Romiplostim is effective in boosting platelet counts in most children with chronic immune thrombocytopenia (ITP) and can lead to a treatment-free response in almost a quarter of children.
A long-term extension study enrolled 66 children from previous phase 1/2 and phase 3 studies comparing romiplostim with placebo. The children were recruited from 28 sites in the US, Canada, Spain and Australia.
Most had already received romiplostim in the previous trials but for 15 patients, who had only previously received placebo, it was their first exposure to the drug.
Romiplostim was administered weekly, starting at 1 μg/kg and adjusted to a maximum of 10 μg/kg based on platelet count.
The thrombopoietin (TPO) receptor agonist was also adjusted downwards for platelet counts in the range 200-400 x109/L and could be withheld completely for counts >400 x109/L.
Patients could also still receive other ITP medications at a stable dose and schedule and rescue medications in severe thrombocytopenia <10 x109/L or with bleeding.
The study found the average weekly romiplostim dose was 4.8 ug/kg over a median treatment duration of 2.6 years.
“Most patients (72%) had a platelet response ≥75% of the time and over half (58%) had a platelet response ≥90% of the time,” the study found.
And the response could be maintained for a median of 12 months in a significant minority of patients.
“Fifteen patients (23%) achieved a treatment-free response when romiplostim was withheld, and maintained platelet counts ≥50 x109/L with no ITP medications for ≥24 weeks.”
A post-hoc analysis of the data showed that younger age at first dose and platelet count increasing to ≥200×109/L in the first four weeks were both independently associated with developing treatment-free response.
“The correlation of treatment-free response with early very good response in the first 4 weeks of romiplostim treatment suggests either that these patients were uniquely sensitive to romiplostim or possibly that they just had milder disease.”
Adverse effects included headache and contusion. Most bleeding events were either not serious or not thought to be related to treatment. No intracranial haemorrhage or arterial/venous thromboembolic events were reported.
No patients developed anti-TPO neutralising antibodies.
While only available on the PBS in Australia for adults with ITP, the researchers concluded that romiplostim was ‘well tolerated and generally maintained its efficacy’ even in children with ITP of more than six months duration who were not responsive to other therapies.
“Development of treatment-free response in almost one quarter of patients suggests that maintenance with romiplostim in children will not always be a ‘life-long treatment’.”
“The continued, steady development of treatment-free response in patients treated for 3 or more years is encouraging as well.”