Airway deposition and clearance and systemic pharmacokinetics of amiloride following aerosolization with an ultrasonic nebulizer to normal airways

Sustained inhibition of ENaC in CF: Potential RNA-based therapies for mutation-agnostic treatment

Disruption of the equilibrium between ion secretion and absorption processes by the airway epithelium is central to many muco-obstructive lung diseases including cystic fibrosis (CF). Besides correction of defective folding and function of CFTR, inhibition of amiloride-sensitive epithelia sodium channels (ENaC) has emerged as a bona fide therapeutic strategy to improve mucociliary clearance in patients with CF. The short half-life of amiloride-based ENaC blockers and hyperosmotic therapies have led to the development of novel RNA-based interventions for targeted and sustained reduction of ENaC expression and function in preclinical models of CF. This review summarizes the recent advances in RNA therapeutics targeting ENaC for mutation-agnostic treatment of CF.

Drivers of absolute systemic bioavailability after oral pulmonary inhalation in humans

There are few studies in humans dealing with the relationship between physico-chemical properties of drugs and their systemic bioavailability after administration via oral inhalation route (Fpulm). Getting further insight in the determinants of Fpulm after oral pulmonary inhalation could be of value for drugs considered for a systemic delivery as a result of poor oral bioavailability, as well as for drugs considered for a local delivery to anticipate their undesirable systemic effects. To better delineate the parameters influencing the systemic delivery after oral pulmonary inhalation in humans, we studied the influence of physico-chemical and permeability properties obtained in silico on the rate and extent of Fpulm in a series of 77 compounds with or without marketing approval for pulmonary delivery, and intended either for local or for systemic delivery. Principal component analysis (PCA) showed mainly that Fpulm was positively correlated with Papp and negatively correlated with %TPSA, without a significant influence of solubility and ionization fraction, and no apparent link with lipophilicity and drug size parameters. As a result of the small sample set, the performance of the different models as predictive of Fpulm were quite average with random forest algorithm displaying the best performance. As a whole, the different models captured between 50 and 60% of the variability with a prediction error of less than 20%. Tmax data suggested a significant positive influence of lipophilicity on absorption rate while charge apparently had no influence. A significant linear relationship between Cmax and dose (R² = "0.79) highlighted that Cmax was primarily dependent on dose and absorption rate and could be used to estimate Cmax in humans for new inhaled drugs.

Airway Epithelial Lining Fluid and Plasma Pharmacokinetics of Inhaled Fluticasone Propionate and Salmeterol Xinafoate in Mechanically Ventilated Pigs

Background: The lower respiratory tract of the landrace pig has close anatomical and physiological similarities with that of the human, and hence, for inhalation studies this species is well suited for biopharmaceutical research. Methods: The objective of this study was to evaluate pharmacokinetics in pigs following one dose of Diskus™ Seretide™ forte device, labeled 500/50 fluticasone propionate (FP) and salmeterol xinafoate (SX), respectively. The PreciseInhale™ (PI) instrument was used to actuate the inhaler for in vitro testing and aerosol dosing to pigs. In vitro, the aerosol was characterized with a cascade impactor with respect to mass median aerodynamic diameter, geometric standard deviation, and fine particle dose. In vivo, dry powder inhalation exposure was delivered as a short bolus dose, to anesthetized and mechanically ventilated landrace pigs. In addition to plasma PK, PK assessment of airway epithelial lining fluid (ELF) was used in this study. ELF of the depth of three to fourth airway generation of the right lung was accessed using standard bronchoscopy and a synthetic absorptive matrix. Results and Conclusions: Dry powder inhalation exposures with good consistency and well characterized aerosols to the pig lung were achieved by the use of the PreciseInhale™ instrument. Drug concentrations of ELF for both FP and SX were demonstrated to be four to five orders of magnitude higher than its corresponding systemic plasma drug concentrations. Clinical PK following inhalation of the same dose was used as benchmark, and the clinical study did demonstrate similar plasma PK profiles and drug exposures of both FP and SX as the current pig study. Two factors explain the close similarity of PK (1) similiar physiology between species and (2) the consistency of dosing to animals. To conclude, our study demonstrated the utility and translational potential of conducting PK studies in pigs in the development of inhaled pharmaceuticals.

Chitosan in Non-Viral Gene Delivery: Role of Structure, Characterization Methods, and Insights in Cancer and Rare Diseases Therapies

Non-viral gene delivery vectors have lagged far behind viral ones in the current pipeline of clinical trials of gene therapy nanomedicines. Even when non-viral nanovectors pose less safety risks than do viruses, their efficacy is much lower. Since the early studies to deliver pDNA, chitosan has been regarded as a highly attractive biopolymer to deliver nucleic acids intracellularly and induce a transgenic response resulting in either upregulation of protein expression (for pDNA, mRNA) or its downregulation (for siRNA or microRNA). This is explained as the consequence of a multi-step process involving condensation of nucleic acids, protection against degradation, stabilization in physiological conditions, cellular internalization, release from the endolysosome ("proton sponge" effect), unpacking and enabling the trafficking of pDNA to the nucleus or the siRNA to the RNA interference silencing complex (RISC). Given the multiple steps and complexity involved in the gene transfection process, there is a dearth of understanding of the role of chitosan's structural features (Mw and degree of acetylation, DA%) on each step that dictates the net transfection efficiency and its kinetics. The use of fully characterized chitosan samples along with the utilization of complementary biophysical and biological techniques is key to bridging this gap of knowledge and identifying the optimal chitosans for delivering a specific gene. Other aspects such as cell type and administration route are also at play. At the same time, the role of chitosan structural features on the morphology, size and surface composition of synthetic virus-like particles has barely been addressed. The ongoing revolution brought about by the recent discovery of CRISPR-Cas9 technology will undoubtedly be a game changer in this field in the short term. In the field of rare diseases, gene therapy is perhaps where the greatest potential lies and we anticipate that chitosans will be key players in the translation of research to the clinic.

Influence of Inhaled Amiloride on Lung Fluid Clearance in Response to Normobaric Hypoxia in Healthy Individuals

Wheatley, Courtney M., Sarah E. Baker, Bryan J. Taylor, Manda L. Keller-Ross, Steven C. Chase, Alex R. Carlson, Robert J. Wentz, Eric M. Snyder, and Bruce D. Johnson. Influence of inhaled amiloride on lung fluid clearance in response to normobaric hypoxia in healthy individuals. High Alt Med Biol 18:343-354, 2017.

AIM

To investigate the role of epithelial sodium channels (ENaC) on lung fluid clearance in response to normobaric hypoxia, 20 healthy subjects were exposed to 15 hours of hypoxia (fraction of inspired oxygen [FiO₂] = 12.5%) on two randomized occasions: (1) inhaled amiloride (A) (1.5 mg/5 mL saline); and (2) inhaled saline placebo (P). Changes in lung fluid were assessed through chest computed tomography (CT) for lung tissue volume (TV), and the diffusion capacity of the lungs for carbon monoxide (DLCO) and nitric oxide (DLNO) for pulmonary capillary blood volume (V_C). Extravascular lung water (EVLW) was derived as TV-V_C and changes in the CT attenuation distribution histograms were reviewed.

RESULTS

Normobaric hypoxia caused (1) a reduction in EVLW (change from baseline for A vs. P, -8.5% ± 3.8% vs. -7.9% ± 5.2%, p < 0.05), (2) an increase in V_C (53.6% ± 28.9% vs. 53.9% ± 52.3%, p < 0.05), (3) a small increase in DLCO (9.6% ± 29.3% vs. 9.9% ± 23.9%, p > 0.05), and (4) CT attenuation distribution became more negative, leftward skewed, and kurtotic (p < 0.05).

CONCLUSION

Acute normobaric hypoxia caused a reduction in lung fluid that was unaffected by ENaC inhibition through inhaled amiloride. Although possible amiloride-sensitive ENaC may not be necessary to maintain lung fluid balance in response to hypoxia, it is more probable that normobaric hypoxia promotes lung fluid clearance rather than accumulation for the majority of healthy individuals. The observed reduction in interstitial lung fluid means alveolar fluid clearance may not have been challenged.

Hypoxia and sterile inflammation in cystic fibrosis airways: mechanisms and potential therapies

Cystic fibrosis is one of the most common autosomal recessive genetic diseases in Caucasian populations. Diagnosisvianewborn screening and targeted nutritional and antibiotic therapy have improved outcomes, however respiratory failure remains the key cause of morbidity and mortality. Progressive respiratory disease in cystic fibrosis is characterised by chronic neutrophilic airway inflammation associated with structural airway damage leading to bronchiectasis and decreased lung function. Mucus obstruction is a characteristic early abnormality in the cystic fibrosis airway, associated with neutrophilic inflammation often in the absence of detectable infection. Recent studies have suggested a link between hypoxic cell death and sterile neutrophilic inflammation in cystic fibrosis and other diseasesviathe IL-1 signalling pathway. In this review, we consider recent evidence regarding the cellular responses to respiratory hypoxia as a potential driver of sterile neutrophilic inflammation in the lung, current knowledge on hypoxia as a pathogenic mechanism in cystic fibrosis and the potential for current and future therapies to alleviate hypoxia-driven sterile inflammation.

The Relationship of Mucus Concentration (Hydration) to Mucus Osmotic Pressure and Transport in Chronic Bronchitis

RATIONALE

Chronic bronchitis (CB) is characterized by persistent cough and sputum production. Studies were performed to test whether mucus hyperconcentration and increased partial osmotic pressure, in part caused by abnormal purine nucleotide regulation of ion transport, contribute to the pathogenesis of CB.

OBJECTIVES

We tested the hypothesis that CB is characterized by mucus hyperconcentration, increased mucus partial osmotic pressures, and reduced mucus clearance.

METHODS

We measured in subjects with CB as compared with normal and asymptomatic smoking control subjects indices of mucus concentration (hydration; i.e., percentage solids) and sputum adenine nucleotide/nucleoside concentrations. In addition, sputum partial osmotic pressures and mucus transport rates were measured in subjects with CB.

MEASUREMENTS AND RESULTS

CB secretions were hyperconcentrated as indexed by an increase in percentage solids and total mucins, in part reflecting decreased extracellular nucleotide/nucleoside concentrations. CB mucus generated concentration-dependent increases in partial osmotic pressures into ranges predicted to reduce mucus transport. Mucociliary clearance (MCC) in subjects with CB was negatively correlated with mucus concentration (percentage solids). As a test of relationships between mucus concentration and disease, mucus concentrations and MCC were compared with FEV1, and both were significantly correlated.

CONCLUSIONS

Abnormal regulation of airway surface hydration may slow MCC in CB and contribute to disease pathogenesis.

NVP-QBE170: an inhaled blocker of the epithelial sodium channel with a reduced potential to induce hyperkalaemia

BACKGROUND AND PURPOSE

Inhaled amiloride, a blocker of the epithelial sodium channel (ENaC), enhances mucociliary clearance (MCC) in cystic fibrosis (CF) patients. However, the dose of amiloride is limited by the mechanism-based side effect of hyperkalaemia resulting from renal ENaC blockade. Inhaled ENaC blockers with a reduced potential to induce hyperkalaemia provide a therapeutic strategy to improve mucosal hydration and MCC in the lungs of CF patients. The present study describes the preclinical profile of a novel ENaC blocker, NVP-QBE170, designed for inhaled delivery, with a reduced potential to induce hyperkalaemia.

EXPERIMENTAL APPROACH

The in vitro potency and duration of action of NVP-QBE170 were compared with amiloride and a newer ENaC blocker, P552-02, in primary human bronchial epithelial cells (HBECs) by short-circuit current. In vivo efficacy and safety were assessed in guinea pig (tracheal potential difference/hyperkalaemia), rat (hyperkalaemia) and sheep (MCC).

KEY RESULTS

In vitro, NVP-QBE170 potently inhibited ENaC function in HBEC and showed a longer duration of action to comparator molecules. In vivo, intratracheal (i.t.) instillation of NVP-QBE170 attenuated ENaC activity in the guinea pig airways with greater potency and duration of action than that of amiloride without inducing hyperkalaemia in either guinea pig or rat. Dry powder inhalation of NVP-QBE170 by conscious sheep increased MCC and was better than inhaled hypertonic saline in terms of efficacy and duration of action.

CONCLUSIONS AND IMPLICATIONS

NVP-QBE170 highlights the potential for inhaled ENaC blockers to exhibit efficacy in the airways with a reduced risk of hyperkalaemia, relative to existing compounds.

Gaining the Upper Hand on Pulmonary Drug Delivery

Asthma, Chronic Obstructive Pulmonary Disease (COPD) and Cystic Fibrosis (CF) are all pulmonary diseases which are characterized by chronic inflammation and an increase in mucus production. Excess mucus in the airways correlates with pathophysiology such as a decline in lung function and prolonged bacterial infections. New drugs to treat these chronic respiratory diseases are currently being developed and include both inhaled and orally administered compounds. Whilst oral drugs may be easier to administer, they are more prone to side-effects due to higher bioavailability. Inhaled compounds may show reduced bioavailability, but face their own unique challenges. For example, thick mucus in the respiratory tracts of asthma, CF and COPD patients can act as a physical barrier that impedes drug delivery. Mucus also contains a high number of enzymes and proteases that may degrade compounds before they reach their site of action. Furthermore, some classes of drugs are rapidly absorbed across the respiratory epithelia into systemic circulation, which may limit their duration of action and/or cause off-target effects. This review discusses some of the different treatment options that are currently available and the considerations that need to be taken into account to produce new therapies for the treatment of chronic respiratory diseases.

Regulation of airway mucosal hydration

Ion channels control the hydration status of the airway epithelium through apical anion secretion and cation absorption, which is accompanied by osmotically obligated water. The key channels in this process are the cystic fibrosis (CF) transmembrane conductance regulator (CFTR), which is principally responsible for Cl(-) secretion by airway epithelial cells, and the epithelial Na(+) channel (ENaC), which is responsible for the absorption of Na ions. In CF, defective CFTR-mediated Cl(-) secretion and an accompanying upregulation in ENaC-mediated Na absorption results in a reduction in airway surface liquid volume, leading to poorly hydrated mucus and impaired mucociliary clearance. Restoration of normal airway hydration by modulation of ion channel activity represents an important therapeutic strategy for CF. CFTR corrector and potentiator compounds are being developed with the aim of recovering normal Cl(-) secretion. Ca(2+)-activated Cl(-) channels (CaCCs) are expressed by the respiratory epithelia and are reported to be functionally upregulated in CF and offer a 'surrogate' pathway for Cl(-) secretion. TMEM16A has recently been described as a CaCC in the airway epithelium and, as such, represents an alternative target for restoring Cl(-) secretion in CF. An alternative therapeutic strategy for CF is to inhibit ENaC, thereby blocking excessive Na absorption. This can be achieved by direct blockade of ENaC or inhibition of the channel-activating proteases (CAPs), whose activity regulates ENaC function. This review will describe the regulation of airway mucosal hydration by ion channels and the efforts currently underway to restore normal mucosal hydration in disease patients by modulating the function of these channels.

Adduction of amiloride hydrochloride in urea through a modified technique for the dissolution enhancement

Amiloride hydrochloride is a potassium-sparing diuretic since it favors sodium excretion and potassium reabsorption. In the present study, urea, a well-known adductor for linear compounds was successfully employed for inclusion of amiloride hydrochloride-a substituted cyclic organic compound through a modified technique. Formation of urea inclusion compounds was confirmed by FTIR, DSC and XRD. The minimum amount of rapidly adductible endocyte (RAE) required for adduction of amiloride hydrochloride in urea was estimated by a modified Zimmerschied calorimetric method. Urea-AH-RAE inclusion compounds containing varying proportions of guests were prepared and their thermal behavior studied by DSC. The inclusion compounds were also found to exhibit high content uniformity and markedly improved dissolution profile as demonstrated by increased dissolution efficiency. Studies reveal the possibility of exploiting co-inclusion of the drug in urea host lattice for the dissolution enhancement.

Design, synthesis, and structure-activity relationships of novel 2-substituted pyrazinoylguanidine epithelial sodium channel blockers: drugs for cystic fibrosis and chronic bronchitis

Amiloride (1), the prototypical epithelial sodium channel (ENaC) blocker, has been administered with limited success as aerosol therapy for improving pulmonary function in patients with the genetic disorder cystic fibrosis. This study was conducted to synthesize and identify more potent, less reversible ENaC blockers, targeted for aerosol therapy and possessing minimal systemic renal activity. A series of novel 2-substituted acylguanidine analogues of amiloride were synthesized and evaluated for potency and reversibility on bronchial ENaC. All compounds tested were more potent and less reversible at blocking sodium-dependent short-circuit current than amiloride. Compounds 30-34 showed the greatest potency on ENaC with IC(50) values below 10 nM. A regioselective difference in potency was found (compounds 30, 39, and 40), whereas no stereospecific (compounds 33, 34) difference in potency on ENaC was displayed. Lead compound 32 was 102-fold more potent and 5-fold less reversible than amiloride and displayed the lowest IC(50) value ever reported for an ENaC blocker.

Vanilloid type 1 receptor and the acid-sensing ion channel mediate acid phosphate activation of muscle afferent nerves in rats

Reflex cardiovascular responses to contracting skeletal muscle are mediated by mechanical and metabolic stimulation of thin-fiber muscle afferents. Diprotonated phosphate (H2PO4-) excites those thin-fiber nerves and evokes the muscle pressor reflex. The receptors mediating this response are unknown. Thus we examined the role played by purinergic receptors, vanilloid type 1 receptors (VR1), and acid-sensing ion channels (ASIC) in mediating H2PO4- -evoked pressor responses. Phosphate and blocking agents were injected into the arterial blood supply of the hindlimb muscles of 53 decerebrated rats. H2PO4- (86 mM, pH 6.0) increased mean arterial pressure by 25 +/- 2 mmHg, whereas monoprotonated phosphate (HPO4(2-), pH 7.5) had no effect. Pyridoxalphosphate-6-azophenyl-2',4'-disulfonic acid (a purinergic receptor antagonist, 2 mM) did not block the response. However, capsazepine (a VR1 antagonist, 1 mg/kg) attenuated the reflex by 60% and amiloride (an ASIC blocker, 6 microg/kg) by 52%. Of note, the H2PO4- -induced pressor response was attenuated by 87% when both capsazepine and amiloride were injected before the H2PO4-. In conclusion, VR1 and ASIC mediate the pressor response due to H2PO4-. The H2PO4- -evoked response was greater when VR1 and ASIC blockers were given simultaneously than when the respective blockers were given separately. Our laboratory's previous study has shown that H+ stimulates ASIC (but not VR1) on thin-fiber afferent nerves in evoking the reflex response. Thus VR1 and ASIC are likely to play a coordinated and interactive role in processing the muscle afferent response to H2PO4-. Furthermore, the physiological mechanisms mediating the response to H+ and H2PO4- are likely to be different.

Novel, mechanism-based therapies for cystic fibrosis

PURPOSE OF REVIEW

Cystic fibrosis results from disruption of the biosynthesis or function of the cystic fibrosis transmembrane conductance regulator. Cystic fibrosis transmembrane conductance regulator plays a critical role in the regulation of epithelial ion transport. Restoration of cystic fibrosis transmembrane conductance regulator function should improve the cystic fibrosis phenotype.

RECENT FINDINGS

Recent investigations affording a better understanding of the mechanism of dysfunction of mutant cystic fibrosis transmembrane conductance regulators, as well as the roles of cystic fibrosis transmembrane conductance regulator in regulating epithelial ion transport, have led to development of therapeutic strategies based on repair or bypass of mutant cystic fibrosis transmembrane conductance regulator dysfunction. The former strategy, coined 'protein repair therapy,' is aimed at improving or restoring the function of mutant cystic fibrosis transmembrane conductance regulators, whereas the latter approach aims to augment epithelial ion transport to compensate for the absent function mutant cystic fibrosis transmembrane conductance regulator.

SUMMARY

Strategies to improve mutant cystic fibrosis transmembrane conductance regulator function or to bypass mutant cystic fibrosis transmembrane conductance regulator function hold great promise for development of novel therapies aimed at correcting the underlying pathophysiology of cystic fibrosis.

Ionic interaction of amiloride and uridine 5'-triphosphate in nebulizer solutions

Combination therapy using nebulized amiloride hydrochloride and uridine-5'-triphosphate (UTP) trisodium salt aerosols has been investigated for the treatment of cystic fibrosis (CF). Amiloride in aqueous solution precipitates in the presence of UTP, reducing drug concentrations. Interactions between these drugs and NaCl in solution were studied using phase-solubility techniques monitored by UV spectrophotometry. Elemental analyses were employed for precipitate characterization. Amiloride solubility was reduced by more than 85% in saline. Amiloride solubility decreased with increasing UTP concentration, resulting in formation of a precipitated complex. The theoretical molar ratio of complexes range from 1-3 amiloride:1 UTP. At most concentrations only 3 amiloride:1 UTP complex was observed in precipitate. This is a reflection of low Ksp for the 3:1 complex of 2.92 x 10(-11) M4 compared with 2.09 x 10(-4) M2 for amiloride alone. Equilibration over excess bulk solid resulted in higher solubility estimates and different phase solubility diagrams than solubility studies utilizing precipitation technique. This may be explained by the absence of amiloride in the solid state and its impact on complex equilibria with UTP. The solubility suppressing effects of UTP and saline were largely additive. A number of ionic interactions increase complex solubility profile of amiloride hydrochloride in the presence of UTP and NaCl.

Evaluation of Second Generation Amiloride Analogs as Therapy for Cystic Fibrosis Lung Disease

Epithelial sodium channel (ENaC) blockers have been proposed as a therapy to restore mucus clearance (MC) in cystic fibrosis (CF) airways. The therapeutic effects of the first generation ENaC blocker, amiloride, in CF patients, however, were minimal. Because the failure of amiloride reflected both its low potency and short duration of action on airway surfaces, we investigated whether the increased potency of benzamil and phenamil would produce more favorable pharmacodynamic properties. In vitro potency, maximal efficacy, rate of recovery from maximal block of ENaC, and rate of drug absorption were compared for amiloride, benzamil, and phenamil in cultured human and ovine bronchial epithelial cells. In both human and ovine bronchial epithelia, the rank order of potency was benzamil > phenamil >> amiloride, the maximal efficacy was benzamil = phenamil = amiloride, the recovery to baseline sodium transport was phenamil < benzamil << amiloride, and the rate of drug absorption was phenamil > benzamil >> amiloride. Based on greater potency, benzamil was compared with amiloride in in vivo pharmacodynamic studies in sheep, including tracheal mucus velocity (TMV) and MC. Benzamil enhanced MC and TMV, but acute potency or duration of effect did not exceed that of amiloride. In conclusion, our data support the hypothesis that ENaC blocker aerosol therapy increases MC. However, rapid absorption of benzamil from the mucosal surface offset its greater potency, making it equieffective with amiloride in vivo. More potent, less absorbable, third generation ENaC blockers will be required for an effective aerosol CF pharmacotherapy.

Inhibition of carcinogen-bioactivating cytochrome P450 1 isoforms by amiloride derivatives

We examined the effects of amiloride derivatives, especially 5-(N-ethyl-N-isopropyl)amiloride (EIPA), on the activity of cytochrome P450 (CYP) 1 isoforms, known to metabolize carcinogenic polycyclic aromatic hydrocarbons (PAHs), such as benzo(a)pyrene (BP), into mutagenic metabolites and whose cellular expression can be induced through interaction of PAHs with the arylhydrocarbon receptor. EIPA was found to cause a potent and dose-dependent inhibition of CYP1-related ethoxyresorufine O-deethylase (EROD) activity in both liver cells and microsomes. It also markedly reduced activity of human recombinant CYP1A1 enzyme through a competitive mechanism; activities of other human CYP1 isoforms, i.e. CYP1A2 and CYP1B1, were also decreased. However, EIPA did not affect BP-mediated induction of CYP1A1 mRNA and protein levels in rat liver cells, likely indicating that EIPA does not block activation of the arylhydrocarbon receptor by PAHs. Inhibition of CYP1 activity by EIPA was associated with a decreased metabolism of BP, a reduced formation of BP-derived DNA adducts and a diminished BP-induced apoptosis in liver cells. The present data suggest that amiloride derivatives, such as EIPA, may be useful for preventing toxicity of chemical carcinogens, such as PAHs, through inhibition of CYP1 enzyme activity.

Harvesting airway surface liquid: a comparison of two techniques

The quantity and composition of airway surface liquid (ASL) are essential to host defense. To date, attempts to harvest ASL and measure its composition have yielded conflicting results. We investigated the physical principles underlying two techniques that were proposed for harvesting ASL: filter paper pledgets and polyethylene catheters. We compared the force and pressure generation and the kinematics of capillarity-induced fluid uptake with both techniques. Both have significant limitations for harvesting ASL, generating physiologically significant pressures (filter paper, 60.4 Pa; polyethylene, 14.3 Pa) that could potentially compromise epithelial integrity. Furthermore, filter paper generates a force 85-fold higher than the polyethylene catheter, which is associated with a very high rate of uptake of liquid and a large total amount of liquid relative to ASL thickness. While the PE catheter harvests liquid more gently, it is only effective when ASL surface tension is below 31 mN/m. These limitations likely account for some of the variability in reported ASL composition, and highlight the need for improved methods for harvesting ASL.

Desiccation and hypertonicity of the airway surface fluid and thermally induced asthma

To determine whether drying and hypertonicity of the airway surface fluid (ASF) are involved in thermally induced asthma, nine subjects performed isocapnic hyperventilation (HV) (minute ventilation 62.2 +/- 8.3 l/min) of frigid air (-8.9 +/- 3.3 degrees C) while periciliary fluid was collected endoscopically from the trachea. Osmolality was measured by freezing-point depression. The baseline 1-s forced expiratory volume was 73 +/- 4% of predicted and fell 26.4% 10 min postchallenge (P > 0.0001). The volume of ASF collected was 11.0 +/- 2.2 microl at rest and remained constant during and after HV as the airways narrowed (HV 10.6 +/- 1.9, recovery 6.5 +/- 1.7 microl; P = 0.18). The osmolality also remained stable throughout (rest 336 +/- 16, HV 339 +/- 16, and recovery 352 +/- 19 mosmol/kgH(2)O, P = 0.76). These data demonstrate that airway desiccation and hypertonicity of the ASF do not develop during hyperpnea in asthma; therefore, other mechanisms must cause exercise- and hyperventilation-induced airflow limitation.

Altering airway surface liquid volume: inhalation therapy with amiloride and hyperosmotic agents

The thin layer of liquid lining the entire respiratory tract is the first line of defense against the continuous insult of inhaled bacteria and noxious chemicals. Many chronic obstructive diseases of the airway may reflect decreased airway surface liquid, which results from imbalances in ion transport and mucus production. Reduction in the thickness of airway surface liquid leads to reduced mucociliary clearance rates, causing mucus accumulation and infection in the airway. In this chapter, two inhalation therapies to replenish airway surface liquid and enhance mucociliary clearance are discussed: (1) aerosolized hyperosmotic agents; and (2) aerosolized sodium channel blockers. The advantages and disadvantages of each therapy are discussed, as well as future directions for improving airway surface liquid volume by inhalation pharmacotherapy.

Influence of hyperpnea on airway surface fluid volume and osmolarity in normal humans

To determine the effect of hyperpnea on the characteristics of periciliary liquid, we collected airway surface fluid (ASF) and measured its osmolarity in 11 normal people while they breathed dry, frigid air (-17 +/- 1.2 degrees C) at minute ventilations (VE) of 10, 40, and 80 l/min through a heat exchanger. The ASF was collected at the fifth tracheal ring by absorption onto filter paper pledgets inserted via fiber-optic bronchoscopy. Hyperpnea had no influence on the amount of ASF recovered (ASF volume at a VE of 10 l/min = 12.0 +/- 2.0 microl; at 80 l/min = 8.8 +/- 1.5 microl; P = 0.28) or its osmolarity (at a VE of 10, 40, and 80 l/min = 326 +/- 15, 323 +/- 11, and 337 +/- 12 mosM, respectively; P = 0.65). These findings demonstrate that the tracheal mucosa of normal subjects does not dessicate during hyperpnea and that hypertonicity of the periciliary fluid does not develop even at high levels of ventilation.

Ion composition of airway surface liquid of patients with cystic fibrosis as compared with normal and disease-control subjects

To test whether a major contribution of airways epithelial ion transport to lung defense reflects the regulation of airway surface liquid (ASL) ionic composition, we measured ASL composition using the filter paper technique. On nasal surfaces, the Cl- concentration (approximately 125 meq/liter) was similar to plasma, but the Na+ concentration (approximately 110 meq/liter) was below plasma, and K+ concentration (approximately 30 meq/liter) above plasma. The resting ASL osmolarity [2(Na+ + K+); 277 meq/liter] approximated isotonicity. There were no detectable differences between cystic fibrosis (CF) and normal subjects. In the lower airways, the Na+ concentrations were 80-85 meq/liter, K+ levels approximately 15 meq/liter, and Cl- concentrations 75-80 meq/liter. Measurements of Na+ activity with Na(+)-selective electrodes and osmolality with freezing point depression yielded values consistent with the monovalent cation measurements. Like the nasal surfaces, no differences in cations were detected between CF, normal, or chronic bronchitis subjects. The tracheobronchial ASL hypotonicity was hypothesized to reflect collection-induced gland secretion, a speculation consistent with observations in which induction of nasal gland secretion produced hypotonic secretions. We conclude that there are no significant differences in ASL ion concentrations between CF, normal, and chronic bronchitis subjects and, because ASL ion concentrations exceed values consistent with defensin activity, the failure of CF lung defense may reflect predominantly factors other than salt-dependent defensins.