Macromolecular prodrugs. XX. Factors influencing model dextranase-mediated depolymerization of dextran derivatives in vitro.

Abstract

Endo-dextranase-mediated depolymerization of dextran and dextran derivatives under various experimental conditions in vitro was determined. By a simultaneous determination of Mn and MW of dextrans treated with the enzyme in aqueous buffer, an initial increase of the polydispersity of the polysaccharide sample was observed, indicating that dextranase cleaved the dextran molecules into chains which differed significantly in length. A pH optimum of 5 for the enzyme action was found. However, in the pH range 5-8, which prevails in the colon, the initial depolymerization rates differed by a factor of less than 2. Dextranase treatment of a dextran sample resulted in a constant increase of the concentration of terminal reducing glucose residues per time unit suggesting, that the initial depolymerization reaction followed zero-order kinetics. For degrees of substitution below 12 the efficacy of dextranase fragmentation of dextran conjugates decreased almost linearly with increasing DS. The chemical nature of the attached drug did not significantly affect the depolymerization rates. Maximally depolymerized dextran derivatives were obtained by the combined action of dextranase and various alpha-glucosidases. Treatment of such solutions with: a) model esterases b) 80% plasma and c) 20% liver homogenate did not give rise to an acceleration of the initial drug regeneration, as compared to identical experiments carried out in pure buffer solution (pH 7.4 and 37 degrees C).