No abstract

Multidrug-resistant pathogens have become a major public health concern. There is a great need for the development of novel antibiotics with alternative mechanisms of action for the treatment of life-threatening bacterial infections. Antimicrobial peptides, a major class of antibacterial agents, share amphiphilicity and cationic structural properties with cell-penetrating peptides (CPPs). Herein, several amphiphilic cyclic CPPs and their analogues were synthesized and exhibited potent antibacterial activities against multidrug-resistant pathogens. Among all the peptides, cyclic peptide [R4W4] (1) showed the most potent antibacterial activity against methicillin-resistant Staphylococcus aureus [MRSA, exhibiting a minimal inhibitory concentration (MIC) of 2.67 μg/mL]. Cyclic [R4W4] and the linear counterpart R4W4 exhibited MIC values of 42.8 and 21.7 μg/mL, respectively, against Pseudomonas aeruginosa. In eukaryotic cells, peptide 1 exhibited the expected cell penetrating properties and showed >84% cell viability at a concentration of 15 μM (20.5 μg/mL) in three different human cell lines. Twenty-four hour time-kill studies evaluating [R4W4] with 2 times the MIC in combination with tetracycline demonstrated bactericidal activity at 4 and 8 times the MIC of tetracycline against MRSA (MIC = 0.5 μg/mL) and 2–8 times the MIC against Escherichia coli (MIC = 2 μg/mL). This study suggests that when amphiphilic cyclic CPPs are used in combination with an antibiotic such as tetracycline, they provide significant benefit against multidrug-resistant pathogens when compared with the antibiotic alone.

Many of the reported arginine-rich cell-penetrating peptides (CPPs) for the enhanced delivery of drugs are linear peptides composed of more than seven arginine residues to retain the cell penetration properties. Herein, we synthesized a class of nine polyarginine peptides containing 5 and 6 arginines, namely, R5 and R6. We further explored the effect of acylation with long chain fatty acids (i.e., octanoic acid, dodecanoic acid, and hexadecanoic acid) and cyclization on the cell penetrating properties of the peptides. The fluorescence-labeled acylated cyclic peptide dodecanoyl-[R5] and linear peptide dodecanoyl-(R5) showed approximately 13.7- and 10.2-fold higher cellular uptake than that of control 5,6-carboxyfluorescein, respectively. The mechanism of the peptide internalization into cells was found to be energy-dependent endocytosis. Dodecanoyl-[R5] and dodecanoyl-[R6] enhanced the intracellular uptake of a fluorescence-labeled cell-impermeable negatively charged phosphopeptide (F′-GpYEEI) in human ovarian cancer cells (SK-OV-3) by 3.4-fold and 5.5-fold, respectively, as shown by flow cytometry. The cellular uptake of F′-GpYEEI in the presence of hexadecanoyl-[R5] was 9.3- and 6.0-fold higher than that in the presence of octanoyl-[R5] and dodecanoyl-[R5], respectively. Dodecanoyl-[R5] enhanced the cellular uptake of the phosphopeptide by 1.4–2.5-fold higher than the corresponding linear peptide dodecanoyl-(R5) and those of representative CPPs, such as hepta-arginine (CR7) and TAT peptide. These results showed that a combination of acylation by long chain fatty acids and cyclization on short arginine-containing peptides can improve their cell-penetrating property, possibly through efficient interaction of rigid positively charged R and hydrophobic dodecanoyl moiety with the corresponding residues in the cell membrane phospholipids.

Phosphopeptides are valuable reagent probes for studying protein-protein and protein-ligand interactions. The cellular delivery of phosphopeptides is challenging because of the presence of the negatively charged phosphate group. The cellular uptake of a number of fluorescent-labeled phosphopeptides, including F′-GpYLPQTV, F′-NEpYTARQ, F′-AEEEIYGEFEAKKKK, F′-PEpYLGLD, F′-pYVNVQN-NH2, and F′-GpYEEI (F′ = fluorescein) was evaluated in the presence or absence of a [WR]4, a cyclic peptide containing alternative arginine (R) and tryptophan (W) residues, in human leukemia cells (CCRF-CEM) after 2 h incubation using flow cytometry. [WR]4 improved significantly the cellular uptake of all phosphopeptides. PEpYLGLD is a sequence that mimics the pTyr1246 of ErbB2 that is responsible for binding to the Chk SH2 domain. The cellular uptake of F′-PEpYLGLD was enhanced dramatically by 27-fold in the presence of [WR]4 and was found to be time-dependent. Confocal microscopy of a mixture of F′-PEpYLGLD and [WR]4 in live cells exhibited intracellular localization and significantly higher cellular uptake compared to that of F′-PEpYLGLD alone. Transmission Electron Microscopy (TEM) and Isothermal Calorimetric (ITC) were used to study the interaction of PEpYLGLD and [WR]4. TEM results showed that the mixture of PEpYLGLD and [WR]4 formed noncircular nanosized structures with width and height of 125 and 60 nm, respectively. ITC binding studies confirmed the interaction between [WR]4 and PEpYLGLD. The binding isotherm curves, derived from sequential binding models, showed an exothermic interaction driven by entropy. These studies suggest that amphiphilic peptide [WR]4 can be used as a cellular delivery tool of cell-impermeable negatively charged phosphopeptides.

There are few synthetic graft alternatives to treat large long-bone defects resulting from trauma or disease that do not incorporate osteogenic or osteoinductive factors. The aim of this study was to test the additional benefit of including a permeable collagen membrane guide in conjunction with a preformed porous hydroxyapatite bone graft to serve as an improved osteoconductive scaffold for bone regeneration. A 10-mm-segmental long-bone defect model in the rabbit radius was used. The hydroxyapatite scaffolds alone or with a collagen wrap were compared as experimental treatment groups to an empty untreated defect as a negative control or a defect filled with autologous bone grafts as a positive control. All groups were evaluated after 4 and 8 weeks of in vivo implantation using microcomputed tomography, mechanical testing in flexure, and histomorphometry. It was observed that the use of the wrap resulted in an increased bone volume regenerated when compared to the scaffold-only group (59% greater at 4 weeks and 27% greater after 8 weeks). Additionally, the increase in density of the regenerated bone from 4 to 8 weeks in the wrap group was threefold than that in the scaffold group. The use of the collagen wrap showed significant benefits of increased interfacial bone in-growth (149% greater) and periosteal remodeling (49%) after 4 weeks compared to the scaffold-alone with the two groups being comparable after 8 weeks, by when the collagen membrane showed close-to-complete resorption. While the autograft and wrap groups showed significantly greater flexural strength than the defect group after 8 weeks, the scaffold-alone group was not significantly different from the other three groups. It is most likely that the wrap shows improvement of function by acting like a scaffold for periosteal callus ossification, maintaining the local bone-healing environment while reducing fibrous infiltration (15% less than scaffold only at 4 weeks). This study indicates that the use of a collagen membrane with a hydroxyapatite structural graft provides benefits for bone tissue regeneration in terms of early interfacial integration.