Theophylline has been used in the treatment of asthma and chronic obstructive pulmonary disease (COPD) since the 1930s although its popularity has declined due to the introduction of long acting β2-adrenoceptor agonists and glucocorticosteroids, either alone or in combination. Theophylline is often used with glucocorticosteroids as second- or third-line therapy where it has proven anti-asthmatic activity in asthma (Sullivan et al., 1994; Weinberger and Hendeles, 1996; Lim et al., 2000) and in combination with long acting bronchodilator drugs in COPD (Rennard, 2004). As an p.o formulation, this drug offers the advantage of improved compliance; however, its perceived lack of efficacy, the necessity to monitor plasma levels, coupled with numerous side effects, known drug interactions, and the effect of smoking on plasma clearance (Boswell-Smith et al., 2006a) have provided an impetus to discover a better theophylline. Moreover, whereas glucocorticosteroids are of clinical utility in asthma (Barnes, 2006a), they are of limited use in COPD (Rennard, 2004). There is clearly an unmet clinical need for the development of disease-modifying drugs in COPD because other than cigarette smoke cessation, current drug therapy does not prevent the accelerated decline in lung function.
The mechanism of action of theophylline, which explains its clinical effect is not entirely certain, but several have been proposed. It was originally shown that theophylline inhibited the activity of a cyclic 3′, 5′ nucleotide PDE with a Ki of 100 μM (Butcher and Sutherland, 1962). This might account for its beneficial effects clinically, as an increase in the intracellular levels of cyclic AMP can reduce the activation of a wide range of inflammatory and lung resident cells. There are presently 11 known families of PDE and at least 21 isoforms with numerous splice variants that are characterized by differences in structure, substrate specificity, inhibitor selectivity, tissue and cell distribution, regulation by kinases, protein–protein interaction and subcellular distribution (Houslay et al., 2005; Bender and Beavo, 2006). However, targeting PDE4, the enzyme responsible for metabolizing cyclic AMP has been the focus for the development of drugs that could prove beneficial in the treatment of respiratory diseases such as asthma (Torphy, 1998; Houslay et al., 2005). It is therefore of interest that plasma levels achieved with a dose of theophylline that demonstrated significant anti-inflammatory activity (Sullivan et al., 1994) was well below the Ki for PDE inhibition and suggested that PDE4 inhibition alone does not completely explain this drugs clinical effectiveness (Barnes et al., 2005). Nevertheless, highly potent and selective PDE4 inhibitors have been developed to target inflammatory airway disease. This strategy is not unique and is exemplified with the development and clinical success of a PDE5 inhibitor, sildenafil, for the treatment of erectile dysfunction (Boolell et al., 1996).
PDE4 catalyses the hydrolysis of cyclic AMP, which terminates the downstream signalling of this second messenger. There are four gene families (A–D), although there is added complexity with over 20 splice variants (Houslay et al., 2007). Hydrolysis of cyclic AMP is a common feature of this family, and it is clear that these isoforms can be targeted to different domains within the extracellular compartment and their activity differentially regulated by kinases, suggesting that these isoforms have specific functions in the control of cellular activity (Houslay et al., 2007). X-ray crystallography has resolved the catalytic domain of these enzymes, which are comprised of three important domains, consisting of a bivalent metal-binding pocket (Zn2+, Mg2+), which forms a complex with the phosphate moiety of cyclic AMP, a pocket containing glutamine (Q pocket), which forms hydrogen bonds with the nucleotide (purine) moiety of cyclic AMP, and a solvent pocket. PDE4 inhibitors occupy this active site through a number of important interactions and prevent cyclic AMP metabolism. These include indirect binding to the metal ions by the formation of hydrogen bonding to water, whereas hydrophobic interactions between the planar ring structure of these inhibitors and hydrophobic amino-acid residues such as phenylalanine and isoleucine serve to ‘clamp’ the inhibitor within the active site and hydrogen bond interaction between the aromatic ring structure of these inhibitors and the invariant glutamine residue in the Q pocket, the site which is normally occupied by the nucleotide moiety of cyclic AMP (Xu et al., 2000; Card et al., 2004; Wang et al., 2007).
There are considerable challenges to the synthesis of subtype selective inhibitors due to the high degree of sequence and structural homology within the catalytic domains of the PDE4 subtypes (Xu et al., 2000; Card et al., 2004; Wang et al., 2007). One possibility might be to exploit subtle differences between the interaction of these inhibitors to the catalytic active site, or alternatively, by non-active site inhibition by targeting the N-terminal region of the enzyme, which contain phosphorylation sites and/or protein-binding sequences and so indirectly interfere with PDE4 activity (Card et al., 2004; Wang et al., 2007).
With well over 100 mediators including prostaglandins, leukotrienes, chemokines, cytokines, proteases and growth factors and numerous cell types including mast cells, neutrophils, eosinophils, macrophages, DCs and lymphocytes implicated in the pathogenesis of asthma and COPD (Barnes, 2006b; Holgate, 2007), suggests that a chemical strategy designed to target a single mediator or cell type is unlikely to be successful particularly as many of these mediators and cell types have overlapping and complementary roles in disease pathology. PDE4 is expressed in a number of cell types that are considered suitable drug targets for the treatment of respiratory diseases such as asthma and COPD (Table 1). It might reasonably be argued that targeting PDE4 could potentially suppress the function of numerous cell types; however, it is well known that other PDE enzymes are also expressed in these cells and the contribution of other PDEs to cell function (for example, PDE3, PDE7) in the context of benefits in respiratory diseases is being explored (Smith et al., 2004; Boswell-Smith et al., 2006b). It would seem prudent to develop subtype selective PDE4 inhibitors in attempts to maximize therapeutic benefit at the expense of adverse effects, whereas there is also the possibility that nonselective PDE inhibitors might offer a better approach in targeting multiple target cells in the disease process. Indeed, it has been suggested that clozapine is a better anti-psychotic than newer generation atypical anti-psychotics because this drug targets numerous receptors, and as such, has been described as a ‘magic shotgun’, for the treatment of Schizophrenia (Roth et al., 2004).
|Cell type||PDE4 Subtypea||Other PDE's||Biological consequence of PDE4 inhibition||Reference|
|T lymphocytes CD4 CD8||B>A||3, 7||Inhibition of proliferation and cytokine release||(Gantner et al., 1997b; Hatzelmann and Schudt, 2001; Smith et al., 2003; Peter et al., 2007)|
|B cells||B, D>A||7||Increased proliferation||(Gantner et al., 1998; Smith et al., 2003)|
|Eosinophils||A, D>B||7||Inhibition of superoxide anion generation; delayed apoptosis||(Hatzelmann and Schudt, 2001; Smith et al., 2003; Parkkonen et al., 2007)|
|Neutrophils||A, D>B||7||Inhibition of superoxide anion and neutrophil elastase release||(Hatzelmann and Schudt, 2001; Smith et al., 2003; Jones et al., 2005)|
|Monocyte||B >A, D||7||Inhibition of TNF-α release||(Hatzelmann and Schudt, 2001; Smith et al., 2003; Heystek et al., 2003; Jones et al., 2005)|
|Macrophages||A, B, D||1, 3, 7||Inhibition of TNF-α releaseb||(Gantner et al., 1997a; Hatzelmann and Schudt, 2001; Smith et al., 2003; Barber et al., 2004)|
|DCs||A>B, D||1, 3||Inhibition of TNF-α release||(Hatzelmann and Schudt, 2001; Heystek et al., 2003)|
|Mast cells||Little if any mast cell stabilization||(Weston et al., 1997; Shichijo et al., 1998)|
|Airway epithelial cells||1–3, 4, 5, 7, 8||Increased production of PGE2; inhibition of IL-6 production||(Fuhrmann et al., 1999; Haddad et al., 2002)|
|Endothelial cells||2, 3, 4, 5||Inhibition of adhesion molecule expression||(Jones et al., 2005; Sanz et al., 2007)|
|Fibroblasts||A, B >D||1, 4, 5, 7||Inhibition of fibroblast chemotaxis; inhibition of pro-MMP1, 2 release||(Kohyama et al., 2002; Smith et al., 2003; Martin-Chouly et al., 2004; Dunkern et al., 2007)|
|Sensory nervesc||D||1, 3||Inhibition of neuropeptide release||(Spina et al., 1995)|
Asthma and PDE4
Increased PDE4 function due to either increased protein expression or activity might provide a plausible mechanism to account for the pathogenesis of asthma. The expression of a novel and distinct cyclic AMP-PDE was isolated from monocytes obtained from individuals with atopic dermatitis. This enzyme had increased PDE activity and as a consequence monocyte function was increased and thought to be the underlying basis for the pathology associated with atopic dermatitis (Chan et al., 1993). However, soluble PDE4 activity was not increased in a range of peripheral blood leukocytes from atopic patients of either mild or severe severity (Gantner et al., 1997b). Similarly, increased total PDE catalytic activity was observed in peripheral blood monocytes from individuals with mild asthma, whereas this was associated, paradoxically, with reduced PDE4 activity (Landells et al., 2000) and the expression of PDE4A–D was not increased in peripheral blood cluster of differentiation (CD)4 positive T lymphocytes in patients with mild asthma (Jones et al., 2007). Together these studies indicate that the underlying pathogenesis of mild asthma cannot be attributed to enhanced PDE4 expression or activity.
Numerous preclinical studies in models of allergic pulmonary inflammation have repeatedly documented the ability of PDE4 inhibitors to inhibit two important characteristic features of asthma, namely, the recruitment of eosinophils to the airways and bronchial hyperresponsiveness (Torphy, 1998; Spina, 2003). One disadvantage of these studies is the inability to ascertain the role of PDE4 isoforms because of the nonselective nature of the PDE4 inhibitors currently under development. The use of genetically modified mice has revealed some interesting findings. Airway inflammation characterized by recruitment of eosinophils to the airways of mice deficient in PDE4D was no different to wild-type controls (Hansen et al., 2000). This indicated that other PDE4 subtypes contributed in the metabolism of intracellular cyclic AMP, as cell recruitment to the airways was inhibited when animals were treated with nonselective PDE4 inhibitors (Kung et al., 2000; Kanehiro et al., 2001). However, airway obstruction caused by methacholine was enhanced in wild-type allergic mice but was abolished in PDE4D gene-deficient mice. These mice were hyporesponsive to this stimulus, even in the absence of allergic sensitization and appeared to be related to an increase in dilator prostaglandin production in the airways of these gene-deficient mice (Hansen et al., 2000; Mehats et al., 2003). However, this effect was specific for methacholine because the enhanced airway obstruction in response to serotonin was unaffected by the removal of PDE4D (Hansen et al., 2000). This study highlighted the potential complimentary role of PDE4 isoforms in regulating allergic airway inflammation, and the need to target more than one PDE4 isoform because inhibition of the inflammatory response, hyperresponsiveness and airway remodelling in allergic wild-type mice was observed following exposure to PDE4 inhibitors such as rolipram and roflumilast (Kung et al., 2000; Kanehiro et al., 2001; Kumar et al., 2003).
The numerous preclinical studies reporting the anti-inflammatory potential of PDE4 inhibitors in models of allergic inflammation and in human cells in vitro have been, to some degree, corroborated in clinical trials in asthmatic patients. Twice daily treatment for 9.5 days with the PDE4 inhibitor CDP840 inhibited the development of the late phase response in asthmatic patients by 30% (Harbinson et al., 1997). A similar degree of inhibition of the late phase response was observed following once daily treatment for 7–10 days with roflumilast (van Schalkwyk et al., 2005). This late phase response is used by clinicians to model the inflammatory component following an allergic insult to the airways. In both studies, the effects of drug treatment on the acute allergen bronchoconstriction was modest and is consistent with the lack of demonstrable action of PDE4 inhibition on mast cell function (Table 1) and highlight the role of other PDE enzymes, namely PDE3 in the context of airway smooth muscle relaxation (Boswell-Smith et al., 2006b). Bronchial hyperresponsiveness was not reduced by these drugs, although a later study purported to show modest protection against allergen-induced bronchial hyperresponsiveness (Louw et al., 2007), which might suggest that PDE4 may not be a suitable target for this particular phenomenon, or that higher doses are required to provide complementary and persistent inhibition of the enzyme and hence attenuation of bronchial hyperresponsiveness. It is of interest that roflumilast has a plasma half-life of 16 h following a single p.o administration, and is metabolized by CYP3A4 to the active N-oxide metabolite, which has considerably greater bioavailability with a half-life of 20 h that would favour prolonged enzyme exposure (David et al., 2004). This favourable pharmacokinetic profile would be anticipated to produce long periods of PDE4 inhibition. There was a significant reduction in the activity of circulating monocytes in patients maintained on roflumilast for 4 weeks, whereas the magnitude of this change was small, resulting in approximately 1.3-fold reduction in tumour necrosis factor-α production by monocytes in response to endotoxin challenge in vitro (Timmer et al., 2002). Therefore, it is questionable whether total PDE4 inhibition can be achieved in cells within the airway tissue compartment at the dose used in clinical studies.
Side effects most commonly reported were headache, nausea and diarrhoea of a mild-to-moderate severity (Harbinson et al., 1997; van Schalkwyk et al., 2005) and suggest that unless the risk/benefit ratio can be improved, then this may well hamper the use of this drug in asthma. This is a worthy aim to pursue in the light of a clinical study reporting comparable clinical efficacy between roflumilast and beclomethasone diproprionate in persistent asthma (Bousquet et al., 2006).
COPD and PDE4
Chronic obstructive pulmonary disease unlike asthma is caused by cigarette smoking, although in developing countries smoke derived from burning biomass fuels is also a predisposing factor. The nature of the inflammatory response is distinct from asthma and is characterized by the activation of macrophages, airway epithelial cells, which in turn, secrete a range of chemokines and lipid mediators resulting in the recruitment of neutrophils and CD8+ T lymphocytes. The secretion of a range of proteases from neutrophils (elastase, MMP9, cathepsins) and macrophages (MMP12) is thought to contribute towards airway fibrosis of the small airways, increased mucus secretion and destruction of the alveolar wall (Barnes, 2006b) These pathological changes give rise to the symptoms of cough, mucus secretion, difficult breathing and emphysema. Many of the cell types implicated in this disease process express PDE4 (Table 1).
The expression of PDE4A–D in peripheral blood neutrophils and CD8T cells is not altered in patients with mild COPD (Jones et al., 2007). However, the expression of PDE4A4 and total cyclic AMP PDE activity was significantly increased in macrophages purified from bronchoalveolar lavage fluid from patients with mild-to-moderate COPD compared with healthy patients or smokers who did not present with COPD (Barber et al., 2004). Of the 12 PDE4 variants analysed, only the activity of PDE4A4 was increased and suggested that local events/processes within the lung of patients with COPD specifically upregulated this variant (Barber et al., 2004). However, the functional consequence of this change remains to be established in light of findings showing that PDE4 inhibition has modest effect in suppressing tumour necrosis factor-α production from human macrophages derived from cultured monocytes, and the contribution of PDE3 and PDE7 in regulating function in this cell type cannot be ignored (Gantner et al., 1997a; Smith et al., 2004).
There are a limited number of preclinical in vivo models of COPD. However, the recruitment of neutrophils to the airways can be readily induced using the bacterial wall component, endotoxin, although it is widely appreciated that this stimulus can only model neutrophil recruitment to the airways. The recruitment of these cells to the airways of wild-type mice was inhibited by around 50% in PDE4B and PDE4D-deficient mice, and a greater degree of inhibition was observed when wild-type mice were treated with rolipram (Ariga et al., 2004). This once again highlighted the complimentary roles of PDE4 isoforms in regulating neutrophil recruitment to the airways. Similarly, smoking induced neutrophil recruitment to the airways, release of chemokines and emphysematous changes to the lung were attenuated by PDE4 inhibitors (Martorana et al., 2005; Leclerc et al., 2006). Together, these studies highlight the utility of inhibiting PDE4 in cell types implicated in this disease.
A number of phase III clinical trials have assessed the potential utility of PDE4 inhibitors in the treatment of COPD (Rabe et al., 2005; Rennard et al., 2006; Calverley et al., 2007). All three studies report modest but significant improvements in spirometry over placebo, quality of life scores and reduction in the number of exacerbations in the severest group of COPD patients. The mechanism of the improvement in spirometry is unlikely to be due to relaxation of airway smooth muscle because this drug class has weak bronchodilator activity. It is possible that this improvement is due to an anti-inflammatory action of the drugs (Table 1), although no biomarker of inflammation was measured in these studies. However, separate studies have addressed whether PDE4 inhibitors are anti-inflammatory in COPD. Both roflumilast (Grootendorst et al., 2007) and cilomilast (Gamble et al., 2003) reduced the number of inflammatory cells such as neutrophils and lymphocytes recruited to the airways and the levels of two biochemical markers of this disease, namely interleukin-8 and neutrophil elastase. The magnitude of the change in the number of these inflammatory cells and concentration of mediators was between 30 and 50% and might underlie their beneficial action in the phase III clinical trials. However, the biomarker study also highlights a recurring theme that complementary inhibition of PDE4 was not achievable because of dose-limiting side effects; or alternatively, other PDE isoforms (for example, PDE3, 7, see Table 1) in these same inflammatory processes may also require targeting for a full anti-inflammatory action to be revealed in this disease.
The most common side effect reported with roflumilast included diarrhoea (9%), headache (6%) and nausea (5%) (Rabe et al., 2005; Calverley et al., 2007), which was of the same order of magnitude as that reported with cilomilast, although abdominal pain and vomiting were also reported for this drug (Rennard et al., 2006). The adverse effects appeared to disappear with continued use but were a major reason why patients discontinued with the study during the first 3–4 weeks of treatment. No cardiovascular liabilities were noted.
PDE4 inhibitors: unwanted effects
Nausea is a commonly reported side effect associated with theophylline and therefore not surprisingly, PDE4 inhibitors also produce a similar constellation of adverse events and are a major drawback for the therapeutic use of these drugs. The mechanism responsible for this side effect has been investigated in an attempt to discover non-emetic PDE4 inhibitors.
The direct recording of neuronal activity within the area postrema conclusively demonstrated that substances known to cause nausea (for example, apomorphine) caused the excitation of neurones within the area postrema of dogs (Carpenter et al., 1988). Neuronal activity within the area postrema was also increased following the systemic administration of 8-bromo cyclic AMP or following elevation of endogenous levels of cyclic AMP within neurones by forskolin, an activator of AC (Carpenter et al., 1988). Elevated levels of cyclic AMP within the area postrema enhanced the emetogenic response. Dogs treated with theophylline and the PDE4 selective inhibitor, 4-(3-butoxy-4-methoxyphenyl)methyl-2-imidazolidone (Ro 20–1724) reduced the emetic threshold of the D2 agonist, apomorphine (Carpenter et al., 1988). Similarly, the i.c.v. administration of highly potent PDE4 inhibitors also induced emesis in the ferret (Robichaud et al., 1999), and the emetic response to systemically administered PDE4 inhibitors is reduced by anti-emetic agents including the 5HT3-antagonist, ondansetron, and the neurokinin 1 antagonist, (+)-(2S,3S)-3-(2-[11C]methoxybenzylamino)-2-phenylpiperidine (CP-99994) (Robichaud et al., 1999, 2001).
Many studies have documented the expression of PDE4D within the area postrema, nucleus tractus solitaris and nodose ganglion neurones in various species including man and implicated this isoform in nausea and vomiting (Cherry and Davis, 1999; Takahashi et al., 1999; Perez-Torres et al., 2000; Lamontagne et al., 2001). However, it should also be recognized that detectable transcript for PDE4B was also found within the nucleus tractus solitaris and area postrema in humans and rodents, respectively, and could be involved in the emetic response (Perez-Torres et al., 2000). As rodents do not possess an emetic reflex, it is not possible to directly investigate the role of different isoforms of PDE4 in emesis. However, a surrogate biological response that measures the reversal of anaesthesia induced by α2-adrenoceptor agonists has been used to study the role of PDE4 subtypes in emesis (Robichaud et al., 2001, 2002a). It was previously shown that the ability of PDE4 inhibitors to induce emesis in the ferret was inhibited by the α2-selective agonist, clonidine (Robichaud et al., 2001) and suggested that raising cyclic AMP within central noradrenergic terminals by PDE4 inhibitors promoted emesis, and this could be attenuated by α2-adrenoceptor-mediated inhibition of AC. The hypnotic action of xylazine was reversed in rodents treated with PDE4 inhibitors (Robichaud et al., 2001) and therefore used as a surrogate for emesis. Deletion of PDE4D and not PDE4B reduced the duration of anaesthesia induced by xylazine, compared with wild-type mice, and second, the ability of PDE4 inhibitors to shorten xylazine-induced anaesthesia was impaired in PDE4D but not PDE4B knockout mice (Robichaud et al., 2002b). Together, these studies suggested that PDE4 inhibitors with low affinity for PDE4D should have reduced emetic potential.
However, it is not entirely clear whether PDE4D inhibition alone is the sole basis of emesis as there are examples of PDE4 inhibitors that document in vivo anti-inflammatory activity but are not emetogenic (Gale et al., 2002) and there are examples of PDE4 inhibitors that have little emetogenic activity but potent anti-inflammatory activity in preclinical studies (Aoki et al., 2000, 2001). Similarly, the emetic profile of various PDE4 inhibitors in the ferret cannot be explained by their selectivity for PDE4D or to differences in PDE4D inhibitor potency (Figure 1). One possible explanation might be that some PDE4 inhibitors preferentially partition within the CNS and hence the degree of PDE4D inhibition in area postrema neurones might explain the differences in the emetic potential of these drugs (Aoki et al., 2001; Robichaud et al., 2002a). However, the concentration of the ‘low emetic’ PDE4 inhibitor, CT-2450 within the CNS was comparable to that achieved by the most potent emetic PDE4 inhibitor, PMNPQ (Figure 1). Second, the area postrema is not completely behind the blood–brain barrier (Gross et al., 1990), and therefore accessible to free drug within the circulation. It would seem unlikely that differential partitioning of these inhibitors within the CNS is a likely explanation for their ability to induce emesis. However, it remains to be established whether the magnitude of PDE4 inhibition within the area postrema differs between the various PDE4 inhibitors.
A number of preclinical studies have highlighted a number of disadvantages to targeting PDE4, include the development of mesenteric vasculitis, immunosuppression (Spina, 2004), heart failure and arrhythmia (Lehnart et al., 2005). However, none of these events appear to be realized in phase II and phase III clinical trials undertaken with cilomilast and roflumilast. Similarly, slow release theophylline has been used for decades in the treatment of asthma and COPD and has not been associated with a number of these potentially adverse events (Ohta et al., 2004). It has been suggested that PDE4 inhibitors may have proinflammatory properties (McCluskie et al., 2006). This conclusion was based on the finding that roflumilast at very high doses (100 mg/kg) promoted the recruitment of neutrophils to the airways and this correlated with the release of interleukin-8 from cultured endothelial cells in vitro, although the concentrations required to achieve these effects were at least 1000 times greater than the ED50 and EC50 values reported for roflumilast against several in vivo biomarkers of inflammation and cell function in vitro, respectively (Bundschuh et al., 2001; Hatzelmann and Schudt, 2001). It is unlikely that the plasma concentrations required to produce this purported proinflammatory effect could be achieved even with chronic dosing. Similarly, another study has shown that PDE4 inhibitors, at concentrations that are pharmacologically relevant, delay apoptosis of neutrophils and eosinophils, an effect that increased when combined with β2-adrenoceptor agonists (Parkkonen et al., 2007). However, to what extent these findings translate into the clinic, particularly, if combined with bronchodilator drugs, remains to be established. The clinical evidence suggests that PDE4 inhibitors suppress and not exacerbate inflammation in the airways (Gamble et al., 2003; Grootendorst et al., 2007).
PDE4 inhibitors and the future
Although there is cause for optimism concerning the potential therapeutic utility of PDE4 inhibitors for the treatment of respiratory diseases such as asthma and COPD, it is clear that further improvements are required. Strategies at improving the risk to benefit ratio will be important, if this drug class is to be widely used. The therapeutic window for anti-inflammatory action of these drugs and side effects such as nausea and emesis is probably not wide enough for cilomilast, and may limit the use of roflumilast in asthma. There are PDE4 inhibitors currently in development, which appear to lack significant emetic action (for example, oglemilast) (Glenmark Pharmaceuticals, 2005) and IPL512602 (Inflazyme pharmaceuticals, 2005), although the molecular basis for this has not been published.
Most PDE4 inhibitors under development are designed for p.o administration, however, the inhaled route would deliver PDE4 inhibitor directly to target cells within the lung and thereby minimize systemic absorption as in the case of AWD 12-281 (N-(3,5-dichloropyrid-4-yl)-[1-(4-fluorobenzyl)-5-hydroxy-indole-3-yl]-glyoxylic acid amide (Kuss et al., 2003) or UK-500001 (Phillips et al., 2007; Vestbo et al., 2007), although clinical trials in respiratory disease have thus far been disappointing. Nonetheless, the development of a potent, long acting PDE4 inhibitor through the inhaled route would offer a solution to the issues of emesis and nausea. Another approach might be the use of antisense oligodeoxynucleotides targeting PDE4, which could be delivered by the inhaled route, and in view of the positive results obtained in the successful targeting of the adenosine A1 receptor in a rabbit model of allergic inflammation (Nyce and Metzger, 1997), illustrates the potential of this approach.
Another reason why targeting PDE4 alone may not fully resolve airway inflammation is the fact that other PDE types exist in structural and inflammatory cells in the lung (Table 1) and therefore, targeting multiple PDE enzymes may be required for optimal anti-inflammatory action. For example, the macrophage is viewed as a critical cell type in the pathogenesis of COPD (Barnes, 2006b); however, the activity of these cells is only inhibited to a small degree by PDE4 inhibitors (Hatzelmann and Schudt, 2001) and the potential functional involvement of PDE3 and PDE7 in these cells cannot be completely ignored. The inhibitory action of PDE4 inhibitors on the cellular activity of CD8+ T lymphocytes and macrophages was significantly increased in the presence of PDE7 selective inhibitors (Smith et al., 2004). Similarly, combined PDE3 and PDE4 inhibitor in a single molecule offers the advantage of delivering a bronchodilator and anti-inflammatory substance. Moreover, it is likely that retention of the inhibitor within the lung may be required to maintain anti-inflammatory activity within the airways (Boswell-Smith et al., 2006b).