Pharmacologic Studies of a Prodrug of Mitomycin C in Pegylated Liposomes (Promitil(®)): High Stability in Plasma and Rapid Thiolytic Prodrug Activation in Tissues.

PURPOSE: Pegylated liposomal (PL) mitomycin C lipid-based prodrug (MLP) has recently entered clinical testing. We studied here the preclinical pharmacology of PL-MLP.
METHODS: The stability, pharmacokinetics, biodistribution, and other pharmacologic parameters of PL-MLP were examined. Thiolytic cleavage of MLP and release of active mitomycin C (MMC) were studied using dithiothreitol (DTT), and by incubation with tissue homogenates.
RESULTS: MLP was incorporated in the bilayer at 10% molar ratio with nearly 100% entrapment efficiency, resulting in a formulation with high plasma stability. In vitro, DTT induced cleavage of MLP with predictable kinetics, generating MMC and enhancing pharmacological activity. A long circulation half-life of MLP (10-15 h) was observed in rodents and minipigs. Free MMC is either extremely low or undetectable in plasma. However, urine from PL-MLP injected rats revealed delayed but significant excretion of MMC indicating in vivo activation of MLP. Studies in mice injected with H3-cholesterol radiolabeled PL-MLP demonstrated relatively greater tissue levels of H3-cholesterol than MLP. MLP levels were highest in tumor and spleen, and very low or undetectable in liver and lung. Rapid cleavage of MLP in various tissues, particularly in liver, was shown in ex-vivo experiments of PL-MLP with tissue homogenates. PL-MLP was less toxic in vivo than equivalent doses of MMC. Therapeutic studies in C26 mouse tumor models demonstrated dose-dependent improved efficacy of PL-MLP over MMC.
CONCLUSIONS: Thiolytic activation of PL-MLP occurs in tissues but not in plasma. Liposomal delivery of MLP confers a favorable pharmacological profile and greater therapeutic index than MMC.