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Introduction Molecular MRD assays targeting NPM1 mutant (mut) transcripts have an established role for monitoring treatment efficacy in patients with NPM1mut AML. Approximately 25% of NPM1mut patients show persistent MRD level in the bone marrow (BM) at the end of treatment (EOT), which is associated with a higher risk of relapse (Ivey, NEJM 2016; Kronke, JCO 2011). Molecular persistence at low copy number (MP-LCN) is defined by the European LeukemiaNet (ELN) as MRD positivity in patients in morphological complete remission (CR) with Methods Consecutive patients with newly diagnosed NPM1mut AML receiving at least 2 cycles of intensive chemotherapy were included if MRD remained detectable at EOT. UK patients were all enrolled in the NCRI AML17 and AML19 studies. Australian patients were retrospectively identified. BM samples were analyzed by either RT-qPCR (n=75) (Ivey, NEJM 2016) or ultra-deep next-generation sequencing (UD-NGS, n=5, excluded from Cox model). We applied the ELN definitions for molecular progression or relapse (considered together as molecular failure [MF]), MP-LCN and complete molecular remission (CRMRD-). Kaplan Meier survival estimates were calculated from EOT to death (OS), and/or morphologic relapse (RFS), and/or MF (EFS). The optimal NPM1mut expression threshold was pre-determined by recursive partitioning. Variables with p Results A total of 80 patients had NPM1mut detectable at a median of 37 days (range 19-90; 12 [15%] were >60 days) from last course of chemotherapy; characteristics summarized in Table 1. The majority of patients (89%) had received 3-4 cycles of chemotherapy. At EOT, the median NPM1mut transcript level by RT-qPCR was 11 copies (range 0.3-8699; 3 were >2000) and median reduction from baseline was 4.7 log (range 2.1-6.0). A total of 34 patients (45%; 4 not monitored) fulfilled the criteria for MP-LCN. With a median follow-up time of 24 mo (range 8-77), 27 (34%) spontaneously achieved a sustained CRMRD- for >6 months, 31 (39%) had MF and 13 (16%) had morphologic relapse without preceding MF (due to rapid relapse [n=8], lack of monitoring [n=3], extramedullary or NPM1 wild-type relapse [n=1 each]) (Figure 1). Nine patients (11%) had persisting MP-LCN (included transient MRD negativity): median interval between EOT to the last known positive sample was 12 mo (range 3-32), and all patients currently remain alive and without morphologic relapse by the data cut-off date July 13, 2020. Next, we examined the variables associated with MF and/or morphologic relapse. In univariate analysis, factors associated (p Conclusions We describe the natural history of patients with NPM1 mutated AML who remain MRD positive after completing intensive chemotherapy. Almost half either spontaneously achieve MRD-negativity or have stable low-level expression without relapse with minimum follow up of 8 mo. Patients with both FLT3-ITD and Disclosures Tiong: Pfizer: Consultancy; Servier: Consultancy; Amgen: Consultancy, Honoraria. Dillon:Abbvie: Honoraria, Research Funding; Novartis: Honoraria; Menarini: Honoraria; Astellas: Honoraria; Pfizer: Honoraria, Research Funding; Jazz Pharmaceuticals: Honoraria; Amgen: Honoraria, Research Funding. Bajel:Abbvie: Honoraria; Pfizer: Honoraria; Amgen: Honoraria, Speakers Bureau; Novartis: Honoraria; Astellas: Honoraria. Russell:Pfizer: Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Speakers Bureau; Astellas: Membership on an entity's Board of Directors or advisory committees; Jazz: Research Funding, Speakers Bureau. Wei:MacroGenics: Consultancy, Honoraria; Pfizer: Honoraria; Servier: Consultancy, Honoraria, Research Funding; Walter and Eliza Hall Institute: Other: former employee and receives a fraction of its royalty stream related to venetoclax; Celgene: Consultancy, Honoraria, Research Funding; AbbVie Inc.: Consultancy, Honoraria, Research Funding; Genentech: Honoraria; Astra Zeneca: Honoraria, Research Funding; Janssen: Consultancy, Honoraria; Astellas: Consultancy, Honoraria; Novartis: Consultancy, Honoraria, Research Funding; Amgen: Consultancy, Honoraria, Research Funding.

Introduction/ Background Traditionally, MM harbouring t(11;14) is considered standard risk disease. In the era of novel therapies, however, pts with t(11;14) are reported to have inferior outcomes to other standard risk pts. Evidence of response to the BCL-2 inhibitor, Venetoclax (Ven), has further increased interest in understanding outcomes of t(11;14) pts. In this study, we aimed to describe the historical outcomes of t(11;14) MM and response to Ven in a real-world cohort of Australian pts. Methods This was a retrospective, multicentre study conducted by members of the Australasian Leukaemia and Lymphoma Group, Myeloma Working Party. Cases were identified by interrogation of cytogenetics/FISH database from 2010 to 2019 inclusive. Baseline patient and disease characteristics, treatment exposure and outcomes were extracted from hospital medical records. Descriptive statistics, and survival analyses were performed as appropriate. Results Seventy-four pts [median age 65 years (yrs), range (43-85)] were identified across seven centres. 43% pts had ISS Stage III MM with 88% harbouring additional cytogenetic abnormalities, incl. 13% with gain in 1q and 12% with del(17p). The majority (81%) of pts received proteasome inhibitor (PI)-based 1 st line therapy with 60% having an upfront autologous stem cell transplant (ASCT) and 54% having immunomodulatory drug (IMiD)-based maintenance therapy. Two patients received an allogeneic stem cell transplant after ASCT. The overall response rate (ORR) was 86% with 38% achieving very good partial response (VGPR) or better. The median progression free survival-1 (PFS-1) was 1.91 yrs (95% CI 1.73-2.56) [PI-based, n=60, PFS 1.84yrs (95% CI 1.61-2.41) vs IMiD-based, n=5, PFS 4.58yrs (95% CI 1.16-5.51), HR 0.68 p=0.45] The median overall survival (OS) was 5.35 yrs (95% CI 4.12-6.56). Second and third line therapy was predominantly IMiD-based (see Table 1) with recent introduction of anti-CD38 monoclonal antibodies (mAbs). Median PFS-2 was 0.77 yrs (95% CI 0.39-0.98) while median PFS-3 was 0.65 yrs (95% CI 0.34-1.16). Eleven pts (median 3 prior lines of therapy) were given Ven [Six pts in combination with PI, three with PI and mAbs and two with dexamethasone]. ORR to Ven was 55% with 45% achieving VGPR or better. Median PFS with Ven was 0.54 yrs (85% CI 0.05-2.17). Median PFS for patients with 1-4 lines (n=6) was 1.22 years and median PFS for patients with 5 lines or more (n=5) was 0.54 years, HR 0.56 (95% CI 0.12-2.5). Conclusion For pts harbouring t(11;14), the duration of response to PI-based 1 st line therapy is suboptimal even with the use of ASCT consolidation in the majority of patients. Despite multiple prior lines of therapy, Venetoclax shows promising results and every effort should be made for early identification of this cytogenetic lesion. Further studies are required to examine the impact of novel triplet and quadruplet combination therapy and use of venetoclax early in the disease course of this sub-group of patients. Figure 1 Figure 1. Disclosures Talaulikar: Takeda: Honoraria, Research Funding; Amgen: Honoraria, Research Funding; Jansenn: Honoraria, Research Funding; Roche: Honoraria, Research Funding; EUSA Pharma: Honoraria, Research Funding. Gibbs: Janssen, Celgene, Amgen, Takeda, BMS and Pfizer: Consultancy, Honoraria; AbbVie: Consultancy.

Introduction EBV reactivation post allogeneic stem cell transplantation (alloSCT) occurs due to profound suppression of T-cell number or function and subsequent reactivation of latent host or recipient EBV infection through loss of immune-surveillance. The relationship of EBV reactivation to patient outcomes has not been reported in patients who have received T-cell depletion (TCD) with Thymoglobulin. This analysis assessed the outcomes in patients who had previously received an alloSCT with TCD using Thymoglobulin and who developed EBV reactivation that either required treatment (EBV Treatment) vs those who did not require treatment (EBV No Treatment) vs those who had no evidence of EBV reactivation (No EBV). Methods This was a retrospective review of all patients who received TCD with Thymoglobulin prior to alloSCT at the Royal Melbourne Hospital between 2006-2017. Eligible patients received reduced intensity conditioning (RIC) or myeloablative conditioning (MAC) for acute leukemia (AL) or myelodysplastic syndrome (MDS). EBV was monitored weekly via semi-quantitative PCR . Preemptive rituximab was given based on peak viral loads, rate of rise in viral loads or biopsy proven post-transplant lymphoproliferative disorder (PTLD). Categorical and continuous baseline characteristics of the cohorts were compared using chi-squared and Kruskal-Wallis tests respectively. EBV Treatment was treated as a time-dependent variable and evaluated for its relationship to overall survival (OS) using Cox proportional modelling, adjusted for age and high or very high Disease Risk Index (DRI) and grade III-IV acute GVHD. Survival estimates were performed by Kaplan-Meier product-limit method. Cumulative incidence (CI) of relapse and non-relapse mortality (NRM) were calculated by the Fine-Gray method. Results Of 165 eligible patients, 101 (61%) had an episode of EBV reactivation at a median of 46 days post-transplant (range 10-221 days). 24 received therapy for EBV. Of these, 4 had biopsy-proven PTLD, and 23 received rituximab alone with 1 receiving R-CHOP for PTLD. An undetectable EBV viral load was achieved in 20 out of 24. The baseline characteristics of the subgroups are presented in Table 1. The median survival of the whole cohort was 79 months with an estimated 2-year OS of 60% (95%CI 56-68) and a median follow up of time of 81 months in survivors. The median survival of the EBV Treatment group was 14 months, compared with 121 months and 81.1 months in the EBV No Treatment and No EBV cohorts respectively (Figure 1). Table 2 lists causes of death per group with relapse and complications of poor graft function (PGF) being the leading causes of death in the EBV Treatment group. The incidence of grade III-IV acute GVHD at 100 days was similar between groups as was the incidence of any chronic GVHD at 6 and 12 months. EBV Treatment was significantly associated with poorer OS even when adjusting for age, high/very high DRI and presence of acute GVHD (HR 2.47 CI 1.46-4.17; p=0.0007). The EBV Treatment group had a significantly higher 2 year CI of NRM compared to the EBV Treatment and No EBV groups (38% vs 11% vs 18% p=0.006) but similar incidence of relapse (Figure 2) . Conclusion In this cohort of AL/MDS patients undergoing TCD alloSCT, those who required treatment for EBV reactivation experienced poorer survival due to increased NRM. This increase in NRM may reflect impaired immune reconstitution of which significant EBV reactivation is a manifestation. Further studies to understand the underlying immune defects post alloSCT that predispose patients to significant EBV reactivation and associated complications is warranted to identify immune recovery strategies that may be employed in this poor prognosis group. Disclosures No relevant conflicts of interest to declare.

T-cell reconstitution after allogeneic haematopoietic stem cell transplantation (alloSCT) is critical for protection against infection and to mediate the graft versus leukemia (GVL) effect against hematological malignancies including acute myeloid leukemia (AML). T-cell reconstitution post-alloSCT is significantly impacted by exogenous factors including T-cell depleting strategies, immunosuppressive medications and the prohibitive effects of graft versus host disease (GVHD) and infection. The early T-cell repertoire post-alloSCT is oligoclonal and clinical events such as infection and GVHD may adversely impact recovery of a diverse TCR repertoire. The objectives of this study were to investigate clinical determinants of TCR diversity at day 100 after alloSCT and the impact of TCR diversity on risk of early AML relapse after alloSCT. Methods Twenty-nine patients who underwent HLA-matched sibling or unrelated donor alloSCT were included in this cohort comprising 16 patients with AML relapse at day 100 to 180 post-alloSCT and 13 control patients who did not relapse post-alloSCT. All patients received unmanipulated peripheral blood or bone marrow stem cells. Anti-thymocyte globulin was administered to all patients receiving unrelated donor stem cells as per institutional practice. Surveillance for cytomegalovirus (CMV) viremia in peripheral blood (plasma) was monitored twice weekly using polymerase chain reaction (PCR) and pre-emptive therapy with intravenous ganciclovir or oral valganciclovir was commenced in patients with plasma viral load of 400 copies/mL or greater. Peripheral blood samples were obtained at day 100 (early time-point). Eleven patients had follow-up samples 1-2 years post-transplant (late time-point). T-cells were isolated using immunomagnetic separation. Following DNA extraction, TCRβ loci deep amplicon sequencing was performed using LymphoTrack TRB. Sequence assembly, annotation and error correction was performed by MiXCR. TCR diversity was quantified using inverse Simpson's diversity index (1/D). Results TCRβ sequencing of the entire cohort of 29 patients was performed with a mean of 454516 sequence reads per patient. Median time from transplant for the early post-alloSCT timepoint was 99 days. Median TCR repertoire diversity (1/D) early post-transplant was 104.3 (IQR 46.5-398.4). TCR diversity was significantly greater in patients who received T-cell replete transplants from matched sibling donors compared with T-cell depleted transplants from unrelated donors (siblings 130.1 [IQR 54-1017] vs unrelated donors 64 [IQR 28.9-96.9]; P=0.04). Early TCR diversity was significantly reduced in recipients who were CMV seropositive prior to transplant compared with seronegative patients (77.5 [IQR 42.4-127.5] vs 718.8 [IQR 75.5-1884]; P=0.01). Twenty patients (69%) developed CMV viremia, defined as any detectable CMV virus in peripheral blood, prior to day 100 post-alloSCT. Early TCR diversity was significantly reduced in patients with CMV viremia within the first 100 days post-alloSCT (83.5 [IQR 42.4-131.5] vs 964.4 [IQR 71.7-2399]; P=0.02). There was no significant difference in TCR diversity at day 100 in patients who had prior acute GVHD compared to those who did not. There was no significant difference in early TCR diversity at the time of AML relapse compared with patients who remained in remission (78.4 [IQR 38-799.2] vs 132.8 [IQR 59.5-753.6]; P=0.22), suggesting that a restricted TCR repertoire early post-transplant is not a mechanism of AML relapse. Eleven patients had serial samples analysed at early (day 100) and late (between 1-2 years post-transplant) timepoints. All patients remained free of leukemia relapse between these two timepoints. Patients with early CMV viremia (prior to day 100) continued to have a significantly reduced TCR diversity late post-transplant compared with patients who did not have early CMV reactivation (33.2 [IQR 27.3-61.4] vs 3868 [IQR 1421-4565]; P=0.006), indicating that early CMV viremia had a persistent effect on post-transplant T-cell recovery extending to 1-2 years post-transplant. Conclusion T-cell depletion and CMV viremia are key determinants of early TCR repertoire diversity post-alloSCT. CMV viremia has persistent and deleterious effects on TCR repertoire late post-transplant. TCR diversity does not impact early AML relapse post-alloSCT. Disclosures Ritchie: Amgen: Consultancy, Honoraria, Research Funding; Pfizer: Consultancy; BMS: Research Funding; Takeda: Research Funding; Beigene: Research Funding; Imago: Research Funding; Novartis: Honoraria; Sanofi: Honoraria. Koldej:NanoString Technologies: Other: Travel grant. Blombery:Novartis: Consultancy; Janssen: Honoraria; Invivoscribe: Honoraria.