In-vitro activity of RP 59500, a semisynthetic streptogramin, against staphylococci and streptococci

The in-vitro activity of RP 59500, an injectable streptogramin derived from pristinamycin, was determined by agar dilution and compared with that of pristinamycin. Two hundred and sixty-one recent clinical isolates of Gram-positive cocci were tested against both antibiotics. The two compounds displayed similar activities. The MIC90s of RP 59500 ranged from 0.5 to 2 mg/L in 114 strains of Staphylococcus aureus showing various phenotypes of antibiotic resistance (penicillin-susceptible; penicillin-resistant and methicillin-susceptible; methicillin-resistant; erythromycin-resistant, either inducible or constitutive; quinolone-resistant). Similar results were obtained with coagulase-negative staphylococci. RP 59500 was consistently active against streptococci, with MIC90s of 0.25, 0.25 and 0.50 mg/L for Streptococcus pyogenes (n = 20), Streptococcus agalactiae (n = 20) and Streptococcus pneumoniae (n = 20), respectively. Enterococcus faecalis (n = 20) appeared to be notably less susceptible (MIC90, 8 mg/L). In view of this consistent activity against all staphylococci and streptococci tested, including multiply resistant isolates, RP 59500 merits further investigation.