Studies on anti-MRSA parenteral cephalosporins. II. Synthesis and antibacterial activity of 7beta-[2-(5-amino-1,2,4- thiadiazol-3-yl)-2(Z)-alkoxyiminoacetamido]-3-(substituted imidaz

In an effort to discover a novel cefozopran (CZOP) derivative having excellent antibacterial activity against methicillin-resistant Staphylococcus aureus (MRSA), we performed chemical modification of the alkoxyimino moiety and imidazo1,2-b]pyridazinium group of CZOP. Among the prepared compounds, the cyclopentyloxyimino derivative 7beta-2-(5-amino-1,2,4-thiadiazol-3-yl)-2(Z)-cyclopentyloxyiminoacetamido]-3-(3,6-diaminoimidazo1,2-b]pyridazinium-1-yl)methyl-3-cephem-4-carboxylate (20 g) showed the most potent anti-MRSA activity, reflecting its high affinity (IC50 = 1.6 microg/ml) for penicillin binding protein 2' (PBP2'), although its anti-MRSA activity was slightly inferior to that of vancomycin (VCM). In experimental systemic infection in mice, however, 20 g showed activity comparable to that of VCM against MRSA. In addition, 20 g showed activity similar or slightly inferior to that of CZOP against Pseudomonas aeruginosa both in vitro and in vivo. Considering its favorable antibacterial activity profile, 20 g was considered to be the most promising CZOP derivative for further studies.