Solid-Phase Synthesis and Antibacterial Activity of Cyclohexapeptide Wollamide B Analogs

Abstract

Herein we report the antibacterial structure-activity relationships of cyclic hexapeptide wollamide analogs derived from solid-phase library synthesis. Wollamide B, a cyclic hexapeptide natural product, has been previously found to have activity against Mycobacterium bovis. To further evaluate its antimycobacterial/antibacterial potential, 27 peptides including wollamides A/B, and desotamide B, were synthesized and subsequently tested against a panel of clinically significant bacterial pathogens. Biological evaluation revealed that the cyclic scaffold, amide functionality in position I, tryptophan residue in position V, and the original stereochemistry pattern of the core scaffold were key for antituberculosis and/or antibacterial activity. In addition, against M. tuberculosis and Gram-positive bacteria, residues in position II and/or VI greatly impacted antibacterial activity and selectivity. Wollamides A (3) and B (2) along with their corresponding II (l-Leu) analog 10 retained the most promising antituberculosis activity, with the lowest minimum inhibitory concentration (MIC) against virulent M. tuberculosis H37Rv (MIC = 1.56 μg/mL), as well as desirable selectivity indices (>100). Importantly, the antimicrobial activities of wollamides A and B do not result from disruption of the bacterial membrane, warranting further investigation into their mechanism of action.

Structure-Activity Relationships of Wollamide Cyclic Hexapeptides with Activity against Drug-Resistant and Intracellular Mycobacterium tuberculosis

“…2) Together with the SAR data presented here, this observation supports the proposition that wollamides act via a specific and well-defined molecular target, where activity or binding can be negated by even minor changes to the wollamide structure (i.e., changes to a single amino acid residue or a single stereocenter). Preferential activity of wollamides against mycobacteria compared to that of Gram-positive and Gram-negative bacteria (8,10,12) further supports the hypothesis that a specific wollamide molecular target exists. Within the infected host, M. tuberculosis bacteria vary greatly with respect to replication, metabolic status, and susceptibility to antibiotics (14)(15)(16).…”

“…Wollamide-producing Streptomyces species are exceptionally rare and to the best of our knowledge cannot be sourced from existing repositories (i.e., the ATCC). Our initial identification of the synthetically accessible antimycobacterial cyclic hexapeptide wollamides (8) have led to reports on the chemical synthesis of wollamides (9,11,12) and SAR investigations against M. tuberculosis H37Rv (11,12). The present study assessed 36 new synthetic wollamides and compared their structure-activity relationships to those of 46 known cyclic peptide wollamide derivatives.…”

“…Preliminary structure-activity relationship (SAR) studies, comparing the naturally produced wollamides and desotamides, highlighted the importance of amino acid residue VI (i.e., D-Orn over Gly) and suggested that compound 2 held promise. The solid-phase synthesis of compound 2 was subsequently described (9,10), as were SAR studies for some synthetic wollamides, assessing in vitro growth-inhibitory activity against M. tuberculosis H37Rv (11,12). Here, we report the synthesis and antimycobacterial activity of 36 new wollamide analogues and compare their antimycobacterial activity with that of 46 known wollamides.…”

The Mechanisms of Action of Cationic Antimicrobial Peptides Refined by Novel Concepts from Biophysical Investigations

“…Whereas molecules whose interactions are with proteinaceous receptors can be made inefficient by one or a few changes in amino acid sequence, polypeptides that act by disrupting the lipid bilayer physicochemical properties are less likely to become inactivated by resistance (Rollins-Smith et al 2002). Indeed the amphipathic nature of AMPs has been found essential and can be achieved by helical (Sansom 1991;Bechinger 1997), cyclic (Cao et al 2018Laurencin et al 2018;Tsutsumi et al 2018;Zhao et al 2018), and/or β-sheet arrangements (Hong and Su 2011; Rautenbach et al 2016a, b;Sychev et al 2017;Usachev et al 2017). Thus the insights gained from the studies of cationic amphipathic antimicrobial peptides have stimulated the design of a number of small amphipathic molecules (Arnusch et al 2012;Ghosh et al 2014), pseudopeptides (Porter et al 2002;Patch and Barron 2003;Kuroda and DeGrado 2005;Violette et al 2006;Makovitzki et al 2008;Scott et al 2008;Rotem and Mor 2009;Palermo and Kuroda 2010;Laurencin et al 2018), and polymers (Rank et al 2017) with potent antimicrobial properties.…”

Biophysical Investigations Elucidating the Mechanisms of Action of Antimicrobial Peptides and Their Synergism

“…In contrast, molecules that specifically target proteinaceous receptors can be made inefficient by mutagenesis of one or a few sites, and it is much less likely that bacteria develop resistance to compounds whose primary target is the destruction of the physico-chemical properties of the lipid membrane [ 15 ]. Membrane-active peptides exhibit a wide range of structural features some being helical in their bilayer-associated state [ 16 , 17 ], others forming cyclic [ 18 , 19 , 20 , 21 ] and/or β-sheet arrangements [ 22 , 23 , 24 , 25 , 26 ]. Indeed, following the insights gained from the studies of cationic amphipathic antimicrobial peptides a number of small amphipathic molecules [ 27 , 28 ], pseudopeptides [ 21 , 29 , 30 , 31 , 32 , 33 , 34 , 35 , 36 ], and polymers [ 37 ] have been designed and investigated, and found to also exhibit potent antimicrobial activities.…”