WASHINGTON, June 9 (Reuters) - Actos, a pill to treat diabetes, can prevent development of the disease in people with early symptoms, U.S. researchers reported on Monday.

A study sponsored by Japan's largest drug maker, Takeda Pharmaceutical Co Ltd <4502.T>, showed use of its drug Actos reduced by 81 percent the number of people with so-called pre-diabetes who developed type-2 diabetes, compared to people taking a placebo.

"The clinical response in this study is next to astronomical -- not 100 percent but obviously highly significant," said Dr. Ralph DeFronzo of the University of Texas Health Science Center at San Antonio, who led the study.

Actos and its rival Avandia from GlaxoSmithKline <GSK.L> are insulin sensitizers, belonging to a class of drugs called thiazolidinediones, TZDs or glitazones, which help the body better use insulin.

Type-2 diabetes is caused as the body gradually loses its ability to respond to insulin, a condition called insulin resistance. It is associated with overeating and a lack of exercise, although genes and other factors play a role.

As insulin works less and less well, levels of glucose, or sugar, rise in the blood, damaging blood vessels and organs. The beta cells in the pancreas begins to lose their ability to make insulin.

Doctors can see the disease developing as a patient's average blood sugar goes up over time. Diet and exercise can often prevent this, but experts have also been hoping that drugs such as Actos and Avandia might prevent it.

Studies have previously shown that Avandia, known generically as rosiglitazone, can prevent progression to diabetes. Actos also appears to do so, DeFronzo told a meeting of the American Diabetes Association.

In his study of 602 people with pre-diabetes, those who took Actos recovered part of their insulin production and their bodies became more sensitive to insulin.

He said 10 patients developed diabetes while taking Actos, compared to 45 taking placebo.

"The drug is the best insulin sensitizer we have and it also preserves beta cell function," DeFronzo said in a statement.

Actos, known generically as pioglitazone, has also been shown to help prevent the build-up of fatty deposits in arteries.

Nearly 21 million Americans have diabetes, which causes about 5 percent of all deaths globally every year. The International Diabetes Federation estimates that 300 million people worldwide have pre-diabetes and 230 million have diabetes. (Reporting by Maggie Fox; editing by Will Dunham and Mohammad Zargham)

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