Encapsulation of mitoxantrone into pegylated SUVs enhances its antineoplastic efficacy

Abstract

Mitoxantrone (MIT) was encapsulated into 60, 80 and 100 nm pegylated hydrogenated soy phosphatidylcholine/cholesterol (HSPC/chol) vesicles using a transmembrane (NH 4) 2SO 4 gradient. In-vitro release studies revealed that small-sized formulation had fast drug-release rate. Acute toxicity studies performed in c57 mice proved that all pegylated liposomal MIT (plm) formulations could be well-tolerated at a dose of 9 mg/kg, significantly compared to severe toxicity induced by free mitoxantrone (f-M). In KM mice, plm60 was at least 2- to 3-fold less toxic than f-M. After intravenous injection, plm60 was slowly eliminated from plasma relative to f-M, resulting in about 6459-fold increase in AUC and its plasma kinetics exhibited dose dependence. In S-180 bearing KM mice, plm60 preferentially accumulated into tumor zone, with a ∼12-fold increase in AUC and ∼10-fold increase in C max Furthermore, the accumulation of plm60 in almost all normal tissues markedly decreased. The antitumor efficacy of plm60 was also considerably enhanced. In L1210 ascitic tumor model, plm60 was the most efficacious which led to a ∼70% long-term survival, significantly compared to 16–33% survival rate in plm80, plm100 and f-M groups at the same dose level (4 mg/kg). The antitumor efficacy of plm60 was more encouraging in L1210 liver metastasis model. At a dose of 6 mg/kg, ∼90% animals receiving plm60 treatment could survive 60 days; however, in f-M group at the same dose, all the mice died at ∼14 days post inoculation. Similarly, plm60 could effectively inhibit the growth of RM-1 tumor in BDF1 mice, resulting in marked increase in tumor doubling time at different dose levels relative to f-M. The improved antineoplastic effects could be ascribed to its small vesicle size, which allowed more drug release after the accumulation into tumor zone. Theoretical considerations revealed that the reduction of vesicle size could increase the specific area of MIT/sulfate precipitate inside the vesicle and the release constant K, which is inversely proportional to vesicle volume ( K = pA m k 2 k 2 ′ / ( [ H + ] i 2 V i ) ) .