Genentech and partner Biogen Idec have released data which shows that their oncology blockbuster Rituxan has failed to prove effective as a treatment for multiple sclerosis.

The firms said that a Phase II/III study of Rituxan (rituximab) for primary-progressive multiple sclerosis did not meet its primary endpoint as measured by the time to confirmed disease progression during the 96-week treatment period. Genentech’s chief medical officer Hal Barron said “we are disappointed in the outcome of the primary endpoint, but not surprised given the significant clinical challenges presented by PPMS”.

He added that there was some evidence of biologic activity “and we will continue to review all the data to better understand the role of B cells in MS.” Data from the 439-patient study showed that the incidence of serious adverse events were 16.4% in the Rituxan arm versus 13.6% with placebo, with an incidence of serious infections of 4.5% compared with 1.0% respectively.

Michael Panzara, chief medical officer at Biogen’s neurology strategic business unit, acknowledged that “while the primary results are not what we had hoped, we continue to believe in the potential of B cell therapy for patients living with MS”. He added that PPMS is widely considered to be a difficult form of the disease to treat “and historically no therapy has proven efficacy in this disease state”.

PPMS affects 10% of the MS population, the firms said, and is evident by the slow and continuous progression of the disease. Sufferers can experience plateaus in the progression of their disability but generally do not experience “distinct periods of relapse or remissions”.

The news is a blow but it is debatable how big a market there is for an MS indication for Rituxan, which is approved as a treatment for non-Hodgkin's lymphoma and rheumatoid arthritis. Analyst estimates put the market potential at around $200-$250 million in the USA, but neither Genentech or Biogen’s stock is expected to suffer too much given the low expectations held for the PPMS trial in the first place.