Old Cancer Drug Candidate Irosustat Shows Promise Against Drug-Resistant Malaria

Malaria remains one of the most stubborn infectious disease challenges of our time, and the parasites that cause it are steadily outmaneuvering the drugs designed to kill them. Now, a team of researchers from Kangwon National University and collaborating institutions in Korea, Vietnam, and China reports that a compound originally developed for a completely different purpose—irosustat, a steroid sulfatase inhibitor once explored in oncology—can halt the growth of drug-resistant malaria parasites, including strains resistant to both chloroquine and artemisinin. The findings, published in the journal Parasites & Vectors, point to an unusual strategy: rather than attacking the parasite directly, the drug appears to work by altering the biochemistry of the human red blood cells that the parasite calls home.

The rationale behind the study rests on a molecule called cholesterol sulfate, a sulfated derivative of cholesterol that circulates in the bloodstream and is incorporated into the membranes of red blood cells. Cholesterol sulfate is known to influence membrane stability, enzyme activity, and cell signaling, and its concentrations are regulated in part by steroid sulfatases—enzymes that strip sulfate groups from sulfated steroids. Irosustat was designed to block these enzymes, thereby raising circulating levels of cholesterol sulfate and other sulfated steroids. Because the malaria parasite Plasmodium falciparum spends a critical portion of its life cycle inside erythrocytes and depends heavily on the lipid composition of its host cell membrane, the researchers hypothesized that elevating cholesterol sulfate might create an environment hostile to parasite development.

To test this idea, the team first evaluated the antimalarial activity of cholesterol sulfate itself in vitro against an impressive panel of laboratory-adapted P. falciparum strains. The panel included chloroquine-sensitive lines 3D7 and CamWT, chloroquine-resistant strains K1 and Dd2, the artemisinin-resistant line CamWTC580Y(+), and a doubly resistant strain, Dd2R539T(+), which carries mutations conferring resistance to both chloroquine and artemisinin. The results were striking: cholesterol sulfate inhibited parasite growth across all strains, with half-maximal inhibitory concentrations (IC50 values) ranging from the nanomolar to the low-micromolar range. Importantly, the compound showed comparable potency against drug-sensitive and drug-resistant parasites alike, suggesting that its mechanism of action is independent of the resistance pathways that have compromised existing therapies.

The kinetics of parasite killing also caught the researchers’ attention. Cholesterol sulfate exhibited what the authors describe as slow clearance, meaning that parasite suppression developed gradually rather than through rapid, direct toxicity. This profile is consistent with a mechanism that subtly disrupts the host environment over time rather than poisoning a specific parasite enzyme. In combination experiments, cholesterol sulfate displayed synergistic interactions with both artemisinin and chloroquine, the two cornerstones of current antimalarial therapy. Synergy implies that the compound and the conventional drugs enhance each other’s effects, a property that could allow lower doses of existing drugs to be used and could help delay the emergence of further resistance if the approach eventually reaches the clinic.

The most conceptually novel part of the study involved a host-directed pretreatment experiment. The researchers incubated uninfected human red blood cells with either cholesterol sulfate or irosustat, then thoroughly washed the compounds away before exposing the treated cells to parasites. Remarkably, red blood cells that had been pretreated and washed still impaired subsequent parasite growth. In these washout assays, cholesterol sulfate pretreatment produced inhibition with nanomolar IC50 values, while irosustat pretreatment yielded low-micromolar inhibition against both the drug-sensitive 3D7 strain and the multidrug-resistant Dd2R539T(+) line. This result suggests that the compounds durably modify the erythrocyte itself—most plausibly by enriching its membrane with cholesterol sulfate—creating a lasting antiplasmodial environment that persists even after the drug is gone. A host-targeted mechanism of this kind is particularly attractive from an evolutionary standpoint, because the parasite cannot easily mutate a target that resides in the host cell rather than in its own genome.

Safety is always the central question for any host-directed therapy, since manipulating human cell biochemistry risks harming the host as well as the pathogen. The team therefore examined whether the compounds damage red blood cells, measuring hemolytic activity—the rupture of erythrocytes—after exposure. The compounds showed no significant hemolysis at the concentrations tested, providing early reassurance that cholesterol sulfate modulation can be achieved without compromising the integrity of the very cells being protected. The authors also note that irosustat has a favorable oral pharmacokinetic profile, an attribute inherited from its earlier development as an oral anticancer agent, which could simplify dosing in malaria-endemic regions where health infrastructure is limited.

Encouraged by the in vitro data, the researchers moved into an animal model. They used mice infected with a chloroquine-resistant strain of the rodent malaria parasite Plasmodium yoelii and administered irosustat orally using a modified single-dose four-day test, a standard suppressive protocol in antimalarial drug development. As a monotherapy at 40 milligrams per kilogram by the oral route, irosustat suppressed parasitemia by more than 80 percent. When combined with chloroquine at 40 and 10 milligrams per kilogram respectively, suppression exceeded 90 percent. Beyond parasite counts, treated animals showed significantly improved survival rates, longer mean survival days, better maintenance of body weight, and improved clinical outcomes compared with untreated controls. The fact that efficacy was achieved via oral administration is noteworthy, as oral bioavailability is a practical prerequisite for any antimalarial intended for widespread field use.

The study was conducted with appropriate ethical oversight, with human sample protocols approved by the Institutional Ethical Committee of Kangwon National University Hospital and animal experiments approved by the university’s Institutional Animal Care and Use Committee. The work was supported by the National Research Foundation of Korea and the Gangwon RISE program, reflecting a sustained national investment in tropical medicine research. The research team, led by corresponding authors Jeong Taeg Seo of Yonsei University and Eun-Taek Han of Kangwon National University, with Nguyen Van Truong and Tae-Gyu Ahn as co-first authors, spans departments of tropical medicine, obstetrics and gynecology, pharmacology, physiology, pathogenic biology, and oral biology—an interdisciplinary breadth that mirrors the drug-repurposing logic at the heart of the project.

The broader significance of this work lies in the convergence of two pressing needs. First, malaria control is threatened by the spread of artemisinin resistance in Southeast Asia and Africa, and by the failure of chloroquine decades earlier; new agents with mechanisms unrelated to existing drugs are urgently needed. Second, drug repurposing offers a faster and cheaper route to new therapies than de novo discovery, because candidate molecules may already have established safety, manufacturing, and pharmacokinetic data. Irosustat, which was developed to block steroid sulfatase in hormone-dependent cancers such as breast cancer, embodies this logic: its original clinical purpose failed to reach fruition, but its ability to raise cholesterol sulfate levels turns out to be exactly the kind of host manipulation that malaria parasites appear unable to tolerate.

Considerable work remains before irosustat or any cholesterol-sulfate-modulating strategy could reach patients. The precise molecular mechanism by which elevated cholesterol sulfate impairs the parasite—whether through membrane perturbation, interference with the parasite’s lipid scavenging, disruption of fatty acid biosynthesis pathways, or effects on host cell signaling—still needs to be fully delineated. Dosing, toxicity, and pharmacodynamic studies in more advanced models will be required, and efficacy against P. falciparum itself must ultimately be demonstrated in humans rather than inferred from the P. yoelii mouse model. Nevertheless, the authors conclude that future optimization of irosustat may enhance its antiplasmodial activity and position it as a promising candidate for antimalarial drug development. In a field where every new resistance mutation is met with alarm, the idea that a drug could fortify the host cell against invasion—rather than chase the parasite’s ever-mutating targets—offers a genuinely fresh line of attack, and one that the malaria research community will be watching closely.

Subject of Research: Repurposing the steroid sulfatase inhibitor irosustat as a host-targeted antimalarial that modulates cholesterol sulfate against drug-resistant Plasmodium falciparum

Article Title: Repurposing irosustat for malaria: host-targeted antiplasmodial activity via cholesterol sulfate modulation against drug-resistant Plasmodium falciparum

Article References: Van Truong, N., Ahn, T.-G., Nguyen, T.-K., Thau, N. S., Chu, T.-T. H., Na, S.-H., Park, W.-S., Chun, W.-J., Lu, F., Han, J.-H., Moon, S. J., Seo, J. T., & Han, E.-T. (2026). Repurposing irosustat for malaria: host-targeted antiplasmodial activity via cholesterol sulfate modulation against drug-resistant Plasmodium falciparum. Parasites & Vectors. https://doi.org/10.1186/s13071-026-07643-3

Image Credits: AI Generated

DOI: 10.1186/s13071-026-07643-3

Keywords: irosustat, cholesterol sulfate, malaria, Plasmodium falciparum, drug resistance, drug repurposing, host-directed therapy, antimalarial activity, red blood cells, artemisinin resistance, chloroquine resistance, Parasites & Vectors