April 30 (Reuters) - The first trial over claims that

blockbuster heartburn drug Zantac, once sold by GSK and

other companies, causes cancer is set to begin this week in

Chicago.

Jury selection began on Tuesday before Judge Daniel Trevino

of the Circuit Court of Cook County, with lawyers' opening

statements expected to follow. GSK is the sole defendant in the

trial, after other companies settled.

The plaintiff, 89-year-old Illinois resident Angela Valadez,

alleges in her lawsuit that she developed colorectal cancer as a

result of taking over-the-counter Zantac and generic versions of

it from 1995 to 2014. Like other plaintiffs suing over the drug,

she says its active ingredient, ranitidine, as it ages turns

into a cancer-causing substance called NDMA.

"We're happy that she's getting her day in court," said

Ashley Keller a lawyer for Valadez.

Her case is one of tens of thousands against GSK and other

companies including Pfizer ( PFE ), Sanofi and

Boehringer Ingelheim, which have worried investors in recent

years. It will offer the first test of whether the cancer claims

will persuade a jury, since all cases previously set for trial

settled or were dropped.

The drugmakers have maintained that Zantac does not cause

cancer or contain significant amounts of NDMA under normal

conditions.

"We are confident in our position based on the facts and

science and look forward to presenting our case at trial," GSK

said in a statement.

First approved in 1983, Zantac became the world's best

selling medicine in 1988 and one of the first-ever drugs to top

$1 billion in annual sales. It was originally marketed by a

forerunner of GSK, which faces the largest potential liability,

and later sold successively to the other companies.

In 2020, the U.S. Food and Drug Administration asked

drugmakers to pull Zantac and its generic versions off the

market after NDMA was found in samples of the drug. Thousands of

lawsuits began piling up in federal and state courts.

The defendants notched a significant win in 2022, when a

judge dismissed about 50,000 claims centralized in federal court

in Florida. That judge concluded that the opinions of the

plaintiffs' expert witnesses that Zantac can cause cancer were

not supported by sound science.

Some, but not all, of the claimants in those cases are

appealing the ruling to the Atlanta, Georgia-based 11th U.S.

Circuit Court of Appeals.

Another judge is currently weighing the fate of about 72,000

cases in Delaware state court, where the drugmakers similarly

argue that plaintiffs' expert testimony should be kept out.

Some other cases were previously settled, including several

individual cases just before trial, and about 4,000 state court

lawsuits outside of Delaware against Sanofi. Pfizer ( PFE ) and

Boehringer Ingelheim have not announced any large-scale

settlements.

A new version of Zantac now on the market has a different

active ingredient and does not contain ranitidine.