Synthesis and biological evaluation of Bicyclic Peptides containing Arginine and Tryptophan residues

Two bicyclic peptides, [W(WR)4K]-GFLG-[W(WR)8E] and [W(WR)4K]-[W(WR)4E], composed of alternate tryptophan (W) and arginine (R) residues in each monocyclic peptide building block connected through a lysine linker or a short cathepsin B tetrapeptide ligand, were synthesized. The minimum inhibitory concentration (MIC) values for a number of peptides were evaluated against different Gram-positive and Gram-negative bacterial strains. [W(WR)4K]-GFLG showed antibacterial activity against Gram-positive methicillin-resistant S. aureus (ATCC BAA-1556) and S. aureus (ATCC 29213) bacteria with MIC values of 16 µg/mL. The cytotoxicity of monocyclic peptide [W(WR)4K]-GFLG was evaluated against human liver (HepG2), kidney (HEK293), and lung (A549) cells after 24 h incubation, showing a CC50 > 300 µM. Furthermore, bicyclic peptides [W(WR)4K]-GFLG-[W(WR)8E] and [W(WR)4K]-[W(WR)4E] did not show any significant cytotoxicity at a concentration of 25 µM against human leukemia adenocarcinoma (CCRF-CEM) cells after 3 h incubation. These peptides were found to be effective molecular transporters for the delivery of a negatively charged phosphopeptide, enhancing the cellular delivery of a fluorescein (F´)-labeled phosphopeptide, F´-GpYEEI (F´-PP) (5 µM), by 6.5- and 4.5-fold, respectively, in CCRF-CEM cells after 3 h incubation when used at a concentration of 25 µM. These data suggest that these peptides can be used as cellular delivery tools.