Tolerance to the bronchoprotective effect of beta2-agonists: comparison of the enantiomers of salbutamol with racemic salbutamol and placebo

Past, present and future uses of methacholine testing

Methacholine challenge testing is a valuable diagnostic and research tool used by clinicians to assist in the diagnosis of asthma, and by researchers to understand disease pathophysiology and assess novel therapeutic efficacy. The use of methacholine challenge in asthma relates to its direct effect on airway smooth muscle (i.e., bronchoconstriction) as a measure of airway hyperresponsiveness, a cardinal feature of asthma. Airway hyperresponsiveness has been documented in other airway disorders, including chronic obstructive pulmonary disease, cystic fibrosis and allergic rhinitis; however, there is little clinical application of methacholine challenge in these conditions as a diagnostic or disease management tool. The authors will review the aspects of methacholine challenge testing, as they relate to asthma, and point out its usefulness in clinical research. A brief review of past (historical) uses and speculation as to the future uses of methacholine challenge will also be discussed.

Levalbuterol versus albuterol

Albuterol has been used for more than 40 years to treat acute asthma exacerbations as a racemic mixture of isomers: the active form, (R)-albuterol, or levalbuterol, and (S)-albuterol, classically considered inert. The single-isomer formulation, levalbuterol, has been synthesized recently and used therapeutically when the racemate is deemed less desirable. Basic investigations indicate that racemic albuterol and levalbuterol can produce effects that favor asthma remediation, including corticosteroid amplification and reduction of inflammatory mediators; in contrast, (S)-albuterol produces opposite effects. With inhalation of racemic albuterol, circulating (S)-albuterol persists 12 times longer than levalbuterol, suggesting potential for paradoxical effects observed clinically. Although mainly consistent with basic findings, clinical studies suggest no overwhelming superiority of levalbuterol over racemic albuterol; however, levalbuterol's effects may be greatest in moderate to severe asthma patients, especially with racemic albuterol overuse. Recent adoption of the hydrofluoroalkane formulation has narrowed the cost gap between levalbuterol and racemic albuterol metered-dose inhalers, but it remains for the nebulized formulations. Thus, physician selection of these drugs has remained dependent on experience, pharmaceutical knowledge, and established prescribing habits combined with cost factors, formulary structures, and availability, such that racemic albuterol is still used significantly compared with levalbuterol to treat acute asthma exacerbations.

Three generations of ongoing controversies concerning the use of short acting beta-agonist therapy in asthma: a review

An increase in asthma mortality in 1960s noted by British authors stirred a debate about the use of beta-adrenergic therapy that has persisted in the medical literature. The cause appears to be isoproterenol and fenoterol overuse. A second debate evolved around the possible deleterious, pro-inflammatory effects, of the albuterol distomer. Most clinical studies showed improved bronchodilatation, but limited benefits from using levalbuterol. Recently, genotyping has uncovered a single nucleotide polymorphism at codon 16 that appears to affect the long term response to both regular and as needed use of albuterol, calling for a new genotype based therapeutic approach in asthma.

Role of arformoterol in the management of COPD

Formoterol is a beta2-agonist that has both short and long acting bronchodilator effects. Beta2-agonists used as bronchodilators have been synthesized as racemates that comprise (R,R) and (S,S)-enantiomers. Compounds that are beta2-selective derive their bronchodilator effect from an interaction between the (R,R)-enantiomer and the beta2-adrenoceptor. Arformoterol is the (R,R)-enantiomer and is distinguished from the more commonly used racemic (RR/S,S)-diasteriomer of formoterol. Overall literature on the use of arformoterol in COPD is very preliminary. There is some in vitro data that demonstrate significant bronchodilation and inhibition of inflammation with arformoterol, and these effects may be more pronounced than those caused by racemic formoterol. There are limited clinical trial data that demonstrate that arformoterol produces significant improvement in lung function in COPD; however, many of the subjects involved had marked baseline airway reversibility. Arformoterol has been very well tolerated in clinical trials and could potentially be used only once every 24 hours (due to its prolonged effect). It can only be given in nebulized form. Arformoterol can potentially be given with other inhaled medications.

Comparison of the dose response to levalbuterol with and without pretreatment with S-albuterol after methacholine-induced bronchoconstriction

STUDY OBJECTIVE

To determine the effect of S-albuterol on the dose response to levalbuterol in patients with moderate bronchoconstriction induced by a methacholine challenge.

DESIGN

Prospective, randomized, double-blind, placebo-controlled, crossover study.

SETTING

University-affiliated clinical trial center.

PATIENTS

Twenty-two adults with mild, stable asthma.

INTERVENTION

At the screening visit, patients were switched from their beta2-agonist to ipratropium bromide for use as an as-needed rescue therapy. At the baseline visit 2-6 days later, the provocative concentration of methacholine to induce a 30% decrease in forced expiratory volume in 1 second (FEV(1) PC(30)) was determined, followed by a nebulized racemic albuterol dose-response study with three doses of albuterol, to familiarize patients with the procedures. At visits 2 and 3, patients were randomly assigned to receive nebulized normal saline placebo or S-albuterol 5 mg before the methacholine challenge and were administered three escalating doses of levalbuterol after the challenge.

MEASUREMENTS AND MAIN RESULTS

Area under the curve for FEV(1) over 40 minutes (AUC(0-40)) after administration of levalbuterol was the primary outcome, with slope of FEV(1) as the secondary outcome. In addition, the fraction of exhaled nitric oxide (FeNO) was measured before and after the challenges. In the 17 patients who met criteria for completion, no deleterious effect for S-albuterol was found for FEV(1) PC(30), AUC(0-40) FEV(1), or the FEV(1) slope(0-40). However, S-albuterol reduced the provocative concentration of methacholine to induce a 20% decrease in FEV(1) (PC(20) 0.52 +/- 2.06 vs 0.39 +/- 1.58 mg/ml, placebo vs S-albuterol, p=0.044) but did not affect FeNO.

CONCLUSION

A single high dose of S-albuterol did not alter the bronchodilator response to levalbuterol. The effect on bronchial responsiveness requires further study.

Levalbuterol for asthma: a better treatment?

The purpose of this review is to determine whether the proinflammatory actions identified in vitro for (S)-albuterol provide a clinically significant therapeutic advantage for levalbuterol over racemic albuterol. Clinical trials evaluating the bronchodilation in chronic and acute asthma provide conflicting evidence. Older trials suggested an advantage for levalbuterol; however, the newer trials have failed to confirm those advantages. Although (S)-albuterol produces increased bronchial hyperresponsiveness in vitro and in animal models, this has not been consistently confirmed in clinical trials; however, the heterogeneity of the trials precludes definitive conclusions. Current clinical trials do not provide evidence of a substantial advantage of levalbuterol over racemic albuterol although the data are insufficient to determine whether subsets of the patient population might benefit from single isomer therapy.

Risk versus benefit considerations for the beta(2)-agonists

Short-acting beta(2)-agonists are the mainstay of therapy for acute bronchospasm associated with asthma and chronic obstructive pulmonary disease, whereas long-acting beta(2)-agonists are used in maintaining disease control in these respiratory disorders. This review describes and compares the pharmacology of the beta(2)-agonists and explains how these differences translate into differences in efficacy and beta(2)-adrenergic-mediated adverse effects. Questions commonly asked by clinicians regarding the efficacy and safety of short- and long-acting beta(2)-agonists include issues about cardiovascular effects, tolerance to their bronchodilator and bronchoprotective effects, blunting of albuterol response by long-acting beta(2)-agonists, potential masking of worsening asthma control, and the role of long-acting beta(2)-agonists as adjunctive therapy with inhaled corticosteroids in maintaining asthma control. Pharmacogenetics may play a role in determining which patients may be at risk for a reduced response to a beta(2)-agonist. The continued use of racemic albuterol, which contains a mixture of R-albuterol and S-albuterol, has been questioned because of data from preclinical and clinical studies suggesting that S-albuterol causes proinflammatory effects and may increase bronchial hyperreactivity. The preclinical and clinical effects of these two stereoisomers are reviewed. Data describing the efficacy and safety of levalbuterol (R-albuterol) and racemic albuterol are presented.

(R)-albuterol for asthma: pro [a.k.a. (S)-albuterol for asthma: con]

Is there scientific evidence to support the replacement of the beta-agonist racemic albuterol with levalbuterol--that is, (R)-albuterol? The argument presented further refines the question as "Do we wish to continue to treat asthma with a mixture of albuterol, of which half is an agent with no known benefit--that is, (S)-albuterol--and which may exacerbate the disease?"

Levalbuterol versus racemic albuterol in the treatment of acute exacerbation of asthma in children

OBJECTIVE

To compare levalbuterol and racemic albuterol for the treatment of acute exacerbation of asthma in pediatric population.

DESIGN

Prospective, double-blind, randomized research trial in a pediatric emergency department of an urban tertiary care hospital.

PARTICIPANTS

Children 5 to 21 years with a history of asthma presenting to the emergency department in acute exacerbation.

INTERVENTIONS

As per a computer-generated randomization sequence, patients received either 1.25 mg of levalbuterol or albuterol 2.5 mg via nebulization along with ipratropium hydrochloride. Patients received 3 back-to-back treatments as needed every 20 minutes, maximum of 3; 2 mg/kg of oral prednisone was administered to the patients after the second treatment. Baseline respiratory parameters such as oxygen saturations, respiratory rates, and peak flow rates were measured and repeated after every treatment.The decision for further treatments and or hospitalization was made by the treating emergency department physician as per his/her clinical judgement of the respiratory parameters at the end of 3 treatments.

RESULTS

Seventy patients completed the study. Most of the patients were in moderate severity of asthma exacerbation. All patients in both groups showed improvement in oxygen saturations, respiratory rates, and peak flow rates. However, no statistically significant difference was observed in the 2 groups regarding the respiratory parameters (P > 0.05).

CONCLUSION

Levalbuterol is not more efficacious than racemic albuterol in improving respiratory parameters in children presenting with acute exacerbation of asthma.

Beta2-agonist eutomers: a rational option for the treatment of asthma?

Beta2-adrenoceptor agonists (beta2-agonists) such as albuterol (salbutamol) and terbutaline and their long-acting analogs salmeterol and formoterol are widely used as bronchodilators in the treatment of asthma. They are chiral drugs historically marketed as racemic mixtures of an active (eutomer) and essentially inactive (distomer) stereoisomer. Despite their obvious therapeutic value and widespread use, beta2-agonists have been implicated, somewhat controversially, in causing an increase in asthma mortality and a deterioration of asthma control by a mechanism that remains elusive. Inherent toxicity of the distomers has been widely touted as an explanation and has given rise to pressure for the replacement of the racemates with pure eutomer formulations (the so-called chiral or racemic switch). This has culminated in the recent introduction into clinical practice of the single active stereoisomer of albuterol (levalbuterol) and the promise of other pure beta2-agonist eutomer formulations to follow. This article examines the evidence on which these chiral switches are based. Clinical studies designed to reveal negative effects of beta2-agonists have searched for reductions in lung function, increases in airway responsiveness to bronchoconstrictor mediators and worsening of asthma control. Crossover studies administering the pure stereoisomers and racemate of albuterol have not shown a clear superiority of the pure eutomer formulation over the racemate in terms of either bronchial hyperresponsiveness, tachyphylaxis to bronchoprotective effects or improvements in lung function. Clinical toxicity of beta2-agonist distomers on any aspect of asthmatic lung function has also not been demonstrated in the relatively short-term inhalational studies (single dose or repeated dose studies <1 week) that have been carried out. In animal studies, the administration of beta2-agonist racemates and distomers has been shown to enhance bronchial hyperresponsiveness but only in ovalbumin-sensitized animals where the relevance to humans is questionable. The pharmacokinetics and metabolism of beta(2)-agonist stereoisomers appear to be essentially similar whether administered as single stereoisomers or as racemates. Levalbuterol may be slightly more potent than an equivalent dose given as racemate, but there is some evidence that it forms a small amount of the distomer in vivo which detracts somewhat from its purported benefits over use of the racemate. Whilst there remains a clear need for studies of longer duration with sensitive clinical endpoints to evaluate the benefits of beta2-agonist eutomers and to investigate distomer toxicity, the chiral switch for beta2-agonists in general, and for albuterol in particular, does not appear to be justified on the basis of the evidence available to date.

What is new with the beta2-agonists: issues in the management of asthma

OBJECTIVE

To review the more recent literature addressing the issue of whether beta2-agonists can worsen asthma and/or increase the risk of severe exacerbations and death from asthma.

DATA SOURCES

PubMed was searched (2001-December 2004), along with the Food and Drug Administration and Cochrane Library Web sites. In addition, the bibliographies of recent reviews of the subject were assessed.

STUDY SELECTION AND DATA EXTRACTION

Randomized clinical trials, retrospective and prospective cohort studies, and meta-analyses published in the past 3 years were reviewed. Studies assessing the potential for beta2-agonists to worsen outcomes in asthma as well as long-term studies assessing asthma outcomes that included an arm with regular administration of short- or long-acting inhaled beta2-agonists (LABAs) were selected. Worsening asthma was defined as a decline in lung function, an increase in bronchial hyperresponsiveness, exacerbations, or death. Studies older than 3 years selected from the bibliographies of the primary articles that addressed background perspective were also included where appropriate.

DATA SYNTHESIS

The studies fell into 3 primary categories with some overlap: those assessing toxicity of the S-enantiomer of albuterol, those evaluating the risk of specific genotypes regarding worsening asthma, and those assessing asthma outcomes with LABA therapy.

CONCLUSIONS

The current data on the use of beta2-agonists continue to support the national and international guidelines for the treatment of asthma. That is, short-acting inhaled beta2-agonists should only be used as needed for symptoms and prevention of exercise-induced bronchospasm, and LABAs should only be used regularly as adjunctive therapy with inhaled corticosteroids in patients whose asthma is not controlled with low to medium doses of the inhaled corticosteroid.

Comparison of racemic albuterol and levalbuterol for treatment of acute asthma

OBJECTIVE

To determine whether levalbuterol resulted in fewer hospital admissions than racemic albuterol when used for treatment of acute asthma. Study design A randomized, double-blind, controlled trial was conducted in the emergency department (ED) and inpatient asthma care unit of an urban tertiary children's hospital. Children age 1 to 18 years (n=482) provided a total of 547 enrollments. Patients received a nebulized solution of either 2.5 mg racemic albuterol or 1.25 mg levalbuterol every 20 minutes (maximum six doses). Patients admitted to the asthma care unit were treated in a standardized fashion by using the same blinded drug assigned in the ED. Hospitalization rate was the primary outcome.

RESULTS

Hospitalization rate was significantly lower in the levalbuterol group (36%) than in the racemic albuterol group (45 %, P=.02). The adjusted relative risk of admission in the racemic group compared with the levalbuterol group was 1.25 (95% confidence interval, 1.01-1.57). Hospital length of stay was not significantly shorter in the levalbuterol group (levalbuterol, 44.9 hours; racemic albuterol, 50.3 hours; P=.63). No significant adverse events occurred in either group.

CONCLUSIONS

Substituting levalbuterol for racemic albuterol in the ED management of acute asthma significantly reduced the number of hospitalizations.

Management of the acute exacerbation of asthma

All patients with asthma are at risk of having asthma exacerbations characterized by worsening symptoms, airflow obstruction, and an increased requirement for rescue bronchodilators. The goals of managing an asthma exacerbation are prompt recognition and rapid reversal of airflow obstruction to avert relapses and future episodes. Short-acting beta-agonists, oxygen, and corticosteroids form the basis of management of acute asthma exacerbation, but a role is emerging for anticholinergics and newer agents such as levalbuterol and formoterol. Initiation or intensification of long-term controller therapy, treatment of comorbid conditions, avoidance of likely triggers, and timely follow-up care prevent setbacks. Acceptance of current treatment guidelines by physicians and adherence to the recommended clinical regimens by patients are essential for effective management of asthma. The physician should strive to establish a constructive relationship with the patient by addressing the patient's concerns, reaching agreement on the goals of therapy, and developing a written action plan for patient self-management.

Managing outpatient asthma exacerbations

Asthma is a chronic inflammatory disease that renders individuals prone to acute exacerbations. Several allergic and nonallergic triggers can incite an asthma exacerbation. The goals of managing an asthma exacerbation are prompt recognition, rapid reversal of airflow obstruction, prevention of relapses, and forestalling future episodes. A written asthma home-management plan is essential to minimize the severity of exacerbations. Short-acting b-agonists, oxygen, and corticosteroids are fundamental to early intervention in acute asthma exacerbation. Anticholinergics and magnesium sulfate can help nonresponders. Newer agents such as levalbuterol and long-acting b-agonists might be future additions to our armamentarium of drugs to treat acute exacerbations. Initiation or intensification of long-term controller therapy, treatment of co-morbid conditions, and avoidance of possible triggers along with prompt follow-up can help prevent relapses. Listening to patient preferences and concerns to enhance adherence and regular follow-up care can help prevent future episodes.

Preserved bronchial dilatation after salbutamol does not guarantee protection against bronchial hyperresponsiveness

Racemic salbutamol, a beta2-adrenoceptor agonist used for dilatation of airways, has recently been shown to induce lessened relaxation of bronchial smooth muscle and partial loss of bronchoprotection, seen as increased hyperresponsiveness, after regular treatment. The racemate undergoes stereo-selective disposition, giving higher plasma levels of S-salbutamol than that of bronchodilating R-salbutamol, thus raising S : R ratios after repeated administration. Our aim was to evaluate whether increased bronchial hyperresponsiveness (BHR) could be found even after 1 day of repeated salbutamol inhalations, with beta2-receptor-induced bronchial smooth muscle relaxation remaining and whether this would be associated with plasma levels of either enantiomer. Fifteen patients with stable asthma, aged 19-54 years, were included in a randomized, cross-over study. An indirect bronchial challenge method was used [voluntary isocapnic hyperventilation of cold air (IHCA)], and airway condition tested by means of impulse oscillometry. Racemic salbutamol was inhaled three times during a 6-h period. IHCA was performed and plasma concentrations of enantiomers were measured 4 h after the last dose. Tests were also performed without preceding drug treatment. beta2-Agonist-produced bronchial dilatation and protection persisted in the majority of the 15 patients 4 h after repeated inhalations of salbutamol during 1 day. In only two of the 15 patients we could trace increased BHR after salbutamol. Neither dilatation nor protection could be linked to plasma levels of either R- or S-salbutamol. The underlying mechanisms of BHR remain unknown and are dissociated from beta2-receptor-mediated dilatation.

Modulation of T-cell function by (R)- and (S)-isomers of albuterol: anti-inflammatory influences of (R)-isomers are negated in the presence of the (S)-isomer

BACKGROUND

beta(2)-Adrenergic agonists interact with specific receptors on T lymphocytes to mediate anti-inflammatory activities. However, anti-inflammatory effects are not observed when beta(2)-adrenergic agonists are administered in vivo as racemates.

OBJECTIVE

We hypothesized that anti-inflammatory influences are mediated by the (R)-isomer and are masked in the additional presence of the (S)-isomer.

METHODS

Antigen-specific T-cell lines were generated in the presence of recombinant human IL-2 and tetanus with or without varying concentrations of (R)- and (S)-isomers of albuterol alone or in combination. Parallel lines were generated in the presence of propranolol. Cells were briefly pulsed with PHA and evaluated for proliferation, apoptosis, and cytokine secretion.

RESULTS

(R)-Albuterol significantly inhibited T-cell proliferation (77.0% +/- 9.7% of control at 10(-8) mol/L and 61.1% +/- 9.0% at 10(-7) mol/L). No influence was observed with (S)-albuterol alone. However, the addition of (S)-albuterol to (R)-albuterol mediated a dose-dependent increase in proliferation. At equivalent concentrations of the 2 isomers, proliferation was unchanged from the control, whereas at 10(-6) mol/L (S)-albuterol, proliferation was enhanced. Both the inhibitory effects of (R)-albuterol alone and the stimulating influence of (R)- plus (S)-albuterol were blocked in the additional presence of propranolol. (R)-Albuterol at 10(-8) mol/L inhibited IL-2 and IFN-gamma production. Racemic albuterol (10(-8) mol/L each) had no influence on cytokine production; however, the combination of 10(-8) mol/L (R)-albuterol with 10(-6) (S)-albuterol stimulated production of IL-2 and IL-13. No effects were observed on apoptosis or cell viability.

CONCLUSION

These studies confirm the beta-adrenergic receptor-specific anti-inflammatory effects of (R)-albuterol. The racemate had minimal influences on proliferation or cytokine production. The presence of excess (S)-albuterol resulted in proinflammatory influences. We hypothesize that the (S)-isomer functions as an inverse agonist to switch the function of the beta(2)-adrenergic receptor.

Levalbuterol hydrochloride

Racemic albuterol, a commonly used bronchodilator, is an exact 50:50 mixture of two enantiomers, R- and S-albuterol. Concern regarding increased mortality associated with the use of this beta-2 (beta 2) agonist triggered the study of both of these enantiomers separately. In vitro studies suggest that the two enantiomers have different binding affinities for beta-adrenoreceptors, may exert opposing effects on inflammation, demonstrate different effects on mucocilary transport, and display differing pharmacokinetics. Clinical studies comparing both enantiomers are few, of short duration, and often in small patient populations, and their results vary. R-albuterol has greater bronchodilatory effects than the racemate and may have anti-inflammatory properties. S-albuterol has markedly less affinity for the beta-adrenoreceptor. It was found to cause bronchoconstriction in animal models, but neither bronchoconstrictive nor pro-inflammatory effects have been conclusively demonstrated in human studies. The data available at present, while suggestive, are insufficient to conclusively recommend R-albuterol over the racemate. Further basic research and investigations in humans comparing both enantiomers at increasing doses over longer time periods are required to clarify the precise roles of R- and S-albuterol.

Single-isomer beta-agonists

A new generation of bronchodilators is being developed for acute asthma management-single-isomer beta-agonists. These drugs consist only of the active bronchodilatory isomer (eutomer); they do not have the inactive and potentially harmful isomer (distomer) that is present in marketed racemic beta-agonists. Clinical studies comparing the effectiveness of (R)-albuterol (levalbuterol) with racemic albuterol established a strong rationale for using single-isomer beta-agonists in place of the racemic mixture: reduced dosages provide equivalent bronchodilatory effects with fewer beta-mediated side effects. Higher dosages achieve superior bronchodilation in episodes of severe asthma and may reduce costs of emergency department treatment.

Novel drugs for treating asthma

The health burden of asthma is increasing globally at an alarming rate, providing a strong impetus for the development of new therapeutics, particularly drugs that may prevent development of the disease. Currently available inhaled bronchodilators and anti-inflammatory drugs are effective in most asthmatic patients, but this palliative therapy requires long-term daily administration. Despite considerable efforts by the pharmaceutical industry, it has been difficult to develop novel therapeutic agents, the leukotriene antagonists being the only new class of asthma treatments to be licensed in the past 30 years. It is clearly important to understand more about the underlying mechanisms of asthma and about how currently used drugs work before rational improvements in therapy can be expected. There are numerous therapies in clinical development that combat the inflammation found in asthma, specifically targeting eosinophils, IgE, adhesion molecules, cytokines (interleukin-4, -5, -13) and chemokines, inflammatory mediators, and cell signaling (kinase inhibitors). In particular, there is the obvious need for new therapy for severe asthma that is poorly controlled by high-dose corticosteroids as well as agents to counter acute emergency asthma. A long-term goal is to develop disease-modifying immunotherapy that could be introduced in childhood to alter the natural history of asthma. Thanks to the extensive efforts of the pharmaceutical industry, we can expect the introduction of a range of novel therapies for asthma in the near future.

The pharmacokinetics of levosalbutamol: what are the clinical implications?

Salbutamol (albuterol) is a beta2-adrenoceptor agonist used as a bronchodilator for the treatment of asthma and as a uterine relaxant for the suspension of premature labour. Salbutamol has been marketed as a racemic mixture, although beta2-agonist activity resides almost exclusively in the (R)-enantiomer. The enantioselective disposition of salbutamol and the possibility that (S)-salbutamol has adverse effects have led to the development of an enantiomerically pure (R)-salbutamol formulation known as levosalbutamol (levalbuterol). Salbutamol is metabolised almost exclusively by sulphotransferase (SULT) 1A3 to an inactive metabolite. (R)-Salbutamol is metabolised up to 12 times faster than (S)-salbutamol. This leads to relatively higher plasma concentrations of (S)- salbutamol following all routes of administration, but particularly following oral administration because of extensive metabolism by the intestine. Enantiomer concentrations are similar for the first hour following an inhaled dose, reflecting the fact that salbutamol in the lung probably undergoes little metabolism. Subsequently, (S)-salbutamol predominates due to absorption and metabolism of the swallowed portion of the inhaled dose. Following oral or inhaled administration of enantiomerically pure salbutamol, a small amount (6%) is converted to the other enantiomer, probably by acid-catalysed racemisation in the stomach. Tissue binding of salbutamol is not enantioselective and plasma protein binding is relatively low. Both enantiomers are actively excreted into the urine. Compared with healthy individuals, patients with asthma do not have substantially different pharmacokinetics of the salbutamol enantiomers, but they do appear to have less drug delivered to the lung following inhaled administration because of their narrowed airways. Levosalbutamol elicits an equal or slightly larger response than an equivalent dose of the racemic mixture. This is probably due to competitive inhibition between the enantiomers at beta-adrenoceptors. Pharmacokinetic-pharmacodynamic relationships for levosalbutamol show relatively large interindividual variations. Functionally significant genetic polymorphisms have been identified for beta2-adrenoceptors, SULT1A3 and organic action transporters, all of which affect the disposition or action of levosalbutamol. Animal, in vitro and some clinical studies have reported deleterious effects of (S)-salbutamol on smooth muscle contractility or lung function. However, well-designed clinical studies in patients with asthma have failed to find evidence of significant toxicity associated with (S)-salbutamol. The clinical consequences of relatively higher plasma concentrations of (S)-salbutamol following administration of racemate remain unclear, but in the absence of clear evidence of toxicity the clinical superiority of levosalbutamol over racemic salbutamol appears to be small.

A population analysis of nebulized (R)-albuterol in dogs using a novel mixed gut-lung absorption PK-PD model

PURPOSE

The objectives of this study were to 1) construct a pharmacokinetic-pharmacodynamic (PK-PD) model, and 2) determine the PKs and PDs of (R)-albuterol when given by nebulization to 8 dogs for 7 consecutive days.

METHODS

Four doses were evaluated (0.002, 0.02, 0.1, and 0.4 mg/kg/ day). Blood samples were obtained after drug administration on days 1 and 7. Heart rates (HR) were obtained during treatment days 1, 4 and 7. All (R)-albuterol plasma concentrations were fitted using a mixed gut-lung absorption 2-compartment PK model. Day-1, 4, and 7 HR data were co-modeled using a direct response model with Hilltype equations, including a necessary tolerance phenomenon. The population PK-PD analysis was performed with an iterative 2-stage methodology (IT2S).

RESULTS

No chiral inversion was seen, and double absorption peaks on the plasma concentration versus time curves were observed in the majority of dogs. These were hypothesized to be the result of combined gut and lung absorption of (R)-Albuterol. Results indicated that 67% (range: 57-89%) of (R)-albuterol systemic exposure after nebulized administration is due to gut absorption. Mean population PK parameters were KaGI (10+/-5.7 h(-1)), KaLUNG (21+/-9.5 h(-1)), CLc/F (0.6+/-0.2 L/h/kg), CLd/F (1.4+/-0.5 L/h/kg), Vc/F (1.4+/-0.9 L/kg), and Vp/F (4.8+/-2.4 L/kg). (R)-albuterol administration was associated with an increase in the dogs heart rates. A tolerance effect related to the cumulative dose was observed and modeled.

CONCLUSIONS

The presented PK-PD model appears to differentiate gut from lung absorption when (R)-albuterol is given by 15-minute nebulization to dogs. These results agree with the accepted hypothesis that most of the systemic exposure of (R)-albuterol after nebulized administration is due to gut absorption.

Levalbuterol nebulizer solution: is it worth five times the cost of albuterol?

Albuterol is a 50:50 mixture of R-albuterol, the active enantiomer, and S-albuterol, which appears to be inactive in humans. The Food and Drug Administration recently approved levalbuterol, the pure R-isomer, as a preservative-free nebulizer solution. Published studies indicate that it is neither safer nor more effective than an equimolar dose of racemic albuterol (levalbuterol 1.25 mg = albuterol 2.5 mg). However, these studies were conducted in patients with stable asthma (at the top of the dose-response curve), whereas a nebulized bronchodilator most likely would be used by patients with an acute exacerbation. Because such patients, in the hospital setting, often require higher doses of albuterol, the manufacturer's recommended dose of levalbuterol is likely to be too low for rescue therapy. Levalbuterol may cost as much as 5 times more than racemic albuterol, depending on purchase method. We conclude that levalbuterol offers no advantage over albuterol but is likely to be more costly.

Novel therapy for asthma

The health burden of asthma is increasing globally at an alarming rate, providing a strong impetus for the development of new therapeutics. Currently available inhaled bronchodilators and anti-inflammatory drugs are effective in most asthmatics, but this palliative therapy requires long-term daily administration. Despite considerable efforts by the pharmaceutical industry, it has been difficult to develop novel therapeutic agents; the leukotriene antagonists and synthesis inhibitors being the only new class of asthma treatments to have been licensed in the last 30 years. It is clearly important to understand more about the underlying mechanisms of asthma and about how current drugs work before rational improvements in therapy can be expected. There are numerous therapies in clinical development that combat the inflammation found in asthma, specifically targeting eosinophils, IgE, adhesion molecules, cytokines and chemokines, inflammatory mediators and cell signalling. In particular, there is the obvious need for new therapy for severe asthma that is poorly controlled by high doses of corticosteroids, as well as agents to counter acute emergency asthma. A long-term goal is to develop disease-modifying immunotherapy, that could be introduced in childhood to alter the natural history of asthma. Thanks to the extensive efforts of the pharmaceutical industry, in the near future we can expect the introduction of a range of novel therapies for asthma.