(Adds details on rival drugs, first U.S. biosimilar Zarzio)

By Ransdell Pierson

Feb 5 (Reuters) - U.S. Food and Drug Administration staff members on Friday said clinical data from Celltrion Inc's biosimilar form of blockbuster arthritis drug Remicade indicated it is "highly similar" to the branded product in terms of safety, purity and potency.

FDA scientists released their supportive findings ahead of a scheduled meeting on Tuesday of an independent medical advisory panel to the agency. The panel will decide whether to recommend approval of the biosimilar, called Remsima, to treat rheumatoid arthritis and inflammatory bowel conditions such as Crohn's disease.

South Korea's Celltrion and U.S. partner Pfizer Inc want to sell Remsima in the United States as a cut-price copy of Johnson & Johnson and Merck & Co's Remicade, or infliximab, which had global 2014 sales of almost $10 billion.

"In considering the totality of the evidence, the data submitted by Celltrion show that (Remsima) is highly similar to U.S.-licensed Remicade" in treatment of rheumatoid arthritis and a related condition called ankylosing spondylitis, the FDA briefing paper said. It cited only "minor differences in clinically inactive components."

Celltrion presented the FDA with data on the use of Remsima against rheumatoid arthritis and ankylosing spondylitis, but the agency staff said it would be reasonable to assume that it is also effective and safe for other conditions for which Remicade is approved, including Crohn's and ulcerative colitis.

Celltrion already sells the moncolonal antibody in dozens of other countries.

Remsima would become the second approved biosimilar in the United States, following Novartis AG's September introduction of Zarzio. That drug is a version of Amgen Inc's Neulasta antibody, which boosts infection-fighting white blood cells in patients who have undergone chemotherapy.

Antibody medicines, which are grown in living cells, are among some of the world's biggest-selling medicines. They are used to treat diseases ranging from cancer to eye disorders.

The potential of biosimilars to wrest sales from original blockbuster brands was a key factor behind Pfizer's decision last year to buy hospital products company Hospira. The deal gave Pfizer U.S. marketing rights to Remsima and rights to other biosimilars.

Remicade, like Amgen's Enbrel and AbbVie Inc's $14 billion-a-year Humira, reduces inflammation by blocking a protein called Tumor Necrosis Factor. The injectable drugs, however, can increase the risk of infection.

(Reporting by Ransdell Pierson; Editing by Lisa Von Ahn)