HLA-Identical Sibling-Matched, CD34+ Selected, T Cell Depleted Peripheral Blood Stem Cells Following Myeloablative Conditioning for First or Second Remission Acute Myeloid Leukemia (AML): Results of Blood and Marrow Transplant Clinical Trials Network (BMT CTN) Protocol 0303.

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Abstract 655Allogeneic hematopoietic cell transplantation (HCT) is the most effective means to prevent relapse in patients (pts) with AML in complete remission (CR). However, quality of life and overall survival (OS) are often affected by both acute and chronic graft versus host disease (GVHD). GVHD is most effectively prevented by ex vivo T cell depletion (TCD) of the allograft, but has been limited in its use by logistical difficulties, lack of an FDA-approved method, and concerns regarding potential risk of graft rejection, post transplant infections, and leukemic relapse. Most reported TCD studies represent single centers, multiple disease types and processing methods with varying degrees of TCD, all of which affect outcome. Therefore we designed a trial using a single processing method providing extensive TCD that did not require post transplant GVHD prophylaxis involving adult pts with AML in first or second CR. We hypothesized that the undesired side effects of TCD HCT would be reduced if combined with a conditioning regimen that was highly immunosuppressive and anti-leukemic. The primary objective was to achieve a disease-free survival (DFS) rate at 6 months (mos) post transplant that exceeded 75%. Secondary objectives included assessments of engraftment, transplant related mortality (TRM), GVHD, relapse, and performance of a single TCD method (CD34+ cell selection using the Miltenyi CliniMACS device) at participating centers. From 10/2005 to 12/2008, 47 pts were enrolled and 44 transplanted at 8 different centers. Median age was 48.5 years (range 21-59) with 28 female and 16 male pts. Of 37 AML CR1 pts, 49% had an unfavorable cytogenetic or molecular risk profile. The conditioning regimen consisted of hyperfractionated total body irradiation (1375cGy in 11 fractions) with partial lung shielding, thiotepa (10mg/kg), cyclophosphamide (120mg/kg), and rabbit antithymocyte globulin (2.5mg/kg). The donors, all HLA-identical siblings, were given G-CSF for mobilization and scheduled to undergo at least 2 leukapheresis procedures to ensure a graft with a high CD34+ cell content. All allografts were CD34-enriched and were targeted to contain ≥ 5×10e6 CD34+ cells/kg and < 1.0×10e5 CD3+ cells/kg. The median CD34+ and CD3+ doses achieved were 8.1 × 10e6/kg (range 2.4-46.2) and 0.07 × 10e5/kg (range 0.01-0.85), respectively. The majority (81%) of pts received the targeted CD34+ cell dose and no pt received > 1.0×10e5 CD3+ cells/kg. No pharmacological GVHD prophylaxis was given post transplant. There were no significant toxicities related to infusion of the CD34 enriched allografts. The most common grade 3-5 regimen-related toxicities included grades 3 or 4 mucositis (39%) and grades 3-5 pulmonary abnormalities (11%). Only 1 pt experienced hepatic veno-occlusive disease. All pts engrafted rapidly with a median time to neutrophil recovery (ANC > 500/ul) of 11 days (range 9-19). There was 1 secondary graft failure. The assessed outcomes are shown below.Estimate (95% Confidence Interval)Outcome100 Days6 Months12 MonthsAcute GVHD II-IV20.5% (8.7 – 23.3%)Acute GVHD III-IV4.5% (0 – 10.6%)Chronic GVHD17.7% (5.8-29.6%)Extensive Chronic GVHD7.6% (0-15.7%)TRM17.8% (5.8-29.8%)Overall Relapse18.2% (5.9-30.5%)Relapse 1st CR9.6% (0- 19.8%%)Relapse 2nd CR64.3% (27.5-100%)DFS81.3% (66.1-90.2%)64.0% (46.5-77.1%)DFS 1st CR89.2% (73.7-95.8%)72.1% (53.0-84.6%)OS74.3% (57.3-85.4%)The absolute peripheral CD4+ cell count remained on average below 200/ul until day +365. Donor cell chimerism increased in the CD3+ cell compartment through day +365. There were 14 deaths. The most common causes of death were relapse (N=5) and pulmonary toxicity (N=4). The median follow-up of survivors is 489 days (range 96-776). There was no difference in OS or DFS for pts above or below the median age of 48.5 years. We conclude that TCD HCT following myeloablative chemoradiotherapy can be performed in a multi-center setting using a single TCD method without additional post transplant prophylaxis with excellent DFS and OS, consistent engraftment, low TRM, and low incidence of relapse even in pts with unfavorable risk AML in CR1. The low incidences of acute and chronic GVHD in the absence of post transplant prophylaxis were particularly encouraging. A follow-up study of TCD HCT in AML recipients of unrelated donor allografts is being planned by the BMT CTN Disclosures:No relevant conflicts of interest to declare.