The safety and efficacy of Nplate in pediatric patients 1 year and older with ITP for at least 6 months were assessed in two double-blind, placebo-controlled clinical trials.

Study 5 (NCT01444417)

In Study 5, patients refractory or relapsed after at least one prior ITP therapy with a platelet count ≤ 30 x 109/L were stratified by age and randomized (2:1) to receive Nplate (n = 42) or placebo (n = 20). The starting dose for all ages was 1 mcg/kg weekly. Over a 24-week treatment period, dose was titrated up to a maximum of 10 mcg/kg weekly of either Nplate or placebo in an effort to maintain a target platelet count of ≥ 50 × 109/L to 200 × 109/L.

The median age of the patients was 9.5 years (range 3 to 17) and 57% were female. Approximately 58% of patients had a baseline count ≤ 20 x 109/L, which was similar between treatment arms. The percentage of patients with at least 2 prior ITP therapies (predominantly immunoglobulins and corticosteroids) was 81% in the group treated with Nplate and 70% in the group treated with placebo. One patient in each group had undergone splenectomy.

Study 5 results are shown in Table 6. The efficacy of Nplate in this trial was measured by the proportion of patients receiving Nplate achieving a durable platelet response and the proportion of patient achieving an overall platelet response. A durable platelet response was defined as achieving at least 6 weekly platelet counts ≥ 50 × 109/L during weeks 18 through 25 of treatment. A transient platelet response was defined as a weekly platelet count ≥ 50 × 109/L for 4 or more times during weeks 2 through 25, but without durable platelet response. An overall platelet response was defined as a durable or a transient platelet response. Platelet responses were excluded for 4 weeks after receiving rescue medications.

Study 6 (NCT00515203)

In study 6, patients diagnosed with ITP at least 6 months prior to enrollment with a platelet count ≤ 30 x 109/L were stratified by age and randomized (3:1) to receive Nplate (n = 17) or placebo (n = 5). The starting dose for all ages was 1 mcg/kg weekly. Over a 12-week treatment period dose was titrated up to a maximum of 10 mcg/kg weekly of either Nplate or placebo in an effort to maintain a target platelet count of ≥ 50 × 109/L to 250 × 109/L. 

The median age of the patients was 10 years (range 1 to 17 years) and 27.3% of patients were female. Approximately 82% of patients had a baseline count ≤ 20 x 109/L, which was similar between treatment arms. The percentage of patients with at least 2 prior ITP therapies (predominantly IVIG and corticosteroids) was 88% in the group treated with Nplate and 100% in the group treated with placebo. Six patients in the Nplate group and 2 patients in the placebo group had undergone splenectomy. 

The efficacy of Nplate in this trial was measured by the proportion of patients who achieved a platelet count of ≥ 50 × 109/L for 2 consecutive weeks and by the proportion of patients who achieved an increase in platelet count of ≥ 20 × 109/L above baseline for 2 consecutive weeks. Platelet responses within 4 weeks following rescue medications use were excluded. Of the 17 patients who received romiplostim, 15 achieved a platelet count of ≥ 50 × 109/L for 2 consecutive weeks (88.2%, 95% CI: 63.6%, 98.5%).

The same 15 patients also achieved an increase in platelet count of ≥ 20 × 109/L above baseline for 2 consecutive weeks during the treatment period (88.2%, 95% CI: 63.6%, 98.5%). None of the patients treated with placebo achieved either endpoint. 

Study 7 (NCT02279173) Long-Term Pediatric Study

In study 7, patients diagnosed with ITP at least 6 months prior to enrollment and who received at least 1 prior ITP therapy or were ineligible for other ITP therapies were enrolled to a study to evaluate efficacy for up to 3 years. Nplate was administered weekly for up to 3 years by subcutaneous injection starting at a dose of 1 mcg/kg with weekly increments to a maximum dose of 10 mcg/kg to reach a target platelet count between 50 × 109/L and 200 × 109/L. The median age of the patients was 10 years (range 1 to 17 years) and the median and maximum duration of treatment were 156 weeks and 163 weeks, respectively. Among the 203 patients, the mean (SD) and median percentage of time with a platelet response (platelet count ≥ 50 × 109/L) within the first 6 months of initiation of Nplate without rescue medication use for the past 4 weeks was 50.6% (37) and 50.0%, respectively. Sixty (29.6%) subjects overall received rescue medications. Rescue medications (i.e., corticosteroids, platelet transfusions, IVIG, azathioprine, anti-D immunoglobulin, and danazol) were permitted.