Assessment of Romiplostim Immunogenicity in Pediatric Patients in Clinical Trials and in a Global Postmarketing Registry.

Abstract Development of first-generation thrombopoietins (TPOs) was halted due to antibodies that neutralized endogenous TPO, causing protracted thrombocytopenia in some patients. The second-generation TPO receptor agonist romiplostim, having no homology to TPO, was developed to circumvent potential immunogenicity. We examined development of binding and neutralizing antibodies to romiplostim and TPO among pediatric patients with primary immune thrombocytopenia (ITP) in 5 trials and a global postmarketing registry. In the trials, 25/280 (8.9%) patients developed anti-romiplostim binding antibodies. The first positive result was detected 67 weeks (median) after starting romiplostim; median romiplostim dose was 8 µg/kg and median platelet count 87 x 109/L. Most patients who developed anti-romiplostim binding antibodies (18/25 [72%]) had ≥ 90% platelet assessments showing response. Anti-romiplostim neutralizing antibodies developed in 8/280 (2.9%) patients; this was unrelated to romiplostim dose, and most patients who developed anti-romiplostim neutralizing antibodies (7/8 [88%]) had platelet response. Nine of 279 (3.2%) patients developed anti-TPO binding antibodies, and 1 (0.4%) developed transient anti-TPO neutralizing antibodies. In 8 patients who developed anti-romiplostim neutralizing antibodies, no TPO cross-reactivity was observed. In the postmarketing registry, 3/19 (15.8%) patients had anti-romiplostim binding antibodies; 1 (5.3%) had anti-romiplostim neutralizing antibodies. These results show that immunogenicity to romiplostim occurs infrequently in children with ITP and is generally not associated with loss of platelet response or other negative clinical sequelae.