Characterization of lipophilic gemcitabine prodrug - Liposomal membrane interaction by differential scanning calorimetry

Gemcitabine is an anticancer agent rapidly deaminated to the inactive metabolite 2',2'-difluorodeoxyuridine. Its stability as well as bioavailability can be increased by making prodrugs. A series of lipophilic prodrugs of gemcitabine were synthesized by linking the 4-amino group with valeroyl, lauroyl, and stearoyl linear acyl derivatives. We studied, by the differential scanning calorimetry technique, and compared the interaction of pure gemcitabine and its prodrugs with dimyristoylphosphatidylcholine and distearoylphosphatidylcholine vesicles with the aim of demonstrating if the gemcitabine prodrug is more able than the pure gemcitabine to interact with lipid vesicles employed both as model biomembranes and as carriers in the transport of antitumor drugs. These studies, carried out by static and kinetic calorimetric measurements, give evidence that the increase of the prodrug's lipophilic character improves the interaction with lipid bilayers, favoring the absorption through the lipid barriers and allowing the liposomes to work (when the prodrug is inserted inside the vesicles) as a lipophilic carrier which is able to deliver the drug near the cell surface. The use of different prodrugs modified in their lipophilic character, of different kinds of vesicles (multilamellar and unilamellar), and of different kinds of vesicles forming phospholipids permitted us to determine the better equilibrium between invesicle solubility and through-vesicle diffusion of the drug, important in the preformulative studies of antitumor carriers based on phospholipid formulations. Such studies suggest that the prodrug lipophilic tail should modulate the transport and the release of gemcitabine inside the cellular compartments, and the efficiency of the liposomal system is related to the length of the prodrug's acyl chain which has to match the phospholipid acyl chain allowing or retarding the migration through the lipid release device.