Drug disposition in three dimensions: an update on stereoselectivity in pharmacokinetics

The Effects of 16-HETE Enantiomers on Hypertrophic Markers in Human Fetal Ventricular Cardiomyocytes, RL-14 Cells

BACKGROUND

Cytochrome P450 (CYP) metabolizes arachidonic acid to produce bioactive metabolites such as EETs and HETEs: mid-chain, subterminal, and terminal HETEs. Recent studies have revealed the role of CYP1B1 and its associated cardiotoxic mid-chain HETE metabolites in developing cardiac hypertrophy and heart failure. Subterminal HETEs have also been involved in various physiological and pathophysiological processes; however, their role in cardiac hypertrophy has not been fully defined.

OBJECTIVE

The objective of the current study is to determine the possible effect of subterminal HETEs, R and S enantiomers of 16-HETE, on CYP1B1 expression in vitro using human cardiomyocytes RL-14 cells.

METHODS

In the study, RL14 cell line was treated with vehicle and either of the 16-HETE enantiomers for 24 h. Subsequently, the following markers were assessed: cell viability, cellular size, hypertrophic markers, CYP1B1 gene expression (at mRNA, protein, and activity levels), luciferase activity, and CYP1B1 mRNA and protein half-lives.

RESULTS

The results of the study showed that 16-HETE enantiomers significantly increased hypertrophic markers and upregulated CYP1B1 mRNA and protein expressions in RL-14 cell line. The upregulation of CYP1B1 by 16-HETE enantiomers occurs via a transcriptional mechanism as evidenced by transcriptional induction and luciferase reporter assay. Furthermore, neither post-transcriptional nor post-translational modification was involved in such modulation since there was no change in CYP1B1 mRNA and protein stabilities upon treatment with 16-HETE enantiomers.

CONCLUSION

The current study provides the first evidence that 16R-HETE and 16S-HETE increase CYP1B1 gene expression through a transcriptional mechanism.

Quantum Chemical Investigation of the Interaction of Thalidomide Monomeric, Dimeric, Trimeric, and Tetrameric Forms with Guanine DNA Nucleotide Basis in DMSO and Water Solution: A Thermodynamic and NMR Spectroscopy Analysis

Thalidomide (TLD) was used worldwide as a sedative, but it was revealed to cause teratogenicity when taken during early pregnancy. It has been stated that the (R) enantiomer of TLD has therapeutic effects, while the (S) form is teratogenic. Clinical studies, however, demonstrated the therapeutic efficacy of thalidomide in several intractable diseases, so TLD and its derivatives have played an important role in the development and therapy of anticancer drugs. Therefore, it is important to know the molecular mechanism of action of the TLD, although this is still not clear. In what molecular interactions are concerned, it is known that drug molecules can interact with DNA in different ways, for example, by intercalation between base pairs. Furthermore, the ability of the TLD to interact with DNA has been confirmed experimentally. In this work, we report a theoretical investigation of the interaction of the R and S enantiomers of TLD, in its monomeric, dimeric, trimeric, and tetrameric forms, with guanine (GUA) DNA nucleotide basis in solution using density functional theory (DFT). Our initial objective was to evaluate the interaction of TLD-R/S with GUA through thermodynamic and spectroscopic study in dimethyl sulfoxide (DMSO) solvent and an aqueous solution. Comparison of the experimental 1H nuclear magnetic resonance (NMR) spectrum in DMSO-d6 solution with calculated DFT-PCM-DMSO chemical shifts revealed that TLD can undergo molecular association in solution, and interaction of its dimeric form with a DNA base ((TLD)2-GUA and (TLD)2-2GUA, for example) through H-bond formation is likely to take place. Our results strongly indicated that we must consider the plausibility of the existence of TLD associations in solution when modeling the complexation of the TLD with biological targets. This is new information that may provide further insight into our understanding of drug binding to biological targets at the molecular level.

Updates on drug designing approach through computational strategies: a review

The drug discovery and development (DDD) process in pursuit of novel drug candidates is a challenging procedure requiring lots of time and resources. Therefore, computer-aided drug design (CADD) methodologies are used extensively to promote proficiency in drug development in a systematic and time-effective manner. The point in reference is SARS-CoV-2 which has emerged as a global pandemic. In the absence of any confirmed drug moiety to treat the infection, the science fraternity adopted hit and trial methods to come up with a lead drug compound. This article is an overview of the virtual methodologies, which assist in finding novel hits and help in the progression of drug development in a short period with a specific medicinal solution.

Potential Stereoselective Binding of Trans-(±)-Kusunokinin and Cis-(±)-Kusunokinin Isomers to CSF1R

Breast cancer cell proliferation and migration are inhibited by naturally extracted trans-(−)-kusunokinin. However, three additional enantiomers of kusunokinin have yet to be investigated: trans-(+)-kusunokinin, cis-(−)-isomer and cis-(+)-isomer. According to the results of molecular docking studies of kusunokinin isomers on 60 breast cancer-related proteins, trans-(−)-kusunokinin was the most preferable and active component of the trans-racemic mixture. Trans-(−)-kusunokinin targeted proteins involved in cell growth and proliferation, whereas the cis-(+)-isomer targeted proteins involved in metastasis. Trans-(−)-kusunokinin targeted CSF1R specifically, whereas trans-(+)-kusunokinin and both cis-isomers may have bound AKR1B1. Interestingly, the compound’s stereoisomeric effect may influence protein selectivity. CSF1R preferred trans-(−)-kusunokinin over trans-(+)-kusunokinin because the binding pocket required a ligand planar arrangement to form a π-π interaction with a selective Trp550. Because of its large binding pocket, EGFR exhibited no stereoselectivity. MD simulation revealed that trans-(−)-kusunokinin, trans-(+)-kusunokinin and pexidartinib bound CSF1R differently. Pexidartinib had the highest binding affinity, followed by trans-(−)-kusunokinin and trans-(+)-kusunokinin, respectively. The trans-(−)-kusunokinin-CSF1R complex was found to be stable, whereas trans-(+)-kusunokinin was not. Trans-(±)-kusunokinin, a potential racemic compound, could be developed as a selective CSF1R inhibitor when combined.

Enantiospecific pharmacokinetics of intravenous dexmedetomidine in beagles

The goal of this study was to investigate the pharmacokinetic (PK) behaviour of dexmedetomidine in dogs administered as a pure enantiomer versus as part of a racemic mixture. Eight unmedicated intact purpose‐bread beagles were included. Two intravenous treatments of either medetomidine or dexmedetomidine were administered at 10‐ to 14‐day intervals. Atipamezole or saline solution was administered intramuscularly 45 min later. Venous blood samples were collected into EDTA collection tubes, and the quantification of dexmedetomidine and levomedetomidine was performed by chiral LC–MS/MS. All dogs appeared sedated after each treatment without complication. Plasma concentrations of levomedetomidine were measured only in the racemic group and were 51.4% (51.4%–56.1%) lower than dexmedetomidine. Non‐compartmental analysis (NCA) was performed for both drugs, while dexmedetomidine data were further described using a population pharmacokinetic approach. A standard two‐compartment mammillary model with linear elimination with combined additive and multiplicative error model for residual unexplained variability was established for dexmedetomidine. An exponential model was finally retained to describe inter‐individual variability on parameters of clearance (Cl₁) and central and peripheral volumes of distribution (V₁, V₂). No effect of occurrence, levomedetomidine or atipamezole could be observed on dexmedetomidine PK parameters. Dexmedetomidine did not undergo significantly different PK when administered alone or as part of the racemic mixture in otherwise unmedicated dogs.

Enantioselectivity of Pentedrone and Methylone on Metabolic Profiling in 2D and 3D Human Hepatocyte-like Cells

Pentedrone and methylone can express stereoselectivity in toxicokinetic and toxicodynamic processes. Similarly, their chiral discrimination in metabolism, which was not yet evaluated, can result in different metabolic profiles and subsequent hepatotoxic effects. Therefore, the aim of this work was to assess, for the first time, both the hepatic cytotoxic and metabolic profile of pentedrone and methylone enantiomers using physiologically relevant in vitro models. The hepatotoxicity of these compounds was observed in a concentration-dependent manner in human stem-cell-derived hepatocyte-like cells (HLCs) cultured under 3D (3D-HLCs) and 2D (2D-HLCs) conditions. Enantioselectivity, on the other hand, was only shown for pentedrone (1 mM) in 3D-HLCs, being R-(−)-pentedrone the most cytotoxic. Furthermore, the metabolic profile was initially evaluated in human liver microsomes (HLM) and further demonstrated in 3D-HLCs and 2D-HLCs applying a gas chromatography coupled to a mass spectrometer (GC–MS) technique. Methylone and pentedrone showed distinct and preferential metabolic routes for their enantiomers, resulting in the production of differentiated metabolites; R-(+)-methylone and R-(−)-pentedrone are the most metabolized enantiomers. In conclusion, the results demonstrated enantioselectivity for pentedrone and methylone in the metabolic processes, with enantioselectivity in cytotoxicity for pentedrone.

Improved Photodynamic Activity of Phthalocyanine by Adjusting the Chirality of Modified Amino Acids

Herein, four zinc phthalocyanines (ZnPcs) with chiral lysine modification were synthesized. We found that the chirality of lysine and the chiral structure position strongly influence the properties of ZnPcs. Among the four ZnPcs, d-lysine-modified ZnPc through -NH₂ on C_ε [denoted N(ε)-d-lys-ZnPc] showed superior properties, including tumor enrichment, cancer cell uptake, and tumor retention capability, compared to the other three ZnPcs. Thus, chiral molecule modification is a simple and effective strategy to regulate the abovementioned properties to achieve a satisfactory antitumor outcome of drugs.

An Investigation of the Predominant Structure of Antibiotic Azithromycin in Chloroform Solution through NMR and Thermodynamic Analysis

Azithromycin (AZM) is a well-known macrolide-type antibiotic that has been used in the treatment of infections and inflammations. Knowledge of the predominant molecular structure in solution is a prerequisite for...

Enantioresolution and Binding Affinity Studies on Human Serum Albumin: Recent Applications and Trends

The interaction between proteins and drugs or other bioactive compounds has been widely explored over the past years. Several methods for analysis of this phenomenon have been developed and improved. Nowadays, increasing attention is paid to innovative methods, such as high performance affinity liquid chromatography (HPALC) and affinity capillary electrophoresis (ACE), taking into account various advantages. Moreover, the development of separation methods for the analysis and resolution of chiral drugs has been an area of ongoing interest in analytical and medicinal chemistry research. In addition to bioaffinity binding studies, both HPALC and ACE al-low one to perform other type of analyses, namely, displacement studies and enantioseparation of racemic or enantiomeric mixtures. Actually, proteins used as chiral selectors in chromatographic and electrophoretic methods have unique enantioselective properties demonstrating suitability for the enantioseparation of a large variety of chiral drugs or other bioactive compounds. This review is mainly focused in chromatographic and electrophoretic methods using human serum albumin (HSA), the most abundant plasma protein, as chiral selector for binding affinity analysis and enantioresolution of drugs. For both analytical purposes, updated examples are presented to highlight recent applications and current trends.

Enantioselective Separation of Chiral N1-Substituted-1H-pyrazoles: Greenness Profile Assessment and Chiral Recognition Analysis

Two commercialized polysaccharide-based chiral stationary phases, Lux cellulose-2 and Lux amylose-2, were examined for their chiral recognition ability on a set of 18 biologically active racemic 4,5-dihydro-1H-pyrazole derivatives by applying normal and polar organic elution modes. The results showed that all compounds were baseline-resolved with at least one of the used elution modes. The cellulose-based column was superior using polar organic mobile-phase compositions with analysis times close to 5 min and resolutions up to 18, while the enantiomer-resolving ability of amylose-based columns was greater using the normal elution mode with analysis times close to 30 min and resolutions up to 30. The competition between the analytes and the mobile phase constituents on H-bond interactions with the stationary phase has been discussed, and the impact of this competition on chiral recognition has been investigated. It was found that the polar organic mode is very beneficial for short run times and sharp peaks. The developed enantioselective high-performance liquid chromatography (HPLC) methods will be applied to monitor the stereoselective synthesis of compounds 1-18 or to develop preparative HPLC techniques for compounds 1-18, followed by stereospecific pharmacological studies for each enantiomer separately. Greenness profile assessment of the different elution solvents was carried out using the AGREE metric approach.

Worsening of the Toxic Effects of (±)Cis-4,4'-DMAR Following Its Co-Administration with (±)Trans-4,4'-DMAR: Neuro-Behavioural, Physiological, Immunohistochemical and Metabolic Studies in Mice

4,4’-Dimethylaminorex (4,4’-DMAR) is a new synthetic stimulant, and only a little information has been made available so far regarding its pharmaco-toxicological effects. The aim of this study was to investigate the effects of the systemic administration of both the single (±)cis (0.1–60 mg/kg) and (±)trans (30 and 60 mg/kg) stereoisomers and their co-administration (e.g., (±)cis at 1, 10 or 60 mg/kg + (±)trans at 30 mg/kg) in mice. Moreover, we investigated the effect of 4,4′-DMAR on the expression of markers of oxidative/nitrosative stress (8-OHdG, iNOS, NT and NOX2), apoptosis (Smac/DIABLO and NF-κB), and heat shock proteins (HSP27, HSP70, HSP90) in the cerebral cortex. Our study demonstrated that the (±)cis stereoisomer dose-dependently induced psychomotor agitation, sweating, salivation, hyperthermia, stimulated aggression, convulsions and death. Conversely, the (±)trans stereoisomer was ineffective whilst the stereoisomers’ co-administration resulted in a worsening of the toxic (±)cis stereoisomer effects. This trend of responses was confirmed by immunohistochemical analysis on the cortex. Finally, we investigated the potentially toxic effects of stereoisomer co-administration by studying urinary excretion. The excretion study showed that the (±)trans stereoisomer reduced the metabolism of the (±)cis form and increased its amount in the urine, possibly reflecting its increased plasma levels and, therefore, the worsening of its toxicity.

Deuterium-Stabilized (R)-Pioglitazone (PXL065) Is Responsible for Pioglitazone Efficacy in NASH yet Exhibits Little to No PPARγ Activity

The antidiabetic drug pioglitazone is, to date, the most efficacious oral drug recommended off-label for the treatment of nondiabetic or diabetic patients with biopsy-proven nonalcoholic steatohepatitis (NASH). However, weight gain and edema side effects have limited its use for NASH. Pioglitazone is a mixture of two stereoisomers ((R)-pioglitazone and (S)-pioglitazone) that interconvert in vitro and in vivo. We aimed to characterize their individual pharmacology to develop a safer and potentially more potent drug for NASH. We stabilized the stereoisomers of pioglitazone with deuterium at the chiral center. Preclinical studies with deuterium-stabilized (R)-pioglitazone (PXL065) and (S)-pioglitazone demonstrated that (R)-pioglitazone retains the efficacy of pioglitazone in NASH, including reduced hepatic triglycerides, free fatty acids, cholesterol, steatosis, inflammation, hepatocyte enlargement, and fibrosis. Although both stereoisomers inhibit the mitochondrial pyruvate carrier, PXL065 shows limited to no peroxisome proliferator-activated receptor gamma (PPARγ) activity, whereas (S)-pioglitazone appears responsible for the PPARγ activity and associated weight gain. Nonetheless, in preclinical models, both stereoisomers reduce plasma glucose and hepatic fibrosis to the same extent as pioglitazone, suggesting that these benefits may also be mediated by altered mitochondrial metabolism. In a phase 1a clinical study, we demonstrated safety and tolerability of single 7.5-mg, 22.5-mg, and 30-mg doses of PXL065 as well as preferential exposure to the (R)-stereoisomer in comparison to 45-mg pioglitazone. Conclusion: PXL065 at a dose lower than 22.5 mg is predicted to exhibit efficacy for NASH equal to, or greater than, 45-mg pioglitazone without the potentially detrimental weight gain and edema. The development of PXL065 for NASH represents a unique opportunity to leverage the therapeutic benefits of pioglitazone, while reducing or eliminating PPARγ-related side effects.

Enantioselectivity in Drug Pharmacokinetics and Toxicity: Pharmacological Relevance and Analytical Methods

Enzymes, receptors, and other binding molecules in biological processes can recognize enantiomers as different molecular entities, due to their different dissociation constants, leading to diverse responses in biological processes. Enantioselectivity can be observed in drugs pharmacodynamics and in pharmacokinetic (absorption, distribution, metabolism, and excretion), especially in metabolic profile and in toxicity mechanisms. The stereoisomers of a drug can undergo to different metabolic pathways due to different enzyme systems, resulting in different types and/or number of metabolites. The configuration of enantiomers can cause unexpected effects, related to changes as unidirectional or bidirectional inversion that can occur during pharmacokinetic processes. The choice of models for pharmacokinetic studies as well as the subsequent data interpretation must also be aware of genetic factors (such as polymorphic metabolic enzymes), sex, patient age, hepatic diseases, and drug interactions. Therefore, the pharmacokinetics and toxicity of a racemate or an enantiomerically pure drug are not equal and need to be studied. Enantioselective analytical methods are crucial to monitor pharmacokinetic events and for acquisition of accurate data to better understand the role of the stereochemistry in pharmacokinetics and toxicity. The complexity of merging the best enantioseparation conditions with the selected sample matrix and the intended goal of the analysis is a challenge task. The data gathered in this review intend to reinforce the importance of the enantioselectivity in pharmacokinetic processes and reunite innovative enantioselective analytical methods applied in pharmacokinetic studies. An assorted variety of methods are herein briefly discussed.

Pharmacokinetics of Mephedrone Enantiomers in Whole Blood after a Controlled Intranasal Administration to Healthy Human Volunteers

Mephedrone, which is one of the most popular synthetic cathinones, has one chiral centre and thus exists as two enantiomers: R-(+)-mephedrone and S-(−)-mephedrone. There are some preliminary data suggesting that the enantiomers of mephedrone may display enantioselective pharmacokinetics and exhibit different neurological effects. In this study, enantiomers of mephedrone were resolved via chromatographic chiral recognition and the absolute configuration was unambiguously determined by a combination of elution order and chiroptical analysis (i.e., circular dichroism). A chiral liquid chromatography tandem mass spectrometry method was fully validated and was applied to the analysis of whole blood samples collected from a controlled intranasal administration of racemic mephedrone hydrochloride to healthy male volunteers. Both enantiomers showed similar kinetics, however, R-(+)-mephedrone had a greater mean C_max of 48.5 ± 11.9 ng/mL and a longer mean half-life of 1.92 ± 0.27 h compared with 44.6 ± 11.8 ng/mL and 1.63 ± 0.23 h for S-(−)-mephedrone, respectively. Moreover, R-(+)-mephedrone had a lower mean clearance and roughly 1.3 times greater mean area under the curve than S-(−)-mephedrone. Significant changes in the enantiomeric ratio over time were observed, which suggest that the analytes exhibit enantioselective pharmacokinetics. Even though the clinical significance of this finding is not yet fully understood, the study confirms that the chiral nature, and consequently the enantiomeric purity of mephedrone, can be a crucial consideration when interpreting toxicological results.

Chiral Serum Pharmacokinetics of 4-Fluoroamphetamine after Controlled Oral Administration: Can (R)/(S) Concentration Ratios Help in Interpreting Forensic Cases?

Over the last two decades, misuse of 4-fluoroamphetamine (4-FA) became an emerging issue in many European countries. Stimulating effects last for 4-6 hours and can impact psychomotor performance. The metabolism of amphetamine-type stimulants is stereoselective and quantification of (R)- and (S)-enantiomers has been suggested for assessing time of use. To date no data on enantioselective pharmacokinetics is available for 4-FA in serum samples. An enantioselective liquid chromatography-tandem mass spectrometry (LC-MS-MS) method was developed using a chiral Phenomenex® Lux 3 μm AMP column. Validation of the method showed satisfactory selectivity, sensitivity, linearity (0.5-250 ng/mL), precision and accuracy. Recreational stimulant users orally ingested two doses (100 mg, n=12, and 150 mg, n=5) of 4-FA. Blood samples were drawn prior to application and over a period of 12 hours after ingestion and analyzed for 4-FA enantiomers. Peak concentrations and corresponding times did not differ significantly between the enantiomers (mean (R)/(S)-ratio at tmax 1.05, 0.85-1.16). With mean 12.9 (8.3-16.1) hours, apparent elimination half-lives (t1/2) were significantly (p < 0.01) longer for (R)-4-FA than for (S)-4-FA (6.0 hours; range 4.4-10.2 hours) and independent of the dose given. Over time, (R)/(S)-concentration-ratios were linearly increasing in all subjects to maximum ratios of 2.00 (1.08-2.77) in the last samples (after 12 hours). The slopes of the (R)/(S)-ratio exhibited marked inter-individual differences (0.023 to 0.157 h-1, mean 0.095 h-1). Ratios higher than 1.60 only appeared earliest after a minimum of 6 hours and therefore suggest the absence of acute drug effects. Different elimination half-lives of enantiomers lead to constantly increasing (R)/(S)-concentration-ratios. Consequently, ratios of 4-FA enantiomers in serum are a promising indicator for assessment of the time of drug consumption.

The role of stereochemistry of antibiotic agents in the development of antibiotic resistance in the environment

Antibiotic resistance (ABR) is now recognised as a serious global health and economic threat that is most efficiently managed via a 'one health' approach incorporating environmental risk assessment. Although the environmental dimension of ABR has been largely overlooked, recent studies have underlined the importance of non-clinical settings in the emergence and spread of resistant strains. Despite this, several research gaps remain in regard to the development of a robust and fit-for-purpose environmental risk assessment for ABR drivers such as antibiotics (ABs). Here we explore the role the environment plays in the dissemination of ABR within the context of stereochemistry and its particular form, enantiomerism. Taking chloramphenicol as a proof of principle, we argue that stereoisomerism of ABs impacts on biological properties and the mechanisms of resistance and we discuss more broadly the importance of stereochemistry (enantiomerism in particular) with respect to antimicrobial potency and range of action.

Chiral Separations in Preparative Scale: A Medicinal Chemistry Point of View

Enantiomeric separation is a key step in the development of a new chiral drug. Preparative liquid chromatography (LC) continues to be the technique of choice either during the drug discovery process, to achieve a few milligrams, or to a scale-up during the clinical trial, needing kilograms of material. However, in the last few years, instrumental and technical developments allowed an exponential increase of preparative enantioseparation using other techniques. Besides LC, supercritical fluid chromatography (SFC) and counter-current chromatography (CCC) have aroused interest for preparative chiral separation. This overview will highlight the importance to scale-up chiral separations in Medicinal Chemistry, especially in the early stages of the pipeline of drugs discovery and development. Few examples within different methodologies will be selected, emphasizing the trends in chiral preparative separation. The advantages and drawbacks will be critically discussed.

Unexpected differences in the pharmacokinetics of N-acetyl-DL-leucine enantiomers after oral dosing and their clinical relevance

The enantiomers of many chiral drugs not only exhibit different pharmacological effects in regard to targets that dictate therapeutic and toxic effects, but are also handled differently in the body due to pharmacokinetic effects. We investigated the pharmacokinetics of the enantiomers of N-acetyl-leucine after administration of the racemate (N-acetyl-DL-leucine) or purified, pharmacologically active L-enantiomer (N-acetyl-L-leucine). The results suggest that during chronic administration of the racemate, the D-enantiomer would accumulate, which could have negative effects. Compounds were administered orally to mice. Plasma and tissue samples were collected at predetermined time points (0.25 to 8 h), quantified with liquid chromatography/mass spectrometry, and pharmacokinetic constants were calculated using a noncompartmental model. When administered as the racemate, both the maximum plasma concentration (Cmax) and the area under the plasma drug concentration over time curve (AUC) were much greater for the D-enantiomer relative to the L-enantiomer. When administered as the L-enantiomer, the dose proportionality was greater than unity compared to the racemate, suggesting saturable processes affecting uptake and/or metabolism. Elimination (ke and T1/2) was similar for both enantiomers. These results are most readily explained by inhibition of uptake at an intestinal carrier of the L-enantiomer by the D-enantiomer, and by first-pass metabolism of the L-, but not D-enantiomer, likely by deacetylation. In brain and muscle, N-acetyl-L-leucine levels were lower than N-acetyl-D-leucine, consistent with rapid conversion into L-leucine and utilization by normal leucine metabolism. In summary, the enantiomers of N-acetyl-leucine exhibit large, unexpected differences in pharmacokinetics due to both unique handling and/or inhibition of uptake and metabolism of the L-enantiomer by the D-enantiomer. Taken together, these results have clinical implications supporting the use of N-acetyl-L-leucine instead of the racemate or N-acetyl-D-leucine, and support the research and development of only N-acetyl-L-leucine.

Stereoselective pharmacokinetic and pharmacodynamic analysis of a CNS-active sulphamoylphenyl carbamate derivative

3-Methylpentyl(4-sulphamoylphenyl)carbamate (MSPC) came as the most potent compound out of a new series of carbamates composed of phenyl-ethanol or branched aliphatic alcohols, and 4-benzenesulphonamide-carbamic acid. In this study, the anticonvulsant activity and pharmacokinetics (PKs) of MSPC-two individual enantiomers were comparatively analysed in rats as well as their carbonic anhydrase (CA) inhibition. The anticonvulsant activity of MSPC enantiomers was evaluated at the rat-maximal electroshock (MES) test, and their CA inhibition evaluated. (R)-MSPC had a 29% higher clearance and consequently, a lower plasma exposure area under the curve (AUC) than (S)-MSPC and racemic-MSPC. Nevertheless, (R)-MSPC had a better brain permeability than its (S)-enantiomer with brain-to-plasma-(AUC)-ratio (BPR) of 2.07 ((R)-enantiomer), 1.85 (racemate), and 0.79 ((S)-enantiomer). As a whole body (in vivo) pharmacodynamic (PD) measure, MSPC-anticonvulsant maximal electroshock seizure (MES) activity was less enantioselective than MSPC-CA inhibition. The lack of significant differences between racemic-MSPC and its individual enantiomers suggest that their anticonvulsant activity might be due to multiple mechanisms of action.

Modulation of CRMP2 via (S)-Lacosamide shows therapeutic promise but is ultimately ineffective in a mouse model of CLN6-Batten disease

CLN6-Batten disease is a rare neurodegenerative disorder with no cure, characterized by accumulation of lipofuscin in the lysosome, glial activation, and neuronal death. Here we test the therapeutic efficacy of modulating collapsin response mediator protein 2 (CRMP2) activity via S-N-benzy-2-acetamido-3-methoxypropionamide ((S)-Lacosamide) in a mouse model of CLN6-Batten disease. Promisingly, mouse neuronal cultures as well as Cln6 patient fibroblasts treated with varying concentrations of (S)-Lacosamide showed positive restoration of lysosomal associated deficits. However, while acute in vivo treatment enhanced glial activation in 3-month-old Cln6 mutant mice, chronic treatment over several months did not improve behavioral or long-term survival outcomes. Therefore, modulation of CRMP2 activity via (S)-Lacosamide alone is unlikely to be a viable therapeutic target for CLN6-Batten disease.

Enantioselective Plasma Pharmacokinetic Study of a Novel Anti- Sichistosomiasis Agent P96 in Rat by Liquid Chromatography-tandem Mass Spectrometry

Background:

2-Cyclopentanecarbonyl-1,2,3,6,7,11b-hexahydro-pyrazino[2,1- a]isoquinolin- 4-one (P96), was found to be a novel drug candidate with one chiral center to treat schistosomiasis caused by Schistosoma japonicum. </P><P> Objective: To study pharmacokinetic characteristics, a simple, rapid and sensitive liquid chromatography- tandem mass spectrometry (LC-MS/MS) method was developed and fully validated for the quantification analysis of P96 in rat plasma.

Methods:

Chromatographic separation was performed on a C18 column with gradient eluted mobile phase composed of acetonitrile and water at a flow rate of 0.5 mL/min. Detection was performed on a triple-quadrupole tandem mass spectrometer using positive mode electrospray ionization in the multiple reactions monitoring (MRM) mode.

Results:

Excellent linearity was observed in the range of 3-900 ng/mL with the lower limit of quantification of 3 ng/mL in rat plasma for P96. The intra- and inter-day precisions exhibited less than 6.6%. Mean recoveries ranged from 96.9% to 102.4%. This method was applied to investigate the enantioselective differences on the pharmacokinetics between (R,S)-P96 and its enantiomers in rats after oral administration. The enantioselective differences of (R)-P96, (S)-P96 and (R,S)-P96 were found and compared.

Conclusion:

The established method was found to be accurate, precise, and sensitive and can be applied to investigate the stereoselective differences on pharmacokinetics between rac-P96 and its enantiomers.

Comparative toxicity and toxicokinetic studies of oxiracetam and (S)-oxiracetam in dogs

Oxiracetam (ORT) is known as a derivative of piracetam in the family of nootropics for treating memory impairment and cognition disorders. Given the chiral toxicological concerns surrounding ORT and the absence studies of (S)-ORT, the toxicity and toxicokinetics of (S)-ORT, and comparative toxicology of oxiracetam were systematically investigated in dogs following acute and 13-week repeated oral dosing. The animal toxicity mainly manifested as loose stools in both the acute and the 13-week studies. The no-observed-adverse-effect level is proposed to be 100 mg/kg. The 13-week toxicokinetics study indicated that, in the (S)-ORT group, the time to peak concentration was delayed, elimination half-life extended, and apparent volume of distribution increased compared with the ORT group. The clearance rate increased at low- and mid-doses, but decreased in the high-dose group and was accompanied by drug accumulation. Compared with the same dose of ORT, (S)-ORT had a lower clearance rate and longer elimination half-life.

Characterization of Novel 18F-Labeled Phenoxymethylpyridine Derivatives as Amylin Imaging Probes

Deposition of islet amyloid consisting of amylin constitutes one of pathological hallmarks of type 2 diabetes mellitus (T2DM), and it may be involved in the development and progression of T2DM. However, the details about the relationship between the deposition of islet amyloid and the pathology of T2DM remain unclear, since no useful imaging tracer enabling the visualization of pancreatic amylin is available. In the present study, we synthesized and evaluated six novel ¹⁸F-labeled phenoxymethylpyridine (PMP) derivatives as amylin imaging probes. All ¹⁸F-labeled PMP derivatives showed not only affinity for islet amyloid in the post-mortem T2DM pancreatic sections but also excellent pharmacokinetics in normal mice. Furthermore, ex vivo autoradiographic studies demonstrated that [¹⁸F]FPMP-5 showed intense labeling of islet amyloids in the diabetes model mouse pancreas in vivo. The preclinical studies suggested that [¹⁸F]FPMP-5 may have potential as an imaging probe that targets amylin aggregates in the T2DM pancreas.

Biotransformation and oxidative stress responses in rat hepatic cell-line (H4IIE) exposed to racemic ketoprofen (RS-KP) and its enantiomer, dexketoprofen (S(+)-KP)

Pharmaceuticals such as racemate ketoprofen (RS-KP) and its enantiomer, dexketoprofen (S(+)-KP) are highly detectable non-steroidal anti-inflammatory drugs (NSAIDs) in the aquatic environment and therefore are designated as one of the most emerging groups of pollutants that can affect environmental and human health. The potential impact of these pharmaceuticals was assessed for the first time in vitro using a rat hepatocellular carcinoma cell line (H4IIE). Cells were exposed to low and high concentrations of these drugs. Cytotoxicity was determined by MTT reduction assay; CYP1A1 transcriptional and enzymatic levels together with canonical oxidative stress responsive markers (GPx, GR, GST and CAT) were also investigated. Cells exposed to RS-KP and S(+)-KP did not show cytotoxicity effect at the concentrations tested. However, this study highlighted differences between RS-KP and S(+)-KP in most of the evaluated markers, showing compound-, concentration- and time-specific effect patterns which suggest a potential stereo-selective toxicity of these drugs.

Ginsenoside Rg3 stereoisomers differentially inhibit vascular smooth muscle cell proliferation and migration in diabetic atherosclerosis

Ginsenoside 20(R/S)‐Rg3, as a natural peroxisome proliferator‐activated receptor gamma (PPARγ) ligand, has been reported to exhibit differential biological effects. It is of great interest to understand the stereochemical selectivity of 20(R/S)‐Rg3 and explore whether differential PPARγ activation by Rg3 stereoisomers, if it exists, could lead to differential physiological outcome and therapeutic effects in diabetic atherosclerosis. Here, we investigated the binding modes of 20(R/S)‐Rg3 stereoisomers in the PPARγ ligand‐binding domain (PPARγ‐LBD) using molecular modelling and their effects on smooth muscle cell proliferation and migration induced by advanced glycation end products (AGEs). The results revealed that 20(S)‐Rg3 exhibited stronger antiproliferative and antimigratory effects due to stronger PPARγ activation. To validate the in vitro results, we used a mice model with diabetic atherosclerosis and obtained that 20(S)‐Rg3 markedly reduced the plaque size secondary to reducing the proliferation and migration of VSMCs, while the plaques were more stable due to improvements in other plaque compositions. The results shed light on the structural difference between Rg3 stereoisomers that can lead to significant differential physiological outcome, and the (S)‐isomer seems to be the more potent isomer to be developed as a promising drug for diabetic atherosclerosis.

Stereoselective in vitro metabolism of rhynchophylline and isorhynchophylline epimers of Uncaria rhynchophylla in rat liver microsomes

1. The objective was to investigate the underlying mechanism of the stereoselectivity in the metabolism of rhynchophylline (RIN) and isorhynchophylline (IRN) epimers in rat liver microsomes (RLM). 2. After incubation, eight metabolites of RIN (M1-5) and IRN (M6-8) reacted at A- and C-ring were identified using LC-Q-TOF/MS. Metabolic pathways included oxidation, hydroxylation, N-oxidation and dehydrogenation. In addition, hydroxylation at A-ring was the major metabolic pathway for RIN whereas the oxidation at C-ring was the major one for IRN. 3. Enzyme kinetics showed that the intrinsic clearance (CL_int) for IRN elimination was 1.9-fold higher than RIN and the degradation half-life (T_1/2) of RIN was 4.7-fold higher than that of IRN, indicating IRN was more favorable to be metabolized than RIN in RLM. 4. Data from chemical inhibition study demonstrated CYP3A was the predominant isoform involved in the metabolic elimination of both epimers, as well as the formation of M1-8. 5. In conclusion, data revealed that due to the spatial configurations at C-7 position, RIN and IRN epimers possessed different hepatic metabolic pathways and elimination rates which were mainly mediated by CYP3A.

The pharmacokinetics of racemic MDPV and its (R) and (S) enantiomers in female and male rats

BACKGROUND

These studies investigated the serum pharmacokinetic (PK) profile of racemic (3,4)-methylenedioxypyrovalerone [(R,S)-MDPV)] and its (R)- and (S)-enantiomers in female and male Sprague Dawley rats.

METHODS

Intravenous (R,S)-MDPV (3 and 5.6mg/kg) and single enantiomer of (R)- and (S)-MDPV (1.5mg/kg) were administered to both sexes for PK studies. Intraperitoneal (ip) bioavailability was determined at 3mg/kg (R,S)-MDPV. Locomotor activity studies were conducted after ip treatment with saline and 0.3-5.6mg/kg of (R,S)-MDPV.

RESULTS

PK values after iv (R,S)-MDPV showed a significant (p<0.05) sex-dependent differences in the volume of distribution at steady state (Vd_ss) for (R)- and (R,S)-MDPV at both (R,S)-MDPV doses. The female S/R enantiomeric ratios for area under the concentration time curve (AUC_inf) and clearance were significantly lower and higher, respectively, than values determined in males. Importantly, there was no evidence of in vivo inversion of (R)-MDPV or (S)-MDPV to its antipode. There were, however, significant sex-dependent differences in volume of distribution after administration of the (R)-enantiomer. Bioavailability studies of ip (R,S)-MDPV showed greater variability and significantly greater bioavailability in male rats. Accordingly, there was a significantly greater maximal distance traveled measurement in male rats at a 3.0mg/kg dose.

CONCLUSION

PK sex differences in (R,S)-MDPV and enantiomers were most apparent in volume of distribution, which could be caused by differences in drug blood and tissue protein binding. The increased magnitude and variance in ip bioavailability in male compared to female rats could lead to sex-dependent differences in the pharmacological action caused by active enantiomer (S)-MDPV.

Differences of first-pass effect in the liver and intestine contribute to the stereoselective pharmacokinetics of rhynchophylline and isorhynchophylline epimers in rats

ETHNOPHARMACOLOGICAL RELEVANCE

Uncaria rhynchophylla (Miq.) Miq. ex Havil., is a plant species used in traditional Chinese medicine to treat cardiovascular and central nervous system diseases. Rhynchophylline (RIN) and isorhynchophylline (IRN), a pair of epimers, are major alkaloids isolated from U. rhynchophylla and exhibit diverse pharmacological effects. Our previous study demonstrated that the pharmacokinetics of these epimers existed stereoselectivity after oral administration; however, the specific mechanism remains unknown and merits investigation.

AIM OF THE STUDY

In the present study, the aim was to elucidate the mechanism underlying stereoselective pharmacokinetic characteristics of RIN and IRN in rats.

MATERIALS AND METHODS

The total (F), hepatic (F_h) and intestinal (F_a·F_g) bioavailabilities of each epimer were measured using portal vein cannulated rats following different dosing routes (intravenous, intraportal and intraduodenal) to assess individual contributions of the liver and intestine in stereoselective pharmacokinetics. Then the differences of first-pass metabolism in the liver and intestine between two epimers were evaluated by in vitro incubation with rat liver microsomes, intestinal S9 and gastrointestinal (GI) content solutions, respectively. Meanwhile, the membrane permeability and efflux by P-glycoprotein (P-gp) were examined by in situ single-pass intestinal perfusion with and without P-gp inhibitor verapamil. The configurational interconversion at different pH values and the excretions via feces and urine were also examined.

RESULTS

Pharmacokinetic data showed that the total bioavailability of RIN was 5.9 folds higher than that of IRN (23.4% vs. 4.0%). The hepatic availability of RIN was 4.6 folds higher than that of IRN (46.9% vs. 10.3%), whereas the intestinal availability of RIN (48.1%) was comparable to that of IRN (42.7%). In addition, intestinal perfusion showed that IRN possessed higher intestinal permeability than RIN and co-perfusion with verapamil could affect absorption process of RIN but not IRN. Conversely, the metabolism rate of IRN in rat liver microsomes was significantly faster than that of RIN, resulting in a lower systemic exposure of IRN after oral administration. The degradation in GI lumen and epimerization between two epimers also existed but had small contributions. Additionally, the excretions of both epimers via feces and urine were negligible.

CONCLUSIONS

Taken together, different first-pass metabolism in the liver was the major factor responsible for the stereoselective pharmacokinetics of RIN and IRN.

Characterizing QT interval prolongation in early clinical development: a case study with methadone

Recently, we have shown how pharmacokinetic–pharmacodynamic (PKPD) modeling can be used to assess the probability of QT interval prolongation both in dogs and humans. A correlation between species has been identified for a drug‐specific parameter, making it possible to prospectively evaluate nonclinical signals. Here, we illustrate how nonclinical data on methadone can be used to support the evaluation of dromotropic drug effects in humans. ECG and drug concentration data from a safety pharmacology study in dogs were analyzed using nonlinear mixed effects modeling. The slope of the PKPD model describing the probability of QT interval prolongation was extrapolated from dogs to humans and subsequently combined with methadone pharmacokinetic data as input for clinical trial simulations. Concentration versus time profiles were simulated for doses between 5 and 500 mg. Predicted peak concentrations in humans were then used as reference value to assess the probability of an increase in QT interval of ≥5 and ≥10 ms. Point estimates for the slope in dogs suggested low probability of ≥10 ms prolongation in humans, whereas an effect of approximately 5 ms increase is predicted when accounting for the 90% credible intervals of the drug‐specific parameter in dogs. Interspecies differences in drug disposition appear to explain the discrepancies between predicted and observed QT prolonging effects in humans. Extrapolation of the effects of racemic compound may not be sufficient to describe the increase in QT interval observed after administration of methadone to patients. Assessment of the contribution of enantioselective metabolism and active metabolites is critical.

Chiral Derivatives of Xanthones: Investigation of the Effect of Enantioselectivity on Inhibition of Cyclooxygenases (COX-1 and COX-2) and Binding Interaction with Human Serum Albumin

Searching of new enantiomerically pure chiral derivatives of xanthones (CDXs) with potential pharmacological properties, particularly those with anti-inflammatory activity, has remained an area of interest of our group. Herein, we describe in silico studies and in vitro inhibitory assays of cyclooxygenases (COX-1 and COX-2) for different enantiomeric pairs of CDXs. The evaluation of the inhibitory activities was performed by using the COX Inhibitor Screening Assay Kit. Docking simulations between the small molecules (CDXs; known ligands and decoys) and the enzyme targets were undertaken with AutoDock Vina embedded in PyRx—Virtual Screening Tool software. All the CDXs evaluated exhibited COX-1 and COX-2 inhibition potential as predicted. Considering that the (S)-(−)-enantiomer of the nonsteroidal anti-inflammatory drug ketoprofen preferentially binds to albumin, resulting in lower free plasma concentration than (R)-(+)-enantiomer, protein binding affinity for CDXs was also evaluated by spectrofluorimetry as well as in in silico. For some CDXs enantioselectivity was observed.

Itraconazole cis-diastereoisomers activate aryl hydrocarbon receptor AhR and pregnane X receptor PXR and induce CYP1A1 in human cell lines and human hepatocytes

Triazole antimycotic itraconazole contains in its structure three chiral centres; therefore, it forms eight stereoisomers. Commercial preparations of itraconazole are a mixture of four cis-diastereoisomers. There is much evidence that efficacy, adverse effects, and toxicity of chiral drugs may be stereospecific. Therefore, we have prepared 4 pure cis-diastereoisomers of itraconazole and investigated their effects on transcriptional activities of xenoreceptors aryl hydrocarbon receptor AhR and pregnane X receptor PXR. Gene reporter assays showed that itraconazole dose-dependently activated both AhR and PXR, and the activation of AhR but not of PXR was enantiospecific. Itraconazole diastereoisomers transformed AhR and PXR into their DNA-binding forms, as demonstrated by electromobility shift assays. Cytochrome P450 CYP1A1 mRNA and protein were induced by itraconazole diastereoisomers in human hepatoma cells HepG2, human skin cells HaCaT, and in primary human hepatocytes. The expression of CYP3A4 in human intestinal LS180 cells was not influenced by itraconazole, but we observed downregulation of CYP3A4 in human hepatocytes. Collectively, we show that itraconazole is a dual activator of AhR and PXR, with differential effects on the target genes for xenoreceptors. The enantiospecific pattern was observed only in gene reporter assays for AhR. The data presented here might be of toxicological and clinical importance.

Synthesis and Monkey-PET Study of (R)- and (S)-18F-Labeled 2-Arylbenzoheterocyclic Derivatives as Amyloid Probes with Distinctive in Vivo Kinetics

This study describes an effective strategy to improve pharmacokinetics of Aβ imaging agents, offering a novel class of (R)- and (S)-¹⁸F-labeled 2-arylbenzoheterocyclic derivatives which bear an additional chiral hydroxyl group on the side chain. These ligands displayed binding abilities toward Aβ aggregates with K_i values ranging from 3.2 to 195.6 nM. Chirality-related discrepancy was observed in biodistribution, and (S)-2-phenylbenzoxazole enantiomers exhibited vastly improved brain clearance with washout ratios higher than 20. Notably, (S)-[¹⁸F]28 possessed high binding potency (K_i = 7.6 nM) and exceptional brain kinetics (9.46% ID/g at 2 min, brain_2min/brain_60min = 27.8) that is superior to well-established [¹⁸F]AV45. The excellent pharmacokinetics and low nonspecific binding of (S)-[¹⁸F]28 were testified by dynamic PET/CT scans in monkey brains. In addition, (S)-[¹⁸F]28 clearly labeled Aβ plaques both in vitro and ex vivo. These results might qualify (S)-[¹⁸F]28 to detect Aβ plaques with high signal-to-noise ratio.

A clinical and microbiological study on the enantiomers of delmopinol

Objective The clinical part of this study aimed to investigate whether the racemate of delmopinol [(±)-delmopinol] is equivalent to its two enantiomers [(+)-delmopinol and (-)-delmopinol] with respect to efficiency and to determine and compare their pharmacokinetic properties. The purpose of the pre-clinical part was to elucidate possible differences in antimicrobial efficiency. Materials and methods The compounds were tested clinically in a double-blind, randomized, cross-over study comprising three treatment periods of 4 days each. The antimicrobial efficacy of the enantiomers was compared in vitro with respect to planktonic and biofilm bacteria of different species. Results No statistically significant differences in prevention of plaque formation were observed. Except for a somewhat higher systemic exposure in terms of AUC and Cmax indicated for (-)-delmopinol compared to (+)-delmopinol, the pharmacokinetic properties were similar. The most common adverse event was a transient anaesthetic feeling in the mouth. This event was reported with the same frequency for all three test solutions. The enantiomers showed similar antimicrobial effects on planktonic bacteria and their biofilms. Conclusions The enantiomers were found to be equally effective with respect to inhibition of plaque development and only minor differences were observed with respect to their pharmacokinetic properties. No differences could be observed in the adverse events reports. There is, therefore, no reason to use one of the enantiomers of delmopinol instead of the racemate. This was further supported by the antimicrobial tests. It is suggested that the combined action of cationic and neutral delmopinol is important for its effect on biofilms.

Significance and challenges of stereoselectivity assessing methods in drug metabolism

Stereoselectivity in drug metabolism can not only influence the pharmacological activities, tolerability, safety, and bioavailability of drugs directly, but also cause different kinds of drug-drug interactions. Thus, assessing stereoselectivity in drug metabolism is of great significance for pharmaceutical research and development (R&D) and rational use in clinic. Although there are various methods available for assessing stereoselectivity in drug metabolism, many of them have shortcomings. The indirect method of chromatographic methods can only be applicable to specific samples with functional groups to be derivatized or form complex with a chiral selector, while the direct method achieved by chiral stationary phases (CSPs) is expensive. As a detector of chromatographic methods, mass spectrometry (MS) is highly sensitive and specific, whereas the matrix interference is still a challenge to overcome. In addition, the use of nuclear magnetic resonance (NMR) and immunoassay in chiral analysis are worth noting. This review presents several typical examples of drug stereoselective metabolism and provides a literature-based evaluation on current chiral analytical techniques to show the significance and challenges of stereoselectivity assessing methods in drug metabolism.

Enantiospecific effects of chiral drugs on cytochrome P450 inhibition in vitro

1. The aim of this work was to examine the differences in the inhibitory potency of individual enantiomers and racemic mixtures of selected chiral drugs on human liver microsomal cytochromes P450. 2. The interaction of enantiomeric forms of six drugs (tamsulosin, tolterodine, citalopram, modafinil, zopiclone, ketoconazole) with nine cytochromes P450 (CYP3A4, CYP2E1, CYP2D6, CYP2C19, CYP2C9, CYP2C8, CYP2B6, CYP2A6, CYP1A2) was examined. HPLC methods were used to estimate the extent of the inhibition of specific activity in vitro. 3. Tamsulosin (TAM) and tolterodine (TOL) inhibited CYP3A4 activity with an enantiospecific pattern. The inhibition of CYP3A4 activity differed for R-TAM (Ki 2.88 ± 0.12 µM) and S-TAM (Ki 14.22 ± 0.53 µM) as well as for S-TOL (Ki 1.71 ± 0.03 µM) and R-TOL (Ki 4.78 ± 0.17 µM). Also, the inhibition of CYP2C19 by ketoconazole (KET) cis-enantiomers exhibited enantioselective behavior: the (+)-KET (IC50 23.64 ± 6.25 µM) was more potent than (-)-KET (IC50 66.12 ± 12.6 µM). The inhibition of CYP2C19 by modafinil (MOD) enantiomers (R-MOD IC50 = 51.79 ± 8.58 µM, S-MOD IC50 = 48.62 ± 9.74 µM) and the inhibition of CYP2D6 by citalopram (CIT) enantiomers (R-CIT IC50 = 68.17 ± 5.70 µM, S-CIT IC50 = 62.63 ± 7.89 µM) was not enantiospecific. 4. Although enantiospecific interactions were found (TAM, TOL, KET), they are probably not clinically relevant as the plasma levels are generally lower than the drug concentration needed for prominent inhibition (at least 50% of CYP activity).

Enantiospecific determination of naftopidil by RRLC-MS/MS reveals stereoselective pharmacokinetics and tissue distributions in rats

Naftopidil (NAF) is used as a racemate to treat benign prostatic hyperplasia (BPH) and to prevent prostate cancer. However, racemic NAF has low bioavailability; therefore, it is commonly administered at higher clinical dosages compared to other therapeutic BPH drugs. Differences in interactions between individual enantiomers and biological macromolecules may result in variations in pharmacokinetics and dispositions. This study aimed to investigate the pharmacokinetics, bioavailability and tissue distributions of NAF enantiomers in rats after intragastric administration of the individual enantiomers. A rapid and sensitive liquid chromatography coupled with triple-quadrupole mass spectrometric method (RRLC-MS/MS) was developed and validated for determination of NAF enantiomers in rat plasma, tissues, urine and feces. After intragastric administration, S(-)-NAF in plasma [maximum concentration (C(max)) = 186.4 ng/mL, area under the curve from 0 h to 24h (AUC(0-24 h)) = 877.9 ng h/mL] was significantly higher than that of R(+)-NAF (C(max) = 133.2 ng/mL, AUC(0-24 h) = 602.1 ng h/mL). Moreover, S(-)-NAF bioavailability was twice that of R(+)-NAF. R(+)-NAF distributions in the prostate, liver, and kidney were significantly higher than S(-)-NAF distributions (R/S ratios of 3.16, 1.33, and 2.90, respectively). These data reveal the stereoselective pharmacokinetic profiles of the two enantiomers in rats.

Rapid metabolite discovery, identification, and accurate comparison of the stereoselective metabolism of metalaxyl in rat hepatic microsomes

Metabolite identification and quantitation impose great challenges on risk assessment of agrochemicals, as many metabolite standards are generally unavailable. In this study, metalaxyl metabolites were identified by time-of-flight mass spectrometry and semiquantified by triple quadrupole tandem mass spectrometry with self-prepared (13)C-labeled metalaxyl metabolites as internal standards. Such methodology was employed to characterize the stereoselective metabolism of metalaxyl in rat hepatic microsomes successfully. Metabolites derived from hydroxylation, demethylation, and didemethylation were identified and semiquantified. The results indicated that (+)-S-metalaxyl eliminated preferentially as the enantiomer fraction was 0.32 after 60 min incubation. The amounts of hydroxymetalaxyl and demethylmetalaxyl derived from (-)-R-metalaxyl were 1.76 and 1.82 times higher than that of (+)-S-metalaxyl, whereas didemethylmetalaxyl derived from (+)-S-metalaxyl was 1.44 times larger than that from (-)-R-metalaxyl. This study highlights a new quantitation approach for stereoselective metabolism of chiral agrochemicals and provides more knowledge on metalaxyl risk assessment.

Bioanalysis of chiral compounds during drug development using a tiered approach

Significant differences in the pharmacodynamic activity and pharmacokinetic properties could exist for a pair of enantiomeric drugs. In order to evaluate the activity, toxicity, absorption, distribution, metabolism, and excretion properties of the individual enantiomers, and any potential for chiral inversion caused by the biotransformation process, chiral bioanalytical assays are necessary for individual enantiomers and/or their metabolites for in vivo samples. However, development and validation of chiral quantitative assays are highly challenging in comparison to typical nonchiral assays. Therefore, a tiered approach should be used to address specific needs arising in different scenarios of chiral drug development, including development of racemate or fixed-ratio (nonracemic) enantiomers, development of a single enantiomer, racemic switches, and quantitation of enantiomeric metabolites. The choice of a nonchiral quantitative assay, a chiral qualitative assay, or a chiral quantitative assay should be based on the development strategy and on the molecular properties of the drug candidate.

Antibacterial inhibitors of Gram-positive thymidylate kinase: structure-activity relationships and chiral preference of a new hydrophobic binding region

Thymidylate kinase (TMK), an essential enzyme in bacterial DNA biosynthesis, is an attractive therapeutic target for the development of novel antibacterial agents, and we continue to explore TMK inhibitors with improved potency, protein binding, and pharmacokinetic potential. A structure-guided design approach was employed to exploit a previously unexplored region in Staphylococcus aureus TMK via novel interactions. These efforts produced compound 39, with 3 nM IC50 against S. aureus TMK and 2 μg/mL MIC against methicillin-resistant S. aureus (MRSA). This compound exhibits a striking inverted chiral preference for binding relative to earlier compounds and also has improved physical properties and pharmacokinetics over previously published compounds. An example of this new series was efficacious in a murine S. aureus infection model, suggesting that compounds like 39 are options for further work toward a new Gram-positive antibiotic by maintaining a balance of microbiological potency, low clearance, and low protein binding that can result in lower efficacious doses.