* Drugs could be used to fight dangerous forms of TB

* Compounds already proven in humans

By Julie Steenhuysen

CHICAGO, July 2 (Reuters) - Drugs used to treat Parkinson's disease show promise as a new way to stem the rise of drug-resistant forms of tuberculosis, U.S. researchers said on Thursday.

They said computer models and lab experiments suggest the drugs tolcapone or Tasmar made by Valeant Pharmaceuticals <VRX.N>, and entacapone or Comtan made by Novartis AG <NOVN.VX> have the potential to treat multiple-drug-resistant and extensively drug-resistant strains of TB.

Computer programs predicted the chemically similar drugs should interfere with the TB bacillus, and tests in lab dishes using the drug Comtan confirmed it, the researchers said.

About 1.8 million people die worldwide each year from tuberculosis and a third of the world's population -- 2 billion people -- is infected, according to the World Health Organization.

Many people unknowingly have latent infections that can turn active if their immune system becomes weakened with other infections, such as HIV.

The WHO says that of 9 million new TB cases annually, about 490,000 are multiple-drug resistant TB or MDR-TB and about 40,000 are extensively drug resistant or XDR-TB.

"Given the continuing emergence of M. tuberculosis strains that are resistant to all existing, affordable drug treatments, the development of novel, effective and inexpensive drugs is an urgent priority," said Sarah Kinnings, a graduate student at the University of California, San Diego, who led the study.

Kinnings and colleagues used computer models and lab experiments to look for established drugs that might be of use in treating resistant forms of TB.

They found that the active component in both Comtan and Tasmar -- which are used to boost the effectiveness of the Parkinson's drug levodopa -- can also block the multiple-drug resistant tuberculosis bacterium.

The drugs block a brain chemical called COMT, in turn stopping it from breaking down Parkinson's drugs. But their molecular structure also allows them to block a compound that TB bugs need to build their protective cell wall.

Tasmar can damage the liver but Comtan is safer and could be used against TB, said Philip Bourne of the University of California, San Diego, who worked on the study published in the Public Library of Science journal PLoS Computational Biology.

"We have computational and experimental data to support this repositioning," Bourne, a professor of pharmacology, said in a statement.

He said the drugs are known to be safe in humans, and lab tests suggest they may be effective at blocking the bacterium.

Globally, cases of drug-resistant tuberculosis are being recorded at the highest rates ever seen, according to the World Health Organization. They are a particular threat in Russia and other former Soviet republics, India, China and South Africa.

(Editing by Maggie Fox and Vicki Allen)

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