Last year, scientists elsewhere noted the drug's ability to kill fungi. Now, a Johns Hopkins research team has found that amiodarone kills by disrupting cells' interior and exterior calcium balance.
Importantly, this is completely different from the way in which commonly used antifungal agents miconazole and fluconazole stop fungal growth, say the researchers. In their experiments, a combination of amiodarone and either of the "azoles" killed dramatically more fungi than expected.
"This antiarrhythmic drug, in low doses, combined with azoles looks very promising in the lab and appears to be worth taking a look at clinically," says Rajini Rao, Ph.D., associate professor of physiology in the Johns Hopkins School of Medicine's Institute for Basic Biomedical Sciences. "The azoles prevent fungi from growing, but don't actually kill them -- the immune system is supposed to do that. Amiodarone kills them instead of keeping them dormant."
While fungal infections in general are easy to get rid of, in patients with depressed immune systems or with conditions like cystic fibrosis that increase susceptibility to chronic or recurring fungal infections, prolonged use of azoles can lead to the fungus becoming resistant to treatment. Finding a way to treat severe and life-threatening fungal infections with an option that boosts the effects of azoles without relying on the strength of the immune system is attractive, says Rao, who studies how calcium is brought into cells.
"Importantly, amiodarone and miconazole or fluconazole were synergistic, killing more fungus together than expected by just adding their individual results," says Rao. "What we've learned about amiodarone's effects on cellular calcium levels may one day help explain its side effects, too."
The drug is currently approved by the Food and Drug Administration to treat life-threatening and severe disturbances in the heart's natural rhythm, known as arrhythmias. At higher doses or given for a long time, amiodarone's side effects can include damage to the lungs and thyroid. Rao stresses that amiodarone has not been tested in animals or in people to see if it can fight a fungal infection the way it can in a laboratory dish.
In the current study, visiting medical student Soma Sen Gupta and graduate student Van-Khue Ton tested amiodarone on a collection of yeast mutants, each one missing a different gene. Yeast without genes critical to counteracting the drug's activity were particularly susceptible to amiodarone. Genes whose products regulate the transport of calcium into and out of the cell, and those responsible for controlling intracellular stores of calcium, were most important, the researchers discovered.
In another set of experiments, undergraduate student Veronica Beaudry and Kyle Cunningham, Ph.D., an associate professor of biology at The Johns Hopkins University, discovered that amiodarone first causes calcium to pour into the cell's interior from the outside and then causes its release from storage spaces inside the cell. This one-two punch makes the cells begin a death spiral, says Rao.
While yeast isn't life threatening, some fungi can be pretty nasty in people with depressed immune systems. In laboratory experiments, low doses of amiodarone combined with fluconazole killed about 95 percent of two pathogenic fungi, Candida albicans and Cryptococcus neoformans. Fluconazole by itself cut growth of the fungi by 25 percent, and amiodarone alone by just 10 percent, the researchers report.
The experiments were funded by the National Institute of General Medical Science, a Burroughs Wellcome Student Elective Prize, and the American Heart Association Mid-Atlantic Affiliate. Authors on the study are Sen Gupta, Ton, Beaudry, Samuel Rulli, Cunningham and Rao. Sen Gupta has returned to the Lucy Cavendish College at the University of Cambridge. Rulli is now at the National Cancer Institute.
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