A Study of Efgartigimod IV in Participants From 12 Years to Less Than 18 Years of Age With Chronic Immune Thrombocytopenia (ITP)

• Efgartigimod IV (Biological): Intravenous infusion of efgartigimod
• Placebo IV (Other): Intravenous infusion of placebo

Primary outcomes

• Efgartigimod serum concentrations in the DBTP [Up to 24 weeks]
• Total IgG levels in the DBTP [Up to 24 weeks]

Secondary outcomes

• Efgartigimod serum concentrations over time during the DBTP [Up to 24 weeks]
• Percent change from baseline in total IgG levels in serum over time during the DBTP [Up to 24 weeks]
• Incidence of AEs, SAEs and AEs leading to IMP discontinuation [Up to 136 weeks]: SAE: Serious adverse event; AE: adverse event
• Sustained platelet count response between study weeks 19 and 24 of the DBTP and in OLTP1 for participants receiving placebo in the DBTP [Up to 48 weeks]: Sustained platelet count defined as achieving platelet counts of ≥50 × 10\^9/L for at least 4 of the 6 study visits
• Extent of disease control during the DBTP and during the first 24 weeks of OLTP1 for those participants receiving placebo in the DBTP [Up to 48 weeks]: Extend of disease defined as the number of cumulative weeks with a platelet count of ≥50 × 10\^9/L
• Changes from baseline for platelet counts over time [Up to 76 weeks]
• Incidence of bleeding, assessed by the Modified Buchanan and Adix Bleeding Score for pediatric ITP [Up to 76 weeks]: The Modified Buchanan and Adix Bleeding Score for pediatric ITP is a semiquantitative assessment tool that measures bleeding signs and symptoms, comprising a score based on a scale of 0 to 5, each representing a different level of severity (0 = no risk; 5 = highest severity).
• Incidence of ADA and Nab against efgartigimod in serum [Up to 76 weeks]: ADA: anti-drug antibodies; Nab: neutralizing antibodies
• Change from baseline in EQ-5D-5L [Up to 76 weeks]: The European Quality of Life 5 Dimensions 5 Level (EQ-5D-5L) is a questionnaire comprised of 5 dimensions. Participants are asked to select the statement in each dimension which best describes their health on the day they complete the questionnaire. Responses in each dimension are coded as a 1-digit number ranging from 1 (no problems) to 5 (extreme problems).
• Change from baseline in KIT Child Self-Report and KIT Parent Impact Report [Up to 76 weeks]: The Kids' ITP Tools (KIT) comprises two disease- specific tools: a self-report form for children aged 12 and older, and a parent impact form. Respondents provide insights into their disease experience using a 1-week recall period. The instrument generates a total score, calculated by summing the items and converting them to a scale of 0 to 100, where higher scores reflect a better disease-specific quality of life (QoL).
• Change from baseline in peds FACIT-F [Up to 76 weeks]: In all participants, the pediatric Functional Assessment of Chronic Illness Therapy-Fatigue (FACIT-F) will use to assess health-related quality of life (HRQoL) before and after treatment. The FACIT-F questionnaire has a score range of 0 to 52, where 0 represents the worst possible fatigue and 52 indicates no fatigue.

Sex: All, ages: 12 Years to 17 Years

Inclusion Criteria:

* Is aged 12 to less than 18 years when completing the informed consent process
* Has a documented duration of primary ITP of more than 12 months on the date the informed consent process is complete
* Has documented prior ITP treatment with at least 1 of the following treatments: corticosteroids, IVIg, anti-D immunoglobulin, thrombopoietin receptor agonist (TPO-RAs), or rituximab.
* Has documented prior response, defined as 1 platelet count of ≥50 × 10\^9/L to at least 1 of the following ITP treatments: prednisone, other or nonspecified corticosteroids, IVIg, or anti-D immunoglobulin
* Has documented insufficient response to a prior ITP treatment with corticosteroids, IVIg, anti-D immunoglobulin, TPO-RAs, rituximab, or splenectomy
* Has documented mean platelet count of less than 30 x10\^9/L

Exclusion Criteria:

* Secondary ITP according to the following definition by the International Working Group (IWG): all forms of immune-mediated thrombocytopenia except primary ITP
* Nonimmune thrombocytopenia
* ITP-associated critical or severe bleeding
* History of hereditary thrombocytopenia

NIH grants: 2025