A pH-dependent charge reversal peptide for cancer targeting

Naturally occurring cationic antimicrobial peptides exhibit not only antimicrobial activity, but also anticancer activity and are expected to be new weapons in cancer treatment. The selectivity for cancer cells over normal cells is at least partly due to the more negative surface of cancer cells. A lower pH in tumor tissue (pH 6.2-6.9) than that in normal tissues (pH 7.3-7.4) has also been utilized to develop anticancer agents. However, cytotoxicity against normal cells at physiological pH is often an issue. Furthermore, acidic regions can be found in some normal tissues such as the kidneys. Therefore, existing approaches to cancer targeting are not fully satisfactory. In this study, we designed a peptide, HE (GIHHWLHSAHEFGEHFVHHIMNS-amide), with a charge that reverses from -1.5 at pH 7.4 to +6 at pH 5.5 for cancer targeting at low pH based on the antimicrobial peptide magainin 2 by introducing 6 His, an additional Glu, and an amidated terminal. HE interacted with cancer-mimicking negatively charged liposomes in a pH-dependent fashion with a midpoint with a pH of 6.5 just above the membrane surface. The peptide killed human renal adenocarcinoma ACHN cells at pH 6.0, but not at pH 7.4, and was nontoxic against human normal glomerular mesangial cells even at this low pH. Thus, the novel peptide may be a promising lead peptide for cancer therapy, although this derivatization resulted in weakened cytotoxicity.