Preclinical studies show innovative malaria prodrug targets liver, increasing efficacy while reducing toxicity

Preclinical studies show innovative malaria prodrug targets liver, increasing efficacy while reducing toxicity

With malaria still a constant and often fatal threat to billions of people around the world, new therapies to combat the infection are urgently needed. This is complicated by the multiple phases in the parasite's life cycle.

A new report in Scientific Advances reports on tafenoquine, a prodrug that could meet the criteria for a mass eradication campaign.

The malaria situation

There have been nearly 250 million malaria infections worldwide caused by the malaria parasitePlasmodiumThis resulted in over 600,000 deaths in 2021. African leaders say we may be facing the biggest malaria-related health emergency in 20 years.

WhileP. falciparumis the deadliest parasite,P. vivaxcauses the most cases because it has the largest habitat. This means that around 3.3 billion people are at riskP. vivaxInfection worldwide. They live in the Americas, India, Southeast Asia and the Western Pacific.

As habitat expansion is expected, the situation will only get worse over the next few decades.P. vivaxgoes through a hypnozoite stage in which the parasite rests in the liver cells. At this stage it is invulnerable to standard malaria therapies.

Hypnozoites not only pose the risk of relapses, but are also key to continuing the chain of transmission in these individuals even after they have been treated for malaria. The need for new drugs suitable for mass administration and eradication campaigns is clear.

Only two 8-aminoquinoline (8-AQ) drugs, primaquine and tafenoquine (TQ), are approved for the radical cure of malaria. This term refers to removalP. vivaxHypnozoites from all liver cells.

The problem with tafenoquine

Tafenoquine is an oral medication that is taken as a single dose, as opposed to the 14-day treatment required for primaquine. However, it is not suitable for people with glucose-6-phosphate dehydrogenase (G6PD) deficiency, a common enzyme defect, or for people whose G6PD status is unknown.

This deficiency, which affects about 400 million people worldwide - nearly 17% in some regions, offers some protection against severe malaria but also complicates treatment with tafenoquine.

In affected individuals, the drug can cause toxic oxidation in red blood cells, leading to severe hemolytic anemia, kidney failure and, in severe deficiency, possibly death. Ironically, it is the same oxidizing metabolites that cause these effects that also enable tafenoquine to kill malaria parasites.

Given the uneven access to G6PD testing, particularly in low-resource areas that also have high malaria prevalence, tafenoquine is unsuitable for many who need it. Additionally, those with a deficiency serve as a reservoir for the parasite, hindering mass eradication efforts.

To address these challenges, research has been conducted on prodrugs to potentially even slightly expand the therapeutic range. Previous studies suggest that these modifications could make tafenoquine safe for use in people with G6PD deficiency.

“While the approved dose of 300 mg TQ was dose-limiting in the G6PD-deficient cohort, a dose of 100 mg did not result in hemotoxicity.” However, a TQ dose of 300 mg may not be enough to produce radical healing.

A possible solution

Researchers developed a polymeric prodrug to improve the therapeutic index of tafenoquine (TQ) administered subcutaneously (SC). This modification results in lower peak blood concentrations and reduces the risk of hemolytic anemia.

The prodrug was also designed to optimize transport through liver cells. The aim is to achieve a radical cure with a single dose while minimizing the production of hemotoxic metabolites in the liver.

The prodrug is intended to remain stable in the bloodstream, but is broken down in the body by cathepsin enzymes. Given the lack of non-8-aminoquinoline (non-8-AQ) options for radical remedies and the practicality of SC administration in mass eradication efforts, this development could represent a significant advance.

In comparative studies, this prodrug was found to be more effective against Plasmodium berghei sporozoites than oral TQ and demonstrated reduced hemolysis in a humanized G6PD-deficient mouse model.

A major hurdle in the development of radical cure drugs is the lack of animal models that accurately mimic the effects of anti-hypnozoite drugs on Plasmodium vivax. Currently, the only primate model available uses Plasmodium cynomolgi hypnozoites, and there are significant metabolic and pharmacological differences between human and primate responses to TQ.

Therefore, the study used primary non-human primate hepatocytes with P. cynomolgi hypnozoites to evaluate the prodrug. In addition, the investigation evaluated cost of goods sold (COGS) and manufacturability to determine the feasibility of mass production of the prodrug.

What did the study show?

By modifying the blood-stable linker in the prodrug, the researchers increased the stability of the prodrug fourfold when administered subcutaneously.

The modified optimized pSVCTQ prodrug was easily cleaved in the liver cells.

Surprisingly, it targeted the liver, with hepatocyte exposure significantly higher than oral TQ. At the same time, it showed selectivity with a significantly lower maximum concentration in the plasma.

Two important TQ metabolites were also selectively increased in the liver compared to blood compared to oral TQ. The exposure of the liver cells to the prodrug was therefore comparable to that after oral administration.

A dose-dependent activity was observed, with complete elimination of parasites at 10 mg/kg, thus being superior to oral administration of TQ. The higher liver exposure could therefore be proven as the underlying mechanism.

Accordingly, hemotoxicity was also reduced by more than twofold with pSVCTQ using the industry standard for evaluation, a humanized G6PD-deficient mouse model. The prodrug binds to membrane receptors on the cell surface to enter the cell through endocytosis, along with the amount of ASGPR receptors changing over time.

COGS could be reduced to 36% by redesigning the prodrug, making the product more attractive to resource-poor settings. Its manufacturability could be improved.

Conclusions

“These results demonstrate how the polymer could be developed and optimized with COGS requirements and health equity in mind, and not just on the basis of therapeutic index.”

The prodrug is likely to improve the prospects of mass extinction. In addition, the results could be used to develop additional therapies for multiple internal organs.

“Taken together, these results confirm that the liver-targeted TQ prodrug design platform is an important therapeutic approach to address the spectacularly unmet need for malaria therapeutics for radical cure.”

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