Monobactams--structure-activity relationships leading to SQ 26,776

129

Azthreonam (SQ 26,776) is a member of a new class of monocyclic ,B-lactam antibiotics. In Escherichia coli, azthreonam caused filamentation at its lowest effective concentration (0.2 ,ug/ml), a morphological effect identical to that observed with cephalothin. The penicillin-binding protein (PBP) profile indicated a very high affinity for PBP3 (complete binding at 0.1 pug/ml), a moderate affinity for PBPla (complete binding at 10 ,ug/ml), and poor affinities for PBPlb, PBP2, PBP4, and PBP5/6 (complete binding at .100 Fg/ml). Accordingly, azthreonam had poor activity against Streptomyces R61 DD-carboxypeptidase (50% inhibition, >100 ,ug/ml) and E. coli peptidoglycan transpeptidase (50% inhibition, 100 ,ug/ml). Azthreonam also showed very high affinity for PBP3 (complete binding at 0.1 ,ug/ ml) in Proteus vulgaris, Enterobacter cloacae, Klebsiella pneumoniae, and Pseudomonas aeruginosa. In all four organisms, its PBP profile was similar to that observed in E. coli. It is concluded that azthreonam, although of novel structure, has a mode of action similar to that of cephalosporins, affecting specifically septation in E. coli and most likely other gram-negative bacteria.Monobactams are monocyclic ,B-lactam antibiotics characterized by the 2-oxoazetidine-1-sulfonic acid moiety ( Fig. 1) which have been recently isolated from gram-negative bacteria (25). Although the naturally occurring compounds have generally weak antibacterial activity, synthetic derivatives are potent antibiotics (2, 24) with biological properties similar to those of bicyclic P-lactam antibiotics (24).Azthreonam (SQ 26,776) is a synthetic monobactam specifically active against gram-negative aerobic bacteria and stable to most P-lactamases (11,24). To investigate its mode of action and that of related compounds, we have examined its effect on penicillin-binding proteins (PBPs), penicillin-sensitive enzymes, and cell morphology. The effect has been subsequently compared with that produced by a typical cephalosporin, cephalothin. Ghuysen (University of Liege, Belgium). All other organisms were from the Squibb Culture Collection and, unless indicated otherwise, were clinical isolates.PBPs. Organisms were grown in brain heart infusion medium at 37°C on a gyrotory shaker (280 rpm; New Brunswick Scientific Co., New Brunswick, N.J.) for 16 h and subsequently (10%o inoculum) for 4 h (7). Cells were harvested by centrifugation and sonicated for 3 min (except for Staphylococcus aureus, which was sonicated for 9 min), and membranes were solubilized with 2% Triton X-100 as previously described (7). They were then incubated (100 ,ug of protein, deter-

mentioning

confidence: 99%

Mode of action of azthreonam

Georgopapadakou¹,

Smith²,

Sykes³

1982

129

“…The naturally occurring compounds have generally weak antibacterial activity, but synthetic derivatives are potent antibiotics (3), with stability to 1-lactamases equal to or better than that of thirdgeneration cephalosporins (6). A synthetic monobactam, azthreonam, is currently being developed for clinical use (25).…”

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confidence: 99%

Binding of monobactams to penicillin-binding proteins of Escherichia coli and Staphylococcus aureus: relation to antibacterial activity

Georgopapadakou¹,

Smith²,

Cimarusti³

et al. 1983

A series of novel monocyclic beta-lactam antibiotics having side chains related to penicillin, piperacillin, azlocillin, and cefotaxime were examined with respect to binding to essential penicillin-binding proteins (PBPs) in Escherichia coli and Staphylococcus aureus. In the penicillin series, there was poor binding to all essential PBPs of E. coli (greater than 100 micrograms/ml) but good binding to PBPs 1, 2, and 3 of S. aureus (approximately 1 microgram/ml). In the piperacillin and azlocillin series, there was good binding to PBP 3 of E. coli (0.1 microgram/ml) and PBPs 1, 2, and 3 of S. aureus (approximately 1 microgram/ml). In the cefotaxime series, there was generally good binding to PBP 3 of E. coli (0.1 micrograms/ml) but poor binding to PBPs 1, 2, and 3 of S. aureus (greater than or equal to 100 micrograms/ml). With a few exceptions in the cefotaxime series, antibacterial activity paralleled essential PBP binding. Binding studies with radioactively labeled compounds revealed no additional essential monobactam-binding proteins in the two organisms. The studies suggest that monobactams are intrinsically active against both gram-positive and gram-negative bacteria; the activity spectrum of a given monobactam is determined by the binding to essential PBPs, which in turn is determined by the nature of the substituents on the beta-lactam nucleus.

“…Studies done at the Squibb Institute for Medical Research, where the compound was synthesized, demonstrated that azthreonam has a high degree of beta-lactamase stability and excellent activity against aerobic and facultative gramnegative bacteria (2,8).…”

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confidence: 99%

In vitro activity of azthreonam, a monobactam antibiotic

Jacobus¹,

Ferreira²,

Barza³

1982

We studied the activity of azthreonam (SQ 26,776) Antibiotic susceptibility studies. For aerobic and facultative bacteria, susceptibility to antibiotics was determined by a broth dilution microtiter technique (1). Mueller-Hinton broth (GIBCO Diagnostics, Madison, Wis.) was used, and the inoculum was adjusted to contain 10' colony-forming units (CFU) per ml, resulting in a concentration of 5 x 10' organisms per well.The minimal inhibitory concentration (MIC) was determined as previously described (1).Anaerobic bacteria were studied by a modified agar dilution technique (9). To avoid confluent growth with spreading clostridia, the medium was altered to contain 2% instead of 1.5% agar when these organisms were tested.