By Deena Beasley

June 3 (Reuters) - A copycat version of Roche Holding AG's ROG.S breast cancer drug Herceptin worked as well, and as safely, as the original biotechnology treatment in the first-ever study to compare such a "biosimilar" to a brand-name drug.

Results from the late-stage study of Mylan NV's drug, known as MYL-14010, versus Herceptin were presented on Friday at the annual meeting of the American Society of Clinical Oncology. Herceptin is used in about 25 percent of breast cancer patients whose tumors generate a protein called HER-2.

Biotech drugs are made from living cells, making it impossible to manufacture exact copies, unlike treatments manufactured from chemicals which can have generic versions after their patents expire. As a result, regulators have accepted the notion of approving copycat biotech treatments that are similar enough in safety and efficacy.

Biosimilars have been on the market in Europe since 2006, some at steep discounts of more than 50 percent to the original treatments. A U.S. regulatory process for biosimilars was established in 2010 and two such copycats have since been approved.

In the new study, nearly 70 percent of trial patients responded to MYL-14010, meaning their tumors shrank, compared with a 64 percent response rate for Herceptin. Serious side effects occurred in 38 percent of patients treated with the biosimilar and 36 percent of patients given Herceptin, also known as trastuzumab.

"Trastuzumab has markedly improved survival of women with HER2-positive breast cancer, but many women around the world can't benefit from trastuzumab due to its high cost," lead study author Dr Hope Rugo, professor of medicine at the University of California San Francisco, said in a statement.

Herceptin costs around $5,500 a month in the United States, generating worldwide sales of more than $6.5 billion for Roche last year. Mylan's version was launched in India in 2014, but the company declined to comment on where the drug is currently sold or whether it is seeking U.S. approval.

Novartis last September began selling Zarxio, its form of Amgen Inc's white blood cell-boosting product Neupogen in the United States. Zarxio is priced at a discount of about 15 percent to the original drug.

In April, U.S. regulators approved Inflectra, a version of Johnson & Johnson's arthritis drug Remicade developed by Celltrion Inc and Pfizer Inc PFE.N .

Express Scripts , the largest manager of drug benefit plans for U.S. employers and insurers, estimates the country could save $250 billion between 2014 and 2024, assuming 11 specific biosimilars reach the market.

(Reporting By Deena Beasley; Editing by Michele Gershberg and David Gregorio) ((deena.beasley@thomsonreuters.com; 213 955 6746; Reuters Messaging: deena.beasley.thomsonreuters.com@reuters.net))