Paul Roepe: Testing the Killing Power of Anti-Malarial Drugs
In 2004, Paul Roepe, PhD, described his groundbreaking research as “stupidly simple,” and this year, he used the phrase again to describe another sweet piece of science.
“To a chemist, there’s nothing major about these discoveries — they just needed to be thought of,” laughs Roepe, professor of biochemistry and cellular & molecular biology, and co-director of the Georgetown Center for Infectious Disease.
Roepe, an internationally recognized expert in drug resistant malaria, is being modest about his contributions. Among other achievements, he helped develop a highly effective, much faster, and less expensive test for screening the potency of potential anti-malarial drugs. His test uses a fluorescent dye to quantify parasitic DNA in human red blood cells, measuring the ability of a drug to shut down growth of the protozoa. Earlier tests had added expensive radioactive elements to red blood cell culture to measure the incorporation of this radioactivity in parasitic DNA.
This assay, recently adopted by the U.S. military as well as by malaria research labs around the world, is able to rapidly measure whether a new formula will effectively slow the growth of malarial parasites.
Designing new therapies to treat the infection is vital, he says. Today there are about 10 anti-malarial drugs being used around the world, yet most cannot efficiently and effectively treat all of the 500 million people who are infected with the parasite each year. Nearly two million of them die from the disease, and most of these victims are children.
Moreover, infections by some strains of the mosquito-borne disease are never cured, and resistance to drugs designed to treat them is growing at an alarming rate. There are more than 160 species of malarial parasites that infect vertebrates, and the five that infect humans range from the lethal (Plasmodium falciparum), to the chronically debilitating (Plasmodium vivax).
Despite the success of the assay, Roepe has long realized its use is limited because laboratory analysis doesn’t seem to match up well with clinical experience. “Just because a drug works well in cell culture doesn’t mean it works well in humans,” he says.
He says that the standard for measuring anti-malarial drug potency has long been the “inhibitory concentration 50” or “IC50” value, meaning that the growth of the parasite slows by 50 percent or more, given the continuous presence of the drug in laboratory dishes, relative to growth in the absence of drug. This IC50 test has long been used to distinguish between malaria that is either sensitive or resistant to chloroquine, a drug that has historically been the main agent used to treat or prevent malaria.
But two years ago, he said he had an epiphany. He realized that malaria drugs work differently in their human host. Using chloroquine on malaria that is sensitive to the drug reduces the parasitic load from trillions to billions, and it has to do that within hours “or a person will die,” Roepe says. “That tells me that a good malaria drug has to be cytocidal — it has to be able to kill the bugs — as well as being growth inhibitory.
“In fact, for malaria, you could argue that the cell killing effects of the drug are more important than the growth inhibitory effects,” he says. “Even though the distinction between growth inhibitory and cell killing effects for an antimicrobial drug is actually a straightforward concept in microbiology and infectious disease, no one had designed a test to look at how well a drug can kill malarial parasites.”
So Roepe set out to design his second “stupidly simple” assay, the results of which were published in May in the online edition of Molecular & Biochemical Parasitology.
Roepe and his research assistants, Michelle Paguio, PhD, and Kelly Bogle, developed a new assay to calculate the “lethal dose 50” or “LD50” needed to kill 50 percent of chloroquine resistant Plasmodium falciparum. And they found a remarkable thing: malarial parasites are much more resistant to the killing effects of chloroquine than IC50 scores would have predicted.
“If it takes 10 times more drug to slow drug resistant parasite growth, compared to a sensitive parasite, then it takes 150 times as much drug to kill the drug resistant parasite,” Roepe says.
“That shows why we can’t cure malaria in the clinic. We can’t give such high drug doses or we would kill patients,” he says. “Clinicians knew this intuitively, but basic scientists didn’t.”
Roepe’s new — and inexpensive — test can now be used to test how effective investigational drugs now in the drug development pipeline will be.
“It will provide a critical filter that should speed up this pipeline,” Roepe says. “With it, you will know an essential fact about the drug — how well it kills malaria — and may not need to launch expensive animal studies.
“This kind of test is done all the time in other fields, such as in cancer drug development, but somehow we missed this stupidly simple concept in malaria parasitology,” he says. “I can now say, with great authority – duh!!!”
By Renee Twombly, GUMC Communications