Discrete Modelling of Powder Dispersion in Dry Powder Inhalers - A Brief Review

CFD-DEM investigation of the effects of aperture size for a capsule-based dry powder inhaler

Capsule based dry powder inhalers (DPIs) often require piercing of the capsule before inhalation, and the characteristics of the apertures (punctured holes) affect air flow and the release of powders from the capsule. This work develops a numerical model based on the two-way coupling of computational fluid dynamics and discrete element method (CFD-DEM) to investigate the effect of aperture size on powder dispersion in the Aerolizer® device loaded with only carrier particles (lactose). Powders (carrier particles) in the size range 60-140 μm (d50: 90 μm and span: 0.66) were initialized in a capsule which had a circular aperture at each end. Boundary conditions corresponding to an air flow rate of 45 L/min were specified at each inlet to the mixing chamber (i.e., a total flow rate 90 L/min), and a capsule spin speed of ∼ 4050 rpm. The velocity magnitudes inside the capsule were considerably lower than those in the mixing chamber in the vicinity of the rotating capsule, with the exception of the capsules featuring 2.5 mm and 4 mm apertures. Larger apertures reduced the capsule emptying time and increased the particle evacuation velocity; the fluid drag force on the particles issuing from the capsule peaked for an aperture of 1.3 mm. Inside the capsule, particle-particle (PP) collisions were more frequent than particle-wall (PW) collisions due to high concentration of powder, but PP collisions had smaller (median) impact energy than PW collisions. Larger apertures resulted in fewer collisions in the capsule with higher PW and virtually unchanged PP collision energies. Outside the capsule (i.e., in the inhaler mixing chamber), PW collisions occurred more frequently than PP collisions with median collision energies typically two orders of magnitude higher than inside the capsule. Larger apertures resulted in more collisions with slightly reduced collision energy, but this effect plateaued for aperture sizes larger than 1.3 mm. Powder dispersion, expressed as the fine particle fraction (FPF) of the powder, was predicted using an empirical equation based on carrier PW collisions. Therefore, consistent with the model prediction of the effect of aperture sizes on the chamber collision frequency, FPF increased with aperture size but plateaued beyond 1.3 mm.

Quantifying Agglomerate-to-Wall Impaction in Dry Powder Inhalers

PURPOSE

The probability of agglomerate-to-wall collision was quantified using a unique image processing technique applied to high-speed microscopic images. The study aimed to investigate the effects of flow rate and particle size on the percentage of colliding agglomerates detected within an in-house powder dispersion device.

METHOD

The device consists of a swirl chamber and two tangential inlets in various configurations, designed to emulate the geometric features of commercial devices such as the Aerolizer® and Osmohaler®. The test cases were conducted with constant flow rates of 30 SLPM and 60 SLPM. Four powder samples were tested, including carrier Respitose® SV010 (median volume diameter 104 µm, span 1.7) and mannitol of three constituent primary particle sizes (3 µm, 5 µm and 7 µm; span 1.6 - 1.9).

RESULTS

At the lower flow rate of 30 SLPM, collision frequencies were significantly different between powders of different constituent particle sizes, but the effects of powder properties diminished on increasing the flow rate to 60 SLPM. At the higher flow rate, all powders experienced a significant increase in the proportion of colliding particles.

CONCLUSION

Analysis of collision events showed that the probability of collision for each agglomerate increased with agglomerate diameter and velocity. Experimental data of agglomerate-to-wall collision were utilised to develop a logistic regression model that can accurately predict collisions with various powders and flow rates.

Numerical Modeling of Particle Dynamics Inside a Dry Powder Inhaler

The development of novel dry powders for dry powder inhalers (DPIs) requires the in vitro assessment of DPI aerodynamic performance. As a potential complementary method, in silico numerical simulations can provide additional information about the mechanisms that guide the particles and their behavior inside DPIs. The aim of this study was to apply computational fluid dynamics (CFDs) coupled with a discrete phase model (DPM) to describe the forces and particle trajectories inside the RS01^® as a model DPI device. The methodology included standard fluid flow equations but also additional equations for the particle sticking mechanism, as well as particle behavior after contacting the DPI wall surface, including the particle detachment process. The results show that the coefficient of restitution between the particle and the impact surface does not have a high impact on the results, meaning that all tested combinations gave similar output efficiencies and particle behaviors. No sliding or rolling mechanisms were observed for the particle detachment process, meaning that simple bouncing off or deposition particle behavior is present inside DPIs. The developed methodology can serve as a basis for the additional understanding of the particles' behavior inside DPIs, which is not possible using only in vitro experiments; this implies the possibility of increasing the efficiency of DPIs.

Scientific and regulatory activities initiated by the U.S. Food and drug administration to foster approvals of generic dry powder inhalers: Bioequivalence perspective

Regulatory science for generic dry powder inhalers (DPIs) in the United States (U.S.) has evolved over the last decade. In 2013, the U.S. Food and Drug Administration (FDA) published the draft product-specific guidance (PSG) for fluticasone propionate and salmeterol xinafoate inhalation powder. This was the first PSG for a DPI available in the U.S., which provided details on a weight-of-evidence approach for establishing bioequivalence (BE). A variety of research activities including in vivo and in vitro studies were used to support these recommendations, which have led to the first approval of a generic DPI in the U.S. for fluticasone propionate and salmeterol xinafoate inhalation powder in January of 2019. This review describes the scientific and regulatory activities that have been initiated by FDA to support the current BE recommendations for DPIs that led to the first generic DPI approvals, as well as research with novel in vitro and in silico methods that may potentially facilitate generic DPI development and approval.

Additive manufacturing in respiratory sciences - Current applications and future prospects

Additive Manufacturing (AM) comprises a variety of techniques that enable fabrication of customised objects with specific attributes. The versatility of AM procedures and constant technological improvements allow for their application in the development of medicinal products and medical devices. This review provides an overview of AM applications related to respiratory sciences. For this purpose, both fields of research are briefly introduced and the potential benefits of integrating AM to respiratory sciences at different levels of pharmaceutical development are highlighted. Tailored manufacturing of microstructures as a particle design approach in respiratory drug delivery will be discussed. At the dosage form level, we exemplify AM as an important link in the iterative loop of data driven inhaler design, rapid prototyping and in vitro testing. This review also presents the application of bioprinting in the respiratory field for design of biorelevant in vitro cellular models, followed by an overview of AM-related processes in preventive and therapeutic care. Finally, this review discusses future prospects of AM as a component in a digital health environment.

Phage cocktail powder for Pseudomonas aeruginosa respiratory infections

Phage cocktail broadens the host range compared with a single phage and minimizes the development of phage-resistant bacteria thereby promoting the long-term usefulness of inhaled phage therapy. In this study, we produced a phage cocktail powder by spray drying three Pseudomonas phages PEV2 (podovirus), PEV1 and PEV20 (both myovirus) with lactose (80 wt%) and leucine (20 wt%) as excipients. Our results showed that the phages remained viable in the spray dried powder, with little to mild titer reduction (ranging from 0.11 to 1.3 logs) against each of their specific bacterial strains. The powder contained spherical particles with a small volume median diameter of 1.9 µm (span 1.5), a moisture content of 3.5 ± 0.2 wt%., and was largely amorphous with some crystalline peaks, which were assigned to the excipient leucine, as shown in the X-ray diffraction pattern. When the powder was dispersed using the low- and high-resistance Osmohalers, the fine particle fraction (FPF, wt. % of particles < 5 µm in the aerosols relative to the loaded dose) values were 45.37 ± 0.27% and 62.69 ± 2.1% at the flow rate of 100 and 60 L/min, respectively. In conclusion, the PEV phage cocktail powder produced was stable, inhalable and efficacious in vitro against various MDR P. aeruginosa strains that cause pulmonary infections. This formulation will broaden the bactericidal spectrum and reduce the emergence of resistance in bacteria compared with single-phage formulations reported previously.

Role of CFD based in silico modelling in establishing an in vitro-in vivo correlation of aerosol deposition in the respiratory tract

Effective evaluation and prediction of aerosol transport deposition in the human respiratory tracts are critical to aerosol drug delivery and evaluation of inhalation products. Establishment of an in vitro-in vivo correlation (IVIVC) requires the understanding of flow and aerosol behaviour and underlying mechanisms at the microscopic scale. The achievement of the aim can be facilitated via computational fluid dynamics (CFD) based in silico modelling which treats the aerosol delivery as a two-phase flow. CFD modelling research, in particular coupling with discrete phase model (DPM) and discrete element method (DEM) approaches, has been rapidly developed in the past two decades. This paper reviews the recent development in this area. The paper covers the following aspects: geometric models of the respiratory tract, CFD turbulence models for gas phase and its coupling with DPM/DEM for aerosols, and CFD investigation of the effects of key factors associated with geometric variations, flow and powder characteristics. The review showed that in silico study based on CFD models can effectively evaluate and predict aerosol deposition pattern in human respiratory tracts. The review concludes with recommendations on future research to improve in silico prediction to achieve better IVIVC.

Effect of Particle-Wall Interaction and Particle Shape on Particle Deposition Behavior in Human Respiratory System

Dry powder inhalation (DPI) has attracted much attention as a treatment for respiratory diseases owing to the large effective absorption area in a human respiratory system. Understanding the drug particle motion in the respiratory system and the deposition behavior is necessary to improve the efficiency of DPI. We conducted computer simulations using a model coupling a discrete element method and a computational fluid dynamics method (DEM-CFD) to evaluate the particle deposition in human respiratory system. A simple artificial respiratory model was developed, which numerically investigated the effect of particle properties and inhalation patterns on the particle deposition behavior. The DEM-CFD simulations demonstrated that the smaller- and lower-density particles showed higher reachability into the simple respiratory model, and the particle arrival ratio to the deep region strongly depended on the aerodynamic diameter. The particle arrival ratio can be described as an exponential function of the aerodynamic diameter. Furthermore, the exponential relationship between the particle reachability into the depth of the simple respiratory model and the aerodynamic diameter predicted the particle aerodynamic diameter based on the required reachability. The particle shape also had an impact on the particle deposition behavior. The rod-like particles with a larger aspect ratio indicated higher reachability into the depth of the simple respiratory model. This was attributed to the high velocity motion of the particles whose long axis was in the direction of the deep region.

In Silico Methods for Development of Generic Drug-Device Combination Orally Inhaled Drug Products

The development of generic, single‐entity, drug–device combination products for orally inhaled drug products is challenging in part because of the complex nature of device design characteristics and the difficulties associated with establishing bioequivalence for a locally acting drug product delivered to the site of action in the lung. This review examines in silico models that may be used to support the development of generic orally inhaled drug products and how model credibility may be assessed.