Stereoselective sulphate conjugation of salbutamol by human lung and bronchial epithelial cells

Bronchopulmonary pharmacokinetics of (R)-salbutamol and (S)-salbutamol enantiomers in pulmonary epithelial lining fluid and lung tissue of horses

AIMS

Salbutamol is usually administered as a racemic mixture but little is known about the enantioselectivity of salbutamol pharmacokinetics in the lung. This study was designed to investigate enantiomer concentrations in lung tissue after inhaled dosing.

METHODS

Horses (n = 12) received racemic salbutamol 1000 μg via inhalation. Enantioselective ultra performance liquid chromatography-tandem mass spectrometry was used to determine salbutamol concentrations in pulmonary epithelial lining fluid (PELF) sampled 2, 5, 10 and 15 min after administration, in central lung (endoscopic bronchial biopsy) and peripheral lung (percutaneous pulmonary biopsy) tissues (at 20 and 25 min respectively), and in plasma samples.

RESULTS

Mean ± 95% confidence interval (CI) yield of PELF was 57 ± 10 mg. Initial mean ± 95%CI (R)- and (S)-salbutamol PELF concentrations were 389 ± 189 ng g^-1 and 378 ± 177 ng g^-1 respectively, and both reduced approximately 50% by 15 min. Mean ± 95%CI central lung levels of drug were higher than peripheral lung tissue for both (R)-salbutamol (875 ± 945 vs. 49.5 ± 12 ng g^-1 ) and (S)-salbutamol (877 ± 955 vs. 50.9 ± 12 ng g^-1 ) respectively. There was no evidence of enantioselectivity in PELF or central lung but minor (~2%) enantioselectivity was observed in the peripheral lung. Enantioselectivity was clearly evident in plasma with (S):(R) ratio of 1.25 and 1.14 for both area under the concentration-time curve (0-25 min) and C_max respectively.

CONCLUSIONS

PELF sampling in horses offers sufficient yield allowing direct detection of drug and, combined with tissue sampling, is a valuable model to investigate bronchopulmonary pharmacokinetics. Salbutamol did not demonstrate enantioselectivity in PELF or central lung tissue uptake following acute dosing, however, enantioselective plasma concentrations were demonstrated, with minor enantioselectivity in the peripheral lung.

Polarized Airway Epithelial Models for Immunological Co-Culture Studies

Epithelial cells line all cavities and surfaces throughout the body and play a substantial role in maintaining tissue homeostasis. Asthma and other atopic diseases are increasing worldwide and allergic disorders are hypothesized to be a consequence of a combination of dysregulation of the epithelial response towards environmental antigens and genetic susceptibility, resulting in inflammation and T cell-derived immune responses. In vivo animal models have long been used to study immune homeostasis of the airways but are limited by species restriction and lack of exposure to a natural environment of both potential allergens and microflora. Limitations of these models prompt a need to develop new human cell-based in vitro models. A variety of co-culture systems for modelling the respiratory epithelium exist and are available to the scientific community. The models have become increasingly sophisticated and specific care needs to be taken with regard to cell types, culture medium and culture models, depending on the aim of the study. Although great strides have been made, there is still a need for further optimization, and optimally also for standardization, in order for in vitro co-culture models to become powerful tools in the discovery of key molecules dictating immunity and/or tolerance, and for understanding the complex interplay that takes place between mucosa, airway epithelium and resident or infiltrating immune cells. This review focuses on current knowledge and the advantages and limitations of the different cell types and culture methods used in co-culture models of the human airways.

Detection of main urinary metabolites of β2-agonists clenbuterol, salbutamol and terbutaline by liquid chromatography high resolution mass spectrometry

Clenbuterol, terbutaline and salbutamol are B2-agonists drugs included in the list of banned substances of the World Anti Doping Agency (WADA) prohibited in and out of competition. In this article, the excretion of urinary metabolites of clenbuterol, terbutaline and salbutamol have been studied using liquid chromatography electrospray time-of-flight mass spectrometry (LC-TOFMS), after a single therapeutic dose administration in rats. Urine collected was processed with solid-phase extraction prior to LC-TOFMS analyses using electrospray in the positive ion mode and pseudo MS/MS experiments from in-source collision induced dissociation (CID) fragmentation (without precursor ion isolation). The strategy applied for the identification of metabolites was based on the search of typical biotransformations with their corresponding accurate mass shift and the use of common diagnostic fragment ions from the parent drugs. The approach was satisfactory applied, achieving the identification of 11 metabolites (5 from clenbuterol, 4 from salbutamol and 3 from terbutaline), 4 of them not previously reported in urine. Novel metabolites identified in rat urine included N-oxide-salbutamol, hydroxy-salbutamol, methoxy-salbutamol glucuronide and terbutaline N-oxide, which are all reported here for the first time.

Organic cation transporters in the blood-air barrier: expression and implications for pulmonary drug delivery

Studies concerning the impact that hepatic, renal and intestinal transporters have on drug disposition have been frequently reported in the literature. Surprisingly, however, little is known regarding the distribution and function of drug-transporter proteins of the lung epithelium. Many drugs (delivered to the lung) have a net positive charge and, thus, are potential substrates of organic cation transporters; currently marketed compounds (e.g., bronchodilators), as well as novel drug candidates in development, are such substrates. It is the aim of this review to summarize the current state of organic cation-transporter expression analysis in the lung and in in vitro models of bronchial and alveolar barriers. Moreover, activity of selected transporters in lung epithelium in situ and in vitro will be highlighted, and their potential role in pulmonary drug disposition will be addressed. One example included here is the transporter-dependent absorption of beta2-agonists in respiratory epithelial cells.

Sulfation of ractopamine and salbutamol by the human cytosolic sulfotransferases

Feed additives such as ractopamine and salbutamol are pharmacologically active compounds, acting primarily as β-adrenergic agonists. This study was designed to investigate whether the sulfation of ractopamine and salbutamol may occur under the metabolic conditions and to identify the human cytosolic sulfotransferases (SULTs) that are capable of sulfating two major feed additive compounds, ractopamine and salbutamol. A metabolic labelling study showed the generation and release of [(35)S]sulfated ractopamine and salbutamol by HepG2 human hepatoma cells labelled with [(35)S]sulfate in the presence of these two compounds. A systematic analysis using 11 purified human SULTs revealed SULT1A3 as the major SULT responsible for the sulfation of ractopamine and salbutamol. The pH dependence and kinetic parameters were analyzed. Moreover, the inhibitory effects of ractopamine and salbutamol on SULT1A3-mediated dopamine sulfation were investigated. Cytosol or S9 fractions of human lung, liver, kidney and small intestine were examined to verify the presence of ractopamine-/salbutamol-sulfating activity in vivo. Of the four human organs, the small intestine displayed the highest activity towards both compounds. Collectively, these results imply that the sulfation mediated by SULT1A3 may play an important role in the metabolism and detoxification of ractopamine and salbutamol.

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.

Inhibitory effects of herbal extracts on the activity of human sulfotransferase isoform sulfotransferase 1A3 (SULT1A3)

Sulfotransferase 1A3 (SULT1A3) is a phase II detoxifying enzyme of xenobiotics predominantly expressed in the intestinal epithelium. Recent increase in the use of herbal extracts as dietary supplements may lead to an increase in the possibility of dietary supplement-drug interactions. The purpose of the present study was to investigate the effects of 18 herbal extracts on SULT1A3 activity and the possibility of interaction between medicinal drugs and herbal extracts. We examined the inhibitory potencies of 18 herbal extracts on the sulfation of dopamine, a typical substrate of SULT1A3, and ritodrine, a beta(2) stimulant, by human recombinant SULT1A3. The sulfation of dopamine was inhibited by extracts of banaba, green tea, Rafuma, grape seed, peanut seed coat, gingko biloba leaf, St. John's wort, gymnema and milkthistle. The IC(50) values of these herbal extracts were lower than the putative gastrointestinal concentration when the recommended dose was ingested. On the other hand, chlorella extract and rutin showed no inhibitory effects and wheat, mulberry and siberian ginseng had IC(50) values exceedingly higher than the putative gastrointestinal concentration. The inhibitory profiles of herbal extracts for the sulfation of ritodrine were comparable to those for the sulfation of dopamine. In conclusion, the extracts of herbs such as banaba and green tea potently inhibited SULT1A3 activity. These extracts may increase the bioavailability of drugs whose bioavailabilities were limited by the function of SULT1A3 on the intestinal epithelium.

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.

Beta2-agonists and bronchial hyperresponsiveness

Bronchial hyperresponsiveness (BHR) is a characteristic feature of asthma, and individuals with this disease respond to a range of physiological and chemical insults that are otherwise innocuous to healthy subjects, suggesting that the mechanisms underlying this phenomenon are characteristic of the asthma phenotype. BHR can be increased following exposure to environmental allergens in suitably sensitized individuals, pollutants, and certain viruses and can also be exacerbated by exposure to certain drugs, including nonsteroidal anti-inflammatory agents and beta-blockers. Although beta2-agonists administered acutely remain the treatment for the symptoms of asthma, paradoxically, regular treatment with these drugs can result in an increase in BHR, and this has been suggested to contribute to the increase in asthma morbidity and mortality that has been reported by numerous investigators. This article highlights our current understanding of this phenomenon and examines the potential mechanisms responsible for this effect.

(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?"

Evaluation of levalbuterol metered dose inhaler in pediatric patients with asthma: a double-blind, randomized, placebo- and active-controlled trial

OBJECTIVE

To evaluate the efficacy and safety of levalbuterol metered dose inhaler (MDI) in children aged 4-11 years (n = 173).

RESEARCH DESIGN AND METHODS

Multicenter, randomized, double-blind 28-day study of QID levalbuterol 90 microg, racemic albuterol 180 mug, and placebo (2:1:1 ratio). Serial spirometry was performed on Days 0, 14, and 28. The primary endpoint was the double-blind average peak percent (%) change in FEV(1) from visit pre-dose; the primary comparison was with placebo. Secondary endpoints included the area under the FEV(1) percent change from pre-dose curve and peak % predicted FEV(1). Safety endpoints included adverse events, laboratory tests, rescue medication use, and electrocardiograms.

RESULTS

Levalbuterol significantly improved the least square mean peak percent change in FEV(1) compared with placebo (levalbuterol 25.6% +/- 1.3% [p < 0.001]; racemic albuterol 21.8% +/- 1.8% [p = ns]; placebo 16.8% +/- 1.9%). Results for levalbuterol were similar for the other spirometry endpoints (p < 0.05 vs. placebo). No levalbuterol-treated patients had a peak percent change in FEV(1) < 10% (compared with 15.8% of racemic albuterol-treated patients and 30.3% of placebo-treated patients). The incidence of adverse events was 43.4% for levalbuterol, 56.4% for racemic albuterol, and 51.4% for placebo. The rate of discontinuation was 1.3% for levalbuterol, 2.6% for racemic albuterol, and 8.6% for placebo. The rate of asthma attacks (10.5%, 12.8%, 14.3%, respectively) was similar among treatments. Levalbuterol and racemic albuterol both reduced rescue medication use (p < 0.01 vs. placebo) and produced changes in ventricular heart rate and QT(c-F) that were similar to placebo.

CONCLUSIONS

In this study, levalbuterol administered via MDI significantly improved airway function in comparison with placebo in asthmatic children aged 4-11 years with a safety profile that was similar to placebo.

Population pharmacokinetics of (R)-albuterol and (S)-albuterol in pediatric patients aged 4-11 years with asthma

OBJECTIVE

To characterize the population pharmacokinetics (PK) of (R)- and (S)-albuterol in pediatric asthmatics using a model that supports a sparse blood sampling strategy.

METHODS

The data for this analysis were collected from patients enrolled in a randomized, double-blind, multicenter, placebo- and active-controlled study evaluating the safety and efficacy of levalbuterol in asthmatic children aged 4-11 years. Patients received either levalbuterol 0.31 mg, levalbuterol 0.63 mg, racemic albuterol 1.25 mg, or racemic albuterol 2.5 mg via nebulizer. Separate population pharmacokinetic models were developed for (R)- and (S)-albuterol using the NOMNEM((R)) computer program. Covariate models were developed to identify significant predictors of inter-patient variability.

RESULTS

A total of 995 samples and 262 patients were used for the (R)-albuterol population PK model while a total of 496 samples and 128 patients were used for the (S)-albuterol population PK model. The apparent clearance of (R)-albuterol was much more rapid than that of (S)-albuterol (approximately four-fold higher), and the apparent volume of distribution was much larger for (R)-albuterol (in part due to pre-systemic metabolism) than for (S)-albuterol (approximately four-fold higher).

CONCLUSIONS

In this study of pediatric patients, the models were able to demonstrate using two to four samples per patient that the apparent clearance and volume of distribution of (R)-albuterol were several fold higher than that of (S)-albuterol. The pharmacokinetics of (R)-albuterol were similar after administration of levalbuterol or racemic albuterol and were linear over the examined dose range (0.31-0.63 mg nebulized dose). The presence of (S)-albuterol did not significantly alter the pharmacokinetics of (R)-albuterol, suggesting that effects of (S)-albuterol may be due to the intrinsic pharmacology of this isomer.

The correlation of urinary levels of albuterol and its metabolites isomers following inhalation from a dry powder inhaler and in vitro particle size characterisation

This study was designed to analyse the two enantiomers of abuterol in urine after the inhalation of a single dose of racemic albuterol from three dry powder inhalers by human volunteers. Urine samples were collected over 24h and analysed by HPLC-with fluorescence detection. Albuterol and its metabolites in urine could only have resulted from pulmonary absorption because gastrointestinal absorption was prevented. Unchanged albuterol and its conjugated metabolites were detected in the urine of healthy volunteers at much higher levels than in the urine of the asthmatics. Also, the amount of S-(+)-isomer excreted in urine was higher than that of the R-(--)-isomer. These differences did not arise as a consequence of either the formulation or the inter-conversion of two isomers in the urine. There is a relationship between the improvement of mid-expiratory flow (FEF(25-75)) and the amount of R-(--)-albuterol remaining to be excreted. The elimination rate constants of the parent drug in healthy volunteers of both R-(--)- and S-(+)-isomers were higher than those of the respective conjugated metabolites. The mean S/R ratio of the parent drug was about unity initially and increased to about 1.5 in the urine collected between 12 and 24h. The values of S/R ratio of the conjugated metabolites in the healthy volunteers were in the range 1.2-2.4, with the value increasing over the time of collection before reaching a plateau. This also occurred with the asthmatics, but the ratios were higher, in the range of 2.0-4.5. In summary, the urinary level of albuterol following in vivo inhalation was found to correlate with in vitro deposition data from the dry powder inhaler.

Levalbuterol hydrochloride

Racemic salbutamol (racemic albuterol) ameliorates symptoms of asthma by activating beta-adrenoceptors on nerve, smooth muscle and inflammatory cells within the airways. Racemic salbutamol comprises equal proportions of 2 isomers: (S)-salbutamol and (R)-salbutamol, with the latter being exclusively responsible for activation of beta-adrenoceptors. Accordingly, within racemic salbutamol it is (R)-salbutamol that efficiently relieves obstruction of asthmatic airways and affords highly effective protection from bronchoconstrictor stimuli, including allergens. During regular use of racemic salbutamol, there is a progressive decline of protective efficacy and a corresponding intensification of airway responsiveness. This decline is largely absent during regular use of (R)-salbutamol. Consequently, bronchodilator responses to sub-maximal doses of (R)-salbutamol exceed responses to the equivalent dose of (R)-salbutamol given as the racemate. For example, in asthmatics with baseline FEVs <or= 60%, 1.25 mg of nebulised (R)-salbutamol achieved a maximal 52% change in FEV while 2.5 mg of racemic salbutamol only achieved a 38% change in FEV. Since extrapulmonary effects (e.g., tremor, heart rate) of beta agonists are related to dose and limit the use of beta agonist therapy, (R)-salbutamol at 0.63 mg provides uncompromised efficacy with marked reduction of side-effects. In addition to quantitative differences, the constituent isomers of salbutamol also exhibit qualitative differences. Thus, (R)-salbutamol inhibits activation of human eosinophils in vitro whereas, under the same conditions and concentrations, (S)-salbutamol augments activation of these cells. This property of (S)-salbutamol may explain why eosinophilia in induced sputum from subjects with allergic asthma is increased by regular use of racemic salbutamol. Similarly, the capacity of (R)-salbutamol to suppress hyperresponsiveness of the airways can be contrasted with the capacity of (S)-salbutamol to intensify hyperresponsiveness. This action of (S)-salbutamol would explain why regular use of racemic salbutamol intensifies the bronchoconstrictor response to antigen in subjects with allergic asthma. Taken together, these findings imply that replacement of racemic salbutamol by (R)-salbutamol will diminish, or even eliminate, the anomalous actions that have curtailed the efficacy of racemic salbutamol. Pharmacokinetically, (R)-salbutamol exhibits near absolute conformational stability (i.e., no conversion to (S)-salbutamol). If in vitro anti-inflammatory actions of (R)-salbutamol are also manifest in asthmatic airways, (R)-salbutamol could provide a novel approach to asthma therapy which combines bronchodilation and bronchoprotection with anti-inflammatory efficacy.

Levalbuterol: pharmacologic properties and use in the treatment of pediatric and adult asthma

LEARNING OBJECTIVES

To review the rationale supporting the use of levalbuterol [(R)-albuterol] for the treatment of pediatric and adult asthma.

DATA SOURCES

Peer-reviewed articles, selected abstracts from studies presented at recent professional meetings, and the Xopenex [levalbuterol, (R)-albuterol; Sepracor, Marlborough, MA] Summary Basis of Approval and package insert.

STUDY SELECTION

Institutional review board-approved clinical study protocols.

RESULTS

Levalbuterol is a single isomer beta2-agonist that differs from racemic albuterol by elimination of (S)-albuterol. Levalbuterol is an effective bronchodilator whose primary mechanism of action is unimpeded by (S)-albuterol. Thus, when compared with racemic albuterol, clinically comparable bronchodilation can be achieved with doses that substantially lessen beta-mediated side effects. In chronic or acute treatment of asthma, this favorable therapeutic profile cannot apparently be duplicated by increasing or decreasing the dose of racemic albuterol or by the addition of anticholinergic agents such as ipratropium bromide.

CONCLUSIONS

Levalbuterol seems to provide efficacy and safety advantages in pediatric and adult patients suffering from asthma. Its use may afford a cost benefit as well. More clinical studies are required to extend these observations for use in the treatment of other pulmonary diseases in both adults and children and to determine levalbuterol's impact on long-term therapy of respiratory diseases.

Single-isomer levalbuterol: a review of the acute data

Levalbuterol, the pure (R)-isomer of racemic albuterol, is a new therapeutic option for patients with asthma. Racemic albuterol comprises a 50:50 mixture of (R)- and (S)-albuterol, with (R)-albuterol conferring all of the bronchodilator effects of the racemate. Numerous preclinical and in vitro studies have indicated that (S)-albuterol is not an inert isomer, but may have proinflammatory effects. Results from clinical trials in adults and children with asthma have demonstrated that 0.63 mg levalbuterol provides effective bronchodilation with lower b-mediated side effects compared with 2.5 mg racemic albuterol. In the emergency department, levalbuterol provided greater bronchodilation and significantly reduced hospital admissions compared with racemic albuterol. Recent studies have supported that levalbuterol use in acute settings may reduce the cost of asthma treatment by decreasing the total treatments and subsequent respiratory therapy resources. Levalbuterol provides heath care professionals with a safe, effective, and potentially cost-saving alternative to racemic albuterol for the treatment of patients with asthma.

Stereoselective pharmacokinetics and pharmacodynamics of anti-asthma agents

OBJECTIVE

To review the previously published studies on pharmacokinetics and pharmacodynamics of chiral drugs used in the treatment of asthma.

DATA SOURCES

Primary and review articles were identified with a MEDLINE search (1980-May 2001) and through secondary sources.

STUDY SELECTION AND DATA EXTRACTION

All English-language studies and reviews obtained from the MEDLINE search pertaining to stereoselective pharmacokinetics and pharmacodynamics of chiral anti-asthma drugs were assessed.

DATA SYNTHESIS

Several anti-asthma drugs (e.g., beta(2)-adrenergic agonists, leukotriene modifiers) are chiral and marketed as racemates, which consist of equal proportions of 2 enantiomers. Significant stereoselectivity has also been reported in pharmacodynamics and pharmacokinetics of the beta(2)-agonists. The enantiomers of beta(2)-agonists in the R configuration are primarily responsible for the bronchodilating effects of the racemate. The plasma concentrations of the enantiomers of anti-asthma drugs may differ as a reflection of stereoselectivity in clearance, volume of distribution, and route of administration.

CONCLUSIONS

Stereoselectivity in the pharmacokinetics of anti-asthma drugs may complicate the relationship between dose and/or plasma concentration of racemic drug versus effect relationship. An appreciation of the stereoselective pharmacokinetics and pharmacodynamics of chiral anti-asthma drugs may optimize the use of these agents in asthmatic patients.

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.

Biotransformation in vitro of the 22R and 22S epimers of budesonide by human liver, bronchus, colonic mucosa and skin

The pharmacological effects of glucocorticoids are greatly influenced by their pharmacokinetic properties. In the present report, the in vitro biotransformation of the 22R and 22S epimers of the topical steroid budesonide was studied in the S-9 fraction of human liver, bronchus, skin and colonic mucosa. The disappearance of unchanged epimers of budesonide was measured during 90 min of incubation by high performance liquid chromatography. The rate of disappearance was high in human liver while little biotransformation occurred in bronchial tissue and colonic mucosa, and none was detected in the skin. A marked decay of the initial concentration of unchanged budesonide epimers was noticed after 2 h incubation in cultured human hepatocytes, while only a small decrease was observed after 24 h incubation in cultured human airway smooth muscle cells and BEAS-2B cells. The 22R epimer of budesonide suffered greater in vitro biotransformation than the 22S epimer in human hepatic, bronchial and colonic tissues. These findings extend those of other studies, and confirm that the high therapeutic ratio of budesonide is due to negligible local biotransformation combined with high level of liver metabolism for locally absorbed budesonide.

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.

Stereochemical composition of clenbuterol residues in edible tissues of swine

A gas chromatography-mass spectrometry method was developed to measure the stereochemical residues of clenbuterol derivatives in edible tissues of swine. Clenbuterol present in tissue extracts was derivatized with phosgene to form clenbuterol oxazolidin-3-one, which was then separated into component enantiomers using a dimethyl beta-cyclodextrin capillary gas chromatographic column. Purified clenbuterol stereoisomers, isolated using published liquid chromatographic techniques, were used to determine stereoisomer elution order, stereoisomer racemization potential, and accuracy of the method. The stereochemical composition of clenbuterol could be measured at tissue concentrations of <2 ppb using the method. The dextrorotatory stereoisomer was the predominant clenbuterol stereoisomer present in edible tissues of hogs slaughtered after withdrawal periods of 0, 3, and 7 days, with a (+)/(-) isomer ratio of about 3:1. The prevalence of the dextrorotatory stereoisomer in edible tissues of hogs at all withdrawal periods suggests that stereoselective processes are occurring during the absorption, distribution, metabolism, and (or) excretion of clenbuterol. The effect of clenbuterol dose on its stereochemical composition in edible tissues is unknown but will be an area of further investigation.

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.

Enantioselective sulfation of beta 2-receptor agonists by the human intestine and the recombinant M-form phenolsulfotransferase

The beta 2-receptor agonist class of drugs is metabolized in humans almost exclusively by sulfate conjugation. The objective of this investigation was to determine the influence of chemical structure on the stereoselectivity of the sulfoconjugation of these chiral drugs. The pure enantiomers of six beta 2-agonists, including those clinically most widely used, were all effectively sulfated both by the cytosol of the human intestine and the recombinant human M-form phenolsulfotransferase (PST). Whereas the apparent Km values (Km,app) for the sulfation of the individual drug enantiomers by the intestinal cytosol varied widely, ranging from 4.8 microM for (S)-isoproterenol to 889 microM for (S)-albuterol, these Km,app values were highly correlated with those obtained with M-PST (correlation coefficient 0.994). In contrast, the M-PST Vmax,app values were similar for all drug enantiomers, ranging from 276 to 914 pmol min-1 mg-1 protein, implying that substrate binding to M-PST by far is the main determinant of the sulfation activity. For isoproterenol, the Km,app for M-PST was 6.1 times higher for the active (R)- than for the inactive (S)-enantiomer. For other beta 2-agonists, the stereoselectivity decreased towards unity as the Km,app increased. However, for albuterol, containing a hydroxymethyl substituent at the aromatic ring, the stereoselectivity was dramatically reversed, with 10 times higher Km,app for the inactive (S)- than for the active (R)-enantiomer.

Enantiomeric disposition of inhaled, intravenous and oral racemic-salbutamol in man--no evidence of enantioselective lung metabolism

Aims To establish whether enantioselective metabolism of racemic (rac )-salbutamol occurs in the lungs by determining its enantiomeric disposition following inhalation, in the absence and presence of oral charcoal, compared with that following the oral and intravenous routes. Methods Fifteen healthy subjects (eight male) were randomized into an open design, crossover study. Plasma and urine salbutamol enantiomer concentrations were measured for 24 h following oral (2 mg) with or without oral charcoal (to block oral absorption), inhaled (MDI; 1200 microg) with or without oral charcoal and intravenous (500 microg) rac-salbutamol. Systemic exposure (plasma AUC(0,infinity) and urinary excretion (Au24h ) of both enantiomers were calculated, and relative exposure to (R)-salbutamol both in plasma (AUC(R)-/AUC(S)- ) and urine (Au(R)-/Au(S)- ) was derived for each route. Relative exposure after the inhaled with charcoal and oral routes were compared with the intravenous route. Results AUC(R)-/AUC(S)- [geometric mean (95% CI)] was similar following the intravenous [0.32 (0.28, 0.36)] and inhaled with charcoal rates [0.29 (0.24, 0.36); P=0.046], but was far lower following oral dosing [0.05 (0.03, 0.07); P<0.001]. Similar results were found when relative exposure was analysed using Au24h. Conclusions These results show no evidence of significant enantioselective presystemic metabolism in the lungs, whilst confirming it in the gut and systemic circulation, indicating that the (R)- and (S)-enantiomers are present in similar quantities in the airways following inhaled rac-salbutamol.

Human airway epithelial cell lines for in vitro drug transport and metabolism studies

The pharmaceutical industry relies on appropriate in vitro models for the evaluation of drug absorption and metabolism. Despite increasing interest in drug delivery via the lung, there is currently no widely accepted cell culture model of the airway epithelium. This review considers the airway epithelium, the culture of airway epithelial cells and the need for cell lines which can model the airway epithelium. Three of the most promising human bronchial cell lines, 16HBE14o-, Calu-3 and BEAS-2B, are reviewed, with emphasis on their recent application for the study of drug transport, drug metabolism and gene delivery. Current limitations and future directions for the development of these cell lines as models of the airway epithelium are discussed.

New millennium bronchodilators for asthma: single-isomer beta agonists

Racemic beta2 agonists are composed of a 50:50 mixture of R and S isomers. The R isomer exhibits virtually all the bronchodilation, whereas the S isomers are generally considered inert. However, (S)-albuterol was shown to enhance bronchial reactivity to methacholine, eosinophil activation, and histamine-induced influx of fluid, proteins, and neutrophils into the airspaces. Actions such as these may compress the potency and foreshorten the duration of (R)-albuterol. Accordingly, pure (R)-albuterol provides bronchodilation at lower doses than racemate, allowing for fewer beta-adrenergic-mediated side effects. In addition, differential metabolism may allow for the progressive accumulation of (S)-albuterol. This logic is applicable to long-acting beta2 agonists: the therapeutically active (R,R)-formoterol is currently being developed in the United States, and preliminary results suggest rapid improvements in FEV1 with up to 24-hour duration of action. These combined observations with the R isomers of beta2 agonists suggest that potential improvements in therapeutic indices can be achieved with isomerically pure versions of existing racemic drugs.

X-ray crystal structure of human dopamine sulfotransferase, SULT1A3. Molecular modeling and quantitative structure-activity relationship analysis demonstrate a molecular basis for sulfotransferase substrate specificity

Humans are one of the few species that produce large amounts of catecholamine sulfates, and they have evolved a specific sulfotransferase, SULT1A3 (M-PST), to catalyze the formation of these conjugates. An orthologous protein has yet to be found in other species. To further our understanding of the molecular basis for the unique substrate selectivity of this enzyme, we have solved the crystal structure of human SULT1A3, complexed with 3'-phosphoadenosine 5'-phosphate (PAP), at 2.5 A resolution and carried out quantitative structure-activity relationship (QSAR) analysis with a series of phenols and catechols. SULT1A3 adopts a similar fold to mouse estrogen sulfotransferase, with a central five-stranded beta-sheet surrounded by alpha-helices. SULT1A3 is a dimer in solution but crystallized with a monomer in the asymmetric unit of the cell, although dimer interfaces were formed by interaction across crystallographic 2-fold axes. QSAR analysis revealed that the enzyme is highly selective for catechols, and catecholamines in particular, and that hydrogen bonding groups and lipophilicity (cLogD) strongly influenced K(m). We also investigated further the role of Glu(146) in SULT1A3 using site-directed mutagenesis and showed that it plays a key role not only in defining selectivity for dopamine but also in preventing many phenolic xenobiotics from binding to the enzyme.

Epithelium smooth muscle regulation and interactions

In the 1980s, studies in the vascular field revealed that the endothelium was not simply a metabolic and physical barrier, but liberated substances that could modulate the function of underlying vascular smooth muscle. Investigators in the respiratory field also found that the airway epithelium was more than a physical barrier to airborne insults. The epithelium is composed of at least eight different cell types that have a range of functions, including ciliary motility and mucous secretion, and contain enzymes for liberating arachidonic acid metabolites and peptides. The epithelium also contains degradative enzymes for a number of peptides and biological amines. It was also recognized that the epithelium released substances that, like their vascular counterparts, could regulate the function of a number of cell types, including nerves and airway smooth muscle. These studies document the importance the epithelium plays in the regulation of human airway smooth muscle.

Pharmacokinetics and extrapulmonary beta 2 adrenoceptor activity of nebulised racemic salbutamol and its R and S isomers in healthy volunteers

BACKGROUND

Racemic salbutamol remains one of the most commonly used bronchodilators in the treatment of reversible airways obstruction. Data from animal and human studies suggest that the S-isomer, whilst contributing no bronchodilator activity, may induce increased bronchial hyperreactivity and may explain the adverse effects of regular racemic salbutamol on asthmatic disease control. The purpose of this study was to evaluate the dose-response effects of racemic (+/-) salbutamol and its R(-) and S(+) isomers in terms of pharmacokinetics and pharmacodynamics at extrapulmonary beta 2 adrenoceptors when given by the inhaled route to healthy volunteers.

METHODS

Twelve healthy volunteers of mean age 20.6 years were studied in a double blind, placebo controlled, crossover design comparing cumulative doubling doses of nebulised R-salbutamol (R) and S-salbutamol (S) isomers (200 micrograms/400 micrograms/800 micrograms/1600 micrograms/3200 micrograms) and racemic salbutamol (RS) (400 micrograms/800 micrograms/1600 micrograms/3200 micrograms/6400 micrograms). Doses were administered at 20 minute intervals (t0/t20/t40/t60/t80) and measurements were made of extrapulmonary beta 2 responses as an increase in finger tremor and heart rate and fall in plasma potassium at baseline and each dose level (t0/t20/t40/t60/t80/t100). Plasma levels of salbutamol were measured at 15 minutes after each dose with a further sample at 30 minutes after the last dose (t110).

RESULTS

Pharmacodynamics showed dose related beta 2 responses for R-salbutamol and RS-salbutamol but not for the S isomer, and a plateau in response was not reached within the administered dose range. No differences in responses were found between R-salbutamol and RS-salbutamol when compared on a 1:2 microgram basis. The effects of the S isomer were indistinguishable from those of placebo. For all beta 2 responses there were differences between R-salbutamol and S-salbutamol (for t100 response as change from placebo); tremor (log units): R 0.74 vs S 0.03 (95% CI 0.39 to 1.03); fall in potassium (mmol/ 1): R 0.35 vs S -0.02 (95% CI 0.03 to 0.71). Pharmacokinetics showed consistently higher levels for S-salbutamol than R-salbutamol at 15 minutes after each dose, with R-salbutamol already being cleared and S-salbutamol reaching peak levels at 30 minutes after the last dose (at t110). There were higher plasma levels of R-salbutamol and S-salbutamol following administration of the respective isomers alone compared with their levels after administration of the racemate, suggesting an influence of each isomer on the clearance of the opposite isomer when given as a racemate.

CONCLUSIONS

The S-isomer of salbutamol has no detectable activity at extrapulmonary beta 2 adrenoceptors whilst exhibiting higher plasma levels than the R-isomer, in keeping with greater clearance of R-salbutamol than S-salbutamol. Inhalation of R-salbutamol and RS-salbutamol produced dose-related beta 2 responses which were equivalent when compared on a 1:2 microgram basis, despite higher plasma levels of R-salbutamol after administration of the R isomer than after administration of the racemate. Further dose ranging studies are required at steady state to evaluate the pharmacokinetics of R- and S-salbutamol and their relative effects on bronchial hyperreactivity when given on a regular basis to asthmatic subjects.

Pharmacological importance of stereochemical resolution of enantiomeric drugs

Drug enantiomers have identical properties in an achiral environment, but should be considered as different chemical compounds. This is because they often differ considerably in potency, pharmacological activity and pharmacokinetic profile, since the modules with which they interact in biological systems are also optically active. Within biological systems, the metabolism of one isomer may be via a different pathway or occur at a different rate from that of the other isomer. Preferential binding of one isomer to plasma proteins may cause differences in circulating free drug and hence alter concentrations at active sites. Interactions of both isomers may differ at the active sites through which pharmacological action is mediated. Actions and levels of activity of the stereoisomers in vivo may also differ. All the pharmacological activity may reside in a single enantiomer, whereas several possibilities exist for the other enantiomer-- it may be inactive, have a qualitatively different effect, an antagonistic effect or produce greater toxicity. Two isomers may have nearly identical qualitative pharmacological activity, qualitatively similar pharmacological activity but quantitatively different potency, or qualitatively different pharmacological activity. To avoid adverse effects and optimise the therapeutic value of enantiomeric drugs, it is necessary that methods for the resolution of racemates be evolved and devolved to determine isomeric purity, establish the effectiveness of isomers of the drug, and detect the presence of an enantiomer with lower therapeutic activity and undesirable adverse effects. Even if a drug is given as a pure enantiomer, methods to discriminate between enantiomers are required because racemisation can occur both in vitro and in vivo. Methods developed for resolution of drug enantiomers should facilitate routine testing of single isomers and their metabolites, studies of pharmacological, toxicological and clinical effectiveness, routine analysis of racemates, pure enantiomers or intermediates in manufacturing processes, and investigation of the potential for inversion of an enantiopure drug substance during the early stages of drug development and therapeutic drug monitoring.