In vitro and in vivo antibacterial activity of KT3777, a new orally active carbacephem.

KT3777 is a novel carbacephem antibiotic structurally identical to cefaclor (CCL), except that the sulfur atom of position 1 of the cephem nucleus has been replaced by carbon. KT3777 was investigated for in vitro and in vivo antibacterial activities in comparison with CCL, cephalexin (CEX) and amoxicillin. The MIC50 of KT3777 ranged from 0.2 to 3.13 micrograms/ml for clinical isolates of Staphylococcus aureus, Streptococci, Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Haemophilus influenzae, and Neisseria gonorrhoeae. KT3777 possessed an antibacterial spectrum and potency similar to that of CCL. However, against E. coli and K. pneumoniae, KT3777 was about twice as active as CCL. KT3777 was more active than CEX against all strains tested. Killing-curve studies demonstrated bactericidal activity of KT3777 at concentrations above the MIC. KT3777 showed good affinity for penicillin-binding proteins 1A, 1Bs, 3 and 4 of E. coli NIHJ JC-2. The protective effect of KT3777 against systemic infections in mice was comparable to that of CCL with a few exceptions and about 3 to 7 times greater than that of CEX. KT3777 also proved effective against localized infections such as acute pneumonia and ascending urinary tract infections in mice.