Revisiting the interaction of the radical anion metabolite of nitrofurantoin with glutathione

Stimulation of de novo glutathione synthesis by nitrofurantoin for enhanced resilience of hepatocytes

Toxicity is not only a function of damage mechanisms, but is also determined by cellular resilience factors. Glutathione has been reported as essential element to counteract negative influences. The present work hence pursued the question how intracellular glutathione can be elevated transiently to render cells more resistant toward harmful conditions. The antibiotic nitrofurantoin (NFT) was identified to stimulate de novo synthesis of glutathione in the human hepatoma cell line, HepG2, and in primary human hepatocytes. In intact cells, activation of NFT yielded a radical anion, which subsequently initiated nuclear-factor-erythroid 2-related-factor-2 (Nrf2)-dependent induction of glutamate cysteine ligase (GCL). Application of siRNA-based intervention approaches confirmed the involvement of the Nrf2-GCL axis in the observed elevation of intracellular glutathione levels. Quantitative activation of Nrf2 by NFT, and the subsequent rise in glutathione, were similar as observed with the potent experimental Nrf2 activator diethyl maleate. The elevation of glutathione levels, observed even 48 h after withdrawal of NFT, rendered cells resistant to different stressors such as the mitochondrial inhibitor rotenone, the redox cycler paraquat, the proteasome inhibitors MG-132 or bortezomib, or high concentrations of NFT. Repurpose of the antibiotic NFT as activator of Nrf2 could thus be a promising strategy for a transient and targeted activation of the endogenous antioxidant machinery. Graphical abstract.

Single-step synthesis and in vitro anti-mycobacterial activity of novel nitrofurantoin analogues

The emergence of drug-resistant tuberculosis (DR-TB) as well as the requirement for long, expensive and toxic drug regimens impede efforts to control and eliminate TB. Therefore, there's a need for effective and affordable anti-mycobacterial agents which can shorten the duration of therapy and are active against Mycobacterium tuberculosis (Mtb) in both active and latent phases. Nitrofurantoin (NFT) is a hypoxic agent with activity against a myriad of anaerobic pathogens and, like the first-line TB drug, rifampicin (RIF), kills non-replicating bacilli. However, the poor ability of NFT to cross host cell membranes and penetrate tissue means that it does not reach therapeutic concentrations. To improve TB efficacy of NFT, a series of NFT analogues was synthesized and evaluated in vitro for anti-mycobacterial activity against the laboratory strain, Mtb H37Rv, and for potential cytotoxicity using human embryonic kidney (HEK-293) and Chinese hamster ovarian (CHO) cells. The NFT analogues showed good safety profiles, enhanced anti-mycobacterial potency, improved lipophilicity, as well as reduced protein binding affinity. Analogue 9 which contains an eight carbon aliphatic chain was the most active, equipotent to isoniazid (INH), a major front-line agent, with MIC₉₀ = 0.5 μM, 30-fold more potency than the parent drug, nitrofurantoin (MIC₉₀ = 15 μM), and 100-fold more selective towards mycobacteria. Therefore, 9 was identified as a validated hit for further investigation in the urgent search for new, safe and affordable TB drugs.

Phototransformation of 5-nitro-2-furaldehyde in aqueous solution. A laser flash photolysis and product analysis study

Laser flash photolysis of 5-nitro-2-furaldehyde (NFA) in solution shows a short-lived transient absorption with λ_max = 475 ± 5 nm, which is relatively insensitive to solvent polarity and is assigned to the lowest triplet state of NFA (³NFA*). In water, the ³NFA* absorption decays to a long-lived absorption, the study of which, at different times after the end of the laser pulse, reveals it to be due to a furyloxyl radical (λ_max ≈ 375 nm) and to the radical anion NFA˙^- (λ_max ≈ 400 nm). These radicals were produced independently to confirm the assignment. The lifetime of ³NFA* depends both on the solvent and the ground-state concentration of NFA. An (n, π*) nature is attributed to ³NFA* on the basis of the propensity of ³NFA* to abstract a hydrogen-atom from the solvent. Kinetic evidence for triplet excimer formation was obtained from the self-quenching of ³NFA* in solvents where the triplet decay is slower. The effect of acidity on the triplet lifetime is discussed with respect to an electron-transfer self-quenching mechanism, assisted by the triplet excimer which is proposed to dissociate into radical ions. Chromatographic and spectroscopic analysis of the photolysed aqueous solution of NFA enabled the identification of 5-hydroxymethylene-2(5H)-furanone, nitrite ion and an unknown substance as the major photoproducts. Conclusive evidence is presented that the observed 5-hydroxymethylene-2(5H)-furanone is formed from the furyloxyl radical. It is shown that the unknown substance can also be obtained from both the photoreduction of NFA in propan-2-ol and chemical reduction of NFA by Fe(s) in water (along with 5-amino-2-furaldehyde). Based on ¹H- and ¹³C-NMR (with 2-D HMQC) and vibrational absorption spectroscopy, a tentative structure is proposed for the substance of t_R 3.69 minutes obtained as a photoreduction product of NFA in water. Inorganic anions are shown to be one-electron oxidised by ³NFA* (as indicated by the observation of both the radical anion of NFA and the inorganic radical) with second-order rate constants being dependent on E of the inorganic radical. The implications of the results from complete quenching of ³NFA* by inorganic anions, and subsequent product analysis, for the phototransformation mechanism of NFA in water are discussed.

A comparison of oral and intravenous pimonidazole in canine tumors using intravenous CCI-103F as a control hypoxia marker

PURPOSE

Pimonidazole HCl is widely used in immunohistochemical analyses of hypoxia in normal and malignant tissues. The present study investigates oral administration as a means of minimizing invasiveness.

METHODS AND MATERIALS

Twelve dogs with confirmed malignancy received 0.5 g/m2 of pimonidazole HCl: 6 by mouth and 6 by i.v. infusion. All dogs received i.v. CCI-103F as a control. Plasma levels of pimonidazole, pimonidazole N-oxide, and CCI-103F were measured. Tumor biopsies were formalin fixed, paraffin embedded, sectioned, immunostained, and analyzed for pimonidazole and CCI-103F binding. pH dependence for pimonidazole and CCI-103F binding was studied in vitro.

RESULTS

Pimonidazole and CCI-103F binding in carcinomas and sarcomas was strongly correlated for both oral and i.v. pimonidazole HCl (r2=0.97). On average, the extent of pimonidazole binding exceeded that for CCI-103F by a factor of approximately 1.2, with the factor ranging from 1.0 to 1.65. Binding of both markers was pH dependent, but pimonidazole binding was greater at all values of pH.

CONCLUSIONS

Oral pimonidazole HCl is effective as a hypoxia marker in spontaneously arising canine tumors. Selective cellular uptake and concomitant higher levels of binding in regions of hypoxia at the high end of pH gradients might account for the greater extent of pimonidazole binding.

Staphylococcus aureus NfrA (SA0367) is a flavin mononucleotide-dependent NADPH oxidase involved in oxidative stress response

The NfrA protein, a putative essential oxidoreductase in the soil bacterium Bacillus subtilis, is induced under heat shock and oxidative stress conditions. In order to characterize the function of an homologous NfrA protein in Staphylococcus aureus, an nfrA deletion strain was constructed, the protein was purified, the enzymatic activity was determined, and the transcriptional regulation was investigated. The experiments revealed that NfrA is not essential in S. aureus. The purified protein oxidized NADPH but not NADH, producing NADP in the presence of flavin mononucleotide, suggesting that NfrA is an NADPH oxidase in S. aureus. In addition, the NfrA enzyme showed nitroreductase activity and weak disulfide reductase activity. Transcription was strongly induced by ethanol, diamide, and nitrofurantoin. Hydrogen peroxide induced nfrA transcription only at high concentrations. The expression of nfrA was independent of the alternative sigma factor sigma(B). Furthermore, the transcriptional start site was determined, which allowed identification of a PerR box homologous sequence upstream of the nfrA promoter. The observations presented here suggest that NfrA is a nonessential NADPH oxidoreductase which may play a role in the oxidative stress response of S. aureus, especially in keeping thiol-disulfide stress in balance.