Revealing Dynamics of Accumulation of Systemically Injected Liposomes in the Skin by Intravital Microscopy

Accumulation
of intravenously injected cytotoxic liposomes in the
skin induces serious toxicity. We used single time point and longitudinal
intravital microscopy to understand skin accumulation dynamics of
non-PEGylated and PEGylated liposomes after systemic injection into
mice. Non-PEGylated egg phosphatidylcholine (PC) liposomes showed
short circulation half-life (1.3 h) and immediate aggregation in the
blood, with some aggregates lodging in skin microvasculature soon
after the injection. At 24 h, and more prominently at 48 h postinjection,
liposomes appeared in dermal and subdermal cells. PEGylated egg PC
liposomes showed long circulation half-life (22 h) and no aggregation
in the blood. PEGylated liposomes started to accumulate in the skin
microvasculature as soon as 5 min after the injection. Within 3 h
postinjection, PEGylated liposomes accumulated in extravascular cells
in the dermis and subdermis. Liposomes were present in the skin for
at least 7 days postinjection. A regulatory approved PEGylated liposomal
doxorubicin (LipoDox) and empty liposomes of the same composition
as LipoDox showed similar skin distribution as PEGylated egg PC liposomes,
suggesting that this phenomenon is relevant to liposomes of different
lipid composition. Decorating liposomes with shorter PEGs (350 or
700) in addition to PEG 2000 did not decrease the deposition. Outside
the capillaries, liposomes partially colocalized with CD45-, F4/80+
cells. The accumulation of liposomes was not due to prior neutrophil/platelet
binding and transport across endothelium. Moreover, our studies have
excluded a role of complement in the skin accumulation of liposomes.
Further understanding of mechanisms of this important phenomenon can
improve the safety of liposomal nanocarriers.