This is a multicenter, randomized, double-blind, placebo-controlled Phase II clinical study. The primary objective is to evaluate the safety and efficacy of SG301 SC Injection in participants with ITP, while the secondary objectives are to assess the pharmacokinetic, pharmacodynamic and immunogenicity profiles of SG301 SC Injection in ITP participants.

A total of 60 ITP participants are planned to be enrolled in this study and randomly assigned at a 1:1:1 ratio to the low dose group, high dose group and placebo group, with approximately 20 participants per group. All participants enrolled in this study may receive concomitant background therapies at stable doses for at least 4 weeks prior to the first study drug administration, and is expected to be maintained throughout the study period.

Inclusion Criteria:

1. Aged 18-75 years, any gender.

2. Voluntarily signed informed consent and able to comply with study procedures.

3. Clinically diagnosed with primary immune thrombocytopenia (ITP) per 2019 ASH Guidelines, international consensus statements, or 2025 Chinese Guidelines for Adult ITP Diagnosis and Treatment; disease duration ≥ 3 months.

4. Experienced treatment failure or relapse after prior first-line therapy (glucocorticoids and/or IVIG).

5. ≥2 platelet assessments ≥3 days apart before first dosing: mean platelet count <30×10⁹/L, no single value >35×10⁹/L.

6. ECOG performance status 0-2.

7. On stable-dose maintenance therapy for ≥4 weeks pre-first dose, with stability expected throughout the trial.

8. Screening lab parameters meeting criteria below:

   1. Liver function: AST and ALT ≤3×ULN; total bilirubin ≤1.5×ULN.
   2. Renal function: eGFR ≥30 mL/min (CKD-EPI equation).
   3. CBC (no RBC transfusions, IL-11, EPO or colony-stimulating factors within 2 weeks before testing): hemoglobin ≥90 g/L, absolute neutrophil count ≥1.5×10⁹/L.

9. Participants or partners with childbearing potential must use effective contraception during the study and for 6 months after last study drug administration.

Exclusion Criteria:

1. Prior treatment targeting CD38, including but not limited to monoclonal antibodies, bispecific antibodies and antibody-drug conjugates.
2. Secondary thrombocytopenia or concurrent autoimmune hemolytic anemia.
3. History of thromboembolism within 12 months prior to randomization, or severe bleeding events (hemoptysis, massive gastrointestinal hemorrhage, intracranial hemorrhage, etc.).
4. Anticoagulants or antiplatelet agents (e.g., aspirin) administered within 4 weeks before randomization.
5. Emergency ITP therapies (methylprednisolone, platelet transfusion, IVIG, TPO-RA, etc.) received within 4 weeks before randomization.
6. Oral immunosuppressants other than azathioprine, cyclosporine A and mycophenolate mofetil (e.g., tacrolimus, sirolimus) within 4 weeks pre-randomization; anti-CD20 mAbs (e.g., rituximab) within 3 months pre-randomization.
7. Splenectomy performed within 6 months before randomization.
8. Diagnosis of myelodysplastic syndrome; history of malignancy within 5 years pre-randomization (excluding cervical carcinoma in situ, basal cell skin carcinoma, etc.).
9. Participation in another clinical trial within 4 weeks or 5 half-lives pre-randomization (whichever is longer), except placebo-controlled trials.
10. History of severe recurrent or chronic infection; active systemic infection requiring IV antibiotics, antivirals, antiparasitics or antifungals within 4 weeks pre-randomization (excluding prophylaxis); acute infection; superficial skin infection within 1 week pre-randomization.
11. Major surgery within 12 weeks pre-randomization, unhealed wounds, ulcers or fractures, or planned major surgery during the study.
12. Severe cardiovascular or cerebrovascular diseases, including but not limited to: myocardial infarction, unstable angina or stroke within 6 months pre-randomization; NYHA Class III/IV heart failure; screening QTcF >480 ms or familial long QT syndrome; uncontrolled hypertension (resting SBP ≥160 mmHg and/or DBP ≥95 mmHg).
13. HIV infection, active hepatitis B or hepatitis C.
14. Confirmed active syphilis or Mycobacterium tuberculosis infection.
15. Known hypersensitivity to monoclonal antibodies (prior Grade ≥3 allergic reactions per CTCAE v6.0) or excipients of investigational product.
16. Live or attenuated live vaccines administered within 4 weeks pre-randomization or planned during the study.
17. History of solid organ transplantation (heart, lung, kidney, liver, etc.) or hematopoietic stem cell/bone marrow transplantation.
18. Lactating females.
19. Any condition judged by the investigator to interfere with subject compliance or trial participation.