Rigel Pharmaceuticals (RIGL) - Get Report said Tuesday that its experimental drug, fostamatinib, met the response rate primary endpoint in a phase III study of patients with an autoimmune platelet disease.

The study showed that 18% of patients treated with fostamatinib achieved a stable platelet response compared to none receiving a placebo. The benefit was statistically significant, Rigel said.

All the patients enrolled in the study were diagnosed with chronic/persistent immune thrombocytopenia, or ITP, a disease in which the patient's own immune system attacks and destroys platelets, a component of blood needed for clotting and healing.

Fostamatinib works by blocking Syk, a protein involved in the destruction of blood platelets by the immune system.

Rigel is conducting a second phase III study of fostamatinib in chronic ITP with results expected later this year. If the second study is also positive, the company intends to file for the drug's approval in the first quarter of 2017.

The 18% response rate for fostamatinib in the phase III study was on the lower end of investor expectations. Investors were looking for a response rate in the high teens to mid-20% range. J.P. Morgan biotech analyst Anupam Rama called the Rigel result a "grind it out win" more than a home run.

There were no new, significant safety concerns reported in the phase III study. The most frequently reported adverse events were gastrointestinally related, Rigel said.

J.P. Morgan's Rama forecasts that peak sales of fostamatinib could reach $360 million if the drug is approved. His model assumes the drug is used to treat chronic ITP patients who do not respond to other drugs approved for the disease.

Rigel shares were up 10% to $2.91 in Tuesday premarket trading.

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