The in vitro and in vivo antimicrobial activities of dihydrofolate reductase (DHFR) inhibitors are inhibited in the presence of free thymidine in the growth milieu and in rodent efficacy models. However, for thymidine kinase (TK) deficient mutant bacteria, the presence of free thymidine does not impact the activity of DHFR inhibitors, and these mutants were used to assess the in vivo efficacy of the DHFR inhibitor, iclaprim. The efficacies of iclaprim, trimethoprim, and vancomycin were evaluated in a systemic mouse infection model. Female CD-1 mice were infected intraperitoneally (IP) with wild-type Staphylococcus aureus ATCC 25923 (MSSA) or AW 6 (MRSA) or their corresponding isogenic TK-deficient mutant S. aureus strains AH 1246 and AH 1252. Iclaprim showed potent antibacterial activity against both the TK-deficient mutant S. aureus strains, with PD50 values of 1.8 and < 0.5 mg/kg, respectively, for strains AH 1246 and AH 1252. In contrast, poor antibacterial activity was observed against corresponding wild-type (TK competent) S. aureus strains, with PD50 values of 10.8 and 2.2 mg/kg, respectively, for strains ATCC 25923 and AW 6. This study confirms that thymidine plays an important antagonistic role when determining the efficacy of DHFR inhibitors in vivo. This is the first study to show that iclaprim is active against TK-deficient S. aureus strains in a systemic mouse infection model, and that TK-deficient mutants may be used to evaluate iclaprim’s activity in rodent models in vivo.
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This study was funded by Motif BioSciences, Inc., New York, NY, USA.
Conflict of interest
DBH is an employee of Motif BioSciences. TM is an employee of NeoSome Life Sciences.
This research involved animals. All procedures in this research were in compliance with the Animal Welfare Act, the Guide for the Care and Use of Laboratory Animals, and the Office of Laboratory Animal Welfare.
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Huang, D.B., Park, J.H. & Murphy, T.M. Iclaprim activity against wild-type and corresponding thymidine kinase–deficient Staphylococcus aureus in a mouse protection model. Eur J Clin Microbiol Infect Dis 38, 409–412 (2019). https://doi.org/10.1007/s10096-018-3440-2