Efficacy of iclaprim against wild-type and thymidine kinase-deficient methicillin-resistant Staphylococcus aureus isolates in an in vitro fibrin clot model

The effects of iclaprim on exotoxin production in methicillin-resistant and vancomycin-intermediate Staphylococcus aureus

PURPOSE

Extracellular protein toxins contribute to the pathogenesis of Staphylococcus aureus infections. The present study compared the effects of iclaprim and trimethoprim - two folic acid synthesis inhibitors - with nafcillin and vancomycin on production of Panton-Valentine leukocidin (PVL), alpha haemolysin (AH) and toxic-shock syndrome toxin I (TSST-1) in methicillin-resistant and vancomycin-intermediate S. aureus (MRSA and VISA, respectively).

METHODOLOGY

Northern blotting and RT-PCR were used to assess gene transcription; toxin-specific bioassays were used to measure protein toxin production.

RESULTS

As shown previously, sub-inhibitory concentrations (sub-MIC) of nafcillin increased and prolonged MRSA toxin gene transcription and enhanced PVL, TSST-1 and AH production. Sub-inhibitory doses of iclaprim and trimethoprim delayed maximal AH gene (hla) transcription and suppressed AH production; both drugs delayed, but neither reduced, maximal TSST-1 production. Trimethoprim significantly increased lukF-PV expression and PVL production compared to both untreated and iclaprim-treated cultures. Higher concentrations of iclaprim and trimethoprim markedly suppressed MRSA growth, mRNA synthesis and toxin production. In VISA, iclaprim, vancomycin and nafcillin variably increased tst and hla expression, but only nafcillin increased toxin production. Despite its ability to increase hla expression, iclaprim was the most potent inhibitor of AH production.

CONCLUSIONS

We conclude that, due to its ability to suppress toxin production, iclaprim should be effective against severe staphylococcal infections caused by toxin-producing MRSA and VISA strains, especially given its ability to concentrate at sites of infection such as skin and skin structures and the lung.

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.

Identification of the In Vivo Pharmacokinetics and Pharmacodynamic Driver of Iclaprim

The neutropenic murine thigh infection model was used to define the pharmacokinetic/pharmacodynamic index linked to efficacy of iclaprim against Staphylococcus aureus ATCC 29213 and Staphylococcus pneumoniae ATCC 10813. The 24-h area under the curve (AUC)/MIC index was most closely linked to efficacy for S. aureus (R², 0.65), while both the 24-h AUC/MIC and the percentage of time that drug concentrations remain above the MIC (%T>MIC) were strongly associated with effect (R², 0.86 for both parameters) for S. pneumoniae.

Defeating Antibiotic- and Phage-Resistant Enterococcus faecalis Using a Phage Cocktail in Vitro and in a Clot Model

The deteriorating effectiveness of antibiotics is propelling researchers worldwide towards alternative techniques such as phage therapy: curing infectious diseases using viruses of bacteria called bacteriophages. In a previous paper, we isolated phage EFDG1, highly effective against both planktonic and biofilm cultures of one of the most challenging pathogenic species, the vancomycin-resistant Enterococcus (VRE). Thus, it is a promising phage to be used in phage therapy. Further experimentation revealed the emergence of a mutant resistant to EFDG1 phage: EFDG1^r. This kind of spontaneous resistance to antibiotics would be disastrous occurrence, however for phage-therapy it is only a minor hindrance. We quickly and successfully isolated a new phage, EFLK1, which proved effective against both the resistant mutant EFDG1^r and its parental VRE, Enterococcus faecalis V583. Furthermore, combining both phages in a cocktail produced an additive effect against E. faecalis V583 strains regardless of their antibiotic or phage-resistance profile. An analysis of the differences in genome sequence, genes, mutations, and tRNA content of both phages is presented. This work is a proof-of-concept of one of the most significant advantages of phage therapy, namely the ability to easily overcome emerging resistant bacteria.

Efficacy evaluation of iclaprim in a neutropenic rat lung infection model with methicillin-resistant Staphylococcus aureus entrapped in alginate microspheres

The objective of this study was to demonstrate the efficacy of iclaprim in a neutropenic rat lung infection model with methicillin-resistant Staphylococcus aureus (MRSA) entrapped in alginate beads. An inoculum of 5.25 × 10⁵ colony-forming units (CFU)/mL of S. aureus strain AH1252 was administered intratracheally to rats with prepared alginate bacteria suspensions. Beginning 2 h post-infection, rats received: (1) iclaprim 80 mg/kg (n = 16); (2) iclaprim 60 mg/kg (n = 16), or (3) vancomycin 50 mg/kg (n = 24), for 3 days via subcutaneous (SC) injection every 12 h. Twelve hours after the last treatment, rats were euthanized and lungs collected for CFU determination. Iclaprim administered at 80 mg/kg or 60 mg/kg or vancomycin 50 mg/kg SC twice a day for 3 days resulted in a 6.05 log₁₀ CFU reduction (iclaprim 80 mg/kg compared with control, p < 0.0001), 5.11 log₁₀ CFU reduction (iclaprim 60 mg/kg compared with control, p < 0.0001), and 3.42 log₁₀ CFU reduction, respectively, from the controls (p < 0.0001). Iclaprim 80 mg/kg and 60 mg/kg resulted in 2.59 and 1.69 log₁₀ CFU reductions, respectively, from vancomycin-treated animals (80 mg/kg iclaprim vs. vancomycin, p = 0.0005; 60 mg/kg iclaprim vs. vancomycin, p = 0.07). Animals receiving iclaprim, vancomycin, and controls demonstrated 100%, 91.7%, and 48.3% survival, respectively. In this neutropenic rat S. aureus lung infection model, rats receiving iclaprim demonstrated a greater CFU reduction than the controls or those receiving vancomycin.

Synergistic Interaction Between Phage Therapy and Antibiotics Clears Pseudomonas Aeruginosa Infection in Endocarditis and Reduces Virulence

Background.

Increasing antibiotic resistance warrants therapeutic alternatives. Here we investigated the efficacy of bacteriophage-therapy (phage) alone or combined with antibiotics against experimental endocarditis (EE) due to Pseudomonas aeruginosa, an archetype of difficult-to-treat infection.

Methods.

In vitro fibrin clots and rats with aortic EE were treated with an antipseudomonas phage cocktail alone or combined with ciprofloxacin. Phage pharmacology, therapeutic efficacy, and resistance were determined.

Results.

In vitro, single-dose phage therapy killed 7 log colony-forming units (CFUs)/g of fibrin clots in 6 hours. Phage-resistant mutants regrew after 24 hours but were prevented by combination with ciprofloxacin (2.5 × minimum inhibitory concentration). In vivo, single-dose phage therapy killed 2.5 log CFUs/g of vegetations in 6 hours (P < .001 vs untreated controls) and was comparable with ciprofloxacin monotherapy. Moreover, phage/ciprofloxacin combinations were highly synergistic, killing >6 log CFUs/g of vegetations in 6 hours and successfully treating 64% (n = 7/11) of rats. Phage-resistant mutants emerged in vitro but not in vivo, most likely because resistant mutations affected bacterial surface determinants important for infectivity (eg, the pilT and galU genes involved in pilus motility and LPS formation).

Conclusions.

Single-dose phage therapy was active against P. aeruginosa EE and highly synergistic with ciprofloxacin. Phage-resistant mutants had impaired infectivity. Phage-therapy alone or combined with antibiotics merits further clinical consideration.