May 2 (Reuters) - U.S. attorneys for a woman who claims

her colon cancer was caused by the now discontinued heartburn

drug Zantac on Thursday told a jury in Chicago that

pharmaceutical companies GSK and Boehringer Ingelheim knew the

product could cause cancer if it was not handled properly but

failed to warn the public.

Mikal Watts, who is representing 89-year-old Angela Valadez,

said the companies knew that Zantac's active ingredient,

ranitidine, would turn into a cancer-causing substance called

NDMA as it aged or was exposed to extreme temperatures, but did

not ensure it was properly handled by transporters, distributors

and stores.

Valadez's case is one of tens of thousands against GSK,

Boehringer Ingelheim and other pharmaceutical companies, which

have worried investors in recent years. It will offer the first

test of whether the cancer claims in the long-running litigation

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

settled or were dropped.

GSK and Boehringer Ingelheim are the only defendants in the

trial, after other companies settled.

Watts told the jury the pills would change color as they

degraded, but the companies would cover it up.

"They know we're not going to take a product that looks bad,

so they put a paint job on it," Watts said.

Attorneys for GSK, which developed the active ingredient in

Zantac but later sold the brand to other companies, and

Boehringer Ingelheim, which sold the drug from 2006 until 2017,

countered that Zantac has been repeatedly proven to be safe and

effective and that no scientific or medical study had connected

Zantac to cancer.

There is "no objective evidence linking Ms. Valadez's cancer

to Zantac," GSK's attorney Tarek Ismail told the jury. "No

genetic test, lab report, imaging study... nothing whatsoever."

Valadez, who said she took Zantac every day for at least 18

years, had a host of risk factors that made her more likely to

develop colon cancer, Ismail said.

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 and later sold successively to 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.

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

active ingredient and does not contain ranitidine.

The companies 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.

More than 70,000 Zantac cases remain pending in the U.S.,

many of them in Delaware state court where a judge is

considering similar arguments from drugmakers 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 French drugmaker Sanofi SA

.