1.  Field of the Invention
The present invention relates to novel compounds which are both phosphodiesterase 4 (PDE4) enzyme inhibitors and muscarinic M3 receptor antagonists.  More particularly, the present invention relates to compounds of formula (I) as below described, methods of preparing such compounds, compositions containing them and therapeutic uses thereof. 
2.  Discussion of the Background
Chronic obstructive pulmonary disease (COPD) is a respiratory disorder characterized by progressive, not fully reversible, airflow limitation associated with an abnormal pulmonary inflammatory response to noxious particles or gases.  For this reason, bronchial relaxation and inflammatory response suppression represent a mechanistic approach to the treatment of COPD that might improve symptoms such as dyspnea, wheezing, chest tightness, cough and mucus secretion, improve health status and reduce exacerbations. 
Nowadays, the drug therapy options for COPD fall into 2 general classes: bronchodilators, (β2-adrenoceptor agonists, antimuscarinic agents and methylxanthines) and antiinflammatory agents (glucocorticosteroids and selective phosphodiesterase-4 (PDE4) inhibitors).  Bronchodilator drugs are the current mainstay of treatment for symptoms' relief. 
As anticholinergic bronchodilators, the efficacy of muscarinic M3 antagonists is based on the fact that the major reversible component of airflow narrowing in COPD patients is the increase of acetylcholine (ACh) released to airway smooth muscle, by the bronchial postganglionic vagal efferent in some pathological conditions.  Therefore, compounds that antagonize the action of ACh at muscarinic receptors are able to counteract the bronchoconstriction and thus improve lung function in these patients. 
Muscarinic antagonists block the effects of ACh at muscarinic receptors.  Currently, there are five known muscarinic receptor subtypes (M1-M5); human airway smooth muscle contains M1, M2 and M3 receptors.  M1 receptors facilitate neurotransmission through parasympathetic ganglia and are weakly expressed on submucosal glands in human airways.  The M2 receptors are located on the smooth-muscle fibers.  Some studies have suggested a small role of M2 mediating the inhibition of airway smooth-muscle relaxation caused by adenylyl cyclase activation by compounds such as beta agonists.  In addition, presynaptic M2 receptors are found on postganglionic parasympathetic nerves that project to airway smooth muscle and mucus-producing cells.  These presynaptic M2 autoreceptors provide a negative feedback mechanism, which, when stimulated, inhibit further release of ACh.  Postsynaptic M3 receptors are known to mediate both contraction of smooth muscle in the respiratory tract and mucus secretion, making them a major target for symptomatic relief of COPD.  Consequently, in the airways, the major effects of muscarinic antagonists are bronchodilation and reduction of mucus secretion via blockage of ACh-induced effects in the parasympathetic nervous system. 
Given the distribution of muscarinic receptors, systemically available agents that bind to muscarinic receptors outside of the respiratory tract have the potential to produce unwanted side effects such as tachycardia, dry mouth, urinary retention and constipation.  Whereas dry mouth is the most common systemic anticholinergic side effect associated with the use of antimuscarinic antagonists as a result of the systemic blockade of M1 and M3 receptors the most potentially serious systemic effect is tachycardia, which results from the blockade of cardiac M2 receptors. 
Inhaled anticholinergic antimuscarinic drugs approved for the treatment of COPD include ipratropium bromide (Atrovent®), oxitropium bromide (Oxivent®) and tiotropium bromide (Spiriva®).  Both ipratropium and oxitropium are short-acting agents.  In contrast, tiotropium bromide is the only long-acting antimuscarinic agent (LAMA) currently marketed for COPD, proved to be suitable for once-daily administration as a dry powder.  Several others newer LAMAs are newly registered for the treatment of COPD, including aclidinium bromide and glycopyrrolate bromide, or are currently in phase III development, including umeclidinium. 
Although bronchodilators are quite effective to improve symptoms, they do not address the underlying chronic inflammation or the changes in airway structure. 
Standard treatment with glucocorticosteroids as antiinflammatory agents has demonstrated limited efficacy.  However, among the antiinflammatory agents currently being developed, PDE4 inhibitors proved to be effective in attenuating the responses of various inflammatory cells, through their ability to elevate cAMP levels. 
PDE4 is the predominant PDE expressed in neutrophils and T cells, suggesting that PDE4 inhibitors would be effective in controlling inflammation in COPD.  Inhibition of PDE4 in inflammatory cells influences various specific responses, such as the production and/or release of pro-inflammatory mediators including cytokines and reactive oxygen species, with a well-documented efficacy in animal models mimicking certain aspects of asthma and COPD, as well as inflammatory bowel disease, atopic dermatitis, psoriasis and rheumatoid arthritis. 
The selective PDE4 inhibitor, roflumilast (Daxas®) is an approved phosphodiesterase-4 inhibitor for the treatment of COPD associated with chronic bronchitis and a history of exacerbations.  Roflumilast inhibits lung inflammation and emphysema in a smoking model of COPD in mice.  In COPD patients, oral roflumilast given over 4 weeks significantly reduces the numbers of neutrophils (by 36%) and CXCL8 concentrations in sputum.  In clinical trials roflumilast (500 mg once daily) given over 12 months improved lung function in COPD patients to a small extent but had little effect in reducing exacerbations or improving quality of life.  More recently roflumilast has been shown to significantly improve FEV 1 (by approximately 50 mL) and reduce exacerbation (by about 15%) in patients with severe disease who have frequent exacerbations and mucus hypersecretion.  Roflumilast provides clinical benefit when added to salmeterol or tiotropium and so may be used as an additional treatment in patients with severe disease. 
However, the clinical utility of PDE4 inhibitors has so far been compromised by the occurrence of mechanism-associated side effects, including headache, nausea and emesis, which often limited the maximally tolerated dose.  This problem could be overcome by inhaled delivery and designing compounds with a potentially more advantageous therapeutic window. 
Since bronchial relaxation and inflammatory response suppression represent a mechanistic approach to the treatment of COPD, the combination of muscarinic M3 antagonism with selective PDE4 inhibition may lead to a new class of drugs, combining both bronchodilating and antiinflammatory properties in one molecule, which may open new perspectives in the management of COPD. 
Thus, there remains