A Study of Mezagitamab in Adults With Chronic Primary Immune Thrombocytopenia

Primary immune thrombocytopenia (ITP) is a condition where the immune system mistakenly destroys platelets, which are cells that help stop bleeding. This leads to a low number of platelets, making it easier to bruise or bleed. The main aim of this study is to learn whether mezagitamab, when given just under the skin (subcutaneously \[SC\]), is effective in keeping the platelet count of adults with ITP stable when compared to a placebo. A placebo looks like medicine but doesn't have any active ingredients in it. The participants will be treated with mezagitamab for up to 6 months. During the study, participants will visit their study clinic several times. Participants who complete the TAK-079-3002 study or do not have any response to study treatment by week 16 (according to study criteria) will be given the opportunity to participate in a continuation study to receive open label mezagitamab (if they are eligible and the site is able to open the continuation study).

Key Inclusion Criteria: 1. The participant has been diagnosed with ITP that has persisted for at least 12 months. 2. The participant's diagnosis of ITP is supported by a prior response to an ITP therapy (not including a thrombopoietin receptor agonist \[TPO-RA\]), defined as having achieved a platelet count ≥50,000/μL. 3. The participant has evidence of insufficient response or intolerance to at least 1 currently available first-line therapy for treatment of ITP (for example, corticosteroids), and at least 1 currently available second-line therapy for treatment of ITP (for example, TPO-RA, rituximab, fostamatinib, mycophenolate). Insufficient response to previous treatment is defined as failure to achieve a sustained platelet count of at least 50,000/μL or doubling of baseline platelet count after an appropriate course of prior ITP treatment. Intolerance is defined as a documented side effect causing discontinuation of the therapy. 4. The participant has a mean platelet count of less than (\