INVITRO ACTIVITY OF RO-23-9424, A DUAL-ACTION CEPHALOSPORIN, COMPARED WITH ACTIVITIES OF OTHER ANTIBIOTICS

The in vitro activity of Ro 23-9424, which is desacetyl-cefotaxime linked to fleroxacin, was compared with the activities of cefotaxime, desacetyl-cefotaxime, fleroxacin, ofloxacin, and ciprofloxacin. It inhibited the majority of members of the family Enterobacteriaceae, except for some Serratia marcescencs, Citrobacter freundii, and Enterobacter cloacae strains, at ≤ 0.25 μg/ml and had an MIC for 90% of strains tested (MIC90) of 8 μg/ml against Pseudomonas aeruginosa. Most group A, B, C, and G streptococci and Streptococcus pneumoniae were inhibited at ≤ 0.25 μg/ml. Ninety percent of the staphylococci were inhibited at ≤ 4 μg/ml, except for some methicillin-resistant Staphylococcus aureus isolates. The MIC90s of Ro 23-9424 for Enterococcus faecalis and Listeria monocytogenes were ≥ 16 μg/ml. Ninety percent of Clostridium perfringens isolates were inhibited by ≤ 2 μg/ml, whereas Bacteroides fragilis had an MIC90 of 32 μg/ml. There was a minimal inoculum size effect. The MICs and MBCs were either identical or within a twofold dilution. The MICs of Ro 23-9424 for Escherichia coli, Klebsiella pneumoniae, Serratia marcescens, Enterobacter cloacae, Citrobacter freundii, Pseudomonas aeruginosa, and Staphylococcus aureus increased 16- to 128-fold after 2 weeks of transfer in the presence of Ro 23-9424, showing that the presence of two agents does not prevent resistance.