Determination of drug solubility in aerosol propellants

A Novel Apparatus for the Determination of Solubility in Pressurized Metered Dose Inhalers

The accurate solubility of salbutamol sulfate, budesonide, and formoterol fumarate dihydrate in hydrofluoroalkane propellant 134a at 25 degrees C for 24 h, are reported. The authors describe a novel reusable in-line pressurized solubility apparatus containing an integral filter holder and a continuous decrimpable valve for the determination of drug/excipients solubility in pressurized metered dose inhalers. The solubility was determined by high-performance liquid chromatography. Solubility of salbutamol sulfate was determined as being below the detection limits while budesonide and formoterol fumarate dihydrate solubility were 23.136 +/- 2.951 microg x g(-1) and 0.776 +/- 1.023 microg x g(-1), respectively (n = 3). This novel solubility apparatus offers an improved ease of use and potential higher analytical throughput.

Surfactant design for the 1,1,1,2-tetrafluoroethane-water interface: ab initio calculations and in situ high-pressure tensiometry

In situ high-pressure tensiometry and ab initio calculations were used to rationally design surfactants for the 1,1,1,2-tetrafluoroethane-water (HFA134a|W) interface. Nonbonded pair interaction (binding) energies (E(b)) of the complexes between HFA134a and candidate surfactant tails were used to quantify the HFA-philicity of selected moieties. The interaction between HFA134a and an ether-based tail was shown to be predominantly electrostatic in nature and much more favorable than that between HFA134a and a methyl-based fragment. The interfacial activity of (i) amphiphiles typically found in FDA-approved pressurized metered-dose inhaler (pMDI) formulations, (ii) a series of nonionic surfactants with methylene-based tails, and (iii) a series of nonionic surfactants with ether-based tails was investigated at the HFA134a|W interface using in situ tensiometry. This is the first time that the tension of the surfactant-modified HFA134a|W interface has been reported in the literature. The ether-based surfactants were shown to be very interfacially active, with tension decreasing by as much as 27 mN.m(-)(1). However, the methyl-based surfactants, including those from FDA-approved formulations, did not exhibit high activity at the HFA134a|W interface. These results are in direct agreement with the E(b) calculations. Significant differences in interfacial activity are noted for surfactants at the 2H,3H-perfluoropentane (HPFP)|water and HFA134a|W interfaces. Care should be taken, therefore, when results from the mimicking solvent (HPFP) are extrapolated to HFA134a-based systems. The results shown here are of relevance in the selection of surfactants capable of forming and stabilizing reverse aqueous aggregates in HFA-based pMDIs, which are promising formulations for the systemic delivery of biomolecules to and through the lungs.

Novel hydrofluoroalkane suspension formulations for respiratory drug delivery

Due to the poor solvent properties of hydrofluoroalkanes, suspension is often the only formulation option for respiratory drug delivery. Research in this area has focussed mainly on two main themes over the past 5 years: new design of stabilisers and particle engineering. Among the most important advances, the introduction of secondary particulate systems and the establishment of porous particles as a viable delivery system must be mentioned. Other noteworthy developments include new classes of stabilisers and surface tailoring approaches. Work has been underpinned by new theoretical insights, via the introduction of atomic force microscopy to measure particle interactions, and the development of the surface tension component approach to predict them. Future areas of development include the formulation of nanoparticles and of non-inhalation therapies in non-pressurised hydrofluoroalkanes. All these aspects are reviewed in this article.

Characterization of suspension-based metered dose inhaler formulations composed of spray-dried budesonide microcrystals dispersed in HFA-134a

PURPOSE

To assess the physicochemical characteristics and aerosol properties of suspensions of lipid-coated budesonide microcrystals dispersed in HFA-134a.

METHODS

Lipid-coated budesonide microcrystals were prepared by spray-drying an emulsion-based feedstock. Physicochemical characteristics of spray-dried particles were assessed by electron microscopy, laser diffraction, and differential scanning calorimetry. Purity and content were determined by reverse-phase HPLC. Particle aggregation and suspension stability were assessed visually, and aerosol performance was assessed by Andersen cascade impaction and dose content uniformity.

RESULTS

Spray-drying of micronized budesonide microcrystals in the presence of phospholipid-coated emulsion droplets results in the production of low-density lipid-coated microcrystals with low surface energy. These spray-dried particles form stable suspensions in HFA-134a. This translates into good uniformity in the metered dose across the contents of the inhaler and acceptable aerodynamic particle size distributions (MMAD = 3.2 to 3.4 microm). The formulation was observed to maintain its performance over 6 months at 40 degrees C/75% RH and 16 months at 25 degrees C/60% RH. No effect of storage orientation was observed on the content of first sprays following storage (i.e., no Cyr effect). The fine particle dose was found to be linear out to suspension concentrations of about 2% wt/vol, and FPD(4.7 microm) values approaching 400 microg can be delivered in a single inhalation.

CONCLUSIONS

Engineered particles comprised of lipid-coated microcrystals may provide an acceptable alternative formulation technology for metered dose inhalers in the new hydrofluoroalkane propellants.

A comparison of two methods to determine the solubility of compounds in aerosol propellants

A new on-line reverse phase HPLC method for determining the solubility of compounds in propellant based metered dose inhaler (MDI) formulations was compared with a conventional method. The new method employs a direct injection from a MDI vial into the needle injector port of a manual injector. To evaluate the two methods, beclomethasone dipropionate (BDP), 5,5-diphenyl hydantoin and 3,3'-diindolylmethane, were used as model compounds in propellant HFA-134a. Comparison was performed by analyzing known and unknown concentrations of BDP in various combinations of HFA-134a and ethanol. In addition, the solubility of 5,5-diphenyl hydantoin and 3,3'-diindolylmethane were determined in HFA-134a using both the new and the conventional methods. The two methods were found to be in good agreement with each other, with the new direct injection technique offering enhanced precision and accuracy along with considerable reduction in analysis time.

Novel method for the determination of solubility in aerosol propellants

A new on-line reverse-phase high-performance liquid chromatography method to determine the solubility of compounds in propellant-based metered-dose inhaler (MDI) formulations was developed. The new method uses a direct injection from an MDI vial into the port of a manual injector. The MDI vials were coupled with a filtration and injection assembly to filter the excess compound, and deliver the filtrate of the MDI vials to the injector port of the manual injector. A backpressure regulator was connected to the manual injector, to maintain the propellant in the liquid state after injection from the MDI vial. Phase separation studies were conducted to investigate the miscibility of hydrofluoroalkane 134a with different mobile-phase solvent compositions. Characterization of the new direct injection method was done by evaluating sample presentation and chromatography variables to determine the robust nature of the method. Beclomethasone dipropionate in ethanol/hydrofluoroalkane 134a was used as a model system for development. A chemically diverse set of nine compounds was used to evaluate solubility measurement reproducibility. Solubilities were determined at two different temperatures (25 and 37 degrees C) and the average relative standard deviation for all of the solubility measurements was <4%.

The influence of formulation variables on the performance of alternative propellant-driven metered dose inhalers

There are a multitude of formulation factors to consider when developing a pMDI. Evaluation of each of these variables has been performed over the years, but there has been an abundance of different approaches in the determination of the effects on device performance. Thus, although much is known about pMDI on the empirical level, a systematic approach has clearly been missing. With the ratification of the Montreal Protocol and the introduction of alternative propellant systems, the opportunity to establish relationships between different levels of testing, such as in vitro measurements and in vivo outcomes, and in vivo assessments and clinical outcomes, has arrived. This review outlines research efforts that have focused on the formulation of propellant-driven metered dose inhalers using alternative propellants. These formulation factors, including device characteristics, are reviewed with respect to the performance of MDIs.

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.

Influence of water on the solubility of two steroid drugs in hydrofluoroalkane (HFA) propellants

The objective of this research work was to investigate the influence of water level, temperature, and propellant composition on the solubility of two hydrophobic steroid drugs, triamcinolone acetonide (TAA) and beclomethasone diapropionate (BDP). pMDIs containing TAA or BDP, spiked water, and propellant blend with different ratios of HFA 134a and HFA 227 were prepared. The contents of the prepared pMDIs were filtered through a 0.22 mm Acrodisc, syringe filter into a receiving canister after the pMDIs were equilibrated at 15 degrees C, 25 degrees C, 30 degrees C, and 40 degrees C. The drug concentration in the receiving canisters was determined by HPLC and the drug solubility in the propellant blend was calculated. Also, the drug crystal collected on the filter from the donor pMDIs were characterized by x-ray diffraction. The solubility of TAA and BDP varied with propellant composition at all experimental temperatures investigated. The solubility of TAA and BDP increased as the temperature was increased at all propellant compositions and water levels studied, but decreased as the water level in the propellant system was increased at all compositions and temperatures. The x-ray diffraction results indicated that the water in the propellant system had no significant influence on the crystal characteristics of TAA in HFA propellant system, but had a significant impact on the crystal characteristics of BDP was higher than TAA at all propellant compositions, experimental temperatures and water levels investigated. The solubility of TAA and BDP was not only influenced by propellant composition and storage temperature, but also depended on the water level in the propellant system. As a consequence, the crystallinity of the drugs formulated in HFA propellant was influenced by the temperature, propellant composition and the water level in the propellant system. The impact of these factors on the crystallinity of formulated drugs.

Moisture uptake and its influence on pressurized metered-dose inhalers

The objective of this study was to investigate moisture ingress into pressurized metered dose inhalers (pMDIs) containing hydrofluoroalkane (HFA) propellants and the consequences of this ingress. Moisture ingress into the pMDIs containing tetrafluoroethane (HFA 134a) or heptafluoropropane (HFA 227) was evaluated and modeled. The influence of water level in pMDIs on the stability of pMDIs containing triamicinolone acetonide (TAA) and beclomethasone dipropionate (BDP) in terms of particle growth, fine particle fraction, and drug solubility in the propellant system was evaluated using scanning electron microscopy, particle size analysis, single-stage impaction, and HPLC. The water level in HFA-containing pMDIs increased during storage and the process obeyed a diffusion model. HFA 134a had a greater tendency to take up moisture from the environment than did HFA 227. Unlike TAA, the propensity for particle growth of the suspended BDP in HFA propellants was significantly depressed by the increase in water level in the pMDIs. As a result, the fine particle fraction of the emitted BDP aerosols significantly increased as the water level in the HFA propellant was increased. Moisture ingress into pMDIs containing HFAs occurred during storage. The influence of the increased water level in pMDIs on the physical stability of the pMDI formulation and the dose delivery performance was a function of the composition of the internal lining of the container, the type of drug and propellant, and storage temperature.

Prodrug to probe solution HFA pMDI formulation and pulmonary esterase activity

A novel salbutamol prodrug was synthesised. Solubility in HFA-134a and susceptibility to rat lung homogenate, blood and plasma esterase enzymes were investigated. Whereas salbutamol had a very low solubility in HFA-134a, the prodrug was found to be miscible in all proportions. In lung homogenate, the prodrug hydrolysed with a half-life of 45 min, re-generating approximately 17% of expected salbutamol after 8 h incubation. The use of a solution pMDI for pulmonary delivery of the salbutamol prodrug is predicted to result in liberation of salbutamol in the lungs following in vivo hydrolysis by lung esterases.

Study of solubility of steroids in hydrofluoroalkane propellants

The solubility of prednisone, hydrocortisone 21-acetate, hydrocortisone, dexamethasone, betamethasone 17-valerate, and danazol in hydrofluoroalkane (HFA) 134a and HFA 227 was determined at 5 degrees C and 25 degrees C. It was found that the solubility of steroid in HFA propellants was related to the melting point and the lipophilicity of the steroid. The solubility of the steroids in the binary system of HFA propellants and ethanol also was investigated in the study. Ethanol significantly increased the solubility of the steroids in HFA propellant. The magnitude of increase was related to the solubility of the corresponding steroid in ethanol alone.

Drug-surfactant-propellant interactions in HFA-formulations

The required replacement of chlorofluorocarbon (CFC) with hydrofluoroalkane (HFA) propellants has challenged formulators of pressurized metered dose inhalers in several major respects. Conventional (CFC soluble) surfactants are effectively insoluble in the major CFC replacement candidates, HFA 134 and HFA 227ea, in the absence of co-solvents. While these ethane and propane derivatives have comparable boiling points and vapor pressures to dichlorodifluoromethane (CFC 12), their increased polarity demands that formulators use either alternative (soluble) surfactants, or co-solvents along with traditional surfactants, in order to stabilize pressurized suspension products. The use of either approach is complicated by the existence of many competing patents and the fact that the science in the area is empirical; predictive theoretical approaches are frustrated by the lack of an adequate database. Technical developments in this area must also take into account the need to avoid crystal growth and/or adhesion of micronized, suspended drugs to internal container surfaces, problems which may be catalyzed by some combinations of surfactant type/concentration, vehicle(s) and physical form/type(s) of drug substance. For some drugs, it appears simpler to use co-solvents with HFA propellants to dissolve the drug, avoiding the need for suspension stabilization. This article presents an overview of the present state of the art with respect to the formulation of MDIs.

Application of co-grinding to formulate a model pMDI suspension

The objective of this study was to investigate the effect of co-grinding the model drug, triamcinolone acetonide (TAA), with a polymeric surfactant on the in vitro performance of a model pMDI suspension system. The physicochemical properties of TAA after co-grinding with the surfactant, Pluronic F77, were determined by laser light diffraction, helium pycnometry and equilibrium solubility measurements. TAA-surfactant interaction was investigated by differential scanning calorimetry and Fourier transform infrared spectroscopy (FTIR). The suspension characteristics of pMDI formulations prepared with co-ground TAA and surfactant were investigated by determining their in situ sedimentation, rheological profiles and vapor pressure. The performance characteristics of the pMDI formulations were determined by cascade impaction and dose delivery through-the-valve (DDV) measurements. It was found that the presence of Pluronic F77 decreased the solubility of TAA in the propellant medium. Co-grinding TAA particles with Pluronic F77 influenced the particle size distribution, sedimentation and flocculation characteristics of the pMDI suspension formulation. The addition of Pluronic F77 decreased the viscosity of the pMDI formulation. Formulating the suspension pMDI system with co-ground TAA and Pluronic F77 decreased the mass median aerodynamic diameter (MMAD) of the emitted aerosol and increased the percent respirable fraction (%RF). The co-ground TAA and Pluronic F77 pMDI suspension formulation exhibited greater physical stability which was due to the influence of the co-grinding technique on the physicochemical properties of the TAA particle surface and the propellant dispersion medium. The changes induced by co-grinding with Pluronic F77 improved the performance characteristics of a pMDI suspension formulation by stabilizing the suspension and influencing the flocculation characteristics. Co-grinding is a process which may be useful when developing new pMDI systems containing HFA propellants.

Influence of micronization method on the performance of a suspension triamcinolone acetonide pressurized metered-dose inhaler formulation

The purpose of this study was to investigate the influence of micronization technique on performance and stability of the model drug formulated in a suspension-based pressurized metered-dose inhaler (pMDI). The model drug, triamcinolone acetonide (TAA), was subjected to ball milling or air-jet milling prior to formulation of the pMDI. The dose delivery characteristics of the emitted aerosol cloud were monitored for the ball-milled, air-jet-milled, and unmicronized TAA pMDI formulations prior to and after storage at 25 and 40 degrees C. Cascade impaction was used to determine the aerodynamic particle size distribution of the emitted dose. Both micronization techniques reduced the drug particle size distribution and the polydispersity of the drug particles to a similar extent, but the ball-milling technique reduced the crystallinity of the drug to a greater degree compared to the air-jet-milling technique. The air-jet-milled and unmicronized TAA pMDI displayed similar aerodynamic particle size distributions of the emitted aerosol and respirable fractions over the storage period. The ball-milled TAA resulted in a pMDI formulation with the smallest aerodynamically sized particles and the highest respirable fraction compared to the air-jet-milled or unmicronized TAA pMDI formulations. The micronization techniques significantly influenced the dose delivery characteristics as a result of different initial particle size distributions, amorphous contents, and surface energies.

Surfactant dissolution and water solubilization in chlorine-free liquified gas propellants

The initial water content of a group of 15 pharmaceutically and toxicologically acceptable surfactants showed a tendency to increase with the surfactant hydrophilic-lipophilic balance (HLB) value. Surfactant solubility was determined in chlorine-free "alternative propellants" (n-butane, propane, dimethyl ether [DME], 1,1,1,2-tetrafluoroethane (HFA-134a), and 1,1,1,2,3,3,3-heptafluoropropane [HFA-227ea], and trichloromonofluoromethane [CFC-11] in the absence of cosolvents such as ethanol. Water-soluble surfactants such as Carbowax, Sentry, PEG 300, Tween 20, and Brij 30, with high HLB values showed appreciable solubility in HFA-134a and HFA-227ea. In systems containing > or = 80% propellant by weight, each single-phase propellant-surfactant blend was screened for its ability to solubilize iodine and dissolve or solubilize water with increasing surfactant concentration. This screening was performed to investigate the possibility of formulating high-volatility, single-phase systems with increased polarity and solvency from these conventional excipients and vehicles. Ternary-phase diagrams show the regions of apparent single and multiple phase behavior in each system. Despite the increased polarity of the hydrofluoroalkanes (HFAs), appreciable water solubility was seen only with these surfactants in DME and in the hydrocarbons (HCs) n-butane and propane.

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.