Comparative antimicrobial activity of O-demethylfortimicin A, a derivative of fortimicin A.

The in vitro antimicrobial activity of O-demethylfortimicin A (ODMF), a derivative of fortimicin A, was compared with those of fortimicin A and gentamicin against a spectrum of 256 organisms. All three antibiotics were active in low concentrations against all strains of Enterobacteriaceae, Acinetobacter sp., and Staphylococcus aureus, with ODMF most active against Proteus mirabilis, indole-positive Proteus, and Providencia and gentamicin most active against other species. Activity of each of the antibiotics against group D streptococci was poor. The overall activity of ODMF was superior to that of fortimicin A for all groups of organisms examined and was most pronounced, approximately three-fold, against strains of Pseudomonas aeruginosa. Both ODMF and fortimicin A were resistant to the action of several aminoglycoside-inactivating enzymes, with the exception of 3-N-acetyltransferase-I. ODMF and fortimicin A showed similar rapid bactericidal effects at multiples of the minimum inhibitory concentration and equivalent synergistic activity against enterococci when combined with penicillin G. The combination of carbenicillin with ODMF, fortimicin A, or gentamicin was synergistic for approximately 80% of the P. aeruginosa strains tested. Inactivation of ODMF and fortimicin A when combined with carbenicillin in vitro was minimal or absent, whereas gentamicin was substantially inactivated under similar conditions. ODMF, fortimicin A, and gentamicin exhibited protective activity in mice infected with Escherichia coli, Klebsiella pneumoniae, Proteus vulgaris, S. aureus, or P. aeruginosa. Gentamicin was the most active, followed by ODMF and fortimicin A. The superior in vitro activity of ODMF compared with fortimicin A against P. aeruginosa was confirmed in vivo.