French healthcare company Sanofi said it would stop further work on amcenestrant, once seen to have large potential against breast cancer, after a second trial failure dealt a major blow to its drug development prospects.

The move puts more pressure on Sanofi to bolster its drug development pipeline as it grows increasingly dependent on its multi-billion best seller, exzecma and asthma treatment Dupixent, and after it fell far behind in the race to develop COVID-19 vaccines.

A trial dubbed AMEERA-5, which was testing amcenestrant on women with newly-diagnosed advanced breast cancer, was stopped early because an independent monitoring panel found no signs of it working."All other studies of amcenestrant, including in early-stage breast cancer (AMEERA-6), will be discontinued," Sanofi said.

The shares dropped 1.5% shortly after the market open at 0700 GMT, for the biggest loss on the French blue chip index CAC40

In March, Sanofi's shares were hit when the company announced disappointing results of a breast cancer study involving previously treated women.

The company has previously said the drug candidate's biggest commercial opportunity was as a treatment early after diagnosis.

"This was a flagship drug in pipeline and important oncology asset," wrote Credit Suisse analyst Jo Walton.

Sanofi said the drug, which was used in combination with Pfizer's established Ibrance in the study, "did not meet the prespecified boundary for continuation," when compared to a group of patients in the trial on standard hormone therapy.

Amcenestrant belongs to a drug class known as selective oestrogen receptor degraders (SERD) to fight tumours that grow in response to oestrogen, which are estimated to account for up to 80% of all breast cancer cases.

The market opportunity for oral SERDs has attracted a range of drugmakers. Sanofi's rivals including Roche and AstraZeneca are working on similar pills, known as giredestrant and camizestrant, respectively.

The drugs are designed to work like AstraZeneca's established injectable breast cancer drug Faslodex, while offering a less burdensome route of administration.

Separately, the plaintiff in the first lawsuit over the heartburn drug Zantac, which has also been weighing on the French company's shares recently, on Tuesday agreed to drop his case.

Sanofi was among a range of companies selling Zantac, which U.S. regulators pulled from the market in 2020.

(Editing by Sudip Kar-Gupta, Edmund Blair, Elaine Hardcastle)