Evaluation of the beta 2 adrenoceptor agonist/antagonist activity of formoterol and salmeterol

Recent Advances in β2-Agonists for Treatment of Chronic Respiratory Diseases and Heart Failure

β₂-Adrenoceptor (β₂-AR) agonists are widely used as bronchodilators. The emerge of ultralong acting β₂-agonists is an important breakthrough in pulmonary medicine. In this review, we will provide mechanistic insights into the application of β₂-agonists in asthma, chronic obstructive pulmonary disease (COPD), and heart failure (HF). Recent studies in β-AR signal transduction have revealed opposing functions of the β₁-AR and the β₂-AR on cardiomyocyte survival. Thus, β₂-agonists and β-blockers in combination may represent a novel strategy for HF management. Allosteric modulation and biased agonism at the β₂-AR also provide a theoretical basis for developing drugs with novel mechanisms of action and pharmacological profiles. Overlap of COPD and HF presents a substantial clinical challenge but also a unique opportunity for evaluation of the cardiovascular safety of β₂-agonists. Further basic and clinical research along these lines can help us develop better drugs and innovative strategies for the management of these difficult-to-treat diseases.

Tremor and β(2)-adrenergic agents: is it a real clinical problem?

Tremor is one of the most characteristic adverse effects following administration of β(2)-adrenergic agonists. It is reported by around 2-4% of patients with asthma taking a regular β(2)-adrenergic agonist and is induced by both short-acting and long-acting agents. Tremor associated with β(2)-adrenergic agonists is dose-related and may occur more commonly with oral dosing. The exact mechanism for tremor induction by β(2)-adrenergic agonists is still unknown, but there is some evidence that β(2)-adrenergic agonists act directly on muscle. An early explanation of the tremor was that β(2)-adrenoceptor stimulation shortens the active state of skeletal muscle, which leads to incomplete fusion and reduced tension of tetanic contractions. More recently, tremor has been correlated closely with hypokalaemia. A possible diverse impact of different modes of administration of β(2)-adrenergic agonists on tremorogenic responses has been suggested but solid evidence is still lacking. In any case, the desensitization of β(2)-adrenoceptors that occurs during the first few days of regular use of a β(2)-adrenergic agonist accounts for the commonly observed resolution of tremor after the first few doses. Therefore, tremor is not a really important adverse effect in patients under regular treatment with a β(2)-adrenergic agonist.

Beta-adrenoceptor responses of the airways: for better or worse?

Beta2-adrenoceptor agonists are the first-line treatment of asthma and chronic obstructive pulmonary disease (COPD), in which a short-acting beta2-adrenoceptor agonist is used as required for relief of bronchoconstriction. A long-acting beta2-adrenoceptor agonist may be added to an inhaled corticosteroid as step 3 in the management of chronic asthma. Long-acting beta2-adrenoceptor agonists may also be added in treatment of COPD. This review examines the beneficial and detrimental effects of beta2-adrenoceptor agonists. The beneficial effects of beta2-adrenoceptor agonists are mainly derived from their bronchodilator activity which relieves the bronchiolar narrowing and improves air flow. The potential anti-inflammatory actions of stabilizing mast cell degranulation and release of inflammatory and bronchoconstrictor mediators, is considered. Other potential beneficial responses include improvements in mucociliary clearance and inhibition of extravasation of plasma proteins that is involved in oedema formation in asthma. The side effects of beta2-adrenoceptor agonists are primarily related to beta2-adrenoceptor-mediated responses at sites outside the airways. Of major concern has been the development of tolerance and this is discussed in relation to incidence of increased morbidity and mortality to asthma over the past three decades. A clinical aspect of beta2-adrenoceptor pharmacology in recent years has been the recognition of genetic polymorphism of the receptor and how this affects responses to and tolerance to beta2-adrenoceptor agonists. A controversial feature of beta2-adrenoceptor agonists is their stereoisomerism and whether the inactive (S)-isomer of salbutamol had detrimental actions in the commercially used racemate. The consensus is that despite these adverse properties, beta2-adrenoceptor agonist remains the most useful pharmacological agents in the management of asthma and COPD.

Cross tolerance to salbutamol occurs independently of beta2 adrenoceptor genotype-16 in asthmatic patients receiving regular formoterol or salmeterol

BACKGROUND

The development of tolerance following the use of long acting beta(2) agonists in asthmatic patients with either the homozygous arginine (Arg-16) or glycine (Gly-16) genotypes is poorly documented, especially in relation to the acute reliever response to salbutamol in constricted airways. A study was undertaken to evaluate the Arg-16 and Gly-16 genotypes for the acute salbutamol response following methacholine bronchial challenge between the first and last doses of formoterol (FM) and salmeterol (SM) combination inhalers.

METHODS

Parallel groups of 10 matched homozygous Arg-16 and 10 homozygous Gly-16 patients completed a randomised, double blind, double dummy, crossover study. Following a 1 week washout period, patients received treatment for 2 weeks with either inhaled budesonide (BUD) 200 micro g + FM 6 micro g (two puffs twice daily) or inhaled fluticasone propionate (FP) 250 micro g + SM 50 micro g (one puff twice daily). After washouts and randomised treatments (1 hour after the first and last inhalation) a methacholine challenge was performed followed by salbutamol 200 micro g, with recovery over 30 minutes (the primary outcome).

RESULTS

Washout values for forced expiratory volume in 1 second (FEV(1)), methacholine hyperreactivity, and salbutamol recovery were similar for both treatments and genotypes. Pre-challenge FEV(1) values for both genotypes did not differ significantly between the first and last doses of each treatment. Salbutamol recovery as mean (SE) area under the 30 minute time-response curve was significantly delayed (p<0.05) equally in both genotype and treatment groups. There were no differences in salbutamol recovery in either genotype or treatment group.

CONCLUSION

Acute salbutamol recovery in methacholine constricted airways was significantly delayed to a similar degree in both genotypes due to cross tolerance induced by FM or SM.

Comparison of combination inhalers vs inhaled corticosteroids alone in moderate persistent asthma

AIMS

Inhalers combining long acting beta2-adrenoceptor agonists (LABA) and corticosteroids (ICS) are indicated at Step 3 of current asthma guidelines. We evaluated the relative effects of LABA + ICS combination vs ICS alone on pulmonary function, bronchoprotection, acute salbutamol recovery following methacholine bronchial challenge, and surrogate inflammatory markers in patients with moderate persistent asthma.

METHODS

Twenty-nine patients with mean FEV1 (+/- SEM) of 78 +/- 3% predicted completed a randomized, double-blind, double-dummy, cross-over study. Patients received either 4 weeks of budesonide 400 microg + formoterol 12 microg (BUD + FM) combination twice daily followed by 1 week of BUD 400 microg alone twice daily, or 4 weeks of fluticasone propionate 250 microg + salmeterol 50 microg (FP + SM) combination twice daily followed by 1 week of FP 250 microg alone twice daily. Measurements were made at baseline and following each randomized treatment.

RESULTS

FEV1 increase from pretreatment baseline as mean (+/- SEM) % predicted was significantly higher (P < 0.05) for BUD + FM (8 +/- 1%) vs BUD (2 +/- 1%), and for FP + SM (8 +/- 1%) vs FP (2 +/- 1%). The fall in FEV1 following methacholine challenge as percentage change from prechallenge baseline FEV1 was not significantly different in all four groups; BUD + FM (22 +/- 1%), BUD (24 +/- 1%), FP + SM (23 +/- 1%) and FP (23 +/- 1%). Salbutamol recovery over 30 min following methacholine challenge as area under curve (AUC %.min) was significantly blunted (P < 0.05) with BUD + FM (486.7 +/- 35.5) vs BUD (281.1 +/- 52.8), and with FP + SM (553.1 +/- 34.1) vs FP (368.3 +/- 46.7). There were no significant differences between respective combination inhalers or between respective ICS alone. Decreases in exhaled nitric oxide (NO) and serum eosinophilic cationic protein (ECP) from baseline were not significantly different between treatments.

CONCLUSIONS

Combination inhalers improve pulmonary function without potentiating anti-inflammatory effects on exhaled NO and serum ECP as compared with ICS alone, but delay acute salbutamol recovery after bronchoconstriction.

Second-line controller therapy for persistent asthma uncontrolled on inhaled corticosteroids: the step 3 dilemma

The asthma syndrome is characterised by airway inflammation with associated bronchial hyperresponsiveness (BHR) and reversible airflow obstruction. Therapy has benefited from an enhanced understanding of the pathophysiology of asthma and the resulting guidelines that emphasise the pivotal role of anti-inflammatory inhaled corticosteroids (ICS) as first-line therapy. Most patients with mild-to-moderate asthma can be adequately controlled on low-to-medium dosages of ICS alone. For patients with moderate-to-severe asthma who are not adequately controlled by ICS, it is unclear which medication should be added on. The two principal drugs under consideration are long-acting beta(2)-agonists (LABAs) and leukotriene antagonists (LTAs). Although both LABAs and LTAs are both effective at improving lung function, reducing symptoms and decreasing exacerbations, important differences exist that may determine the selection of one over the other in particular circumstances. LABAs and LTAs are equally effective at reducing exacerbations and improving symptoms and quality of life when used as add-on therapy. LABAs tend to be more effective bronchodilators than LTAs. Although LABAs stabilise the airway smooth muscle, they do not affect the underlying inflammatory process. Their long-term use also leads to subsensitivity of response to both LABAs and short-acting beta(2)-agonists (SABAs). The subsensitivity of response to SABAs is more pronounced in the presence of acute bronchoconstriction, which could be relevant during an acute attack. When combined with an ICS, LTAs provide additive non-steroidal anti-inflammatory properties and alleviate associated BHR, but do not induce subsensitivity of response. Not only is the efficacy of LTAs maintained over time, but also they do not affect the response to SABAs as reliever therapy. LTAs also have beneficial effects in patients who have concomitant allergic rhinitis, thus treating the unified airway. The choice between LABA and LTA as add-on therapy will therefore be determined by the needs of the individual patient in terms of providing anti-inflammatory versus bronchodilatory control. For patients with poor lung function where bronchodilatation is required, then an LABA would seem to be a logical choice. For the patient whose lung function is less impaired, with evidence of ongoing BHR where bronchoprotection is needed (e.g. exercise, allergen, cold air), or when there is concomitant allergic rhinitis, then an LTA would be more suitable.

Effects of formoterol (Oxis Turbuhaler) and ipratropium on exercise capacity in patients with COPD

Although long-acting inhaled beta 2-agonists improve various outcome measures in COPD, no double-blind study has yet shown a significant effect of these drugs on exercise capacity. In a randomized, double-blind, placebo-controlled, crossover study, patients received formoterol (4, 5, 9, or 18 micrograms b.i.d. via Turbuhaler), ipratropium bromide (80 micrograms t.i.d. via pMDI with spacer), or placebo for 1 week. Main endpoint was time to exhaustion (TTE) in an incremental cycle ergometer test. Secondary endpoints were Borg dyspnoea score during exercise, lung function, and adverse events. Thirty-four patients with COPD were included, mean age 64.8 years, FEV1 55.6% predicted, reversibility 6.1% predicted. All doses of formoterol, and ipratropium significantly improved TTE, FEV1, FEF25-75%, FRC, IVC, RV and sGAW compared with placebo. A negative dose-response relationship was observed with formoterol. Ipratropium increased time to exhaustion more compared with formoterol, 18 micrograms, but not with formoterol, 4.5 and 9 micrograms. No changes in Borg score were found. There was no difference in the adverse event profile between treatments. In conclusion, 1 week of treatment with formoterol and ipratropium significantly improved exercise capacity and lung function compared with placebo. However, a negative dose-response relation for formoterol was unexpected and needs further investigation.

Decreased bronchodilating effect of salbutamol in relieving methacholine induced moderate to severe bronchoconstriction during high dose treatment with long acting β2 agonists

BACKGROUND

In vitro the long acting β2 agonist salmeterol can, in contrast to formoterol, behave as a partial agonist and become a partial antagonist to other β2 agonists. To study this in vivo, the bronchodilating effect of salbutamol was measured during methacholine induced moderate to severe bronchoconstriction in patients receiving maintenance treatment with high dose long acting β2agonists.

METHODS

A randomised double blind crossover study was performed in 19 asthmatic patients with mean forced expiratory volume in one second (FEV1) of 88.4% predicted and median concentration of methacholine provoking a fall in FEV1 of 20% or more (PC20) of 0.62 mg/ml at entry. One hour after the last dose of 2 weeks of treatment with formoterol (24 μg twice daily by Turbuhaler), salmeterol (100 μg twice daily by Diskhaler), or placebo a methacholine provocation test was performed and continued until there was at least a 30% decrease in FEV1. Salbutamol (50 μg) was administered immediately thereafter, followed by ipratropium bromide (40 μg) after a further 30 minutes. Lung function was monitored for 1 hour after provocation.

RESULTS

There was a significant bronchodilating and bronchoprotective effect after 2 weeks of active treatment. The dose of methacholine needed to provoke a fall in FEV1 of ⩾30% was higher after pretreatment with formoterol (2.48 mg) than with salmeterol (1.58 mg) or placebo (0.74 mg). The difference between formoterol and salmeterol was statistically significant: 0.7 doubling dose steps (95% CI 0.1 to 1.2, p=0.016). The immediate bronchodilating effect of subsequently administered salbutamol was significantly impaired after pretreatment with both drugs (p<0.0003 for both). Three minutes after inhaling salbutamol the increase in FEV1 relative to the pre-methacholine baseline was 15.8%, 7.3%, and 5.5% for placebo, formoterol and salmeterol, respectively (equivalent to increases of 26%, 14%, and 12%, respectively, from the lowest FEV1after methacholine). At 30 minutes significant differences remained, but 1 hour after completing the methacholine challenge FEV1had returned to baseline values in all three treatment groups.

CONCLUSION

Formoterol has a greater intrinsic activity than salmeterol as a bronchoprotective agent, indicating that salmeterol is a partial agonist compared with formoterol in contracted human airways in vivo. Irrespective of this, prior long term treatment with both long acting β2agonists reduced the bronchodilating effect of an additional single dose of salbutamol equally, indicating that the development of tolerance or high receptor occupancy overshadowed any possible partial antagonistic activity of salmeterol. Patients on regular treatment with long acting β2 agonists should be made aware that an additional single dose of a short acting β2 agonist may become less effective.

Decreased bronchodilating effect of salbutamol in relieving methacholine induced moderate to severe bronchoconstriction during high dose treatment with long acting beta2 agonists

BACKGROUND

In vitro the long acting beta2 agonist salmeterol can, in contrast to formoterol, behave as a partial agonist and become a partial antagonist to other beta2 agonists. To study this in vivo, the bronchodilating effect of salbutamol was measured during methacholine induced moderate to severe bronchoconstriction in patients receiving maintenance treatment with high dose long acting beta2 agonists.

METHODS

A randomised double blind crossover study was performed in 19 asthmatic patients with mean forced expiratory volume in one second (FEV1) of 88.4% predicted and median concentration of methacholine provoking a fall in FEV1 of 20% or more (PC(20)) of 0.62 mg/ml at entry. One hour after the last dose of 2 weeks of treatment with formoterol (24 microg twice daily by Turbuhaler), salmeterol (100 microg twice daily by Diskhaler), or placebo a methacholine provocation test was performed and continued until there was at least a 30% decrease in FEV1. Salbutamol (50 microg) was administered immediately thereafter, followed by ipratropium bromide (40 microg) after a further 30 minutes. Lung function was monitored for 1 hour after provocation.

RESULTS

There was a significant bronchodilating and bronchoprotective effect after 2 weeks of active treatment. The dose of methacholine needed to provoke a fall in FEV1 of > or = 30% was higher after pretreatment with formoterol (2.48 mg) than with salmeterol (1.58 mg) or placebo (0.74 mg). The difference between formoterol and salmeterol was statistically significant: 0.7 doubling dose steps (95% CI 0.1 to 1.2, p=0.016). The immediate bronchodilating effect of subsequently administered salbutamol was significantly impaired after pretreatment with both drugs (p<0.0003 for both). Three minutes after inhaling salbutamol the increase in FEV1 relative to the pre-methacholine baseline was 15.8%, 7.3%, and 5.5% for placebo, formoterol and salmeterol, respectively (equivalent to increases of 26%, 14%, and 12%, respectively, from the lowest FEV1 after methacholine). At 30 minutes significant differences remained, but 1 hour after completing the methacholine challenge FEV1 had returned to baseline values in all three treatment groups.

CONCLUSION

Formoterol has a greater intrinsic activity than salmeterol as a bronchoprotective agent, indicating that salmeterol is a partial agonist compared with formoterol in contracted human airways in vivo. Irrespective of this, prior long term treatment with both long acting beta2 agonists reduced the bronchodilating effect of an additional single dose of salbutamol equally, indicating that the development of tolerance or high receptor occupancy overshadowed any possible partial antagonistic activity of salmeterol. Patients on regular treatment with long acting beta2 agonists should be made aware that an additional single dose of a short acting beta2 agonist may become less effective.

Formoterol Fumarate Inhalation Powder

Each month, subscribers to The Formulary® Monograph Service receive five to six researched monographs on drugs that are newly released or are in late Phase III trials. The monographs are targeted to your Pharmacy and Therapeutics Committee. Subscribers also receive monthly one-page summary monographs on the agents that are useful for agendas and pharmacy/nursing in-services. A comprehensive target drug utilization evaluation (DUE) is also provided each month. The monographs are published in printed form and on diskettes that allow customization. Subscribers to the The Formulary Monograph Service also receive access to a pharmacy bulletin board called The Formulary Information Exchange (The F.I.X). All topics pertinent to clinical pharmacy are discussed on The F.I.X.

Through the cooperation of The Formulary, Hospital Pharmacy publishes selected reviews in this column. If you would like information about The Formulary Monograph Service or The F.I.X., call The Formulary at 800-322-4349. The July 2001 Formulary monographs are on ima-tinib mesylate, alemtuzumab, parecoxib sodium, pramlintide acetate, and busesonide modified-release capsules. The DUE is on galantamine.

In vivo effect of albuterol on methacholine-contracted bronchi in conjunction with salmeterol and formoterol

BACKGROUND

It has been shown in vitro that prior treatment with salmeterol and formoterol antagonizes the relaxant effect of albuterol in carbachol-contracted human bronchi.

OBJECTIVES

The primary aim of this study was to evaluate whether there is a potential in vivo interaction between long- and short-acting beta2-agonists in the presence of increased airway tone induced by methacholine. In addition, a post hoc analysis was made to evaluate the effects of beta2-adrenoceptor polymorphisms.

METHODS

Sixteen asthmatic subjects (mean age [+/-SD], 39 [13] years; FEV1, 81% [17%] of predicted value), all taking inhaled corticosteroids and having methacholine PD20 values of less than 500 micrograms, were randomized in double-blind, double-dummy, cross-over fashion to receive single doses of inhaled placebo, inhaled formoterol 12 micrograms, or inhaled salmeterol 50 micrograms followed 12 hours later by a single dose of inhaled albuterol 400 micrograms (low dose) or 1600 micrograms (high dose). Methacholine challenges were performed on each of 6 separate occasions 1 hour after albuterol.

RESULTS

There was a greater numerical difference in geometric mean PD20 values between low- and high-dose albuterol after placebo dosing (671 micrograms vs 1080 micrograms, a 1.61-fold difference; P <.05) compared with low- and high-dose albuterol after formoterol dosing (660 micrograms vs 799 micrograms, a 1. 21-fold difference; P =.4), or after salmeterol dosing (568 micrograms vs 847 micrograms, a 1.49-fold difference; P =.055). PD20 values with high-dose albuterol in combination with formoterol or salmeterol were numerically lower than those found with high-dose albuterol in combination with placebo, but they were not significantly different. There was a significant difference between PD20 values with low-dose albuterol after dosing with formoterol (PD20 = 660 micrograms, a 1. 6-fold difference; P <.05) or with salmeterol (PD20 = 568 micrograms, a 1.9-fold difference; P <.05) compared with PD20 with high-dose albuterol after placebo dosing (PD20 = 1080 micrograms). Post hoc polymorphism analysis for pooled pretreatment with formoterol and salmeterol (excluding placebo pretreatment) showed significantly (P <.05) lower PD20 values with homozygous glycine-16 compared with heterozygous glycine/arginine-16 and significantly (P <.05) lower PD20 values with homozygous glutamate-27 compared with either heterozygous glutamate/glutamine-27 or homozygous glutamine-27.

CONCLUSION

Compared with placebo, both salmeterol and formoterol caused a significant degree of antagonism of albuterol-induced bronchorelaxation in methacholine-contracted bronchi in vivo. This interaction could be caused by prolonged occupancy of airway beta2-adrenoceptors by long-acting beta2-agonists or by early tachyphylaxis 12 hours after a single-dose exposure. The degree of albuterol protection was also related to beta2-adrenoceptor polymorphism.

Long-acting inhaled beta2-agonists in asthma therapy

OBJECTIVE

To review the pharmacology of the long-acting inhaled beta2-agonists, salmeterol and formoterol, summarize results of their clinical trials, evaluate their safety records, and discuss their roles in the treatment of asthma.

DATA SOURCES

Preclinical and clinical studies involving salmeterol or formoterol were identified by a MEDLINE search, weekly computerized literature updates, and manual searches. Studies of satisfactory quality were chosen for review.

DATA SYNTHESIS

Salmeterol and formoterol are potent and selective beta2-adrenoceptor agonists with durations of action >12 h. Their major differences are that formoterol has a rapid onset of action and is a partial agonist of high intrinsic efficacy, whereas salmeterol has a delayed onset and is a partial agonist of low intrinsic efficacy. Twice daily use of either drug results in improved lung function, reduced symptoms, and a better quality of life. These agents protect against exercise-induced asthma for 12 h and eliminate nighttime awakening in most patients. Limited tolerance develops, especially to their bronchoprotective effects, but their improvement of lung function is sustained.

CONCLUSIONS

Regular use of salmeterol or formoterol provides subjective and objective amelioration of asthma in patients experiencing excessive symptoms or physiologic impairment despite the regular administration of low doses of inhaled corticosteroids (equivalent to approximately 500 microg/d of beclomethasone). Intermittent use of either long-acting beta2-agonist can provide prolonged protection against exercise-induced asthma or nighttime symptoms. Patients should be instructed to continue taking inhaled steroids when long-acting beta2-agonists are administered on a regular schedule and to not take long-acting beta2-agonists between regularly scheduled doses. Used properly, they are effective and safe adjunctive agents in the treatment of asthma.

Formoterol. An update of its pharmacological properties and therapeutic efficacy in the management of asthma

Formoterol, a selective beta 2-adrenoceptor agonist, produces effective dose-proportional bronchodilation, which persists for up to 12 hours, in patients with reversible obstructive respiratory disease. Bronchodilation is significant within minutes of inhalation, maximal within 2 hours, and at therapeutic doses is equivalent to that produced by standard doses of traditional beta 2-agonists. In single-dose studies comparing the two long-acting beta 2-agonists formoterol and salmeterol, significant bronchodilation is achieved more rapidly with formoterol than salmeterol. Duration of bronchodilation is similar with both drugs. The therapeutic efficacy of inhaled formoterol has been equal to or greater than that of salbutamol (albuterol), fenoterol and terbutaline in both short and long term clinical trials. Formoterol reduces symptoms of nocturnal asthma and reduces the need for rescue medication compared with salbutamol. Recent studies have shown that the addition of inhaled formoterol 12 or 24 micrograms twice daily to existing inhaled corticosteroid regimens improves lung function and reduces asthma symptoms compared with placebo. In one well designed study, the frequency of severe exacerbations of asthma over 12 months was decreased by adding formoterol to existing regimens of inhaled corticosteroids. Tolerance to the bronchodilator response of formoterol has not been observed in long term clinical trials. Because of its long duration of action, formoterol offers significant therapeutic advantages over shorter-acting beta 2-agonists in the treatment of nocturnal and exercise-induced asthma. Formoterol is effective in preventing exercise-induced asthma in adults and children and confers significantly more protection than salbutamol when administered 3 and 12 hours before exercise. In general, inhaled formoterol is well tolerated. The most commonly reported adverse effects, tremor and palpitations, are those traditionally associated with the use of beta 2-agonists. Oral formoterol and high doses of inhaled formoterol are associated with more adverse events than are the recommended doses of 6 to 24 micrograms. Formoterol is currently recommended for use as an alternative to increasing inhaled steroid dosage in patients whose symptoms are inadequately controlled despite therapy with low to moderate doses of inhaled steroids and intermittent short-acting beta 2-agonists, and results of recent studies support therapeutic guidelines. Long term clinical studies comparing formoterol and salmeterol have not yet been published. Further studies to evaluate the earlier use of formoterol in patients with mild to moderate asthma are needed to determine the role and long term safety of formoterol in the management of asthma.

Dose-response of inhaled drugs in asthma. An update

The demographic characteristics of patients used in clinical trials (such as the severity of airway obstruction) can significantly influence the results of dose-response studies, emphasising the need to evaluate effects on the steep part of the dose-response curve. Differences in inhaler devices can also influence study outcomes, as for inhaled drugs both airway efficacy and adverse effect profiles are primarily determined by lung deposition and hence bioavailability. Dose-response studies with short- and long-acting beta 2-agonists show an excellent therapeutic ratio at conventional doses used in everyday clinical practice (i.e. 2 to 4 puffs). Dose-related systemic effects of beta 2-agonist occur at higher doses, for salbutamol (albuterol) > 500 micrograms. Fenoterol is a beta 2-agonists with higher intrinsic activity than salbutamol and produces greater systemic effects at higher than conventional doses on a microgram equivalent basis, although even at 4000 micrograms such differences are unlikely to be clinically relevant. No differences between fenoterol and salbutamol have been shown in terms of bronchodilator potency on a microgram equivalent basis. The long-acting beta 2-agonist salmeterol, as a partial agonist, has the potential to attenuate the acute bronchodilator response to a higher activity beta 2-agonist such as salbutamol or fenoterol, although there is no evidence to date on whether this is relevant in the setting of acute asthma. When comparing inhaled corticosteroids, attention should be focused on their respective risk-benefit ratios for antiasthmatic versus systemic activity. In terms of detecting systemic activity, it is important to use sensitive measures, such as urinary cortisol excretion, rather than insensitive parameters, such as a single morning plasma cortisol measurement between 0800h and 1000h. For fluticasone, a greater in vitro potency results in only marginal differences in antiasthmatic efficacy, particularly on the flatter part of the dose-response curve above 1000 micrograms/day in adults and 400 micrograms/day in children. However, the same enhanced potency translates directly into commensurate differences in systemic adverse effects on the steep part of the systemic dose-response curve above 1000 micrograms/day in adults and 400 micrograms/day in children, respectively. Furthermore, with repeated twice-daily administration, a longer elimination half-life and prolonged systemic tissue retention due to enhanced lipophilicity will result in greater systemic activity observed at steady-state in long term administration studies. This dissociation of airway and systemic dose-response curves results in a J-shaped curve for benefit: risk ratio, with a watershed area above 1000 microgram/day in adults. This fall in the benefit: risk ratio is likely to be greater for fluticasone than for budesonide or beclomethasone. Further studies are needed to clearly define the dose-response relationships of higher potency steroids such as fluticasone, particularly on the steep part of the curve (for clinical efficacy), using the appropriate back-titration design along with sensitive measures of antiasthmatic and systemic activity.

Evaluation of partial beta-adrenoceptor agonist activity

A partial beta-adrenoceptor (beta-AR) agonist will exhibit opposite agonist and antagonist activity depending on the prevailing degree of adrenergic tone or the presence of a beta-AR agonist with higher intrinsic activity. In vivo partial beta-AR agonist activity will be evident at rest with low endogenous adrenergic tone, as for example with chronotropicity (beta 1/beta 2), inotropicity (beta 1) or peripheral vasodilatation and finger tremor (beta 2). beta-AR blocking drugs which have partial agonist activity may exhibit a better therapeutic profile when used for hypertension because of maintained cardiac output without increased systemic vascular resistance, along with an improved lipid profile. In the presence of raised endogenous adrenergic tone such as exercise or an exogenous full agonist, beta-AR subtype antagonist activity will become evident in terms of effects on exercise induced heart rate (beta 1) and potassium (beta 2) responses. Reduction of exercise heart rate will occur to a lesser degree in the case of a beta-adrenoceptor blocker with partial beta 1-AR agonist activity compared with a beta-adrenoceptor blocker devoid of partial agonist activity. This may result in reduced therapeutic efficacy in the treatment of angina on effort when using beta-AR blocking drugs with partial beta 1-AR agonist activity. Effects on exercise hyperkalaemia are determined by the balance between beta 2-AR partial agonist activity and endogenous adrenergic activity. For predominantly beta 2-AR agonist such as salmeterol and salbutamol, potentiation of exercise hyperkalaemia occurs. For predominantly beta 2-AR antagonists such as carteolol, either potentiation or attenuation of exercise hyperkalaemia occurs at low and high doses respectively. beta 2-AR partial agonist activity may also be expressed as antagonism in the presence of an exogenous full agonist, as for example attenuation of fenoterol induced responses by salmeterol. Studies are required to investigate whether this phenomenon is relevant in the setting of acute severe asthma.

Effects of prior treatment with salmeterol and formoterol on airway and systemic beta 2 responses to fenoterol

BACKGROUND

Previous studies have shown that both salmeterol and formoterol act as partial beta 2 receptor agonists in terms of antagonising the extrapulmonary responses to fenoterol in normal subjects. The aim of the present study was to extend previous observations in evaluating the effect of prior treatment with salmeterol and formoterol on bronchodilator responses to fenoterol, a full beta 2 receptor agonist, in patients with asthma.

METHODS

Ten stable asthmatic patients of mean (SE) age 37 (3.7) years and forced expiratory volume in one second (FEV1) 59.5 (4.1)% of predicted completed the study. One hour after inhaling single doses of placebo, salmeterol 25 micrograms, or formoterol 12 micrograms, dose-response curves to repeated doses of inhaled fenoterol were constructed (cumulative doses of 100-3200 micrograms). Measurements of airway and systemic beta 2 receptor mediated responses were made at baseline, after inhalation of placebo, salmeterol, or formoterol, and after each dose of fenoterol.

RESULTS

Salmeterol and formoterol produced significant bronchodilation compared with placebo (mean difference and 95% CI compared with placebo): FEV1, salmeterol 0.41 (95% CI 0.13 to 0.69) 1, formoterol 0.47 (95% CI 0.19 to 0.75) 1. Salmeterol and formoterol had no significant effect on systemic responses compared with placebo. There were no significant differences in peak airway responses to fenoterol after treatment with salmeterol or formoterol compared with placebo (mean (pooled SE)): FEV1, placebo 2.84 (0.03) 1, salmeterol 2.87 (0.03) 1, and formoterol 2.88 (0.03) 1. There were no significant differences in the area under the dose-response curve for any of the parameters during the dose-response curve following treatment with salmeterol or formoterol compared with placebo. There was no difference in the slope of the dose-response curves to fenoterol for FEV1 or forced expiratory flow (FEF25-75) after treatment with salmeterol or formoterol compared with placebo, although there was a significant (p < 0.05) attenuation of the slope in the dose-response curve for the peak expiratory flow rate (PEFR).

CONCLUSIONS

Prior treatment with low doses of salmeterol or formoterol does not significantly alter bronchodilator dose-response curves to repeated doses of fenoterol in stable asthmatic patients.