Novel Small Multilamellar Liposomes Containing Large Quantities of Peptide Nucleic Acid Selectively Kill Breast Cancer Cells

Peptide nucleic acid (PNA) may be used in various biomedical applications; however, these are currently limited, due to its low solubility in aqueous solutions. In this study, a methodology to overcome this limitation is demonstrated, as well as the effect of PNA on cell viability. We show that extruding a mixture of natural phospholipids and short (6–22 bases), cytosine-rich PNA through a 100 nm pore size membrane under mild acidic conditions resulted in the formation of small (60–90 nm in diameter) multilamellar vesicles (SMVs) comprising several (3–5) concentric lipid membranes. The PNA molecules, being positively charged under acidic conditions (due to protonation of cytosine bases in the sequence), bind electrostatically to negatively charged phospholipid membranes. The large membrane surface area allowed the encapsulation of thousands of PNA molecules in the vesicle. SMVs were conjugated with the designed ankyrin repeat protein (DARPin_9-29), which interacts with human epidermal growth factor receptor 2 (HER2), overexpressed in human breast cancer. The conjugate was shown to enter HER2-overexpressing cells by receptor-mediated endocytosis. PNA molecules, released from lysosomes, aggregate in the cytoplasm into micron-sized particles, which interfere with normal cell functioning, causing cell death. The ability of DARPin-functionalized SMVs to specifically deliver large quantities of PNA to cancer cells opens a new promising avenue for cancer therapy.