In vitro activity of dihydrofolate reductase inhibitors and other antibiotics against Gram-positive pathogens collected globally between 2004 and 2016

The activity of the diaminopyrimidine dihydrofolate reducatase inhibitor, iclaprim, against Toxoplasma gondii in an in vitro model: a pilot study

The objective of this pilot study was to examine the activity of iclaprim, a diaminopyrimidine dihydrofolate reducatase inhibitor, in an in vitro infection model of infection with Toxoplasma gondii. Toxoplasma growth was assessed by enzyme linked immunoassay (ELISA) performed directly on the fixed cultures using a peroxidase labeled monoclonal antibody directed against the SAG-l surface protein of T. gondii. For each well, the results were expressed as optical density (OD) values. Iclaprim inhibited T. gondii growth at concentrations between 0.1 and 10 mg/L; the IC₅₀ was estimated at 0.26 mg/L (95% confidence interval 0.22-0.33). Iclaprim was about 10 times more active than trimethoprim, which had an IC₅₀ of 2.3 mg/L. Iclaprim demonstrated synergistic effects at concentrations of 0.02, 0.05 and 0.1 mg/L when combined with subinhibitory concentrations of sulfamethoxazole (0.1 or 0.02 mg/L). These results show that iclaprim is a potent inhibitor of T. gondii growth in vitro. In addition, iclaprim exhibited synergy in vitro when tested in the presence of sulfamethoxazole. Iclaprim should be further investigated as an agent for the treatment or prophylaxis of toxoplasmosis.

Evaluation of in vitro activity of iclaprim in combination with other antimicrobials against pulmonary pathogens: a pilot study

In this pilot study, the in vitro antimicrobial activity of iclaprim, a diaminopyrimidine, tested in combination with other antimicrobials against recent and common Gram-positive and Gram-negative respiratory pathogens, was examined by the checkerboard method. The range of minimal inhibitory concentrations (MICs) for iclaprim against all bacteria tested in the study was 0.03 to >128 µg ml⁻¹. Iclaprim exhibited synergy with sulfamethoxazole against 11 of the 16 bacterial strains tested, with mean fractional inhibitory concentration index (FICI) values of 0.2–0.5. Synergy with sulfamethoxazole was demonstrated against all Gram-positive bacteria and selected Gram-negative bacteria. Neither synergy nor antagonism was observed for combinations of iclaprim with ampicillin, meropenem, tetracycline, levofloxacin, aztreonam, piperacillin/tazobactam, colistin, cefepime or gentamicin against any of the bacterial strains tested. The significant reduction in the MIC values observed with the combination of iclaprim and sulfamethoxazole demonstrates that this regimen could be effective against common Gram-positive and selected Gram-negative respiratory bacteria.

Iclaprim activity against wild-type and corresponding thymidine kinase-deficient Staphylococcus aureus in a mouse protection model

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.