Folate-mediated targeting of polymeric conjugates of gemcitabine.

The synthesis of two new macromolecular prodrugs for active tumor targeting was set up. Gemcitabine (2'-deoxy-2',2'-difluorocytidine) was conjugated to alpha,beta-poly(N-2-hydroxyethyl)-DL-aspartamide (PHEA) through succinyl or diglycolyl hydrolysable spacers. The targeting agent folic acid was attached to the macromolecular backbone through the aminocaproic spacer. The two conjugates [PHEA-(5'-succinylgemcitabine)-1'-carboxypentyl-folamide and PHEA-(5'-diglycolyl-gemcitabine)-1'-carboxypentyl-folamide], were purified and extensively characterised by spectroscopic (UV, IR and NMR) and chromatographic analyses to determine the correct chemical structure, the purity degree and the reaction yield. In vitro studies demonstrated that the drug release depends on the spacer arm (diglycolyil or succinyl) and incubation pH. After 30 h incubation at pH 7.4, mimicking the plasma and extracellular compartments, the gemcitabine release from the succinyl and diglycolyl derivatives was 28 and 31%, respectively. After 30 h incubation at pH 5.5, mimicking the lisosomial compartment, the drug released from both bioconjugates was lower than 13%. In plasma, the polymer conjugation increased the drug stability and provided for a sustained drug release. In vitro citotoxicity studies performed using human nasopharyngeal epidermal carcinoma KB cells demonstrated that PHEA-(5'-succinylgemcitabine)-1'-carboxypentyl-folamide displays an higher dose dependent cytotoxic effect with respect to PHEA-(5'-diglycolyl-gemcitabine)-1'-carboxypentyl-folamide.