Suppression of Phagocytic Activity Leads to the Efficient Surface Modification of Macrophages with Liposomes for Developing a Biomimetic Drug Delivery System

Macrophages selectively infiltrate the lesion sites of several diseases, including cancers, and, thus, have attracted attention as a biomimetic drug delivery carrier. To achieve the efficient drug loading of macrophages with minimal cytotoxicity, drugs are preferably encapsulated into nanoparticles, such as liposomes, and modified on the surface of macrophages rather than being incorporated into cells. However, liposomes are rapidly taken up by macrophages after binding to the cell surface because of their strong phagocytic activity. To overcome this, we herein attempted to modify the surface of macrophages with liposomes by suppressing their phagocytic activity using a pretreatment with anionic liposomes. We confirmed that 1,2-distearoyl-sn-glycero-3-phospho-(1′-rac-glycerol) (DSPG)- and cholesterol-rich anionic liposomes were efficiently taken up by RAW264.7 murine macrophage-like cells. Furthermore, the cellular uptake of anionic liposomes by RAW264.7 cells was higher in the absence of fetal bovine serum (FBS) than in its presence. Moreover, the viability of RAW264.7 cells was maintained above 90% when cells were incubated with anionic liposomes for 3 h, whereas viability was markedly decreased after a 24-h incubation. Based on these results, we pretreated RAW264.7 cells by an incubation with DSPG- and cholesterol-rich liposomes for 3h in the absence of FBS. This pretreatment significantly inhibited the internalization of other liposomes, which subsequently bound to the cell surface. Therefore, we succeeded in modifying the surface of macrophages with liposomes, and liposome-modified macrophages have potential as a biomimetic active drug delivery carrier.