- The high toxicity of most chemotherapeutic drugs and their inactivation by multidrug resistance phenotypes motivated extensive search for drugs with new modes of action. We designed a short cationic diastereomeric peptide composed of d- and l-leucines, lysines, and arginines that has selective toxicity toward cancer cells and significantly inhibits lung metastasis formation in mice (86%) with no detectable side effects. Its ability to depolarize the transmembrane potential of cancer cells at the same rate (within minutes) and concentration (3 micro m), at which it shows biological activity, suggests a killing mechanism that involves plasma membrane perturbation. Confocal microscopy experiments verified that the cells died as a result of acute injury, swelling, and bursting, suggesting necrosis. Biosensor binding experiments and attenuated total reflectance-Fourier transform infrared spectroscopy using model membranes have substantiated its high selectivity toward cancer cells. Although this is an initial study that looked at tumor formation rather than the ability of the peptides to reduce established tumors, the simple sequence of the peptide, its high solubility, substantial resistance to degradation, and inactivation by serum components might make it a good candidate for future anticancer treatment.