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

Abstract

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 pH5. We show that crystalline doxorubicin dissolution can be described by a first order rate constant of 1.0×10cms at 37°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 3×10cms at 37°C. Based on these results we discuss implications for the mechanism of action, taking Doxil pharmacokinetics into account.

Predicting drug delivery efficiency into tumor tissues through molecular simulation of transport in complex vascular networks

“…Temporal release of drugs from liposomes roughly follows an exponential decay of drug released over time, as measured in vitro [54], [58], [59] and described via mathematical models [60]. The half-life of doxorubicin release from doxil, measured via dialysis, is 118.4±18.8 h [61].…”

“…To investigate how the kinetics of doxorubicin leakage from doxil liposomes affects tumor accumulation we ran simulations with different leakage rates. In vitro experiments by Russell et al found that doxorubicin leakage from doxil liposomes can be described by a first-order rate constants proportional to the liposome surface area to volume ratio [54]. Using their mean observed rate of 2.2 × 10 −12 cm s −1 for commercial 100 nm diameter doxil liposomes with an estimated surface area to volume ratio of 16.7 nm, the reference leakage rate was determined to be ~4.8 x 10 -3 h -1 .…”

“…This shows that the slow leakage rate of doxil results in higher tumor accumulation by protecting the doxorubicin payload from being cleared. A surprising result was that intermediate rates (4.8 h -1 ), which are 1000 times higher than commercial doxil [54], can offer a marginally increased tumor delivery, but also result in higher doxorubicin loads in peripheral tissues, which may outweigh the benefit of improved tumor delivery ( Figure 8C).…”

“…The half-life of doxorubicin release from doxil, measured via dialysis, is 118.4±18.8 h [61]. Another study measuring liposomal doxorubicin release kinetics shows a maximal release of 20-30% of the doxorubicin over the course of 2 weeks, indicating liposomal release is much slower than the clearance rate of doxil [54], [62], suggesting that the majority of the doxorubicin will remain inside the doxil liposomes until it is delivered into the tissue. Figure 8 shows, that as suggested by El Kareh et al and Charrois et al [61], [63], the doxorubicin leakage rate from doxil directly affects the amount of drug delivered to the tumor.…”

Securing the Payload, Finding the Cell, and Avoiding the Endosome: Peptide‐Targeted, Fusogenic Porous Silicon Nanoparticles for Delivery of siRNA

“…Fusogenic liposomes directly fuse with the cell membrane, bypassing endocytotic uptake pathways . While they have shown promise as delivery vehicles for ribonucleic acid interference (RNAi) therapeutics, liposomes suffer from a generally low carrying capacity for nucleic acid therapeutics and leakage of their payloads either during storage or in vivo . We recently demonstrated a delivery system that harnessed together a fusogenic lipid and a solid porous silicon nanoparticle (pSiNP) core.…”