Properties of HPMA copolymer–doxorubicin conjugates with pH-controlled activation: Effect of polymer chain modification

Various conjugates of anticancer drug doxorubicin (DOX) covalently attached via hydrolytically degradable hydrazone bond to water-soluble N-(2-hydroxypropyl)methacrylamide (HPMA) copolymer drug carriers were synthesized. Three types of precursors containing either positively or negatively charged groups or a hydrophobic substituent were employed. In vitro incubation of the conjugates in buffers showed relative stability at pH 7.4 (modelling blood) and a fast DOX release at pH 5 (modelling intracellular environment). The presence of carboxylic groups in the copolymer structure resulted in an increase in the DOX release rate of 15–20% while no effect of the introduction of positively charged groups was observed if compared with the unmodified conjugate. Self-assembling of the oleoyl groups-containing conjugate led into formation of polymeric micelles with high apparent molecular weight (Mw = 170 000) in aqueous solution and resulted in a decrease in the DOX release rate of ∼ 20%. The cytostatic activity of the conjugates tested on several cancer cell lines was comparable with that of free DOX·HCl, depending on the sensitivity of a particular cell line to DOX. All the conjugates showed a much higher antitumour activity in vivo than the free drug tested in mice bearing EL4 T-cell lymphoma and treated using the therapeutic regime of drug administration. The highest activity (100% long-term survivors) exhibited polymer–DOX conjugate containing negatively charged GFLG sequences.