No abstract

This work shows once again that novel CTX-M enzymes with an expanded activity towards ceftazidime through a single amino acid substitution can be identified from clinical isolates. Thus, detection of CTX-M enzymes can no longer be based solely on the resistance phenotypes of clinical isolates towards ceftazidime and cefotaxime.

The enzyme aminoimidazole ribonucleotide (AIR) carboxylase catalyzes the synthesis of the purine intermediate, 4-carboxy-5-aminoimidazole ribonucleotide (CAIR). Previously, we have shown that the compound 4-nitro-5-aminoimidazole ribonucleotide (NAIR) is a slow, tight binding inhibitor of the enzyme with a K i of 0.34 nM. The structural attributes and the slow, tight binding characteristics of NAIR implicated this compound as a transition state or reactive intermediate analog. However, it is unclear what molecular features of NAIR contribute to the mimetic properties for either of the two proposed mechanisms of AIR carboxylase. In order to gain additional information regarding the mechanism for the potent inhibition of AIR carboxylase by NAIR, a series of heterocyclic analogs were prepared and evaluated. We find that all compounds are weaker inhibitors than NAIR and that CAIR analogs are not alternative substrates for the enzyme. Surprisingly, rather subtle changes in the structure of NAIR can lead to profound changes in binding affinity. Computational investigations of enzyme intermediates and these inhibitors reveal that NAIR displays an electrostatic potential surface similar to a proposed reaction intermediate. The result indicates that AIR carboxylase is likely sensitive to the electrostatic surface of reaction intermediates and thus compounds which mimic these surfaces should possess tight binding characteristics. Given the evolutionary relationship between AIR carboxylase and N 5 -CAIR mutase, we believe that this concept extends to the mutase enzyme as well. The implications of this hypothesis for the design of selective inhibitors of the N 5 -CAIR mutase are discussed.

This work shows that CTX-M-12, which confers high-level resistance to cefotaxime but not to ceftazidime, has emerged in Korea. The bla(CTX-M-12) gene was associated with an upstream ISEcp1 insertion sequence.

The in vitro antibacterial activities of LB 10827, a new oral cephalosporin, against common respiratory tract pathogens were compared with those of six ␤-lactams (cefdinir, cefuroxime, cefprozil, penicillin G, amoxicillinclavulanate, and ampicillin), two quinolones (trovafloxacin and ciprofloxacin), and one macrolide (clarithromycin). The MIC of LB 10827 at which 90% of the penicillin-resistant strains of Streptococcus pneumoniae tested were inhibited was 0.5 g/ml, and the drug was 4-to 32-fold more active than the compared ␤-lactams. The potent activity of LB 10827 against Haemophilus influenzae and Moraxella catarrhalis was retained, and the presence of ␤-lactamase in both strains had little effect on the in vitro activity of the compound. Time-kill studies revealed that LB 10827 had bactericidal activity against these respiratory pathogens. This agent reduced original counts of all pathogens tested by >3 log 10 CFU/ml at the MIC, and the regrowth was completely prevented for 12 h. The potent in vitro antibacterial activity of LB 10827 against respiratory pathogens has been proved in both mouse pneumonia and neutropenic rat models. These results strongly suggest that this agent has potential for the treatment of respiratory tract infections.The respiratory pathogens Streptococcus pneumoniae, Haemophilus influenzae, and Moraxella catarrhalis cause a wide spectrum of pediatric and adult infections, including acute otitis media, sinusitis, pneumonia, bacteremia, meningitis, and acute exacerbations of chronic bronchitis. S. pneumoniae is the most common community-acquired respiratory tract pathogen; it causes as many as 20% of all cases of communityacquired pneumonia annually, leading to significant morbidity and mortality rates (2, 5). Recent data indicate that approximately one-third of S. pneumoniae isolates in the United States have some level of resistance to penicillin, and up to 40% of H. influenzae isolates and almost all M. catarrhalis isolates produce ␤-lactamase, which mediates resistance to penicillins and certain cephalosporins (6). Furthermore, the emergence of strains of S. pneumoniae resistant to penicillin as well as extended-spectrum cephalosporins, macrolides, and quinolones has become a considerable concern in many parts of the world (1, 3). LB 10827 (Fig. 1) is a new, nonester-type oral cephalosporin which has an excellent in vitro activity against the major respiratory pathogens and is currently under preclinical trial (S. H. Oh, E. J. Ryu, K. S. Paek, M. Y. Kim, S. H. Lee, and C. S. Lee, Abstr. 39th Intersci. Conf. Antimicrob. Agents Chemother., abstr. 397, 1999). In this study, we assessed the in vitro and in vivo activities of LB 10827 against these three common bacterial respiratory tract pathogens. MATERIALS AND METHODSBacterial strains. The bacterial strains used in this study were obtained from various community hospitals and medical centers in Korea between 1998 and 1999. All strains were stored frozen at Ϫ70°C.Antimicrobial agents. LB 10827 was synthesized at LG Chem., and other anti...