S-alkyl dithioformates as 1,3-dipolarophiles. Generation of C(2)-unsubstituted penems.

S-Alkyl dithioformates, generated by a cycloreversion process, react as 1,3-dipolarophiles with beta-lactam-based azomethine ylids to provide, after (net) elimination of MeSH, C(2)-unsubstituted penems. The overall cycloreversion/cycloaddition sequence was accelerated by microwave irradiation. [reaction: see text]

SAR of biphenyl carboxamide ligands of the human melanin-concentrating hormone receptor 1 (MCH R1): discovery of antagonist SB-568849.

We report here the discovery of a class of MCH R1 ligands based on a biphenyl carboxamide template. A docked-in model is presented indicating key interactions in the putative binding site of the receptor. Parallel high throughput synthetic techniques were utilised to allow rapid exploration of the structure-activity relationship around this template, leading to compound SB-568849 which possessed good receptor affinity and selectivity. This compound proved to be an antagonist with stability in vivo, an acceptable brain-blood ratio and oral bioavailability.

Characterization of monomeric L1 metallo-beta -lactamase and the role of the N-terminal extension in negative cooperativity and antibiotic hydrolysis.

The L1 metallo-beta-lactamase from Stenotrophomonas maltophilia is unique among this class of enzymes because it is tetrameric. Previous work predicted that the two regions of important intersubunit interaction were the residue Met-140 and the N-terminal extensions of each subunit. The N-terminal extension was also implicated in beta-lactam binding. Mutation of methionine 140 to aspartic acid results in a monomeric L1 beta-lactamase with a greatly altered substrate specificity profile. A 20-amino acid N-terminal deletion mutant enzyme (N-Del) could be isolated in a tetrameric form but demonstrated greatly reduced rates of beta-lactam hydrolysis and different substrate profiles compared with that of the parent enzyme. Specific site-directed mutations of individual N terminus residues were made (Y11S, W17S, and a double mutant L5A/L8A). All N-terminal mutant enzymes were tetramers and all showed higher K(m) values for ampicillin and nitrocefin, hydrolyzed ceftazidime poorly, and hydrolyzed imipenem more efficiently than ampicillin in contrast to wild-type L1. Nitrocefin turnover was significantly increased, probably because of an increased rate of breakdown of the intermediate species due to a lack of stabilizing forces. K(m) values for monomeric L1 were greatly increased for all antibiotics tested. A model of a highly mobile N-terminal extension in the monomeric enzyme is proposed to explain these findings. Tetrameric L1 shows negative cooperativity, which is not present in either the monomer or N-terminal deletion enzymes, suggesting that the cooperative effect is mediated via N-terminal intersubunit interactions. These data indicate that while the N terminus of L1 is not essential for beta-lactam hydrolysis, it is clearly important to its activity and substrate specificity.

Identification of a series of tricyclic natural products as potent broad-spectrum inhibitors of metallo-beta-lactamases.

This work describes the discovery and characterization of a novel series of tricyclic natural product-derived metallo-beta-lactamase inhibitors. Natural product screening of the Bacillus cereus II enzyme identified an extract from a strain of Chaetomium funicola with inhibitory activity against metallo-beta-lactamases. SB236050, SB238569, and SB236049 were successfully extracted and purified from this extract. The most active of these compounds was SB238569, which possessed K(i) values of 79, 17, and 3.4 microM for the Bacillus cereus II, Pseudomonas aeruginosa IMP-1, and Bacteroides fragilis CfiA metallo-beta-lactamases, respectively, yet none of the compounds exhibited any inhibitory activity against the Stenotrophomonas maltophilia L-1 metallo-beta-lactamase (50% inhibitory concentration > 1,000 microM). The lack of activity against angiotensin-converting enzyme and serine beta-lactamases demonstrated the selective nature of these compounds. The crystal structure of SB236050 complexed in the active site of CfiA has been obtained to a resolution of 2.5 A. SB236050 exhibits key polar interactions with Lys184, Asn193, and His162 and a stacking interaction with the indole ring of Trp49 in the flap, which is in the closed conformation over the active site groove. SB236050 and SB238569 also demonstrate good antibacterial synergy with meropenem. Eight micrograms of SB236050 per ml gave rise to an eightfold drop in the MIC of meropenem for two clinical isolates of B. fragilis producing CfiA, making these strains sensitive to meropenem (MIC < or = 4 microg/ml). Consequently, this series of metallo-beta-lactamase inhibitors exhibit the most promising antibacterial synergy activity so far observed against organisms producing metallo-beta-lactamases.