Shares of Berkeley, Calif., biotech Xoma ( XOMA) fell sharply in the premarket session after phase II tests studying Raptiva as a rheumatoid arthritis treatment were ended by it and Genentech ( DNA).

The studies were called off after showing Raptiva was having no overall clinical benefit in arthritis patients receiving it. The treatment continues to be evaluated by the FDA as a treatment for psoriasis.

Xoma was recently off 55 cents, or 10%, to $4.90 on Instinet.