Abstract: HCT entails substantial TRM justifying continued efforts to better risk stratify patients. Single institutional studies have suggested several clinically available biomarkers of CRP, ferritin and albumin influence TRM if not overall survival (OS). We sought to confirm the independent prognostic value of these biomarkers in a large multi-institutional cohort. The study population consisted of 784 adults with AML in remission (80%) or MDS (20%) undergoing unrelated donor HCT between 2008 and 2010 and available cryopreserved samples through the Center for International Blood and Marrow Transplant Research (CIBMTR) repository. CRP and ferritin were centrally quantified by ELISA from cryopreserved plasma whereas albumin levels obtained from center reported data. Correlation studies for biomarkers required log transformation of CRP and ferritin to produce a normal distribution. Multivariate models were fitted separately for each biomarker applying protocol specified thresholds generated from the literature of CRP 〉 10 mg/L, ferritin 〉 2500 ng/mL, albumin 〈 3.5 g/dL on TRM. Further analysis explored optimal cutpoints for this cohort for all significant clinical variables for TRM. HCT characteristics included a median age of 50 (range 18-78) years, HCT-CI (co-morbidity index) 3 or more in 35%, single allele/antigen mismatch (7/8) in 23%, PB as stem cell source in 83%, and myeloablative conditioning in 72%. Biomarker data were available in 783, 781 and 695 cases for CRP, ferritin and albumin respectively. The median values and ranges for each biomarker were as follows: 5.0 mg/L (0.3 - 316) for CRP, 1148 ng/mL (51 – 14,298) for ferritin and 3.6 g/dL (0.6-5.3) for albumin. Log transformed CRP and ferritin showed a modest correlation (r=0.35, P 〈 0.001), and log CRP was marginally associated with albumin (r=-0.12, P=0.002). HCT-CI had no correlation to CRP and albumin. Higher ferritin was associated with HCT-CI (P=0.014) and disease (P 〈 .001). In the entire population, TRM and overall survival (OS) at 2 years were 23% and 54%, respectively. The table shows the independent association of each biomarker with TRM and OS. Ferritin had no association with these outcomes but only 12% had levels above 2500 by ELISA. The optimal cutpoints for TRM were defined as follows: CRP 〉 3.67 (HR=1.75, P=0.001); albumin 〈 3.4 (HR 1.70, P 〈 0.001); and ferritin as a continuous variable (HR 1.30, P = 0.008). A risk score was created modeling optimal biomarker thresholds to predict TRM based on the following formula: risk score = 0.44633*I(CRP 〉 3.67)+0.18330*ln(Ferritin)+0.44730*I(albumin 〈 3.4), where “I” is an indicator function and represent 1 or 0 depending on the presence of the abnormal biomarker level in parenthesis. The risk score included adjustment for HCT-CI. The biomarkers did not influence the incidence of relapse or graft-versus-host disease with any of these models. Elevated CRP and ferritin and decreased albumin prior to HCT were associated with impaired transplant survival, primarily through higher TRM. Integration of a biomarker panel in clinical practice once validated can enhance risk-stratification, improve adjustment for comparative studies and point towards the design of biomarker driven trials. Abstract 422. Table: Multivariate analysis of TRM and OS for biomarkers, and biomarker panel risk score* Abnormal, N (%) Transplant related mortality Overall Survival Biomarker HR 95% CI P HR 95% CI P Protocol defined CRP 〉 10 mg/L 184 (24) 1.52 1.11 – 2.07 0.008 1.22 0.98 – 1.53 0.072 Ferritin 〉 2500 ng/mL 93 (12) 1.26 0.84 – 1.88 0.27 1.15 0.86 – 1.54 0.35 Albumin 〈 3.5 g/dL 290 (42) 1.48 1.10 – 2.0 0.010 1.39 1.13 – 1.72 0.002 Optimal threshold, combined model ** CRP 〉 3.67 mg/L 497 (64) 1.56 1.11 – 2.19 0.010 1.24 1.00 – 1.55 0.054 Log(Ferritin), linear n/a 1.20 0.99 – 1.46 0.07 1.20 1.04 – 1.38 0.010 〈 3.4 g/dL 203 (29.5) 1.58 1.16 – 2.15 0.004 1.37 1.10 – 1.72 0.005 Biomarker Score ** N Low ( 〈 1.5) 200 1.0 - - 1.0 Intermediate (1.5-2.0) 331 1.66 1.10-2.49 0.015 1.38 1.06-1.80 0.017 High ( 〉 2.0) 159 2.72 1.77-4.19 〈 0.001 2.01 1.51-2.70 〈 0.001 * biomarkers adjusted for significant covariates of BMI, disease, mismatch, and CMV serostatus and stratified on conditioning regimen ** combined model includes all biomarkers and HCT-CI Disclosures No relevant conflicts of interest to declare.

Abstract: Background: Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare, aggressive hematologic malignancy arising from plasmacytoid dendritic cells. It commonly presents as skin lesions with or without bone marrow, lymph node, central nervous system (CNS), or systemic involvement. Limited retrospective data have shown durable remissions after allogeneic (allo) hematopoietic cell transplantation (HCT). Methods: In this retrospective single-center analysis, we evaluated outcomes of 31 BPDCN patients treated with allo-HCT between September 2000 and June 2023 at our institution. Results: Median age of our cohort was 52 years [range 16-76 y]. Patient characteristics are summarized in the attached Table. Only 6 patients (19%) had their disease confined to skin and 4 (13%) had CNS/CSF involvement at diagnosis. Four patients (13%) had prior hematologic malignancies (HM), and 9 patients (29%) had a cytogenetic abnormality at baseline. Thirteen (42%) patients, who were evaluated by a next-generation sequencing Leukemia Mutation Panel, had TET2 mutation. Median time from diagnosis to HCT was 6.2 (3.2-42.4) months. Eight (26%) patients received tagraxofusp, matched unrelated donor was the most common donor type (39%), and fludarabine + busulfan-based conditioning was used in 17 (55%) patients. Post-transplant cyclophosphamide (PTCy) was used for GVHD prophylaxis in 23 (74%) patients. Median follow up was 23.9 (0.9-119.9) months. One-hundred day and 1-year non-relapse mortality was 9.7% and 23.1%, respectively. Grade 2 acute GVHD was seen in 9 patients (29%), and no grade 3-4 GVHD has been seen so far. Limited or extensive chronic GVHD was seen in 6 (21%) of 28 evaluable patients. Only 6 of the 31 (19%) allo-HCT patients progressed after HCT. Two-year PFS and OS was 56% and 67%, respectively. Patients receiving allo-HCT in first remission had significantly better median PFS (not reached vs. 11.3 months; p=0.034) and OS (not reached vs. 12.6 months; p=0.045). Conclusions: These results demonstrate the safety and efficacy of allo-HCT in BPDCN. Patients undergoing allo-HCT in first remission had significantly better outcomes. Prospective studies are needed to better define the role of allo-HCT in BPDCN.

Abstract: Acute Graft-vs.-Host Disease (aGVHD) is the major cause of mortality and morbidity following allogeneic hematopoietic cell transplantation (AHCT). Steroids are the standard initial therapy yet only ~50-60% will achieve a response. Strategies to improve response and minimize steroid-exposure are needed. We report the results of a randomized trial of extracorporeal photopheresis (ECP) plus steroids vs. steroids- alone as initial therapy for aGVHD. Methods: This was a Phase II, randomized, adaptive Bayesian design. Patients with histologically confirmed aGVHD and who received 〈 72 hours of steroids were randomized to receive 2mg/kg methylprednisolone (MP) with or without ECP. ECP schedule was: days 1-14 (8 sessions), days 15-28 (6 sessions) and days 29-56 (8 sessions). The primary endpoint was treatment success at day 56 post-randomization defined as being alive, in remission, achieving a GVHD response without additional therapy, and on 〈 1mg/kg of MP at day 28 and 〈 0.5mg/kg on day 56. Randomization probabilities were based upon the probabilities of response in each arm, stratified by skin-only vs. visceral involvement. The primary endpoint was evaluated using a Bayesian predictive probability approach; i.e., at the end of the study, if the posterior probability that the success rate for either arm was greater than that for the other arm was 〉 0.80, that arm would be declared the preferred treatment. Pts and Results: Between 2008 and 2014 eighty-one (81) pts were randomized to ECP + MP (51 pts) or MP-alone (30 pts). Median age was 54 yrs (range, 17-75); 42% had AML/MDS and 62% were male. HCT was performed with myeloablative conditioning in 69% and 63% had an unrelated donor. At enrollment, 90% of patients had aGVHD grade II, 10% had grade III/IV. Organ Involvement was: skin (86%), upper GI (22%), lower GI (22%) and liver (10%). The treatment arms were balanced for all baseline characteristics. The probability that ECP+MP had a higher success rate than MP-alone was 0.815 which exceeded the protocol-specified threshold of 80%, declaring ECP-arm the preferred treatment. The ECP-arm was more beneficial in pts with skin-only aGVHD (72% vs. 57% response rate) whereas visceral-organ involvement response rates were similar (47% vs. 43%). By day 56, 43% of the evaluable patients randomized to the ECP were on physiologic doses of steroids (≤0.1mg/kg) versus 30% for MP-alone arm (p=0.34). To study the immunologic effects of ECP, we examined the recovery of T-cell subsets by multi-parameter flow cytometry in a subset of pts randomized to the ECP+MP and MP-alone arm at enrollment and study conclusion. ECP+MP was associated with more robust recovery of CD4+ and CD8+ cells and significantly higher absolute number of regulatory T-cells (p=0.043), figure 1. Conclusions: The results of this randomized, phase II trial indicate that the addition of ECP to steroids results in higher GVHD response and facilitates steroid-tapering in pts with newly diagnosed aGVHD. This combination appears most efficacious for pts with skin-involvement. Correlative studies implicate expansion of regulatory T-cells as a possible mechanism of action through which ECP exerts its immunologic effect. Figure 1. Patient Characteristics and Outcomes Figure 1. Patient Characteristics and Outcomes Figure 2. Mean Absolute Regulatory T-cell Count in subset of patients randomized to ECP+MP (red) vs. MP-alone (green) p-value= 0.043. Figure 2. Mean Absolute Regulatory T-cell Count in subset of patients randomized to ECP+MP (red) vs. MP-alone (green) p-value= 0.043. Disclosures Alousi: Therakos, Inc: Research Funding. Off Label Use: Extracorporeal Photopheresis is FDA approved for the treatment of cutaneous lymphoma. It is commonly used in the treatment of patients with acute and chronic graft-versus-host disease. There are no FDA approved therapies for this indication and all therapies are given as an "off-label" indication..

Abstract: Introduction: Maintenance therapy is considered standard of care after upfront autologous hematopoietic stem cell transplantation (auto-HCT) for patients with newly diagnosed multiple myeloma (MM). However, there is paucity of data on the use of maintenance after a salvage auto-HCT, whether first or second. Aim: We designed a clinical trial for relapsed MM patients who underwent salvage auto-HCT after disease relapse to receive post-transplant maintenance with daratumumab (dara), with the addition of pomalidomide (pom) in a later amendment. Methods: We hypothesized that maintenance therapy with dara +/- pom will improve the progression-free survival (PFS) after a salvage auto-HCT. Patients with prior exposure to dara or pom were included, but patients refractory to either drug were excluded. The trial was planned to enroll a total of 56 patients. Patients received dara SQ weekly for weeks 1-8, every 2 weeks for weeks 9-24, and monthly from week 25 until progression. Pom was given at 2 mg PO from day 1-21 every 28 days. Patients received herpes zoster prophylaxis for the duration of the trial. The primary endpoint of this phase II trial was PFS. Disease response was assessed according to the International Myeloma Working Group (IMWG) uniform response criteria. Minimal residual disease (MRD) was measured by multiparametric flow cytometry (10 -5) in the bone marrow at day 90-180 after transplant. Results: The trial was terminated due to poor accrual after 13/56 patients were enrolled between May 2019 and August 2022. Median age at salvage auto-HCT was 64 (range: 43-76) years, and 7 patients (54%) were male. Median prior lines of treatment was 2 (range: 2-4). Six patients (46%) had a prior auto-HCT, and 7 (54%) had a hematopoietic stem cell comorbidity index (HCT-CI) of & gt;3. Median time from diagnosis to salvage auto-HCT was 58.3 (range 7.6-132) months. Five patients (38%) had high-risk cytogenetic abnormalities (1q+ in all 5). Eight (61%) patients received melphalan alone, 4 (31%) received busulfan/melphalan, and 1 (8%) received gemcitabine/busulfan/melphalan/panobinostat as their conditioning regimen (Table 1). Three patients (23%) started maintenance with dara alone, while the remaining 10 (77%) patients received dara + pom. At study entry, 6 patients (46%) had a complete response (CR), 4 (31%) in very good partial response (VGPR), 2 (15%) in partial response (PR), and 1 (8%) had stable disease (SD). At day 100 post maintenance, 9 (69%) were in CR, while 1 (8%) each had VGPR, PR, SD and progressive disease (PD), respectively. For the 12 patients that did not progress at day 100, best response to dara +/- pom maintenance was CR in 10 (83%), and VGPR and SD in 1 (8%) each. With a median follow up of 26.1 months (range: 14.2-37.7) from the start of maintenance, 3 patients had progressed but there are no deaths so far. Median PFS from the date of auto-HCT was not reached, and from start of maintenance 28.5 (95% CI 26.3-NA) months. 2-year PFS was 92% (Figure 1). Most common adverse events were neutropenia (n=11, 84%; none grade & gt;3), grade 1-2 fatigue (n=6, 46%), and grade ≤3 bacterial infection, grade ≤3 viral infection, grade 1-2 diarrhea and grade 1 thrombocytopenia (n=5, 38% each). Conclusion: Maintenance therapy with dara, with or without pom, is safe and feasible after a salvage auto-HCT for MM, with 83% achieving a CR. After a median follow up exceeding 2 years, the median PFS was 28.5 months. The trial was terminated due to poor accrual.

Abstract: Introduction: Despite the successes of treatment with tyrosine kinase inhibitors (TKIs) in patients with CML, allogeneic hematopoietic stem cell transplantation (ASCT) continues to be a potentially curative option for patients with advanced disease of who fail TKI therapy. Here we analyzed outcomes of patients with advanced CML (aCML) (beyond first chronic phase-CP1) who received an ASCT at our institution to identify factors associated with improved survival. Methods:207 consecutive patients withaCML treated at The University of Texas MD Anderson Cancer Center after year 2000 were included. The median age was 44 years (range 2-70 years), 135 (65%) were male, 77% had less than 5% bone marrow (BM) blasts, 129 (65%) had persistentPh-chromosome positive, and 176 patients (85%) were less than a major molecular response at transplant. Forty of 114 tested patients (35%) had resistant BCR-ABL mutations and 10 patients (8.7%) had T315I mutation at transplant. Conditioning regimen wasmyeloablative (MAC) in 140 patients (68%). Donors were matched related (MRD), matched unrelated (MUD),haploidentical (HAPLO), mismatched unrelated (MMUD), umbilical cord blood (UCB) and 1Ag mismatched related (MMRD) in 79 (38%), 75 (36%), 18 (9%), 17 (8%), 11 (5%) and 7 (3%) patients, respectively. The median time from diagnosis to transplant was 27 months (range 1-318 months) while the median follow-up duration was 20 months (range 1-194 months). Results:At 30 days post-transplant, 180 of 200 tested patients (90%) and 134 of 201 tested patients (67%) achieved a complete cytogenetic and at least a major molecular response, respectively. The response to transplant by day 30assessment correlated significantly with the disease status before transplant. A higher percentage of patients who experienced cytogenetic response before transplant experienced molecular response post-transplant (77%) compared with those who did not (61%; p=0.027). For the entire group, the 1-year cumulative incidence (CI) of acute GVHD (aGVHD) grade II-IV and grade III-IV were 41% and 15%, respectively; 5-year CI of extensive chronic GVHD (cGVHD) was 31%.Haploidentical transplant patients had lesscGVHD compared with HLA matched donor transplants (14% vs. 32% for HLA matched transplants). The CI of non-relapse mortality at 100 days and 1 year was 14% and 30%, respectively. Sixty-five patients (31%) had molecular relapse after transplant, which correlated with the degree of disease control before transplant. The CI of cytogenetic and molecular relapse at 5 years was 22% and 31%, respectively. Overall the 5-year survival (OS), progression free survival (PFS) and GVHD-free, relapse-free survival (GRFS)was49%, 34%, and 22%, respectively. Adjusting for all significant measures, percentage of BM blasts before transplant and donor type were significantly associated with PFS and GRFS (Table1, Figure 1).Haplodenticaltransplant patients had longer PFS and GRFS compared with other donor types including matched related or unrelated donor (5-year GRFS for HAPLO, MRD, MUD, MMUD, UCB and MMRD were 53%, 19%, 23%, 29%, 9%, and 0%, respectively; p=0.033) (Table 1, Figure 1). Cytogenetic and molecular response before transplant as well as year of transplant did not predict survival after transplant. Conclusions: ASCT is curative for a proportion of patients withaCML. PFS and GRFS are favorably influenced by percentage of BM blasts and donor type, withhaploidentical donor having at least as good outcomes as HLA matched donors, while molecular and cytogenetic response before transplant do not appear to correlate with survival post-transplant. Table 1 Multivariable analysis for PFS and GRFS Table 1. Multivariable analysis for PFS and GRFS Figure 1 PFS and GRFS based on percentage of BM blasts before transplant and donor types Figure 1. PFS and GRFS based on percentage of BM blasts before transplant and donor types Disclosures Cortes: Arog: Research Funding; Teva: Research Funding; Pfizer: Consultancy, Research Funding; Novartis: Consultancy, Research Funding; BMS: Consultancy, Research Funding; Ariad: Consultancy, Research Funding; Astellas: Research Funding; Ambit: Research Funding. Champlin:Intrexon: Equity Ownership, Patents & Royalties; Ziopharm Oncology: Equity Ownership, Patents & Royalties. Ciurea:Spectrum Pharmaceuticals: Other: Advisory Board; Cyto-Sen Therapeutics: Equity Ownership.

Abstract: Background: Cardiac involvement by light chain amyloidosis (AL) is generally associated with an unfavorable outcome. Bortezomib-based induction, and high-dose melphalan followed by autologous hematopoietic stem cell transplantation (auto-HCT) in eligible patients is associated with best long-term outcomes. We report the outcome of cardiac AL patients who underwent auto-HCT at our institution. Methods: We retrospectively reviewed all patients with cardiac AL who received auto-HCT between January 1997 and December 2018 at our institution. Hematologic and cardiac organ responses were evaluated according to the Consensus Guidelines for AL (R Comenzo et al. Leukemia 2012). Revised Mayo staging system was used for cardiac staging (S Kumar et al. JCO 2012). Progression free survival (PFS) and overall survival (OS) were calculated from the date of transplant. Survival was estimated using Kaplan Meier method and compared using log rank test. Cox proportional hazard models were used for adjusted survival analysis. Results: 57 patients were identified and baseline characteristics summarized in Table 1. Thirty eight patients (67%) at diagnosis and 17 (30%) at auto-HCT were evaluable by the revised Mayo staging system. Eleven (19%), 14 (25%), 17 (30%), and 13 (23%) patients had stage 1, 2, 3 and 4 disease, respectively, while the stage was unknown in 2 (3%) patients. Twenty-four (42%) patients received induction with a combination of cyclophosphamide, bortezomib, and dexamethasone (CyBorD), 14 (25%) received bortezomib and dexamethasone, and 2 (3%) received other bortezomib-based induction (Table 1). Based on hematologic response criteria, 3 (5%), 15 (27%) and 22 (39%) patients achieved complete response (CR), a very good partial response (VGPR), or partial response (PR) to induction, with an overall response rate (ORR) of 71%. All patients underwent peripheral blood stem cell (PBSC) mobilization with filgrastim, with or without plerixafor. Thirty-nine (68%) patients received melphalan 200mg/m2 and 18 (32%) received melphalan 140mg/m2 as preparative regimen. Nineteen patients (33%) received maintenance therapy post auto-HCT. One-hundred day and 1-year post auto-HCT non-relapse mortality rate was 5% (3 patients). Best post auto-HCT hematologic ORR was 92%, with 19 (34%), 20 (35%), and 13 (23%) patients achieving CR, VGPR and PR, respectively. Based on the consensus guidelines for cardiac response in AL using NT-proBNP or NYHA class, 51 patients (89%) had a cardiac organ response at their last evaluation (Table 2). Median follow up in surviving patients was 32.9 months (range 5.1 - 140.6). The 3-year PFS was 53.5% [95% CI 38.6-68.4%], and 3-year OS was 67.8% [53.9-81.7%] . On univariate analysis, melphalan 200 vs. 140 (p=0.017, HR 0.387 95%CI 0.178- 0.844) was associated with a better PFS, but none of the variables had an impact on PFS or OS on a multivariate Cox regression analysis, perhaps due to a small sample size. Conclusion: In this retrospective analysis we showed that in transplant-eligible patients with advanced cardiac AL, high-dose melphalan and auto-HCT is associated with a low (5%) NRM, an organ response rate of almost 90%, and a 3-year OS of almost 70%. Disclosures Bashir: Takeda: Other: Advisory Board, Research Funding; Acrotech: Research Funding; Celgene: Research Funding; StemLine: Research Funding; KITE: Other: Advisory Board; Purdue: Other: Advisory Board; Amgen: Other: Advisory Board. Nieto:Secura Bio: Other: Grant Support; Astra Zeneca: Other: Grant Support; Novartis: Other: Grant Support; Affimed: Consultancy, Other: Grant Support. Hosing:NKARTA Inc.: Consultancy. Popat:Bayer: Research Funding; Novartis: Research Funding. Lee:Takeda: Consultancy, Research Funding; Amgen: Consultancy, Research Funding; GlaxoSmithKline: Consultancy, Research Funding; Janssen: Consultancy, Research Funding; Celgene: Consultancy, Research Funding; Genentech: Consultancy; Genentech: Consultancy; Daiichi Sankyo: Research Funding; Sanofi: Consultancy; Regeneron: Research Funding. Patel:Takeda: Consultancy, Research Funding; Precision Biosciences: Research Funding; Oncopeptides: Consultancy; Poseida: Research Funding; Janssen: Consultancy, Research Funding; Nektar: Consultancy, Research Funding; Cellectis: Research Funding; Celgene: Consultancy, Research Funding; Bristol Myers Squibb: Consultancy, Research Funding. Manasanch:Merck: Research Funding; Sanofi: Honoraria; GSK: Honoraria; Takeda: Honoraria; Quest Diagnostics: Research Funding; Adaptive Biotechnologies: Honoraria; JW Pharma: Research Funding; Novartis: Research Funding; BMS: Honoraria; Sanofi: Research Funding. Thomas:X4 Pharma: Research Funding; Ascentage: Membership on an entity's Board of Directors or advisory committees, Research Funding; BMS: Research Funding; Genentech: Research Funding; Xencor: Research Funding; Pharmacyclics: Other: Advisory Boards. Kaufman:Karyopharm: Honoraria; Janssen: Research Funding; Bristol Myers Squibb: Research Funding. Orlowski:Sanofi-Aventis, Servier, Takeda Pharmaceuticals North America, Inc.: Honoraria, Membership on an entity's Board of Directors or advisory committees; Laboratory research funding from BioTheryX, and clinical research funding from CARsgen Therapeutics, Celgene, Exelixis, Janssen Biotech, Sanofi-Aventis, Takeda Pharmaceuticals North America, Inc.: Research Funding; Amgen, Inc., AstraZeneca, BMS, Celgene, EcoR1 Capital LLC, Forma Therapeutics, Genzyme, GSK Biologicals, Ionis Pharmaceuticals, Inc., Janssen Biotech, Juno Therapeutics, Kite Pharma, Legend Biotech USA, Molecular Partners, Regeneron Pharmaceuticals, Inc.,: Honoraria, Membership on an entity's Board of Directors or advisory committees; STATinMED Research: Consultancy; Founder of Asylia Therapeutics, Inc., with associated patents and an equity interest, though this technology does not bear on the current submission.: Current equity holder in private company, Patents & Royalties. Champlin:Takeda: Patents & Royalties; Genzyme: Speakers Bureau; DKMS America: Membership on an entity's Board of Directors or advisory committees; Cytonus: Consultancy; Omeros: Consultancy; Johnson and Johnson: Consultancy; Actinium: Consultancy. Qazilbash:Bioclinica: Consultancy; Amgen: Research Funding; Angiocrine: Research Funding; Bioline: Research Funding; Janssen: Research Funding.

Abstract: Background: Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is an aggressive hematologic malignancy with historical overall survival (OS) of 8-14 months from diagnosis. Tagraxofusp is a targeted therapy directed to CD123. The US FDA has approved tagraxofusp for the treatment of BPDCN in adult and pediatric patients 2 years and older. CD123 is ubiquitously expressed at high levels on plasmacytoid dendritic cells (pDCs), the cell of origin of BPDCN, and targeting of CD123 with tagraxofusp resulted in high response rates, durable remissions, and a significant proportion of patients bridged to hematopoietic cell transplant (HCT) in first remission (45%). While HCT has the potential to prolong remission of patients with hematologic cancers, relapses after transplantation still occur in patients with certain risk factors including minimal residual disease (MRD). This study is designed to assess the safety and efficacy of tagraxofusp monotherapy as a maintenance treatment in patients with BPDCN post-HCT. The study is open for enrollment and expected to recruit approximately 20 patients. NCT04317781 Rationale: Available data suggest long-term remission can be expected in approximately 50% of patients with BPDCN that undergo allo-HCT. Accordingly, strategies are needed to further reduce the risk of relapse post-HCT. Methods: The study is a Phase 2, single-center, open-label, single-arm clinical trial of tagraxofusp in patients with BPDCN that have undergone HCT. Planned enrollment is 20 patients. Treatment with tagraxofusp will be initiated between day 45 and 120 post-HCT, and dosed days 1-3 in cycles 1-4, and days 1-2 in cycles 5 and beyond. Each treatment cycle is 28 days. Duration of therapy for patients in complete response (CR) or clinical CR (CRc) post-transplant is 24 cycles, with patients who are MRD-positive or remain at high risk of relapse, eligible for continued treatment for as long as they derive benefit. In patients whose post-HCT disease status is either CR with incomplete blood count recovery (CRi) or partial response (PR), treatment will continue until disease progression or unacceptable toxicity. Major Inclusion/Exclusion Criteria : Age ≥18; & gt;30 days post-transplant without active or chronic infections; Karnofsky PS ≥60%; Lansky ≥60; adequate organ function including LVEF ≥ lower limit of normal (LLN); creatinine ≤1.5 mg/dL; albumin ≥3.2 g/dL; bilirubin ≤1.5 upper limit of normal (ULN); AST/ALT ≤2.5 times ULN; and ANC ≥1000. For allo-HCT, no ≥ grade 2 visceral (gut or liver) acute graft versus host disease (GVHD) and no ≥ grade 3 or any other acute GVHD (chronic GVHD allowed at investigator discretion). Key exclusions: Persistent clinically significant non-hematologic toxicities ≥ grade 2; CNS involvement; receiving chemotherapy, radiotherapy, or other anti-cancer therapy within 14 days of first dose of study drug; uncontrolled infection; HIV/Hepatitis B and/or C; and clinically significant cardiopulmonary disease. Endpoints: Primary endpoints include safety and tolerability of tagraxofusp in patients with BPDCN post-HCT, with survival as secondary outcomes. Disclosures Bashir: Amgen: Other: Advisory Board; Purdue: Other: Advisory Board; Acrotech: Research Funding; StemLine: Research Funding; KITE: Other: Advisory Board; Takeda: Other: Advisory Board, Research Funding; Celgene: Research Funding. Shpall:Magenta: Membership on an entity's Board of Directors or advisory committees; Celgene: Membership on an entity's Board of Directors or advisory committees; Adaptimmune: Membership on an entity's Board of Directors or advisory committees; Takeda: Other: Licensing Agreement; Zelluna: Membership on an entity's Board of Directors or advisory committees; Novartis: Membership on an entity's Board of Directors or advisory committees. Popat:Bayer: Research Funding; Novartis: Research Funding. Pemmaraju:Stemline Therapeutics: Honoraria, Research Funding; LFB Biotechnologies: Honoraria; Roche Diagnostics: Honoraria; Pacylex Pharmaceuticals: Consultancy; Blueprint Medicines: Honoraria; Samus Therapeutics: Research Funding; Novartis: Honoraria, Research Funding; Celgene: Honoraria; Plexxikon: Research Funding; Affymetrix: Other: Grant Support, Research Funding; AbbVie: Honoraria, Research Funding; Daiichi Sankyo: Research Funding; SagerStrong Foundation: Other: Grant Support; DAVA Oncology: Honoraria; MustangBio: Honoraria; Cellectis: Research Funding; Incyte Corporation: Honoraria. Champlin:Takeda: Patents & Royalties; Actinium: Consultancy; Omeros: Consultancy; Genzyme: Speakers Bureau; DKMS America: Membership on an entity's Board of Directors or advisory committees; Johnson and Johnson: Consultancy; Cytonus: Consultancy. Qazilbash:Bioline: Research Funding; Bioclinica: Consultancy; Amgen: Research Funding; Angiocrine: Research Funding; Janssen: Research Funding.

Abstract: Systemic mastocytosis (SM) is a rare hematologic neoplasm characterized by abnormal growth and accumulation of tissue mast cells (MC) in various organ systems, including bone marrow (BM). Indolent and advanced forms of SM have been described. Whereas patients with ISM have a normal or near normal life-expectancy, patients with advanced SM, including those suffering from mast cell leukemia (MCL) have a poor prognosis. In these patients, neoplastic MC are usually resistant against conventional drugs and various targeted drugs. In rapidly progressive aggressive SM (ASM) and MCL, polychemotherapy followed by allogeneic hematopoietic stem cell transplantation (alloHCT) has been proposed. However, outcome of alloHCT in advanced SM is unknown, and it also remains uncertain whether clinically relevant graft-versus-SM (GVSM) effects may occur in these patients, as only sporadic case reports have been published. We performed a retrospective multi-center analysis to evaluate the outcome of alloHCT in patients with advanced SM. Fifty-four advanced SM patients receiving SCT in 32 transplantation centers in Europe and America were identified between 1990 and 2013. The median patient age was 45 years. Donors were: HLA identical siblings (31), unrelated donors (URD) (15), umbilical cord blood donors (UCB) (2), and haploidentical donors (1). In 5 patients, stem cell source was not defined (5). Thirty-four patients received myeloablative conditioning (MAC) and 18 received reduced intensity conditioning regimens (RIC). In 2 patients, conditioning regimen was not specified. Indications for alloHCT were SM with an associated clonal hematologic non-mast cell lineage disease (SM-AHNMD) (n=32), MCL (n=13, including one with MCL-AHNMD), 8 with ASM and 1 with myelomastocytic leukemia (MML). The most prevalent AHNMD was acute myeloid leukemia (AML, n=16). With follow-up of 35-6180 (median 365) days, SM responses (defined as ≥50% decrease in BM mast cells ± decrease in serum tryptase ± regression of other organ manifestations) were observed in 39 patients (72%), including complete responses (CR) documented in 12 patients (22%). Eleven patients had stable disease, whereas 4 patients (7%) progressed immediately after alloHCT (primary resistance). In addition, 10 patients progressed (5 of them within 100 days) after an initial response. Progression was most frequently seen in MCL patients (n=6, 50%). In the AHNMD group, only 8 patients relapsed/progressed (25%). The overall survival (OS) and SM progression-free survival (PFS) at 1 year were 63% and 50% for all patients, 77% and 68% for SM-AHNMD, 63% and 50% for ASM, and 25% and 17% for MCL, respectively. The strongest predictive variable associated with inferior survival was a diagnosis of MCL. Other factors associated with poor outcome were: Karnofsky performance status ≤70%, ≥2 SM regimens given before alloHCT (e.g., steroids, cladribine, chemotherapy, tyrosine kinase inhibitor), donor source (alternative donors-UCB and haploidentical compared to sibling or URD), SM progression within the first 100 days, normal cytogenetics (compared to t(8;21) (q22;q22), and RIC (compared to MAC). The following variables were not associated with poor outcome: patient and donor age, recipient-donor sex match status, graft source (BM vs. peripheral stem cells), BM mast cell percentage at time of alloHCT, and CR status of AML or SM response at time of alloHCT. This largest multi-center analysis of results in advanced SM provides evidence for clinical efficacy of alloHCT, presumably because of a GVSM effect of alloHCT (achieving CR, and response to donor-lymphocyte infusions and RIC alloHCT). However, responses varied among different SM categories: while patients with SM-AHNMD enjoyed excellent outcomes, the OS for MCL patients in general, was poor. Nevertheless it is remarkable that 3 of 13 patients with MCL – an otherwise fatal disease with a median survival of & lt;12 months – became long-term survivors after alloHCT. In summary, our results support development of a prospective alloHCT study, with the aim to optimize therapy and to improve overall outcome in advanced SM. Based on our pilot study, alloHCT should also be considered in practice for eligible and fit patients with SM-AHNMD (if treatment of AHNMD component needs alloHCT or SM component is aggressive), rapidly progressing ASM, MCL and MML when a suitable HLA-matched sibling donor or URD is available. Disclosures: Vercellotti: Sangart Inc.: Research Funding; Seattle Genetics: Research Funding. Akin:Novartis: Consultancy. Valent:Novartis: Consultancy, Honoraria, Research Funding.