Preclinical Properties of N⁴-Hexadecyl- and N⁴- Octadecyl-1-β-D-Arabinofuranosylcytosine in Liposomal Preparations

Abstract

The long chain N⁴-alkyl compounds N⁴-hexadecyl- and N⁴-octadecyl-1-β-D-arabinofuranosylcytosine (NHAC, NOAC) are new lipophilic derivatives of 1-β-D-arabinofuranosylcytosine (ara-C). They have the strongest antitumor activities in the L1210 leukemia model among the series of N⁴-alkyl substituents ranging from C6 to C22. Long circulating liposome formulations of NHAC and NOAC containing poly(ethylene)glycol modified lipids (PEG-liposomes) have a better antitumor activity compared to formulations with plain liposomes. However, the examination of the interactions of both liposomal formulations containing NHAC with blood components revealed that the drug is transferred from the liposomes to blood components at fast rates, regardless of the liposome composition. NHAC is mainly bound to albumin, to the high and low density lipoproteins HDL and LDL and to erythrocytes. Thus, the liposomes serve mainly as pharmaceutical formulation to permit the solubilisation of the lipophilic N⁴-alkyl compounds and their application in aqueous formulations. Compared to ara-C, NHAC is highly protected against deamination to arabinofuranosyluracil (ara-U) in human plasma and liver microsomes, resulting in a >40 and 6-fold reduction of ara-U formation, respectively. The cytotoxicity of NHAC in leukemia cells is independent of both the nucleoside transporter mechanism and the deoxycytidine (dCyd) kinase activity as demonstrated by coincubations of NHAC with dipyridamole and/or dCyd in U-937 leukemia cells. The total cellular drug uptake of NHAC is significantly higher than ara-C uptake and not affected by blocking with dipyridamole. In the U-937 cells, the intracellular half-life of ara-CTP formed from NHAC is 3 times longer than that from ara-C but ara-CTP formation and DNA incorporation from NHAC are significantly lower compared to ara-C and may therefore contribute only marginally to the cytotoxic effects of NHAC. These results indicate that the N⁴-alkyl-ara-C derivatives might exert their antitumor activity by other, yet unknown mechanisms and thus be able to overcome ara-C resistance and have cytotoxic activity in solid tumors.