Perry said U.S. special forces told him one of their men was killed inside the compound and they were trying to extract another serviceman.

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The extract contains oleic acid, a compound that can bind with the stone and protect it, according to Dr. Karen Wilson of the Cardiff School of Chemistry at Cardiff University in Wales.

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But the only firm that achieved any success was Novartis, which in 1998 developed Coartem, an antimalarial drug that combines artemisinin, an extract from the sweet wormwood plant, with a compound called lumefantrine.

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