Duke Researchers Identify The Mechanisms By Which Drug Combinations Kill Fungal Infections

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http://www.sciencedaily.com/releases/2002/02/020207075321.htm

Source: Duke University Medical Center (http://www.mc.duke.edu/)

Date: Posted 2/12/2002

Duke Researchers Identify The Mechanisms By Which Drug Combinations Kill

Fungal Infections

DURHAM, N.C. - Molecular biologists in the Mycology Research Unit at Duke

University Medical Center have traced cellular pathways that are targeted to

enhance the action of drugs used to treat fungal infections in people with

compromised immune systems, such as those undergoing organ and bone marrow

transplants. The discovery of how the immune-suppressing drugs enhance the

action of antifungal drugs could lead to the development of new drugs

designed to treat fungal infections such as systemic yeast infections and

often-deadly cryptococcal infections.

Led by Dr. ph Heitman, who is a Medical Institute

investigator at Duke, the research team explored the mechanism behind the

effectiveness of administering the azole class of antifungal drugs such as

fluconazole (Diflucanâ), with cyclosporin or FK506, two drugs commonly used

to suppress rejection of transplanted organs and tissues.

The study is published in the Feb. 15 issue of the EMBO Journal, a journal

published by the European Molecular Biology Organization.

Cyclosporin and FK506 block rejection of transplanted organs by inhibiting

immune-system signaling pathways that activate T-cells. These drugs block

calcineurin, an enzyme that plays a critical role in activating immune

cells. Suppression of the immune system is necessary for transplanted organs

to survive, but renders patients susceptible to infections by bacteria,

viruses and fungi.

" The azoles are very nontoxic in humans, but the problem is they don't kill

fungal cells. Instead, these drugs act by inhibiting fungal cell growth. The

cells stop growing, but they don't die. As a result, a lot of fungal

isolates become resistant to the azoles, " Heitman said.

The study was funded by grants from the National Institute of Allergy and

Infectious Diseases.

Unlike bacteria or viruses, fungi are eukaryotic cells that resemble cells

of the human body. Fungal infections can thus be difficult to treat, Heitman

said. The majority of transplant recipients suffer one or more infections

and these infections are a significant cause of morbidity and mortality.

Cryptococcal infections occur to two to three out of every 100 transplant

recipients and is associated with a 50 percent mortality rate. Candida

albicans, which can cause thrush, esophagitis or vaginitis, is the most

common fungal infection and is common in hospital settings.

Heitman said these findings open the door to the clinical use of drug

combinations and could lead to the identification of additional drug

targets.

" There's an ongoing need to develop better antifungal drugs. There is an

increasing population of people who are at risk and some infections prove

very difficult to treat, " Heitman said.

" There are relatively few drugs to treat these infections, some have serious

side effects, and drug resistant isolates have emerged. For some fungal

infections, such as aspergillus infections in the lungs and cryptococcal

infections in the brain, there is a 50 percent risk of death. These studies

reveal new ways in which existing drugs can be combined to combat fungal

infections and improve therapy, " he said.

Combining fluconazole with either cyclosporin or FK506 was previously found

by other researchers to potently kill fungal cells in the test tube, and had

been proposed as an approach to combat difficult to treat fungal infections.

The multi-drug antifungal treatment concept was tested in mice in 2000 by a

research team led by Dominque Sanglard, an investigator at the CHUV Hospital

in Lausanne, Switerzerland, and an international scholar of the

Medical Institute. Their studies demonstrated that the combination of

cyclosporin and fluconazole successfully eradicated fungal infections, but

the cellular targets and the mechanism of drug action were not understood.

Elucidating the molecular targets could open the door for new or improved

drugs, Heitman said.

" The big question was, 'What are the molecules that cyclosporin and FK506

target to allow the azole drugs to kill the fungal cell?' One previous

proposal was that cyclosporin inhibits the pumps that extrude the

fluconazole drug from the cell. We found that this is not the mechanism of

action. Instead, both cyclosporin and FK506 enter the fungal cell and

inhibit calcineurin, which is their well-established target.

" We discovered that calcineurin is a component of a fungal stress response

that allows the cell to survive assault on the membrane by the azoles.

Inhibition of calcineurin cripples this stress response and now allows the

azoles to kill rather than simply maim the fungal cells, " he said.

The researchers at Duke, which also included Elena Cardenas and

McCusker from the departments of genetics and microbiology, and Dr.

Perfect from the department of medicine, used molecular genetic approaches

to pinpoint calcineurin as the target of the synergistic drug dynamic duo.

When a mutation was introduced into calcineurin that prevents binding to

FK506, this conferred resistance to the drug combination. They also made

loss of function mutations in which they removed calcineurin from the cell.

The cells were viable, but they now died when exposed to azole drugs that

fail to kill wild-type cells. Thus, these studies reveal new ways in which

existing drugs can be combined to combat fungal infections and improve

therapy.

Note: This story has been adapted from a news release issued by Duke

University Medical Center for journalists and other members of the public.

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http://www.sciencedaily.com/releases/2002/02/020207075321.htm