This study is a prospective, multicenter, open-label, single-arm cohort study to evaluate the effectiveness and safety of romiplostim in patients with immune thrombocytopenia (ITP) who have not responded to oral thrombopoietin receptor agonists (TPO-RAs) such as eltrombopag, hetrombopag, or avatrombopag.

ITP is a blood disorder in which the immune system attacks and destroys the body's own platelets, leading to low platelet counts and an increased risk of bleeding. While oral TPO-RAs are effective for many patients, approximately 20-30% of patients do not respond adequately or lose response over time. For these patients, alternative treatments are urgently needed.

Participants in this study will discontinue their current oral TPO-RA and receive romiplostim as a weekly subcutaneous injection. The starting dose is 3 µg/kg, and the dose may be adjusted weekly based on platelet counts (maximum 10 µg/kg). The total treatment and follow-up period is 24 weeks.

The primary outcome measure is the durable platelet response rate at week 24, defined as maintaining platelet counts at ≥50×10⁹/L for at least two scheduled visits during weeks 22-24 without rescue therapy. Secondary outcomes include the sustained response rate at week 12, complete response rate at week 24, time to first response, improvement in bleeding events, and quality of life assessment.

Safety outcomes include monitoring for thromboembolic events, bone marrow fibrosis, hepatotoxicity, injection site reactions, and other adverse events according to CTCAE v5.0.

Approximately 60 participants will be enrolled across multiple centers in China. This study aims to provide evidence for romiplostim as a treatment option for ITP patients who have failed oral TPO-RAs.

Inclusion Criteria:

Age ≥18 years Diagnosed with primary immune thrombocytopenia (ITP) according to current guidelines

Failed prior oral TPO-RA therapy (eltrombopag, hetrombopag, or avatrombopag), defined as:

Platelet count persistently <30×10⁹/L after ≥4 weeks of treatment at the maximum recommended or tolerated dose, OR Prior achieved response (platelet ≥50×10⁹/L) then lost response with platelet count <30×10⁹/L on two consecutive visits (≥1 week apart), OR Intolerance to oral TPO-RA leading to discontinuation (with documented reason) Baseline platelet count <30×10⁹/L (at least two measurements, ≥5 days apart) ECOG performance status 0-2 Willing and able to receive weekly subcutaneous injections and comply with study procedures For women of childbearing potential: negative pregnancy test at screening and agreement to use effective contraception during the study and for 3 months after the last dose Able to provide written informed consent

Exclusion Criteria:

Secondary ITP (e.g., associated with autoimmune diseases, lymphoproliferative disorders, drug-induced, or infections such as HIV/HCV/HBV) Known hypersensitivity to romiplostim or any of its excipients Prior treatment with romiplostim Received rituximab or other B-cell depleting therapy within 6 months prior to screening Received immunosuppressants (e.g., cyclosporine, mycophenolate mofetil, azathioprine, danazol) within 4 weeks prior to screening Splenectomy within 8 weeks prior to screening Use of corticosteroids (prednisone >10 mg/day or equivalent) or IVIG within 2 weeks prior to screening History of bone marrow fibrosis (MF grade ≥2) or known reticulin fibrosis History of arterial or venous thromboembolic event (e.g., myocardial infarction, stroke, deep vein thrombosis, pulmonary embolism) within 6 months prior to screening Known thrombophilic conditions (e.g., antiphospholipid syndrome, protein C/S deficiency, ATIII deficiency) Severe hepatic impairment (ALT or AST >3×ULN, or total bilirubin >1.5×ULN) Severe renal impairment (creatinine clearance <30 mL/min) New York Heart Association (NYHA) functional class III or IV heart failure Malignancy within 5 years prior to screening (except non-melanoma skin cancer, cervical carcinoma in situ, or ductal carcinoma in situ of the breast) Pregnancy or breastfeeding Participation in another interventional clinical trial within 30 days prior to screening Any other condition that, in the investigator's judgment, would make the participant unsuitable for the study