Development of Methionyl-tRNA Synthetase Inhibitors as Antibiotics for Gram-Positive Bacterial Infections

Abstract

Antibiotic-resistant bacteria are widespread and pose a growing threat to human health. New antibiotics acting by novel mechanisms of action are needed to address this challenge. The bacterial methionyl-tRNA synthetase (MetRS) enzyme is essential for protein synthesis, and the type found in Gram-positive bacteria is substantially different from its counterpart found in the mammalian cytoplasm. Both previously published and new selective inhibitors were shown to be highly active against Gram-positive bacteria with MICs of ≤1.3 μg/ml against Staphylococcus, Enterococcus, and Streptococcus strains. Incorporation of radioactive precursors demonstrated that the mechanism of activity was due to the inhibition of protein synthesis. Little activity against Gram-negative bacteria was observed, consistent with the fact that Gram-negative bacterial species contain a different type of MetRS enzyme. The ratio of the MIC to the minimum bactericidal concentration (MBC) was consistent with a bacteriostatic mechanism. The level of protein binding of the compounds was high (>95%), and this translated to a substantial increase in MICs when the compounds were tested in the presence of serum. Despite this, the compounds were very active when they were tested in a Staphylococcus aureus murine thigh infection model. Compounds 1717 and 2144, given by oral gavage, resulted in 3- to 4-log decreases in the bacterial load compared to that in vehicle-treated mice, which was comparable to the results observed with the comparator drugs, vancomycin and linezolid. In summary, the research describes MetRS inhibitors with oral bioavailability that represent a class of compounds acting by a novel mechanism with excellent potential for clinical development.

Keywords: Enterococcus; Gram-positive bacteria; Staphylococcus aureus; antibiotic resistance; drug discovery; methionyl-tRNA synthetase.

FIG 1

Chemical structures of MetRS inhibitors.

FIG 2

Macromolecular synthesis experiments. The level of incorporation of radiolabeled precursors into S. aureus (ATCC 29213) was measured in 30-min incubations in the presence of established antibiotics or MetRS inhibitors. Dashed vertical lines, MICs.

FIG 3

Efficacy of MetRS inhibitors in a neutropenic mouse model of S. aureus thigh infection. Error bars are SEMs. The stasis level was determined from untreated mice sacrificed at 1 h postinfection (p.i.). Vanco, vancomycin.