Benzothiazole Emerges as a Promising Weapon Against Drug-Resistant Malaria

Malaria remains one of the most stubborn infectious disease challenges of our time, and the arsenal of drugs available to fight it is steadily eroding. In 2023 alone, the world recorded an estimated 263 million cases and 597,000 deaths, with the African region shouldering 95 percent of the burden. Five species of the Plasmodium parasite infect humans, and among them Plasmodium falciparum is the deadliest. The current first-line treatment, artemisinin-based combination therapy, is increasingly compromised by partial resistance that first emerged in western Cambodia in 2008 and has since spread to parts of Africa and South America. Older drugs such as chloroquine and the sulfadoxine-pyrimethamine combination have long been undermined by widespread resistance. Against this backdrop, a new open-access review published in Discover Chemistry by Khyatirupa Sarma, Jun Moni Kalita and colleagues at institutions in Guwahati, India, makes a detailed case for an underappreciated chemical scaffold: benzothiazole, a fused bicyclic ring containing one nitrogen and one sulfur atom, which the authors argue deserves recognition as a privileged nucleus for inhibiting the parasite’s dihydrofolate reductase enzyme.

The target at the heart of the review is dihydrofolate reductase, or DHFR, an enzyme that has long been validated as essential to the parasite’s survival. DHFR catalyses the reduction of dihydrofolate to tetrahydrofolate using NADPH as an electron donor, a conversion that underpins the synthesis of purines, thymidylate and certain amino acids required for DNA replication and cell proliferation. When inhibitors occupy the enzyme’s active site, tetrahydrofolate levels collapse, nucleotide production grinds to a halt and the parasite can no longer divide. Established antifolate drugs such as pyrimethamine and cycloguanil exploit exactly this vulnerability, competitively binding the parasite enzyme and blocking the DHF-to-THF conversion. The problem is that resistance to these drugs has emerged through point mutations in the DHFR gene that alter the active site and reduce inhibitor binding affinity, which is why the search for new scaffolds capable of tolerating or evading these mutations has become so urgent.

The structural biology of the parasite enzyme explains both the promise and the difficulty of this endeavour. Plasmodium falciparum DHFR exists as a bifunctional enzyme coupled with thymidylate synthase, forming a homodimer with extensive interactions between the TS domains. Like all DHFR enzymes, it features a polypeptide fold built around an eight-stranded mixed beta-sheet flanked by alpha-helices. Crystallographic studies of the enzyme complexed with cofactors and inhibitors have revealed the intricate network of interactions at the active site, with residues such as Asp54 and Phe58 engaged in hydrogen bonding and pi-pi interactions that stabilise inhibitor binding. The enzyme is remarkably similar to its Plasmodium vivax counterpart, with roughly 66 percent sequence identity and the capacity to be superimposed with a root mean square deviation of about 0.87 angstroms, raising the possibility that inhibitors designed against one species could serve against both.

Resistance to classical antifolates has been mapped at atomic resolution, and the story is a textbook example of molecular evolution. X-ray crystallography shows that pyrimethamine and cycloguanil form key hydrogen bonds with conserved active-site residues including Asp54 and Ile164, along with a critical water-mediated hydrogen bond to the Ser108 side chain. The S108N mutation disrupts this precise interaction network: the asparagine substitution sterically occludes the inhibitor’s binding mode and eliminates the favourable hydrogen bond with the drug, while the natural substrate continues to be accommodated through alternative conformations. In the quadruple-mutant form of the enzyme, the steric hindrance imposed by Asn-108 is severe enough to defeat rigid inhibitors entirely. This structural insight has driven the design of next-generation antifolates with built-in flexibility, and the review devotes considerable attention to these efforts.

Among the next-generation inhibitors, WR99210, a 4,6-diamino-1,2-dihydro-1,3,5-triazine derivative, stands out. Its flexible (2,4,5-trichlorophenoxy)propoxy side chain allows the molecule to overcome the steric barrier imposed by Asn-108 in the quadruple-mutant enzyme, maintaining potency against both wild-type and pyrimethamine-resistant forms of PfDHFR. A related compound, PS-15, is metabolically converted in the body to WR99210 and is thought to offer improved absorption or reduced toxicity while preserving the active drug in vivo. The clinical candidate P218 takes the strategy further, binding tightly to both wild-type and mutant enzymes with high selectivity for the parasite protein and forming an additional hydrogen bond with the main chain of Cys15, an interaction that is largely insensitive to the resistance mutations. These molecules illustrate the design principle the review advocates: atomic-level precision in maintaining critical interactions within a rapidly evolving active site.

So where does benzothiazole fit in? The authors argue that this scaffold offers a distinct mechanism of action independent of the pathways compromised in artemisinin resistance, which involve mutations in PfKelch13 and enhanced efflux via PfCRT. Benzothiazole derivatives can bind free heme and inhibit heme polymerisation, disrupting the parasite’s haemoglobin detoxification pathway and leading to toxic heme accumulation, while also showing DHFR inhibition during erythrocytic infection. This dual capability means benzothiazole-based compounds could offer synergy potential in novel combinations and retain activity against multidrug-resistant strains. The review systematically surveys the in vitro and in vivo antiplasmodial activity of these compounds across multiple Plasmodium species, along with the synthetic chemistry and structure-activity relationships that govern their potency.

The synthetic accessibility of benzothiazole is one of its practical strengths. The classic route, first reported by Hugershoff in 1900, condenses 2-aminothiophenol with aldehydes or carboxylic acids, proceeding through imine formation, cyclodehydration to a benzothiazoline and air oxidation to the aromatic product, with yields typically between 70 and 90 percent. Oxidative cyclisation approaches using iodine, bromine, DDQ or copper catalysis offer milder conditions, while visible-light photocatalysis with ruthenium tris-bipyridine and oxygen as the terminal oxidant exemplifies modern efficiency. Green chemistry strategies push yields to 85 to 98 percent using microwave irradiation over zinc chloride or silica-supported reagents, solvent-free grinding with montmorillonite clay, or glycerol as a bio-based solvent. Post-synthetic functionalisation at the C-2 and C-6 positions, including palladium-catalysed direct arylation and nucleophilic substitution on 2-chlorobenzothiazoles, enables the construction of diverse compound libraries for biological screening.

The biological data compiled in the review show genuine promise. In work by Oyeneyin and colleagues published in 2024, quinoline-urea-benzothiazole hybrids were evaluated against P. falciparum, with compound 13 showing the greatest beta-haematin inhibition activity at an IC50 of 0.201 micromolar, far outperforming chloroquine at 18.26 micromolar and comparing favourably with artemether-lumefantrine and pyrimethamine. In a series of substituted 2-aminobenzothiazoles studied by Thakkar and colleagues, a meta-nitro derivative emerged as the most potent DHFR inhibitor with an IC50 of 0.0312 millimolar, while para-chloro and para-bromo analogues also showed appreciable activity, suggesting that halogen substitution at the para position contributes favourably to interactions with the DHFR active site. Earlier studies by Hout and colleagues found that certain benzothiazoles act on multiple parasite life-cycle stages, including mature and young schizonts, with some compounds disrupting mitochondrial membrane potential, while Sarkar and colleagues demonstrated that benzothiazole-hydrazone conjugates show superior antimalarial and iron-chelating action in murine models, with a 3,4-dihydroxy benzothiazole showing the highest efficacy among hydroxy-substituted variants.

The structure-activity relationships distilled from these studies point to clear design rules. The amino group at the C-2 position is essential for hydrogen bonding with DHFR active-site residues, particularly Asp54, while hydrophobic residues such as Ile14 and Leu164 stabilise the ligand through van der Waals contacts. Introducing aryl or heteroaryl groups at C-2 increases hydrophobicity and enables pi-pi stacking with Phe58, and electron-withdrawing substituents such as halogens or nitro groups on the aryl ring, especially at the para position, enhance binding affinity through improved electronic interactions. Hybridisation of benzothiazole with pharmacophores such as pyrimidine, triazine and triazole via acetamide or urea linkers achieves multiple simultaneous binding contacts, and bioisosteric replacements in the linker region can improve selectivity. Substitutions at positions C-4 through C-7 modulate the molecule’s electronic distribution and shape, facilitating a better fit into the enzyme’s hydrophobic pocket. Docking and structural studies suggest the benzothiazole core forms hydrogen bonds with Asp58 while hydrophobic interactions with Phe58, Ile14 and Leu164 create a favourable environment that enhances both selectivity and potency.

The authors are candid about the challenges that remain. The DHFR enzyme is prone to mutations at its binding site, and if benzothiazole-based antifolates target the same structural elements as older drugs, their efficacy could be similarly compromised. In vivo validation of promising derivatives lags behind encouraging in vitro results, and pharmacokinetic properties such as lipophilicity must be optimised to balance membrane penetration with solubility. Toxicity is a real concern: the antitumour benzothiazole 5F203 formed adducts in the lungs and liver that persisted in mouse lungs, and in a clinical trial involving 50 patients, lung and liver toxicity were observed without a maximum tolerated dose being defined. The path forward, the review concludes, lies in rational scaffold optimisation at C-2, robust target validation, hybrid designs that simultaneously disrupt folate metabolism, heme detoxification and other validated targets such as PfATP6 and PfPI4K, and combination strategies that raise the barrier to resistance. If those hurdles can be cleared, benzothiazole-based hybrids may well represent a leading frontier in the next generation of antimalarial drug discovery.

Subject of Research: Benzothiazole derivatives as inhibitors of Plasmodium falciparum dihydrofolate reductase for antimalarial drug discovery

Article Title: Review on benzothiazole as a privileged nucleus for Plasmodium DHFR inhibition with structure guided comprehensive analysis

Article References: Sarma, K., Ahmed, N., Banu, Z. W., Patowary, L., Gogoi, D., Devi, N. B., & Kalita, J. M. (2026). Review on benzothiazole as a privileged nucleus for Plasmodium DHFR inhibition with structure guided comprehensive analysis. Discover Chemistry, 3(1), Article 446. https://doi.org/10.1007/s44371-026-00905-0

Image Credits: AI Generated

DOI: 10.1007/s44371-026-00905-0

Keywords: malaria, benzothiazole, dihydrofolate reductase, antimalarial drugs, drug resistance, structure-activity relationship, Plasmodium falciparum, molecular docking, heme detoxification, antifolates, medicinal chemistry, drug discovery