Chirality of antidepressive drugs: an overview of stereoselectivity

Citalopram is a commonly utilized antidepressive agent of the SSRI family [16, 17]. The structure of citalopram or (RS)-1-[3-(dimethylamino)propyl]-1-(4-fluorophenyl)-3H-2-benzofuran-5-carbonitrile is presented in Figure 1. Citalopram has 1 asymmetric center and was originally marketed as a racemate (equimolar ratio of (R)- and (S)-citalopram). As found in numerous studies in vitro and in vivo, (S)-citalopram is pharmacologically more active, with (S)-desmethylcitalopram [(S)-d-citalopram] being active to a smaller extent [18]. Investigations have also indicated (R)-citalopram to be therapeutically dormant and to reduce the activity of (S)-citalopram [19]. After administering (RS)-citalopram, the plasma level of (S)-citalopram is roughly a third of the overall drug, although it is uncertain whether the relatively frequent removal of (S)-citalopram is attributed to the stereoselective activity of cytochrome P450 (CYP) enzymes in the liver [20,21,22].

Figure 1

An enantioselective study of citalopram and its metabolites in femoral blood from about 50 autopsies using a chiral high-performance LC (HPLC) technique indicated that the S:R ratio for citalopram was 0.67 ± 0.25 (mean ± standard deviation [SD]) and that for desmethylcitalopram was 0.68 ± 0.20 (mean ± SD). Further, it was shown that the S:R ratios increase with increasing concentrations of citalopram [23]. “Chiral switch” is an expression used to describe the substitution of an already-accepted-and-sold racemic drug mixture with a pure enantiomer [24, 25]. Citalopram was originally marketed as a racemate, but the difference between the physiological actions of the individual enantiomers resulted in a chiral switch to escitalopram, the pharmacologically active S-(+)-enantiomer of racemic citalopram [26, 27]. Presently, citalopram is among the antidepressive agents that are available on the market as both racemates and a single enantiomer.

The metabolism of citalopram by demethylation produces an effective metabolite, desmethylcitalopram, which is around 6 times less effective than (S)-citalopram; moreover, the metabolite (R)-desmethylcitalopram is about 4-fold more effective than (R)-citalopram. Further, the N-demethylation of desmethylcitalopram to didesmethylcitalopram (DDCIT) has a high inhibitory efficacy and achieves a greater reduction in plasma levels than the parent drug and its major metabolites (Figure 1) [26, 28]. Application of (S)-citalopram has many benefits compared with (RS)-citalopram, such as enhanced efficacy, lower dosage, and reduction of negative effects of (R)-citalopram; this is an ideal model for the advantages of chiral switch [18]. Depending on the amount of (S)-citalopram administered (whether as (S)-citalopram or as (RS)-citalopram), a specified dosage of (S)-citalopram was anticipated to be 2 times the dosage of (RS)-citalopram [29, 30]. Nevertheless, the outcomes of clinical trials indicated preferable results while using (S)-citalopram [31].

An influence of the interaction of (R)-citalopram on (S)-citalopram, with greater affinity toward the principal binding site and its detachment from the serotonin transporter through an allosteric mechanism, can describe the potential antagonism of (R)-citalopram to the effect of (S)-citalopram. The behavior of serotonin in coupling to 2 sites (the main and allosteric sites) on the transporter may be accountable for the greater binding and, thus, the higher hindrance of the neurotransmitter by (S)-citalopram, because (S)-citalopram can be regarded as an allosteric serotonin reuptake inhibitor [32].

In addition, (R)-citalopram is a substrate for CYP2D6, and, therefore, hereditary polymorphism and variance in drug quantities are avoided by the administration of (S)-citalopram [33]. Findings of randomly assigned, double-blind, placebo-controlled, regulated clinical trials demonstrate that (S)-citalopram has a higher efficacy than (RS)-citalopram at dose levels that were anticipated to be comparable to (RS)-citalopram at doses that resulted in less drug termination [34]. Clinical studies indicate that (S)-citalopram, may have a faster onset than (RS)-citalopram and that the single enantiomer tends to be safer and more effective than the racemates [35, 36]. However, there has been no definitive proof that individuals with severe depression who adapt well to the racemate benefit by shifting to (S)-citalopram.

Fluoxetine belongs to the important class of antidepressive agents, the SSRIs. Fluoxetine or N-methyl-3-phenyl-3-[4-(trifluoromethyl)phenoxy]propan-1-amine was the first SSRI therapy introduced and has been used to treat major depressive disorder, panic disorder, obsessive–compulsive disorder, premenstrual dysphoric disorder, and nervous bulimia [37, 38]. Fluoxetine contains a chiral center (Figure 2). Unlike citalopram, fluoxetine could not be developed as a single-enantiomeric form and is still sold as a racemate. Both (R)- and (S)-fluoxetine display similar activity in blocking serotonin reuptake; however, the 2 enantiomers are metabolized differently [39, 40]. Administration of (R)-fluoxetine leads to less-variable plasma concentrations of fluoxetine and its active metabolites than (RS)-fluoxetine. The (R)-enantiomer and its metabolites inhibit CYP2D6 to a lesser extent than the (S)-enantiomer and its metabolites [41]. The racemate of fluoxetine has been observed to be a safe and efficient anti-depressive agent for >15 years, and (R)-fluoxetine is not used due to safety issues.

Figure 2

The metabolism of fluoxetine via N-demethylation by the CYP enzyme system in the liver is stereoselective, leading toward an active chiral metabolite, norfluoxetine. Norfluoxetine has a pharmacological profile comparable to that of fluoxetine, but achieves different plasma concentrations and leads to the pharmacological effect seen with fluoxetine therapy [42]. Unlike the 2 enantiomers of fluoxetine, the metabolites of (S)-fluoxetine [(S)-norfluoxetine] and (R)-fluoxetine [(R)-norfluoxetine] show different efficacies as compared with SSRIs: (S)-norfluoxetine is more effective than (R)-norfluoxetine. Plasma levels of (S)-norfluoxetine in adult patients were higher than levels of (R)-norfluoxetine in patients treated with (RS)-fluoxetine [42]. (R)-Fluoxetine and (S)-fluoxetine present distinct metabolic processes, because (R)-fluoxetine clearance is around 4-fold higher than that of (S)-fluoxetine clearance [43]. The half-lives for the elimination of (S)-fluoxetine and (S)-norfluoxetine are influenced by the variation in CYP2D6 activity to a higher degree than that of the (R)-enantiomers [42, 43]. The stereo-selective metabolism of fluoxetine and norfluoxetine in vitro indicate that N-demethylation is also involved in metabolic processes; in addition, CYP2D6, CYP2C9, and—to a smaller extent—CYP2C19 are also involved in N-demethylation, with a preference for R-norfluoxetine. Conversely, CYP2D6 exhibits greater interaction toward (S)-norfluoxetine than (R)-norfluoxetine [42, 44]. Therefore, clinical trials have been conducted to evaluate the safety and effectiveness of (R)-fluoxetine. However, administration of higher doses of (R)-fluoxetine in Phase II clinical trials resulted in a small, but significant, effect on cardiac repolarization, and so the investigations were terminated [37, 45]. (S)-Fluoxetine has also been investigated for migraine prophylaxis in clinical trials, but has not obtained U.S. Food and Drug Administration (FDA) regulatory approval to date [37]. Perinatal SSRI exposure produces brain-wide differences in the functional activity of the unborn child during adolescence and adulthood [46].

In 1991, sertraline was approved by the U.S. FDA for the treatment of severe depression and, in 2003, approved for the management of social anxiety disorder [47]. Sertraline has been reported to be successful in the treatment of generalized anxiety disorder for nearly 20 years [48, 49]. Sertraline or (1S,4S)-4-(3,4-dichlorophenyl)-N-methyl-1,2,3,4-tetrahydronaphthalen-1-amine has been utilized for the diagnosis and treatment of major depression, panic, obsessive–compulsive, and social anxiety disorders [50].

The structure of sertraline comprises 2 stereocenters and thus has 4 stereoisomers, but it is sold as a single enantiomer because only the stereoisomer with the 1S,4S configuration is clinically effective [51]. Across its preparation, substantial amounts of (−)-cis-(1R,4R)-N-methyl-4-(3,4-dichlorophenyl)-1,2,3,4-tetrahydro-1-naphthalenemine hydrochloride, trans-(1S,4R), and (1R,4S)-N-methyl-4-(3,4-dichlorophenyl)-1,2,3,4-tetrahydro-1-naphthalenemine hydrochloride are produced, which are considered as impurities (Figure 3). The stereoisomer used for the treatment has a configuration (+)-cis-(1S,4S).

Figure 3

Studies conducted to determine the enantiomers of sertraline in Wistar rat plasma 6 h after oral administration of rac-sertraline indicated the presence of trans isomers (1S,4R) and (1R,4S) in the plasma. This confirmed that only the cis-(1S,4S) and cis-(1R,4R) enantiomers were present in Wistar rats when rac-sertraline was administered. However, in the rat plasma, no sertraline metabolites were found [52]. A pharmacokinetic study of sertraline found the kinetics to be stereoselective in terms of the area under the curve (AUC_0–∞) (P = 0.68), clearance (P = 0.68), and maximum serum concentration of a drug (C_max) (P = 0.43). Wistar male rats displayed greater plasma levels of cis-(1S,4S)-sertraline. Further, higher AUC_0–∞ was observed for cis-(1S,4S)-sertraline than for the cis-(1R,4R)-sertraline enantiomer. The disposition of sertraline is enantioselective in male Wistar rats, with a (+):(−) plasma concentration ratio (AUC) near 1.04. The C_max was also greater for (+)-cis-(1S,4S)-sertraline than for (−)-cis-(1R,4R)-sertraline [52].

Paroxetine is another SSRI used in the treatment of all kinds of depressive disorders. The paroxetine enantiomer used in therapeutics is (−)-trans-(3S,4R)-3-(1,3-benzodioxol-5-hydroxymethyl)-4-(4-fluorophenyl)piperidine (Figure 4); it is composed of 2 asymmetric centers and can form 4 stereoisomers, of which 2 have trans and 2 have cis conformations. Paroxetine is sold as an individual enantiomer, that is, the (−)-trans-(3S,4R) stereoisomer, which is therapeutically more active than the other stereoisomers [53]. However, in drug preparations, its inactive enantiomer, (+)-trans-paroxetine, may also be present [54].

Figure 4

Venlafaxine was first manufactured in the early 1980s and is known to suppress the reuptake of serotonin and noradrenaline with low efficacy [54]. Venlafaxine has been demonstrated to function in animal models of anxiety in vivo and has less interaction with postsynaptic muscarinic or histaminergic receptors [54]. Venlafaxine is expected to have a higher tolerability profile than tricyclic antidepressive agents and is among the most widely used drugs in the SNRI category [44,45,46,47,48,49,50,51,52,53]. Venlafaxine is used as an antidepressive agent and is administered as a racemic mixture consisting of the enantiomers (S)- and (R)-venlafaxine in equal quantities [55]. Enantiomers of venlafaxine or 1-[2-(dimethylamino)-1-(4-methoxyphenyl)ethyl]cyclohexan-1-ol (Figure 5) display different therapeutic behaviors, and they affect neurotransmitters differently. The (R)-form of venlafaxine is a potent inhibitor of the reuptake of serotonin noradrenaline, whereas the (S)-form is more selective in suppressing the reuptake of serotonin only [56, 57]. However, both enantiomeric forms exhibit practical therapeutic activity in treating depression, although they interact with the 2 neurotransmitters in different ways [58].

Figure 5

(S)-Venlafaxine displays greater potency toward CYP2D6 as an inhibitor than (R)-venlafaxine in vitro; it is selectively metabolized at clinical doses by CYP2D6 in humans, whereas the opposite occurs at high doses [59, 60]. Venlafaxine is primarily metabolized in the liver to O-desmethylvenlafaxine, N-desmethylvenlafaxine, N,O-didesmethylvenlafaxine, and potent metabolites, which are also chiral compounds. The major metabolic route in humans is the O-demethylation of racemic venlafaxine to O-desmethylvenlafaxine, while N-demethylation of the alkylamino side chain in N-desmethylvenlafaxine and the removal of both O-methyl and N-methyl groups in N,O-didesmethylvenlafaxine are the minor routes. Among these, the O-desmethyl derivative maintains similar pharmacological interaction as the parent drug, whereas the N-desmethyl derivative is also effective, although it is a weaker inhibitor of serotonin and norepinephrine reuptake than venlafaxine [61]. The (R)- and (S)-enantiomeric forms of the metabolites preserve their parent drug's respective effectiveness to inhibit serotonin and norepinephrine reuptake [61].

Duloxetine or (+)-S-N-methyl-3-naphthalene-1-yloxy-3-thiophen-2-ylpropan-1-amine (Figure 6) is an important SNRI used to treat major depression, general anxiety, fibromyalgia, and chronic pain [62, 63]. It may also be used to treat certain medical problems such as urinary incontinence, in addition to the treatment of psychiatric disorders [64]. It has a chiral center and is applied as a single enantiomer in medication, namely, as (S)-duloxetine. Both the enantiomeric forms of duloxetine effectively inhibit norepinephrine and serotonin reuptake, although the (S)-form is 2-fold more active than the (R)-form and is sold as a single-enantiomeric form for clinical therapy [65, 66]. Studies on the binding of duloxetine enantiomers to human serum albumin indicate that the binding constant for the (R)-enantiomer is greater than that for rac-duloxetine. Moreover, the effect of pH on the binding constants showed that at pH 7.4, both (R)-enantiomer and rac-duloxetine showed similar binding, but at higher pH (8.5), (R)-duloxetine showed better binding ability than racduloxetine [67].

Figure 6

The direct enantioseparation of duloxetine and its R-enantiomer has been achieved by HPLC using hydroxypropyl-β-cyclodextrin as a chiral selector and vancomycin as a chiral stationary phase (Chirobiotic V), with a limit of detection of 0.06 μg per mL [68].

Milnacipran is an SNRI accepted for the treatment of fibroma by the U.S. FDA and for the treatment of depression by agencies elsewhere [69]. Milnacipran or (2-(aminomethyl)-N,N-diethyl-1-phenylcyclopropane-1-carboxamide) possesses 2 chiral centers in its structure (Figure 7) and is available as an equimolar mixture of the cis isomers with the following conformation: d-milnacipran (1S,2R) and l-milnacipran (1R,2S). Pharmacokinetic studies in humans [70, 71] have demonstrated a median peak plasma time (t_max) of 2 h. Milnacipran is steadily absorbed from the gastrointestinal system. The absolute oral bioavailability of milnacipran is large (85%), and diet does not affect absorption. The plasma half-life is around 8 h, and approximately 50%–60% of milnacipran is excreted unaltered in the urine. When administered orally, nearly the entire dose (about 93%) is eliminated steadily in the urine.

Figure 7

Elimination of the milnacipran carbamoyl O-glucuronide metabolite in urine (about 19% of the dose) is primarily as a metabolite of l-milnacipran carbamoyl O-glucuronide (about 17% of the dose); approximately 8% of the dose excreted in urine is a metabolite of N-desethylmilnacipran. No significant quantities of extra metabolites are secreted in the urine. Comparable plasma levels of milnacipran and l-milnacipran carbamoyl O-glucuronide metabolite have been detected after medication, and the highest possible plasma level of l-milnacipran carbamoyl O-glucuronide metabolite at 4 h after dosing was 234 ng per mL. Low levels of N-desmethylmilnacipran and d-milnacipran carbamoyl O-glucuronide metabolites (<25 ng per mL of milnacipran) have been reported in the plasma [70].

Reboxetine or (2RS)-2-[(RS)-(2-ethoxyphenoxy)-phenylmethyl]morpholine is the most effective NRI medication used to manage anxiety. Reboxetine has 2 chiral centers (Figure 8) and can therefore appear as a racemate; nevertheless, during enantioselective synthesis, mostly (R,R)-, and (S,S)-reboxetine are formed [72]. Therefore, it is available as a racemate in clinical preparations. However, the 2 enantiomers show substantially different pharmacological properties [73, 74]. For instance, the C_max and AUC_0–tz of (R,R)-reboxetine are greater than those of (S,S)-reboxetine [75]. Conversely, several reports indicate that (S,S)-reboxetine is a more active NRI than (R,R)-reboxetine [76] and is accountable for the vasomotor and adverse effects of reboxetine on the heart [77].

Figure 8

Receptor-binding models in vitro and in vivo indicate that (S,S)-reboxetine is the most active NRI [78], even though its plasma levels are almost half after either injectable or oral administration [79]. The activity is not a consequence of stereoselective metabolism, because CYP3A4 acts on both the enantiomers to about the same extent in vitro [80, 81]. The enantiomers exhibit identical half-lives, irrespective of the route of administration [74, 81]. The difference in plasma levels between the enantiomeric forms is likely due to better renal clearance of (S,S)-reboxetine [74, 82]. The predicted distribution of reboxetine enantiomers following an injectable dose is approximately 2.4-times higher. The difference indicates that enantioselective disposition of reboxetine can result from an association or redistribution with enantioselective tissue. Reboxetine is available as a racemic drug with an explicitly active enantiomer, whereby its enantioselective nature has impaired the accurate description of the dosage–effect interaction [77].

Bupropion ((±)-2-(tert-butyl amino)-1-(3-chlorophenyl)propane-1-one) is an important member of the NDRI class and has 1 chiral center (Figure 9). It is used clinically as a racemic mixture [81, 82] and as its pure enantiomers [83]. The (R)-enantiomer is more effective than the (S)-enantiomer [84]. Bioactivation of bupropion is catalyzed predominantly by stereoselective CYP2B6 and CYP2B6-catalyzed bupropion hydroxylation [85].

Figure 9

Investigations in vitro have established that bupropion may be a noncompetitive, functional inhibitor of nicotinic acetylcholine receptors [86]. The stereoselective pharmacokinetics of bupropion and its metabolites, namely, (R)-bupropion:(S)-bupropion and (1R,2R)-:(1S,2S)-threohydrobupropion, and their ratios are probably indicative of greater presystemic metabolism of (S)- versus (R)-bupropion by carbonyl reductases. Remarkably, the apparent renal clearance of (2S,3S)-hydroxybupropion is nearly 10 times greater than that of (2R,3R)-hydroxybupropion [87]. Bupropion is metabolized into 3 active metabolites, namely, hydroxybupropion, erythrohydrobupropion, and threohydrobupropion (Figure 9), which further metabolize into ineffective metabolites and are eventually eliminated via the urinary tract [88]. Hydroxybupropion is the main metabolite of bupropion detected in human plasma [89] and is obtained by the hydroxylation of its tert-butyl group and amino alcohol isomers and the consequent morpholino ring formation [90]. The C_max and AUC_0–1 for (S)-bupropion were found to be 3 and about 2 times higher respectively, than those of (R)-bupropion [90]; whereas, the C_max and AUC_0–∞ for (R,R)-hydroxybupropion were 11 and 7 times higher respectively, than for (S,S)-hydroxybupropion. Further, the AUC ratio of metabolite to parent plasma was found to be 12–15 times higher [84]. By contrast, a study of bupropion in patients with kidney disease found that its pharmacokinetics is stereoselective. In the context of kidney disease, the time for rac-bupropion, (R)-bupropion, and (S)-bupropion to display t_max was 4 h [90]. While the t_max for hydroxybupropion and (R,R)-hydroxybupropion was found to be around 16 h, (S,S)-bupropion showed a t_max of 8.8 h and demonstrated a faster absorption [89]. The AUC of (R)- and (S)-bupropion were 30% and 70% of the AUC of racbupropion, respectively; thus, indicating that (S)-bupropion is the main enantiomer distributed in normal patients [84, 87]. The AUC of (R,R)-hydroxybupropion and (S,S)-hydroxybupropion were 95% and 5% of the AUC of hydroxybupropion, respectively; thus presenting (R,R)-hydroxybupropion as the prime metabolite [90].