Targeted Intravenous Busulfan and Fludarabine Allogeneic Transplantation Conditioning in Acute Myeloid Leukemia.

Abstract 2290

Poster Board II-267

Reduced and intermediate intensity conditioning with allogeneic hematopoietic cell transplantation (HCT) offers promise to effectively control hematologic malignancies, while limiting treatment related toxicity and mortality (TRM). We aimed to examine the efficacy of IV targeted Busulfan and Fludarabine (IV-Bu/Flu) in a large series of adults with exclusively acute myelogenous leukemia (AML). One hundred adults (median age 48) with AML (CR1 49, CR2 25, REL1 8, REL2 1, PIF 16, untreated 1) were treated with Busulfan 130-145 mg/m2/day for four days with pharmacokinetic targeting on the final two days to achieve an area under the curve (AUC) of 5300 (+/-10%) μmol*min/L/day and Fludarabine 40mg/m2/day for 4 days, followed by transplantation of G-CSF mobilized peripheral blood stem cells (PBSC) (N=98) or unstimulated bone marrow (BM) (N=2) from allogeneic donors (MRD 38, MUD 38, MMUD 24). Acute GVHD prophylaxis consisted of tacrolimus/methotrexate (N = 77), tacrolimus/mycophenolate mofetil (N = 22), or tacrolimus/sirolimus (N = 1). Median time to neutrophil and platelet engraftment was 16 and 12 days, respectively. Non-relapse mortality was 3% at 100 days, and 15% by 1 year. The cumulative incidence of relapse was 41%. Overall survival (OS) was 59% (95% CI: 48.1 – 67.5) at 1 year, and 42% (95% CI: 30.8-53.3) at 4 years. OS at 4 years for primary AML in CR1, secondary AML in CR1, CR2, and PIF were 52.9%, 40.1%, 41.2%, and 57.5% respectively; none with relapsed disease survived to 4 years (log-rank p = 0.0014). Progression-free survival (PFS) was 53% (95% CI: 42.8 – 62.2) at 1 year, and 32.3% (95% CI: 21.8 – 43.2) at 4 years. PFS at 4 years for primary AML in CR1, secondary AML in CR1, CR2, and PIF were 44.1%, 33.4%, 33.9%, and 33.1%, respectively, while none with relapsed disease at transplant reached this endpoint (p = 0.0264). On multivariable modeling, remission status at HCT (relapsed disease HR 14.85 (95% CI: 2.12 - 104.2), p = 0.007), moderate/severe cGVHD (HR 0.281, 95% CI: 0.10 - 0.76; p = 0.013), and day 90 bone marrow (BM) chimerism ≥ 90% (HR 0.245, 95% CI: 0.08 - 0.79; p = 0.018) predicted overall survival, and day 90 BM chimerism ≥ 90% (HR of 0.18 (95% CI: 0.08 - 0.45), p = 0.0002) predicted PFS. The following were not significantly related with OS or PFS: age, cytogenetics, donor relation, number of induction cycles, aGVHD prophylaxis regimen, maximum aGVHD grade, WBC at diagnosis, time in first CR, or % BM blasts prior to transplant. Day 90 BM chimerism and cGVHD were significantly related with relapse. Maximum grade of aGVHD predicted non-relapse mortality. These data support the low TRM and efficacy of IV-Bu/Flu in a large series of exclusively AML patients, and demonstrate the impact of day 90 bone marrow chimerism as an important prognostic factor. Further efforts to mitigate relapse risk after HCT are warranted, particularly in those with advanced disease at time of transplant.