Impact of Non-JAK2 Molecular Mutations and Cryptic SNP Lesions in Myelofibrosis Patients Treated with Ruxolitinib

The identification of the JAK2 V617F mutation in myeloproliferative neoplasms (MPN) paved the way for the pivotal studies that led to the FDA approval of a JAK 1/2 inhibitor, ruxolitinib (rux) in patients (pts) with myelofibrosis (MF). Improvement in splenomegaly and debilitating disease-related symptoms were the primary clinical responses observed with rux. Although JAK2 mutational status did not impact response/survival in MF pts, cytogenetics had an impact on prognosis. In a related myeloid neoplasm specifically myelodysplastic syndromes, molecular mutations ( TET2/DNMT3A ) predict for better therapeutic response to DNA methyltransferase inhibitors. We hypothesized that somatic mutations and single nucleotide polymorphism array (SNP-A) lesions are frequent in MF pts treated with rux and may affect their clinical outcomes. To further investigate the predictive and prognostic impact of SNP-A lesions and somatic mutations in MF pts in the rux era, we studied 54 MF pts who received at least 12 weeks of rux therapy (tx) using a modified dose escalation approach (Tabarroki A et al. 55 th ASH; Abstract 1586). Clinical (total symptom score [TSS], spleen size), cytogenetic (metaphase cytogenetics [MC], SNP-A), hematologic and survival data were collected before and 12-weeks post rux tx. Categorical data were analyzed using X 2 test. A p -value of TET2, CBL, LNK, DNMT3A, TP53, SF3B1, U2AF1, SRSF2, ASXL1, EZH2, JAK2, CALR, and IDH1/2 was performed. The median age of the cohort was 66 yrs (41-89); male/female: 28/26. The median follow-up time after initiation of tx was 17 months. The median overall follow-up of the cohort from the time of diagnosis was 35 months. Using DIPSS-plus, pts were stratified as high (24, 44%), int-2 (22, 41%) and int-1 (8, 15%) risk groups. Baseline median WBC=9.4k/μL, Hgb=10.2g/dL, PLT=212k/μl, TSS=20, and median palpable spleen size=13cm. Post-tx median WBC=9.9k/μL, Hgb=10.1g/dL, PLT=150k/μL, TSS=4, and spleen size=6cm. MC identified cytogenetic abnormalities in 24/54 (44.4%) pts. The most frequent chromosomal defects included del(20), +8, and +9. Serial MC was available for 20 pts and no cytogenetic evolution was identified. SNP-A data were available for 29 pts, of which 28 pts had SNP-A lesions. The most commonly involved chromosomes were 9 (15.1%), 20 (14.1%), and 14 (8.5%). Compared to MC analysis, additional SNP-A lesions were found in 66% of pts. Of note 39% of the pts had normal karyotypes but with pathologic SNP-A lesions; another 27% had pathologic SNP-A lesions besides the abnormal MC. Serial SNP-A analysis was available in 10 pts who while on rux tx did not develop any additional/new SNP-A lesion. There was no difference in spleen response rates or TSS between those who carried SNP-A lesions versus those who did not. Molecular analysis was possible for 34 pts. The most frequent somatic mutations observed involved JAK2 (70.6%), ASXL1 (24%), CALR (24%), SRSF2 (15%), and U2AF1 (9%). Pts with int-2 and high DIPSS plus scores were more likely to carry at least 1 mutation in any gene compared to pts with int-1 scores (int-2 vs int-1, p= .05; high vs int-1, p =.06). After a median follow-up of 35 months from diagnosis, 95% of the pts were still alive. 3 pts died from disease progression: 1 had a sole SRSF2 mutation, 1 had an SRSF2 plus CALR mutation, and 1 had a TET2 plus TP53 mutation. SRSF2 mutant pts had more severe thrombocytopenia pre-rux tx (91 vs. 203k/μL; p =.04). ASXL1 mutant pts had increased spleen sizes pre-rux (21 vs. 15cm, p =.06), but had similar response post-rux (10 vs. 8cm, p =0.6) compared to the wild-type. SRSR2 mutant pts had higher DIPSS-plus score (4.4 vs. 3; p= .05). Our study showed that MF pts treated with a rux modified dose escalation approach resulted in meaningful clinical and splenic responses regardless of molecular mutation status. Frequently found cryptic SNP-A lesions in MF pts may explain their poorer outcomes compared to pts with other MPNs. The fact that pts did not acquire additional/new SNP-A lesions during rux tx may be one of the mechanisms of improved survival in these pts. ASXL1 , CALR , SRSF2 and U2AF1 were the most frequent non- JAK molecular mutations in MF pts treated with rux and were more frequent in high risk pts. Further studies are necessary to elucidate the clinical/ biological effects of these mutations in MF pts treated with rux. Disclosures Tiu: Incyte: Membership on an entity9s Board of Directors or advisory committees, Speakers Bureau; Novartis: Speakers Bureau; Gilead: Membership on an entity9s Board of Directors or advisory committees.