Antitumoral activity of PEG–gemcitabine prodrugs targeted by folic acid

Gemcitabine, 2′,2′-difluoro-2′-deoxycytidine (dFdC), is an antitumor agent effective in the treatment of several solid tumors but its use is hampered by short plasma half-life, rapid metabolism and low selectivity towards tumor tissue. To overcome these limits, bioconjugates of gemcitabine were studied using poly(ethylene glycol) as polymeric carrier. Two types of conjugates were prepared, non-targeted and folic acid targeted conjugates. The formers were obtained starting from mPEG–OH of 5 and 20 kDa with linear or branched structure. The folic acid targeted conjugates, differing for the drug loading, were prepared exploiting a heterobifunctional PEG that allowed a consecutive coupling of the targeting agent and the drug. Folic acid was chosen as targeting agent because its receptor is often over-expressed in many tumors. To increase the polymer drug payload, the bicarboxylic amino acid, aminoadipic acid, was used. All conjugates were able to release the drug in a pH-dependent manner with no role of enzymes. The pharmacokinetic profiles are strictly related to the polymer molecular weight and the folic acid targeting increased 2–3 times the affinity towards the cells over-expressing folic acid receptors. These results are promising and encourage in vivo studies on these conjugates that act as polymeric prodrugs.