Inclusion criteria:
- Ability to understand the requirements of the trial and provide written informed consent, willing and able to comply with the trial protocol procedures
- Is at least the local age of consent for clinical studies when signing the ICF.
- Confirmed diagnosis of primary ITP made at least 3 months before randomization and based on the American Society of Hematology Criteria, and no known etiology for thrombocytopenia
- Diagnosis supported by a response to a prior ITP therapy (other than TPO-RAs), in the opinion of the investigator
- Mean platelet count of <30×10E9/L from at least 3 documented, qualifying counts within the 3 preceding months where at least 2 of the qualifying counts must be taken during the screening period: 1 platelet count collected during the screening period and the predose platelet count on the day of randomization (visit 1). If the third count is not available from the 3 preceding months, this third platelet count can be obtained during the screening period.
- A documented history of a platelet count of <30×10E9/L before screening
- At the start of the trial, the participant either takes concurrent ITP treatment(s) and has received at least 1 prior therapy for ITP in the past, or the participant does not take treatment for ITP (see note) but has received at least 2 prior treatments for ITP. Participants receiving permitted concurrent ITP treatment(s) at baseline must have been stable in dose and frequency for at least 4 weeks before randomization.
Permitted concurrent ITP medications include corticosteroids, danazol, vinca alkaloids, oral immunosuppressants, dapsone, fostamatinib, and/or oral TPO-RAs.
-Agree to use contraceptive measures consistent with local regulations and the protocol
Exclusion criteria:
- Secondary ITP/thrombocytopenia associated with another condition, eg, lymphoma, chronic lymphocytic leukemia, viral infection, hepatitis, induced or alloimmune thrombocytopenia, thrombocytopenia associated with myeloid dysplasia, or hematopoietic stem cell transplant
- Use of anticoagulants (eg, vitamin K antagonists, direct oral anticoagulants) within 4 weeks prior to randomization
- Use of any transfusions within 4 weeks prior to randomization
- Use of Ig (IV, SC, or intramuscular route) or plasmapheresis (PLEX) within 4 weeks prior to randomization
- Use of romiplostim within 4 weeks prior to randomization
- Undergone splenectomy less than 4 weeks prior to randomization
- Use of an investigational product within 3 months or 5 half-lives (whichever is longer) before the first dose of the IMP
- Use of any monoclonal antibody or Fc fusion proteins, other than those previously indicated, within 6 months before the first dose of the IMP (eg, anti-CD20)
- At the screening visit, clinically significant laboratory abnormalities as follows: Hemoglobin ≤9 g/dL - OR - International normalized ratio >1.5 or activated partial thromboplastin time >1.5×upper limit of normal - OR - total IgG level <6 g/L
- History of malignancy unless deemed cured by adequate treatment with no evidence of recurrence for ≥3 years before the first administration of IMP. Participants with the following cancer can be included at any time: Adequately treated basal cell or squamous cell skin cancer, Carcinoma in situ of the cervix, Carcinoma in situ of the breast or Incidental histological finding of prostate cancer (TNM stage T1a or T1b)
- Uncontrolled hypertension, defined as a repeated elevated blood pressure exceeding 160 mmHg (systolic) and/or 100 mmHg (diastolic) despite appropriate treatments
- History of any major thrombotic or embolic event (eg, myocardial infarction, stroke, deep venous thrombosis, or pulmonary embolism) within 5 years prior to randomization
- History of coagulopathy or hereditary thrombocytopenia or a family history of thrombocytopenia
- Evidence of an active clinically significant bleeding of an organ or internal mucosal bleeding, other than expected in ITP, that warrants emergent treatment or therapeutic procedure based on the investigator's judgment (eg, intracranial hemorrhage, pulmonary hemorrhage, bleeding with ongoing need for packed red blood cell transfusion)
- Estimated high risk of a clinically significant bleeding of an organ or internal mucosal bleeding, other than expected in ITP, that warrants emergent treatment or therapeutic procedure according to the investigator's judgment
- Clinical evidence of other significant serious diseases, have had a recent major surgery, or who have any other condition in the opinion of the investigator, that could confound the results of the trial or put the participant at undue risk
- Positive serum test at screening for an active viral infection with any of the following conditions: Hepatitis B virus (HBV) that is indicative of an acute or chronic infection, unless associated with a negative HBV DNA test, Hepatitis C virus (HCV) based on HCV-antibody assay (unless associated with a negative HCV RNA test), Human immunodeficiency virus (HIV) based on test results that are associated with an acquired immunodeficiency syndrome (AIDS)-defining condition or a CD4 count ≤200 cells/mm3
- Known hypersensitivity reaction to efgartigimod, rHuPH20, or 1 of its excipients
- Previously participated in a clinical trial with efgartigimod and have received at least 1 administration of the IMP
- Pregnant or lactating or intends to become pregnant during the trial
- Clinically significant uncontrolled active or chronic bacterial, viral, or fungal infection at screening
- Any other known autoimmune disease that, in the opinion of the investigator, would interfere with an accurate assessment of clinical symptoms of ITP or put the participant at undue risk
- Clinical evidence of significant unstable or uncontrolled acute or chronic diseases other than ITP (eg, cardiovascular, pulmonary, hematologic, gastrointestinal, endocrine, hepatic, renal, neurological, malignancy, infectious diseases, uncontrolled diabetes) despite appropriate treatments which could put the participant at undue risk
- Current or history of (ie, within 12 months of screening) alcohol, drug, or medication abuse
- Received a live/live-attenuated vaccine less than 4 weeks before screening. The receipt of any inactivated, sub-unit, polysaccharide, or conjugate vaccine at any time before screening is not considered an exclusion criterion