Tachykinin receptor antagonists for asthma and COPD

The role of airway secretions in COPD: pathophysiology, epidemiology and pharmacotherapeutic options

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, beta2-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-alpha 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.

Structure–activity relationships of 1-alkyl-5-(3,4-dichlorophenyl)-5-{2-[3-(substituted)-1-azetidinyl]-ethyl}-2-piperidones. Part 2: Improving oral absorption

A series of piperidone analogues of 1b-q, seeking replacements for the polar sulfamide moiety in clinical candidate UK-224,671 1a, possessing reduced H-bonding potential as a strategy to improve oral absorption, were prepared. These studies led to the successful identification of 1n, which demonstrated equivalent pharmacology and metabolic stability to 1a, and greatly improved oral absorption as assessed in rat PK studies.

Mucociliary dysfunction in COPD: effect of current pharmacotherapeutic options

Chronic obstructive pulmonary disease (COPD) is a major cause of morbidity and mortality worldwide. COPD comprises multiple components which, as well as a systemic component, include pulmonary inflammation, airway remodelling and mucociliary dysfunction. The latter features contribute to the development of chronic, progressive airflow limitation. The mucociliary dysfunction component of COPD is due to mucus hypersecretion coupled with a decrease in mucus transport, and represents an important pathophysiological feature requiring appropriate treatment. Current international guidelines do not recommend the use of mucolytics in the treatment of stable COPD. In contrast, bronchodilators are central to symptomatic management of COPD, and include beta(2)-adrenoceptor agonists, anti-cholinergics and methylxanthines. Interestingly, long-acting beta(2)-agonists (LABAs), rather than short-acting beta(2)-agonists, have the potential to improve the mucociliary component of COPD, in addition to providing symptomatic treatment by their bronchodilator action. Combination therapy with a LABA and an inhaled corticosteroid has the potential to more fully address the multicomponent nature of COPD by providing important anti-inflammatory activity, which may indirectly further improve mucociliary clearance. Theoretically, anti-cholinergics are likely to have mixed effects on mucociliary function, but clinically these effects have been difficult to demonstrate. Finally, a number of novel targets for the treatment of airway mucociliary dysfunction have been identified, and targeting agents are currently in development.

Targeting tachykinins for the treatment of obstructive airways disease

The tachykinin family of peptides are distributed throughout the nervous system and are thought to play a critical role in inflammation and immunomodulation. Tachykinins have been implicated in the pathogenesis of many diseases and disease processes including inflammatory pain, emesis, depression, Parkinson's disease and inflammatory bowel syndrome. In the airways of animals, substance P and neurokinin A are released from a subset of airway sensory nerves, and evoke vasodilatation, bronchoconstriction, mucus secretion, leukocyte recruitment, airways hyperreactivity and cough. These observations have led to suggestions that tachykinins may also be viable targets for the treatment of obstructive airways disease. Clinical trials in humans assessing the utility of tachykinin receptor antagonists such as nepadutant and saredutant for the treatment of asthma are limited, and the results for the most part have been inconclusive. Several new tachykinin receptor antagonists have been recently designed to target multiple tachykinin receptor subtypes and to readily penetrate into the central nervous system. Future clinical trials with these compounds should help to shed some light on the role of tachykinins in obstructive airways disease.

Naphtho[2,1-b][1,5] and [1,2-f][1,4]oxazocines as selective NK1 antagonists

Previously we reported on the synthesis and properties of a series of highly potent piperidinyl 2-subsituted-3-cyano-1-naphthamide NK1 antagonists that includes 3 and 4. Here we report our efforts to alleviate a troublesome atropisomeric property of those derivatives by introduction of a tethering bridge that, in addition, could be used to lock the resulting cyclic derivatives in a purported NK1 pharmacophore conformation. Using 3 as a starting point, the naphtho[2,1-b][1,5]oxazocine, 17, was found to contain the optimal ring tether size (8) for retaining NK1 activity, was more NK1 versus NK2 selective, and reduced the number of atropisomers from four to two. Cyclic derivatives 29 and 32, which exist as essentially single atropisomers in the purported pharmacophore conformation, were prepared in the closely related naphtho[1,2-f][1,4]oxazocine series as part of an effort to use mono methyl substitution of the tethering bridge as a conformation stabilizing factor. Both 29 and 32 were found to be less active as NK1 antagonists than the non-methylated parent 28 possibly due to methyl group destabilization of receptor interaction. We discuss the above findings in the context of a previously proposed NK1 pharmacophore model and present a further refinement of that model.

Therapy for chronic obstructive pulmonary disease in the 21st century

Chronic obstructive pulmonary disease (COPD) is a common, smoking-related, severe respiratory condition characterised by progressive, irreversible airflow limitation. Current treatment of COPD is symptomatic, with no drugs capable of halting the relentless progression of airflow obstruction. Better understanding of the airway inflammation, oxidative stress and alveolar destruction that characterise COPD has delineated new disease targets, with consequent identification of novel compounds with therapeutic potential. These new drugs include aids to smoking cessation (e.g. bupropion) and improvements to existing therapies, for example long-acting rather than short-acting bronchodilators, as well as combination therapy. New antiproteases include acyl-enzyme and transition state inhibitors of neutrophil elastase (e.g. sivelestat and ONO-6818), matrix metalloprotease inhibitors (e.g. batimastat), cathepsin inhibitors and peptide protease inhibitors (e.g. DX-890 [EPI-HNE-4] and trappin-2). New antioxidants include superoxide dismutase mimetics (e.g. AEOL-10113) and spin trap compounds (e.g. N-tert-butyl-alpha-phenylnitrone). New anti-inflammatory interventions include phosphodiesterase-4 inhibitors (e.g. cilomilast), inhibitors of tumour necrosis factor-alpha (e.g. humanised monoclonal antibodies), adenosine A(2a) receptor agonists (e.g. CGS-21680), adhesion molecule inhibitors (e.g. bimosiamose [TBC1269]), inhibitors of nuclear factor-kappaB (e.g. the naturally occurring compounds hypoestoxide and (-)-epigallocatechin-3-gallate) and activators of histone deacetylase (e.g. theophylline). There are also selective inhibitors of specific extracellular mediators such as chemokines (e.g. CXCR2 and CCR2 antagonists) and leukotriene B(4) (e.g. SB201146), and of intracellular signal transduction molecules such as p38 mitogen activated protein kinase (e.g. RWJ67657) and phosphoinositide 3-kinase. Retinoids may be one of the few potential treatments capable of reversing alveolar destruction in COPD, and a number of compounds are in clinical trial (e.g. all-trans-retinoic acid). Talniflumate (MSI-1995), an inhibitor of human calcium-activated chloride channels, has been developed to treat mucous hypersecretion. In addition, the purinoceptor P2Y(2) receptor agonist diquafosol (INS365) is undergoing clinical trials to increase mucus clearance. The challenge to transferral of these new compounds from preclinical research to disease management is the design of effective clinical trials. The current scarcity of well characterised surrogate markers predicts that long-term studies in large numbers of patients will be needed to monitor changes in disease progression.

Pulmonary mucus: Pediatric perspective

Airway mucus hypersecretion is a clinical feature of a number of childhood diseases, including asthma and bronchitis-associated conditions. However, compared with adults, there is relatively scarce information concerning mucus pathophysiology in respiratory diseases in children. The available evidence indicates many similarities between adult and childhood respiratory hypersecretory conditions, including goblet-cell hyperplasia and submucosal gland hypertrophy, and airway mucus plugging in asthma. Consequently, it is likely that treatments that are effective in adults would be effective in children. Numerous therapeutic targets are linked to the pathophysiology of airway mucus hypersecretion in experimental models and adults with respiratory disease. Whether or not these same targets are relevant in children is for the most part unclear. These targets include the inflammatory cells mediating the inflammatory response that generates the hypersecretory phenotype, and highly specific cellular elements such as epidermal growth factor receptor tyrosine kinase and calcium-activated chloride (CACL) channels. Identification of these factors is linked with the development of different classes of pharmacotherapeutic molecules directed at these targets. Compounds with a broader spectrum of anti-inflammatory activity are likely to be more effective than compounds with restricted activity. However, certain highly specific targets, such as human CACL1 channels, appear to be strongly associated with the development of an airway hypersecretory phenotype. Data from current clinical trials in adults with blockers of these specific targets are awaited with great interest. The hope is that, if effective, pediatric trials with these compounds could be initiated with a view to alleviation of the clinical impact of airway mucus hypersecretion in children. A significant challenge to the therapeutic progression of these new compounds is effective delivery to the airways in children, with the research effort into development of new compounds matched by advances in inhaler design.

4-Amino-2-(aryl)-butylbenzamides and Their conformationally constrained analogues. Potent antagonists of the human neurokinin-2 (NK(2)) receptor

A library, evaluating a range of piperazines, piperidines and acyclic amines, as replacements for the 4-hydroxy-4-phenylpiperidine moiety in lead (1b) was prepared. These efforts identified the 4-((N)-benzimidazolone)piperidine analogue (2a) which was further optimised using classical single-compound synthesis to yield the 3-((N)-morpholino)azetidine (2j). Conformationally constrained analogues of (2j), generally offered no potency advantage in this particular series.

Advances in the management of chronic obstructive pulmonary disease

Chronic obstructive pulmonary disease (COPD) is a progressive and irreversible airflow limitation with extreme economic and social burden. It is estimated that over the next two decades, it will become the 5(th) most prevalent disease and the 3(rd) most common cause of death in the world. A better understanding of the pathogenesis of airway inflammation and alveolar destruction allows for the development of new therapeutic targets. Tobacco smoking is the most important risk factor in the development of COPD, thus making smoking cessation of the outermost importance. This article provides a critical review of present therapy for COPD. In addition to conventional treatment (bronchodilators, corticosteroids and antibiotics) and smoking cessation therapies, novel approaches with potential benefit are discussed.

Peripheral tachykinin receptors as potential therapeutic targets in visceral diseases

More than 10 years of intensive preclinical investigation of selective tachykinin (TK) receptor antagonists has provided a rationale to the speculation that peripheral neurokinin (NK)-1, -2 and -3 receptors may be involved in the pathophysiology of various human diseases at the visceral level. In the airways, despite promising effects in animal models of asthma, pilot clinical trials with selective NK-1 or -2 receptor antagonists in asthmatics have been ambiguous, whereas the potential antitussive effects of NK-1, -2 or -3 antagonists have not yet been verified in humans. In the gastrointestinal (GI) tract, irritable bowel syndrome (IBS) and pancreatitis are appealing targets for peripherally-acting NK-1 and -2 antagonists, respectively. In the genito-urinary tract, NK-1 receptor antagonists could offer some protection against nephrotoxicity and cytotoxicity induced by chemotherapeutic agents, whereas NK-2 receptor antagonists appear to be promising new agents for the treatment of neurogenic bladder hyperreflexia. Finally, there is preclinical evidence for hypothesising an effect of NK-3 receptor antagonists on the cardiovascular disturbance that characterises pre-eclampsia. Other more speculative applications are also mentioned.

Airway hypersecretion in allergic rhinitis and asthma: new pharmacotherapy

Mucus hypersecretion is a prominent feature of allergic rhinitis and asthma. Biologic targets for suppression of hypersecretion range from the inflammatory cells that initiate airway inflammation, to specific cellular elements such as calcium-activated chloride (CLCA) channels, epidermal growth factor receptor tyrosine kinase, and antiapoptotic factors (eg, Bcl-2). Identification of these targets is driving development of new pharmacotherapeutic compounds. Aside from specific instances in which a single mediator has a major impact on hypersecretion--for example, histamine in rhinitis--it is likely that compounds with broad-spectrum anti-inflammatory activity are more effective than compounds with restricted activity. However, certain highly specific targets, such as CLCA channels, seem to be intimately associated with development of a hypersecretory phenotype. Data from clinical trials with blockers of these targets are awaited with great interest, not only for disease management but also to determine the clinical benefit of selective inhibition of airway hypersecretion.

Structure-activity relationships of 1-alkyl-5-(3,4-dichlorophenyl)- 5-[2-[(3-substituted)-1-azetidinyl]ethyl]-2-piperidones. 1. Selective antagonists of the neurokinin-2 receptor

The design, synthesis, and pharmacological evaluation of a novel class of neurokinin-2 (NK2) antagonists 1-alkyl-5-(3,4-dichlorophenyl)-5-[2-[(3-substituted)-1-azetidinyl]ethyl]-2-piperidones (5-44) are described. These compounds are formally derived from 2 by incorporating the metabolically vulnerable N-methylamide function into a more stable six-membered ring lactam 4, resulting in increased stability in human liver microsome (HLM) preparations relative to 2 (T1/2(HLM) of 30 min vs <10 min for 2). This series was further optimized by replacing the 4,4-disubstituted piperidine functionality found in 4 with simple 3-substituted azetidines. This series, exemplified by 1-benzyl-5-(3,4-dichlorophenyl)-5-[2-[3-(4-morpholinyl)-1-azetidinyl]ethyl]-2-piperidone 5, was found to possess excellent functional potency for the NK2 receptor in the Rabbit pulmonary artery (RPA) assay (pA2 = 9.3) and increased in vitro metabolic stability (T1/2(HLM) = 70 min) relative to 4. Metabolic route identification studies revealed that N-benzyl oxidation was a major route in this relatively lipophilic lead (log D = 3.2). Further exploration of the N-lactam substituent SAR targeting reduced lipophilicity to attenuate P-450 metabolism revealed that incorporation of a cyclopropylmethyl group in this region of the molecule gave a balance of good potency and high metabolic stability. For example, the significantly less lipophilic analogue 29 (log D = 2.3) possessed both good functional potency (RPA, pA2 = 8.1) and high in vitro metabolic stability (T1/2(HLM) = 120 min). Optimization in this N-cyclopropylmethyllactam series by modification of the nature of the azetidine 3-substituent as a strategy to further increase potency and moderate log D led to the identification of sulfamide analogue 33, which possessed both excellent metabolic stability in vitro (T1/2(HLM) >120 min) and high potency in both RPA (pA2 = 8.9) and human bladder smooth muscle (pK(b) = 8.9) functional assays. In addition, NK2 antagonist 33 (IC50 = 4 nM) showed excellent selectivity over both the related human neurokinin receptors h-NK1 (IC50 = 7.9 microM) and h-NK3 (IC50 = 1.8 microM) in radioligand binding studies.