Harrington, Alexandra M.

ABSTRACT The epidemiology of myelodysplastic syndromes/neoplasms (MDS) is challenging to define due to inconsistent reporting, complex diagnostic procedures, and evolving diagnostic criteria. Using the National MDS Natural History Study—a prospective cohort with centrally adjudicated histopathology and genetic variant review—we characterized the landscape of MDS across the United States and identified differences across demographics. Among 2115 participants, 64% (1346) had an MDS spectrum condition, including MDS (24%), MDS/myeloproliferative neoplasm (5%) and precursor conditions—clonal cytopenia of undetermined significance (22%) and idiopathic cytopenia/dysplasia of undetermined significance (13%). The median age was 74 years, and participants were predominantly male (66%), White (91%), and Non‐Hispanic (92%). Myeloid‐associated variants were detected in 68% of participants, most commonly in TET2, DNMT3A, ASXL1, SF3B1, and SRSF2 . Black, compared to White, participants were younger at diagnosis (69 vs. 74 years, p = 0.01), had equal or increased prevalence of higher‐risk MDS, lower hemoglobin, and higher peripheral blood blasts, yet were less likely to receive MDS‐directed therapy (14% vs. 42%, p = 0.008). Black and Hispanic participants had fewer detectable gene mutations than White participants. Females had lower variant allele frequencies and fewer RNA splicing gene mutations than males. After multivariable adjustment, TP53 mutations, MDS diagnosis, and higher‐risk disease were associated with worse progression‐free and overall survival; age was also associated with overall survival. Black race trended toward improved progression‐free survival. These findings highlight the need for enhanced understanding of MDS pathogenesis across patient groups and refined prognostic tools to improve personalized management of MDS spectrum conditions. Trial Registration: ClinicalTrials.gov identifier: NCT02775383

Abstract Patients at risk for progression from myeloid precursor states (idiopathic cytopenia of undetermined significance [ICUS] and clonal cytopenia of undetermined significance [CCUS]) to myelodysplastic syndrome (MDS) are not well defined. Epidemiologic risk factors, clonal changes for MDS development, and evolution from one MDS state to another were available in the MDS Natural History Study (NHS), a prospective cohort enrolled at diverse US sites. Extensive clinical data and biospecimens were collected at baseline and follow-up. A total of 1177 (56%) individuals with MDS or MDS-related precursor conditions (ICUS/idiopathic dysplasia of undetermined significance [IDUS], CCUS, and MDS/myeloproliferative neoplasm overlap) were enrolled and diagnosed in the MDS NHS. The median age was 74 years, with 89% aged ≥60 years, and most were male patients. Median follow-up time was 1.6 years and was similar among ICUS/IDUS, CCUS, and MDS. During follow-up period, 68% of participants terminated the study, 35% died, and 38% had disease progression. Using a refined clinical definition of progression, the greatest proportion of progression was observed among higher-risk-MDS (116 [73%]), followed by lower-risk-MDS (170 [52%]); fewer progression events were observed among participants with CCUS (65 [22%]) and ICUS/IDUS (46 [17%]). Predictors of progression are delineated, including female sex, higher quantity of mutations, the presence of TP53 mutation, and poor/very poor cytogenetic score. Based on these prospective data, guidelines for clinical management including monitoring and surveillance are outlined. The MDS NHS provides real-world data to illustrate how clinical and genotypic differences inform the classification, disease course, and approach to therapy, with informed monitoring guidelines for patients. This trial was registered at www.ClinicalTrials.gov as NCT02775383.

Abstract Introduction Defining the epidemiology of myelodysplastic syndromes/neoplasms (MDS) and precursor conditions, such as clonal cytopenia of undetermined significance (CCUS), is challenging due to inconsistent reporting, complex diagnostics, and evolving criteria. The National MDS Natural History Study (NHS) overcomes these challenges through central pathology review, prospective follow-up, and a nationally representative cohort enrolled at the time of suspected MDS, enabling robust analysis across the myeloid disease spectrum. We analyzed clinical, genetic, and pathologic features in NHS participants by race, ethnicity, and sex to characterize disease variation across populations. Methods The NHS (NCT02775383) enrolled individuals undergoing evaluation for suspected MDS from June 2016–July 2024 at 102 US sites. Participants underwent bone marrow assessment with centralized, adjudicated histopathology review. Those meeting WHO 2016 criteria for MDS, MDS/myeloproliferative neoplasm (MPN), CCUS, idiopathic cytopenia/dysplasia of undetermined significance (ICUS/IDUS), or oligoblastic AML were followed longitudinally. Demographics, comorbidities, exposures, and clinical data were collected at baseline and every 6 months. Deep DNA sequencing was performed on baseline marrow, with pathogenic variants manually curated (53 genes). Statistical analyses assessed associations using chi-squared, Fisher's exact, Kruskal-Wallis, logistic regression, and Cox models. A p-value <0.05 was considered significant. Results Among 2,021 participants, 1,174 (58%) were diagnosed with MDS or precursor conditions. Median age was 74 years (yrs) and 66% were male. Most identified as Non-Hispanic White (NHW, 92%), with 4% Black and 4% Hispanic. The most common diagnoses were MDS (42%) and CCUS (37%). Pathogenic somatic mutations were identified in 84% (774/918) of MDS/CCUS, with TET2 (26% of 918), DNMT3A (21%), and ASXL1 (16%) most frequent. Median follow-up was 1.8yrs and consistent across race, ethnicity, and sex (p≥0.6). Black, compared to NHW, participants were younger (69 vs 74 yrs, p=0.006) and trended toward more females (47% vs 34%, p=0.09) and more MDS diagnoses (51% vs 41%, p=0.07). Black participants showed more advanced disease (e.g., lower hemoglobin, higher peripheral blood blasts, higher variant allele frequencies (VAFs) of pathogenic mutations), yet received less MDS-directed treatment (14% vs 41%, p=0.01). Black, compared to NHW, participants had more mutations in RUNX1, TP53, BCOR, and DNMT3A, but fewer in TET2 and ASXL1. Hispanic, compared to Non-Hispanic, participants had fewer known pathogenic mutations, leading to more ICUS (19% vs 5%, p=0.02). Females, compared to males, were younger at diagnosis (72 vs 75yrs, p<0.001), had more ICUS/IDUS (20% vs 10%, p<0.001), fewer pathogenic mutations, especially in splicing/chromatin modifier genes (U2AF1, SRSF2, ASXL1, ZRSR2), more DNMT3A mutations, and lower VAFs (0.27 vs 0.33, p<0.001). Median overall survival (OS) was 6.4yrs, and shorter in MDS (4.2yrs) than CCUS (not reached). Worse OS was associated with age (adjusted hazard ratio (aHR) 1.06), MDS (vs CCUS; aHR 1.89), higher-risk disease by IPSS-R/CCRS (aHR 2.15), and TP53 mutations (aHR 2.17) (all p<0.001). A trend toward improved OS in Black participants was attenuated after adjustment (aHR 0.79, p=0.6). Progression (e.g., worsening of disease, increase in transfusion dependency, death) occurred in 53% of participants. Worse progression-free survival (PFS) was associated with MDS diagnosis (aHR 2.59), higher-risk disease by IPSS-R/CCRS (aHR 2.17), and TP53 mutations (aHR 1.86) (all p<0.001). Despite more advanced disease and less MDS therapy, Black participants trended toward improved PFS (aHR 0.62, p=0.14), while age was borderline significant (aHR 1.01, p=0.05). Conclusions For the first time, we show that among individuals evaluated for suspected MDS at sites across the US, 58% had MDS or precursor conditions. Differences across demographics underscore the heterogeneity of MDS. Black participants presented with more advanced MDS, received less treatment, but trended toward improved outcomes, particularly PFS, even after adjustment, suggesting biological rather than access-to-care differences driving disparities. These findings highlight the complex interplay of sex, race, and genetics in shaping MDS outcomes and reinforce the need for diverse, large-scale studies to improve risk stratification and personalized care.

Background: Outcomes of adults with relapsed/refractory T-cell acute lymphoblastic leukemia (T-ALL) has remained poor. CD38 is a transmembrane glycoprotein that is highly expressed in T-ALL and is potentially targetable with CD38 blocking agents [Naik J et al. Haematologica 2019, Bride KL et al. Blood 2018, Tembhare et al. J Immunother Cancer 2020]. XmAb18968 is a novel CD38-CD3 bi-specific T-cell engager with Fc domain modified to minimize Fcγ receptor binding and non-selective T-cell activation resulting in reduced cytokine release without compromising target cell killing. We designed a phase I clinical trial to assess the safety and tolerability of XmAb18968 in patients with T-ALL. Methods: We conducted a multicenter, open label, phase I dose escalation study of XmAb18968 (NCT05038644). Patients aged ≥18 years with relapsed/refractory T-ALL or T-lymphoblastic lymphoma (T-LBL), ECOG PS 0-2 and adequate organ function were eligible. Major exclusion criteria are hematopoietic cell transplantation within 6 months of enrollment, prior therapy with daratumumab in the last 6 months and active acute graft-versus-host disease. Dose escalation was conducted using 3+3 design, and 4 separate dose levels were studied [0.8mg, 1mg, 1.3mg, 1.5mg]. XmAb18968 was administered intravenously on days 1, 8, 15, 22 on a 28-day cycle. During cycle 1 alone, day 1 dose was fractionated and administered over 2 days to minimize the risk of significant cytokine release syndrome (CRS). Treatment was continued until progression or unacceptable toxicity. The primary objective was to determine the safety and dose limiting toxicity (DLT) and secondary objective was to determine the preliminary efficacy. Correlative studies included pharmacokinetics, characterizing changes in serum cytokines and phenotypic expression of activated T-cells and leukemic cells, correlation of the phenotypic expression with changes in transcriptome at the single-cell level, proteomics evaluation of cytokine secretion at the single-cell level and correlation of response with N-glycan profiling, quantitative site-occupancy, and direct glycopeptide analysis. Results: Four patients with relapsed/refractory T-ALL have been enrolled so far at dose level 1 (0.8mg) with 3 of them being evaluable for response (one patient came off study before DLT period). Median age was 62 years (34-74), all response evaluable patients had early T-precursor ALL (ETP-ALL), and one patient (25%) was previously treated with allogeneic HCT, CD7 CAR-T therapy and daratumumab. Grade 3 or higher adverse events are summarized in Table 1. CRS grades 1 was noted in 25% of patients, no grade ≥3 CRS or neurotoxicity (any grade) was seen. Of the 3 evaluable total patients, 2 patients had achieved complete response (CR) after 1 cycle of therapy (one patient had prior allogeneic HCT, CD7 CAR-T therapy and daratumumab) (Table 2). The two responders remain on treatment. Conclusions: XmAb18968, a novel CD-38 bispecific antibody appears safe and is showing response at the starting dose level in T-ALL. The study is currently enrolling additional patients with T-ALL. Updated analysis with additional patients and details on correlative studies examining mechanisms of therapeutic efficacy will be reported at the main meeting.

Background: Outcomes of adults with relapsed/refractory acute myeloid leukemia (RR-AML) have remained poor. CD38 is a transmembrane glycoprotein that is highly expressed in AML and is potentially targetable with CD38 blocking agents [Naik J et al. Haematologica 2019, Tembhare et al. J Immunother Cancer 2020]. XmAb18968 is a novel CD38-CD3 bi-specific T-cell engager with Fc domain modified to minimize Fcγ receptor binding and non-selective T-cell activation resulting in reduced cytokine release without compromising target cell killing. We designed a phase I clinical trial to assess the safety and tolerability of XmAb18968 in patients with RR-AML. Methods: We conducted a multicenter, open label, phase I dose escalation study of XmAb18968 (NCT05038644). Patients aged ≥18 years with relapsed/refractory AML, ECOG PS 0-2 and adequate organ function were eligible. Major exclusion criteria are hematopoietic cell transplantation within 6 months of enrollment, active acute graft-versus-host disease, and acute promyelocytic leukemia. Dose escalation was conducted using 3+3 design and 4 separate dose levels were studied [0.8mg, 1mg, 1.3mg, 1.5mg]. XmAb18968 was administered intravenously on days 1, 8, 15, 22 on a 28-day cycle. During cycle 1 alone, day 1 dose was fractionated and administered over 2 days to minimize the risk of significant cytokine release syndrome (CRS). Treatment was continued until progression or unacceptable toxicity. The primary objective was to determine the safety and dose limiting toxicity (DLT) and secondary objective was to determine the preliminary efficacy. Measurable residual disease (MRD) status was assessed using multicolor flow cytometry with 0.01% sensitivity. Correlative studies included pharmacokinetics, characterizing changes in serum cytokines and phenotypic expression of activated T-cells and leukemic cells, correlation of the phenotypic expression with changes in transcriptome at the single-cell level, proteomics evaluation of cytokine secretion at the single-cell level and correlation of response with N-glycan profiling, quantitative site-occupancy, and direct glycopeptide analysis. Results: Thirteen patients with RR-AML were enrolled, 12 of whom were evaluable (1 patient came off study before DLT period). Median age was 63 years (45-77), 77% with adverse risk disease, 38% with secondary AML, and received median two (range 2-7) prior lines of therapy. Twelve of 13 patients (92%) were previously treated with venetoclax based regimen and 31% had prior allogeneic HCT. Adverse risk mutations included TP53 (31%), RUNX1 (23%), and ASXL1 (15%). Grade 3 or higher adverse events included anemia (8%), neutropenia (15%), and thrombocytopenia (23%) (Table 1). CRS grades 1-2 were noted in 62% of patients, no grade ≥3 CRS or neurotoxicity (any grade) was seen. There was no dose limiting toxicity (DLT). Of the 12 evaluable total patients, clinically meaningful response was noted in two patients who were MRD positive by flow cytometry prior to study entry and achieved MRD negative CR (Table 2). These two patients who achieved MRD negative CR subsequently proceeded to allogeneic HCT. Median time to response was 1 cycle. Median OS of the cohort was 6.2 months (1.8-NR). Conclusions: XmAb18968 is safe and well tolerated in patients with RR-AML. Dose escalation yielded MRD negative CR in RR-AML patients who were MRD positive at study entry. Details on correlative studies examining mechanisms of therapeutic efficacy and resistance will be reported in the main meeting.

TPS7070 Background: Outcomes of adults with relapsed/refractory T-cell acute lymphoblastic leukemia (T-ALL) and acute myeloid leukemia (AML) have remained poor. CD38 is a transmembrane glycoprotein with several important functions including its role in immune escape of tumor cells [Chillemi A et al. Front Immunol 2017, Furano A et al. J Immunol 1990]. Several studies have demonstrated that CD38 is expressed in T-ALL and AML and is targetable with CD38 blocking agents [Naik J et al. Haematologica 2019, Bride KL et al. Blood 2018, Tembhare et al. J Immunother Cancer 2020]. XmAb18968 is a novel CD38-CD3 bi-specific T-cell engager with Fc domain modified to minimize Fcγ receptor binding and non-selective T-cell activation resulting in reduced cytokine release without compromising target cell killing. We hypothesize that targeting CD38 in relapsed/refractory T-ALL and AML would be safe and feasible using XmAb18968. Methods: This is an investigator-sponsored multi-institutional phase I study evaluating the safety and tolerability of XmAb18968. Patients aged 18 years or above with relapsed/refractory T-ALL or AML (including measurable residual disease relapse), CD38 expression ≥ 20% by flow cytometry, and adequate organ function will be eligible. Major exclusion criteria are hematopoietic cell transplantation within 6 months of enrollment, active acute graft-versus-host disease, and acute promyelocytic leukemia. The primary objective is to determine the recommended phase 2 dose (RP2D) and toxicity profile of XmAb18968. The secondary objectives include determination of response rates, duration of response, survival and pharmacokinetics. Exploratory objectives include correlation of responses with genomic profile, characterizing changes in serum cytokines and phenotypic expression of activated T-cells and leukemic cells, correlation of the phenotypic expression with changes in the transcriptome at the single-cell level, proteomics evaluation of cytokine secretion at the single-cell level and correlation of response with N-glycan profiling, quantitative site-occupancy and direct glycopeptide analysis. The study will follow 3+3 design. Dose escalation will proceed in two separate groups: Group A for subjects with T-ALL and Group B for subjects with AML. Patients will be entered sequentially to each dose level [0.8mg cohort, 1mg cohort, 1.3mg cohort, 1.5mg cohort). The observation period for dose-escalation will be 28-days. RP2D will be defined as the highest dose level at which none of the 3 treated subjects, or no more than 1 of the 6 treated subjects’ experiences a dose limiting toxicity. A minimum of 24 and a maximum of 60 patients will be enrolled. The study is being conducted at 6 sites in United States and is currently open for enrollment. Clinical trial information: NCT05038644.

Abstract Background: Outcomes of patients with relapsed/refractory AML (RR-AML) have remained poor. Therapy with venetoclax based combinations in this setting leads to CR/CRi rates of 21-49% (DiNardo CD et al. Am J Hematol 2018, Aldoss I et al. Haematologica 2018, Stahl M et al. Blood Adv 2020). Preclinical studies with BCL-2 inhibitors indicate potential mechanisms of drug resistance including overexpression of the anti-apoptotic protein MCL-1 (Konopleva et al. Cancer Cell. 2006). Pevonedistat is a first in class inhibitor of Nedd8 activating enzyme that induces the pro-apoptotic protein NOXA leading to neutralization of MCL-1 and apoptosis. Preclinical studies evaluating the combination of pevonedistat and venetoclax against AML cell lines have demonstrated a synergistic effect (Knorr KL et al. Cell Death Differ. 2015). Hence, we designed a phase I study to assess the safety and tolerability of adding pevonedistat to the combination of azacitidine and venetoclax in patients with RR-AML. Study design and methods: We conducted a phase I study with the combination of pevonedistat, venetoclax and azacitidine in patients with RR-AML. Patients aged 18 years or above with morphologically documented RR-AML, ECOG performance status 0-2 and adequate organ function were eligible. Major exclusion criteria were isolated extramedullary relapse, hematopoietic cell transplantation (HCT) within 100 days of enrollment, and active acute GVHD. Previous therapy with hypomethylating agents (HMA) or venetoclax was not an exclusion criterion. The dose escalation phase was conducted using 3+3 design. Treatment included azacitidine (75 mg/m 2 daily x 7 days), venetoclax (400 mg daily x 28 days), and pevonedistat in escalating doses (10-20 mg/m 2 IV days 1,3,5 of each cycle) with a cycle length of 28 days. Pevonedistat was given at 10 mg/m 2 dose in cohort 1, 15 mg/m 2 dose in cohort 2 and 20 mg/m 2 dose in cohort 3. The primary endpoint is to determine the recommended phase 2 dose (RP2D) and toxicity profile of pevonedistat, azacitidine and venetoclax. Other endpoints included determination of response rates, duration of response, survival, pharmacokinetics, correlation of response rates with AML genomic profile, correlation of pretreatment levels of BCL2, BCLXL, MCL1, BAX or BAK with response, determination of changes in NOXA (PMAIP1) mRNA and protein expression pre-and post-pevonedistat treatment, evaluation of BH3 mimetic profiling on bone marrow samples by flow cytometry and assessing the sensitivity of leukemia and leukemic stem/progenitor cells to pevonedistat ex vivo. Results: Thirteen patients participated in the dose escalation phase, 12 of whom were evaluable. Median age was 69 years (61-91), 30.8% had secondary/therapy related AML, 69.2% with adverse risk disease, 53.8% previously received venetoclax/HMA and 23.1% had relapse after prior allogeneic hematopoietic cell transplantation (HCT) (Table 1). Seven patients were enrolled into cohort 1 (pevonedistat 10 mg/m 2 dose) of which one was not evaluable, three patients enrolled into cohort 2 (pevonedistat 15 mg/m 2 dose), and three patients enrolled into cohort 3 (pevonedistat 20 mg/m 2 dose). Grade 3 or higher AEs included febrile neutropenia (23%), infection (15%), anemia (38%), neutropenia (54%), thrombocytopenia (38%). There was 1 dose limiting toxicity (DLT) in cohort 1 (atrial fibrillation) that triggered cohort expansion. However, subsequent patients did not experience DLT despite planned dose escalation. Of the 12 evaluable total patients, CR/CRi was observed in 5 (41.6%) patients. Notably, patients with RR-AML who were venetoclax/HMA naïve had a CR/CRi 83.3% (5/6 patients). The response rates for each cohort are summarized in Table 2. Three of the five patients with CR/CRi (60%) achieved MRD negativity by flow cytometry. Four patients who achieved CR/CRi proceeded to allogeneic HCT after therapy. Median OS of the cohort was 5.4 months (1.8-14) and median OS was not reached in patients with CR/CRi. Conclusions: The addition of pevonedistat to venetoclax and azacitidine backbone is safe and well tolerated in patients with RR-AML. Dose escalation yielded encouraging efficacy in venetoclax/HMA naïve RR-AML patients. The study is currently in the dose expansion phase. Details on correlative studies examining mechanisms of therapeutic efficacy and resistance will be reported in the main meeting. Figure 1 Figure 1. Disclosures Guru Murthy: Cardinal health: Honoraria; TG Therapeutics: Other: Advisory board meeting; DAVA Oncology: Honoraria; CancerExpertNow: Honoraria; Qessential: Honoraria; Techspert: Consultancy; Curio Sciences: Honoraria; Guidepoint: Consultancy. Abedin: AltruBio: Research Funding; Helsinn: Research Funding; Amgen: Honoraria; Pfizer: Research Funding; Agios: Honoraria; Actinium: Research Funding; Astellas Pharma Inc.: Research Funding. Litzow: Jazz: Other: Advisory Board; Pluristem: Research Funding; Actinium: Research Funding; Amgen: Research Funding; Astellas: Research Funding; AbbVie: Research Funding; Omeros: Other: Advisory Board; Biosight: Other: Data monitoring committee. Atallah: Novartis: Consultancy, Honoraria, Research Funding; Pfizer: Consultancy, Research Funding; BMS: Honoraria, Speakers Bureau; Takeda: Consultancy, Research Funding; Abbvie: Consultancy, Speakers Bureau; Amgen: Consultancy.

Abstract Background: Outcomes of adults with relapsed/refractory T-cell acute lymphoblastic leukemia or lymphoma (T-ALL or T-LBL respectively) and acute myeloid leukemia (AML) have remained poor despite the availability of newer agents. CD38 is a 45 kDa transmembrane glycoprotein expressed by lymphoid and myeloid cells with several important functions including a role in immune escape from tumor cells [Chillemi A et al. Front Immunol 2017, Furano A et al. J Immunol 1990]. Several studies have demonstrated that CD38 is expressed in tumor cells of patients with T-ALL and AML and potentially targetable using CD38 blocking agents [Naik J et al. Haematologica 2019, Bride KL et al. Blood 2018, Tembhare et al. J Immunother Cancer 2020, Farber M et al. ASH 2018]. XmAb18968 is structurally distinct CD3-CD38 bi-specific T-cell engager with a full fragment crystallizable (Fc) domain modified to minimize Fcγ receptor binding and non-selective activation of T cells and other immune effector cells. XmAb18968 has optimized relative affinities for both CD3 and CD38 which results in reduced cytokine release without compromising target cell killing. We hypothesize targeting CD38 in relapsed/refractory- T-cell ALL/LBL and AML is safe and feasible using XmAb18968, a novel CD3-CD38 bispecific antibody. Study design and methods: This is an investigator-sponsored multi-institutional phase I study evaluating the safety and tolerability of XmAb18968. Patients aged 18 years or above with T-ALL, T-LBL or AML (including undifferentiated leukemia and bi phenotypic leukemia), in relapsed/refractory status (at least one line of prior therapy) including measurable residual disease relapse, CD38 expression ≥ 20%, and adequate organ function will be eligible. Major exclusion criteria are hematopoietic cell transplantation (HCT) within 6 months of enrollment, active acute GVHD, veno-occlusive disease, and acute promyelocytic leukemia. The study is planned to be conducted at 6 sites in United States. The primary endpoint is to determine the recommended phase 2 dose (RP2D) and toxicity profile of XmAb18968. The secondary endpoints include determination of response rates, duration of response, survival and pharmacokinetics. Exploratory endpoints include correlation of responses with genomic profile, assessment of serum cytokine response, identifying the changes in phenotypic expression of activated T-cells and leukemic cells, correlation of the phenotypic expression with changes in the transcriptome at the single-cell level, proteomics evaluation of cytokine secretion at the single-cell level and correlation of response with N-glycan profiling, quantitative site-occupancy and direct glycopeptide analysis. The study will follow 3+3 design for dose escalation and de-escalation in case of dose limiting toxicity (DLT). Dose escalation will proceed in two separate groups: Group A for subjects with T-ALL and T-LBL and Group B for subjects with AML. Patients will be entered sequentially to each dose level, starting with the first dose level. The DLT observation period for dose-escalation will be 28-days. RP2D will be defined as the highest dose level at which none of the first 3 treated subjects, or no more than 1 of the first 6 treated subjects experience a DLT. A minimum of 24 and a maximum of 60 patients will be needed for the dose escalation phase. Disclosures Guru Murthy: Curio Sciences: Honoraria; Techspert: Consultancy; Qessential: Honoraria; CancerExpertNow: Honoraria; DAVA Oncology: Honoraria; TG Therapeutics: Other: Advisory board meeting; Cardinal health: Honoraria; Guidepoint: Consultancy. Johnson: Miltenyi Biotec: Research Funding. Abedin: AltruBio: Research Funding; Amgen: Honoraria; Agios: Honoraria; Helsinn: Research Funding; Pfizer: Research Funding; Astellas Pharma Inc.: Research Funding; Actinium: Research Funding. Stock: Pfizer: Consultancy, Honoraria, Research Funding; amgen: Honoraria; agios: Honoraria; jazz: Honoraria; kura: Honoraria; kite: Honoraria; morphosys: Honoraria; servier: Honoraria; syndax: Consultancy, Honoraria; Pluristeem: Consultancy, Honoraria. Shah: Precision Biosciences: Consultancy; Kite, a Gilead Company: Consultancy, Honoraria, Other: Expenses, Research Funding; Pharmacyclics/Janssen: Honoraria, Other: Expenses; Acrotech/Spectrum: Honoraria; BeiGene: Consultancy, Honoraria; Amgen: Consultancy; Pfizer: Consultancy, Other: Expenses; Novartis: Consultancy, Other: Expenses; Bristol-Myers Squibb/Celgene: Consultancy, Other: Expenses; Adaptive Biotechnologies: Consultancy; Servier Genetics: Other; Jazz Pharmaceuticals: Research Funding; Incyte: Research Funding. Leonard: Amgen: Research Funding; Takeda: Membership on an entity's Board of Directors or advisory committees. Pratz: Agios: Consultancy; BMS: Consultancy, Honoraria; Novartis: Consultancy; Astellas: Consultancy, Honoraria, Research Funding; Cellgene: Consultancy, Honoraria; Millenium: Research Funding; University of Pennsylvania: Current Employment; Abbvie: Consultancy, Honoraria, Research Funding. Luger: Syros: Honoraria; Agios: Honoraria; Daiichi Sankyo: Honoraria; Jazz Pharmaceuticals: Honoraria; Brystol Myers Squibb: Honoraria; Acceleron: Honoraria; Astellas: Honoraria; Pfizer: Honoraria; Onconova: Research Funding; Celgene: Research Funding; Biosight: Research Funding; Hoffman LaRoche: Research Funding; Kura: Research Funding. Badar: Pfizer Hematology-Oncology: Membership on an entity's Board of Directors or advisory committees. Atallah: Novartis: Consultancy, Honoraria, Research Funding; Amgen: Consultancy; Abbvie: Consultancy, Speakers Bureau; Takeda: Consultancy, Research Funding; BMS: Honoraria, Speakers Bureau; Pfizer: Consultancy, Research Funding.

Introduction: The NHLBI National MDS Study (NCT02775383) is a prospective cohort study conducted at 92 community hospitals and 29 academic centers. It enrolls patients undergoing work up for suspected MDS to understand the genetic, epigenetic, and biological factors associated with the initiation and progression of the disease. Previously untreated, cytopenic participants undergo both local and centralized pathology review and are assigned a diagnosis, including MDS, MDS/MPN, AML with blasts < 30%, and "Other". Emerging data suggests that Next Generation Sequencing (NGS), along with cytogenetics and clinical variables, may improve MDS diagnostic precision. Given that our study relies on central review (with additional tertiary pathology review used to adjudicate disagreements), we examined whether targeted gene sequencing data could be used to increase the agreement between local and central pathologic diagnosis of MDS vs. Other. Methods: Peripheral blood and bone marrow (BM) biopsy specimens from cytopenic patients, along with clinical history, CBC, and other results including karyotyping, FISH and pathology reports from local pathologists were reviewed by central pathologists. The updated 2016 WHO classifications were used to diagnose MDS. Targeted exon sequencing of 96 genes was performed using BM specimens. A subset of 648 individuals that were classified as MDS (n=212) or Other (n=436, including 90 CCUS and 89 individuals with other cancers) by pathology assessments were selected. A mean coverage of 1,317X was achieved and variants had a minimum variant allele frequency (VAF) of 2% (except FLT3). Variants for 596 subjects were manually reviewed to retain likely disease-causing variants to build a binary classifier (MDS vs. Other) using the maximum VAF per gene as input (Figure 1). Subjects diagnosed with MDS or Other by both central and local pathology were used for training, validation, and testing, and were considered "gold standard" (GS) cases (n=546). These subjects were split into 4 random groups with equal proportions of MDS cases. 75% of the GS cases were used to train and validate lasso-regularized logistic regression models using 3-fold cross validation. ROC curve analysis was carried out using the remaining 25% of GS cases (Test Set 1) on the best model to identify an optimal probability cut off point for classifying subjects as MDS. Model performance was then tested on 50 subjects for which the central and local pathology diagnosis disagreed (Test Set 2), as well as on 52 additional subjects irrespective of agreement (Test Set 3). Results : The best performing logistic regression model retained 7 genes as most informative in a discriminating diagnosis of MDS from Other based on their VAFs, in order of impact: TP53, SF3B1, U2AF1, ASXL1, TET2,STAG2, and SRSF2. We used this model to assign probabilities for each of the subjects in Test Set 1 and to estimate the performance using ROC analysis (Figure 1), resulting in a high area under the curve (AUC) of 0.89. We chose a probability cut-off of ≥0.17, being associated with a high percentage of correct classification of MDS with a sensitivity and specificity of 0.90 and 0.81, respectively. Among the cohort of 50 subjects with a discordant local and central pathology diagnosis (Test Set 2), the classifier accurately reassigned 37 subjects (accuracy = 74%) from the local to the central pathology. The blinded tertiary pathology reviewer agreed with central in all Test Set 2 cases. This included 24/34 MDS cases that had been labeled as Other by local pathology (positive predictive value [PPV]=0.89). 3/16 final pathology-classified Other cases were mis-classified as MDS by the local pathologist (negative predictive value [NPV] = 0.57). Next, we assessed the ability of the model to predict MDS vs. Other for 52 additional independent subjects using the third pathologist's diagnosis to break any ties (Test Set 3). The classifier correctly predicted 15/21 MDS cases (PPV=0.83) and misclassified 6/31 Others as MDS (NPV=0.82). The overall accuracy was 83%. Conclusions: We identified that VAFs for 7 genes can correctly re-classify subjects as either MDS or Other in 74% of cases that were misclassified between local and central pathology review. Further assessment on an independent cohort showed an accuracy of 83% of the model. Taken together, these data suggest that complementing pathology reviews with targeted sequencing of 7 genes could improve MDS diagnosis. Disclosures Lindsley: MedImmune: Research Funding; Jazz Pharmaceuticals: Consultancy, Research Funding; Bluebird Bio: Consultancy; Takeda Pharmaceuticals: Consultancy. Bejar:Aptose Biosciences: Current Employment; AbbVie/Genentech: Honoraria; Astex/Otsuka: Honoraria; Takeda: Honoraria, Research Funding; Celgene/BMS: Honoraria, Research Funding; Daiichi-Sankyo: Honoraria; Forty-Seven/Gilead: Honoraria; Genoptix/NeoGenomics: Honoraria. DeZern:MEI: Consultancy; Astex: Research Funding; Abbvie: Consultancy; Celgene: Consultancy, Honoraria. Foran:H3Biosciences: Research Funding; Aptose: Research Funding; Kura Oncology: Research Funding; Trillium: Research Funding; Takeda: Research Funding; Revolution Medicine: Consultancy; Xencor: Research Funding; Agios: Honoraria, Research Funding; Aprea: Research Funding; Actinium: Research Funding; Servier: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees; 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; Boehringer Ingelheim: Research Funding. Gore:Abbvie: Consultancy, Honoraria, Research Funding. Komrokji:Acceleron: Honoraria; Incyte: Honoraria; Abbvie: Honoraria; Agios: Speakers Bureau; BMS: Honoraria, Speakers Bureau; Jazz: Honoraria, Speakers Bureau; Geron: Honoraria; Novartis: Honoraria. Maciejewski:Alexion, BMS: Speakers Bureau; Novartis, Roche: Consultancy, Honoraria. Padron:Novartis: Honoraria; BMS: Research Funding; Incyte: Research Funding; Kura: Research Funding. Starczynowski:Captor Therapeutics: Consultancy; Tolero Therapeutics: Research Funding; Kurome Therapeutics: Consultancy, Current equity holder in private company, Research Funding. Sekeres:BMS: Consultancy; Takeda/Millenium: Consultancy; Pfizer: Consultancy.

AbstractSolid organ and hematopoietic stem cell transplantation may be complicated by the development of post‐transplant lymphoproliferative disorders (PTLDs). The World Health Organization categorizes PTLDs into four entities including non‐destructive, monomorphic, polymorphic, and classical Hodgkin lymphoma types. The most common PTLDs are B‐cell lymphomas, with T‐cell lymphomas accounting for only a few cases. Cutaneous T‐cell lymphomas are rarer still in post‐transplant patients with primary cutaneous peripheral T‐cell lymphoma being an extraordinarily rare subtype in this population. PTLDs may be aggressive and are often associated with high morbidity and mortality. Advances in medicine have led to increased awareness of PTLDs and improved diagnostic tools which assist in the diagnosis of these conditions. However, the clinical and histopathologic heterogeneity of PTLDs may make diagnosis a challenge. In the transplant patient population, the cutaneous manifestations of the lymphoproliferative disease may mimic other conditions, such as eczematous dermatitis and graft‐vs‐host disease. Herein, we report a case of post‐transplant primary cutaneous peripheral T‐cell lymphoma not otherwise specified (PTCL‐NOS) in a pediatric heart transplant patient and describe the clinical presentation and diagnostic histopathologic features.