W.J.M. Underberg

Cyclodextrins in the Pharmaceutical Field

AbstractCyclodextrins (CyDs) are cyclic oligosaccharides, containing a minimum of six D-(+)-glycopyranose units attached by α-1, 4-linkages produced by the action of the cyclodextrin-trans-glycosidase enzyme on a medium containing starch. CyDs are somewhat cone-shaped. The outside of CyDs is hydrophilic and the inside of the cavity is hydrophobic in character. If a molecule fits entirely or at least partially into the cavity, an inclusion complex may be formed. In general, hydrophobic molecules, rather than hydrophilic ones, have a higher affinity to the CyD cavity in aqueous solutions. The CyD complexes thus formed are stabilized by various intermolecular forces, such as hydrophobic interaction, van der Waals forces, hydrogen bonding, release of high energy water molecules in complex formation and release of strain energy in the macromolecular CyD ring. Orally administered CyDs have shown to be harmless, because insignificant amounts are absorbed.Parenterally administered natural CyDs may cause severe ne...

Aspects of the degradation kinetics of doxorubicin in aqueous solution

Abstract The chemical stability of doxorubicin in aqueous solution has been investigated utilizing a stability-indicating HPLC assay with UV-VIS and fluorescence detection. The degradation kinetics were studied as function of pH (1–11), buffer composition (acetate, phosphate, carbonate), ionic strength (μ= 0.1–0.4), temperature (30–70°C) and drug concentration (1–20 μg/ml). A pH-rate profile, obtained from first-order kinetic plots, corrected for buffer and ionic strength influences, was constructed. The pH profile shows maximum stability for doxorubicin at about pH 4. A theoretical description of the pH-rate profile is presented. In addition, some attention has been given to the degradation mechanism of doxorubicin in alkaline solution. The proposed mechanism is partly based on a comparison of the chemical stability of doxorubicin with daunorubicin and five related anthracycline agents.

Derivatization trends in capillary electrophoresis

This survey gives an overview of recent derivatization protocols, starting from 1996, in combination with capillary electrophoresis (CE). Derivatization is mainly used for enhancing the detection sensitivity of CE, especially in combination with laser‐induced fluorescence. Derivatization procedures are classified in tables in pre‐, on‐ and postcapillary arrangements and, more specifically, arranged into functional groups being derivatized. The amine and reducing ends of saccharides are reported most frequently, but examples are also given for derivatization of thiols, hydroxyl, carboxylic, and carbonyl groups, and inorganic ions. Other reasons for derivatization concern indirect chiral separations, enhancing electrospray characteristics, or incorporation of a suitable charge into the analytes. Special attention is paid to the increasing field of research using on‐line precapillary derivatization with CE and microdialysis for in vivo monitoring of neurotransmitter concentrations. The on‐capillary derivatization can be divided in several approaches, such as the at‐inlet, zone‐passing and throughout method. The postcapillary mode is represented by gap designs, and membrane reactors, but especially the combination of separation, derivatization and detection on a chip is a new emerging field of research. This review, which can be seen as a sequel to our earlier reported review covering the years 1991—1995, gives an impression of current derivatization applications and highlights new developments in this field.

Derivatization in capillary electrophoresis

In recent years capillary electrophoresis (CE) has been developed into a versatile separation technique, next to gas and liquid chromatography (LC), well suited for the determination of a wide variety of e.g., pharmaceutical, biomedical and environmental samples. The main advantages of CE over chromatographic separation techniques are its simplicity and efficiency. It is well recognized, however, that the sensitivity and selectivity of the detection are relatively weak points of CE. One way to overcome these limitations is the conversion (derivatization) of the analytes into product(s) with more favourable detection characteristics. Although, in principle, almost any detection mode can be combined with a derivatization procedure, in practice, fluorescence monitoring is favoured in most cases. This paper aims to give a short overview on the various reagents that can be used for pre-, post- and on-column derivatization in CE. First, a short introduction is given on CE as an analytical technique, followed by a discussion of the pros and cons of the various modes of derivatization, a comparison of derivatizations in CE with derivatizations in LC, the principles of fluorescence and prerequisites for a good fluorophore and the potential of using diode lasers in combination with a labelling procedure. With respect to the derivatization reagents the emphasis is on the labelling of amino, aldehyde, keto, carboxyl, hydroxyl and sulfhydryl groups.

High-performance liquid chromatographic analysis of the new antitumour drug SK&F 104864-A (NSC 609699) in plasma☆

A high-performance liquid chromatographic (HPLC) assay is described for the determination of the new, investigational antitumour drug SK&F 104864-A and its lactone ring opened form (SK&F 105992). The analytical methodology reported here involves a protein precipitation step with methanol as sample pretreatment procedure. The instability of the drug necessitates that the plasma fraction is obtained within 5 min after blood sampling by centrifugation, immediately followed by protein precipitation with cold methanol (-30 degrees C). The methanolic extract can be stored at -30 degrees C for several days without deterioration of the analyses. Stability data of the drug and its lactone ring opened metabolite in plasma and after methanolic extraction are discussed. The parent drug and the metabolite are separated by reversed-phase ion-pair liquid chromatography on a LiChrosorb RP-18 column, using methanol-water eluent (pH 6.0) with sodium dioctylsulphosuccinate (DOSS) as ion-pairing agent and fluorescence detection. The proposed method has been validated and, subsequently, implemented in a phase I clinical trial for pharmacokinetic evaluation of the new cytotoxic agent.

On-line sample preconcentration in capillary electrophoresis, focused on the determination of proteins and peptides.

This overview highlights the possibilities of on‐ or in‐line preconcentration procedures in combination with a CZE separation, focused on the determination of peptides and proteins. The discussed methods, including sample stacking, field‐amplified injection, isotachophoresis, solid phase extraction, membrane preconcentration, electroextraction, supported liquid membranes, hollow fibers, immunoaffinity, and molecularly imprinted polymers technology preconcentration are categorized in electrophoresis‐based and chromatography‐based preconcentration. The chromatography‐based preconcentration is subdivided in low‐specificity and high‐specificity methods. A number of preconcentration methods are available, however, this paper demonstrates that various compounds in different media (aqueous solutions, urine, and plasma) require different preconcentration systems. The preconcentration techniques of first choice in general seem to be solid‐phase extraction and membrane preconcentration, because of their high concentration ability, multiapplicability, relative simplicity and clean‐up capability. For the future, hollow fibers seem to hold a great potential as preconcentration technique, yielding high concentration factors, using simple designs. New techniques, such as hollow fibers, molecularly imprinted polymers technology and supported liquid membranes may have the potential to supersede the conventional preconcentration techniques in some cases. The larger the arsenal of preconcentration techniques becomes, the more efficiently peptides and proteins may be analyzed in the future. These techniques, in some cases, require pre‐cleanup procedures, to ensure the purity of the samples to concentrate.

Fluorescence detection in high performance liquid chromatography

Abstract The principles and applications of fluorescence detection and fluorescence introducing reagents and methods in HPLC are reviewed. The design and requirements for fluorescence detectors, flow cells and excitation sources and the conversion of non-fluorescent compounds into fluorescent products by pre-column and post-column derivatization reactions are discussed. For the applications the emphasis is on drug analysis, where possible in biological fluids (serum, urine, etc.). The last paragraphs are divided in a number of sections in which newly developed and some scarcely used reagents are mentioned shortly; a more complete treatment is given of the reagents and labels most frequently used in the derivatization of certain functional groups. In this discussion the methods of derivatization as well as the selectivity, stability, fluorescence behaviour of the reagents/labels and derivatives and the reaction conditions are included. An up-to-date survey of the applications of fluorescence detection in l...

Equilibrium kinetics of the new experimental anti-tumour compound SK&F 104864-A in aqueous solution☆

The equilibrium kinetics of lactone ring hydrolysis in the new experimental anti-tumour compound SK&F 104864-A. (S)-dimethylaminomethyl-10-hydroxycamptothecin hydrochloride, have been studied. Only one product is formed, SK&F 105992. A stability-indicating HPLC method has been optimized to perform the analysis. The pH is the main factor influencing equilibrium; at pH greater than or equal to 10 the lactone ring is quantitatively opened while at pH values less than or equal to 4 the lactone form is exclusively present. Other parameters, such as buffer ions and ionic strength, do not influence equilibrium. Complexation with dimethyl-beta-cyclodextrin stabilizes the lactone form. Other cyclodextrins do not show this stabilization.

Capillary electrophoresis as a versatile tool for the bioanalysis of drugs - a review

This review article presents an overview of current research on the use of capillary electrophoretic techniques for the analysis of drugs in biological matrices. The principles of capillary electrophoresis and its various separation and detection modes are briefly discussed. Sample pretreatment methods which have been used for clean-up and concentration are discussed. Finally, an extensive overview of bioanalytical applications is presented. The bioanalyses of more than 200 drugs have been summarised, including the applied sample pretreatment methods and the achieved detection limits.

Hydrolysis of Partially Saturated Egg Phosphatidylcholine in Aqueous Liposome Dispersions and the Effect of Cholesterol Incorporation on Hydrolysis Kinetics

Abstract— Hydrolysis kinetics of partially hydrogenated egg phosphatidylcholine (PHEPC) were studied as a function of pH, temperature, buffer concentration, ionic strength, and the effect of cholesterol incorporation. Results showed that PHEPC has a maximum stability at around pH 6·5. General acid base catalysis was observed for acetate, HEPES and Tris buffers. Increasing the ionic strength of the buffer solutions did not influence the hydrolysis kinetics. The relationship between the observed hydrolysis rate constants and the temperature could adequately be described by the Arrhenius equation. Incorporation of cholesterol did not affect the hydrolysis kinetics. This result indicates that the hydrolysis kinetics of PHEPC do not depend on the changes in bilayer rigidity induced by cholesterol incorporation. Cholesterol is stable under the experimental conditions used in this study; no changes were observed in cholesterol concentration over the experimental time interval.