Could a Novel ‘Trojan Horse’ Prodrug Approach be Effective in Treating Malaria?

By Nancy McCann

The Findings

Researchers in the John Laboratory at Children’s Hospital of Philadelphia showed that a human red blood cell enzyme, acylpeptide hydrolase (APEH), is internalized by the parasite that causes severe malaria, Plasmodium falciparum, and is responsible for the activation of antimalarial prodrugs within the parasite. The findings could be leveraged to design host enzyme-activated prodrugs with high barriers to drug resistance to better combat the deadly disease.

Why It Matters

New antimalarial drugs are urgently needed as malaria is a global health threat, with pregnant women and children at highest risk. There are approximately 250 million cases of malaria each year and about 600,000 deaths, with an estimated 66.6% of those deaths among children under the age 5.

Although the current malaria treatment called artemisinin combination therapies (ACT) has been effective at killing the parasite quickly and responsible for many lives saved, resistance to ACT has emerged in Southeast Asia and Africa, where most malaria cases occur.

Mutations in prodrug activating enzymes are a common mechanism for antimicrobial drug resistance. The researchers noted that this novel finding could help design “resistance-proof” prodrugs because the parasite would be unable to mutate a host enzyme, which decreases the likelihood that drug resistance could develop by this mechanism.

“This might eventually lead to the development of parasite- or bacteria-specific prodrugs that are less reliant on specific enzymes,” said first author Sesh Sundararaman, MD, PhD, Attending Physician in Pediatric Infectious Disease and a member of the John Laboratory.

The challenge with developing new drugs to treat malaria is that the parasite is hard to reach. It lives inside a membrane container within red blood cells and is surrounded by two membranes of its own — a well-insulated killing machine.

“Prodrugging is an enticing strategy because these drugs have methods for getting through the layers of protection offered by membranes of the parasite and host cells, as well as a drug ‘warhead’ that can more effectively kill the parasite,” said senior author Audrey Odom John, MD, PhD, Chief of the Division of Infectious Diseases. “We’ve been working on prodrugs that might be effective for treating malaria, but in doing so, we’ve also needed to learn what kinds of enzymes within the parasite are capable of activating the prodrug, as that information is critical to understanding the nature of the target for future antimalarial strategies.”

Who Conducted the Study

In addition to Drs. John and Sundararaman, CHOP contributors to the study include fellow lab members Kelsey O’Brien and Ellora Daley.

How They Did It

CHOP researchers set out to understand how antimalarial prodrugs, which are used to improve a drug’s ability to be absorbed or reach its target, are activated, in the hopes of identifying a way to treat malaria more effectively. Prodrugs are inactive and must be activated, typically by an enzyme, to achieve their desired effect; however, many potential drugs fail in development because they are poorly absorbed in the gastrointestinal tract or cleared too rapidly from the body.

Sesh Sundararaman, MD, PhD

By using a novel fluorogenic ester library, which colorfully highlights changing chemical compounds, they characterized the structure activity relationship of APEH, and compared it to that of the malaria-causing parasite’s esterases.

What they unexpectedly found was that it is not the parasite that breaks down the prodrug and releases the antibiotic. The red blood cell enzyme APEH is taken into the parasite’s cytoplasm where it retains enzymatic activity. APEH is the major activating enzyme of multiple antimalarial prodrugs known as lipophilic ester prodrugs.

Quick Thoughts

“The malaria parasite has developed resistance to every antimalarial drug that has been clinically used,” Dr. Odom John said. “We haven’t lost ACTs everywhere, but we will lose them. And when we do, we need to have a pipeline of drugs to replace them, or children will die.”

What’s Next

The research team plans to continue with their Trojan horse-approach to understanding these mechanisms, which could provide novel insights into parasite biology and host parasite interactions, and potentially unveil new therapeutic targets or methods of parasite specific drug delivery.

“What we discovered will help us begin to try to tailor the way we design prodrugs,” Dr. John said.

Where the Study Was Published

The study was featured in the Proceedings of the National Academy of Science..
Funding was provided by the PIDS-St. Jude Children’s Research Hospital Fellowship Award in Basic and Translational Science, the National Institutes of Health grants R01AI171514, R01AI123433, T32AI141393, the Doris Duke Foundation Paragon of Research Excellence Award, the Indiana Academy of Sciences Senior Research Grant and CHOP.