Leakage kinetics of the liposomal chemotherapeutic agent Doxil: The role of dissolution, protonation, and passive transport, and implications for mechanism of action

Doxil, a liposomal formulation of the chemotherapeutic drug doxorubicin, is FDA-approved for multiple indications. Doxil liposomes are designed to retain doxorubicin in circulation, minimize clearance by the mononuclear phagocyte system, and limit uptake in healthy tissue. Although pharmacokinetic data and survival statistics from clinical trials provide insight into distribution and efficacy, many details of the mechanism of action remain unresolved, despite the importance in translating liposome-based drug delivery systems to other molecules and cargo. Therefore, the objective of this study is to quantitatively assess the kinetics of doxorubicin leakage from Doxil liposomes. In contrast to previous studies, we consider three processes: dissolution of solid doxorubicin, protonation/deprotonation of soluble doxorubicin, and passive transport of neutral doxorubicin across the lipid bilayer of the liposomes. Experiments were performed for Doxil, Doxil-like liposomes, and Doxil-like liposomes with reduced cholesterol and pegylation. To mimic physiological conditions, we also performed experiments in serum and under slightly acidic conditions at pH 5. We show that crystalline doxorubicin dissolution can be described by a first order rate constant of 1.0 x 10(-9) cm s(-1) at 37 degrees C. Doxorubicin leakage can be described by first order rate constant for transport across the lipid bilayer with values in the range from 1 to 3 x 10(-12) cm s(-1) at 37 degrees C. Based on these results we discuss implications for the mechanism of action, taking Doxil pharmacokinetics into account.