Old drug, new discovery: Scientists find new use for old antimalarial drug

For most of human history, drug discovery was often based on chance. Various natural substances, mostly from plants, were ingested or applied, the effects of which were judged by both observation and superstition. Through trial and error, some lasting remedies emerged: morphine from poppy seeds, quinine from the bark of cinchona trees, and aspirin from the bark of willow trees.

Even in today's world, when drug development is led by biology and its possibilities are expanded by synthetic chemistry, chance plays a role. Sometimes ancient remedies are lost and rediscovered. Sometimes new tools reveal hidden potential in old drugs.

When a team of researchers and collaborators at Stanford Medicine recently set out to find a drug for cardiac fibrosis – a stiffening of heart tissue that underlies many cardiovascular diseases – their high-tech search led them to a derivative of Artemisinin, a drug with an ancient history and a storied past. Artemisinin has long been used in traditional Chinese medicine to treat malaria, but was forgotten until the 1960s.

A forgotten cure

At the time, both the United States and China were desperate to find a better drug to treat malaria, which was incapacitating troops in Vietnam. The mosquito-borne parasite that causes the disease had developed resistance to existing drugs, and malaria infected half of some U.S. military units in Vietnam. At times, more soldiers were taken out of action because of malaria than because of combat injuries.

Military research programs in both countries tested thousands of compounds with little success.

But then in 1969, the Chinese government commissioned a young scientist named Tu Youyou to find a cure in traditional Chinese medicine. She combed through traditional literature that dates back thousands of years. After years of false starts, vats of toxic chemicals, and even testing the drug on itself, Tu's team landed on a shrubby herb known as sweet wormwood Artemisia annua.

Sweet wormwood was among the first herbs Tu's team tested, but their initial extractions were ineffective. Only after a close re-reading of a fourth century text – A handbook with recipes for emergencies – Did Tu know that although most herbal preparations were boiled, the ancient instructions called for unheated sweet wormwood tea? Dip a handful of the herb in water, it says, then “squeeze out the juice and drink it all.”

When Tu revised extraction methods using lower temperatures, the results were dramatic. Extracted artemisinin cured 100% of all malaria infections, first in rodents and monkeys, then in human clinical trials.

Artemisinin and its derivatives proved to be groundbreaking in the treatment of malaria and are still widely used today. In 2015, Tu received the Nobel Prize in Physiology or Medicine for her discovery.

New tools of the trade

Half a century later, a team led by Joseph Wu, MD, PhD, a professor of cardiology and radiology, approached the search for a drug for cardiac fibrosis using techniques that would have been difficult to imagine in the 1960s.

Trial and error in drug development now takes the form of high-throughput screenings, allowing thousands of compounds to be tested simultaneously in thousands of tiny experiments on thousands of genetically engineered cells.

The researchers combed through a library of around 5,000 compounds, consisting of a wide range of drugs in use and development.

“It is a comprehensive library that targets almost all major signaling pathways,” said Dr. Hao Zhang, a lecturer at the Stanford Cardiovascular Institute and co-lead author of the study published Oct. 15 Cell.

Fibrosis is a natural response to injury and occurs throughout the body. For example, when skin is cut, cells known as fibroblasts secrete more extracellular matrix, the network of proteins and other molecules that holds cells together, and form tough scar tissue to close the wound.

“Essentially, the extracellular matrix is ​​like a glue,” said Wu, the study’s senior author, director of the Stanford Cardiocular Institute and professor Simon H. Stertzer.

However, in many diseases and with increasing age, the fibrosis becomes overactive.

It is estimated that 45% of deaths in the developed world are due to fibrosis-related organ failure.

Hao Zhang

“It is estimated that 45% of deaths in the developed world are due to fibrosis-related organ failure,” Zhang said.

In the heart, cardiac fibroblasts can produce too much extracellular matrix in response to an injury, such as a heart attack. Too much glue causes the heart to become stiff and increases the likelihood that it will fail.

No medications have been approved by the U.S. Food and Drug Administration to treat cardiac fibrosis. “There are actually a lot of basic research papers describing antifibrotic drugs, but these drugs failed in clinical trials,” Zhang said.

A major obstacle has been the difficulty of obtaining enough human cardiac fibroblasts to test potential drugs.

The researchers overcame this problem by deriving cardiac fibroblasts from human-induced pluripotent stem cells, generating thousands of cellular test subjects. They have also genetically engineered them so that they fluoresce when stimulated. In their first round of screening, they stimulated cardiac fibroblasts and looked for compounds that inhibit fibrosis. Using a machine learning algorithm, they selected 20 compounds with promising antifibrotic activity. Through another round of screening, they eliminated those that showed toxicity to other types of heart cells.

These techniques took several months to perfect, Zhang said, but the screening, carried out by automated robots, was carried out relatively quickly.

We can test 1,000 compounds in three to five days.

Hao Zhang

“We can test 1,000 compounds in three to five days,” he said.

Out of 5,000 candidates, one prevailed – a derivative of artemisinin called artesunate.

Old drug, new tricks

The researchers found that the potent antimalarial drug can, through a completely different mechanism, partially reverse cardiac fibrosis in cells from patients with the disease and in heart tissue grown in the laboratory. It also improved heart function in mice with heart failure.

When fighting malaria, artesunate binds to the heme protein in red blood cells, producing reactive oxygen species that help destroy the parasite. However, in cardiac fibrosis, artesunate targets a separate molecular pathway involving myeloid differentiation factor 2 and Toll-like receptor 4 to inhibit the expression of fibrotic genes.

Since artesunate is already approved by the FDA for malaria, researchers hope it can soon be tested in clinical trials. They plan to further develop the drug into pill form – it is currently given as an injection – and study its effect on fibrosis in other organs.

Without the wide net enabled by today's high-throughput screening and extensive compound libraries, researchers would have had little reason to fish out artesunate from the sea of ​​drug candidates. However, artesunate exists in today's compound libraries only because of Tu's rediscovery 50 years ago – “It's in the library because it's on the market,” Zhang said – and instructions from the fourth century.

Illustration: Emily Moskal/Stanford Medicine