In our study derivatives of a short cationic peptide derived from the human host defense peptide Lactoferricin (LFcin) were optimized in their activity and selectivity towards cancer cells. The negatively charged membrane lipid phosphatidylserine (PS) serves as a target for these peptides since PS is specifically exposed by cancer cells (1).Calorimetric and permeability studies showed that N-acylation and even more the repeat sequences of derivatives of hLFcin leads to strongly improved interaction with the cancer mimic PS, whereas the healthy mimic phosphatidylcholine (PC) is only slightly affected by the lipopeptide. Tryptophan fluorescence of selective peptides revealed peptide penetration only into the PS membrane interface and circular dichroism showed change of structure by increase of amount of β-sheets just in the presence of the cancer mimic. We demonstrated that hLFcin derivatives also exhibit anticancer activity in vitro against several cancer cell lines, correlating with selective activity against the cancer model PS. Active and selective peptides induced apoptosis only in cancer cells, whereas melanocytes and fibroblasts remained unaffected at same concentrations, yielding specificity for cancer cells higher than 100-fold for some peptides. Currently, first in vivo studies are under progress.The data indicate the need of high affinity to the target PS, a minimum length and positive net charge, an adequate but moderate hydrophobicity, and capability of adoption of a defined structure exclusively in presence of the target membrane for high antitumor activity.Acknowledgment: Austrian Science Foundation FWF (grant no. P20760, P24608)(1) Riedl et al. BBA 1808: 2638-2645, 2011.