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The biological activity of the selected peptides from the combinatorial library in bacteria, fungi and mammalian cells is studied to characterize the selected peptides and to compare the activity of antimicrobial peptides (AMPs) in vitro and in vivo. The analysis has shown that the methods focusing on screening peptide libraries in vitro for membranes with the appropriate interfacial activity has enabled the design, selection and discovery of novel, potent and broad-spectrum membrane-active antibiotics.

A nonclassical approach is used for biomolecular engineering and design of membrane-active peptides. The composition-space peptide libraries coupled with function-based high-throughput screens has led to the discovery of diverse, soluble and highly potent membrane-permeabilizing peptides.

Beta sheet pore forming peptides are characterized and the structure function relationships in lipid vesicles are compared to their activity in biological membranes. Fluorescence spectroscopy and quenching showed that the peptides reside mainly on the bilayer surface and are partially exposed to water, rather than in a membrane-spanning state.

Cycloviolacin O2 (CyO2), a cyclotide from Viola odorata (Violaceae) has antitumor effects and causes cell death by membrane permeabilization. In the breast cancer line, MCF-7 and its drug resistant subline MCF-7/ADR, the cytotoxic effects of CyO2 (0.2-10 microM) were monitored in the presence and absence of doxorubicin (0.1-5 microM) using cell proliferation assays to establish its chemosensitizing abilities. SYTOX Green assays were Sperformed to verify membrane permeabilization and showed cellular disruption correlates with cyclotide chemosensitization. Fluorescence microscopy studies demonstrated increased cellular internalization of doxorubicin in drug resistant cells when coexposed to CyO2. Interestingly, CyO2 did not produce significant membrane disruption in primary human brain endothelial cells, which suggested cyclotide specificity toward induced pore formation in highly proliferating tumor cells. Furthermore, three novel cyclotides (psyle A, C and E) from Psychotria leptothyrsa (Rubiaceae) were also monitored for cytotoxic activity. The cyclotides displayed potent cytotoxicity (IC50 = 0.64->10 microM), and coexposure to cyclotides significantly enhanced doxorubicin induced toxicity (IC50 = 0.39-0.76 microM). This study documents several cyclotides with robust cytotoxicity that may be promising chemosensitizing agents against drug resistant breast cancer.