Treatment of murine L1210 lymphoid leukemia and melanoma B16 with lipophilic cytosine arabinoside prodrugs incorporated into unilamellar liposomes.

Lipophilic prodrugs of I-beta-D-arabinofuranosyl cytosine (Ara-C), namely N4- and 5'-oleyl-I-beta-D-arabinofuranosyl cytosine (N4-oleyl-ara-C, 5'-oleyl-ara-C) and N4-palmitoyl-I-beta-D-arabinofuranosyl cytosine (N4-palm-ara-C) were incorporated into liposomes of various lipid compositions. The phospholipid vesicles were prepared by controlled dialysis of lipid/prodrug/detergent micelles yielding homogeneous and stable unilamellar liposomes. The liposome size ranged from 70 to 120 nm depending on the lipid composition and the amounts of prodrug incorporated. Depending on the total amount of micellized Ara-C prodrugs, a maximal incorporation rate of 250 micrograms prodrug per mg egg phosphatidylcholine was achieved. The incorporation efficiency was in the range of 85 to 97%. The in vivo antitumor activity of the liposome preparations against L1210 lymphoid leukemia was clearly superior by factors of 2-8 depending on the therapy schedule and route of drug application. The treatment of melanoma B16 with free Ara-C as well as with the prodrug-liposomes exhibit rather weak antitumor activity. Drug application by means of prodrug-liposomes yields equal or higher tumor-inhibitory effects at drug concentrations which were 2-4 times lower than those of free Ara-C. Although drug tolerance and myelosuppression studies with Swiss albino mice revealed that the prodrug-liposomes were 5-10 times more toxic than free Ara-C, a substantial improvement of efficiency could be observed. The incorporation of the ara-C prodrugs into liposomes provides protection against fast degradation and systemic elimination which might be an explanation for the improved antitumor effect of these preparations.