For most of human history, drug discovery has often relied on serendipity. Various natural substances, usually from plants, were ingested or applied, their effects judged by both observation and superstition. Through trial and error, some enduring remedies emerged: morphine from poppy seeds, quinine from the bark of cinchona trees and aspirin from the bark of willow trees.
Even in modern times, with drug discovery guided by biology and its possibilities expanded by synthetic chemistry, serendipity plays a role. Sometimes ancient remedies are lost and rediscovered. Sometimes new tools reveal hidden potential in old drugs.
Recently, when a team of Stanford Medicine researchers and collaborators 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 had long been used in traditional Chinese medicine as a treatment for malarial fevers, but it fell into obscurity until the 1960s.
A forgotten remedy
In that era, both the United States and China were desperate to find a better drug to treat malaria, which was disabling troops in Vietnam. The mosquito-borne parasite that causes the disease had gained resistance to existing drugs, and malaria was infecting half of some U.S. military units in Vietnam. At times, more soldiers were taken out of action from malaria than from combat injuries.
Military research programs in both countries tested thousands of compounds with little success.
But then, in 1969, the Chinese government assigned a young scientist named Tu Youyou to search for a cure in traditional Chinese medicine. She scoured the traditional literature dating back millennia. After years of false starts, vats of toxic chemicals, and even testing the drug on herself, Tu's team landed on a shrubby herb known as sweet wormwood, or Artemisia annua.
Sweet wormwood had been among the first herbs Tu's team tested, but their initial extractions were ineffective. Only after a close rereading of a fourth-century text -- A Handbook of Prescriptions for Emergencies -- did Tu realize that while most herbal concoctions were boiled, the ancient instructions implied an unheated tea made from sweet wormwood. Immerse a handful of the herb in water, it directed, then "wring out the juice and drink it all."
When Tu revised the extraction methods using lower temperatures, the results were dramatic. Extracted artemisinin cured 100% of malaria infections, first in rodents and monkeys, then in human clinical trials.
Artemisinin and its derivatives proved to be game-changing in the treatment of malaria and are still widely used today. In 2015, Tu shared 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 of radiology, approached their search for a cardiac fibrosis drug with techniques hardly imaginable in the 1960s.
Trial and error in drug discovery now takes the form of high-throughput screening, 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 some 5,000 compounds consisting of a wide range of drugs in use and in development.
"It's a comprehensive library designed to target almost all the important signaling pathways," said Hao Zhang, MD, an instructor at the Stanford Cardiovascular Institute and co-lead author of the study published Oct. 15 in Cell.
Fibrosis is a natural response to injury and occurs throughout the body. When skin is cut, for example, cells known as fibroblasts exude more extracellular matrix, the network of proteins and other molecules that holds cells together, forming sturdy scar tissue to close the wound.
"Essentially, the extracellular matrix is like a glue," said Wu, who is the senior author of the study, the director of the Stanford Cardiovascular Institute and the Simon H. Stertzer, MD, Professor.
But in many diseases and with advanced age, fibrosis becomes overactive.
It's estimated that 45% of deaths in the developed world are due to fibrosis-induced organ failure.
Hao Zhang
"It's estimated that 45% of deaths in the developed world are due to fibrosis-induced organ failure," Zhang said.
In the heart, cardiac fibroblasts can churn out too much extracellular matrix in response to an injury, such as a heart attack. Too much glue, and the heart stiffens and is more likely to fail.
No drugs have been approved by the U.S. Food and Drug Administration to treat cardiac fibrosis. "There are actually many basic research papers describing anti-fibrotic drugs, but these drugs failed in clinical trials," Zhang said.
A major stumbling block has been the difficulty of acquiring enough human cardiac fibroblasts on which 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 further genetically engineered them to fluoresce when stimulated. In their first round of screening, they stimulated the cardiac fibroblasts and looked for compounds that inhibited fibrosis. They used a machine-learning algorithm to select 20 compounds with promising antifibrotic activity. Through another round of screening, they eliminated those that showed toxicity to other types of heart cells.
Perfecting these techniques took several months, Zhang said, but the screening, conducted by automated robots, was done in a relative flash.
We can screen 1,000 compounds within three to five days.
Hao Zhang
"We can screen 1,000 compounds within three to five days," he said.
From 5,000 contenders, one came out on top -- a derivative of artemisinin called artesunate.
Old drug, new tricks
The researchers found that through an entirely different mechanism, the potent antimalarial drug could partially reverse cardiac fibrosis in cells taken from patients with the condition and in lab-grown heart tissue. It also improved heart function in mice with heart failure.
In fighting malaria, artesunate binds to the heme protein in red blood cells to generate reactive oxygen species, which help destroy the parasite. But with 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.
Because artesunate is already FDA-approved for malaria, the researchers hope it can soon be tested in clinical trials. They plan to develop the drug into pill form -- it's currently given as an injection -- and to study its effect on fibrosis in other organs.
Without the wide net enabled by today's high-throughput screening and expansive compound libraries, researchers would have had little reason to fish out artesunate from the sea of drug candidates. Yet, artesunate exists in today's compound libraries only because of Tu's rediscovery 50 years ago -- "It's in the library because it is on the market," Zhang said -- and instructions relayed from the fourth century.
Illustration: Emily Moskal/Stanford Medicine (olgamoopsi and scisettialfio via Getty Images)
