Adding Hydroxyurea in Combination with Ruxolitinib Improves Clinical Responses in Hyperproliferative Forms of Myelofibrosis

Background

Ruxolitinib (Ruxo), an orally bioavailable and selective inhibitor of JAK1 and JAK2, significantly reduces splenomegaly and disease-related symptoms in patients with myelofibrosis (MF). However, no clear survival benefit has been demonstrated, which may in part reflect suboptimal drug exposure related to lower dosages needed to minimize hematological toxicity, specifically cytopenias. Furthermore, the optimal management of specific conditions such as leukocytosis or thrombocytosis in patients under ruxolitinib therapy is still undefined. In these cases, combining ruxolitinib with a cytoreductive agent like hydroxyurea (HU) might improve hematological response.

Aims:

To evaluate the efficacy and safety of Ruxo and HU combination.

Methods

This observational multicenter study, conducted from April 2012 to April 2017, enrolled 20 adult patients with a confirmed diagnosis of primary myelofibrosis (PMF), post-polycythemia vera (PPV-MF), or post-essential thrombocythemia (PET-MF) with hyperproliferative manifestations of the disease not controlled by Ruxo therapy. All patients we enrolled into the study had received Ruxo at a starting dose based on baseline platelet count. Patients were included into the study when Ruxo proved to be unable to reduce WBC and/or platelet count to within normal range (WBC<10.0x10⁹/L and platelets <400x10⁹/L). The starting dose of HU was based on clinician choice and modulated on the basis of WBC and platelet count and on the maximum Ruxo tolerated dose. Responses were defined, according to ELN criteria and side effects were assessed and graded according to NCI-CTCAE version 4.03.

Results

The addition of HU was started after a median time of ruxolitinib monotherapy of 6.5 months (range 1-49.6 months): 17 patients (85%) started HU treatment due to lack of WBC control, whereas the remaining 3 patients (15%) started for the lack of platelet control. While on Ruxo monotherapy, ten patients (50%) needed a dose reduction due to hematological toxicity and three patients temporarily discontinued Ruxo due to severe thrombocytopenia (15%).

After 14.5 months (range 2-195) median time of combination therapy in 8 patients the Ruxo daily doses increased (mean increase = 5.1 mg BID), in 9 the dose was unchanged despite the addition of HU, and only in three cases the ruxolitinib dose had to be reduced (mean reduction = 3.4 mg BID). Overall, the mean daily dose of Ruxo administered to the whole cohort of patients increased by 1.8 mg BID (P=.013), and only one patient had to discontinue Ruxo, due to severe thrombocytopenia. Non-hematological adverse events were not observed.

Among the 17 patients who started combination therapy to control WBC count, 14 (82.3%) obtained a WBC response, whereas, among the three patients who started HU in association with Ruxo in order to reduce platelets count, two (66.6%) achieved a platelet response. In addition, 12 patients of the whole study cohort had clinical improvement in MF-associated symptomatic burden, with 9 of them showing spleen and symptoms response and the remaining three only spleen response. Thus, a clinical response of any type was obtained in 8 patients (40%) during Ruxo monotherapy and in 17 patients (85%) during Ruxo-HU combination (P=.003). Of note, all 8 patients in which Ruxo dose was increased during combination had at least one type of drug-related clinical response.

Conclusion

In conclusion, the combination of HU with Ruxo yielded a high clinical response rate and increased Ruxo exposure in patients with hyperproliferative forms of MF. The association was very well tolerated, and the hematological toxicities mostly and were generally manageable with dose reductions and/or supportive treatment.