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 (<)30,000/μL.
5. If the participant is receiving allowed standard-of-care treatment for ITP at screening, and continued use is intended, treatment may continue during the trial if the dose, and frequency have been stable for at least 4 weeks before receiving the first dose of IMP (i.e., Day 1), and are expected to remain stable throughout the trial.
6. If the participant is an individual with potential for pregnancy, the participant is not pregnant as confirmed by negative human chorionic gonadotropin during screening, and before the first dose of trial intervention.

Key Exclusion Criteria:

1. The participant has secondary ITP.

2. The participant has had any thrombotic or embolic event within 12 months before signing the informed consent form (ICF).

3. The participant has had a splenectomy.

4. The participant has active infection with hepatitis B virus, hepatitis C virus, or human immunodeficiency virus (HIV).

5. History of malignancy (including myelodysplastic syndrome) within 5 years of signing the ICF, except for treated non-melanoma skin cancer or cervical carcinoma in situ.

6. In the opinion of the investigator, the participant has a serious medical or psychiatric illness that could potentially interfere with the completion of treatment according to this protocol.

7. The participant has received anti-cluster of differentiation (CD) 20 treatment within 12 months before screening, and either of the following applies:

   1. The last dose was received within 6 months before screening.
   2. The last dose was received between 6 and 12 months before screening, and the participant has a cluster of differentiation 19 positive (CD19+) count below the lower limit of normal.

8. The participant has received any monoclonal or polyclonal antibody for immunomodulation within 6 months before Day 1.

9. The participant has any prior exposure to mezagitamab or has been exposed to another investigational agent within 4 weeks or 5 half-lives, whichever is longer, before Day 1.

10. The participant has used anticoagulants (e.g., vitamin K antagonists, direct oral anticoagulants) within 3 weeks prior to the first dose of trial treatment.

11. The participant has received a live or live-attenuated vaccine within 4 weeks prior to the first dose of trial treatment or has any live or live-attenuated vaccine planned during the trial.

12. The participant has used the following immunosuppressive agents as specified prior to the first dose of trial treatment: alkylating agents (e.g., cyclophosphamide) within 8 weeks, vinca alkaloids (e.g., vincristine) within 4 weeks, sulfones (e.g., dapsone) within 3 weeks, antiproliferative agents: (e.g., mycophenolate mofetil, and azathioprine) within 2 weeks, and calcineurin inhibitors: (e.g., cyclosporine) within 2 weeks.

13. The participant has used intravenous immunoglobulin (IVIg), SC immunoglobulin, recombinant human thrombopoietin, anti-D immunoglobulin treatment, or efgartigimod within 4 weeks before signing the ICF or it is expected that any treatment for thrombocytopenia other than the participant's standard-of-care ITP therapy (e.g., rescue therapy, administration of blood products) may be used between screening, and Day 1.

14. The participant has a history of severe allergic or anaphylactic reactions to recombinant proteins or excipients used in the mezagitamab/placebo formulation.

Other protocol defined inclusion/exclusion criteria may apply.

Takeda provides access to the de-identified individual participant data (IPD) for eligible studies to aid qualified researchers in addressing legitimate scientific objectives (Takeda's data sharing commitment is available on https://clinicaltrials.takeda.com/takedas-commitment?commitment=5). These IPDs will be provided in a secure research environment following approval of a data sharing request, and under the terms of a data sharing agreement.

IPD from eligible studies will be shared with qualified researchers according to the criteria and process described on https://vivli.org/ourmember/takeda/. For approved requests, the researchers will be provided access to anonymized data (to respect patient privacy in line with applicable laws and regulations) and with information necessary to address the research objectives under the terms of a data sharing agreement.