Synthesis and anti-staphylococcal activity of novel bacterial topoisomerase inhibitors with a 5-amino-1,3-dioxane linker moiety

Novel bacterial type II topoisomerase inhibitors (NBTIs) constitute a promising new class of antibacterial agents. We report a series of NBTIs with potent anti-staphylococcal activity and diminished hERG inhibition. Dioxane-linked compound 9 demonstrated MICs ≤1?μg/mL against both methicillin-susceptible (MSSA) and -resistant Staphylococcus aureus (MRSA), accompanied by reduced hERG inhibition as compared to cyclohexane- or piperidine-linked analogs.     

Design, synthesis and antibacterial activities of a series of new 2-oxaisocephems

A series of 2-oxaisocephems with a thio-substituted methyl group at the 3-position and a [2-(5-amino-1,2,4-thiadizol-3-yl)-2-(Z)-alkoxyimino]acetamido moiety at 7-position were synthesized and tested for their antibacterial activities. The analogs 17c and 17f have well-balanced potency and significantly enhanced activity as compared with the reference compound ceftazidime.     

Correlation of carboxylic acid pKa to protein binding and antibacterial activity of a novel class of bacterial translation inhibitors

Previously we reported the discovery and initial optimization of a novel anthranilic acid derived class of antibacterial agents which suffered from extensive protein binding. This report describes efforts directed toward understanding the relationship of the acidity of the carboxylic acid with the extent of protein binding. The pK(a) of the acid was modified via the synthesis of a number of anthranilic acid analogs which vary the aromatic ring substituent at the 4-position. The pK(a) and HSA binding constants have been determined for each of the analogs. Our results indicate a correlation between pK(a) and HSA K(d). The physical properties and antibacterial activities will be discussed as well as how these results help address the protein binding issue with this series of compounds.     

Preparation of novel antibacterial agents. Replacement of the central aromatic ring with heterocycles

Discovery of novel antibacterial agents is a significant challenge. We have recently reported on our discovery of novel antibacterial agents in which we have rapidly optimized potency utilizing a parallel chemistry approach. These advanced leads suffer from high affinity for human serum albumin (HSA). In an effort to decrease the affinity for HSA we have prepared a series of heterocyclic analogs, which retained antibacterial activity and demonstrated reduced affinity for HSA.     

Myxopyronin B analogs as inhibitors of RNA polymerase, synthesis and biological evaluation

A series of myxopyronin B analogs has been prepared via a convergent synthetic route and were tested for in vitro inhibitory activity against DNA-dependent RNA polymerase and antibacterial activity against E. coli and S. aureus. The parent lead compound proved to be very sensitive to even small changes. Only the achiral desmethyl myxopyronin B (1a) provided enhanced potency.     

Design and synthesis of diketopiperazine and acyclic analogs related to the caprazamycins and liposidomycins as potential antibacterial agents

A systematic simplification methodology of a class of 6'-N-alkyl-5'-O-aminoribosyl-glycyluridine antibiotics was shown to produce potential antibacterial agents having a novel mechanism of action. Diketopiperazines and acyclic analogs of the caprazamycins (CPZs) and liposidomycins (LPMs) were efficiently synthesized, and their antibacterial activity was evaluated. The diketopiperazine analog 11a and the acyclic analogs 12a and 16a having a palmitoyl group as a lipophilic side chain exhibited moderate antibacterial activities with MICs of 12.5-50 microg/mL. This approach could provide ready access to a range of analogs for the development of potential antibacterial agents.     

Analogs of nitrofuran antibiotics are potent GroEL/ES inhibitor pro-drugs

In two previous studies, we identified compound 1 as a moderate GroEL/ES inhibitor with weak to moderate antibacterial activity against Gram-positive and Gram-negative bacteria including Bacillus subtilis, methicillin-resistant Staphylococcus aureus, Klebsiella pneumonia, Acinetobacter baumannii, and SM101 Escherichia coli (which has a compromised lipopolysaccharide biosynthetic pathway making bacteria more permeable to drugs). Extending from those studies, we developed two series of analogs with key substructures resembling those of known antibacterials, nitroxoline (hydroxyquinoline moiety) and nifuroxazide/nitrofurantoin (bis-cyclic-N-acylhydrazone scaffolds). Through biochemical and cell-based assays, we identified potent GroEL/ES inhibitors that selectively blocked E. faecium, S. aureus, and E. coli proliferation with low cytotoxicity to human colon and intestine cells in vitro. Initially, only the hydroxyquinoline-bearing analogs were found to be potent inhibitors in our GroEL/ES-mediated substrate refolding assays; however, subsequent testing in the presence of an E. coli nitroreductase (NfsB) in situ indicated that metabolites of the nitrofuran-bearing analogs were potent GroEL/ES inhibitor pro-drugs. Consequently, this study has identified a new target of nitrofuran-containing drugs, and is the first reported instance of such a unique class of GroEL/ES chaperonin inhibitors. The intriguing results presented herein provide impetus for expanded studies to validate inhibitor mechanisms and optimize this antibacterial class using the respective GroEL/ES chaperonin systems and nitroreductases from E. coli and the ESKAPE bacteria.     

Synthesis and in vitro antitubercular evaluation of novel sansanmycin derivatives

Tuberculosis (TB) is a major health problem worldwide. A series of novel sansanmycin derivatives were designed, semi-synthesized and evaluated for their activity against drug-susceptible Mycobacterium tuberculosis strain H(37)Rv with sansanmycin A (SSA) as the lead. Among these analogs tested, compound 1d possessing an isopropyl group at the amino terminal afforded an increased antimycobacterial activity with a MIC value of 8 μg/mL in comparison with SSA. Importantly, it was active for rifampicin- and isoniazid-resistant M. tuberculosis strain isolated from patients in China. These promising results offer an opportunity for further exploration of this novel class of analogs as antitubercular agents.     

Acyl-sulfamates target the essential glycerol-phosphate acyltransferase (PlsY) in Gram-positive bacteria

PlsY is the essential first step in membrane phospholipid synthesis of Gram-positive pathogens. PlsY catalyzes the transfer of the fatty acid from acyl-phosphate to the 1-position of glycerol-3-phosphate to form the first intermediate in membrane biogenesis. A series of non-metabolizable, acyl-sulfamate analogs of the acyl-phosphate PlsY substrate were prepared and evaluated as inhibitors of Staphylococcus aureus PlsY and for their Gram-positive antibacterial activities. From this series phenyl (8-phenyloctanoyl) sulfamate had the best overall profile, selectively inhibiting S. aureus phospholipid biosynthesis and causing the accumulation of both long-chain fatty acids and acyl-acyl carrier protein intermediates demonstrating that PlsY was the primary cellular target. Bacillus anthracis was unique in being more potently inhibited by long chain acyl-sulfamates than other bacterial species. However, it is shown that Bacillus anthracis PlsY is not more sensitive to the acyl-sulfamates than S. aureus PlsY. Metabolic profiling showed that B. anthracis growth inhibition by the acyl-sulfamates was not specific for lipid synthesis illustrating that the amphipathic acyl-sulfamates can also have off-target effects in Gram-positive bacteria. Nonetheless, this study further advances PlsY as a druggable target for the development of novel antibacterial therapeutics, through the discovery and validation of the probe compound phenyl (8-phenyloctanoyl) sulfamate as a S. aureus PlsY inhibitor.     

Synthesis and antibacterial activity of 5-substituted oxazolidinones

A series of 5-substituted oxazolidinones with varying substitution at the 5-position of the oxazolidinone ring were synthesized and their in vitro antibacterial activity was evaluated. The compounds demonstrated potent to weak antibacterial activity. A novel compound (PH-027) demonstrated potent antibacterial activity, which is comparable to or better than those of linezolid and vancomycin against antibiotic-susceptible standard and clinically isolated resistant strains of gram-positive bacteria. Although the presence of the C-5-acetamidomethyl functionality at the C-5 position of the oxazolidinones has been widely claimed and reported as a structural requirement for optimal antimicrobial activity in the oxazolidinone class of compounds, our results from this work identified the C-5 triazole substitution as a new structural alternative for potent antibacterial activity in the oxazolidinone class.     

Addressing the stability of C-capped dipeptide efflux pump inhibitors that potentiate the activity of levofloxacin in Pseudomonas aeruginosa

Synthetic optimization of a biologically labile class of dipeptides that function as efflux pump inhibitors to potentiate the antibacterial agent levofloxacin in Pseudomonas aeruginosa has led to the discovery of a related series of compounds that are completely stable in a variety of biological matrices. Other than the stability profile, the in vitro profile of the new series is essentially identical to that observed with the original one. A prototypical compound from the new series demonstrates potentiation in an in vivo model of infection.     

Synthesis and biological evaluation of novel (L)-alpha-amino acid methyl ester, heteroalkyl, and aryl substituted 1,4-naphthoquinone derivatives as antifungal and antibacterial agents

A series of (S)-N-(1,4-naphthoquinon-2-yl)-alpha-amino acid methyl esters 3-9, 2-N,N-dialkylamino-1,4-naphthoquinones 10-11 and 2-hydroxy-3-(2'-mercaptoimidazolyl)-1,4-naphthoquinones and their cyclic analogs 12-15 were synthesized and evaluated for antifungal and antibacterial activities. The structure-activity relationships of these compounds were studied and the results show that the compounds 9b and 13c exhibited in vitro antifungal activity against Candida albicans, Cryptococcus neoformans, and Sporothrix schenckii, whereas compound 6a showed in vitro antibacterial activity against Streptococcus faecalis, K. pneumoniae, Escherichia coli, and Staphylococcus aureus.     

The structure-activity relationships of mansonone F, a potent anti-MRSA sesquiterpenoid quinone: SAR studies on the C6 and C9 analogs

For the systematic SAR study on mansonone F, a series of C6 and C9 analogs of mansonone F have been synthesized and their anti-MRSA activities were evaluated. Most of the analogs exhibited good or excellent anti-MRSA activities. In particular, the 6-n-butylmansonone F showed fourfold higher antibacterial activities compared to that of vancomycin.     

Synthesis and characterization of quinoline-based thiosemicarbazones and correlation of cellular iron-binding efficacy to anti-tumor efficacy

Iron chelators have emerged as a potential anti-cancer treatment strategy. In this study, a series of novel thiosemicarbazone iron chelators containing a quinoline scaffold were synthesized and characterized. A number of analogs show markedly greater anti-cancer activity than the 'gold-standard' iron chelator, desferrioxamine. The anti-proliferative activity and iron chelation efficacy of several of these ligands (especially compound 1b), indicates that further investigation of this class of thiosemicarbazones is worthwhile.     

Synthesis and structure-activity relationships of antifungal crassinervic acid analogs

A series of analogs of the natural antifungal compound crassinervic acid, a constituent of Piper crassinervium, were synthesized to gain insight into the most relevant structural features affecting the activity of the parent molecule. Most compounds were prepared by aldol reaction of methyl 3-acetyl-4-hydroxybenzoate with a series of ketones, followed by reduction, hydrolysis, and oxidation. The antifungal activities of crassinervic acid and its analogs was assessed against Cladosporium cladosporioides by using the method of bioautography. The bioassay results allowed us to depict structure?activity relationships for this class of compounds.     

Discovery of benzimidazole pyrrolidinyl amides as prolylcarboxypeptidase inhibitors

A series of benzimidazole pyrrolidinyl amides containing a piperidinyl group were discovered as novel prolylcarboxypeptidase (PrCP) inhibitors. Low-nanomolar IC₅₀’s were achieved for several analogs, of which compound 9b displayed modest ex vivo target engagement in eDIO mouse plasma. Compound 9b was also studied in vivo for its effect on weight loss and food intake in an eDIO mouse model and the results will be discussed.     

Substituted heterocyclic analogs as selective COX-2 inhibitors in the flosulide class

Substituted heterocyclic analogs in the Flosulide class were investigated as potential selective cyclooxygenase-2 inhibitors. 6-(4-Ethyl-2-thiazolylthio)-5-methanesulfonamido-3H-isobe nzofuran-1-one 14 was found to be the optimal compound in the series with superior in vitro and in vivo activities.     

Antibacterials: are the new entries enough to deal with the emerging resistance problems?

Fifty years of making analogs based on less than ten antibacterial scaffolds has resulted in the development and marketing of over 100 antibacterial agents but, with the exception of the oxazolidinone core, no new scaffolds have emerged in the past 30 years to address emerging resistance problems. As the support for antibacterial research shifts away from large pharmaceutical companies, a wave of biotechnology companies have pursued a diverse choice of targets resulting in several novel classes of agent in late-stage development. Although critical for certain resistance niche needs, these agents are unlikely to provide the solution to the requirement for a major novel scaffold class of antibacterials.     

3,5-Dihydro-imidazo[4,5-d]pyridazin-4-ones: a class of potent DPP-4 inhibitors

Systematic variations of the xanthine scaffold in close analogs of development compound BI 1356 led to the class of 3,5-dihydro-imidazo[4,5-d]pyridazin-4-ones which provided, after substituent screening, a series of highly potent DPP-4 inhibitors.     

Synthesis and Structure?Activity Relationships of Antifungal Crassinervic Acid Analogs

A series of analogs of the natural antifungal compound crassinervic acid, a constituent of Piper crassinervium, were synthesized to gain insight into the most relevant structural features affecting the activity of the parent molecule. Most compounds were prepared by aldol reaction of methyl 3‐acetyl‐4‐hydroxybenzoate with a series of ketones, followed by reduction, hydrolysis, and oxidation. The antifungal activities of crassinervic acid and its analogs was assessed against Cladosporium cladosporioides by using the method of bioautography. The bioassay results allowed us to depict structure? activity relationships for this class of compounds.