Pegylated liposomal mitoxantrone is more therapeutically active than mitoxantrone in L1210 ascitic tumor and exhibits dose-dependent activity saturation effect

Our previous studies have proved that encapsulation of mitoxantrone into pegylated SUVs (plm60-s) could enhance its antineoplastic efficacy (Li et al., 2008b). However, why plm60-s is more therapeutically active than free mitoxantrone (MIT), and whether pharmacokinetics and activity of plm60-s exhibits dose-dependency are left unknown. In studies with L1210 ascitic tumor-bearing mice in which the dose of MIT was elevated from 2 to 8 mg/kg, a saturation of antineoplastic efficacy was observed after plm60-s, and not after free MIT therapy. Total MIT concentrations in plasma, liver and ascitic fluids after plm60-s increased linearly with escalated doses. The released MIT concentrations in ascitic fluid increased continuously before reaching the peak at a dose of 6 mg/kg and then decreased. In vitro release experiments using ascitic fluid as release medium revealed that at high concentrations of plm60-s the release of drug was inhibited. At a dose of 4 mg/kg, the areas under the curve (AUC) of released MIT in ascitic fluid after plm60-s were higher than those after free MIT, which might be responsible for the enhanced efficacy of plm60-s. These observations may be used to choose a dose regimen of plm60-s to ensure optimal efficacy and to expound the reasons why plm60-s was more therapeutically active.

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