Steric aspects of agonism and antagonism at beta-adrenoceptors: synthesis of and pharmacological experiments with the enantiomers of formoterol and their diastereomers

Engineered Imine Reductase Catalyzed Enantiodivergent Synthesis of Alkylated Amphetamines

Less steric ketones exhibited low stereoselectivity toward M5 due to their difficulty in restricting the free rotation of the imine intermediate. An engineered enantio-complementary imine reductase from M5 was obtained with catalytic activity. We identified four key residues that play essential roles in controlling stereoselectivity. Two mutants, I149Y-W234L (up to 99%S ee) and L200M-F260M (up to 99%R ee), were achieved, showing excellent stereoselectivity toward the tested substrates, offering valuable biocatalysts for synthesizing alkylated amphetamines.

8-Hydroxyquinolin-2(1H)-one analogues as potential β2-agonists: Design, synthesis and activity study

β₂-Agonists that bind to plasmalemmal β₂-adrenoceptors causing cAMP accumulation are widely used as bronchodilators in chronic respiratory diseases. Here, we designed and synthesized a group of 8-hydroxyquinolin-2(1H)-one analogues and studied their β₂-agonistic activities with a cellular cAMP assay. Compounds B05 and C08 were identified as potent (EC₅₀ < 20 pM) and selective β₂-agonists among the compounds tested. They behaved as partial β₂-agonists in non-overexpressed HEK293 cells, and possessed rapid smooth muscle relaxant actions and long duration of action in isolated guinea pig tracheal strip preparations. In summary, B05 and C08 are β₂-agonists with potential applicability in chronic respiratory diseases.

Chiral Switch Drugs for Asthma and Allergies: True Benefit or Marketing Hype

Enantiomers are one of several possible molecular configurations present in a drug that has at least 1 chiral center. A drug containing 2 or more enatiomers is called a racemic mixture. Enatiomers are being developed from racemic mixtures as drugs in their own right often to extend patent protection of highly popular drugs. However, the therapeutic advantages of single enantiomer drugs developed for respiratory use such as levalbuterol, arformoterol, and levocetirizine over their racemate has been disappointing. In addition, single enantiomer drugs may be several fold more expensive than the racemic drug. New single enantiomer drugs, which are stable (no interconversion back to the racemate) and have fewer adverse effects and a more predictable pharmacodynamic or pharmacokinetic profile would confer a therapeutic advantage and thus would be beneficial for clinical use.

Differential effects of formoterol on thrombin- and PDGF-induced proliferation of human pulmonary arterial vascular smooth muscle cells

Background

Increased pulmonary arterial vascular smooth muscle (PAVSM) cell proliferation is a key pathophysiological component of pulmonary vascular remodeling in pulmonary arterial hypertension (PH). The long-acting β₂-adrenergic receptor (β₂AR) agonist formoterol, a racemate comprised of (R,R)- and (S,S)-enantiomers, is commonly used as a vasodilator in chronic obstructive pulmonary disease (COPD). PH, a common complication of COPD, increases patients’ morbidity and reduces survival. Recent studies demonstrate that formoterol has anti-proliferative effects on airway smooth muscle cells and bronchial fibroblasts. The effects of formoterol and its enantiomers on PAVSM cell proliferation are not determined. The goals of this study were to examine effects of racemic formoterol and its enantiomers on PAVSM cell proliferation as it relates to COPD-associated PH.

Methods

Basal, thrombin-, PDGF- and chronic hypoxia-induced proliferation of primary human PAVSM cells was examined by DNA synthesis analysis using BrdU incorporation assay. ERK1/2, mTORC1 and mTORC2 activation were determined by phosphorylation levels of ERK1/2, ribosomal protein S6 and S473-Akt using immunoblot analysis.

Results

We found that (R,R) and racemic formoterol inhibited basal, thrombin- and chronic hypoxia-induced proliferation of human PAVSM cells while (S,S) formoterol had lesser inhibitory effect. The β₂AR blocker propranolol abrogated the growth inhibitory effect of formoterol. (R,R), but not (S,S) formoterol attenuated basal, thrombin- and chronic hypoxia-induced ERK1/2 phosphorylation, but had little effect on Akt and S6 phosphorylation levels. Formoterol and its enantiomers did not significantly affect PDGF-induced DNA synthesis and PDGF-dependent ERK1/2, S473-Akt and S6 phosphorylation in human PAVSM cells.

Conclusions

Formoterol inhibits basal, thrombin-, and chronic hypoxia-, but not PDGF-induced human PAVSM cell proliferation and ERK1/2, but has little effect on mTORC1 and mTORC2 signaling. Anti-proliferative effects of formoterol depend predominantly on its (R,R) enantiomer and require the binding with β₂AR. These data suggest that (R,R) formoterol may be considered as potential adjuvant therapy to inhibit PAVSM cell proliferation in COPD-associated PH.

Inhalation by design: novel ultra-long-acting β(2)-adrenoreceptor agonists for inhaled once-daily treatment of asthma and chronic obstructive pulmonary disease that utilize a sulfonamide agonist headgroup

A novel series of potent and selective sulfonamide derived β(2)-adrenoreceptor agonists are described that exhibit potential as inhaled ultra-long-acting bronchodilators for the treatment of asthma and chronic obstructive pulmonary disease. Analogues from this series mediate very long-lasting smooth muscle relaxation in guinea pig tracheal strips. The sulfonamide agonist headgroup confers high levels of intrinsic crystallinity that could relate to the acidic sulfonamide motif supporting a zwitterionic form in the solid state. Optimization of pharmacokinetic properties was achieved through targeted introduction of a phenolic moiety to support rapid phase II clearance, thereby minimizing systemic exposure following inhalation and reducing systemically mediated adverse events. Compound 38 (PF-610355) is identified as a clinical candidate from this series, with in vivo duration of action studies confirming its potential for once-daily use in humans. Compound 38 is currently in advanced phase II clinical studies.

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).

Effect of nebulized arformoterol on airway function in COPD: results from two randomized trials

RATIONALE

Arformoterol, a single isomer long-acting beta(2)-agonist, was developed as an inhalation solution for the maintenance treatment of bronchoconstriction in COPD.

METHODS

The pulmonary function efficacy of nebulized arformoterol (15 micro g BID, 25 micro g BID, 50 micro g QD) and salmeterol MDI (42 micro g BID) versus placebo was assessed in 1456 subjects (mean FEV(1) 1.2L, mean predicted 41%). Data were pooled from 2 identical, 12-week, double-blind, randomized trials. The percent change in trough FEV(1), percent change in FEV(1) average AUC((0 - 12 hrs)) and peak percent change FEV(1) from predose were analyzed.

RESULTS

Improvement in trough FEV(1) averaged over 12 weeks was greater for arformoterol and salmeterol versus placebo (mean differences from placebo [95% CI] arformoterol-15 micro g BID: 11.4% [8.4, 14.3]; 25 micro g BID: 15.4% [12.2, 18.6]; 50 micro g QD: 10.9% [7.9, 13.9]); salmeterol: (11.6% [8.8, 14.4]). Greater improvements versus placebo occurred after the first dose (mean differences between arformoterol and placebo for trough FEV(1): 13-19%; FEV(1) AUC((0 - 12 hrs)): 19-24%; peak percent change: 20-25%) and at week 12 (trough FEV(1): 10-13%; FEV(1) AUC((0 - 12 hrs)): 6-13%; peak percent change: 7-14%); all 95% CIs excluded zero. Increases in FEV(1) AUC((0 - 12 hrs)) and peak percent change were greater for arformoterol than for salmeterol (95% CIs excluded zero). After 12 weeks, 78-87% of arformoterol subjects had > or = 10% increases in FEV(1) from pre-dose (56% salmeterol, 44% placebo); the median time to response was 3-13 minutes (142 minutes salmeterol).

CONCLUSIONS

In these trials, COPD subjects administered nebulized arformoterol demonstrated significant and sustained improvement in lung function over 12 weeks.

Role of arformoterol in the management of COPD

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

The discovery of indole-derived long acting beta2-adrenoceptor agonists for the treatment of asthma and COPD

The design and profile of a series of indole containing long acting beta(2)-adrenoceptor agonists is described. Evaluation of these analogues using an in vitro guinea pig trachea tissue model demonstrates that analogues within this series have salmeterol-like duration of action with potential for long duration of action in humans.

Nebulized arformoterol in patients with COPD: a 12-week, multicenter, randomized, double-blind, double-dummy, placebo- and active-controlled trial

OBJECTIVE

The aim of this study was to assess the efficacy and tolerability of nebulized arformoterol tartrate (a selective, long-acting beta(2)-adrenergic agonist that is the [R,R] isomer of formoterol) and salmeterol xinafoate versus placebo in patients with chronic obstructive pulmonary disease (COPD).

METHODS

This 12-week, multicenter, randomized, double-blind, double-dummy, placebo- and active-controlled trial was conducted at 60 centers across the United States. Male and female patients aged >or=35 years with physician-diagnosed COPD received arformoterol (15 microg BID, 25 microg BID, or 50 microg QD via nebulizer), salmeterol (42 microg BID via metered dose inhaler), or placebo. Pulmonary function was assessed by spirometry; dyspnea, by the Transitional Dyspnea Index (TDI); and health status, by the St. George's Respiratory Questionnaire (SGRQ). Adverse events (AEs) were assessed by site personnel at all clinic visits (screening, first dose at week 0, and at weeks 3, 6, 9, 12, and follow-up). COPD exacerbations were defined as worsening respiratory status requiring a change in medication or an unscheduled provider visit.

RESULTS

A total of 717 patients received study medication. The demographic composition of all treatment arms was similar. The mean age was 62.9 years, 58% were men, and mean baseline forced expiratory volume in 1 second (FEV(1)) was 1.2 L (41% predicted). Mean improvement in trough FEV(1) over 12 weeks was significantly greater with all 3 arformoterol doses (15 microg BID, +16.9%; 25 microg BID, +18.9%; 50 microg QD, +14.9%) and for salmeterol (+17.4%) relative to placebo (+6.0%; P < 0.001). There were significantly greater improvements in the mean percentage change in FEV(1) AUC(0-12h) from the predose value over 12 weeks (15 microg BID, 12.7%, 25 microg BID, 13.9%, 50 microg QD, 18.9%; salmeterol, 9.8%) versus placebo (2.7%; P <or= 0.001); all doses of arformoterol were statistically different from salmeterol for this end point (P <or= 0.024). At week 12, TDI focal scores were significantly greater with all arformoterol doses compared with placebo (mean [95% CI]: 15 microg BID, 0.97 [0.25-1.69]; 25 microg BID, 1.08 [0.3-1.86]; 50 microg QD, 1.04 [0.32-1.771), suggesting treatment-associated improvement in dyspnea, however, the difference between salmeterol and placebo was not statistically significant (0.36 [-0.40 to 1.12]). Improvements in health status, as measured using SGRQ total scores, were -2.6 to -3.6 U in the arformoterol groups, -4.4 U for salmeterol, and -1.2 U for placebo; 95% CI of differences versus placebo suggested significant improvement for the arformoterol 25 microg BID and salmeterol groups. There was a similar frequency of AEs and COPD exacerbations across all groups, including placebo.

CONCLUSIONS

In this trial, patients with moderate to severe COPD administered nebulized arformoterol over 12 weeks were observed to have significant and sustained improvements in airway function and dyspnea compared with placebo. The results also suggest that all doses of arformoterol, including the lowest dose (15 microg BID), were effective. Overall, nebulized arformoterol was well tolerated.

Comparative Molecular Field Analysis of the Binding of the Stereoisomers of Fenoterol and Fenoterol Derivatives to the β2Adrenergic Receptor

Stereoisomers of fenoterol and six fenoterol derivatives have been synthesized and their binding affinities for the beta2 adrenergic receptor (Kibeta2-AR), the subtype selectivity relative to the beta1-AR (Kibeta1-AR/Kibeta2-AR) and their functional activities were determined. Of the 26 compounds synthesized in the study, submicromolar binding affinities were observed for (R,R)-fenoterol, the (R,R)-isomer of the p-methoxy, and (R,R)- and (R,S)-isomers of 1-naphthyl derivatives and all of these compounds were active at submicromolar concentrations in cardiomyocyte contractility tests. The Kibeta1-AR/Kibeta2-AR ratios were >40 for (R,R)-fenoterol and the (R,R)-p-methoxy and (R,S)-1-naphthyl derivatives and 14 for the (R,R)-1-napthyl derivative. The binding data was analyzed using comparative molecular field analysis (CoMFA), and the resulting model indicated that the fenoterol derivatives interacted with two separate binding sites and one steric restricted site on the pseudo-receptor and that the chirality of the second stereogenic center affected Kibeta2 and subtype selectivity.

Enantioselective separation and online affinity chromatographic characterization of R,R- and S,S-fenoterol

BACKGROUND

rac-Fenoterol is a beta2-adrenoceptor agonist (beta2-AR) used in the treatment of asthma. It has two chiral centers and is marketed as a racemic mixture of R,R'- and S,S'-fenoterol (R-F and S-F). Here we report the separation of the R-F and S-F enantiomers and the evaluation of their binding to and activation of the beta2-AR.

METHODS

R-F and S-F were separated from the enantiomeric mixture by chiral chromatography and absolute configuration determined by circular dichroism. Beta2-AR binding was evaluated using frontal affinity chromatography with a stationary phase containing immobilized membranes from HEK-293 cells that express human beta2-AR and standard membrane binding studies using the same membranes. The effect of R-F and S-F on cardiomyocyte contractility was also investigated using freshly isolated adult rat cardiomyocytes.

RESULTS

Chiral chromatography of rac-fenoterol yielded separated peaks with an enantioselectivity factor of 1.21. The less retained peak was assigned the absolute configuration of S-F and the more retained peak R-F. Frontal chromatography using membrane-bound beta2-AR as the stationary phase and rac-3H-fenoterol as a marker ligand showed that addition of increasing concentrations of R-F to the mobile phase produced concentration-dependent decreases in rac-3H-fenoterol retention, while similar addition of S-F produced no change in rac-3H-fenoterol retention. The calculated dissociation constant of R-F was 472 nM and the number of available binding sites 176 pmol/column, which was consistent with the results from the membrane binding study 460 +/- 55 nM (R-F) and 109,000 +/- 10,400 nM (S-F). In the cardiomyocytes, R-F increased maximum contractile response from (265 +/- 11.6)% to (306 +/- 11.8)% of resting cell length (P < 0.05) and reduced EC50 from -7.0 +/- 0.270 to -7.1 +/- 0.2 log[M] (P < 0.05), while S-F had no significant effect.

DISCUSSION

Previous studies have shown that rac-fenoterol acts as an apparent beta2-AR/G(s) selective agonist and fully restores diminished beta2-AR contractile response in cardiomyocytes from failing hearts of spontaneously hypertensive rats (SHR). Here we report the separation of the enantiomers of rac-fenoterol and that R-F is the active component of rac-fenoterol. Further evaluation of R-F will determine if it has enhanced selectivity and specificity for beta2-AR/G(s) activation and if it can be used in the treatment of congestive heart failure.

Cardiac implications for the use of beta2-adrenoceptor agonists for the management of muscle wasting

There are proposals for the implementation of beta(2)-adrenoceptor agonists for the management of muscle wasting diseases. The idea has been initiated by studies in animal models which show that beta(2)-adrenoceptor agonists cause hypertrophy of skeletal muscle. Their use in clinical practice will also need an understanding of possible effects of activation of human heart beta(2)-adrenoceptors. Consequences could include an increased probability of arrhythmias in susceptible patients.

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.

Long-chain formoterol analogues: an investigation into the effect of increasing amino-substituent chain length on the beta2-adrenoceptor activity

The synthesis of a series of long-chain formoterol analogues in which the terminal ether residue of the beta-phenethyl-amino-substituent has been extended beyond the methyl ether residue present in the parent compound are described. Evaluation of these analogues as beta(2)-adrenoceptor agonists was used to provide an insight into the factors controlling the magnitude and duration of receptor activation.

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.

Optimization and validation of a gas chromatographic method for analysis of (RS,SR)-diastereoisomeric impurity in formoterol fumarate

A gas chromatographic method for the determination of formoterol (RS,SR)-diastereoisomer, a process impurity, in formoterol fumarate was optimized and validated. The method involves silylation of formoterol fumarate with N-(trimethylsilyl)imidazole in N,N-dimethylformamide at room temperature in an autosampler vial to produce trimethylsilyl derivatives of the enantiomers prior to GC analysis. The optimized silylation and separation conditions, respectively, produced good yield and resolution of the analytes. The method appears to be convenient and fast, and permits accurate determination of (RS,SR)-diastereoisomer in formoterol fumarate with adequate precision (R.S.D. = 3.0%, n = 9) and sensitivity (DL < 0.01%) when compared with the official liquid chromatographic limit test method of Pharmeuropa. The method was successfully applied to quality control of commercial formoterol fumarate for their (RS,SR)-diastereoisomer contents.

Three-dimensional pharmacology, a subject ranging from ignorance to overstatements

Stereoselectivity has been known to play a role in drug action for 100 years or more. Nevertheless, chiral drugs have been developed and used as racemates, neglecting the fact that they comprise mixtures of two or more compounds which may have quite different pharmacological properties. A very limited access to pure enantiomers in the past has been responsible for this unsatisfactory state of affairs. During the last 20 years, significant achievements have made it possible to perform stereoselective synthesis and analysis. Today, novel chiral drugs are as a rule developed as single enantiomers. Yet, studies of old racaemic drugs are still designed, performed and published without mention of the fact that two or more compounds are involved. In recent years, a number of old racaemic drugs have been re-evaluated and re-introduced into the clinical area as the pure, active enantiomer (the eutomer). While in principle correct, the clinical benefit of this shift from a well established racaemate to a pure enantiomer often seems to be limited and sometimes exaggerated. Racaemic drugs with a deleterious enantiomer that does not contribute to the therapeutic effect (the distomer), may have been sorted out in the safety evaluation process. However, in the future any pharmacological study of racaemic drugs must include the pure enantiomers. This will generate new, valuable information on stereoselectivity in drug action and interaction.

Stereoselective urinary excretion of formoterol and its glucuronide conjugate in human

AIMS

Formoterol is an inhaled beta2-adrenoceptor agonist used as a racemic mixture of the active (R; R)- and inactive (S; S)-enantiomers (rac-formoterol). Glucuronidation is an important route of metabolism in humans which occurs faster for (S; S)-formoterol in human liver microsomes. The aim of this study was to investigate the stereoselectivity of urinary excretion of formoterol and its glucuronide conjugate after oral dosing with rac-formoterol.

METHODS

Seven nonsmoking volunteers (six males, one female) were included in the study. After an overnight fast, a single 60 micro g oral dose of rac-formoterol fumarate dihydrate was ingested. Urine samples were collected at 1 h intervals for the first 4 h, and at 6, 8, 12 and 24 h after dosing. Formoterol enantiomers were analysed by chiral h.p.l.c. assay and formoterol glucuronides were determined as formoterol enantiomers after enzymatic cleavage with beta-glucuronidase.

RESULTS

The female subject displayed a different pattern of metabolism and statistical analysis was therefore limited to data for the six males. The median (range) of the total urinary excretion of formoterol was 37.8% (20.9-51.2%) of the dose. The medians (ranges) of the amounts of (R; R)- and (S; S)-formoterol and of (R; R)- and (S; S)-formoterol glucuronide excreted were 2.1 (1.0-2.9), 3.5 (2.6-3.8), 21.0 (13.1-31.0) and 10.3 (4.2-14.6)%, respectively, of the dose. Unchanged (S; S)-formoterol excretion was significantly greater than that of unchanged (R; R)-formoterol and (R; R)-formoterol glucuronide excretion was significantly greater than that of (S; S)-formoterol glucuronide. The total RR-formoterol (unchanged drug plus glucuronide) excreted was significantly greater than the total (S; S)-formoterol.

CONCLUSIONS

Our study demonstrates that the urinary excretion of formoterol in male humans after oral administration of rac-formoterol is stereoselective with preferential excretion of the active (R; R)-formoterol as unchanged drug and glucuronide. The different pattern of metabolism in the female subject provides impetus for further studies of the effect of gender on the stereoselective metabolism and pharmacokinetics of formoterol.

Biological actions of formoterol isomers

Racemic beta(2) agonists, composed of equal amounts of (R)- and (S)-isomers, can display anomalous actions that compromise their effectiveness as asthma therapies. Loss of efficacy during regular use is characteristic of isoprenaline, albuterol and terbutaline and has in part been attributed to the biological effects of the (S)-isomer. This hypothesis was applied to the (R,R)- and (S,S)-isomers of formoterol. (R,R)-formoterol had 1000-times greater affinity (2.9 nm) to the human beta(2) adrenoceptor than (S,S)-formoterol (3100 nm), with receptor binding modulating intracellular cAMP levels. The minimum lethal intravenous (IV) dose was determined to be 100 mg/kg for (R,R)- and 50 mg/kg for (S,S)-formoterol, suggesting that the toxicity of (S,S)-formoterol may not be related to the binding of beta(2) adrenoceptors. In tissues pretreated with (S,S)-formoterol but not with (R,R)- or racemic formoterol contractions to high concentrations of carbachol were exaggerated. In vivo experiments with sensitized guinea pigs demonstrated that (R,R)-formoterol inhibited both histamine and antigen-induced bronchoconstriction with greater potency than (R,R/S,S)-formoterol while (S,S)-formoterol was ineffective. Metabolic radiolabeling experiments of (R,R)-, (S,S)- or (R,R/S,S)-formoterol with crude human liver phenolsulfotransferase (PST) determined the V(max)/K(m) values to be (0.151), (0.74) and (0.143), respectively. The reciprocal plot illustrates a 2-fold reduction in sulfation rate when (R,R)-formoterol is present as a single isomer. The data presented here suggest that (R,R)-formoterol binds to the beta(2) adrenoceptor and inhibits the contraction of bronchial tissues by spasmogens. However, (S,S)-formoterol exhibits properties inconsistent as an asthma therapeutic and may antagonize the actions of (R,R)-formoterol.

Beta-adrenoceptor agonists and asthma--100 years of development

Inhaled beta(2)-adrenoceptor agonists are by far the most effective and safe bronchodilators currently available. They have not been surpassed by any other bronchodilating principle. The way to this position has been long and started with the first successful treatment of acute, severe asthma with s.c. injections of adrenaline 100 years ago. Over the years, synthetic congeners of adrenaline have been produced and tested for their pharmacological properties. During the first decades, little attention was given airway smooth muscle. The discovery of isoprenaline in 1940 was the first major step towards selective bronchodilation. This compound became a key tool for the classification of adrenoceptors into alpha and beta. Salbutamol and terbutaline were the first to show a significant attenuation of the cardiostimulant effect and confirmed the subdivision of beta-adrenoceptors into beta(1) and beta(2). Much effort was made to eliminate the next dose-limiting side effect, skeletal muscle tremor but in vain. Prolonged duration of action was achieved in three ways: with bambuterol, an orally active carbamate ester prodrug of terbutaline, salmeterol, an inhaled beta(2)-adrenoceptor agonist emerging from a purposeful research project, and formoterol which was found, accidentally, to have a long duration of action when inhaled. Throughout the 20th century, beta-adrenoceptor agonists have been developed and marketed as racemates. The pharmacological activity usually resides in the (R)-enantiomer. Despite claims for the opposite, there is so far no compelling evidence that the presence of the less active (S)-enantiomer is of any harm to the patient. One hundred years of experience of structural modifications of adrenaline has shown that the possibilities to modify the properties of this endogenous prototype appear to be unlimited.

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.

Stereoselective glucuronidation of formoterol by human liver microsomes

AIMS

Formoterol is a beta2-adrenoceptor agonist marketed as a racemic mixture of the active (R; R)- and inactive (S; S)-enantiomers (rac-formoterol). The drug produces prolonged bronchodilation by inhalation but there is significant interpatient variability in duration of effect. Previous work has shown that in humans formoterol is metabolized by conjugation with glucuronic acid but little is known about the stereoselectivity of this reaction. The aim of the present study was to investigate the glucuronidation of formoterol enantiomers in vitro by human liver microsomes.

METHODS

The kinetics of formation of formoterol glucuronides during incubation of racemate and of single formoterol enantiomers with human liver microsomes (n=9) was characterized by chiral h.p.l.c. assay.

RESULTS

The kinetics of glucuronidation of the two formoterol enantiomers obeyed the Michaelis-Menten equation. Glucuronidation of formoterol was stereoselective and occurred more than two times faster for (S; S)-formoterol than for (R; R)-formoterol. In incubations with single formoterol enantiomers, the median (n=9) Km values for (R; R)-glucuronide and (S; S)-glucuronide were 827.6 and 840.4 microm, respectively, and the median V max values were 2625 and 4304 pmol min-1 mg-1, respectively. Corresponding values determined in incubations with rac-formoterol were 357.2 and 312.1 microm and 1435 and 2086 pmol min-1 mg-1 for (R; R)- and (S; S)-glucuronide, respectively. Interindividual variation was large with the ratio of V max/Km (S; S/R; R) ranging from 0.57 to 6.90 for incubations with rac-formoterol.

CONCLUSIONS

Our study demonstrates that glucuronidation of formoterol by human liver microsomes is stereoselective and subject to high interindividual variability. These findings suggest that clearance of formoterol in humans is subject to variable stereoselectivity which could explain the variation in duration of bronchodilation produced by inhaled formoterol in patients with asthma.

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.

Characterisation of beta2-adrenoceptors, using the agonist [11C]formoterol and positron emission tomography

The agonist radioligand N-[2-hydroxy-5-[1-hydroxy-2-[[2-(4-[11C]-methoxyphenyl)-1-methylethyl]am ino]ethyl]phenyl]formamide ([11C]formoterol) was synthesised in order to test its ability to visualise pulmonary beta2-adrenoceptors in vivo, with positron emission tomography (PET). Formoterol was labelled via reaction of a dibenzyl-protected precursor with [11C]CH3I. Subsequent deprotection with Pd/C and H2 yielded [11C]formoterol in 5-15% (corrected for decay) and the specific activity ranged from 5.5-22.2 TBq mmol (150-600 Ci mmol(-1)), 60-70 min after end of bombardment. Biodistribution studies with [11C]formoterol were performed in male Wistar rats which were either untreated or predosed with (D,L)-propranolol hydrochloride (2.5 mg kg(-1), beta-adrenoceptor antagonist), erythro-DL-1-(7-methylindan-4-yloxy)-3-isopropylaminobuta n-2-ol hydrochloride (ICI 118551, 0.15 mg kg(-1), beta2-adrenoceptor antagonist), isoprenaline (15 mg kg(-1), non-subtype selective beta-adrenoceptor agonist) or (+/-)-(2-hydroxy-5-[2-((2-hydroxy-3-(4-((1-methyl-4-trifluoromethyl)1H-i midazol-2-yl-)phenoxy)propyl)amino)ethoxy]benzamide)monomethane sulfonate (CGP 20712A, 0.15 mg kg(-1), beta1-adrenoceptor antagonist). Lungs, heart, liver and plasma were analysed for radioactive metabolites. The kinetics of [11C]formoterol in the lungs of male Wistar rats were investigated by means of a dynamic PET study. The biodistribution studies showed significant specific binding in tissues known to contain beta2-adrenoceptors (lungs, spleen, and heart). Binding in these organs was blocked by ICI 118551 and isoprenaline, but not by CGP 20712A. [11C]Formoterol was rapidly metabolised in rats but lungs and heart did not substantially take up the labelled metabolites. The binding of [11C]formoterol in various tissues of rats is consistent with the beta2-selectivity of formoterol. Whether [11C]formoterol selectively binds to the high affinity state of beta2-adrenoceptors remains to be elucidated. [11C]Formoterol is potentially useful for studying beta2-adrenoceptors with PET and this radioligand may provide new insights in the mechanisms underlying prolonged sympathomimetic action.

Simultaneous measurement of Ca2+ transients and changes in the cell volume and microviscosity of the plasma membrane in smooth muscle cells. Evaluation of the effect of formoterol

The effects of the beta 2-adrenoceptor agonist formoterol (50 nM) on the angiotensin II (20 nM)-induced Ca2+ response and changes in the cell volume and microviscosity of the plasma membrane of vascular smooth muscle cells were studied. Applied as a model substance for the stimulation of the phosphoinositide-phospholipase C pathway, angiotensin II has been used to simulate the bronchospasm of smooth muscle in asthma. Our results demonstrated that angiotensin II-induced smooth muscle contraction primarily involves an InsP3-mediated release of Ca2+ from intracellular stores and, to a minor extent, an enhanced influx of Ca2+ through the plasma membrane. Both the Ca2+ response and the contractile reaction were strongly antagonized by pretreatment of the cells with 50 nM formoterol. The protective effect of formoterol on smooth muscle contractions is proposed to be mainly related to a direct stimulation of beta 2-adrenoceptor-coupled cAMP generation. Moreover, it is predicted that the interaction between the beta 2-adrenoceptor glycoprotein and adenylate cyclase will be enhanced following a formoterol-associated decrease in the microviscosity of the plasma membrane.

Some pharmacodynamic aspects on long-acting beta-adrenoceptor agonists

1. Formoterol and salmeterol are the first members of a new generation of long-acting beta(2)-adrenoceptor agonists for inhalation. The discovery of the long effect duration of formoterol was made by chance, while the development of salmeterol appeared to follow a purposeful research strategy. 2. Preclinical evaluation predictive of the clinical duration of effect of long-acting bronchodilators is not straightforward. Experiments in vitro may give false positive results, while experiments in vivo may show false negative results. 3. Once the principle of a long duration of effect was established, a number of novel long-acting beta(2)-adrenoceptor agonists of various chemical structure have emerged. 4. There are two alternative models for the explanation of the long duration of effect: the exosite binding explaining the mode of action of salmeterol, and the more general diffusion microkinetic model applicable for both formoterol and salmeterol. 5. Long-acting beta-adrenoceptor agonists with a relatively low efficacy like salmeterol may, under certain circumstances, inhibit competitively the relaxing effect of agonists with higher efficacy like formoterol and salbutamol. 6. Like all other beta(2)-adrenoceptor agonists in current clinical use, formoterol and salmeterol comprise racemic mixtures. Only the RR- and R-enantiomers are pharmacologically active. The experimental compounds TA-2005 and picumeterol have been developed as pure RR- and R-enantiomers, respectively.

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

Experiments were made on isolated tissues from guinea-pig to test the hypothesis that the distomers of rac-beta 2-adrenoceptor agonists induce airway hyperreactivity. Tracheal strip preparations were contracted with carbachol. Both rac- and (R;R)-formoterol (2 and 1 mumol/l, respectively) produced an immediate relaxation, followed by a slow recovery of tone. (S;S)-Formoterol (2 mumol/l) had no effect on smooth muscle tone. Similar results were obtained with the enantiomers of terbutaline. In other strip preparations of the trachea or the main bronchi, cholinergic or nonadrenergic/noncholinergic (NANC) excitatory responses were evoked by electrical field-stimulation. The eutomers, (R;R)-formoterol and (R)-terbutaline, inhibited concentration-dependently both cholinergic and NANC-induced contractions. The distomers, (S;S)-formoterol and (S)-terbutaline, showed qualitatively the same effects but were about 1,000 times less potent than the corresponding eutomer. In a third series of experiments, either enantiomer of formoterol was administered to an electrically stimulated vagus nerve-trachea tube preparation. The nerve-induced contractions were inhibited by both enantiomers, but (S;S)-formoterol was about 1,000 times less potent than (R;R)-formoterol. For both enantiomers of formoterol, about tenfold higher concentrations was required to obtain the same degree of inhibition when given intratracheally as compared with administration in the external medium. There was no indication in any of the experimental approaches that (S;S)-formoterol or (S)-terbutaline might enhance the response to cholinergic or NANC-related stimuli.