Safety and Efficacy of Ruxolitinib in Patients with Low Platelets Enrolled in a Phase 3b Expanded-Access Study in Myelofibrosis (MF)

BACKGROUND:

Ruxolitinib is a potent JAK1/JAK2 inhibitor that has demonstrated durable reductions in splenomegaly and MF-related symptoms and improved survival compared with placebo and best available therapy in the 2 large phase 3 COMFORT studies. Those studies required baseline platelet (PLT) counts ≥ 100 × 10⁹/L, limiting safety and efficacy data in patients (pts) with lower PLTs; however, thrombocytopenia is frequent in MF. Ongoing phase 2 clinical trials in pts with low PLTs have shown ruxolitinib to be generally well tolerated and to provide efficacy benefits for these pts. To gather additional safety and efficacy data in pts with low PLTs, recruitment for JUMP (JAK Inhibitor Ruxolitinib in Myelofibrosis Patients), a phase 3b expanded-access trial for countries with no access to ruxolitinib outside a clinical trial, was extended to pts with baseline PLT counts ≥ 50 × 10⁹/L. Here, we present safety and efficacy data from a planned interim analysis of ruxolitinib in pts with baseline PLT count ≥ 50 to < 100 × 10⁹/L.

METHODS:

Eligible pts had high-, intermediate-2, or intermediate-1-risk MF, with a palpable spleen (≥ 5 cm from the costal margin) and baseline PLT counts ≥ 50 × 10⁹/L. The starting doses of ruxolitinib was 5 mg bid for pts with baseline PLTs ≥ 50 to < 100 × 10⁹/L. The primary endpoint was assessment of safety and tolerability of ruxolitinib based on the frequency, duration, and severity of adverse events (AEs). An interim analysis of the safety and efficacy of ruxolitinib in pts with PLTs ≥ 50 to < 100 × 10⁹/L at baseline (low-PLT group) was planned for when the first 50 low-PLT pts had completed 6 mo of therapy; this analysis includes results from the first 6 mo of treatment. The final analysis will be performed after all pts have completed 24 mo of treatment or ended treatment due to commercial availability.

RESULTS:

At data cutoff (01 January 2014), 50 pts with low PLTs had been treated for 6 mo and are included in this analysis. Pt characteristics were median age, 68.5 years; median palpable spleen length, 15.5 cm below the costal margin; and female, 50.0%. Median (range) baseline hemoglobin (Hb) was 98 g/L (57-149 g/L) and PLT count was 87 × 10⁹/L (68 to 98 × 10⁹/L). Most pts (76.0%) remained on or completed treatment as per protocol at the time of data cutoff. The primary reasons for discontinuation included adverse events (AEs; n = 9) and disease progression (n = 2). The median daily dose was 11.8 mg/day (range, 5.9-40.0 mg/day).

Of evaluable pts at week 24, 38.2% (13/34 pts) achieved a ≥ 50% reduction from baseline in palpable spleen length; 38.2% had reductions of ≥ 25% to < 50%. Overall, 44.7% of pts achieved a ≥ 50% reduction from baseline in spleen length at any time. Clinically meaningful improvements in symptoms, as assessed using the FACT-Lymphoma Total Score (the range for the minimally important difference is 6.5 to 11.2 points), were seen as early as week 4 (mean change from baseline, 8.2) and were durable through week 12 (9.6).

The most common hematologic grade ≥ 3 AEs were anemia (28.0%) and thrombocytopenia (30%; 4% grade 4); 3 pts discontinued due to thrombocytopenia and 1 due to anemia. Four pts had grade 1/2 hemorrhages (1 conjunctival, 1 gastric, and 2 epistaxis) and 2 pts had grade 3/4 (1 intestinal and 1 esophageal varices). PLT counts decreased slightly from baseline (mean change at nadir, −5.9 × 10⁹/L); 3 pts required PLT transfusions. The mean change from baseline to nadir in Hb was −13.4 g/L. Rates of nonhematologic grade ≥ 3 AEs were low overall (1 pt each), with the exception of pyrexia (6.0%), septic shock (4.0%), and arthralgia (4.0%); 44.0% of pts had ≥ 1 dose modification and 28.0% had ≥ 1 dose interruption.

CONCLUSIONS:

In this cohort of pts with low PLTs, ruxolitinib demonstrated an AE profile consistent with that previously reported in pts with normal PLTs. As compared to a previous analysis of pts with median baseline PLT counts of 248 × 10⁹/L from this same study (Al-Ali et al. EHA 2014), pts in this low-PLT cohort experienced similar improvements in symptoms (mean change from baseline at week 12, 9.6 vs 11.8) but lower achievement of a ≥ 50% reduction from baseline in spleen length within the first 24 weeks of treatment (44.7% vs 69%), likely due to the lower median daily dose (11.8 mg/day vs 36.8 and 24.0 mg/day for pts with starting doses of 20 and 15 mg bid, respectively). Taken together, these data suggest that low-dose ruxolitinib is generally safe and efficacious in pts with PLTs ≥ 50 to < 100 × 10⁹/L.