The significance of this finding lies in the peculiar epidemiology of sleeping sickness. Two subspecies of Trypanosoma brucei infect humans: Trypanosoma brucei gambiense, which accounts for more than 95 percent of all reported cases and is found primarily in West and Central Africa, and Trypanosoma brucei rhodesiense, which circulates in East and Southern Africa and also infects livestock and wild animals. Although gambiense sleeping sickness dominates the global case count, elimination of the disease as a public health problem requires confronting both parasites. A drug that works only against one subspecies would leave an entire region of the continent without a simple cure, undermining the ambitious elimination targets set by the World Health Organization and its partners.

Acoziborole, developed through a partnership led by the Drugs for Neglected Diseases initiative, represents a genuine breakthrough in chemotherapy for neglected diseases. Unlike earlier treatments for gambiense sleeping sickness, which required intravenous infusions of toxic compounds such as melarsoprol, or lengthy courses of oral drugs like fexinidazole, acoziborole is designed to be given as a single oral dose. The drug has already completed clinical development as a one-dose cure for Trypanosoma brucei gambiense infection, a milestone that could transform treatment programs by allowing community-level administration without the need for hospitalization or complex follow-up. But a single-dose strategy only delivers full elimination if it can be deployed against both subspecies of the parasite.

To address this gap, a team of researchers from the Swiss Tropical and Public Health Institute and the Drugs for Neglected Diseases initiative conducted an in-depth preclinical evaluation of acoziborole against bloodstream forms of all three Trypanosoma brucei subspecies, with particular attention to the rhodesiense parasite. The study, authored by Monica Cal, Sonja Keller, Romina Rocchetti, Stéphanie Braillard, Jean-Robert Ioset, Pascal Mäser and Marcel Kaiser, was designed to strengthen the drug’s preclinical dossier by systematically characterizing its potency, speed of action and killing profile across diverse parasite strains, including clinical isolates that had never been exposed to the compound in the laboratory.

The results were encouraging on every measure. Acoziborole displayed consistent and high activity against all tested strains of Trypanosoma brucei rhodesiense, with half-maximal inhibitory concentrations, or IC50 values, clustered around 600 nanomolar. This means that extremely low concentrations of the drug are sufficient to halve the growth of the parasite in vitro, a hallmark of a potent antitrypanosomal agent. Crucially, the activity was uniform across different strains, including a multidrug-resistant field isolate, suggesting that resistance mechanisms circulating in natural parasite populations are unlikely to compromise the drug’s effectiveness. For a disease whose control has repeatedly been set back by drug toxicity and emerging resistance, this consistency is a valuable asset.

Speed of kill is another critical parameter for any drug intended to cure a rapidly progressive infection, and here acoziborole also outperformed its closest oral competitor. The researchers found that acoziborole acts faster than fexinidazole, the ten-day oral treatment that has recently been introduced for both forms of sleeping sickness. More striking still, acoziborole exhibited irreversible cidality against Trypanosoma brucei rhodesiense at concentrations of 4.3 micromolar or higher, equivalent to 1.6 micrograms per milliliter, after just 15 hours of incubation. Irreversible cidality means that even after the drug is removed from the culture medium, the parasites cannot recover and resume growth. A compound that permanently disables the parasite within hours of exposure offers a substantial margin of safety in clinical practice, where drug levels in the blood fluctuate over time.

One nuance emerged from the side-by-side comparison of the two human-infective subspecies. At very short exposure times of eight hours or less, Trypanosoma brucei gambiense appeared to be more sensitive to acoziborole than its rhodesiense counterpart. In a conventional drug, such a difference in early kill kinetics might raise concerns about whether the same dosing regimen would suffice for both forms of the disease. However, the researchers argue that this discrepancy is clinically irrelevant, and the reasoning rests on a remarkable pharmacological property of the compound itself.

Acoziborole possesses an extremely long elimination half-life in the human body, meaning it persists in the bloodstream at active concentrations for weeks or even months after a single oral dose. This slow clearance is precisely what makes a one-dose cure possible: rather than requiring repeated administrations to maintain therapeutic levels, the drug lingers and continues to act on any parasites it encounters. Because patients effectively live with sustained drug exposure for extended periods, the modest difference in how quickly the two subspecies succumb during the first hours of contact becomes immaterial. Over the long residence time of acoziborole in the body, both parasites face the same relentless chemical assault, and both are expected to be eliminated.

Taken together, the laboratory findings support a confident conclusion: acoziborole is expected to be as effective against rhodesiense sleeping sickness, often abbreviated r-HAT, as it is against gambiense sleeping sickness, or g-HAT, in patient populations. This is more than a technical footnote. Rhodesiense sleeping sickness follows a more acute course than the gambiense form, with a shorter incubation period and faster progression to neurological disease, and it has historically been treated with the toxic arsenical melarsoprol, which itself kills a significant fraction of patients. A safe, single-dose oral alternative for this eastern form of the disease would eliminate one of the last justifications for using a nineteenth-century poison in modern medicine.

The broader context makes the timing of this work especially compelling. The incidence of sleeping sickness has fallen to a historic low, thanks to decades of sustained surveillance, vector control and treatment efforts, and the World Health Organization has recognized the disease as one of the very few infections targeted for elimination as a public health problem. Yet the endgame of any elimination campaign is the hardest phase: cases become rare, scattered and hard to find, and health systems in endemic regions struggle to justify maintaining specialized diagnostic and treatment capacity. A single-dose oral drug that works against both parasite subspecies could be administered presumptively or with minimal testing at the community level, closing the last gaps in coverage. With its clinical success against gambiense infection now backed by preclinical evidence of comparable activity against rhodesiense parasites, acoziborole stands as the most promising tool yet for delivering the final blow against a disease that has plagued tropical Africa for generations.

Subject of Research: Preclinical efficacy profiling of the antitrypanosomal drug acoziborole against Trypanosoma brucei rhodesiense and other African trypanosomes

Article Title: Winning the endgame against sleeping sickness: Efficacy profiling of acoziborole against African trypanosomes with the focus on Trypanosoma brucei rhodesiense

Article References: Cal, M., Keller, S., Rocchetti, R., Braillard, S., Ioset, J.-R., Mäser, P., & Kaiser, M. (2026). Winning the endgame against sleeping sickness: Efficacy profiling of acoziborole against African trypanosomes with the focus on Trypanosoma brucei rhodesiense. PLOS Neglected Tropical Diseases, 20(10), e0014762. https://doi.org/10.1371/journal.pntd.0014762

Image Credits: AI Generated

DOI: 10.1371/journal.pntd.0014762

Keywords: sleeping sickness, acoziborole, African trypanosomiasis, Trypanosoma brucei rhodesiense, Trypanosoma brucei gambiense, tsetse fly, neglected tropical diseases, drug resistance, fexinidazole, elimination, preclinical testing, antiparasitic drugs

T. brucei leads a double life, cycling between the bloodstream of mammals and the gut of the tsetse fly. Within the mammalian host, the parasite exists mainly in two forms: the long slender form, which divides rapidly and drives parasitaemia upward, and the short stumpy form, a non-dividing stage produced through a quorum-sensing mechanism once parasite density climbs. The stumpy form is the parasite’s ticket to the next stage of its life cycle, primed to differentiate into procyclic forms once it reaches the fly. Yet stumpy cells are also fragile. Earlier work showed they survive only around four days in the bloodstream, and during the remission of the first parasitaemic peak in mice they disappear far faster than their natural turnover alone could explain. Something else, the researchers suspected, was accelerating their demise.

The team’s first clue came from body temperature. In C57BL/6J mice infected with pleomorphic T. brucei, parasitaemia first became detectable around three days post-infection, and by day four the slender forms had begun differentiating into stumpy cells just as fever set in. Both parasite load and body temperature peaked on days four and five, when stumpy forms made up nearly three-quarters of the population. Parasitaemia then collapsed to undetectable levels by day seven, coinciding with the return of body temperature to baseline. A Pearson correlation analysis confirmed a strong positive relationship between parasitaemia and temperature during the first week of infection, with a correlation coefficient of 0.678. A multiple linear regression model, which explained 67.54 percent of the variance in late-phase parasite levels, singled out the temperature window spanning days three to five as a significant predictor of subsequent parasite decline.

To test whether heat itself could kill stumpy cells, the researchers purified them from infected mice and cultured them at different temperatures. At 27 degrees Celsius, the temperature of the tsetse fly, the cells survived and even began differentiating into procyclic forms. At 37 degrees, normal mammalian body temperature, they underwent their expected natural turnover over roughly three to four days. But at 39 degrees, a fever temperature previously recorded in infected mice, mortality climbed steeply. Notably, long slender forms tolerated prolonged heat exposure far better; only the stumpy cells declined sharply within the first 48 hours. Fever, the results suggest, places a disproportionate burden on the very stage of the parasite that is already winding down.

The interplay with antibodies proved equally revealing. Plasma from mice at 21 days post-infection, rich in effective antibodies, suppressed stumpy cells in culture but could not eliminate them completely, unlike its effect on slender forms. Plasma from mice at day six, when the first peak collapses, showed no killing activity against either form. Yet when elevated temperature and day-21 plasma were combined, more than 99.5 percent of stumpy cells were eradicated within 24 hours, while either treatment alone left 15 to 18 percent alive. This synergy indicates that the well-documented IgM and complement-mediated clearance of the first parasitaemic peak is only part of the story. The parasite’s own heat sensitivity, amplified by fever, appears to be an underappreciated partner in that dramatic remission.

What happens inside a heat-stressed stumpy cell? The researchers found that apoptosis-like events, detected through phosphatidylserine exposure, rose from almost nothing at 27 degrees to 3.72 percent after 24 hours at 37 degrees and 30.63 percent at 39 degrees. Slender forms showed essentially no such events at any temperature. Gene expression analysis pointed to the mitochondrion as the driver: Metacasp-3, a mitochondrial-associated apoptosis gene, was upregulated roughly twofold at 37 degrees. Fluorescence imaging of mitochondria stained with MitoTracker revealed progressive damage, with punctate dye aggregation appearing in about 43 percent of cells after 24 hours at 37 degrees and over 95 percent after the same period at 39 degrees. Transmission electron microscopy confirmed the structural toll, showing fenestrated mitochondrial membranes and multivesicular structures suggestive of a stress-induced degradative pathway resembling autophagy.

Mitochondrial dysfunction went hand in hand with oxidative stress. Using MitoSOX staining, the team measured mitochondrial reactive oxygen species accumulating in stumpy cells exposed to heat: 15.30 percent of cells showed oxidative stress after 24 hours at 37 degrees, rising to 44.58 percent at 39 degrees, while cells kept at 27 degrees remained largely unaffected. The reactive oxygen species co-localized with the damaged mitochondria, reinforcing the link between heat, mitochondrial injury, and cell death. Mitochondrial membrane potential, a key indicator of mitochondrial health, collapsed in parallel, falling to 51.36 percent of baseline at 37 degrees and just 19.38 percent at 39 degrees. Slender forms, by contrast, showed neither mitochondrial oxidative stress nor membrane potential loss under the same conditions.

Untargeted metabolomics added a metabolic dimension to the picture. Comparing stumpy cells incubated for 12 hours at 37 versus 39 degrees, the researchers detected 306 significantly altered metabolites among more than 22,000 measured. The most striking change was a sixfold accumulation of succinate at fever temperature, alongside elevated glucose-6-phosphate and 3-phosphoglycerate, pointing to disrupted energy production through glycolysis and the TCA cycle. The team attributes the succinate surge to mitochondrial fumarate reductase, an enzyme known to generate reactive oxygen species and highly expressed in stumpy cells. Lipid metabolism was also disturbed: myristic acid, a key component of the parasite’s surface GPI anchors, accumulated at 39 degrees, while carnitine, needed for fatty acid degradation, dropped significantly, suggesting the cells could neither properly remodel nor dispose of their lipids under febrile stress.

The findings carry broader implications for how trypanosome infections unfold. Stumpy cells lack complexes III and IV of the electron transport chain and maintain their membrane potential through glycolysis-derived ATP and a reversed ATP synthase, a precarious arrangement the authors describe as dancing on a knife-edge. This metabolic mismatch between a partially activated TCA cycle and a limited antioxidant arsenal, including lower levels of iron superoxide dismutase than in slender forms, may be an evolutionary trade-off that pre-adapts the parasite for the insect gut at the cost of vulnerability to mammalian fever. It may also explain why trypanosomes take refuge in the cooler, peripheral tissues of the skin and adipose tissue, and why livestock species with naturally higher baseline body temperatures often show low parasitaemia yet remain reservoirs for transmission.

The authors caution that they were unable to create a fever-suppressed mouse model to prove causation directly, though earlier veterinary studies found that anti-inflammatory drugs raised parasitaemia in infected animals, indirectly supporting the fever-clearance hypothesis. They also note that natural infections initiated by tsetse bites produce lower first peaks than experimental injections, so fever’s role there may be more modest, though still relevant before specific antibodies appear. Because fever is among the most common clinical signs in human patients, and because stumpy cells resist antibody-mediated elimination better than slender forms do, the study proposes that temperature-driven clearance complements immunity throughout infection. Understanding the molecular basis of this thermo-sensitivity, the researchers conclude, could open new avenues for controlling a disease that still threatens millions across sub-Saharan Africa.

Subject of Research: Heat-induced degeneration of short stumpy Trypanosoma brucei forms and the role of fever in clearing the first parasitaemic peak

Article Title: Thermo-related degeneration of stumpy forms of Trypanosoma brucei, the pathogen of African sleeping sickness

Article References: Thermo-related degeneration of stumpy forms of Trypanosoma brucei, the pathogen of African sleeping sickness. (n.d.). https://doi.org/10.1007/s44307-025-00081-9

Image Credits: AI Generated

DOI: 10.1007/s44307-025-00081-9

Keywords: Trypanosoma brucei, sleeping sickness, short stumpy form, fever, mitochondrial damage, oxidative stress, apoptosis-like cell death, metabolomics, glycolysis, TCA cycle, parasitaemia, tsetse fly