A Randomized, Double-Blind, Placebo-Controlled Phase 1/2 Study to Determine the Safety and Efficacy of Romiplostim in Children with Chronic Immune (Idiopathic) Thrombocytopenic Purpura (ITP).

ITP is an autoimmune disorder characterized by thrombocytopenia due to accelerated destruction as well as suboptimal platelet production. Childhood ITP is most commonly an acute illness; however, chronic ITP (duration > 6 months) develops in 20%–30% of ITP cases. Romiplostim, a peptibody protein designed to increase platelet production, is approved for treating chronic ITP in adults. The objective of this study was to evaluate the safety and efficacy of romiplostim in the treatment of thrombocytopenia in children with chronic ITP.

ITP patients aged 12 months to <18 years with persistent severe thrombocytopenia for at least six months before enrollment (mean of 2 platelet counts ≥ 30 × 10⁹/L at baseline) were included in this study. Patients were randomized (3:1) to receive romiplostim or placebo and stratified by age: 12 months - <3 years (N=4), 3 - <12 years (N=8), and 12 - <18 years (N=8). Treatment for a 12 week period was followed by a 4 week pharmacokinetic (PK) assessment period for responding patients (those who achieved a platelet count of >20 × 10⁹/L above baseline for 2 consecutive weeks without rescue therapy at any point during the treatment period). Treatment was initiated at 1 μg/kg once weekly by subcutaneous injection. The dose was adjusted in 2 μg/kg increments every two weeks, to a maximum dose of 10 μg/kg/week based on weekly platelet counts. The incidence of adverse events (AEs) during the 12-week treatment period and the number of patients achieving platelet counts >50 × 10⁹/L for 2 consecutive weeks during the treatment period, or achieving an increase in platelet count >20 × 10⁹/L above baseline for 2 consecutive weeks during the treatment period was recorded.

A total of 22 (romiplostim, 17; placebo, 5) patients were randomized; 16 (73%) were boys and 6 (27%) were girls. Eight patients had undergone splenectomy. The mean age was 9.5 (SD: 5.1) years, with 4 subjects aged 12 months - <3 years, 10 aged 3 - <12 years, and 8 aged 12 - <18 years. The median baseline platelet count was 13 × 10⁹/L (range 2 to 29 × 10⁹/L) and the median duration of ITP was 2.4 years (range 0.6 to 14 years). All patients completed the study. Sixteen of 17 patients in the romiplostim arm (94%) and 5/5 in the placebo arm (100%) had at least 1 AE during the treatment period. The most common AEs were (romiplostim, placebo, respectively) headache (35%, 40%), epistaxis (35%, 20%), cough (12%, 40%), and vomiting (12%, 40%). Serious AEs were experienced by 1 patient in the romiplostim arm (moderate influenza and sepsis) and none in the placebo arm. AEs considered to be treatment related were reported for 3 (18%) and 1 (20%) subjects in the romiplostim and placebo arms, respectively; none of the treatment-related AEs were serious or of ≥3 grade severity. No patients died during the study and none tested positive for neutralizing antibodies to romiplostim or thrombopoietin. The same group of patients in the romiplostim-treated arm (15/17, 88.2%, 95% CI: 63.6%, 98.5%) achieved both efficacy endpoints during the treatment period. The median platelet count in the romiplostim-treated arm after 6 weeks of treatment was ≥50 × 10⁹/L. The median weekly platelet count in the placebo arm remained stable at approximately 10 × 10⁹/L. None of the placebo-treated patients achieved either platelet count endpoint. Rescue medication was administered to 2/17 (12%) of romiplostim- and 2/5 (40%) of placebo-treated patients during the 12 week treatment period. Twelve (71%) and 2 (40%) subjects in the romiplostim and placebo arms, respectively, experienced bleeding events. The majority of bleeding events (15/17) in the romiplostim arm occurred in the first 6 weeks of treatment. Most bleeding events (14/17) in the romiplostim arm and all bleeding events in the placebo arm occurred when the platelet count was < 30 × 10⁹/L. A total of 14 patients treated with romiplostim entered the PK assessment period. The romiplostim serum concentration results were not different among the 3 age cohorts. The mean weekly dose of romiplostim in the treatment period was 3.4 (SD: 1.6) μg/kg.

Treatment with romiplostim appeared to be well tolerated in pediatric ITP patients, with no new safety concerns observed in this study as compared to adults with chronic ITP. Romiplostim was effective in treating thrombocytopenia in children with chronic ITP.

Buchanan:Amgen Inc.: Research Funding. Off Label Use: Use of romiplostim, a thrombopoietin mimetic, in treatment of thrombocytopenia in pediatric ITP patients. . Bomgaars:Novartis: Research Funding. Bussel:Eisai, Inc: Research Funding; Sysmex: Research Funding; Shionogi: Membership on an entity's Board of Directors or advisory committees; Ligand: Membership on an entity's Board of Directors or advisory committees, Research Funding; Immunomedics: Research Funding; Amgen: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Research Funding; GlaxoSmithKline: Equity Ownership, Membership on an entity's Board of Directors or advisory committees, Research Funding; Cangene: Research Funding; Genzyme: Research Funding; Scienta: Speakers Bureau. Nie:Amgen Inc.: Employment, Equity Ownership. Eisen:Amgen Inc.: Employment, Equity Ownership. Berger:Amgen Inc.: Employment, Equity Ownership.