Evaluating the Pharmacokinetics and Pharmacodynamics of Romiplostim in Patients With Immune Thrombocytopenia (ITP)

Inclusion Criteria: * Diagnosed with immune thrombocytopenia (ITP) for at least 6 months prior to signing the informed consent form (ICF). * Subject is ≥ 18 years old and ≤ 70 years old while signing the ICF. * Patients who have no response or relapsed after splenectomy. Or patients who have not been splenectomised and have completed at least 1 prior treatment for ITP. Excluded having previously received recombinant human thrombopoietin (rHuTPO) or any other thrombopoietin receptor agonist. * The mean of 3 scheduled platelet counts taken during the screening period must be: \< 30 ×10\^9/L, with none \>35×10\^9/L. Exclusion Criteria: * Any known history of bone marrow stem cell disorder. Any abnormal bone marrow findings other than typical of ITP. * Any active malignancy. If prior history of cancer other than basal cell carcinoma or cervical carcinoma in situ, no treatment or active disease within 5 years prior to signing the ICF. * Had received Eltrombopag, recombinant human thrombopoietin (rHuTPO) or other myeloproliferative leukemia (MPL) stimulation product. * Received hematopoietic growth factors (e.g., granulocyte colony-stimulating factor, macrophage colony-stimulating factor, erythropoietin, interleukin-11) for any reason within 4 weeks prior to signing the ICF. * Received any anti-malignancy agents (e.g., cyclophosphamide, 6-mercaptopurine, vincristine, vinblastine, interferon-alfa) for any reason within 8 weeks prior to signing the ICF. * Received any monoclonal antibody drugs (e.g., rituximab) for any reason within 14 weeks prior to signing the ICF. * Less than 4 weeks since end of any clinical trials about therapeutic drug or device for any indication prior to signing the ICF. * Pregnant or breast feeding. * In the opinions of the principal investigator or investigators, the patients are not suitable for participation in this trial.