Molecularly Engineered Surfactin Analogues Induce Nonapoptotic-Like Cell Death and Increased Selectivity in Multiple Breast Cancer Cell Types

[Image: see text] Surfactin, a negatively charged amphiphilic lipopeptide biosurfactant, is synthesized by the bacterium Bacillus subtilis. It consists of a cyclic heptapeptide and an 11–15C β-hydroxy fatty acid. To probe how the modification of the molecular skeleton of surfactin influences its selectivity and activity against breast cancer, six synthetic surfactins were generated. Modifications were accomplished by conjugating amine-functionalized molecules to the Glu and Asp carboxyl moieties of the heptapeptide. The resulting synthetic surfactins provided a diverse series of molecules with differences in charge, size, and hydrophilicity. After purification and structural analysis, insights into biological activity and specificity were generated for each compound. Dose-dependent growth inhibition was determined for four tumorigenic breast cancer cell lines in monolayer and spheroid morphologies, as well as nontumorigenic fibroblasts and sheep erythrocytes, which were utilized to determine selectivity indices. Results indicated that two compounds, which have amplified anionic charge, had increased activity on breast cancer, with reduced activity on nontumorigenic fibroblasts and erythrocytes. Cationic derivative surf-ethylenediamine has increased activity on all cell lines tested. Novel correlations between dose–response activities and physicochemical properties of all compounds determined that there is a significant correlation between the critical micelle concentration and activity against multiple cell lines.