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Data on safety and efficacy of ruxolitinib treatment in myeloproliferative neoplasms (MPN) with concomitant thrombocytopenia are scarce and limited to patients with myelofibrosis (MF). It was demonstrated that 11–16% of MF patients had platelet (PLT) count \50 9 10/l at diagnosis and the rate of thrombocytopenia increases with disease progression. Thrombocytopenia in MF may be due to ineffective megakaryocytopoiesis as a result of bone marrow fibrosis or results from platelet sequestration and destruction in enlarged spleen [1]. Immune dysregulation with the production of immune complexes or antiplatelet antibodies may account for low platelet count in single cases [2]. It is noteworthy that thrombocytopenia remains an exceptional finding in polycythemia vera (PV). Ruxolitinib is the first and only approved JAK inhibitor for the treatment of intermediateand high-risk MF as well as for PV patients with inadequate response to or are intolerant of hydroxyurea. It should be mentioned that thrombocytopenia was identified as the dose-limiting toxicity for ruxolitinib in MF and was the most common cause of dose modifications. Grade 3/4 thrombocytopenia was rarely seen in PV patients receiving ruxolitinib [3]. There has been a limited experience with ruxolitinib in thrombocytopenic patients with MPN, and all available data were derived from studies in MF. Based on these preliminary results, the treatment with low dose of ruxolitinib (10 mg daily) was well tolerated and resulted in spleen reduction with PLT count improvement. To date, only few MF patients with PLT count of\50 9 10/l were successfully treated with ruxolitinib [1, 4]. A 25-year-old female initiated a diagnostic workup due to progressive thrombocytopenia. Seven years earlier, she was diagnosed with polycythemia vera (PV) with detectable H538–K539delinsL mutation in the exon 12 of the JAK2 gene. Since her PV diagnosis she was phlebotomized every other month and receiving hydroxyurea and interferon with no effect. During the follow-up, she developed an isolated mild thrombocytopenia (110 9 10/ l). She did not present a prior history of immune thrombocytopenia or any other autoimmune disorders. On her current admission the white blood cell (WBC) count was 28.1 9 10/l with the predominance of segmented cells in differential (78%). No myeloid precursors were found. Red blood cell (RBC) and PLT counts were 5.98 9 10/ll and 21 9 10/l, respectively. A massive splenomegaly (40 cm) was found on ultrasonography. On trephine biopsy her marrow cellularity was 100%. She had reticullin fibrosis of grade I and megakaryocytes were numerous with normal maturation. Blast cells were not seen. Cytogenetics detected normal diploid karyotype. She was found to have antiplatelet antibodies and received steroids with no PLT increase. Then, she was started with ruxolitinib at reduced dose of 15 mg daily due to grade 4 thrombocytopenia. A slow increase in PLT count was observed with concomitant spleen reduction. Five months later, her PLT count was 44 9 10/l, and ruxolitinib dose was increased to 30 mg daily. At 15 months of ruxolitinib treatment, her PLT count was 93 9 10/l with further spleen reduction (17 cm on ultrasound). Figure 1. However, her RBC and WBC counts were elevated, and she required regular phlebotomies. A repeated trephine biopsy & Grzegorz Helbig ghelbig@o2.pl

Resolution of thrombocytopenia, but not polycythemia after ruxolitinib for polycythemia vera with detectable mutation in the exon 12 of the JAK2 gene