Advances in b2-Agonist Therapy: Making a Difference in Acute and Chronic Asthma Treatment

Treatment of asthma with b₂-adrenoceptor agonists began fully a century ago when patients first inhaled powder derived from calves’ adrenal glands. The pioneers of these agents did not know, however, that in giving their patients epinephrine from the adrenal medullary cells, they were also giving them steroids from the cortex. This combination of agents — inhaled low-dose steroids and low-dose b₂-agonists — remains first-line therapy for moderate asthma and for mild asthma that is refractory to inhaled steroids alone. Yet observation indicates that there is substantial interindividual variation in clinical responses to these agents; and for some patients, continuous use of b₂-agonists may be detrimental and even fatal. This suggests that there may be underlying genetic differences in subpopulations of patients that influence the clinical response. Correlation of clinical data with the presence or absence of specific polymorphisms on the b₂-adrenoceptor appear to support this hypothesis. More detailed haplotyping may facilitate the identification of patients who will and will not benefit from b₂-agonist therapy.

This program was supported by an unrestricted educational grant from Sepracor Inc.

Pharmacologic Properties of Available b₂-adrenoceptor Agonists

Inhaled b₂-adrenoceptor agonists (henceforth “b-agonists”) currently available for clinical use differ in five important pharmacologic parameters: (i) onset of action, (ii) duration of action, (iii) receptor selectivity, (iv) intrinsic efficacy, and (v) stereoselectivity. Burton F. Dickey, MD, FCCP (Baylor College of Medicine) compared the most commonly prescribed b-agonists — albuterol, formoterol, and salmeterol — with respect to these parameters.

Salmeterol has a slow onset of action compared with albuterol and formoterol. The duration of action of formoterol and salmeterol is more than 12 hours compared with 4 to 6 hours for albuterol. The principal factor that determines onset and duration of action is drug lipophilicity. All of these drugs approach the receptor, which is embedded in the lipophilic membrane, through an aqueous medium, and their molecular structure determines their lipophilicity. Of the three drugs, albuterol is the most highly lipophilic, resulting in rapid binding with the receptor but also in rapid washing away. Formoterol has a large side chain that makes it moderately lipophilic. Thus it arrives at the receptor easily, accounting for its rapid onset of action, and it dissolves into the membrane in the vicinity of the receptor and remains for an extended time, explaining its prolonged duration of action. Salmeterol has a greatly extended side chain, enters the lipid membrane rapidly, but has difficulty approaching the receptor. However, it remains in the vicinity of the receptor for an extended time. This sequence determines salmeterol’s slow onset but long duration of action. Importantly, the apparent duration of action of any of these agents may be reduced under conditions of receptor desensitization or functional antagonism, as occurs with exercise or an acute asthma attach. 

Intrinsic efficacy is most readily conceptualized in terms of two conformations of the b₂-receptor: active and inactive. The empty receptor is generally inactive, but in the presence of an agonist, the receptor shifts to the active conformation, more so when stimulated by a full agonist than by a partial agonist. 

Epinephrine and isoproterenol rank highest among the b-agonists in intrinsic efficacy. Albuterol is only 5% as intrinsically efficacious, yet it is used routinely in emergency situations. Of the two principal b-agonists used in maintenance therapy, salmeterol is a very weak partial agonist with approximately 2% of the intrinsic efficacy of epinephrine. Formoterol is still under study, but appears to be nearly a full agonist. 

The critical issue, however, is whether or not differences in intrinsic efficacy are clinically significant. Fenoterol has very high intrinsic efficacy and may have a role in emergency treatment despite its having gained a bad reputation by having been associated with deaths in an Australian trial in which it was used without concurrent inhalation steroids. Whereas the increase in FEV1 induced by albuterol plateaus early with repetitive dosing, that of fenoterol continues to rise. Similarly, in comparison with salmeterol, with which the increase in FEV1 levels off despite dosage multiplication under conditions of functional antagonism, bronchoprotection associated with formoterol continues to improve in a dose-dependent manner. An individual who is in remission from asthma and who has plentiful b₂-receptors in the airway smooth muscle, will get a full clinical response from a complete or partial agonist even if only a few receptors are activated. But during an exacerbation, the signal transduction pathway is antagonized, resulting in a partial response to a complete agonist and little or no response to a partial agonist. 

High intrinsic efficacy is not entirely beneficial, however. The receptor conformation that initiates downstream signal transduction and increases in cAMP and smooth muscle relaxation, also stimulates the desensitization machinery. Thus a full b-agonist drives a higher rate of desensitization. With formoterol, for example, the benefit observed on day 1of therapy diminishes in the next few days, though it eventually plateaus. The best way to determine the clinical significance of differential desensitization would be a head-to head randomized and controlled study stratifying for b-receptor alleles. 

Most chemical syntheses yield equal concentrations of mirror-image isomers, with each pair comprising a racemate. It is possible, however, to synthesize some compounds and purify the active compound, or enantiomer. (The inactive counterpart is the distomer.) Only the enantiomer of a b-agonist binds with high affinity to the adrenoceptor and can induce the conformational changes that the natural compound and the drug are intended to do. Several studies have investigated the significance of this stereoselectivity in asthma. In a placebo- controlled study comparing the albuterol racemate and the active stereoisomer, FEV1 area under the curve increased more after inhalation of the enantiomer. 

Stereoselectivity also appears to influence desensitization of the receptor. One aspect of desensitization is internalization of the receptor in response to binding the agonist. In studies using flourescetly-labeled receptor proteins, strong agonists such as isoproterenol and formoterol drove nearly all the receptors off the cell surface and into intracellular compartments. In contrast, weak agonists such as salmeterol and ephedrine induced very little internalization. The result with albuterol, a moderate agonist, was intermediate. The formoterol enantiomer had an internalizing effect equal to that of epinephrine, while the distomer actually induced an increase in receptor protein on the cell surface, suggesting that it is an adverse agonist. 

The full significance of stereoselectivity is unknown, but clinical data suggest that in the presence of the distomer of at least some b-agonists, the efficacy of the enantiomer is diminished. 

Short-acting b-Agonists in the Treatment of Asthma

Short-acting b-agonists are the most effective medications available for the relief of acute bronchospasm. There has been a traditional assumption that regular administration would increase the time spent with improved function and, therefore, improve symptoms. Some epidemiological data suggest, however, that long-term regular use of these agents increases the risk of asthma morbidity and mortality. Elliot Israel, MD, FCCP (Harvard Medical School) explored this issue by posing two questions: “Does chronic use of b-agonists improve or worsen asthma control?”; and “Are there populations in which chronic use of b-agonists should be avoided?” 

The initial reaction to data implicating regular b-agonist therapy in increased risk was that regular use is a marker for disease severity and that the drugs themselves are not a risk factor. That convention was first challenged in a double-blind crossover study using fenoterol. Patients were randomized to regular b-agonist medication, intermittent medication, or placebo. The primary outcome variable was an asthma control index consisting of a formula made up of morning and evening peak flow, symptoms, and the use of rescue medication. The trial data indicated that 70% of patients derived greater benefit from intermittent therapy and 30% responded better to regular use (Sears M. Lancet. 1990;336(8728):1391). 

A study by Taylor and colleagues confirmed these results in a 24-week crossover study, and found in addition that the exacerbation rate was higher in patients while taking b-agonists regularly. Smaller studies have demonstrated that with regular use, patients had decreased protection against allergens and methacholine and a mean shortfall in FEV1 of approximately 5% relative to patients taking b-agonists intermittently. However, in the 16-week b-Agonist Study (BAGS) with a 4-week run-out using albuterol, there were no significant differences between regular and intermittent users in morning peak volume flow, symptoms, quality of life, exacerbation rate, or supplemental use of albuterol. Regular users did, however, experience a temporary but statistically significant increase in responsiveness to methacholine. Regular users took an average of 9.5 puffs per day compared with 1.5 puffs per day for intermittent users. The investigators concluded that regular use is neither detrimental nor more beneficial than inter- mittent use (Drazen JM, Israel E, et al. N Engl J Med. 1996;335:841). Similar conclusions resulted from the TRUST trial (Dennis S et al. Lancet. 2000; 365:1675).

Nelson and colleagues conducted a study in which patients were treated with either the albuterol racemate or its enantiomer. After 4 weeks, patients treated with the racemate had a decline from baseline of FEV1 that was not observed in patients taking the enantiomer or in patients given placebo, suggesting that the distomer component of the racemate may be detrimental. Dr. Israel introduced the hypothesis that interindividual variation in response to b-agonists may be due to the presence of polymorphisms in the b2-adrenoceptor, and that some polymorphisms may predispose patients to adverse effects from regular use of b-agonists (Israel E et al. Am J Respir Crit Care Med. 2000;162:75 and Int Arch Allergy Immunol. 2001;124:183). Stephen B. Liggett, MD discussed this concept in detail subsequently (see infra). 

Because there is no incremental benefit from using b-agonists regularly, there appears to be little sense in exposing patients to the potential risks suggested in epidemiological data. Dr. Israel concluded, however, that the population of concern consists of patients with asthma who, during exacerbations, may inhale b-agonists hourly. During exacerbations, it may be necessary to introduce alternate therapies for individuals who are genetically predisposed to risk. 
 

Long-acting b-Agonists in the Treatment of Asthma

Long-acting b-agonists such as formoterol and salmeterol provide round-the-clock bronchodilation and some protection against stimulation by exercise and allergens. James E. Fish, MD, FCCP (Jefferson Medical College) and his co-investigators in the NHLBI Asthma Clinical Research Network explored these agents as monotherapy in mild persistent asthma. This was a trial comparing the efficacy of salmeterol and an inhaled steroid (triamcinolone) in which all patients were treated with the inhaled steroid during the run-in and were then randomized to triamcinolone, salmeterol, or placebo. There were no significant differences in morning (premedication) peak flow volume or symptoms, but patients treated with the steroid had significantly fewer exacerbations than did salmeterol-treated patients or controls. During the run-in period, sputum eosinophil counts fell abruptly and remained low in patients randomized to triamcinolone, but they rose again in both the salmeterol and placebo trial arms, indicating that while salmeterol was treating lung function, it was not treating the underlying inflammatory reaction. Salmeterol and placebo were not significantly different in this regard (Lazarus SC, JAMA. 2001;285: 2583). In a similar trial, patients given salmeterol monotherapy did not do as well as patients treated with inhaled steroids or steroid plus salmeterol. Dr. Fish concluded from these two studies that “in the population of mild persistent asthma, salmeterol should never be used as monotherapy.” Mild persistent asthma is best treated with low-dose inhaled steroids. 

For some patients, however, low-dose inhaled steroids are insufficient to relieve symptoms. Should the steroid dosage be increased, or is combination therapy preferable for these patients? Dr. Fish reviewed two randomized trials in which patients were treated with either high-dose steroids or salmeterol and inhaled steroids at standard dosages. In one of these studies, patients had somewhat more severe asthma. Nevertheless, in both studies, patients treated with low-dose steroid in combination with the long-acting b-agonist had superior physiologic and symptomatic outcomes (Greening AP. Lancet. 1994;344:219; Woolcock A et al. Am J Respir Crit Care Med. 1996;153:1481). The same result was reported from a trial in which formeterol plus low dose inhaled steroid was compared with high-dose steroid monotherapy. A meta-analysis of studies comparing these two treatment strategies observed significantly superior results from combination therapy. A trial comparing low-dose steroid/salmeterol with low-dose steroid/montelukast (a leukotriene receptor antagonist) found superior results with salmeterol (Nelson HS et al. J Allergy Clin Immunol. 2000; 106:1088).

Based on this trial evidence, Dr. Fish concluded that “for moderate persistent asthma, there is very clear evidence suggesting that the addition of a long-acting b-agonist to a low-dose inhaled steroid is optimal treatment.” 

 

Future Advances in b-Agonist Therapeutic Strategies

Stephen B. Liggett, MD, FCCP (University of Cincinnati) discussed the pharmacogenetics of the b₂-receptor as a basis for understanding the interindividual variation in clinical response to b-agonists. Approximately 60% of this variability is genetically determined, as is approximately 85% of the variability in response to corticosteroids. (These are upper-limit estimates.)

For more that a decade, Dr. Liggett’s laboratory has been investigating genetic variation in the b₂-receptor. He and his colleagues have identified, among other things, the sites that are particularly important to the binding of agents such as albuterol, those for the coupling of 
G-protein that activates adenyl cyclase and increases cAMP, and those involved in phosphorylation of the receptor by various kinases that regulate desensitization. They have also identified a number of differences in amino acid sequence within the human population including polymorphic variants that induce changes in the encoded amino acid. These involve arginine or glycine at position 16, glutamine or glutamic acid at position 27, and threonine or isoleucine at position 164. 

Liggett and Green discovered several years ago that if you introduce b₁-receptor amino acid sequence into the fourth transmembrane spanning domain of the b₂-receptor, the long-acting properties of salmeterol are lost (Green A, Liggett SB. J Biol Chem. 1996;271: 24029). Because of the importance of this location, they looked at the Ile 164 polymorphic receptor, which is located in the same spanning domain, and found that it has an impaired ability to stimulate adenyl cyclase compared with the wild-type receptor in the presence of full, moderate, or weak agonists. On the polymorphic Ile 164 receptor, the exosite binding of salmeterol in the fourth transmembrane spanning domain has a 50% reduction in cells expressing this polymorphic form of receptor. In kinetic washout studies, the functional response of the Ile 164 polymorphic receptor to salmeterol deteriorates rapidly compared with the wild-type receptor. 

Based on these findings, Dr. Liggett hypothesized that a patient with the Ile 164 polymorphism would have a weak initial response to any long-term b-agonist, and that the duration of the response would be reduced by approximately 50% in those receiving salmeterol. Of note, in the in vitro studies, when the same assays were conducted with formoterol, no difference was observed between the polymorphic and the wild-type receptors, because the binding of formeterol does not involve the same exosite binding region. Thus Ile 164 patients should have normal duration of action. Correlation of these experimental findings with clinical data has yet to be undertaken. 

Polymorphisms of the b₂-receptor play a role in tachyphylaxis. In a trial in which patients with mild asthma were randomized to regular albuterol, intermittent albuterol, or placebo, the decrease in morning peak expiratory flow rate was significantly more in patients with arginine-16 than in patients with glycine-16. Moreover, individuals with arginine-16 had a significantly greater fall in flow rate if they took albuterol regularly rather than intermittently. These outcomes suggest that tachyphylaxis to albuterol is present in individuals with arginine-16 but not glycine-16 and that the arginine-16 genotype coupled with regular administration is predictive of tachyphylaxis. In addition, individuals who were homozygous for arginine-16 and took albuterol regularly experienced the most rapid decline in expiratory peak flow rate. There was no loss of asthma control, however, in regular users of albuterol who were homozygous for glycine-16. 

Knowing the amino acid composition of a single common polymorphic site may be helpful for predicting patient responsiveness to a medication, but it can be misleading. The only sure method of making clinical correlations is to know all of the polymorphisms of the gene and to determine haplotypes. Thirteen different single nucleotide polymorphisms on the b₂-adrenergic receptor gene have approximately 8,000 possible haplotypic combinations, only twelve of which actually occur in the human population. The vast majority of polymorphisms is found in just four or five groups, though there are marked differences in frequency among races. (For example, haplotype 1 is rare in Caucasians, but appears in approximately 25% of the African-American population, perhaps explaining different treatment responses to b-agonists.) In an initial trial with 131 patients, Dr. Liggett and his colleagues were able to differentiate clinical responses to albuterol by haplotype pairs, with individuals with the 4-4 haplotype having significantly poorer responses than those with the 4-6 haplotype or the 2-2 haplotype. Sub-cloning of haplotypes 4 and 2 and transfecting cells revealed that cells of the 4 haplotype had reduced expression of b₂receptor mRNA and patients had a poor FEV1 response to b-agonist therapy. In contrast, cells with the 2-2 haplotype pair had increased expression of receptor mRNA and patients had an increased response to b-agonist therapy. 

These experiments and the correlation of findings to clinical data on treatment responses illustrate the potential future role of pharmacogenetics as a criterion when selecting treatment modalities for patients with asthma.