Maleimide is a stable and easy-to-handle moiety that rapidly and covalently conjugates thiol groups of cysteine residues in proteins or peptides. Herein, we use maleimide to modify the surface of liposomes in order to obtain an advanced drug delivery system. Employing a small amount (0.3 mol%) of maleimide-polyethylene glycol (PEG) to modify the surface of the liposomes M-GGLG-liposomes, composed of 1,5-dihexadecyl N,N-diglutamyl-lysyl-L-glutamate (GGLG)/cholesterol/poly(ethylene glycol) 1,2-distearoyl-sn-glycero-3-phosphoethanolamine (PEG5000-DSPE)/maleimide-PEG5000-Glu2C18 at a molar ratio of 5:5:0.03:0.03, drug delivery efficiency was remarkably improved both in vitro and in vivo compared to unmodified liposomes (GGLG-liposomes, composed of GGLG/cholesterol/PEG5000-DSPE/PEG5000-Glu2C18 at a molar ratio of 5:5:0.03:0.03). Moreover, this modification did not elicit any detectable increase in cytotoxicity. The maleimide-modification did not alter the physical characteristics of the liposomes such as size, zeta potential, pH sensitivity, dispersibility and drug encapsulation efficiency. However, M-GGLG-liposomes were more rapidly (≥2-fold) internalized into HeLa, HCC1954, and MDA-MB-468 cells compared to GGLG-liposomes. In vivo, M-GGLG-liposomes encapsulating doxorubicin (M-GGLG-DOX-liposomes) also showed a more potent antitumor effect than GGLG-DOX-liposomes and the widely used 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC)-DOX-liposomes after two subcutaneous injections around breast cancer tissue in mice. The biodistribution of liposomes in this model was observed using an in vivo imaging system, which showed that M-GGLG-liposomes were present for significantly longer at the injection site compared to GGLG-liposomes. The outstanding biological functions of the maleimide-modified liposomes as a novel drug delivery system make them ideally suited to a wide range of applications.
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