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Published on: November 5, 2021
Evaluating 5-nitrofurans as trypanocidal agents
Christopher Bot1, Belinda S Hall, Guzmán Alvarez
1Queen Mary Pre-Clinical Drug Discovery Group, School of Biological and Chemical Sciences, Queen Mary University of London, London, United Kingdom.
Abstract:
The nitroheterocycle nifurtimox, as part of a nifurtimox-eflornithine combination therapy, represents one of a limited number of treatments targeting Trypanosoma brucei, the causative agent of human African trypanosomiasis. The mode of action of this prodrug involves an initial activation reaction catalyzed by a type I nitroreductase (NTR), an enzyme found predominantly in prokaryotes, leading to the formation of a cytotoxic unsaturated open-chain nitrile metabolite. Here, we evaluate the trypanocidal activities of a library of other 5-nitrofurans against the bloodstream form of T. brucei as a preliminary step in the identification of additional nitroaromatic compounds that can potentially partner with eflornithine. Biochemical screening against the purified enzyme revealed that all 5-nitrofurans were effective substrates for T. brucei NTR (TbNTR), with the preferred compounds having apparent kcat/Km values approximately 50-fold greater than those of nifurtimox. For several compounds, in vitro reduction by this nitroreductase yielded products characterized by mass spectrometry as either unsaturated or saturated open-chain nitriles. When tested against the bloodstream form of T. brucei, many of the derivatives displayed significant growth-inhibitory properties, with the most potent compounds generating 50% inhibitory concentrations (IC50s) around 200 nM. The antiparasitic activities of the most potent agents were demonstrated to be NTR dependent, as parasites having reduced levels of the enzyme displayed resistance to the compounds, while parasites overexpressing TbNTR showed hypersensitivity. We conclude that other members of the 5-nitrofuran class of nitroheterocycles have the potential to treat human African trypanosomiasis, perhaps as an alternative partner prodrug to nifurtimox, in the next generation of eflornithine-based combinational therapies.
Insights
New 5-nitrofuran compounds show potent activity against Trypanosoma brucei, the parasite causing African sleeping sickness. These nitroheterocycles, activated by a specific nitroreductase enzyme, could become next-generation partners in eflornithine combination therapies.
Area of Science:
- Medicinal Chemistry
- Parasitology
- Drug Discovery
Background:
- Human African trypanosomiasis (HAT) treatment options are limited, with nifurtimox-eflornithine combination therapy being a key strategy.
- Nifurtimox, a nitroheterocycle prodrug, requires activation by a type I nitroreductase (NTR) to generate cytotoxic metabolites.
- Identifying new nitroaromatic compounds is crucial for developing improved HAT therapies.
Purpose of the Study:
- To evaluate the trypanocidal activities of various 5-nitrofuran derivatives against Trypanosoma brucei.
- To identify novel nitroaromatic compounds that can serve as potential partners with eflornithine in HAT treatment.
- To understand the role of Trypanosoma brucei nitroreductase (TbNTR) in the activation and efficacy of these compounds.
Main Methods:
- Biochemical screening of 5-nitrofurans using purified TbNTR to determine substrate efficacy (kcat/Km).
- Mass spectrometry to characterize the in vitro reduction products of selected nitrofurans by TbNTR.
- In vitro testing of 5-nitrofuran derivatives against bloodstream-form T. brucei to assess growth-inhibitory properties (IC50).
- Genetic manipulation of T. brucei to alter TbNTR levels (knockdown and overexpression) and assess drug resistance/hypersensitivity.
Main Results:
- All tested 5-nitrofurans were substrates for TbNTR, with some exhibiting significantly higher catalytic efficiency (kcat/Km) than nifurtimox.
- In vitro reduction yielded unsaturated and saturated open-chain nitriles, consistent with the proposed activation mechanism.
- Many 5-nitrofuran derivatives demonstrated potent trypanocidal activity, with IC50 values as low as approximately 200 nM.
- Antiparasitic activity was confirmed to be TbNTR-dependent, with reduced enzyme levels conferring resistance and overexpression leading to hypersensitivity.
Conclusions:
- Other 5-nitrofuran derivatives possess significant potential as trypanocidal agents.
- These compounds can be activated by TbNTR, similar to nifurtimox.
- The 5-nitrofuran class represents a promising source for next-generation nitroaromatic prodrugs to partner with eflornithine in HAT combination therapy.
