Homologated aza analogs of arabinose as antimycobacterial agents

Synthetic Arabinomannan Heptasaccharide Glycolipids Inhibit Biofilm Growth and Augment Isoniazid Effects in Mycobacterium smegmatis

Biofilm formation, involving attachment to an adherent surface, is a critical survival strategy of mycobacterial colonies in hostile environmental conditions. Here we report the synthesis of heptasaccharide glycolipids based on mannopyranoside units anchored on to a branched arabinofuranoside core. Two types of glycolipids-2,3-branched and 2,5-branched-were synthesized and evaluated for their efficacies in inhibiting biofilm growth by the non-pathogenic mycobacterium variant Mycobacterium smegmatis. Biofilm formation was inhibited at a minimum biofilm growth inhibition concentration (MBIC) of 100 μg mL^-1 in the case of the 2,5-branched heptasaccharide glycolipid. Further, we were able to ascertain that a combination of the drug isoniazid with the branched heptasaccharide glycolipid (50 μg mL^-1 ) potentiates the drug, making it three times more effective, with an improved MBIC of 30 μg mL^-1 . These studies establish that synthetic glycolipids not only act as inhibitors of biofilm growth, but also provide a synergistic effect when combined with significantly lowered concentrations of isoniazid to disrupt the biofilm structures of the mycobacteria.

Synthesis and anti-tuberculosis activity of glycitylamines

A series of glycitylamines, which were prepared in few steps from readily available carbohydrates, were tested for their ability to inhibit tuberculosis growth in an Alamar Blue BCG colourimetric assay. Several derivatives, including (2R,3R)-1-(hexadecylamine)pent-4-ene-2,3-diol, (2R,3R)-1-(hexadecylmethylamino)pent-4-ene-2,3-diol and 5-deoxy-5-hexadecylmethylamino-D-arabinitol, were prepared in good to excellent (44-90%) overall yield and exhibited micromolar (20-41μM) inhibitory activity that was similar to the first line tuberculosis (TB) drug ethambutol (39μM) in the same assay. The ease and low cost of the synthesis of the glycitylamines offers definite advantages for their use as potential TB drugs.

Synthesis and evaluation of 1,2-trans alkyl galactofuranoside mimetics as mycobacteriostatic agents

The strong interaction of an octyl chain with M. smegmatis cells was paired with high specificity of the galactofuranose ring against mycobacteria growth.

Chemo-enzymatic synthesis and glycosidase inhibitory properties of DAB and LAB derivatives

A chemo-enzymatic strategy for the preparation of 2-aminomethyl derivatives of (2R,3R,4R)-2-(hydroxymethyl)pyrrolidine-3,4-diol (also called 1,4-dideoxy-1,4-imino-D-arabinitol, DAB) and its enantiomer LAB is presented. The synthesis is based on the enzymatic preparation of DAB and LAB followed by the chemical modification of their hydroxymethyl functionality to afford diverse 2-aminomethyl derivatives. This strategy leads to novel aromatic, aminoalcohol and 2-oxopiperazine DAB and LAB derivatives. The compounds were preliminarily explored as inhibitors of a panel of commercial glycosidases, rat intestinal disaccharidases and against Mycobacterium tuberculosis, the causative agent of tuberculosis. It was found that the inhibitory profile of the new products differed considerably from the parent DAB and LAB. Furthermore, some of them were active inhibiting the growth of M. tuberculosis.

Regioselective glycosylation method using partially protected arabino- and galactofuranosyl thioglycosides as key glycosylating substrates and its application to one-pot synthesis of oligofuranoses

We describe in this paper the development of a novel regioselective furanosylation methodology using partially protected furanosyl thioglycosides as central glycosylating building blocks and its application in the efficient one-pot synthesis of a series of linear and branched-type arabino- and galactofuranoside fragments structurally related to the cell wall polysaccharides of Mycobacterium tuberculosis , Streptococcus pneumoniae serostype 35A, and sugar beet.

Preparation and antitubercular activities of alkylated amino alcohols and their glycosylated derivatives

A series of N- and C-alkylated amino alcohols and of their protected galactopyranosyl derivatives was synthesized and evaluated for antitubercular activity. Five of these compounds displayed good activity, with a MIC below 12.5mug/mL. The presence of the carbohydrate slightly affected the antibacterial activity.

Antituberculosis drugs: ten years of research

Tuberculosis is today amongst the worldwide health threats. As resistant strains of Mycobacterium tuberculosis have slowly emerged, treatment failure is too often a fact, especially in countries lacking the necessary health care organisation to provide the long and costly treatment adapted to patients. Because of lack of treatment or lack of adapted treatment, at least two million people will die of tuberculosis this year. Due to this concern, this infectious disease was the focus of renewed scientific interest in the last decade. Regimens were optimized and much was learnt on the mechanisms of action of the antituberculosis drugs used. Moreover, the quest for original drugs overcoming some of the problems of current regimens also became the focus of research programmes and many new series of M. tuberculosis growth inhibitors were reported. This review presents the drugs currently used in antituberculosis treatments and the most advanced compounds undergoing clinical trials. We then provide a description of their mechanism of action along with other series of inhibitors known to act on related biochemical targets. This is followed by other inhibitors of M. tuberculosis growth, including recently reported compounds devoid of a reported mechanism of action.

Synthesis of putative chain terminators of mycobacterial arabinan biosynthesis

The synthesis of a variety of arabinose derivatives that have been modified at C-5 was achieved from d-arabinose. The 5-fluoro and 5-methoxy compounds were converted into the corresponding farnesyl phosphodiesters as putative chain terminators of mycobacterial arabinan biosynthesis. Biological testing of these materials revealed no effective anti-mycobacterial activity.

Synthesis and antitubercular activities of bis-glycosylated diamino alcohols

Conjugate addition of diamines to glycosyl olefinic esters 1a and 1b followed by reduction of resulting bis-glycosyl beta-amino esters (2-7 and 14-19) with lithium aluminium hydride led to the respective glycosyl amino alcohols (8-13 and 20-25) in moderate to good yields. All the compounds were evaluated for antitubercular activity against Mycobacterium tuberculosis H(37)Ra and H(37)Rv. Few of the compounds exhibited antitubercular activity with MIC as low as 6.25-3.12microg/mL in virulent and avirulent strains. Compound 13 was found to be active against MDR strain and showed mild protection in mice.

Synthesis and antimycobacterial activities of glycosylated amino alcohols and amines

Reduction of glycosyl beta-amino esters (6-14 and 25-30) with lithium aluminum hydride resulted in glycosyl amino alcohols (15-23 and 31-36) in good yields. However, reductive amination of glycosyl aldehydes (1-3) with different amines in presence of sodium borohydride resulted in good to moderate yields of glycosyl amines (37-41). All the compounds were evaluated for antitubercular activity against Mycobacterium tuberculosis H37Ra and H37Rv. Compounds 18, 21, 35 and 36 exhibited antitubercular activities with MIC ranging from 6.25 to 3.12 microg ml(-1).

Fighting tuberculosis: An old disease with new challenges

Tuberculosis (TB) caused by Mycobacterium tuberculosis remains a leading cause of mortality worldwide into 21st century. The mortality and spread of this disease has further been aggravated because of synergy of this disease with HIV. A number of anti-TB drugs are ineffective against this disease because of development of resistance strains. Internationally efforts are being made to develop new anti-tubercular agents. A number of drug targets from cell wall biosynthesis, nucleic acid biosynthesis, and many other biosynthetic pathways are being unraveled throughout the world and are being utilized for drug development. In this review, socioeconomic problems in developing countries, efforts to control this disease in different individuals, the targets (known already and newly discovered), existing anti-tubercular agents including natural products and lead molecules, and the future prospects to develop new anti-TB agents are described.

Sulfone and phosphinic acid analogs of decaprenolphosphoarabinose as potential anti-tuberculosis agents

Mycobacteria biosynthesize a cell wall structure that is rich in polysaccharides containing arabinofuranose residues. The source of these arabinofuranose residues is decaprenolphosphoarabinose (1), the donor substrate for mycobacterial arabinosyltransferases. We have previously demonstrated that an analog of 1, C-phosphonate 7, prevented the growth of mycobacteria and this compound is currently undergoing testing for efficacy in tuberculosis-infected mice. We describe here the synthesis and testing of additional analogs of 1 that contain either a sulfone (8-14) or phosphinic acid (15-19) moiety in place of the phosphodiester functionality. Screening of these compounds in vitro against Mycobacterium tuberculosis strain H(37)Rv revealed that while some of these compounds possessed low to modest activity, none was as potent as 7.

Synthesis of galactopyranosyl amino alcohols as a new class of antitubercular and antifungal agents

The galactopyranosyl amino alcohols (3-16) were synthesised by regioselective oxirane ring opening of compound 2 with variety of amines and screened for antitubercular and antifungal activities. One of the compounds (16) showed potent activity against Mycobacterium tuberculosis H37 Rv in vitro and also displayed activity in MDR TB. The compound (16) was found to be superior to ethambutol clinically used anti TB drug in in vitro screen.

Synthesis of octyl arabinofuranosides as substrates for mycobacterial arabinosyltransferases

A panel of octyl oligosaccharides comprised of arabinofuranose rings have been synthesized via efficient and readily scaleable routes. The key glycosylation reactions involved the coupling of octyl glycoside acceptors with the appropriate thioglycosides using N-iodosuccinimide and silver triflate activation. These syntheses were undertaken to provide substrates suitable for use in assays of mycobacterial arabinosyltransferases.

Synthesis of an arabinofuranosyl disaccharide photoaffinity probe for arabinosyltransferase activity in Mycobacterium tuberculosis

(5-Azidonaphthalene-1-sulfonamidoethyl)-5-O-(alpha-arabinofuranosyl)-alpha-D-arabinofuranoside 1 was synthesized as a photoaffinity probe for the determination of arabinosyl transferase activity and for the identification of binding and functional sites in Mycobacterium tuberculosis.

Synthesis of glycosylated beta-amino acids as new class of antitubercular agents

A series of glycosylated beta-amino acids was prepared and evaluated against Mycobacterium tuberculosis, M. avium, M. fortuitum and M. smegmatis. The compounds were designed to mimic the enzyme D-alanine racemase and glycosyl transferase involved in the biosynthesis of essential cell wall peptidoglycan and arabinogalactan. Though most of the compounds exhibited little activity, however, one showed significant activity against all the strains in cell culture and activity was confirmed by BACTEC method.

Oligofuranosides containing conformationally restricted residues: synthesis and conformational analysis

The synthesis of a panel of arabinofuranosyl oligosaccharide analogues (5-13) in which one ring is locked into either the E(3) or OE conformation is described. The E(3)-locked scaffolds 15 and 16 required for the synthesis of 5-10 were prepared in one step from known 1,5-anhydroalditols. A number of routes were explored for the preparation of the OE-locked monosaccharide derivative 17 needed for the preparation of 11-13. The successful synthesis of 17 was achieved in 17 steps from D-arabinose. Subsequent analysis of 5-13 by 1H NMR spectroscopy demonstrated that the locked residue does not exert any detectable influence upon the conformers populated by adjacent conformationally unrestricted furanose rings.

Studies on n-octyl-5-(alpha-D-arabinofuranosyl)-beta-D-galactofuranosides for mycobacterial glycosyltransferase activity

The mycobacterial cell wall is a potential target for new drug development. Herein we report the preparation and activity of several n-octyl-5-(alpha-D-arabinofuranosyl)-beta-D-galactofuranoside derivatives. A cell-free assay system has been utilized for determination of the ability of disaccharide analogues to act as arabinosyltransferase acceptors using [14C]-DPA as the glycosyl donor. In addition, in vitro inhibitory activity has been determined in a colorimetric broth microdilution assay system against MTB H37Ra and three clinical isolates of Mycobacterium avium complex (MAC). One of these disaccharides showed moderate activity against MTB. The biological evaluation of these disaccharides suggests that more hydrophobic analogues with a blocked reducing end showed better activity as compared to a totally deprotected disaccharide that more closely resembles the natural substrates in cell wall biosynthesis.

Arabinofuranosides from mycobacteria: synthesis of a highly branched hexasaccharide and related fragments containing beta-arabinofuranosyl residues

The synthesis of 11 oligosaccharides (4-14) containing beta-arabinofuranosyl residues is reported. The glycans are all fragments of two polysaccharides, arabinogalactan and lipoarabinomannan, which are found in the cell wall complex of mycobacteria. In the preparation of the targets, the key step was a low-temperature glycosylation reaction that installed the beta-arabinofuranosyl residues with good to excellent stereocontrol.

Studies on (beta,1-->5) and (beta,1-->6) linked octyl Gal(f) disaccharides as substrates for mycobacterial galactosyltransferase activity

The emergence of multi-drug resistant (MDR) strains of Mycobacterium tuberculosis (MTB) and the continuing pandemic of tuberculosis emphasizes the urgent need for the development of new anti-tubercular agents with novel drug targets. The recent structural elucidation of the mycobacterial cell wall highlights a large variety of structurally unique components that may be a basis for new drug development. This publication describes the synthesis, characterization, and screening of several octyl Galf(beta,1-->5)Galf and octyl Galf(beta,1-->6)Galf derivatives. A cell-free assay system has been utilized for galactosyltransferase activity using UDP[14C]Galf as the glycosyl donor, and in vitro inhibitory activity has been determined in a colorimetric broth microdilution assay system against MTB H37Ra and three clinical isolates of Mycobacterium avium complex (MAC). Certain derivatives showed moderate activities against MTB and MAC. The biological evaluation of these disaccharides suggests that more hydrophobic analogues with a blocked reducing end showed better activity as compared to totally deprotected disaccharides that more closely resemble the natural substrates in cell wall biosynthesis.

Ethambutol analogues as potential antimycobacterial agents

A range of new ethambutol analogues was synthesised and their inhibitory potencies were probed with Mycobacterium smegmatis. Interestingly, apparently even minor deviation from the structure of the parent compound resulted in reduced antimycobacterial activity.

Studies on beta-D-Gal(f)-(1-->4)-alpha-L-Rha(p) octyl analogues as substrates for mycobacterial galactosyl transferase activity

The biochemically unique structures of sugar residues in the outer cell wall of Mycobacterium tuberculosis (MTB) make the pathways for their biosynthesis and utilization attractive targets for the development of new and selective anti-tubercular agents. A cell-free assay system for galactosyltransferase activity using UDP[14C]Gal as the glycosyl donor, as well as an in vitro colorimetric broth micro-dilution assay system, were used to determine the activities of three beta-D-gal(f)(1-->4)-alpha-L-rham(p) octyl disaccharides as substrates and antimycobacterial agents respectively. The cell-free enzymatic studies using compounds 8 and 10 suggested that these disaccharides bind to and are effective substrates for a putative mycobacterial galactosyltransferase. The modified acceptor 8 was found to be a slower but prolonged binder as compared to the less substituted analogue 10 as evidenced by their Km and Vmax values. Moderate antimycobacterial activity was observed with compounds 8 and 9 against MTB H37Ra and three clinical isolates of Mycobacterium avium complex (MAC).

Ethambutol-sugar hybrids as potential inhibitors of mycobacterial cell-wall biosynthesis

Ethambutol is an established front-line agent for the treatment of tuberculosis, and is also active against Mycobacterium avium infection. However, this agent exhibits toxicity, and is considered to have low potency. The action of ethambutol on the mycobacterial cell wall, particularly the arabinan, and comparison of the structure of ethambutol with several of the cell-wall saccharides, suggested that ethambutol-saccharide hybrids might lead to agents with a more selective mechanism of action. To this end, eight ethambutol-saccharide hybrids were synthesized and screened against M. tuberculosis and several clinical isolates of M. avium.