not seen in the SI-product (1.407+/-0.048m(2)/g). The appearance of an endothermic melting peak at 77 degrees C and X-ray patterns representing ibuprofen in drug-loaded granules can be attributed to the amount of ibuprofen loaded in its crystalline form. A significant increase in drug dissolution was achieved by either drug-loading procedures compared to the unprocessed ibuprofen. In this study, the CPD technique, a supercritical fluid process avoiding the use of toxic or organic solvents was successfully applied to load drug into solid carriers, thereby improving the water-solubility of the drug.

Influence of Perfluorinated End Groups on the SFRD of [Pt(cod)Me(CnF2n+1)] onto Porous Al2O3 in CO2 under Reductive Conditions

The optimized synthesis of a range of cyclooctadiene-stabilized Pt complexes that contained different perfluoro-alkane chains, [Pt(cod)Me(Cn F2n+1 )], is presented. These metal-organic compounds were employed in the so-called supercritical fluid reactive deposition (SFRD) in CO2 under reductive conditions to generate metallic nanoparticles on aluminum oxide as a porous support. Thus, Al2 O3 -supported Pt nanoparticles with a narrow particle-size distribution were obtained. At a reduction pressure of 15.5 MPa and a temperature of 353 K, particle diameters of d50 =2.3-2.8 nm were generated. Decreasing the pressure during the reduction reaction led to slightly larger particles whilst decreasing the amount of organometallic precursor in CO2 yielded a decrease in the particle size from x50 =3.2 nm to 2.6 nm and a particle-size distribution of 2.2 nm. Furthermore, substitution of the CH3 end group by the Cn F2n+1 end groups led to a significant drop in Pt loading of about 50 %. Within the series of perfluorinated end groups that were considered, the Pt complex that contained a branched perfluoro-isopropyl group showed the most-interesting results when compared to the control precursor, [Pt(cod)Me2 ] (1).