THE INVITRO ACTIVITY OF NEW STREPTOGRAMINS, RP 59500, RP 57669 AND RP 54476, ALONE AND IN COMBINATION

RP 59500 is a 30: 70 mixture of RP 57669 and RP 54476. The activity of RP 59500 and its two components against Gram-positive and Gram-negative organisms was compared with that of clarithromycin, roxithromycin, azithromycin and rokita-mycin. RP 59500 inhibited 90% of erythromycin-susceptible and resistant Staphylococcus aureus and coagulase-negative staphylococci at 1 mg/L (range 0-06-2 mg/L). Both indudbly and constitutively-resistant strains of S. aureus, as well as strains resistant to rifampicin, gentamicin and ciprofloxacin, were inhibited. Streptococcus pyogenes, induding erythromycin-resistant isolates, and group C and G streptococd were inhibited by 0.5 mg/L. Streptococcus pneumoniae and viridans group streptococd were inhibited by 1 mg/L. The MIC90 was 4 mg/L for Haemophilus influenzae and 1 mg/L for Moraxella catarrhalis. RP 59500 did not inhibit Enterobacteriaceae or Pseudomonas aeruginosa. The activity of RP 59500 against streptococd was less than that of the four other macrolides. Clostridium perfringens strains were highly susceptible, as were Bacteroides spp. RP 59500, when combined with ciprofloxacin, cefotaxime or gentamidn, did not have altered activity against susceptible spedes or alter the activity of the other component of the combination against susceptible spedes. MBCs in serum were increased two- to four-fold for S. pyogenes, S. pneumoniae and S. aureus, compared with MBCs in broth, but RP 59500 was as active at pH 6 as at pH 7, and there was not an appreciable inoculum effect RP 59500 has potential use as an agent against indu-dbly and constitutively erythromydn-resistant isolates of Gram-positive spedes and selected anaerobic organisms. © 1992 Oxford University Press.