In Vitro Activity of CEM-101 against Streptococcus pneumoniae and Streptococcus pyogenes with Defined Macrolide Resistance Mechanisms

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Abstract
CEM-101 had MIC ranges of 0.002 to 0.016 μg/ml against macrolide-susceptible pneumococci and 0.004 to 1 μg/ml against macrolide-resistant phenotypes. Only 3 strains with erm (B), with or without mef (A), had CEM-101 MICs of 1 μg/ml, and 218/221 strains had CEM-101 MICs of ≤0.5 μg/ml. CEM-101 MICs were as much as 4-fold lower than telithromycin MICs against all strains. For Streptococcus pyogenes , CEM-101 MICs ranged from 0.008 to 0.03 μg/ml against macrolide-susceptible strains and from 0.015 to 1 μg/ml against macrolide-resistant strains. Against erm (B) strains, erythromycin, azithromycin, and clarithromycin MICs were 32 to >64 μg/ml, while 17/19 strains had telithromycin MICs of 4 to 16 μg/ml; CEM-101 MICs were 0.015 to 1 μg/ml. By comparison, erm (A) and mef (A) strains had CEM-101 MICs of 0.015 to 0.5 μg/ml, clindamycin and telithromycin MICs of ≤1 μg/ml, and erythromycin, azithromycin, and clarithromycin MICs of 0.5 to >64 μg/ml. Pneumococcal multistep resistance studies showed that although CEM-101 yielded clones with higher MICs for all eight strains tested, seven of eight strains had clones with CEM-101 MICs that rose from 0.004 to 0.03 μg/ml (parental strains) to 0.06 to 0.5 μg/ml (resistant clones); for only one erm (B) mef (A) strain with a parental MIC of 1 μg/ml was there a resistant clone with a MIC of 32 μg/ml, with no detectable mutations in the L4, L22, or 23S rRNA sequence. Among two of five S. pyogenes strains tested, CEM-101 MICs rose from 0.03 to 0.25 μg/ml, and only for the one strain with erm (B) did CEM-101 MICs rise from 1 to 8 μg/ml, with no changes occurring in any macrolide resistance determinant. CEM-101 had low MICs as well as low potential for the selection of resistant mutants, independent of bacterial species or resistance phenotypes in pneumococci and S. pyogenes .

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