OBJECTIVES: Retapamulin is the first agent of the pleuromutilin class formulated 
as a topical antibacterial for treating skin infections. The aim of this study 
was to determine the antimicrobial activity of retapamulin by determining the 
minimal inhibitory concentration (MIC) values of this new drug and comparators 
against a wide range of anaerobic bacteria of human origin.
METHODS: The in vitro activity of retapamulin and six comparators (amoxicillin, 
amoxicillin/clavulanic acid, ceftriaxone, imipenem, clindamycin and 
metronidazole) was evaluated against 232 anaerobic clinical isolates. MICs were 
determined by the CLSI reference agar dilution method (M11-A6).
RESULTS: Ceftriaxone, clindamycin and amoxicillin/clavulanic acid resistance 
rates were 54%, 42% and 9.6%, respectively, within the Bacteroides fragilis 
group. Despite high resistance rates to various antibiotics, retapamulin 
inhibited 37/52 (71%) strains of the B. fragilis group and 85/87 (98%) of the 
other Gram-negative bacilli at a concentration of 2 mg/L or less. All the 
investigated strains of Clostridium perfringens were inhibited by 1 mg/L 
retapamulin. Three strains of C. difficile and one strain of C. clostridioforme 
demonstrated decreased susceptibility to retapamulin. Based on inhibitory 
concentrations, retapamulin was more active than clindamycin, metronidazole and 
ceftriaxone against Propionibacterium acnes and anaerobic Gram-positive cocci, 
as all isolates were inhibited by <or=2 mg/L.
CONCLUSIONS: At <or=2 mg/L, retapamulin inhibited 90% of all 232 anaerobes 
tested, whereas overall resistance rates for the comparators were as follows: 
co-amoxiclav, 2%; metronidazole, 12%; clindamycin, 15% and ceftriaxone, 20%. The 
broad anaerobic spectrum demonstrated by retapamulin in vitro is attractive. 
Pending further clinical investigation, retapamulin may offer an alternative 
treatment for anaerobic skin infections in this era of increasing resistance.