A critical evaluation of the relevant parameters for drug redispersion from adhesive mixtures during inhalation

The role of capsule aperture size on the dispersion of carrier-based formulation at different air flowrates

The effect of capsule aperture size on the aerosol performance of lactose blend formulation was studied using Foradil® (containing 12 μg of formoterol fumarate (FF¹) and 24 mg of lactose) dispersed with a powder inhaler Aerolizer® at increasing air flowrates. Apertures sizes of 0.4, 1.0, 1.5, 2.5, and 4.0 mm were introduced at the opposite ends of the capsule. The formulation was dispersed into a Next Generation Impactor (NGI) at 30, 60 and 90 L/min, with the fine particle fractions (FPF_rec and FPF_em) measured by chemical assay of FF and lactose using high-performance liquid chromatography. Particle size distribution (PSD) of FF particles dispersed in wet media was also characterized by laser diffraction. FPF_rec showed a stronger dependency on the flowrate than the capsule aperture size. The most efficient dispersion was achieved at 90 L/min. At a given flowrate, FPF_em remained broadly constant across different aperture sizes. The laser diffraction studies demonstrated the presence of large agglomerates.

Twin-Screw Continuous Mixing Can Produce Dry Powder Inhalation Mixtures for Pulmonary Delivery

The feasibility of twin-screw corotating extruder as a continuous process mixer to prepare dry powder inhalation (DPI) powders was investigated. Interactive mixtures of 1% micronized budesonide, 0.3% magnesium stearate and 98.7% alpha-lactose monohydrate were manufactured using a Leistritz Nano-16 extruder at various processing conditions. One set of GFM (grooved mixing) elements were included in the screw profile to provide distributive mixing of conveyed powders with the goal of resulting in a homogeneous mixture. Residence time in the twin-screw mixer was modelled to quantify mixing efficiency. Comparative powders were also prepared using either low or high-shear batch mixing to compare the effect of mixing methods on the properties of the budesonide dry powder inhalation formulation. Twin screw mixing results in homogeneous mixtures with aerosol performance comparable to that of high-shear batch mixing. Scanning electron microscopy confirmed that twin screw mixing produces particles with morphology like that of low and high-shear batch mixing. X-ray diffraction (XRD) analysis verified that there was no form change of the drug due to twin-screw processing. Statistical regression was used to probe the relationship between twin screw mixing process parameters such as screw speed and feed rate and aerosol performance. The twin screw mixing process was found to be robust, as no significant differences in aerosol performance were found for various processing parameters.

Porous particles and novel carrier particles with enhanced penetration for efficient pulmonary delivery of antitubercular drugs

This study aimed to design dry powder inhaler formulations using a hydrophilic polymeric polysaccharide, phytoglycogen (PyG), as a multi-functional additive that increases the phagocytic activity of macrophage-like cells and enhances pulmonary delivery of drugs. The safety and usefulness of PyG were determined using in vitro cell-based studies. Dry powder inhaler formulations of an antitubercular drug, rifampicin, were fabricated by spray drying with PyG. The cytotoxicity, effects on phagocytosis, particle size, and morphology were evaluated. The aerosolization properties of the powder formulations were evaluated using an Andersen cascade impactor (ACI). Scanning electron microscope images of the particles on each ACI stage were captured to observe the deposition behavior. PyG showed no toxicity in A549, Calu-3, or RAW264.7 cell lines. At concentrations of 0.5 and 1 g/L, PyG facilitated the cellular uptake of latex beads and the expression of pro-inflammatory cytokine genes in RAW264.7 cells. Formulations with outstanding inhalation potential were produced. The fine particle fraction (aerodynamic size 2-7 µm) of the porous particle batch reached nearly 60%, whereas in the formulation containing wrinkled carrier particles, the extra-fine particle fraction (aerodynamic particle size < 2 μm) was 25.0% ± 1.7%. The deposition of porous and wrinkled particles on individual ACI stages was distinct. The inclusion of PyG dramatically improved the inhalation performance of porous and wrinkled powder formulations. These easily inhaled immunostimulatory carrier particles may advance the state of research by enhancing the therapeutic effect and alveolar delivery of antitubercular drugs.

On the relationship between blend state and dispersibility of adhesive mixtures containing active pharmaceutical ingredients

The objectives of this investigation were to study the evolution in blend state of adhesive mixtures containing the active pharmaceutical ingredients (APIs) salbutamol, budesonide and AZD5423 and to study the relationship between blend state and dispersibility of the mixtures, as assessed by the fine particle fraction (FPF). A series of adhesive mixtures of varied fines concentration were prepared for each API using the same type of carrier. Based on visual examination and powder mechanics, blend states were identified and summarized as blend state maps for each API. The dispersibility of the mixtures was studied using a Fast Screening Impactor (FSI) equipped with a ScreenHaler. The evolution in blend state differed between the APIs in terms of the width of the blend states. The structure of the adhesion layer also differed between the APIs, from relatively uniform to a heterogeneous layer with small agglomerates dispersed on the carrier surface. All three APIs expressed a similar type of bended relationship between FPF and fines concentration. However, the initial rate of increase and the fines concentration of the plateau differed between the APIs. The adhesive mixtures of all APIs followed the three main states in terms of structural evolution and the overall shape of the FPF-fines concentration profiles could be explained by the evolution in blend state. It is proposed that the structure of the adhesion layer is an important factor explaining the differences in blend state - blend dispersibility relationships between the APIs.

Controlling the performance of adhesive mixtures for inhalation using mixing energy

A concept of mixing energy, ME, has been developed and applied to blending of adhesive mixtures for inhalation in a high shear blender. Six different systems were investigated, four of which included a coating agent. For blends containing a coating agent, it is shown that the applied ME is key to the control of two important functional mechanisms: i) coating of the carrier by the coating agent, and ii) the dispersibility of the active pharmaceutical ingredient (API). The mass of the carrier was identified to be the mass which is relevant to the forces acting during mixing. The dispersibility in terms of the fine particle fraction (FPF) can be expressed as the product of two exponentials which both are functions of ME. The first factor accounts for the initial increase in FPF, while the second accounts for the decrease observed at extensive mixing. For adhesive mixtures without a coating agent, a similar decrease in FPF is observed when high forces are applied during mixing. Mechanistic interpretation of the behavior is provided.

Quantification of surface composition and surface structure of inhalation powders using TOF-SIMS

A multivariate TOF-SIMS methodology has been developed and applied to quantify surface composition and chemical distribution for dry powder blends. Surface properties are often critical to the behavior of powder formulations, especially in the case of dry powders for inhalation, as surface properties directly affect inter-particulate forces and, hence, the dispersibility of the formulation. The mass spectrum at each pixel was fit to a linear combination of reference spectra obtained by non-negatively constrained alternating least squares. From the pixel compositions, average surface coverage and a range of other image features were calculated. Two kinds of systems have been examined: 1) binary blends of lactose particles and coating agents, and 2) blends of different inhalation drugs with carrier lactose. For both kinds of systems, detailed insight into the surface composition and structure could be derived. For the former study, TOF-SIMS results were compared with a complementary surface analysis technique, XPS.

Linking carrier morphology to the powder mechanics of adhesive mixtures for dry powder inhalers via a blend-state model

The aim of this study was to investigate how the carrier morphology affects the expression of blend states in adhesive mixtures as a function of surface coverage ratio (SCR) and to identify where transitions between the different states occur. Adhesive mixtures of five lactose carriers with varying contents of lactose fines, corresponding to blends with different SCR ranging from 0 to 6, were produced by low-shear mixing. The powder mechanics of the mixtures were characterized by bulk density, compressibility and permeability. The appearance of the carriers and blends was studied by scanning electron microscopy, light microscopy and atomic force microscopy. The size and morphology of the carriers had a crucial impact on the evolution of the blend state, and affected the powder mechanical properties of the mixtures. It was found that smaller carriers with little or no surface irregularities were more sensitive to additions of fines resulting in self-agglomeration of fines at relatively low SCR values. On the contrary, carriers with irregular surface structures and larger sizes were able to reach higher SCR values before self-agglomeration of fines occurred. This could be attributed to an increased deagglomeration efficiency of irregular and larger carriers and to fines predominantly adhering to open pores.

Assessing Aerosol Performance of a Dry Powder Carrier Formulation with Increasing Doses Using a Novel Inhaler

This study aims to investigate the implications of loaded formulation mass on aerosol performance using a reservoir novel dry powder inhaler containing a custom dosing cup to deliver carrier-based formulation to the lungs. A 3D printed dosing cup with volume size of 133.04 mm³ was manufactured to allow for the progressive loading of different carrier formulation masses of 1% beclomethasone dipropionate BDP (w/w) formulation (10 to 60 mg, with increments of 10 mg), in a novel customizable DPI device. Scanning electron micrographs were used to investigate BDP detachment from carrier particles post-aerosolisation and particle deposition on the USP induction port. The subsequent aerosol performance analysis was performed using the next generation impactor (NGI). Incrementally increasing the loading mass to 60 mg led to decreases in BDP detachment from carrier particles, resulting in significant decreases in aerosol performance. Increases in loading dose mass led to progressively decreased detachment of BDP from the carrier and the overall aerosol performance in comparison to the initial mass of 10 mg. These results are likely to be due to a decrease in void volume within the dosing cup with increased loading mass leading to altered airflow, decreased impaction forces and the possibility of a significant quantity of large carrier particles introducing a 'sweeping' effect on the inhaler inner surface. This study has shown that despite the decreased BDP detachment from the carrier and decreased aerosol performance, the dose delivered to the lung still increased due to the higher loaded dose.

Technological and practical challenges of dry powder inhalers and formulations

In the 50 years following the introduction of the first dry powder inhaler to the market, several developments have occurred. Multiple-unit dose and multi-dose devices have been introduced, but first generation capsule inhalers are still widely used for new formulations. Many new particle engineering techniques have been developed and considerable effort has been put in understanding the mechanisms that control particle interaction and powder dispersion during inhalation. Yet, several misconceptions about optimal inhaler performance manage to survive in modern literature. It is, for example still widely believed that a flow rate independent fine particle fraction contributes to an inhalation performance independent therapy, that dry powder inhalers perform best at 4 kPa (or 60 L/min) and that a high resistance device cannot be operated correctly by patients with reduced lung function. Nevertheless, there seems to be a great future for dry powder inhalation. Many new areas of interest for dry powder inhalation are explored and with the assistance of new techniques like computational fluid dynamics and emerging particle engineering technologies, this is likely to result in a new generation of inhaler devices and formulations, that will enable the introduction of new therapies based on inhaled medicines.

Drug content effects on the dispersion performance of adhesive mixtures for inhalation

The drug content in adhesive mixtures for inhalation is known to influence their dispersion performance, but the direction and magnitude of this influence depends on other variables. In the past decades several mechanisms have been postulated to explain this finding and a number of possible interacting variables have been identified. Still, the role of drug content in the formulation of adhesive mixtures for inhalation, which includes its significance as an interacting variable to other parameters, is poorly understood. Therefore, the results from a series of drug detachment experiments are presented in which the effect of drug content and its dependence on flow rate, the mixing time and the type of drug is studied. Furthermore, it is investigated whether the effect depends on the range within which the drug content is changed. Quantitative and qualitative multiple order interactions are observed between these variables, which may be explained by a shifting balance between three different mechanisms. The results therefore demonstrate that accounting for (multiple order) interactions between variables has to be part of quality by design activities and the rational design of future experiments.

Mixing time effects on the dispersion performance of adhesive mixtures for inhalation

This paper deals with the effects of mixing time on the homogeneity and dispersion performance of adhesive mixtures for inhalation. Interactions between these effects and the carrier size fraction, the type of drug and the inhalation flow rate were studied. Furthermore, it was examined whether or not changes in the dispersion performance as a result of prolonged mixing can be explained with a balance of three processes that occur during mixing, knowing drug redistribution over the lactose carrier; (de-) agglomeration of the drug (and fine lactose) particles; and compression of the drug particles onto the carrier surface. For this purpose, mixtures containing salmeterol xinafoate or fluticasone propionate were mixed for different periods of time with a fine or coarse crystalline lactose carrier in a Turbula mixer. Drug detachment experiments were performed using a classifier based inhaler at different flow rates. Scanning electron microscopy and laser diffraction techniques were used to measure drug distribution and agglomeration, whereas changes in the apparent solubility were measured as a means to monitor the degree of mechanical stress imparted on the drug particles. No clear trend between mixing time and content uniformity was observed. Quantitative and qualitative interactions between the effect of mixing time on drug detachment and the type of drug, the carrier size fraction and the flow rate were measured, which could be explained with the three processes mentioned. Generally, prolonged mixing caused drug detachment to decrease, with the strongest decline occurring in the first 120 minutes of mixing. For the most cohesive drug (salmeterol) and the coarse carrier, agglomerate formation seemed to dominate the overall effect of mixing time at a low inhalation flow rate, causing drug detachment to increase with prolonged mixing. The optimal mixing time will thus depend on the formulation purpose and the choice for other, interacting variables.

A critical view on lactose-based drug formulation and device studies for dry powder inhalation: which are relevant and what interactions to expect?

Many years of research have not led to a profound knowledge of the mechanisms involved in the formulation and dispersion of carrier based mixtures for inhalation. Although it is well understood that the mixing is a key process in DPI carrier based formulation, there remains a limited understanding of how blending processes affect in-process material properties and the resulting distribution of the drug in the final dosage form. A great number of variables are considered relevant to the interfacial forces in adhesive mixtures, but their effects have mostly been investigated individually, without taking account of the influence they may have on each other. Interactions may be expected and without proper choices made and definitions given for all the variables involved, conclusions from studies on adhesive mixtures are of less relevance. By varying any of the variables that are not subject of the study, an opposite effect may be obtained. Currently, there is a strong focus on exploring techniques for the characterisation of drug and carrier surface properties that are believed to have an influence on the interparticulate forces in adhesive mixtures. For a number of surface properties it may be questioned whether they are really the key parameters to investigate however. Their orders of magnitude are subordinate to the effects they are supposed to have on the drug-to-carrier forces. Therefore, they seem rather indicators of other variability and their influence may be dominated by other effects. Finally, the relevance of inhaler design is often ignored. By using powerful inhalers, the effect of many variables of current concern may become less relevant. Carrier properties that are considered disadvantageous at present may even become desirable when a more appropriate type of dispersion force is applied. This can be shown for the effect of carrier surface rugosity when inertial separation forces are applied instead of the more widely applied lift and drag forces. Therefore, inhaler design should be taken into consideration when evaluating studies on adhesive mixtures. It should also become an integral part of powder formulation for inhalation.

Adhesion forces in interactive mixtures for dry powder inhalers – Evaluation of a new measuring method

Dry powder inhalers mostly contain carrier based formulations where micronized drug particles are adhered to coarse carrier particles. The performance of the dry powder inhaler depends on the inhaler device, the inhalation manoeuvre and the formulation. The most important factor influencing the behaviour of the formulation is the adhesion force acting between the active ingredient and the carrier particles, which can be measured using different methods, for example the centrifuge technique or atomic force microscopy. In this study the tensile strength method, usually applied to determine cohesion forces between powder particles of one material, is optimized for adhesion force measurements between powder particles of unlike materials. Adhesion force measurements between the carrier materials lactose or mannitol and the drug substance salbutamol sulphate using the tensile strength method and the atomic force microscopy show higher values with increasing relative humidity. Consequently, the fine particle fraction determined using the Next Generation Impactor decreases with increasing relative humidity as a result of the enhanced interparticle interactions.

Modifying Drug Release and Tablet Properties of Starch Acetate Tablets by Dry Powder Agglomeration

In this study three model drugs (N-acetyl-D-glucosamine (NAG), anhydrous caffeine, and propranolol hydrochloride) were agglomerated with starch acetate (SA) by mixing the binary powders on a stainless steel (SS) plate. Agglomeration was induced by triboelectrification of the particles during mixing, and it was evaluated as a method to achieve controlled drug release rate. These agglomerates, mixed with different amounts of a disintegrant, were compressed into tablets whose dissolution characteristics were determined. Triboelectric measurements showed that when the drugs were in contact with SS, charges of the opposite polarity were generated to SA (+) and caffeine and NAG (-) promoting adhesion. Instead, propranolol HCl was charged with the same polarity as SA. SEM micrographs showed that smaller caffeine particles, in spite of their larger negative charge, agglomerated less efficiently with SA than larger NAG particles. This emphasizes the importance of particle size in the agglomeration process. Propranolol HCl did not form agglomerates with SA since their particle sizes and charges were identical. As a result, agglomeration of powders prior to tablet compression allows for modification and control of the release rate of the drugs from the SA matrix tablets as well as the tensile strength of the tablets.

The effect of carrier surface treatment on drug particle detachment from crystalline carriers in adhesive mixtures for inhalation

In this study, the effect of lactose carrier surface treatment on drug particle detachment during inhalation has been investigated. Crystals of marketed brands of alpha lactose monohydrate brands normally exhibit a certain surface rugosity and contain natural fines and impurities on their surface, which influence the drug-to-carrier interaction in adhesive mixtures for inhalation. Submersion treatment may change these surface characteristics. Two different sieve fractions (63-90 and 250-355microm) were submerged in mixtures of ethanol and water (96 and 80% v/v, respectively). Microscopic observation and laser diffraction analysis revealed that neither the shape nor the size of the carrier particles was changed by the submersion treatment. However, the specific surface area and the amount of impurities appeared to decrease substantially after submersion, and the magnitude of the decrease was different for the different ethanol-water mixtures. The reduction in specific surface area was attributed particularly to the removal of the adhering lactose fines from the carrier surface. Mixtures with budesonide (in a wide range of carrier payloads) were prepared before and after treatment. Drug particle detachment from the various mixtures was studied with a sieve test and with a cascade impactor analysis at 30 and 60l/min. Two different types of inhalers were used, one generating lift- and drag-forces (ISF inhaler) and one generating inertial forces (test inhaler), respectively. The cascade impactor and sieve test experiments showed that an increase in carrier surface smoothness results in a reduced drug particle detachment during inhalation, which was independent of the type of inhaler used. This reduction could be attributed to the removal of the adhering lactose fines which may provide shelter for the drug particles from press-on forces during mixing.

Electrostatic characterisation of inhaled powders: effect of contact surface and relative humidity

Electrostatic charge accumulation on drug and excipient powders arising from interparticulate collisions or contacts between particles and other solid surfaces often leads to agglomeration and adhesion problems during the manufacture and use of dry powder inhaler (DPI) formulations. The aim of this work was to investigate the role of triboelectrification in particle interactions between micronised drug (salbutamol sulphate or ipratropium bromide monohydrate) and excipient (alpha-lactose monohydrate, 63-90 microm) during mixing in cylindrical vessels constructed from stainless steel, polypropylene and acetal under selected relative humidity (rh) conditions (0-86%). The charge was found to depend on both the nature of the powders and the mixing vessel surface. In addition, coating the vessels with drug or excipient removed the influence of the vessel material on charge generation, thus providing a technique to investigate interactions between the drug and excipient substances. A triboelectric series of all materials used, placed ipratropium at the positive end and polypropylene at the negative end. Micronised drug profoundly altered the charging properties of lactose in drug (1.46%, w/w)/lactose DPI formulations. An increase in rh in the range 0-86% produced a corresponding decrease in charge and adhesion values for each drug, lactose and DPI formulation during triboelectrification with each mixing vessel surface. The results provide increased knowledge of the role of electrostatics in DPI technology.

Air classifier technology (ACT) in dry powder inhalation

In this study, the in vitro fine particle deposition from a multi dose dry powder inhaler (Novolizer) with air classifier technology has been investigated. It is shown that different target values for the fine particle fraction (fpf<5 microm) of the same drug can be achieved in a well-controlled way. This is particularly relevant to the application of generic formulations in the inhaler. The well-controlled and predictable fpf is achieved through dispersion of different types of formulations in exactly the same classifier concept. On the other hand, it is shown that air classifier-based inhalers are less sensitive to the carrier surface and bulk properties than competitive inhalers like the Diskus. For 10 randomly selected lactose carriers for inhalation from four different suppliers, the budesonide fpf (at 4 kPa) from the Novolizer varied between 30 and 46% (of the measured dose; R.S.D.=14.2%), whereas the extremes in fpf from the Diskus dpi were 7 and 44% (R.S.D.=56.2%) for the same formulations. The fpf from a classifier-based inhaler appears to be less dependent of the amount of lactose (carrier) fines (<15 microm) in the mixture too. Classifier-based inhalers perform best with coarse carriers that have relatively wide size distributions (e.g. 50-350 microm) and surface discontinuities inside which drug particles can find shelter from press-on forces during mixing. Coarse carrier fractions have good flow properties, which increases the dose measuring accuracy and reproducibility. The fpf from the Novolizer increases with increasing pressure drop across the device. On theoretical grounds, it can be argued that this yields a more reproducible therapy, because it compensates for a shift in deposition to larger airways when the flow rate is increased. Support for this reasoning based on lung deposition modelling studies has been found in a scintigraphic study with the Novolizer. Finally, it is shown that this inhaler produces a finer aerosol than competitor devices, within the fpf<5 microm, subfractions of particles (e.g. <1, 1-2, 2-3, 3-4 and 4-5 microm) are higher.

Trends in the technology-driven development of new inhalation devices

Inhalation technology diverges rapidly along various lines. A variety of technological solutions are currently under development to overcome the many problems related to adequate aerosol generation both for dry powder inhalation systems and for liquid inhalation systems. Many of the improvements are related to the fine particle fraction in the generated aerosol, particularly its dependency on the patients' inspiratory flow profile and the velocity of the aerosol.: