HR 756, the syn isomer of a new methoxyimino cephalosporin with unusual antibacterial activity.

HR 756, the syn derivative of 7-[(2-(2-amino-4-thiazolyl)-2-methoxyimino)acetamido]cephalosporanic acid, is a new semisynthetic cephalosporin. It was 80 times more active than the anti derivative against beta-lactamase-producing strains of gram-negative bacteria. The range of inhibitory concentrations of HR 756 against gram-negative bacteria, including Haemophilus influenzae, susceptible or resistant to penicillins and cephalosporins was from 0.01 to 0.1 mug/ml. This activity was consistently higher than those observed with cephalothin, cephaloridine, cephalexin, and cefazolin. Nevertheless, some strains of Enterobacter cloacae were resistant. HR 756 showed very similar activity to that of ampicillin against group A streptococci and Streptococcus pneumoniae.