Montserrat Gallardo

Physicochemical properties of enrofloxacin

The physicochemical properties of enrofloxacin, a fluoroquinolone that inhibits the activity of bacterial DNA gyrase, are described. Its spectral, solubility and related physicochemical characteristics are discussed. The dissociation behaviour of enrofloxacin was examined by UV spectrophotometry at 25 degrees C in a series of buffers ranging from pH 1 to 10. The corresponding macro- and microscopic dissociation constants were calculated. The apparent n-octanol-water partition coefficients were measured from pH 2 to 10.

An autocatalytic reaction as a model for the kinetics of the aggregation of β‐amyloid

Alzheimers disease is the commonest form of senile dementia, affecting almost 20 million people worldwide. This neurodegenerative disorder is characterized by amyloid deposition in senile plaques, composed primarily of fibrils of an aggregated peptide, β‐amyloid. Fibrillation of β‐amyloid is a nucleation‐dependent polymerization process, which is controlled by two kinetics parameters: the nucleation rate and the elongation or growth rate. As the kinetics of fibrillation is strongly dependent on the presence of trace amounts of fibrils, we suggest that the aggregation of β‐amyloid is a model of autocatalytic reaction. A mathematical analysis, permitting quantitative monitoring of the kinetics of fibrillogenesis of β‐amyloid, nucleation, and elongation constants, is presented. The model was checked by applying it to the aggregation of the fragment 1–40 of the β‐amyloid. Understanding of these rate constants may facilitate the study of the effect of substances used for controlling fibril creation and growth. The disaggregating effect of dodecyl trimethylammonium bromide, a cationic surfactant, was easily quantified by means of the model.

Physical stability of liposomes bearing hemostatic activity.

The physical stability of six liposome systems designed as platelet substitutes was determined on storage at 4 degrees C over a 3-month period under quiescent conditions. Liposomes used were large unilamellar vesicles. Correlation of the n-average mean diameter, polydispersity, zeta-potential and the presence of aminophospholipid on liposome surface (in those preparations which contain phosphatidylethanolamine (PE) and phosphatidylserine (PS)) led to the conclusion that liposomes that mimicked the composition of platelets were the most stable. When a net charge was present in the vesicles (liposomes with PS), the likelihood of aggregation was extremely low. In the period studied, a proportion of 25% of charged lipid (PS) conferred sufficient electrostatic stabilization to prevent vesicle fusion. An increase in this charge did not modify the stability characteristics. PE-containing liposomes behaved in a particular way: when PE content was 50%, the stability of the preparation was limited to 1 month; whereas if the content was 25%, the zeta-potential rose with time, as did the presence of PE in the liposome surface.

Electrophoretic properties of dodecyltrimethylammonium bromide micelles in KBr solution.

Charge in ionic micelles determines the trends of their stability and their practical applications. Charge can be calculated from zeta potential (ζ) measurements, which, in turn, can be obtained by Doppler microelectrophoresis. In this study, the electrophoretic properties of dodecyltrimethylammonium bromide (DTAB) in KBr aqueous solution (0—6 mM) were determined by Doppler microelectrophoresis. At very low surfactant concentrations (up to 6 mM), ζ potential was quite constant and due to the ionized monomers (DTA+). Above 6 mM, ζ potential increased to a maximum at surfactant concentrations still below the critical micellar concentration (CMC). This increase could be explained by a formation of nonmicellar aggregates of DTAB. Then, above the CMC, ζ potential underwent an abrupt reduction, which was dependent qualitatively and quantitatively on KBr concentration, and which could be due to an increase of the number of counterions adsorbed on the micelle surface. Calculation of effective micellar charge from ζ potential gave the surface charge density. Comparing this value with the theoretical, obtained from geometrical considerations, a fraction of 0.29 of charged micellar headgroups was obtained when DTAB was in aqueous solution, which is consistent with the value obtained by conductivity measurements.

Design and applications of a new fluorimetric assay of thioguanine in liposomes.

This paper describes the systematic design of a modification that overcomes the difficulties encountered with the original fluorimetric method for thioguanine. It allows the determination of thioguanine alone, as well as in a lipid medium (specifically in liposomes). It consists of an earlier lipid extraction based on liquid-solid extraction cartridges; then the oxidation of thioguanine in the lipid-free solution is performed by adding hydrogen peroxide in pH 4.7 acetate buffer at 50 degrees C for 30 min. A central composite design was used to finally optimize the method. The assay is precise (RSD values were 1.8 and 2.1% for intra-day and between-day precision, respectively). The detection limit was 0.05 microM and the limit of quantitation 0.08 microM.

Factors influencing the encapsulation of thioguanine in DRV liposomes

Thioguanine, a chemotherapeutic drug employed in the treatment of leukemia, is, like other amphiphilic drugs, very permeant and presents a high ability to escape from the liposomal membrane. For this reason, it is very important to know which parameters can enhance the relatively low encapsulation of thioguanine. In this way, the influence of four factors (pH, content of cholesterol, charge of lipids, and time of sonication) on the encapsulation of such drug in dehydration-rehydration liposomes was studied using a 24 factorial design. In this study, the maximal encapsulation efficiency obtained was 14.07 mmol/mol of lipid and time of sonication was the unique factor whose influence on the encapsulation was statistically significant (P < 0.05). The energy input concomitant to higher periods of sonication seems to facilitate the entrapment of the drug into the bilayer. Among the two-way interactions, time charge and pH-charge presented levels of significance less than 0.05. In liposomes with negative charge (10% phosphatidic acid), time of sonication barely influenced on the encapsulation, but in stearylamine-containing liposomes, higher times of sonication were necessary to achieve a better yield of encapsulation. More complex was the effect of the pH-charge interaction. At pH 4.7, liposomes with phosphatidic acid favoured the encapsulation, while at pH 7.4, liposomes with stearylamine encapsulated more drug. The different extent of encapsulation in the function of pH and charge only could be explained by the different charge born by the positive and negative liposomes depending on the pH.

Adsorption of cyanuric chloride-activated polyethylene glycol on liposomes

Abstract One method for obtaining sterically stabilized liposomes consists of mixing an excess of cyanuric chloride-activated polyethylene glycol 5000 (activated PEG) with liposomes bearing distearoylphosphatidylethanolamine. Quantitative analysis of activated PEG present in extruded liposomes suggested that it can be found grafted as well as coated on the liposomal surface. Penetration kinetics at constant area and surface activity have shown the ability of activated PEG to adsorb on liposomes. This suggests that the interaction between activated PEG and lipid is primarily hydrophobic, and the triazine group of activated PEG must be embedded in the lipid. Microelectrophoretic measurements are consistent with the process of adsorption observed: Incorporation of PEG reduces the corresponding zeta potentials, but the presence of PEG in the bulk increases them in absolute terms. An experimental value lying between such extremes indicates that PEG is adsorbed.

Prediction of stability of cefazolin sodium in perfusion fluids

Abstract The stability of cefazolin sodium in commonly used intravenous solutions was studied by accelerated degradation. The methods of analysis were thin layer chromatography and an hydroxylamine method for cephalosporins. The shelf life of all the samples studied allows its clinical utilisation.