Primary immune thrombocytopenia is an autoimmune disorder characterised by decreased platelet counts and increased bleeding risk. Corticosteroids have been the standard initial treatment of primary immune thrombocytopenia for more than 30 years. The aim of this randomized controlled trial is to compare the efficacy and safety of high-dose dexamethasone in treating new-diagnosed primary immune thrombocytopenia (ITP) in di-cycle and tri-cycle.

In this multicentre, open-label, randomized controlled trial, about 118 new-diagnosed ITP patients will be enrolled from five tertiary medical centres in China. Eligible participants are randomly assigned (1:1) to 2 groups: group DEX2 and group DEX3. In group DEX2, dexamethasone was administered orally at 40 mg per day for two cycles (days1-4, and days 11-14). In group DEX3, dexamethasone was administered orally at 40 mg per day for three cycles (days1-4, days 11-14, and days 21-24). The clinical effect, onset time, duration of efficacy and adverse reactions were observed to compare the efficacy and safety of two different plans.

Inclusion Criteria:

• Participant must be at least 18 years of age at the time of the screening.
• Participant may be male or female.
• Participant has a confirmed diagnosis of newly diagnosed ITP according to the 2019 International Working Group assessment at screening, and has a baseline platelet count of less than 30 × 10^9 cells per L or had bleeding manifestations, or both.

Exclusion Criteria:

• Participant has evidence of a secondary cause of immune thrombocytopenia (e.g. leukemia, lymphoma, common variable immune- deficiency, systemic lupus erythematosus, autoimmune thyroid disease, past medical history of untreated H. pylori infection) or to drug treatments (e.g. heparin, quinine, antimicrobials, anticonvulsants) or participant has a multiple immune cytopenia, e.g. Evan's syndrome.
• Participant has clinically life-threatening bleeding (e.g. central nervous system bleeding, menorrhagia with significant drop in hemoglobin).
• Participant has a history of coagulopathy disorders other than ITP.
• Participant has a history of arterial or venous thromboembolism (e.g. stroke, transient ischemic attach, myocardial infarction, deep vein thrombosis or pulmonary embolism) within the 6 months prior to randomization or requires anticoagulant treatment.
• Participant has 12-lead ECG with changes considered to be clinically significant upon medical review at baseline.
• Participant has severe renal impairment (glomerular filtration rate less than 45ml/min/1.73 m2).
• Participant has 3 × upper limit of normal of any of the following: alanine aminotransferase, aspartate aminotransferase, or alkaline phosphatase.
• Participant with any of the following conditions: severe immunodeficiency, active or previous malignancy, human immunodeficiency virus (HIV), hepatitis B or C virus infection, pregnancy or lactation.