Caplacizumab in paediatric immune thrombotic thrombocytopenic purpura (iTTP): the UK TTP Registry experience.

Paediatric thrombotic thrombocytopenic purpura (TTP) is an ultra-rare disease. Immune TTP (iTTP) is driven by anti-ADAMTS13 autoantibodies causing an imbalanced VWF:ADAMTS13 axis, and rarer still in children, but potentially life-threatening. Caplacizumab is licensed for iTTP treatment in adults and adolescents aged 12 years and older who weigh 40kg plus. There is a need to clarify whether caplacizumab can be used in younger children. We retrospectively describe caplacizumab use in 16 patients under 18 years of age from the UK TTP Registry, including 4 children aged less than 12 years. For patients weighing less than 40kg (n=3), caplacizumab was dosed at 5mg od. The youngest patient was 33 months old at diagnosis. Plasma exchange (PEX) was used in 15 patients, with a median of 5 exchanges required before platelet count normalisation (range 2-9). One patient was managed without plasma exchange. All patients achieved normalisation of platelet count (median 5.5 days, range 3-28) and ADAMTS13 activity (median 35 days, range 8 to 149), with a median hospital admission of 11 days (range 5 to 26). There were no refractory patients. One patient relapsed at 9 months post presentation. Bleeding requiring VWF supplementation and reduction of caplacizumab use occurred in one patient with severe epistaxis, with no significant intracranial or gastrointestinal bleeding. We demonstrate the efficacy and safety of caplacizumab in the paediatric population, synonymous with the adult trial data: primarily, reduction of PEX compared with the pre-caplacizumab era. This has implications for intensification and duration of admission, particularly relevant in paediatric care.