Chirality of bioactive agents in protein binding storage and transport processes

New two-stage pH combined with dissolved oxygen control strategy for cyclic β-1,2 glucans synthesis

On the basis of a novel two-stage pH combined with dissolved oxygen (DO) control strategy in fed-batch fermentation, this research addresses the influence of pH on cyclic β-1,2-glucans (CβGs) biosynthesis and melanin accumulation during the production of CβGs by Rhizobium radiobacter ATCC 13,333. Under these optimal fermentation conditions, the maximum cell concentration and CβGs concentration in a 7-L stirred-tank fermenter were 7.94 g L^-1 and 3.12 g L^-1, which were the maximum production reported for R. radiobacter. The melanin concentration of the fermentation broth was maintained at a low level, which was beneficial to the subsequent separation and purification of the CβGs. In addition, a neutral extracellular oligosaccharide (COGs-1) purified by the two-stage pH combined with DO control strategy fermentation medium was structurally characterized. Structural analyses indicated that COGs-1 was a family of unbranched cyclic oligosaccharides composed of only β-1,2-linked D-glucopyranose residues with degree of polymerization between 17 and 23, namely CβGs. This research provides a reliable source of CβGs and structural basis for further studies of biological activity and function. KEY POINTS: • A two-stage pH combined with DO control strategy was proposed for CβGs production and melanin biosynthesis by Rhizobium radiobacter. • The final extracellular CβGs production reached 3.12 g L^-1, which was the highest achieved by Rhizobium radiobacter. • The existence of CβGs could be detected by TLC quickly and accurately.

Development, Robustness by Design Expert and Validation of a Method for Enantiomeric Impurity Content Determination in Pretomanid Drug Substance and Pharmaceutical Dosage Form

For the purpose of identifying enantiomeric impurities in the drug substance and pharmaceutical dosage forms of the novel anti-TB medication pretomanid, an RP-high-performance liquid chromatography method was devised. To ensure the robustness of the optimized approach, analytical quality by design was used. Studies on factor screening and risk evaluation helped pinpoint the critical method parameters (CMPs); resolution (R1), analyte retention time (R2) and tailing factor (R3) are those terms. Pareto charts, half-normal plots, 3D surface plots, 2D contour plots and 3D cube plots were used to study the effects of solo and interactive CMPs on critical analytical attributes. Analysis of variance (ANOVA) was used to verify the technique parameters' confirmation of significance (P = 0.05). With a Chiral Cel OJ-3R (150 × 4.6 mm, 3 µm) and a mobile phase consisting of 20 mM of ammonium trifluoroacetate, pH = 2.5, and acetonitrile in a gradient mode, chromatographic separation was accomplished. At 30°C and 330 nm, the column's temperature and wavelength, respectively, were recorded. The procedure is stability-indicating and is LC-MS compatible. According to the International Conference on Harmonization tripartite guidelines, the method demonstrated appropriate specificity, sensitivity, linearity, accuracy, precision and robustness. The LOD and LOQ were, respectively, 0.09 and 0.3 μg/mL. With a correlation coefficient of >0.990, it was discovered that the established method for enantiomeric impurity was linear over the concentration range of 0.3-2.25μg/mL. The approach exhibits adequate accuracy (%recovery = 85-115%), robustness (%RSD = 5.0) and precision (%RSD = 5.0). The method was also shown to be stability-indicating and was shown to provide effective separation in the presence of degradation products through the use of forced degradation tests.

Chiral separation of calcium channel antagonists by SFC and HPLC using different immobilized chiral stationary phases

Supercritical fluid chromatography and high-performance liquid chromatography techniques are popular for the chiral separations of different drugs and pharmaceuticals. Therefore, this article describes a comparative study of the chiral separation of some calcium channel antagonists such as verapamil, gallopamil, and nisoldipine. The columns used were Chiralpak IG and Chiralpak ID (250 mm × 4.6 mm, 5.0 μm). The separation was achieved by using a variety of mobile phases in both techniques. The retention, separation, and resolution factors in SFC were in the range of 1.36-7.30, 1.09-1.72, and 1.16-3.47, while these values in the case of HPLC were 1.03-2.42, 1.12-1.35, and 0.49-2.46. The complete resolution of gallopamil and verapamil was achieved successfully. The chiral recognition was controlled by hydrogen bondings, π-π interactions, dipole induced dipole interactions, van der Waal forces, and steric effects. SFC was found to be a better technique than HPLC because of quick separation, good separation power, economic, environment-friendly, and green technology.

Development of an effective novel validated stability-indicating HPLC method for the resolution of brivaracetam stereoisomeric impurities

Reverse-phase high-performance liquid chromatography method has been developed for the determination of brivaracetam stereoisomeric impurities such as (R,S)-brivaracetam, (R,R)-brivaracetam, and (S,S)-brivaracetam with good resolution using the chiral column, Chiral PAK IG-U (100 × 3.0 mm; 1.6 μm). The method is simple, stability-indicating, and compatible with LC-MS. The separation was achieved with the mobile phase consisted of 10 mM ammonium bicarbonate along with acetonitrile in an isocratic mode. The column temperature and wavelength were monitored at 40°C and 215 nm, respectively. The method showed adequate specificity, sensitivity, linearity, accuracy, precision, and robustness inline to ICH tripartite guidelines. The limit of detection and quantification limits were 0.3 and 0.8 μg ml^-1 , respectively, for all stereoisomeric impurities and brivaracetam. The developed method was found to be linear over the concentration range of 0.8-5.6 μg ml^-1 for stereoisomeric impurities with a correlation coefficient >0.999. The method was precise (%RSD < 5.0), robust, and accurate (with 85%-115% recovery). The values of retention times of stereoisomeric impurities, (R,S)-brivaracetam, (R,R)-brivaracetam, and (S,S)-brivaracetam, were 4.9, 5.4, and 6.6 min, respectively, and resolution among the impurities were 2.0, 3.3, and 4.7, respectively. In addition, forced degradation studies were performed to prove that method was stability-indicating. The enrichment of isomeric impurity, (R,R)-brivaracetam, was observed under basic stress conditions of brivaracetam and proposed a plausible mechanism to enhance that isomeric impurity. As well, a good separation among brivaracetam and its stereoisomeric impurity peaks was observed in the presence of degradation products and process-related impurities.

Automated Synthesis of (rac)-, (R)-, and (S)-[18 F]Epifluorohydrin and Their Application for Developing PET Radiotracers Containing a 3-[18 F]Fluoro-2-hydroxypropyl Moiety

To introduce the 3-[¹⁸ F]fluoro-2-hydroxypropyl moiety into positron emission tomography (PET) radiotracers, we performed automated synthesis of (rac)-, (R)-, and (S)-[¹⁸ F]epifluorohydrin ([¹⁸ F]1) by nucleophilic displacement of (rac)-, (R)-, or (S)-glycidyl tosylate with ¹⁸ F^- and purification by distillation. The ring-opening reaction of (R)- or (S)-[¹⁸ F]1 with phenol precursors gave enantioenriched [¹⁸ F]fluoroalkylated products without racemisation. We then synthesised (rac)-, (R)-, and (S)- 2-{5-[4-(3-[¹⁸ F]fluoro-2-hydroxypropoxy)phenyl]-2-oxobenzo[d]oxazol-3(2H)-yl}-N-methyl-N-phenylacetamide ([¹⁸ F]6) as novel radiotracers for the PET imaging of translocator protein (18 kDa) and showed that (R)- and (S)-[¹⁸ F]6 had different radioactivity uptake in mouse bone and liver. Thus, (rac)-, (R)-, and (S)-[¹⁸ F]1 are effective radiolabelling reagents and can be used to develop PET radiotracers by examining the effects of chirality on their in vitro binding affinities and in vivo behaviour.

Preclinical Evaluation of [(18)F]THK-5105 Enantiomers: Effects of Chirality on Its Effectiveness as a Tau Imaging Radiotracer

PURPOSE

Noninvasive imaging of tau and amyloid-β pathologies would facilitate diagnosis of Alzheimer's disease (AD). Recently, we have developed [(18)F]THK-5105 for selective detection of tau pathology by positron emission tomography (PET). The purpose of this study was to clarify biological properties of optically pure [(18)F]THK-5105 enantiomers.

PROCEDURES

Binding for tau aggregates in AD brain section was evaluated by autoradiography (ARG). In vitro binding assays were performed to evaluate the binding properties of enantiomers for AD brain homogenates. The pharmacokinetics in the normal mouse brains was assessed by ex vivo biodistribution assay

RESULTS

The ARG of enantiomers showed the high accumulation of radioactivity corresponding to the distribution of tau deposits. In vitro binding assays revealed that (S)-[(18)F]THK-5105 has slower dissociation from tau than (R)-[(18)F]THK-5105. Biodistribution assays indicated that (S)-[(18)F]THK-5105 eliminated faster from the mouse brains and blood compared with (R)-[(18)F]THK-5105.

CONCLUSION

(S)-[(18)F]THK-5105 could be more suitable than (R)-enantiomer for a tau imaging agent.

Enantiomeric resolution of multiple chiral centres racemates by capillary electrophoresis

Enantiomeric resolution of multichiral centre racemates is an important area as some multichiral centre racemates are of great medicinal importance. However, enantioseparation of such types of racemates is a challenging task. Amongst many analytical techniques, capillary electrophoresis is a powerful technique and may be used to resolve such racemates. Only few papers are available describing enantiomeric resolution of such racemates. Therefore, efforts have been made to describe the enantiomeric resolution of multichiral centre racemates by capillary electrophoresis. This article discusses the importance of multichiral racemates, the need for capillary electrophoresis in enantiomeric resolution and chiral resolution of multichiral centre racemates using various chiral selectors. Further, attempts have been made to discuss the future challenges and prospects of enantiomeric resolution of multichiral racemates. The various chiral selectors used for the purpose are chiral crown ether, cyclodextrins, polysaccharides, macrocyclic glycopeptide antibiotics and ligand exchange.

Structure-Activity Relationship of 2-Arylquinolines as PET Imaging Tracers for Tau Pathology in Alzheimer Disease

Abnormal deposition of amyloid-β and hyperphosphorylated tau protein in the brain are the pathologic hallmark of Alzheimer disease (AD). Noninvasive detection of the lesions is considered an effective tool for early diagnosis and staging of AD. In the past decade, we developed 2-arylquinoline (2-AQ) derivatives as PET tau tracers. In this study, we synthesized new derivatives and evaluated their properties.

METHODS

Fifteen 2-AQ derivatives were labeled with (18)F, and their binding to tau lesions was evaluated by autoradiography using AD brain sections. The binding affinity for the AD brain homogenates was assessed by an in vitro competitive binding assay with (18)F-THK-5105. (18)F-labeled derivatives were injected into mice via the tail vein, and their pharmacokinetics over the first 120 min after injection were evaluated by an ex vivo biodistribution study. Tracer metabolism analysis was also assessed in mice.

RESULTS

The average logP value was 2.80. This study revealed that 2-AQ derivatives having (18)F-labeled side chains on benzene or position 7 of the quinoline showed significantly lower binding affinity for tau than 6-substituted quinoline derivatives. The 2-AQ derivatives labeled with (18)F-fluoroethoxy, (18)F-fluoropropoxy, and (18)F-fluoro-polyethyleneglycol groups displayed slow clearance from blood or a high accumulation in bone, whereas derivatives labeled with the (18)F-(3-fluoro-2-hydroxy)propoxyl group did not. (18)F-THK-5151 had outstanding tau binding properties and pharmacokinetics. Furthermore, the properties of its optically pure (S)-enantiomer (THK-5351) were superior to those of the (R)-enantiomer (THK-5451), particularly in terms of its clearance from the brain and its resistance to defluorination in mice.

CONCLUSION

The structure-activity relationship study of 2-AQ derivatives revealed the optimal structural features for tau imaging agents. On the basis of these results, (18)F-THK-5351 ((S)-(18)F-THK-5151) was selected as a potential agent for tau imaging.

18F-THK5351: A Novel PET Radiotracer for Imaging Neurofibrillary Pathology in Alzheimer Disease

Imaging of neurofibrillary pathology in the brain helps in diagnosing dementia, tracking disease progression, and evaluating the therapeutic efficacy of antidementia drugs. The radiotracers used in this imaging must be highly sensitive and specific for tau protein fibrils in the human brain. We developed a novel tau PET tracer, (18)F-THK5351, through compound optimization of arylquinoline derivatives.

METHODS

The in vitro binding properties, pharmacokinetics, and safety of (18)F-THK5351 were investigated, and a clinical study on Alzheimer disease (AD) patients was performed.

RESULTS

(18)F-THK5351 demonstrated higher binding affinity for hippocampal homogenates from AD brains and faster dissociation from white-matter tissue than did (18)F-THK5117. The THK5351 binding amount correlated with the amount of tau deposits in human brain samples. Autoradiography of brain sections revealed that THK5351 bound to neurofibrillary tangles selectively and with a higher signal-to-background ratio than did THK5117. THK5351 exhibited favorable pharmacokinetics and no defluorination in mice. In first-in-human PET studies in AD patients, (18)F-THK5351 demonstrated faster kinetics, higher contrast, and lower retention in subcortical white matter than(18)F-THK5117.

CONCLUSION

(18)F-THK5351 is a useful PET tracer for the early detection of neurofibrillary pathology in AD patients.

Chiral discrimination of β-3-homo-amino acids using the kinetic method

Chiral discrimination of seven enantiomeric pairs of β-3-homo-amino acids was studied by using the kinetic method and trimeric metal-bound complexes, with natural and unnatural α-amino acids as chiral reference compounds and divalent metal ions (Cu(2+) and Ni(2+) ) as the center ions. The β-3-homo-amino acids were selected for this study because, first of all, chiral discrimination of β-amino acids has not been extensively studied by mass spectrometry. Moreover, these β-3-homo-amino acids studied have different aromatic side chains. Thus, the emphasis was to study the effect of the side chain (electron density of the phenyl ring, as well as the difference between phenyl and benzyl side chains) for the chiral discrimination. The results showed that by the proper choice of a metal ion and a chiral reference compound, all seven enantiomeric pairs of β-3-homo-amino acids could be differentiated. Moreover, it was noted that the β-3-homo-amino acids with benzyl side chains provided higher enantioselectivity than the corresponding phenyl ones. However, increasing or decreasing the electron density of the aromatic ring by different substituents in both the phenyl and benzyl side chains had practically no role for chiral discrimination of β-3-homo-amino acids studied. When copper was used as the central metal, the phenyl side chain containing reference molecules (S)-2-amino-2-phenylacetic acid (L-Phg) and (S)-2-amino-2-(4-hydroxyphenyl)-acetic acid (L-4'-OHPhg) gave rise to an additional copper-reduced dimeric fragment ion, [Cu(I) (ref)(A)](+) . The inclusion of this ion improved noticeably the enantioselectivity values obtained.

HPLC separation of amino acid enantiomers and small peptides on macrocyclic antibiotic-based chiral stationary phases: a review

The search for new and effective chiral selectors capable of separating a wide variety of enantiomeric compounds is an ongoing process. In the past decade, macrocyclic antibiotics have proved to be an exceptionally useful class of chiral selectors for the separation of enantiomers of biological and pharmacological importance by means of HPLC, TLC and electrophoresis. More chiral analytes have been resolved through the use of glycopeptides than with all the other macrocyclic antibiotics combined (ansamycins, thiostrepton, aminoglycosides, etc.). The glycopeptides avoparcin, teicoplanin, ristocetin A and vancomycin have been extensively used as chiral selectors in the form of chiral bonded phases in HPLC, and HPLC stationary phases based on these glycopeptides have been commercialized. Teicoplanin, vancomycin, their analogs and ristocetin A seem to be the most useful glycopeptide HPLC bonded phases for the enantioseparation of proteins and unusal native and derivatized amino acids. In fact, the macrocyclic glycopeptides are to some extent complementary to one another: where partial enantioresolution is obtained with one glycopeptide, there is a high probability that baseline or better separation can be obtained with another. This review sets out to characterize the physicochemical properties of these antibiotics and their application in the enantioseparations of amino acids. The mechanism of separation, the sequence of elution of the stereoisomers and the relation to the absolute configuration are also discussed.

Enantioanalysis of Bisoprolol in Human Plasma with a Macrocyclic Antibiotic HPLC Chiral Column Using Fluorescence Detection and Solid Phase Extraction

A sensitive, enantioselective, high-performance liquid chromatographic (HPLC) method was developed and validated to determine S-(-)- and R-(+)-bisoprolol in human plasma. Baseline resolution was achieved using the teicoplanin macrocyclic antibiotic chiral stationary phase (CSP) known as Chirobiotic T with a polar ionic mobile phase (PIM) consisting of methanol-glacial acetic acid-triethylamine (100 : 0.02 : 0.025, v/v/v) at a flow rate of 1.5 ml/min and fluorescence detection set at 275 nm for excitation and 305 nm for emission. All analyses with S-(-)-atenolol as the internal standard were conducted at ambient temperature. The assay involved the use of a solid-phase extraction procedure for human plasma samples prior to HPLC analysis. The C18 cartridge gave good recovery rates for both enantiomers without any interference. The method was validated over the range of 20-200 ng/ml for each enantiomer concentration. Recovery rates for S-(-)- and R-(+)-bisoprolol enantiomers were in the range of 95-102%. The method proved to be precise (within-run precision expressed as % RSD ranged from 1.0-6.2% and between-run precision ranged from 0.9-6.7%) and accurate (within-run accuracies expressed as percentage error ranged from 0.2-4.8% and between-run accuracies ranged from 0.3-1.7%). The limit of quantitation and limit of detection for each enantiomer in human plasma were 20 and 5 ng/ml, respectively.

Enantioselective determination of arotinolol in human plasma by HPLC using teicoplanin chiral stationary phase

A sensitive enantioselective high-performance liquid chromatography (HPLC) method was developed and validated to determine S-(+)- and R-(-)-arotinolol in human plasma. Baseline resolution was achieved by using teicoplanin macrocyclic antibiotic chiral stationary phase (CSP) known as Chirobiotic T with a polar organic mobile phase consisting of methanol:glacial acetic acid:triethylamine, 100:0.1:0.1, (v/v/v) at a fl ow rate of 0.8 mL/min and UV detection set at 317 nm. Human plasma was spiked with stock solution of arotinolol enantiomers and labetalol as the internal standard. The assay involved the use of liquid-liquid extraction procedure with ethyl ether under alkaline condition for human plasma sample prior to HPLC analysis. Recoveries for S-(+)- and R-(-)-arotinolol enantiomers were in the range 93-103% at 200-1400 ng/mL level. Intra-day and inter-day precision calculated as %RSD was in the ranges 1.3-3.4 and 1.9-4.5% for both enantiomers, respectively. Intra-day and inter-day accuracies calculated as percentage error were in the ranges 1.2-3.5 and 1.5-6.2% for both enantiomers, respectively. Linear calibration curves in the concentration range 100-1500 ng/mL for each enantiomer showed a correlation coefficient (r) of 0.9998. The limit of quantitation (LOQ) and limit of detection (LOD) for each enantiomer in human plasma were 100 and 50 ng/mL (S/N = 3), respectively.

Species-dependency in chiral-drug recognition of serum albumin studied by chromatographic methods

Stereoselective binding of benzodiazepine and coumarin drugs to serum albumin from human and six mammalian species were studied by chiral chromatographic techniques. The applied methods were affinity chromatography on the albumins immobilized on Sepharose 4B, high-performance liquid chromatography (HPLC) separation on columns based on human serum albumin (HSA) and bovine serum albumin (BSA), and chiral HPLC analysis of ultrafiltrates of solutions containing the racemic drug and the native protein. Substantial differences in preferred configurations and conformations were detected among the species. The binding stereoselectivity of the 2,3-benzodiazepine drug, tofisopam, in human, is opposite to that in all other species. In the binding of 1,4-benzodiazepines, dog albumin is very similar to HSA. Highly preferred binding of (S)-phenprocoumon was found with dog albumin.

Labeling and evaluation of N-[11C]methylated quinoline-2-carboxamides as potential radioligands for visualization of peripheral benzodiazepine receptors

The novel quinoline-2-carboxamide derivatives N-[methyl-11C]-3-methyl-4-phenyl-N-(phenylmethyl)quinoline-2-carboxamide ([11C]4), (+/-)-N-[methyl-11C]-3-methyl-N-(1-methylpropyl)-4-phenylquinoline-2-carboxamide ([11C]5), and (+/-)-N-[methyl-11C]-3-methyl-4-(2-fluorophenyl)-N-(1-methylpropyl)quinoline-2-carboxamide ([11C]6) were labeled with carbon-11 (t1/2 = 20.4 min, beta+ = 99.8%) as potential radioligands for the noninvasive assessment of peripheral benzodiazepine type receptors (PBR) in vivo with positron emission tomography (PET). The radiosynthesis consisted of N-methylation of the desmethyl precursors 3-methyl-4-phenyl-N-(phenylmethyl)quinoline-2-carboxamide (4a), (+/-)-3-methyl-N-(1-methylpropyl)-4-phenylquinoline-2-carboxamide (5a), and (+/-)-4-(2-fluorophenyl)-3-methyl-N-(1-methylpropyl)quinoline-2-carboxamide (6a) with either [11C]methyl iodide or [11C]methyl triflate in the presence of tetrabutylammonium hydroxide or potassium hydroxide in dimethylformamide. The radioligands [11C]4, [11C]5, and [11C]6 were synthesized with over 99% radiochemical purity in 30 min, 30 +/- 5% radiochemical yield, calculated at the end of synthesis (EOS) non-decay-corrected, and 2.5 +/- 1.2 Ci/micromol of specific radioactivity. Inhibition studies in rats following intravenous pre-administration of 1-(2-chlorophenyl)-N-methyl-N-(1-methylpropyl)-3-isoquinolinecarboxamide (PK 11195, 1) showed high specific binding to PBR of [11C]4, [11C]5, and [11C]6 in heart, lung, kidney, adrenal gland, spleen, and brain. The biological data suggest that [11C]5, [11C]6, and particularly [11C]4 are promising radioligands for PBR imaging in vivo with PET.

Isomerism and anaesthetic drugs

Isomers are two or more different substances with the same molecular formula (i.e., the same number of different types of atoms). There are two main types of isomerism: 1) structural isomerism, and 2) steroisomerism. Structural isomers (e.g., enflurane and isoflurane) have different molecular structures, and usually behave like different drugs. Occasionally, structural isomers are interconvertible (i.e., they are tautomers or dynamic isomers); this occurs with the barbiturates and midazolam. Steroisomers have identical structures, but a different configuration or spatial arrangement. Stereiosomerism in drugs is often due to chirality or "handedness"; i.e., the presence of right-handed (R)- and left-handed (S)- forms of drugs which are nonsuperimposable mirror images ("enantiomers"). Approximately 60% of anaesthetic agents are chiral drugs; some of these are administered as single enantiomers. However, many synthetic chiral drugs are equal mixtures of (R)- and (S)-isomers, and there are often important differences in their activity and pharmacokinetics. Halothane, enflurane, and isoflurane are chiral drugs with different anaesthetic potencies. Similar differences occur with intravenous anaesthetics; thus, (S) (+)-ketamine causes fewer psychotic emergence reactions, less agitated behaviour, and better intraoperative amnesia and analgesia than its enantiomer. Some local anaesthetics are administered as chiral mixtures; the (S)-isomers have a longer action because of enhanced vasoconstriction. (S)-prilocaine is more slowly metabolized than its enantiomer, while (S)-bupivacaine may produce less cardiotoxicity than (R)-bupivacaine. These differences suggest that some anaesthetic drugs (particularly ketamine and chiral local anaesthetics) should be administered as single enantiomers. In recent years, their synthesis has been greatly simplified, and almost all new drugs may soon be introduced in this form.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis of the enantiomers of [N-methyl-11C]PK 11195 and comparison of their behaviours as radioligands for PK binding sites in rats

The enantiomers of [N-methyl-11C]PK 11195, a radioligand for PET studies of PK (peripheral benzodiazepine) binding sites, have been prepared from the newly synthesized N-desmethyl-enantiomers by 11C-methylation with N.C.A. [11C]iodomethane. The brain uptake and retention of each enantiomer was compared with that of the racemic radioligand after i.v. administration into normal rats and into rats with focal cortical lesions. No significant differences in the uptakes of the enantiomers were observed in regions devoid of PK binding sites. However, the R-enantiomer was retained to a significantly greater extent than the S-enantiomer in olfactory bulbs-tubercles, which contain some PK binding sites, and also in 9-day-old focal cortical lesions, which are greatly enriched in PK binding sites associated with macrophage infiltration. The observed differences are consistent with the approximately 2-fold greater affinity of the R-enantiomer for PK binding sites reported in vitro and imply that the use of this enantiomer would have advantages over the use of the racemate currently used for PET studies.

Study of interaction of carprofen and its enantiomers with human serum albumin--II. Stereoselective site-to-site displacement of carprofen by ibuprofen

The site-to-site displacement of carprofen, a site II-specific drug, bound to human serum albumin (HSA) by ibuprofen, another site II-specific drug, was qualitatively and quantitatively studied by circular dichroism (CD) and equilibrium dialysis (ED). Carprofen gives rise to different CD spectra at lower (1:1) and higher (3:1) molar ratios to HSA, indicating different mechanisms for the binding of this drug to its high and low affinity sites on HSA. Ibuprofen at a 5:1 molar ratio to HSA displaces carprofen at a molar ratio of 1:1 to HSA from its high affinity binding site (site II) to its low affinity site (site I), as shown by production of the CD spectrum similar to that obtained in the case of the carprofen-HSA complex at a molar ratio 3:1. As revealed by the ED experiments, the free fraction of carprofen at a molar ratio of 1:2 to HSA (2 x 10(-5) M) was not initially increased by the addition of ibuprofen at a lower concentration, but at a higher concentration (6 x 10(-5) M), the free fraction was increased by only 90%. When site I was sufficiently blocked by a site I-specific drug like warfarin or phenylbutazone (6 x 10(-5) M), there was about a 4-fold increase in the free fraction of carprofen caused by ibuprofen. This site-to-site displacement demonstrated by carprofen was found to be stereospecific as indicated by the highest interaction between the S(+)-enantiomers of carprofen and ibuprofen. Moreover, the displacement of carprofen occurred at the azapropazone region rather than the warfarin region of site I on HSA.

Stereochemical selectivity in the induction of cytochrome P450IVA1 (P452)-dependent fatty acid hydroxylation and peroxisome proliferation

Induction of hepatic microsomal cytochrome P450IVA1 and peroxisomal enzymes of the beta-oxidation spiral were observed when male Long Evans hooded rats were administered optically pure enantiomeric forms and a racemic mixture of a clofibrate analogue [2-[4-(4-chlorophenyl)benzyloxy]-2-phenylacetic acid] at a dose level of 80 mg/kg for 3 days. The R(-)-enantiomer was found to be a more potent inducer of microsomal cytochrome P450IVA1 and its associated lauric acid 12-hydroxylase activity than its corresponding S(+)-antipode. This difference in potency was reflected by a eudismic ratio (R/S activity ratio) of approximately 3, whereas the racemic mixture exhibited a potency intermediary between the two isomers. An identical enantiomeric selectivity was observed for the phenomenon of peroxisome proliferation as judged by induction of cyanide-insensitive palmitoyl CoA oxidation and the bifunctional protein of the peroxisomal beta-oxidation spiral. The highest potency was shown by the R(-)-isomer resulting in approximately a 3-6-fold increase over the control value. These increases was paralleled by an increase in total carnitine acetyl transferase activity with a eudismic ratio of approximately 4. In addition, immunochemical detection by Western blotting analysis for both the microsomal cytochrome P450IVA1 isozyme and the peroxisomal bifunctional protein was in agreement with the above modulation of catalytic activities. These results are therefore not inconsistent with the hypothesis that cytochrome P450IVA1 induction and peroxisome proliferation are intimately linked. Whether the observed stereochemical selectivity resides in xenobiotic recognition or disposition still remains to be determined.

Enantioselective steady-state kinetics of unbound disopyramide and its dealkylated metabolite in man

Disopyramide is provided as a racemic mixture of R and S enantiomers, which have different pharmacodynamic and pharmacokinetic characteristics. Five volunteers were given racemic disopyramide 100 mg and 200 mg t.d.s. in a cross-over design. Plasma and urine concentrations of disopyramide and its active metabolite monodesisopropyl-disopyramide (MND) were determined at steady state by an enantioselective HPLC method. Unbound drug in plasma was measured after ultrafiltration. There was enantioselective clearance of unbound disopyramide (0.39 l.h-1.kg-1 for R-disopyramide and 0.58 l.h-1.kg-1 for S-disopyramide after 100 mg t.d.s.). The enantioselectivity was due to differences in the metabolism of disopyramide to MND and in further non-renal clearance, and the renal clearance of disopyramide was not enantioselective. The in vivo protein binding of disopyramide, which was saturable for both enantiomers, was also enantioselective. The difference in binding of the two enantiomers was explained by a difference in apparent binding capacity rather than in apparent binding affinity. The renal clearance of S-MND was significantly higher than R-MND (0.29 and 0.19 l.h-1.kg-1, respectively, after 100 mg t.d.s.). The renal clearance of MND also showed a tendency to saturation at higher concentrations.

Measurement of free drug in phase I: is it useful?

Early investigation of protein binding of a new drug is mandatory. The following questions have to be answered: is unbound fraction constant over tested concentrations? Which proteins are involved? What are the binding parameters? Can the drug compete with other therapeutic agents for the binding sites or in other words can drug displacements be predicted? What is the interindividual variability in protein binding? Is the binding stereoselective? All this information is necessary in predicting the pharmacokinetic behaviour of the drug and in assisting in the design of future pharmacokinetic protocols in phases II and III. The use of free drug concentration should also be considered when comparing the bioavailability of regular vs sustained release dosage forms of drugs exhibiting concentration-dependent binding and when studying concentration-effect relationships.

Importance of enantiomeric purity and its control by thin-layer chromatography

Methods for the direct resolution of enantiomers are important and are necessary for pharmaceutical and biomedical analysis, synthetic and mechanistic studies and various other fields. The present paper deals with the results of recent approaches, such as ligand exchange, ion exchange and steric interactions, providing direct resolution of enantiomers of a variety of compounds by thin-layer chromatography. General aspects of various methods for analysis of enantiomeric purity and resolution have been compared.

Stereochemical features of 1,4-benzodiazepin-2-ones bound to human serum albumin: difference CD and UV studies

The stereochemistry of an achiral (Diazepam) and two chiral (3-methyl and 3-succinyloxy substituted) 1,4-benzodiazepin-2-ones interacting with human serum albumin (HSA) has been investigated by making use of difference absorption (UV) and circular dichroism (CD) spectroscopies. Evidence is obtained for a higher affinity with HSA for one of the two possible conformations of the seven-membered benzodiazepine ring. The red shift revealed by the absorption difference spectrum between the free and the bound drug accounts for the CD difference spectra observed.

Different binding of propranolol enantiomers to human alpha 1-acid glycoprotein

The binding of propranolol enantiomers to human alpha 1-acid glycoprotein was studied using high performance liquid chromatography in order to provide insight into binding models and to describe individual binding parameters of both enantiomers. The binding of (-)-propranolol was shown to be saturable with one major binding site (n = 0.81, k = 2.73 x 10(5)/M). The saturation process achieved its upper asymptotic value at drug/protein molar ratio of approximately 1. In the case of the opposite (+)-enantiomer the binding isotherm did not show evidence of saturation even at higher drug/protein molar ratios (up to 50). The individual binding parameters for (+)-enantiomer were n = 0.38, k = 3.4 x 10(6)/M and n'k' = 1.39 x 10(4)/M for the saturable and nonsaturable binding component, respectively. At drug/protein molar ratio 2 the circular dichroism measurements confirmed the existence of different binding models for individual propranolol enantiomers.

Chirality: pharmacokinetics and pharmacodynamics in 3 dimensions

1. Biological macromolecules are able to distinguish between enantiomeric substrates. A three-point interaction between the drug enantiomers and the macromolecule (Easson-Stedman hypothesis) can frequently account for this selectivity. 2. Significant pharmacodynamic differences between enantiomers are more the rule than the exception. 3. Pharmacokinetic differences between enantiomers are, in general, not as great as the pharmacodynamic differences. However, stereoselective protein binding, metabolism and renal clearance are still very important aspects of understanding drug disposition and the time course of drug action. 4. There may be pharmacokinetic and pharmacodynamic enantiomer-enantiomer interactions. Consequently, the activity and disposition of a racemic drug may not be the simple sum of the activities and disposition of the individual enantiomers. 5. Enantiomers have been used as sensitive 3-dimensional probes to establish structure-activity relationships, to provide insights into genetic polymorphism of drug metabolism, and to provide insights into other aspects of drug disposition. 6. A need for a 3-dimensional understanding of pharmacodynamics and pharmacokinetics is implicit in the asymmetric nature of biological environments.

Receptor affinity, neurochemistry and behavioral characteristics of the enantiomers of thioridazine: evidence for different stereoselectivities at D1 and D2 receptors in rat brain

The binding characteristics of the enantiomers of thioridazine were assessed in the brain of the rat using competitive radioreceptor assays with tritiated ligands selective for dopamine D1 (SCH-23390), D2 (spiperone), norepinephrine alpha-1 (prazosin) and muscarinic (quinuclinidinyl benzilate) receptors. (+)-Thioridazine was shown to have 2.7 and 4.5 times higher affinity than (-)-thioridazine for D2 and alpha-1 receptors, respectively. In contrast, (-)-thioridazine had 10 times higher affinity for the D1 receptor. Both enantiomers showed similar affinities for the muscarinic receptor. In a second experiment, thioridazine, dopamine, norepinephrine, serotonin and their metabolites were assayed in the brain of the rat after acute administration of the enantiomers of thioridazine and the assessment of catalepsy. (+)-Thioridazine was 4.1 times as potent as (-)-thioridazine in elevating the turnover of dopamine in the striatum, but neither enantiomer affected the other monoamines. The concentration of thioridazine and its metabolites in the brain, for a given dose, was similar for both enantiomers. (-)-Thioridazine induced slightly more catalepsy than (+)-thioridazine and appeared to be more toxic at large doses. While racemic thioridazine had an intermediate effect between that of its two enantiomers in the binding and neurochemical assays, it appeared to induce more catalepsy than either enantiomer, suggesting a synergistic effect in this behavioral assay. It was concluded that (+)- and (-)-thioridazine act as partially selective D2 and D1 antagonists, respectively. Therefore, clinical administration of only one enantiomer of thioridazine, rather than the currently prescribed racemate, may result in an improved therapeutic profile and so be worthy of further investigation.

Selective effect of clonazepam and (S)-uxepam on the binding of warfarin enantiomers to human serum albumin

Both clonazepam and (S)-uxepam selectively increase the binding of (S)-warfarin to human serum albumin. By liquid affinity chromatography, improved resolution of rac-warfarin was achieved.

Stereoselective disposition of the geometric isomers of a novel lipoxygenase cyclo-oxygenase inhibitor in dog and photochemical interconversion of its isomers

A sensitive (10 ng/mL) and specific high-performance liquid chromatographic (HPLC) assay, with electrochemical (EC) detection, for the geometric isomers of 3-hydroxy-N-(2-phenyl-2-(2-thienyl)ethenyl-5-(trifluoromethyl)benzo(b) thiophene-2-carboxamide in dog and human plasma has been developed. Both isomers strongly absorb light, leading to an efficient E in equilibrium Z photoisomerization. After iv administration of a single isomer (Z) to a dog, only the Zisomer was detected in plasma; no in vivo conversion to the E isomer was observed. However, when a mixture of the E and Z isomers (58.6:41.4) was administered in the same manner to the same dog, the E:Z ratio decreased significantly to 47.5:52.5 six hours after drug administration, indicating stereoselective disposition of the isomers. The elimination of the E isomer was found to be faster than that of the Z isomer.

Determination of drug enantiomers in biological samples by coupled column liquid chromatography and liquid chromatography-mass spectrometry

Liquid chromatography-mass spectrometry (LC-MS) and coupled column chromatography can be used to overcome problems likely to occur in direct separation and determination of drug enantiomers in biological samples. This is exemplified here with the direct separation and determination of terbutaline in human plasma at the nmol/l level. A beta-cyclodextrin column with an aqueous mobile phase was used for chiral separation. For coupled column chromatography, the concentration of each enantiomer was calculated from the enantiomeric area ratio and the racemate concentration. A deuterium-labelled internal standard was used in the LC-MS experiments.

The stereoselectivity of the 'single drug binding site' of human alpha 1-acid glycoprotein (orosomucoid)

The stereoselective binding of six pairs of basic, one pair of acidic drug enantiomers, and one pair of diastereomers for human alpha 1-acid glycoprotein was investigated by means of competition experiments against [3H]propranolol- or [14C]nicardipine-labelled binding sites using equilibrium dialysis to separate free from bound marker ligand. The affinity constants (Ka) for association of [3H]propranolol and [14C]nicardipine with alpha 1-AGP were 1.2 +/- 0.6 X 10(5) M-1 and 3.4 +/- 1.4 X 10(5) M-1, respectively, and control binding amounted to 57 +/- 7 and 91 +/- 2%, respectively. The following selectivity factors, calculated as the ratio of the higher over the lower enantiomer concentrations displacing 15% of control radiomarker binding (IC15-value), were obtained against propranolol and nicardipine: (-)/(+) propranolol: 1.9 and 1.7.; (+)-/(-)-disopyramide: 2.8 and 1.4; (+)-/(-)-verapamil: 1.6 and 1.9; (+)-(S)-/(-)-(R)-202-791, a dihydropyridine derivative: 2.6 and 2.0; (-)-/(+)-asocainol: 1.7 and 3.0; (+)-/(-)-tilidine: 1.1 and approximately equal to 2; (-)-(S)-/(+)-(R)- warfarin: 1.6 and 2.4; (+/-)-cis/(+/-)-trans-trans-tilidine: 1.7 and 1.8. When the calculation of radioligand-free fractions is also taken into account, it is apparent that only the tilidine isomers show no selectivity at propranolol-marked, and the disopyramide isomers at nicardipine-marked alpha 1-AGP-binding sites, in all other cases, a weak selectivity is detectable, which is, however, far below the values obtained for most neurotransmitter receptors. It is concluded that the single drug binding site of alpha 1-AGP is only slightly stereoselective and that the stereoselective binding of the drugs investigated is probably of no clinical consequence.

The biliary excretion of acenocoumarol in the rat: stereochemical aspects

Within 24 h, 50% of a single dose of the acenocoumarol enantiomers was recovered in bile and 20% in urine of Wistar rats. The elimination products were mainly (greater than 90%) the 6- and 7-hydroxyacenocoumarol as conjugates in the bile but free in the urine. Only R-acenocoumarol, free and conjugated, was excreted in bile. There were no gross differences between the enantiomers in metabolic pattern or in the amount of metabolites formed. A significant difference was observed for the biliary excretion of the 7-hydroxy metabolite; the ratio of free and conjugated 7-hydroxyacenocoumarol was three times higher for the S- than for the R-isomer. An unknown third metabolite was recovered in bile in higher amounts with the S- than with the R-acenocoumarol. Only traces of this metabolite were recovered from urine. The data show an extensive biliary excretion of acenocoumarol and demonstrate stereoselective mechanisms in the excretion processes.