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Title: A Trojan-Horse Strategy for Antimicrobial Therapy
Other Titles: Een Trojaans paard-strategie voor antimicrobiële therapie
Authors: Gadakh, Bharat
Issue Date: 3-Jul-2014
Abstract: Over the last seven decades, antibiotics have been essential components of modern medicines and this is one of the leading causes for our increased life expectancy. However, due to development of resistance to the existing antibiotics, there is an urgent need to look for new analogues or new bacterial targets which are essential for the survival of pathogens. Recently, aminoacyl-tRNA synthetases (aaRSs) have emerged as promising and clinically validated targets. Aminoacyl-tRNA synthetases are a family of enzymes which play a key role in the translation process and are responsible for charging tRNA with the correct amino acid. Although these enzymes are conserved during evolution, yet some structural divergence has occurred allowing selective inhibition of the bacterial aaRSs over their human orthologs. Because of their crucial role in protein synthesis and the possibility of selective inhibition, these enzymes have been considered as a prime antibiotic target. Numerous aaRS inhibitors have been reported in the literature, either from natural or synthetic origin. However, none of them (except mupirocin) has been developed yet into a clinically useful antibiotic. Last year, a new drug application (NDA) for Tavaborole (a benzoxaborole derivative) from Anacor Pharmaceuticals has been accepted by the US FDA for the treatment of onychomycosis. More recently, inhibitors based on the reaction intermediate (aa-AMP) have been designed and evaluated for antibacterial activity. Among them, aminoacyl-sulfamoyl adenosine (aaSA) analogues proved to be the strongest inhibitors of the corresponding aaRSs in vitro. However, these analogues could not progress further due to their lack of selectivity and poor cell penetration. In 1998, Cubist Pharmaceuticals reported a series of aryl-tetrazole containing sulfamates as selective aaRS inhibitors. Despite of excellent activity and high selectivity, these analogues could not be pursued further due to their poor cell penetration and high serum albumin binding.Elaborating on these findings, we attempted to improve the in vivo efficacy of the aryl-tetrazole containing sulfamates by coupling them with a siderophore (trihydroxamate or biscatecholate). Although no antibacterial activity was observed in whole-cell assay screening, trihydroxamate-based siderophore-drug conjugates (SDCs) did show nice in vitro activity in cell extracts (except DABN extracts which lack peptidase activity). Therefore, we concluded that the SDCs are efficiently metabolized by broad-specificity peptidases to release the active moiety. Thus, failure of uptake is the main reason for the inactivity of these trihydroxamate-based SDCs. Most probably, the iron-transport system may be selective for adenylates or very closely resembling derivatives. In addition, the biscatecholate-based SDC was not metabolized by broad-specificity peptidases as the SDC was protected at its N-terminal. Thus failure to release the active moiety is the likely reason for the absence of whole-cell activity of this latter SDC.Visual inspection of the compounds reported by Cubist Pharmaceuticals, and of albomycin and mupirocin reveals that these compounds vary from aaSA analogues in having either a heterocyclic base or a modified pyrimidine base or no base at all, the latter as found in mupirocin. However, these analogues displayed excellent activity and good selectivity against their respective aaRSs. These observations prompted us to investigate the pharmacophoric importance of the adenine base in aaSA analogues. In chapter 3, different natural and unnatural base containing isoleucyl-sulfamoyl nucleosides were designed, synthesized and evaluated for their antibacterial activity. To our surprise, the order of the in vitro inhibitory activity found was uracil (U)> adenine (A) = cytosine (C)> 4-aminobenzimidazole (4-ABI)> 4-nitrobenzimidazole (4-NBI)> guanine (G). Moreover, hexapeptidyl conjugates of these analogues were prepared in an attempt to promote the uptake of these analogues. Unfortunately, only transient antibacterial activity was noticed for purine derivatives (A, I, 4-ABI), while no activity was observed for the in vitro strongly active uracil derivative. This observation again supports our hypothesis that the YejABEF transporter is selective for peptidyl-adenylate analogues and closely resembling derivatives.Part of our synthetic efforts focused on improvement of the in vivo efficacy of aaSA analogues by combining them with a peptide carrier (either a siderophore or a McC hexapeptide). During these studies, we consistently observed the formation of a cycloadenosine derivative as a side product. In an effort to reduce this side reaction, we proposed the synthesis of aminoacyl-sulfonamides (aaSoAs) as a potentially more stable alternative for aaSA analogues. Towards this end, we designed and synthesized several aaSoAs. We further compared the activity of these aaSoAs along with the intermediate sulfamate (SA) and sulfonamide (SoA) cores with their corresponding aaSA analogues. It was shown however that these analogues are not able to inhibit the corresponding aaRSs. Only the AspSoA analogue to our surprise showed some selective antibacterial activity against E. coli wt. Although, sulfamate (SA) is not active against the tested aaRSs, it did display a broad-spectrum of activity in whole-cell assays. However, the mode of action of this sulfamate core structure remains to be determined.
Publication status: published
KU Leuven publication type: TH
Appears in Collections:Medicinal Chemistry (Rega Institute)

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