Beta 2 receptor agonist: metabolism and pharmacology

Sports doping: emerging designer and therapeutic β2-agonists

Beta2-adrenergic agonists, or β2-agonists, are considered essential bronchodilator drugs in the treatment of bronchial asthma, both as symptom-relievers and, in combination with inhaled corticosteroids, as disease-controllers. The use of β2-agonists is prohibited in sports by the World Anti-Doping Agency (WADA) due to claimed anabolic effects, and also, is prohibited as growth promoters in cattle fattening in the European Union. This paper reviews the last seven-year (2006-2012) literature concerning the development of novel β2-agonists molecules either by modifying the molecule of known β2-agonists or by introducing moieties producing indole-, adamantyl- or phenyl urea derivatives. New emerging β2-agonists molecules for future therapeutic use are also presented, intending to emphasize their potential use for doping purposes or as growth promoters in the near future.

Concerted actions of the catechol O-methyltransferase and the cytosolic sulfotransferase SULT1A3 in the metabolism of catecholic drugs

Catecholic drugs had been reported to be metabolized through conjugation reactions, particularly methylation and sulfation. Whether and how these two Phase II conjugation reactions may occur in a concerted manner, however, remained unclear. The current study was designed to investigate the methylation and/or sulfation of five catecholic drugs. Analysis of the spent media of HepG2 cells metabolically labeled with [(35)S]sulfate in the presence of individual catecholic drugs revealed the presence of two [(35)S]sulfated metabolites for dopamine, epinephrine, isoproterenol, and isoetharine, but only one [(35)S]sulfated metabolite for apomorphine. Further analyses using tropolone, a catechol O-methyltransferase (COMT) inhibitor, indicated that one of the two [(35)S]sulfated metabolites of dopamine, epinephrine, isoproterenol, and isoetharine was a doubly conjugated (methylated and sulfated) product, since its level decreased proportionately with increasing concentrations of tropolone added to the labeling media. Moreover, while the inhibition of methylation resulted in a decrease of the total amount of [(35)S]sulfated metabolites, sulfation appeared to be capable of compensating the suppressed methylation in the metabolism of these four catecholic drugs. A two-stage enzymatic assay showed the sequential methylation and sulfation of dopamine, epinephrine, isoproterenol, and isoetharine mediated by, respectively, the COMT and the cytosolic sulfotransferase SULT1A3. Collectively, the results from the present study implied the concerted actions of the COMT and SULT1A3 in the metabolism of catecholic drugs.

Pharmacology and therapeutics of bronchodilators

Bronchodilators are central in the treatment of of airways disorders. They are the mainstay of the current management of chronic obstructive pulmonary disease (COPD) and are critical in the symptomatic management of asthma, although controversies around the use of these drugs remain. Bronchodilators work through their direct relaxation effect on airway smooth muscle cells. at present, three major classes of bronchodilators, β(2)-adrenoceptor (AR) agonists, muscarinic receptor antagonists, and xanthines are available and can be used individually or in combination. The use of the inhaled route is currently preferred to minimize systemic effects. Fast- and short-acting agents are best used for rescue of symptoms, whereas long-acting agents are best used for maintenance therapy. It has proven difficult to discover novel classes of bronchodilator drugs, although potential new targets are emerging. Consequently, the logical approach has been to improve the existing bronchodilators, although several novel broncholytic classes are under development. An important step in simplifying asthma and COPD management and improving adherence with prescribed therapy is to reduce the dose frequency to the minimum necessary to maintain disease control. Therefore, the incorporation of once-daily dose administration is an important strategy to improve adherence. Several once-daily β(2)-AR agonists or ultra-long-acting β(2)-AR-agonists (LABAs), such as indacaterol, olodaterol, and vilanterol, are already in the market or under development for the treatment of COPD and asthma, but current recommendations suggest the use of LABAs only in combination with an inhaled corticosteroid. In addition, some new potentially long-acting antimuscarinic agents, such as glycopyrronium bromide (NVA-237), aclidinium bromide, and umeclidinium bromide (GSK573719), are under development, as well as combinations of several classes of long-acting bronchodilator drugs, in an attempt to simplify treatment regimens as much as possible. This review will describe the pharmacology and therapeutics of old, new, and emerging classes of bronchodilator.

Inhibition of tracheal vascular extravasation by liposome-encapsulated albuterol in rats

PURPOSE

To develop a liposome-based system for systemic delivery of anti-inflammatory drugs to airways and other inflamed tissues.

METHODS

Postcapillary venular gap junctions open during airway inflammation and allow fluid accumulation and permit molecules (e.g. complement, kininogen) to enter tissues, initiating inflammatory cascades. Beta-adrenergic agonists prevent inflammatory plasma extravasation, but because of their deleterious side effects, they are not used intravenously. When sterically stabilized "stealth" liposomes are injected i.v., they remain in the circulation for long periods. Inflammatory mediators [e.g., substance P(SP)] open postcapillary venular gaps and allow liposomes and their contents to be deposited selectively in the inflamed tissue.

RESULTS

We hypothesized that liposomes encapsulating a beta-adrenergic agonist, such as albuterol, would deposit selectively in inflamed airway tissue, where the drug would slowly leak out of the liposomes, resulting in closure of the gaps, thus preventing subsequent inflammatory extravasation. To test this hypothesis, we delivered albuterol-loaded liposomes i.v. in rats. Then we injected SP to open the venular gaps and allow accumulation of the drug-loaded liposomes in airway tissue. We examined whether this treatment resulted in inhibition of subsequent plasma extravasation induced by SP. The results indicate that liposome-encapsulated albuterol inhibits subsequent extravasation, presumably by leaking out of liposomes in airway tissue. This inhibition occurs for prolonged periods of time and with limited side effects compared to the effect of free albuterol.

CONCLUSIONS

We conclude that liposomes loaded with appropriate drugs, by migrating to inflamed tissue and subsequently inhibiting inflammatory cascades, may be of therapeutic value in inflammatory diseases.

Isoetharine versus albuterol for acute asthma: greater immediate effect, but more side effects

PURPOSE

To compare the magnitudes of the immediate effects of the nebulized beta-agonists isoetharine and albuterol in the treatment of acute severe asthma.

PATIENTS AND METHODS

Fifty-one adults presenting with severe asthma exacerbations (forced expiratory volumes in the first second of exhalation [FEV1] <40% of predicted) to the emergency department were randomized (double-blind) to receive hourly inhaled nebulization treatment with either isoetharine (5 mg) or albuterol (2.5 mg). The FEV1 was measured immediately before and after each nebulized treatment. Any side effects were recorded.

RESULTS

Immediately after the first nebulized treatment, the isoetharine group improved its mean FEV1 (+/-SEM) by a significantly greater amount than did the albuterol group: 60% +/- 11% versus 39% +/- 5%, respectively (P <0.05). One hour later the mean FEV1 were equivalent. This pattern repeated itself after the second hourly treatment. The two groups did not differ in any outcome parameters (FEV1 at discharge, number of nebulized treatments required, the number of inpatient admissions, number of clinical relapses after discharge). More patients treated with isoetharine had side effects (36% versus 4% for albuterol, P <0.01), 1 of whom required discontinuation from the study.

CONCLUSIONS

Both medications were equally effective in alleviating bronchospasm. The immediate effect of isoetharine was significantly greater, but equalized that of albuterol within an hour after treatment. There were more side effects with isoetharine.

Bronchodilator, oxygen level, and temperature effects on ascites incidence in broiler chickens

Oxygen level, ambient temperature, and bronchodilator (metaproterenol) effects on performance, ascites incidence, and hematology were investigated using commercial broilers. In Experiment 1, two atmospheric oxygen concentrations (17.6, 20.6%) and two ambient temperatures (26.7, 32.2 C) were examined in a factorial arrangement. The low-oxygen (17.6%), ambient temperature combination (26.7 C) reduced (P < .01) weight gain whereas the high-oxygen ambient temperature combination enhanced (P < .05) such performance. Ascites incidence for chicks housed at 32.2 C was higher (P < .01) for birds maintained at 17.6% oxygen but oxygen concentration did not impact ascites incidence at 26.7 C. White blood cell counts were elevated in the low-oxygen (17.6%) group at both 26.7 and 32.2 C, and hematocrit was positively (R = .84; P < .05) correlated with ascites incidence. In Experiment 2, two oxygen levels (17.6, 20.6%) at constant 26.7 C were examined with and without metaproterenol, a bronchodilator. Metaproterenol reduced (P < .01) ascites incidence at 17.6% oxygen and completely prevented ascites at 20.6% oxygen. The data suggest that therapeutic development may be enhanced by using hematocrit as an indices of ascites and further that studies directed at bronchodilation have potential to alleviate ascites.

Effects of beta 2-adrenergic agonists on isolated guinea pig lung mast cells

The mast cell protective effects of the newly developed long-acting beta 2-agonists salmeterol and formoterol were compared with those of conventionally used beta 2-agonists, non-specific beta-agonists, disodium cromoglycate (DSCG) and theophylline. With the exception of DSCG, all the test agents inhibited ovalbumin-induced histamine release from enzymically dispersed guinea pig lung mast cells in a dose-dependent fashion. At the maximum concentration tested, theophylline produced the highest level of protection, inhibiting up to 90% of ovalbumin-induced histamine release whereas DSCG produced only 10% inhibition. The maximum inhibition produced by all the beta 2-agonists tested was around 45%. While salmeterol was equipotent with salbutamol, formoterol was at least a 100-fold more potent. Hence the present study confirmed the previously reported mast cell stabilizing actions of conventional beta 2-agonists and extended the observation to the newly developed long-acting analogues.

Airway cooling and rewarming. The second reaction sequence in exercise-induced asthma

To determine if a relationship exists among the magnitude and rate of airway rewarming, and the severity of bronchial obstruction in thermally induced asthma, we had seven subjects perform three- to four-point stimulus response curves with isocapnic hyperventilation of frigid air with and without pretreatment with inhaled norepinephrine. The latter was employed to alter the heat supplied to the airway walls by producing vasoconstriction. 1-s forced expiratory volume (FEV1) was measured before and 5 min after the cessation of each bout of hyperpnea and before and after norepinephrine. On a separate day, the subjects repeated the above challenges while the temperatures of the airstream in the intrathoracic airways were measured. Prenorepinephrine, FEV1 progressively decreased in a stimulus response fashion as ventilation rose, while norepinephrine shifted this curve to the right. As the level of ventilation increased, the size of the temperature difference between the cooling of hyperpnea and the rewarming of recovery followed suit, and their magnitude was linearly related to the severity of bronchial narrowing. Reducing the mucosal blood supply of the airways with norepinephrine limited rewarming and attenuated the obstructive response. These data demonstrate that the airway narrowing that develops following hyperpnea and the magnitude of the thermal differences are related, and that alterations in blood supply directly affect bronchial heat flux and influence obstruction.

Comparison of albuterol to isoproterenol as a bronchodilator for use in pulmonary function testing

We compared the effectiveness of albuterol with isoproterenol as a bronchodilator for use in pulmonary function testing. A total of 180 patients presenting for routine pulmonary function testing were randomly assigned to receive 5 mg of either albuterol or isoproterenol by compressed air nebulizer. Forced expiratory maneuvers were performed before, 5 min after, and 10 min after bronchodilator administration. The average increase in FEV1 and FVC did not differ between drugs. Also, the fraction of patients achieving a clinically significant bronchodilator response did not differ between drugs. Importantly, there was no significant difference between average 5 and 10 min postbronchodilator values for FEV1 or FVC for either bronchodilator, suggesting that a peak response was reached by 5 min. These results show no advantage of isoproterenol over albuterol in terms of potency or speed of action. Given the well-known cardiovascular side effects of isoproterenol, albuterol is the preferable agent for use in pulmonary function testing.

Care of the asthmatic oral and maxillofacial surgery patient

It is estimated that asthma affects 6 to 9 million people in the United States. The nature of this disease makes it a special concern to the oral and maxillofacial surgeon. Appropriate management of the asthmatic patient with regard to anesthesia and surgical procedures of the oral and maxillofacial region is discussed.

Bronchodilators for patients with ischemic heart disease. How to avoid complications

Treatment of chronic obstructive pulmonary disease and asthma with bronchodilating agents is more complex when the patient has ischemic heart disease. Drs Myers and Bogden recommend some precautions to help avoid potentially serious cardiac side effects in patients at risk.

Twelve-hour bronchodilation in asthma with a single aerosol dose of the anticholinergic compound glycopyrrolate

Anticholinergic aerosols provide effective bronchodilation in some patients with obstructive lung disease. Glycopyrrolate is a quaternary ammonium anticholinergic compound that is poorly absorbed from mucus membranes, thus reducing anticholinergic side effects. In 20 adult patients with asthma, we evaluated bronchodilation to a single administration of metered-dose glycopyrrolate aerosol (GA) to ascertain its onset and duration of action along with evaluation of safety. In this randomized, double-blind, placebo-controlled, single-dose, crossover trial, bronchodilation was evaluated on five separate occasions to either placebo or a GA dose of 80, 240, 480, or 960 micrograms. Baseline spirometry for each patient on each visit was similar (mean FEV1 +/- SD of 62.2 +/- 13.6% predicted). After aerosol dosing, spirometry was measured at 30 minutes and then at hourly intervals up to 12 hours. Compared to placebo, metered-dose aerosols of 240, 480 and 960 micrograms elicited significantly greater bronchodilation at each test time. Furthermore, significant bronchodilation was noted within 30 minutes of dosing and was sustained for at least 12 hours. Bronchodilation with the 480 and 960 micrograms dose was equal, and both were greater than 240 micrograms. A subset of four asthma patients with baseline FEV1 values less than 50% predicted did not have a bronchodilating response with GA. No notable side effects occurred. Thus, a single aerosol dose of GA provides clinically significant, safe 12-hour bronchodilation in patients with asthma without severe airway obstruction (i.e., FEV1 greater than 50% predicted).

The effects of epithelium removal on the sensitivity of guinea-pig isolated trachealis to bronchodilator drugs

Mechanical removal of the epithelium increased the sensitivity of tracheal strips to isoprenaline, sodium nitroprusside, and to adenosine (only in the presence of inhibitors of its uptake and metabolism). Epithelium removal was without effect on sensitivity to salbutamol or papaverine. Preincubation of tracheal strips with an inhibitor of extraneuronal uptake, corticosterone (50 microM), had no effect on tissue sensitivity to either salbutamol or papaverine. However, the steroid both increased sensitivity to isoprenaline, and abolished the effect of epithelium removal on sensitivity to this catecholamine. These results suggest that in the guinea-pig, the tracheal epithelium is a major source of extraneuronal uptake for catecholamines. Furthermore, the increase in trachealis sensitivity to isoprenaline following epithelium removal is probably due to loss of these sites of extraneuronal uptake. The fact that sensitivity to salbutamol, papaverine and adenosine (in the absence of metabolic inhibitors) was not increased by denuding the epithelium indicates that loss of a diffusion barrier to drugs is not the mechanism of increased sensitivity. Adenosine (and possibly nitroprusside) may cause the epithelium to release a smooth muscle excitatory factor. Thus, removal of the epithelium attenuates this excitatory influence and enhances smooth muscle responsiveness to adenosine. These results provide further evidence that the epithelium has an important role in modulating the sensitivity of guinea-pig trachealis to drugs.

Adverse reactions to beta 2-agonist bronchodilators

Beta 2-Agonists are safe and effective bronchodilator drugs. Their major adverse effects of skeletal muscle tremor, tachycardia and various metabolic effects are mediated by beta-adrenoceptor stimulation and are reversible. Skeletal muscle tremor is the most frequent dose-limiting side effect. It may be reduced by commencing treatment with a low dose and if it persists another beta 2-agonist may be tried. Other side effects such as cardiac arrhythmias and reduction in PaO2 are a serious potential problem in some susceptible asthmatics. However, they are infrequent or of a mild degree and are generally outweighed by the good control of asthma produced by beta 2-agonists. Side effects from beta 2-agonist therapy can be minimised by use of the inhaled route which selectively delivers the drug to the airways. Furthermore, selective tolerance develops to their side effects. The dose of a beta 2-agonist should be assessed on the basis of therapeutic effect and the level of tolerance to its side effects. Recommended doses of beta 2-agonists used for long term therapy do not cause clinically significant desensitisation of airway beta-adrenoceptors, although this may become a relevant problem in patients who are regularly receiving very high doses. Intravenous beta 2-agonists have a place in the treatment of severe asthma not responding to nebuliser therapy. In this life-threatening situation with severe airflow obstruction, monitoring of heart rate, PaO2, plasma potassium and the electrocardiogram should be mandatory and supplemental oxygen given so that serious adverse effects are presented.

Treatment of acute asthma. Is combination therapy with sympathomimetics and methylxanthines indicated?

The role of bronchodilator regimens combining a sympathomimetic and a methylxanthine in the treatment of acute exacerbations of asthma remains controversial. This report describes the outcome of 157 emergency room visits for asthma in which patients were randomly assigned to single-drug therapy with intravenous aminophylline, subcutaneous epinephrine, or inhaled isoproterenol or to one of three regimens combining a sympathomimetic and a methylxanthine. The increase in one-second forced expiratory volume after one hour of treatment with the two-drug combinations (0.79 +/- 0.07 liter) was significantly greater than for epinephrine alone (0.57 +/- 0.08 liter; p less than 0.05) but did not differ significantly from that occurring with therapy with isoproterenol alone (0.72 +/- 0.09 liter; p = NS). This disparity reflects the greater bronchodilation effected by isoproterenol as a single agent than by epinephrine, in the dosing schedules and routes of administration chosen. Among patients presenting with severe airflow obstruction (one-second forced expiratory volume 35 percent or less of normal), the bronchodilator response to isoproterenol alone was 0.88 +/- 0.14 liter versus 0.51 +/- 0.11 for epinephrine alone (p less than 0.05). It is concluded that the observed benefit derived from use of combination therapy depends on the dosage and potency of the particular sympathomimetic to which a methylxanthine is added, and on the severity of the airflow obstruction at presentation.

Clinical use of beta-adrenergic agonists

In the last decade the treatment of airway obstruction has been greatly facilitated by the introduction of a number of long-acting, highly bronchoselective beta-adrenergic agonists. Many medical professionals feel that these agents should be first-line treatment for asthma. The choice of route of administration and drug to use depends on the desired clinical effect. Most studies in both acute and chronic situations demonstrate that the inhaled route produces at least as much bronchodilation as oral or parenteral therapy and does so with substantially fewer side effects. Fears about safety and tachyphylaxis with the modern agents have been unfounded. Failure of self-administered beta-agonists to produce relief from acute episodes of airway obstruction cannot be interpreted as evidence of primary drug resistance and does not predict the emergency room response to these agents. The available evidence suggests that if drug tolerance does develop with beta-agonists, it is small in magnitude, nonprogressive, and not clinically significant.

Aerosol formulations of terbutaline and isoproterenol in theophylline-stabilized asthmatic patients

A metered-dose aerosol formulation of terbutaline sulfate (Brethaire), 0.400 mg four times daily, was compared with an identical formulation of isoproterenol sulfate, 0.150 mg four times daily, in a parallel, double-blind, clinical study completed by 40 adult patients with asthma. All patients had been stabilized on theophylline (serum levels of 10 to 20 micrograms/ml). The effectiveness of isoproterenol peaked between 5 and 15 minutes after administration. The effectiveness of terbutaline peaked between 5 and 120 minutes after administration. In each of five visits spaced over a three-month period, patients receiving terbutaline showed a longer duration of bronchodilatory effect than those receiving isoproterenol, with the greatest difference occurring at 60 and 120 minutes after drug administration.

Effect of outpatient treatment of asthma with beta agonists on the response to sympathomimetics in an emergency room

It has been suggested that tolerance to the bronchodilating effects of sympathomimetics may develop in asthmatic patients after long-term use of these agents. In an emergency room setting, the effects of inhaled and injected sympathomimetic therapy in 58 patients who had pretreated themselves with beta agonists were compared with the results observed in 38 patients who had not used such drugs. The two groups had similar degrees of obstruction on presentation and were also well-matched with respect to the clinical features of their illness. Both populations showed equal responses to treatment; no significant differences were found in either the amount of bronchodilation or the incidence of adverse effects in those who had or had not taken sympathomimetics as outpatients. These findings indicate that drug resistance does not account for outpatient treatment failures with sympathomimetics and that beta agonists can be usefully employed in the treatment of acute asthma, irrespective of a patient's medication history.