Abstract
The successful trajectory of liposome-encapsulated doxorubicin (e.g., Doxil, which has been approved by the U.S. Food and Drug Administration) as an anticancer nanodrug in clinical applications is contradicted by in vitro cell viability data that highlight its reduced efficacy in promoting cell death compared with non-encapsulated doxorubicin. No reports to date have provided a mechanistic explanation for this apparently discordant evidence. Taking advantage of doxorubicin intrinsic fluorescence and time-resolved optical microscopy, we analyze the uptake and intracellular processing of liposome-encapsulated doxorubicin (L-DOX) in several in vitro cellular models. Cell entry of L-DOX was found to lead to a rapid (seconds to minutes), energy- and temperature-independent release of crystallized doxorubicin nanorods into the cell cytoplasm, which then disassemble into a pool of fibril-shaped derivatives capable of crossing the cellular membrane while simultaneously releasing active drug monomers. Thus, a steady state is rapidly established in which the continuous supply of crystal nanorods from incoming liposomes is counteracted by a concentration-guided efflux in the extracellular medium of fibril-shaped derivatives and active drug monomers. These results demonstrate that liposome-mediated delivery is constitutively less efficient than isolated drug in establishing favorable conditions for drug retention in the cell. In addition to explaining previous contradictory evidence, present results impose careful rethinking of the synthetic identity of encapsulated anticancer drugs.