Effects of tumors on inhaled pharmacologic drugs

Lung carcinomas are now the most common form of cancer. Clinical data suggest that tumors are found preferentially in upper airways, perhaps specifically at carina within bifurcations. The disease can be treated by aerosolized pharmacologic drugs. To enhance their efficacies site-specific drugs must be deposited selectively. Since inhaled particles are transported by air, flow patterns will naturally affect their trajectories. Therefore, in Part I of a systematic investigation, we focused on tumor-induced effects on airstreams, in Part II (the following article [p. 245]), particle trajectories were determined. To facilitate the targeted delivery of inhaled drugs, we simulated bifurcations with tumors on carinas using a commercial computational fluid dynamics (CFD) software package (FIDAP) with a Cray T90 supercomputer and studied effects of tumor sizes and ventilatory parameters on localized flow patterns. Critical tumor sizes existed; e.g., tumors had dominant effects when r/R > or = 0.8 for bifurcation 3-4 and r/R > or = 0.6 for bifurcation 7-8 (r = tumor radius and R = airway radius). The findings suggest that computer modeling is a means to integrate alterations to airway structures caused by diseases into aerosol therapy protocols.     

Radon dosimetry applications ofin vitro tracheobronchial-deposition measurements

Deposition efficiencies of monodisperse ammonium fluorescein aerosols have been measured in simulated human lungs made of replica laryngeal casts combined with trachebronchial systems. Other tests, with radiolabelled submicron-sized particles, combined the larynges with replica tracheobronchial casts. The laryngeal casts had internal flow rate-specific geometries. Data indicate thatin vitro bifurcations have ?hot spots? or highly localized deposits, particularly at carinal ridges, suggesting that epithelial cells at airway branching sitesin vitro receive increased exposures to inhaled particulate matter. For dosimetry purposes, therefore, the lung should be likened to a series of Y-shaped airway junctures. The data have risk assessment applications for ambient radon progeny and radioactive airborne particles found in uranium mining and milling operations.     

Systemic pilocarpine increases deposition of and decreases responsiveness to inhaled carbachol in sheep.

The purpose of this study was to determine whether excessive airway secretions could serve as a barrier function against inhaled particulate matter. To increase airway secretions, six conscious sheep were treated with pilocarpine (0.8 mg/kg i.v.). Pilocarpine increased pulmonary resistance (RL) and total aerosol deposition within five breaths (AD5) as determined by the rebreathing of an inert monodisperse aerosol. When RL had returned to baseline, AD5 remained elevated [21 +/- 2% (SE), P < 0.05] and tracheal secretions were increased (237 +/- 77%, P < 0.05) above the values before pilocarpine administration. A carbachol aerosol dose-response curve was carried out at this time and compared with a control carbachol dose-response curve by calculating the dose of carbachol required to increase RL by 400% (PD400). Mean PD400 was increased postpilocarpine by 53 +/- 18 (P < 0.05) and 85 +/- 25% (P < 0.05) when normalized for increased aerosol deposition. Thus, pilocarpine decreased airway responsiveness to inhaled carbachol despite increasing aerosol deposition. The pilocarpine-induced airway hyporesponsiveness to inhaled carbachol is consistent with the hypothesis that excessive secretions have a protective role in the airways.     

Aerosol-derived lung morphometry: comparisons with a lung model and lung function indexes.

This study evaluated the ability of aerosol-derived lung morphometry to noninvasively probe airway and acinar dimensions. Effective air-space diameters (EAD) were calculated from the time-dependent gravitational losses of 1-microns particles from inhaled aerosol boluses during breath holding. In 17 males [33 +/- 7 (SD) yr] the relationship between EAD and volumetric penetration of the bolus into the lungs (Vp) could be expressed by the linear power-law function, log (EAD) alpha beta log (Vp). Our EAD values were consistent with Weibel's symmetric lung model A for small airways and more distal air spaces. As lung volume increased from 57 to 87% of total lung capacity (TLC), EAD at Vp of 160 and 550 cm3 increased 70 and 41%, respectively. At 57% TLC, log (EAD) at 160 cm3 was significantly correlated with airway resistance (r = -0.57, P less than 0.0204) but not with forced expired flow between 25 and 75% of vital capacity. Log (EAD) at 400 cm3 was correlated with deposition of 1-micron particles (r = -0.73, P less than 0.0009). We conclude that aerosol-derived lung morphometry is a responsive noninvasive probe of peripheral air-space diameters.     

Rat airway morphometry measured from in situ MRI-based geometric models

Rodents have been widely used to study the environmental or therapeutic impact of inhaled particles. Knowledge of airway morphometry is essential in assessing geometric influence on aerosol deposition and in developing accurate lung models of aerosol transport. Previous morphometric studies of the rat lung performed ex situ provided high-resolution measurements (50-125 μm). However, it is unclear how the overall geometry of these casts might have differed from the natural in situ appearance. In this study, four male Wistar rat (268 ± 14 g) lungs were filled sequentially with perfluorocarbon and phosphate-buffered saline before being imaged in situ in a 7-T magnetic resonance (MR) scanner at a resolution of 0.2 × 0.2 × 0.27 mm. Airway length, diameter, gravitational, bifurcation, and rotational angles were measured for the first four airway generations from 3D geometric models built from the MR images. Minor interanimal variability [expressed by the relative standard deviation RSD (=SD/mean)] was found for length (0.18 ± 0.07), diameter (0.15 ± 0.15), and gravitational angle (0.12 ± 0.06). One rat model was extended to 16 airway generations. Organization of the airways using a diameter-defined Strahler ordering method resulted in lower interorder variability than conventional generation-based grouping for both diameter (RSD = 0.12 vs. 0.42) and length (0.16 vs. 0.67). Gravitational and rotational angles averaged 82.9 ± 37.9° and 53.6 ± 24.1°, respectively. Finally, the major daughter branch bifurcated at a smaller angle (19.3 ± 14.6°) than the minor branch (60.5 ± 19.4°). These data represent the most comprehensive set of rodent in situ measurements to date and can be used readily in computational studies of lung function and aerosol exposure.     

Effects of bronchodilator particle size in asthmatic patients using monodisperse aerosols.

Aerosol particle size influences airway drug deposition. Current inhaler devices are inefficient, delivering a heterodisperse distribution of drug particle sizes where, at best, 20% reaches the lungs. Monodisperse aerosols are the appropriate research tools to investigate basic aerosol science concepts within the human airways. We hypothesized that engineering such aerosols of albuterol would identify the ideal bronchodilator particle size, thereby optimizing inhaled therapeutic drug delivery. Eighteen stable mildly to moderately asthmatic patients [mean forced expiratory volume in 1 s (FEV1) 74.3% of predicted] participated in a randomized, double-blind, crossover study design. A spinning-top aerosol generator was used to produce monodisperse albuterol aerosols that were 1.5, 3, and 6 microm in size, and also a placebo, which were inhaled at cumulative doses of 10, 20, 40, and 100 microg. Lung function changes and tolerability effects were determined. The larger particles, 6 and 3 microm, were significantly more potent bronchodilators than the 1.5-microm and placebo aerosols for FEV1 and for the forced expiratory flow between exhalation of 25 and 75% of forced vital capacity. A 20-microg dose of the 6- and 3-microm aerosols produced FEV1 bronchodilation comparable to that produced by 200 microg from a metered-dose inhaler. No adverse effects were observed in heart rate and plasma potassium. The data suggest that in mildly to moderately asthmatic patients there is more than one optimal beta2-agonist bronchodilator particle size and that these are larger particles in the higher part of the respirable range. Aerosols delivered in monodisperse form can enable large reductions of the inhaled dose without loss of clinical efficacy.     

A two-component theory of aerosol deposition in lung airways

The deposition of aerosol particles in the human lung airways is due to two distinct mechanisms. One is by direct deposition resulting from diffusion, sedimentation and impaction as the aerosol moves in and out of the lung. The other is an indirect mechanism by which particles are transported mechanically from the tidal air to the residential air and eventually captured by the airways due to intrinsic particle motion. This last mechanism is not well understood at present. Using a trumpet airway model constructed from Weibel's data, a two-component theory is developed. In this theory, the particle concentrations in the airways and the alveoli at a given airway depth are considered to be quantitatively different. This difference in concentrations will cause a net mixing between the tidal and residential aerosol as the aerosol is breathed in and out. A distribution parameter is then introduced to account for the distribution of ventilation. The effect of intrinsic particle motion on the aerosol mixing is also included. From this theory, total and regional deposition in the lung at the steady mouth breathing without pause is calculated for several different respiratory cycles. The results agree reasonably well with the experimental data.     

Nonmicrobial-mediated inflammatory airway diseases—an update

In lungs, airways are in constant contact with air, microbes, allergens, and environmental pollutants. The airway epithelium represents the first line of lung defense through different mechanisms, which facilitate clearance of inhaled pathogens and environmental particles while minimizing an inflammatory response. The innate immune system facilitates immediate recognition of both foreign pathogens and tissue damage through toll-like receptor, which acts as a gateway for all intracellular events leading to inflammation. In the absence of microbial stimulus, the immune system is capable of detecting a wide range of insults against the host. This review focuses on various molecular mechanisms involved in pathophysiology of airway inflammation mediated by environmental factors, cellular stress, and pharmacological and clinical agents.     

Airway deposition of hygroscopic heterodispersed aerosols: results of a computer calculation

A new computer model is developed and used to calculate the deposition of inhaled heterodispersed hygroscopic aerosols for mouth breathing in a Weibel symmetric bronchial tree. The model was first validated by obtaining good agreement with recent experimental and theoretical data on regional and total airway deposition of monodispersed and heterodispersed nonhygroscopic aerosols. The model was then used to obtain predictions of regional and total deposition of heterodispersed hygroscopic aerosol particles (droplets of NaCl solutions). Parameters that were varied in the hygroscopic calculations include initial droplet NaCl concentration, time of inspiration and expiration, volume of aerosol inspired, period of breath holding, and initial inhaled lognormal aerosol mass median diameter and geometric standard deviation. Results of the computer calculations show that increasing heterodispersity tends to flatten and broaden regional deposition curves when fraction of inhaled mass deposited is plotted vs. inhaled mass median aerodynamic particle diameter. Hygroscopicity is shown to increase tracheobronchial and pulmonary airway deposition with hypertonic NaCl solution aerosols showing increases over isotonic and nonhygroscopic aerosols of up to 200%.     

The influence of a carbachol aersol on the tracheobronchial deposition of 99mTc tagged particles.

The tracheobronchial deposition of inhaled 99mTc tagged teflon particles of 6 mum (specific density 2g/cm3) was determined in rabbits by comparing the particle content in free dissected parts of the tracheobronchial tree with that in the whole lung. There was a singificantly larger deposition of particles in the proximal parts of the airways in rabbits exposed to an aerosol of the bronchoconstrictor compound carbachol than in control rabbits exposed to distilled water alone. The resistance to insufflation of a constant volume of air increased during the exposure to the carbachol aerosol, indicating bronchoconstriction. There was reproducible interindividual differences in bronchoconstrictor response to the carbachol aerosol. They were attributed to interindividual differences either in deposition of carbachol or in bronchial muscle sensitivity to carbachol. It is concluded that bronchoconstriction might serve as a defensive measure in causing a more proximal deposition of inhaled particles.     

In Vitro Surfactant and Perfluorocarbon Aerosol Deposition in a Neonatal Physical Model of the Upper Conducting Airways

Objective

Aerosol delivery holds potential to release surfactant or perfluorocarbon (PFC) to the lungs of neonates with respiratory distress syndrome with minimal airway manipulation. Nevertheless, lung deposition in neonates tends to be very low due to extremely low lung volumes, narrow airways and high respiratory rates. In the present study, the feasibility of enhancing lung deposition by intracorporeal delivery of aerosols was investigated using a physical model of neonatal conducting airways.

Methods

The main characteristics of the surfactant and PFC aerosols produced by a nebulization system, including the distal air pressure and air flow rate, liquid flow rate and mass median aerodynamic diameter (MMAD), were measured at different driving pressures (4–7 bar). Then, a three-dimensional model of the upper conducting airways of a neonate was manufactured by rapid prototyping and a deposition study was conducted.

Results

The nebulization system produced relatively large amounts of aerosol ranging between 0.3±0.0 ml/min for surfactant at a driving pressure of 4 bar, and 2.0±0.1 ml/min for distilled water (H₂Od) at 6 bar, with MMADs between 2.61±0.1 µm for PFD at 7 bar and 10.18±0.4 µm for FC-75 at 6 bar. The deposition study showed that for surfactant and H₂Od aerosols, the highest percentage of the aerosolized mass (∼65%) was collected beyond the third generation of branching in the airway model. The use of this delivery system in combination with continuous positive airway pressure set at 5 cmH₂O only increased total airway pressure by 1.59 cmH₂O at the highest driving pressure (7 bar).

Conclusion

This aerosol generating system has the potential to deliver relatively large amounts of surfactant and PFC beyond the third generation of branching in a neonatal airway model with minimal alteration of pre-set respiratory support.     

Comparison of theoretical and experimental particle diffusion data within human airway casts

The dose delivered to airway cells is a critical factor whether one is addressing the therapeutic (i.e., positive) effects of inhaled pharmacologic agents or the toxic (i.e., negative) effects of pollutants. In this study, theoretical models describing particle deposition have been compared with experimental data from the literature. In the simulations, airways can be either roughor smooth-walled to be consistent with human lungs which can be either lined by cartilaginous rings (i.e., rough) or muscle (i.e., smooth). Particle motion for rough-walled airways within generations I=1–6 is calculated using the formula proposed by Martonen et al. (1). For smooth-walled airways within generations I=7–10, particle motion is calculated using the formula proposed by Martonen et al. (2). Theoretical predictions of particle deposition efficiencies are not only in agreement with the overall best fit empirical correlation presented by Cohen and Asgharian (3) over a wide range of dimensionless diffusion parameters, but also match individual experimental measurements (only available in I=1–6) with regard to effects of the parameters of particle size, flow rate, and airway dimensions. The mean difference in the ratio of experimental-to-theoretical particle diffusion values is 0.9 for a flow rate of 18 L/min and 1.1 for a flow rate of 34 L/min (i.e., the difference is only about 10%) within the upper airways of the casts (airway generations I=1–6), the mean difference for the whole casts was much greater. This may be attributed to the uncertainty of flow conditions in the peripheral airways as a result of the trimmmed nature of the casts. Overall, the findings suggest that the model can be a valuable component of aerosol therapy and risk assessment protocols, especially to address effects of enhanced deposition of pharmacologic drugs and radionuclides at sites within the human tracheobronchial tree.     

Pulmonary defense mechanisms in Boa constrictor

We studied aerosol deposition and the response to inhaled particles and irritants in lungs of Boa constrictor. Snakes which breathed submicrometric particles radiolabeled with 99mTc retained 41.4 +/- 9.9% of the aerosol in the trachea, 42.5 +/- 8.8% in the anterior faveolar regions, and 8.7 +/- 4.1% in the posterior saccular regions of the lungs. Low activity recovered in the gastrointestinal tract over a 5-h period following aerosol exposure indicated slow clearance of inhaled particles. In contrast to mammalian lungs, there are no macrophages resident on the surface of boa lungs, and uningested particles persist for up to 4 days without being phagocytized. Particles and irritant stimuli (Fe2O3, endotoxin, and N-formylmethionylphenylalanine) elicited only eosinophilic granulocytes that were not phagocytic. The numbers of these cells peaked at 24 h following exposure and declined gradually over the next 7 days. Lavage fluid from stimulated snake lungs contained many large lamellar figures continuous with tubular myelin, a form of surfactant. Very little of this material was recovered from control lungs. Response to inhaled particles and lung injury in boas increased surfactant release, elicited eosinophilic granulocytes, but did not recruit phagocytic mononuclear cells.     

Lung tumor induction in Syrian hamsters with internally deposited particulate Pu: A synthesis based on microscopic dose distribution

Syrian hamsters inhaled a monodisperse aerosol of 238PuO2 and were serially sacrificed to study the microscopic distribution of particles, tissue at risk and dose as a function of time after exposure. The distribution of dose and tissue at risk around single particles in lung and the changes in distribution of particles with time have been reported previously. In the present paper, these measurements are applied to the computation of tissue-at-risk and radiation-dose-rate distributions within the lungs of Syrian hamsters. Based on these results, airway epithelium is irradiated at the same levels as other lung tissue and does not require separate consideration on the basis of dose to tissue. Incorporation of the measured microscopic radiation dose distribution into existing dose-effect models allowed data on lung tumor induction in Syrian hamsters from several laboratories to be adequately described by a model fit to data from a single laboratory.     

Flow simulation in the human upper respiratory tract

Computer simulations of airflow patterns within the human upper respiratory tract (URT) are presented. The URT model includes airways of the head (nasal and oral), throat (pharyngeal and laryngeal), and lungs (trachea and main bronchi). The head and throat morphology was based on a cast of a medical school teaching model; tracheobronchial airways were defined mathematically. A body-fitted three-dimensional curvilinear grid system and a multiblock method were employed to graphically represent the surface geometries of the respective airways and to generate the corresponding mesh for computational fluid dynamics simulations. Our results suggest that for a prescribed phase of breath (i.e., inspiration or expiration), convective respiratory airflow patterns are highly dependent on flow rate values. Moreover, velocity profiles were quite different during inhalation and exhalation, both in terms of the sizes, strengths, and locations of localized features such as recirculation zones and air jets. Pressure losses during inhalation were 30-35% higher than for exhalation and were proportional to the square of the flow rate. Because particles are entrained and transported within airstreams, these results may have important applications to the targeted delivery of inhaled drugs.     

Lung protease/anti-protease network and modulation of mucus production and surfactant activity

Lung epithelium guarantees gas-exchange (performed in the alveoli) and protects from external insults (pathogens, pollutants…) present within inhaled air. Both functions are facilitated by secretions lining airway surface liquid, mucus (in the upper airways) and pulmonary surfactant (in the alveoli). Mucins, the main glycoproteins present within the mucus, are responsible for its rheologic properties and participate in lung defense mechanisms. In parallel, lung collectins are pattern recognition molecules present in pulmonary surfactant that also modulate lung defense. During chronic airways diseases, excessive protease activity can promote mucus hypersecretion and degradation of lung collectins and therefore contribute to the pathophysiology of these diseases. Importantly, secretion of local and systemic anti-proteases might be crucial to equilibrate the protease/anti-protease unbalance and therefore preserve the function of lung host defense compounds and airway surface liquid homeostasis. In this review we will present information relative to proteases able to modulate mucin production and lung collectin integrity, two important compounds of innate immune defense. One strategy to preserve physiological mucus production and collectin integrity during chronic airways diseases might be the over-expression of local ‘alarm’ anti-proteases such as SLPI and elafin. Interestingly, a cross-talk between lung collectins and anti-protease activity has recently been described, implicating the presence within the lung of a complex network between proteases, anti-proteases and pattern recognition molecules, which aims to keep or restore homeostasis in resting or inflamed lungs.     

Patient-Specific Modeling of Regional Antibiotic Concentration Levels in Airways of Patients with Cystic Fibrosis: Are We Dosing High Enough?

Background

Pseudomonas aeruginosa (Pa) infection is an important contributor to the progression of cystic fibrosis (CF) lung disease. The cornerstone treatment for Pa infection is the use of inhaled antibiotics. However, there is substantial lung disease heterogeneity within and between patients that likely impacts deposition patterns of inhaled antibiotics. Therefore, this may result in airways below the minimal inhibitory concentration of the inhaled agent. Very little is known about antibiotic concentrations in small airways, in particular the effect of structural lung abnormalities. We therefore aimed to develop a patient-specific airway model to predict concentrations of inhaled antibiotics and to study the impact of structural lung changes and breathing profile on local concentrations in airways of patients with CF.

Methods

In- and expiratory CT-scans of children with CF (5–17 years) were scored (CF-CT score), segmented and reconstructed into 3D airway models. Computational fluid dynamic (CFD) simulations were performed on 40 airway models to predict local Aztreonam lysine for inhalation (AZLI) concentrations. Patient-specific lobar flow distribution and nebulization of 75 mg AZLI through a digital Pari eFlow model with mass median aerodynamic diameter range were used at the inlet of the airway model. AZLI concentrations for central and small airways were computed for different breathing patterns and airway surface liquid thicknesses.

Results

In most simulated conditions, concentrations in both central and small airways were well above the minimal inhibitory concentration. However, small airways in more diseased lobes were likely to receive suboptimal AZLI. Structural lung disease and increased tidal volumes, respiratory rates and larger particle sizes greatly reduced small airway concentrations.

Conclusions

CFD modeling showed that concentrations of inhaled antibiotic delivered to the small airways are highly patient specific and vary throughout the bronchial tree. These results suggest that anti-Pa treatment of especially the small airways can be improved.     

Three-dimensional visualization and morphometry of small airways from microfocal X-ray computed tomography

Physiological morphometry is a critical factor in the flow dynamics in small airways. In this study, we visualized and analyzed the three-dimensional structure of the small airways without dehydration and fixation. We developed a two-step method to visualize small airways in detail by staining the lung tissue with a radiopaque solution and then visualizing the tissue with a cone-beam microfocal X-ray computed tomographic (CT) system. To verify the applicability of this staining and CT imaging (SCT) method, we used the method to visualize small airways in excised rat lungs. By using the SCT method to obtain continuous CT images, three-dimensional branching and merging bronchi ranging from 500 to 150 microm (the airway generation=8-16) were successfully reconstructed. The morphometry of the small airways (diameter, length, branching angle and gravity angle between the gravity direction and airway vector) was analyzed using the three-dimensional thinning algorithm. The diameter and length exponentially decreased with the airway generation. The asymmetry of the bifurcation decreased with generation and one branching angle decided the other pair branching angle. The SCT method is the first reported method that yields faithful high-resolution images of soft tissue geometry without fixation and the three-dimensional morphometry of small airways is useful for studying the biomechanical dynamics in small airways.     

Methodology for the measurement of mucociliary function in the mouse by scintigraphy.

The objective of the study was to develop a scintigraphic method for measurement of airway mucociliary clearance in small laboratory rodents such as the mouse. Previous investigations have characterized the secretory cell types present in the mouse airway, but analysis of the mucus transport system has been limited to in vitro examination of tissue explants or invasive in vivo measures of a single airway, the trachea. Three methods were used to deposit insoluble, radioisotopic colloidal particles: oropharyngeal aspiration, intratracheal instillation, and nose-only aerosol inhalation. The initial distribution of particles within the lower respiratory tract was visualized by gamma-camera, and clearance of particles was followed intermittently over 6 h and at the conclusion, 24 h postdelivery. Subsets of mice underwent lavage for evidence of tissue inflammation, and others were restudied for reproducibility of the methods. The aspiration and instillation methods of delivery led to greater distributions of deposited activity within the lungs, i.e., approximately 60--80% of the total respiratory tract radioactivity, whereas the nose-only aerosol technique attained a distribution of 32% to the lungs. However, the aerosol technique maximized the fraction of particles that cleared the airway over a 24-h period, i.e, deposited onto airway epithelial surfaces and cleared by mucociliary function such that lung retention at 24 h averaged 57% for delivery by aerosol inhalation and > or =80% for the aspiration or intratracheal instillation techniques. Particle delivery methods did not cause lung inflammation/injury with use of inflammatory cells and chemoattractant cytokines as criteria. Scintigraphy can discern particle deposition and clearance from the lower respiratory tract in the mouse, is noninvasive and reproducible, and includes the capability for restudy and lung lavage when time course or chronic treatments are being considered.