In vitro/in vivo comparisons in pulmonary drug delivery

Quality by Design in Pulmonary Drug Delivery: A Review on Dry Powder Inhaler Development, Nanotherapy Approaches, and Regulatory Considerations

Dry powder inhalers (DPIs) are state-of-the-art pulmonary drug delivery systems. This article explores the transformative impact of nanotechnology on DPIs, emphasizing the Quality Target Product Profile (QTPP) with a focus on aerodynamic performance and particle characteristics. It navigates global regulatory frameworks, underscoring the need for safety and efficacy standards. Additionally, it highlights the emerging field of nanoparticulate dry powder inhalers, showcasing their potential to enhance targeted drug delivery in respiratory medicine. This concise overview is a valuable resource for researchers, physicians, and pharmaceutical developers, providing insights into the development and commercialization of advanced inhalation systems.

Dry powder inhaler design and particle technology in enhancing Pulmonary drug deposition: challenges and future strategies

OBJECTIVES

The efficient delivery of drugs from dry powder inhaler (DPI) formulations is associated with the complex interaction between the device design, drug formulations, and patient's inspiratory forces. Several challenges such as limited emitted dose of drugs from the formulation, low and variable deposition of drugs into the deep lungs, are to be resolved for obtaining the efficiency in drug delivery from DPI formulations. The objective of this study is to review the current challenges of inhaled drug delivery technology and find a way to enhance the efficiency of drug delivery from DPIs.

METHODS/EVIDENCE ACQUISITION

Using appropriate keywords and phrases as search terms, evidence was collected from the published articles following SciFinder, Web of Science, PubMed and Google Scholar databases.

RESULTS

Successful lung drug delivery from DPIs is very challenging due to the complex anatomy of the lungs and requires an integrated strategy for particle technology, formulation design, device design, and patient inhalation force. New DPIs are still being developed with limited performance and future device design employs computer simulation and engineering technology to overcome the ongoing challenges. Many issues of drug formulation challenges and particle technology are concerning factors associated with drug dispersion from the DPIs into deep lungs.

CONCLUSION

This review article addressed the appropriate design of DPI devices and drug formulations aligned with the patient's inhalation maneuver for efficient delivery of drugs from DPI formulations.

Nebulization and In Vitro Upper Airway Deposition of Liposomal Carrier Systems

Liposomal carrier systems have emerged as a promising technology for pulmonary drug delivery. This study focuses on two selected liposomal systems, namely, dipalmitoylphosphatidylcholine stabilized by phosphatidic acid and cholesterol (DPPC-PA-Chol) and dipalmitoylphosphatidylcholine stabilized by polyethylene glycol and cholesterol (DPPC-PEG-Chol). First, the research investigates the stability of these liposomal systems during the atomization process using different kinds of nebulizers (air-jet, vibrating mesh, and ultrasonic). The study further explores the aerodynamic particle size distribution of the aerosol generated by the nebulizers. The nebulizer that demonstrated optimal stability and particle size was selected for more detailed investigation, including Andersen cascade impactor measurements, an assessment of the influence of flow rate and breathing profiles on aerosol particle size, and an in vitro deposition study on a realistic replica of the upper airways. The most suitable combination of a nebulizer and liposomal system was DPPC-PA-Chol nebulized by a Pari LC Sprint Star in terms of stability and particle size. The influence of the inspiration flow rate on the particle size was not very strong but was not negligible either (decrease of Dv50 by 1.34 μm with the flow rate increase from 8 to 60 L/min). A similar effect was observed for realistic transient inhalation. According to the in vitro deposition measurement, approximately 90% and 70% of the aerosol penetrated downstream of the trachea using the stationary flow rate and the realistic breathing profile, respectively. These data provide an image of the potential applicability of liposomal carrier systems for nebulizer therapy. Regional lung drug deposition is patient-specific; therefore, deposition results might vary for different airway geometries. However, deposition measurement with realistic boundary conditions (airway geometry, breathing profile) brings a more realistic image of the drug delivery by the selected technology. Our results show how much data from cascade impactor testing or estimates from the fine fraction concept differ from those of a more realistic case.

Pulmonary inhalation for disease treatment: Basic research and clinical translations

Pulmonary drug delivery has the advantages of being rapid, efficient, and well-targeted, with few systemic side effects. In addition, it is non-invasive and has good patient compliance, making it a highly promising drug delivery mode. However, there have been limited studies on drug delivery via pulmonary inhalation compared with oral and intravenous modes. This paper summarizes the basic research and clinical translation of pulmonary inhalation drug delivery for the treatment of diseases and provides insights into the latest advances in pulmonary drug delivery. The paper discusses the processing methods for pulmonary drug delivery, drug carriers (with a focus on various types of nanoparticles), delivery devices, and applications in pulmonary diseases and treatment of systemic diseases (e.g., COVID-19, inhaled vaccines, diagnosis of the diseases, and diabetes mellitus) with an updated summary of recent research advances. Furthermore, this paper describes the applications and recent progress in pulmonary drug delivery for lung diseases and expands the use of pulmonary drugs for other systemic diseases.

Using Dry Dispersion Laser Diffraction to Assess Dispersibility in Spheronized Agglomerate Formulations

In this study, dry dispersion laser diffraction was used to study the dispersibility of spheronized agglomerate formulations and identify geometric particle size metrics that correlated well with aerodynamic particle size distribution (APSD). Eleven unique batches of agglomerates were prepared for both laser diffraction and cascade impaction testing. Correlations between the particle size distribution (PSD) and aerodynamic particle size distribution (APSD) metrics for the eleven agglomerate batches were determined in a semi-empirical manner. The strongest correlation between APSD and PSD was observed between the impactor-sized mass (%ISM) and the cumulative PSD fraction <14.5 µm. The strongest correlation with fine particle fraction (FPF) was observed with the cumulative PSD fraction <0.99 micron (R-squared = 0.974). In contrast to the other APSD metrics, good correlations were not found between the mass median aerodynamic diameter (MMAD) and the cumulative PSD fractions. Overall, the implementation of laser diffraction as a surrogate for cascade impaction has the potential to streamline product development. Laser diffraction measurements offer savings in labor and turnaround time compared to cascade impaction.

Tracheomalacia Reduces Aerosolized Drug Delivery to the Lung

Rationale: Neonates with respiratory issues are frequently treated with aerosolized medications to manage lung disease or facilitate airway clearance. Dynamic tracheal collapse (tracheomalacia [TM]) is a common comorbidity in these patients, but it is unknown whether the presence of TM alters the delivery of aerosolized drugs. Objectives: To quantify the effect of neonatal TM on the delivery of aerosolized drugs. Methods: Fourteen infant subjects with respiratory abnormalities were recruited; seven with TM and seven without TM. Respiratory-gated 3D ultrashort echo time magnetic resonance imaging (MRI) was acquired covering the central airway and lungs. For each subject, a computational fluid dynamics simulation modeled the airflow and particle transport in the central airway based on patient-specific airway anatomy, motion, and airflow rates derived from MRI. Results: Less aerosolized drug reached the distal airways in subjects with TM than in subjects without TM: of the total drug delivered, less particle mass passed through the main bronchi in subjects with TM compared with subjects without TM (33% vs. 47%, p = 0.013). In subjects with TM, more inhaled particles were deposited on the surface of the airway (48% vs. 25%, p = 0.003). This effect becomes greater with larger particle sizes and is significant for particles with a diameter >2 μm (2-5 μm, p ≤ 0.025 and 5-15 μm, p = 0.004). Conclusions: Neonatal patients with TM receive less aerosolized drug delivered to the lungs than subjects without TM. Currently, infants with lung disease and TM may not be receiving adequate and/or expected medication. Particles >2 μm in diameter are likely to deposit on the surface of the airway due to anatomical constrictions such as reduced tracheal and glottal cross-sectional area in neonates with TM. This problem could be alleviated by delivering smaller aerosolized particles.

A new method for investigating bioequivalence of inhaled formulations: A pilot study on salbutamol

Purpose: An efficient, cost-effective and non-invasive test is required to overcome the challenges faced in the process of bioequivalence (BE) studies of various orally inhaled drug formulations. Two different types of pressurized meter dose inhalers (MDI-1 and MDI-2) were used in this study to test the practical applicability of a previously proposed hypothesis on the BE of inhaled salbutamol formulations. Methods: Salbutamol concentration profiles of the exhaled breath condensate (EBC) samples collected from volunteers receiving two inhaled formulations were compared employing BE criteria. In addition, the aerodynamic particle size distribution of the inhalers was determined by employing next generation impactor. Salbutamol concentrations in the samples were determined using liquid and gas chromatographic methods. Results: The MDI-1 inhaler induced slightly higher EBC concentrations of salbutamol when compared with MDI-2. The geometric MDI-2/MDI-1 mean ratios (confidence intervals) were 0.937 (0.721-1.22) for maximum concentration and 0.841 (0.592-1.20) for area under the EBC-time profile, indicating a lack of BE between the two formulations. In agreement with the in vivo data, the in vitro data indicated that the fine particle dose (FPD) of MDI-1 was slightly higher than that for the MDI-2 formulation. However, the FPD differences between the two formulations were not statistically significant. Conclusion: EBC data of the present work may be considered as a reliable source for assessment of the BE studies of orally inhaled drug formulations. However, more detailed investigations employing larger sample sizes and more formulations are required to provide more evidence for the proposed method of BE assay.

Imaging Aerosol Deposition with Two-Dimensional Gamma Scintigraphy

Several imaging modalities have been employed to quantify lung dose and the distribution of the dose of orally inhaled aerosols in vivo. Two-dimensional (2D, or planar) imaging using gamma scintigraphy is the most widely used of these modalities. Two-dimensional gamma scintigraphy studies are accomplished using a single- or dual-headed gamma camera. The formulation to be tested is admixed with the gamma emitting radioisotope ^99mtechnetium, which serves as a surrogate for the drug. This article provides details as to how 2D gamma scintigraphy images should be acquired and analyzed using recently standardized methods. Based on the new guidelines, the investigator should confirm that the drug formulation is unchanged with the addition of the radioisotope, determine the amount of radioactivity needed for inhalation to obtain appropriate radioactivity counts in the lungs, perform quality control procedures for the gamma camera, identify the lung borders of the study subject using a reference image such as an X-ray computed tomography scan, a ventilation scan, or a transmission scan, acquire a lung transmission image to correct for attenuation of radioactivity by lung tissue, instruct the subject how to inhale the radiolabel-drug mixture and record associated breathing parameters, acquire anterior and/or posterior views of the lungs and any other regions of interest (i.e., oropharynx, stomach) and assess the acquired images for total and regional dose to the lungs. Total dose should be assessed after identification of the right lung border and appropriate correction for tissue attenuation. Regional dose should be quantified as a normalized outer/inner deposition ratio (O/I) and expressed as the penetration index (PI). Mass balance should be performed as needed. By following the standardized methods, 2D gamma scintigraphy data from studies in different laboratories may be compared and combined, leading to multi-center studies and more rapid development of new medications and devices for inhaled therapies.

Characterization of dry powder inhaler performance through experimental methods

Experimental methods provide means for the quality control of existing DPIs and for exploring the influence of formulation and device parameters well in advance of clinical trials for novel devices and formulations. In this review, we examine the state of the art of in vitro testing of DPIs, with a focus primarily on the development of accurate in vitro-in vivo correlations. Aspects of compendial testing are discussed, followed by the influence of flow profiles on DPI performance, the characterization of extrathoracic deposition using mouth-throat geometries, and the characterization of regional thoracic deposition. Additional experimental methods that can inform the timing of bolus delivery, the influence of environmental conditions, and the development of electrostatic charge on aerosolized DPI powders are reviewed. We conclude with perspectives on current in vitro methods and identify potential areas for future investigation, including the estimation of variability in deposition, better characterization of existing compendial methods, optimization of formulation and device design to bypass extrathoracic deposition, and the use of novel tracheobronchial filters that aim to provide more clinically relevant measures of performance directly from in vitro testing.

The Study of Spray-Freeze-Drying Technique for Development of Novel Combination pMDIs, Part I: Study on the Preparation Method

Clinically available pressurized metered-dose inhalers (pMDIs) mainly directly use micronized drugs as inhalable microparticles. Although technology for preparing pMDIs has proven to obtain clinically appropriate aerosol performance, the fine particle fraction and delivered dose content uniformity (DDCU) of pMDIs still need to be improved. DDCU problem is usually exacerbated by patients' handling errors prior to taking a dose. In this study, novel phospholipid microparticle inhalation pMDIs were prepared by a spray-freeze-drying process using mometasone furoate and formoterol fumarate dihydrate as model drugs and distearoylphosphatidylcholine as an excipient. Combined with the material composition, the atomization and freeze-drying processes were also studied. Our data showed that both atomization parameters of gas–liquid ratio and freeze-drying curve settings met the requirements of drug design. According to aerodynamic performance in vitro and DDCU evaluation, the performance of the phospholipid microparticle inhalation pMDI was better than that of the micronized drug microparticle pMDI. In conclusion, preparing pMDIs with particle engineering has the potential to ensure accuracy of quantification and to improve the efficiency of drug deposition in lungs in clinical practice.

Cytotoxicity of 2D engineered nanomaterials in pulmonary and corneal epithelium

Two-dimensional (2D) engineered nanomaterials are widely used in consumer and industrial goods due to their unique chemical and physical characteristics. Engineered nanomaterials are incredibly small and capable of being aerosolized during manufacturing, with the potential for biological interaction at first-contact sites such as the eye and lung. The unique properties of 2D nanomaterials that make them of interest to many industries may also cause toxicity towards epithelial cells. Using murine and human respiratory epithelial cell culture models, we tested the cytotoxicity of eight 2D engineered nanomaterials: graphene (110 nm), graphene oxide (2 um), graphene oxide (400 nm), reduced graphene oxide (2 um), reduced graphene oxide (400 nm), partially reduced graphene oxide (400 nm), molybdenum disulfide (400 nm), and hexagonal boron nitride (150 nm). Non-graphene nanomaterials were also tested in human corneal epithelial cells for ocular epithelial cytotoxicity. Hexagonal boron nitride was found to be cytotoxic in mouse tracheal, human alveolar, and human corneal epithelial cells. Hexagonal boron nitride was also tested for inhibition of wound healing in alveolar epithelial cells; no inhibition was seen at sub-cytotoxic doses. Nanomaterials should be considered with care before use, due to specific regional cytotoxicity that also varies by cell type. Supported by U01ES027288 and T32HL007013 and T32ES007059.

Preparation and Characterization of Inhalable Ivermectin Powders as a Potential COVID-19 Therapy

Background: Ivermectin has received worldwide attention as a potential COVID-19 treatment after showing antiviral activity against SARS-CoV-2 in vitro. However, the pharmacokinetic limitations associated with oral administration have been postulated as limiting factors to its bioavailability and efficacy. These limitations can be overcome by targeted delivery to the lungs. In this study, inhalable dry powders of ivermectin and lactose crystals were prepared and characterized for the potential treatment of COVID-19. Methods: Ivermectin was co-spray dried with lactose monohydrate crystals and conditioned by storage at two different relative humidity points (43% and 58% RH) for a week. The in vitro dispersion performance of the stored powders was examined using a medium-high resistance Osmohaler connecting to a next-generation impactor at 60 L/min flow rate. The solid-state characteristics including particle size distribution and morphology, crystallinity, and moisture sorption profiles of raw and spray-dried ivermectin samples were assessed by laser diffraction, scanning electron microscopy, Raman spectroscopy, X-ray powder diffraction, thermogravimetric analysis, differential scanning calorimetry, and dynamic vapor sorption. Results: All the freshly spray-dried formulation (T0) and the conditioned samples could achieve the anticipated therapeutic dose with fine particle dose of 300 μg, FPF_recovered of 70%, and FPF_emitted of 83%. In addition, the formulations showed a similar volume median diameter of 4.3 μm and span of 1.9. The spray-dried formulations were stable even after conditioning and exposing to different RH points as ivermectin remained amorphous with predominantly crystalline lactose. Conclusion: An inhalable and stable dry powder of ivermectin and lactose crystals was successfully formulated. This powder inhaler ivermectin candidate therapy appears to be able to deliver doses that could be safe and effective to treat the SARS-COV-2 infection. Further development of this therapy is warranted.

Fine Particle Fraction: The Good and the Bad

Fine particle fraction (FPF) is defined in general terms as the fraction or percentage of the drug mass contained in an aerosol cloud that may be small enough to enter the lungs and exert a clinical effect. An aerodynamic diameter of 5 μm represents the approximate border between "fine" and "coarse" particles, but there is no universally agreed upon definition of FPF in terms of an aerodynamic particle size range. FPF alone does not adequately describe a heterodisperse aerodynamic particle size distribution, and it needs to be combined with another measure or measures indicating the width of the distribution. When determined using techniques specified in United States and European Pharmacopeias, FPF is measured by cascade impactors that have straight-sided ninety degree inlets through which air is drawn at a constant rate. It is not the purpose of in vitro tests to predict in vivo behavior, and FPF is primarily a measure of aerosol quality. Despite this, FPF broadly predicts the amount of drug from an inhaler device depositing in the lungs, but it systematically overestimates whole lung deposition and may not correctly predict the relative lung depositions for two inhalers of different types. The relationship between FPF and both drug pharmacokinetics and clinical response is incompletely understood at the present time, and more studies are needed to investigate these relationships. Modifications to impactor technologies, including inlets that mimic the human extrathoracic airways and the use of realistic breathing patterns, would be expected to improve the predictive power of in vitro tests for drug delivery in vivo.

Advancements in Particle Engineering for Inhalation Delivery of Small Molecules and Biotherapeutics

Dry powder inhalation formulations have become increasingly popular for local and systemic delivery of small molecules and biotherapeutics. Powder formulations provide distinct advantages over liquid formulations such as elimination of cold chain due to room temperature stability, improved portability, and the potential for increasing patient adherence. To become a viable product, it is essential to develop formulations that are stable (physically, chemically and/or biologically) and inhalable over the shelf-life. Physical particulate properties such as particle size, morphology and density, as well as chemical properties can significantly impact aerosol performance of the powder. This review will cover these critical attributes that can be engineered to enhance the dispersibility of inhalation powder formulations. Challenges in particle engineering for biotherapeutics will be assessed, followed by formulation strategies for overcoming the hurdles. Finally, the review will discuss recent examples of successful dry powder biotherapeutic formulations for inhalation delivery that have been clinically assessed.

Nanoparticle-Based Drug Delivery System: The Magic Bullet for the Treatment of Chronic Pulmonary Diseases

Chronic pulmonary diseases encompass different persistent and lethal diseases, including chronic obstructive pulmonary disease (COPD), idiopathic pulmonary fibrosis (IPF), cystic fibrosis (CF), asthma, and lung cancers that affect millions of people globally. Traditional pharmacotherapeutic treatment approaches (i.e., bronchodilators, corticosteroids, chemotherapeutics, peptide-based agents, etc.) are not satisfactory to cure or impede diseases. With the advent of nanotechnology, drug delivery to an intended site is still difficult, but the nanoparticle's physicochemical properties can accomplish targeted therapeutic delivery. Based on their surface, size, density, and physical-chemical properties, nanoparticles have demonstrated enhanced pharmacokinetics of actives, achieving the spotlight in the drug delivery research field. In this review, the authors have highlighted different nanoparticle-based therapeutic delivery approaches to treat chronic pulmonary diseases along with the preparation techniques. The authors have remarked the nanosuspension delivery via nebulization and dry powder carrier is further effective in the lung delivery system since the particles released from these systems are innumerable to composite nanoparticles. The authors have also outlined the inhaled particle's toxicity, patented nanoparticle-based pulmonary formulations, and commercial pulmonary drug delivery devices (PDD) in other sections. Recently advanced formulations employing nanoparticles as therapeutic carriers for the efficient treatment of chronic pulmonary diseases are also canvassed.

In vitro-in vivo correlation of cascade impactor data for orally inhaled pharmaceutical aerosols

In vitro-in vivo correlation is the establishment of a predictive relationship between in vitro and in vivo data. In the context of cascade impactor results of orally inhaled pharmaceutical aerosols, this involves the linking of parameters such as the emitted dose, fine particle dose, fine particle fraction, and mass median aerodynamic diameter to in vivo lung deposition from scintigraphy data. If the dissolution and absorption processes after deposition are adequately understood, the correlation may be extended to the pharmacokinetics and pharmacodynamics of the delivered drugs. Correlation of impactor data to lung deposition is a relatively new research area that has been gaining recent interest. Although few in number, experiments and meta-analyses have been conducted to examine such correlations. An artificial neural network approach has also been employed to analyse the complex relationships between multiple factors and responses. However, much research is needed to generate more data to obtain robust correlations. These predictive models will be useful in improving the efficiency in product development by reducing the need of expensive and lengthy clinical trials.

Bioequivalence assessment of pharmaceutical aerosol products through IVIVC

Many pharmaceutical developers of generic orally inhaled products (OIPs) are facing significant issues in passing the regulatory requirement to show pharmacokinetic (PK) bioequivalence (BE) to the originator product. The core of the issue is that no reliable in vitro-in vivo correlation (IVIVC) is available to guide their development. In this paper, several issues are identified and means to improve the data used for developing an IVIVC are discussed. The article also presents an "IVIVC-free" approach for developing a formulation matching the originator's PK performance.

A Quality by Design Framework for Capsule-Based Dry Powder Inhalers

Capsule-based dry powder inhalers (cDPIs) are widely utilized in the delivery of pharmaceutical powders to the lungs. In these systems, the fundamental nature of the interactions between the drug/formulation powder, the capsules, the inhaler device, and the patient must be fully elucidated in order to develop robust manufacturing procedures and provide reproducible lung deposition of the drug payload. Though many commercially available DPIs utilize a capsule-based dose metering system, an in-depth analysis of the critical factors associated with the use of the capsule component has not yet been performed. This review is intended to provide information on critical factors to be considered for the application of a quality by design (QbD) approach for cDPI development. The quality target product profile (QTPP) defines the critical quality attributes (CQAs) which need to be understood to define the critical material attributes (CMA) and critical process parameters (CPP) for cDPI development as well as manufacturing and control.

Aerosol delivery via noninvasive ventilation: role of models and bioanalysis

Non-invasive ventilation (NIV) is external support for respiration to assist breathing in case of respiratory failure (either hypercapnic or hypoxemic) without patient intubation. Nowadays, medicated aerosols are normally delivered to mechanically ventilated patients by nebulizers and pressurized metered-dose inhaler (pMDI) attached to adapter or spacer that fit into the ventilated circuit. Studies with obstructive lung disease patients have shown that aerosol delivery during mechanical ventilation is possible and of benefit. There are several models for investigating the aerosol delivery and deposition during mechanical ventilation such as in vitro, in vivo, and ex vivo models, these models depend on the technique used for quantitative or qualitative measurement of the deposited aerosol. In vitro models could be used for calculating the total emitted doses from different aerosol-generating devices or for aerodynamic characterization of the deposited inhaled medications. In vivo models dependents of extracting drugs from biological samples for measuring its concentration and bioavailability (pharmacokinetic model) or be dependent on the imaging technique of the radioactive aerosol. Applying different methods to predict aerosol efficiency before starting NIV and to quantify aerosol delivery during NIV are promising approaches that guide clinicians to avoid treatment failure before and during patient therapy.

In vitro-in vivo correlations (IVIVCs) of deposition for drugs given by oral inhalation

Conventional in vitro tests to assess the aerodynamic particle size distribution (APSD) from inhaler devices use simple right-angle inlets ("mouth-throats", MTs) to cascade impactors, and air is drawn through the system at a fixed flow for a fixed time. Since this arrangement differs substantially from both human oropharyngeal airway anatomy and the patterns of air flow when patients use inhalers, the ability of in vitro tests to predict in vivo deposition of pharmaceutical aerosols has been limited. MTs that mimic the human anatomy, coupled with simulated breathing patterns, have yielded estimates of lung dose from in vitro data that closely match those from in vivo gamma scintigraphic or pharmacokinetic studies. However, different models of MTs do not always yield identical data, and selection of an anatomical MT and representative inhalation profiles remains challenging. Improved in vitro - in vivo correlations (IVIVCs) for inhaled drug products could permit increased reliance on in vitro data when developing new inhaled drug products, and could ultimately result in accelerated drug product development, together with reduced research and development spending.

Effects of Inhaled Corticosteroids and Particle Size on Risk of Obstructive Sleep Apnea: A Large Retrospective Cohort Study

Background: Inhaled corticosteroids (ICS) produce local effects on upper airway dilators that could increase the risk of developing obstructive sleep apnea (OSA). Given that the particle size of ICS changes their distribution, the particle size of ICS may impact the risk of developing OSA. Objectives: In this large retrospective study, we explore the relationship of ICS use and OSA in patients with asthma. In addition, we seek to determine if this relationship is affected by the particle size of ICS. Methods: Using electronic health records, we established a cohort of 29,816 asthmatics aged 12 and older with a diagnosis of asthma documented by ICD-9 or ICD-10 codes between January 2011 and August 2016. We performed analyses of variance and multivariate logistic regression analysis to determine the effects ICS on the diagnosis of OSA with sub-analysis by particle size of ICS. Results: Uncontrolled asthmatics showed increased odds of receiving a diagnosis of OSA whether when looking at ACT scores (adjusted odds ratio (aOR) 1.60, 95% CI 1.32–1.94) or PFT results (aOR 1.45, 95% CI 1.19–1.77). Users of ICS also had increased odds of OSA independent of asthma control (aOR 1.58, 95% CI 1.47–1.70). Notably, users of extra-fine particle ICS did not have significantly increased odds of having OSA compared to non-users of ICS (aOR 1.11, 95% CI 0.78–1.58). Conclusions: Use of ICS appears to be an independent risk factor for OSA. Notably, extra-fine particle size ICS do not appear to be associated with an increased risk of OSA.

Significant Unresolved Questions and Opportunities for Bioengineering in Understanding and Treating COVID-19 Disease Progression

COVID-19 is a disease that manifests itself in a multitude of ways across a wide range of tissues. Many factors are involved, and though impressive strides have been made in studying this novel disease in a very short time, there is still a great deal that is unknown about how the virus functions. Clinical data has been crucial for providing information on COVID-19 progression and determining risk factors. However, the mechanisms leading to the multi-tissue pathology are yet to be fully established. Although insights from SARS-CoV-1 and MERS-CoV have been valuable, it is clear that SARS-CoV-2 is different and merits its own extensive studies. In this review, we highlight unresolved questions surrounding this virus including the temporal immune dynamics, infection of non-pulmonary tissue, early life exposure, and the role of circadian rhythms. Risk factors such as sex and exposure to pollutants are also explored followed by a discussion of ways in which bioengineering approaches can be employed to help understand COVID-19. The use of sophisticated in vitro models can be employed to interrogate intercellular interactions and also to tease apart effects of the virus itself from the resulting immune response. Additionally, spatiotemporal information can be gleaned from these models to learn more about the dynamics of the virus and COVID-19 progression. Application of advanced tissue and organ system models into COVID-19 research can result in more nuanced insight into the mechanisms underlying this condition and elucidate strategies to combat its effects.

Development of a Carrier-Free Dry Powder Ofloxacin Formulation With Enhanced Aerosolization Properties

Tuberculosis (TB) is a serious infectious disease that affects more than new 10 million patients each year. Many of these cases are resistant to first-line drugs so second-line ones, like fluoroquinolones, need to be incorporated into the therapeutic. Ofloxacin (OF) is a fluoroquinolone which demonstrates high antibiotic activity against the bacteria that causes TB (M. tuberculosis). In this work, ionic complexes, composed by hyaluronic acid (HA) and OF, with different neutralization degrees, were prepared and processed by spray drying (SD) to obtain powders for inhalatory administration. Combining a formulation with high neutralization degree, high SD atomization air flowrate and the use of a high-performance collection cyclone, very good process yields were obtained. Carrier-free formulations with a loading of 0.39-0.46 g_OF/g_powder showed excellent emitted, fine particle, and respirable fractions for capsule loadings of 25 and 100 mg. The ionic complexes demonstrated higher mucoadhesion than pure OF and HA. The best formulation did not affect CALU-3 cell viability up to a dose 6.5 times higher than the MIC₉₀ reported to treat multi-drug resistant TB.

Controller Inhalers: Overview of Devices, Instructions for Use, Errors, and Interventions to Improve Technique

Inadequate inhaler technique in persistent asthma is frequently reported. However, there is little consensus on inhaler checklists, and critical elements of technique are not uniformly described. In addition, inhaler error rates and risk factors for poor technique are variable across studies. This Clinical Commentary Review summarizes the literature on inhaler design, use, and interventions to improve technique. Our aim is to help clinicians identify patients with poor inhaler technique, recognize the most important errors, and correct technique using evidence-based interventions.

Cascade Impactor Equivalence Testing: Comparison of the Performance of the Modified Chi-Square Ratio Statistic (mCSRS) with the Original CSRS and EMA's Average Bioequivalence Approach

The performances of three statistical approaches for assessing in vitro equivalence was evaluated with a set of 55 scenarios of realistic test (T) and reference (R) cascade impactor (CI) profiles (originally employed by the Product Quality Research Institute to evaluate the chi-square ratio statistic: CSRS) by comparing the outcomes against experts' opinion (surrogate for the truth). The three methods were (A) a stepwise aerodynamic particle size distribution (APSD) equivalence test integrating population bioequivalence (PBE) testing of impactor-sized mass (ISM) with the CSRS (PBE-CSRS approach), previously suggested by the USFDA; (B) the combination of PBE testing of single actuation content and ISM with the newly suggested modified CSRS (PBE-mCSRS approach), a method employing reference variance scaling; and (C) EMA's average bioequivalence (ABE approach). Based on Monte-Carlo simulations, both PBE-CSRS and ABE approaches resulted in high misclassification rates, the former with highest false-pass rate and the latter with highest false-fail rate at both ≥ 50% and ≥ 80% classification threshold values (the % of simulations or experts necessary to judge a given scenario as equivalent). Based on DeLong's tests, the PBE-mCSRS approach showed significantly better overall agreement with experts' opinion compared to the other approaches. Comparison of CSRS with mCSRS (both without PBE) suggested that the more discriminatory characteristics of the mCSRS method is based on the integration of variance scaling into the mCSRS method. Contrary to the ABE approach, the application of PBE-mCSRS approach for assessing APSD profiles of three dry powder inhaler (DPI) formulations supported the pharmacokinetic bioequivalence assessment of these formulations.

Overview of Brazilian Requirements for Therapeutic Equivalence of Orally Inhaled and Nasal Drug Products

Brazil has established a framework for provision of generic pharmaceuticals including for orally inhaled and nasal drug products (OINDP) to its populace. This includes the development of guidelines or "resolutions" and normative instructions describing the Brazilian medicines agency's (Anvisa) expectations for demonstrating OINDP therapeutic equivalence. The Anvisa regulatory framework for OINDP therapeutic equivalence, challenges, and comparisons with the US Food and Drug Administration and European Medicines Agency approaches are assessed and discussed.

Aerosol Therapy for Pneumonia in the Intensive Care Unit

Antibiotic aerosolization in patients with ventilator-associated pneumonia (VAP) allows very high concentrations of antimicrobial agents in the respiratory secretions, far more than those achievable using the intravenous route. However, data in critically ill patients with pneumonia are limited. Administration of aerosolized antibiotics might increase the likelihood of clinical resolution, but no significant improvements in important outcomes have been consistently documented. Thus, aerosolized antibiotics should be restricted to the treatment of extensively resistant gram-negative pneumonia. In these cases, the use of a vibrating-mesh nebulizer seems to be more efficient, but specific settings and conditions are required to improve lung delivery.

A new hypothesis to investigate bioequivalence of pharmaceutical inhalation products

BACKGROUND

This short communication reports a new hypothesis regarding bioequivalence of inhalation products which can potentially provide a reliable means to compare pharmaceutical aerosol formulations and inhalers.

METHODS

Available methods regarding the bioequivalence studies, inhaled drugs and advantages of exhaled breath condensate (EBC) samples were reviewed to develop this hypothesis.

RESULTS

It is postulated that two inhalation products providing the same drug concentrations in airway lining fluid (ALF) could be considered bioequivalent. The use of EBC tests which reflect ALF composition can be recommended as an alternative to current testing methods for consideration of bioequivalence.

CONCLUSION

The methods based on EBC analysis can potentially be applied to bioequivalence study of inhalation products and could reflect drug concentration in ALF. However, experimental studies would be necessary to support or refute this hypothesis on the novel application of EBC to bioequivalence in the future. Graphical abstract In vitro (cascade impactor) and In vivo (EBC concentration) corrolation for inhaled drugs.

Delivery Technologies for Orally Inhaled Products: an Update

Orally inhaled products have well-known benefits. They allow for effective local administration of many drugs for the treatment of pulmonary disease, and they allow for rapid absorption and avoidance of first-pass metabolism of several systemically acting drugs. Several challenges remain, however, such as dosing limitations, low and variable deposition of the drug in the lungs, and high drug deposition in the oropharynx region. These challenges have stimulated the development of new delivery technologies. Both formulation improvements and new device technologies have been developed through an improved understanding of the mechanisms of aerosolization and lung deposition. These new advancements in formulations have enabled improved aerosolization by controlling particle properties such as density, size, shape, and surface energy. New device technologies emerging in the marketplace focus on minimizing patient errors, expanding the range of inhaled drugs, improving delivery efficiency, increasing dose consistency and dosage levels, and simplifying device operation. Many of these new technologies have the potential to improve patient compliance. This article reviews how new delivery technologies in the form of new formulations and new devices enhance orally inhaled products.

Radiolabeling an Electronic Cigarette Aerosol Using Technetium Carbon Ultrafine Particles

BACKGROUND

Electronic cigarettes (ECIGs) are widely used, but their health effects are not well known. ECIG exposure is difficult to quantify, and a direct measurement of deposition would be beneficial to in vivo and in vitro toxicity studies. The aim of this study was to demonstrate effective radiolabeling of an ECIG.

METHODS

A technetium-99m-labeled carbon ultrafine (TCU) aerosol was generated and introduced to a fourth-generation ECIG before nucleation and aerosol formation. The aerosolized e-liquid was a commercially available strawberry flavor containing 1.2% nicotine in a 55% propylene glycol and 45% vegetable glycerine base. An ECIG power setting of 100 W was selected. Mass and radioactivity were measured on each stage within a Sierra Cascade Impactor at 14 L/min to verify the labeling technique using the calculated aerodynamic diameters. A strong positive correlation (R² > 0.95) between the percent activity and percent mass deposition on each stage provides a reliable validation of colocation.

RESULTS

Unlabeled ECIG aerosol from the chosen e-liquid produced a mass median aerodynamic diameter (MMAD) of 0.85 μm. An ECIG labeled with TCU produced an aerosol with an activity median aerodynamic diameter of 0.84 μm and an MMAD of 0.84 μm. The relative mass versus radioactivity on each plate was highly correlated (average R² = 0.973, p < 0.001).

CONCLUSION

A TCU radiolabel was generated and shown to associate with the mass of an aerosol produced by a typical commercially available ECIG. Thus, the radioactivity of the deposited aerosol may be used to determine ECIG aerosol deposition for the future in vivo and in vitro dosimetry studies of the third- and fourth-generation ECIGs.

Carrier free indomethacin microparticles for dry powder inhalation

The present studies were designed to evaluate inhalatory microparticles carrying indomethacin (IN) for potential local (specific and non-specific bronchial inflammatory asthma responses) and systemic treatments (joint inflammation, rheumatoid arthritis and osteoarthritis pain) by optimizing microparticle properties, characterizing their lung deposition, drug release, evaluating cytotoxicity and also pharmacological effect in vitro. The acidic groups of IN were complexed with the cationic groups of the polyelectrolyte polylysine in order to increase the drug water compatibility. The polylysine/indomethacin ratio was fixed and the pH was adjusted in different formulations. Microparticles were obtained by spray drying using a relatively high atomization air flowrate (742 L/min) and a high-performance cyclone in order to optimize the production of microparticles with adequate attributes for inhalatory delivery. The produced microparticles exhibited high process yield and IN loading, volumetric mean diameters smaller than 5 μm and narrow particle size distributions. According to demonstrated aerosolization performance, the powders were suitable for inhalatory indomethacin local and systemic treatments. Emitted fraction was higher than 90%, the MMAD was around 3 μm and the GSD lower than 3. The respirable fraction for particles with aerodynamic diameters smaller than 5 μm was around 29% while for particles with aerodynamic diameters smaller than 3 μm the value was around 17%. The addition of lactose as carrier worsened the aerodynamic performance of the microparticles. The developed powdered systems got wet and dissolved quickly and presented higher release rates respect to pure IN in simulated lung physiological conditions. Furthermore, the assays performed in RAW 264.7 cell line showed that the microparticles exhibited the same anti-inflammatory capability as the pure drug. The developed particles did not affect the RAW 264.7 cell viability. In conclusion, a promising powder formulation for DPIs has been developed to treat, locally and systemically, inflammatory diseases.

Scientific Rationale for Determining the Bioequivalence of Inhaled Drugs

In recent years, pathways for the development and approval of bioequivalent inhaled products have been established for regulated markets, including the European Union (EU), and a number of orally inhaled products (OIPs) have been approved in the EU solely on the basis of in vitro and pharmacokinetic data. This review describes how these development pathways are structured and their implications for the treatment of airway diseases such as asthma. The EU guidance follows a stepwise approach that includes in vitro criteria as the first step. If all in vitro criteria are not met, the second step is based on pharmacokinetic evaluations, which include assessments of lung and systemic bioavailability. If all pharmacokinetic criteria are not met, the third step is based on clinical endpoint studies. In this review, the scientific rationale of the European Medicines Agency guidance for the development of bioequivalent OIPs is reviewed with the focus on the development of bioequivalent OIPs in the EU. Indeed, we discuss the advantages and disadvantages of the weight-of-evidence and stepwise approaches. The evidence indicates that the EU guidance is robust and, unlike clinical endpoint studies, the pharmacokinetic studies are far more sensitive to measure the minor differences, i.e. deposition and absorption rates, in drug delivery from the test and reference products and, thus, should be best suited for assessing bioequivalence. The acceptance range of the 90% confidence intervals for pharmacokinetic bioequivalence (i.e. 80-125% for both the area under the plasma concentration-time curve and maximum plasma concentration) represent appropriately conservative margins for ensuring equivalent safety and efficacy of the test and reference products.

Effects of Formulation Variables on Lung Dosimetry of Albuterol Sulfate Suspension and Beclomethasone Dipropionate Solution Metered Dose Inhalers

The performance of pressurized metered dose inhalers (MDIs) is affected by formulation and device variables that impact delivered dose, aerodynamic particle size distribution, and consequently lung deposition and therapeutic effect. Specific formulation variables of relevance to two commercially available products-Proventil® HFA [albuterol sulfate (AS) suspension] and Qvar® [beclomethasone dipropionate (BDP) solution]-were evaluated to determine their influence on key performance attributes measured experimentally with in vitro cascade impaction studies. These commercial MDIs, utilized as model systems, provided mid-points for a design of experiments (DoE) plan to manufacture multiple suspension and solution MDI formulations. The experimental results were utilized as input variables in a computational dosimetry model to predict the effects of MDI formulation variables on lung deposition. For the BDP solution DoE MDIs, increased concentrations of surfactant oleic acid (0-2% w/w) increased lung deposition from 24 to 46%, whereas changes in concentration of the cosolvent ethanol (7-9% w/w) had no effect on lung deposition. For the AS suspension DoE MDIs, changes in oleic acid concentration (0.005-0.25% w/w) did not have significant effects on lung deposition, whereas lung deposition decreased from 48 to 26% as ethanol concentration increased from 2 to 20% w/w, and changes in micronized drug volumetric median particle size distribution (X₅₀, 1.4-2.5 μm) increased deposition in the tracheobronchial airways from 5 to 11%. A direct correlation was observed between fine particle fraction and predicted lung deposition. These results demonstrate the value of using dosimetry models to further explore relationships between performance variables and lung deposition.

Formulation techniques for high dose dry powders

Delivery of drugs to the lungs via dry powder inhaler (DPI) is a promising approach for the treatment of both local pulmonary conditions and systemic diseases. Though DPIs are widely used for the pulmonary deposition of potent bronchodilators, anticholinergics, and corticosteroids, there is growing interest in the utilization of this delivery system for the administration of high drug doses to the lungs, as made evident by recent regulatory approvals for anti-microbial, anti-viral and osmotic agents. However, the formulation of high dose DPIs carries several challenges from both a physiological and physicochemical standpoint. This review describes the various formulation techniques utilized to overcome the barriers associated with the pulmonary delivery of high dose powders.

Aerosolization of Zn-DTPA Decorporation Agent Using Jet and Ultrasonic Nebulizers

BACKGROUND

Chelating agents such as diethylenetriamine pentaacetic acid (DTPA) can be used as a decorporation drug in the zinc (Zn) form to treat internal radioactive contamination after exposure to plutonium or americium in a nuclear accident. Although Zn-DTPA is normally administered intravenously, inhalation of Zn-DTPA in aerosol form is a better route for direct delivery to the lungs. This work investigates the feasibility of synthesizing Zn-DTPA from three common chemicals and aerosolizing it using a jet or ultrasonic nebulizer.

METHODS

The particle size distribution (PSD) of this decorporation agent at different concentrations were tested in vitro using two different methods: inertial impaction and aerodynamic time of flight. The particles were generated using either a jet nebulizer or an ultrasonic nebulizer. Two parameters, namely the mass median aerodynamic diameter and the geometric standard deviation, were assessed to determine the PSD of the generated aerosols. These parameters were obtained for different concentrations of Zn-DTPA using both nebulizers.

RESULTS AND CONCLUSIONS

Zn-DTPA was successfully synthesized for decorporation purposes. Aerosol particles within the inhalable range were successfully generated by both nebulizers from four different concentrations of Zn-DTPA. It was found that the medication concentration did not affect the PSD of Zn-DTPA. The ultrasonic nebulizer was observed to produce a slightly larger aerosol particle size and required slightly longer treatment periods to deliver an effective dose to the lungs when compared with the jet nebulizer. Both nebulizers can be sustainably run to administer the agent for effective decorporation treatment of a large population after any major nuclear accident.

Dry powder aerosols to co-deliver antibiotics and nutrient dispersion compounds for enhanced bacterial biofilm eradication

The purpose of this study was to formulate a dry powder for inhalation containing a combination treatment for eradication of Pseudomonas aeruginosa bacterial biofilms. Dry powders containing an antibiotic (ciprofloxacin hydrochloride, CH) and nutrient dispersion compound (glutamic acid, GA) at a ratio determined to eliminate the biofilms were generated by spray drying. Leucine was added to the spray dried formulation to aid powder flowability. A central composite design of experiments was performed to determine the effects of solution and processing parameters on powder yield and aerodynamic properties. Combinations of CH and GA eradicated bacterial biofilms at lower antibiotic concentrations compared to CH alone. Spray dried powders were produced with yields up to 43% and mass mean aerodynamic diameters (MMAD) in the respirable range. Powder yield was primarily affected by variables that determine cyclone efficiency, i.e. atomizer and solution flow rates and solution concentration; while MMAD was mainly determined by solution concentration. Fine particle fractions (FPF)<4.46μm and <2.82μm of the powders ranged from 56 to 70% and 35 to 46%, respectively. This study demonstrates that dry powder aerosols containing high concentrations of a combination treatment effective against P. aeruginosa biofilms could be developed with high yield, aerodynamic properties appropriate for inhalation, and no loss of potency.

Application of a One-Dimensional Computational Model for the Prediction of Deposition from a Dry Powder Inhaler

BACKGROUND

Accurate prediction of the regional deposition of inhaled dry powders as a function of powder properties and breathing pattern is a long-term research goal for pulmonary drug delivery. In the present work, deposition along the respiratory tract of dry powders of Fluticasone propionate and Salmeterol is predicted.

METHODS

A one-dimensional particle transport and deposition model is used, whose novelty is in the treatment of the alveolar space of each airway generation as an efficient mixing chamber. This assumption has been supported by simulations and measurements during the last 20 years. The model is applied to two popular pulmonary tree geometries, to investigate the effect of particle size on localized deposition and to estimate the uncertainty due to variations in airway size.

RESULTS AND CONCLUSIONS

Application of the model for the specific particle size distribution measured by a cascade impactor in the marketed product ELPENhaler, predicts the whole lung deposition (WLD), as well as the split between pulmonary (PU) and tracheobronchial (TB) deposition. Introduction in the model of modified particle size distributions with increased fractions of fine particles, indicates that the fine-particle dose is a satisfactory predictor of WLD but not of the PU/TB ratio.

Inhaled Treprostinil Drug Delivery During Mechanical Ventilation and Spontaneous Breathing Using Two Different Nebulizers

OBJECTIVES

To determine the feasibility of delivering inhaled treprostinil during mechanical ventilation and spontaneous unassisted ventilation using the Tyvaso Inhalation System and the vibrating mesh nebulizer. We sought to compare differences in fine particle fraction, and absolute inhaled treprostinil mass delivered to neonatal, pediatric, and adult models affixed with a face mask, conventional, and high-frequency ventilation between Tyvaso Inhalation System and with different nebulizer locations between Tyvaso Inhalation System and vibrating mesh nebulizer.

DESIGN

Fine particle fraction was first determined via impaction with both the Tyvaso Inhalation System and vibrating mesh nebulizer. Next, a test lung configured with neonatal, pediatric, and adult mechanics and a filter to capture medication was attached to a realistic face model during spontaneous breathing or an endotracheal tube during conventional ventilation and high-frequency oscillator ventilator. Inhaled treprostinil was then nebulized with both the Tyvaso Inhalation System and vibrating mesh nebulizer, and the filter was analyzed via high-performance liquid chromatography. Testing was done in triplicate. Independent two-sample t tests were used to compare mean fine particle fraction and inhaled mass between devices. Analysis of variance with Tukey post hoc tests were used to compare within device differences.

SETTING

Academic children's hospital aerosol research laboratory.

MEASUREMENTS AND MAIN RESULTS

Fine particle fraction was not different between the Tyvaso Inhalation System and vibrating mesh nebulizer (0.78 ± 0.04 vs 0.77 ± 0.08, respectively; p = 0.79). The vibrating mesh nebulizer delivered the same or greater inhaled treprostinil than the Tyvaso Inhalation System in every simulated model and condition. When using the vibrating mesh nebulizer, delivery was highest when using high-frequency oscillator ventilator in the neonatal and pediatric models, and with the nebulizer in the distal position in the adult model.

CONCLUSIONS

The vibrating mesh nebulizer is a suitable alternative to the Tyvaso Inhalation System for inhaled treprostinil delivery. Fine particle fraction is similar between devices, and vibrating mesh nebulizer delivery meets or exceeds delivery of the Tyvaso Inhalation System. Delivery for infants and children during high-frequency oscillator ventilator with the vibrating mesh nebulizer may result in higher than expected dosages.

Computationally efficient analysis of particle transport and deposition in a human whole-lung-airway model. Part II: Dry powder inhaler application

Pulmonary drug delivery is becoming a favored route for administering drugs to treat both lung and systemic diseases. Examples of lung diseases include asthma, cystic fibrosis and chronic obstructive pulmonary disease (COPD) as well as respiratory distress syndrome (ARDS) and pulmonary fibrosis. Special respiratory drugs are administered to the lungs, using an appropriate inhaler device. Next to the pressurized metered-dose inhaler (pMDI), the dry powder inhaler (DPI) is a frequently used device because of the good drug stability and a minimal need for patient coordination. Specific DPI-designs and operations greatly affect drug-aerosol formation and hence local lung deposition. Simulating the fluid-particle dynamics after use of a DPI allows for the assessment of drug-aerosol deposition and can also assist in improving the device configuration and operation. In Part I of this study a first-generation whole lung-airway model (WLAM) was introduced and discussed to analyze particle transport and deposition in a human respiratory tract model. In the present Part II the drug-aerosols are assumed to be injected into the lung airways from a DPI mouth-piece, forming the mouth-inlet. The total as well as regional particle depositions in the WLAM, as inhaled from a DPI, were successfully compared with experimental data sets reported in the open literature. The validated modeling methodology was then employed to study the delivery of curcumin aerosols into lung airways using a commercial DPI. Curcumin has been implicated to possess high therapeutic potential as an antioxidant, anti-inflammatory and anti-cancer agent. However, efficacy of curcumin treatment is limited because of the low bioavailability of curcumin when ingested. Hence, alternative drug administration techniques, e.g., using inhalable curcumin-aerosols, are under investigation. Based on the present results, it can be concluded that use of a DPI leads to low lung deposition efficiencies because large amounts of drugs are deposited in the oral cavity. Hence, the output of a modified DPI has been evaluated to achieve improved drug delivery, especially needed when targeting the smaller lung airways. This study is the first to utilize CF-PD methodology to simulate drug-aerosol transport and deposition under actual breathing conditions in a whole lung model, using a commercial dry-powder inhaler for realistic inlet conditions.

The Impact of Inspiratory Flow Rate on Drug Delivery to the Lungs with Dry Powder Inhalers

Current marketed dry powder inhalers utilize the energy from patient inspiration to fluidize and disperse bulk powder agglomerates into respirable particles. Variations in patient inspiratory flow profiles can lead to marked differences in total lung dose (TLD), and ultimately patient outcomes for an inhaled therapeutic. The present review aims to quantitate the flow rate dependence in TLD observed for a number of drug/device combinations using a new metric termed the Q index. With this data in hand, the review explores key attributes in the design of the formulation and device that impact flow rate dependence. The review also proposes alternative in vitro methods to assess flow rate dependence that more closely align with in vivo observations. Finally, the impact of variations in flow rate on lung function for inhaled bronchodilators is summarized.