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A F Fell

Publications and source records attributed to A F Fell.

At least 19 recordsLinked to original sources
PubMed2004

High-resolution method for regulatory control of Echinacea species in Nutraceuticals by CD-MEKC.

One problem in the international regulatory control of Echinacea, a therapeutic Nutraceutical, is recognition of caffeoyl solutes and alkamides in different products. Cyclodextrin-modified micellar electrokinetic chromatography (CD-MEKC) has been applied to Echinacea spp. in combination with pattern recognition of some caffeoyl solutes. A novel metric based on relative migration time (RMT) data has been developed in CE to address the problem of variable reported migration time. The CD-MEKC method of Gotti's group using hydroxypropyl-beta-cyclodexrin (HP-beta-CD; 100 mM) with sodium dodecyl sulphate (SDS; 110 mM), in a triacid background electrolyte (10 mM, pH 8) under 19 kV was adapted to identify two key hydrophilic solutes: chlorogenic acid and cichoric acid present in all commercial products. Two internal markers were taken as reference points to calculate the RMT of any target peak: RMT=t(m (target))/t(m (marker)). The RMT method was robust to temperature change from 20 to 40 degrees C, but sensitive to pH. The lateral shift and reproducibility of the target peak t(m (target)) were significantly reduced by this novel transformation. In the worst cases migration time variability ranged up to 12% (n=6); the RMT algorithm reduced this to less than 1%. In general, the RMT transformation reduced the variability of migration time data by a factor of 2-5. For systematic comparison of electrophoretic profiles for test sample and standard, a new pattern recognition algorithm permits sequential peak-by-peak comparison using specified segments of the electropherograms for comparison of test and Echinacea purpurea (root product) as a standard. This algorithm was capable of rapidly characterising the similarity of target peaks in a test sample relative to those in the reference standard. Combination of the RMT algorithm and pattern recognition in CE is expected to offer a robust approach to international regulatory characterisation and control of Nutraceuticals.

Chromatography, Micellar Electrokinetic Capillary↗
PubMed1999

Separation of the voriconazole stereoisomers by capillary electrophoresis and liquid chromatography.

A chiral capillary electrophoresis (CE) method has been developed for the direct separation of the four stereoisomers of a new broad spectrum antifungal agent, voriconazole. Cyclodextrin (CD) modified micellar electrokinetic chromatography employing, alpha-CD, beta-CD, gamma-CD, hydroxypropyl-beta-CD and hydroxyethyl-beta-CD was not sufficiently selective for the four neutral stereoisomers. Three anionic sulphobutyl-ether-beta-CD (SBE-beta-CD) electrolyte additives, each having a defined degree of substitution (DS) (6.5, 4.5 and 1.0) were subsequently examined. The complete CE separation of all four stereoisomers was obtained when using the medium substituted additive DS = 4.5. In liquid chromatography (LC), two approaches were examined for the direct chiral separation of the stereoisomers of voriconazole: (a) use of the neutral and anionic CD mobile phase additives and (b) a vancomycin chiral stationary phase. The CD additives were shown to be extremely selective for two stereoisomers of voriconazole (active drug and its enantiomer) but unable to discriminate between the opposite two stereoisomers. The converse was observed, however, when the vancomycin chiral stationary phase was employed.

Antifungal Agents↗
PubMed1998

High-performance liquid chromatography study of stereospecific microsomal enzymes catalysing the reduction of a potential cytostatic drug, oracin. Interspecies comparison.

One of the main metabolites of oracin (I) ¿6-[2-(2-hydroxyethyl)aminoethyl]-5,11-dioxo-5,6-dihydro-11H-indeno[1,2- c] isoquinoline¿, a potential cytostatic drug, is 11-dihydrooracin (II) ¿(+),(-)-6-[2-(2-hydroxyethyl)aminoethyl]-5-oxo-11-hydroxy-5,6-dihydro-1 1H- indeno[1,2-c]isoquinoline¿, a metabolite formed by the reduction of oracin's pro-chiral centre on C 11. This metabolite has been found in all laboratory species in vitro and in vivo and it constitutes the main metabolite in man. The stereospecificity of reducing enzymes participating in the oracin biotransformation pathway was investigated using microsomal preparations from standard laboratory animals. Enzyme stereospecificity has been defined as preferential formation by the enzyme of the (+) or (-) stereoisomer of II. Significant interspecies differences were observed in the stereospecificity of the respective biotransformation enzymes. HPLC quantitative determinations of both enantiomers were performed using a Chiralcel OD-R column as chiral stationary phase with excellent resolution and stability.

Animals↗
PubMed1997

Separation of the stereoisomers of the main metabolite of a non-steroidal anti-inflammatory drug, flobufen, by chiral high-performance liquid chromatography.

The major metabolite of a novel non-steroidal anti-inflammatory drug, DL-4-(2',4'-difluorobiphenyl-4-yl)-2-oxo-2-methylbutanoic acid (flobufen, I), namely 4-(2',4'-difluorobiphenyl-4-yl)-2-methyl-gamma-butyrolactone (4-dihydroflobufen lactone, III), has four stereoisomers consisting of two racemic pairs of enantiomers. Of three chiral stationary phases tested, Cyclobond I beta-RSP (Astec) (beta-cylodextrin derivatized with R,S-hydroxypropyl) was best able to separate the (+2)(--) racemate, with a liquid phase containing acetonitrile as modifier and triethylamine acetate as buffer. Using the Box-Wilson Central Composite Design for three factors, an optimum combination of pH and concentrations of the modifier and buffer was eventually obtained. A chromatographic response function based on a combination of the Kaiser peak separation function, Pi, and retention time of the second eluting enantiomer, tRL, served as a response criterion for the process of optimization. The optimum conditions developed for the (+2)(--) racemate were also found to be suitable for separating the (+-)(-+) racemate, for which earlier studies had shown the separation to be more facile. Separation of the four stereoisomers of III, for which the chiral chromatographic system optimized in this study is proposed as the second stage, is targeted at a biochemical study of the stereoisomeric metabolism of I.

Anti-Inflammatory Agents, Non-Steroidal↗
PubMed1997

Chiral recognition in liquid chromatography utilising chargeable cyclodextrins for resolution of doxazosin enantiomers.

The chromatographic resolution of rac-doxazosin using reversed-phase high performance liquid chromatography (HPLC) with the chargeable chiral mobile phase additive, carboxymethyl-beta-cyclodextrin (CM-beta-CD), is described. The effects of different modifiers (acetonitrile, methanol and tetrahydrofuran), pH, temperature, and cyclodextrin concentration were investigated to a) assess the key chromatographic parameters for subsequent chemometric optimisation, and b) explore the enantioselective mechanism. Assuming a 1:1 complex between each doxazosin enantiomer and CM-beta-CD, studies of the relationship between the capacity factors (k') and functions of CM-beta-CD concentration indicate that the mechanisms for retention and chiral selectivity are comparable with those proposed earlier by Sybilska et al. Stability constants (KG) calculated for rac-doxazosin complexed with CM-beta-CD (647 +/- 55 and 594 +/- 45 M-1 for each enantiomer respectively) are significantly larger than those calculated for the barbiturates complexed with beta-CD (ca. 101-108 M-1). Investigations on pH indicate an ionic or ino-pair interaction between the anionic CM-beta-CD and the cationic doxazosin enantiomers. A central composite design was used to optimise the key chromatographic parameters: pH, methanol (v/v) and CM-beta-CD concentration. The Kaiser peak separation index, Pi, was used for the response function. The predicted response for this chiral separation has been compared with that observed experimentally and samples of the four-dimensional response surface have been assessed for their value in showing robustness.

Adrenergic alpha-Antagonists↗
PubMed1997

Use of 1H-NMR spectroscopy to determine the enantioselective mechanism of neutral and anionic cyclodextrins in capillary electrophoresis.

One-dimensional (ID) and two-dimensional (2D) 1H nuclear magnetic resonance (NMR) techniques have been used to investigate the chiral recognition process in capillary electrophoresis (CE) for seven different cyclodextrins (CDs) with the calcium channel blocker amlodipine as a model compound. These include five neutral CDs (alpha-CD, beta-CD, gamma-CD, hydroxypropyl-beta-CD and hydroxyethyl-beta-CD) and two anionic CDs (sulphobutyl-ether-beta-CD and carboxymethyl-beta-CD) where mixtures of amlodipine with each of the seven CDs were examined by 1D NMR in deuterated phosphate buffer at pD 3.4. The resonance shift of signals with added CD, relative to the CD-free position (shift displacement, delta delta) and shift non-equivalence (delta delta *) of enantiomeric signals shifted relative to each other after addition of CD were examined for non-overlapped protons of amlodipine. The possible correlations of NMR shift non-equivalence data with chiral separation in CE for amlodipine have been critically assessed. Qualitative differences in the 1D NMR shifts and enhanced enantioselectivity in CE were observed for amlodipine with sulphobutyl-ether-beta-CD. Further experiments on the through-space interactions using 2D rotating frame nuclear Overhauser effect spectroscopy (ROESY) indicated that there was no association between internal glucopyranose hydrogen atoms and the aromatic hydrogens of amlodipine. This gives evidence for the aromatic ring not being included in this CD. Moreover, data from spin-lattice relaxation times (T1) measured for amlodipine in the free state and after addition of the anionic sulphobutyl-ether-beta-CD indicate that the aromatic moiety of amlodipine is not included into the sulphobutyl-ether-beta-CD cavity. There is evidence that it interacts with the sulphobutyl side chains, and may adopt a preferred orientation outside the sulphobutyl-ether-beta-CD toroid itself.

Amlodipine↗
PubMed1996

Determination of plasma concentrations of epirubicin and its metabolites by high-performance liquid chromatography during a 96-h infusion in cancer chemotherapy.

In order to determine epirubicin and its metabolites at low concentrations (< 38 ng/ml) in small plasma samples, a fast reliable method based on a precipitation pre-treatment and sensitive reversed-phase isocratic HPLC has been developed and validated for epirubicin in the range 5-100 ng/ml. The R.S.D. was 5-9% over this concentration range. For human serum containing 25 ng/ml of epirubicin, the inter- and intra-day variation was < 10%. Recoveries of the metabolites epirubicinol, 7-deoxydoxorubicinone and 7-deoxydoxorubicinolone at 20 ng/ml ranged from 94-104%. The assay has been used to study human plasma samples taken during a 96-h infusion of epirubicin in a patient with multiple myeloma. The combined levels of the unseparated metabolites, epirubicin glucuronide and epirubicinol glucuronide, were semiquantitatively determined after treatment with beta-glucuronidase. The metabolites epirubicinol and 7-deoxydoxorubicinolone, but not 7-deoxydoxorubicinone, were also detected and measured.

Antibiotics, Antineoplastic↗
PubMed1996

Method development in liquid chromatography with a charged cyclodextrin additive for chiral resolution of rac-amlodipine utilising a central composite design.

A negatively charged derivative of beta-cyclodextrin, sulphobutyl ether-beta-cyclodextrin (SBE-beta-CD), was examined as a chiral mobile phase additive in reversed-phase high-performance liquid chromatography for the enantiomeric resolution of the calcium channel blocker rac-amlodipine. Theoretical and practical aspects are discussed for setting up a central composite design applicable to any analytical method. These include the correct location of factor points for maintaining orthogonality within the design and the augmentation of centrepoint experiments to allow a larger factor space by increasing the distance of axial star points. Optimised separation was achieved using a reverse-phase column with eluent comprising: acetonitrile (ACN)-potassium dihydrogen phosphate (pH 3.93) containing 2.66 mM SBE-beta-CD (26.5:73.5% v/v) at a flow rate of 1.0 ml/min. This yielded a Kaiser peak separation index, Pi = 0.96, at tR2 = 52 min with satisfactory reproducibility, relative standard deviation values: tR1, 0.39%; tR2, 0.47% (n = 5). These experimental results were in excellent agreement with those predicted by the SAS software package for a chromatographic response function model. Multiple regression analysis in four dimensions, with three response models based on Rs, Pi, and a function of Pi, produced response surfaces which revealed zones of optimum robustness and illustrated the interactions involved between the key chromatographic factors. Putative proposals for a mechanism involving the interaction of each of the positively charged enantiomers with the negatively charged cyclodextrin are also discussed. These examine the possibility of ion-pairing and inclusion phenomena to account for the excellent resolution observed.

Amlodipine↗
PubMed1994

Determination of altretamine in human plasma with high-performance liquid chromatography.

A fast, simple, and sensitive isocratic HPLC method has been developed and validated for the determination of the anticancer drug altretamine in human plasma. Spiked serum samples and clinical plasma samples are extracted with acetonitrile at 4 degrees C and the precipitate removed by filtration. The plasma sample volume required (ca. 0.2 ml) is small and the total analysis time is less than 15 min per sample (including batch-wise pre-treatment). Recovery of altretamine is 99 to 106% for pooled human serum spiked with altretamine in the range 200 ng/ml to 10 mg/ml. In this concentration range, the R.S.D. varies from 1 to 8%. The limit of quantitation is ca. 150 ng/ml for an R.S.D. of 10%. The intra-day R.S.D. for human samples spiked at 5 mg/ml varied between 1.7 and 4%; the inter-day R.S.D. at this concentration was ca. 3%. A preliminary study with one patient receiving 260 mg/m2 by mouth indicated that the peak altretamine concentration was significantly lower after a standard breakfast than in the fasting state.

Altretamine↗
PubMed1994

Photodegradation studies on chloroquine phosphate by high-performance liquid chromatography.

A method based on high-performance liquid chromatography (HPLC) was developed to study degraded chloroquine samples produced after exposure to sunlight in the Sudan. The method was also used to investigate chloroquine photodegradation after irradiation by UV and sunlight at ambient temperature. The study showed that the photodecomposition of chloroquine was pH and solvent dependent. Moreover, the extent of reaction was found to increase in the absence of oxygen. At pH 8, where the reaction rate was high, the photodecomposition was found to follow pseudo-first-order reaction kinetics. The HPLC method developed was also employed to analyse chloroquine and its degradation products in two commercially available brands of chloroquine injections which had been stored under local conditions in the Sudan. A number of degradation products were separated and examined by photodiode array spectroscopy and preparative TLC.

Antimalarials↗
PubMed1993

Strategies for peak-purity assessment in liquid chromatography.

One of the key requirements for the validation of chromatographic methods is to determine the purity of a chromatographic peak. Statistical modelling of the chromatographic process suggests that overlapping components are highly probable in a chromatogram. Hence extensive efforts have been directed at the development of sensitive, reliable and robust methods to assess peak purity. This is especially the case in the pharmaceutical industry, where liquid chromatography (LC) is widely utilized and the demands on method validation are justifiably high. On-line multiwavelength absorptiometric detection is often used to generate the additional data required to facilitate peak-purity assessment in LC. This approach, using photodiode-array technology, is directly compatible with the aqueous-based reversed-phase LC solvents used extensively in drug analysis. Consequently, this work highlights many of the peak-purity algorithms, which may be applied using LC with diode-array detection. The relative merits of the individual techniques are discussed, and a rationale is developed for their application.

Algorithms↗
PubMed1993

LC studies on the potential interaction of paraben preservatives with sorbitol and glycerol.

Pharmaceutical formulations often contain one or more paraben preservatives in conjunction with a polyol such as sorbitol or glycerol. In one of these experimental formulations a number of unknown polar peaks have been detected near the solvent front by reversed-phase LC. These degradation products were not attributable to the active drug component or a hydrolysis product. The possibility of an interaction between the polyols and paraben preservatives has been explored using a three-variable, two-level, factorial design to determine the relative significance of the factors involved in the formation of these unknown peaks. The factors examined were pH, temperature and the ratio of polyol to paraben. This study has shown that pH and temperature are key factors affecting the formation of these unknown peaks. On the basis of these results suitable conditions can be suggested for minimizing the production of these unknown peaks. It seems clear that a number of pharmaceutical formulations containing a polyol and a paraben would present potential problems for assay validation on storage owing to the formation of these degradation products, particularly if the drug component is polar and elutes near the solvent front.

Chromatography, Liquid↗
PubMed1992

Evaluation of the anti-tumour action and acute toxicity of kosins from Hagenia abyssinica.

The kosins are phloroglucinol derivatives isolated from female flowers of Hagenia abyssinica (Rosaceae) and were tested for possible cytotoxic activity in vitro and in vivo against a panel of three transplantable murine adenocarcinomas of the colon of varying growth characteristics and morphology (MAC system). Significant reductions in colony formation were observed in vitro in MAC 15A tumour following 1, 3, 6 and 24 h exposure to all kosins (alpha-kosin, kosotoxin and protokosin). The kosins (kosotoxin and protokosin) were also found to be cytotoxic against MAC tumour cells in vivo in some cases. Kosotoxin was subjected to preliminary toxicity studies in mice. It showed no observable toxicity up to 200 mg kg-1 orally and was found to be toxic at doses in excess of 50 mg kg-1 (i.p.). A single dose of 100 mg kg-1 (i.p.) was lethal for 100% of the animals.

Administration, Oral↗
PubMed1991

A multiwavelength approach to the selection of absorbance ratios for the assessment of chromatographic peak purity.

The routine use of diode-array detectors (DAD), based on the linear photodiode array device, has transformed the practice of UV-vis detection in liquid chromatography (LC). Multiwavelength detection is widely employed to generate absorbance ratios as a relatively non-specific method for characterizing peak purity in LC. If several wavelength pairs are selected the selectivity of the absorbance ratio method and its sensitivity to an interfering impurity can be increased, however, these attributes still depend on the selection of suitable pairs of wavelengths. This paper presents a novel approach to the selection of absorbance ratios for the assessment of peak purity in LC, utilizing a matrix derived from all the spectral data collected. As with single absorbance ratios, the absorbance ratio matrix (ARM) generated (containing all possible finite absorbance ratios) is characteristic for the analyte and independent of the analyte concentration. Moreover, the ARM technique eliminates the need to select "appropriate wavelength pairs", for sensitive discrimination of small spectral differences, when used for peak purity assessment. The ARM is found to give comparably high sensitivity to the presence of co-eluting species, as compared with the use of the wavelength pair selected on the basis of the conventional optimization criteria.

Chemistry Techniques, Analytical↗
PubMed1991

Chromatographic and spectroscopic analysis of bound and unbound phenolic acids in Lagenaria breviflora fruit.

The fruit pulp of Lagenaria breviflora Robert (Cucurbitaceae), used in Nigeria as an anti-bacterial and anti-fertility drug, was found to contain phenolic acids. Isolation and characterization of these compounds was based on column chromatography, TLC, PC, UV, IR and GC-MS. While p-hydroxybenzoic and vanillic acids were found to occur as free and bound acids in the pulp, ferulic acid was found to occur only as an ester. An optimized HPLC procedure for the quantitative analysis of these acids was developed, featuring short retention, times, high sensitivity and excellent resolution. The concentration of these phenols in the fruit mesocarp was established.

Chromatography, High Pressure Liquid↗
PubMed1991

Diode array detection and simultaneous quantitation of the coeluting atenolol-related synthetic route impurities, PPA-Diol.

The diode array multichannel detector is used to acquire spectral information at specified intervals in the elution profile of atenolol and its related impurities for post-chromatographic data analysis. The applicability of a number of peak homogeneity testing methods, including spectral normalization, absorbance ratio, chromatographic derivatives, and spectral suppression (SS), are assessed for suitability in simultaneous determinations of the coeluting atenolol-related synthetic impurities, PPA-Diol. Spectral suppression displays a superior performance in comparison to all the other techniques in that both qualitative and quantitative information are acquired on the system.

Acetamides↗