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Clioquinol, whose full chemical name is iodochlorhydroxyquin, is a drug used for treating fungus and protozoan infection. It works by blocking certain enzymes that are responsible for DNA replication and thereby stopping any fungus or protozoan, as in the case of amebic dysentery, from growing. If taken internally in large doses, it can cause harm to the nervous system tissues.
For the most part, clioquinol is used only topically. It is often combined with hydrocortisone cream to treat infections of the skin. This medication can help provide relief from the itching, redness, and overall discomfort that these types of infections produce.
Originally, clioquinol was used primarily as a drug for its antiprozoan properties. The drug has been largely discontinued, or at least severely restricted, in some countries as a result of an outbreak of subacture myelo-optic neuropathy (SMON) in Japan. It was estimated that somewhere between 10,000 and 30,000 people contracted this disease during the outbreak, which caused paralysis, blindness, and even death during a twenty-year period from the mid-1950s to the mid-1970s. A Tokyo court ruled in 1978 that clioquinol was responsible for the epidemic.
There has been no definitive explanation as to why clioquinol caused SMON, leading some to believe that it was another factor that contributed to the high incidence of the disease. One particular argument was that it was used 20 years prior to the outbreak without any problems being reported. There are many theories as to the cause, including improper dosage, interaction with another compound, and interaction with another virus.
Despite the controversy surrounding clioquinol, there has been a resurgence of interest, primarily for its possible role in preventing neurodegenerative diseases. It has been shown to stop the progression of Alzheimer’s disease in clinical trials. Researchers believe the drug stops the progression because it helps chelate zinc and copper ions. It has also shown positive results in helping with Huntington’s disease. Further studies were done on animals at McGill University in Canada, which showed it could reverse progression not just of Huntington’s and Alzheimer’s, but also Parkinson's.
It is currently manufactured in a variety of countries throughout the world. Denmark has used it for protozoan infections. The US has used it indirectly, in that an Australian company has the rights to produce the drug for use in that country, primarily for use with Alzheimer’s disease. It is also manufactured in India, primarily for topical use.
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