LONDON, June 10 (Reuters) - New clinical data has bolstered the case for using cheaper copies of expensive biotech drugs by showing that patients with rheumatic diseases can be safely and effectively switched from original-brand medicine to a lower-cost version.

While the first copies of a blockbuster antibody drug for rheumatoid arthritis are now on sale in Europe, such so-called biosimilars have so far been used mainly in new patients, given doubts about how existing patients might react to a switch.

An independent Finnish study, however, found that patients with rheumatoid arthritis and ankylosing spondylitis experienced comparable clinical effectiveness and safety after switching from Merck and Johnson & Johnson's branded Remicade, or infliximab, to a cheaper biosimilar copy.

The full results of the study will be presented on Wednesday by Tuulikki Sokka of Finland's Jyvaskyla Central Hospital at the European League Against Rheumatism annual congress in Rome.

"This post-market study shows that patients with various rheumatic conditions who are stable on Remicade treatment don't experience a loss of effectiveness or unexpected side effects when switched to biosimilar infliximab," Sokka said.

She believes the data, involving 39 patients with an average four years experience of Remicade, who were given a biosimilar for 11 months, should improve doctors' confidence and help reduce healthcare spending.

The potential for biosimilars to take business from original brands is grabbing investors' attention and was a key factor in Pfizer's decision in February to buy injectable drugmaker Hospira for about $15 billion.

Hospira sells a biosimilar copy of infliximab under a partnership deal with South Korean firm Celltrion.

The arrival of biosimilars threatens companies heavily reliant on biotech drugs, like AbbVie and Roche , and Citigroup has predicted a transfer of at least $110 billion of value from innovator companies to copycat producers in the next decade.

The big unknown is how quickly biosimilars will be adopted in clinical practice, which in turn hinges in large part on the price discounts on offer.

Merck said at its first quarter results in April that copycat versions of Remicade were being offered at discounts of about 45 percent to the brand's price in Europe, steeper than the company's initial expectations of 30 percent.

(Reporting by Ben Hirschler. Editing by Jane Merriman)