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Background: Patients ≥55 years of age with high-risk acute myeloid leukemia (AML) may benefit from an allogeneic hematopoietic cell transplantation (HCT), which is often the only potentially curative therapy available. However, complete remission (CR) is usually a pre-requisite for most centers to perform HCT, as CR predicts an optimal outcome. Many older patients with relapsed/refractory (R/R) AML are not transplanted as they do not achieve the required CR. In addition, due to their advanced age, they are unable to tolerate the myeloablative conditioning required for eradication of disease, while the more tolerable reduced intensity conditioning often exhibits high rates of relapse. The SIERRA trial is a prospective, randomized, phase 3 trial for older patients with R/R AML to address this unmet need. We hypothesized that a targeted delivery of radiation to the marrow, achieved with a limit of 24 Gray (Gy) radiation dose to the liver, via an infusion of a therapeutic dose of Iomab-B enables successful engraftment despite active disease in the marrow. Methods: Eligible patients were ≥55 years with active R/R AML (≥5% blasts), adequate organ function, and related/unrelated 8/8 HLA-matched donors. Patients were randomized (1:1) to the Iomab-B or Conventional Care (CC) arm. Patients randomized to Iomab-B received an outpatient dosimetric dose of Iomab-B followed by nuclear medicine imaging to determine the personalized therapeutic dose that would deliver radiation to the marrow while limiting the delivery of radiation to the liver to 24 Gy. HCT is performed 12-14 days following the infusion of a therapeutic dose of Iomab-B, and a non-myeloablative conditioning backbone of fludarabine (30 mg/m2 x 3) and low-dose Total Body Irradiation (TBI 2 Gy x 1 dose). Patients on the CC arm received investigator's choice of salvage therapy, including newly approved targeted agents, and could proceed to physician's choice of conditioning and HCT if they achieved CR. If CC patients did not achieve CR, the study allowed them to cross-over and receive Iomab-B-based conditioning followed by allogeneic HCT. Results: Preliminary data were available from 106 patients (Table 1). 53 patients (median age 64) were randomized to Iomab-B and received allogeneic transplant. All Iomab-B patients engrafted, despite a pre-therapy median of 26% marrow blasts. After randomization, 83% (44/53) CC patients failed salvage therapy, including 45% (24/53) who received targeted agents. 26 of 44 (60%) CC patients, with a median 33% marrow blasts, crossed over and received Iomab-B followed by allogeneic HCT. Patient age, donor type (MRD, MUD), bone marrow cellularity, blast percentage, type of donor, stem cell dose, administered Iomab-B activity (mCi), and absorbed radiation dose to the marrow (Gy), were analyzed for time to engraftment among each group. Median time to neutrophil and platelet engraftment were 14 days (range 9-22) and 17 days (range 4-39) respectively, with 91% of evaluable patients achieving full donor chimerism (>95% by day 100, Table 1). Neither the radiation dose delivered to the marrow (median 14.7 Gy; range, 4.6-32 Gy; Table 1) nor the total administered activity (median 632 mCi; range, 354-1027 mCi) showed correlation with the time to either neutrophil (p value=0.525) or platelet engraftment (p value=0.952). Regression analyses, considering all of the variables individually, did not indicate a statistically significant correlation (p > 0.1) between days to engraftment and radiation dose delivered to the marrow. These results were consistent for the Iomab-B cross-over group as well. Furthermore, the marrow absorbed dose did not show a significant correlation with % chimerism at day 28 in patients on the Iomab-B arm or in cross-over patients. Conclusion: Dosimetry to determine a patient-specific therapeutic dose of Iomab-B of 24 Gy to the liver provided absorbed marrow radiation doses that allowed for reliable engraftment after allogeneic HCT in older patients with R/R AML, despite a heavy leukemia burden with median bone marrow blasts of 26% prior to HCT (Iomab-B arm). There was no relationship between total administered activity or radiation dose delivered to the marrow with the speed of engraftment. The SIERRA trial is currently enrolling patients (www.sierratrial.com or clinicaltrials.gov NCT02665065). Disclosures Gyurkocza: Actinium: Research Funding. Nath:Daiichi Sankyo: Consultancy, Honoraria; Actinium: Consultancy, Honoraria; Astellas: Consultancy, Honoraria. Stiff:Kite, a Gilead Company: Research Funding; Gamida Cell: Research Funding; Atara: Research Funding; Unum: Research Funding; Macrogenics: Research Funding; Delta-Fly: Research Funding; Amgen: Research Funding. Hari:Takeda: Consultancy; BMS: Consultancy; GSK: Consultancy; Amgen: Consultancy; Janssen: Consultancy; Incyte Corporation: Consultancy. Al-Kadhimi:Actinium Pharmaceuticals Inc.: Research Funding; Genzyme: Research Funding; Bluebird Bio: Current equity holder in publicly-traded company; Gilead Science: Current equity holder in publicly-traded company; Moderna: Current equity holder in publicly-traded company; Novavax: Current equity holder in publicly-traded company. Abboud:BMS: Current equity holder in publicly-traded company; AbbVie Inc: Current equity holder in publicly-traded company; Forty Seven Inc: Research Funding; Abbott Labs: Current equity holder in publicly-traded company; AlloVir: Research Funding; Ryvu: Research Funding; Actinium Pharmaceuticals Inc.: Research Funding; Johnson & Johnson: Current equity holder in publicly-traded company; Pfizer: Research Funding. Magalhaes-Silverman:Incyte: Research Funding; Kadmon Holdings: Research Funding; Actinium Pharmaceuticals: Research Funding. Foran:BMS: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; H3Biosciences: Research Funding; Agios: Honoraria, Research Funding; Xencor: Research Funding; Trillium: Research Funding; Takeda: Research Funding; Kura Oncology: Research Funding; Aptose: Research Funding; Aprea: Research Funding; Actinium: Research Funding; Boehringer Ingelheim: Research Funding; Abbvie: Research Funding; Revolution Medicine: Consultancy; Servier: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Schuster:Amgen, Abbvie, Gilead, Takeda, Celgene, Pharmacyclics, Astellas, Verastem, Merck, Novartis, Takeda, Genentech,, Seattle Genetics: Other: Personal Fees; Karyopharm: Membership on an entity's Board of Directors or advisory committees. Kebriaei:Amgen: Other: Research Support; Pfizer: Other: Served on advisory board; Ziopharm: Other: Research Support; Jazz: Consultancy; Novartis: Other: Served on advisory board; Kite: Other: Served on advisory board. Levy:Karyopharm: Consultancy, Honoraria, Research Funding; Sanofi: Consultancy, Honoraria, Research Funding; Seattle Genetics: Consultancy, Honoraria, Research Funding; Takeda: Consultancy, Honoraria, Research Funding; Janssen: Consultancy, Honoraria, Research Funding; Bristol Meyers Squibb: Consultancy, Honoraria, Research Funding; AstraZeneca: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding; AbbVie: Consultancy, Honoraria, Research Funding; BeiGene: Consultancy, Research Funding, Speakers Bureau; Baylor University Med Center: Current Employment. Giralt:ACTINUUM: Consultancy, Research Funding; MILTENYI: Consultancy, Research Funding; KITE: Consultancy; TAKEDA: Research Funding; CELGENE: Consultancy, Honoraria, Research Funding; JAZZ: Consultancy, Honoraria; AMGEN: Consultancy, Research Funding; OMEROS: Consultancy, Honoraria; NOVARTIS: Consultancy, Honoraria, Research Funding. Liang:Actinium Pharmaceuticals: Current Employment. Berger:Actinium Pharmaceuticals Inc.: Current Employment, Current equity holder in publicly-traded company. Reddy:Actinium Pharmaceuticals Inc.: Current Employment, Current equity holder in publicly-traded company.

Introduction: Failure to engraft donor cells is a devastating complication after allogeneic hematopoietic cell transplantation (HCT) while full engraftment remains a challenge in some recipients. Therapeutic approaches, such as recombinant hematopoietic growth factors, salvage only a minority of patients leaving affected patients at risk for recurrent infections and multiple blood products transfusions. PLX-R18 cells have been shown to mitigate bone marrow failure in rodent and non-human primate models. Further, these human placenta-derived mesenchymal-like adherent stromal cells secrete many cytokines after intramuscular (IM) injection that have been shown to stimulate growth and migration of bone marrow cells in vitro, e.g. MCP-1, IL-6, and IL-8. The initial results of the first cohort of this phase I trial were previously reported (ASH 2018 abstract 3379). Methods: Enrollment for this multi-center, open label, dose-escalation, phase I trial (ClinicalTrials.gov: NCT03002519) began in October 2017. Eligibility criteria included patients age ≥18 years who underwent either autologous (auto) or allogeneic (allo) HCT but suffered from incomplete hematopoietic recovery at least 3 months post-transplant, defined as failure to maintain either hemoglobin (Hb) >8g/dL and/or neutrophil count (ANC) >1,000/μL and/or platelet (PLT) count >50,000/μL. Other eligibility criteria included no evidence of underlying malignancy at time of enrollment, no active infection and no evidence of grade 3-4 acute or severe chronic GvHD. Subjects were enrolled into 3 dose-escalating cohorts: low-dose at 1M cells/kg (n=3), intermediate-dose at 2M cells/kg (n=6), and high-dose at 4M cells/kg (n=10). The specified dose of PLX-R18 cells was injected IM in multiple locations and given twice, 1 week apart. Cohorts were recruited sequentially, and progression to the next cohort was based on the safety assessment of its predecessor. Adverse event grading used the NCI CTCAE criteria. As of July 2020, all patients in cohorts 1 and 2 (3 & 6 patients, respectively) and 10 patients in cohort 3 were treated. Data were analyzed using the lmerTest package (ver. 3.1.0) in R (ver. 3.6.1) to produce baseline-adjusted least square means. As study is in progress and data collection ongoing, not all subjects have data on all parameters for all time points. Results: Data on 19 subjects (29-75 yrs) with either allo (n=17) or auto (n=2) HCT are reported herein. Not all patients reached the 9 month time point. Four patients died while on study and 2 withdrew consent within the 1 year follow up period. All deaths and other severe adverse events (SAEs) occurred due to progression of primary disease and were assessed as not related by investigators and sponsor. Most frequent related AEs were injection-site reactions. The table summarizes number of patients who improved from below the critical level, as defined above, at day 0 to above the critical level for a sustained period. Only those patients that were below the critical blood levels at day 0 are shown. To date, the majority of such patients improved to above the critical level after treatment. For example, 9 patients had ANC <1000/μL at day 0. Of the 4 patients for which data currently exists, all improved to ≥1000/μL at 9 months. The plots show changes in Hb, ANC and PLT up to 9 months following treatment. The dashed lines represent respective critical levels. Overall, there was improvement in the 3 parameters, with the greatest improvement observed in patients receiving the predicted optimal dose of 4M cells/kg. In this cohort, Hb levels increased from 9.29±0.5 to 13.19±1.2 (p=0.0019), ANC increased from 1.09±0.6 to 5.13±1.7 (p=0.018) and PLT increased from 45.7±10 to 175.9±28 (p=0.000012). Conclusions: Treatment with PLX-R18 is safe, well-tolerated and clearly effective in some patients. The most frequent AEs were reversible injection site reactions, all without sequelae. An overall improvement was observed in most patients, and among the high-dose cohort clinically impressive improvements in Hb, ANC and PLT were observed. The trial continues to follow up on patients. Upon completion, a large phase II trial, using the optimal dose, is considered for the same indication. Other attractive clinical areas for PLX-R18 therapy include acute radiation syndrome, bone marrow failure, aplastic anemia, myelofibrosis and with HCT as a "pan" cytokine to enhance rapid count recovery in all cells lines. Figure Disclosures Lazarus: Pluristem ltd: Consultancy. McGuirk:Astellas: Research Funding; Bellicum Pharmaceutical: Research Funding; Gamida Cell: Research Funding; Pluristem Ltd: Research Funding; Kite Pharmaceuticals: Consultancy, Honoraria, Research Funding, Speakers Bureau; Allo Vir: Consultancy, Honoraria, Research Funding; Juno Therapeutics: Consultancy, Honoraria, Research Funding; Novartis: Research Funding; Fresenius Biotech: Research Funding. Rowley:AbbVie: Current equity holder in publicly-traded company; FATE Therapeutics: Consultancy. Ofir:Pluristem: Current Employment. Shani:Pluristem ltd: Current Employment. Rowe:Pluristem ltd: Consultancy.

Introduction Germline predisposition syndromes (GPS) are inherited disorders associated with germinal aberrations that increase the risk of malignancies. While aberrations in certain genes increase the risk for all types of malignancies (Tp53, ATM, CDKN2A, CHEK2), there is a growing list of genes associated specifically with hematological malignancies (GATA2, RUNX1, DDX41, ETV6, ANKRD26). At our institution, we have established a hematology GPS clinic to diagnose and manage GPS and with this report, detail our experience with 130 patients. Methods GPS were investigated in pediatric and adult patients with one or more first degree relatives with hematological/visceral malignancies or in those with antecedent thrombocytopenia (ANKRD26, RUNX1, ETV6), or with specific syndromic features (short telomere syndromes/STS, GATA2 haploinsufficiency, Fanconi anemia/FA, Shwachman-Diamond syndrome/SDS). Depending on the phenotype, specific functional assays such as flow-FISH for telomere length assessment and chromosomal breakage assays were ordered. After informed consent and genetic counselling, germline testing was carried out on peripheral blood mononuclear cell, skin fibroblast, or hair follicle-derived DNA. A custom-designed marrow failure NGS panel (200 genes) was used in most cases and interrogation of variants, in silico studies, and functional assays were carried out as previously described (Mangaonkar et al MC Proc 2019). Copy number variations were identified by aCGH. At the time of progression/worsening cytopenias, bone marrow/lymph node biopsies and NGS (next generation sequencing) were carried out where indicated. Results 130 patients with germline predisposition have been identified to date. The spectrum of disorders seen include STS 29 (22%), FA 17 (13%), GATA2 16 (12%), Diamond Blackfan anemia/DBA 13 (10%), RUNX1-FPD 12 (9%), ATM deletions/mutations 11 (8%), ANKRD26 6 (5%), SDS 5 (4%), DDX41 4 (3%), MPL 3 (2%), CHEK2, MECOM, Tp53 mutations 2 (2%) each, and CBL, CEPBA, ELANE, NF1, CDKN2A, CSF3R, ETV6, and GATA1 mutations, 1 (1%) each. Evidence for clonal evolution (CCUS) and hematological malignancies were seen in 51 (39%) patients, involving all the aforementioned genes/syndromes with the exception of DBA, CBL, ETV6, MPL, CSF3R, and GATA1. Seven (64%) of 11 patients with germline ATM deletions/mutations developed lymphoid malignancies; homozygous ATM (Follicular NHL-1, Burkitt lymphoma-1, T-ALL-1, T-LPD-1) and heterozygous ATM (T-PLL-1, DLBCL-1, CLL-1). Clonal evolution occurred in 11 (69%) of 16 GATA2 haploinsufficient patients (CCUS-2, MDS-3, CMML-1, AML-5) and in 7 (58%) of 12 RUNX1-FPD patients (CCUS-1, MDS-1, MDS/MPN-3, AML-2). Five of 29 (17%) STS patients had clonal progression (CCUS-2, MDS-2, AML-1), and 5 (29%) of 17 FA patients progressed to MDS-2 or AML-3. JMML was seen in one patient with a germline NF1 mutation, while 1 (20%) of 5 SDS patients progressed to AML. NGS data at progression was available in 24 (55%) of 44 myeloid/CCUS progressions, with somatic truncating ASXL1 mutations being most frequent (29%), followed by RAS pathway mutations (15%). AML/MDS progressions in STS, FA, and SDS were universally associated with complex/monosomal karyotypes, translating to refractory disease. Seventeen (39%) of 44 patients with myeloid predisposition underwent allogenic HCT (GATA2-7, FA-3, RUNX1-FPD-3, STS-2, NF1-1, Tp53-1), with 10 (59%) being alive at last follow up (Table 1). Conclusion We demonstrate the spectrum of germline aberrations associated with predisposition to hematological malignancies and outline the phenotypic heterogeneity of clonal transformation. The advent of NGS allows identification of clonal progression earlier than morphological changes, with mutations in ASXL1 and RAS pathway genes being commonly implicated. This study supports the universal development of dedicated germline predisposition clinics. Disclosures Pruthi: CSL Behring: Honoraria; Genentech Inc.: Honoraria; Bayer Healthcare: Honoraria; HEMA Biologics: Honoraria; Instrumentation Laboratory: Honoraria; Merck: Honoraria.

Background: Chronic myelomonocytic leukemia (CMML) is a rare chronic myeloid malignancy with a poor prognosis. There are few epidemiologic studies on CMML. We studied population-based outcomes in CMML using the Surveillance, Epidemiology, and End Results (SEER) and the National Cancer Database (NCDB). Methods: The SEER 18 registries and NCDB Participant User File were used to identify patients with ICD-O-3 diagnosis code 9945/3 from 2004-2015. Incidence was identified using SEER and age-adjusted to the U.S. 2000 standard population. Causes of death were obtained and CMML-related death was defined as death from any myeloid disorder (CMML, acute monocytic leukemia, acute myeloid leukemia [AML], chronic myeloid leukemia, other myeloid/monocytic leukemia, and aleukemic, subleukemic, and NOS) to avoid misattribution. Relative survival (RS) was defined as the ratio of the proportion of observed survivors in a cohort of CMML patients to the proportion of expected survivors in a comparable set of individuals that did not have CMML, adjusting for the general survival of the US population for race, sex, age, and time when the diagnosis was established. Time to AML was calculated using the left-truncated life tables session and a 3 month latency period was used to prevent misattribution. Per the NCDB's system on classifying treatment, hydroxyurea, decitabine, and azacitidine are considered chemotherapy. The Kaplan-Meier method was used to calculate overall survival (OS), and Cox regression model was used to identify predictors of survival. Variables significant in univariate analysis (age, Charlson Deyo score [CDS], insurance type, and treatment facility type) were included in a multivariate analysis. Statistical analyses were performed using SAS version 9.0. Results: In the SEER database (n=4437), the median age at diagnosis was 76 years (interquartile range [IQR] 68-83) and 2,783 patients (63%) were male. Incidence rates (1 case/1,000,000 individuals) were as follows: overall 4.4, male 6.6, female 2.9, Non-Hispanic White 4.9, and Non-Hispanic Black 3.3. The incidence did not significantly change from 2004 to 2015. With a median follow up of 5.8 years (95% CI: 5.5-6.3), the median OS was 1.3 years (95% CI: 1.3-1.4). 3635 patients (82%) died. Among those who died, 2016 patients (55%) deaths were CMML-related. When comparing CMML patients to the US general population, 3,156 patients were matched to the expected survival tables. In the general U.S. population, the expected survival for 12, 24, 36, 48 and 60 months was 95.6%, 91.3%, 87.1%, 83.1%, and 79.2%, respectively. In contrast, the RS for patients with CMML at the same time points was 63.6%, 46.2%, 35.2%, 28.8%, and 23.1% (Figure 1). 229 patients (5.2%) progressed to AML. The median time to AML was 1.2 years (IQR 0.6-2.3). In NCDB (n=6403), the median age at diagnosis was 74 years (IQR 66-82). With the median follow up of 6 years (95% CI 3.5-9), the median OS was 1.3 years (95% CI 0.4-3.3). The distribution of CDS score 0, 1, and >1 was 4518 (71%), 1193 (18%), and 692 (11%) respectively. 4687 (77%) had government insurance, while 1399 (23%) had private insurance. 3750 (60%) were treated at a non-academic center. The year of diagnosis was associated with improved OS (HR 0.99, 95% CI: 0.98-0.99). Patients who were diagnosed in 2012-2015 had improved OS compared to patients diagnosed in 2004-2007 (HR 0.89, 95% CI 0.83-0.95) with OS at 1- and 5- years being 60% and 16% (2012-2015) vs. 56% and 17% (2008-2011) vs. 53% and 15% (2004-2007) respectively. 3029 (48%) patients received chemotherapy, while 270 (4%) patients received stem cell transplant as the first-line therapy. The median time to chemotherapy from diagnosis was 19 days (IQR 6-43). OS at 1-, 5-, and 10-years was 57%, 18%, and 4% (no chemotherapy), 56%, 14%, and 7% (received chemotherapy), 79%, 44%, and 38% (received stem cell transplant), and 56%, 15%, 5% (no stem cell transplant). OS curves are shown in Figure 2. Factors independently predicting inferior OS were age ≥ 65 years at the time of diagnosis (p<0.001), CDS ≥ 1 (p<0.001), government insurance (p<0.001), and treatment at a non-academic center (p<0.001) (Table). Conclusion: Despite an improvement in survival over the years, CMML continues to be a cause of significant morbidity and mortality. Older age, CDS ≥ 1, government insurance, and treatment at non-academic facilities were predictive of inferior survival. Disclosures Shah: Dren Bio: Consultancy.

Background: Despite recently approved novel targeted therapies, most patients with high-risk acute myeloid leukemia (AML) still require allogeneic hematopoietic cell transplantation (HCT) to achieve long-term survival. However, older patients with relapsed/refractory (R/R) AML are often unable to undergo HCT due to the toxicity of myeloablative pre-conditioning. Since higher doses of chemotherapy and radiation are associated with prolonged myelosuppression or infectious complications, targeted radiotherapy with Iomab-B, employed in the SIERRA trial, was designed to minimize toxicities in a patient population unable to tolerate standard high-dose myeloablative HCT pre-conditioning. We hypothesize that Iomab-B, which spares non-hematologic organs such as the GI tract, exhibits a favorable toxicity profile with respect to mucositis, febrile neutropenia (FN), and sepsis. Methods: The SIERRA trial is a prospective trial for patients ≥55 years of age with R/R AML (≥5% blasts) and 8/8 HLA-matched related or unrelated donors. Patients were randomized (1:1) to Iomab-B or Conventional Care (CC). Those in the Iomab-B group received a dosimetric infusion of Iomab-B followed by nuclear imaging to determine the personalized therapeutic dose to be delivered to the bone marrow and to other sites of disease. Therapeutic dose calculations, utilizing the Olinda software (Version 2.1, Hermes Medical) to determine the delivery of a maximum of 24 Gray (Gy) dose to the liver, were performed based on the imaging results. HCT is performed 12-14 days following the infusion of a therapeutic dose of Iomab-B, and a non-myeloablative conditioning backbone of fludarabine (30 mg/m2 x 3) and low-dose Total Body Irradiation (TBI 2 Gy x 1 dose). CC patients received the investigator's choice of salvage therapy. Safety data was evaluated from randomization and up to 100 days after HCT in the patients randomized to the Iomab-B group. We specifically evaluated AE variables of mucositis (Gr 2-4), FN (Gr3,4) and sepsis. An analysis was performed with both total activity of Iomab-B as well as with the radiation dose delivered to the bone marrow, stomach wall, small and large intestinal walls. For each of the 3 AEs of interest, a multivariate analysis was performed, allowing each of the predictor variables to be included in the model. Results: A total of 106 patients were randomized at the time of analysis. For those randomized to Iomab-B, data is available for 45 patients that received a therapeutic dose of Iomab-B and HCT. The median dose of Iomab-B administered was 603 mCi (range 313-1027 mCi) and the median radiation dose delivered to the marrow in the Iomab-B group was 14.7 Gy (range 4.6 - 32 Gy). All patients (median age 64) treated with a therapeutic dose of Iomab-B achieved neutrophil engraftment after a median of 14 days post-HCT (range 9-22), despite presenting with a median 26% marrow blasts prior to initiation of therapy. Rates of mucositis, FN, and sepsis are shown in Table 1. No correlation between AEs (FN, mucositis, sepsis) and administered Iomab-B activity (p=0.08), nor with dose delivered to GI tract (p=0.09) was observed. Furthermore, these AEs were not related to the dose of radiation received by the small intestine (median 2.7 Gy range 1.1-6.8 Gy) large intestine (median 2.6 Gy range 0.9-6.5 Gy), nor an average of all 4 GI sites (median 2.8 Gy range 1.6-6.8 Gy). Conclusions: Unlike cases involving treatment with cytotoxic chemotherapy and external beam radiation where mucositis, infections and sepsis increase along with the administered dose, targeted myeloablative conditioning with Iomab-B resulted in low incidences of severe mucositis, FN, and sepsis. These AEs do not appear to be related to the total dose of Iomab-B administered nor to the specific radiation dose delivered to the bone marrow. Furthermore, a low dose of radiation (2.8 Gy) was received by the GI tract compared to the larger dose of radiation (14.7 Gy) delivered to the marrow. Taken together, these results represent potentially significant improvements in the safe delivery of myeloablative therapy with high-dose targeted radioimmunotherapy, prior to HCT in older patients with R/R AML. This SIERRA trial is currently enrolling patients (www.sierratrial.com or clinicaltrials.gov NCT02665065). Disclosures Gyurkocza: Actinium: Research Funding. Nath:Actinium: Consultancy, Honoraria; Astellas: Consultancy, Honoraria; Daiichi Sankyo: Consultancy, Honoraria. Stiff:Kite, a Gilead Company: Research Funding; Gamida Cell: Research Funding; Atara: Research Funding; Unum: Research Funding; Delta-Fly: Research Funding; Macrogenics: Research Funding; Amgen: Research Funding. Hari:Amgen: Consultancy; GSK: Consultancy; Janssen: Consultancy; BMS: Consultancy; Takeda: Consultancy; Incyte Corporation: Consultancy. Al-Kadhimi:Actinium Pharmaceuticals Inc.: Research Funding; Genzyme: Research Funding; Bluebird Bio: Current equity holder in publicly-traded company; Gilead Science: Current equity holder in publicly-traded company; Moderna: Current equity holder in publicly-traded company; Novavax: Current equity holder in publicly-traded company. Abboud:Ryvu: Research Funding; Actinium Pharmaceuticals Inc.: Research Funding; Pfizer: Research Funding; Johnson & Johnson: Current equity holder in publicly-traded company; BMS: Current equity holder in publicly-traded company; Abbott Labs: Current equity holder in publicly-traded company; AbbVie Inc: Current equity holder in publicly-traded company; AlloVir: Research Funding; Forty Seven Inc: Research Funding. Magalhaes-Silverman:Kadmon Holdings: Research Funding; Actinium Pharmaceuticals: Research Funding; Incyte: Research Funding. Foran:Xencor: Research Funding; Trillium: Research Funding; Takeda: Research Funding; Kura Oncology: Research Funding; Aptose: Research Funding; Aprea: Research Funding; Actinium: Research Funding; Boehringer Ingelheim: Research Funding; Abbvie: Research Funding; BMS: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Servier: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; Revolution Medicine: Consultancy; Agios: Honoraria, Research Funding; H3Biosciences: Research Funding. Schuster:Amgen, Abbvie, Gilead, Takeda, Celgene, Pharmacyclics, Astellas, Verastem, Merck, Novartis, Takeda, Genentech,, Seattle Genetics: Other: Personal Fees; Karyopharm: Membership on an entity's Board of Directors or advisory committees. Kebriaei:Pfizer: Other: Served on advisory board; Kite: Other: Served on advisory board; Amgen: Other: Research Support; Ziopharm: Other: Research Support; Novartis: Other: Served on advisory board; Jazz: Consultancy. Levy:Karyopharm,Takeda, BMS: Consultancy, Honoraria, Speakers Bureau. Giralt:KITE: Consultancy; NOVARTIS: Consultancy, Honoraria, Research Funding; OMEROS: Consultancy, Honoraria; ACTINUUM: Consultancy, Research Funding; MILTENYI: Consultancy, Research Funding; TAKEDA: Research Funding; CELGENE: Consultancy, Honoraria, Research Funding; JAZZ: Consultancy, Honoraria; AMGEN: Consultancy, Research Funding. Liang:Actinium Pharmaceuticals: Current Employment. Berger:Actinium Pharmaceuticals Inc.: Current Employment, Current equity holder in publicly-traded company. Reddy:Actinium Pharmaceuticals Inc.: Current Employment, Current equity holder in publicly-traded company.

Somatic variants in TET2 and DNMT3A are founding mutations in hematological malignancies that both affect the epigenetic regulation of DNA methylation. Although the proteins antagonistically regulate the epigenetic mark of 5-methylcytosine (5-mC), where DNMT3A catalyzes addition of 5-mC while TET2 oxidizes 5-mC as a first step in DNA demethylation, mutations in both genes appear in a similar spectrum of human hematopoietic malignancies. Mutations in both genes often co-occur with activating mutations in oncogenic tyrosine kinases (OTKs) such as FLT3ITD, BCR-ABL1, JAK2V617F, MPLW515L or mutations affecting related signaling pathways such as NRASG12D and CALRdel52. Moreover, while the mutations exert divergent effect on primitive hematopoietic progenitor cells, they lead to similar disease phenotypes, suggesting the roles of these mutations in hematopoietic malignancies may relate to mechanisms outside of DNA methylation. OTK-positive malignant cells accumulate high numbers of spontaneous and drug-induced DNA double-strand breaks (DSBs) in comparison to normal cells, but they manage to survive because of their enhanced/altered ability to repair these breaks. DSBs, the most lethal DNA lesions, are repaired by two major mechanisms, BRCA1/2-dependent homologous recombination (HR) and DNA-PK -mediated non-homologous end-joining (D-NHEJ). Both HR and D-NHEJ repair DSBs in proliferating cells, while D-NHEJ plays a major role in quiescent cells. PARP1 -dependent alternative NHEJ (Alt-NHEJ) serves as back-up in both proliferating and quiescent cells. The existence of these redundant pathways creates the opportunity to employ a phenomenon called "synthetic lethality", which was originally applied to eliminate cancer cells with mutations in BRCA1 and BRCA2 by PARP inhibitor (PARPi). Our previous report [M. Nieborowska-Skorska et al., Gene expression and mutation-guided synthetic lethality eradicates proliferating and quiescent leukemia cells. J Clin Invest 127, 2392-2406 (2017)] suggested that certain leukemias are sensitive to PARPi-triggered synthetic lethality. Here we show that TET2 and DNMT3A mutations exert divergent roles in regulating DNA repair activities in leukemia cells expressing OTKs. Malignant TET2-deficient cells display downregulation of BRCA1 and LIG4 resulting in reduced activity of HR and D-NHEJ, respectively, and rely on Alt-NHEJ to protect them from the toxic effects of replication stress and drug-induced DSBs. Conversely, DNMT3A-deficient cells favor HR/D-NHEJ owing to downregulation of PARP1 and reduction of Alt-NHEJ. Consequently, malignant TET2-deficient cells are sensitive to PARPi treatment in vitro and in vivo, whereas DNMT3A-deficient cells were resistant. Disruption of TET2 dioxygenase activity and/or TET2 - Wilms tumor 1 (WT1) binding ability were responsible for DNA repair defects and sensitivity to PARPi associated with TET2 deficiency. Moreover, mutation or deletion of WT1 mimicked the effect of TET2 mutation on DSB repair activity and sensitivity to PARPi. Our findings reveal that TET2 and WT1 mutations may serve as biomarkers of synthetic lethality triggered by PARPi, which should be explored therapeutically. Disclosures Valent: Allcyte GmbH: Research Funding; Pfizer: Honoraria; Cellgene: Honoraria, Research Funding. Tallman:Abbvie: Research Funding; Cellerant: Research Funding; Orsenix: Research Funding; ADC Therapeutics: Research Funding; BioSight: Membership on an entity's Board of Directors or advisory committees, Research Funding; Glycomimetics: Research Funding; Rafael: Research Funding; Amgen: Research Funding; Bioline rx: Membership on an entity's Board of Directors or advisory committees; Daiichi-Sankyo: Membership on an entity's Board of Directors or advisory committees; KAHR: Membership on an entity's Board of Directors or advisory committees; Rigel: Membership on an entity's Board of Directors or advisory committees; Delta Fly Pharma: Membership on an entity's Board of Directors or advisory committees; Oncolyze: Membership on an entity's Board of Directors or advisory committees; Jazz Pharma: Membership on an entity's Board of Directors or advisory committees; Roche: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; UpToDate: Patents & Royalties. Martinelli:Roche: Consultancy; Pfizer: Consultancy, Research Funding, Speakers Bureau; Incyte: Consultancy; Jazz: Consultancy; Janssen: Consultancy; Daichii Sankyo: Consultancy, Research Funding; Celgene: Consultancy, Speakers Bureau; Amgen: Consultancy; AbbVie: Consultancy, Research Funding. Vassiliou:Kymab Ltd - Monoclonal antibody company. Currently not working in myeloid cancers or clonal haematopoiesis.: Consultancy.

Acute leukemia is a constellation of rapidly progressing diseases that affect a wide range of patients regardless of age or gender. Traditional treatment options for patients with acute leukemia include chemotherapy and hematopoietic cell transplantation. The advent of cancer immunotherapy has had a significant impact on acute leukemia treatment. Novel immunotherapeutic agents including antibody-drug conjugates, bispecific T cell engagers, and chimeric antigen receptor T cell therapies have efficacy and have recently been approved by the US Food and Drug Administration (FDA) for the treatment of patients with acute leukemia. The Society for Immunotherapy of Cancer (SITC) convened a panel of experts to develop a clinical practice guideline composed of consensus recommendations on immunotherapy for the treatment of acute lymphoblastic leukemia and acute myeloid leukemia.

Background. Experience with sequential hematopoietic stem cell transplant (HSCT) and kidney transplant (KT) is limited. Methods. We conducted a retrospective observational study of adult patients who underwent both HSCT and KT at our center, with a median follow-up of 11 y. Results. In our 54 patients cohort (94% autologous HSCT), 36 (67%) patients received HSCT first followed by KT, while 18 (33%) received KT before HSCT. In both groups, AL amyloidosis represented 50% of hematologic diagnosis. Only 4 patients expired due to hematologic disease relapse (2 patients in each group) and only 3 allografts were lost due to hematologic disease recurrence (HSCT first n = 1 and KT first n = 2). Overall 1, 5, and 10 y death-censored graft survival rates were 94%, 94%, and 94%, respectively, for the HSCT first group and 89%, 89%, and 75%, respectively, for the KT first group. Overall 1, 5, and 10 y patients survival rates were 100%, 97% and 90%, respectively, for the HSCT first group and 100%, 76%, and 63%, respectively, for the KT first group. Conclusions. Our study supports safety of sequential KT and HSCT, with improved overall patient survival compared to recipients of HSCT remaining on dialysis and good long-term kidney allograft outcome.

Abstract Comparative outcomes of allogeneic hematopoietic cell transplantation (HCT) for BCR-ABL1− myeloproliferative neoplasms (MPNs) in blast phase (MPN-BP) vs de novo acute myeloid leukemia (AML), and AML with prior myelodysplastic syndromes (MDSs; post-MDS AML), are unknown. Using the Center for International Blood and Marrow Transplant Research (CIBMTR) database, we compared HCT outcomes in 177 MPN-BP patients with 4749 patients with de novo AML, and 1104 patients with post-MDS AML, using multivariate regression analysis in 2 separate comparisons. In a multivariate Cox model, no difference in overall survival (OS) or relapse was observed in patients with MPN-BP vs de novo AML with active leukemia at HCT. Patients with MPN-BP in remission had inferior OS in comparison with de novo AML in remission (hazard ratio [HR], 1.40 [95% confidence interval [CI], 1.12-1.76]) due to higher relapse rate (HR, 2.18 [95% CI, 1.69-2.80]). MPN-BP patients had inferior OS (HR, 1.19 [95% CI, 1.00-1.43]) and increased relapse (HR, 1.60 [95% CI, 1.31-1.96]) compared with post-MDS AML. Poor-risk cytogenetics were associated with increased relapse in both comparisons. Peripheral blood grafts were associated with decreased relapse in MPN-BP and post-MDS AML (HR, 0.70 [95% CI, 0.57-0.86]). Nonrelapse mortality (NRM) was similar between MPN-BP vs de novo AML, and MPN-BP vs post-MDS AML. Total-body irradiation–based myeloablative conditioning was associated with higher NRM in both comparisons. Survival of MPN-BP after HCT is inferior to de novo AML in remission and post-MDS AML due to increased relapse. Relapse-prevention strategies are required to optimize HCT outcomes in MPN-BP.