(Adds analyst comments, updates share prices)
TOKYO, March 18 (Reuters) - Shares in Chugai Pharmaceutical Co (4519.T), the Japanese subsidiary of Swiss drugmaker Roche Holding AG (ROG.VX), fell 5.9 percent after Chugai reported 15 deaths among users of its Actemra arthritis drug.
Japan was the first country to approve Actemra for rheumatoid arthritis. Roche has seen the drug as a potential blockbuster.
Actemra won approval in Europe in January, but has yet to be approved in the United States. [ID:nLL414111] [ID:nWNAB9431]
Chugai shares closed down 94 yen at 1,497 yen, underperforming the benchmark Nikkei average .N225, which gained 0.3 percent, after it reported the deaths among 4,915 users of the treatment.
Roche shares were down 1.2 percent at 145.30 euros.
"The fact that the causal relationship was not denied means there is a possibility that Actemra may have had an impact," said Chugai spokesman Masayuki Yamada.
One analyst, however, said a death rate of 0.3 percent was not a surprise and would not affect the outlook for the drug's marketability.
The data "may seem like negative news at first glance, but we believe it will not affect significantly the potential of the drug", Merrill Lynch analyst Ritsuo Watanabe said in a note to clients.
Chugai said in a monthly report on Actemra's side effects that it could not deny the possibility of a link between the deaths and the use of Actemra.
It said there had been 221 cases of serious side effects.
Known chemically as tocilizumab, Actemra is an anti-interleukin-6 receptor antibody and works in a different way than the existing multibillion-dollar-selling class of biotech drugs for arthritis that block an inflammatory protein called tumor necrosis factor (TNF).
Roche's pharmaceuticals chief, William Burns, has projected peak sales of the drug at $2 billion to $4 billion a year, though analysts forecast lower sales. [ID:nN06452658]