Novel betulin-containing polyelectrolyte conjugates

Conjugates of anionic polyelectrolytes (PE) with betulin (PE-B) were synthesized by cross-linking the carboxyl groups of temperature responsive poly(N-isopropylacrylamide-co-acrylic acid) (CP1) and poly(N-vinylpyrrolidone-co-acrylic acid) (CP2) copolymers, to the hydroxyl groups of betulin using thionyl chloride as the coupling agent. Water-insoluble CP1-B and poorly soluble CP2-B covalent conjugates were formed in a single step by mixing solutions of the selected components. Water-soluble bovine serum albumin (BSA)-betulin covalent conjugates (BSA*B) were synthesized by carbodiimide coupling. Betulin-containing polyelectrolyte conjugates (PEC-B) were prepared by complexing BSA*B conjugates with synthetic polycations - poly(N-alkyl-4-vinylpyridinium bromide). A single immunization of mice with CP1-B and CP2-B, covalent conjugated with protein containing PEC-B and BSA*B systems without traditional adjuvants increased the specific immune response to betulin. CP2-B, as a soft and wet biocompatible delivery system, in contrast to the CPl-B conjugates, displayed an essentially higher betulin-specific immunogenic activity. The immunogenicity of BSA*B conjugates decreased rapidly with time while the PE-BSA*B complexes generated betulin-specific antibodies for 160 days.