Novartis advanced its bid to copies Roche's multi-billion dollar antibody drug rituximab on Monday, starting phase II trials for its own version of the blockbuster medicine.

Drugs firms are racing to come up with versions of Roche's drug Rituxan, which raked in 6.1 billion Swiss francs ($6.33 billion) for it in 2009. Known as MabThera in Europe, it is used to treat non-Hodgkin's lymphoma (NHL) and rheumatoid arthritis.

The Swiss drugmaker's generics unit Sandoz started mid-stage trials in rheumatoid arthritis patients for the so-called biosimilar version of the drug, Sandoz said on Monday.

Biosimilars - generic copies of biotech drugs used to treat complex diseases like cancer - are becoming increasingly important means of building revenues as global pharmaceutical companies face the loss of patent protection on many of their established drugs.

"The launch of the phase II program for biosimilar rituximab clearly underlines Sandoz's leading position in the area of biosimilars and is good news for Novartis," Vontobel analyst Andrew Weiss said.

The Novartis unit is the market leader and has nearly 50% market share within the global regulated biosimilar market and three marketed products.

Sandoz's news comes after Spectrum Pharmaceuticals said last week it had signed a deal to develop a biosimilar version of the drug, which loses patent protection in the coming years.

Israel's Teva Pharmaceuticals Industries said last year it was launching a second clinical trial using a copies of Rituxan to treat non-Hodgkin's lymphoma. It already has a study under way comparing the original version of the drug with its copies, known as TL011, in patients with rheumatoid arthritis.

Worldwide sales of all biologic drugs reached $130 billion in 2009, according to IMS Health, and industry analysts believe the potential market for biosimilar copies could be worth tens of billions of dollars by the second half of the decade.