Predicting physical stability in pressurized metered dose inhalers via dwell and instantaneous force colloidal probe microscopy

Particle Surface Roughness Improves Colloidal Stability of Pressurized Pharmaceutical Suspensions

PURPOSE

The effects of particle size and particle surface roughness on the colloidal stability of pressurized pharmaceutical suspensions were investigated using monodisperse spray-dried particles.

METHODS

The colloidal stability of multiple suspensions in the propellant HFA227ea was characterized using a shadowgraphic imaging technique and quantitatively compared using an instability index. Model suspensions of monodisperse spray-dried trehalose particles of narrow distributions (GSD < 1.2) and different sizes (MMAD = 5.98 μm, 10.1 μm, 15.5 μm) were measured first to study the dependence of colloidal stability on particle size. Particles with different surface rugosity were then designed by adding different fractions of trileucine, a shell former, and their suspension stability measured to further study the effects of surface roughness on the colloidal stability of pressurized suspensions.

RESULTS

The colloidal stability significantly improved (p < 0.001) from the suspension with 15.5 μm-particles to the suspension with 5.98 μm-particles as quantified by the decreased instability index from 0.63 ± 0.04 to 0.07 ± 0.01, demonstrating a strongly size-dependent colloidal stability. No significant improvement of suspension stability (p > 0.1) was observed at low trileucine fraction at 0.4 % where particles remained relatively smooth until the surface rugosity of the particles was improved by the higher trileucine fractions at 1.0 % and 5.0 %, which was indicated by the substantially decreased instability index from 0.27 ± 0.02 for the suspensions with trehalose model particles to 0.18 ± 0.01 (p < 0.01) and 0.03 ± 0.01 (p < 0.002) respectively.

CONCLUSIONS

Surface modification of particles by adding shell formers like trileucine to the feed solutions of spray drying was demonstrated to be a promising method of improving the colloidal stability of pharmaceutical suspensions in pressurized metered dose inhalers.

Characterization of the suspension stability of pharmaceuticals using a shadowgraphic imaging method

A new shadowgraphic imaging method and an associated instrument for analyzing the physical stability of pharmaceutical suspensions are introduced in this paper. The new suspension tester consists mainly of a high-resolution camera that takes sequential shadowgraphic images of emulsions or suspensions and a 2D collimated LED for simultaneous whole-sample illumination in bright field. A built-in ultrasonic bath provides controlled initial agitation to the samples of interest. Sequential images acquired by the experimental setup were used to derive normalized transmission profiles from which an instability index was developed for quantitative stability comparison between samples. Instrument performance was verified by measuring the stability of a series of oil-in-water emulsions prepared with surfactant mixtures of different ratios. The new instrument correctly determined the required hydrophilic-lipophilic balance for sunflower oil to be 7.0. The stability of a pressurized suspension of spray dried lipid (DSPC) particles was monitored for 5 days after propellant filling. Although stable for the first 24 h, the lipid suspension was found to decrease in stability from day 1 to day 4. Morphological and spectroscopic analysis revealed that the suspended DSPC particles had reformed into large thin sheets of lipid, thereby causing the gradual stability decrease during the aging study. The effects of initial agitation on the stability of suspensions were demonstrated by agitating a suspension of micronized fluticasone propionate in propellant using a wrist action shaker and an ultrasonic bath respectively. A significant improvement of suspension stability was achieved by replacing the wrist action shaker method with ultrasonic agitation. Simultaneous illumination of the complete suspension, a high image acquisition rate, and controlled initial agitation are features that make this new suspension tester a suitable and more reliable instrument for investigating the stability of pressurized pharmaceutical suspensions.

Lorentz contact resonance spectroscopy for nanoscale characterisation of structural and mechanical properties of biological, dental and pharmaceutical materials

Scanning probe microscopy has been widely used to obtain topographical information and to quantify nanostructural properties of different materials. Qualitative and quantitative imaging is of particular interest to study material-material interactions and map surface properties on a nanoscale (i.e. stiffness and viscoelastic properties). These data are essential for the development of new biomedical materials. Currently, there are limited options to map viscoelastic properties of materials at nanoscale and at high resolutions. Lorentz contact resonance (LCR) is an emerging technique, which allows mapping viscoelasticity of samples with stiffness ranging from a few hundred Pa up to several GPa. Here we demonstrate the applicability of LCR to probe and map the viscoelasticity and stiffness of 'soft' (biological sample: cell treated with nanodiamond), 'medium hard' (pharmaceutical sample: pMDI canister) and 'hard' (human teeth enamel) specimens. The results allowed the identification of nanodiamond on the cells and the qualitative assessment of its distribution based on its nanomechanical properties. It also enabled mapping of the mechanical properties of the cell to demonstrate variability of these characteristics in a single cell. Qualitative imaging of an enamel sample demonstrated variations of stiffness across the specimen and precise identification of enamel prisms (higher stiffness) and enamel interrods (lower stiffness). Similarly, mapping of the pMDI canister wall showed that drug particles were adsorbed to the wall. These particles showed differences in stiffness at nanoscale, which suggested variations in surface composition-multiphasic material. LCR technique emerges as a valuable tool for probing viscoelasticity of samples of varying stiffness's.

A 'soft spot' for drug transport: modulation of cell stiffness using fatty acids and its impact on drug transport in lung model

The impact of a polyunsaturated fatty acid, arachidonic acid (AA), on membrane fluidity of epithelial cells and subsequent modulation of the drug transport was investigated. Membrane fluidity was assessed using molecular force microscopy. Calu-3 human bronchial epithelial cells were cultured on Transwell® inserts and the cell stiffness was assessed in the absence of fatty acids or in the presence of 30 μM AA. The morphology of the epithelial cells was distinctly different when AA was present, with the cell monolayer becoming more uniform. Furthermore the cell stiffness and variation in stiffness was lower in the presence of AA. In the fat-free medium, the median cell stiffness was 9.1 kPa which dropped to 2.1 kPa following exposure to AA. To further study this, transport of a common β2-agonist, salbutamol sulphate (SS) was measured in the presence of AA and in a fat free medium. The transport of SS was significantly higher when AA was present (0.61 ± 0.09 μg versus 0.11 ± 0.003 μg with and without AA respectively). It was evidenced that AA play a vital role in cell membrane fluidity and drug transport. This finding highlights the significance of the dietary fatty acids in transport and consequentially effectiveness of medications used to treat pulmonary diseases such as asthma.