Patients are administered R788 for 24 weeks (double-blind period), followed by R788 for up to 52 weeks (open-label period). Patients who have completed open-label period and meet the criteria are eligible to continue R788 treatment over a 3 year period (extension - period).

Patients are administered Placebo for 24 weeks (double-blind period), followed by R788 for up to 28 weeks (open-label period). Patients who have completed open-label period and meet the criteria are eligible to continue R788 treatment over a 3 year period (extension - period).

Oral administration

The percentage of subjects who achieved stable platelet response (defined as a platelet count of ≥50000/μL at 4 or more of the 6 visits from Weeks 14 to 24)

Period from the first measurement day on which a platelet count of ≥50000/μL for at least 28 consecutive days was achieved to the first measurement day on which platelet count fell below 50000/μL for at least 28 consecutive days

R788 will be administered orally for 24 weeks in the Period I.

R788 placebo will be administered orally for 24 weeks in the Period I.

The percentage of subjects who achieved stable platelet response (defined as a platelet count of ≥50000/μL at 4 or more of the 6 visits from Weeks 14 to 24)

R788 will be administered orally for 24 weeks. On Day 1, 100 mg of the investigational product will be administered. The dose of 100 mg of the study drug will be administered and this must be the only administration of the study drug on Day 1, irrespective of the time of administration. The investigational product will be administered twice daily beginning on the next day (=100 mg bid). If the platelet count at Week 4 or later is below 50000/μL and the investigational product has been well tolerated, the dose of investigational product will be increased from 100 mg bid to 150 mg bid at the discretion of the investigator. The dose during Period I will be adjusted in accordance with the platelet count and tolerance to the investigational product.