Although a combination of i.v.
busulfan (Bu) and
fludarabine (Flu) is a safe, reduced-toxicity conditioning program for
acute myelogenous leukemia/
myelodysplastic syndromes (AML/MDS), recurrent
leukemia posttransplantation remains a problem. To enhance the conditioning regimen's antileukemic effect, we decided to supplant Flu with
clofarabine (Clo), and assayed the interactions of these
nucleoside analogs alone and in combination with Bu in Bu-resistant human cell lines in vitro. We found pronounced synergy between each
nucleoside and the
alkylator but even more enhanced cytotoxic synergy when the
nucleoside analogs were combined prior to exposing the cells to Bu. We then designed a 4-arm clinical trial in patients with
myeloid leukemia undergoing allogeneic
stem cell transplantation (allo-SCT). Patients were adaptively randomized as follows: Arm I-Clo:Flu 10:30 mg/m(2), Arm II-20:20 mg/m(2), Arm III-30:10 mg/m(2), and Arm IV-single-agent Clo at 40 mg/m(2). The
nucleoside analog(s) were/was infused over 1 hour once daily for 4 days, followed on each day by Bu, infused over 3 hours to a pharmacokinetically targeted daily area under the curve (AUC) of 6000 μMol-min ± 10%. Fifty-one patients have been enrolled with a minimum follow-up exceeding 100 days. There were 32 males and 19 females, with a median age of 45 years (range: 6-59). Nine patients had
chronic myeloid leukemia (CML) (BC: 2, second AP: 3, and
tyrosine-kinase inhibitor refractory first chronic phase [CP]: 4). Forty-two patients had AML: 14 were induction failures, 8 in first
chemotherapy-refractory relapse, 7 in untreated relapse, 3 in second or subsequent relapse, 4 were in second complete remission (CR), and 3 in second CR without platelet recovery (CRp), 2 were in high-risk CR1. Finally, 1 patient was in first CRp.
Graft-versus-host disease (GVHD) prophylaxis was
tacrolimus and mini-methorexate (MTX), and those who had an unrelated or 1
antigen-mismatched donor received low-dose
rabbit-ATG (Thymoglobulin™). All patients engrafted. Forty-one patients had active
leukemia at the time of transplant, and 35 achieved CR (85%). Twenty of the 42 AML patients and 5 of 9 CML patients are alive with a projected median overall survival (OS) of 23 months. Marrow and blood (T cell) chimerism studies at day +100 revealed that both in the lower-dose Clo groups (groups 1+2) and the higher-dose Clo groups (groups 3+4), the patients had a median of 100% donor (T cell)-derived
DNA. There has been no secondary graft failure. In the first 100 days, 1 patient died of
pneumonia, and 1 of liver GVHD. We conclude that (1) Clo ± Flu with i.v. Bu as pretransplant conditioning is safe in high-risk
myeloid leukemia patients; (2)
clofarabine is sufficiently immunosuppressive to support allo-SCT in
myeloid leukemia; and (3) the median OS of 23 months in this high-risk patient population is encouraging. Additional studies to evaluate the antileukemic efficacy of Clo ± Flu with i.v. Bu as pretransplant conditioning therapy are warranted.