With malaria still posing a constant and often deadly threat to billions worldwide, new therapies are urgently required to combat the infection. This is made more difficult by the multiple stages of the parasite’s lifecycle.

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

Study: Liver-targeted polymeric prodrugs delivered subcutaneously improve tafenoquine therapeutic window for malaria radical cure. Image Credit: nechaevkon/Shutterstock.com

The malaria situation

The world saw nearly 250 million malaria infections caused by the malarial parasite Plasmodium, with over 600,000 deaths occurring as a result in 2021. The greatest malaria-related public health emergency is possibly about to occur within 20 years, according to the African leaders.

While P. falciparum is the most lethal parasite, P. vivax causes the most cases because it has the largest habitat. About 3,300 million people are thus at risk of P. vivax infection globally. They live in the American continents, India, South-East Asia, and the Western Pacific regions.

With the habitat expected to expand, the situation will only worsen over the next few decades. P. vivax passes through a hypnozoite stage, in which the parasite becomes dormant within the liver cells. In this stage, it is invulnerable to standard malaria therapies.

Hypnozoites not only carry the risk of relapses but are key to the continuation of the chain of transmission from such individuals even after they have been treated for malaria. The need for new drugs that are suitable for mass administration and eradication campaigns is obvious.

Only two 8-aminoquinoline (8-AQ) drugs, primaquine and tafenoquine (TQ), have been approved for the radical malaria cure. This term refers to removing P. vivax hypnozoites from all liver cells.

The problem with tafenoquine

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

This deficiency, which affects approximately 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 induce toxic oxidation in red blood cells, leading to severe hemolytic anemia, renal failure, and potentially death in cases of severe deficiency. Ironically, the same oxidizing metabolites that cause these effects are also what allow tafenoquine to kill malaria parasites.

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

To address these challenges, research into prodrugs has been conducted to potentially broaden the therapeutic margin, even slightly. Prior studies suggest that these modifications may make tafenoquine safe for use in individuals with G6PD deficiency.

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

A potential solution

The researchers developed a polymeric prodrug to enhance the therapeutic index of tafenoquine (TQ) administered subcutaneously (SC). This modification results in lower peak blood concentrations, reducing hemolytic anemia risk.

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

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

In comparative studies, this prodrug proved more effective against Plasmodium berghei sporozoites than oral TQ and exhibited reduced hemolysis in a humanized mouse model of G6PD deficiency.

A major hurdle in developing drugs for radical cures is the lack of animal models that accurately mimic the effect of anti-hypnozoite drugs on Plasmodium vivax. Currently, the only available primate model uses Plasmodium cynomolgi hypnozoites, and significant metabolic and pharmacologic differences exist between human and primate responses to TQ.

Therefore, the study utilized primary nonhuman primate hepatocytes with P. cynomolgi hypnozoites to evaluate the prodrug. Additionally, the research assessed the cost of goods sold (COGS) and manufacturability to determine the feasibility of producing the prodrug on a mass scale.

What did the study show?

By modifying the blood-stable linker in the prodrug, the researchers increased the stability of the prodrug fourfold on SC administration.

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

Surprisingly, it targeted the liver, with hepatocyte exposure significantly higher than oral TQ. Simultaneously, it exhibited selectivity, with a significantly lower maximum concentration in the plasma.

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

A dose-dependent activity was seen, with complete elimination of the parasites at 10 mg/kg, superior to oral TQ. The underlying mechanism was thus shown to be the higher liver exposure.

Correspondingly, the hemotoxicity was also reduced more than twofold with pSVCTQ, using the industry standard for evaluation, a humanized mouse model with G6PD deficiency. The prodrug binds to membrane receptors on the cell surface to enter the cell by endocytosis, along with the level of ASGPR receptors that vary over time.

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

Conclusions

“These results show how the polymer could be engineered and optimized for COGS requirements and health equity, rather than by the therapeutic index alone.”

The prodrug should improve the prospects of mass eradication. Moreover, the results could be used to design other therapies for multiple internal organs.

“Together, these results validate the liver-targeted TQ prodrug design platform as an important therapeutic approach to the spectacularly unmet need for radical cure malaria therapeutics.”