Activity Of a JAK1/JAK2 Inhibitor In a Patient With KIT-Mutated Systemic Mastocytosis (SM) Associated With Myelofibrosis

KIT mutations, particularly the D816V mutation, are frequently found in patients with SM and play a central role in the pathogenesis of the disorder. A recent publication demonstrated pre-clinical evidence that inhibition of JAK-STAT signaling may inhibit KITD816V-mediated cell growth (Pardanani A et al, Leukemia 2010, 24:1378-1380), but there is no clinical data on the efficacy of JAK inhibitors in patients with MS.

To describe the case of a patient with KIT-mutated SM associated with myelofibrosis that was successfully treated with the JAK1/JAK2 inhibitor ruxolitinib

A 67-years old male patient was referred to our center with an outside diagnosis of primary myelofibrosis (PMF). His main complaints included fatigue and pruritus. On physical examination, liver was enlarged 2 cm from right costal margin (RCM) and spleen was enlarged 8 cm from left costal margin (LCM). His complete blood count (CBC) showed Hb 13.9 g/dL, white blood cell count (WBC) was 22.41x10⁹/L, and a platelet count of 702x10⁹/L. His bone marrow (BM) biopsy was compatible with the diagnosis of PMF. He was found to be negative for the JAK2V617F-mutation. The patient was started on ruxolitinib at a dose of 20 mg twice daily. Four weeks from start of therapy, his spleen had decreased to 3 cm from LCM. The CBC at 4 weeks showed Hb 16 g/dL, WBC 12.8x10⁹/L, platelets 459x10⁹/L. After 16 weeks of therapy, the patient was feeling much better, and his spleen was down to <1 cm from LCM and liver was non-palpable. The CBC showed Hb 15.8 g/dL, WBC 23.44x10⁹/L, platelet count 422x10⁹/L. As of the time of this writing, the patient is still being treated with ruxolitinib and maintains his response.

Since the patient was negative for the JAK2V617F-mutation and had presented with a clinical response to ruxolitinib, we proceeded with whole exome sequencing (WES) in order to detect mutations that might predict for response to ruxolitinib. Paired DNA (sorted CD66b-granulocytes/skin biopsy) was subjected to WES, sequence-aligned by Bowtie v2, and putative somatic variants detected via SomaticSniper (Washington University). Following in-house filters to decrease false-positive findings, a total of 124 putative somatic point mutations were detected in the CD66b-granulocytes sample. Two known oncogenic KIT mutations were found, D816V and K642E. The D816V mutation had a variant allele frequency (VAF) of 45%, indicating that it was present in the predominant clone, while the K642E mutation had a VAF of 28%, suggesting that it was subclonal and acquired later in the course of disease. No other tyrosine kinase or associated receptors (e.g. MPL) mutations were found. Since KIT mutations have not been previously reported in patients with PMF, we reviewed all clinical and laboratory data on the patient’s diagnosis. Tryptase staining of the BM biopsy slides showed aggregates of spindle-shaped mast cells that also stained positive for CD2 and CD25. A tryptase level was found to be elevated at 150 ng/ml. The KITD816V mutation was validated by Sanger sequencing. There were no lytic bone lesions, diarrhea, cytopenias or other “C” findings suggestive of aggressive SM. We changed the diagnosis from PMF to SM associated with myelofibrosis.  Even though the patients has features of myelofibrosis, no mutations commonly found in PMF were detected by WES, while the KITD816V mutation is probably the main driver of the disease due to its high allele frequency and known oncogenic role in SM. Currently, we are performing correlative studies to evaluate STAT3/5 phosphorylation in patient’s samples taken before and after therapy with ruxolitinib.

We believe this is the first case of a patient with KIT-mediated SM that was treated with a JAK inhibitor. The improvement in symptoms and other signs of myeloproliferation seen in this case suggest that JAK inhibition might be a therapeutic avenue in such patients, as expected from published pre-clinical data. A clinical trial evaluating JAK inhibitors in patients with SM should be conducted to further delineate the activity of this class of drugs in these patients.

Santos:Novartis: Consultancy, Research Funding, Speakers Bureau. Off Label Use: Ruxolitinib for systemic mastocytosis.