Comparative antimicrobial activities of ribostamycin, gentamicin, ampicillin and lincomycin in vitro and in vivo

The antimicrobial activity of ribostamycin, a unique aminoglycoside antibiotic possessing a neutral sugar component, was compared with those of gentamicin, ampicillin and lincomycin in vitro and in vivo. Ribostamycin showed comparable or slightly weaker in vitro activity than the reference antibiotics against Gram-positive bacteria. Against Gram-negative bacteria, ribostamycin was less active than gentamicin, but comparable to or more active than ampicillin. Lincomycin was less active or inactive to Gram-negative bacteria. Ribostamycin was active against some gentamicin-resistant bacteria, especially K. pneumoniae possessing the aminoglycoside-modifying enzymes AAC(3)-l and AAD(2"). The in vivo activity of ribostamycin was weaker than that of gentamicin, but comparable to that of ampicillin and lincomycin against Gram-positive bacteria, and superior to that of ampicillin against Gram-negative bacteria. The in vivo activity of ribostamycin was characterized by (i) and ED50 value not so affected by the challenge inoculum as that of ampicillin; (ii) a lower ED50 value by bolus administration than that by divided administration of the same dosage; and (iii) a lower ED50 value than that expected from the MIC value as compared with that of ampicillin and lincomycin. These characteristics are explained by the rapid and potent bactericidal activity of ribostamycin at high inoculum and high drug concentration, assisted by high serum concentration in mice.