Synthesis and anti-infective activities of novel nitrofurantoin derivatives
Tuberculosis (TB) continues to be in the top ten lethal diseases. In 2018, it was responsible for 1.5 million deaths. Mycobacterium tuberculosis (Mtb) is the bacterium responsible for TB infection in humans. It is estimated that a third of the world’s population is infected with latent TB (dormant infection), which serves as a reservoir for new active TB infections to occur. In latent TB, the mycobacteria survive under anaerobic and nutrient deficient conditions. Current anti-mycobacterial treatments are lengthy (a minimum of six to nine months depending on drug susceptibility) and involve a cocktail of drugs. The opportunistic development of resistant Mtb strains, the toxicity and adverse effects of current regimens, as well as latent TB, put importunate urgency on the need for drugs with activity against all active and latent infections. Furthermore, effective and low cost oral drugs that can reduce treatment duration and achieve mycobacterial clearance would have a positive impact on patient compliance which would reduce the spread of the disease as well as progression of drug susceptible to drug resistant Mtb strains. Leishmaniasis disease burden tallies up to 350 million people at risk of infection, between 700 000 - 1 million new infection cases and up to 30 000 fatalities, annually. Of the many leishmanial species that can infect humans, L. major, and Leishmania (L) donovani responsible for cutaneous and visceral leishmaniasis, respectively, present the biggest concerns. Current anti-leishmanial therapies are unsatisfactory as they: (i) provide variable curative results, (ii) are too expensive, (iii) are impractical and inaccessible (since they are administered by intravenous and intramuscular injections), (iv) are not effective for all Leishmania species, and (v) are toxic. Therefore, there is a need for affordable oral drugs that would be effective and curative against both promastigote (infective) and amastigote (clinical and symptomatic) forms of leishmaniasis. In 2017, malaria burden accounted for 219 million new cases and 435 000 deaths. Plasmodium (P.) falciparum which is responsible for the largest percentage of malaria morbidity and mortality has the ability to quickly develop drug resistance. Artemisinins are the cornerstone of malaria treatment. Indeed, the treatment of both uncomplicated and complicated malaria infections relies on artemisinin drugs. Artemisinins modulate parasite oxidative stress through the generation of reactive oxygen species (ROS) and reduce the levels of antioxidants and glutathione in the parasite which lead to its death. However, partial resistance to artemisinins has been reported in South-East Asia. The threat of widespread artemisinin resistance and the lack of alternative antimalarial drugs, amplify the urgency for the discovery and development of new chemotherapeutic agents that can (i) quickly and efficiently clear parasitaemia, (ii) block transmission, and (iii) deter the emergence of drug resistant strains, either by novel mode of action or by disrupting multiple crucial metabolic processes in the parasite. In this study, nitrofurantoin (NFT) was investigated as a potential anti-infective nitroaromatic agent against mycobacterial, leishmanial and plasmodial infections. NFT is a staple urinary tract infection (UTI) drug that has multi-target activity. It overwhelms pathogens by attacking multiple critical metabolic pathways, including DNA replication, translation, transcription, as well as the Krebs cycle. Furthermore, it is active under both aerobic and anaerobic conditions. The anti-pathogenic effect occurs following a step-wise process involving activation by azoreduction, followed by nitroreduction. Azoreduction yields stable metabolites that have the ability to covalently bind to cellular proteins. Nitroreduction, on the other hand, occurs either by type I (anaerobic) or II (aerobic) reduction of the nitro group in the presence of pathogenic NADPH-cytochrome P450 reductases, producing toxic anti-pathogenic hydroxylamines and oxidative stress, respectively. Akin to rifampicin, a first-line TB drug, NFT is active against both replicating and non-replicating mycobacteria. Despite the success of combination therapy involving nifurtimox (an NFT sister drug) in the treatment of human African trypanosomiasis (HAT) and its use in the treatment of Chagas’ disease, nitroaromatic compounds, in general, have not been used for the treatment of leishmaniasis. Similar to HAT and Chagas, leishmaniasis is also a kinetoplastid disease. NFT has similar anti-pathogenic effects as artemisinins in that it generates ROS. However, nitroaromatics, including NFT, have rarely been investigated for malaria treatment. Furthermore, cases of NFT resistance in pathogens are rare, likely due to its multi-activity, as well as effectiveness under both aerobic and anaerobic conditions. Triazoles are the building blocks for different anti-infective drugs; hence they are often used in molecular hybridisation drug design. Chemical properties such as strong dipole moments, bioisosteric effects, hydrogen bonding and their high affinity to bind with biological targets amplify their importance in medicinal chemistry. In this study, aryl and n-alkyl NFT analogues (Series 1) as well as NFT-triazole hybrids (Series 2 to 4) were investigated as potential anti-infective agents. The NFT analogues were synthesised using single step nucleophilic substitution reactions. The NFT-triazole hybrids were synthesised by molecular hybridisation with aliphatic (Series 2 and 3) and aryl (series 4) linkers between NFT and 1,2,3-triazole, using click chemistry. All compounds were isolated by recrystallisation. The analogues had improved solubility and safety profiles as well as potent anti-infective activity, as evident from improved lipophilicity, low cytotoxicity, enhanced anti-mycobacterial, anti-leishmanial and anti-plasmodial potency. Analogue 113, a twelve carbon aliphatic chain was the most active compound across all the tested pathogens. The NFT-triazole hybrids had pronounced cytotoxicity, with aryl-linked hybrids being the most toxic to mammalian cells. Furthermore, the hybrids showed varying and poor anti-mycobacterial and anti-plasmodial activity. The hybrids, particularly series 2, performed better as potential anti-leishmanial agents, predominantly against L. major, the causal agent for cutaneous leishmaniasis. In summary, molecular derivatisation of NFT was an effective strategy for improving its drug-like properties such as solubility and lipophilicity. Furthermore, NFT analogues performed best as potential anti-infective agents, as evident from good safety profiles, and potent anti-infective activity against all three tested pathogens, compared to the parent drug (NFT), and the NFT-triazole hybrids. Analogue 113 was identified as an anti-infective hit for further investigation in the urgent search for new, safe and affordable drugs. By contrast molecular hybridisation did not yield a favourable outcome. The introduction of the triazole produced hybrids with pronounced toxicity towards mammalian cells. Thus, 1,2,3-triazole did not stand as a viable bioactive companion for enhancement of the potency and safety profiles of nitrofurantoin.
- Health Sciences