Stereoisomers and drug toxicity. The value of single stereoisomer therapy

Determination of the Handedness of Urea Inclusion Compounds

Since the discovery of urea inclusion compounds (UICs) in 1940, the handedness of this chiral helical system has not been established experimentally. Here three UIC systems containing only light atoms were studied. The optical rotations were first measured, and the absolute structures of the enantiomorphic domains of three UICs were determined by single crystal X-ray diffraction (SCXRD). The correlation between the optical rotation and the absolute configuration of the UICs was finally established, showcasing the power of absolute structure determination by SCXRD, which is essential in structural chemistry and pharmaceutical research.

Chiral Separation of Cannabichromene, Cannabicyclol, and Their Acidic Analogs on Polysaccharide Chiral Stationary Phases

Until recently, chirality has not been a major focus in the study of cannabinoids, as most cannabinoids of interest, such as cannabidiol and tetrahydrocannabinol, exist as a single isomer from natural sources. However, this is changing as more cannabinoids are identified, and compounds such as cannabichromene and cannabicyclol are emerging as potential investigatory candidates for varying indications. Because these molecules are chiral, the separation and study of the individual enantiomers' biological and physiological effects should therefore be of interest. The purpose of this study was to identify analytical separation conditions and then adapt those conditions to preparative separation. This was accomplished with a column-screening approach on Daicel's immobilized polysaccharide chiral stationary phases using non-traditional mobile phases, which included dichloromethane, ethyl acetate, and methyl tert-butyl ether under high-performance liquid chromatography conditions. CHIRALPAK^® IK was found to separate all four compounds well with mobile phases containing hexane-dichloromethane (with or without an acidic additive). From these methods, the separation productivities were calculated to better visualize the separation scalability, which shows that the kilogram-scale separations of each are feasible.

Host Defence Peptides: A Potent Alternative to Combat Antimicrobial Resistance in the Era of the COVID-19 Pandemic

One of the greatest challenges facing the medical community today is the ever-increasing trajectory of antimicrobial resistance (AMR), which is being compounded by the decrease in our antimicrobial armamentarium. From their initial discovery to the current day, antibiotics have seen an exponential increase in their usage, from medical to agricultural use. Benefits aside, this has led to an exponential increase in AMR, with the fear that over 10 million lives are predicted to be lost by 2050, according to the World Health Organisation (WHO). As such, medical researchers are turning their focus to discovering novel alternatives to antimicrobials, one being Host Defence Peptides (HDPs). These small cationic peptides have shown great efficacy in being used as an antimicrobial therapy for currently resistant microbial variants. With the sudden emergence of the SARS-CoV-2 variant and the subsequent global pandemic, the great versatility and potential use of HDPs as an alternative to conventional antibiotics in treating as well as preventing the spread of COVID-19 has been reviewed. Thus, to allow the reader to have a full understanding of the multifaceted therapeutic use of HDPs, this literature review shall cover the association between COVID-19 and AMR whilst discussing and evaluating the use of HDPs as an answer to antimicrobial resistance (AMR).

Antisense Oligonucleotide Therapy: From Design to the Huntington Disease Clinic

Huntington disease (HD) is a fatal progressive neurodegenerative disorder caused by an inherited mutation in the huntingtin (HTT) gene, which encodes mutant HTT protein. Though HD remains incurable, various preclinical studies have reported a favorable response to HTT suppression, emphasizing HTT lowering strategies as prospective disease-modifying treatments. Antisense oligonucleotides (ASOs) lower HTT by targeting transcripts and are well suited for treating neurodegenerative disorders as they distribute broadly throughout the central nervous system (CNS) and are freely taken up by neurons, glia, and ependymal cells. With the FDA approval of an ASO therapy for another disease of the CNS, spinal muscular atrophy, ASOs have become a particularly attractive therapeutic option for HD. However, two types of ASOs were recently assessed in human clinical trials for the treatment of HD, and both were halted early. In this review, we will explore the differences in chemistry, targeting, and specificity of these HTT ASOs as well as preliminary clinical findings and potential reasons for and implications of these halted trials.

Chiral liquid chromatography-tandem mass spectrometry analysis of superwarfarin rodenticide stereoisomers - Bromadiolone, difenacoum and brodifacoum - In human plasma

Superwarfarins are second-generation long-acting anticoagulant rodenticides that can cause unintended human and wildlife toxicity due, in part, to their prolonged half-lives. Commercially available superwarfarin rodenticides are synthesized as racemates with two asymmetric carbons, producing four stereoisomers. To support studies of human plasma half-lives of individual superwarfarin stereoisomers, a method was developed based on LC-MS/MS to separate and quantify stereoisomers of the commercially important superwarfarins bromadiolone, difenacoum and brodifacoum. Human plasma samples were prepared using protein precipitation and centrifugation. Chiral-phase HPLC separation was carried out on-line with tandem mass spectrometric quantitative analysis of the eluting stereoisomers using selected-reaction monitoring with positive ion electrospray on a triple quadrupole mass spectrometer. All four stereoisomers of each superwarfarin were resolved within 12.5 min with calibration curves spanning 2-3 orders of magnitude and lower limits of quantitation between 0.87 and 2.55 ng/mL. This method was used to determine the half-lives of superwarfarin stereoisomers in plasma from patients who had inhaled synthetic cannabinoid products contaminated with superwarfarins. These data may be used to guide the development of safer next generation anticoagulant rodenticides stereoisomers.

Enantiomeric trans β-aryl-δ-iodo-γ-lactones derived from 2,5-dimethylbenzaldehyde induce apoptosis in canine lymphoma cell lines by downregulation of anti-apoptotic Bcl-2 family members Bcl-xL and Bcl-2

For many years, studies focused on developing new natural or synthetic compounds with antineoplastic activity have attracted the attention of researchers. An interesting group of such compounds seem to be those with both lactone moiety and an aromatic ring which, in addition to antimicrobial or antiviral activity, also exhibit antitumor properties. The study shows antitumor activity of two enantiomeric trans isomers of 5-(1-iodoethyl)-4-(2',5'-dimethylphenyl)dihydrofuran-2-one. Our aim was to determine their antitumor activity manifested as an ability to induce apoptosis in selected canine cancer cell lines as well as to evaluate differences in their strength depending on the configuration of their stereogenic centers. The enantiomers (+)-(4R,5S,6R)-1 and (-)-(4S,5R,6S)-2 were found to induce classical caspase-dependent apoptosis through downregulation of the expression of anti-apoptotic proteins Bcl-xL and Bcl-2. Although the mechanism of apoptosis induction was the same for both enantiomers, they differed in their strength, as stronger antineoplastic activity in vitro was exhibited by isomer (+)-(4R,5S,6R)-1.

Differential effects of the enantiomers of tamsulosin and tolterodine on P-glycoprotein and cytochrome P450 3A4

The pregnane X receptor (PXR) is a transcription factor regulating P-glycoprotein (P-gp; ABCB1)-mediated transport and cytochrome P450 3A4 (CYP3A4)-mediated metabolism of xenobiotics thereby affecting the pharmacokinetics of many drugs and potentially modulating clinical efficacy. Thus, pharmacokinetic drug-drug interactions can arise from PXR activation. Here, we examined whether the selective α1-adrenoreceptor blocker tamsulosin or the antagonist of muscarinic receptors tolterodine affect PXR-mediated regulation of CYP3A4 and of P-gp at the messenger RNA (mRNA) and protein level in an enantiomer-specific way. In addition, the effect of tamsulosin and tolterodine on P-gp activity was evaluated. We used quantitative real-time PCR, gene reporter assay, western blotting, rhodamine efflux assay, and calcein assay for determination of expression, activity, and inhibition of P-glycoprotein. The studied compounds significantly and concentration-dependently increased PXR activity in the ABCB1-driven luciferase-based reporter gene assay. We observed much stronger induction of ABCB1 mRNA by S-tamsulosin as compared to the R or racemic form. R or racemic form of tolterodine and R-tamsulosin concentration-dependently increased P-gp protein expression; the latter also enhanced P-gp efflux function in a rhodamine-based efflux assay. R-tamsulosin and all forms of tolderodine slightly inhibited P-gp. The effect on CYP3A4 expression followed the same pattern but was much weaker. Taken together, tamsulosin and tolterodine are demonstrated to interfere with P-gp and CYP3A4 regulation in an enantiomer-specific way.

Artificial evolution of graphene oxide chemzyme with enantioselectivity and near-infrared photothermal effect for cascade biocatalysis reactions

It is highly desirable and challenging when the chemzyme can be not only simply duplicating and imitating the properties of natural enzymes, but also introducing additional new features for practical applications. Herein, we report a zinc-finger-protein like α-helical chiral metallo-supramolecular complex ([Fe2L3](4+)) functionalized graphene oxide (GO-COOH) as a peroxidase mimic. This artificial enzyme integrates the characteristics of both chiral metallo-supramolecular complex and GO-COOH, and shows excellent catalytic activity. More intriguingly, the novel chemzyme turn out to have enantioselectivity and near-infrared photothermal effect. To the best of our knowledge, this is the first example that the chemzyme has such new features. Based on these properties, we have demonstrated three examples for the applications of our designed enzyme: 1) Intracellular H2O2 detection in PC12 cells against Alzheimer's disease; 2) Discrimination between the chiral drug, Levodopa (L-dopa), the gold standard for treating Parkinson's disease and its enantiomer, D-dopa. This is important because L-dopa is the most effective drug at present used to combat Parkinson's disease while D-dopa is inactive and can even cause side effects, thus for drug efficacy it must be free of D-dopa in the formulation; 3) Remote control of enzyme cascade biocatalysis reactions using high transparent, bio-friendly near-infrared (NIR) light. NIR allows remote activation with relatively high spatial and temporal precision. Our work will provide new insights into design and construction of novel chemzyme with more advanced features beyond intrinsic enzyme property.

Chiral toxicology: it's the same thing...only different

Chiral substances possess a unique architecture such that, despite sharing identical molecular formulas, atom-to-atom linkages, and bonding distances, they cannot be superimposed. Thus, in the environment of living systems, where specific structure-activity relationships may be required for effect (e.g., enzymes, receptors, transporters, and DNA), the physiochemical and biochemical properties of racemic mixtures and individual stereoisomers can differ significantly. In drug development, enantiomeric selection to maximize clinical effects or mitigate drug toxicity has yielded both success and failure. Further complicating genetic polymorphisms in drug disposition, stereoselective metabolism of chiral compounds can additionally influence pharmacokinetics, pharmacodynamics, and toxicity. Optically pure pharmaceuticals may undergo racemization in vivo, negating single enantiomer benefits or inducing unexpected effects. Appropriate chiral antidotes must be selected for therapeutic benefit and to minimize adverse events. Enantiomers may possess different carcinogenicity and teratogenicity. Environmental toxicology provides several examples in which compound bioaccumulation, persistence, and toxicity show chiral dependence. In forensic toxicology, chiral analysis has been applied to illicit drug preparations and biological specimens, with the potential to assist in determination of cause of death and aid in the correct interpretation of substance abuse and "doping" screens. Adrenergic agonists and antagonist, nonsteroidal anti-inflammatory agents, SSRIs, opioids, warfarin, valproate, thalidomide, retinoic acid, N-acetylcysteine, carnitine, penicillamine, leucovorin, glucarpidase, pesticides, polychlorinated biphenyls, phenylethylamines, and additional compounds will be discussed to illustrate important concepts in "chiral toxicology."

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.

Antibiotic safety assessment

Antibiotics usually have positive risk-benefit ratios, their adverse effects being generally mild and reversible on treatment cessation. However, severe adverse drug reactions (ADR), associated with significant mortality and morbidity have resulted in the withdrawal of several active antibiotics, including new fluoroquinolones. Adverse reactions to antibiotics are often poorly documented. The purpose of this article is to examine current tools for investigating and preventing antibiotic toxicity and to suggest future lines of investigation. Structure/ADR relationships have been investigated with various antibiotics (beta-lactams, macrolides, quinolones, etc.) in an attempt to reduce the risk of adverse reactions. Some reactions can be linked to the drug's stereochemical composition. In the case of quinolones for instance, particularly ofloxacin and its derivatives, experimental data show that individual enantiomers have different toxicities. Another major factor that influences the risk of ADRs in a given population is metabolic variability, due to genetic differences in the relevant drug-metabolizing enzymes. Idiosyncratic antibiotic toxicity can be caused by a chemically reactive metabolite. Recent advances in molecular biology, and especially in individual genomic characterization (DNA chip technology, etc.), could in future be useful for identifying patients who are at a special risk of ADR. Finally, certain pharmacokinetic parameters (AUC, Cmax, etc.) can be used to predict adverse effects.

Recent applications of stereoselective chromatography

Some recent applications of stereoselective chromatography in the fields of clinical pharmacy, drug analysis, food, and natural products are reviewed. The review is documented with up-to-date literature, which will assist further expansion of research in these areas.

The pharmacokinetics of levosalbutamol: what are the clinical implications?

Salbutamol (albuterol) is a beta2-adrenoceptor agonist used as a bronchodilator for the treatment of asthma and as a uterine relaxant for the suspension of premature labour. Salbutamol has been marketed as a racemic mixture, although beta2-agonist activity resides almost exclusively in the (R)-enantiomer. The enantioselective disposition of salbutamol and the possibility that (S)-salbutamol has adverse effects have led to the development of an enantiomerically pure (R)-salbutamol formulation known as levosalbutamol (levalbuterol). Salbutamol is metabolised almost exclusively by sulphotransferase (SULT) 1A3 to an inactive metabolite. (R)-Salbutamol is metabolised up to 12 times faster than (S)-salbutamol. This leads to relatively higher plasma concentrations of (S)- salbutamol following all routes of administration, but particularly following oral administration because of extensive metabolism by the intestine. Enantiomer concentrations are similar for the first hour following an inhaled dose, reflecting the fact that salbutamol in the lung probably undergoes little metabolism. Subsequently, (S)-salbutamol predominates due to absorption and metabolism of the swallowed portion of the inhaled dose. Following oral or inhaled administration of enantiomerically pure salbutamol, a small amount (6%) is converted to the other enantiomer, probably by acid-catalysed racemisation in the stomach. Tissue binding of salbutamol is not enantioselective and plasma protein binding is relatively low. Both enantiomers are actively excreted into the urine. Compared with healthy individuals, patients with asthma do not have substantially different pharmacokinetics of the salbutamol enantiomers, but they do appear to have less drug delivered to the lung following inhaled administration because of their narrowed airways. Levosalbutamol elicits an equal or slightly larger response than an equivalent dose of the racemic mixture. This is probably due to competitive inhibition between the enantiomers at beta-adrenoceptors. Pharmacokinetic-pharmacodynamic relationships for levosalbutamol show relatively large interindividual variations. Functionally significant genetic polymorphisms have been identified for beta2-adrenoceptors, SULT1A3 and organic action transporters, all of which affect the disposition or action of levosalbutamol. Animal, in vitro and some clinical studies have reported deleterious effects of (S)-salbutamol on smooth muscle contractility or lung function. However, well-designed clinical studies in patients with asthma have failed to find evidence of significant toxicity associated with (S)-salbutamol. The clinical consequences of relatively higher plasma concentrations of (S)-salbutamol following administration of racemate remain unclear, but in the absence of clear evidence of toxicity the clinical superiority of levosalbutamol over racemic salbutamol appears to be small.

Designing chiral libraries for drug discovery

Identification of new biological targets combined with combinatorial medicinal chemistry has enormous potential to facilitate the discovery of new therapeutics. However, these new targets might require the use of different or more complex screening libraries. This complexity might be achieved through the use of new templates or the identification of new chemistries. One approach is the use of chiral libraries that have been designed using three-dimensional pharmacophore and shape descriptors to provide maximal structure-activity information after screening. The advantages of using a designed chiral library in primary screening and the information one should obtain are discussed.

Simultaneous enantiomeric determination of dansyl-D,L-phenylalanine by fluorescence spectroscopy in the presence of alpha-acid glycoprotein

Few techniques are amenable to real-time analysis of enantiomers. In this paper, total complexation by alpha-acid glycoprotein (AGP) is shown to discriminate between enantiomers of dansyl-D,L-phenylalanine (DPs) by changing the local environment of the D and L enantiomers (DDP and DLP, respectively) from hydrophilic to hydrophobic. DDP and DLP show the same native fluorescence at lambda ex/lambda em = 200/544 nm in the absence of AGP, but show shifted emissions with a component at lambda ex/lambda em = 220/497 nm in the presence of AGP and in lipophilic solutions. The conditions for an analytical determination have been optimized, and the method has been used to measure the enantiomeric composition of DDP/DLP mixtures with concentration ratios varying over 2 orders of magnitude. The mechanism of chiral recognition for DDP and DLP by AGP is discussed and should be equally applicable to other dansyl-derivative amino acid enantiomers. The association constants for AGP with DDP and with DLP have been determined to be 1.33 x 10(2) L g-1 and 2.29 x 10(2) L g-1, respectively.

Pharmacological importance of stereochemical resolution of enantiomeric drugs

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

Stereochemical aspects of pharmacotherapy

Over the past 15 years stereoselectivity has become a well-recognized consideration in clinical pharmacology. Drugs that have an asymmetric center or plane of symmetry within their molecular structure are said to be chiral. They are available as pairs of nonsuperimposable mirror images, called enantiomers, that share essentially the same physicochemical properties. These three-dimensional structural differences, however, can translate into enantiospecific pharmacologic or pharmacokinetic properties, which may be important in understanding the clinical pharmacology of chiral drugs. Most chiral drugs are available as the racemate, in which equal proportions of the two enantiomers are administered concurrently. The pharmacologic and disposition properties of many chiral drugs are documented to be stereospecific, and this has influenced the regulatory requirements for the approval of new drug candidates. Due to this influence on new drug development, the possible issues surrounding racemic drugs will undoubtedly affect the types of pharmaceuticals that are used clinically in the next century. Accordingly, considerable advances have been made in producing optically pure drug. It should be emphasized, however, that stereochemically pure drugs are not necessarily superior to the respective racemates.