By John Miller

ZURICH (Reuters) – Swiss drugmaker Roche aims to broaden the use of its Kadcyla breast cancer treatment as rivals crowd into the market with biosimilar copies of its older mainstay Herceptin.

Roche said on Tuesday it had applied for U.S. Food and Drug Administration approval for Kadcyla, a five-year-old drug, for post-surgical use in women with a form of early stage breast cancer who still show signs of disease after treatment with Herceptin and chemotherapy.

Kadcyla, which generated sales of 979 million Swiss francs ($981 million) last year, is already approved for people with metastatic HER2-positive breast cancer.

Expanding use of Kadcyla, which works by attaching Herceptin to a chemotherapy drug developed by ImmunoGen Inc to interfere with cancer cell growth, could help Roche defend its turf in treating women whose cancer contains increased amounts of a protein known as HER2.

Herceptin is Roche’s biggest-selling drug, at about $7 billion last year, but its patent has expired and its European sales last year tumbled 16 percent. The company sees possible U.S. competition in the second half of 2019 from rivals including Celltrion and Teva, Pfizer and Mylan.

“We are working closely with the FDA to bring Kadcyla to people with HER2-positive early breast cancer who have residual disease after neoadjuvant therapy as early as possible,” Sandra Horning, Roche’s chief medical officer, said.

Approval of Kadcyla for such patients would help Roche secure a big share of Herceptin sales now made to patients following surgery, an analyst said.

“With Kadcyla, the approximately 60 percent share of sales generated by Herceptin as an adjuvant would be secured for several years,” wrote Michael Nawrath, of Zuercher Kantonalbank, in a note.

“This prospect is an underrated asset in the market,” Nawrath said.

Roche shares were up 0.5 percent after the announcement, in line with the rise of the Stoxx Europe 600 Health Care Index.

Of people treated with Kadcyla, 88.3 percent did not see their breast cancer return after three years – compared to 77 percent treated after surgery with Herceptin – according to a clinical study Roche is using to underpin its approval case with the FDA.

The FDA has granted the medicine a speedy review, Roche said, pointing to a decision in coming months.

($1 = 0.9983 Swiss francs)

(Reporting by John Miller; Editing by Michael Shields and Susan Fenton)