Intravenous busulfan in children prior to stem cell transplantation: study of pharmacokinetics in association with early clinical outcome and toxicity

Impact of prophylactic echinocandin on the development of neurological complications in patients receiving busulfan-containing conditioning regimens for stem cell transplantation: A single-center retrospective study

BACKGROUND

Although neurotoxicity is a major adverse event associated with busulfan, little information is available regarding the association between drug interactions and neurological symptoms during busulfan-based regimens. This study evaluated the association between prophylactic echinocandins and neurological complications in patients receiving busulfan-containing conditioning regimens for stem cell transplantation.

METHODS

We retrospectively included consecutive patients who administered intravenous busulfan as a conditioning regimen at our facility between 2007 and 2022. Prophylactic echinocandin use was defined as the use of an echinocandin antifungal drug to prevent invasive fungal disease in SCT recipients. The primary outcome was the incidence of neurological complications within 7 days of busulfan initiation and was compared between the echinocandin group (patients received prophylactic echinocandin) and nonechinocandin group (patients received prophylactic antifungal drugs other than echinocandin and those without antifungal prophylaxis).

RESULTS

Among the 59 patients included in this study, the incidence of neurological complications in the echinocandin (n = 26) and nonechinocandin groups (n = 33) was 30.8% and 63.6%, respectively. We observed a negative association between prophylactic echinocandin use and the development of neurological complications after adjusting for the propensity score for receiving prophylactic echinocandins (adjusted odds ratio 0.294, 95% confidence interval 0.090 to 0.959). We observed a lower incidence of neurological complications in the echinocandin group than in the nonechinocandin group.

CONCLUSION

Our results suggested that the choice of antifungal prophylaxis is associated with busulfan neurotoxicity.

Population Pharmacokinetic Modeling for Twice-Daily Intravenous Busulfan in a Large Cohort of Pediatric Patients Undergoing Hematopoietic Stem Cell Transplantation-A 10-Year Single-Center Experience

Reaching target exposure of busulfan-based conditioning prior to hematopoietic stem cell transplantation is vital for favorable therapy outcomes. Yet, a wide inter-patient and inter-occasion variability in busulfan exposure has been reported, especially in children. We aimed to identify factors associated with the variability of busulfan pharmacokinetics in 124 consecutive patients transplanted at the University Children's Hospital Zurich between October 2010 and February 2020. Clinical data and busulfan plasma levels after twice-daily intravenous administration were analyzed retrospectively by population pharmacokinetic modeling. The volume of distribution correlated with total body water. The elimination rate constant followed an age-dependent maturation function, as previously suggested, and correlated with the levels of serum albumin. Acute lymphoblastic leukemia reduced busulfan clearance by 20%. Clearance significantly decreased by 17% on average from the start to the third day of busulfan administration, in agreement with other studies. An average reduction of 31% was found in patients with hemophagocytic lymphohistiocytosis and X-linked lymphoproliferative disease. In conclusion, we demonstrate that in addition to known factors, underlying disease and serum albumin significantly impact busulfan pharmacokinetics in pediatric patients; yet, substantial unexplained variability in some patients remained. Thus, we consider repeated pharmacokinetic assessment essential to achieve the desired target exposure in twice-daily busulfan administration.

Role of therapeutic drug monitoring of intravenous Busulfan for prevention of sinusoidal obstructive syndrome in children

BACKGROUND

Therapeutic drug monitoring (TDM) of intravenous busulfan (Bu) has been recommended for safe engraftment and decreased toxicity in children undergoing hematopoietic stem cell transplantation (HSCT). This study aims to compare HSCT-related outcomes, such as acute or chronic graft-versus-host disease (GvHD), sinusoidal obstructive syndrome (SOS), event-free survival (EFS), and overall survival (OS) in children with and without TDM for busulfan.

METHODS

This retrospective study conducted between February 2012 and February 2021 at our Bone Marrow Transplantation Unit included 172 patients (34% girls) with a median age of 4.70 years (IQR 2.41-10.01). Group A consisted of 46 patients whose Bu doses were adjusted according to actual body weight, and group B consisted of 126 patients whose Bu dose adjustments made according to TDM.

RESULTS

Totally, 32 patients (19%) developed moderate or severe SOS. The incidence of SOS was significantly higher in the group without TDM (29% vs. 15%, p = .041). A multivariable analysis showed that the presence of acute GvHD and one alkylating drug-containing conditioning regimen compared with two or three were associated with SOS (p = .03 and p = .002, respectively). In patients with TDM, cumulative Bu dose and area under curve also were not associated with SOS. Other HSCT-related outcomes such as acute or chronic GvHD, relapse and graft rejection rates, OS and EFS rates did not differ between the groups.

CONCLUSIONS

TDM and making dose adjustments with Bayesian forecasting over four days of Bu therapy optimizes exposure and reduces the risk of SOS in children undergoing HSCT.

Decreased Systemic Busulfan Exposure After Oral Dosing With Concomitant Levetiracetam Compared With Phenytoin

BACKGROUND

Busulfan (Bu) conditioning used in hematopoietic stem cell transplantation may induce seizures, and prophylactic antiepileptic treatment is recommended. Following updated guidelines, in August 2019, the adult hematopoietic stem cell transplantation department of the Rambam Health Care Campus (Haifa, Israel) switched the antiepileptic prophylaxis protocol from phenytoin to oral levetiracetam during oral Bu conditioning. The aim of this study was to compare the pharmacokinetic parameters of Bu after oral dosing between patients receiving phenytoin and those receiving levetiracetam prophylaxis.

METHODS

This study was a retrospective cohort study in adults undergoing myoablative conditioning with oral Bu between August 2018 and August 2020. Bu pharmacokinetic parameters (AUC0-6, C0, Cmax, and Tmax) were compared in patients treated with phenytoin comedication (during the year before the change in policy) and levetiracetam comedication (during the year after the change). Potential confounders were accounted for including age, azole comedication, and body weight.

RESULTS

There were no significant differences in demographic and clinical parameters or weight-corrected Bu dose between the phenytoin group (n = 28) and the levetiracetam group (n = 25). There was no difference in the rate of voriconazole comedication, but fluconazole was more common in the phenytoin group (P = 0.026). The median AUC0-6 was significantly lower in the levetiracetam group (949 μM*min; IQR = 806 to 1101 μM*min) than in the phenytoin group (1208 μM*min; IQR = 1087 to 1389 μM*min; P < 0.001). This is a clinically significant difference of 258 μM*min (21%). Azole use was not associated with Bu exposure.

CONCLUSIONS

The findings suggest that, after treatment with oral Bu, oral levetiracetam comedication is associated with reduced systemic exposure compared with phenytoin comedication, possibly because of decreased bioavailability.

Characterization of Drug-Drug Interactions on the Pharmacokinetic Disposition of Busulfan in Pediatric Patients during Hematopoietic Stem Cell Transplantation Conditioning

AIM

The study objective was to develop a population pharmacokinetic model for busulfan to comprehensively examine drug-drug interactions (DDIs) in pediatric patients undergoing hematopoietic stem cell transplantation. Currently, there is limited evidence to substantiate potential DDIs with busulfan.

METHODS

This retrospective study population was comprised of 250 patients receiving, on average, 0.8 mg/kg intravenous busulfan as pretreatment. All model analyses were conducted using non-linear mixed effects modeling in Pumas v2.0. The metabolic pathways of primary interest were glutathione conjugation and cytochrome P450 activity. Concomitant medications were categorized as CYP inhibitors, inducers, or GST depleters and included in the model as conditional covariates. A bootstrap simulation and visual predictive check were conducted to qualify the final model.

RESULTS

The final one-compartment model incorporates covariates of weight and age in relation to their effects on both total body clearance and volume of distribution. The estimated typical values of clearance and volume were 1.138 L/hr (CI: 1.095 - 1.179 L/hr) and 3.527 L (CI: 3.418 - 3.621 L), respectively. No significant changes in clearance were observed when medications that alter proposed hepatic and metabolic pathways of busulfan were coadministered.

CONCLUSION

To the best of our knowledge, this is the largest single center study of busulfan in children and the first to quantify the maturation effect of both clearance and volume. This study could not demonstrate a difference in busulfan clearance when comparing patients who received medications that alter the GST, CYP3A4, or CYP2C9 pathway to those who did not.

Total Body Irradiation Forever? Optimising Chemotherapeutic Options for Irradiation-Free Conditioning for Paediatric Acute Lymphoblastic Leukaemia

Total-body irradiation (TBI) based conditioning prior to allogeneic hematopoietic stem cell transplantation (HSCT) is generally regarded as the gold-standard for children >4 years of age with acute lymphoblastic leukaemia (ALL). Retrospective studies in the 1990's suggested better survival with irradiation, confirmed in a small randomised, prospective study in the early 2000's. Most recently, this was reconfirmed by the early results of the large, randomised, international, phase III FORUM study published in 2020. But we know survivors will suffer a multitude of long-term sequelae after TBI, including second malignancies, neurocognitive, endocrine and cardiometabolic effects. The drive to avoid TBI directs us to continue optimising irradiation-free, myeloablative conditioning. In chemotherapy-based conditioning, the dominant myeloablative effect is provided by the alkylating agents, most commonly busulfan or treosulfan. Busulfan with cyclophosphamide is a long-established alternative to TBI-based conditioning in ALL patients. Substituting fludarabine for cyclophosphamide reduces toxicity, but may not be as effective, prompting the addition of a third agent, such as thiotepa, melphalan, and now clofarabine. For busulfan, it's wide pharmacokinetic (PK) variability and narrow therapeutic window is well-known, with widespread use of therapeutic drug monitoring (TDM) to individualise dosing and control the cumulative busulfan exposure. The development of first-dose selection algorithms has helped achieve early, accurate busulfan levels within the targeted therapeutic window. In the future, predictive genetic variants, associated with differing busulfan exposures and toxicities, could be employed to further tailor individualised busulfan-based conditioning for ALL patients. Treosulfan-based conditioning leads to comparable outcomes to busulfan-based conditioning in paediatric ALL, without the need for TDM to date. Future PK evaluation and modelling may optimise therapy and improve outcome. More recently, the addition of clofarabine to busulfan/fludarabine has shown encouraging results when compared to TBI-based regimens. The combination shows activity in ALL as well as AML and deserves further evaluation. Like busulfan, optimization of chemotherapy conditioning may be enhanced by understanding not just the PK of clofarabine, fludarabine, treosulfan and other agents, but also the pharmacodynamics and pharmacogenetics, ideally in the context of a single disease such as ALL.

Comparison of Two Analytical Methods for Busulfan Therapeutic Drug Monitoring

BACKGROUND AND OBJECTIVES

Busulfan (Bu) is an old drug, but is still well recommended as an alkylating agent during conditioning therapy, before hematopoietic stem cell transplantation. Although its dose administration is standardized and based on patient weight, therapeutic drug monitoring is required in order to maintain its exposure [as area under the concentration-time curve (AUC) from 0 to infinity AUC_0-∞] within a narrow therapeutic range and, if necessary, to adjust the dose with as short a lead time as possible. The aim of the study is to evaluate the agreement (as calculated AUC) between a gold standard analytical method and a new one that is faster and easier.

METHODS

We analyzed 221 plasma samples from 37 children (0.25-16 years; 4-62.5 kg) and 11 adults (21-59 years; 45-80 kg), corresponding to 52 AUC values (ng h/mL). The drug exposure was calculated, simultaneously, by two validated analytical methods. The reference method was a high-performance liquid chromatography (HPLC) assay combined with an ultraviolet detector (UV). The test method had a triple quadrupole mass spectrometer (MS) as detector; the clean-up procedures of the samples were different and faster.

RESULTS

The agreement between the two methods (reference and test) was evaluated in terms of Bu exposure differences based on Lin's concordance correlation coefficient (CCC) and represented by the Bland-Altman plot. The CCC between the AUC of the two methods was excellent (0.868; 95% CI: 0.802-0.935). The precision of the measures (expressed by Pearson's italic "r") was 0.872, and the accuracy (accounted by the bias correction factor) was 0.996.

CONCLUSIONS

We can conclude that the HPLC-MS/MS assay represents a very good alternative to the reference.

Busulfan systemic exposure and its relationship with efficacy and safety in hematopoietic stem cell transplantation in children: a meta-analysis

Background

Busulfan (Bu) is a key component of several conditioning regimens used before hematopoietic stem cell transplantation (HSCT). However, the optimum systemic exposure (expressed as the area under the concentration-time curve [AUC]) of Bu for clinical outcome in children is controversial.

Methods

Research on pertinent literature was carried out at PubMed, EMBASE, Web of science, the Cochrane Library and ClinicalTrials.gov. Observational studies were included, which compared clinical outcomes above and below the area under the concentration-time curve (AUC) cut-off value, which we set as 800, 900, 1000, 1125, 1350, and 1500 μM × min. The primary efficacy outcome was notable in the rate of graft failure. In the safety outcomes, incidents of veno-occlusive disease (VOD) were recorded, as well as other adverse events.

Results

Thirteen studies involving 548 pediatric patients (aged 0.3–18 years) were included. Pooled results showed that, compared with the mean Bu AUC (i.e., the average value of AUC measured multiple times for each patient) of > 900 μM × min, the mean AUC value of < 900 μM × min significantly increased the incidence of graft failure (RR = 3.666, 95% CI: 1.419, 9.467). The incidence of VOD was significantly decreased with the mean AUC < 1350 μM × min (RR = 0.370, 95% CI: 0.205–0.666) and < 1500 μM × min (RR = 0.409, 95% CI: 0182–0.920).

Conclusions

In children, Bu mean AUC above the cut-off value of 900 μM × min (after every 6-h dosing) was associated with decreased rates of graft failure, while the cut-off value of 1350 μM × min were associated with increased risk of VOD, particularly for the patients without VOD prophylaxis therapy. Further well-designed prospective and multi centric randomized controlled trials with larger sample size are necessary before putting our result into clinical practices.

A semi-mechanistic model based on glutathione depletion to describe intra-individual reduction in busulfan clearance

Aim

To develop a semi‐mechanistic model, based on glutathione depletion and predict a previously identified intra‐individual reduction in busulfan clearance to aid in more precise dosing.

Methods

Busulfan concentration data, measured as part of regular care for allogeneic hematopoietic cell transplantation (HCT) patients, were used to develop a semi‐mechanistic model and compare it to a previously developed empirical model. The latter included an empirically estimated time effect, where the semi‐mechanistic model included theoretical glutathione depletion. As older age has been related to lower glutathione levels, this was tested as a covariate in the semi‐mechanistic model. Lastly, a therapeutic drug monitoring (TDM) simulation was performed comparing the two models in target attainment.

Results

In both models, a similar clearance decrease of 7% (range −82% to 44%), with a proportionality to busulfan metabolism, was found. After 40 years of age, the time effect increased with 4% per year of age (0.6–8%, P = 0.009), causing the effect to increase more than a 2‐fold over the observed age‐range (0–73 years). Compared to the empirical model, the final semi‐mechanistic model increased target attainment from 74% to 76%, mainly through better predictions for adult patients.

Conclusion

These results suggest that the time‐dependent decrease in busulfan clearance may be related to gluthathione depletion. This effect increased with older age (>40 years) and was proportional to busulfan metabolism. The newly constructed semi‐mechanistic model could be used to further improve TDM‐guided exposure target attainment of busulfan in patients undergoing HCT.

Limited Sampling Strategies Supporting Individualized Dose Adjustment of Intravenous Busulfan in Children and Young Adults

BACKGROUND

Therapeutic drug monitoring (TDM) for busulfan supports dose adjustment during conditioning for stem cell transplantation. The authors aimed to develop and validate limited sampling strategies (LSS) of 4-5 samples for a precise estimation of the area under concentration (AUC)-time curve of busulfan, in plasma as an alternative to an intensive sampling strategy (ISS) requiring 9-10 samples.

METHODS

ISS TDM data from 297 patients (≤18 years of age) were used. AUCLSS was calculated using the trapezoidal rule and multiple linear regression (MLR). Unlike more complex modeling methods, MLR does not require sophisticated software or advanced training of personnel. MLR coefficients were estimated in the development subset containing randomly selected 50% of the records and were then used to calculate the AUCLSS of the remaining records (the validation subset). The agreement between dose adjustment recommendations (DAR) based on ISS and LSS, in the validation subset, was evaluated by a Bland-Altman analysis. A DAR deviating from an ISS-based reference by <15% was deemed acceptable.

RESULTS

Twelve LSSs were acceptable. Sampling at 0, 120, 180, and 240 minutes after the start of the second infusion (LSS15) yielded the best performance, with DAR deviating from the reference by <10% for 95% of cases; the AUCLSS was determined as follows: AUCLSS = 74.7954 × C(0) + 81.8948 × C(120) + 38.1771 × C(180) + 138.1404 × C(240) + 54.1837. This LSS and LSS13 performed similarly well in an independent external validation.

CONCLUSIONS

MLR-based estimates of AUCLSS provide DARs that deviate minimally from the reference. LSSs allow the reduction of patient discomfort, a ∼50% reduction of TDM-related workload for nursing staff and blood loss and a ∼25% reduction in laboratory workload. These benefits may encourage wider use of busulfan TDM, supporting safe and efficacious personalized dosing.

Pharmacokinetics-adapted Busulfan-based myeloablative conditioning before unrelated umbilical cord blood transplantation for myeloid malignancies in children

Unrelated umbilical cord blood transplantation (UCBT) is an alternative to provide transplants in children with acute leukemia or myelodysplastic syndrome who lack a related donor. Intravenous Busulfan (Bu) combined with therapeutic drug monitoring-guided dosing has been increasingly used, with more predictable bioavailability and better outcomes comparing to oral Bu. There is still an important variation in Bu pharmacokinetic between patients that is associated with an increased risk of toxicity and graft failure. The objective of the study was to analyze the impact of first-dose pharmacokinetic adapted myeloablative conditioning regimen of intravenous Bu on the different outcomes after transplantation. Data of 36 children who underwent allogeneic HSCT with Bu plus a second alkylating agent at Sainte Justine Hospital in Montreal, Canada, between December 2000 and April 2012 were analyzed. For children with high risk myeloid malignancies receiving an UCBT, first dose Bu pharmacokinetic seems to be a significant prognostic factor, influencing neutrophil (100% vs 67.9%) and platelet recovery (95.5% vs 70.5%), non-relapse mortality (0% vs 18.6%), EFS (64% vs 28.6%) and OS (81.3% vs 37.5%) for a first-dose steady-state concentration (Css) <600ng/mL vs >600ng/mL, respectively. These data reinforce the importance of Busulfan therapeutic drug monitoring-guided dosing in pediatric HSCT patients, particularly in the context of UCBT.

A rapid and simple LC-MS/MS method for personalized busulfan dosing in pediatric patients undergoing hematopoietic stem cell transplantation (HSCT)

BACKGROUND

Busulfan is commonly used as a conditioning regimen before hematopoietic stem cell transplantation (HSCT). There is a big inter-individual variability in busulfan exposure and the narrow therapeutic index, especially in pediatric population. Therefore, to achieve therapeutic efficacy and safety concurrently, personalized busulfan dosing, guided by pharmacokinetic study with serial plasma samples, is needed a few hours afterwards.

METHODS

A fast, sensitive, and accurate method for busulfan measurement was developed, validated, and implemented with liquid-chromatography-mass spectrometry (HPLC-MS/MS). The sample preparation procedure involves only protein precipitation and dilution, and the HPLC-MS/MS method takes 3 min/sample. The assay was linear from 10 ng/ml to 7500 ng/ml (R² = 0.99). Recoveries were above 90%. The precision was determined at 3 levels (30, 300 and 4000 ng/ml): the intra-day variability (%CV) ranged from 1.4% to 2.5% (n = 20); the inter-day variability ranged from 2.2% to 5.5% (n = 20). The accuracy of the HPLC-MS/MS test was evaluated with an old HPLC-fluorescence method (n = 84), and a Correlation Coefficient (R) of 0.99 was observed.

CONCLUSIONS

The analytical performance of the assay allows for timely dose adjustment and has been implemented in clinical service for better clinical outcome.

Clarifying busulfan metabolism and drug interactions to support new therapeutic drug monitoring strategies: a comprehensive review

INTRODUCTION

Busulfan (Bu) is an alkylating agent with a limited therapeutic margin and exhibits inter-patient variability in pharmacokinetics (PK). Despite decades of use, mechanisms of Bu PK-based drug-drug interactions (DDIs), as well as the negative downstream effects of these DDIs, have not been fully characterized. Areas covered: This article provides an overview of Bu PK, with a primary focus on how known and potentially unknown drug metabolism pathways influence Bu-associated DDIs. In addition, pharmacogenomics of Bu chemotherapy and Bu-related DDIs observed in the stem cell transplant clinic (SCT) are summarized. Finally the increasing importance of Bu therapeutic drug monitoring is highlighted. Expert opinion: Mechanistic studies of Bu metabolism have shown that in addition to GST isoenzymes, other oxidative enzymes (CYP, FMO) and ABC/MDR drug transporters likely contribute to the overall clearance of Bu. Despite many insights, results from clinical studies, especially in polypharmacy settings and between pediatric and adult patients, remain conflicting. Further basic science and clinical investigative efforts are required to fully understand the key factors determining Bu PK characteristics and its effects on complications after SCT. Improved TDM strategies are promising components to further investigate, for instance DDI mechanisms and patient outcomes, in the highly complex SCT treatment setting.

Prospective validation of a novel dosing scheme for intravenous busulfan in adult patients undergoing hematopoietic stem cell transplantation

The objective of this study was to externally validate a new dosing scheme for busulfan. Thirty-seven adult patients who received busulfan as conditioning therapy for hematopoietic stem cell transplantation (HCT) participated in this prospective study. Patients were randomized to receive intravenous busulfan, either as the conventional dosage (3.2 mg/kg daily) or according to the new dosing scheme based on their actual body weight (ABW) (23×ABW^0.5 mg daily) targeting an area under the concentration-time curve (AUC) of 5924 µM·min. Pharmacokinetic profiles were collected using a limited sampling strategy by randomly selecting 2 time points at 3.5, 5, 6, 7 or 22 hours after starting busulfan administration. Using an established population pharmacokinetic model with NONMEM software, busulfan concentrations at the available blood sampling times were predicted from dosage history and demographic data. The predicted and measured concentrations were compared by a visual predictive check (VPC). Maximum a posteriori Bayesian estimators were estimated to calculate the predicted AUC (AUC_PRED). The accuracy and precision of the AUC_PRED values were assessed by calculating the mean prediction error (MPE) and root mean squared prediction error (RMSE), and compared with the target AUC of 5924 µM·min. VPC showed that most data fell within the 95% prediction interval. MPE and RMSE of AUC_PRED were -5.8% and 20.6%, respectively, in the conventional dosing group and −2.1% and 14.0%, respectively, in the new dosing scheme group. These fi ndings demonstrated the validity of a new dosing scheme for daily intravenous busulfan used as conditioning therapy for HCT.

Should busulfan therapeutic range be narrowed in pediatrics? Experience from a large cohort of hematopoietic stem cell transplant children

Busulfan, the corner stone of hematopoietic stem cell transplantation regimens, has a narrow therapeutic window. Therapeutic drug monitoring (TDM)-guided dosing to reach the conventional area under the concentration-time curve (AUC) target range of 900-1500 μmol min/L is associated with better outcomes. We report our experience with busulfan TDM in a large cohort of children. The aims were to investigate the relevance of using a more restricted therapeutic range and investigate the association between busulfan therapeutic range and clinical outcome. This study includes 138 children receiving 16 doses of intravenous busulfan, with the first dose assigned based on weight and doses adjusted to a local AUC target range of 980-1250 μmol min/L. Busulfan TDM combined with model-based dose adjustment was associated with an increased probability of AUC target attainment, for both target range: 90.8% versus 74.8% for the conventional target range and 66.2% versus 43.9% for the local target range (P<0.001). The median follow-up was 56.2 months. Event-free survival was 88.5%, overall survival was 91.5% and veno-occlusive disease occurred in 18.3% of patients. No difference was observed for clinical outcomes depending on the selected target range. Pharmacokinetic monitoring and individualization of busulfan dosage regimen are useful in improving target attainment, but using a restricted target range has no impact on clinical outcomes.

Single Daily Busulfan Dosing for Infants with Nonmalignant Diseases Undergoing Reduced-Intensity Conditioning for Allogeneic Hematopoietic Progenitor Cell Transplantation

Busulfan (Bu) is widely used in conditioning regimens for infants undergoing allogeneic hematopoietic progenitor cell transplantation (HPCT), but the best approach to administer Bu in this population is still unknown. Here, we report a single-center experience of the use of a test dose to guide dose adjustment of intravenous (i.v.) Bu therapy in infants. Between 2004 and 2013, 33 infants younger than 1 year with nonmalignant conditions received allogeneic peripheral blood or cord blood HPCT after a reduced-intensity conditioning (RIC) regimen consisting of fludarabine, antithymocyte globulin, and 2 single daily doses of i.v. Bu. Pharmacokinetic results of a test dose of i.v. Bu (.8 mg/kg) were used to determine the dose of 2 single daily i.v. Bu regimen doses, adjusted to target an area under the curve (AUC) of 4000 μMol*minute per day in a first cohort (n = 12) and 5000 μMol*minute in a second cohort (n = 21). The mean Bu clearance in our infant patients was found to be 3.67 ± 1.03 mL/minute/kg, and the test dose clearance was highly predictive of the regimen dose clearance. The mean AUC achieved after the first single daily regimen dose was 3951 ± 1239 in the AUC 4000 cohort and 4884 ± 766 for the AUC 5000 cohort. No patient in either cohort developed hepatic sinusoidal obstructive syndrome or seizures attributable to Bu. Primary graft failure occurred in 4 patients and secondary graft failure occurred in 3, predominantly in the AUC 4000 cohort (6 of 7). Among the engrafted patients (n = 28), 16 achieved full donor chimerism and 9 patients attained stable mixed chimerism. Overall survival of patients at 6 years after transplantation was 59.5% for the AUC 4000 cohort and 85.4% for the AUC 5000 cohort, with primary graft failure in the first cohort being a major contributor to morbidity. Logistic regression analysis showed that the risk of graft failure increased significantly if cord blood hematopoietic progenitor cells were used or if total Bu exposure was below 4000 μMol*minute per day for 2 days. The difference in clinical outcomes between the 2 cohorts supports the conclusion that targeting a higher Bu AUC of 5000 μMol*minute per day for 2 days improves donor engraftment in infants with nonmalignant conditions undergoing RIC HPCT without increasing toxicity. Measuring i.v. Bu pharmokinetics using a test dose allows timely adjustment of single daily regimen doses and optimization of total Bu exposure, resulting in an effective and safe regimen for these infants.

Clinical comparison of weight- and age-based strategy of dose administration in children receiving intravenous busulfan for hematopoietic stem cell transplantation

Bu, combined with TDM-guided dosing, is associated with fewer graft failures/relapses and lower toxicity in pediatric HSCT. We aimed this retrospective study for comparison of weight- and age-based dosing in terms of clinical outcomes such as time to engraftment, early complications, EFS, OS, and toxicity profiles in children receiving iv Bu. Sixty-one children who underwent HSCT from April 2010 to February 2013 by means of a Bu-based conditioning regimen and completed 100 days after transplantation at Ankara Children?s Hematology and Oncology Hospital Bone Marrow Transplantation Unit were enrolled in this study. SOS and neutropenic fever occurred more frequently in the weight-based dosing group. We found a statistically significant correlation between Bu dose and the incidence of SOS (r = 0.26, p = 0.04). Multivariate analysis showed only weight-based dosing of Bu was a significant predictor of SOS (HR = 9.46; p = 0.009). However, no relationship was found between two groups in terms of hemorrhagic cystitis, engraftment syndrome, acute or chronic GvHD, time to engraftment, chimerism, TRM, OS, and EFS rates. Weight-based dosing of Bu may cause higher incidence of SOS and early infectious complications at the places where TDM of Bu cannot be performed.

Optimizing drug therapy in pediatric SCT: focus on pharmacokinetics

Given age-related differences in drug metabolism and indications for hematopoietic SCT (HSCT), personalized drug dosing of the conditioning regimen and post-transplant immunosuppression may reduce graft rejection, relapse rates and toxicity in pediatric HSCT recipients. This manuscript summarizes the pharmacokinetic/dynamic data of HSCT conditioning and post-grafting immunosuppression, presented at the First Annual Pediatric Bone Marrow Transplant Consortium (PBMTC) meeting in April 2013. Personalized dosing of BU to a target plasma exposure reduces graft rejection in children and improves relapse/toxicity rates in adults. Current weight-based dosing achieves the target BU exposure in only a minority (24.3%) of children. The initial BU dose should be based on the European Medicines Agency nomogram or population pharmacokinetic models to improve the numbers of children achieving the target exposure. There are limited pharmacokinetic data for treosulfan, CY, fludarabine and alemtuzumab as HSCT conditioning in children. For post-grafting immunosuppression, mycophenolic acid (MPA) clearance may be increased in younger children (<12 years). The preferred MPA pharmacokinetic monitoring parameters and target range are still evolving in HSCT recipients. Multi-institutional trials incorporating properly powered pharmacokinetic/dynamic studies are needed to assess the effect of variability in the plasma exposure of drugs/metabolites on clinical outcomes in pediatric HSCT recipients.

Thirteen years' experience of pharmacokinetic monitoring and dosing of busulfan: can the strategy be improved?

BACKGROUND

A busulfan concentration monitoring and dosing service has been provided by Christchurch Hospital since 1998. This study aimed to see (1) the percentage of patients with an area under the concentration time curve (AUC) outside the target range and had dose adjustment, (2) how busulfan clearance (CL) relates to body weight, and (3) if fewer samples could be used to predict doses.

METHODS

Blood samples were taken from patients after oral administration, usually at 0.5, 1, 1.5, and 6 hours, and after the start of a 2-hour intravenous (IV) infusion of busulfan, at 1, 2, 2.5, 3, 6, and 8 hours. Dose adjustment was made based on the AUC compared with the target range. The relationship of CL and body weight for the IV group was used to develop a revised IV dosing schedule. The bias and imprecision of AUCs estimated using fewer sampling points were examined to see if sampling could be economized.

RESULTS

Data were available for 150 patients but for 6 patients, data were incomplete and excluded. Of the remaining 144 patients (256 sample sets, 209 oral, 47 IV, 62% with repeats), 38% (IV) and 35% (oral) of patients had AUCs within the target range after the first dose. Dose adjustment was made in 47% and 34% of patients dosed IV and orally, respectively, after which there was a trend to more patients achieving the target AUC. A nonlinear relationship was found between CL and body weight. The initial IV dosing schedule was revised to take this into account. Sampling for busulfan concentration measurement at 3 points (2.5, 4, 8 hours) or 2 points (2.5, 8 hours) after the start of the infusion enabled accurate and precise estimates of AUC₀₋₂₄.

CONCLUSIONS

Around two thirds of patients treated with busulfan were outside the target AUC range after the first dose. Dose adjustment was made in 37% of patients. The relationship between CL and body weight was used to revise the initial IV dosing schedule. Sampling for AUC estimation could be reduced to 2 time points after IV dosing.

Association between busulfan exposure and outcome in children receiving intravenous busulfan before hematopoietic stem cell transplantation

BACKGROUND AND OBJECTIVE

Intravenous (IV) busulfan (Bu) combined with therapeutic drug monitoring-guided dosing is associated with better event-free survival (EFS), lower transplant-related mortality. But optimal target steady-state concentration (Css) of Bu in children undergoing hematopoietic stem cell transplantation (HSCT) remains unclear. This study aimed to evaluate the relation between Css of Bu and clinical outcomes in children receiving Bu before HSCT.

METHODS

This study includes 75 children receiving IV Bu in 16 doses, with first dose assigned based on age. Bu first-dose pharmacokinetic parameters were estimated from Bu plasma concentrations measured at 6 time points by high-performance liquid chromatography. Doses were adjusted at the fifth dose to a target Css of 600-900 ng/mL. Cumulative incidence of overall survival (OS), EFS, transplant-related mortality, acute graft-versus host disease (aGVHD), and other toxicities in relation to Css of Bu were analyzed using Kaplan-Meier curves in univariate and Cox's proportional hazards model in multivariate analysis.

RESULTS

After the first dose, median Css was 578 (325-1227) ng/mL. Forty-one patients had Bu IV dose increased by > 10%. Neutrophil and platelet recoveries, grade 2-4 aGVHD, and nonrelapse mortality (NRM) incidences were 90%, 91%, 12%, and 13%, respectively. Relapse incidence was 33%. Incidence of veno-occlusive disease, hemorrhagic cystitis, and lung toxicities were 13%, 24%, and 7%, respectively. OS and EFS were 70% and 58%. First-dose Bu Css >600 ng/mL was associated with a higher NRM (P < 0.001) and grade 2-4 aGVHD (P = 0.04), a lower EFS (P < 0.001), and OS (P = 0.001).

CONCLUSIONS

This study demonstrated a significant association between the first-dose pharmacokinetics of Bu and NRM, OS, and EFS. Bu therapeutic drug monitoring provides information that potentially influences outcomes of HSCT in pediatric patients.

Fludarabine and exposure-targeted busulfan compares favorably with busulfan/cyclophosphamide-based regimens in pediatric hematopoietic cell transplantation: maintaining efficacy with less toxicity

Busulfan (Bu) is used as a myeloablative agent in conditioning regimens before allogeneic hematopoietic cell transplantation (allo-HCT). In line with strategies explored in adults, patient outcomes may be optimized by replacing cyclophosphamide (Cy) with or without melphalan (Mel) with fludarabine (Flu). We compared outcomes in 2 consecutive cohorts of HCT recipients with a nonmalignant HCT indication, a myeloid malignancy, or a lymphoid malignancy with a contraindication for total body irradiation (TBI). Between 2009 and 2012, 64 children received Flu + Bu at a target dose of 80-95 mg·h/L, and between 2005 and 2008, 50 children received Bu targeted to 74-80 mg·h/L + Cy. In the latter group, Mel was added for patients with myeloid malignancy (n = 12). Possible confounding effects of calendar time were studied in 69 patients receiving a myeloablative dose of TBI between 2005 and 2012. Estimated 2-year survival and event-free survival were 82% and 78%, respectively, in the FluBu arm and 78% and 72%, respectively, in the BuCy (Mel) arm (P = not significant). Compared with the BuCy (Mel) arm, less toxicity was noted in the FluBu arm, with lower rates of acute (noninfectious) lung injury (16% versus 36%; P = .007), veno-occlusive disease (3% versus 28%; P = .003), chronic graft-versus-host disease (9% versus 26%; P = .047), adenovirus infection (3% versus 32%; P = .001), and human herpesvirus 6 infection reactivation (21% versus 44%; P = .005). Furthermore, the median duration of neutropenia was shorter in the FluBu arm (11 days versus 22 days; P < .001), and the patients in this arm required fewer transfusions. Our data indicate that Flu (160 mg/m(2)) with targeted myeloablative Bu (90 mg·h/L) is less toxic than and equally effective as BuCy (Mel) in patients with similar indications for allo-HCT.

Comparison of intravenous with oral busulfan in allogeneic hematopoietic stem cell transplantation with myeloablative conditioning regimens for pediatric acute leukemia

Recent reports revealed that intravenous (iv) busulfan (BU) may not only reduce early nonrelapse mortality (NRM) but also improve overall survival (OS) probability in adults. Therefore, we retrospectively compared outcomes for 460 children with acute leukemia who underwent hematopoietic stem cell transplantation with either iv-BU (n = 198) or oral busulfan (oral-BU) (n = 262) myeloablative conditioning. OS at 3 years was 53.4% ± 3.7% with iv-BU and 55.1% ± 3.1% with oral-BU; the difference was not statistically significant (P = .77). OS at 3 years in 241 acute lymphoblastic leukemia and 219 acute myeloid leukemia patients was 56.4% ± 5.5% with iv-BU and 54.6% ± 4.1 with oral-BU (P = .51) and 51.0% ± 5.0% with iv-BU and 55.8% ± 4.8% with oral-BU (P = .83), respectively. Cumulative incidence of relapse at 3 years with iv-BU was similar to that with oral-BU (39.0% ± 3.6% and 36.4% ± 3.1%, respectively; P = .67). Cumulative incidence of NRM at 3 years was 16.6% ± 2.7% with iv-BU and 18.3% ± 2.5% with oral-BU (P = .51). Furthermore, multivariate analysis showed no significant survival advantage with iv-BU. In conclusion, iv-BU failed to show a significant survival advantage in children with acute leukemia.

Children's Oncology Group's 2013 blueprint for research: neuroblastoma

Estimated 5-year survival rates for patients with non-high-risk and high-risk neuroblastoma are 90% and 50%, respectively. Recent clinical trials have shown excellent outcomes with reduced therapy for non-high-risk disease. For patients with high-risk neuroblastoma treated with chemoradiotherapy, surgery, and stem cell transplantation, the addition of anti-disialoganglioside (GD2) immunotherapy plus cytokines improves survival. Upcoming trials will study the incorporation of targeted radionuclide therapy prior to myeloablative chemotherapy into high-risk treatment. Phase 2 trials will investigate druggable target(s) including mTOR inhibition and GD2-directed therapy in combination with chemotherapy for patients with recurrent neuroblastoma, and ALK inhibition for those with ALK-aberrant tumors.

Increased risk of gastrointestinal acute GVHD following the addition of melphalan to busulfan/cyclophosphamide conditioning

Risk factors associated with the development of aGVHD in the gastrointestinal tract have not been studied in depth. We retrospectively assessed 25 pediatric patients with MDS and JMML and compared the treatment outcome of two different conditioning regimens. Seventeen children (68%) underwent conditioning with busulfan (Bu), cyclophosphamide (Cy), and melphalan (Mel) and eight children (32%) with Bu and Cy. Gastrointestinal aGVHD stages II-IV (day 0-100) were observed in 47% (eight of 17) of the patients in the BuCyMel group and in none (0 of 8) in the BuCy group (p < 0.05). In patients who developed gastrointestinal aGVHD stages III-IV, a 24-h variation in the Bu concentration with a nighttime peak was noted. HC and liver aGVHD stages II-IV were observed in 47% (eight of 17) and 35% (six of 17) after BuCyMel conditioning and in 0% (0 of 17) and 12.5% (one of eight) after BuCy conditioning. The overall survival rate was 53% (nine of 17) in the BuCyMel group and 62.5% (five of eight) in the BuCy group. In conclusion, the addition of melphalan to the BuCy conditioning regimen resulted in severe gastrointestinal complications and did not improve overall survival.

Variation in prescribing patterns and therapeutic drug monitoring of intravenous busulfan in pediatric hematopoietic cell transplant recipients

Personalizing intravenous (IV) busulfan doses in children using therapeutic drug monitoring (TDM) is an integral component of hematopoietic cell transplant. The authors sought to characterize initial dosing and TDM of IV busulfan, along with factors associated with busulfan clearance, in 729 children who underwent busulfan TDM from December 2005 to December 2008. The initial IV busulfan dose in children weighing ≤12 kg ranged 4.8-fold, with only 19% prescribed the package insert dose of 1.1 mg/kg. In those children weighing >12 kg, the initial dose ranged 5.4-fold, and 79% were prescribed the package insert dose. The initial busulfan dose achieved the target exposure in only 24.3% of children. A wide range of busulfan exposures were targeted for children with the same disease (eg, 39 target busulfan exposures for the 264 children diagnosed with acute myeloid leukemia). Considerable heterogeneity exists regarding when TDM is conducted and the number of pharmacokinetic samples obtained. Busulfan clearance varied by age and dosing frequency but not by underlying disease. The authors- group is currently evaluating how using population pharmacokinetics to optimize initial busulfan dose and TDM (eg, limited sampling schedule in conjunction with maximum a posteriori Bayesian estimation) may affect clinical outcomes in children.

Pharmacology of dimethanesulfonate alkylating agents: busulfan and treosulfan

INTRODUCTION

Among the dimethanesulfonates, busulfan, in combination with other alkylating agents or nucleoside analogues, is the cornerstone of high-dose chemotherapy. It is used, and followed hematopoietic stem cell transplantation, for the treatment of various hematologic malignancies and immunodeficiencies. Treosulfan, which is a hydrophilic analogue of busulfan, was the first dimethanesufonate registered for the treatment of ovarian cancer. Recently, treosulfan has been investigated for the treatment of hematologic malignancies in combination with the same second agents before hematopoietic stem cell transplantation.

AREAS COVERED

This work reviews the pharmacological data of these two dimethanesulfonates alkylating agents. Specifically, the article looks at their chemistry, metabolism, anticancer activity, and their pharmacokinetics and pharmacodynamics.

EXPERT OPINION

Busulfan has been investigated widely for more than three decades leading to a large and precise handling of this agent with numerous studies on activity and pharmacokinetics and pharmacodynamics. In contrast, the behavior of treosulfan is still under investigation and not fully described. The complexity of treosulfan's metabolism and mechanism of action gives rise to the need of a deeper understanding of its pharmacological activity in a context of high-dose chemotherapy. Specifically, there is a great need to better understand its pharmacokinetics/pharmacodynamics relationship.

Levetiracetam for busulfan-induced seizure prophylaxis in children undergoing hematopoietic stem cell transplantation

Anti-seizure prophylaxis is routinely utilized during busulfan administration for HSCT. We evaluated the feasibility and efficacy of levetiracetam in children undergoing HSCT. A total of 28 children and young adults received levetiracetam during HSCT and the outcomes and costs were compared to a historical, but similar cohort of 25 patients who had received fosphenytoin. Levetiracetam was well tolerated and was efficacious in preventing seizures. Cost of drug, administration, and monitoring were also similar among the two groups. Due to non-induction of the hepatic cytochrome P450 enzymes, levetiracetam may lead to better dose assurance of busulfan in targeted dose regimens for HSCT.

A novel method for quantification of sulfolane (a metabolite of busulfan) in plasma by gas chromatography-tandem mass spectrometry

The role of busulfan (Bu) metabolites in the adverse events seen during hematopoietic stem cell transplantation and in drug interactions is not explored. Lack of availability of established analytical methods limits our understanding in this area. The present work describes a novel gas chromatography-tandem mass spectrometric assay for the analysis of sulfolane (Su) in plasma of patients receiving high-dose Bu. Su and Bu were extracted from a single 100 μL plasma sample by liquid-liquid extraction. Bu was separately derivatized with 2,3,5,6-tetrafluorothiophenolfluorinated agent. Mass spectrometric detection of the analytes was performed in the selected reaction monitoring mode on a triple quadrupole instrument after electronic impact ionization. Bu and Su were analyzed with separate chromatographic programs, lasting 5 min each. The assay for Su was found to be linear in the concentration range of 20-400 ng/mL. The method has satisfactory sensitivity (lower limit of quantification, 20 ng/mL) and precision (relative standard deviation less than 15 %) for all the concentrations tested with a good trueness (100 ± 5 %). This method was applied to measure Su from pediatric patients with samples collected 4 h after dose 1 (n = 46), before dose 7 (n = 56), and after dose 9 (n = 54) infusions of Bu. Su (mean ± SD) was detectable in plasma of patients 4 h after dose 1, and higher levels were observed after dose 9 (249.9 ± 123.4 ng/mL). This method may be used in clinical studies investigating the role of Su on adverse events and drug interactions associated with Bu therapy.

Phase II study of dose-modified busulfan by real-time targeting in allogeneic hematopoietic stem cell transplantation for myeloid malignancy

We aimed to evaluate the efficacy and safety of allogeneic hematopoietic stem cell transplantation with targeted oral busulfan (BU) and cyclophosphamide (CY) in a phase II study. Busulfan (1.0 mg/kg) was given initially in six doses. Based on the estimated concentration at steady state after the first dose of BU, subsequent (7th-16th) doses were adjusted to obtain a targeted overall concentration at steady state of 700-900 ng/mL. The primary endpoint was 1-year overall survival (OS). Fifty patients were registered and 46 (median age, 53 years; range, 18-62 years) received planned transplant, including 24 with AML, 16 with myelodysplastic syndrome, and six with CML. Fourteen patients were categorized as standard risk. Nineteen patients received transplant from human leukocyte antigen-identical siblings, 27 from unrelated donors. The BU dose required reduction in 32 patients and escalation in six patients. One-year OS was 65% (95% confidence interval, 50-77%). Cumulative incidence of hepatic sinusoidal obstruction syndrome was 11%. One-year transplant-related mortality was 18%. Both OS and transplant-related mortality were favorable in this study, including patients of older age and with high risk diseases. Individual dose adjustment based on BU pharmacokinetics was feasible and effective in the current phase II study. This trial is registered in the University Hospital Medical Information Network Clinical Trial Registry System (UMIN-CTR, ID:C000000156).

Pharmacokinetic behavior and appraisal of intravenous busulfan dosing in infants and older children: the results of a population pharmacokinetic study from a large pediatric cohort undergoing hematopoietic stem-cell transplantation

BACKGROUND

Intravenous (IV) busulfan (Bu) dosing approved in Europe based on 5 body weight (BW) strata has been validated for targeting Bu exposures in children undergoing hematopoietic stem-cell transplantation and with mostly malignant diseases. The authors conducted an observational study aiming to investigate the behavior and ontogeny of IV Bu pharmacokinetic (PK) disposition, and to reevaluate the consistency of the BW-based dosing in very young children with rare diseases.

METHODS

The observational study comprised 115 patients, mostly infants with immunodeficiencies and metabolic inherited disorders and with altered liver function and/or iron overload. Additional data (90 children, mostly malignant diseases) were pooled with the first data set. The overall data (205 children aged from 10 days to 15 years) were analyzed using population PK modeling.

RESULTS

The BW remained the main determinant of IV Bu PK, and no further covariate effect was identified. Bu clearance (CL) variability was best described by BW allometric functions. Increase of drug CL with the child's growth was faster in younger children. This pattern is likely related to the maturation of GSTA1 enzymes during infancy and was accounted for in the model by estimating a higher BW allometric exponent in children <9 kg compared with that in children ≥9 kg. IV Bu PK was not modified in children with altered liver function and/or iron overload, and no disease specific difference was observed. Bu dosing either adjusted according to the final model or with the approved EU labeling yields similar targeting performances. For both dosing strategies, the percent of patients achieving the therapeutic area under the curve window (900-1500 μmole·min/L were 60% and 70%-90% in children <9 and ≥9 kg, respectively.

CONCLUSIONS

A population PK model accounting for the highest Bu CL in the youngest patients was validated on training and evaluation data sets. The BW-based dosing strategy recommended in Europe proved to be consistent on a large paediatric cohort representative of the population heterogeneity observed in hematopoietic stem-cell transplantation.

Identifying developmental toxicity pathways for a subset of ToxCast chemicals using human embryonic stem cells and metabolomics

Metabolomics analysis was performed on the supernatant of human embryonic stem (hES) cell cultures exposed to a blinded subset of 11 chemicals selected from the chemical library of EPA's ToxCast™ chemical screening and prioritization research project. Metabolites from hES cultures were evaluated for known and novel signatures that may be indicative of developmental toxicity. Significant fold changes in endogenous metabolites were detected for 83 putatively annotated mass features in response to the subset of ToxCast chemicals. The annotations were mapped to specific human metabolic pathways. This revealed strong effects on pathways for nicotinate and nicotinamide metabolism, pantothenate and CoA biosynthesis, glutathione metabolism, and arginine and proline metabolism pathways. Predictivity for adverse outcomes in mammalian prenatal developmental toxicity studies used ToxRefDB and other sources of information, including Stemina Biomarker Discovery's predictive DevTox® model trained on 23 pharmaceutical agents of known developmental toxicity and differing potency. The model initially predicted developmental toxicity from the blinded ToxCast compounds in concordance with animal data with 73% accuracy. Retraining the model with data from the unblinded test compounds at one concentration level increased the predictive accuracy for the remaining concentrations to 83%. These preliminary results on a 11-chemical subset of the ToxCast chemical library indicate that metabolomics analysis of the hES secretome provides information valuable for predictive modeling and mechanistic understanding of mammalian developmental toxicity.

Therapeutic drug monitoring is essential for intravenous busulfan therapy in pediatric hematopoietic stem cell recipients

Busulfan is widely used for myeloablative conditioning in HSCT. Intravenous busulfan has been introduced to reduce interindividual variability in plasma levels especially in pediatric patients. TDM of intravenous busulfan was performed in 34 pediatric HSCT patients with malignant (n = 9) and non-malignant (n = 25) diseases (50% of patients <three yr) in a single-center analysis (2006-2009). Intravenous busulfan was administered twice daily in a four-h infusion according to recommended weight-based doses. Busulfan drug levels were measured, and pharmacokinetic analysis was performed. The targeted busulfan exposure was aimed to range between AUC of 9000-12, 000 ng/mL/h. In 23/34 patients (68%), the busulfan dose had to be adjusted at least once. In 16/23 patients (70%), the dose had to be increased in a range of 7 to 33%, while in 7/23 patients (30%), the dose had to be decreased by 7-20%. The need of dose adjustment was not related to weight, age, or underlying disease. Seven out of 34 patients (21%) <20 months experienced VOD despite that their total AUCs were within the target AUC. TDM of intravenous busulfan is essential to increase the efficacy and safety of busulfan-based conditioning protocols in pediatric HSCT recipients.

Safety and efficacy of targeted busulfan therapy in children undergoing myeloablative matched sibling donor BMT for sickle cell disease. Bone Marrow Transplant. 2011;46:27-33. [PMID: 20305698 DOI: 10.1038/bmt.2010.60]

Busulfan influences engraftment and toxicities during hematopoietic stem cell transplantation (HSCT). We report our single-institution experience of targeted busulfan therapy for myeloablative, matched sibling donor (MSD) HSCT for sickle cell disease (SCD) and assess the relationships of busulfan levels to engraftment and toxicities. Twenty-seven patients with SCD underwent MSD HSCT from 1993 to 2007, 25 with busulfan measurements. The conditioning regimen was busulfan (initial dose 0.875 mg/kg for 16 doses), CY and antithymocyte globulin. Busulfan area under curve (AUC) was determined with the first dose, and dose adjustments were made to target the desired AUC range. Median AUC was 963 μmol min/L (range 780-1305 μmol min/L). Engraftment occurred in all cases: 21 (84%) full donor chimerism (> 95% donor cells), 4 (16%) partial donor chimerism. Hepatic veno-occlusive disease (VOD) occurred in 8 (32%) patients. Lower AUC was seen with partial donor chimerism (862 ± 73 μmol min/L) versus full donor chimerism (AUC 1018 ± 122 μmol min/L) (P = 0.022). VOD was not associated with busulfan AUC (P = 0.153). Of 25 patients, 24 survived with median follow-up of 4.9 years. Our experience shows that targeting busulfan AUC above the range used in previous multicenter trials appears safe and may contribute to sustained engraftment in SCD.

Novel pharmacokinetic behavior of intravenous busulfan in children with thalassemia undergoing hematopoietic stem cell transplantation: a prospective evaluation of pharmacokinetic and pharmacodynamic profile with therapeutic drug monitoring

We prospectively studied the pharmacokinetics (PK) and clinical outcomes of intravenous busulfan (Bu) in 71 children with preexisting liver damage who underwent hematopoietic stem cell transplantation for thalassemia. Intravenous Bu was administered every 6 hours as part of a conditioning regimen with PK-based dose adjustment to target a conservative area under the concentration-versus-time curve (AUC) range (900-1350 microMol*min). The first-dose Bu clearance (CL) was significantly higher than the subsequent daily CL that remained unchanged in the ensuing days. One-third of patients required dose escalation based on dose 1 AUC, whereas dose reduction was needed in the subsequent days. At doses 5, 9, and 13, 78%, 81%, and 87% of patients, respectively, achieved the target range of AUC. A population PK analysis confirmed that the first-dose CL was 20% higher and that body weight was the most important covariate to explain PK variability. Patients with variant GSTA1*B had a 10% lower Bu CL than wild-type. These results suggest that the disease-specific behavior of intravenous Bu PK should be considered for PK-guided dose adjustment in patients with thalassemia, and the use of a conservative AUC range resulted in low toxicity, good engraftment, and good survival rate.

Evaluating pharmacokinetics and pharmacodynamics of intravenous busulfan in pediatric patients receiving bone marrow transplantation

BU is a commonly used conditioning agent in BMT. However, it is a narrow therapeutic index drug which shows a strong correlation between AUC and both efficacy and toxicity. Studies in pediatric patients have suggested that children less than four yr of age have a greater clearance and thus lower AUC at standard adult doses. The goal of this retrospective analysis was to evaluate any age-related pharmacokinetic and pharmacodynamic differences in pediatric patients who received BU as a conditioning agent. From 2003 to 2006, 21/77 pediatric patients who received BMT were reviewed. There were 15 males and six females with a mean age of six yr old. Diagnoses of leukemia (n = 11), Hodgkin's lymphoma (n = 3), myelodysplastic syndrome (n = 2), and other (n = 5) were included. Sixteen patients received BU + cyclophosphamide while five patients received BU + another agent. There were 20 allogeneic and one autologous transplants among which 16 were human leukocyte antigen matched and five were mismatched. Average BU clearance in patients younger than four yr old (n = 8) was 4.1 +/- 1.0 mL/min/kg vs. 3.1 +/- 0.7 mL/min/kg in patients older than four yr old (n = 13) (p = 0.02). The corresponding averages for AUC were 998 +/- 226 microm x min vs. 1155 +/- 183 microm x min (p = 0.12). No patients younger than four yr old developed VOD while five of the older patients did (p = 0.044). There were no significant differences in terms of engraftment and acute GvHD. There were significant age-related pharmacokinetic differences in pediatric patients less than four yr of age receiving BU for conditioning prior to BMT. There was a decrease in drug toxicity seen in these patients.

Busulfan in hematopoietic stem cell transplant setting

This paper focuses primarily on the data published in the last decade about the pharmacokinetics and pharmacodynamics of oral and intravenous (i.v.) busulfan, therapeutic drug monitoring and clinical outcome in hematopoietic stem cell transplant (HCT) patients. Busulfan is commonly used in HCT as it is toxic to the marrow. Busulfan is available as oral or i.v. formulation. The most common significant toxicity of busulfan is sinusoidal obstruction syndrome. Even with the introduction of i.v. busulfan, variability in the systemic concentrations of busulfan after weight-based dosing and the association between busulfan plasma exposure and outcome in HCT patients have led to the continued use of therapeutic drug monitoring of busulfan. New strategies for personalizing busulfan dosing are being studied to maximize the use of busulfan for optimal disease control with the least toxicity to HCT patients. One such strategy currently being evaluated is if busulfan clearance can be accurately predicted by genetic polymorphism of glutathione S-transferase (GST), with the currently available data suggesting that GST polymorphisms cannot be used to personalize busulfan dosing.

Influence of GST gene polymorphisms on busulfan pharmacokinetics in children

Busulfan (BU) is a key compound in conditioning myeloablative regimens for children undergoing hematopoietic stem cell transplantation (HSCT). There are wide interindividual differences in BU pharmacokinetics, which increase the risk of veno-occlusive disease, graft rejection and disease relapse. As BU is mainly metabolized by glutathione S-transferase (GST), it is hypothesized that functional polymorphisms in GST genes may explain in part the variability in BU pharmacokinetics. We analyzed polymorphisms in GSTA1 (C-69T, A-513G, G-631T, C-1142G), GSTM1 (deletion) and GSTP1 (A1578G, C2293T) genes in 28 children undergoing HSCT. All patients had individualized dosing based on pharmacokinetics after the first dose of intravenous BU. GSTM1-null individuals had higher drug exposure (P(Cmax)=0.008; P(AUC)=0.003; P(Css)=0.02) and lower clearance (P(CL)=0.001). Multivariate regression models showed that, other than the drug dose and age, the GSTM1 genotype was the best predictor of first-dose pharmacokinetic variability. GSTM1-null patients also received lower cumulative BU doses (P=0.02). No association was found between BU exposure and major GSTA1 or GSTP1 gene variants. In children, GSTM1 polymorphism seems to modify BU pharmacokinetics after intravenous drug administration.

Vascular disorders of the liver

This guideline has been approved by the American Association for the Study of Liver Diseases (AASLD) and represents the position of the association.

The use of pharmacokinetic models in paediatric onco-haematology: effects on clinical outcome through the examples of busulfan and cyclosporine

Overall survival after allogeneic haematopoietic stem cell transplantation (HSCT) is reduced by the high rate of transplantation-related mortality (TRM), especially because of liver veno-occlusive disease (VOD) or acute graft-vs.-host disease (GVHD) because of the toxicity or inefficacy of busulfan and cyclosporine (CsA), respectively. Results of clinical outcome of previous studies performed to optimize busulfan and CsA therapy by controlling their pharmacokinetic variability by means of maximum a posteriori (MAP) Bayesian individualization of both drugs are presented. The 90-day VOD-free survival was significantly higher in patients with individualized busulfan doses: 97% vs. 76%. Monitoring CsA trough blood concentrations allowed us to obtain a successful GVHD outcome (mild or moderate GVHD and graft vs. leukaemia effect (GVL) in malignant diseases and no GVHD (in non-malignant ones) in the majority of our patients. Severe GVHD occurred in <5% of patients. TRM in children can be significantly decreased by using population pharmacokinetic models and MAP Bayesian individualization of dose regimens for drugs such as CsA and busulfan.

Glutathione S-transferase polymorphisms are not associated with population pharmacokinetic parameters of busulfan in pediatric patients

High busulfan exposure is associated with increased toxicity, for example veno-occlusive disease, whereas low exposure results in less efficacy such as lower engraftment rates. Despite adjusting dose to body weight, interindividual variability in pharmacokinetics and thus drug exposure remained rather large. In this report, the contribution of genetic polymorphisms in the glutathione-S-transferases (GST) isozymes GSTA1, GSTM1, GSTP1, and GSTT1 to the pharmacokinetics of busulfan is studied retrospectively. Seventy-seven children, undergoing myeloablative conditioning for allogeneic hematopoietic stem cell transplantation, were treated with busulfan (Busulvex) during 4 days, receiving busulfan either in one single dose or dived in four doses every 6 hours. Genetic variants of GSTA1, GSTM1, GSTP1, and GSTT1 were determined by pyrosequencing. Pharmacokinetic parameters were estimated by using nonlinear mixed-effect modeling (NONMEM). Subsequently, a combined population pharmacokinetic-pharmacogenetic model was developed describing the pharmacokinetics of busulfan taking into account the GST polymorphisms. In the presented pediatric population, body weight appeared to be the most important covariate and explained a major part of the observed variability in the pharmacokinetics of busulfan. None of the studied polymorphisms in the genes encoding GSTA1 GSTM1, GSTP1, and GSTT1 nor combinations of genotypes were significant covariates. It was concluded that in children, variability in pharmacokinetics of busulfan could not be related to polymorphisms in GST.