Amphiphilic Cell-Penetrating Peptides Containing Natural and Unnatural Amino Acids as Drug Delivery Tools and Antimicrobial Agents

Cell-penetrating peptides containing arginine as positively charged residues and tryptophan or diphenylalanine as hydrophobic residues were synthesized. The synthesis was accomplished through the Fmoc solid-phase peptide synthesis in the presence of HBTU and DIPEA. The side-chain protected linear peptides were cleaved from the resin and cyclized in the presence of DIC and HOAt in the solution phase overnight. MALDI-TOF mass spectrometry was used to characterize the peptides. The cytotoxicity of the synthesized peptides was determined in CCRF-CEM (human, lymphoblast peripheral blood), and HEK-293 (human, embryonic epithelial kidney healthy) cells using the MTS assay. A concentration of 10 µM was found to have minimal cytotoxic effects. CCRF-CEM and SK-OV-3 (human, epithelial ovary adenocarcinoma) cells were employed to measure the cellular uptake of fluorescence-labeled phosphopeptide (F´-GpYEEI), stavudine, lamivudine, emtricitabine, and siRNA in the presence of peptides with flow cytometry. The cellular uptake studies showed that [DipR]5 significantly improved the uptake of F´-GpYEEI, by approximately 110-folds after 3 h incubation. Confocal microscopy demonstrated the improved delivery of F´-GpYEEI (6-folds), which was mostly localized in the cytosol, in the presence of [DipR]5 for MDA-MB-231 cells. To determine the mechanism of the cellular uptake, the physical mixture of F´-GpYEEI and [DipR]5 was used to measure the fluorescence in the treated CCRF-CEM cells and in a concentration- and time-dependent manner. A study was conducted in the presence of endocytosis inhibitors such as nystatin, chlorpromazine, chloroquine, and methyl β-cyclodextrin suggesting that the uptake for [DipR]5 may not be endocytosis-dependent. Circular dichroism, dynamic light scattering, and transmission electron microscopy were utilized to determine the secondary structure, particle sizes, and morphology of peptides, respectively. The data indicates that [DipR]5 and Fluorescence-labeled [DipR]5 can act as a molecular transporter of several compounds including the cell-impermeable phosphopeptide and siRNA specifically. The antibacterial activities of peptides were evaluated against pathogens. The minimum inhibitory concentrations (MIC) of peptides were determined by micro-broth dilution protocol against Methicillin-Resistant Staphylococcus aureus, Klebsiella pneumoniae, Pseudomonas aeruginosa, Escherichia coli, Staphylococcus aureus, Enterococcus faecium, Enterococcus faecalis, Streptococcus pneumoniae, and Bacillus subtilis. [DipR]5 and((DipR)2(WR)3) showed promising MIC values of 0.39-6.25 µM against Gram-positive bacteria strains and 3.13-25 µM against Gram-negative bacteria strains