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Heterogeneity is the disease-defining epithet of myelodysplastic syndromes (MDS), a clonal disorder of hematopoietic stem and progenitor cells. During the last decade, significant progress has been made to better understand the diversity of clinical, molecular, cellular, and immunological factors that are bound to the prognosis and outcomes of patients with MDS. Despite the rapid generation of all of this biological information, how to implement it has fallen short. Redefining clinical tools to use this new information remains a challenge. The holistic integration of novel, high-impact individual risk parameters such as patient-reported outcomes or mutational and immunological data into conventional risk stratification systems may further refine patient subgroups, improve predictive power for survival, and provide a next-generation classification and prognosis system for patients with MDS. Dichotomic treatment strategies in patients with MDS according to their patient and disease profiles highlight the importance of precise risk stratification, which may be complemented by the definition of granular cohorts of patients with myeloid neoplasms and a druggable target (ie, IDH1/2 mutations) across conventional blast thresholds.

Background: Clonal hematopoiesis (CH) describes the presence of genetic alterations and expansion of clonal cell populations in the peripheral blood (PB) of individuals without clinical manifestation of a hematologic malignancy. CH is a common, age-related state. However, individuals carrying CH are at greater risk for hematologic cancer. It has been shown that presence of TP53- and other high-risk mutations, variant allele frequency (VAF) >10% or multiple mutations in CH carriers may confer a higher risk of preleukemic development and transformation to myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML). Nevertheless, further insights into the role of CH in clonal evolution towards leukemia and elaborating features that are predictive of leukemic progression are needed. Aims: We have recently shown that CH is common in individuals undergoing hip replacement surgery (Hecker, Hartmann et al., Blood 2021). Here, we compared mutational spectrum in these CH individuals to MDS and secondary AML (sAML) cohorts. Additionally, we analyzed spatial and lineage distribution of CH and longitudinally monitored CH and MDS clones over time. Methods: Samples from individuals without known hematologic disease undergoing hip replacement surgery (n=288 samples from 261 individuals), patients with MDS (n=92) or sAML (n=123) were screened for variants in 68 leukemia-associated genes using a targeted sequencing approach (VAF cut off, 1%). Follow-up (FU) PB samples were available for 21 individuals with CH and 16 untreated low-risk MDS patients, 6-24 months after screening. n=5 CH bone marrow (BM) samples carrying six ASXL1-mutations were sorted for seven different cell fractions. Results: At screening, variants were detected in 127/261 (49%) healthy individuals, 84/92 (91%) MDS and 117/123 (95%) sAML patients, with median VAFs of 2.7% (ranging from 1-44%), 18.8% (1.1-87%) and 37.1% (1-99%), respectively. Individuals with CH had a median number of 1 variant per individual, whereas median detected variants per patient increased with clonal evolution with 3 variants in the MDS and 4 variants per patient in the sAML cohort. CH, MDS and sAML showed entity-specific mutation profiles (Figure 1A). Most variants in CH affected epigenetic modifiers, while mutations in splicing factors, signaling pathways and transcription factors increased with clonal progression. During FU, untreated low-risk MDS patients more frequently gained additional mutations compared to CH individuals (7/16 vs 2/21, respectively; p=0.024). However, we did not observe significant changes in clone sizes over time. In 17 hip replacement individuals both femoral heads were removed simultaneously, allowing paired analysis of two different hematopoietic sites. CH prevalence in this subgroup was 70.6% (12/17). Ten individuals showed identical mutation patterns in BM obtained from the right and left femoral head, with little differences in VAFs (Table 1). In contrast, two other individuals showed significant differences in variant detection comparing one to the other side: ASXL1-mutations were only present in one hip sample, whereas all the other variants were detectable in both sides (p To further characterize ASXL1 mosaicism across hematopoietic lineages and differentiation stages, we sorted and sequenced n=5 CH BM samples carrying six ASXL1-mutations for seven different hematopoietic cell fractions. In all cases ASXL1-mutations were identified in CD34 +CD38 -CD90 + hematopoietic stem cells (HSC). VAFs of ASXL1-mutations were differentially distributed (p=0.0049, Kruskal Wallis test): detected VAFs were significantly higher in HSC and common myeloid progenitors (CMP) compared to VAFs in T-cells (p=0.045 and p=0.011, respectively; Dunn´s multiple comparison test; Figure 1B). Conclusions: CH, MDS and sAML show characteristic mutation profiles that remained stable over a 6-24-month period. Gains of additional variants and clonal expansion associated with disease progression. Cellular distribution analysis of ASXL1 CH variants revealed characteristic repartition patterns within the hematopoietic differentiation tree. Additionally, differences in variant detection between cellular BM compartments indicates spatial heterogeneity of CH clones warranting further investigation. J.S.H. and L.H. contributed equally. Figure 1 Figure 1. Disclosures Platzbecker: Geron: Honoraria; Takeda: Honoraria; AbbVie: Honoraria; Celgene/BMS: Honoraria; Janssen: Honoraria; Novartis: Honoraria. Goetze: BMS/Celgene: Other: Advisory Board, Research Funding; Abbvie: Other: Advisory Board.

AK+NG eq. cont. Introduction Management of multiple myeloma (MM) improved dramatically in the last decade. However, prognosis is still poor for high-risk MM patients experiencing early relapse (ER) within 12 months after primary diagnosis. While some patients achieve long lasting remission with first-line triple combinations including immunomodulatory drugs (IMiDs) and proteasome inhibitors (PI) followed by high dose therapy plus autologous stem cell transplantation (ASCT), ER has been recognized as an independent risk factor of resistance to rescue treatments and shorter overall survival (OS). Individual treatment response duration is influenced by several disease-specific and patient-related features. Despite the availability of several MM prognostic scores, the prediction and prevention of ER in MM is still challenging. Our goal was to construct a holistic MM ER prediction model based on machine learning (ML) algorithms. Methods In this study, we applied ML algorithms to sociodemographic, clinical, and cytogenetic data associated with MM ER collected by the Multiple Myeloma Research Foundation (MMRF) CoMMpass consortium. Our model included the following covariates at baseline: gender, age, BMI, ECOG, creatinine, hemoglobin, platelets, neutrophils, calcium, beta-2-microglobuline, albumin, first-line therapy classification (doublet versus triplet, IMiD- versus PI-based versus combination), ASCT within the first year of primary diagnosis, high-risk cytogenetics as well as best response during first year of therapy. Our core patient characteristics in relation to ER are summarized in Table 1. Only complete case documentations were included in the data set (N = 563). To reduce dimensionality and overfitting, continuous data was converted to ordinal data bins according to standard clinical reference ranges (e.g. American Board of Internal Medicine (ABIM) reference ranges for laboratory tests). To ensure that only relevant patient characteristics are used for ER prediction, the Boruta algorithm (version 7.0.0) was used for feature selection using R 4.0.2. Feature independence was assessed by the calculation of correlation between the selected features using the Spearman correlation coefficient for nominal features and the point-biserial correlation coefficient for the comparison of nominal and dichotomous features. Because only few patients experienced ER, the Synthetic Minority Oversampling Technique (SMOTE) was used to correct for imbalance in the target variable. For classification, 14 different ML algorithms were evaluated with the PyCaret 2.0.0 package for Python 3.8.6, using 70% (N=394) of the data as the training set and 30% (N=169) for testing. Hyperparameter tuning and final evaluation was performed for the model with the highest accuracy. Results The feature selection algorithm identified four features as relevant to predict ER in newly diagnosed MM patients enrolled in the MMRF CoMMpass study (Figure 1). Interestingly, factors that usually guide first-line treatment decision like ECOG and kidney function as well as type of induction therapy had no significant impact on risk for ER. The four most important features selected were ´first year best response´, followed by ´ASCT during first year of diagnosis´, ´age´ and ´beta-2-microglobuline´. To rule out bias that might have been introduced by e.g. selecting fitter patients for more intensive regimen, feature independency was assessed as described above. No correlation was identified between the selected features except a weak correlation between age and ASCT with a point-biserial correlation coefficient of -0.35 (Figure 2). The best performing model to predict ER considering the reported relevant features was Gradient Boosting Classification. The final model resulted in an accuracy of 73%, whereas 82% and 69% of the patients in the test data were correctly classified regarding ER and non-ER, respectively. The corresponding AUROC curve is displayed in Figure 3. Conclusions Based on ML algorithms, we identified four features associated with ER in patients with newly diagnosed MM. Our results contribute to the early identification of patients with high risk of ER and thus will guide treatment decision in the future. Moreover, our presented features underline the importance of depth of response to first-line treatment and the role of ASCT to prevent ER even in the era of novel agent-based induction therapies. Figure 1 Figure 1. Disclosures Hochhaus: Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Bristol-Myers Squibb: Research Funding; Pfizer: Research Funding; Incyte: Research Funding. Platzbecker: Janssen: Honoraria; Celgene/BMS: Honoraria; Novartis: Honoraria; AbbVie: Honoraria; Takeda: Honoraria; Geron: Honoraria. Merz: Sanofi: Honoraria; Takeda: Honoraria; BMS: Honoraria; Celgene: Honoraria; Amgen: Honoraria; Hexal: Honoraria; GSK: Honoraria; Janssen: Honoraria; onkowissen.de: Honoraria.

*AK, PP shared first-, UP, HM, LA shared senior authors Introduction: Hypomethylating agents (HMAs) are the standard of care for patients with higher-risk myelodysplastic syndrome (HR-MDS) or acute myeloid leukemia (AML), not eligible for intensive chemotherapy or allogeneic stem cell transplantation. However, the majority of patients do not respond to these agents. Patients failing HMA therapy still have a dismal outcome with very limited treatment options available. Bemcentinib (BEM) is a selective small molecule inhibitor of AXL, a surface membrane protein kinase receptor mediating resistance to chemotherapeutic agents and decreased antitumor immune response. AXL is overexpressed on leukemic cells, especially in the stem cell compartment, and represents a potential novel target in patients with MDS and AML. Methods: The multicenter phase 2 BERGAMO trial evaluated the safety and efficacy of BEM in patients with HR-MDS or AML, refractory to or relapsing after at least six or four cycles of either azacitidine (AZA) or decitabine (DAC), respectively. Patients received an initial loading dose of 400mg BEM orally administered on d1-3 of cycle 1 and a maintenance dose of 200mg BEM on d4-28 of cycle 1 as well as on d1-28 in subsequent 28-day cycles. The primary endpoint was overall response rate (CR, CRi, PR or SD) assessed after 4 treatment cycles. All patients who achieved CR, CRi, PR or SD after 4 cycles of BEM were considered as responders and allowed to continue treatment for a total of up to 9 cycles. Non-responding patients stopped treatment after 4 cycles. Secondary endpoints of the trial included a translational project evaluating the role of biomarkers and response. Results: Between 2018 and 2020, 58 patients in 10 European centers were screened (MDS=27/AML=31) with a median age of 78 years (range, 62-86 years). 45 patients (MDS=21, AML=24) received at least one cycle of BEM and were eligible for safety and efficacy analysis after the first 4 months of treatment. Thirty six (80%) out of 45 patients were considered relapsed after a response to HMA, 8 (18%) had primary resistance and 1 patient (2%) had HMA intolerance. The median number of prior AZA or DAC cycles was 13. In addition, 18 patients (40%) (MDS=9/AML=9) were red blood cell transfusion-dependent. Treatment with BEM was generally well tolerated with only low rates of grade 3-4 hematologic toxicities (n=7). Forty-seven serious adverse events (SAE) occurred in 22 patients (MDS=9/AML=13), mostly due to infectious complications (n=12), hematological toxicity (n=4) or gastrointestinal disturbance (n=6). Until the first 4 months of BEM treatment, the median number of BEM treatment cycles received was 1.5 (range, 0.5-4). During study, 9 patients (20%) died due to infection (MDS=1/AML=3), deterioration of general condition (MDS=1/AML=2) and progressive disease (MDS=0/AML=2). BEM treatment is currently ongoing in four patients (MDS=3/AML=1). The primary endpoint was met in 9 of 45 patients (20.0%). Within the MDS cohort 33.3% achieved a response (n=7/21; CR=1, CRi= 3, SD=3), while 8.3% of patients (n=2/24) with AML achieved stable disease(SD). The 4 MDS patients achieving CR/CRi had a normal karyotype and a median baseline IPSS-R score of 4 (range 3-5). Among the MDS non-responders 4 patients had a complex karyotype, median IPSS-R score was 5 (range 3-5). Median baseline bone marrow blast count was 15% vs. 13% and median pretreatment hemoglobin-, platelet- and WBC levels were 9.2 g/dl (range 7.9-11.8 g/dl), 51 G/L(range 26-120 G/L) and 1.9x109/l (range 1.18-8.9 x109/l) vs. 9.6 g/dl (range 7.9-11.3 g/dl), 35 G/L (range 0-167 G/L) and 1.45x109/l (range 0.82-6.97 x109/l) comparing MDS responders (CR/CRi) vs MDS non-responders, respectively. Preliminary data evaluating soluble AXL (sAXL) in blood plasma at baseline showed that 66.6 % (n=2/3) of MDS patients achieving CR/CRi displayed sAXL levels below a defined threshold, while this was only seen in 20% (n=2/10) of MDS non-responders. Conclusion: These results of the prospective phase II BERGAMO trial confirm, that single agent BEM is generally safe and well tolerated in this patient population with an unmet medical need. Importantly, BEM showed clinical efficacy in a subgroup of MDS patients. These encouraging clinical results are being further explored by an in depth translational research program aiming to identify additional molecular and biological prognostic factors associated with response. Disclosures Kubasch: Celgene: Research Funding; Shire: Research Funding; Novartis: Research Funding. Cluzeau:Celgene: Membership on an entity's Board of Directors or advisory committees; Amgen: Consultancy, Membership on an entity's Board of Directors or advisory committees; Astellas: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Agios: Membership on an entity's Board of Directors or advisory committees; Menarini: Consultancy; Abbvie: Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees; Jazz Pharma: Consultancy, Membership on an entity's Board of Directors or advisory committees. Götze:Celgene: Research Funding. Teipel:janssen: Honoraria. Jentzsch:Novartis: Honoraria; JAZZ Pharmaceuticals: Honoraria. Giagounidis:Novartis: Membership on an entity's Board of Directors or advisory committees; AMGEN: Membership on an entity's Board of Directors or advisory committees. McPherson:BerGenBio ASA: Current Employment. van de Loosdrecht:novartis: Honoraria; celgene: Honoraria. Fenaux:Jazz: Honoraria, Research Funding; BMS: Honoraria, Research Funding; Novartis: Honoraria, Research Funding; Abbvie: Honoraria, Research Funding. Platzbecker:BMS: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; AbbVie: Consultancy, Honoraria; Geron: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; Amgen: Honoraria, Research Funding. Medyouf:Bergenbio: Consultancy, Research Funding. Ades:Celgene/BMS: Research Funding; novartis: Research Funding; takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; jazz: Membership on an entity's Board of Directors or advisory committees, Research Funding; Abbvie: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Research Funding. OffLabel Disclosure: Bemcentinib is currently undergoing Phase II clinical trials for non-small-cell lung cancer (NSCLC), triple-negative breast cancer (TNBC), acute myeloid leukemia /myelodysplastic syndrome (AML/MDS), melanoma and metastatic pancreatic cancer. In 2019, FDA have granted fast track designation to bemcentinib for the treatment of elderly patients with acute myeloid leukemia (AML) whose disease has relapsed.

*UP, LA contributed equally Introduction A significant proportion of lower risk (LR)-MDS patients present with thrombocytopenia, being associated with shortened survival and higher risk of progression to acute myeloid leukemia (AML). Treatment options for patients with LR-MDS and severe thrombocytopenia remain limited apart from transfusion support. Romiplostim (ROM), a thrombopoietin receptor agonist (TPO-RA) has shown safety and efficacy dependent on endogenous TPO levels as well as platelet transfusion history in a poorly defined subset of LR-MDS patients (Giagounidis et al. Cancer 2014, Sekeres et al. BJH 2014). Methods The multicenter phase 2 EUROPE trial investigated potential biomarkers of response (e.g. TPO levels, molecular markers) to single agent ROM in LR-MDS patients with severe thrombocytopenia. Patients were eligible if platelet counts were ≤30 G/L or ≤50 G/L in case of bleeding history. The primary efficacy endpoint was the rate of hematologic improvement of platelets (HI-P, according to IWG 2006 criteria) lasting for at least 8 weeks after 16 weeks of ROM (750µg SC qw) treatment. At the time of screening, patients were assigned into 3 different cohorts based on their previous platelet transfusion events (PTE) as well as centrally assessed TPO serum levels (A: TPO Results From 2015 to 2019, a total of 79 patients were included at 29 trial sites in Germany, France and the Czech Republic. Patients' median age was 74 years (range 42-93), median baseline platelet count was 25.5 G/L (range 3-50 G/L) and they were stratified into cohort A (n=51) or B+C (n=28), respectively. The primary endpoint was met with 34 out of 79 (43%) patients responding (HI-P), with response being markedly higher in cohort A (49%, n=25) vs. cohort B and C (32%, n=9) (p=0.145). Ten (13%) and eight (10%) patients had additional neutrophil (HI-N) and erythroid (HI-E) responses, respectively. During treatment, six patients had transient increases in peripheral blasts to more than 10% and one patient progressed to AML after one month of ROM. Although a higher number of responders was observed in group A, neither TPO level at screening (p=0.21), nor number of pretreatment PTE (p=0.12) were significantly associated with response to ROM treatment. Thus, our findings do not confirm that baseline TPO levels and number of pretreatment PTE alone allow reliable prediction of response to ROM. With the aim to identify new molecular patterns correlating with response, we performed a targeted NGS analysis for somatic variants in 54 candidate genes in 75 patients at baseline and in 44 patients after 16 weeks of ROM. Responders (R) more frequently exhibited mutations like SRSF2 (R=39%, NR=17%), RUNX1 (R=24%, NR=14%) and TET2 (R=30%, NR=29%), whereas non-responders (NR) exhibited mutations like DNMT3A (R=12%, NR=21%), U2AF1(R=9%, NR=14%) or ASXL1 (R=6%, NR 17%) more frequent. The percentages of patients with a response to ROM were similar regardless of total number of baseline somatic mutations. Comparing responders vs. non-responders, we found no significant changes of variant allelic burden of variants detected pre- and post-ROM (Fig. 1). We identified the presence of a SRSF2 mutation as a significant predictor of response to ROM treatment (p=0.031, logistic regression). Mutated SRSF2 was significantly more frequent in responders (39%) compared to non-responders (17%) (p=0.036, Fisher's exact test) (Fig. 2A,B). We used logistic regression with stepwise backward selection to assess the influence of the presence of ASXL1, DNMT3A, RUNX1, TET2 and SRSF2 mutations on response. Our final regression model excludes the non-significant ASXL1, DNMT3A, RUNX1 and TET2 mutations and includes the significant SRSF2 mutation, resulting in an overall accuracy of 64.0% for a correct ROM response prediction in this patient cohort. Conclusion: This prospective study did not confirm a significant association between response to ROM, pretreatment PTE burden and endogenous TPO levels. Instead, patients with a mutated SRSF2 displayed a significantly higher response to ROM treatment. This may allow personalized treatment approaches in patients with LR-MDS and severe thrombocytopenia. In this study, extended treatment with ROM did not lead to a significant increase in AML cases. Disclosures Kubasch: Shire: Research Funding; Celgene: Research Funding; Novartis: Research Funding. Giagounidis:AMGEN: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Götze:Celgene: Research Funding. Cony-Makhoul:Novartis: Consultancy; Pfizer: Consultancy; Incyte Biosciences: Speakers Bureau; BMS: Consultancy; BMS: Speakers Bureau. Laribi:takeda: Research Funding; novartis: Honoraria, Research Funding; amgen: Research Funding; abbvie: Honoraria, Research Funding. Park:Takeda: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Other: Travel expenses. Metzeler:Astellas: Honoraria; Otsuka Pharma: Consultancy; Pfizer: Consultancy; Jazz Pharmaceuticals: Consultancy; Novartis: Consultancy; Celgene: Consultancy, Honoraria, Research Funding; Daiichi Sankyo: Honoraria. Thiede:AgenDix GmbH: Other: Co-owner and CEO. Schlenk:PharmaMar: Research Funding; Daiichi Sankyo: Membership on an entity's Board of Directors or advisory committees, Other: Travel, Accomodations, Expenses, Research Funding, Speakers Bureau; Novartis: Speakers Bureau; Roche: Research Funding; AstraZeneca: Research Funding; Pfizer: Membership on an entity's Board of Directors or advisory committees, Research Funding, Speakers Bureau. Fenaux:Novartis: Honoraria, Research Funding; Abbvie: Honoraria, Research Funding; Jazz: Honoraria, Research Funding; BMS: Honoraria, Research Funding. Platzbecker:Novartis: Consultancy, Honoraria, Research Funding; Amgen: Honoraria, Research Funding; BMS: Consultancy, Honoraria; AbbVie: Consultancy, Honoraria; Geron: Consultancy, Honoraria; Takeda: Consultancy, Honoraria; Janssen: Consultancy, Honoraria, Research Funding. Ades:Abbvie: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Research Funding; takeda: Honoraria, Membership on an entity's Board of Directors or advisory committees; novartis: Research Funding; Celgene/BMS: Research Funding; jazz: Membership on an entity's Board of Directors or advisory committees, Research Funding. OffLabel Disclosure: Romiplostim is a thrombopoietin receptor agonist indicated for the treatment of thrombocytopenia in patients with chronic immune thrombocytopenia (ITP). Limitations of Use: Romiplostim is not indicated for the treatment of thrombocytopenia due to myelodysplastic syndrome (MDS) or any cause of thrombocytopenia other than chronic ITP.

Introduction: Myelodysplastic syndromes (MDS) are characterized by ineffective hematopoiesis and peripheral cytopenia. In about half of patients with lower-risk (LR) MDS, thrombocytopenia is present at the time of diagnosis and associated with shortened survival and an increased risk of progression to acute myeloid leukemia (AML). The thrombopoietin receptor agonist (TPO-RA) romiplostim has shown safety and marked efficacy in a still poorly-defined subset of LR-MDS patients with thrombocytopenia. Methods: The EUROPE multicenter phase 2 trial within the EMSCO network investigated the impact of biomarkers like endogenous thrombopoietin (TPO) level and platelet transfusion events (PTE) on the efficacy of romiplostim (750µg SC qw) treatment in patients with LR-MDS (IPSS low/int-1). Patients were eligible if baseline bone marrow blast count was Results: From 2015 to 2018, a total of 68 patients were included in 20 trial sites in Germany, France and Czech Republic. Patients displayed a median age of 74 years and a median platelet count of 25 G/L (range 1-50 G/L) and were stratified into cohort A (n=47), B (n=17) or C (n=4), respectively. All patients received at least one cycle of romiplostim with a median weekly dose of 750μg and a median of 15 cycles of romiplostim until week 16. Reasons for premature study discontinuation before week 16 were investigator/patient decision (n=8), adverse events (n=5), disease progression (n=4) or death (n=1). There were 9 reported severe treatment-related adverse events in seven patients including pulmonary embolism (n=1), subacute stroke (n=1), mucocutaneous hemorrhage (n=1), asthenia (n=1), suspicion of anti-romiplostim antibodies (n=1), progression to AML (n=1) and varicella zoster infection (n=1). Two patients had transient increases in peripheral blasts to more than 10% and 1 patient progressed to AML after 1 month of treatment. HI-P was observed in 26 of 68 (38%) patients, while response was ongoing in 24 of them beyond week 16. Moreover, rate of HI-P lasting for at least 8 weeks was notably higher in cohort A (45%, n=21/47) compared to patients in cohort B and C (24%, n=5/21) (p=0.11). Median peak increase of PLT count in responding patients was 199 G/L in cohort A and 83 G/L in cohort B (p=0.25) and was observed in median after 7 weeks (range 3-16). In addition, responses occurred also in 2 patients in the neutrophil (HI-N) and in 7 patients in the erythroid (HI-E) lineage according to IWG 2006 criteria (Table 1). Explorative analysis showed a correlation between pretreatment platelet transfusion requirement and endogenous TPO-levels (spearman-test, p=0.034). Median pretreatment endogenous TPO-level was lower in responders compared to non-responders (82 vs. 103 pg/ml, p=0.15). Higher response rates occurred in patients with lower TPO-levels ( Conclusion: This prospective study confirms that romiplostim treatment is highly effective in a subgroup of LR-MDS patients, but neither baseline platelet transfusion requirements nor baseline TPO levels were significantly associated with clinical response to romiplostim. Further translational analyses are ongoing to elucidate potential biomarkers of response. Disclosures Platzbecker: Celgene: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria. Götze:AbbVie: Membership on an entity's Board of Directors or advisory committees. Cony-Makhoul:Pfizer: Consultancy; Novartis: Consultancy; Incyte Biosciences: Honoraria, Speakers Bureau; BMS: Consultancy, Honoraria, Speakers Bureau. Park:Celgene: Membership on an entity's Board of Directors or advisory committees, Research Funding. Thiede:Daiichi Sankyo: Honoraria; Novartis: Consultancy, Honoraria, Research Funding, Speakers Bureau; AgenDix GmbH: Employment, Equity Ownership; Diaceutics: Membership on an entity's Board of Directors or advisory committees. Ades:Helsinn Healthcare: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Takeda: Membership on an entity's Board of Directors or advisory committees; Silence Therapeutics: Membership on an entity's Board of Directors or advisory committees; Agios: Membership on an entity's Board of Directors or advisory committees; Jazz: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Astellas: Membership on an entity's Board of Directors or advisory committees; Amgen: Research Funding. OffLabel Disclosure: Romiplostim is formally not licensed for the treatment of thrombocytopenia due to myelodysplastic syndrome (MDS).

Background: Measurable residual disease (MRD) can identify patients (pts) with myelodysplastic syndrome (MDS) or acute myeloid leukemia (AML) in complete hematological remission (CR) at high risk of relapse even after allogeneic hematopoietic stem cell transplantation (HSCT). We have recently shown in 53 pts treated within the first cohort of the RELAZA2 trial that pre-emptive therapy with azacitidine (AZA) at the time of MRD-positivity (MRDpos) can successfully prevent imminent hematological relapse (Platzbecker et al. Lancet Oncol. 2018). We now report on the results of the second cohort of 41 pts undergoing MRD-guided treatment in the RELAZA2 trial (ClinicalTrials.gov NCT01462578) by the Study Alliance Leukemia (SAL). Methods: Between 2015 and 2018, 166 MDS/AML pts were screened and centrally monitored for MRD in bone marrow or peripheral blood at monthly intervals for a period of 2 years prospectively in 9 centers in Germany. Of these 166, 41 pts with either advanced MDS (n=6) or AML (n=35) in CR after either conventional chemotherapy only (n=13) or consecutive allogeneic HSCT (n=28) developed MRD above a threshold defining imminent hematological relapse. Still being in morphological CR, these pts pre-emptively received 6 cycles of AZA (75mg/m2, s.c. days 1-7), which was followed by a risk-adapted AZA-maintenance therapy based on MRD-response for up to 18 additional months. Pts developing a hematological relapse went off study. MRD was detected by either the quantification of NPM1 mutation level (n=19), leukemia-specific fusion genes DEK-NUP214 (n=1) or RUNX1/RUNX1T1 (n=2) or a sensitive donor chimerism analysis of sorted CD34(+)/CD117(+) peripheral blood cells (n=28) in pts undergoing allogeneic HSCT. Here, we report the analysis of the primary endpoint of the 41 pts in the second cohort as well as the data for the entire 94 pts who entered the treatment phase of the RELAZA-2 study. Results: At a median of 110 days (range 28-476) after start of screening, 41 (25%) out of 166 prospectively screened pts became MRDpos as defined by either a decrease of CD34(+)/CD117(+) donor chimerism to 1% (NPM1 n=18) while being still in hematological CR. All of these MRDpos pts started AZA-based pre-emptive treatment to prevent imminent hematological relapse. Six months after start of MRD-guided therapy, 25 out of 41 pts were still in CR (61%, 95%-CI 45-76%, p Conclusion: These multicenter prospective data provide further strong evidence that continuous MRD monitoring is feasible and can identify MDS/AML pts at high risk of hematological relapse. Pre-emptive MRD-guided therapy with AZA is an effective treatment to prevent or at least substantially delay hematologic relapse in these pts. Disclosures Platzbecker: Novartis: Consultancy, Honoraria; Celgene: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria. Wolf:Celgene: Honoraria, Research Funding; Abbvie: Honoraria. Krämer:Daiichi-Sankyo: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Research Funding; Roche: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Bayer: Research Funding. Bug:Gilead Sciences: Membership on an entity's Board of Directors or advisory committees, Other: Travel grants; Jazz Pharmaceuticals: Honoraria; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: travel grants; Hexal: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Membership on an entity's Board of Directors or advisory committees; Sanofi: Other: travel grants; Celgene Neovii: Other: travel grant. Götze:AbbVie: Membership on an entity's Board of Directors or advisory committees. Stelljes:Novartis: Honoraria; Amgen: Honoraria; Jazz Pharmaceuticals: Honoraria; Pfizer: Consultancy, Honoraria, Research Funding; MDS: Consultancy. Subklewe:AMGEN: Consultancy, Honoraria, Research Funding; Gilead: Consultancy, Honoraria, Research Funding; Miltenyi: Research Funding; Oxford Biotherapeutics: Research Funding; Janssen: Consultancy; Roche: Consultancy, Research Funding; Pfizer: Consultancy, Honoraria; Celgene: Consultancy, Honoraria; Morphosys: Research Funding. Hänel:Celgene: Other: advisory board; Novartis: Honoraria; Takeda: Other: advisory board; Amgen: Honoraria; Roche: Honoraria. Dührsen:Gilead: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Alexion: Honoraria; Takeda: Consultancy, Honoraria; Teva: Honoraria; Celgene: Research Funding; Roche: Honoraria, Research Funding; AbbVie: Consultancy, Honoraria; Janssen: Honoraria; Amgen: Consultancy, Honoraria, Research Funding; CPT: Consultancy, Honoraria. Müller-Tidow:MSD: Membership on an entity's Board of Directors or advisory committees. Thiede:Novartis: Consultancy, Honoraria, Research Funding, Speakers Bureau; AgenDix GmbH: Employment, Equity Ownership.

Background: PPM1D is a serine/threonine phosphatase that inactivates p53 tumor suppressor pathway. Recently, PPM1D mutations have been described in clonal hematopoiesis and are more frequently found in therapy-related MDS than in primary MDS (15% vs. 3%). Del(5q), is the most prevalent cytogenetic abnormality in MDS. A high proportion of MDS del(5q) patients respond to lenalidomide, but almost 40% of them progress to AML. Scharenberg et al. identified recurrent mutations in a limited number of genes i.e. TP53, RUNX1, and TET2 in a longitudinal cohort of 35 MDS del(5q) patients that progressed to AML. The clinical impact and occurrence of PPM1D mutations in MDS del(5q) patients remains unknown. Aim: To determine the clinical impact of PPM1D mutations in MDS del (5q) patients on lenalidomide resistance and AML progression. Methods: We studied a cohort of 243 patients with MDS or AML following MDS and 5q deletion diagnosed according to the 2008 WHO classification. Patients were cytogenetically characterized by chromosome banding analysis and followed for disease progression, treatment and survival. From 22 del(5q) patients treated with lenalidomide, follow-up (FU) material was available before and after treatment. Molecular analysis for mutations in all 6 exons of PPM1D was performed by Sanger and/or a next-generation sequencing panel covering mutations in 46 genes frequently mutated in MDS, including TP53 and CSNK1A1. Results: At the time of diagnosis 14 PPM1D mutations were detected in 13 of 243 (5.3%) MDS patients with del(5q), 12 of which were found in the previously described hotspot region of PPM1D between amino acids 427 and 542. Six patients had nonsense mutations, 3 patients had frameshift mutations (one patient with 2 frameshift mutations), and 4 patients had missense mutations. TP53 mutations were found in 34 of 243 (14%) MDS patients with del(5q). Three TP53 mutated patients, two with complex karyotype, carried an additional PPM1D mutation. Co-occurrence of PPM1D and CSNK1A1 mutations was not observed in any patient. In total, 71 of 243 patients were treated with lenalidomide and had available information about treatment response. Eleven patients (15.5%) did not respond to lenalidomide and 17 patients (24%) progressed to AML. Nine of 71 (12.6%) patients were TP53 (n=5, 7%) or PPM1D mutated (n=4, 5.6%). For 22 of 71 patients who either achieved a complete remission (n=5), developed resistance to lenalidomide followed by MDS progression (n=7) or AML transformation (n=10), FU samples were available before and after lenalidomide treatment. Of the 5 patients with complete remission 4 patients displayed no mutations, while 1 patient was PPM1D- and ASXL1-mutated with a variant allele frequency (VAF) of 27.6% and 12.1%, respectively, prior to lenalidomide treatment. After 76 months on lenalidomide, both mutations had disappeared. Of the 17 patients with lenalidomide resistance/AML progression, 5 patients (29.4%) carried mutations either in PPM1D (n=2) or in TP53 (n=3) prior to lenalidomide treatment, with a mean VAF of 15.3% and 13.5%, respectively. The 2 PPM1D-mutated patients progressed to AML 59.4 and 79.6 months after diagnosis. None of the 3 initially TP53-mutated patients progressed to AML. All 3 TP53-mutated patients co-expressed SF3B1 mutations. At the time of lenalidomide resistance/AML progression, we observed 2 known and 1 novel PPM1D mutation in a patient previously wildtype for PPM1D and TP53, 3 known and 6 novel TP53 mutations in 5 patients previously wildtype for PPM1D and TP53, and 1 novel TP53 mutation in a patient who was previously found mutated in PPM1D. Thus, at the time of lenalidomide resistance or AML progression 10 of 17 patients (58.8%) were mutated for PPM1D (n=3, 18%) and/or TP53 (n=9, 53%; 2 of 9 co-expressed PPM1D mutations). At the time of lenalidomide resistance/AML progression, VAF increased from 10.2% to 23.3% for PPM1D and from 4% to 16.9% for TP53 mutations, indicating expansion of the mutated clone under the selective pressure of lenalidomide. Conclusion: PPM1D mutations are recurrently found in MDS del(5q) patients at a frequency of 5.3% and may be coexpressed with TP53 mutations in 5q- MDS/AML cells. Frequency at resistance/AML progression was 18% for PPM1D and 53% for TP53 mutated patients, respectively. Our findings indicate an association of PPM1D mutations in addition to the previously described TP53 mutations with lenalidomide resistance and AML progression. Disclosures Meggendorfer: MLL Munich Leukemia Laboratory: Employment. Krönke:Celgene: Honoraria. Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Thiede:Novartis: Honoraria, Research Funding; AgenDix: Other: Ownership. Germing:Janssen: Honoraria; Novartis: Honoraria, Research Funding; Celgene: Honoraria, Research Funding. Kobbe:Amgen: Honoraria, Research Funding; Roche: Honoraria, Research Funding; Celgene: Honoraria, Other: Travel Support, Research Funding. Koenecke:Amgen: Consultancy; Roche: Consultancy; abbvie: Consultancy; BMS: Consultancy. Sperr:Novartis: Honoraria; Pfizer: Honoraria; Daiichi Sankyo: Honoraria. Valent:Novartis: Honoraria; Pfizer: Honoraria; Incyte: Honoraria. Ganser:Novartis: Membership on an entity's Board of Directors or advisory committees. Haferlach:MLL Munich Leukemia Laboratory: Employment, Equity Ownership. Platzbecker:Celgene: Research Funding. Heuser:BergenBio: Research Funding; StemLine Therapeutics: Consultancy; Astellas: Research Funding; Novartis: Consultancy, Honoraria, Research Funding; Bayer Pharma AG: Consultancy, Research Funding; Pfizer: Consultancy, Honoraria, Research Funding; Janssen: Consultancy; Karyopharm: Research Funding; Daiichi Sankyo: Research Funding; Sunesis: Research Funding; Tetralogic: Research Funding.

Introduction Recently, progress has been made in the treatment of patients with higher risk myelodysplastic syndromes (HR MDS) and acute myeloid leukemia (AML). Nevertheless, patients failing hypomethylating agents (HMA) have a dismal prognosis and very limited treatment options. Targeting CD123 on leukemic stem cells (LSC) is one promising approach in MDS and AML. Talacotuzumab (TAL, JNJ-56022473) is an IgG1 monoclonal antibody targeting CD123 preferentially via antibody-dependent cellular cytotoxicity (ADCC) mediated by natural killer cells (NKs). Aim The SAMBA trial, a phase II study of the German and French MDS study groups within the EMSCO network assessed the overall hematological response rate after 3 months of single agent TAL treatment in AML or HR MDS patients failing hypomethylating agents (HMAs). Methods TAL was given IV at a dose of 9 mg/kg once every two weeks for a total of 6 infusions, responders received up to 20 additional infusions. After the first 3 months, overall hematological response rate (either CR, PR, marrow-CR, HI, SD) was evaluated by bone marrow biopsy. The study was accompanied by an immune monitoring via flow cytometric analysis to investigate the distribution of T- and NK cells in peripheral blood (PB) and bone marrow (BM) at the time of screening and during therapy in comparison with healthy, age-matched controls. Results 24 patients (19 AML and 5 HR MDS) with a median age of 77 (range 71-90) years, who either failed to achieve complete- (CR), partial response (PR), hematological improvement (HI) or relapsed after HMA therapy were included in the study. After TAL administration, 14 patients could be assessed for response after 4 infusions and 10 patients after 6 infusions. The overall response rate (ORR) was 20.8% including 1 complete remission (CRi), 1 patient with hematologic improvement (HI-E) and additionally 3 patients with disease stabilization. The median duration of response in these patients was 3 months (range 3-14 months). Two patients are still on treatment, one patient despite losing objective response (14 months) and one patient with disease stabilization (13 months). The median overall survival for the entire cohort of patients was 3.2 months (range 0.4-11.2 months). In 10 patients (41.6%), therapy with TAL resulted in grade 3/4 infusion related side effects (pneumonia, n=1; infusion-related reaction, n=8; septic shock, n=1). Before treatment initiation, patients had lower levels of CD56dim NK-cells in PB (82% vs. 89% of NK-cells; p=0.069) expressing significantly more inhibiting NK-cell receptors like KIR2DL2 (8.8% vs. 3.2% of NK-cells; p We could not detect any difference in NK-cell levels in responding patients compared to non-responders. Interestingly, pre-treatment expression (MFI and percentage) of CD123 on immature myeloid derived suppressor cells (iMDSC) was higher in responders than in non-responders (p Conclusion Single agent TAL has limited efficacy in patients with advanced myeloid malignancies failing HMA. Expression of CD123 on immature MDSCs might serve as a biomarker of response for future anti-CD123 targeted approaches. Disclosures Götze: Celgene: Honoraria, Research Funding; JAZZ Pharmaceuticals: Honoraria; Novartis: Honoraria; Takeda: Honoraria, Other: Travel aid ASH 2017. Krönke:Celgene: Honoraria. Middeke:Roche: Membership on an entity's Board of Directors or advisory committees; Abbvie: Membership on an entity's Board of Directors or advisory committees; Janssen: Membership on an entity's Board of Directors or advisory committees, Research Funding. Fenaux:Celgene: Honoraria, Research Funding; Janssen: Honoraria, Research Funding; Jazz: Honoraria, Research Funding; Otsuka: Honoraria, Research Funding. Schlenk:Pfizer: Research Funding, Speakers Bureau. Ades:JAZZ: Honoraria; Takeda: Membership on an entity's Board of Directors or advisory committees; silent pharma: Consultancy; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding. Platzbecker:Celgene: Research Funding.