Synergistic Effect of Imatinib and Ruxolitinib in a Patient with JAK2V617F positive Myelofibrosis and Concomitant BCR-ABL1 positive Chronic Myeloid Leukemia
Authors: Tantravahi Srinivas K. , Kim Soo Jin , Gilreath Jeffrey A , Pomicter Anthony D. , Tashi Tsewang , Deininger Michael W. , Prchal Josef T. 
Published: October 2019

Abstract
Introduction: JAK2 V617Fmutation is present in majority of polycythemia vera (PV) and in 50-60% of patients with primary myelofibrosis (MF). JAK1/2 inhibitor ruxolitinib (RUX) is approved for treatment of both PV and intermediate/high risk MF. Two independent randomized studies showed the benefit of RUX in MF with significant reduction in spleen volume and improvement in constitutional symptoms compared to best supportive care. Clonal evolution may occur in PV and MF upon acquisition of new mutations and transformation to acute myeloid leukemia. Rarely, patients with JAK2V617Fpositive PV or MF have also concomitant BCR-ABL1 fusion gene with phenotype of chronic myelogenous leukemia (CML). Imatinib, a BCR-ABL1 inhibitor, induces long-term cytogenetic and molecular remissions in CML. Single agent imatinib has shown no clinical benefit in MF patients despite in vitro efficacy (Gaikwad et al Exp. Hematol. 2007). We describe here a patient with JAK2V617F positive post-PV MF who later developed concomitant BCR-ABL+ CML. Splenomegaly was initially resolved by RUX but returned coincidently with the diagnosis of CML and was again normalized by imatinib alone. Massive splenomegaly reappeared along with elevated hematocrit on imatinib and was resolved by the addition of low dose RUX.
Case report: A 71-year old woman initially presented in September 2011 with isolated erythrocytosis and splenomegaly. A diagnosis of PV was established based on the presence of the JAK2V617Fmutation and hypercellular marrow. Bone marrow karyotype was normal (Table 1). The patient was randomized to pegylated interferon arm in the MPD Consortium study. A complete hematologic response was achieved after nearly 2 years on maximal dose 180 mcg weekly. In early 2015, she developed constitutional symptoms, and progressive splenomegaly. Progression to post-PV MF was confirmed on bone marrow evaluation. The patient was taken off study and started on RUX 20 mg BID. She developed severe anemia 4 months later requiring dose reduction to 10 mg BID. The anemia markedly improved, while JAK2V617F allele burden remained high at 88% with normal cytogenetics. In December 2017, the patient developed anemia, fatigue and rapid regrowth of spleen. Marrow karyotype showed Philadelphia (Ph) chromosome in 25% of the cells analyzed, PCR was positive for BCR-ABL1 p210, and JAK2V617F allele burden was reduced to 56.4%. In January 2018, she started imatinib 400 mg daily and RUX was discontinued due to toxicity concerns. In April 2018, her hematocrit increased and massive splenomegaly returned. Restart of initial RUX 20 mg BID was followed by severe pancytopenia which normalized when RUX was reduced to 5 mg BID with continuing imatinib regimen. After 8 weeks, splenomegaly completely resolved (Figure 1). A complete cytogenetic response and major molecular response were achieved after 3 months and 6 months of combination therapy respectively.
Methods: Mononuclear cells from peripheral blood collected in June 2018 were plated in methylcellulose without erythropoietin or other cytokines. After 14 days, colonies were plucked, with ½ of each colony used for JAK2V617F allele-specific PCR and ½ prepared as cytospins for BCR-ABL fluorescent in situ hybridization (FISH) with the Vysis LSI BCR/ABL Dual Color, Dual Fusion Translocation Probe from Abbott Laboratories.
Results: Analysis of 3-6 EPO-independent colonies of each CFU-E, G, M, GM, GEMM, BFU-E showed no BCR-ABL1 positive cells, while a single CFU-GEMM colony was positive and also homozygous for JAK2V617F. Frozen marrow cells from the time of the original diagnosis are being analyzed.
Conclusion: The recurrence of splenomegaly in this patient following single agent RUX indicates emergence of novel BCR-ABL1+ clone. While single agent imatinib temporarily normalized the splenomegaly and leukocytosis for several months, MF phenotype reoccurred. Splenomegaly resolved completely in 8 weeks with the addition of very low dose RUX to standard dose imatinib, providing the first clinical observation of in vivo synergism of RUX and imatinib. We are investigating serial samples during her course, which will be presented at the meeting. The analysis of a single clone demonstrate that BCR-ABL1 mutation did not occur in previously normal dormant stem cell but in JAK2V617F positive progenitors and represent in this pts a novel subclone of previous PV/MF stem cell.

Disclosures
Deininger: Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees; Blueprint: Consultancy.