Biodegradable ion-exchange microspheres based on modified polylysines

Abstract

Poly- l-lysine was synthesized via a triethylamine initiated ring-opening polymerization of Z- l-lysine- N α-carboxyanhydride, followed by deprotection of the ϵ-amino group. Subsequently the polylysine was sulfamated using a pyridinium-sulfate complex to obtain polymers with varying degrees of sulfamation ranging from 0 to 100%. Cytotoxicity of these materials was tested using tetrazolium metabolism (MTT) assays with B16F10 and P388 cell lines. Cytotoxicity of sulfamated polylysines with a degree of sulfamation of 80% and higher was significantly reduced as compared with the native polylysines. In both cell lines, LD 50 of the sulfamated materials was higher than 5 mg/ml, which was the highest dose tested. LD 50 of the native polylysines was lower than 0.1 mg/ml in the case of B16F10 and lower than 0.01 mg/ml in the case of P388 cells. Sulfamated polylysines with a degree of sulfamation of 80% were used to prepare microspheres (SPLMS). The microspheres were stabilized using glutaral-dehyde or oxidized dextran as a crosslinking agent. The swelling ratio (defined as V swollen/V dried) of the SPLMS in aqueous media decreased with increasing ionic strength and crosslink density. The pH (ranging from 3 to 11 ) had no influence on the swelling ratio of SPLMS. The maximal swelling ratio was approximately 35 (SPLMS crosslinked with 0.5% glutaraldehyde in distilled water). SPLMS could be loaded with adriamycin up to a payload of 60%, which was not influenced by the crosslinking method. The adriamycin release was controlled by the ionic strength of the release medium: no drug was released in non-ionic medium such as distilled water, while 80% of the drug was released in phosphate buffered saline. This effect of the change in ionic strength could be applied to prepare a microsphere suspension in non-ionic medium such as 5% glucose solution, which does not contain free adriamycin. The drug would only be release after intra-arterial administration of this suspension, due to the presence of the blood.