Annexin A5-functionalized liposomes for targeting phosphatidylserine-exposing membranes

Long-circulating liposomes functionalized with cell-targeting elements and loaded with bioactive compounds present high interest as drug delivery nanosystems. We present here the synthesis and physicochemical characterization of liposomes containing PEGylated lipids covalently linked to oriented Annexin-A5 (Anx5) proteins, and we show that Anx5-functionalized liposomes are able to target phosphatidylserine (PS)-exposing membranes. The covalent coupling of Anx5 to liposomes is almost quantitative, which is mainly due to the high accessibility of the reacting groups. The influence of Anx5 functionalization on liposome aggregation was investigated by dynamic light scattering, showing that Anx5-functionalized liposomes are stable below a threshold density of 250 Anx5 molecules per liposome. Anx5-functionalized liposomes bind PS-containing membranes with very high efficacy, which is mainly due to the controlled orientation of the Anx5 at the liposome surface. A striking result, obtained by quartz crystal microbalance with dissipation monitoring, is that one single Anx5 molecule is able to anchor a liposome to a PS-containing supported membrane. Finally, we show by fluorescence microscopy that Anx5-functionalized liposomes bind PS-exposing apoptotic K562 cells with high specificity. This study demonstrates that Anx5-functionalized liposomes bind specifically to PS membranes and are thus potential candidates to deliver drug or imaging agents to sites of apoptosis or thrombosis.