Investigation of vancomycin and related substances by liquid chromatography/ion trap mass spectrometry.

Liquid chromatography (LC) methods compatible with mass spectrometry (MS) that are suitable for impurity profiling of vancomycin mixtures have not been described in the literature. The mobile phases of the existing methods contain non-volatile additives and/or solvents that give problems in combination with MS. In this paper, a reversed-phase LC/tandem mass spectrometry method is described for the investigation of vancomycin and related substances. The LC method uses a Zorbax Extend C18 column (250 x 4.6 mm i.d.), 5 microm, and a mobile phase consisting of methanol, water and ammonium acetate solution (pH 9.0). This method allows us to separate vancomycin and its impurities. Mass spectral data are acquired on an LCQ ion trap mass spectrometer equipped with an electrospray interface operated in the positive and negative ion modes. The LCQ is ideally suited for identification of impurities and related substances because it provides on-line LC/MSn capability, which allows efficient identification without time-consuming isolation and purification procedures. Using this method, the fragmentation of vancomycin and known derivatives was studied and the structures of six substances occurring in commercial samples were elucidated.

Analysis of unknown compounds in azithromycin bulk samples with liquid chromatography coupled to ion trap mass spectrometry.

A selective reversed-phase liquid chromatography/mass spectrometry (LC/MS(n)) method is described for the identification of azithromycin impurities and related substances in commercial azithromycin samples. Mass spectral data are acquired on an LCQ ion trap mass spectrometer equipped with an atmospheric pressure chemical ionization interface operated in positive ion mode. The LCQ provides on-line LC/MS(n) capability, making it ideally suited for identification purposes. In comparison with UV detection, this hyphenated technique provides as its main advantage efficient identification of novel substances without time-consuming isolation and purification procedures. Using this technique, six novel related substances detected in commercial azithromycin samples have been studied.

Population, acid-base, and redox properties of N-acetylcysteine conformers.

Rotamers of N-acetyl-L-cysteine (NAC, the most popular mucolytic drug) are characterized in terms of populations, site- and conformer-specific acid-base properties, reducing strength, and molecular pharmacology. A new, general relationship between the bulk- and rotamer-specific basicities is introduced. NAC at high pH predominantly exists in a trans thiolate-carboxylate rotameric form, whereas protonation promotes the occurrence of intramolecular hydrogen bond-forming isomers. Distribution curves of the rotamers are depicted as a function of pH. Rotamer-dependent thiolate basicities differ by up to 0.5 log k units. Carboxylate basicities show slight conformation-dependence only. The membrane-penetrating capabilities from various compartments of the body are assessed on the basis of the pH-dependent charge of the molecule. The thiol-disulfide half-cell potential is calculated, using the correlation between the thiolate basicity and oxidizability. The oxidation-reduction properties of NAC are compared to those of other biological thiols in their definite microscopic forms. The pharmacokinetic behavior is interpreted in terms of the physicochemical parameters, providing molecular/submolecular explanation for several therapeutic properties of NAC.

[Characterization of the kinetics of ester hydrolysis at the submolecular level]. / Az észterhidrolízis kinetikájának szubmolekuláris jellemzése.

Ester-type compounds are important drugs as directly binding active principles, but also, as prodrugs that provide the acting agent after hydrolytic decomposition. The profound characterisation of the kinetics of ester hydrolysis is therefore equally important to interpret and design metabolic processes and prodrug-->drug transitions. The extramolecular factors (temperature, ionic strength, solvent etc.) influencing the rate constant values have extensively been studied. Contrary to that, few data can be found on the effect and magnitude of the intramolecular factors (substituent effect, neighbour-group protonation). This paper reports the introduction and determination of microscopic rate constants of ester hydrolysis, a new physicochemical parameter, which is defined in the sense of the protonation state of the basic site(s) adjacent the ester group. The microscopic rate constants of phenylalanine-methyl-ester hydrolysis are determined and interpreted. Also, the principles to characterise the rate of monobasic double esters at the submolecular level, exemplified by cocaine, are established.