Maximal concentration of intravenous busulfan as a determinant of veno-occlusive disease: a pharmacokinetic-pharmacodynamic analysis in 293 hematopoietic stem cell transplanted children

Therapeutic drug monitoring in acute lymphoblastic leukemia-a deep dive into pharmacokinetics, -dynamics, and -genetics of antileukemic drugs

INTRODUCTION

Therapeutic drug monitoring (TDM) is important to optimize drug exposure and minimize toxicity for the individual patient.

AREAS COVERED

This narrative review covers the pharmacokinetics (PK), -dynamics (PD) and -genetics of classic chemotherapeutic drugs used in frontline therapy for acute lymphoblastic leukemia (ALL), including anthracyclines, asparaginase, busulfan, cyclophosphamide, cytarabine, glucocorticoids, methotrexate, nelarabine, thiopurines, tyrosine kinase inhibitors, and vincristine. Furthermore, novel immunotherapies including blinatumomab, inotuzumab ozogamicin, and chimeric antigen receptor T-cells that are rapidly moving into frontline therapy are addressed. This review focuses on TDM already used in clinical practice as well as the unused potential and feasibility of TDM. Finally, important factors affecting PK/PD such as obesity and transition to adolescence and young adulthood are discussed.

EXPERT OPINION

Investigation of TDM as standard of care for antileukemic agents is highly warranted to personalize curative yet toxic anticancer regimens within frontline ALL treatment. Some of the drugs have been used in ALL treatment regimens for decades, but a wide range of new compounds are being introduced, some like blinatumomab reaching standard-of-care designation. Not least, optimized drug efficacy and reduction of the risk of serious toxicities may render TDM implementation cost-effective.

Comparison of busulfan pharmacokinetics between four-times-daily and once-daily administration in pediatric patients: a preliminary prospective observational trial

Therapeutic drug monitoring (TDM) of busulfan (BU) is useful for achieving the target area under the curve (AUC) because its effective plasma-concentration range is narrow. This preliminary observational study evaluated the pharmacokinetic (PK) parameters of BU administered four times daily or once daily to pediatric patients. The plasma concentrations were measured at both the test dose and first dose, and the doses on day 1 and days 2-4 were determined based on each TDM. A comparison of PK parameters between four-times-daily and once-daily administration was performed for both the test dose and first dose of BU. Of the 11 patients, five received four-times-daily BU and six received once-daily BU. The Vd for once-daily administration was higher than that for four-times-daily administration for the first dose but not for the test dose. The ratio of actual AUC for the first dose to estimated AUC guided by the test dose was lower with once-daily administration than with four-times-daily administration. These results indicate that the PK parameters of BU administered once daily are challenging to predict based on the TDM of the test dose. TDM should be considered on day 1 to achieve the target AUC, especially with once-daily administration.

How I treat sickle cell disease with gene therapy

ABSTRACT

In 2023, 2 different gene therapies were approved for individuals with severe sickle cell disease (SCD). The small number of patients treated on the pivotal clinical trials that led to these approvals have experienced dramatic short-term reductions in the occurrence of painful vaso-occlusive crises, but the long-term safety and efficacy of these genetic therapies are yet to be ascertained. Several challenges and treatment-related concerns have emerged in regard to administering these therapies in clinical practice. This article discusses the selection and preparation of individuals with SCD who wish to receive autologous gene therapy, as well as the salient features of the care needed to support them through a long and arduous treatment process. I specifically focus on postinfusion care, as it relates to immune monitoring and infection prevention. Compared with allogeneic hematopoietic cell transplantation, delivering autologous gene therapy to an individual with SCD has distinct nuances that require awareness and special interventions. Using clinical vignettes derived from real-life patients, I provide perspectives on the complex decision-making process for gene therapy for SCD based on currently available data and make recommendations for evaluating and supporting these patients.

A practical guide to therapeutic drug monitoring in busulfan: recommendations from the Pharmacist Committee of the European Society for Blood and Marrow Transplantation (EBMT)

Busulfan (Bu) is an important component of many conditioning regimens for allogeneic hematopoietic cell transplantation. The therapeutic window of Bu is well characterized, with strong associations between Bu exposure and the clinical outcome in adults (strongest evidence in myelo-ablative setting) and children (all settings). We provide an overview of the literature on Bu as well as a step-by-step guide to the implementation of Bu therapeutic drug monitoring (TDM). The guide covers the clinical, pharmacological, laboratory and administrative aspects of the procedure. Through this document, we aim to support centers in implementing TDM for Bu to further enhance the success rates of HCT and improve patient outcomes. The Pharmacist Committee of the European Society for Blood and Marrow Transplantation (EBMT) encourages all centers to perform TDM for Bu in the aforementioned indications.

Risk Factors for Sinusoidal Obstruction Syndrome After Hematopoietic Stem Cell Transplantation in Children and Young Adults: A Systematic Review and Meta-Analysis

OBJECTIVE AND BACKGROUND

Sinusoidal obstruction syndrome (SOS) is a life-threatening complication in hematopoietic stem cell transplantation (HSCT) patients. However, the related risk factors in pediatric and young adult HSCT recipients remain unclear. Thus, we conducted this meta-analysis to identify potential risk factors for SOS in children and young adults undergoing HSCT.

METHOD

We acquired related articles through searching PubMed, EMBASE, and the Cochrane Library up to May 31, 2024. We calculated odds ratios (ORs) and corresponding 95% confidence intervals (CIs) to identify potential risk factors.

RESULTS

A total of 12 studies with 7644 HSCT recipients were included. Bone marrow transplantation (OR = 1.35, 95% CI: 1.03-1.77, I2 = 0%), busulfan (BU) (OR = 3.63, 95% CI: 1.78-7.38, I2 = 70%), and fludarabine (FLU) (OR = 1.55, 95% CI: 1.09-2.21, I2 = 16%) were risk factors for SOS after HSCT in children and young adults.

CONCLUSION

Bone marrow transplantation and the use of BU or FLU might be risk factors for SOS after HSCT in children and young adults.

Model-Informed Precision Dosing of Intravenous Busulfan in Thai Pediatrics Undergoing Hematopoietic Stem Cell Transplantation

BACKGROUND

Conditioning bifunctional agent, busulfan, is commonly used on children before hematopoietic stem cell transplantation. Currently, at the Ramathibodi hospital, Bangkok, Thailand, initial dosing is calculated according to age and body surface area, and 7 samples per day are used for therapeutic drug monitoring (TDM). This study aimed to identify the best strategies for individual dosages a priori from patient characteristics and a posteriori based on TDM.

METHODS

The pharmacokinetic data set consisted of 2018 plasma concentrations measured in 135 Thai (n = 135) pediatric patients (median age = 8 years) and were analyzed using a population approach.

RESULTS

Body weight, presence of malignant disease, and genetic polymorphism of Glutathione S-transferase Alpha-1 (GSTA1) were predictors of clearance. The optimum sampling times for TDM concentration measurements were 0.25, 2, and 5 hours after a 3-hour infusion. This was sufficient to obtain a Bayesian estimate of clearance a posteriori. Simulations showed the poor performance of a priori formula-based dose calculations with 90% of patients demonstrating a 69%-151% exposure interval around the target. This interval shrank to 85%-124% if TDM was carried out only at day 1 and to 90%-116% with TDM at days 1 and 3.

CONCLUSIONS

This comprehensive study reinforces the interest of TDM in managing interindividual variability in busulfan exposure. Therapeutic drug monitoring can reliably be implemented from 3 samples using the Bayesian approach, preferably over 2 days. If using the latter is not possible, the formulas developed herein could present an alternative in Thai patients.

Optimal Once-Daily Busulfan Administration in Pediatric Patients: A Simulation-Based Investigation of Intravenous Infusion Times

Purpose

Pediatric patients receiving hematopoietic stem cell transplantation undergo regular administration of intravenous busulfan as a conditioning regimen. Once-daily regimen of busulfan has been proposed as a more convenient alternative to the traditional regimen, but it may increase the risk of toxicity such as veno-occlusive disease (VOD). The study aims to evaluate the pharmacokinetics (PKs) of once-daily regimens and investigate appropriate intravenous infusion times to reduce the risk of toxicity.

Patients and methods

Once-daily busulfan dosing regimens for pediatric patient were reviewed and selected including EMA- and FDA-based once-daily dosing regimens. We generated busulfan PK data of virtual pediatric patients using a previously developed population PK model. PK profiles and proportion of patients achieving the referenced maximum concentration (Cmax) and exposure to busulfan were used to evaluate the appropriateness of both infusion time and dosing regimens.

Results

Predicted PK profiles and exposure of busulfan showed relatively similar distributions for all once-daily dosing regimens. Most patients exceeded the referenced Cmax possibly associated with a high risk of VOD with all once-daily regimens when applied with 3 hours of infusion.

Conclusion

While intravenous infusion of once-daily busulfan is typically administered over 3 hours, our findings emphasize the necessity of considering sufficient infusion times to ensure safe drug utilization and prevent toxicity, which will aid in optimal busulfan use in pediatric oncology.

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.

Population Pharmacokinetic Model of Intravenous Busulfan in Hematopoietic Cell Transplantation: Systematic Review and Comparative Simulations

BACKGROUND

Busulfan is commonly used in the chemotherapy prior to hematopoietic cell transplantation (HCT). Busulfan has a narrow therapeutic window and a well-established exposure-response relationship with important clinical outcomes. Model-informed precision dosing (MIPD) based on population pharmacokinetic (popPK) models has been implemented in the clinical settings. We aimed to systematically review existing literature on popPK models of intravenous busulfan.

METHODS

We systematically searched Ovid MEDLINE, EMBASE, Cochrane Library, Scopus, and Web of Science databases from inception to December 2022 to identify original popPK models (nonlinear mixed-effect modeling) of intravenous busulfan in HCT population. Model-predicted busulfan clearance (CL) was compared using US population data.

RESULTS

Of the 44 eligible popPK studies published since 2002, 68% were developed predominantly in children, 20% in adults, and 11% in both children and adults. The majority of the models were described using first-order elimination or time-varying CL (69% and 26%, respectively). All but three included a body-size descriptor (e.g., body weight, body surface area). Other commonly included covariates were age (30%) and GSTA1 variant (15%). Median between-subject and between-occasion variabilities of CL were 20% and 11%, respectively. Between-model variabilities in predicted median CL were < 20% in all of the weight tiers (10-110 kg) in the simulation based on US population data.

CONCLUSION

Busulfan PK is commonly described using a first-order elimination or time-varying CL. A simple model with limited covariates were generally sufficient to attain relatively small unexplained variabilities. However, therapeutic drug monitoring may still be necessary to attain a narrow target exposure.

Individualizing busulfan dose in specific populations and evaluating the risk of pharmacokinetic drug-drug interactions

INTRODUCTION

Busulfan is an alkylating agent widely used in the conditioning of hematopoietic stem cell transplantation possessing a complex metabolism and a large interindividual and intra-individual variability, especially in children. Combined with the strong rationale of busulfan PK/PD relationships, factors altering its clearance (e.g., weight, age, and GST-A genetic polymorphism mainly) can also affect clinical outcomes.

AREAS COVERED

This review aims to provide an overview of the current knowledge on busulfan pharmacokinetics, its pharmacokinetics variabilities in pediatric populations, drug-drug interactions (DDI), and their consequences regarding dose individualization. This review was based on medical literature up until October 2021.

EXPERT OPINION

To ensure effective busulfan exposure in pediatrics, different weight-based nomograms have been established to determine busulfan dosage and provided improved results (65 - 80% of patients correctly exposed). In addition to nomograms, therapeutic drug monitoring (TDM) of busulfan measuring plasmatic concentrations to estimate busulfan pharmacokinetic parameters can be used. TDM is now widely carried out in routine practices and aims to ensure the targeting of the reported therapeutic windows by individualizing busulfan dosing based on the clearance estimations from a previous dose.

Evaluation of different pharmacokinetically guided IV busulfan exposure ranges on adult patient outcomes after hematopoietic stem cell transplantation

Conditioning intensity contributes significantly to outcomes in allogeneic hematopoietic stem cell transplantation (allo-HSCT). We evaluated two myeloablative conditioning dosing ranges of intravenous (IV) busulfan (Bu) in combination with fludarabine in 70 patients. In 2015, our practice changed to target busulfan area under the curve (AUC) of ≥ 19.7 mg*h/L. We assessed responses in patients receiving busulfan AUCs of < 19.7 mg*h/L (Low-Bu) and ≥ 19.7 mg*h/L (High-Bu). At 18-month median follow-up, no differences in overall survival (OS) and relapse-free survival (RFS) were found between Low-Bu and High-Bu groups (p = 0.35 and p = 0.29, respectively). Relapses occurred in 25.7% of patients. No differences in median time to relapse were noted. Minimal residual disease (MRD)-positive patients had a shorter median OS and RFS than MRD-negative patients. No differences were found in OS and RFS between Low-Bu and High-Bu groups in MRD-positive patients (p = 0.86 and p = 0.83, respectively), or MRD-negative patients (p = 0.56 and p = 0.38, respectively). Non-relapsed mortality (NRM) at 100 days was 3.4% vs. 4.1% in the Low-Bu vs. High-Bu groups. There were no significant differences in the incidence of acute-graft-versus-host disease (aGVHD) (71.4% vs. 63.4%) or chronic GVHD (cGVHD) (48.3% vs. 43.9%) between the groups. The cumulative incidence of grades III-IV aGVHD was 24.1% in Low-Bu group and 22.4% in High-Bu group. In conclusion, targeting a busulfan AUC of > 19.7 mg*h/L with fludarabine does not appear to add an advantage in OS and RFS.

Impact of valproic acid on busulfan pharmacokinetics: In vitro assessment of potential drug-drug interaction

Busulfan (Bu) is an alkylating agent commonly used at high doses in the preparative regimens of hematopoietic stem cell transplantation (HSCT). It has been shown that such high doses of Bu are associated with generalized seizures which are usually managed by prophylactic antiepileptic drugs (AEDs) such as valproic acid (VPA). Being a strong enzyme inhibitor, VPA may inhibit Bu metabolism and thus increase its potential toxicity. Despite its clinical relevance, the potential interaction between Bu and VPA has not yet been evaluated. The aim of the present study was to assess and evaluate the potential drug-drug interaction (DDI) between Bu and VPA. This study was carried out by incubating Bu in laboratory-prepared rat liver-subcellular fractions including S9, microsomes, and cytosol, alone or in combination with VPA. The liver fractions were prepared by differential centrifugation of the liver homogenate. Analysis of Bu was employed using a fully validated LC-MS/MS method. The validation parameters were within the proposed limits of the international standards guidelines. Bu metabolic stability was assessed by incubating Bu at a concentration of 8 μg/ml in liver fractions at 37°C. There were significant reductions in Bu levels in S9 and cytosolic fractions, whereas these levels were not significantly (P ˃ 0.05) changed in microsomes. However, in presence of VPA, Bu levels in S9 fraction remained unchanged. These results indicated, for the first time, the potential metabolic interaction of Bu and VPA being in S9 only. This could be explained by inhibiting Bu cytosolic metabolism by the interaction with VPA either by sharing the same metabolic enzyme or the required co-factor. In conclusion, the present findings suggest, for the first time, a potential DDI between Bu and VPA in vitro using rat liver fractions. Further investigations are warranted in human-derived liver fractions to confirm such an interaction.

The first successful bone marrow transplantation in Vietnam for a young Vietnamese boy with chronic granulomatous disease: a case report

Background

Chronic granulomatous disease (CGD) is an inborn error of immunity (IEI) disorder that results from defects in the respiratory burst activity in phagocytes, leading to the inability to kill bacterial and fungal microorganisms. CGD patients usually have a high incidence of morbidity such as infections and autoinflammatory diseases and a high mortality rate. Allogeneic bone marrow transplantation (BMT) is the only definitive cure for patients who suffer from CGD.

Case presentation

We report the first transplant case of chronic granulomatous disease in Vietnam. A 25-month-old boy with X-linked CGD underwent bone marrow transplantation from his 5-year-old, full-matched human leukocyte antigen (HLA)-carrier sibling after myeloablative conditioning regimen with busulfan 5.1 mg/kg/day for 4 days, fludarabine 30 mg/m²/day for 5 days, and rATG (Grafalon-Fresenius) 10 mg/kg/day for 4 days. Neutrophil was engrafted on day 13 posttransplant, donor chimerism was 100% on day 30 with the dihydrorhodamine-1,2,3 (DHR 123) flow cytometric assay test that reached 38% of the normal 45 days posttransplant. Five months after transplant, the patient was free of infection with stable DHR 123 assay at 37%, and donor chimerism remained 100%. No sign of a graft-versus-host disease had been observed posttransplant.

Conclusion

We suggest that bone marrow transplantation is a safe and effectual cure for CGD patients, especially for patients with HLA-identical siblings.

Implementation and Cross-Validation of a Pharmacokinetic Model for Precision Dosing of Busulfan in Hematopoietic Stem Cell Transplanted Children

Busulfan, a drug used in conditioning prior to hematopoietic stem cell transplantation (HSCT) in children, has a narrow therapeutic margin. The model-informed precision dosing (MIPD) of busulfan is desirable, but there is a lack of validated tools. The objective of this study was to implement and cross-validate a population pharmacokinetic (PK) model in the Tucuxi software for busulfan MIPD in HSCT children. A search of the literature was performed to identify candidate population PK models. The goodness of fit of three selected models was assessed in a dataset of 178 children by computing the mean error (ME) and root-mean-squared error of prediction (RMSE). The best model was implemented in Tucuxi. The individual predicted concentrations, the area under the concentration-time curve (AUC), and dosage requirements were compared between the Tucuxi model and a reference model available in the BestDose software in a subset of 61 children. The model from Paci et al. best fitted the data in the full dataset. In a subset of 61 patients, the predictive performance of Tucuxi and BestDose models was comparable with ME values of 6.4% and -2.5% and RMSE values of 11.4% and 13.6%, respectively. The agreement between the estimated AUC and the predicted dose was good, with 6.6% and 4.9% of the values being out of the 95% limits of agreement, respectively. To conclude, a PK model for busulfan MIPD was cross-validated and is now available in the Tucuxi software.

Interest of the preventive and curative use of defibrotide on the occurrence and severity of sinusoidal obstruction syndrome after hematopoietic stem cell transplant in children

Defibrotide (DF) is indicated for the treatment of severe sinusoidal obstruction syndrome (SOS) following hematopoietic stem cell transplantation (HSCT), but its prophylactic use against SOS is not recommended yet. This study describes the impact of the preventive and curative use of DF on reducing the incidence and severity of SOS in children. Patients aged 0-19 years, who received allogenic HSCT after myeloablative conditioning regimen with busulfan or total body irradiation in our comprehensive cancer center, between 2013 and 2017, were included. The Baltimore or modified Seattle criteria were used for SOS diagnosis. SOS was graded using the 2017 European Society for Blood and Marrow Transplantation classification defining severity criteria of SOS in children. SOS occurrence tended to decrease with prophylactic DF, but no significant difference was observed in terms of severity. When not treated with preventive DF, 50% (19/38) of the patients with SOS were graded severe to very severe, but only 37% (7/19) had organ dysfunction. Curative DF was administered at a median of 2 days post-HSCT, for a median of 6.5 days. The absence of fatal SOS supports the use of early curative DF with acceptable toxicities and questions the optimal duration of DF treatment.

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.

The Correlation Between Busulfan Exposure and Clinical Outcomes in Chinese Pediatric Patients: A Population Pharmacokinetic Study

Aims: The aims of the study were to 1) establish a population pharmacokinetic (Pop-PK) model for busulfan in Chinese pediatric patients undergoing hematopoietic stem cell transplantation (HSCT) and then estimate busulfan exposure and 2) explore the association between busulfan exposure and clinical outcomes.

Methods: A total of 128 patients with 467 busulfan concentrations were obtained for Pop-PK modeling using nonlinear mixed effect model (NONMEM) software. Sixty-three patients who received the 16-dose busulfan conditioning regimen were enrolled to explore the correlations between clinical outcomes and the busulfan area under the concentration–time curve (AUC) using the Cox proportional hazards regression model, Kaplan–Meier method and logistic regression.

Results: The typical values for clearance (CL) and distribution volume (V) of busulfan were 7.71 L h⁻¹ and 42.4 L, respectively. The allometric normal fat mass (NFM) and maturation function (Fmat) can be used to describe the variability in CL, and the fat-free mass (FFM) can be used to describe the variability in V. Patients with AUCs of 950–1,600 µM × min had 83.7% (95% CI: 73.3–95.5) event-free survival (EFS) compared with 55.0% (95% CI: 37.0–81.8) for patients with low or high exposure (p = 0.024). The logistic regression analysis results showed no association between transplant-related toxicities and the busulfan AUC (p > 0.05).

Conclusions: The variability in busulfan CL was related to the NFM and Fmat, while busulfan V was related to the FFM. Preliminary analysis results suggested that a busulfan AUC of 950–1,600 µM × min was associated with better EFS in children receiving the 16-dose busulfan regimen.

Impact of busulfan pharmacokinetics on outcome in adult patients receiving an allogeneic hematopoietic cell transplantation

Busulfan (Bu) is widely used in conditioning regimens before allogeneic hematopoietic cell transplantation, with variable metabolism due to interindividual differences of pharmacokinetics (PK). The purpose of this study was to correlate pharmacokinetics and clinical outcomes. Lower-AUC, in range-AUC and higher-AUC were defined as ±25% of the targeted Bu-AUC. In 2019, we changed Bu dosing from 4×/day (Bu-4) to 1×/day (Bu-1) for ease of application. AUC-target range was reached in 46% of patients; 40% were in low-AUC and 14% in high-AUC. Among all toxicities, viral and fungal infections were significantly more frequent in high-AUC compared with low-AUC (20% vs. 8%; p = 0.01 and 37% vs. 17%; p = 0.03). Bu-1 showed lower PK values (66% vs. 36% of Bu-4 in low-AUC; p < 0.01) and higher incidence of mucositis (p = 0.02). Long-term outcomes at 2 years showed a higher non-relapse mortality (NRM) (p < 0.01) and higher relative risk of death in the high-AUC group compared to the other groups. Cumulative incidence of relapse and acute/chronic GvHD were not significantly different. The optimal cut-off in Bu-AUC associated with low NRM was 969 µmol/l*min (ROC AUC 0.67, sensitivity 0.86 and specificity 0.47) for Bu-4. In conclusion, low-AUC BU-PK seems of benefit regarding NRM and survival.

Therapeutic Drug Monitoring of Conditioning Agents in Pediatric Allogeneic Stem Cell Transplantation; Where do We Stand?

Allogeneic hematopoietic stem cell transplantation (HSCT) is an established curative treatment that has significantly improved clinical outcome of pediatric patients with malignant and non-malignant disorders. This is partly because of the use of safer and more effective combinations of chemo- and serotherapy prior to HSCT. Still, complications due to the toxicity of these conditioning regimens remains a major cause of transplant-related mortality (TRM). One of the most difficult challenges to further improve HSCT outcome is reducing toxicity while maintaining efficacy. The use of personalized dosing of the various components of the conditioning regimen by means of therapeutic drug monitoring (TDM) has been the topic of interest in the last decade. TDM could play an important role, especially in children who tend to show greater pharmacokinetic variability. However, TDM should only be performed when it has clear added value to improve clinical outcome or reduce toxicity. In this review, we provide an overview of the available evidence for the relationship between pharmacokinetic parameters and clinical outcome or toxicities of the most commonly used conditioning agents in pediatric HSCT.

Clinical outcomes of individualized busulfan-dosing in hematopoietic stem cell transplantation in Chinese children undergoing with therapeutic drug monitoring

To identify relationships between busulfan (Bu) exposure and outcomes of a cohort pediatric patients receiving hematopoietic stem cell transplantation (HSCT), along with a targeted busulfan-based conditioning regimen. We retrospectively evaluated targeted busulfan concentrations in 53 pediatric patients (age 0.4-16 years) who received busulfan 4 times daily according to recommended weight-based doses in a single-center analysis between 2018 and 2020. In this trial, individual busulfan pharmacokinetics were performed following dose 5 of the conditioning regimen. Twenty four of 53 patients (45.3%) studies did not require dose adjustments. Equal number of patients (24/53) required one dose adjustments while two-dose adjustment applied for 5 of 53 (9.4%). Twenty-one percent of the patients exhibited ll-lV aGVHD. The incidence of veno-occlusive disease (VOD) was in 3.8% of the 53 patients, while incidence of hemorrhagic cystitis (II-III) reached to 9.7%. Engraftment was successful in 98% of the 53 patients with relapse in 2% of cases. The probability of overall survival and disease-free survival at day 100 was 96% and 94%, respectively. In conclusion, therapeutic drug monitoring (TDM) and individualization of Bu dosage are essential to improve the efficacy and safety of busulfan-based regimen in Chinese pediatric HSCT recipients.

Hematopoietic cell transplant in pediatric acute myeloid leukemia after similar upfront therapy; a comparison of conditioning regimens

The impact of conditioning regimen prior to hematopoietic cell transplant (HCT) in pediatric AML-patients is not well studied. We retrospectively analyzed the impact of Busulfan-Cyclophosphamide (BuCy), Busulfan-Cyclophosphamide-Melphalan (BuCyMel) and Clofarabine-Fludarabine-Busulfan (CloFluBu) in pediatric AML-patients, with similar upfront leukemia treatment (NOPHO-DBHconsortium), receiving an HCT between 2010 and 2015. Outcomes of interest were LFS, relapse, TRM and GvHD. 103 patients were included; 30 received BuCy, 37 BuCyMel, and 36 CloFluBu. The 5-years LFS was 43.3% (SE ± 9.0) in the BuCy group, 59.2 % (SE ± 8.1) after BuCyMel, and 66.7 % (SE ± 7.9) after CloFluBu. Multivariable Cox regression analysis showed a trend to lower LFS after BuCy compared to CloFluBu (p = 0.07). BuCy was associated with a higher relapse incidence compared to the other regimens (p = 0.06). Younger age was a predictor for relapse (p = 0.02). A strong correlation between Busulfan Therapeutic Drug Monitoring (TDM) and lower incidence of aGvHD (p < 0.001) was found. In conclusion, LFS after BuCyMel and CloFluBu was comparable, lower LFS was found after BuCy, due to higher relapse incidence. CloFluBu was associated with lower incidence of aGvHD, suggesting lower toxicity with this type of conditioning. This finding is also explained by the impact of Busulfan monitoring.

NRF2 -617 C/A Polymorphism Impacts Proinflammatory Cytokine Levels, Survival, and Transplant-Related Mortality After Hematopoietic Stem Cell Transplantation in Adult Patients Receiving Busulfan-Based Conditioning Regimens

Busulfan (BU) is widely used in conditioning regimens prior to hematopoietic stem cell transplantation (HSCT). The exposure-escalated BU directed by therapeutic drug monitoring (TDM) is extremely necessary for the patients with high-risk hematologic malignancies in order to diminish relapse, but it increases the risk of drug-induced toxicity. BU exposure, involved in the glutathione- (GSH-) glutathione S-transferases (GSTs) pathway and proinflammatory response, is associated with clinical outcomes after HSCT. However, the expression of genes in the GSH-GSTs pathway is regulated by NF-E2-related factor 2 (Nrf2) that can also alleviate inflammation. In this study, we evaluated the influence of NRF2 polymorphisms on BU exposure, proinflammatory cytokine levels, and clinical outcomes in HSCT patients. A total of 87 Chinese adult patients receiving twice-daily intravenous BU were enrolled. Compared with the patients carrying wild genotypes, those with NRF2 -617 CA/AA genotypes showed higher plasma interleukin (IL)-6, IL-8 and tumor necrosis factor (TNF)-α levels, poorer overall survival (OS; RR = 3.91), and increased transplant-related mortality (TRM; HR = 4.17). High BU exposure [area under the concentration-time curve (AUC) > 9.27 mg/L × h)] was related to BU toxicities. Furthermore, NRF2 -617 CA/AA genotypes could significantly impact TRM (HR = 4.04; p = 0.0142) and OS (HR = 3.69; p = 0.0272) in the patients with high BU AUC. In vitro, we found that high exposure of endothelial cell (EC) to BU, in the absence of Nrf2, elicited the hyperstimulation of NF-κB-p65, accompanied with the elevated secretion of proinflammatory cytokines, and led to EC death. These results showed that NRF2 -617 CA/AA genotypes, correlated with high proinflammatory cytokine levels, could predict inferior outcomes in HSCT patients with high BU AUC. Thus, NRF2 -617 CA/AA genotyping combined with TDM would further optimize personalized BU dosing for sufficient efficacy and safety endpoint.

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.

Review of the Pharmacokinetics and Pharmacodynamics of Intravenous Busulfan in Paediatric Patients

We aimed to review the pharmacokinetics (PK) of intravenous busulfan in paediatric patients, identify covariate factors influencing exposure, investigate evidence of changes in PK behaviour over time, and correlate exposure with efficacy and toxicity outcomes. A literature review was undertaken of original research published between 2007 and 2019, investigating the PK and pharmacodynamics (PD) of intravenous busulfan in patients ≤ 18 years of age. The review identified 41 publications characterising the PK, and 45 publications describing the PD, of busulfan. Median typical clearance (CL) was 0.22 L/h/kg and median typical volume of distribution was 0.69 L/kg. Patient weight, age, glutathione-S-transferase A1 (GSTA1) genotype and busulfan dosing day/time were the most commonly identified factors affecting CL. Of nine studies investigating changes in CL, seven reported reduced CL over the 4-day course of treatment. Exposure monitoring methods and therapeutic targets were heterogeneous across studies. Relationships between busulfan exposure and patient outcomes were observed in five studies. One study observed a cumulative area under the concentration-time curve over all days of treatment of between 78 and 101 mg/L·h, and two studies observed an average concentration at first dose of < 600 ng/mL improved overall survival, transplant-related mortality, or relapse. One study observed increased sinusoidal obstructive syndrome with maximum busulfan concentration > 1.88 ng/mL. Patient weight, age and GSTA1 genotype are important covariates to consider when individualising busulfan therapy. Reduced busulfan CL over time may need to be accounted for, particularly in patients not receiving phenytoin co-therapy. Standardised monitoring of busulfan exposure over the entire course of treatment and further investigation of the role of busulfan metabolites and pharmacogenomics is warranted.

Intra-individual Pharmacokinetic Variability of Intravenous Busulfan in Hematopoietic Stem Cell-Transplanted Children

BACKGROUND

Busulfan therapeutic drug monitoring (TDM) is necessary to better achieve the target exposure in children before hematopoietic stem cell transplantation (HSCT). However, TDM-based dosing may be challenging if intra-individual pharmacokinetic variability (also denoted inter-occasion variability [IOV]) occurs during therapy.

OBJECTIVES

The objectives of this study were to describe and quantify busulfan IOV in children, and to investigate its potential determinants.

METHODS

We performed a new analysis of published data from children who received intravenous busulfan over 4 days before HSCT. We calculated individual pharmacokinetic parameters on each day of therapy using a published population pharmacokinetic model of busulfan and analyzed their changes. Population estimation of IOV was also performed with non-linear mixed effects (NLME) modeling. Potential predictors of significant decrease in busulfan clearance (CL) were assessed by using machine learning approaches.

RESULTS

IOV could be assessed in 136 children. Between day (D) 1 and D2, most patients (80%) experienced a decrease in busulfan CL, with a median change of - 7.9%. However, both large decreases (minimum, -  48.5%) and increases in CL (maximum, + 44%) were observed. Over D1-D3 of therapy, mean CL significantly decreased (-  15%), with a decrease of ≥ 20% in 22% of patients. Some patients also showed unstable CL from day to day. NLME modeling of IOV provided a coefficient of variation of 10.6% and 13.1% for volume of distribution (V_d) and CL, respectively. Some determinants of significant decreases in busulfan CL were identified, but predictive performance of the models was limited.

CONCLUSIONS

Significant busulfan intra-individual variability may occur in children who receive a HSCT and is hardly predictable. The main risk is busulfan overexposure. Performing TDM repeatedly over therapy appears to be the best way to accurately estimate busulfan exposure and perform precision dosing.