Getting drugs into the brain is hard. Maybe a parasite can do the job

A mind-bending parasite may one day deliver drugs to the brain.

Toxoplasma gondii is a single-celled parasite that famously makes mice lose their fear of cats, but also can cause deadly foodborne illnesses (SN: 1/14/20). Now, researchers have engineered the parasite to deliver large therapeutic proteins to the brains of mice and into human brain cells grown in lab dishes, an international team of scientists reports July 29 in Nature Microbiology.

Such proteins and the genes that produce them are often too big for viruses — the most common courier for gene therapy — to carry (SN: 10/20/23). If the parasite can be made safe for human use, the technique may eventually help treat a variety of neurological conditions.

While critics doubt that the parasitic villain can ever be turned into a helpful hero, some researchers are intrigued by the idea.

Microbes such as bacteria and parasites are usually viewed as bad guys, says Sara Molinari, a bacterial synthetic biologist at the University of Maryland in College Park who was not involved with the work. But microbes have evolved “pretty sophisticated relationships with our bodies,” she says. “The idea that we can leverage this relationship to instruct them to do good things for us is actually groundbreaking.”

The challenge of crossing the blood-brain barrier

Current methods of delivering therapies to the brain often produce unpredictable results or have a hard time penetrating the protective shield known as the blood-brain barrier, says Shahar Bracha, a bioengineer and neuroscientist at MIT (SN: 5/2/23).

As a graduate student at Tel Aviv University, Bracha was looking for a better way to get drugs and therapeutic proteins into the brain. Those include proteins that can replace missing or nonfunctional ones in people with degenerative and developmental genetic diseases that affect the nervous system, such as Parkinson’s disease and Rett syndrome.  

Then she heard about T. gondii making mice behave recklessly. “It seems like that parasite has solved everything that we need for drug delivery,” Bracha says.

The parasite, which people can get from foods such as raw meat, undercooked shellfish, unwashed fruits and vegetables, as well as from cat feces or contaminated soil, has evolved to cross the blood-brain barrier. Once there, it can infect brain cells and live quietly inside them for up to a lifetime. It can also pump large proteins into brain cells it touches without invading the cells itself.

A ‘crazy idea’

Could T. gondii be turned into a therapeutic tool?

“In the beginning, it was kind of like, ‘Oh, I wonder. Crazy idea,’” Bracha says. “But the more I read about this idea, the more I could figure out an actual plan to test it.”

Bracha and colleagues in Israel teamed up with T. gondii researcher Lilach Sheiner at the University of Glasgow in Scotland to engineer a potentially helpful version of the parasite.

When Anita Koshy, an infectious diseases researcher at the University of Arizona College of Medicine in Tucson who studies T. gondii, first heard someone float the idea of the parasite as a therapy she thought, “It’s a terrible idea. Who’s going to agree to that?” But several years later, when Sheiner approached her for advice, Koshy’s thinking had evolved and she got on board with the project, she says.

If you take the long view and learn to “de-risk” T. gondii, the parasite has some evolutionary aspects that make it appealing, she says.

Toxoplasma gondii risks

As parasites go, T. gondii is already relatively safe for most people with healthy immune symptoms. About a quarter of healthy people worldwide have antibodies in their blood indicating that they have been infected with T. gondii at some point. The U.S. Centers for Disease Control and Prevention estimates that more than 40 million people in the United States carry the parasite.

But the parasite isn’t harmless. In the United States, it’s a leading cause of death from foodborne illness, and can damage the brain, eyes and other organs and cause hearing loss in people who develop severe disease.

Those with weakened immune systems have a higher risk of developing severe disease when exposed to T. gondii. Pregnant people run the risk of preterm birth and pregnancy loss. In addition, the parasite can cause a variety of problems for the baby including blindness, hearing loss, epilepsy and jaundice. More than 200,000 cases of toxoplasmosis are diagnosed each year in the United States, with about 5,000 requiring hospitalization. An estimated 750 people each year die from the disease.

Koshy’s own previous research indicates that brain cells the parasite injects a payload into eventually die.

If researchers want to use the parasite for drug delivery, they will need to learn how it causes disease and disable those mechanisms without harming T. gondii’s ability to quietly infect the brain.

“This may be like trying to deliver pastries with a bazooka.”

Sebastian Lourido, parasitologist

Parasitologist Sebastian Lourido of the Whitehead Institute in Cambridge, Mass., says it may be impossible to make T. gondii safe while retaining all the qualities that would allow it to act as a cargo van. For instance, the parasite hitches rides inside immune cells and breaks through the blood-brain barrier destroying those cells as it goes.

If scientists disable T. gondii’s ability to kill cells and subvert the immune system, the parasite may never be able to reach its destination to unload its cargo. “It’s difficult to imagine how you just engineer it away,” he says.

Building a delivery vehicle

As a first step, the team began by co-opting two organelles T. gondii uses to secrete its own proteins into host cells. One organelle, the rhoptry, is used to inject proteins into brain cells the parasite touches, in an approach known as kiss-and-spit.

In order to deliver proteins to the right place, the researchers had to write the molecular equivalent of an address on them. They did that by attaching the protein they wanted delivered to a protein the rhoptry was already shooting into cells. The piggyback proteins were produced in the rhoptries but the parasite didn’t spit enough of the proteins into neurons grown in lab dishes for the researchers to detect.

That failure could be because the kiss-and-spit mechanism is too harsh for the fused proteins to survive, Lourido says. “This may be like trying to deliver pastries with a bazooka.”

The team also tinkered with dense granules, another type of T. gondii organelle that secretes proteins once the parasite is safely inside the host cell. The researchers fused proteins, including one called MeCP2, to a dense-granule protein called GRA16. MeCP2 is mutated in people who have Rett syndrome, a genetic disorder that includes seizures and developmental delays (SN: 9/3/21).

The parasite had no problem producing MeCP2-GRA16 and unloading it into mammalian and human nerve cells and into brain organoids grown in lab dishes (SN: 10/16/19).

The researchers also injected T. gondii parasites engineered to make MeCP2-GRA16 proteins into the abdomens or bloodstreams of mice. From there, the parasites made their way to the rodents’ brains and began pumping the fused proteins into brain cells. The mice developed no symptoms, indicating that neither the infection nor the fusion protein triggered dangerous immune system reactions.

But the parasite has not been neutered. It could potentially still cause severe disease in people, though Lourido notes that relatively few of the engineered parasites reached the brain in mice. It’s common for engineered organisms to be weakened by researchers’ manipulations, Molinari of the University of Maryland says.

While the experiments were only a partial success, they were encouraging enough for some of the researchers to form an Israel-based company called Epeius Pharma, to develop T. gondii as a protein delivery system. But Bracha and her colleagues emphasize that they’re just taking the first steps toward turning the parasite into a delivery vehicle and are nowhere near using it for medical treatments.

The team still has significant hurdles to overcome before the benefits of intentionally giving people a brain parasite could ever outweigh the risks, Koshy says. She isn’t involved with Epeius but says her colleagues moving the research forward have “the kind of persistence you need to take something that everyone says is going to fail and actually make it successful.”