June 28, 2016
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Mechanism Of Action

Busulfan is a bifunctional alkylating agent in which two labile methanesulfonate groups are attached to opposite ends of a four-carbon alkyl chain. In aqueous media, busulfan hydrolyzes to release the methanesulfonate groups. This produces reactive carbonium ions that can alkylate DNA. DNA damage is thought to be responsible for much of the cytotoxicity of busulfan.


The pharmacokinetics of BUSULFEX were studied in 59 patients participating in a prospective trial of a BUSULFEXcyclophosphamide preparatory regimen prior to allogeneic hematopoietic progenitor stem cell transplantation. Patients received 0.8 mg/kg BUSULFEX every six hours, for a total of 16 doses over four days. Fifty-five of fifty-nine patients (93%) administered BUSULFEX maintained AUC values below the target value (less than1500 μM•min).

Table 3: Steady State Pharmacokinetic Parameters Following Busulfex® (busulfan) Infusion (0.8 mg per kg; N=59)

  Mean CV (%) Range
Cmax (ng per mL) 1222 18 496-1684
AUC (μM•min) 1167 20 556-1673
CL (mL per min per kg)1 2.52 25 1.49-4.31
1 Clearance normalized to actual body weight for all patients.

BUSULFEX pharmacokinetics showed consistency between dose 9 and dose 13 as demonstrated by reproducibility of steady state Cmax and a low coefficient of variation for this parameter.


Busulfan achieves concentrations in the cerebrospinal fluid approximately equal to those in plasma. Busulfan primarily binds to albumin (Mean ± standard deviation=32.4 ± 2.2%).


Busulfan is predominantly metabolized by conjugation with glutathione, both spontaneously and by glutathione S-transferase (GST) catalysis. This conjugate undergoes extensive oxidative metabolism in the liver.


Following administration of 14C-labeled busulfan to humans, approximately 30% of the radioactivity was excreted into the urine over 48 hours; negligible amounts were recovered in feces.

Specific Populations

Pediatric Patients: In a pharmacokinetic study of BUSULFEX in 24 pediatric patients, the population pharmacokinetic (PPK) estimates of BUSULFEX for clearance (CL) and volume of distribution (V) were determined. For actual body weight, PPK estimates of CL and V were 4.04 L/hr per 20 kg (3.37 mL per min per kg; interpatient variability 23%); and 12.8 L per 20 kg (0.64 L per kg; interpatient variability 11%).

Clinical Studies

Documentation of the safety and efficacy of busulfan as a component of a conditioning regimen prior to allogeneic hematopoietic progenitor cell reconstitution is derived from two sources:

  1. analysis of a prospective clinical trial of BUSULFEX that involved 61 patients diagnosed with various hematologic malignancies, and
  2. the published reports of randomized, controlled trials that employed high-dose oral busulfan as a component of a conditioning regimen for transplantation, which were identified in a literature review of five established commercial databases.
Prospective Clinical Trial of BUSULFEX

The prospective trial was a single-arm, open-label study in 61 patients who received BUSULFEX as part of a conditioning regimen for allogeneic hematopoietic stem cell transplantation. The study included patients with acute leukemia past first remission (first or subsequent relapse), with high-risk first remission, or with induction failure; chronic myelogenous leukemia (CML) in chronic phase, accelerated phase, or blast crisis; primary refractory or resistant relapsed Hodgkin's disease or non-Hodgkin's lymphoma; and myelodysplastic syndrome. Forty-eight percent of patients (29/61) were heavily pretreated, defined as having at least one of the following: prior radiation, greater than or equal to 3 prior chemotherapeutic regimens, or prior hematopoietic stem cell transplant. Seventy-five percent of patients (46/61) were transplanted with active disease.

Patients received 16 BUSULFEX doses of 0.8 mg per kg every 6 hours as a two-hour infusion for 4 days, followed by cyclophosphamide 60 mg per kg once per day for two days (BuCy2 regimen). All patients received 100% of their scheduled BUSULFEX regimen. No dose adjustments were made. After one rest day, allogeneic hematopoietic progenitor cells were infused. The efficacy parameters in this study were myeloablation (defined as one or more of the following: absolute neutrophil count [ANC] less than 0.5x109/L, absolute lymphocyte count [ALC] less than 0.1x109/L, thrombocytopenia defined as a platelet count less than 20,000/mm³ or a platelet transfusion requirement) and engraftment (ANC greater than or equal to 0.5x109/L).

All patients (61/61) experienced myeloablation. The median time to neutropenia was 4 days. All evaluable patients (60/60) engrafted at a median of 13 days post-transplant (range 9 to 29 days); one patient was considered non-evaluable because he died of a fungal pneumonia 20 days after BMT and before engraftment occurred. All but 13 of the patients were treated with prophylactic G-CSF. Evidence of donor cell engraftment and chimerism was documented in all patients who had a chromosomal sex marker or leukemic marker (43/43), and no patient with chimeric evidence of allogeneic engraftment suffered a later loss of the allogeneic graft. There were no reports of graft failure in the overall study population. The median number of platelet transfusions per patient was 6, and the median number of red blood cell transfusions per patient was 4.

Twenty-three patients (38%) relapsed at a median of 183 days post-transplant (range 36 to 406 days). Sixty-two percent of patients (38/61) were free from disease with a median follow-up of 269 days post-transplant (range 20 to 583 days). Forty-three patients (70%) were alive with a median follow up of 288 days post-transplant (range 51 to 583 days). There were two deaths before BMT Day +28 and six additional patients died by BMT Day +100. Ten patients (16%) died after BMT Day +100, at a median of 199 days post-transplant (range 113 to 275 days).

Oral Busulfan Literature Review

Four publications of randomized, controlled trials that evaluated a high-dose oral busulfan-containing conditioning regimen (busulfan 4 mg/kg/d x4 days + cyclophosphamide 60 mg/kg/d x2 days) for allogeneic transplantation in the setting of CML were identified. Two of the studies (Clift and Devergie) had populations confined to CML in chronic phase that were randomized between conditioning with busulfan/cyclophosphamide (BU/CY) and cyclophosphamide/total body irradiation (CY/TBI). A total of 138 patients were treated with BU/CY in these studies. The populations of the two remaining studies (Ringden and Blume) included patients with CML, acute lymphoblastic leukemia (ALL), and acute myelogenous leukemia (AML). In the Nordic BMT Group study published by Ringden, et al., 57 patients had CML, and of those, 30 were treated with BU/CY. Patients with CML in chronic phase, accelerated phase, and blast crisis were eligible for this study. The participants with CML (34/122 patients) in a SWOG study published by Blume, et al., had disease beyond first chronic phase. Twenty of those CML patients were treated with BU/CY, and the TBI comparator arm utilized etoposide instead of cyclophosphamide.

Table 4 summarizes the efficacy analyses reported from these 4 studies.

Table 4: Summary of efficacy analyses from the randomized, controlled trials utilizing a high dose oral busulfan-containing conditioning regimen identified in a literature review.

Clift, 1994 CML Chronic Phase;
3 year Overall Survival 3 year DFS
Relapse Time to Engraftment (ANC greater than or equal to 500)
80% 80% 71% 68% 13% 13% 22.6 days 22.3 days
Devergie, 1995 CML Chronic Phase;
5 year Overall Survival (p=0.5) 5 year DFS (p=0.75) Relapse (Relative Risk analysis BU/CY:CY/TBI) (p=0.04) Time to Engraftment (ANC greater than or equal to 500)
60.6% ±11.7% 65.8% ±12.5% 59.1% ±11.8% 51.0% ±14% 4.10 (95% CI =1.00-20.28) None Given None Given
Ringden, 1994 CML, AML, ALL;
3 year Overall Survival (p < 0.03) 3 year Relapse Free Survival (p=0.065) Relapse (p=0.9) Time to Engraftment (ANC greater than 500)
62% 76% 56% 67% 22% 26% 20 days 20 days
Blume, 19931 CML, AML, ALL; Relative Risk Analysis BU/CY: Etoposide/TBI
RR of Mortality DFS RR of Relapse (Relative Risk analysis BU/CY: Eto/TBI) Time to Engraftment
0.97 (95% CI=0.64-1.48) Not Given 1.02 (95% CI=0.56-1.86) Not Given
1 Eto = etoposide.
TBI was combined with etoposide in the comparator arm of this study.
BU = Busulfan
CY = Cyclophosphamide
TBI = Total Body Irradiation
DFS = Disease Free Survival
ANC = Absolute Neutrophil Count

Last reviewed on RxList: 1/28/2015
This monograph has been modified to include the generic and brand name in many instances.

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