β₂-adrenoceptor agonists: current and future direction

Abstract

Despite the passionate debate over the use of β₂-adrenoceptor agonists in the treatment of airway disorders, these agents are still central in the symptomatic management of asthma and COPD. A variety of β₂-adrenoceptor agonists with long half-lives, also called ultra long-acting β₂-adrenoceptor agonists (ultra-LABAs; indacaterol, olodaterol, vilanterol, carmoterol, LAS100977 and PF-610355) are currently under development with the hopes of achieving once-daily dosing. It is likely that the once-daily dosing of a bronchodilator would be a significant convenience and probably a compliance-enhancing advantage, leading to improved overall clinical outcomes. As combination therapy with an inhaled corticosteroid (ICS) and a LABA is important for treating patients suffering from asthma, and a combination with an inhaled long-acting antimuscarinic agent (LAMA) is important for treating COPD patients whose conditions are not sufficiently controlled by monotherapy with a β₂-adrenoceptor agonist, some novel once-daily combinations of LABAs and ICSs or LAMAs are under development.

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This article is part of a themed issue on Respiratory Pharmacology. To view the other articles in this issue visit http://dx.doi.org/10.1111/bph.2011.163.issue-1

Current direction

β₂-adrenoceptor agonists, mainly long-acting β₂-adrenoceptor agonists (LABAs) such as formoterol and salmeterol, are an important pharmacological approach to induce bronchodilation in patients suffering from chronic obstructive pulmonary disease (COPD) (Celli and MacNee, 2004; Rabe et al., 2007) and asthma (National Asthma Education and Prevention Program, 2007; Bateman et al., 2008a), although controversy has reigned over their regular use prescribed as monotherapy, at least in the treatment of asthma (Wijesinghe et al., 2008). In effect, with chronic or high-dose exposure, β₂-adrenoceptor agonists demonstrate proinflammatory effects. In vitro, they can enhance the Th2 inflammatory pathway by inhibiting interleukin (IL)-12 and interferon (IFN)-γ (Panina-Bordignon et al., 1997; Agarwal and Marshall, 2000). In vivo, pretreatment with β₂-adrenoceptor agonists increases the severity of the late asthmatic reaction (Lai et al., 1989), continuous treatment is associated with an increase in sputum eosinophils, notably when the patient is not taking concomitant inhaled corticosteroids (ICSs) (Aldridge et al., 2000; Lazarus et al., 2001), and most studies demonstrate that β₂-adrenoceptor agonists increase airway hyperresponsiveness (Taylor, 2009). Intriguingly, in mice harbouring overexpressed β₂-adrenoceptors (equivalent to continuous receptor stimulation), there is enhanced airway smooth muscle contractility (Taylor et al., 1996).

Nonetheless, there is solid evidence suggesting that asthmatic patients in clinical practice treated with a single inhaler containing an ICS plus a LABA experience fewer asthma exacerbations than similar patients treated with ICSs alone (Hirst et al., 2010). It is intriguing that, while the use of LABAs (with or without ICS) increased over the last decade, asthma mortality declined in major western European countries (Chatenoud et al., 2009). Moreover, a rather recent analysis of more than 40 000 asthmatics (Rodrigo et al., 2009) found that regular LABA use as monotherapy reduced acute exacerbations requiring oral corticosteroids by 20%, and withdrawals due to acute exacerbations by 32%. Additionally, this analysis did not identify any detrimental effect of LABAs on acute exacerbations requiring hospitalization or on life-threatening episode. However, the Food and Drug Administration (FDA) still recommends that LABAs be reserved for patients whose asthma cannot be adequately managed with asthma-controller medication such as an ICS, and long-term use of LABAs should be limited to patients who require prolonged use of ICSs (Chowdhury and Dal Pan, 2010).

Although regular monotherapy with β₂-adrenoceptor agonists should clearly be avoided, and the concomitant use of ICS does not mean that adverse effects of β₂-adrenoceptor agonists are a nonissue (Taylor, 2009), β₂-adrenoceptor agonists remain the most effective bronchodilators available for the immediate relief of asthma symptoms and, as such, they are still an important component of asthma treatment (Cazzola and Matera, 2007), however, taking into account that in acute asthma, a full agonist (high intrinsic efficacy) offers a clinical advantage over a partial agonist (low intrinsic efficacy) but with the potential of inducing dose-dependent adverse effects (Hanania et al., 2010a). In any case, LABAs in combination with ICSs will continue to remain the main focus of treatment of asthma (Chung et al., 2009).

A volume of published evidence supports the role of LABAs in the treatment of stable COPD (Cazzola et al., 1997; Rossi et al., 2008). Physiological studies have shown that β₂-adrenoceptor agonists dilate the airways and reduce air trapping, and this leads to improved lung function and improved exercise tolerance for patients (Di Marco et al., 2003; O'Donnell et al., 2004). Clinical trials clearly show that short- and long-acting β₂-adrenoceptor agonists improve dyspnoea and quality of life and reduce respiratory exacerbations in patients with COPD (Appleton et al., 2006). Interestingly, LABAs, rather than short-acting β₂-adrenoceptor agonists, have also the potential to improve the mucociliary component of COPD (Rogers, 2005).

For patients whose conditions are not sufficiently controlled by monotherapy, combining bronchodilators of different classes, in particular an inhaled β₂-adrenoceptor agonist with an inhaled antimuscarinic agent, seems a convenient way of delivering treatment and obtaining superior results (Cazzola and Molimard, 2010). It is reasonable to postulate that targeting bronchoconstriction through two distinct mechanisms (sympathomimetic and anticholinergic) should maximize the bronchodilator response and help to overcome inter- and intra-patient variability in bronchomotor tone associated with COPD (Cazzola and Molimard, 2010). The nature of interaction between the two systems is not yet fully understood, but there is enough evidence to suggest that combining β₂-adrenoceptor agonists and antimuscarinic agents is pharmacologically useful for two reasons (Cazzola and Molimard, 2010). Firstly, the addition of a β₂-adrenoceptor agonist decreases the release of acetylcholine (ACh) through the modulation of cholinergic neurotransmission by prejunctional β₂-adrenoceptors, and thereby amplifies the bronchial smooth muscle relaxation induced by the muscarinic antagonist. Secondly, the addition of a muscarinic antagonist can reduce bronchoconstrictor effects of ACh, whose release has been modified by the β₂-adrenoceptor agonist, and thereby amplify the bronchodilation elicited by the β₂-agonist through the direct stimulation of smooth muscle β₂-adrenoceptors. Another possibility is the fact that the antimuscarinic agent and not the β₂-adrenoceptor agonist can suppress mucus/fluid secretions; hence, surface tension changes which would collapse the airways do not occur.

The LABA/long-acting antimuscarinic agent (LAMA) combination appears to play an important role in maximizing bronchodilation, with studies to date indicating that combining different classes of bronchodilators results in significantly greater improvements in lung function and other outcomes compared with individual drugs used alone, and that these combinations are well tolerated in patients with moderate to severe COPD (Cazzola and Tashkin, 2009).

For patients with more severe COPD and a history of exacerbations, current guidelines recommend the addition of an ICS to a LABA (Celli and MacNee, 2004; Rabe et al., 2007), but evidence from clinical trials indicates that combining a LABA with an ICS and an antimuscarinic agent may provide clinical benefits additional to those associated with these treatments alone in patients with COPD (Cazzola et al., 2007; Singh et al., 2008).

It has been suggested that the use of β₂-adrenoceptor agonists might lead to an increased risk for adverse events in patients suffering from COPD (Salpeter et al., 2004). However, TORCH trial has unequivocally demonstrated that long-term use of LABAs over a period of 3 years is safe and is associated with a slightly lower risk of mortality compared with placebo (Calverley et al., 2007). Moreover, a review and meta-analysis of five trials that compared salmeterol against placebo and four trials that compared formoterol against placebo documented that LABAs by themselves did not significantly alter total mortality in COPD (Kliber et al., 2010). Nonetheless, LABAs may have adverse cardiovascular effects, which are amplified in COPD patients with concomitant cardiac disorders (Cazzola et al., 1998b). In these patients, LABAs should be used with some cautions (Cazzola et al., 2005a). At the present level of our knowledge, the use of LABAs in COPD patients, even patients with cardiac disease comorbidities, does not put ‘caveats’ similar to the use of LABAs in asthma (Rossi et al., 2008). This might be considered surprising to some extent, because COPD patients are older than patients with asthma and have a larger prevalence of cardiac disorders.

The central role of LABAs as bronchodilators in the treatment of asthma and COPD indicates important marketing opportunities. On the other hand, non-adherence to medication plans is a major obstacle to successful management of asthma and COPD (Bender, 2002). In general, adherence to treatment with inhalants is poor because of the complex procedures required to use them, as well as the tedious frequent dosing (Jones et al., 2003). Being able to increase adherence to treatment is, therefore, a main medical need. Since there is a well-established belief that the only limits set for the development of a long-lasting bronchodilator with a new product profile are medical needs and marketing opportunities (Cazzola and Matera, 2009), the great interest within the pharmaceutical industry in the discovery of a third-generation once-daily β₂-adrenoceptor agonist to be used as a part of a combination therapy for the treatment of asthma or also as single agent for the therapy of COPD is not surprising.

Future direction

Once-daily LABAs

Anyone planning to develop a once-daily LABA, which can also be called ultra-LABA (Cazzola et al., 2005b), must very carefully consider the pharmacological characteristics of the β₂-adrenoceptor agonist component to understand how it will fit into treatment strategies, and whether it should only be used in combination with other drugs. Evidence shows that current LABAs can be improved upon. Ideally, there is a need for a new LABA that is fast acting and has true 24 h duration of action, providing consistent improvements in the symptoms that matter to patients (Table 1) (Cazzola and Matera, 2008). Such an ultra-LABA would provide flexibility to prescribers and could be used alone or in combination with a once-daily long-acting muscarinic antagonist. Obviously, an ideal ultra-LABA should be well tolerated with a favourable safety profile. Thus, a new entry to the market must ensure that potential cardiac effects are minimized, especially taking into account that mainly COPD patients are often older and may have cardiovascular comorbidities. Currently available LABAs have been associated with a potential antagonism to the bronchodilator effect of a rescue medication (fast-acting β₂-adrenoceptor agonists), due to competition for the same receptors (Cockcroft and Swystun, 1996; Cazzola et al., 1998a). While the clinical relevance of this phenomenon in COPD is unclear, it is important that there should be minimal risk of this occurring with a new ultra-LABA. Further, patient compliance and treatment persistence could be improved with a once-daily treatment that provides immediate and sustained bronchodilation.

Table 1.

Designing a new LABA for COPD

Criteria for a new β2-adrenoceptor agonist could include:

•Longer duration of action (compared with existing LABAs)

True 24 h sustained bronchodilator efficacy

Allowing once-daily dosing

•Fast onset of action

•Superior efficacy compared with existing LABAs

•Favourable safety and tolerability profile

•Efficient and convenient device

Breath actuated

With effective feedback to indicate successful inhalation

LABA, long-acting β₂-adrenoceptor agonist.

A variety of β₂-adrenoceptor agonists with longer half-lives are currently undergoing development, with the hope of achieving once-daily dosing. These agents include indacaterol – which received European regulatory approval in November 2009 and has already been launched in several countries – olodaterol, vilanterol, carmoterol, LAS100977 and PF-610355 (Figure 1; Table 2). It must be mentioned that only indacaterol has been studied extensively, while for the other ultra-LABAs, information is still scarce, often limited to conference abstracts.

Figure 1.

Chemical structures of salmeterol, formoterol and emerging ultra-LABAs.

Table 2.

Functional properties of emerging β₂-adrenoceptor agonists against the three human β-adrenoceptor subtypes

Agonist	β1		β2		β3		Selectivity for β2 over β1
	pEC50	IA	pEC50	IA	pEC50	IA		Reference
Indacaterol	6.60 ± 0.24	16 ± 2	8.06 ± 0.02	73 ± 1	6.72 ± 0.13	113 ± 7	1.46	Battram et al., 2006
Olodaterol	7.55 ± 0.08	52 ± 8	9.93 ± 0.07	88 ± 2	6.57 ± 0.08	81 ± 2	2.38	Bouyssou et al., 2010a
Vilanterol	6.4 ± 0.1		9.4 ± 0.0		6.1 ± 0.2		3.0	Procopiou et al., 2010
Carmoterol			10.19 ± 0.15	88.6 ± 4.1				Rosethorne et al., 2010

pEC₅₀ is the negative logarithm of the molar drug concentration that produces a cAMP response equal to 50% of its maximal response. IA is the percentage of isoprenaline-induced maximal response. Selectivity for β₂ over β₁ expressed as pEC₅₀ at β₂-adrenoceptor – pEC₅₀ at β₁-adrenoceptor.

Indacaterol

Indacaterol, also known as QAB149, is a novel, chirally pure inhaled ultra-LABA. Within a series of 8-hydroxyquinoline 2-aminoindan derived β₂-adrenoceptor agonists, lipophilicity was used as the basis for the design and rationalization of their onset and duration of action profiles, as assessed by a guinea pig tracheal-strip assay. In addition to lipophilicity, potency and intrinsic efficacy have also been shown to be contributing factors in regulating these in vitro time course profiles (Baur et al., 2010). The 5,6-diethyl substituted indan analogue indacaterol was selected from these studies.

Extensive preclinical studies involving indacaterol have been performed both in vitro and in vivo, and have documented that it offers unique rapid onset of action and true 24 h bronchodilating effect (Cazzola et al., 2010a).

Indacaterol appears to have a high intrinsic activity at human β₂-adrenoceptors in vitro. The mean maximum effect (E_max) for indacaterol was 73% of the maximum effect of isoprenaline, compared to 90%, 38% and 47% for formoterol, salmeterol and salbutamol respectively (Battram et al., 2006). Similar to formoterol, indacaterol was a very weak agonist at the β₁-adrenoceptor (mean E_max = 16% of the maximal effect of isoprenaline) and a full agonist at the β₃-adrenoceptor (mean E_max = 113%) (Battram et al., 2006). Studies with isolated human bronchi and small airway lung slices showed that indacaterol behaves as a high efficacy β₂-adrenoceptor agonist, with an onset of action that is not significantly different from that of formoterol and salbutamol but significantly faster than that of salmeterol, and a significantly longer duration of action than both formoterol and salmeterol (Naline et al., 2007; Sturton et al., 2008). In particular, a study that compared the properties of indacaterol with salmeterol, formoterol and salbutamol on small airways in precision-cut lung slices from human contracted with carbachol (Sturton et al., 2008) confirmed that the onset of action is fast for salbutamol, formoterol and indacaterol, whereas it is significantly slower for salmeterol, and showed that indacaterol and formoterol had a higher intrinsic efficacy than salbutamol and salmeterol. It was also shown that indacaterol, in contrast with salmeterol, does not antagonize the bronchorelaxant effect of a fast-acting β₂-adrenoceptor agonist (Naline et al., 2007).

Interestingly, no tachyphylaxis has been demonstrated for indacaterol, although significant improvement in protection against 5-hydroxytryptamine-induced bronchoconstriction has been documented after 5 day dosing of indacaterol and formoterol (compared with a single treatment), but not with salmeterol, at least in guinea pig (Battram et al., 2006). The fact that indacaterol behaves as a nearly full β₂-adrenoceptor agonist could explain why indacaterol does not induce tachyphylaxis and also does not antagonize the bronchorelaxant effect of a fast-acting β₂-adrenoceptor agonist. Although low-efficacy agonists may cause less receptor desensitization at equal occupancy, they require more receptors to generate a subsequent response so they will be more sensitive to loss of functional receptors (Charlton, 2009). High-efficacy agonists, in contrast, may cause a greater loss of receptors, but are more tolerant to this, as they have ‘spare receptors’, resulting in a loss in potency but not necessarily maximal effect and are, therefore, less sensitive to loss of receptors through desensitization (Charlton, 2009).

Preclinical data also suggest that, for a given degree of bronchodilator activity, indacaterol has a greater cardiovascular safety margin than formoterol or salmeterol (Battram et al., 2006).

The faster onset of action and longer duration of action of indacaterol compared with some other β₂-adrenoceptor agonists may be related to lipid membrane interactions (Lombardi et al., 2009). Thus, indacaterol and salmeterol show no major but several minor differences in their steady state and kinetic interactions with lipid membranes. The sum of these small differences, including higher partitioning of indacaterol into the microenvironment of the receptor and its faster membrane permeation, is likely to contribute to its faster onset and longer duration of therapeutic action. A striking difference was observed in the effect of the two compounds on membrane fluidity. While indacaterol did not alter membrane fluidity, salmeterol drastically increased membrane fluidity. This may affect the function of the receptor reducing the intrinsic efficacy of the compound (Lombardi et al., 2009). It has also been suggested that lipid rafts, which are areas of cell membranes where β₂-adrenoceptors are held together in close contact with signalling molecules and effectors, and calveolae, which are a special type of lipid raft being small (50–100 nm) invaginations of the plasma membrane, in airway smooth muscle might play a role in long duration of action of indacaterol (Lombardi et al., 2009). Indacaterol has twofold higher affinity for raft micro-domains compared to salmeterol, and this might contribute to the difference in duration of action. Recently, it has also be suggested that indacaterol utilizes higher intrinsic efficacy to offset the high lipophilicity that is important for achieving long duration of action (Rosethorne et al., 2010). In fact, in primary human bronchial smooth muscle cells, indacaterol displays a similar intrinsic activity to formoterol which, combined with comparable lipophilicity, translates to a fast rate of cyclic adenosine monophosphate (cAMP) accumulation, which plays a key role in β₂-adrenoceptor-induced smooth muscle relaxation in the airways.

After inhalation, indacaterol is rapidly absorbed into the systemic circulation with a median T_max of 15 min. It has linear and dose-proportional pharmacokinetics (PK), and steady state is reached within 12 days of once-daily dosing at doses of 150, 300 and 600 µg (Perry et al., 2010).

Although the already mentioned issue about the long-term safety of LABAs in asthma mainly when used as monotherapy, several short-term studies have explored the effect of indacaterol in asthmatic patients (Cazzola et al., 2010b). In particular, two large 7 day dose finding trials examined the effect of indacaterol 100, 200, 300 400 or 600 µg once daily, via single-dose dry powder inhaler (SDDPI) (Kanniess et al., 2008a), indacaterol 50, 100, 200 or 400 µg via multiple-dose dry powder inhaler (MDDPI) and 400 µg via SDDPI once daily (LaForce et al., 2008) in patients with persistent asthma respectively. Once-daily dosing with indacaterol provided sustained 24 h bronchodilation in patients with moderate to severe asthma, with a satisfactory overall safety profile. In the first study, mean forced expiratory volume in the 1st second (FEV₁) for indacaterol doses 200 µg or more on day 7 was higher than placebo predose and at all postdose time points (Kanniess et al., 2008a). In the second study, all doses of indacaterol provided rapid onset, sustained 24 h bronchodilator efficacy on once-daily dosing from day 1, with no loss of efficacy after 7 days of treatment, although indacaterol 200 µg appeared to be the optimum dose, offering the best efficacy/safety balance (LaForce et al., 2008).

In COPD, single doses of indacaterol 150 and 300 µg demonstrate a fast onset of action similar to that for salbutamol and faster than that for salmeterol-fluticasone (Balint et al., 2010). Moreover, once-daily indacaterol 150 µg is at least as effective as tiotropium, with a faster onset of action (within 5 min) on the first day of dosing (Vogelmeier et al., 2010).

The efficacy of indacaterol in the maintenance treatment of COPD in adults has been assessed in large, randomized, double-blind, parallel-group, placebo-controlled, multicentre phase III trials [Chapman et al., (2011); Dahl et al., 2010; Donohue et al., 2010; Feldman et al., 2010; Kornmann et al., 2010; Laforce et al., 2010]. The analysis of these trials (Moen, 2010) shows that indacaterol 150 and/or 300 µg once daily was more effective than tiotropium, formoterol or salmeterol for improving trough FEV₁ values versus placebo. COPD exacerbations were significantly reduced versus placebo for indacaterol 150 µg or 300 µg once daily. In a 52 week study (Dahl et al., 2010), once-daily treatment with indacaterol prolonged the time to first COPD exacerbation and was effective in reducing incidence and frequency of COPD exacerbations, with no significant difference between indacaterol and formoterol. Patients treated with indacaterol had a significantly higher percentage of days with no use of as-needed rescue salbutamol than placebo recipients in all large studies. Moreover, the percentages of days with no rescue medication were significantly (P < 0.05) higher in the indacaterol groups than the active comparator groups in all studies. In general, indacaterol appeared to have greater effects on most COPD symptoms than tiotropium, formoterol or salmeterol, although differences between indacaterol and active comparators were not consistently statistically significant. Indacaterol also provided significant and clinically relevant better health-related quality of life. In all studies designed to investigate whether indacaterol has the same tolerability of LABAs already in the market, indacaterol was well tolerated at all doses and with a good overall safety profile. The most common treatment-emergent adverse events were COPD worsening and nasopharyngitis. Study investigators also recorded the incidence of cough. Cough was, however, mild in severity, transient in nature and duration, and tended to decline with the duration of treatment (Cazzola et al., 2010a). Decreases in serum potassium levels to <3.0 mM were rare (≤0.5% of patients) in all treatment groups including placebo, and also increases in corrected QT (QTc) interval values of greater than 60 ms were unusual (≤0.7% of patients).

Olodaterol

Olodaterol, previously known as BI 1744 CL, is a novel, enantiopure inhaled β₂-adrenoceptor agonist. It was identified from a series of 6-hydroxy-4H-benzo[1,4]oxazin-3-ones as potent agonist of the human β₂-adrenoceptor with a high β₁/β₂-selectivity (Bouyssou et al., 2010b). In vitro, olodaterol showed a potent, nearly full agonistic response at the human β₂-adrenoceptor [median effective concentration required to induce a 50% effect (EC₅₀) = 0.1 nM; intrinsic activity = 88% compared with isoprenaline] and, unlike formoterol and salmeterol that exerted either a full-agonistic or a partial agonistic profile for all β-adrenoceptors, a significant selectivity profile (219- and 1622-fold against the human β₁- and human β₃-adrenoceptors respectively) (Bouyssou et al., 2010a). On isolated human bronchi, olodaterol dose-dependently reversed the constriction induced by different stimuli, such as histamine, ACh, and EFS with an efficacy not statistically different from the nearly full agonist formoterol under all conditions (Bouyssou et al., 2010a). Olodaterol and formoterol exhibited similar potencies and E_max at resting tone and in the presence of contracting stimuli. Formoterol induced significant β₂-adrenoceptor desensitization in vitro, whereas olodaterol preserved the β₂-adrenoceptor signalling capacity even after long-term pre-incubation (Naline et al., 2010).

In vivo, antagonistic effects of single doses of olodaterol and formoterol were measured against ACh challenges in anesthetized guinea pigs and dogs for up to 24 h by using the Respimat® Soft Mist™ inhaler (Boehringer Ingelheim, Ingelheim, Germany). In both models, olodaterol provided bronchoprotection over 24 h. Formoterol applied at an equally effective dose did not retain efficacy over 24 h. In both models, olodaterol showed a rapid onset of action comparable with formoterol (Bouyssou et al., 2010a). Interestingly, olodaterol has a biphasic dissociation profile from the human β₂-adrenoceptors, with the slow component (approx. 30–40% of the total β₂-adrenoceptor pool) showing a half life of dissociation of more than 12 h, providing a rationale for its long duration of action in human therapy (Schnapp et al., 2009).

Initial studies achieved their objective by providing proof of concept of the 24 h bronchoprotective effect of olodaterol in patients with intermittent asthma (O'Byrne et al., 2009) or COPD (van Noord et al., 2008) and the 24 h bronchodilation following 4 weeks once-daily administration in COPD (van Noord et al., 2009).

Vilanterol

Vilanterol, also known as GSK-642444, is the triphenylacetate salt of the 2,6-dichlorobenzyl analogue of a series of novel β₂-adrenoceptor agonists obtained by the incorporation of an oxygen atom at the homobenzylic position of the right-hand side phenyl ring of (R)-salmeterol (Procopiou et al., 2010). The triphenylacetate salt was found to have suitable properties for inhaled administration.

Vilanterol is a potent, selective, β₂-adrenoceptor agonist in human functional cellular assays. Vilanterol has a greater intrinsic efficacy than salmeterol and a greater potency than indacaterol and salbutamol. In addition, it has been shown using human recombinant β_1/2/3-adrenoceptor cAMP assays that it has significantly greater β₂-adrenoceptor selectivity than formoterol, indacaterol and salbutamol (Procopiou et al., 2010; Barrett et al., 2010a). In isolated, electrically stimulated, guinea pig trachea strips, vilanterol [negative logarithm of EC₅₀ (pEC₅₀) = 8.6] was equipotent with formoterol (pEC₅₀ = 8.6) and more potent than salmeterol (pEC₅₀ = 6.7) or indacaterol (pEC₅₀ = 7.0) and was shown to be antagonized in a competitive manner by propranolol (Ford et al., 2010a). Vilanterol, formoterol and indacaterol had a more rapid onset of activity when compared with salmeterol. On removal of vilanterol, the tissue showed no recovery after 1 h, suggesting a long duration of action in contrast with isoprenaline (rapid and full recovery) or formoterol (slow but continuous recovery) (Ford et al., 2010a). In human tissue pre-constricted with 0.1 µM histamine, 1 nM vilanterol was shown to have a significantly faster onset of action (3.1 min) than 1 nM salmeterol (8.3 min) (Barrett et al., 2010b). Vilanterol had a significantly longer duration of action compared to salmeterol as vilanterol still demonstrated a significant bronchodilator effect at 22 h, but salmeterol did not.

Vilanterol inhibited histamine-induced bronchoconstriction when administered to conscious guinea pigs as a nebulized solution, and was equipotent to salmeterol (Ford et al., 2010a). At equieffective doses, the duration of action was similar to salmeterol (10 h) and longer than formoterol (<3 h); however unlike salmeterol, vilanterol duration increased in a dose-dependent manner.

Interestingly, vilanterol is a metabolically labile LABA, which undergoes conversion in human microsomes to metabolites with significantly lower β₂-adrenoceptor activity and exhibits low systemic exposure in vivo after inhaled dosing (Ford et al., 2010b).

Vilanterol has been tested both in asthmatic and COPD patients. In mild to moderate persistent asthma patients, single doses of inhaled vilanterol (25–100 µg) produced a rapid and prolonged bronchodilation over 24 h, suggesting the potential for once-daily administration (Kempsford et al., 2010a). All doses were well tolerated with no clinically significant unwanted systemic effects. Vilanterol (25–100 µg) produced rapid bronchodilation in COPD patients, which was maintained over 24 h at all doses (Kempsford et al., 2010b). Following dosing with vilanterol, there were no serious adverse events or withdrawals due to adverse events and no clinically significant laboratory, vital sign or 12-lead electrocardiogram (ECG), QTc or Holter ECG abnormalities. Vilanterol was rapidly absorbed into plasma (median T_max at 10 min) with systemic exposure increasing in an approximately dose proportional manner across the vilanterol dose range (25–100 µg). A 28 day study in patients ≥12 years with persistent asthma on maintenance ICS showed that once-daily vilanterol was well tolerated and resulted in a prolonged duration of action of at least 24 h at doses ≥12.5 µg (Lötvall et al., 2010a).

A 28 day dose-ranging (3, 6.25, 12.5, 25 or 50 µg) study of vilanterol in patients with COPD demonstrated statistically significant improvements in trough FEV₁ for all doses compared with placebo (P < 0.001) (92 mL, 98 mL, 110 mL, 137 mL and 165 mL respectively) (Hanania et al., 2010b). The time to an increase of ≥100 mL in FEV₁ on Day 1 was significantly shorter for all vilanterol arms compared with placebo (P < 0.001) (median time of 6 min in the 25 and 50 µg groups). The same study documented that vilanterol was safe (Hanania et al., 2010c). The incidence of adverse events was low (≤3%) with no apparent treatment or dose relationship. There was no clinically relevant effect on systolic or diastolic blood pressure, pulse rate or blood glucose or potassium levels.

Carmoterol

Carmoterol {CHF 4226, TA 2005; 8-hydroxy-5-[ (1R)-1-hydroxy-2-[N-[ (1R)-2-(p-methoxy-phenyl)-1-methylethyl]-amino]-ethyl]-carbostyril hydrochloride}, a pure (R,R)-isomer, is a non-catechol β₂-adrenoceptor agonist with a p-methoxyphenyl group on the amine side chain and a 8-hydroxyl group on the carbostyril aromatic ring (Kikkawa et al., 1991), possessing structural elements from both formoterol and procaterol. It binds very firmly to β₂-adrenoceptors (Voss et al., 1992), a property shared by some other agonists which, like carmoterol, are based on a carbostyril skeleton (Standifer et al., 1989). In studies using chimeric β₂-adrenoceptors, the methoxyphenyl group in carmoterol has been found to be critical to the β₂-adrenoceptor selectivity of the molecule (Kikkawa et al., 1998).

Carmoterol has been demonstrated to be a highly potent and selective β₂-adrenoceptor agonist [it has 53 times higher affinity for the β₂-adrenoceptors than for the β₁-adrenoceptors (Voss et al., 1994), and is about five times more selective for the β₂-adrenoceptors present in the tracheal preparation than those mediating chronotropic response in the right atrium (Kikkawa et al., 1998) ]. Moreover, it displays a fast onset and long duration of activity both in in vitro and in vivo experimental conditions (Kikkawa et al., 1991; Voss et al., 1992; Kikkawa et al., 1994). Interestingly, the persistence of action of carmoterol at the human recombinant β₂-adrenoceptors is similar to that of salmeterol, and longer than that of indacaterol, which is longer than that of formoterol (Summerhill et al., 2008). In a study that measured the rate of accumulation of cAMP in primary human bronchial smooth muscle cells and compared this with measures of intrinsic activity in the same systems, carmoterol displayed a similar intrinsic activity to formoterol which, combined with comparable lipohilicity, translated to a fast rate of cAMP accumulation (Rosethorne et al., 2010).

Carmoterol is more potent than other LABAs in methacholine precontracted guinea-pig tracheal smooth muscle (Kikkawa et al., 1991; Voss et al., 1992; Voss, 1994). Carmoterol has a similar onset of action compared to salbutamol and formoterol, and a faster onset of action compared to salmeterol. Furthermore, the duration of tracheal smooth muscle relaxation is longer for carmoterol compared to both formoterol and salmeterol (Voss, 1994).

In asthmatic patients, the PK of carmoterol is proportional to the dose, and nonlinear accumulation of the drug after repeated dosing treatments is negligible (Cazzola et al., 2010b). Interestingly, using Modulite™ technology (Chiesi Farmaceutici, Parma, Italy) that utilizes hydrofluoroalkane (HFA) 134a as propellant, a lung deposition of carmoterol as high as 41% of the nominal dose can be reached (Haeussermann et al., 2006).

In patients with persistent asthma carmoterol 2 µg administered once daily is as effective as formoterol 12 µg twice daily (Kottakis et al., 2006). Safety and tolerability results are similar between carmoterol and formoterol (Nandeuil et al., 2006).

In COPD, a single 4 µg – but not 1 or 2 µg – dose of carmoterol had an effect on 24 h trough FEV₁ that was better than that of two 50 µg doses of salmeterol given 12 h apart (Kanniess et al., 2008b). After a 2 week treatment, once-daily doses of 2 and 4 µg carmoterol resulted in placebo-adjusted improvements compared to baseline in trough FEV₁ of 94 and 112 mL, respectively, whereas 50 µg salmeterol twice-daily resulted in an increase of 78 mL (Make et al., 2008). There were no significant changes in ECG results, blood pressure or serum potassium or glucose levels compared with salmeterol or placebo (Bateman et al., 2008b). No tolerance to the bronchodilatory effects of carmoterol or salmeterol was observed over the 2 weeks of treatment (Rossing et al., 2008).

LAS100977

LAS100977 is a novel once-daily LABA. Its in vitro pharmacological profile has recently been reported (Aparici et al., 2010). In radioligand displacement assays by using human β₁-, β₂- and β₃-adrenoceptors expressed in cell lines, it showed the highest β₂-adrenoceptor affinity (0.6 nM) in comparison to reference compounds. LAS100977 was 10 times more potent than salmeterol and similar to formoterol and indacaterol. LAS100977 onset of action (10 min) was faster than salmeterol and indacaterol (19 min and 14 min, respectively) and slower than formoterol (6 min), whereas its duration of action (670 min) was longer than formoterol and salmeterol (77 min and 230 min, respectively) and comparable to indacaterol (450 min). LAS100977 demonstrated higher β₂/β₁-adrenoceptor selectivity than formoterol and indacaterol in both binding (64-fold) and tissue functional studies (10 750-fold). In anaesthetized dogs, LAS100977 inhibited ACh-induced bronchoconstriction more potently and with a longer duration of action than salmeterol (Miralpeix et al., 2010). LAS100977 also had a higher therapeutic index than salmeterol, suggesting a reduced potential for cardiac side effects in man.

In healthy subjects, LAS100977 at doses of 5, 10, 25 and 50 µg produced an increase in specific airway conductance (sGaw) at the 24 h post dose time point compared to the respective pre-dose value in contrast to placebo (Timmer et al., 2010). In addition, airways resistance (Raw) decreased for LAS100977 at the 24 h post dose time point compared to the pre-dose value. The most frequent drug-related adverse events were palpitations and tremors, which were both of mild to moderate in intensity. Two preliminary clinical studies have documented 24 h duration of action. In the first study, single different once-daily doses (5, 10 and 25 µg) of LAS100977 induced a significant increase in FEV₁ compared with both placebo and salmeterol 50 µg twice-daily with a rapid onset of effect with improvements in lung function at 5 min post-dose in patients with mild to moderate asthma (Beier et al., 2010). Tachycardia and tremor were seen in higher doses (Beier et al., 2010). In another study, LAS100977 also demonstrated sustained efficacy during multiple-dose administrations with no significant increase in cardiovascular adverse events (Cazzola et al., 2010b).

PF-610355

PF-610355 is a member of a novel series of potent and selective sulfonamide derived β₂-adrenoceptor agonists (Glossop et al., 2010). The sulfonamide agonist headgroup confers high levels of intrinsic crystallinity that could relate to the acidic sulfonamide motif supporting a zwitterionic form in the solid state. Optimization of PK properties was achieved through targeted introduction of a phenolic moiety to support rapid phase II clearance, thereby minimizing systemic exposure following inhalation and reducing systemically mediated adverse events. It is intriguing that plasma PK of orally inhaled PF-610355 are consistent and exhibit a sustained plateau after single/multiple doses, but plasma exposure is reduced in asthmatic patients, compared to healthy volunteers (Li et al., 2009). In healthy subjects, duration of action of PF-610355 450 µg on airways determined by plethysmography was superior to salmeterol 50 µg by 9.77 h indicating the potential for sustained pharmacological effect in the lung (Macintyre et al., 2009). A preliminary trial has documented that in asthmatic patients, PF-610355 elicits a clear dose-response for peak and trough (32 h post-dose) FEV₁. At doses of 368, 736 and 1472 µg, it produced higher peak FEV₁ than salmeterol 50 µg (Ward et al., 2009).

Novel combinations with ultra-LABAs

A range of once-daily LABAs (ultra-LABAs) and LAMAs fixed-dose combinations, including QVA149 [the combination of indacaterol and the LAMA NVA237 (glycopyrronium) ], olodaterol + tiotropium, and vilanterol + GSK-573719, are in clinical development as fixed combinations (Cazzola and Matera, 2008; 2009;). There is documentation that a 7-day treatment with QVA149 (indacaterol 300 µg/glycopyrronium 50 µg) once daily via single dose dry powder inhaler is more effective than indacaterol 300 and 600 µg (van Noord et al., 2010). Moreover, QVA149 at the dosage of 600/100 µg, 300/100 µg or 150/100 µg has a safe cardiovascular profile, with no different 24-h mean heart rate at day 14 between QVA149 and placebo, nor between QVA149 and indacaterol, and no clinically relevant differences in QTc intervals observed among treatment groups on days 1, 7 and 14 (van de Maele et al., 2010). Addition of olodaterol enhanced the beneficial effect of the tiotropium monotherapy on ACh-induced bronchoconstriction with a longer duration of action in anaesthetized dogs (Bouyssou et al., 2010c). It has also been shown that olodaterol/tiotropium (10/5 µg) fixed-dose combination administered using the Respimat® Soft Mist™ inhaler is more effective than tiotropium 5 µg in COPD patients, with superior bronchodilation over 24 h following 4 weeks once-daily dosing (Maltais et al., 2010).

Drugs with a bifunctional mechanism of action, combining both muscarinic antagonist and β₂-adrenoceptor agonist pharmacology in a single molecule, which are known as dual-acting muscarinic antagonist/β₂-adrenoceptor agonist (MABA) bronchodilators, are a different interesting approach (Norman, 2006). MABAs have the advantage of delivering a fixed ratio into every region of the lung, reducing the complexity of combination inhalers (Cazzola and Matera, 2009).

TEI3252 is a novel bifunctional bronchodilator that, in experimental setting, showed bronchoprotective activities against acethylcholine and histamine stimulation in a dose-dependent manner at the dose range of 1–5 µg·kg⁻¹ (Sugiyama et al., 2010). Its efficacy was long lasting compared with existing bronchodilators, such as tiotropium and indacaterol. On the other hand, the inhibitory effect on salivation was not observed even at the dose up to 100 µg·kg⁻¹. This finding suggests that TEI3252 has a reduced side effect.

Evaluation of THRX-200495, a single bifunctional molecule that possesses both muscarinic antagonist and β₂-adrenoceptor agonist pharmacology, in a guinea pig model of bronchoconstriction revealed a matched muscarinic antagonist and β₂-adrenoceptor agonist potency [median dose that causes 50% inhibition (ID₅₀) = 11.4 and 11.2 µg·mL⁻¹, respectively], with similar onset of action and potent dual pharmacology (MABA ID₅₀ = 3.5 µg·mL⁻¹) lasting for >24 h (McNamara et al., 2009).

GSK-961081 (formerly TD-5959) is a further novel bifunctional molecule. It conferred potent 24 h bronchoprotection in guinea pigs through a dual mechanism involving antagonism of muscarinic receptors and agonism of β₂-adrenoceptors. Dual pharmacology yielded bronchoprotection that was two- to fivefold more potent (MABA ID₅₀ = 6.4 mg·mL⁻¹) than either ipratropium or salbutamol alone (Pulido-Rios et al., 2009). In phase I randomized double-blind placebo-controlled single and multiple-dose studies that enrolled healthy volunteers, GSK-961081 was generally well tolerated and demonstrated evidence of bronchodilation over 24 h after a single dose and after seven consecutive daily doses and, consequently, has entered into phase II (Cazzola and Matera, 2009). In a phase II study, GSK-961081 dosed at both 400 and 1200 µg once daily showed bronchoprotection at day 14 that was at least equivalent to that of 50 µg salmeterol twice-daily plus 18 µg tiotropium once-daily, as measured by changes in FEV₁ (Cazzola and Matera, 2009). Both the time to peak effect and maximum bronchodilation of GSK-961081 were numerically better than salmeterol plus tiotropium, although the study was not powered to compare the results to the salmeterol plus tiotropium control.

PF-3429281 is another inhaled dual antimuscarinic/β₂-adrenoceptor agonist. In anaesthetized guinea pigs, it caused a dose-related inhibition of ACh-induced bronchoconstriction (Philip et al., 2010). This was 26-fold weaker than tiotropium and equipotent with salmeterol. Infusion of propranolol throughout the experiment blocked the effects of salmeterol on ACh-induced bronchoconstriction, whereas both tiotropium and PF-3429281 were unaffected. Data generated in vitro using guinea pig isolated trachea suggest that the duration of action of the β₂ component is longer than the M₃ one (Patel et al., 2010). In an anaesthetized dog model of bronchoconstriction, PF-3429281 had an equivalent potency to ipratropium bromide and a superior therapeutic index and duration of action compared to salmeterol (Wright et al., 2010).

Novel combinations of LABAs and ICSs under development

As combination therapy with an ICS and a LABA is considered an important approach for treating patients suffering from asthma and also severe COPD patients with frequent exacerbations (Celli and MacNee, 2004; National Asthma Education and Prevention Program, 2007; Rabe et al., 2007; Bateman et al., 2008a), there is a strong interest in developing a once-daily combination therapy, again in an attempt to simplify the treatment, and also to overcome the loss of patent protection. The awareness that new ICSs, such as ciclesonide, fluticasone furoate and mometasone furoate, which can be used as a once-daily dosing, have been developed or are in development, have further supported the development of new ultra-LABAs that can be used on a once-a-day basis (Cazzola and Matera, 2009), although a combination of formoterol and mometasone furoate administered on a twice-daily basis has been developed and successfully tested in asthma (Maspero et al., 2010).

A new inhaled therapy will combine indacaterol with mometasone (QMF-149) or indacaterol and QAE-397, a novel corticosteroid in phase II development for the treatment of asthma (Cazzola and Matera, 2009). In particular, two trials have investigated the safety and tolerability of QMF-149. The first was designed to evaluate the bronchodilatory efficacy of QMF-149 delivered via a MDDPI (Twisthaler) in adult patients with persistent asthma using open-label salmeterol/fluticasone (50/250 µg twice daily) as an active control (Cazzola and Matera, 2009), whereas the second one investigated the safety and tolerability of 14 days treatment with QMF-149 500/800 µg in patients with mild to moderate asthma (Cazzola and Matera, 2009). The results of these trials have not been released yet.

A next-generation, once-daily combination consisting of vilanterol and fluticasone furoate is another combination under development. A small study that enrolled 60 COPD (GOLD Stage II–III) patients documented that this combination had greater improvements than placebo in trough FEV₁ after a 4 week treatment with a good safety and tolerability profile (Lötvall et al., 2010b).

A positive interaction of carmoterol with budesonide in the control of bronchoconstriction induced by acetaldehyde in the guinea pigs has been documented (Rossoni et al., 2005). Intriguingly, carmoterol/budesonide was twofold more effective than the formoterol/budesonide combination. The finding suggests that carmoterol/budesonide, by optimizing each other's beneficial pharmacological potential, may represent a new fixed combination in asthma. In effect, in mild or moderate asthmatic patients, the systemic exposure to carmoterol 2 µg and budesonide 400 µg was not increased with the combination in comparison with each component administered alone (Poli et al., 2009). Moreover, a prolonged bronchodilatation was observed with carmoterol/budesonide combination (Poli et al., 2009). The fixed combination carmoterol/budesonide formulated as HFA 134a pMDI (Chiesi Modulite™ HFA technology) administered once a day in asthmatic patients maintained the bronchodilator effect over 24 h and was as effective as formoterol/budesonide Turbuhaler twice a day in patients with moderate or severe persistent asthma (Woodcock et al., 2009).

Intravenous β₂-adrenoceptor agonist

An interesting new option is the development of a β₂-adrenoceptor agonist to be administered intravenously. Bedoradrine (MN-221) is a novel, highly selective β₂-adrenoceptor agonist under development for the treatment of acute exacerbation of asthma and COPD. Bedoradrine was calculated to be 832- and 126-fold more selective for β₂-adrenoceptors than for β₁- and β₃-adrenoceptors, respectively, which indicates that it is very highly selective for β₂-adrenoceptor in this assay system (Inoue et al., 2009). However, it was initially considered for the treatment of preterm labour because it is 1590-fold more specific for the uterus than for the trachea (Inoue et al., 2009).

The PK and pharmacodynamics of bedoradrine were investigated using data from a single i.v. dose study in stable moderate to severe COPD patients (Sadler et al., 2010). Patients receiving doses of 600 and 1200 µg showed superior response to those receiving 300 µg. Patients receiving doses of 600 and 1200 µg showed superior response to those receiving 300 µg. At 1200 µg, the mean peak FEV₁ increase was about 55% of maximal lending support to this dose.

A basic study provided evidence that, while both salbutamol and bedoradrine induced an increase in heart rate independently, adverse effects on heart rate were not observed upon combination in dogs. Other cardiovascular parameters (QTc and monophasic action potential) were not adversely affected in the salbutamol + bedoradrine combination. These findings are consistent with some other data implicating bedoradrine as a partial agonist at β₁-adrenoceptors (Johnson et al., 2010).

A study that evaluated the safety and tolerability of bedoradrine at doses of 150 to 900 µg via intravenous infusion in patients with mild to moderate stable asthma documented that it was safe and well tolerated, with dose-dependent improvements in FEV₁ (Matsuda et al., 2010). A preliminary small trial showed that bedoradrine added to standard therapy for severe acute asthma exacerbations was safe and appeared to provide additional clinical benefit (Nowak et al., 2010).

In a small group of COPD patients, bedoradrine, at doses of 300, 600 or 1200 µg i.v., appeared to improve lung function at all dose levels and reached statistical significance at both 600 and 1200 µg as compared to placebo (Pearle et al., 2010). FEV₁ (L) increased as compared to baseline by an average of 21.5% (P = 0.0025) for the 1200 µg dose, 16.2% (P = 0.02) for the 600 µg dose, and 9.2% (P = NS) for the 300 µg dose compared to a decrease of 4.0% for placebo. Bedoradrine was generally well tolerated by all patients.

Conclusion

This review clearly indicates that, despite the passionate and sometimes harsh debate over the use of β₂-adrenoceptor agonists in the treatment of airway disorders, the interest of pharmaceutical companies in the field is still high. However, it is difficult to determine whether this is due to the success that this class of bronchodilators still has, or just the fact that we are not yet able to identify a new class of bronchodilators that can control the bronchial muscle tone without being burdened by the risks of β₂-adrenoceptor agonists. Whatever the case may be, pending new true bronchodilators, we believe that the possibility of administering β₂-adrenoceptor agonists on the once-daily basis is an advantage because it improves convenience and compliance, controls airflow over a complete 24 h period and allows the combination with other classes of drugs, such as antimuscarinic agents and ICSs, that are fundamental for treating asthma and/or COPD and now are administered on the once-daily basis. In any case, it is still too early to indicate which of the new ultra-LABAs will meet with greater success. In fact, excluding indacaterol, which has now entered the market having completed the pivotal phases, we have too little information on other drugs to make a prediction based on the pharmacological and clinical evidence.

Glossary

Abbreviations

ACh

acetylcholine

cAMP

cyclic adenosine monophosphate

COPD

chronic obstructive pulmonary disease

EC₅₀

median effective concentration required to induce a 50% effect

ECG

electrocardiogram

EFS

electric field stimulation

E_max

the maximum possible effect for the agonist

FDA

Food and Drug Administration

FEV₁

Forced Expiratory Volume in One Second

HFA

hydrofluoroalkane

ICS

inhaled corticosteroid

IFN

interferon

IL

interleukin

LABA

long-acting β₂-adrenoceptor agonist

LAMA

long-acting antimuscarinic agent

mL

milliliter

pEC₅₀

negative logarithm of EC₅₀

PK

pharmacokinetics

QTc

corrected QT interval

Conflict of interests

Mario Cazzola has received honoraria for speaking and consulting and/or financial support for attending meetings from Abbott, AstraZeneca, Boehringer Ingelheim, Chiesi Farmaceutici, Dey, GSK, Menarini Farmaceutici, Mundipharma, Novartis, Nycomed, Pfizer, Sanovel, Sigma Tau and Valeas.

Luigino Calzetta has received honoraria for writing a report from Boehringer Ingelheim.

Maria Gabriella Matera has received honoraria for consulting and/or financial support for attending meetings from AstraZeneca, Boehringer Ingelheim, Chiesi Farmaceutici, GSK, Novartis, Nycomed and Pfizer.

No funding has been provided for this article.

Supporting Information

Teaching Materials; Fig 1 as PowerPoint slide.