Pre-Emptive Use of Rituximab in Epstein-Barr Virus Reactivation: Incidence, Predictive Factors, Monitoring, and Outcomes.

Post-transplant lymphoproliferative disease (PTLD) is a fatal complication of hematopoietic cell transplantation (HCT) associated with the Epstein-Barr virus (EBV). Multiple factors such as transplant type, graft-versus-host disease (GVHD), human leukocyte antigens (HLA) mismatch, patient age, and T-lymphocyte-depleting treatments increase the risk of PTLD. EBV reactivation in hematopoietic cell transplant recipients is monitored through periodic quantitative polymerase chain reaction (Q-PCR) tests. However, substantial uncertainty persists regarding the clinically significant EBV levels for these patients. Guidelines recommend initiating EBV monitoring no later than four weeks post-HCT and conducting it weekly. Pre-emptive therapies, such as the reduction of immunosuppressive therapy and the administration of rituximab to treat EBV viral loads are also suggested. In this study, we investigated the occurrence of EBV-PTLD in 546 HCT recipients, focusing on the clinical manifestations and risk factors associated with the disease. We managed to identify 67,150 viral genomic copies/mL as the cutoff point for predicting PTLD, with 80% sensitivity and specificity. Among our cohort, only 1% of the patients presented PTLD. Anti-thymocyte globulin (ATG) and GVHD were independently associated with lower survival rates and higher treatment-related mortality. According to our findings, prophylactic measures including regular monitoring, pre-emptive therapy, and supportive treatment against infections can be effective in preventing EBV-related complications. This study also recommends conducting EBV monitoring at regular intervals, initiating pre-emptive therapy when viral load increases, and identifying factors that increase the risk of PTLD. Our study stresses the importance of frequent and careful follow-ups of post-transplant complications and early intervention in order to improve survival rates and reduce mortality.

Context-dependent T-cell Receptor Gene Repertoire Profiles in Proliferations of T Large Granular Lymphocytes.

T cell large granular lymphocyte (T-LGL) lymphoproliferations constitute a disease spectrum ranging from poly/oligo to monoclonal. Boundaries within this spectrum of proliferations are not well established. T-LGL lymphoproliferations co-occur with a wide variety of other diseases ranging from autoimmune disorders, solid tumors, hematological malignancies, post solid organ, and hematopoietic stem cell transplantation, and can therefore arise as a consequence of a wide variety of antigenic triggers. Persistence of a dominant malignant T-LGL clone is established through continuous STAT3 activation. Using next-generation sequencing, we profiled a cohort of 27 well-established patients with T-LGL lymphoproliferations, aiming to identify the subclonal architecture of the T-cell receptor beta (TRB) chain gene repertoire. Moreover, we searched for associations between TRB gene repertoire patterns and clinical manifestations, with the ultimate objective of discriminating between T-LGL lymphoproliferations developing in different clinical contexts and/or displaying distinct clinical presentation. Altogether, our data demonstrates that the TRB gene repertoire of patients with T-LGL lymphoproliferations is context-dependent, displaying distinct clonal architectures in different settings. Our results also highlight that there are monoclonal T-LGL cells with or without STAT3 mutations that cause symptoms such as neutropenia on one end of a spectrum and reactive oligoclonal T-LGL lymphoproliferations on the other. Longitudinal analysis revealed temporal clonal dynamics and showed that T-LGL cells might arise as an epiphenomenon when co-occurring with other malignancies, possibly reactive toward tumor antigens.

SARS-CoV-2-specific T cell therapy for severe COVID-19: a randomized phase 1/2 trial.

Despite advances, few therapeutics have shown efficacy in severe coronavirus disease 2019 (COVID-19). In a different context, virus-specific T cells have proven safe and effective. We conducted a randomized (2:1), open-label, phase 1/2 trial to evaluate the safety and efficacy of off-the-shelf, partially human leukocyte antigen (HLA)-matched, convalescent donor-derived severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2)-specific T cells (CoV-2-STs) in combination with standard of care (SoC) in patients with severe COVID-19 compared to SoC during Delta variant predominance. After a dose-escalated phase 1 safety study, 90 participants were randomized to receive CoV-2-ST+SoC (n = 60) or SoC only (n = 30). The co-primary objectives of the study were the composite of time to recovery and 30-d recovery rate and the in vivo expansion of CoV-2-STs in patients receiving CoV-2-ST+SoC over SoC. The key secondary objective was survival on day 60. CoV-2-ST+SoC treatment was safe and well tolerated. The study met the primary composite endpoint (CoV-2-ST+SoC versus SoC: recovery rate 65% versus 38%, P = 0.017; median recovery time 11 d versus not reached, P = 0.052, respectively; rate ratio for recovery 1.71 (95% confidence interval 1.03-2.83, P = 0.036)) and the co-primary objective of significant CoV-2-ST expansion compared to SοC (CoV-2-ST+SoC versus SoC, P = 0.047). Overall, in hospitalized patients with severe COVID-19, adoptive immunotherapy with CoV-2-STs was feasible and safe. Larger trials are needed to strengthen the preliminary evidence of clinical benefit in severe COVID-19. EudraCT identifier: 2021-001022-22 .

Differences in the immunoglobulin gene repertoires of IgG versus IgA multiple myeloma allude to distinct immunopathogenetic trajectories.

The analysis of the immunogenetic background of multiple myeloma (MM) has proven key to understanding disease ontogeny. However, limited information is available regarding the immunoglobulin (IG) gene repertoire in MM cases carrying different heavy chain isotypes. Here, we studied the IG gene repertoire in a series of 523 MM patients, of whom 165 and 358 belonged to the IgA and IgG MM groups, respectively. IGHV3 subgroup genes predominated in both groups. However, at the individual gene level, significant (p<0.05) differences were identified regarding IGHV3-21 (frequent in IgG MM) and IGHV5-51 (frequent in IgA MM). Moreover, biased pairings were identified between certain IGHV genes and IGHD genes in IgA versus IgG MM. Turning to the imprints of somatic hypermutation (SHM), the bulk of rearrangements (IgA: 90.9%, IgG: 87.4%) were heavily mutated [exhibiting an IGHV germline identity (GI) <95%]. SHM topology analysis disclosed distinct patterns in IgA MM versus IgG MM cases expressing B cell receptor IG encoded by the same IGHV gene: the most pronounced examples concerned the IGHV3-23, IGHV3-30 and IGHV3-9 genes. Furthermore, differential SHM targeting was also identified between IgA MM versus IgG MM, particularly in cases utilizing certain IGHV genes, alluding to functional selection. Altogether, our detailed immunogenetic evaluation in the largest to-date series of IgA and IgG MM patients reveals certain distinct features in the IGH gene repertoires and SHM. These findings suggest distinct immune trajectories for IgA versus IgG MM, further underlining the role of external drive in the natural history of MM.

Soluble Urokinase-Type Plasminogen Activator Receptor (suPAR) and Growth Differentiation Factor-15 (GDF-15) Levels Are Significantly Associated with Endothelial Injury Indices in Adult Allogeneic Hematopoietic Cell Transplantation Recipients.

Hematopoietic stem cell transplantation-associated thrombotic microangiopathy (HSCT-TMA) and graft-versus-host disease (GvHD) represent life-threatening syndromes after allogeneic hematopoietic stem cell transplantation (allo-HSCT). In both conditions, endothelial dysfunction is a common denominator, and development of relevant biomarkers is of high importance for both diagnosis and prognosis. Despite the fact that soluble urokinase plasminogen activator receptor (suPAR) and growth differentiation factor-15 (GDF-15) have been determined as endothelial injury indices in various clinical settings, their role in HSCT-related complications remains unexplored. In this context, we used immunoenzymatic methods to measure suPAR and GDF-15 levels in HSCT-TMA, acute and/or chronic GVHD, control HSCT recipients, and apparently healthy individuals of similar age and gender. We found considerably greater SuPAR and GDF-15 levels in HSCT-TMA and GVHD patients compared to allo-HSCT and healthy patients. Both GDF-15 and suPAR concentrations were linked to EASIX at day 100 and last follow-up. SuPAR was associated with creatinine and platelets at day 100 and last follow-up, while GDF-15 was associated only with platelets, suggesting that laboratory values do not drive EASIX. SuPAR, but not GDF-15, was related to soluble C5b-9 levels, a sign of increased HSCT-TMA risk. Our study shows for the first time that suPAR and GDF-15 indicate endothelial damage in allo-HSCT recipients. Rigorous validation of these biomarkers in many cohorts may provide utility for their usefulness in identifying and stratifying allo-HSCT recipients with endothelial cell impairment.

Adolescents and young adults (AYA) with acute myeloid leukemia (AML): real-world long-term results and age-specific outcomes.

Opposing acute lymphoblastic leukemia, sparse data about AYAs with acute myeloid leukemia (AML) is available. Overall, 125 AYAs (age 10-35 years) treated during the last two decades were evaluated and compared to 385 older patients. CBF leukemia was more frequent in AYAs (21.6% vs. 8%, p < 0.001); however, many presented high-risk features. AYAs showed improved complete remission rate (CR, 80% vs. 65%, p = 0.01), lower cumulative incidence of relapse and TRM and longer survival (5 year-OS 53% vs. 24%, p < 0.0001), observed mainly in intermediate-risk karyotype. Adolescents displayed even better outcomes (5 year-OS 69%). AlloHCT in CR1 was beneficial for nonadolescent AYAs (5 year-OS 66.7% vs. 44.4% without HCT, p = 0.04). Among 50 APL patients, 19 AYAs experienced better outcomes than older, mainly attributed to reduced treatment-related mortality (TRM, 5% vs. 19%, p = 0.1). We observed an important (>10%) survival gain for AYAs during the last decade. However, AYAs have still unmet needs to obtain optimal cure rates.

Genetic and Functional Evidence of Complement Dysregulation in Multiple Myeloma Patients with Carfilzomib-Induced Thrombotic Microangiopathy Compared to Controls.

BACKGROUND: Carfilzomib, an irreversible proteasome inhibitor approved for the treatment of relapsed/refractory Multiple Myeloma (MM) has been associated with Thrombotic Microangiopathy (TMA). Several pathogenetic mechanisms of carfilzomib-induced TMA have been proposed; however, recently, there has been a shift of focus on the potential contribution of complement dysregulation. Our aim was to explore whether patients with carfilzomib-induced TMA harbor germline variants of complement-related genes, which have been characterized as risk factors for TMA. METHODS: We retrospectively recruited consecutive MM patients with carfilzomib-induced TMA and compared them to MM patients who received ≥4 cycles of carfilzomib and did not develop signs/symptoms of TMA, in a 1:2 ratio. Genomic DNA from peripheral blood was analyzed using next generation sequencing (NGS) with a complement-related gene panel; ADAMTS13 activity and soluble C5b-9 were measured using ELISA. RESULTS: Complement-related variants were more common in patients with carfilzomib-induced TMA compared to non-TMA controls, regardless of patient and treatment characteristics; ADAMTS13 activity and C5b-9 were compatible with the phenotype of complement-related TMA. CONCLUSIONS: We confirmed the previous findings that implicated complement-related genes in the pathogenesis of carfilzomib-induced TMA. Most importantly, by incorporating a control group of non-TMA MM patients treated with carfilzomib-based regimens and functional complement assays, we enhanced the credibility of our findings.

Secondary Acute Myeloid Leukemia (sAML): Similarly Dismal Outcomes of AML After an Antecedent Hematologic Disorder and Therapy Related AML.

Therapy related acute myeloid leukemia (tAML) and secondary AML after an antecedent hematologic disorder (sAML-AHD) are often addressed together, blurring any clinical and prognostic differences. Among 516 AML patients, we compared characteristics and outcomes of 149 patients with "sAML" (sAML-AHD: 104, tAML: 45), uniformly and intensively treated during the last 2 decades at 1 center. Clinical outcomes of the whole "sAML" cohort were significantly inferior compared to de novo AML and in both intermediate and poor cytogenetic risk groups. Adverse karyotype had no effect on survival in tAML, while it was a negative predictor in sAML-AHD. Both groups showed similarly dismal outcome, with low complete remission rates (CR 44% vs. 41%) and median overall survival (OS 7 vs. 10.5 months). Allogeneic hematopoietic cell transplantation (alloHCT) recipients in CR1 had superior median OS (24 vs. 8 months). By multivariate analysis, alloHCT was an independent predictor of outcome, while karyotype was for sAML-AHD only. In conclusion, both "sAML" groups have inferior outcomes after chemotherapy, with adverse karyotype affecting primarily sAML-AHD. Until new treatment approaches are available, only alloHCT offers a survival advantage.

Predictors of Transplant-Associated Thrombotic Microangiopathy in Patients With Overlap or Chronic Graft-vs-Host-Disease.

BACKGROUND: Recent data suggest that novel biologic agents are associated with increased risk of thrombotic microangiopathy (TMA). Ruxolitinib, an approved treatment for graft-vs-host-disease (GVHD), has been associated with thrombocytopenia of unclear etiology. METHODS: We investigated factors and outcomes associated with transplant-associated thrombotic microangiopathy (TA-TMA) in patients with GVHD. We retrospectively enrolled consecutive allogeneic hematopoietic cell transplantation recipients with overlap or chronic GVHD at our Joint Accreditation Committee ISCT-Europe & EBMT-accredited unit (January 2016-June 2019). Ruxolitinib has been administered off-label since 2016. RESULTS: Among 160 patients with GVHD, 18 were diagnosed with TA-TMA. TA-TMA developed at a median of 150 posttransplant days (range, 98-3013). Among pre- and posttransplant factors, TA-TMA was associated only with ruxolitinib administration and severe GVHD. Interestingly, these 2 variables did not correlate with each other. In the multivariate analysis, both were independent predictors of TA-TMA. Time-dependent analysis confirmed ruxolitinib's association with TA-TMA. With a follow-up of 38.4 months (4.6-83.9) in surviving patients, 5-year overall survival was 52.9%, independently predicted by TA-TMA, severe acute GVHD, and CD34+ cells infused. Ruxolitinib was not associated with survival outcomes. CONCLUSIONS: Our data suggest that ruxolitinib and GVHD severity are associated with TA-TMA. Given the expanding use of ruxolitinib in GVHD and ongoing trials on chronic GVHD, further studies are warranted to confirm these findings.

Survival Advantage of Treosulfan Plus Fludarabine Before Allogeneic Hematopoietic Cell Transplantation for Older or Comorbid Patients With Myeloid Malignancies.

We have previously shown an advantage of a myeloablative conditioning regimen with reduced toxicity (Fludarabine 150 mg/m2, Treosulfan 42 g/m2, FluTreo) compared to a reduced-intensity regimen. We aimed to determine long-term safety and efficacy of FluTreo. We prospectively studied consecutive patients who received FluTreo in our center (2014-2019) on the basis of age (≥50 years), hematopoietic cell transplantation comorbidity index (HCT-CI) ≥2, or both. FluTreo recipients were then compared to a historical control group. We studied 68 FluTreo recipients, with a median age of 58.5 years and HCT-CI of 3. We calculated cumulative incidence (CI) of acute (grade 2-4) and moderate/severe chronic graft-versus-host disease (GVHD) (29.9% and 25%, respectively). The 3-year CI of treatment-related mortality was 19.1%, associated only with acute GVHD (P < .001). With a median follow-up of 27.3 (range 5.7-84.5) months in surviving patients, the 3-year overall survival (OS) was 56.6%, and disease-free survival (DFS) was 54.9%. Median survival has not yet been reached. Among pretransplantation and transplantation factors, only HCT-CI was associated with DFS and OS (P = .022 and P = .043, respectively). FluTreo recipients aged ≥50 with HCT-CI ≤ 2 had favorable DFS and OS compared with patients aged ≥50 with HCT-CI ≤2 after myeloablative conditioning. Our real-world study confirms that HCT with FluTreo expands the transplant population with favorable outcomes compared to previously used conditions. The choice of HCT in patients of a rather older age and comorbidity index needs to be revisited.

Patient risk stratification and tailored clinical management of post-transplant CMV-, EBV-, and BKV-infections by monitoring virus-specific T-cell immunity.

Background: Despite routine post-transplant viral monitoring and pre-emptive therapy, viral infections remain a major cause of allogeneic hematopoietic cell transplantation-related morbidity and mortality. Objective: We here aimed to prospectively assess the kinetics and the magnitude of cytomegalovirus-(CMV), Epstein Barr virus-(EBV), and BK virus-(BKV)-specific T cell responses post-transplant and evaluate their role in guiding therapeutic decisions by patient risk-stratification. Study design: The tri-virus-specific immune recovery was assessed by Elispot, in 50 consecutively transplanted patients, on days +20, +30, +60, +100, +150, +200 post-transplant and in case of reactivation, weekly for 1 month. Results: The great majority of the patients experienced at least one reactivation, while over 40% of them developed multiple reactivations from more than one of the tested viruses, especially those transplanted from matched or mismatched unrelated donors. The early reconstitution of virus-specific immunity (day +20), favorably correlated with transplant outcomes. Εxpanding levels of CMV-, EBV-, and BKV-specific T cells (VSTs) post-reactivation coincided with decreasing viral load and control of infection. Certain cut-offs of absolute VST numbers or net VST cell expansion post-reactivation were determined, above which, patients with CMV or BKV reactivation had >90% probability of complete response (CR). Conclusion: Immune monitoring of virus-specific T-cell reconstitution post-transplant may allow risk-stratification of virus reactivating patients and enable patient-tailored treatment. The identification of individuals with high probability of CR will minimize unnecessary overtreatment and drug-associated toxicity while allowing candidates for pre-emptive intervention with adoptive transfer of VSTs to be appropriately selected.

Pretransplant Genetic Susceptibility: Clinical Relevance in Transplant-Associated Thrombotic Microangiopathy.

Transplant-associated thrombotic microangiopathy (TA-TMA) is a life-threatening complication of allogeneic hematopoietic cell transplantation (HCT). We hypothesized that pretransplant genetic susceptibility is evident in adult TA-TMA and further investigated the association of TMA-associated variants with clinical outcomes. We studied 40 patients with TA-TMA, donors of 18 patients and 40 control non-TMA HCT recipients, without significant differences in transplant characteristics. Genomic DNA from pretransplant peripheral blood was sequenced for TMA-associated genes. Donors presented significantly lower frequency of rare variants and variants in exonic/splicing/untranslated region (UTR) regions, compared with TA-TMA patients. Controls also showed a significantly lower frequency of rare variants in ADAMTS13, CD46, CFH, and CFI. The majority of TA-TMA patients (31/40) presented with pathogenic or likely pathogenic variants. Patients refractory to conventional treatment (62%) and patients that succumbed to transplant-related mortality (65%) were significantly enriched for variants in exonic/splicing/UTR regions. In conclusion, increased incidence of pathogenic, rare and variants in exonic/splicing/UTR regions of TA-TMA patients suggests genetic susceptibility not evident in controls or donors. Notably, variants in exonic/splicing/UTR regions were associated with poor response and survival. Therefore, pretransplant genomic screening may be useful to intensify monitoring and early intervention in patients at high risk for TA-TMA.

Intestinal thrombotic microangiopathy: a distinct entity in the spectrum of graft-versus-host disease.

Transplant-associated thrombotic microangiopathy (TA-TMA) is a severe complication of allogeneic hematopoietic cell transplantation (allo-HCT) with multisystem involvement. Cases of TMA in the intestinal vasculature (intestinal TMA/iTMA) have been reported. We hypothesized that iTMA is a distinct entity from TA-TMA. To test this hypothesis, we prospectively recruited allo-HCT recipients with an indication for endoscopy. Among 20 patients, histological features of iTMA, including loss of glands, total denudation of mucosa, apoptosis and detachment of endothelial cells, mucosal hemorrhage, intraluminal fibrin and microthrombi were found in six. Only 2/6 were classified as GVHD/TA-TMA, while the other 4 as GVHD/no TA-TMA. Gastro-intestinal symptoms were similar between the patients with or without iTMA. With a median follow-up of 11.1 (2.1-67.5) months, 1-year overall survival was 22.2% for iTMA, 55% for GVHD and 60% for TA-TMA. On multivariate analysis, independent unfavorable predictors of OS were iTMA (p = 0.048), HLA mismatched donors (p = 0.008) and gastro-intestinal bleeding (p = 0.021). In conclusion, iTMA emerges as a novel distinct entity in patients with GVHD and/or TA-TMA. Distinct histological features may be useful in differential diagnosis of these severe HCT complications. The higher mortality rates of iTMA than TA-TMA highlight the need for further investigation of this condition.

Linking Complement Activation, Coagulation, and Neutrophils in Transplant-Associated Thrombotic Microangiopathy.

Transplant-associated thrombotic microangiopathy (TA-TMA) is a severe and life-threatening complication of hematopoietic cell transplantation (HCT) that often coincides with graft-versus-host-disease (GVHD). Although endothelial damage seems to be the common denominator for both disorders, the role of complement system, neutrophils, and coagulation has not been clarified. In an effort to distinguish the pathogenesis of TA-TMA from GVHD, we evaluated markers of complement activation, neutrophil extracellular trap (NET) release, endothelial damage, and activation of coagulation cascade in the circulation of patients with these two disorders, as well as control HCT recipients without TA-TMA or GVHD. We observed that the terminal complement product C5b-9 levels, the levels of markers of NET formation, and thrombin-antithrombin complex levels were significantly increased in the TA-TMA group compared with patients without complications, whereas there was no significant difference between the GVHD and the control group. On the other hand, the levels of circulating thrombomodulin, an endothelial damage marker, were significantly increased in both TA-TMA and GVHD patients. These findings propose a role for the interplay between complement system, neutrophil activation through NET release, and activation of the coagulation cascade in TA-TMA.

Skewing of the T-cell receptor repertoire in patients receiving rituximab after allogeneic hematopoietic cell transplantation: what lies beneath?

Rituximab is known to affect T cell immune responses. We and others have reported expansions of T large granular lymphocytes (T-LGLs) in lymphoma patients after Rituximab. We report here the immunogenetic profiling of the T cell receptor (TR) gene repertoire in 14 patients who received Rituximab post allo-HCT and explore clinicobiological correlations. All experienced antigenic triggers, CMV, EBV re-activation and chronic GvHD and had been treated with Rituximab. Skewing of TRBV genes was observed: 3 TRBV genes accounted for half of the repertoire. Oligoclonal pattern with expanded clonotypes was common. Patients with oligoclonality exhibited frequently cGvHD. Longitudinal samples in one revealed distinct clonotypes, suggesting clonal drift. T-LGL leukemia of donor origin with mixed chimerism eventually developed. In conclusion, we report development of oligoclonal T-LGLs after Rituximab post allo-HCT, alluding to antigen selection. Persistence of this phenomenon likely reflects strong antigenic stimulation by viruses and/or cGVHD aggravated by Rituximab.

Favorable impact of extracorporeal photopheresis in acute and chronic graft versus host disease: Prospective single-center study.

BACKGROUND: Graft vs host disease (GVHD) is the most severe complication of allogeneic hematopoietic cell transplantation. Conventional immunosuppressive therapy increases morbidity and mortality without improving survival. Extracorporeal photopheresis (ECP) has been introduced as an alternative treatment in steroid-dependent and steroid-refractory disease. STUDY DESIGN AND METHODS: We studied the safety and efficacy of ECP as a second- or third-line treatment in GVHD. RESULTS: ECP was administered in 21 patients with grade III-IV acute GVHD and 88 patients with extensive chronic GVHD, without ECP-related adverse events. Eight patients receiving four or less ECP sessions were not further analyzed. The majority of acute GVHD patients (84%) presented partial (15) or complete (1) response to ECP. Immunosuppression was reduced in 10 of 19 patients and ceased in 1 of 19 patients. One-year cumulative incidence (CI) of transplant-related mortality (TRM) (17.6%) was associated with the lack of response to ECP and steroid refractoriness. With a follow-up of 17.5 (1.8-58.3) months, 1-year overall survival (OS) (52.5%) was independently associated with a higher number of ECP sessions. Regarding chronic GVHD, complete response was achieved in 35 patients, whereas partial response in 25 patients, leading to an overall response rate of 73%. Cutaneous sclerosis manifestations were associated with higher response rates. With a follow-up of 68.1 (5.4-283.1) months, 5-year CI of TRM (24.1%) was associated only with a number of ECP sessions. The 5-year OS (64.5%) was independently associated with number of ECP sessions and cutaneous manifestations. CONCLUSION: Our findings suggest that ECP is safe and effective for GVHD and should be considered early in the course of GVHD, before irreversible end-organ damage has been established.

Survival Advantage and Comparable Toxicity in Reduced-Toxicity Treosulfan-Based versus Reduced-Intensity Busulfan-Based Conditioning Regimen in Myelodysplastic Syndrome and Acute Myeloid Leukemia Patients after Allogeneic Hematopoietic Cell Transplantation.

Treosulfan has been incorporated in conditioning regimens for sustained remission without substantial toxicity and treatment-related mortality (TRM). We aimed to analyze the safety and efficacy of a fludarabine 150 mg/m2 and treosulfan 42 g/m2 (FluTreo) conditioning regimen in medically infirm patients. Outcomes were compared with those of a similar historical group treated with fludarabine 150 mg/m2 to 180 mg/m2, busulfan 6.4 mg/kg, and antithymocyte globulin (ATG) 5 mg/kg to 7.5 mg/kg (FluBuATG). Thirty-one consecutive patients with acute myeloid leukemia (AML; n = 21), myelodysplastic syndrome (MDS; n = 6), or treatment-related AML (n = 4) received FluTreo conditioning. The historical group consisted of 26 consecutive patients treated with FluBuATG. In the FluTreo group, engraftment was prompt in all patients and 74% achieved >99% donor chimerism by day +30. No grades III or IV organ toxicities were noted. One-year cumulative incidences (CI) of acute and chronic graft-versus-host disease (GVHD) were 19.4% and 58.4%. The groups were similar for age, disease risk, lines of treatment, hematopoietic cell transplantation-specific comorbidity index, and acute or chronic GVHD incidence, except that there were more matched unrelated donor recipients in the FluTreo group (P < .001). With 20 (range, 2 to 36) months follow-up for FluTreo and 14 (range, 2 to 136) for FluBuATG, the 1-year cumulative overall survival (OS) probability was 76% versus 57%, respectively (P = .026); 1-year disease-free survival (DFS) was 79% versus 38% (P < .001). In multivariate analysis, the only significantly favorable factor for OS and DFS was FluTreo (P = .010 and P = .012). The CI of relapse mortality was markedly decreased in FluTreo versus FluBuATG (7.4% versus 42.3%, P < .001). In conclusion, the treosulfan-based regimen resulted in favorable OS and DFS with acceptable toxicity and low relapse rates compared with busulfan-based conditioning.