Gilliland, D. Gary

Abstract Background: We recently reported on MPLW515L/K mutations in 1182 patients with myeloid disorders (Blood, First Edition July 25, 2006) and found low frequency occurrence in in agnogenic myeloid metaplasia (AMM) (~5%) and essential thrombocythemia (ET) (~1%), but not polycythemia vera (PV). Of the 20 MPLW515L/K-positive patients in that series, 6 (30%) also carried a low burden of the JAK2V617F mutation. Studies of mutation prevalence in individuals with clonal granulopoiesis as well as familial myeloproliferative disorders (MPD) studies suggest that these mutations (JAK2V617F) may be acquired as secondary genetic events. Aim: to gain additional insight regarding the aforementioned two mutations and MPD pathogenesis by determining (i) concordance between mutation expression and clonal hematopoiesis, (ii) the instructive role of each mutation in determining lineage fate, iii) differences in pattern and lineage restriction of these mutations between various MPD, and (iv) chronology of clonal emergence and clonal dominance for the 2 mutations. Methods: Sixteen MPD patients were studied (13 JAK2V617F+ and 3 MPLW515L/K+); 9 had PV (all JAK2V617F+), 2 ET (both JAK2V617F+), 1 CMML (JAK2V617F+), and 4 AMM (2 MPLW515K+, and one each with MPLW515L and JAK2V617F). CD34+ progenitor cells were plated in methylcellulose and resulting hematopoietic colonies were scored on day 10–12. Colonies were obtained with (all patients) and without (10 patients; 5 with PV, 1 ET, and 4 AMM) cytokine support. Individual colonies were screened for presence of either mutation by DNA sequencing. In addition, MPLW515L/K analysis was performed in T and B lymphocytes in 3 informative cases. Finally, serially stored bone marrow was examined in 3 patients with co-expression of MPL515 and JAK2V617F mutations. Results: A total of 552 hematopoietic colonies from 16 MPD patients were genotyped. We found cohabitation of both mutation-positive and mutation-negative endogenous colonies in PV as well as other MPD. For JAK2V617F+ patients, the majority (75–100%) of erythropoietin-independent erythroid colonies (EEC) but not endogenous granulocyte colonies (20–70%) were mutation-positive. In contrast, no EEC were obtained from progenitors from the 3 MPLW515L/K+ patients. For 2 of the 3 patients, the endogenous myeloid colonies were predominantly mutation-positive (~90%). A mixed allele pattern (homozygous and/or heterozygous) was seen with both mutations regardless of MPD phenotype, but the overall mutant colony burden was higher with MPLW515L/K mutations relative to JAK2V617F. When such testing was possible, MPL515 mutations were detected in T-lymphocytes (2 of 3 patients) as well as B-lymphocytes (2 of 2 patients). Finally, among 3 patients carrying both mutations, serial marrow studies (25 time points over 4–8 years) revealed the occurrence of both mutations at diagnosis and persistent dominance of MPLW515L/K over JAK2V617F over time. Conclusions: Our observations provide direct evidence for clonal hematopoiesis that antedates acquisition of either MPLW515L/K or JAK2V617F that bias towards myeloid and erythroid fates, respectively. Furthermore, both mutations appear to occur early in the disease course and lineage studies suggest a hierarchically earlier stem cell origin for MPLW515L/K as compared to JAK2V617F.

Abstract Background: Scant data exists regarding the burden of fatigue and constitutional symptoms in patients with myeloproliferative disorders (MPDs) (essential thrombocythemia (ET), polycythemia vera (PV), and myelofibrosis with myeloid metaplasia (MMM)). Methods: An Internet based survey of MPD patients regarding clinical course, blood counts, constitutional symptoms, and fatigue (FACT-AN and Brief Fatigue Inventory (BFI)). Results: Patient Demographics: A total of 1179 MPD patients (median age 56; 41.4% male) internationally (76% USA: 50 states, 30 countries, 6 continents) responded to the survey. Self reported diagnoses were PV (35%), ET (26%), or MMM (39%). Clinical History: 1025 patients (87%) had undergone MPD therapy (drugs 70.5%, phlebotomy 44.1%, other 16%). Thrombosis (n=261, 22%), hemorrhage (N=272; 23%), and splenomegaly (N=478 (43%) were frequently reported. Blood tests (available in 917) showed 39% were anemic, 7% transfusion dependent. Symptoms: Fatigue, pruritus, night sweats and bone pain were present across the majority of MPD patients (see table). Additionally it was observed that these symptoms appeared with nearly equal frequency in PV as in MMM patients. However MMM patients displayed more advanced features (fevers, night sweats and weight loss). MPD Associated Constitutional Symptoms in 1179 Patients SYMPTOM PV (N=405) ET (N=304) MMM (N=456) MISSING (N=14) TOTAL (N=1179) P VALUE Fatigue 344 (84.9%) 220 (72.4%) 381 (83.6%) 7 945 (81.1%) <0.0001 Pruritus 265 (65.4%) 120 (39.5%) 228 (50.0%) 3 613 (52.6%) <0.0001 Night Sweats 199 (49.1%) 123 (40.5%) 254 (55.7%) 4 576 (49.4%) 0.0002 Bone Pain 174 (43.0%) 125 (41.1%) 214 (46.9%) 4 513 (44%) 0.2480 Fevers 53 (13.1%) 26 (8.6%) 81 (17.8%) 2 160 (13.7%) 0.0013 Weight Loss 39 (9.6%) 22 (7.2%) 93 (20.4%) 0 154 (13.2%) <0.0001 Spleen Pain 17 (4.2%) 27 (8.9%) 30 (6.6%) 4 74 (6.3%) <0.0001 Fatigue (82% MPD linked) was significantly increased across all MPD subtypes compared to published controls for both the BFI (P<0.0001) and Fact-An (P<0.0001). Additionally 69% curtail social activity because of fatigue, 35% need assistance with activities of daily living, and 11% reported MPD associated medical disability. Fatigue strongly correlated with multiple MPD disease features including: anemia, bone pain, constitutional symptoms, pruritus, splenic pain, smoking, thrombosis, and hemorrhagic events (all p<0.0001). Interestingly, even in “asymptomatic” MPD patients (those without anemia, splenomegaly, or thrombohemorrhagic complications) (n=279)) 24% (41% PV, 31% ET, 28% MMM) described significant fatigue worse than published controls. Conclusions: Fatigue and other constitutional symptoms are a significant burden to MPD patients, despite current therapies employed. Additionally fatigue is also a significant problem even in MPD patients lacking other advanced disease features. Clearly, direct attempts to improve fatigue (through pharmacologic and non-pharmacologic interventions) are necessary.

Abstract Background: Constitutive activation of the JAK-STAT signaling pathway through acquisition of the JAK2V617F mutation plays a key role in the pathogenesis of related myeloproliferative disorders (MPD) including polycythemia vera (PV), agnogenic myeloid metaplasia (AMM), and essential thrombocythemia (ET). Hypothesis: Genetic variation in JAK2 and/or the cytokine receptor genes for erythropoietin (EPOR), thrombopoietin (MPL), and granulocyte colony stimulating factor (GCSFR), influences the expression of a specific disease phenotype (PV, AMM, or ET). Methods: We studied 179 Caucasian MPD patients (PV=84, AMM=58, ET=37) for genetic variation in 4 candidate genes (i.e. JAK2, EPOR, MPL, GCSFR) through single nucleotide polymorphism (SNP) and haplotype analyses. A total of 32 LD (linkage disequilibrium) tag-SNPs with a minimum allele frequency of at least 5% were selected from the HapMap CEU database (JAK2=13, EPOR=4, MPL=5, GCSFR=10). Genotyping was performed using archived DNA from enriched neutrophils. Results: Seventy six (94%), 26 (45%), and 14 (38%) patients with PV, AMM, and ET, respectively, carried the JAK2V617F mutation. Significant differences in allele frequencies was observed at 6 SNP loci (rs10758669, rs3808850, rs7849191, rs7046736, rs10815148, and rs12342421, p-values < 0.0005), all within the JAK2 gene, in comparing our overall study population with the founder Caucasian population in the HapMap database. In the additive genotype-phenotype association analysis adjusted for gender and age at diagnosis, 3 SNP loci in JAK2 (rs7046736, rs10815148, and rs12342421) were found to be significantly, but reciprocally associated with PV (p-values < 0.00006, odds ratio=0.37, 2.82, and 2.39, respectively) and ET (p-values < 0.002, odds ratio=2.50, 0.36, and 0.41, respectively), but not AMM, in terms of the minor allele being ’protective’. These three SNPs were all in high LD, with the ’r2’ measures of LD between 0.59 and 0.71. Furthermore, 2 additional JAK2 SNP loci (rs10758669, p-value = 0.0024 and rs10974947, p-value = 0.0046) were significantly associated with PV (odds ratio=1.88 and 0.47, respectively), but not ET or AMM. Similarly, presence of the minor allele at a single SNP locus in EPOR (rs318699, p-value = 0.0012) was significantly associated with PV only (odds ratio=2.16). For the phenotype-genotype intragene haplotype analyses, the EPOR intragene haplotypes GAAA and GGAA were significantly associated with PV (simulated global p-value = 0.058, simulated individual p-values 0.0013 and 0.0068, haplotype frequency of 35% and 56%, respectively). In addition to EPOR, 6 intragene haplotypes within JAK2 where significantly associated with PV (simulated global p-value < 0.0001, individual simulated p-values < 0.03). Conclusion: The current study demonstrates a genotype-phenotype association that involves JAK2 and EPOR in the setting of PV, but not that of AMM. Some JAK2 SNPs were found to be associated with both PV and ET but in opposite direction. Therefore, genetic variation among MPDs might contribute to phenotypic diversity in the presence of specific mutations.

Abstract BACKGROUND. Several studies have recently reported on the occurrence of a JAK2 V617F mutation in myeloid cells from the majority of patients with polycythemia vera (PV). The clinical relevance of this novel observation is currently under study. Similarly, there is limited information regarding the correlation of JAK2 V617F mutational status and neutrophil PRV-1 transcript level in PV, secondary polycythemia (SP) and other myeloproliferative disorders (MPD). METHODS. In a single institutional study, mutation screening for JAK2 V617F was performed in DNA derived from archived blood granulocytes from 63 consecutive patients with PV in whom current diagnostic criteria were strictly applied and diagnosis confirmed by bone marrow histology. Concomitant quantitative analysis of neutrophil PRV-1 expression was performed in 28 of the 63 patients and the results of both JAK2 V617F and PRV-1 analysis in these 28 patients were compared with those from another 22 patients with essential thrombocythemia (ET), 10 with myelofibrosis with myeloid metaplasia (MMM), 19 with SP, and 11 healthy volunteers. RESULTS. The 63 patients with PV were followed for a median of 33 months (range 3–324) from the time of initial diagnosis. During this period, 3 patients have died including 2 from acute myeloid leukemia (AML). Disease transformation into either AML or MMM has occurred in 2 and 4 patients, respectively. The median time from diagnosis to the time of mutation analysis was 12 months (range 0–306). JAK2 V617F was detected in 58 of the 63 patients with PV (92%; homozygous in 21%). The clinical phenotype of the 5 patients with wild-type allele was otherwise typical for the disease. A statistical comparison between JAK2 V617F heterozygotes (n=45) and homozygotes (n=13) in PV did not reveal significant associations in regards to age, gender, leukocyte or platelet count at diagnosis, disease duration, or the incidences of thrombosis or bleeding. However, JAK2 V617F homozygotes, compared to their heterozygote counterparts, displayed significantly higher hemoglobin level at diagnosis (p=0.001), increased incidence of pruritus (69% vs. 38%; p=0.04), and a higher rate of fibrotic transformation (23% vs. 2%; p=0.009). The overall (homozygous) JAK2 V617F mutational frequencies in the other study patients with ET, MMM, SP, and healthy controls were 55% (0%), 30% (0%), 0%, and 0%, respectively. The corresponding figures for increased PRV-1 expression in these patients were 18%, 20%, 21%, and 9% as compared to 89% in PV. In patients with either ET or MMM, the likelihood of detecting JAK2 V617F was significantly higher in the presence of an increased PRV-1 expression (83% vs. 38%; p=0.05). Similarly, in patients with PV, homozygous as compared to heterozygous JAK2 V617F correlated with higher levels of PRV-1 expression (p=0.07). CONCLUSIONS. The current clinical study demonstrates a mutant allele dose effect on erythrocytosis, PV-characteristic clinical features, and granulocyte PRV-1 expression in PV. Furthermore, compared to the PRV-1 assay, mutation screening for JAK2 V617F displayed greater specificity in distinguishing PV from SP.

Abstract Background: In essential thrombocythemia (ET), homozygosity for the JAK2 V617F mutant allele is rare despite the frequent (23–57%) occurrence of the heterozygous state. This is in direct contrast with the situation in polycythemia vera (PV) where homozygosity for the mutation is seen in 25–33% of the cases. The particular observation is consistent with what has been observed in vitro as well as in animal studies where JAK2 V617F has been associated with induction of erythropoietin hypersensitivity in cell lines and erythrocytosis in mice (James et al. Nature2005;434:1144). Therefore, regardless of whether or not JAK2 V617F represents a primary event in myeloproliferative disorders, it is probable that it promotes a PV phenotype. In the current study, we explore this hypothesis as well as investigate the long-term prognostic relevance of the mutation in a well defined cohort of patients with ET seen at a single institution. Methods: The diagnosis of ET was according to the WHO classification of tumors (Jaffe ES et al. Lyon, IARC press, 2001). All patients were seen at the Mayo Clinic and patient characteristics and subsequent clinical events were determined both retrospectively (chart review) and prospectively (by means of a questionnaire sent both to patients and their primary doctors). In addition date and cause of death were confirmed by probing both the social security index and the national death index. DNA for mutation screening was derived and sequenced from either archived unsorted cells from the bone marrow or prospectively collected peripheral blood granulocytes according to previously published methods (Levine et al. Cancer Cell, 2005;7:387). Results: The study cohort consisted of 150 consecutive patients with ET including 99 females and 51 males (median age 49.8 years; range 16.8–98). Overall, JAK2 V617F was detected in 73 patients (48.7%) but none were homozygous for the mutant allele. At diagnosis, ET patients with JAK2 V617F were significantly older (median age of 53.5 vs. 42.3; p=0.02) and also displayed higher readings of both hemoglobin level (p=0.0002) and leukocyte count (p=0.005). All study patients have now been followed for a median of 11.4 years from diagnosis (range 0–32.7) and follow-up period was similar between the two mutational categories (p=0.53). During this period, 32 patients have died and the median survival for the entire group was 19.3 years; 21.9 and 17.7 years in the absence and presence of the mutation, respectively (p=0.09). Disease transformation into acute leukemia, myelofibrosis, and PV occurred in 6 (4%), 15 (10%), and 8 (5.3%) patients, respectively, and the only significant difference between the two mutational categories in this regard was the higher incidence of transformation into PV in patients with the mutation (p=0.02). Multivariate analysis identified age, hemoglobin level, and thrombosis history at diagnosis but not JAK2 V617F mutational status as independent risk factors for survival. Conclusion: Although the presence of JAK2 V617F in ET appears to promote a PV phenotype, it might not carry treatment-relevant information.

Abstract Background: Little objective data exists regarding the impact that myeloproliferative disorders (MPDs) (essential thrombocythemia (ET), polycythemia vera (PV), and myelofibrosis with myeloid metaplasia (MMM)) have on quality of life. Methods: An Internet based survey of MPD patients regarding clinical course, co-morbidities (through the Charlson Co-Morbidity Index), fatigue (through two validated fatigue instruments (FACT-AN and Brief Fatigue Inventory (BFI)), and exercise tolerance (Godin Leisure-Time Questionnaire). Results: Patient Demographics: A total of 830 MPD patients (median age 56; 60.8% male) from a broad geographic distribution (77% USA: 50 states, 30 countries, 5 continents) responded to the survey. Self reported diagnoses were PV (N=272; 33%), ET (N=221; 27%), or MMM (N=329; 40%) for a median of 5 years (range (0–53). Clinical History: 708 patients (87%) had undergone treatment for their MPD (drugs 84.6%, phlebotomy 49.5%, other 16%). A history of thrombotic (n=195, 24%) or hemorrhagic events (N=214; 26%) were frequently reported. Blood tests (available in 597 a median 1.65 months prior to survey) showed 37% were anemic (7% transfusion dependent). Additionally, 347 (44%) had splenomegaly, painful in 21%. Symptoms: Frequent symptoms reported included fatigue (90%), pruritus (57%), night sweats (54%), bone pain (48%), undesired weight loss (15%), and/or fevers (15%). 41% felt restricted in their activities, and 89 (11%) were medically disabled secondary directly to their MPD. 82% of patients felt their fatigue was linked to their MPD. Additionally 69% curtail social activity because of fatigue, and 41% need some degree of assistance with activities of daily living. Charlson co-morbidity index demonstrated 722 (87%) had minimal co-morbidities strengthening the association between fatigue and their MPD. The burden of fatigue decreased from MMM to PV to ET respectively (p=0.0002). Results from both fatigue assessment tools (FACT-AN and BFI) were highly correlated. Fatigue (FACT-AN fatigue subscale) strongly correlated with multiple MPD disease features including: anemia, bone pain, constitutional symptoms, pruritus, splenic pain, hemorrhagic events (all p<0.0001) and thrombosis(p=0.01). Additionally, currently smoking (as opposed to being a prior smoker) was highly associated with increased fatigue (p<0.0001). Interestingly, even in “asymptomatic” patients (those without anemia, splenomegaly, or thrombohemorrhagic complications (n=187)) 73% (n=128; 40% PV, 32% ET, 27% MMM; p=N.S.) described moderate to severe MPD associated fatigue. Reported physical activity was diminished (Godin Leisure Score) between MPD patients and published norms, with 38% describing minimal to no exercise. Overall, 86% stated fatigue was the major obstacle to exercise. Conclusions: Fatigue is an extremely common problem in patients with MPDs despite current therapies, and a minimal burden of co-morbidities in the majority of patients. Fatigue is also common in patients lacking advanced MPD disease features, or thrombohemorrhagic complications. Clearly, direct attempts to improve fatigue (through pharmacologic and non-pharmacologic interventions) are necessary.

Abstract Background: The occurrence of an activating JAK2 mutation (JAK2 V617F) has been reported in a variable proportion of patients with bcr/abl-negative myeloproliferative disorders including myelofibrosis with myeloid metaplasia (MMM). In the current study, we report mutational frequencies in each of the 3 MMM variants as well as examine the prognostic relevance of the mutation in a large group of patients from a single institution. Methods: Mutation analysis for JAK2 V617F was performed in DNA derived from peripheral blood mononuclear cells (PBMC) from all study patients (n=157). Concomitant analysis was also performed in DNA derived from purified granulocytes (n=57), CD34-positive cells (n=25), and T cells (n=19). Genomic DNA was amplified by PCR and fluorescent dye chemistry sequencing was performed using the same primers used for amplification. Results: The study cohort included 157 consecutive patients with MMM including 117 with agnogenic (AMM), 22 with post polycythemic (PPMM), and 18 with post thrombocythemic (PTMM) myeloid metaplasia. The overall detection rate for JAK2 V617F in PBMC was 51% (homozygous in 5.7%) and was significantly (p=0.0002) higher in PPMM (91%; homozygous in 18%) compared to either AMM (45.3%; homozygous in 2.6%) or PTMM (38.9%; homozygous in 11.1%). In AMM (n=117), the presence of JAK2 V617F was significantly associated with an older age at diagnosis (p=0.03), and a history of both thrombosis (p=0.01) and pruritus (p=0.005). No other statistical associations were evident and the presence of the mutation did not predict leukemic transformation. Furthermore, multivariate analysis for overall survival of patients with AMM identified only age and the Dupriez prognostic score as independent prognostic factors; JAK2 V617F had no prognostic significance. Finally, the mutation was not found in T cells and its detection rate in either granulocytes or CD34-positive cells was similar to that in PBMC. Conclusion: JAK2 V617F is a myeloid lineage-specific event, its incidence in MMM is significantly higher with an antecedent history of polycythemia vera, and its presence in AMM does not affect prognosis but is associated with PV-characteristic clinical features.

Abstract The JAK2V617F mutation is present in the majority of cases of myeloproliferative disease, including polycythemia vera (PV), essential thrombocythemia (ET) and primary myelofibrosis (PMF), and is an attractive candidate for molecularly targeted therapy. However, the potential toxicities of JAK2 inhibition in vivo, and identification of appropriate surrogate endpoints for response, are challenges that may limit clinical usefulness in treatment of these relatively indolent diseases. We report efficacy and assessment of surrogate endpoints for response of a small molecule JAK2 inhibitor, TG101348 in a murine model of polycythemia vera. TG101348 is selective for JAK2 with an in vitro IC50 of ∼3 nM that is ∼334 fold more potent than for inhibition of JAK3. TG101348 showed therapeutic efficacy in the murine model of PV that included a statistically significant reduction in hematocrit, normalization of white blood cell count, a dose dependent reduction/elimination of extramedullary hematopoiesis in the spleen and liver, and marked attenuation of myelofibrosis. Consistent with its selective inhibition of JAK2 and not JAK3, there was no significant change in T-cell number in treated animals. These clinical responses correlated with surrogate endpoints for response, including reduction or elimination of JAK2V617F expressing clones based on quantitative genomic PCR, suppression of JAK2V617F positive endogenous erythroid colony growth of JAK2V617F MPD bone marrow, and inhibition of JAK-STAT signal transduction as assessed by phosphoflow cytometry for phosphorylated STAT5. Thus, TG101348 is efficacious in treatment of a murine model of PV, and surrogate endpoints have been identified that may be of value in clinical trials in humans.

Abstract Introduction: Several groups are developing orally bioavailable small molecule JAK2 inhibitors as an approach to targeted therapy of myeloproliferative disorders. We previously reported that one such inhibitor (TG101209) potently inhibits JAK2V617F in vitro, and was effective in reducing tumor growth in a mouse xenograft model (Leukemia, 2007 Aug;21(8):1658–68). Here, we expand our analysis using TG101348, a highly selective JAK2 inhibitor currently in preclinical development, using cell lines and primary cells harboring JAK2V617F, MPLW515L/K, or JAK2 exon 12 mutations. Methods: Cell proliferation was assessed using the XTT assay. For primary cell experiments, PBMCs were plated in methylcellulose medium at 0, 300nM, 600nM, and 1200nM TG101348, both with and without erythropoietin (Epo). Hematopoietic colonies were scored after 12–16 days culture, and harvested for genotyping. Results: TG101348 inhibits growth of human erythroleukemia (HEL) and Ba/F3-V617F cells (IC50=300nM and 580nM, respectively), as well as Ba/F3-(MPL)W515L cells (IC50=620nM). JAK2:JAK3 selectivity of TG101348 was demonstrated by its 10-fold more potent inhibition of CHRF (JAK2T875N) as compared to CMK (JAK3A572V) cell proliferation. TG101348’s inhibitory effect on ex vivo hematopoietic colony growth was studied in 12 MPD patients (JAK2V617F=9, MPLW515L/K=1, JAK2 exon 12 mutation [N542-E543del]=1, no mutation=1), and 2 healthy controls (Table). Conclusions: (1) TG101348 inhibits colony growth from MPD patients more potently than healthy controls, reflecting its potential therapeutic window. (2) Endogenous colony growth is inhibited at nanomolar concentrations in all cases, with some alleles showing greater sensitivity than others (MPLW515K = exon 12 mutation > JAK2V617F). The corresponding Epo-supported erythroid colonies are less potently inhibited, reflecting the generally lower prevalence of JAK2 mutations in such colonies. (3) EEC growth in the absence of known JAK2 or MPLW515 mutations is also inhibited, indicating a similar dependence on dysregulated JAK-STAT signaling in such cases. (4) The ability of TG101348 to selectively suppress mutation-positive hematopoietic colonies will be discussed. Inhibitor effect on ex vivo hematopoietic colony growth Paient Mutation IC50 (nM) % Mutation-positive colonies Epo No Epo Epo No Epo (EEC) Normal 1 None >1200 n/a n/a n/a Normal 2 None >1200 n/a n/a n.a PV1 V617F 600 <300 5 40 PV2 V617F 600 300 33 50 PV3 V617F 600–1200 300–600 9 70 PV4 V617F 300–600 600 0 15 PV5 V617F 600–1200 300 80 100 PV6 V617F 600 300–600 7 19 PV7 V617F 600–1200 600 32 36 PV8 V617F 600–1200 <300 45 37 PV9 V617F 600 600–1200 20 12 PV10 Exon 12 (N542–E543del) 900 <300 9 60 PMF1 W515K 300–600 <300 100 33 PMF2 None >1200 300 n/a n/a

Abstract Abstract 459 Background: A Phase I study of TG101348, a potent, oral JAK2-selective inhibitor in patients with high- or intermediate-risk primary, post-polycythemia vera (PV) and post-essential thrombocythemia (ET) myelofibrosis (MF) recently completed enrollment. Data from the dose escalation cohort (n=28; 30–800 mg administered as a single daily dose) showed dose-linear plasma exposure, with mean elimination T1/2 at steady state ranging from 16 to 34 hours. The dose-limiting toxicity was reversible asymptomatic grade 3 or 4 amylasemia/lipasemia; the maximum tolerated dose was 680 mg. We now present updated results from an extension study in which subjects continued treatment beyond six 28-day cycles. Result: In the entire study, 59 subjects (median age 64 years) were treated. Forty four patients had PMF, 12 post-PV MF, and 3 post-ET MF; 86% were JAK2V617F-positive. Median palpable spleen size was 18 cm and 21 (36%) patients were transfusion-dependent at study enrollment. Forty three (73%) subjects continued treatment on the extension study; the median (range) cumulative exposure to TG101348 was 380 days (170-767); the number of cycles of treatment completed ranged from 7–29, and the proportion of subjects who completed 12, 18, and 24 cycles of treatment was 72%, 28%, and 14%, respectively. The median (range) treatment dose at data cutoff was 440 mg/day (120 to 680 mg/day; n = 35). Toxicity: TG101348 was well tolerated. Non-hematologic, at least possibly related adverse events included nausea (69%; Grade 3=3%), diarrhea (64%; Grade 3=10%), and vomiting (58%; Grade 3=3%), that were dose-dependent; the symptoms were self-limited or controlled with symptomatic treatment and/or dose reduction. Also observed were asymptomatic increases in serum lipase (27%), AST/ALT (27%/25%), creatinine (24%), and alkaline phosphatase (17%). Grade 3/4 hematological adverse events possibly related to TG101348 included treatment-emergent anemia (35% of patients who were not transfusion dependent at baseline), thrombocytopenia (24%), and neutropenia (10%). Spleen: During cycles 1–6, 42% of patients achieved Clinical Improvement (CI) per the International Working Group for Myeloproliferative Neoplasms Research and Treatment (IWG-MRT) criteria for spleen response. The CI rate for the 43 subjects who continued treatment beyond 6 cycles was 65%, and the median time (range) to CI was 140 days (77-802). After 12 cycles, 36 (95%) of the 38 evaluable patients had some degree of spleen size reduction; 32 (85%), 26 (68%), and 10 (26%) of subjects had a minimum 25%, 50%, and 100% decrease in palpable splenomegaly. JAK2V617F allele burden: Of the 48 JAK2V617F mutation-positive subjects, 23 (48%) had a baseline mutant allele burden of >20%. Of these, 20 subjects with a baseline median allele burden (range) of 62% (23% to 100%) completed at least 6 cycles of treatment. Sixteen subjects (80%) exhibited a median 62% (range 6% to 96%) decrease in JAK2V617F burden, including 9 (45%) with a ≥50% decrease; 4 subjects (20%) had increases (range 18% to 58%). Eighteen subjects (78%) completed 12 cycles of treatment; 13 (72%) had a median 50% decrease (range 29% to 82%), including 7 (39%) subjects with a ≥50% decrease; 5 subjects (28%) had no change or increases (range 0% to 22%) in JAK2V617F. Constitutional symptoms: For patients with data at baseline and cycle 6, early satiety, fatigue, night sweats, cough, and pruritus were reported by 56%, 47%, 24%, 22%, and 19% of subjects, respectively. Improvement (complete resolution) rates observed at cycle 6 were: 75% (40%), 65% (29%), 89% (89%), 88% (75%), and 100% (57%) for the aforementioned symptoms, respectively. Leukocytosis/Thrombocytosis: Leukocytosis was present at baseline in 33 subjects; following 6 and 12 cycles of therapy, 57% (n=28) and 56% (n=25) achieved normal WBC counts. Thrombocytosis was noted at baseline for 10 (17%) subjects; following 6 and 12 cycles of therapy, 90% (n=10) and 88% (n=8) achieved normal platelet counts. Conclusion: The current study provides proof-of-concept that selective JAK2 inhibition has anti-myeloproliferative/anti-clonal activity in myelofibrosis. The majority of subjects treated with TG101348 for more than 6 cycles experienced a rapid, significant, and durable decrease in splenomegaly, normalization of leukocyte/platelet counts, and improvement of disease-related symptoms, all within the context significantly decreased JAK2V617F burden. Disclosures: Off Label Use: There is no FDA approved drug for Myelofibrosis.All the drugs discussed will be off-label. Stone:Novartis: Consultancy. Silverman:TargeGen: Consultancy. Shorr:TargeGen: Employment, Equity Ownership.