The Role of Airway Secretions in COPD: Pathophysiology, Epidemiology and Pharmacotherapeutic Options

Abstract

Often considered an aggravating but otherwise benign component of chronic obstructive pulmonary disease (COPD), airway mucus hypersecretion is now recognised as a potential risk factor for an accelerated loss of lung function in COPD and is a key pathophysiological feature in many patients, particularly those prone to respiratory tract infection. Consequently, it is important to develop drugs that inhibit mucus hypersecretion in these susceptible patients. Conventional therapy including anticholinergics, β₂-adrenoceoptor agonists, alone or in combination with corticosteroids, mucolytics and macrolide antibiotics are not entirely or consistently effective in inhibiting airway mucus hypersecretion in COPD. Novel pharmacotherapeutic targets are being investigated, including inhibitors of nerve activity (e.g., BK_Ca channel activators), tachykinin receptor antagonists, epoxygenase inducers (e.g., benzafibrate), inhibitors of mucin exocytosis (e.g., anti-MARCKS peptide and Munc-18B blockers), inhibitors of mucin synthesis and goblet cell hyperplasia (e.g., EGF receptor tyrosine kinase inhibitors, p38 MAP kinase inhibitors, MEK/ERK inhibitors, hCACL2 blockers and retinoic acid receptor-α antagonists), inducers of goblet cell apoptosis (e.g., Bax inducers or Bcl-2 inhibitors), and purinoceptor P_2Y2 antagonists to inhibit mucin secretion or P_2Y2 agonists to hydrate secretions. However, real and theoretical differences delineate the mucus hypersecretory phenotype in COPD from that in other hypersecretory diseases of the airways. More information is required on these differences to identify therapeutic targets pertinent to COPD which, in turn, should lead to rational design of anti-hypersecretory drugs for specific treatment of airway mucus hypersecretion in COPD.

Key words: :

• Airway inflammation
• Chronic obstructive pulmonary disease
• Goblet cell
• MUC gene
• Mucin
• Erythromycin

Introduction

Airway mucus hypersecretion was steadily demoted to an aggravating but otherwise benign component of chronic obstructive pulmonary disease (COPD) [1&2]. Thus, although included in earlier descriptions of COPD [3&4], the term “mucus hypersecretion” is omitted from more recent definitions [5&6]. However, new epidemiological studies demonstrate that mucus is far from innocent. Consequently, airway mucus hypersecretion is now recognised as a potential risk factor for an accelerated loss of lung function in COPD and is a key pathophysiological feature in many patients (Figure 1). The inclusion of recommendations for mucolytic (i.e., mucus “lysis”) therapy in the latest guidelines for clinical management of COPD [6], in contrast to the lack of such recommendations in guidelines of only 4 years ago [5], is testament to our rapidly changing view of the role of mucus in the pathophysiology of COPD.

Figure 1 Airway mucus hypersecretion in COPD. A) Gross pathology. Luminal mucus (M) partially blocking an extrapulmonary bronchus in a cigarette smoker with chronic sputum production. B) Histopathology: Luminal mucus (M) occluding a small airway in a patient with COPD.

Mucus hypersecretion (commonly associated with the term chronic bronchitis) is 1 of 3 pathophysiological entities that comprise COPD, the other 2 being chronic bronchiolitis (often termed small airways disease) and emphysema (characterised by alveolar destruction and airspace enlargement) [5][7]. The relative contribution of each component to pathophysiology varies between patients, with the impact of mucus hypersecretion on clinical symptoms varying accordingly. In many patients, airway hypersecretion has clinical significance, for example in patients with low lung function or who are prone to chest infections [8] (Figure 2). Consequently, it is important to understand the pathophysiology of mucus hypersecretion in COPD. This in turn should allow identification of therapeutic targets and rational development of pharmacotherapeutic drugs. It should be noted, however, that mucus hypersecretion in response to airway irritation, inflammation or infection is a physiologically protective response, with a balance between beneficial excess secretion and detrimental secretion retention. However, with continued hypersecretion there is presumably a point after which the secretions become excessive and, instead of being protective, become detrimental to airway homeostasis. Consequently, drugs that inhibited the excess secretion but retained the “normal” protective amount of secretion might have benefit over drugs that dried up secretions completely.

Figure 2 Putative schemas for airway mucus pathophysiology in COPD. A) Impact of mucus hypersecretion on lung function. B) ‘Vicious circle’ of mucus hypersecretion and bacterial infection.

The present article: 1) assesses the contribution of airway mucus hypersecretion and impaired mucociliary clearance to pathophysiology of the “bronchitic” component of COPD, 2) considers the epidemiology and changing view of the clinical impact of mucus hypersecretion in COPD, and 3) briefly discusses current therapy and outlines potential novel therapy for this condition. To set these issues in context, the following section gives a condensed summary of airway mucus, mucins and mucin (MUC) genes.

Airway Mucus, Mucins and MUC Genes

In healthy individuals, a film of slimy liquid overlies and protects the airway surface [9]. The liquid is often referred to as “mucus” and is a complex, non-homogeneous, dilute (1–2%) aqueous solution of salts, enzymes and anti-enzymes, oxidants and antioxidants, bacterial products, antibacterial agents, cell-derived mediators and proteins, plasma-derived mediators and proteins, and cell debris such as DNA. The mucus forms a bilayer comprising an upper gel layer and a lower sol layer. A thin layer of surfactant appears to separate the gel and sol [10&11]. Cilia beat in the sol layer, often termed periciliary liquid. Inhaled particles are trapped in the gel layer and, by transportation on the tips of beating cilia, are removed from the airways, a process termed mucociliary clearance. Airway mucus requires an optimal combination of viscosity and elasticity for efficient ciliary interaction. Viscoelasticity is conferred primarily by high molecular weight mucous glycoproteins, termed mucins, which comprise up to 2% by weight of the mucus [12]. Airway mucins are primarily produced in, and secreted by, goblet cells in the surface epithelium [13] and by mucous cells in the submucosal glands [14]. Mature mucins are long, thread-like molecules composed of monomers joined end-to-end by disulphide bridges. These threads form a “tangled network” [15] that contributes to the formation of the mucus gel. The mucin monomers comprise a highly glycosylated linear peptide sequence, termed “apomucin,” that is encoded by specific mucin (MUC) genes. Nineteen human MUC genes are reported to date, namely MUC1, 2, 3A, 3B, 4, 5AC, 5B, 6–9, 11–13 and 16–20 [16-21]. Although a number of these genes are expressed in the airways [12], it is the MUC5AC and MUC5B gene products that are the major gel forming mucins in normal respiratory tract secretions [12], although MUC2 may be upregulated in COPD (see below) (Figure 3).

Figure 3 Putative differences in airway mucus pathophysiology between COPD and asthma. Compared with normal, in COPD there is increased luminal mucus, goblet cell hyperplasia, submucosal gland hypertrophy (with an increased proportion of mucous to serous acini), an increased ratio of mucin (MUC) 5B (low-charge glycoform, lcgf) to MUC5AC, small amounts of MUC2, and respiratory infection (possibly due to reduced bacterial enzymatic shield from reduced serous cell number). Pulmonary inflammation includes macrophages and neutrophils. In asthma, there is increased luminal mucus, epithelial fragility, marked goblet cell hyperplasia, submucosal gland hypertrophy (although without an increased mucous to serous ratio), tethering of mucus to goblet cells, and plasma exudation. Airway inflammation includes T lymphocytes and eosinophils. Many of these differences require more data from greater numbers of subjects.

Mucus Hypersecretory Phenotype in COPD

Airway mucus hypersecretion in COPD has characteristic pathophysiological features. A number of these features, for example sputum production and goblet cell hyperplasia, are common to other hypersecretory respiratory diseases, for example asthma and cystic fibrosis (CF). Other features appear to be associated specifically with COPD (see below). Differences in mucus pathophysiology between COPD and asthma have been discussed previously [22], and are summarised in Figure 3. Presumably, differences in the pulmonary inflammatory “profile” of COPD and asthma (the former essentially a macrophage-driven neutrophilia, the latter a Th2 lymphocyte-driven eosinophilia) [23&24] underlie the differences in hypersecretory phenotype between these two conditions.

Sputum production of up to 100 ml per day in many patients is associated with excessive mucus in the airway lumen (Figures 1 and 4) [25-27]. The increased mucus is associated with goblet cell hyperplasia (Figure 4) [25&26][28] and submucosal gland hypertrophy (Figure 5) [25&26][29&30]. Of particular note is that the gland mucous cells are markedly increased relative to the serous cells [31] (see below for pathophysiological implications of this change in gland mucous cell-serous cell ratio). This is in contrast to asthma where the glands, albeit hypertrophied, are morphologically normal. Gland size in COPD correlates with amount of luminal mucus and daily sputum volume [29][32]. Although not necessarily causal, the latter observation suggests a significant relationship between gland hypertrophy and mucus hypersecretion in COPD.

Figure 4 Airway mucus hypersecretion in COPD. Amount of luminal mucus (mucus occupying ratio, MOR) is significantly greater in both central and distal airways of patients who die with a diagnosis of chronic bronchitis compared with those with emphysema or controls without respiratory disease. L, luminal perimeter; Sb₁, size of bronchus before computer-based image analysis conversion to Sb₂ (Br, bronchial radius); S_m, size of stained area of mucus. Redrawn after Ref. [26].

Figure 5 Airway submucosal gland hypertrophy in COPD. The percentage of the airway wall occupied by the glands (G) is significantly greater in the airways of patients who die with a diagnosis of chronic bronchitis compared with those with emphysema or controls without respiratory disease. Redrawn after Ref. [26].

The above features of hypersecretion are not common to all patients with COPD. Not all patients expectorate, and there is overlap in gland size with healthy non-smokers, and also between sputum producers and non-producers [28][30][33-35]. Goblet cell hyperplasia is not noted in all patients (Figure 6) [26][31]. Interestingly, although goblet cell hyperplasia is associated with degree of airway inflammation, gland size is not [28]. Thus, although considered general features of COPD, mucus hypersecretion and its associated pathophysiological abnormalities do not characterise all patients.

Figure 6 Airway goblet cell mucus in COPD: lack of goblet cell hyperplasia? The percentage of the airway epithelium staining for mucus is not significantly different to that in the airways of patients who die with a diagnosis of chronic bronchitis compared with those with emphysema or controls without respiratory disease. Redrawn after Ref. [26].

The mucin composition of airway mucus is abnormal in COPD. Mucins in sputum are less acidic than normal [36], which may reflect disease-related alterations in glycosylation. MUC5AC and a low charge glycoform of MUC5B are the major mucin species in patients with COPD [37-40]. Intriguingly, the low-charge glycoform appears to be proportionally increased above normal levels [41]. This is a potentially significant observation, in terms of altered mucin glycosylation processing in COPD, that requires confirmation, or otherwise, in a greater number of samples. It is possible that the change in MUC5B glycoforms, coupled with the reduction mentioned above [31] in gland serous cells, a rich source of antimicrobial enzymes such as lysozyme and lactoferrin [42], contributes to the airway bacterial infections that are a clinical feature of many COPD patients [5].

In contrast to normal airways, goblet cells in COPD contain not only MUC5AC but also MUC5B [39][43] and MUC2 [12]. This distribution is different to that in patients with asthma or CF, where MUC5AC and MUC5B show a similar histological distribution to normal controls [44&45]. Although not found consistently [27][38], there is a growing impression that MUC2 is increased in irritated airways, including COPD [12][41][46].

Abnormalties in Mucociliary Clearance in COPD

Abnormal airway mucus in COPD goes in concert with abnormal ciliated cells and cilia. The number of ciliated cells and length of cilia is decreased in patients with chronic bronchitis [47]. Ciliary abnormalities include compound cilia, cilia enclosed within periciliary sheaths, cilia with abnormal axonemes and cilia with intra-cytoplasmic microtubule doublets [48]. These abnormalities coupled with mucus hypersecretion are associated with reduced mucus clearance and airway obstruction [49]. However, differences in methodology [50] and patient selection, especially ensuring exclusion of patients with asthma [51], can confuse interpretation of these studies. Lung clearance is significantly reduced in heavy smokers [52] and in patients with chronic bronchitis [53]. However, it should be noted that forced expirations and cough compensate relatively effectively for decreased mucociliary clearance in patients with chronic bronchitis, although they are much less effective in patients with emphysema where lung elastic recoil is impaired [54&55].

Epidemiology of Mucus Hypersecretion in COPD

The perception of the role of airway mucus hypersecretion in pathophysiology and clinical symptoms in COPD has shifted from being a condition independent of disease progression to now being positively associated with morbidity and mortality [22][56]. Epidemiological studies sampling many hundreds of subjects in the late 1970s to early 1990s found scant evidence for the involvement of mucus in either the mortality or accelerated age-related decline in lung function associated with COPD [1&2][57-60]. In all studies, sputum production, assessed by standardised questionnaire, was the index of mucus hypersecretion. However, the relationship between sputum production and mucus hypersecretion, particularly in the small airways, the main site of airflow obstruction in COPD [61], is unclear. It is also noteworthy that these studies were primarily in occupational cohorts, and were exclusively in men. Nevertheless, the consensus of these studies was that airflow obstruction and mucus hypersecretion were largely independent disease processes.

In contrast, a number of studies over the last 18 years, again with large sample number, and with an emphasis on general population samples rather than occupational cohorts, found positive associations between sputum production and decline in lung function, hospitalisation and death [62-70]. Some of these reports were re-examinations of the same patients, now older, reported previously [62]. Of note is the observation that incidence of death was related to increased risk in patients with phlegm production to die of respiratory infection [8]. Additionally, the association between chronic mucus hypersecretion and frequency of lower respiratory illness extends to an association with an accelerated decline in lung function with successive bouts of pulmonary infection [71]. In summary, although not associated with disease progression in all cases, mucus hypersecretion contributes to morbidity and mortality in many patients with COPD, particularly those prone to infection, those with low lung function [63] and, possibly, as patients age. This highlights the importance of developing drugs that inhibit mucus hypersecretion in these patients.

Pharmacotherapy of Mucus Hypersecretion in COPD

As discussed before, airway mucus contributes to morbidity and mortality in patients in whom the mucus hypersecretory phenotype impacts significantly on pathophysiology and clinical status. Consequently, drugs affecting the bronchitic component of COPD should be beneficial in these patients. However, COPD has specific trigger factors, profile of pulmonary inflammation and mucus hypersecretory phenotype (Figure 3), and specific drugs may be required to fulfil the theoretical requirements for treatment of hypersecretion in COPD (Table 1). The following sections give a condensed summary of different approaches to inhibition of mucus hypersecretion in COPD, starting with conventional pharmacotherapy and followed by consideration of the wide range of potential novel pharmacotherapeutic approaches. It should be noted that drugs that suppress lung inflammation should also indirectly suppress mucus hypersecretion and are, therefore, possibly the most beneficial therapy overall (Figure 7).

Table 1.  Theoretical objectives for pharmacotherapy of mucus hypersecretory pathophysiology in COPD

Overall desired objective	Component objective
Facilitate mucus clearance (short term relief of symptoms)	Reduce viscosity (?increase elasticity)
	Increase ciliary function
	Induce cough
Reverse hypersecretory phenotype (long-term benefit)	Reduce pulmonary inflammation
	Reduce submucosal gland size
	Correct increased gland mucous:serous cell ratio
	Reduce goblet cell number
	Reverse increased lcgf-MUC5B:MUC5AC ratio
	Inhibit MUC2 production

lcgf, low charge glycoform; MUC mucin gene product.

Figure 7 Relationship between neutrophilic inflammation in COPD and generation of a hypersecretory phenotype (e.g., goblet cell hyperplasia). Interactions between inhaled pollutants (e.g., cigarette smoke), macrophages and epithelial cells generate neutrophil chemoattractants, with resultant release of factors that induce mucin synthesis, goblet cell hyperplasia and mucus hypersecretion. The sequence of initiating events can be inhibited at different levels of the inflammatory pathway. PDE, phosphodiesterase.

Conventional Pharmacotherapy

The medications currently used in clinical management of COPD, namely bronchodilators (anticholinergics, β₂-adrenoceptor agonists and methylxanthines) and anti-inflammatories, primarily glucocorticosteroids [5], are not administered necessarily to target airway hypersecretion, but may nevertheless exert some of their beneficial effects via actions on mucus. The activity of these drugs on mucociliary dysfunction in COPD has been recently reviewed in detail [72]. Theoretically, anticholinergics are likely to have beneficial effects on mucociliary function, but clinically these effects have been difficult to demonstrate. Long-acting β₂-agonists (LABAs), rather than short-acting β₂-agonists, might improve the mucociliary component of COPD, in addition to providing symptomatic treatment by their bronchodilator action.

Suppression of inflammation should indirectly suppress mucus hypersecretion. Anti-inflammatory drugs also directly affect the mucus hypersecretory phenotype. For example, in experimental systems, glucocorticosteroids inhibit mucus secretion, MUC gene expression, mucus synthesis and goblet cell hyperplasia [73]. However, in contrast to asthma where they are clinically effective [74], in part due to an anti-hypersecretory action, glucocorticoids have limited effectiveness in stable COPD [5]. This limited effectiveness appears to be due to a relative lack of effect of corticosteroids on the pulmonary inflammation in COPD [75&76]. A limited effect on pulmonary inflammation will, therefore, hinder suppression of the component of hypersecretion that is inflammation driven (for example, by neutrophil elastase). In addition, it may be that the COPD-specific aspects of mucus hypersecretion (for example, increased proportion of gland mucous cells) are similarly limited in their response to corticosteroids. However, it should be noted that combination therapy with a LABA and an inhaled glucocorticosteroid may address the multifactorial nature of COPD by providing not only bronchodilation but also anti-inflammatory activity, which may indirectly further improve mucociliary clearance.

Mucolytics and/or Anti-Oxidants

Decreasing the viscosity of, or “thinning”, viscous airway mucus with mucolytic drugs should be a way of improving mucus clearance, both by mucociliary transport and by cough. This last could be aided by use of expectorant drugs, some of which may in fact increase secretion temporarily to aid the effectiveness of cough in dislodging and expelling mucus. However, although numerous mucolytic drugs are available worldwide, their effectiveness in treatment of stable COPD has not been established [77]. In addition, there are safety issues with a number of mucolytic preparations, for example iodinated glycerol. Consequently, mucolytics are not generally recommended in current guidelines on clinical management of COPD [5]. However, last year's NICE guideline recommended that mucolytic therapy should be considered in patients with a chronic productive cough, and that mucolytic therapy be continued if there was symptomatic improvement in cough and sputum production [6].

These recommendations were based upon data from 3 rigorous meta-analyses that found that treatment for at least 2 months with certain mucolytic drugs, with a heavy bias towards N-acetylcysteine, reduced the number of exacerbations and days of illness [78-80]. The reduction in exacerbations leads to reductions in both direct hospital costs and indirect costs due to sick leave from work. It has been calculated that, in the Swiss health care system, when cost of treatment is balanced against direct and indirect costs, treatment becomes cost effective in patients who would be expected to have 1 exacerbation during the winter months (approximately 6-month interval), and increases proportionally with severity of disease [81].

However, it is not clear whether the beneficial effects of a number of these drugs, in particular N-acetylcysteine [82], are due to their mucolytic or anti-oxidant properties (or both). Nevertheless, there is continuing interest in mucolytic-anti-oxidant compounds for treatment of COPD. For example, a recent clinical trial of the thiol derivative erdosteine [83] found that 8 months' treatment of patients with moderate COPD reduced exacerbation and hospitalization rates and improved health status. The authors suggested that erdosteine is likely to make an important contribution to therapy of patients with symptomatic COPD.

Oxidant stress is considered a pathophysiological feature of COPD [84] and nitric oxide (NO) is elevated in COPD [85]. Oxidants and NO have marked effects on airway mucus and goblet cells [86&87]. Consequently, anti-oxidants and inhibitors of inducible NO synthase (iNOS) may have therapeutic benefit for mucus hypersecretion in COPD. However, at present, the NICE guidelines do not recommend use of the anti-oxidants alpha-tocopherol or beta-carotene in clinical management of COPD [6]. Clearly, suitable clinical investigations with more potent anti-oxidants are warranted.

Macrolide Antibiotics

Although antibiotics are recommended in clinical management of exacerbations of COPD, there is no evidence to recommend prophylactic antibiotic treatment in management of stable COPD [5&6]. However, erythromycin, clarithromycin and flurythromycin are macrolide antibiotics that have a variety of beneficial effects on airway mucus, for example inhibition of mucin secretion in a variety of experimental preparations including human airways in vitro [22][88]. Anecdotally, erythromycin reduces excessive sputum production in patients with airway mucus hypersecretion [89&90]. The mechanism of action of erythromycin is relatively unexplored, but may involve anti-inflammatory effects [91&92] as well as direct inhibitory effects on MUC gene expression, mucin synthesis and mucin secretion [93]. Formal clinical studies of its effects on the pathophysiology of mucus hypersecretion, for example sputum production and lung function, in COPD would be of interest.

Novel Pharmacotherapy

The clinical symptoms of cough and sputum production, coupled with a perception of the importance of mucus hypersecretion in the pathophysiology of a number of severe lung conditions, including COPD, has prompted renewed interest in research into airway hypersecretion and, in concert, in development of drugs targeting mucus and the hypersecretory phenotype in COPD. These drugs and their purported targets have been recently discussed in detail [22][94&95], and will be only briefly highlighted herein (Table 2). It should be noted that the activity of many of these compounds is not as selective for the target as may be thought and, in any event, whether or not any beneficial activity of the drug is due to activity at the target is, for the most part, substantially unproven.

Table 2.  Novel targets for inhibition of the mucus hypersecretory phenotype in COPD

Target	Requirement	Example drug(s)	Reference
Bax	Inducer	—	[96]
Bcl-2	Inhibitor	Antisense oligonucleotides (eg oblimersin sodium)	[97][123]
ErbB (1, 2, 3) tyrosine kinases	Inhibitors (including dual ErbB1/2 inhibitors)	Gefitinib (ZD1839, Iressa), erlotinib, PKI 166, GW 572016, EKB 569	[87][98]
Epoxyeicosanoids	Adjunct therapy	—	—
Epoxygenase	Inducer	Benzafibrate	[105]
hCLCA1	Inhibitor	MSI1956 (talniflumate)	[114]
MAP kinase	Inhibitor	SB203580, SB202190	[112]
MARCKS	Inhibitor	Synthetic peptide to N-terminal region, antisense oligonucleotide	[107]
MEK	Inhibitor	PD98059, U0126	[113]
MUC gene expression	Inhibitor	Antisense oligonucleotide	[122]
Munc18B	Inhibitor	Antisense oligonucleotide	[108]
Nitric oxide (NO)	iNOS inhibitor	GW273629, L-NIL	[86]
Oxidants	Inhibitors	N-acetylcysteine, superoxide dismutase mimetics, spin trap compounds	[99]
P2Y2 purinoceptors	Agonist	INS365	[124]
P2Y2 purinoceptors	Antagonist	? reactive blue 2 derivatives	[126]
PtdIns 3-kinase	Inhibitor	LY294002	[113]
RAR-α	Antagonist	RO-41-5253	[120]

Inhibition of Nerve Activity

There are novel options for inhibiting the effects of cholinergic nerves [100], none of which are used clinically. These include inhibition of neurotransmitter (acetylcholine) release by activation of prejunctional receptors, for example opioid μ and δ receptors, cannabinoid CB₂ receptors or vasoactive intestinal peptide VPAC1 receptors, and activation of large conductance calcium-activated potassium (BK_Ca) channels (Figure 8). There is also the possibility that sensory nerve activation may mediate mucus output in COPD [101]. These nerves can also be inhibited in a similar fashion to cholinergic nerves (see above), and also by inhibition of the effects of their tachykinin neurotransmitters, including substance P and neurokinin A, by tachykinin receptor antagonists [102]. The vanilloid VR-1 receptor mediates activation of sensory nerves and selective VR-1 antagonists, such as capsazepine, are in development [103].

Figure 8 Pharmacotherapy of airway mucus hypersecretion in COPD. The pathophysiological ‘cascade’ from initiating factors to clinical symptoms can be accessed at different levels by ‘antihypersecretory’ pharmacotherapeutic compounds. The precise site(s) of action of many compounds is unclear, and some compounds may act at more than one site. hCLCA, human calcium-activated chloride channel; COX, cyclooxygenase; EGFR, epidermal growth factor receptor; ERK, extracellular signal-regulated kinase; MARCKS, myristoylated alanine-rich C kinase substrate; MEK, mitogen-activated protein kinase kinase; MUC, mucin (gene); NKCC, Na⁺-K⁺-Cl⁻ cotransporter; PI-3K, phosphatidylinositol 3-kinase; RAR, retinoic acid receptor.

Epoxygenase Inducers

Epoxygenases (cytochrome P-450 enzymes) metabolise arachidonic acid and regulate inflammation [104]. Benzafibrate, an inducer of epoxygenase, inhibits airway goblet cell hyperplasia in a rat model of chronic bronchitis [105]. The mechanisms underlying the inhibition include production of anti-inflammatory mediators and reduction in amount of available arachidonic acid. Epoxygenase inducers, or selective eopxyeicosanoids, would be potential therapy for both the inflammation and mucus hypersecretion of COPD.

Inhibitors of Mucin Exocytosis

Inhibition of mucin exocytosis is a therapeutic option for mucus hypersecretion in COPD. However, it should be noted that inhibition of secretion could lead to excessive accumulation of intracellular mucins, with unknown, and potentially detrimental, effects on secretory cell function. Myristoylated alanine-rich C kinase substrate (MARCKS) protein is a key intracellular molecule involved in intracellular movement and exocytosis of mucin granules [106]. Inhibition of MARCKS production by an antisense oligonucleotide down-regulated both mRNA and protein levels of MARCKS and attenuated mucin secretion. Blockade of MARCKS by a synthetic peptide to its N-terminal region (MANS peptide) inhibited mucin secretion by normal human bronchial epithelial cells in vitro [106] and by mouse airway epithelium in vivo [107]. Similar to MARCKS, the Sec1/Munc18 family are critical to exocytosis in airway goblet cells. Experimental induction of Munc18B induces a marked airway hypersecretory phenotype [108]. Inhibition of Munc18B using antisense technology is under investigation.

Inhibitors of Goblet Cell Hyperplasia

Increased MUC gene expression, mucin synthesis and goblet cell hyperplasia appear to be linked processes that are regulated by a number of inflammatory mechanisms. For example, airway epidermal growth factor receptor (EGF-R) expression is induced by experimental procedures pertinent to COPD [87]. It appears that long-term cigarette smoking induces enhanced expression of EGF-R, as well as ErbB3 (another member of the EGF-R family) and MUC5AC [109]. In experimental systems, EGF-R upregulation and signalling via EGF-R tyrosine kinase is a signalling event for induction of mucin synthesis and goblet cell hyperplasia. Inhibitors of EGF-R tyrosine kinase block these responses. One of these, gefitinib (ZD1839, also known as Iressa), is in clinical trial for cancer, but not yet for COPD or similar respiratory diseases.

Unsurprisingly, the p38 mitogen activated protein (MAP) kinase pathway, the MEK/ERK pathway, and the phosphatidylinositol 3-kinase pathway are all involved, to a greater or lesser extent, in intracellular events leading to mucin synthesis and goblet cell hyperplasia [110-113]. Inhibitors of these pathways inhibit mucus hypersecretory endpoints in experimental systems.

Calcium-activated chloride (CLCA) channels also appear to be critically involved in development of an airway hypersecretory phenotype [114]. In mice, suppression of mCLCA3 inhibits goblet cell hyperplasia, whilst overexpression increases goblet cell number [115]. Talniflumate (MSI 1956 or ‘Lomucin’) is a small molecule putative inhibitor of hCLCA1 originally developed by Laboratorios Bago which is currently being developed by Genaera as a mucoregulatory treatment for asthma, CF and COPD [116]. Phase I clinical trials were completed in 2001 and phase II trials in CF are underway in Ireland. The results of these trials are awaited with great interest.

Retinoic acid (vitamin A) is perceived to be of clinical benefit in a variety of clinical conditions. Agonists at the retinoic acid RAR-γ are currently being intensely investigated as inhibitors and ‘reversers’ of alveolar destruction in emphysema [117]. In contrast, the RAR-α receptor appears to be involved in mucin expression [118-120] and in the development and maintenance of a hypersecretory phenotype [120]. RAR-α antagonists such as RO-41-5253 inhibit a number of these activities [121]. Consequently, there is interest in development of selective RAR-α antagonists for mucus hypersecretion in a number of respiratory diseases, including COPD [117].

Finally, antisense technology is also being explored as a new approach to inhibition of goblet cell hyperplasia. For example, an 18-mer MUC antisense oligomer suppressed mucin gene expression and in wood smoke-induced epithelial metaplasia in rabbit airways [122].

Inducers of Goblet Cell Apoptosis

Hyperplastic airway goblet cells in COPD models express the anti-apoptotic factor Bcl-2 [123]. Conversely, the pro-apoptotic factor Bax is crucial for resolution of hyperplasia. Thus, the balance between Bcl-2 and Bax may affect the persistence of goblet cell hyperplasia. Reduction of Bcl-2 expression by antisense oligonucleotides induces a dose-dependent resolution of hyperplasia.

Mucus Inhibition vs. Mucus Hydration

The purine nucleotides, adenosine 5′-triphosphate (ATP) and uridine triphosphate (UTP), increase airway mucin and water secretion via interaction with P_2Y2 purinoceptors [124&125]. Consequently, P_2Y2 antagonists might be effective in inhibiting airway hypersecretion [126]. However, mucus hydration is associated with improvements in mucociliary clearance and stimulation of water secretion may have greater therapeutic potential than inhibition of P_2Y2-mediated mucin secretion [9]. Consequently, there is considerable interest in development of P_2Y2 agonists. In phase I clinical trial, a second generation P_2Y2 agonist, INS365, was safe, well tolerated and significantly enhanced sputum expectoration [124]. However, uncontrolled thinning of airway mucus may have adverse clinical effects.

Summary and Conclusions

Airway mucus hypersecretion and the pathophysiological changes that accompany it, for example goblet cell hyperplasia, are features of many patients with COPD. The impact of airway hypersecretion on morbidity and mortality is now more fully understood, albeit that it may be limited to certain groups of patients, particularly those who are prone to respiratory tract infection. Nevertheless, it is important to develop drugs that inhibit mucus hypersecretion in these susceptible patients. However, before addressing these issues in a rational manner, considerably more information is required on basic mucus physiology and, in particular, mucus pathophysiology. For example, more detail is required concerning the biochemical and biophysical nature of airway mucins in normal healthy subjects. Answers to the questions of whether or not there is an intrinsic abnormality of mucus in COPD, and whether any abnormality is specific for COPD are urgently required. In addition, the factors that regulate MUC gene expression in health and disease, and the relationship between this regulation and development of a hypersecretory phenotype that appears to be specific to the bronchitic component of COPD, need to be determined. The above information could then be used in delineation of therapeutic targets which, in turn, should lead to rational design of anti-hypersecretory drugs for specific treatment of airway mucus hypersecretion in COPD. At present, drugs that suppress lung inflammation should also suppress mucus hypersecretion and may be the most beneficial therapy overall. Of the more selective molecules currently under investigation, it would appear that the EGF signalling cascade mediates generation of the hypersecretory phenotype in a wide variety of experimental preparations and is upregulated in COPD. Consequently, inhibitors of EGF signalling may be prime contenders as antihypersecretory pharmacotherapy. However, considerably more information is required concerning which specific EGF receptors are involved in airway mucus hypersecretion, and whether selective single or dual (or multiple) inhibitors will be most effective.