Overview

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 lower number of platelets, making it easier to bruise or bleed. The main aim of this study is to check how safe mezagitamab is and how well it is tolerated by adults with chronic primary ITP, if given over a longer time. Other aims are to learn how effective treatment with mezagitamab is and how the body processes it (called pharmacokinetics or PK) over a longer time.

Participants of the following previous mezagitamab studies will be invited to join this continuation study: TAK-079-3002 and TAK-079-1004. In this continuation study, participants will receive mezagitamab when certain protocol criteria are met.

During the study, participants will visit their study clinic several times.

Eligibility

• Key Inclusion Criteria:
  1. The participant has completed TAK-079-3002 (end of trial [EOT]) or TAK-079-1004 (EOT).
     • Key Exclusion criteria:

For TAK-079-3002 participants:

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

For TAK-079-1004 participants:

1. The participant has had any thrombotic or embolic event within 12 months before signing the ICF.
2. The participant has had a splenectomy within 3 months before signing the ICF.
3. The participant has active infection with hepatitis B virus (HBV), hepatitis C virus (HCV), or human immunodeficiency virus (HIV).
4. 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.
5. 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.
6. 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 CD19+ count below the lower limit of normal.
7. The participant has received any monoclonal or polyclonal antibody for

   immunomodulation within 6 months before Visit 1.

8. The participant has been exposed to another investigational agent within 4 weeks or 5 half-lives, whichever is longer, before Visit 1.
9. The participant has used anticoagulants (for example, vitamin K antagonists, direct oral anticoagulants) within 3 weeks prior to Visit 1.

10 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.

11. The participant has used the following immunosuppressive agents as specified prior to Visit 1: alkylating agents (for example, cyclophosphamide) within 8 weeks, vinca alkaloids (for example, vincristine) within 4 weeks, sulfones (for example, dapsone) within 3 weeks, antiproliferative agents: (for example, mycophenolate mofetil and azathioprine) within 2 weeks, and calcineurin inhibitors: (for example, cyclosporine) within 2 weeks.

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

Other protocol defined inclusion/exclusion criteria may apply.