A multicenter, randomized, double-blind placebo-controlled study to report the efficacy and safety of all-trans etinoic acid compared to placebo for the treatment of adults with corticosteriod-resistant/relapsed primary immune thrombocytopenia (ITP).

The investigators are undertaking a parallel-group, multicenter, randomized controlled trial of 192 adults with corticosteriod-resistant/relapsed primary ITP. Patients were randomized to all-trans etinoic acid and placebo group. Platelet count, bleeding, and other symptoms were evaluated before and after treatment. Adverse events are also recorded throughout the study.

Inclusion Criteria:

• Primary ITP if aged ⩾18 years;
• With an average of two platelet counts ⩾1 day apart of <30×10^9/L during screening and no single platelet count >35×10^9/L within 2 weeks before study treatment;
• Patients who have previously received at least one first-line standard therapy for ITP (corticosteroid and/or intravenous immunoglobulin) with unsustained efficacy, relapse, intolerance to standard therapy, or insufficient response.

Exclusion Criteria:

• Pregnant or lactating women, and who were possibly pregnant, planning to become pregnant, or who had partners planning to become pregnant;
• With active malignancy or a history of malignant tumor;
• Having experienced severe bacterial, viral, fungal or parasitic infection within the past 4 weeks;
• With a history of symptomatic herpes zoster infection within 12 weeks prior to screening;
• Active or chronic HBV, HCV or HIV infection;
• Evidence of active tuberculosis; or previous evidence of active tuberculosis without appropriate and documented treatment; or household contact with patients with active tuberculosis without appropriate and documented tuberculosis prophylaxis;
• Receipt of live vaccines within the past 12 weeks, or planned live vaccination during the study period;
• Prior ATRA therapy;
• History of solid organ transplant or planned surgery;
• Myelodysplastic syndrome, aplastic anemia or myelofibrosis;
• Patients with other diseases were undergoing treatment with immunosuppressants;
• Clinically significant thromboembolic events within the past 24 weeks, or ongoing anticoagulant treatment, who are deemed ineligible for the study by the investigator;
• History or presence of myocardial infarction, unstable ischemic heart disease, stroke, or NYHA Class IV heart failure;
• History or presence of cardiovascular, respiratory, hepatic, gastrointestinal, endocrine, neurological, neuropsychiatric or any other severe and/or unstable diseases that, in the opinion of the investigator, may pose an unacceptable risk with the investigational product or interfere with the interpretation of study data;
• AST > 2 times the upper limit of normal (ULN), ALT > 2×ULN, TBIL ≥ 1.5×ULN;
• WBC < 2500/µL, neutrophil count < 1200/µL, lymphocyte count < 750/µL, hemoglobin < 9 g/dL;
• eGFR < 50 mL/min/1.73m²;
• Other patients deemed unsuitable for enrollment in this study by the investigator.