Researchers have identified a compound, ML-901, that showed highly specific and potent inhibition of the malaria parasite. ML-901 exerted activity against all stages of the malaria parasite lifecycle with low nanomolar potency and single-dose efficacy in a mouse model of malaria, without toxicity to mammalian cells.

The team reported its findings in the June 2, 2022, online edition of Science.

Around the world, the mosquito-borne Plasmodium falciparum parasite kills about 450,000 people each year, most of them children and pregnant women, while another 200 million people suffer illness because of malaria infections. Current antimalarial treatments, including front-line therapies like artemisinins, are failing with clinically validated resistance being detected against them. New treatments with novel modes of action are, therefore, the need of the hour.

Speaking to BioWorld Science, lead investigator Leann Tilley said that "our team has discovered a new and very surprising way to kill malaria parasites – effectively getting them to self-sabotage. ML-901 kills malaria parasites that are resistant to currently used drugs. It shows rapid action that results in excellent parasite killing, both in cultures and in an animal model of malaria." Tilley is Professor of Biochemistry and Molecular Biology in the Bio21 Molecular Science and Biotechnology Institute, The University of Melbourne. Her research focuses on the use of molecular approaches and imaging techniques to study the malaria parasite and its interactions with its host to develop novel therapies.

For the work now published in Science, Tilley and her team tested a library of sulfamate molecules provided by Takeda Pharmaceutical.

The authors realized that they had inadvertently discovered a way to turn malaria parasites on themselves. According to Tilley, "ML-901 does not work like a standard inhibitor that attaches to its target like a key in a keyhole. ML-901 could be called a pro-inhibitor. It works by an unusual reaction-hijacking mechanism. Once the compound is taken up into the parasite, it hijacks the machinery for charging tRNA molecules with tyrosine. The aminoacyl tRNA synthetase enzyme catalyzes the attachment of ML-901 onto tyrosine, effectively supergluing ML-901 into the enzyme's active site, which jams the machinery. The parasite cannot make proteins; and grinds to a halt."

The authors used targeted mass spectrometry to search for ML-901 conjugates in P. falciparum-infected red blood cells and cultured human cells treated with the drug. ML-901 inhibited protein translation in P. falciparum schizonts by hijacking the active site-bound reaction product of aminoacyl transfer RNA (tRNA) synthetases. The authors tested ML-901 for cytotoxicity against different mammalian cell lines and showed 800- to 5000-fold selectivity toward P. falciparum.

ML-901 retained activity against all tested strains of P. falciparum regardless of their resistance profile and geographical origin, and potently inhibited transmissible male gametes and prevented development of P. falciparum in primary human hepatocytes. In humans, the equivalent tRNA charging enzyme has a different molecular structure and is not susceptible to attack by ML-901 due to differential flexibility of a loop over the catalytic site, thus accounting for the low cytotoxicity in human and mammalian cell lines.

The paper also reports a new basic science discovery, namely, the previously unreported susceptibility of tRNA synthetases to reaction hijacking by a previously overlooked class of nucleoside sulfamates.

Since the reported pyrazolopyrimidine sulfamate chemotype is active against all stages of the malaria parasite lifecycle, Tilley emphasizes that "it could be used for prophylaxis as well as for treating the disease. It is also active against the transmissible stage of the parasite, which is critical to stop the spread of malaria." The rat pharmacokinetic profile of ML-901 shows that the drug exhibits low blood clearance and has a long terminal half-life in blood following intravenous or oral dosing, suggesting ML-901 could be suitable for treating severe malaria via intravenous administration.

"This is just the beginning. We now have the possibility of finding drugs, like ML-901, that target a range of deadly infectious diseases, including multidrug-resistant bacterial infections. The work opens several new drug discovery avenues," Tilley said.

On the basis of these findings, Tilley and her team are now pursuing the development of new antimalarial drug candidates that build on ML-901 as a starting point. The team has initiated a medicinal chemistry program to find even more potent and selective inhibitors. Tilley's team is working to find a compound that is orally bioavailable and can be formulated to be taken as a pill.