Structural requirements for the stability of novel cephalosporins to AmpC beta-lactamase based on 3D-structure

AmpC beta-lactamase is one of the leading causes of Pseudomonas aeruginosa (P. aeruginosa) resistance to cephalosporins. FR259647 is a cephalosporin having a novel pyrazolium substituent at the 3-position and exhibits excellent activity (MIC=1 microg/mL) against the AmpC beta-lactamase overproducing P. aeruginosa FP1380 strain in comparison with the third-generation cephalosporins FK518 [Abstracts of Papers, 30th Interscience Conference on Antimicrobial Agents and Chemotherapy, Atlanta, GA, October 21-24, 1990, Abs. 454; Abstracts of Papers, 30th Interscience Conference on Antimicrobial Agents and Chemotherapy, Atlanta, GA, October 21-24, 1990, Abs. 455; Abstracts of Papers, 30th Interscience Conference on Antimicrobial Agents and Chemotherapy, Atlanta, GA, October 21-24, 1990, Abs. 456; Abstracts of Papers, 30th Interscience Conference on Antimicrobial Agents and Chemotherapy, Atlanta, GA, October 21-24, 1990, Abs. 457] (MIC=16 microg/mL) and ceftazidime (CAZ) (MIC=128 microg/mL). The stability of FR259647 and FK518 to AmpC beta-lactamase was evaluated using MIC assays against both the P. aeruginosa PAO1 strain and a PAO1 mutant strain overproducing AmpC beta-lactamase as a differential assay, which indicates that the main difference derives from their stability to AmpC beta-lactamase. A structural analysis using computer simulations indicated that the difference in stability may be due to steric hindrance of the 3-position substituents causing differential affinity. This steric hindrance may disturb entry of the cephalosporins into the binding pocket. We predicted the possibility of inhibition of entry as a potential means of enhancing stability by conformational analysis. In order to validate this speculation, novel FR259647 derivatives 4-9 were designed, calculated, synthesized, and evaluated. As a result, we demonstrated that their probability of entry correlated with the MIC ratio of the mutant strain to the parent strain and supports the validity of our model.