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The pharmacologic approach to airway clearance: Mucoactive agents

Bruce K. Rubin, MEngr, MD, MBA, FRCPC Professor and Vice-Chair of Pediatrics Professor of Biomedical Engineering, Physiology and Pharmacology Wake Forest University School of Medicine Medical Center Blvd. Winston-Salem, NC 27157-1081

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Outline 1. Introduction 2. Expectorants 3. Medications that change the biophysical properties of secretions 3.1 Mucolytic agents 3.1.1 Classic mucolytics 3.1.2 Peptide Mucolytics 3.1.3 Nondestructive mucolytics 4. Mucokinetic agents 5. Cough clearance promotors 6. Mucoregulatory medications 7. Summary

1. Introduction The airway mucosa responds to infection and inflammation in a variety of ways. This response often includes surface mucous (goblet) cell and submucosal gland hyperplasia and hypertrophy with mucus hypersecretion. Products of inflammation including neutrophil derived DNA and filamentous actin (F-actin), effete cells, bacteria, and cell debris all contribute to mucus purulence and, when this is expectorated it is called sputum. Mucoactive medications are intended to serve one of two purposes; either to increase the ability to expectorate sputum or to decrease mucus hypersecretion. Mucoactive medications have been classified according to their proposed method of action (1). Sputum is expectorated mucus mixed with inflammatory cells, cellular debris, polymers of DNA and F-actin, as well as bacteria. Mucus is usually cleared by airflow and ciliary movement, and sputum is cleared by cough (2). In this review, I will discuss each of these classes of medication, their proposed mechanism of action, and their potential use in treating patients with chronic airways diseases associated with poor mucus clearance and with mucus hypersecretion. Rubin. Mucoactive medications. Page # 3

2. Expectorants

Expectorants are defined as medications that are taken to improve the ability to expectorate purulent secretions. This term is now taken to mean medications that increase airway water or the volume of airway secretions. The most commonly used of these are simple hydration including both bland aerosol administration and oral hydration, iodide containing compounds such as SSKI or iodinated glycerol, glyceryl guaiacolate or guaifenesin, and the more recently developed ion channel modifiers such as the P2Y2 purinergic agonists. Most of these medications or maneuvers are ineffective at adding water to the airway and those that are effective are also mucus secretagogues increasing the volume of both mucus and water in the airways. Despite widespread use, iodinated compounds, guaifenesin, and simple hydration are ineffective as expectorants (3). In fact, over-hydration has been shown to decrease airway mucus clearance in some patients with chronic airway disease, particularly with acute asthma (4). For many years, sputum induction using hyperosmolar saline inhalation has been used to obtain specimens for the diagnosis of pneumonia. As summarized in the Cochrane Database, the long term use of inhaled hyperosmolar saline improves pulmonary function in patients with cystic fibrosis (CF) (5) and inhaled hyperosmolar saline or mannitol is beneficial in non-CF bronchiectasis (6). Although this therapy is readily available and inexpensive, it has been reported that hypertonic saline aerosol is not as effective as dornase alfa in the therapy of CF lung disease (7). Agents that increase transport across ion channels such as the CFTR chloride channel, calcium dependent chloride channel, or agents that increase water transport across the airway aquaporin water channels may increase the hydration of the periciliary fluid and so may aid expectoration. These medications (including gene transfer vectors) are actively being investigated. Early results using UTP to stimulate chloride secretion or amiloride to block epithelial sodium channels were disappointing in that these did not produce a sustained Rubin. Mucoactive medications. Page # 4

improvement in pulmonary function in persons with CF (8) but newer P2Y2 chloride channel activators appear to be more effective (9, 10). In general, expectorant medications have not been consistently demonstrated to be effective for the treatment of airway disease associated with mucus stasis or hypersecretion.

3. Medications that change the biophysical properties of secretions

The principal polymer component of normal airway mucus is mucin glycoprotein. The mucin protein is heavily decorated with oligosaccharide side chains and the elongated glycoproteins linearly polymerize and form a “tangled network” secondary structure. This accounts for the gel structure of normal airway mucus. With chronic inflammation there is thought to be hypersecretion of mucin although this has not been proven. In fact it has been shown that there is mucus hyposecretion in the CF airway (11) and this may predispose the airway to chronic infection with biofilm producing organisms. In sputum, a secondary polymer network comprised of neutrophil derived DNA and F-actin also forms within the airway. This DNA forms rigid polymer chains that copolymerize with cell wall associated actin (12). This secondary polymer network is responsible for many of the abnormal properties of purulent secretions.

3.1 Mucolytics Mucolytic medications depolymerize either the mucin network (classic mucolytics) or the DNA-actin polymer network (peptide mucolytics) and in so doing reduce the viscosity and elasticity of airway secretions. Mucus has viscoelastic properties of both liquids (viscosity) and solids (elasticity). Thus it is a gel and both the viscous (energy loss) and elastic (energy storage) properties are essential for mucus spreading and clearance (13). Mucociliary clearance appears to be dependent upon there being an optimal ratio of viscosity to elasticity (14). Mucolytic agents have the potential to improve mucus rheology thus improving mucociliary or cough clearance, Rubin. Mucoactive medications. Page # 5 but these medications are also potentially able to over liquify secretions and this would decrease clearance (15).

3.1.1 Classic Mucolytics Classic mucolytics depolymerize the mucin glycoprotein oligomers by hydrolyzing the disulfide bonds linking the mucin monomers. This is usually accomplished by free thiol (sulfhydryl) groups hydrolyzing disulfide bonds attached to cysteine residues of the protein core. The best known of these agents is N-acetyl L-cysteine (NAC) which is widely used for the treatment of chronic bronchitis in Europe and Asia. There are few data available from placebo controlled clinical trials of NAC or its derivatives, and these data do not demonstrate that NAC improves mucus clearance or pulmonary function (16). The aerosol is available in the United States but is often poorly tolerated by patients because of its sulfurous odor and because the pH of 2.2 is associated with bronchospasm. NAC is an antioxidant and has been used to treat acetaminophen overdose. The orally available compound is also available in Europe but despite being a potent anti oxidant there are no data demonstrating that this medication is effective in the treatment of chronic airway disease (17). There are a number of similar compounds containing sulfhydryl groups that can effectively depolymerize mucin polymers in vitro. Although many of these are better tolerated than NAC, none have been clearly demonstrated to be effective in improving mucus clearance.

3.1.2 Peptide mucolytics The mucin polymer network is essential for normal mucus clearance. It may be that the classic mucolytics are generally ineffective because they are depolymerizing essential components of the mucous gel. With airway inflammation and inflammatory cell necrosis, a secondary polymer network develops in purulent secretions. In contrast to the mucin network, this pathologic polymer gel serves no obvious purpose in airway protection or mucus clearance. Rubin. Mucoactive medications. Page # 6

The peptide mucolytics are designed specifically depolymerize the DNA polymer (dornase alfa) or the F-actin network (e.g. gelsolin, thymosin beta 4). Dornase alfa has seen wide acceptance as a peptide mucolytic for the treatment of cystic fibrosis airway disease (18). When used as prescribed, its use is associated with improved pulmonary function, decreased antibiotic use, and decreased hospitalization rate for many patients with CF (19). For reasons that are not clear, this medication is not uniformly effective for the treatment of CF airway disease and efficacy does not seem to be related to sputum DNA content. There are limited and anecdotal data suggesting that dornase alfa may be effective in treating some persons with non CF bronchiectasis including some patients with primary ciliary dyskinesia (20). Although dornase alfa was not effective for the therapy of severe chronic bronchitis, there are no published studies evaluating its potential efficacy in patients with milder disease. Both gelsolin and thymosin ß4 have been demonstrated to depolymerize the pathologic DNA/F-actin network in CF sputum. These agents have never been studied in controlled clinical trials.

3.1.3 Nondestructive mucolytics Mucin is a polyionic tangled network and the charged nature of the oligosaccharide side chains help to hold this network together as a gel. Several agents have been proposed that can

“loosen” this network by charge shielding. Such agents include low molecular weight dextran, heparin, and other sugars or glycoproteins (21).

4. Mucokinetic agents

A mucokinetic medication is a drug that increases mucociliary clearance, generally by acting on the cilia. Although a variety of medications such as tricyclic nucleotides, beta agonist bronchodilators, and methylxanthines bronchodilators have all been demonstrated to increase ciliary beat frequency, these agents have only a minimal effect on mucociliary clearance in Rubin. Mucoactive medications. Page # 7 patients with lung disease (22). The reason for this is probably a combination of factors including the limited potential for efficacy in an airway with dysfunctional cilia or denuded of cilia. Most of these agents are also mucus secretagogues which may paradoxically increase the burden of airway secretions. Bronchodilator medications can also increase airway collapse in patients with bronchomalacia by virtue of their ability to relax airway smooth muscle. Therefore, the only persons for whom these medications are recommended are those who have improvement in expiratory airflow following their use. Increased expiratory airflow can enhance the effectiveness of cough (23). Thus bronchodilators might be better considered cough clearance promotors as described below.

5. Cough clearance promotors

Cough becomes a major mechanism for mucus clearance when there is extensive ciliary damage and mucus hypersecretion. Cough clearance depends on expiratory airflow, volume, and force, and the biophysical properties of airway secretions. In general, decreasing the viscoelasticity of airway secretions will not improve cough clearance unless this therapy also releases mucus from adherent entanglements with cilia. As mucus becomes adherent to the epithelium, it becomes far more difficult to expectorate. Patients who appear to benefit from expectorants or mucolytic agents may do so by virtue of these medications releasing mucus from epithelial attachment.

Agents that reduce the adhesivity of airway secretions and thus binding to the epithelium are the abhesives. There is a thin layer of surfactant that serves to separate the periciliary fluid and the cilia from the mucous layer permitting effective ciliary function and preventing secretion adherence to the epithelium. With airway inflammation, there is extensive surfactant hydrolysis by secretory phospholipases A2 (sPLA2) and the generation of lysophospholipids that appear to increase mucus adhesivity (24). It has been shown that aerosolized surfactants are effective abhesives and can significantly improve both cough clearability of secretions and pulmonary function in patients with chronic bronchitis (25). Rubin. Mucoactive medications. Page # 8

Earlier generations of Venturi-driven jet nebulizers made it difficult to efficiently aerosolize surfactant as this medication foams extensively and coats surfaces. Newer aerosol delivery devices permit surfactant to be administered efficiently either as a dry powder or as a wet aerosol. Studies are planned evaluating the potential effectiveness of surfactant for patients with chronic airway diseases using some of these newer modes of drug delivery.

6. Mucoregulatory medications Another approach to reducing the burden of airway secretions is to decrease hypersecretion by goblet cells and submucosal glands. Medications that decrease mucus hypersecretion are referred to as mucoregulatory medications. These medications include anti inflammatory drugs such as corticosteroids which are effective at decreasing the inflammatory stimulus that leads to mucus hypersecretion. Aerosolized indomethacin has also been used in Japan to treat patients with diffuse panbronchiolitis who have impairment due to mucus hypersecretion (26). Anticholinergic medications are also extensively used as mucoregulatory medications. Atropine is routinely given peri-operatively to prevent laryngospasm and to decrease mucus secretion associated with endotracheal intubation. Atropine and its derivatives are mucoregulatory medications in that they do not “dry” secretions but will decrease hypersecretion that is mediated through M3 cholinergic mechanisms. The quaternary ammonium derivatives of atropine including ipratropium bromide and tiotropium do not significantly cross the blood airway barrier and as such, their use is not associated with typical systemic effects of anticholinergic medications such as flushing or tachycardia. Ipratropium bromide is widely used as a bronchodilator medication in patients with chronic bronchitis. Studies have also shown that the long-term use of ipratropium is associated with a reduction in the volume of mucus secretion in patients with chronic bronchitis (27). More specific M3 antagonists hold the promise of improved mucoregulatory efficacy of this class of medications with less risk of adverse effects. Rubin. Mucoactive medications. Page # 9

Some of the more interesting of the mucoregulatory medications are the macrolide antibiotics. These antibiotics were discovered 50 years ago and derivatives of erythromycin A have been widely used for the treatment of bacterial infection. Since the mid 1960’s data have been accumulating demonstrating that these medications also have immunomodulatory properties. This means that they decrease hyperimmunity or inflammation to more normal and beneficial levels. The mechanism for these properties appears to be different from that of the corticosteroids. These immunomodulatory and mucoregulatory properties of macrolide antibiotics have been exploited for the treatment of diffuse panbronchiolitis (DBP), a chronic inflammatory airway disease with great morbidity and mortality when untreated. DPB is primarily seen in Japan and Korea. Its etiology is unknown but the disease results in chronic sinobronchitis with mucus hypersecretion and debilitation. Antibiotics and corticosteroids are ineffective for the treatment of DPB. By virtue of their immunomodulatory and mucoregulatory properties, the macrolide antibiotics have been demonstrated to be the most effective agents for the treatment of DPB (28). Accumulating evidence suggests that the 14- and 15-membered macrolides but not the 16 member macrolides, may also be highly effective for the therapy of CF airway disease (29, 30). The mechanism of action of the macrolides as mucoregulatory agents is under intensive study (31). It is anticipated that the development of macrolide medications without antibiotic properties will significantly extend the spectrum of use of these medications.

7. Summary Airway mucus hypersecretion and mucus retention is a significant problem for patient with chronic airway disease. The burden of asthma, chronic bronchitis, bronchiectasis, CF, and other airway diseases poses one of the most significant public health problems internationally. Medications that can effectively improve mucus clearance would provide relief to millions of persons around the world. Although many medications have been used clinically as mucoactive therapy, there are few data to support any but a handful of these medications. Rubin. Mucoactive medications. Page # 10

Table 1: Mucoactive agents

Mucoactive agent Potential mechanisms of action Expectorants

Hypertonic saline Increases secretion volume and perhaps hydration

Classical mucolytics

N-acetylcysteine Severs disulfide bond linking mucin oligomers Nacystelyn Increases chloride secretion and severs disulfide bonds Peptide mucolytics

Dornase alfa Hydrolyzes DNA polymer with reduction in DNA length

Gelsolin or Thymosin β4 Depolymerizes F-actin Non-destructive mucolytics

Dextran Breaks hydrogen bonds and increases secretion hydration Low molecular weight heparin May break both hydrogen and ionic bonds

Mucoregulatory agents

Anticholinergic agents Decreases volume of stimulated secretions Glucocorticoids Decreases airway inflammation and mucin secretion Indomethacin Decrease airway inflammation Macrolide antibiotics Decreases airway inflammation and mucin secretion

Cough clearance promotors

Bronchodilators Can improve cough clearance by increasing expiratory flow Surfactants Decreases sputum adhesiveness

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Table 2 Airway targets for mucoactive medications

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12. Tomkiewicz RP, Kishioka C, Freeman J, Rubin BK. DNA and actin filament ultrastructure in cystic fibrosis sputum. In: Baum G, ed. Cilia, Mucus and Mucociliary Interactions. New York: Marcel Dekker, 1998:333-341. 13. King M, and Rubin BK. Mucus rheology: Relationship with transport. Chapter 7 in Airway secretion: Physiological Bases for the Control of Mucus Hypersecretion. Ed. T. Takishima. Marcel Dekker, Inc. New York. 1994 pp. 283-314. 14. Puchelle E, Zahm JM, Girard F, Bertrand A, Polu JM, Aug F, Sadoul P. Mucociliary transport in vivo and in vitro. Relations to sputum properties in chronic bronchitis. Eur J Respir Dis 1980; 61:254-264. 15. Rubin BK, MacLeod PM, Sturgess JM, King M. Recurrent respiratory infections in a child with fucosidosis: Is the mucus too thin for effective transport? Pediatr Pulmonol 1991; 10:304-09 16. Grandjean EM, Berthet P, Ruffmann R, and Leuenberger P. Efficacy of oral long-term N- acetylcysteine in chronic bronchopulmonary disease: A meta-analysis of published double-blind, placebo-controlled clinical trials. Clin Ther 2000; 22:209-221. 17. Decramer M, Rutten-van Molken M, Dekhuijzen PN, et al. Effects of N-acetylcysteine on outcomes in chronic obstructive pulmonary disease (Bronchitis Randomized on NAC Cost- Utility Study, BRONCUS): A randomised placebo-controlled trial. Lancet. 2005;365:1552-60. 18. Laube BL, Auci RM, Shields DE, et al. Effect of rhDNase on airflow obstruction and mucociliary clearance in cystic fibrosis. Am J Respir Crit Care Med 1996; 153:752-60.

19. Fuchs HJ, Borowitz DS, Christiansen DH, et al. Effect of aerosolized recombinant human DNase on exacerbations of respiratory symptoms and on pulmonary function in patients with cystic fibrosis. N Engl J Med 1994; 331:637-42. 20. Rubin BK. Who will benefit from DNase? Pediatric Pulmonology 1999; 27:3-4. 21. Feng W, Garrett H, Speert DP, King M. Improved clearability of cystic fibrosis sputum with dextran treatment in vitro. Am J Respir Crit Care Med 1998;157:710-714. 22. Isawa T, Teshima T, Hirano T, Ebina A, Konno K. Effect of oral salbutamol on mucociliary clearance mechanisms in the lungs. Tohoku J Experimental Med 1986;150:51-61 Rubin. Mucoactive medications. Page # 14

23. King M, Brock G, Lundell C. Clearance of mucus by simulated cough. J Appl Physiol 1985; 58: 1776-82. 24. Hite RD, Seeds MC, Jacinto RB, Balasubramanian R, Waite M, Bass D. Hydrolysis of surfactant-associated phosphotidylcholine by mammalian secretory phospholipases A2. Am J Physiol 1998: 275 (Lung Cell Mol Physiol 19): L740-47. 25. Anzueto A, Jubran A, Ohar JA, et al. Effects of aerosolized surfactant in patients with stable chronic bronchitis. A prospective randomized controlled trial. J Am Med Assoc 1997; 278:1426- 31. 26. Tamaoki J, Chiyotani A, Kobayashi K, Sakai N, Kanemura T, Takizawa T. Effect of indomethacin on bronchorrhea in patients with chronic bronchitis, diffuse panbronchiolitis, or bronchiectasis. Am Rev Respir Dis 1992;145:548-52. 27. Tamaoki J, Chiyotani A, Tagaya E, Sakai N, Konno K. Effect of long term treatment with oxitropium bromide on airway secretion in chronic bronchitis and diffuse panbronchiolitis. Thorax 1994;49:545-48. 28. Shinkai M, Rubin BK. A global perspective on macrolide use. Japanese J Antibiotics 2005;58:129-32. 29. Shinkai M, Park CS, Rubin BK. Immunomodulatory effects of macrolide antibiotics. Clin Pulm Med 2005;12:341-349. 30. Jaffé A, Bush A. Anti-inflammatory effects of macrolides in lung disease. Pediatric

Pulmonology 2001;31:464-73. 31. Rubin BK and Tamaoki J. Eds. Antibiotics as Anti-inflammatory and Immunomodulatory Agents. Birkhäuser Verlag AG, Basel. November 2004.