Fludarabine and Oral Busulfan: A Low Toxicity Conditioning Regimen When Plasma Level Targeting and Intravenous Busulfan Is Not Available. A Brazilian Experience.
Authors: Simoes Belinda P. , Souza M. , Mauad Marcos , Paton Eduardo J.A. , Pieroni Fabiano , Stracieri Ana B.P.L. , Oliveira Maria C. , Moraes Daniela , Coutinho Marina , Voltarelli Julio C. , Colturato Vergilio R. 
Published: April 2020

Abstract
Fludarabine in combination with intravenous Busulfan has been used to reduce treatment related mortality (TRM) in allogeneic stem cell transplantation (SCT). Considering that busulfan has erratic bioavailability and unpredictable intestinal absorption, previous investigations have used intravenous busulfan with plasma-targeted levels dosing. In Brazil neither intravenous Busulfan nor routine plasma level measurement of Busulfan are available. In this report we retrospectively analyze 74 patients (median age 32 y.o.) submitted to SCT using a Fludarabine and oral Busulfan based conditioning regimen. Forty three patients had acute myeloid leukaemia (AML, age range 5 to 58) and 31 patients chronic myeloid leukemia (CML, age range 19 to 58). Among the AML patients, 44% were in first remission (CR1), 23% in second remission (CR2) and 33% patients had active disease (AD) at the time of transplant. Among CML patients, 65% were in chronic phase (CML-CP), 29% in accelerated phase (CML-AP) and 6% in blast crisis (CML-BC). Fludarabine 30 mg/m2 and oral Busulfan 4 mg/kg was given for 4 days with Cyclosporine A and Methotrexate GVHD prophylaxis. For patients with unrelated donors, ATG were added. All patients engrafted (median 14 days). At a median follow up time of 180 days non-relapse mortality was 5%. Seventeen patients (23%) died (12 AML and 5 CML patients). Among survivors (57 patients), 56 remain in complete remission. One patient with a refractory AML and a complex karyotype relapsed on day +90. One patient died of severe sinusoidal obstruction syndrome (SOS). Acute GVHD occurred in 14 (19%) patients (10 AML and 4 CML) none of them grade III or IV. Chronic GVHD occurred in 11 patients so far, being the cause of death in 2 patients. Major cause of death was relapse of disease in patients transplanted with active disease. Only three AML patients in CR1 died (chronic GVHD, sepsis and fulminant hepatic failure) in contrast to 60% of patients with active disease (88% relapsed). One patient with CML-CP died with a relapse in CML-BC, the remaining 19 (92%) patients are alive in complete remission. One patient transplanted in CML-AP died (cGVHD and zygomykosis) and all patients transplanted in BC died of their disease. Actuarial 1-year overall survival for CML is 92% and 54% (CP and AP/BC respectively, p=0,04) and for AML 72% and 23% (CR1/CR2 and AD, respectively, p=0,005). These preliminary data show that oral Busulfan and Fludarabine can safely be used without plasma level monitoring. The follow up time is short and the effect on the control of CML cannot be concluded. However AML has been effectively controlled as far as patients were transplanted in remission (AML-CR1 and CR2). For patients in more advanced disease stages this regimen is not optimal. However, this regimen is safe and may improve the results in early stages of AML and CML even in countries with limited resources in health care.