Citations

Abstract Background: Liquid chromatography-tandem mass spectrometry (LC-MS/MS) with electrospray ionization is robust and allows accurate measurement of both low- and high-molecular weight components of complex mixtures. We developed a LC-MS/MS method for the analysis of methylmalonic acid (MMA), a biochemical marker for inherited disorders of propionate metabolism and acquired vitamin B12 deficiency. Methods: We added 1 nmol of the internal standard MMA-d3 to 500 μL of plasma or 100 μL of urine before solid-phase extraction. After elution with 18 mol/L formic acid, the eluate was evaporated, and butyl ester derivatives were prepared with 3 mol/L HCl in n-butanol at 65 °C for 15 min. For separation, we used a Supelcosil LC-18, 33 × 4.6 mm column with 60:40 (by volume) acetonitrile:aqueous formic acid (1 g/L) as mobile phase. The transitions m/z 231 to m/z 119 and m/z 234 to m/z 122 were used in the selected reaction monitoring mode for MMA and MMA-d3, respectively. The retention time of MMA was 2.2 min in a 3.0-min analysis, without interference of a physiologically more abundant isomer, succinic acid. Results: Daily calibrations between 0.25 and 8.33 nmol in 0.5 mL exhibited consistent linearity and reproducibility. At a plasma concentration of 0.12 μmol/L, the signal-to-noise ratio for MMA was 40:1. The regression equation for our previous gas chromatography-mass spectrometry (GC-MS) method (y) and the LC-MS/MS method (x) was: y = 1.030x − 0.032 (Sy|x = 1.03 μmol/L; n = 106; r = 0.994). Inter- and intraassay CVs were 3.8–8.5% and 1.3–3.4%, respectively, at mean concentrations of 0.13, 0.25, 0.60, and 2.02 μmol/L. Mean recoveries of MMA added to plasma were 96.9% (0.25 μmol/L), 96.0% (0.60 μmol/L), and 94.8% (2.02 μmol/L). One MS/MS system used only overnight (7.5 h) replaced two GC-MS systems (30 instrument-hours/day) to run 100–150 samples per day, with reductions of total cost (supplies plus equipment), personnel, and instrument time of 59%, 14%, and 75%, respectively. Conclusions: This method is well suited for large-scale MMA testing (≥100 samples per day) where a shorter analytical time is highly desirable. Reagents are less expensive than the anion-exchange/cyclohexanol-HCl method, and sample preparation of batches up to 100 specimens is completed in less than 8 h and is automated.

ABSTRACT The epidemiologic relatedness of methicillin-resistant Staphylococcus aureus (MRSA) isolates is currently determined by analysis of chromosomal DNA restriction patterns by pulsed-field gel electrophoresis (PFGE). We have evaluated an alternative typing system (MicroSeq StaphTrack Kit; Perkin-Elmer Biosystems) based on the sequence analysis of the chromosomally encoded polymorphic repeat X region of the S. aureus protein A ( spa ) gene. A total of 69 clinical MRSA isolates were divided into 18 groups according to the number and nucleotide sequences of the spa repeats. Molecular typing results obtained both by spa sequencing and from the PFGE patterns were concordant except for one group, which contained 20 isolates recovered over a 2-year period from hospitalized patients at the Mayo Clinic. Although the spa typing patterns were indistinguishable for those isolates, PFGE analysis yielded seven related but distinguishable patterns. Further coagulase gene sequence analysis subtyped those 20 strains into four groups which followed distinct temporal and geographic distributions. During a 2-year epidemic period there were up to 7 fragment changes in PFGE patterns among epidemiologically related isolates, suggesting that PFGE may be unsuitable for long-term typing of strains involved in epidemics. Although more limited than PFGE in discriminatory power, spa sequencing analysis could be used as a screening method for typing of MRSA strains because of the shorter turnaround time, ease of use, and the inherent advantages of sequence analysis, storage, and sharing of information.

ABSTRACT A branched-DNA (bDNA) signal amplification method was used to detect the mecA gene directly from blood culture broth growing staphylococci. BACTEC blood culture bottles with positive growth indices and containing staphylococcus-like organisms as shown by Gram stain were tested for the presence of the mecA gene. Comparison of test results was done among 225 patients (one blood culture from each patient). Compared with PCR, the sensitivity and specificity of the bDNA method are 100 and 99%, respectively. The bDNA test is carried out in a 96-well format and requires approximately 6 h to perform. Our preliminary results suggest that direct detection of the mecA gene by bDNA signal amplification is (i) sensitive enough to detect mecA directly from blood culture bottles without the requirement for subculture and (ii) as sensitive and specific as the PCR-based method.

ABSTRACT The identification of methicillin-resistant staphylococcus isolates in the clinical laboratory has typically been performed by using methods that detect phenotypic expression of resistance determinants. However, these methods may be difficult to interpret and some isolates do not express resistance until selective pressure is administered. Assays that detect genetic determinants are not subject to these limitations and have been effective in distinguishing isolates that are capable of expressing the resistance phenotype. In this study, a novel branched-DNA (bDNA) hybridization assay was used to test for the mecA gene in 416 clinical staphylococcal isolates. The results were compared with those obtained by a PCR-based assay and oxacillin disk diffusion. For 155 Staphylococcus aureus and 261 coagulase-negative Staphylococcus isolates, the bDNA assay and PCR results were 100% concordant. Among the S. aureus isolates, 20 were MecA + and 135 were MecA − . For the coagulase-negative staphylococci, 150 were MecA + and 111 were MecA − . The results from the genotypic detection methods were compared with those obtained by oxacillin disk diffusion. No discrepancies were detected among the S. aureus isolates; however, 10 coagulase-negative isolates were MecA + but oxacillin sensitive and 1 isolate was MecA − but oxacillin resistant. Oxacillin resistance was induced in 6 of the 10 MecA + isolates previously classified as oxacillin sensitive. These results suggest that the bDNA method described here is a sensitive and efficient method for detection of methicillin resistance in staphylococci and that genetic detection methods may be useful for detection of potential methicillin resistance in the clinical laboratory.

Background: Activation induced cytidine deaminase (AICDA or AID) causes somatic hypermutation (SHM) of the immunoglobulin heavy chain variable genes (IGHV). It has been widely hypothesized that aberrant targeting of AICDA creates additional driver mutations and intra-clonal heterogeneity which enables clonal evolution and thus disease progression. This hypothesis predicts that greater AICDA-mediated intraclonal heterogeneity will be associated with shortened progression free survival (PFS). To definitively test this hypothesis that AICDA-mediated heterogeneity is linked to risk of progression, we evaluated available samples from the largest US-based phase 3 trial of untreated follicular lymphoma patients treated with CHOP-based therapies, SWOG trial S0016. Methods: We have previously shown that ultradeep sequencing of two genes, the expressed IGHV and the 5' UTR of the translocated BCL2 allele, provide unrelated measures of AICDA-mediated genetic heterogeneity (Spence, J Immunol. 2014). Furthermore, the number of AICDA-mediated mutations in the 5' UTR of BCL2, but not in IGHV, correlates with the number of AICDA-mediated mutations at other known AICDA sites throughout the genome (such as PIM1, BCL6, RHOA, etc). Genomic DNA was prepared from all available paraffin-embedded specimens. Ultra-deep, amplicon-based sequencing (~10,000x) with internal controls to establish specimen-specific signal-to-noise parameters allowed sensitive and specific detection of sequence variants at 0.2% (FDR 1%). Results: For subjects with (152) and without (419) available specimens, the median duration of follow-up (11.3 years), the number of patients who progressed (60%), and other patient characteristics (demographics, histology, stage, FLIPI) were indistinguishable. Sequencing data that fulfilled strict quality metrics for low frequency variants were obtained for >75% of the specimens (114 for BCL2; 120 for IGHV); further analysis was restricted to these subjects. The extent of intraclonal variation within both the IGHV and the 5' UTR of BCL2 was highly variable. For BCL2, a median of 7 mutations/case were detected (range 0-61); the mutations were highly skewed to the RCYW motif (p<0.001), the preferred target of AICDA. These mutations defined a median of 5 subclones/case (range 1-22). The clonal IGHV was somatically mutated (<0.98 germline identity) in 94% of samples (median 0.89). For IGHV, a median of 113 subclones/case were detected (range 0-1531). Consistent with our prior study, no correlation was seen between the number of mutations detected in the IGHV and BCL2 or the number of subclones defined by IGHV and BCL2. Neither the number of mutations in nor the number of subclones defined by BCL2 or IGHV were prognostic. Cox regression showed no relationship between PFS and mutation number or subclone count based on either IGHV or BCL2 (hazard ratios are indistinguishable from 1; parameters were treated both as a continuous variables and dichotomized as quartiles). Furthermore, receiver operator curves did not suggest a cut point for any of these measures that would predict disease progression at any time (areas under the curve are indistinguishable from 0.5). Conclusions: Several measures of AICDA-mediated mutation and resulting subclonal heterogeneity were not found to be prognostic in previously untreated, advanced stage FL treated with CHOP-based regimens. In contrast, AICDA-mediated mutations have been reported to accrue during transformation from low grade FL to large cell lymphoma. This paradox calls for a more refined model to explain the role of AICDA in the progression of FL. Support: NIH/NCI grants U10CA180888, U10CA180819, U10CA180821, R21CA198072(WRB); and in part by GlaxoSmithKline. Disclosures Rimsza: NanoSting: Patents & Royalties: Named inventor on a patent licensed to NanoSting [Institution]. Leonard:Bayer Corporation: Consultancy; AstraZeneca: Consultancy; Genentech, Inc./F. Hoffmann-La Roche Ltd: Consultancy; Sandoz: Consultancy; ADC Therapeutics: Consultancy; Miltenyi: Consultancy; BeiGene: Consultancy; Sutro Biopharma: Consultancy; Celgene: Consultancy; Nordic Nanovector: Consultancy; Gilead: Consultancy; Karyopharm Therapeutics: Consultancy; Genentech, Inc./F. Hoffmann-La Roche Ltd: Consultancy; Bayer Corporation: Consultancy; Celgene: Consultancy; Epizyme, Inc: Consultancy; Merck: Consultancy; MorphoSys: Consultancy; AstraZeneca: Consultancy; Epizyme, Inc: Consultancy; Akcea Therapeutics: Consultancy; Miltenyi: Consultancy; ADC Therapeutics: Consultancy; BeiGene: Consultancy; Karyopharm Therapeutics: Consultancy; Merck: Consultancy; Sandoz: Consultancy; Gilead: Consultancy; Akcea Therapeutics: Consultancy; MorphoSys: Consultancy; Nordic Nanovector: Consultancy; Sutro Biopharma: Consultancy. Fisher:Celgene: Consultancy; Barclays: Honoraria; AstraZeneca: Consultancy; prIME: Honoraria. Smith:Portola Pharmaceuticals: Research Funding. Friedberg:Bayer: Honoraria, Other: Data & Safety Monitoring Committee; Acerta: Other: Data & Safety Monitoring Committee.

Background Poly(adenosine diphosphate ribose) polymerase (PARP) inhibitors exhibit promising activity against ovarian cancers, but their efficacy can be limited by acquired drug resistance. This study explores the role of autophagy in regulating the sensitivity of ovarian cancer cells to PARP inhibitors. Methods Induction of autophagy was detected by punctate LC3 fluorescence staining, LC3I to LC3II conversion on Western blot analysis, and electron microscopy. Enhanced growth inhibition and apoptosis were observed when PARP inhibitors were used with hydroxychloroquine, chloroquine (CQ), or LYS05 to block the hydrolysis of proteins and lipids in autophagosomes or with small interfering RNA against ATG5 or ATG7 to prevent the formation of autophagosomes. The preclinical efficacy of the combination of CQ and olaparib was evaluated with a patient‐derived xenograft (PDX) and the OVCAR8 human ovarian cancer cell line. Results Four PARP inhibitors (olaparib, niraparib, rucaparib, and talazoparib) induced autophagy in a panel of ovarian cancer cells. Inhibition of autophagy with CQ enhanced the sensitivity of ovarian cancer cells to PARP inhibitors. In vivo, olaparib and CQ produced additive growth inhibition in OVCAR8 xenografts and a PDX. Olaparib inhibited PARP activity, and this led to increased reactive oxygen species (ROS) and an accumulation of γ‐H2AX. Inhibition of autophagy also increased ROS and γ‐H2AX and enhanced the effect of olaparib on both entities. Treatment with olaparib increased phosphorylation of ATM and PTEN while decreasing the phosphorylation of AKT and mTOR and inducing autophagy. Conclusions PARP inhibitor–induced autophagy provides an adaptive mechanism of resistance to PARP inhibitors in cancer cells with wild‐type BRCA, and a combination of PARP inhibitors with CQ or other autophagy inhibitors could improve outcomes for patients with ovarian cancer.

Abstract Background: TP53 is reported to be mutated in about 20% of transformed and/or relapsed follicular lymphoma (FL) specimens but in fewer than 5% of pre-treatment FL specimens. The disparity in the reported prevalence of TP53 mutations at diagnosis versus transformation/relapse suggests that TP53-mutated subpopulations, which are too small for detection, might be present at the time of diagnosis in tumors destined to relapse. To test this hypothesis, we developed a more sensitive assay for TP53 mutations and applied the assay to SWOG S0016, a phase III randomized intergroup trial of CHOP chemotherapy plus rituximab (R-CHOP) compared with CHOP chemotherapy plus 131-iodine tositumomab (radio-immunotherapy (RIT)-CHOP) for previously untreated follicular non-Hodgkin lymphoma. Methods: Formalin Fixed Paraffin Embedded (FFPE) biopsies were available for 213/571 enrollees. 147 samples fulfilled inclusion criteria: 1) sufficient DNA for 3 PCR-reactions each at 250 ng gDNA (~40,000 diploid equivalents; 5'UTR and all exons of TP53 were amplified in 3 multiplexed reactions to obtain consistently sized amplicons); 2) appropriate size distribution of DNA products after library preparation (~300 bp). To maintain specificity with high read-coverage, each PCR included an amplicon of eUCR41 (eukaryotic ultra-conserved region) to provide specimen-specific metrics of DNA-damage and sequencing noise. Sequence reads (Ilumina 300 nt paired end) were merged, mapped to reference (>10,000x coverage), and single nucleotide variants (SNV) were called using LoFreq based on case and base-change error thresholds (from the eUCR41 data). Pathogenicity of each TP53 sequence variant was based on a consensus of the Seshat and IARC databases. Variant Allele Frequencies (VAF) of passenger SNVs (5'UTR of BCl2) allowed normalization to the minimum tumor fraction. Results: A total of 73 SNVs within TP53 were detected in 147 FFPE specimens (80 RIT-CHOP; 67 R-CHOP); the SNVs in 34 patients (23%; 17-30% 95% CI) created missense or nonsense mutations which met criteria for pathogenicity. The VAF for these SNVs ranged from 0.008 to 0.7 (median 0.02) and 82% had a VAF of less than 0.1. Among the 34 patients with a pathogenic TP53 mutation, no relationship was observed between the VAF and time to progression. The group of 34 patients with a pathogenic TP53 mutation had a shortened Progression Free Survival (PFS) (HR=1.68; p=0.035) and Overall Survival (OS) (HR=2.02; p=0.045). However, the relationship between TP53 mutations and PFS depended on treatment: for the R-CHOP arm, the pathogenic TP53 mutations were not associated with PFS (10-year PFS 44% vs 42%); in contrast, for the RIT-CHOP arm, pathogenic TP53 mutations were associated with a shortened PFS (10 year PFS 27% vs 65%; HR=3.2; p=0.001) (see figure). Furthermore, among patients with no detectable pathogenic TP53 mutation, RIT-CHOP was associated with a longer PFS than R-CHOP (10 year PFS 65% vs 44%; HR=0.58; p=0.046). Conclusions: Using specimens from SWOG S0016, we find that pathogenic TP53 mutations are present in 23% of untreated, advanced stage FL specimens, a considerably higher fraction than reports based on less sensitive methods. Long term follow-up of S0016 has shown that the RIT-CHOP arm had a small but statistically significant improvement in the 10-year PFS compared to R-CHOP (Shadman 2018). We now show that the interaction of RIT and TP53-status substantially affects PFS. First, in the cohort without a detectable TP53 mutation, a longer PFS was observed with RIT-CHOP than R-CHOP. Second, in the cohort with a detectable TP53 mutation, a shorter PFS was observed with RIT-CHOP than R-CHOP. This observation is consistent with the well-defined role of TP53 as the mediator of the apoptotic response to ionizing radiation. Therefore, an assay which detects subclonal populations with TP53 mutations defines two cohorts for which the inclusion of radio-immunotherapy had substantial and opposite effects on the risk of progression. Finally, this identification of subclonal TP53 mutations as a predictive biomarker highlights that preserving specimens from clinical trials to mine with new technologies can be key to the advancement of precision medicine. We wish to recognize the late Dr. Oliver Press for his central role in S0016 and his many contributions to SWOG. Support: NIH/NCI grants U10CA180888, U10CA180819, U10CA180821, R21CA198072 (RB); and in part by GlaxoSmithKline. Figure. Figure. Disclosures Rimsza: NanoString: Other: Inventor on the patent for the Lymph2Cx assay. Leonard:Juno: Consultancy; ADC Therapeutics: Consultancy; Pfizer: Consultancy; Gilead: Consultancy; Sutro: Consultancy; Biotest: Consultancy; United Therapeutics: Consultancy; BMS: Consultancy; Bayer: Consultancy; Genentech/Roche: Consultancy; Celgene: Consultancy; MEI Pharma: Consultancy; AstraZeneca: Consultancy; Karyopharm: Consultancy; Novartis: Consultancy. Smith:BMS: Consultancy; Portola: Honoraria. Friedberg:Bayer: Honoraria.

Abstract Background: Patients with chronic myelomonocytic leukemia (CMML) have historically had poor outcomes, with overall response rates (ORR) of ~16% for hypomethylating agents (HMA) in first-line registration studies with a median overall survival (OS) of ~4-7 months in the relapsed/refractory (R/R) setting. Allogeneic stem cell transplant is not an option for the majority, due to older age at diagnosis and comorbidities. Tagraxofusp (Elzonris™, SL-401) is a novel targeted therapy directed to the interleukin-3 receptor-α (CD123), a target expressed on a variety of malignancies. In CMML, CD123 is expressed on malignant progenitor cells as well as microenvironmental plasmacytoid dendritic cell (pDC) infiltrates, now shown to be part of the malignant clone (Solary, EHA 2018). We thus hypothesized that therapeutic targeting of CD123-expressing malignant cells and infiltrating clonal pDCs may offer a novel therapeutic approach. Tagraxofusp has already demonstrated high levels of clinical activity against blastic plasmacytoid dendritic cell neoplasm (BPDCN), a CD123+ malignancy derived from pDCs. Methods: This multicenter, two-stage Phase 1/2 trial is enrolling patients with relapsed/refractory (r/r) CMML or other myeloproliferative neoplasms (MPNs). Primary objectives include assessment of safety, determining optimal dose/regimen, and evaluating efficacy outcomes in patients with r/r CMML. In the Stage 1 dose escalation cohort (completed), tagraxofusp was administered as a daily IV infusion at 7, 9, and 12 mcg/kg/day, on days 1-3 every 21 days (cycle 1-4), every 28 days (cycles 5-7), and every 42 days (cycles 8+). In Stage 2 (ongoing), patients received the optimal dose determined in Stage 1 (12 mcg/kg; no MTD reached). Results: As of July 2018, 18 patients with CMML (CMML-1 [n=10]; CMML-2 [n=8]) received tagraxofusp. 13 patients were treated in second-line setting and 5 patients were treated in third-line and beyond, with HMAs being the most commonly administered prior therapy. Median age was 70 years (range 42-80); 78% patients were male. 53% (9/17) of patients had baseline splenomegaly (range: 2 to 22 cm palpable below left costal margin (BCM) by physical exam). Most common treatment-related adverse events (TRAEs) were hypoalbuminemia and nausea (each 38%), vomiting (31%), fatigue, edema, and thrombocytopenia (each 25%). Most common ≥grade 3 TRAEs were thrombocytopenia (13%) and nausea (6%). Capillary leak syndrome was reported in 3 patients (19%; all grade 2). 100% (8/8) of patients with baseline splenomegaly had a spleen response, including 75% (6/8) who had reduction in splenomegaly of 50% or more. 60% (3/5) of patients with baseline spleen size ≥5cm had reduction in splenomegaly of 50% or more. Two patients treated with tagraxofusp achieved bone marrow CRs. 43% (6/14) of evaluable patients had a treatment duration of 6 months or more, including one at 8+ and one at 14+ months. Conclusions: Tagraxofusp monotherapy resulted in significant reductions in spleen sizes along with bone marrow morphological responses in relapsed/refractory patients with CMML, with a manageable safety profile. Given CD123 expression on both neoplastic myeloid cells and pDCs infiltrates, tagraxofusp may offer a novel targeted approach for patients with CMML, an area of unmet medical need. Enrollment continues, and updated safety and efficacy data will be presented. A registrational trial in this patient population is planned. Clinical trial information: NCT02268253. Disclosures Ali: Incyte Corporation: Membership on an entity's Board of Directors or advisory committees. Gupta:Novartis: Consultancy, Honoraria, Research Funding; Incyte: Research Funding. Schiller:Celator/Jazz Pharmaceuticals: Research Funding; Pharmacyclics: Research Funding. Lee:AstraZeneca: Consultancy; Clinipace: Consultancy; Karyopharm Therapeutics Inc: Consultancy; LAM Therapeutics: Research Funding; Amgen: Consultancy. Yacoub:Cara Therapeutics: Equity Ownership; Ardelyx, INC.: Equity Ownership; Dynavax: Equity Ownership; Inycte: Honoraria, Speakers Bureau; Seattle Genetics: Honoraria, Speakers Bureau; Novartis: Honoraria, Speakers Bureau. Sardone:Stemline Therapeutics: Employment, Equity Ownership. Wysowskyj:Stemline Therapeutics: Employment, Equity Ownership. Shemesh:Stemline Therapeutics: Employment, Equity Ownership. Chen:Stemline Therapeutics: Employment, Equity Ownership. Olguin:Stemline Therapeutics: Employment, Equity Ownership. Brooks:Stemline Therapeutics: Employment, Equity Ownership. Dunn:Stemline Therapeutics: Employment, Equity Ownership. Verstovsek:Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Incyte: Consultancy; Italfarmaco: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees. Khoury:Stemline Therapeutics: Research Funding. Pemmaraju:celgene: Consultancy, Honoraria; novartis: Research Funding; Affymetrix: Research Funding; samus: Research Funding; cellectis: Research Funding; daiichi sankyo: Research Funding; stemline: Consultancy, Honoraria, Research Funding; plexxikon: Research Funding; abbvie: Research Funding; SagerStrong Foundation: Research Funding.

Abstract Background: Patients with myelofibrosis (MF) that have failed or are intolerant to JAK inhibitors (JAKi) have no standard treatment options. Tagraxofusp (Elzonris™; SL-401) is a novel targeted therapy directed to the interleukin-3 receptor-α (CD123), a target expressed on a variety of malignancies, including certain myeloproliferative neoplasms (MPNs) such as MF. CD123 is also expressed on plasmacytoid dendritic cells (pDCs), which reside in the tumor microenvironment and may play a role in promoting growth of MPNs and other malignancies. Therefore, targeting of CD123-expressing malignant cells and/or neighboring pDCs may offer a novel therapeutic approach. Tagraxofusp has demonstrated high levels of clinical activity against blastic plasmacytoid dendritic cell neoplasm (BPDCN), a CD123+ malignancy derived from pDCs. Methods: This multicenter, two-stage Phase 1/2 trial is enrolling patients with MF that were relapsed, refractory, or unable to tolerate JAKi. Primary objectives include assessment of safety, determining optimal dose/regimen, and evaluating efficacy outcomes. In the Stage 1 dose escalation cohort (completed), tagraxofusp was dosed as a daily IV infusion at 7, 9, and 12 mcg/kg/day, on days 1-3 every 21 days (cycle 1-4), every 28 days (cycles 5-7), and every 42 days (cycles 8+). In Stage 2 (ongoing), patients received the optimal dose determined in Stage 1 (12 mcg/kg/day). Results: As of July 2018, 18 patients with MF received tagraxofusp. Five patients were treated in the second line setting and 12 patients were treated in third-line setting, and beyond. Median age was 69 years (range 55-81); 60% patients were female. Of the 18 patients with DIPSS Plus risk group assessment available, 3 patients (17%) were intermediate-1, 10 patients (55%) were intermediate-2, and 5 patients (28%) were high-risk. Median platelet count was 66 K/uL (range 15-579). 70% (14/20) of patients had baseline platelets <100K/uL, of which 6 patients had platelets <50K/uL at study entry. 89% (17/19) of patients had baseline splenomegaly defined as palpable spleen 5 cm or more below costal margin (BCM) by physical exam. Most common treatment-related adverse events (TRAEs) include hypoalbuminemia and thrombocytopenia (each 27%), and alanine aminotransferase increased, anemia, dizziness, fatigue, headache and nausea (each 20%). Most common ≥grade 3 TRAEs include anemia (20%) and thrombocytopenia and fatigue (each 7%). Capillary leak syndrome was reported in 1 patient (5%; grade 3). 50% (6/12) of evaluable patients, with baseline spleen size 5 cm or more BCM, had a spleen response, of which 33% (4/12) had splenomegaly reduction by 33% or more and 25% (3/12) had splenomegaly reduction by 35% or more. 50% (3/6) of patients with spleen response had treatment duration 8+ months, including 8+, 12+ and 14+ months (all 3 ongoing). Four patients with baseline thrombocytopenia had treatment durations 8+ months, 3 of which are ongoing. Conclusions: Tagraxofusp monotherapy demonstrated improvements in splenomegaly, with a manageable safety profile, in patients with relapsed/refractory MF, an area of unmet medical need. Given CD123 expression on myeloid neoplastic cells and pDCs in the tumor microenvironment, tagraxofusp may offer a novel targeted approach for MF patients. Enrollment continues, and updated safety and efficacy data will be presented. Registrational designs are being evaluated. Clinical trial information: NCT02268253. Disclosures Pemmaraju: celgene: Consultancy, Honoraria; abbvie: Research Funding; cellectis: Research Funding; stemline: Consultancy, Honoraria, Research Funding; samus: Research Funding; novartis: Research Funding; Affymetrix: Research Funding; SagerStrong Foundation: Research Funding; plexxikon: Research Funding; daiichi sankyo: Research Funding. Gupta:Incyte: Research Funding; Novartis: Consultancy, Honoraria, Research Funding. Schiller:Pharmacyclics: Research Funding; Celator/Jazz Pharmaceuticals: Research Funding. Lee:LAM Therapeutics: Research Funding; AstraZeneca: Consultancy; Clinipace: Consultancy; Karyopharm Therapeutics Inc: Consultancy; Amgen: Consultancy. Yacoub:Ardelyx, INC.: Equity Ownership; Cara Therapeutics: Equity Ownership; Dynavax: Equity Ownership; Inycte: Honoraria, Speakers Bureau; Seattle Genetics: Honoraria, Speakers Bureau; Novartis: Honoraria, Speakers Bureau. Ali:Incyte Corporation: Membership on an entity's Board of Directors or advisory committees. Sardone:Stemline Therapeutics: Employment, Equity Ownership. Wysowskyj:Stemline Therapeutics: Employment, Equity Ownership. Shemesh:Stemline Therapeutics: Employment, Equity Ownership. Chen:Stemline Therapeutics: Employment, Equity Ownership. Olguin:Stemline Therapeutics: Employment, Equity Ownership. Brooks:Stemline Therapeutics: Employment, Equity Ownership. Dunn:Stemline Therapeutics: Employment, Equity Ownership. Verstovsek:Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau; Italfarmaco: Membership on an entity's Board of Directors or advisory committees; Incyte: Consultancy; Celgene: Membership on an entity's Board of Directors or advisory committees. Khoury:Stemline Therapeutics: Research Funding.