Dec. 11, 2003
Vol. 23 No. 6

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    Researchers discover a new delivery system for antimicrobial medications

    By John Easton
    Medical Center Public Affairs

    A multicenter research team led by a University researcher has discovered how to deliver antimicrobial medications directly to the infectious parasites that cause such diseases as toxoplasmosis, even when the parasites lay hidden and inactive within cysts, where they have been untreatable by any available medicines.

    The study, which was published online Monday, Nov. 17, by the Proceedings of the National Academy of Sciences, demonstrates the first effective, non-toxic method of transporting drugs across multiple membrane barriers and even into cysts of Toxoplasma gondii, the single-celled microorganism that causes toxoplasmosis. It also describes a new therapeutic target within this common parasite.

    “This is a major step forward in developing ways to treat one of mankind’s most common chronic infections,” said Rima McLeod, Professor of Ophthalmology and Visual Sciences and director of the study. “For the first time, we have access to this microbe in its latent stage, a part of its life cycle that was previously inaccessible. We also have a better means of delivering medicines to its active stage, as well a new target for treatment.”

    The authors noted that better approaches to treating toxoplasmosis, which is spread by cats and by eating undercooked meat, are needed. The disease can cause devastating problems for those with weakened immune systems or when transmitted from mother to unborn child.

    About 3,000 infants in the United States are born each year with toxoplasmosis, which causes severe eye damage, mental retardation and death. The cost of caring for these children is estimated to exceed $5 billion per year.

    In addition to active infections, Toxoplasma gondii in its latent stage infects the nervous system of an estimated 3 billion people, including about 30 percent of Americans.

    The new delivery system uses a short chain of eight connected arginines, a naturally occurring amino acid, to ferry a drug across membranes. In 1996, Paul Wender and Jonathan Rothbard at Stanford University led a research team that discovered short sequences of arginine could slip easily through biological membranes, either alone or attached to active molecules.

    While Wender’s team refined this delivery system, McLeod and colleagues searched for new drug targets within Toxoplasma gondii and related parasites from the apicomplexan family, which includes the causes of malaria and cryptosporidiosis.

    This family of microbes relies on enzymes that are not present in animals. Because the microbes require these enzymes to live and animals do not, they make ideal targets for treatment with minimal toxicity.

    One such target is T. gondii -enoyl reductase, an enzyme first identified in McLeod’s lab. The parasites require this enzyme to synthesize fatty acids, necessary for survival.

    In 2001, a research team that included McLeod showed that triclosan, a common antiseptic used in toothpaste, skin creams and mouthwash, can kill the parasites responsible for toxoplasmosis and malaria. In the Proceedings of the National Academy of Sciences paper, the researchers show that triclosan’s antimicrobial effect comes from its ability to inhibit enoyl reductase.

    The problem, however, has been how to get triclosan to the parasite. Even in its active stages, Toxoplasma gondii live in the host’s cells and are inaccessible to drugs. But soon after infection, many of the parasites enter a latent stage, called bradyzoites, causing chronic infection. Bradyzoites infect the central nervous system, including the eyes, often hiding within cysts inside the host’s cells.

    The arginine chains, linked to triclosan, could rapidly cross multiple animal and microbial membranes. They were able to enter the host cell, pass through its internal barriers and cross into cysts, which are surrounded by densely packed animal and parasite constituents.

    Within a cyst, the arginine compound could enter the parasite, cross into its specialized organelles and then release the triclosan in a way that inhibits the target enzyme. Learning the basic structure of this enzyme has enabled the team to attach triclosan to the arginines in the most effective way. The system inhibits the parasite in mice and in tissue culture.

    “We found this quite remarkable,” said McLeod. “No current antimicrobial compound can cross the cyst wall, and development of new small-molecule medicines is hampered considerably by our inability to deliver them inside cells and the organism.”

    The discovery raises the possibility of treating active and latent infection in the eye by applying a lotion containing triclosan or other antimicrobials bound to a transporter that would carry it into the eye.