Dihydrofolate reductase inhibitors as antimicrobial agents and their potentiation by sulfonamides

Metabolic interventions for the prevention and treatment of daptomycin non-susceptibility in Staphylococcus aureus

BACKGROUND

A major developing problem in the treatment of Staphylococcus aureus infections is the emergence of resistance during treatment with daptomycin. Previous metabolomic analyses of isogenic S. aureus strains prior to and after evolution into a daptomycin non-susceptible (DapNS) state provided important metabolic information about this transition (e.g. perturbations of the tricarboxylic acid cycle).

OBJECTIVES

To assess the significance of these metabolic changes, in vitro susceptibility to daptomycin was determined in daptomycin-susceptible (DapS) and DapNSS. aureus strains cultivated with metabolic inhibitors targeting these changes.

METHODS

Only inhibitors that are approved for use in humans were chosen (i.e. fosfomycin, valproate, trimetazidine and 6-mercaptopurine) to assess the importance of metabolic pathways for daptomycin non-susceptibility. The ability of these inhibitors to forestall the emergence of DapNS strains was also assessed.

RESULTS

The combination of daptomycin and fosfomycin synergistically killed both DapS and DapNS strains in vitro and enhanced the in vivo outcome against a DapNS strain in experimental endocarditis. Interestingly, fosfomycin acts on the peptidoglycan biosynthetic enzyme UDP-N-acetylglucosamine enolpyruvyl transferase (MurA); however, it also had a significant effect on the enzymatic activity of enolase, an essential enzyme in S. aureus. While fosfomycin acted synergistically with daptomycin, it failed to prevent the in vitro evolution of daptomycin non-susceptibility. In contrast, trimetazidine, an anti-angina drug that stimulates glucose oxidation, abolished the ability of DapSS. aureus strains to transition to a DapNS state.

CONCLUSIONS

These data reveal that metabolic adaptations associated with DapNS strains can be targeted to prevent the emergence of and/or reverse pre-existing resistance to daptomycin.

Cotrimoxazole and neonatal kernicterus: a review

Sulfamethoxazole (SMX) and trimethoprim (TMP) individually and a combination known as cotrimoxazole (SMX-TMP) are widely used for the treatment of protozoan and bacterial infections. SMX-TMP is also one of the widely used antibiotics administered orally in neonates, along with gentamicin injection, for treating pneumonia and sepsis by home-based healthcare providers in Asian countries. Although the use of this drug has successfully reduced neonate mortality, there is a concern for it causing neurotoxicity. Previous clinical studies with sulfisoxazole have demonstrated occurrence of kernicterus in neonates. This sulfonamide is thought to displace bilirubin from its albumin-binding sites in plasma leading to an elevation of plasma bilirubin, which crosses the blood-brain barrier, reaches central neurons to cause kernicterus. We performed an extensive review of clinical and animal studies with cotrimoxazole, which showed no reported incidences of kernicterus with SMX-TMP use in neonates. EndNote, BasicBiosis, Embase, PubMed and Toxline database searches were conducted using specific keywords yielding 74 full-length articles relevant to the review. This review has taken into account various factors, including the disease itself, direct effects of the drug and its metabolism through conjugation and acetylation through a thorough review of the literature to examine the potentials of SMX-TMP to cause kernicterus in neonates. SMX-TMP in oral doses administered to neonates for 7-10 days is unlikely to cause kernicterus. Also, this review recommends warranting the need of future studies using animal models and clinical studies in humans to address SMX-TMP toxicity.

Modeling the evolution of drug resistance in malaria

Plasmodium falciparum, the causal agent of malaria, continues to evolve resistance to frontline therapeutics such as chloroquine and sulfadoxine-pyrimethamine. Here we study the amino acid replacements in dihydrofolate reductase (DHFR) that confer resistance to pyrimethamine while still binding the natural DHFR substrate, 7,8-dihydrofolate, and cofactor, NADPH. The chain of amino acid replacements that has led to resistance can be inferred in a computer, leading to a broader understanding of the coevolution between the drug and target. This in silico approach suggests that only a small set of specific active site replacements in the proper order could have led to the resistant strains in the wild today. A similar approach can be used on any target of interest to anticipate likely pathways of future resistance for more effective drug development.

High throughput screening identifies novel inhibitors of Escherichia coli dihydrofolate reductase that are competitive with dihydrofolate

This communication describes the high-throughput screen of a diverse library of 50,000 small molecules against Escherichia coli dihydrofolate reductase to detect inhibitors. Sixty-two compounds were identified as having significant inhibitory activity against the enzyme. Secondary screening of these revealed twelve molecules that were competitive with dihydrofolate, nine of which have not been previously characterized as inhibitors of dihydrofolate reductase. These novel molecules ranged in potency (K(i)) from 26 nM to 11 microM and may represent fresh starting points for new small molecule therapeutics directed against dihydrofolate reductase.