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Background Based on early evidence of a high rate of coronavirus mortality in patients with acute myeloid leukaemia (AML) undergoing intensive chemotherapy (IC), the national health service (NHS) in the United Kingdom temporarily made venetoclax available as an alternative therapy, with the aim of reducing both mortality and healthcare resource use. From late April 2020, venetoclax was available to patients aged >16y with NPM1 mutation without FLT3 internal tandem duplication (ITD), patients aged >50y with NPM1, IDH1 or IDH2 mutations (regardless of FLT3 status) and patients aged >60y without favourable-risk cytogenetics. Venetoclax could be given with either azacitidine or low-dose cytarabine (LDAC), with the latter recommended mainly for patients with NPM1 mutation. We report a health-system-wide real world data collection for toxicity and patient outcomes across 65 NHS Hospitals. Methods Each patient was registered on a central NHS database. Clinicians certified that their patient met the above criteria, had not received previous AML treatment, and was fit for induction chemotherapy. Anonymised data were retrospectively collected by treating physicians. Venetoclax dose, duration and toxicity information was requested for the first 4 cycles of therapy. Response definitions were as per European Leukaemia Network (ELN) guidelines. A total of 870 patients have been registered on the scheme, with outcomes reported here for those with follow-up information at a data cut on 1st August 2021. Results There were 301 patients, median age 72y (range 34 - 90) with 62% male. The majority (81%) had an ECOG performance status of 0-1. AML was secondary to a previous haematological disorder in 33%, therapy-related in 10% and de novo in the remaining 57%. MRC cytogenetic risk was intermediate in 70% and adverse in 27%. NPM1 mutations were detected in 28% and FLT3-ITD in 12%. Next-generation sequencing results were available in 86% of patients, which detected mutations in IDH1 or IDH2 in 28%, ASXL1 in 20%, RUNX1 in 17% and TP53 in 12%. The ELN risk was favourable for 23%, intermediate for 30% and adverse for 44%. A majority received venetoclax in combination with azacitidine (85%), with the remaining 15% receiving LDAC. The LDAC cohort was enriched for de novo AML (76% vs 54%) and NPM1-mutated disease (56% vs 23%). Most patients (81%) followed the recommended initial schedule of venetoclax 100mg daily for 28 days in combination with posaconazole or voriconazole. Patients spent a median 14 days in hospital in cycle 1, then a median of 0 days for cycles 2-4. In cycles 1, 2, 3 and 4, the median number of days for recovery of neutrophils to >0.5x10 9/L was 33, 25, 24 and 14 respectively, and the median number of days to recovery of platelets to >50x10 9/L was 22, 3, 0 (no drop below 50) and 0. The composite complete remission (CR) / CR with incomplete haematological recovery (CRi) rate was 70%. MRD data is being collected. The best response was morphological leukaemia free state (MLFS) in 2%, partial remission in 7% and refractory disease in 11%. CR/CRi was higher in de novo (78%) compared to secondary AML (57%, p=0.02); NPM1 mutated (78% vs 67%, p=0.02) and IDH1/IDH2 mutated disease (85% vs 62%, p=0.02). ELN favourable risk patients had the highest CR/CRi rate (85%, intermediate 71%, adverse 60%, p=0.01). Median follow-up was 8.2 months (95%CI 7.8 - 9.0) with median overall survival (OS) 12.8 months (95%CI 10.9 - not reached). Mortality at day 30 was 5.7% and day 60 was 8.4%. 12-month overall survival was 51%, increasing to 71% in those who achieved CR/CRi. Survival was poorer in secondary (HR 1.9, p Conclusion This large real-world study demonstrates CR/CRi and survival rates comparable to those reported in prospective clinical trials. Importantly, during the COVID-19 pandemic, the adoption of venetoclax regimens permitted the great majority of treatment to be delivered as an outpatient with significant resource saving at a time of critically constrained inpatient resources. The data support prospective comparisons of venetoclax-based regimens to IC in fit adults with AML particularly in older patients with de novo AML, NPM1-mutated and IDH-mutated disease. Figure 1 Figure 1. Disclosures Belsham: Celgene: Other: meeting attendance; Abbvie: Other: meeting attendance. Khan: Abbvie: Honoraria; Astellas: Honoraria; Takeda: Honoraria; Jazz: Honoraria; Gilead: Honoraria; Novartis: Honoraria. Khwaja: Pfizer: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Novartis: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Jazz Pharmaceuticals: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Astellas: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Abbvie: Membership on an entity's Board of Directors or advisory committees, Speakers Bureau. Latif: Kite: Consultancy, Honoraria, Speakers Bureau; Jazz: Consultancy, Honoraria; Daiichi Sankyo: Consultancy, Honoraria; Novartis: Consultancy, Honoraria; Amgen: Consultancy, Honoraria; Abbvie: Consultancy, Honoraria; Astellas: Consultancy, Honoraria, Speakers Bureau; Takeda UK: Speakers Bureau. Loke: Pfizer: Honoraria; Amgen: Honoraria; Janssen: Honoraria; Novartis: Other: Travel; Daichi Sankyo: Other: Travel. Murthy: Abbvie: Other: support to attend educational conferences.. Smith: ARIAD: Honoraria; Pfizer: Speakers Bureau; Daiichi Sankyo: Speakers Bureau. Whitmill: Daiichi-sankyo: Other: travel fees; EHA in stockholm: Other: conference support. Craddock: Novartis Pharmaceuticals: Other: Advisory Board ; Celgene/BMS: Membership on an entity's Board of Directors or advisory committees, Research Funding. Dillon: Shattuck Labs: Membership on an entity's Board of Directors or advisory committees; Jazz: Other: Education events; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: educational events; Novartis: Membership on an entity's Board of Directors or advisory committees, Other: Session chair (paid to institution), Speakers Bureau; Menarini: Membership on an entity's Board of Directors or advisory committees; Astellas: Consultancy, Other: Educational Events , Speakers Bureau; Amgen: Other: Research support (paid to institution); Abbvie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Research Support, Educational Events.

Background: Post-transplant lymphoproliferative disease (PTLD) confers a high morbidity and mortality in a vulnerable population. We present the epidemiology and outcomes of PTLD in a large UK cohort of solid organ transplant (SOT) recipients who were transplanted over a 20-year period. Methods: This is a retrospective study of 5365 SOT recipients who had their first transplant between 2000 and 2021 at two UK transplant centres (Addenbrooke's Hospital and Papworth Hospital). We reviewed the records of all patients and found 142 who subsequently developed PTLD. For each type of transplant, we calculated the incidence rate of PTLD and cumulative incidence using a competing risk of death model. Survival was compared with the age-adjusted life expectancy of the UK population using the National life tables and a landmark analysis was performed to compare overall survival (OS) of PTLD patients from the date of diagnosis with the background survival of the transplant population. To compare treatment outcomes, a subset of 90 cases of monomorphic PTLD, DLBCL subtype were identified. 66 were treated with first-line Rituximab monotherapy and 24 received first-line R-Chemotherapy. Demographics, treatment response, and survival data were analysed with univariate and multivariate analysis to identify covariates associated with death in the first year post diagnosis of PTLD. Results: With a median follow-up time of 5.3 years, 142 of 5365 solid organ transplant recipients have developed PTLD (56/1965 kidney, 22/1428 liver, 12/327 simultaneous kidney-pancreas (SPK), 21/113 multivisceral (MVT), 10/778 heart, 15/503 bilateral lung, 3/148 single lung and 3/85 heart and lung). The incidence rate of PTLD was highest in the first year post-transplant in lung and MVT recipients. Cumulative incidence (shown in Figure 1) was 18% at 5 years post-MVT and 1-3% at 5 years following the other SOT types. Cumulative incidence was lowest for liver and heart transplants and was 10% at 20 years post-kidney transplantation. Median OS following SOT was 16 years which is significantly reduced compared with the age-adjusted UK population. There is a relatively high early mortality rate following diagnosis of PTLD and only patients surviving two years post diagnosis regained a similar longer-term survival to the non-PTLD SOT cohort. Treatment with rituximab monotherapy (RM) is now a standard of care for monomorphic PTLD 1. Outcomes for monomorphic patients were compared between those treated with RM (n=66, median follow-up 2.2 y) and R-Chemotherapy (n=24, median follow-up 5.2 y). The two groups were well matched for age and IPI. Of the 66 RM patients, 22 (33%) achieved complete remission with RM and required no further treatment. A further 18 (27%) patients achieved remission following further treatment with chemotherapy/surgery/CTL. 6/66 (9%) patients died of progressive disease (PD), 9/66 (14%) died pre-remission of non-PTLD causes and 11/66 (17%) died in remission of unrelated causes. In the R-Chemotherapy group, 22 patients received R-CHOP and 2 received R-CVP (n=24). 8 (33%) patients are alive and in remission after first line treatment and a further 3 patients (13%) after second line treatment. 2/24 (8%) patients died of PD, 4/24 (17%) died pre-remission of non-PTLD causes and 7/24 (30%) died post-remission of unrelated causes. There is no significant difference in OS between the two groups. Only a minority of deaths were due to PD and death from non-lymphoma causes pre and post remission remain considerably higher than non-PTLD SOT patients up to 2 years post treatment (Figure 1). Multivariate analysis of all 90 monomorphic PTLD patients identified IPI3+ as the strongest pre-treatment variable associating with inferior 1 year OS. Interestingly IPI3+ did not retain this significance when R-chemo patients were analysed alone. Conclusion: With this large SOT dataset we have mapped the cumulative incidence of PTLD over a 20 year period and highlight transplanted organ-specific differences in PTLD incidence over time. Treating monomorphic DLBL patients first-line with RM rather than R-chemotherapy does not appear to compromise OS, but the number of patients dying from non-lymphoma causes pre- and post-treatment remains high with both treatment approaches, with poor OS compared with age-matched non-PTLD SOT recipients. 1Trappe et al. Lancet Oncol; 2012 13(2):196-206 Figure 1 Figure 1. Disclosures Santarsieri: Janssen: Honoraria. Uttenthal: Roche: Other; Takeda: Other; Jazz: Other. Follows: Janssen, Abvie, Roche, AZ: Other.

Background Post-transplant cyclophosphamide (PTCy) is a powerful strategy to prevent occurrence of graft-versus-host disease (GvHD) following allogeneic hematopoietic stem cell transplantation (HSCT). Initially developed in the setting of haploidentical HSCT, PTCy has been increasingly used for fully HLA-matched transplants with favorable results. The purpose of this retrospective multi-center study is to evaluate PTCy-based GvHD prevention for patients with acute leukemia receiving a traditionally prohibitive highly mismatched HSCT and to describe their outcome. Methods This is a registry-based study employing the data set of the Acute Leukemia Working Party (ALWP) of the European Society of Blood and Marrow Transplantation (EBMT). We retrospectively assessed the outcome of adult patients with acute myeloid or lymphoblastic leukemia (AML/ALL), transplanted between 2010 and 2020 with grafts from HLA-mismatched donors with more than 5/10 mismatches using PTCy-based GvHD prophylaxis. Results The study cohort consisted of 59 patients, with a median time of follow up of 20 (95% CI, 14-39) months. The median age was 47 (range, 18-69) years. Forty-four patients had a diagnosis of AML, 14 of ALL and one case of mixed phenotype acute leukemia. At time of transplant, 39 (66%) were in first or second complete remission, 4 (7%) were in later remission and 16 (27%) had active, relapsed or refractory, disease. Conditioning regimens were myeloablative for 54% of cases and peripheral blood was the preferred source of stem cells (64%). All donors were related. Most patients (85%) received a 4/10 HLA-matched transplant, the most commonly mismatched loci were C and DQB1, often with a double mismatch involving the same locus. Two cases of fully mismatched donors were also recorded. PTCy was always associated with other immunosuppressive treatments, especially with the standard combination of calcineurin inhibitors and mycophenolate mofetil. In only 8 cases in vivo T-cell depletion was realized with anti-thymocyte globulin. A large proportion of patients (86%) attained engraftment with a median time of 19 (range, 11-37) days. Only 8 patients did not reach engraftment and all of them died of infection or disease relapse in the first one-hundred days (range, 6-99) after HSCT. Thirty-three patients (58%) did not present any sign of acute GvHD (aGvHD). Cumulative incidence of grade II-IV and grade III-IV aGvHD at day 180 were 30.3% and 14.3%, respectively. At 2 years, the cumulative incidence of chronic GvHD (cGvHD) was 21%, and 7% for extensive cGvHD. Twenty-four patients died during the study period, mostly because of leukemia progression (n=13, 54%), infectious complications (n=6, 25%) and interstitial pneumonia (n=2, 8%). Other causes of death were hemorrhage, GvHD, and another non HSCT-related that accounted for one case (4%) each. At 2 years, the overall survival (OS), leukemia-free survival (LFS), and a GVHD and relapse free survival (GRFS) were 56%, 54% and 47% respectively. Rates of relapse incidence and non-relapse mortality were 28% and 19%, respectively. Conclusion According to this preliminary data overview, transplantation in a highly mismatched framework is possible, without unfavorable OS, LFS and GVHD rates. Despite the important limitation of the retrospective non-controlled nature of this analysis, these findings suggest that PTCy-based strategies could help overcome the barrier of HLA-matching and configure a new setting of transplantation, encouraging more in-depth investigations. Figure 1 Figure 1. Disclosures Labopin: Jazz Pharmaceuticals: Honoraria. Angelucci: Novartis: Honoraria, Membership on an entity's Board of Directors or advisory committees, Speakers Bureau; Celgene BSM: Honoraria, Other: DMC; Blue Bird Bio: Honoraria, Membership on an entity's Board of Directors or advisory committees; Menarini-Stemline: Honoraria, Membership on an entity's Board of Directors or advisory committees, Other: steering commitee, Speakers Bureau; Vertex Pharmaceuticals: Honoraria, Other: DMC; Crispr therapeutics: Honoraria, Other: DMC; Glaxo: 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. Yakoub-Agha: Jazz Pharmaceuticals: Honoraria. Mohty: Takeda: Honoraria; Jazz: Honoraria, Research Funding; Gilead: Honoraria; Adaptive Biotechnologies: Honoraria; Sanofi: Honoraria, Research Funding; Pfizer: Honoraria; Novartis: Honoraria; Janssen: Honoraria, Research Funding; Celgene: Honoraria, Research Funding; Bristol Myers Squibb: Honoraria; Amgen: Honoraria; Astellas: Honoraria.

Allogeneic stem-cell transplantation is an important curative strategy in acute myeloid leukaemia (AML) consequent upon the development of a graft-versus-leukaemia (GVL) effect. Disease relapse is the major cause of transplant failure and novel therapeutic strategies are required in patients with measurable residual disease (MRD) post-transplant. Recognizing that full donor T cell chimerism serves as a biomarker of GVL and that mixed donor T cell chimerism reflects the presence of bi-directional tolerance, we report the first prospective correlation of the impact of post-transplant T cell chimerism and MRD in patients allografted for AML/MDS. Methods: 186 patients from FIGARO, a prospective randomized trial of reduced intensity conditioning regimens in AML/MDS, were alive and relapse free, at the first MRD timepoint and provided post-transplant bone marrow samples for flow cytometric MRD monitoring (day+42, month+3/+6/+9/+12) and peripheral blood samples for T cell chimerism analysis (3-monthly during the first year). Results were not available to clinicians. Flow cytometric MRD incorporating an unsupervised immunophenotypic analysis approach was used on 778 sequential samples. The prognostic significance of post-transplant MRD from each sampling timepoint was assessed by landmark analysis. Results: 29 (15.6%) patients were MRD+ at one or more timepoints in the first year after transplant. The presence of post-transplant MRD was associated with reduced OS (HR:2.19, p=0.0026) and RFS (HR:5.36, p Next, we examined the impact of T-cell chimerism, and post-transplant MRD on outcomes. In patients with mixed-donor T-cell chimerism (MDTC) at month+3 or +6, the presence of post-transplant MRD (preceding or at time of first MDTC) was associated with decreased OS (p=0.001) and RFS. Specifically, 2yr RFS in MRD+ patients was 23.1% (95%CI:5.6-47.5), compared to 63.6% (95%CI:44.9-77.5) in patients who were MRD negative (p=0.004) (Figure). In contrast, in 48 patients with full donor T-cell chimerism (FDTC) at months 3 and 6, only 5 patients were MRD+ post-transplant and their outcomes were equivalent to patients who were MRD negative (FDTC/MRD negative 2yr RFS: 80.9% (95% CI:65.3-90.0)). Downregulation of HLA class II molecules on AML blasts after transplant may result in immune evasion from GVL: we examined whether this had an additional impact on outcome in post-transplant MRD+ patients. Both RFS and OS were significantly lower in MRD+ patients when MRD+ blasts had down-regulated HLA-DR expression. In MRD+ patients, 2yr OS was 20.0% (95%CI:3.1-47.5) in those with HLA-DR down-regulation, versus 57.9% (95%CI 33.2-76.3) in those without (p=0.001). Conclusion We demonstrate that dynamic assessment of post-transplant MRD is an important predictor of outcome in patients allografted for AML/MDS, with additional prognostic value to pre-transplant MRD assessment. Post-transplant MRD and T-cell chimerism results are most informative when combined, underlining the importance of a GVL effect in AML/MDS. This suggests post-transplant strategies which result in FDTC may improve outcomes in patients allografted for AML/MDS. Our data also supports the clinical relevance of monitoring for leukemic immune escape as HLA-DR down-regulation identifies a post-transplant MRD positive subgroup at risk of almost inevitable relapse. Figure 1 Figure 1. Disclosures Loke: Novartis: Other: Travel; Janssen: Honoraria; Amgen: Honoraria; Pfizer: Honoraria; Daichi Sankyo: Other: Travel. Protheroe: Jazz Pharmaceuticals: Honoraria; Astellas: Honoraria; Kite Gilead: Honoraria. Dillon: Amgen: Other: Research support (paid to institution); Astellas: Consultancy, Other: Educational Events , Speakers Bureau; Jazz: Other: Education events; Pfizer: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: educational events; Novartis: Membership on an entity's Board of Directors or advisory committees, Other: Session chair (paid to institution), Speakers Bureau; Menarini: Membership on an entity's Board of Directors or advisory committees; Shattuck Labs: Membership on an entity's Board of Directors or advisory committees; Abbvie: Consultancy, Membership on an entity's Board of Directors or advisory committees, Other: Research Support, Educational Events. Craddock: Novartis Pharmaceuticals: Other: Advisory Board ; Celgene/BMS: Membership on an entity's Board of Directors or advisory committees, Research Funding.

Improving outcomes in multiple myeloma will involve not only development of new therapies but also better use of existing treatments. We performed RNA sequencing on samples from newly diagnosed patients enrolled in the phase 2 PADIMAC (Bortezomib, Adriamycin, and Dexamethasone Therapy for Previously Untreated Patients with Multiple Myeloma: Impact of Minimal Residual Disease in Patients with Deferred ASCT) study. Using synthetic annealing and the large margin nearest neighbor algorithm, we developed and trained a 7-gene signature to predict treatment outcome. We tested the signature in independent cohorts treated with bortezomib- and lenalidomide-based therapies. The signature was capable of distinguishing which patients would respond better to which regimen. In the CoMMpass data set, patients who were treated correctly according to the signature had a better progression-free survival (median, 20.1 months vs not reached; hazard ratio [HR], 0.40; confidence interval [CI], 0.23-0.72; P = .0012) and overall survival (median, 30.7 months vs not reached; HR, 0.41; CI, 0.21-0.80; P = .0049) than those who were not. Indeed, the outcome for these correctly treated patients was noninferior to that for those treated with combined bortezomib, lenalidomide, and dexamethasone, arguably the standard of care in the United States but not widely available elsewhere. The small size of the signature will facilitate clinical translation, thus enabling more targeted drug regimens to be delivered in myeloma.

INTRODUCTION: Allogeneic stem cell transplantation (allo-SCT) is an important curative strategy in adults with high risk acute myeloid leukemia (AML) and myelodysplasia (MDS). Disease relapse represents the major cause of treatment failure and whilst retrospective analyses have identified that pre-transplant measurable residual disease (MRD) is an important predictor of transplant outcome this has never been examined prospectively. The advent of reduced intensity conditioning (RIC) regimens has substantially increased the number of older adults eligible for allo-SCT but the optimal RIC regimen in high risk AML remains unknown. Registry data have demonstrated improved outcomes using a sequential transplant regimen utilizing cytosine arabinoside (araC)/amsacrine (AMSA) cytoreduction followed by a fludarabine (Flu)/busulfan (Bu) based RIC regimen (FLAMSA-Bu). However, although the FLAMSA-Bu regimen is now widely used in adults with high risk AML and MDS its benefit has not been evaluated in a randomized trial. We report the results of a randomized trial evaluating the FLAMSA-Bu regimen compared with standard RIC regimens which also represents the first prospective evaluation of the impact of pre-transplant MRD levels on transplant outcome. PATIENTS AND METHODS: 244 patients (median age 59 yrs) with high risk AML (n=164) or high risk myelodysplasia (n=80) were randomized 1:1 to a control arm determined by investigator's choice of either Flu/B2/ATG (Flu, Bu 3.2 mg/kg x 2 days, ATG 2.5 mg/kg x 2 days); Flu/Mel/Alemtuzumab (A) (Flu, Mel 140 mg/m2, A 50 mg) or Flu/Bu2/A (Flu, Bu 3.2 mg/kg x 2 days, A 50 mg) versus an experimental arm of FLAMSA-Bu (Flu, araC 2g/m2 x 4 days, AMSA 100mg/m2 x 4 days, Bu -total dose 11.2 mg/kg). Patients over the age of 60 received an adjusted FLAMSA-Bu regimen utilising a reduced dose of araC (1mg/m2 x 4 days) and a total Bu dose of 6.4 mg/kg. Patients were transplanted using either an HLA identical sibling (n=49) or matched (10/10 or 9/10) unrelated donor (n=195). All patients received cyclosporine GVHD prophylaxis. 155 patients with AML were in CR1 or CR2 at the time of transplant and 9 had primary refractory disease. MRD was monitored by flow cytometry (applying different-from-normal analysis when no diagnostic/relapse leukemic aberrant immunophenotype was available). Pre-transplant MRD levels were measured up to four weeks prior to transplantation in 201 patients (MRD positive = 80 (40%), MRD negative = 94 (47%), inadequate sample = 27 (13%)). MRD results were not reported to clinicians. The primary outcome was overall survival. RESULTS: Baseline characteristics including CR1/CR2 status, adverse cytogenetics and MRD levels were similar between regimens. Median follow up was 35 months. Transplant outcomes were comparable between patients allografted in the control and FLAMSA-Bu arms. 2 yr overall survival (OS) and cumulative incidence of relapse (CIR) were 61% and 30% respectively in the control arm vs 62% and 26% for the FLAMSA-Bu arm. Transplant related mortality at 100 days was 3.0% in patients allografted using the control regimen vs 14% in patients allografted using the FLAMSA-Bu regimen and 17% vs 21% at 1 year. In the study cohort pre-transplant MRD positivity was associated with both an increased CIR compared to patients testing MRD negative (2 yr CIR 42% vs 19%, p=0.009) and decreased OS (2 yr OS 52% vs 71%, p=0.048). The FLAMSA-Bu regimen failed to improve OS or reduce CIR in either MRD positive or MRD negative patients. CONCLUSIONS: This trial, the largest randomized trial of RIC regimens in AML to date, did not detect any benefit of intensification of the conditioning regimen in adults with high risk AML or MDS. Specifically, the FLAMSA-Bu regimen was not associated with improved transplant outcome in patients who were MRD positive pre-transplant. These data include the first demonstration in a prospective analysis that the presence of pre-transplant MRD measured in real time is associated with reduced OS consequent upon an increased risk of disease relapse. Further randomized studies of novel conditioning regimens in adult AML, crucially with integrated MRD studies, are now required but these results support exploration of alternative strategies, such as pre or post-transplant pharmacological intervention, as the most promising strategy to reduce the risk of disease relapse post allograft. Disclosures Russell: Jazz: Consultancy, Honoraria, Speakers Bureau; DSI: Consultancy, Honoraria, Speakers Bureau; Pfizer Inc: Consultancy, Honoraria, Speakers Bureau; Astellas: Consultancy, Honoraria, Speakers Bureau. Freeman:Jazz Pharmaceuticals: Speakers Bureau. OffLabel Disclosure: We report data using the combination of fludarabine, busulphan, amsacrine and cytosine arabinsoide as a conditioning regimen in patients allografted for high risk acute myeloid leukemia

Background ADAMTS13 circulates in a folded conformation, which is mediated by interactions between the C-terminal CUB domains and its central Spacer domain. Binding of ADAMTS13 to the VWF D4-CK domains disrupts the CUB-Spacer interaction, inducing a structural change that extends ADAMTS13 into an open conformation that enhances catalytic efficiency ~2-fold. This mechanism supports a model in which ADAMTS13 unfolding induces exposure of an exosite in the Spacer domain that interacts with the VWF A2 domain, increasing the affinity between the two molecules, and, therefore, the rate of proteolysis. The D4-CK-mediated conformational activation of ADAMTS13 can be mimicked in vitro with the use of antibodies that disrupt the CUB-Spacer interaction, such as the previously published anti-CUB antibody, Ab17G2. We recently generated a novel, activating antibody against the Spacer domain (Ab3E4). Aim To characterize the mechanism by which the Ab17G2 and Ab3E4 enhance the catalytic efficiency of ADAMTS13. Methods The effects of the Ab17G2 and Ab3E4 on the activity of ADAMTS13 were studied using FRETS-VWF73. The effects of the Ab17G2 and Ab3E4 on the kinetics of VWF96 (VWF G1573-R1668) proteolysis were characterized using an in-house assay. ELISA was used to investigate conformational changes in ADAMTS13 induced by the Ab17G2 and Ab3E4. Results Both Ab17G2 and Ab3E4 enhanced FRETS-VWF73 proteolysis by ~1.7-fold. This result was reproduced using the VWF96 substrate; the Ab17G2 and Ab3E4 enhanced the catalytic efficiency (kcat/Km) of ADAMTS13 by ~1.8- and ~2.0-fold, respectively. The activation was dependent on the conformational extension of ADAMTS13, since the antibodies could not enhance the activity of an ADAMTS13 variant that lacks the TSP2-CUB2 domains (MDTCS). Surprisingly, ADAMTS13 activation was not mediated through exposure of the Spacer or Cys-rich domain exosites as previously proposed, as the Ab17G2 and Ab3E4 efficiently enhanced proteolysis of VWF96 variants in which the Spacer/Cys-rich exosite binding sites were disrupted. Kinetic analysis of VWF96 proteolysis showed that the Ab17G2- and Ab3E4-induced activation of ADAMTS13 is primarily manifest through a ~1.5- to ~2-fold increase in enzyme turnover (kcat). Thus, contrary to the current model, this suggests that the conformational extension of ADAMTS13 influences the functionality of the active site, and not substrate binding affinity (Km). Incubating ADAMTS13 with either Ab17G2 or Ab3E4 exposed a cryptic epitope in the metalloprotease domain that was specifically detected by ELISA, further corroborating that the antibodies induce a conformational change in ADAMTS13 affecting the M domain. Conclusion Antibodies can be used as tools for understanding the structure/function of enzymes. Using activating antibodies against the Spacer and CUB1 domains of ADAMTS13, we show for the first time that the activation of ADAMTS13 following its unfolding is not a result of exposure of a functional exosite in Spacer/Cys-rich domain that increases affinity to VWF. Rather, our data are consistent with an allosteric activation mechanism upon the metalloprotease domain. We propose that ADAMTS13 unfolding causes a conformational change in the active site that further activates the enzyme. We are currently investigating whether the D4-CK-induced enhancement of ADAMTS13 proteolytic activity is also mediated by conformational changes in the active site. Disclosures Vanhoorelbeke: Ablynx: Consultancy; Shire: Consultancy.

The von Willebrand factor (VWF) A2 crystal structure has revealed the presence of a rare vicinal disulfide bond between C1669 and C1670, predicted to influence domain unfolding required for proteolysis by ADAMTS13. We prepared VWF A2 domain fragments with (A2-VicCC, residues 1473-1670) and without the vicinal disulfide bond (A2-ΔCC, residues 1473-1668). Compared with A2-ΔCC, A2-VicCC exhibited impaired proteolysis and also reduced binding to ADAMTS13. Circular dichroism studies revealed that A2-VicCC was more resistant to thermal unfolding than A2-ΔCC. Mutagenesis of C1669/C1670 in full-length VWF resulted in markedly increased susceptibility to cleavage by ADAMTS13, confirming the important role of the paired vicinal cysteines in VWF A2 domain stabilization.