Susceptibilities of 228 penicillin- and erythromycin-susceptible and -resistant pneumococci to RU 64004, a new ketolide, compared with susceptibilities to 16 other agents

The susceptibilities of 228 penicillin- and erythromycin-susceptible and -resistant pneumococci to RU 64004, a new ketolide, were tested by agar dilution, and the results were compared with those for penicillin G, erythromycin, azithromycin, clarithromycin, rokitamycin, clindamycin, pristinamycin, ciprofloxacin, sparfloxacin, trimethoprim-sulfamethoxazole, doxycycline, chloramphenicol, cefuroxime, ceftriaxone, imipenem, and vancomycin, RU 64004 was very active against all strains tested, with MICs at which 90% of the isolates are inhibited (MIC(90)s) of 0.016 mu g/ml for erythromycin-susceptible strains (MIG, less than or equal to 0.25 mu g/ml) and 0.25 mu g/ml for erythromycin-resistant strains (MIC, greater than or equal to 0,5 mu g/ml). All other macrolides had MIC(90)s of 0.03 to 0.25 and greater than or equal to 128 mu g/ml for erythromycin-susceptible and -resistant strains, respectively. Among erythromycin-resistant strains, clindamycin MICs for 28 of 91 (30.7%) were less than or equal to 0,125 mu g/ml. Pristinamycin MICs for all strains were less than or equal to 1,0 mu g/ml. MIC(90)s of ciprofloxacin and sparfloxacin were 4.0 and 0.25 mu g/ml, respectively, and were unaffected by susceptibility to penicillin or erythromycin, Vancomycin and imipenem inhibited all strains at less than or equal to 0.5 and 0.25 mu g/ml, respectively, MICs of cefuroxime and cefotaxime rose with those of penicillin G, MICs of trimethoprim-sulfamethoxazole, doxycycline, and chloramphenicol were variable but were generally higher for penicillin- and erythromycin-resistant strains. RU 64004 is the first member of the macrolide group which has low MICs for erythromycin-resistant pneumococci.