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.