The thrombopoietin receptor agonist romiplostim (NPlate®) is now approved by the U.S. Food and Drug Administration (FDA) to treat pediatric patients 1 year and older who have had immune thrombocytopenia (ITP) for at least 6 months and have not responded sufficiently to corticosteroids, immunoglobulins, or splenectomy.
Romiplostim was originally FDA-approved in 2008 to treat adult patients with chronic ITP who had an insufficient response to the same treatments.
Romiplostim is manufactured by Amgen, Inc.
The FDA based its approval on two double-blind, placebo-controlled clinical trials.
The phase 3 study (NCT01444417) enrolled 62 pediatric patients 1 year and older who had ITP for at least 6 months. They were refractory to or relapsed after at least one prior therapy.
Investigators randomized them 2:1 to receive romiplostim (n=42) or placebo (n=20).
The starting dose was 1 μg/kg weekly for all ages. The dose was titrated up over a 24-week period to a maximum of 10 μg/kg weekly.
Patients were a median age of 9.5 years (range, 3–17), and 57% were female. A little over half (58%) had baseline platelet counts of 20 x 109/L or less, which was similar in both treatment arms.
Eighty-one percent of romiplostim-treated patients had at least two prior ITP therapies, compared with 70% in the placebo group. One patient in each group had undergone splenectomy.
Twenty-two (52%) of the romiplostim-treated patients had durable platelet responses of 50 x 109/L or greater for at least six weekly assessments during weeks 18 through 25 of treatment. Two (10%) patients in the placebo arm achieved durable platelet responses.
Thirty (71%) romiplostim-treated patients achieved an overall platelet response, defined as a durable or transient platelet response. This compared with four (20%) patients in the placebo group.
Romiplostim-treated patients had platelet counts of at least 50 x 109/L for a median of 12 weeks, compared to 1 week for patients in the placebo arm.
All response endpoints were significant at P<0.05.
The phase 1/2 study (NCT00515203) enrolled 22 patients who had ITP for at least 6 months prior to study enrollment and were relapsed from or refractory to prior treatment.
Investigators randomized the patients 3:1 to romiplostim (n=17) or placebo (n=5).
Patients were a median age of 10 years (range, 1–17), and 27.3% were female.
Approximately 82% of patients had baseline platelet counts of 20 x 109/L or less, which was similar between the treatment arms.
Eighty-eight percent of patients in the romiplostim arm had at least two prior ITP therapies, as did 100% in the placebo group.
Six patients in the romiplostim group and two in the placebo group had undergone splenectomy.
Of the 17 patients treated with romiplostim, 15 (88.2%) achieved a platelet count of 50 x 109/L or great for 2 consecutive weeks.
The same 15 patients also achieved an increase in platelet count of 20 x 109/L or greater above baseline for 2 consecutive weeks during the treatment period.
None of the placebo-treated patients achieved either endpoint.
The adverse events profile in pediatric patients was compiled from the two trials and reflects a median drug exposure of 168 days for 59 patients.
The most common adverse events, occurring in 25% or more of romiplostim-treated patients, were contusion (41%), upper respiratory tract infection (31%), and oropharyngeal pain (25%). These occurred with an incidence at least 5% higher than in the placebo group.
The recommended starting dose for pediatric patients is 1 µg/kg based on actual body weight and administered as a weekly subcutaneous injection.
The dose should be adjusted in increments of 1 µg/kg until the patient achieves a platelet count of 50 x 109/L or greater.
The prescribing information recommends reassessing patients’ body weight every 12 weeks.