Abstract: Abstract 999 All-trans retinoic acid (ATRA) is used successfully to treat acute promyelocytic leukemia (APML), however, to date it has not shown promise in treating other AML subtypes. ATRA has been shown to enhance hematopoietic stem cell (HSC) self-renewal (requiring RARγ activation) but promotes differentiation of myeloid progenitors likely through RARα activation. We hypothesized that (1) the lack of success of ATRA in treating other AML subtypes may be due to the potential ability of ATRA to enhance self-renewal of the leukemic stem cell and (2) the use of a specific RARα agonist may have more promise in enhancing AML differentiation. We therefore compared the effects of pharmacological levels (1μM) of ATRA and an RARα-specific agonist, NRX195183, on bone marrow cells harvested from a Cre-inducible conditional AML1-ETO (AE) knock-in murine model. AE cells cultured for 2 weeks with ATRA showed significant reductions in the proportions of mature myeloid cells (Gr1brightCD11b+) by fluorescence activated cell sorting (FACS) (DMSO: 14.2±4.3%, ATRA: 4.0±1.6%, p=0.04, n=4). By 4 weeks of culture, ATRA-treated AE cells had increased blast and reduced maturing myeloid cell proportions (Blasts %: DMSO 70.2 ± 3.0, ATRA 95.3 ± 1.2, p=0.08; Intermediate %: DMSO 14.3 ± 2.6, ATRA 3.8 ± 1.0, p=0.01; Neutrophils %: DMSO 2.3± 1.0, ATRA 0.3 ± 0.2, p=0.07, n=6). Furthermore, ATRA potentiated the clonogenicity of the AE cells after 5 weeks of treatment in vitro (Mean±SEM for colony #/ 5×104 cells: DMSO 505.8±337.0, ATRA 4394±388.9, p=0.001; n=6). In contrast, AE cells cultured for 2 weeks with NRX195183 showed significant increases in the proportions of mature myeloid cells by FACS (DMSO: 15.8±3.5%, NRX195183 26.7±3.0%, p=0.03; n=5). By 4 weeks of culture, NRX195183-treated AE cells had decreased blast and increased maturing myeloid cell proportions (Blasts %: DMSO 82.4±3.0, NRX195183 58.8±9.1, p=0.03; Intermediate %: DMSO 14.5±2.5, NRX195183 29.0±6.8, p=0.07; Neutrophils %: DMSO 1.6±0.8, NRX195183 8.2±4.7 p=ns; DMSO n=8, NRX195183 n=5). Moreover, NRX195183 reduced the clonogenicity of the AE cells after 5 weeks of treatment in vitro (Mean±SEM for colony #/ 5×104 cells DMSO 554.8±252.6, NRX195183 82.6±61.6, p=0.05; n=8). Short-term in vivo transplants of fetal liver cells overexpressing the truncated AE gene, AE9a, into sublethally irradiated recipients revealed similar findings in the NRX195183-treated mice with a decrease in blasts and an increase in mature neutrophils in the peripheral blood on morphological analysis after 4 weeks of treatment (Blasts x106/ml: DMSO 3.1±1.0, NRX195183 0.9±0.3, p=0.08; Neutrophils x106/ml: DMSO 0.5±0.1, NRX195183 0.8±0.1, p=0.04; DMSO n=16, NRX195183 n=11). Taken together, these findings support a model whereby ATRA promotes self-renewal of leukemic blasts whilst NRX195183 has the opposing effect. To understand the mechanism by which ATRA promotes self-renewal in AE cells, we performed genome-wide gene expression analyses on the ATRA- versus control-treated AE cells. This revealed 16 differentially upregulated genes after 24 hours of treatment. Using Ingenuity Pathway Analysis, the top scoring network in the ATRA-treated AE cells was cell-to-cell signalling and interaction (p=1.1E-7-2.4E-3), lipid metabolism (p=2.3E-7-2.0E-3) and small molecule biochemistry (p=2.3E-7-2.1E-3); SERPINE1 and BMP2 were the genes with the highest connectivity within the network interacting with molecules known for their roles in tumorigenesis, including AKT, NF-kβ complex and TGFβ1. SERPINE1 upregulation has been shown to be RARα-mediated whilst BMP2 has been shown to be a RARγ-regulated gene. Interestingly, the specific RARγ agonist, NRX204723, had no effect on the clonogenic potential of these AE progenitors thus raising the hypothesis that both RARα and RARγ activation are required to promote self-renewal of the AE progenitors. Further studies using both RARα/RARγ agonists are warranted to assess if the ATRA effects on AE cells are phenocopied. Collectively, these findings reveal the contrasting roles of specific RARα activation in promoting loss of self-renewal ability and enhancing differentiation in the AE cells whilst ATRA promotes clonogenicity of these cells. This has potential significant implications in AML treatment as specific RARα agonists may be beneficial in improving the efficacy of current treatment modalities to achieve sustained remission in other AML subtypes. Disclosures: No relevant conflicts of interest to declare.

Abstract: Poor graft function (PGF), defined by the presence of multilineage cytopenias in the presence of 100% donor chimerism, is a serious complication of allogeneic stem cell transplant (alloSCT). Inducers or potentiators of alloimmunity such as cytomegalovirus reactivation and graft-versus-host disease are associated with the development of PGF, however, more clinical studies are required to establish further risk factors and describe outcomes of PGF. The pathophysiology of PGF can be conceptualized as dysfunction related to the number or productivity of the stem cell compartment, defects in bone marrow microenvironment components such as mesenchymal stromal cells and endothelial cells, or immunological suppression of post-alloSCT hematopoiesis. Treatment strategies focused on improving stem cell number and function and microenvironment support of hematopoiesis have been attempted with variable success. There has been limited use of immune manipulation as a therapeutic strategy, but emerging therapies hold promise. This review details the current understanding of the causes of PGF and methods of treatment to provide a framework for clinicians managing this complex problem.

Abstract: Introduction Understanding the economic impact of managing allogeneic hematopoietic stem cell transplant (HSCT) recipients with cytomegalovirus (CMV) is important for future planning within institutional transplant programs. CMV remains the most frequent viral infection following HSCT of which the clinical impact on transplant outcomes has been well described. However, much less is known about the impact of CMV on health resource utilisation, re-admissions and hospital costs. In addition to antiviral therapy, there are nursing, medical and pharmacy costs to consider. We therefore undertook a study to evaluate the clinical and economic burden of CMV infection following HSCT in a large Australian transplant centre operating under a universal health care system. Methods A retrospective single centre study at the Royal Melbourne Hospital, Melbourne, Australia was performed on all consecutive allogeneic HSCT recipients between January 2015 to December 2017. CMV pre-emptive monitoring using quantitative CMV plasma viral load was performed twice weekly from time of transplant to 100 days or longer in the presence of graft versus host disease. Clinically significant CMV (csCMV) was defined as patients receiving anti-CMV treatment, often with a plasma CMV viral load & gt;400 IU/ml. Throughout the study period, the first line anti-CMV therapy was ganciclovir; either as oral valganciclovir for outpatient management in asymptomatic patients or IV ganciclovir as an inpatient for patients with concerns about oral absorption. Second-line therapy was IV foscarnet. Hospital costing data for the first and subsequent re-admissions for the first 12 months were obtained from the business intelligence unit. Financial year costing was available for FY2015/2016 to FY2017/2018. Ethics was approved by the Melbourne Health Human Ethics Review Committee (HREC 2017.368). Results A total of 255 patients underwent alloHSCT with a median age of 51 years (IQR 40-59) with the most common underlying diagnoses being AML (41%), ALL (11%) and MDS (11%) (Table 1). Thirty-one percent of transplants used myeloablative conditioning, 54% had unrelated donors and 3% had an umbilical cord source. Pre-transplant recipient CMV seropositivity was 62% (n=158), of whom 139 had detectable CMV viremia and 104 (40.8%) experienced clinically significant CMV (csCMV). The median duration of CMV treatment was 33 days (IQR 21-63). Re-admission to hospital within the first 12 months of HSCT occurred in 78.4%. There was a greater number of admissions observed in csCMV patients compared to no csCMV (median 3 vs 2 admissions, p=0.001) with the duration of admitted days within the first 12 months being significantly greater in csCMV patients compared to no csCMV (median 65 vs 36 days, p & lt;0.00001). The mean total cost of treating patients with csCMV for the first 12 months compared to the total cost for patients not requiring CMV treatment was A$196,822 (US$147,616) and A$114503 (US $85,877) (p & lt;0.0001), respectively. Therefore the crude attributable mean cost of treating csCMV was A$82,319 (US$61,739) per patient for the first 12 months of HSCT. The greatest significant contributory costs were from pharmacy A$17,807 (US$13,355), nursing A$16,944 (US$12,708) and medical A$5,898 (US$4,423). Conclusions The health care cost and resource utilisation of treating CMV infection following an allogeneic HSCT is substantial and places a heavy burden on limited health resources. In this study, patients experiencing csCMV had an increased number and longer total duration of admissions days compared to patients who did not require CMV treatment. Interventions aimed at reducing the burden of CMV in alloHSCT recipients are required. Disclosures Yong: Merck Ltd: Honoraria. Bajel:AbbVie: Membership on an entity's Board of Directors or advisory committees, Other: travel funding. Ritchie:Sanofi: Honoraria; Novartis: Honoraria; Imago: Research Funding; Beigene: Research Funding; Takeda: Research Funding; BMS: Research Funding; Pfizer: Consultancy; Amgen: Consultancy, Honoraria, Research Funding. Slavin:Merck Ltd: Honoraria, Research Funding.

Abstract: Introduction While core-binding factor acute myeloid leukaemia (CBF AML) as defined by t(8;21)(q22;q22) or inv(16)(p13q22)/t(16;16)(p13;q22) has a favourable prognosis, 30-40% of patients still relapse after chemotherapy. Risk factors predictive for relapse include increasing age and white cell count (WCC), poor performance status and adverse-risk cytogenetics. Rising minimal residual disease (MRD) by molecular monitoring and receptor tyrosine kinase (RTK) mutations also predict higher risk of relapse (Yin et. al., Blood 2012; Jourdan et. al., Blood 2013). Aims 1. Identify prognostic markers for CBF AML 2. Determine significance of persistent molecular positivity in complete remission (CR) post-treatment Methods We undertook a retrospective audit from 2001-2012 at 4 tertiary-level hospitals. The inclusion criteria were adult patients 〉 18 years with de novo CBF AML treated with at least intermediate dose cytarabine. Co-variates included in univariate analysis and then multivariate analysis if predictive of overall survival (OS) and relapse-free survival (RFS) included age, WCC, sex, cytogenetics, RTK mutations (KIT and FLT3) and MRD (RUNX1-RUNX1T1 or CBFB-MYH11 bone marrow (BM) qPCR post-induction (MRD1). Univariate analysis was also performed for BM MRD after each consolidation cycle 1-4 (MRD2-5 respectively). Results 70 patients were identified (Male=39, Female=31; inv(16)=30, t(8:21)=40). The median age at diagnosis was 43 years (range 17-73). There were 2 induction deaths and 68 achieved morphological CR. OS was 70.7% and RFS was 59.1% at a median follow-up of 31.4 months. There were 16 deaths (inv(16)=9, t(8;21)=7; median OS 14.8 months; 9 from relapsed AML, 4 treatment-related complications, 2 graft-versus-host disease (GVHD) and 1 unrelated). 24 relapses (22 morphological, 2 molecular) occurred with a median RFS of 9 months (inv(16)=17, t(8;21)=7). RFS was significantly worse for inv(16) vs t(8;21) (39% vs 75%; p=0.0004). The inv(16) cohort had significantly higher median WCC of 33x109/L vs 10.5x109/L t(8;21). Univariate analysis identified age (p=0.032) and WCC 〉 40 (p=0.002) as significant for inferior OS and RFS respectively. The impact of KIT (n=34, Pos=14, p=0.158) and FLT3 (n=27, Pos=2, p=0.152) mutations on OS/RFS were non-significant independent of CBF AML subtype. However, there were systematic differences in KIT and FLT3 data availability in the relapse vs non-relapse cohorts (Absent data: KIT 17/24(71%) vs 19/46(41%); FLT3 17/24(71%) vs 26/46(57%)). MRD analysis by qPCR at the different timepoints (MRD1=37, MRD2=24, MRD3=28, MRD4=23, MRD5=12) by 〈 or ≥3 log reduction in comparison to pre-treatment values was not predictive of OS/RFS. On multivariate analysis excluding RTK mutations, age was the only significant predictor for OS (p=0.032) and WCC 〉 40 for relapse (p=0.025). Standard vs intermediate/high-dose cytarabine in induction had no impact on OS/RFS but ≥3 consolidation cycles of intermediate-dose cytarabine improved OS vs ≤2 cycles (p=0.035). There was no significant difference in the median age of the cohorts receiving consolidation chemotherapy (≤2: n=37, 47 years, ≥3: n=30, 35.5 years). Median BM qPCR values increased from 0 to 11% at relapse for inv(16) at a median duration of 4.3 months (range 1-8) and 0.05 to 178% for t(8;21) at 6 months (range 5-35) respectively. Of the 43 with durable CR, 28 maintained PB or BM qPCR values of 0-0.1% or 〈 10 copy numbers throughout follow-up for 2 years post-completion of consolidation treatment. 6 had qPCR values 〉 0.1% or 〉 50 copy numbers; 5 were t(8;21) achieving PCR negativity at a range of 9-24 months post-completion of consolidation. Of the 24 who relapsed, 15 proceeded to stem cell transplant (13 allograft: inv(16)=8, t(8;21)=5; 2 autograft: inv(16)). There were 6 deaths in the allograft group (inv(16)=3, t(8;21)=3) and none in the autograft group. 3 deaths were related to relapsed disease post-allograft, 2 GVHD and 1 from non-GVHD complications. Conclusions Age is a significant predictor of OS in our CBF AML cohort while WCC is more predictive of relapse risk. The significance of RTK mutations and MRD is limited by data availability. ≥3 consolidation cycles of intermediate-dose cytarabine improved OS in comparison to fewer cycles. Stable low level MRD did not predict relapse. Regular monitoring of PB and BM qPCR values post-completion of treatment is necessary for prediction of subsequent relapse. Figure 1. Figure 1. Disclosures Grigg: Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees; BMS: Honoraria, Membership on an entity's Board of Directors or advisory committees; Roche: Honoraria, Membership on an entity's Board of Directors or advisory committees; Gilead: Honoraria, Membership on an entity's Board of Directors or advisory committees; Merck: Honoraria, Membership on an entity's Board of Directors or advisory committees; Amgen: Honoraria, Membership on an entity's Board of Directors or advisory committees. Szer:Alexion Pharmaceuticals Australasia Pty Ltd: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Pfizer Australia: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Shire Australia: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Sanofi: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.

Abstract: Introduction Poor Graft Function (PGF) characterized by 2 or more lineage cytopenias in the setting of complete donor chimerism is a recognized complication of allogeneic stem cell transplantation (alloSCT). Recurrent CMV viremia jeopardizes blood count recovery in PGF through direct effect of the virus on bone marrow function as well as myelosuppression/organ toxicity due to widely available antivirals such as val/ganciclovir or foscarnet. Methods To assess the clinical effect and resource utilization of recurrent CMV viremia on patients with PGF we performed a retrospective analysis of alloSCT recipients at our Centre from 2018-2019. Patients were classified as having PGF based on the following parameters: 1) Donor myeloid chimerism ≥95% at last assessment, 2) 2 or more lineage cytopenias defined by Hb ≤85g/L, Neutrophils ≤1.0x10 9/L, Platelets ≤60x10 9/L, 3) cytopenias present for 10 days after D30. CMV viral loads were monitored by q PCR twice weekly with reactivation defined as the first reading quantifiable above the threshold of detection of the assay. Recurrent CMV reactivation was defined as detectable CMV after an interval of 4weeks without detectable virus, with patients who reactivated within 4 weeks of an undetectable viral load defined as prolonged persisting infection. Patients who died before D60 or who relapsed within the 1 st 100 days or those who had a prior alloSCT were excluded. Baseline factors in the CMV seropositive PGF group were evaluated by Chi Squared and Kruskall-Wallis test for categorical and continuous variables respectively to establish any associations with recurrent or persisting CMV. Survival analysis of PGF patients was performed by the Kaplan Meier method with patients stratified by the following: Recurrent CMV/Prolonged persisting infection (RP-CMV), Single CMV reactivation (S-CMV), No CMV reactivations (N-CMV) and CMV sero-negativity (S-NCMV). A descriptive analysis of the number of hospital admissions in addition to the initial alloSCT admission (extra admissions) and total hospital days was also performed. Results There were 155 eligible patients of which 38 (24%) fulfilled criteria for PGF. The median follow-up of the cohort was 26 months (95%CI:23.1-28). The median overall survival (OS) of the cohort was not reached with an estimated 2 year OS of 73%. The 2 year OS of PGF patients was 59% compared to 78% in the non PGF group. There were no significant baseline associations found with RP-CMV (Table 1). Figure 1 demonstrates the survival of patients by nature of CMV reactivation. The 2 year OS by CMV reactivation was as follows: RP-CMV 25%, S-CMV 68%, N-CMV 70%, SN-CMV 87%. On univariate analysis, RP-CMV was significantly associated with mortality [HR 5.41; 95%CI 1.80-16.28; P=0.003] . This association remained significant when including Age and grade III-IV GVHD in multivariate analysis: RPCMV [HR 7.57; 95%CI 2.20-25.9;P=0.0013=], Age [HR 1.03; 95% CI 0.99-1.08; P=0.16] and GVHD [HR 2.91; 95%CI 0.87-9.75; P=0.08]. RP-CMV was not a risk factor for mortality within the NPGF population [HR 0.75; 95%CI 0.29-01.97; P=0.56] . Those with RP-CMV experienced an increase in their CMV viral load within a median of 4 days range (2-86) after achievement of an undetectable result. Those with PGF and RP-CMV had an average of 1.4 extra admissions with an average length of stay of 80 days, those with S-CMV reactivation had an average of 1.6 extra admissions with an average length of stay of 35 days, and those with N-CMV had an average of 1 extra admission with an average length of stay of 17 days. Those with SN-CMV had an average of 1 extra admission with an average length of stay of 23 days. CMV reactivation and CMV therapy contributed to 13/17 (76%) total readmissions for those with RP-CMV compared to 5/13 (38%) readmissions for those with S-CMV. Conclusion Patients with PGF and RP-CMV are at high risk of mortality as well as more lengthy hospital admissions of which CMV reactivation and CMV therapy is a potential contributor. Targeted use of novel therapies to prevent CMV reactivation in patients with PGF may improve both bone marrow function and survival leading to less resource utilization. Figure 1 Figure 1. Disclosures Chee: Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees. Koldej: CRISPR Therapeutics: Research Funding. Ritchie: BMS: Research Funding; Takeda: Research Funding; CRISPR Therapeutics: Research Funding; Amgen Inc: Honoraria, Research Funding; CSL: Honoraria; Novartis: Honoraria.

Abstract: Background Cytomegalovirus (CMV) is a common, potentially devastating complication of allogeneic haematopoietic stem cell transplantation (alloHSCT). Universal antiviral prophylaxis strategies including letermovir are effective, but unsubsidised in Australia. Prophylactic ganciclovir or valganciclovir are challenging due to myelotoxicity. Valaciclovir demonstrates anti-CMV activity in high doses, but little current data explore prophylaxis in the alloHSCT setting, particularly in haploidentical transplantation. We aimed to evaluate the clinical efficacy and tolerability of high dose valaciclovir (high dose VALA) as CMV prophylaxis in high risk patients undergoing alloHSCT. Methods This study was completed at the Royal Melbourne Hospital, Melbourne Australia. We performed a retrospective analysis of alloHSCT recipients at high risk of CMV reactivation (defined as recipient and/or donor CMV seropositivity, and undergoing T-cell depletion, haploidentical or umbilical cord stem cell transplantation). Patients transplanted between July 2018 - June 2019, treated with high dose VALA (2g TDS) from day +7 to +100 and beyond were compared to a historical cohort (transplanted between July 2017 - June 2018) on standard dose valaciclovir (std dose VALA) (500mg BD until engraftment then 500mg daily). We compared the rates and time to reach a CMV threshold of 400 IU/ml, at which point pre-emptive CMV therapy was commenced. Tolerability was also evaluated. Results Patient demographics are described in Table 1. Of the standard dose VALA cohort, (median follow-up 259 days), 23/31 (74%) developed a viral load & gt;400 IU/mL, requiring pre-emptive CMV therapy. None had CMV disease. Median time to viral load & gt;400 IU/mL was 39 days (range 13 - 68). Of the high dose VALA cohort (median follow-up 209 days), 11/25 (44%) developed a viral load & gt;400 IU/mL, requiring pre-emptive CMV therapy. Of these 11 cases, 7 patients had viral load & gt;400 IU/mL while on high dose VALA prior to D+100, 3 patients had ceased high dose VALA prior to D+100 due to intolerance and in 1 patient this occurred post D+100 while on high dose VALA. One patient developed CMV (gut) disease following early cessation of high dose VALA, whilst on standard dose VALA. Median time to reactivation & gt;400 IU/mL was 64 days (range 26-170). Time to reactivation & gt;400 IU/ml was significantly different between the standard vs high dose VALA cohorts (mean ± SEM; 37.9 ± 2.7 vs 67.8 ± 11.3 days, **p=0.0015). Median duration of high dose VALA prophylaxis was 50 days (range 11-288). Seven (28%) patients continued high dose VALA to day +100 and beyond. Intolerance led to early cessation in 10 (40%) patients (acute kidney injury, n=6; cytopenia, n=3; both, n=1). Other patients ceased due to requirement for definitive CMV therapy (n=6) and unclear reasons (n=2). Conclusions In high risk alloHSCT recipients, high dose VALA is an effective CMV prophylactic strategy resulting in lower CMV reactivation rates, and delays CMV reactivation. This may reduce requirements for myelotoxic CMV treatment during the early post-engraftment period and need for inpatient admission. CMV infection following high dose VALA cessation remains a risk, particularly when dose reductions have occurred due to toxicity, and intolerance and ongoing monitoring is required. Treatment tolerability remains a limitation. Disclosures No relevant conflicts of interest to declare. OffLabel Disclosure: Valaciclovir, for CMV prophylaxis

Abstract: Background: Myelodysplastic syndromes (MDS) are characterized by ineffective hematopoiesis leading to cytopenias, including anemia and thrombocytopenia. KER-050, a modified activin receptor type IIA inhibitor, is designed to target transforming growth factor-β ligands, including activin A. In preclinical studies, KER-050 promoted the maturation of progenitors across the full spectrum of erythropoiesis and thrombopoiesis and elicited bone anabolic effects. In a Phase 1 study in healthy participants, KER-050 treatment resulted in robust and sustained increases in reticulocytes (RETs), hemoglobin (HGB), and platelets. Increases in the bone formation marker bone specific alkaline phosphatase were also observed. Here we report results of an ongoing Phase 2 study to evaluate whether KER-050 provides therapeutic benefit in MDS patients with anemia. Aims: Evaluate safety, tolerability, pharmacodynamics and efficacy of ascending doses of KER-050 in participants with MDS in an open-label, 2-part Phase 2 study. Methods: IPSS-R very low-to-intermediate risk MDS patients (both RS+ and non-RS) with anemia (HGB & lt;10g/dL or requiring RBC transfusions) are enrolled. In Part 1, ascending dose cohorts receive KER-050 subcutaneously every 4 weeks for 4 doses starting at 0.75mg/kg until a recommended Part 2 dose is determined. Part 2 dose expansion will begin following Part 1, with treatment extended to 2 years. Safety endpoints include incidence of adverse events (AEs); erythroid efficacy endpoints (≥8 weeks duration) include rates of transfusion independence (TI) in transfused participants, reduction in RBC transfusions by ≥4 units or ≥50% reduction in high transfusion burden participants (HTB) and HGB increase ≥1.5g/dL in non-transfused (NT) and low transfusion burden (LTB) participants. Results are reported for efficacy-evaluable participants in cohorts 1 and 2 of Part 1 dose escalation, defined as having ≥8 weeks of HGB and transfusion data. Results: At data cut-off (July 10, 2021) with median follow-up of 140 days (range 1 to 169 days), 17 participants had received ≥1 dose of KER-050 across 3 dose levels: 0.75 mg/kg, 1.5 mg/kg and 2.5 mg/kg. Baseline characteristics are described in Table 1. No related serious AEs, dose-limiting toxicities, or dose modifications were reported. One participant developed grade 2 maculopapular rash after the first dose which was considered treatment related, resolved and did not recur with subsequent doses. No other related AEs were reported. Two discontinued study drug prior to end of treatment: 1 due to participant decision, 1 due to death unrelated to study drug. None developed high risk MDS or AML. In 10 efficacy-evaluable participants, overall erythroid response rate was 60% (n=6/10). 33% (n=1/3) NT participants had a HGB increase of ≥1.5g/dL sustained ≥ 8 weeks. 5 of 7 transfused participants (71%) (n=1/2 LTB and n=4/5 HTB; n=2/3 non-RS and n=3/4 RS+) had erythroid responses sustained ≥8 weeks (range 8-20 weeks, ongoing) and 57% (n=4/7) achieved TI (Figure 1, Panel A). Maximum increase from baseline in RETs observed in transfused responders (TR) (n=5) was 24.6 x10 9/L (mean), range 10.5- 41.6 x10 9/L from day 1-29 with increases in RETs seen after each dose (Panel B). Maximum reduction in serum ferritin in TR was 40.4% (mean), range 10-66%, and maximum increase in soluble transferrin receptor (sTfR) was 52.8% (mean), range 29.8-116.4%. Increases in platelets were observed in TR (Panel C). Mean baseline platelet count for TR was 234 x10 9/L (range 104-401 x10 9/L), and maximum increase from baseline was 130 x10 9/L (mean), range 32-235 x10 9/L. No participants required dose reduction due to thrombocytosis. Summary: Erythroid responses have been observed in RS+ and non-RS MDS patients including reduction in transfusion burden at the initial dose levels. Observed increases in RETs and sTfR and observed decreases in ferritin suggest that KER-050 treatment is potentially associated with increased erythropoiesis. Increases in platelets have been observed in TR. These data support the potential of KER-050 as a treatment for multilineage cytopenias in MDS by potentially targeting multiple stages of hematopoiesis. As of data cut-off, KER-050 has been well tolerated. Dose escalation is ongoing in this Phase 2 study of anemic patients with MDS; data from planned cohorts from Part 1 will be presented. Part 2 dose expansion phase is expected to initiate prior to the meeting. Figure 1 Figure 1. Disclosures Ross: Bristol Myers Squib: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Keros Therapeutics: Consultancy, Honoraria. Arbelaez: Amgen: Other: Travel, Accommodations, Expenses. Chee: Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees. Fong: AbbVie: Consultancy; Amgen: Consultancy; Astellas: Consultancy, Research Funding; Bristol Myers Squibb: Consultancy; Novartis: Consultancy, Speakers Bureau; Phizer: Consultancy; Novotech: Honoraria; Specialised Therapeutics: Honoraria. Hiwase: Novartis: Membership on an entity's Board of Directors or advisory committees; AbbVie: Membership on an entity's Board of Directors or advisory committees. Wight: Jannsen: Honoraria, Other: Travel subsidies; Abbvie: Honoraria, Other: Travel subsidies. Rovaldi: Keros Therapeutics: Current equity holder in publicly-traded company. Furutani: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Gaggi: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Jiang: Keros Therapeutics: Current Employment, Current holder of stock options in a privately-held company. Lachey: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees. Natarajan: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company. Ordonez: Keros Therapeutics: Current Employment, Current equity holder in publicly-traded company, Membership on an entity's Board of Directors or advisory committees.

Abstract: INTRODUCTION: Asciminib is the first BCR-ABL1 inhibitor that potently inhibits kinase activity of the BCR-ABL1 oncoprotein by Specifically Targeting the ABL Myristoyl Pocket (STAMP). The primary analysis from ASCEMBL, a randomized, phase 3 trial, demonstrated that asciminib has superior efficacy and a better safety and tolerability profile than bosutinib (BOS) in patients (pts) with chronic myeloid leukemia in chronic phase (CML-CP) after ≥2 prior ATP-binding tyrosine kinase inhibitors (TKIs). The major molecular response (MMR) rate at wk 24 was 25.5% with asciminib vs 13.2% with BOS; the difference in MMR rates, after adjusting for major cytogenetic response status at baseline, was 12.2% (95% CI, 2.19-22.30; 2-sided P=.029). Grade ≥3 adverse events (AEs) were reported in 50.6% and 60.5% of pts receiving asciminib and BOS, respectively, and AEs leading to discontinuation in 5.8% and 21.1%, respectively. After a median follow-up of 19.2 months (or 7.5 months' additional follow-up since the primary analysis), we report updated efficacy and safety results from ASCEMBL. METHODS: Adults with CML-CP treated with ≥2 prior TKIs were randomized 2:1 to asciminib 40 mg twice daily (BID) or BOS 500 mg once daily (QD). Eligible pts must have experienced treatment failure (lack of efficacy) per 2013 European LeukemiaNet recommendations for second-line TKI therapy or intolerance of the most recent TKI at screening. Pts intolerant of their most recent TKI were eligible only if they had BCR-ABL1 on the International Scale & gt;0.1% at screening. Pts with known BOS-resistant T315I or V299L mutations were ineligible. The cutoff date for the current analysis was January 6, 2021. RESULTS: A total of 233 pts were randomized to receive either asciminib (n=157) or BOS (n=76). At cutoff, all randomized pts had completed their wk 48 visit or had discontinued earlier. Treatment was ongoing in 89 (56.7%) and 17 (22.4%) pts on asciminib and BOS, respectively; the most common reason for treatment discontinuation was lack of efficacy in 37 (23.6%) and 27 (35.5%) pts, respectively (Table 1). By wk 48, the cumulative incidence of MMR was 33.2% with asciminib and 18.6% with BOS, showing that the difference between the 2 treatment arms observed by wk 24 in the primary analysis was maintained with longer follow-up (Figure 1). The cumulative incidence of BCR-ABL1IS ≤1% by wk 48 in patients without this level of response at baseline was 50.8% with asciminib and 33.7% with BOS. The difference in deep molecular responses (MRs) favoring asciminib vs BOS persisted by wk 48: MR 4 (BCR-ABL1IS ≤0.01%) and MR 4.5 (BCR-ABL1IS ≤0.0032%) rates were 14.0% and 9.6% with asciminib and 6.6% and 2.6% with BOS, respectively. With a longer duration of exposure in the asciminib arm, the safety and tolerability profile in the current analysis remains similar to that at the primary analysis. Median duration of exposure was 67.1 wk (range, 0.1-162.1 wk) for asciminib and 29.7 wk (range, 1.0-149.3 wk) for BOS. By the cutoff date, 91.0% of pts on asciminib and 97.4% of pts on BOS reported ≥1 all-grade AE; 54.5% and 67.1% of pts, respectively, reported ≥1 grade ≥3 AE. No additional fatal events were reported since the primary analysis. Fewer pts in the asciminib (7.1%) than BOS (25.0%) arm experienced AEs leading to treatment discontinuation (Table 2). The most common AEs leading to treatment discontinuation included thrombocytopenia (3.2%) and neutropenia (2.6%) in the asciminib arm and increased alanine aminotransferase (5.3%) and neutropenia (3.9%) in the BOS arm. CONCLUSIONS: Resistant/intolerant CML-CP after 2 lines of TKI treatment remains BCR-ABL1 dependent. The novel STAMP inhibitor asciminib demonstrates continued superior efficacy and a limited adverse event profile; myelosuppression, while more prominent, appears to be early and disease related. Secondary resistance based on loss of MMR is rare. Overall, after a median follow-up of 19.2 months, the positive benefit-risk profile of asciminib vs BOS continues to support the use of asciminib as a new therapy in this heavily pretreated pt population. This study is sponsored by Novartis. Figure 1 Figure 1. Disclosures Mauro: Sun Pharma / SPARC: Research Funding; Pfizer: Consultancy; Bristol Myers Squibb: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Takeda: Consultancy. Minami: Takeda: Honoraria; CMIC: Research Funding; Astellas: Honoraria; Ono: Research Funding; Pfizer Japan Inc.: Honoraria; Novartis Pharma KK: Honoraria; Bristol-Myers Squibb Company: Honoraria. Rea: Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees; Incyte: Honoraria, Membership on an entity's Board of Directors or advisory committees; Pfizer: Honoraria, Membership on an entity's Board of Directors or advisory committees. 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. Lomaia: Novartis: Honoraria; Pfizer: Honoraria; BMS: Honoraria; Pharmstandard: Honoraria. Voloshin: Abbvie: Consultancy, Speakers Bureau; Novartis: Consultancy, Speakers Bureau; Astra Zeneca: Consultancy, Speakers Bureau; Pfizer: Consultancy; Biacad: Consultancy, Speakers Bureau. Turkina: Pharmstandart: Speakers Bureau; Novartis Pharma: Speakers Bureau; Bristol Myers Squibb: Speakers Bureau; Pfizer: Speakers Bureau. Kim: Novartis: Research Funding; BMS: Research Funding; Pfizer: Research Funding; ILYANG: Research Funding; Takeda: Research Funding. Apperley: Bristol Myers Squibb, Novartis: Honoraria, Speakers Bureau; Incyte, Pfizer: Honoraria, Research Funding, Speakers Bureau. Cortes: Sun Pharma: Consultancy, Research Funding; Bristol Myers Squibb, Daiichi Sankyo, Jazz Pharmaceuticals, Astellas, Novartis, Pfizer, Takeda, BioPath Holdings, Incyte: Consultancy, Research Funding; Bio-Path Holdings, Inc.: Consultancy, Membership on an entity's Board of Directors or advisory committees; Pfizer: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; Takeda: Consultancy, Research Funding. Fogliatto: AstraZeneca: Speakers Bureau. Kim: Novartis: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding; Pfizer: Honoraria, Research Funding; Paladin: Honoraria, Research Funding; Bristol-Meier Squibb: Research Funding. le Coutre: Pfizer: Honoraria; Novartis: Honoraria; Incyte: Honoraria; Bristol Myers Squibb: Honoraria. Saussele: Novartis: Honoraria, Research Funding; BMS: Honoraria, Research Funding; Incyte: Honoraria, Research Funding; Pfizer: Honoraria; Roche: Honoraria. Hughes: Novartis: Honoraria, Research Funding; BMS: Honoraria, Research Funding; Incyte: Honoraria. Chaudhri: Abbvie: Honoraria; Novartis: Honoraria; Astra Zeneca: Honoraria. Chee: Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees. Garcia Gutierrez: Pfizer: Consultancy, Honoraria, Research Funding; Incyte: Consultancy, Honoraria, Research Funding; BMS: Consultancy, Honoraria, Research Funding; Novartis: Consultancy, Honoraria, Research Funding. Sasaki: Daiichi-Sankyo: Membership on an entity's Board of Directors or advisory committees; Pfizer: Membership on an entity's Board of Directors or advisory committees; Novartis: Consultancy, Research Funding. Kapoor: Novartis: Current Employment, Current equity holder in publicly-traded company. Allepuz: Novartis: Current Employment, Current equity holder in publicly-traded company. Quenet: Novartis: Current Employment. Bédoucha: Novartis: Current Employment. Boquimpani: Pinth Pharma: Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Jansen: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau.

Abstract: Introduction: Poor graft function (PGF) post allogeneic transplantation is defined as peripheral blood cytopenias in the presence of donor chimerism and is increasingly recognised as a specific clinical entity. The criteria for PGF diagnosis are variable with regards to level of donor chimerism level and duration of cytopenias. Patients with this condition are at high risk of complications from bone marrow failure and need to be differentiated from patients with persisting cytopenias that spontaneously recover without serious complications. We retrospectively analysed patients treated at our centre with complete donor chimerism and cytopenias in order to identify markers of poor prognosis and define an at-risk group that will form the basis of further clinical studies. Method: Patients who underwent allogenic stem cell transplant (alloHSCT) at our centre between 2000-2016 were included in this retrospective analysis after fulfilling the criteria for PGF (1) cytopenias in at least 2 lineages (defined as platelets 〈 100x109/L, neutrophils 〈 1.0x109/L, haemoglobin 〈 100g/L)sustained for 30 days and (2) myeloid chimerism 〉 95%. Patients with relapse of their haematological malignancy within 100 days post-alloSCT were excluded. The primary outcome variables were overall survival (OS) and death from PGF. OS was evaluated in landmark analyses from day 100 (D100) post-alloSCT. Variables were investigated for associations with OS using cox proportional hazards models. Relapse and death without PGF were treated as competing risks for the outcome of death from PGF; associations with PGF were investigated using the method of Fine and Gray. Variables statistically significant at α 〈 0.05 were included in a forward step-wise fashion in multivariate models. Previous studies have demonstrated an excess of early mortality from PGF and hence patients were censored if alive at 24 months. Results: Of 908 transplants performed between 2000-2016, 152 patients (16%) fulfilled our criteria for PGF and were alive at day 100 post transplantation. The overall survival of the PGF cohort at 24 months was 65%. There were 102 patients alive at 24 months vs 50 who died within the 24 months period. The early mortality group had a median survival of 6 months (Range 3.3-23). Demographic details of the early mortality vs those who survived to 24 months are listed in Table 1. Univariate analysis of the PGF cohort identified the following variables were significantly associated with inferior OS: non-CMV viral infection, ICU admission during initial 30 days, age as a continuous variable, grade 2-4 acute GVHD and platelet count ≤40x109 /L (P40) at D100. Age and P40 remained significantly associated with OS on multivariate analysis. Hazard ratios and P values are summarised in Table 2. Competing risk analysis revealed, Age, non-CMV viral infections, ICU admission, MPN as an indication for transplantation, P40 at D100 and grade 2-4 GVHD as significant associations for death from PGF. Age, GVHD and P40 remained significantly associated with death from PGF in multivariate analysis. Hazard ratios and P values are summarised in Table 2. Conclusion: Increasing patient age and marked thrombocytopenia at D100 are associated with inferior survival in patients with PGF. These factors identify patients at higher risk of death from PGF which inform a stricter definition of the syndrome and provide a population for future interventional studies. Disclosures Ritchie: Sanofi: Honoraria; Novartis: Honoraria; Imago: Research Funding; Beigene: Research Funding; Takeda: Research Funding; BMS: Research Funding; Pfizer: Consultancy; Amgen: Consultancy, Honoraria, Research Funding. Koldej:NanoString Technologies: Other: Travel grant.