In vitro antibacterial activity of α-methoxyimino acylide derivatives against macrolide-resistant pathogens and mutation analysis in 23S rRNA.

We characterized in vitro activities of α-methoxyimino acylides against macrolide-resistant clinical isolates of Streptococcus pneumoniae, Streptococcus pyogenes and Mycoplasma pneumoniae with ribosome modification or substitution and selected acylide-resistant mutants to clarify the binding point of the acylides. The acylides had low MICs against erm(B) gene-containing S. pneumoniae and S. pyogenes (MIC90s, 1-4 μg ml-1). For M. pneumoniae, although they had poor potencies against macrolide-resistant strains with the A2058G (Escherichia coli numbering) mutation in 23S rRNA (MICs, >32 μg ml-1), one of them showed in vitro activities against macrolide-resistant strains with the A2058U or A2059G mutations (MICs, 0.5-1 μg ml-1). These A2058U and A2059G mutant strains were used for the selection of acylide-resistant mutants. A genetic analysis showed that new point mutations in acylide-resistant mutants were found at G2576 in domain V of 23S rRNA and at Lys90 in L22 ribosomal protein. Furthermore, a molecular modeling study revealed that G2505/C2610, which enables stacking with G2576, might interact with a pyridyl moiety or an α-methoxyimino group at the 3-position of acylides. The α-methoxyimino acylides were shown to possess a tertiary binding point at G2505/C2610 in 23S rRNA. Our results suggest that α-methoxyimino acylides represent significant progress in macrolide antimicrobials.