In the era of precision medicine, the impact of personalized dosing of busulfan is not clear. We undertook a retrospective analysis of 78 patients with myeloid malignancies who received fludarabine and busulfan (FluBu4) with or without measuring Bu pharmacokinetics (Bu PK) and those who received busulfan with cyclophosphamide (BuCy). Fifty-five patients received FluBu4, of whom 21 had Bu PK measured, and 23 patients received BuCy. Total donor cell chimerism showed that the percentage of patients maintaining 100% donor chimerism on day 100 was 66.7%, 38.2%, and 73.9% in the FluBu4 with PK, FluBu4 with no PK, and BuCy, respectively (
Allogeneic stem cell transplant (SCT) which depends on chemotherapy and immunotherapy (graft versus leukemia effect) is the only potential curative treatment for most patients with acute myeloid leukemia (AML), myelodysplastic syndrome (MDS), and other myeloid malignancies. However, despite the advances in allogeneic SCT, disease relapse is still a major cause of death [
Chimerism analysis is an important tool to assess the origin of hematopoietic cells after SCT. Discrimination between donor and recipient cells allows evaluation for engraftment as well as detection of imminent graft rejection but its use as prognostic indicator for relapse is controversial [
Reduced toxicity ablative conditioning regimens are increasingly used in SCT. Busulfan (Bu) has been used for many years as a component of conditioning before SCT and now is being used more and more especially with the intravenous formulation which leads to more predictable delivery and probably improved clinical outcomes compared with oral Bu [
To explore the impact of measuring busulfan pharmacokinetics (Bu PK) in conditioning regimens on early donor chimerism in myeloid malignancies, we undertook a retrospective analysis of patients with myeloid disorders who received 4 days of fludarabine and busulfan (FluBu4) with or without measuring Bu PK and busulfan and cyclophosphamide (BuCy) at our center in the last 10 years.
Patients who underwent their first allogeneic SCT for AML, MDS, or myeloproliferative neoplasms involving myeloablative conditioning with FluBu4 or BuCy at our center between 2005 and 2016 were included in this retrospective analysis. Informed consent was obtained from each patient per institutional guidelines. The institutional review board also reviewed and approved this retrospective analysis.
All patients received 1 of 3 myeloablative conditioning regimens consisting of fludarabine 40 mg/m2 with Bu 3.2 mg/kg daily for 4 days with or without Bu PK dose adjustment or Bu 3.2 mg/kg for 4 days with cyclophosphamide 60 mg/kg for 2 days. The choice of regimen was at the discretion of treating physician.
Posttransplantation graft versus host disease (GVHD) prophylaxis consisted of methotrexate on days 1, 3, 6, and 11 and tacrolimus. Patients receiving unrelated donor transplants received antithymocyte globulin 4.5 mg/kg pretransplantation in divided doses.
Bone marrow donor-recipient total cell chimerism analysis was performed on day 30 and day 100 using a quantitative fluorescence-based short tandem repeat polymerase chain reaction with capillary electrophoresis for polymerase chain reaction product resolution. Data are presented as peaks, and the AUC represents the percentage of host-versus-donor hematopoiesis.
All supportive care measures including prophylactic antibiotics and antifungals were utilized according to institutional protocols. Ursodeoxycholic acid was started with the initiation of conditioning regimen.
Baseline characteristics were summarized by transplant group. Continuous variables were summarized as the mean, standard deviation, and range. Categorical variables were summarized as frequency counts and percentages. Patient and transplantation characteristics were compared using Fisher’s exact and chi-squared test for categorical variables and Mann-Whitney’s test for continuous variables. For overall tests,
In this study, 78 patients were identified and included. Characteristics of the patients are summarized in Table
Summary of patient characteristics by treatment group.
FluBu4 with PK | FluBu4 without PK | BuCy |
| |
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|
|
|
||
(% or range) | (% or range) | (% or range) | ||
Gender | M 14 (67%) | M 24 (70%) | M 12 (52%) | .349 |
F 7 (33%) | F 10 (30%) | F 11 (48%) | ||
Median age at time of SCT | 59 (41–70) | 63 (48–72) | 45 (22–63) | <.001 |
Disease | .006 | |||
Acute myeloid leukemia | 15 (71%) | 15 (44%) | 19 (82%) | |
MPN/MDS | 6 (29%) | 19 (56%) | 4 (18%) | |
Median blasts in bone marrow at time of SCT | 4% (1–40%) | 4% (1–20%) | 4% (1–90%) | .910 |
Cytogenetic risk | .521 | |||
High | 14 (67%) | 17 (50%) | 14 (61%) | |
Intermediate | 6 (29%) | 16 (47%) | 9 (39%) | |
Low | 1 (4%) | 1 (3%) | 0 (0%) | |
Disease risk index | .372 | |||
Intermediate | 9 (43%) | 16 (47%) | 7 (30%) | |
High | 5 (24%) | 13 (38%) | 9 (39%) | |
Very high | 7 (33%) | 5 (15%) | 7 (30%) | |
Antithymocyte globulin use | 12 (57%) | 21 (62%) | 10 (44%) | .38 |
CD34 dose × 106 | 3.9 ± 1.5 | 4.1 ± 1.3 | 4.4 ± 2 | .534 |
Donor type | .401 | |||
Matched related donor | 9 (43%) | 13 (38%) | 12 (52%) | |
Matched unrelated donor | 7 (33%) | 15 (44%) | 10 (43%) | |
Mismatched unrelated donor | 5 (24%) | 6 (18%) | 1 (5%) |
Bu, busulfan; Cy, cyclophosphamide; F, female; Flu, fludarabine; M, male; MDS, myelodysplastic syndrome; MPN; myeloproliferative neoplasm; SCT, stem cell transplant.
Gender, donor type, cytogenetics risk group, disease risk index, median blasts at time of SCT, CD34 dose, and antithymocyte globulin use were comparable (
Median time to neutrophil engraftment was 15, 12, and 14 days in the FluBu4 with PK, FluBu4 with no PK, and BuCy, respectively. Median time to platelet engraftment was 15, 13, and 14 days in the FluBu4 with PK, FluBu4 with no PK, and BuCy, respectively.
We evaluated total donor cell chimerism values on days 30 and 100 after allogeneic SCT as a boxplot (Figure
Total donor chimerism on day 30.
Chimerism results at day 30 |
| |||
---|---|---|---|---|
FluBu4 with PK | FluBu4 with no PK | BuCy | ||
100% donor | 14 | 17 | 16 | .311 |
86%–99% | 7 | 15 | 5 | |
<85% | 0 | 2 | 2 |
Bu, busulfan; Cy, cyclophosphamide; Flu, fludarabine; PK, pharmacokinetics.
Total donor chimerism on day 100.
Chimerism results at day 100 |
| |||
---|---|---|---|---|
FluBu4 with PK | FluBu4 with no PK | BuCy | ||
100% donor | 14 | 13 | 17 | .006 |
86%–99% | 3 | 12 | 0 | |
<85% | 4 | 9 | 6 |
Bu, busulfan; Cy, cyclophosphamide; Flu, fludarabine; PK, pharmacokinetics.
Decreasing and nondecreasing total chimerism between day 30 and day 100.
Decreasing |
Nondecreasing |
|
|
---|---|---|---|
FluBu4 with no PK | 18 (52.9) | 16 (47.1) | .04 |
FluBu4 with PK | 5 (23.8) | 16 (76.2) | |
BuCy | 6 (26.1) | 17 (73.9) |
Bu, busulfan; Cy, cyclophosphamide; Flu, fludarabine; PK, pharmacokinetics.
Effect on the treatment group on being in the 100% chimerism on day 100 group versus being in the 86%–99% or <86% chimerism on day 100, adjusting for age and disease type.
Variable | OR | 95% CI |
|
---|---|---|---|
Treatment (versus BuCy) | |||
FluBu4 with kinetics | 0.73 | 0.17–3.11 | .672 |
FluBu4 without Kinetics | 0.25 | 0.06–1.09 | .066 |
Age at transplant | 1.01 | 0.96–1.06 | .790 |
Disease | 1.53 | 0.57–4.13 | .403 |
Bu, busulfan; Cy, cyclophosphamide; Flu, fludarabine; OR, odds ratio.
Boxplot for day 30 and day 100 chimerism for each of the 3 conditioning regimens. Bu, busulfan; Cy, cyclophosphamide; Flu, fludarabine; PK, pharmacokinetics; SCT, stem cell transplant.
None of the patients in the 3 groups developed hepatic venoocclusive disease. Grade II and grades III-IV acute GVHD were not different between the 3 groups (
The median survival time for all patients was 1.40 years. Relapse rate was 24%, 47%, and 57% for FluBu4 with PK, FluBu4 with no PK, and BuCy, respectively (
Kaplan-Meier curve for overall survival for allogeneic hematopoietic cell transplantation by conditioning regimen (
Myeloablative regimens, BuCy and FluBu4, remain the standard of care for patients undergoing SCT for AML or MDS if they can tolerate it as recently shown by the Blood and Marrow Transplant Clinical Trials Network (BMT CTN 0901) [
Although the intravenous form of Bu bypasses the influence of gastrointestinal enzymes that affects the oral form, there is still a lot of inter- and intraindividual variability in Bu PK [
Many studies were published trying to compare BuCy to FluBu4 [
The MD Anderson Cancer Center group looked retrospectively at the effect of chimerism around day 100 (days +90 to +120) on the risk of relapse after SCT in patients with AML (81%) or MDS (19%) who got FluBu4 [
Regarding relapse, although there was high early 100% chimerism in the BuCy group, there was also high percentage of relapse in the same group. This is probably because that group had the longest follow-up. We started sending busulfan kinetics in our center in 2013. In the BuCy group some patients had follow-up (up to 3432 days) compared to FluBu4 with no PK (up to 2945 days) and the shortest follow-up was for FluBu4 with PK (up to 1373 days) which might explain more reported relapse in the BuCy group. In addition, mechanisms of early relapse are different than those of late relapse, as early relapse can be prevented by modifying the chemotherapy regimen or preemptive therapy with immunotherapy while late relapse might be related to other mechanisms like escape from the powerful cytotoxic effect of human leukocyte antigen-mismatched donor T cells [
Regarding GVHD, there was no difference between the 3 groups in our study, similar to what was found by a meta-analysis by Ben-Barouch et al. In that analysis when they looked at the randomized trials only, the risk grade II–IV acute GVHD was similar between the 2 groups FluBu4 and BuCy [
Our study has limitations since it is a retrospective analysis, including potential selection bias and the presence of other confounding factors that were not measured like fludarabine kinetics and prior therapies. In addition because it is retrospective, we were unable to determine any causation, only association if any. Other limitations include the small number of patients, the short follow-up, and the absence of data on lineage-specific chimerism. The strengths of this study include looking at the kinetics of chimerism and not at just one point of time chimerism and a cohort of adult patients with only myeloid malignancies treated at a single center by the same physicians with uniform peripheral blood stem cell grafts and the widely used Bu based myeloablative preparative regimens with the standard-of-care methotrexate/tacrolimus-based with or without antithymocyte globulin GVHD prophylaxis.
Currently, there are no guidelines on how and when to perform personalized dosing of Bu as part of the conditioning regimen prior to SCT. Chimerism is one of the methods used to monitor disease after SCT. However, interpretation of the results and techniques is not yet standardized. In this small cohort, we found that patients with myeloid disorders who received FluBu4 with Bu PK had a trend for higher donor chimerism similar to BuCy on day 100, while FluBu4 with no PK had a tendency to lose donor chimerism by day 100 and had more patients with less than 95% donor chimerism by day 30. Thus, in the era of precision medicine, the conditioning regimens and personalized dosing may impact early donor chimerism. This is especially important in myeloid disorders. This will need to be examined in larger retrospective multicenter studies like CIBMTR and prospective clinical trials.
The authors declare that there are no conflicts of interest regarding the publication of this article.