A microcalorimetric investigation of the interaction of surfactants with crystalline and partially crystalline salbutamol sulphate in a model inhalation aerosol system

Determination of physical and chemical stability in pressurised metered dose inhalers: potential new techniques

INTRODUCTION

Pressurised metered dose inhalers (pMDIs) are subject to rigorous physical and chemical stability tests during formulation. Due to the time and cost associated with product development studies, there is a need for online techniques to fast screen new formulations in terms of physical and chemical (physico-chemical) stability. The problem with achieving this is that pMDIs are by their definition, pressurised, making the direct observation of physico-chemical properties in situ difficult.

AREAS COVERED

This review highlights the characterisation tools that can enhance the product development process for pMDIs. Techniques investigated include: laser diffraction, Raman spectroscopy, isothermal ampoule calorimetry, titration calorimetry and gas perfusion calorimetry. The operational principles behind each technique are discussed and complemented with examples from the literature.

EXPERT OPINION

Laser diffraction is well placed to analyse real-time physical stability as a function of particle size; however, its use is restricted to suspension pMDIs. Raman spectroscopy can be potentially used to attain both suspension and solution pMDI spectra in real time; however, the majority of experiments are ex-valve chemical composition mapping. Calorimetry is an effective technique in capturing both chemical and physical degradations of APIs in real time but requires redevelopment to withstand pressure for the purposes of pMDI screening.

Influence of the valve lubricant on the aerodynamic particle size of a metered dose inhaler

Presented in this work are the results of a study designed to investigate the impact of valve lubricant (i.e., silicone oil) on the aerodynamic particle size distribution (PSD) of a steroid suspension metered dose inhaler (MDI) containing propellant HFA-227. The objective of this study was to explore whether the valve lubricant, which is often used in MDI products to prevent valve sticking, can enter an MDI product and potentially impact the aerosol spray dynamics. The results of this work have shown that samples containing valves with high silicone levels produced a larger aerodynamic particle size (by cascade impaction) than samples with low-silicone or silicone-free valves. It is postulated that the presence of silicone in the product may increase the propensity for drug aggregation, thereby leading to an increase in the aerodynamic particle size of the emitted aerosol. These findings stress the importance of evaluating the effects of valve lubricant on the aerodynamic PSD in the early formulation development stage of an MDI.

An investigation of the solubility of various compounds in the hydrofluoroalkane propellants and possible model liquid propellants

The aims of this study were to investigate descriptive parameters that may predict the solubility of compounds in the hydrofluoroalkane (HFA) propellants and to identify a model HFA propellant that is liquid at room temperature and atmospheric pressure. The solubility of 32 and 20 compounds chosen to give a wide range of physicochemical properties in HFA-134a and HFA-227, respectively, was measured. The Fedors solubility parameter and a computed log octanol water partition coefficient (CLOGP) were compared with the compounds' solubility in the HFA propellants. A total of 19 and 15 solutes had finite solubilities for HFA-134a and HFA-227, respectively, although the remaining solutes were miscible in all proportions. There was no apparent relation between solubility in HFA and the Fedors solubility parameter. This was not improved by considering the hydrogen-bonding potential of the compounds. When log solubility versus CLOGP was plotted, there was a linear relation for 16 and 12 of the compounds exhibiting a finite solubility in the HFA propellants, although four solutes (phenols) were displaced to the left of the linear relation. The remaining 3 compounds had much lower solubilities than was predicted from their CLOGPs, possibly as a consequence of their crystallinity (high melting points). Of the putative model propellants investigated (i.e., perfluorohexane (PFH), 1H-perfluorohexane [1H-PFH], and 2,2,2-trifluoroethanol), 1H-PFH was the most promising, with a linear relation between solubility in 1H-PFH and solubility in HFA propellant being observed. The solubilities in 1H-PFH were approximately 11 and 26% of those in HFA-134a and HFA-227.

Application of isothermal microcalorimetry in solid state drug development

Microcalorimetry is an analytical technique that has found numerous applications within the pharmaceutical environment. In the realm of pharmaceutics, especially solid state pharmaceutics, the technique has proved to be an invaluable tool. This review addresses the solid state applications of microcalorimetry within the pharmaceutical industry, with a specific focus on stability, compatibility and amorphicity determinations.

Powder dynamic contact angle data in the pharmaceutical industry

Formulation scientists generate physicochemical data that are used in process modelling and in the prediction of end-product quality. In this respect, contact angles are of particular importance as the surface energy of substances influences their processability and bioavailability. Dynamic contact angle (DCA) analysis represents a straightforward wettability assay. Now fully adapted to powders, DCA analysis also proves to be more efficient than other methods for wettability determination.

Characterization of amorphous ursodeoxycholic acid prepared by spray-drying

The objectives of this study were to characterize the amorphous state of ursodeoxycholic acid (UDCA) samples prepared by spray-drying, and to demonstrate the applicability of thermal and water-vapour-adsorption techniques for studying the material. Amorphous UDCA was prepared by spray-drying a solution of the compound in a mixture of ethanol and dichloromethane. The amorphous material was characterized by powder X-ray diffraction, infrared (IR) spectroscopy, isothermal microcalorimetry, differential scanning calorimetry (DSC) and water-vapour adsorption. When the inlet-air temperature of the spray drier was increased beyond 140 degrees C, the intensity of X-ray diffraction peaks from crystalline UDCA decreased and the IR bands in the hydroxyl-stretching and carboxyl-stretching regions changed. Dissolution of intact and spray-dried samples of UDCA prepared at 60 and 100 degrees C was an endothermic process but the dissolution became exothermic with increasing inlet-air temperature. UDCA samples differing in crystallinity were obtained, depending on the inlet-air temperature. A good correlation was obtained between the heat of solution and the heat of crystallization determined from DSC peak area. A good correlation was also obtained between the heat of solution and the crystallinity determined by Ruland's method from X-ray diffraction patterns. The amount of water vapour adsorbed on UDCA samples increased with increasing inlet-air temperature, indicating hydrogen bonding between water molecules and the hydroxyl groups or the carboxyl groups of amorphous UDCA. These results indicate that measurement of adsorption of water vapour and thermal analysis can both be used to evaluate the crystallinity of solid substances.

Drug form selection in albuterol-containing metered-dose inhaler formulations and its impact on chemical and physical stability

New albuterol-containing metered-dose inhaler (MDI) formulations were under development to replace chlorofluorocarbon (CFC) propellants with more environmentally friendly hydrofluoroalkane (HFA) propellants. To achieve good chemical and physical stability of MDI formulations with HFA propellants, different drug forms were evaluated in model formulations (drug, oleic acid, and one of the following: P12/P11, P12/ethanol, P12, P134a/ethanol, P134a). The effects of drug form (base versus sulfate), propellant type (P12 versus P134a), and cosolvent type (P11 or ethanol versus none) on the chemical and physical stability were examined. The chemical stability of the formulations was determined by monitoring the percent drug remaining in the formulations using HPLC. The physical stability of the formulations was followed by visually assessing the suspension appearance, and by determining the mass median diameter (MMD) of the suspended particles using laser diffraction analysis. The drug form has a great impact on the chemical and physical stability of the formulations. The sulfate formulations were chemically stable up to 12 months when stored at 30 degrees C and 40 degrees C/85% relative humidity (RH). Poor chemical stability was observed for the base formulations, except for ethanol-free formulations (P12/P11, P12, and P134a) at 30 degrees C and a P134a formulation at 40 degrees C/85% RH. The chemical instability of albuterol base formulations at 30 degrees C correlates with its solubility. The presence of a cosolvent greatly improved the dispersion characteristics of both sulfate and base formulations. The sulfate formulations in the presence of a cosolvent (P12/P11, P12/ethanol, and P134a/ethanol) showed good physical stability when stored for up to 12 months at 30 degrees C and 40 degrees C/85% RH. The physical stability of the base formulations was not acceptable due to crystal growth/agglomeration in all formulations, except for the P12/P11 formulation. The physical instability of both sulfate and base formulations not only correlates with the drug solubility, but also with particle agglomeration. In conclusion, good chemical and physical stability of albuterol-containing suspension formulations can be achieved with the appropriate choice of drug form and formulation constituents.

Evaluation of amorphous ursodeoxycholic acid by thermal methods

PURPOSE

The purpose of this study was to characterize the amorphous state of ursodeoxycholic acid (UDCA) samples by using isothermal microcalorimetry, X-ray diffraction, infrared (IR) spectroscopy and solid state carbon 13 nuclear magnetic resonance (13C-NMR) spectroscopy, and to demonstrate the application of the thermal methods (microcalorimetry and differential scanning calorimetry (DSC) for studying the amorphous state and clarifying the dissolution mechanism of UDCA.

METHODS

Amorphous UDCA was prepared by grinding and rapid cooling of the melts. The heat of solution of UDCA was measured by an isothermal heat-conduction twin microcalorimeter at 25.0 degrees C. Some physicochemical properties of amorphous UDCA were also studied.

RESULTS

The intensities of X-ray diffraction peaks of crystalline UDCA decreased with an increase in grinding time. The heat levels of solution of crystalline UDCA and UDCA ground for 1 min were endothermic, and became exothermic with an increase in grinding time. A good correlation was obtained between the heat of solution and the heat of crystallization determined from the peak area in DSC. Although no significant difference was observed in X-ray diffraction patterns of amorphous UDCA prepared by the two methods, significant differences were recognized in DSC, IR and 13C-NMR, and the heat of solution indicated different values among the two samples. The stability of amorphous UDCA samples stored under 74.5% relative humidity at 40 degrees C was found to depend upon the preparation methods.

CONCLUSIONS

Different states of amorphous UDCA were obtained depending on the preparation method. The application of thermal methods to evaluate the amorphous state was demonstrated. The mechanism of dissolution of UDCA was discussed from the results of the heat of solution examination.