Great expectations in stereochemistry: focus on antidepressants

Lipase mediated new chemo-enzymatic synthesis of (RS)-, (R)-, and (S)-bunolol

The β-adrenergic receptor blocking agents are an important class of drug molecules. The present study reports a new chemo and chemo-enzymatic synthetic process for (RS)-, (R)-, and (S)-bunolol, one of the potent β-adrenergic receptor blocker. In chemo-enzymatic process, CAL L4777 lipase was employed for enantioselective kinetic resolution to synthesize the enantiopure (R)-alcohol and (S)-ester from the corresponding racemic alcohol. Thereafter, the corresponding (R)-alcohol and deacylated (S)-ester were treated with tert-butylamine to produce (S)- and (R)-bunolol, respectively. In chemical approach, epichlorohydrin (RS-, R-, and S-) was used as a starting material via respective (RS)-, (S)-, and (R)-glycidyl ether as intermediates for synthesis of enantiomeric (RS)-, (R)-, and (S)-bunolol. In comparison between two approaches, it was found that the chemo-enzymatic process was more effective and resulted in enantiomeric excess of 98% with 35% yield.

Chirality of antidepressive drugs: an overview of stereoselectivity

Stereochemistry plays an important role in drug design because the enantiomers of a drug frequently vary in their biological action and pharmacokinetic profiles. Racemates of a drug with either an inactive or an unsafe enantiomer can lead to detrimental effects. The manufacturing industry may still produce racemates, but such decisions must pass through rigorous analyses of the pharmacological and pharmacokinetic characteristics of the particular enantiomer related to the racemates. The pharmacokinetics of antidepressants or antidepressive agents is stereoselective and predominantly favors one enantiomer. The use of pure enantiomers offers (i) better specificity than the racemates in terms of certain pharmacological actions, (ii) enhanced clinical indications, and (iii) optimized pharmacokinetics. Therefore, controlling the stereoselectivity in the pharmacokinetics of antidepressive drugs is of critical importance in dealing with depression and psychiatric conditions. The objective of this review is to highlight the importance of the stereochemistry of antidepressants in the context of the design and development of new chirally pure pharmaceuticals, the potential complications caused by using racemates, and the benefits of using pure enantiomers.

Chiral Discrimination of P-glycoprotein in Parturient Women: Effect of Fluoxetine on Maternal-Fetal Fexofenadine Pharmacokinetics

BACKGROUND AND OBJECTIVE

Fluoxetine, antidepressant widely-used during pregnancy, is a selective inhibitor for P-glycoprotein (P-gp). Fexofenadine, an in vivo P-gp probe, is an antihistamine drug for seasonal allergic rhinitis and chronic urticaria treatment during pregnancy and it is available as a racemic mixture. This study evaluated the chiral discrimination of P-gp investigating the effect of fluoxetine on maternal-fetal pharmacokinetics of fexofenadine.

METHODS

Healthy parturient women received either a single oral dose of 60 mg racemic fexofenadine (Control group; n = 8) or a single oral dose of 40 mg racemic fluoxetine 3 h before a single oral dose of 60 mg racemic fexofenadine (Interaction group; n = 8). Maternal blood and urine samples were collected up to 48 h after fexofenadine administration. At delivery, maternal-placental-fetal blood samples were collected.

RESULTS

The maternal pharmacokinetics of fexofenadine was enantioselective (AUC^0-∞_R-(+)/S-(-) ~ 1.5) in both control and interaction groups. Fluoxetine increased AUC^0-∞ (267.7 vs 376.1 ng.h/mL) and decreased oral total clearance (105.1 vs 74.4 L/h) only of S-(-)-fexofenadine, whereas the renal clearance were reduced for both enantiomers, suggesting that the intestinal P-gp-mediated transport of S-(-)-fexofenadine is influenced by fluoxetine to a greater extent that the R-(+)-fexofenadine. However, the transplacental transfer of fexofenadine is low (~16%), non-enantioselective and non-influenced by fluoxetine.

CONCLUSIONS

A single oral dose of 40 mg fluoxetine inhibited the intestinal P-gp mediated transport of S-(-)-fexofenadine to a greater extent than R-(+)-fexofenadine in parturient women. However, the placental P-gp did not discriminate fexofenadine enantiomers and was not inhibited by fluoxetine.

Pharmacokinetics and Transplacental Transfer of Fluoxetine Enantiomers and Their Metabolites in Pregnant Women

Considering that fluoxetine (FLX) is used to treat depressive states during pregnancy and that it is a cytochrome P450 (CYP)2D6 inhibitor, which is involved in the metabolism of both of its enantiomers, this study aims to describe the enantioselective distribution and metabolism of FLX and of its metabolite norfluoxetine (NorFLX) following a single oral dose. Nine healthy pregnant women received 20 mg FLX at 32 weeks of gestation and later at the day of delivery. The apparent clearance of (S)-(+)-FLX (1.45 vs. 0.66 L/hour/kg) and the area under the plasma concentration vs. time curve (AUC) of the (S)-(+)-NorFLX (AUC_0-∞ 942.7 vs. 498.6 ng hour/mL) were higher (P < 0.05) than those of the respective (R)-(-) enantiomers, indicating that the (S)-(+)-FLX enantiomer is preferentially metabolized to (S)-(+)-NorFLX. The placental transfer (umbilical vein/maternal vein) of FLX and NorFLX is low (30-40%), with the predominant transfer of (S)-(+)-FLX (44 vs. 33%). The distribution of the enantiomers of FLX and NorFLX to amniotic fluid is low (< 10%).

(R)-fluoxetine enhances cognitive flexibility and hippocampal cell proliferation in mice

Fluoxetine is a clinically successful antidepressant. It is a racemic mixture of (R) and (S) stereoisomers. In preclinical studies, chronic treatment with fluoxetine (10 mg/kg) had antidepressant effects correlated with increased hippocampal cell proliferation in adult rodents. However, the contribution of the enantiomers of fluoxetine is largely unknown. We investigated the effects of treatment with (R)- and (S)-fluoxetine on cognitive behavioral paradigms and examined cell proliferation in the hippocampus of C57BL/6J female mice. In a behavioral sequencing task using the IntelliCage system in which discriminated spatial patterns of rewarded and never-rewarded corners were reversed serially, (R)-fluoxetine-treated mice showed rapid acquisition of behavioral sequencing (compared with S-fluoxetine) and cognitive flexibility in subsequent reversal stages in intra- and inter-session analysis. (R)-fluoxetine also increased cell proliferation in the hippocampus, in particular in the suprapyramidal blade of the dentate gyrus. (R)-fluoxetine had superior effects to (S)-fluoxetine in elevated plus maze, forced-swim and tail-suspension tests. These results suggest that (R)-fluoxetine, which has been reported to have a shorter half-life than (S)-fluoxetine, has superior antidepressant effects and more consistently improves spatial learning and memory. This profile offers advantages in depression treatment and may also aid management of the neurocognitive impairments associated with depression.

Pharmacokinetics of mirtazapine: enantioselective effects of the CYP2D6 ultra rapid metabolizer genotype and correlation with adverse effects

Enantiomerically pure drugs and genotyping are promising approaches to achieve optimization in antidepressant therapy. Mirtazapine is a mixed noradrenergic serotoninergic antidepressant used as a racemate. We analyzed pharmacokinetics of its enantiomers in relation to CYP2D6 genotype and in relation to its adverse effects. Mirtazapine was enantioselectively absorbed from the gut with a rate constant of 0.2 min-1 for S+, but 0.08 min-1 for R- mirtazapine. Kinetics of R- mirtazapine was only marginally dependent on CYP2D6 genotype, but total clearance of the S+ enantiomer were 1.3, 2.3, and 3.4 L min-1 in poor, extensive, and ultrarapid metabolizers of CYP2D6 substrates with apparent substantial first-pass metabolism in rapid and ultrarapid metabolizers. Mirtazapine effects on heart rate and blood pressure correlated much more strongly with R- then with S+ concentrations, whereas sedation correlated similarly with both enantiomers. At least concerning some adverse effects, it might be worthwhile to study further mirtazapine enantiospecifically.

Managing depressive and anxiety disorders with escitalopram

This article reviews escitalopram, the S-stereoisomer of the racemic compound citalopram, and a highly selective and potent member of the selective serotonin re-uptake inhibitor class of antidepressants. Escitalopram has a straightforward pharmacokinetic profile, little effect on hepatic metabolism, and is relatively safe in overdose. Similar to other members of the selective serotonin re-uptake inhibitor class, escitalopram (10-20 mg/day) is a well-tolerated and effective treatment of major depressive disorder. Although relatively few head-to-head comparative studies with other antidepressants have been published, pooled analyses of studies using citalopram as the active comparator suggest a modest advantage for the stereoisomer. This advantage, which is more apparent among patients with greater symptom levels, may be attributable to a greater than predicted potency compared with citalopram, presumably as a result of the greater effect of escitalopram at the allosteric binding site of the serotonin transporter. Results of two published studies versus venlafaxine also suggest better tolerability in the context of comparable efficacy. Escitalopram is also approved for the treatment of generalised anxiety disorder (in the US) and social anxiety disorder and panic disorder (in the EU). Pharmacoeconomic models suggest that the greater drug acquisition cost of this patent-protected compound may be offset by greater efficacy (relative to generic citalopram) and tolerability (compared with extended release venlafaxine).

Advances in the enantioseparation of second-generation antidepressant drugs by electrodriven methods

Stereochemistry is steadily increasing in importance in the development of new drugs, and the availability of pure enantiomer drugs can make therapy safer and more efficacious. In particular, almost all second-generation antidepressant drugs possess one or more chiral centres; however, only some of them are administered as single enantiomers. A fundamental part of the quality control of pharmaceutical formulations is the determination of enantiomeric excess and enantiomeric purity; this is also important for the therapeutic drug monitoring of depressed patients. For this purpose, efficient and reliable analytical methods are needed and electrodriven techniques (most of all CE, CEC and MEKC) are very efficient and inexpensive candidates for the role. In this review, the enantioselective electrodriven methods available for the analysis of second-generation antidepressant are presented and discussed. In particular, the following pharmacological classes of antidepressants will be considered: selective serotonin reuptake inhibitors (fluoxetine, citalopram, paroxetine, sertraline); norepinephrine reuptake inhibitors (reboxetine); serotonin and norepinephrine reuptake inhibitors (venlafaxine, milnacipran, duloxetine); and noradrenergic and specific serotonergic antidepressants (mirtazapine).

Single-enantiomer drugs: elegant science, disappointing effects

Most new drugs are marketed as single enantiomers but many older agents are still available in racemic form. As these drugs reach the end of their patent life manufacturers become interested in marketing single enantiomer equivalents. This is called 'chiral switching' and it has been claimed that it will bring clinical benefits in terms of improved efficacy, more predictable pharmacokinetics or reduced toxicity. We reviewed the clinical evidence and prices for three recently marketed single enantiomer versions of widely used racemic drugs: escitalopram, esomeprazole and levosalbutamol. Claims of increased efficacy were based on comparisons of non-equivalent doses and any advantages seemed small and clinically unimportant. Prices of esomeprazole and levosalbutamol were higher than their racemic alternatives and we predict that these prices will remain high despite the market presence of generic versions of the racemates. Patent protection and a perception of superiority based on promotion rather than evidence will maintain price premiums for single enantiomer drugs that are not justified on the basis of clinical performance.

Synthesis and biological evaluation of [11C]talopram and [11C]talsupram: candidate PET ligands for the norepinephrine transporter

PET and SPECT ligands for the norepinephrine transporter (NET) will be important tools for studying the physiology, pathophysiology and pharmacology of the CNS noradrenergic system in vivo. A series of candidate NET ligands were synthesized and characterized in terms of their affinity for human monoamine transporters. The two most promising compounds, talopram and talsupram, were radiolabeled with carbon-11 and evaluated through biodistribution studies in rats and PET imaging studies in a rhesus monkey. Although both compounds displayed high affinity and selectivity for the human NET in vitro, these compounds did not enter the CNS in adequate amounts to be used in PET imaging studies.

Stereochemistry in Drug Action

The importance of stereochemistry in drug action is gaining greater attention in medical practice, and a basic knowledge of the subject will be necessary for clinicians to make informed decisions regarding the use of single-enantiomer drugs. Many of the drugs currently used in psychiatric practice are mixtures of enantiomers. For some therapeutics, single-enantiomer formulations can provide greater selectivities for their biological targets, improved therapeutic indices, and/or better pharmacokinetics than a mixture of enantiomers. This article reviews the nomenclature for describing stereochemistry and enantiomers, emphasizes the potential biological and pharmacologic differences between the 2 enantiomers of a drug, and highlights the clinical experience with single enantiomers of the selective serotonin reuptake inhibitors fluoxetine and citalopram. In some cases, both a mixture of enantiomers and a single-enantiomer formulation of a drug will be available simultaneously. In these cases, familiarity with stereochemistry and its pharmacologic implications will aid the practicing physician to provide optimal pharmacotherapy to his or her patients.