Steric aspects of formoterol and terbutaline: is there an adverse effect of the distomer on airway smooth muscle function?

Differential effects of inhaled R- and S-terbutaline in ovalbumin-induced asthmatic mice

Inhaled terbutaline is commercially available β₂-agonist which consists of equivalent amount of R- and S-enantiomer. In this study, we aimed to investigate the effects of single enantiomers of terbutaline and its racemate in an ovalbumin (OVA)-induced mouse model of asthma via. seven days inhalation and the potential mechanisms involved. In a standard experimental asthma model, BALB/c mice were sensitized and challenged with OVA. R-terbutaline (R-ter), S-terbutaline (S-ter) or racemic terbutaline (rac-ter) was given via. nose-only inhalation for one week. Airway responsiveness to methacholine was measured by the plethysmography in conscious mice. Eosinophils counts in blood and bronchoalveolar (BAL) fluid were determined. The OVA-sIgE in plasma and inflammatory cytokines and mediators in BAL fluid or lung tissue were analyzed by ELISA, qRT-PCR or western blotting. Airway inflammation and remodeling were evaluated with hematoxylin and eosin (HE), periodic acid-Schiff (PAS), and Masson staining. Drug distribution and deposition after inhalation were determined by LC-MS/MS. Our data showed that R-ter efficiently ameliorated asthma responses, including airway hyperresponsiveness, eosinophils influx and IL-5 in BALF, plasma OVA-sIgE and significantly reduced pulmonary inflammation, peribronchial smooth muscle layer thickness, goblet cell hyperplasia, and deposition of collagen fibers, as well as downregulation of p38 MAPK phosphorylation and NF-κB expression. Racemic mixture exhibited diminished effects while S-ter enhanced airway responsiveness to methacholine and exerted pro-asthmatic effects.

Resorcinol-, catechol- and saligenin-based bronchodilating β2-agonists as inhibitors of human cholinesterase activity

We investigated the influence of bronchodilating β2-agonists on the activity of human acetylcholinesterase (AChE) and usual, atypical and fluoride-resistant butyrylcholinesterase (BChE). We determined the inhibition potency of racemate and enantiomers of fenoterol as a resorcinol derivative, isoetharine and epinephrine as catechol derivatives and salbutamol and salmeterol as saligenin derivatives. All of the tested compounds reversibly inhibited cholinesterases with Ki constants ranging from 9.4 μM to 6.4 mM and had the highest inhibition potency towards usual BChE, but generally none of the cholinesterases displayed any stereoselectivity. Kinetic and docking results revealed that the inhibition potency of the studied compounds could be related to the size of the hydroxyaminoethyl chain on the benzene ring. The additional π-π interaction of salmeterol's benzene ring and Trp286 and hydrogen bond with His447 probably enhanced inhibition by salmeterol which was singled out as the most potent inhibitor of all the cholinesterases.

Beta2-adrenergic stimulation increases energy expenditure at rest, but not during submaximal exercise in active overweight men

PURPOSE

β₂-Agonists have been proposed as weight-loss treatment, because they elevate energy expenditure. However, it is unknown what effect β₂-agonists have on energy expenditure in overweight individuals. Furthermore, the influence of β₂-agonist R- and S-enantiomer ratio for the increased energy expenditure is insufficiently explored.

METHODS

Nineteen males were included in the study of which 14 completed. Subjects were 31.6 (±3.5) years [mean (±95% CI)] and had a fat percentage of 22.7 (±2.1)%. On separate days, subjects received either placebo or inhaled racemic (rac-) formoterol (2 × 27 µg). After an overnight fast, energy expenditure and substrate oxidation were estimated by indirect calorimetry at rest and during submaximal exercise. Plasma (R,R)- and (S,S)-formoterol enantiomer levels were measured by ultra-performance liquid chromatograph-mass spectrometry.

RESULTS

At rest, energy expenditure and fat oxidation were 12% (P ≤ 0.001) and 38% (P = 0.006) higher for rac-formoterol than placebo. Systemic (R,R):(S,S) formoterol ratio was correlated with change in energy expenditure at rest in response to rac-formoterol (r = 0.63, P = 0.028), whereas no association was observed between fat percentage and rac-formoterol-induced change in energy expenditure. During exercise, energy expenditure was not different between treatments, although carbohydrate oxidation was 15% higher (P = 0.021) for rac-formoterol than placebo. Rac-formoterol-induced shift in substrate choice from rest to exercise was related to plasma ln-rac-formoterol concentrations (r = 0.75, P = 0.005).

CONCLUSION

Selective β₂-adrenoceptor agonism effectively increases metabolic rate and fat oxidation in overweight individuals. The potential for weight loss induced by β₂-agonists may be greater for R-enantiopure formulations.

Effects of formoterol on contraction and Ca2+ signaling of mouse airway smooth muscle cells

Formoterol, a long-acting beta(2)-receptor agonist, is used to relieve bronchial constriction. However, formoterol is often a racemic formulation, and contains both (R,R)- and (S,S)-enantiomers. Because the activity of each isomer is poorly defined, the mechanisms by which formoterol relaxes smooth muscle cells (SMCs) of intrapulmonary airways are not well understood. Consequently, we compared the effects of (S,S)-, (R,R)-, and racemic formoterol, as well as (R)-albuterol, on the contraction and Ca(2+) signaling of airway SMCs in mouse lung slices with phase-contrast and confocal microscopy. Small airways were contracted with methacholine and the associated SMCs displayed sustained Ca(2+) oscillations and an increase in Ca(2+) sensitivity. These contracted airways displayed a substantial, concentration-dependent relaxation in response to (R,R)-formoterol. Racemic formoterol had a similar potency as (R,R)-formoterol for relaxing airways. By contrast, (S,S)-formoterol only induced a small relaxation. In conjunction with relaxation, (R,R)- and racemic formoterol stopped and decreased the methacholine-induced Ca(2+) oscillations and Ca(2+) sensitivity of the SMCs, respectively, whereas (S,S)-formoterol only decreased the Ca(2+) sensitivity. In these studies, (R,R)- and racemic formoterol had a similar, but much greater, potency than (R)-albuterol for relaxing mice airways. This action was quickly initiated at high concentrations by decreasing the frequency of Ca(2+) oscillations, but was more usually mediated at lower concentrations by decreasing the Ca(2+) sensitivity of the SMCs.

Chiral HPLC analysis of formoterol stereoisomers and thermodynamic study of their interconversion in aqueous pharmaceutical formulations

A chiral HPLC method was validated and successfully applied for the determination of formoterol stereoisomers and their inversion products in an aqueous matrix stored at 5-70 degrees C up to 3 weeks. Analysis was performed on a Chiral-AGP column (100 x 4-mm, 5-microm) using a variable mixture of mobile phase A (50-mM sodium phosphate buffer, pH 7.0) and B (10% v/v IPA) at a flow rate of 1.3 ml min(-1), and UV detection at 242 nm. All four formoterol stereoisomers were adequately resolved with acceptable detection and quantitation limits varying from 0.01-0.04 microg/ml and 0.04-0.1 microg/ml, respectively. The method showed acceptable accuracy (> or = 88%), precision (RSD < or = 8.5%) and good linearity (r(2) > or = 0.9999) over the concentration range investigated. While interconversion at 5+/-3 degrees C and 25+/-2 degrees C/60% RH +/-5% RH was too low to be determined accurately within the study period, chiral inversion of formoterol stereoisomers measured at high temperatures followed the first order rate kinetics and occurred at a single chiral center, resulting in the reversible formation of diastereoisomers, (R,R)<-->(S,R) and (S,S)<-->(R,S). No enantiomerization or diastereomerization occurred. There was no significant difference in inversion of the active components in racemic (R,R/S,S)-formoterol fumarate and the single isomer (R,R)-arformoterol tartrate drug formulations, and both drugs are expected to maintain their stereochemical integrity throughout the proposed shelf-life at the recommended storage condition (5+/-3 degrees C).

Stereoconfiguration of antiallergic and immunologic drugs

OBJECTIVE

To review the concept of chirality and its current role in the pharmacology of antiallergic, antiasthmatic, and immunologic agents.

DATA SOURCES

Ovid MEDLINE and PubMed databases from 1950 to the present time were searched.

STUDY SELECTION

Articles that described the pharmacology of chiral antiallergic, antiasthmatic, and immunologic medications were used for this review.

RESULTS

Stereoselectivity affects the pharmacologic profiles of medications in different ways from class to class and within the classes. This summary illustrates that enantiomers differ not only in potency in receptor binding and physiologic effects but also in pharmacokinetic parameters such as volume of distribution, plasma protein binding, metabolism, and clearance. Different enantiomers may produce unrelated pharmacologic effects as well. This review summarizes the variety of possible effects that different stereoisomers may produce and further underlines the importance of the purification and in-depth analysis of chiral compounds.

CONCLUSION

Chirality plays an important role in pharmacokinetics and pharmacodynamics of various pharmaceutical agents. The importance of stereoisomeric purity in the pharmacologic industry has increased during the past decade as demonstrated by the increased number of studies that examined the in vivo and in vitro effects produced by changes in stereoconfiguration of pharmaceutical agents. This review highlights such effects in certain frequently used medications used in the treatment of asthma and various allergic and immunologic disorders.

The pursuit of precision pharmaceuticals: divergent effects of beta2 agonist isomers

Beta2 agonists are the most commonly used treatment for acute bronchoconstriction. However, during regular use there is a progressive decline of protective efficacy of bronchodilators. This progressive decline has long been considered anomalous because with short-acting beta agonists, there is no corresponding change in bronchodilator efficacy. Airway hyper-responsiveness is itself a feature of asthma and there maybe however, there may be an increase in airway hyper-responsiveness following regular use of beta2 agonist. Airway hyperresponsiveness could diminish the capacity of beta agonists to protect from or result in paradoxical bronchospasm and there effects of racemic salbutamol. There have been reports of increased morbidity and mortality associated with excessive use of beta(2) agonists. As all beta agonists used clinically are racemates composed of 1:1 mixtures of R and S isomers, conducted on the possible involvement of the isomers in hyper-responsiveness. Hyper-responsiveness cannot be attributed to the R isomer, whose capacity to activate beta adrenoceptors will nullify this effect. In contrast, extensive evidence indicated that the S isomer might cause hyper-responsiveness and potential airway inflammation. Further, the S isomer shows a propensity to activate human eosinophils and alter muscarinic M(2) receptor functions. The S isomer, which makes no contribution to therapeutic efficacy and may exacerbate asthma, might therefore be excluded from asthma therapy.

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

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

The effect of formoterol over 24 h in patients with asthma: the role of enantiomers

The single-dose effect of formoterol racemate and enantiomers on bronchodilatation up to 24 h was determined. Forty-six reversible asthmatic patients were randomised to this double blind, crossover study. Formoterol was inhaled by nebulizer (HaloLite); 4.5 and 36 microg of the racemate (rac-formoterol), 2.25 and 18 microg of (R;R)-formoterol, 18 mirog of (S;S)-formoterol, or placebo. Airway and systemic effects were assessed by serial measurements of forced expiratory volume during the first second, FEV1 (24 h), and heart rate (4 h). Rac- and (R;R)-formoterol significantly and dose-dependently increased FEV1 with similar mean maximal effect. (S;S)-formoterol was without significant effects on FEV1 and heart rate. (R;R)- and rac-formoterol were still effective 22-24 h after single high doses, but this was associated with some systemic side effect (increased heart rate) initially. Average 22-24 h FEV1 was 8% (rac-formoterol 36 microg) and 11% ((R;R)-formoterol 18 microg) over placebo, respectively. No significant differences in effects were observed between rac- and (R;R)-formoterol. Thus, the single dose bronchodilatating effect of formoterol resides in (R;R)-formoterol. This study does not indicate a clinically important advantage of (R;R)-formoterol as acute bronchodilator compared to the racemate.

Use of online-dual-column extraction in conjunction with chiral liquid chromatography tandem mass spectrometry for determination of terbutaline enantiomers in human plasma

An online sample extraction chiral bioanalytical method was developed and validated for the quantification of terbutaline, a beta2-selective adrenoceptor agonist, spiked into human plasma by using two extraction columns and a chiral stationary phase (CSP) in conjunction with liquid chromatography tandem mass spectrometry (LC-MS/MS). In this method, two Oasis HLB extraction columns were used in parallel for plasma sample purification and a Chirobiotic T CSP was used for enantiomeric separation. Atmospheric pressure chemical ionization MS/MS was employed in multiple reaction monitoring mode for the detection and quantification. Subsequent to the addition of an internal standard solution, the plasma samples were directly injected onto the system for extraction and analysis. This method allowed the use of one of the extraction columns for purification while the other was being equilibrated. Hence, the time required for reconditioning the extraction columns did not contribute to the total analysis time per sample, which resulted in a shorter run time and higher throughput. A lower limit of quantification of 1.0 ng/mL was achieved using only 50 microliter of human plasma. The method was validated with a dynamic range of 1.0-200 ng/mL. The intra- and interday precision was no more than 11% CV and the assay accuracy was between 94-106%.

Bronchodilating properties of the VIP receptor agonist Ro 25-1553 compared to those of formoterol on the guinea-pig isolated trachea

Ro 25-1553 is a 31-amino acid analogue of vasoactive intestinal peptide (VIP) and has recently been shown to be highly selective for the VPAC(2)-receptor. The bronchodilating property of this compound was evaluated in vitro on preparations of guinea-pig trachea, with the long-acting beta(2)-adrenoceptor selective agonist, formoterol, as a reference. In strip-preparations precontracted with carbachol, Ro 25-1553 caused a concentration-dependent and complete relaxation of the tracheal smooth muscle. Ro 25-1553 was 3-7 times less potent than formoterol on a molar basis, but the efficacy was comparable with that of formoterol. Both compounds showed a rapid onset of action and a similar durability of effect. Ro 25-1553 appeared to interact with formoterol as well as with salmeterol in an additive way. In vagus nerve-trachea tube preparations, when added to the external medium, Ro 25-1553 concentration-dependently and completely inhibited nerve-induced contractions. This occurred in the same concentration range as needed for relaxation of precontracted strips. Ro 25-1553 was active also when administered into the tracheal lumen albeit the concentration had to be increased. The present study supports and extends previous results suggesting that Ro 25-1553 may be a powerful alternative to the beta(2)-adrenoceptor agonists which prevail today.

Comparison of the anti-bronchoconstrictor activities of inhaled formoterol, its (R,R)- and (S,S)-enantiomers and salmeterol in the rhesus monkey

The principle objective of this study was to define the anti-bronchoconstrictor effects of inhaled racemic formoterol and its (R,R)- and (S,S)-enantiomers in a new model of methacholine-induced bronchoconstriction in the rhesus monkey. A second long-acting beta(2)agonist, salmeterol, was included for comparison. Anaesthetized, spontaneously breathing rhesus monkeys were set up for measuring airway resistance. Blood pressure, heart rate and serum potassium concentrations were measured concomitantly to gauge systemic exposure and the potential for side effects. Formoterol, 0.14, 0.34 and 1.15 microg/kg, administered by aerosol, induced rapidly developing, sustained, dose-related inhibition of the bronchoconstrictor responses to aerosolised methacholine (maximum 76%) accompanied by sustained, dose-related tachycardia. (R,R)-formoterol, 0.56 microg/kg, induced anti-bronconstrictor effects and an associated tachycardia which corresponded closely to the effects seen following twice the dose of the racemate. (S,S)-formoterol, 0.54 microg/kg, was inactive. Salmeterol, 1.4 microg/kg, had no significant anti-bronchoconstrictor effect whereas doses of 5.5 and 30 microg/kg produced quantitatively similar but submaximal anti-bronchoconstrictor effects (maximum 47%). Sustained dose-dependent tachycardia was seen with salmeterol over the full dose range. Thus, the anti-bronchoconstrictor activity of formoterol resides in the (R,R) enantiomer and the (S,S) enantiomer does not interfere with the activity when present in the racemic form. Furthermore, the data indicate that the present model of methacholine-induced bronchospasm in the rhesus monkey could be useful in defining the key properties of beta(2)agonist bronchodilators such as relative potency, efficacy, duration of action and potential for systemic side effects.

Determination of the optical purity of (R)-terbutaline by 1H-NMR and RP-LC using chiral derivatizing agent, (S)-(−)-α-methylbenzyl isocyanate

A simple, convenient and precise 1H-NMR and indirect HPLC methods were used for the determination of (S)-terbutaline in (R)-terbutaline. The enantiomers were converted to diastereomeric derivatives using (S)-(-)-alpha-methylbenzyl isocyanate and were successfully separated on an ODS column within 40 min with RS=1.41 and alpha=1.09. Interaction between chiral solutes by the formation of the diastereomeric complexes also led to differentiations of the 1H-NMR spectra of enantiomers and optical purities were determined on the basis of the peak area of the enantiomeric amine proton resonance. The effect of various experimental parameter, such as reaction time, reaction temperature and concentration of chiral derivatizing agent on the derivatization reaction and composition of mobile phase on the ODS column is discussed. Validation data such as recovery, linearity and detection limit are also presented. The results from 1H-NMR and RP-HPLC methods were compared with those from chiral HPLC method and no racemization was found during the experiments. NMR results had agreed with those obtained by indirect HPLC method and two methods could be used as a quality control method for the enantiomeric purity determination of (R)-terbutaline.

Reversed-phase high performance liquid chromatographic separation of the enantiomers of terbutaline by derivatization with 2,3,4,6-tetra-o-acetyl-beta-D-glucopyranosyl isothiocyanate

The enantiomers of the bronchodilator terbutaline were separated by reversed-phase high performance liquid chromatography after derivatization with 2,3,4,6-tetra-O-acetyl-beta-D-glucopyranosyl isothiocyanate(GITC) reagent. The derivatization proceeded quantitatively within 1 h at room temperature. The corresponding diastereomeric thiourea derivatives were well resolved on an ODS column with acetonitrile-acetate buffer as a mobile phase. Elution orders of the diastereomers were confirmed by derivatization of R-(-)-terbutaline and S-(+)-terbutaline which were collected by semi-preparative chiral HPLC using Sumichiral OA-4700 column. The native fluorescence of terbutaline was quenched by derivatization with GITC. The detection limit was 25 ng when monitored at UV 278 nm.

Enantiomers of bronchodilating beta2-adrenoceptor agonists: is there a cause for concern?

All bronchodilating beta2-adrenoceptor agonists in current clinical use are derivatives of adrenaline and are available as racemates. Whereas the vast majority of the pharmacologic and clinical documentation has been made with the racemates, there are a few studies with the individual enantiomers. It thus appears that all established pharmacologic effects of racemic beta2-agonists reside in the (R)-enantiomer, with the (S)-enantiomer being virtually inactive. In recent years the suspicion has been raised that the (S)-enantiomer of the beta2-agonists is responsible for induction of airway hyperreactivity. This suspicion is based primarily on results obtained in guinea pigs exposed to (S)-enantiomers of beta2-agonists. A number of experiments in vitro have been undertaken to find a mechanism of action for these observations in vivo. Most of the results obtained are equivocal. However, the observation that (S)-salbutamol may cause mobilization of intracellular Ca2+, apparently by means of a cholinergic mechanism, deserves further investigation. The clinical studies are focused on the enantiomers of salbutamol. They confirm the preclinical findings that the pulmonary, as well as the extrapulmonary, effects of salbutamol reside in the (R)-enantiomer. The studies available so far do not convincingly show clinically significant airway hyperreactivity after exposure to the (S)-enantiomer. Further studies are needed to settle this issue.