Due to its expected efficacy based on the retrospective data available in ITP, its relatively good safety profile and its very low cost , dapsone could be a good steroid-sparing second-line option for adults with ITP.

This study is a phase III prospective multicenter randomized open trial comparing two treatment strategies:

• Arm A (experimental arm): prednisone at 1 mg/kg for 3 weeks + dapsone at 100 mg per day up to week 52 if an initial response is achieved.
• Arm B (control arm): prednisone alone at 1 mg/kg for 3 weeks followed by monitoring and "standard of care" The aim of the study is to demonstrate the efficacy of dapsone based on the overall response rate (including response and complete response) as a second-line treatment for adults with newly-diagnosed persistent or chronic (modified by amendment 08/11/2016) ITP not achieving a durable response with corticosteroids. The primary endpoint will be the overall response-rate (response or complete response according to standard definitions) in both arms at week 52 (1 year).

The secondary endpoints are the following :

• To assess the safety of dapsone over the study period and especially the incidence of cutaneous reactions.
• To analyze the overall response rate (platelet count > 30 x 109/L with at least a doubling of the pre-treatment count in the absence of any other ITP treatment) in both treatment arms at week 24.
• To compare the rate of complete response and failure in both arms at 24 and 52 weeks.
• To compare time to treatment failure (TTF) in both arms
• To investigate the mechanisms of action of dapsone in ITP in a subgroup of patients (ancillary study)

Inclusion Criteria:

• Age ≥ 18 years
• Diagnosis of primary ITP according to the standard definition and international guidelines.
• Previous transient response to corticosteroids ± intravenous immunoglobulin defined by an increase of the platelet count above 30 x 109/L with at least a twofold increase of the pre-treatment count.
• At least platelet count ≤ 30 x 109/L within the 2 weeks before inclusion with a platelet count at time of inclusion below 50 x 109/L, or platelet count < 50 x 109/L at any time point in patients requiring treatment (i.e., patients with bleeding symptoms, elderly patients with comorbidities and/or patients on aspirin for example, or other reason at the investigator discretion) (modified by amendment 8/11/2016)
• Normal marrow aspirate for patients aged of 60 and above.
• Negative pregnancy test and effective method of contraception for women of childbearing age over the study period.
• Informed consent.
• Patient affiliated to the French National Social Security System

Exclusion Criteria:

• Secondary ITP. Patients with positive antinuclear antibodies and/or positive antiphospholipid antibodies not fulfilling the classification criteria for systemic lupus erythematosus and/or antiphospholipid antibody syndrome will not be excluded.
• Platelet count ≥ 50 x 109/L or between 30 and 50 x 109/L and no bleeding symptoms and no need for treatment (modified by amendment 8/11/2016)
• Severe bleeding manifestations defined a bleeding score ≥ 8
• No previous transient response to corticosteroids ± intravenous immunoglobulin.
• Previous ITP treatment other than corticosteroids and intravenous immunoglobulin (including rituximab and splenectomy).
• Active severe infection or history of severe infection within 4 weeks before inclusion.
• History of allergy to sulfonamides.
• Glucose-6-phosphate dehydrogenase (G6PD) deficiency.
• History of methemoglobinemia
• Hemoglobin level < 11g/dL and/or neutrophil count < 1,500/ gL.
• History of autoimmune (Evans' syndrome) or hereditary haemolytic anemia.
• Liver or kidney function impairment (creatinine clearance < 30 ml/min, ALT, AST >2 times upper normal limit). (modified by amendment 8/11/2017)
• Hepatitis C virus (HCV) Ab, HIV Ab, HBsAg, seropositive status. (modified by amendment 8/11/2017)
• Concomitant medical condition requiring anticoagulation. (modified by amendment 8/11/2017)
• Pregnancy or lactation.