***Background.*** Although spread of multi-drug-resistant Gram-negative pathogens causes serious problems, there are limited therapeutic options because of the lack of new promising antibacterial agents. S-649266 (\'266) is a novel catechol-substituted siderophore cephalosporin antibiotic with potent antibacterial activity against Gram-negative bacteria including drug-resistant pathogens. This study investigated the mechanisms of enhanced activity and β-lactamase stability of \'266.

***Methods.*** MIC was determined by brothmicrodilution methods by using apo-transferrin (apo-T) into the medium and treatment of the medium by Chelex resin. Chelating activity of \'266 and iron was evaluated by Chromeazrol B and uptake of '266 was measured by using calcein fluorescence. The kinetic parameters of \'266 against carbapenemases were determined to evaluate the stability.

***Results.*** The i*n vitro* activity of '266 increased under free iron deficient conditions such as apo-T supplemented medium. These culture conditions mimic the condition in human infection. The evaluation with Chromeazrol B showed the strong chelating activity of \'266 with ferric iron. The rate of iron uptake by *P. aeruginosa* cells were enhanced by addition of '266, suggesting that \'266 had siderophore activity with iron transported efficiently into *P. aeruginosa*. The catalytic efficiency (*k*cat/*Km*) of \'266 against IMP-1, VIM-2, L1 were 5 × 10^3^, 5 × 10^3^, 2 × 10^4^ M^−1^ s^−1^, respectively. These values were approximately 10 to 1,000 fold lower than those of meropenem, ceftazidime, and cefepime. The relative hydrolysis velocity of \'266 against NDM-1 was lower than that of meropenem. No or only a slight hydrolysis of \'266 was observed against KPC-3, P99 and OXA-23, indicating the high stability of \'266 to these β-lactamases.

***Conclusion.*** The in vitro activity of S-649266 was enhanced under free iron deficient conditions which mimic the condition in human infection. S-649266, acting through siderophore receptors was transported into *P.aeruginosa*. S-649266 was more stable against carbapenemase than other cephalosporins and carbapenems. These results showed that \'266 is a promising siderophore cephalosporin to treat multi-drug resistant Gram-negative pathogens.

***Disclosures.*** **M. Tsuji**, Shionogi and Co., Ltd.: Employee, Salary **A. Ito**, Shionogi and Co., Ltd.: Employee, Salary **T. Horiyama**, Shionogi and Co., Ltd.: Employee, Salary **N. Fukuhara**, Shionogi and Co., Ltd.: Employee, Salary **R. Nakamura**, Shionogi and Co., Ltd.: Employee, Salary **Y. Yamano**, Shionogi and Co., Ltd.: Employee, Salary **J. Shimada**, Shionogi and Co., Ltd.: Consultant and Employee, Salary **Y. Ishii**, Toho Univesity Shool of Medicine: Employee, Salary **K. Tateda**, Toho Uneiversity School of Medicine: Employee, Salary

[^1]: **Session:** 40. Antimicrobial Resistance: Novel Agents and Approaches to Gram Negative Infections

[^2]: Thursday, October 9, 2014: 12:30 PM
