Fludarabine and Targeted Busulfan Is Safe and Effective Conditioning Before Hematopoietic Stem Cell Transplantation in Adult Patients with Acute Lymphoblastic Leukemia in First Remission

Chemotherapy for adult patients with acute lymphoblastic leukemia (ALL) is associated with high risk of relapse and an overall 2-year survival of 40 to 50%. Allogenic hematopoietic cell transplantation (HCT) in first complete remission (CR1) decreases the risk of relapse and improves outcome over chemotherapy for adult ALL pts, but non-relapse mortality (NRM) is a drawback especially in older patients. In the MRC UKALL XII/ECOG E2993 trial, the 2 year NRM of patient with an allogenic donor was 19% in standard risk patients and 36% in patients over 35 years or those with high risk leukemia. We tested safety and efficacy of a non-irradiation regimen consisting of fludarabine (FLU) and pharmacokinetically-targeted busulfan (BU) for adults with ALL in CR1.

We report the outcomes of 42 consecutive patients with ALL in CR1, 21 positive for the Philadelphia chromosome (Ph+). All patients were in complete morphologic remission before HCT. The median age was 33 (range: 19–62) years, 19 were females and 23 males. Median time from diagnosis to HCT was 6 (range: 3–45) months. Thirty patients were treated to achieve an average daily BU area under the curve (AUC) of 5300 microM-min for 4 days, and 12 patients were treated on a clinical trial to achieve an average daily BU AUC of 6000 to 7500 microM-min for 4 days. Graft-versus-host disease (GVHD) prophylaxis was tacrolimus in all patients, in combination with either methotrexate (88%) or sirolimus (12%). Twenty (48%) patients received grafts from matched related donor, 16 (38%) from matched unrelated donor and 6 (14%) from a mismatched unrelated donor. The median follow-up of surviving patients is 2 (median 1.2–4.3) years.

Overall survival at 2 years was 66% (95% CI 52%–81%) for all patients, 70% (95% CI 51%–88%) for Ph- and 63% (95% CI 41%–85%) for Ph+ patients (p=0.59). Overall survival did not differ by age, above or below 35 years (p=0.39). Disease-free survival at 2 years was 59% (95% CI 44%–74%) for all patients, 65% (95% CI 45%–84%) for Ph- and 52% (95% CI 28%–74%) for Ph+ pts (p=0.49). The cumulative incidence of relapse at 2-year was 27% (95% CI 16%–45%). The cumulative incidence of acute GVHD grades II–IV was 64% (95% CI 51%–81%) and grades III–IV GVHD was 25% (95% CI 13%–47%). The cumulative incidence of non-relapse mortality (NRM) was 5% (95% CI 1%–18%) at 100 days and 14% (95% CI 7%–30%) at 2 years.

These data show that FLU with myeloablative doses of PK targeted BU is an effective alternative to total body irradiation and etoposide or cyclophosphamide for conditioning patients with ALL without an increased risk of relapse after HCT. The low NRM allows to safely delivering myeloablative chemotherapy and allogenic HCT to older patients. This HCT regimen should be prospectively compared to chemotherapy for adult patients with ALL.

No relevant conflicts of interest to declare.