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Microbial biofilms are organized communities of cells that are associated with a wide spectrum of resistant and chronic infections that lead to the treatment failure. Accordingly, there is an urgent demand to create novel effective therapeutic drugs that can inhibit biofilm formation with new mechanisms of action to surmount the current escalating resistance. In this study, in silico hybrid model was utilized to develop three novel short linear peptides (4, 5, and 6) with potential biofilm inhibiting activities (scores > 1.0). The peptides were composed of cationic and hydrophobic residues. They were synthesized using solid-phase strategy. Synthesized peptides were purified and characterized by reverse-phase high-performance liquid chromatography and matrix-assisted laser desorption/ionization spectroscopy, respectively. They were evaluated using in vitro assay as potential inhibitors of clinically relevant Gram-positive and Gram-negative biofilms. Peptide (4) with five positive charges at physiological pH (4 cationic moieties and W:R = 1:4) showed activity against biofilms of Gram-positive strains (Staphylococcus epidermidis and Listeria monocytogenes). On the other hand, peptide (5) with six positive charges (5 cationic moieties and W:R = 2:2) demonstrated activity against Gram-positive (S. epidermidis) and Gram-negative (Escherichia coli) biofilms. Interestingly, peptide (6), with seven positive charges (6 cationic moieties and W:R = 2:5) revealed higher and broader spectrum of activity against biofilms of Gram-positive (S. epidermidis, S. aureus, L. monocytogenes) and Gram-negative (E. coli).

The hydrophobicity of curcumin creates hurdle towards its use in the anticancer therapy. Herein, we synthesized a curcumin-doxorubicin conjugated cyclic peptide scaffold to improve the solubility of curcumin and create a conjugate containing two anticancer agents. A solid-phase Fmoc/tBu solid phase methodology was used to synthesize a cell-penetrating nuclear targeting peptide with free thiol and amine groups, which was coupled with the activated doxorubicin (Dox) and curcumin, affording Dox-peptide-curcumin conjugate (DPCC) (10). The antiproliferative activity of the conjugate was evaluated in human leukemia carcinoma cell (CCRF-CEM), human ovarian carcinoma cell (SKOV-3), and normal kidney cell line (LLCPK). Cyclic peptide-doxorubicin conjugate (7) and DPCC (10) did not inhibit the proliferation of normal kidney LLCPK cells after 72 h incubation, but were cytotoxic in CCRF-CEM (73% and 41%, respectively) and SKOV-3 (55% and 30%, respectively) cells while Dox was cytotoxic (60–79%) in all three cell lines under similar conditions, suggesting selectivity of these compounds towards cancer cells.

Myocardial toxicity and drug resistance caused by drug efflux are major limitations of doxorubicin (Dox)-based chemotherapy. Dox structure modification could be used to develop conjugates with an improved biological profile, such as antiproliferative activity and higher cellular retention. Thus, Dox thiol conjugates, Dox thiol (Dox-SH), thiol-reactive Dox-SS-pyridine (SS = disulfide), and a Dox-SS-cell-penetrating cyclic peptide, Dox-SS-[C(WR)(4)K], were synthesized. Dox was reacted with Trauťs reagent to generate Dox-SH. The thiol group was activated by the reaction with dithiodipyridine to afford the corresponding Dox-SS-Pyridine (Dox-SS-Pyr). A cyclic cell-penetrating peptide containing a cysteine residue [C(WR)(4)K] was prepared using Fmoc solid-phase strategy. Dox-SS-Py was reacted with the free sulfhydryl of cysteine in [C(WR)(4)K] to generate Dox-SS-[C(WR)(4)K] as a Dox-cyclic peptide conjugate. Cytotoxicity of the compounds was examined in human embryonic kidney (HEK-293), human ovarian cancer (SKOV-3), human fibrosarcoma (HT-1080), and human leukemia (CCRF-CEM) cells. Dox-SH and Dox-SS-pyridine were found to have significantly higher or comparable cytotoxicity when compared to Dox in HEK-293, HT-1080, and CCRF-CEM cells after 24 h and 72 incubation, presumably because of higher activity and retention of the compounds in these cells. Furthermore, Dox-SS-[C(WR)(4)K] showed significantly higher cytotoxic activity in HEK-293, HT-1080, and SKOV-3 cells when compared with Dox after 72 h incubation. Dox-SS-Pyr exhibited higher cellular uptake than Dox-SS-[C(WR)(4)K] in HT-1080 and HEK-293 cells as shown by flow cytometry. Fluorescence microscopy exhibited that Dox-SS-Pyr, Dox-SH, and Dox-SS-[C(WR)(4)K] localized in the nucleus as shown in four cell lines, HT-1080, SKOV-3, MDA-MB-468, and MCF-7. Of note, Dox-SS-[C(WR)(4)K] was significantly less toxic in mouse myoblast cells compared to Dox at the same concentration. Further mechanistic study demonstrated that the level of intracellular reactive oxygen species (ROS) in myoblast cells exposed to Dox-SS-[C(WR)(4)K] was reduced in comparison of Dox when co-treated with FeCl(2). These data indicate that Dox-SH, Dox-SS-Pyr, and Dox-SS-[C(WR)(4)K] have the potential to be further examined as Dox alternatives and anticancer agents.

CN2097 (R7Cs-sCYK[KTE(β-Ala)]V) is a rationally designed peptidomimetic that shows effectiveness in preclinical models for the treatment of neurological disorders, such as Angelman syndrome, traumatic brain injury (TBI) and stroke. Because of its therapeutic activity for the treatment of human CNS disorders, there was an urgent need to develop an efficient strategy for large-scale synthesis of CN2097. The synthesis of CN2097 was accomplished using Fmoc/tBu solid phase chemistry in multiple steps. Two different peptide fragments (activated polyarginine peptide Npys-sCR7 and CYK[KTE(β-Ala)]V) were synthesized, followed by solution phase coupling in water. Activation of the polyarginine (CR7) was achieved in situ during cleavage of protected peptide (C(Trt)R(Pbf)7) from the Rink amide resin using 5 equiv. of 2,2-dithopyridine in TFA:TIS:H2O (95:2.5:2.5, v/v/v) for 4 h. The disulfide coupling was efficient which provided a 60% yield.

Two bicyclic peptides composed of tryptophan and arginine residues were synthesized from monocyclic peptide building blocks and evaluated as cellular delivery agents. [W5G]-(triazole)-[KR5] and [W5E]-(β-Ala)-[KR5] containing triazole and β-alanine linkers improved the cellular delivery of fluorescein (F')-labeled phosphopeptide F'-GpYEEI (F'-PP) by 7.6- and 19.3-fold, respectively, in human ovarian adenocarcinoma cells. However, parent monocyclic peptide [R5 ] and monocyclic peptide [WR]4 only enhanced the cellular uptake of the phosphopeptide by only 1.3- and 3.7-fold, respectively. Confocal microscopy showed that the corresponding fluorescein-labeled bicyclic peptide F'-[KW4E]-(β-Ala)-[KR5] was localized in the cytosol and nucleus. Studying the cellular uptake of F'-[KW4E]-(β-Ala)-[KR5] in the presence of endocytosis inhibitors indicated that the clathrin- and caveolin-dependent endocytosis are the main pathways for cellular uptake. The bicyclic peptide was able to improve antiproliferative activity of doxorubicin by 20 %. These data suggest that this bicyclic peptide can be utilized as a new class of cell-penetrating peptides and cellular delivery tools.