Caracterização reológica e coloidal de xantana biossintetizada a partir de glicose e uso como sistema de liberação controlada de doxiciclina

Abstract

Xanthans are branched polysaccharides of high molecular mass (> 106 Da), consisting of glucose, mannose and glucuronic acid. They are usually obtained by the fermentation of Xanthomonas campestris, which has been considered safe and approved for use in the food and pharmaceutical industries, as thickeners. The present work aimed to characterize xanthan gums biosynthesized from glucose from structural, colloidal and rheological point of view, aiming to evaluate its state of aggregation in aqueous solution. They were produced in duplicate using a medium containing 1.5% glucose, 0.5% K2HPO4, 0.2% NH4Cl, 0.1% NaCl, 0.01% MgSO4 and 0.1% yeast extract inoculated with Xanthomonas campestris pv. manihotis and incubated at 30 oC (72 h), in order to obtain the XantG1 and XantG2 samples. Since biopolymers have been widely used in the pharmaceutical industry, especially in the manufacture of controlled drug delivery systems, this work also aimed to investigate the complex formed through the complexation of xanthan and the cationic antibiotic doxycycline. The pure xanthans had their chemical structures characterized in solid state by FTIR and their thermal profile evaluated by TGA/DTA thermal analysis. Moreover, measurements of electrical conductivity, hydrodynamic diameter (by DLS), Zeta potential and stationary and oscillatory dynamic rheologies were used to investigate their aggregations state at different concentrations. Further, the complexes xanthan/doxycycline (Xant/Dox) were characterized in solid state by infrared spectroscopy and thermal analysis (TGA and DTA), while the complexation process was monitored by zeta potential, DLS, viscosimetric and isothermal calorimetry (ITC) titrations. Stationary rheology studies at 25 oC where used to know the flow profile of the so-produced suspensions after titration. Antimicrobial action of the complexes was evaluated through biological studies. The infrared spectra and the thermal analyzes were very similar for the two pure xanthan samples, showing the reproducibility of the synthesis and purification processes, besides being very similar to those found for other xanthanes in the literature. The electrical conductivity data as a function of time showed chemical stability of the compounds for at least 23 days in aqueous solution. The hydrodynamic diameter, zeta potential, electrical conductivity and rheology measurements showed that xanthanes behave as polyelectrolytes that undergo different levels of aggregation in solution depending on the concentration. The solution of xanthan at 2 g / L showed strong pseudoplasticity due to the existence of molecular entanglements, which justifies its use as a thickener. As for the doxycycline/xanthan complex, its formation was monitored by calorimetric, rheological and zeta potential titrations. The data of isothermal calorimetry showed that xanthan presents two distinct sites of interaction with doxycycline, being one of the exothermic and the other endothermic complexation stages. The rheological titration experiments showed strong reduction of viscosity during titration, suggesting that the doxycycline/xanthan interaction causes a collapse in the structure of the polymers, breaking the entanglements. Finally, the antimicrobial activity of the complexes and precursors was evaluated against Staphylococcus aureus 32388602 by lethal dose and death curve assays. The results showed that the formation of the complexes led to an increase in the antimicrobial activity, through the reduction of the lethal dose and the time of inhibition. This demonstrates that the preparation of xanthan complexes with doxycycline may be a promising alternative for the development of a novel formulation for controlled release of that drug.