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

Saliva as a noninvasive sampling matrix for therapeutic drug monitoring of intravenous busulfan in Chinese patients undergoing hematopoietic stem cell transplantation: A prospective population pharmacokinetic and simulation study

Therapeutic drug monitoring (TDM) of busulfan (BU) is currently performed by plasma sampling in patients undergoing hematopoietic stem cell transplantation (HSCT). Saliva samples are considered a noninvasive TDM matrix. Currently, no salivary population pharmacokinetics (PopPKs) model for BU available. This study aimed to develop a PopPK model that can describe the relationship between plasma and saliva kinetics in patients receiving intravenous BU. The performance of the model in predicting the area under the concentration-time curve at steady state (AUCss ) based on saliva samples is evaluated. Sixty-six patients with HSCT were recruited and administered 0.8 mg/kg BU intravenously. A PopPK model for saliva and plasma was developed using the nonlinear mixed effects model. Bayesian maximum a posteriori (MAP) optimization was used to estimate the model's predictive performance. Plasma and saliva PKs were adequately described with a one-compartment model and a scaled central compartment. Body surface area correlated positively with both clearance and apparent volume of distribution (Vd), whereas alkaline phosphatase correlated negatively with Vd. Simulations demonstrated that the percentage root mean squared prediction error and lower and upper limits of agreements reduced to 10.02% and -16.96% to 22.86% based on five saliva samples. Saliva can be used as an alternative matrix to plasma in TDM of BU. The AUCss can be predicted from saliva concentration by Bayesian MAP optimization, which can be used to design personalized dosing for BU.

Predicting busulfan exposure in patients undergoing hematopoietic stem cell transplantation using machine learning techniques

PURPOSE

This study aimed to establish an optimal model to predict the busulfan (BU) area under the curve at steady state (AUCss) by using machine learning (ML).

PATIENTS AND METHODS

Seventy-nine adult patients (age ≥18 years) who received BU intravenously and underwent therapeutic drug monitoring from 2013 to 2021 at Fujian Medical University Union Hospital were enrolled in this retrospective study. The whole dataset was divided into a training group and test group at the ratio of 8:2. BU AUCss were considered as the target variable. Nine different ML algorithms and one population pharmacokinetic (pop PK) model were developed and validated, and their predictive performance was compared.

RESULTS

All ML models were superior to the pop PK model (R2 = 0.751, MSE = 0.722, 14 and RMSE = 0.830) in model fitting and had better predictive accuracy. The ML model of BU AUCss established through support vector regression (SVR) and gradient boosted regression trees (GBRT) had the best predictive ability (R2 = 0.953 and 0.953, MSE = 0.323 and 0.326, and RMSE = 0.423 and 0.425).

CONCLUSION

All the ML models can potentially be used to estimate BU AUCss with the aim of facilitating rational use of BU on the individualized level, especially models built by SVR and GBRT algorithms.

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.

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.

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.

Busulfan dose recommendation in inherited metabolic disorders: Population pharmacokinetic analysis

Busulfan is a commonly used alkylating agent in the conditioning regimens of hematopoietic cell transplantation (HCT). Population pharmacokinetic (popPK) models enable the description of busulfan PK and optimization of exposure, which leads to improvement of event-free survival after HCT. Prior busulfan popPK analysis has been limited by small numbers in patients with inherited metabolic disorders (IMD). The primary objective was to characterize population PK of busulfan in a large cohort of children and young adults undergoing HCT for IMD. PopPK analysis of busulfan drug concentrations was performed using data from 78 patients with IMD who received intravenous busulfan (every 24 hours, 4 doses) as part of pretransplantation combination chemotherapy. The final model for busulfan drug clearance was then used to estimate individual doses aimed to achieve a target cumulative area under the curve (cAUC) of 80 to 100 mg · h/L. We then compared the probability of cAUC within the range of 80 to 100 mg · h/L by the developed dosing regimen versus conventional regimen. A 1-compartment, linear elimination model best described the PK of busulfan. Significant covariates demonstrated to affect busulfan clearance included total body weight and the time (in days) from busulfan infusion start. The probability of target cumulative AUC attainment by the developed dosing versus the conventional dosing were 47% versus 43% for body weight <12 kg, and 48% versus 36% for body weight ≥12 kg. We described population PK of intravenous busulfan in a large IMD cohort. The proposed dosing regimen based on the developed model can improve the target cAUC attainment of busulfan for IMD.

Clinical pharmacology of cytotoxic drugs in neonates and infants: Providing evidence-based dosing guidance

Cancer in neonates and infants is a rare but challenging entity. Treatment is complicated by marked physiological changes during the first year of life, excess rates of toxicity, mortality, and late effects. Dose optimisation of chemotherapeutics may be an important step to improving outcomes. Body size–based dosing is used for most anticancer drugs used in infants. However, dose regimens are generally not evidence based, and dosing strategies are frequently inconsistent between tumour types and treatment protocols. In this review, we collate available pharmacological evidence supporting dosing regimens in infants for a wide range of cytotoxic drugs. A systematic review was conducted, and available data ranked by a level of evidence (1–5) and a grade of recommendation (A–D) provided on a consensus basis, with recommended dosing approaches indicated as appropriate. For 9 of 29 drugs (busulfan, carboplatin, cyclophosphamide, daunorubicin, etoposide, fludarabine, isotretinoin, melphalan and vincristine), grade A was scored, indicating sufficient pharmacological evidence to recommend a dosing algorithm for infants. For busulfan and carboplatin, sufficient data were available to recommend therapeutic drug monitoring in infants. For eight drugs (actinomycin D, blinatumomab, dinutuximab, doxorubicin, mercaptopurine, pegaspargase, thioguanine and topotecan), some pharmacological evidence was available to guide dosing (graded as B). For the remaining drugs, including commonly used agents such as cisplatin, cytarabine, ifosfamide, and methotrexate, pharmacological evidence for dosing in infants was limited or non-existent: grades C and D were scored for 10 and 2 drugs, respectively. The review provides clinically relevant evidence-based dosing guidance for cytotoxic drugs in neonates and infants.

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Treating cancer in neonates and infants is challenging.

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Dose optimisation of cytotoxic drugs is an important step to improving outcomes.

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Clinical pharmacological evidence supporting dosing regimens in infants was collated.

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All available pharmacological evidence was ranked by a level of evidence.

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A grade of recommendation was derived and a recommended dose per agent provided.

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.

Can Published Population Pharmacokinetic Models of Busulfan Be Used for Individualized Dosing in Chinese Pediatric Patients Undergoing Hematopoietic Stem Cell Transplantation? An External Evaluation

Busulfan is a bifunctional alkylating agent that is widely used before hematopoietic stem cell transplantation (HSCT), in combination with other chemotherapeutic drugs. As of 2020, there is no population pharmacokinetic (popPK) model for busulfan in Chinese pediatric patients. A systemic external evaluation of 11 published popPK models was conducted in Chinese pediatric patients undergoing HSCT. Forty pediatric patients were enrolled in this study, with a total of 183 blood concentrations. The relative prediction error (PE%), median PE%, median absolute PE%, and percentage of PE% within ±20% and ±30% were calculated in prediction-based diagnostics. Simulation-based diagnostics were conducted through a prediction- and variability-corrected visual predictive check and the normalized prediction distribution error. The relative individual prediction error was calculated using Bayesian forecasting with 1 to 3 concentration points. The 1-compartment open linear popPK model, which was built by Su-jin Rhee et al (model H), incorporating the patient's body surface area, age, dosing day, and aspartate aminotransferase as significant covariates had preferable predictability than other popPK models. In prediction-based diagnostics, the median PE%, percentage of PE% within ±20%, and percentage of PE% within ±30% of model H were 8.48%, 45.35%, and 59.56%, respectively. The normalized prediction distribution error of model H showed that it followed the normal distribution. Based on Bayesian forecasting, model H showed good predictive performance. Thus, model H was the most appropriate model that can be used clinically for individualized dosage adjustments in Chinese pediatric HSCT patients.

Precision dosing of intravenous busulfan in pediatric hematopoietic stem cell transplantation: Results from a multicenter population pharmacokinetic study

Busulfan (Bu) is a common component of conditioning regimens before hematopoietic stem cell transplantation (HSCT) and is known for high interpatient pharmacokinetic (PK) variability. This study aimed to develop and externally validate a multicentric, population PK (PopPK) model for intravenous Bu in pediatric patients before HSCT to first study the influence of glutathione‐s‐transferase A1 (GSTA1) polymorphisms on Bu's PK in a large multicentric pediatric population while accounting for fludarabine (Flu) coadministration and, second, to establish an individualized, model‐based, first‐dose recommendation for intravenous Bu that can be widely used in pediatric patients. The model was built using data from 302 patients from five transplantation centers who received a Bu‐based conditioning regimen. External model validation used data from 100 patients. The relationship between body weight and Bu clearance (CL) was best described by an age‐dependent allometric scaling of a body weight model. A stepwise covariate analysis identified Day 1 of Bu conditioning, GSTA1 metabolic groups based on GSTA1 polymorphisms, and Flu coadministration as significant covariates influencing Bu CL. The final model adequately predicted Bu first‐dose CL in the external cohort, with 81% of predicted area under the curves within the therapeutic window. The final model showed minimal bias (mean prediction error, −0.5%; 95% confidence interval [CI], −3.1% to 2.0%) and acceptable precision (mean absolute prediction error percentage, 18.7%; 95% CI, 17.0%–20.5%) in Bu CL prediction for dosing. This multicentric PopPK study confirmed the influence of GSTA1 polymorphisms and Flu coadministration on Bu CL. The developed model accurately predicted Bu CL and first doses in an external cohort of pediatric patients.

Dose individualization of intravenous busulfan in pediatric patients undergoing bone marrow transplantation: impact and in vitro evaluation of infusion lag-time

OBJECTIVES

To apply therapeutic drug monitoring and dose-individualization of intravenous Busulfan to paediatric patients and evaluate the impact of syringe-pump induced Busulfan infusion lag-time after in vitro estimation.

METHODS

76 children and adolescents were administered 2 h intravenous Busulfan infusion every 6 h (16 doses). Busulfan plasma levels, withdrawn by an optimized sampling scheme and measured by a validated HPLC-PDA method, were used to estimate basic PK parameters, AUC, Cmax, kel, t1/2, applying Non-Compartmental Analysis. In vivo infusion lag-time was simulated in vitro and used to evaluate its impact on AUC estimation.

KEY FINDINGS

Mean (%CV) Busulfan AUC, Cmax, clearance and t1/2 for pediatric population were found 962.3 μm × min (33.1), 0.95 mg/L (41.4), 0.27 L/h/kg (33.3), 2.2 h (27.8), respectively. TDM applied to 76 children revealed 6 (7.9%) being above and 25 (32.9%) below therapeutic-range (AUC: 900-1350 μm × min). After dose correction, all patients were measured below toxic levels (AUC < 1500 μm × min), no patient below 900 μm × min. Incorporation of infusion lag-time revealed lower AUCs with 17.1% more patients and 23.1% more younger patients, with body weight <16 kg, being below the therapeutic-range.

CONCLUSIONS

TDM, applied successfully to 76 children, confirmed the need for Busulfan dose-individualization in paediatric patients. Infusion lag-time was proved clinically significant for younger, low body-weight patients and those close to the lower therapeutic-range limit.

Therapeutic drug monitoring of intravenous busulfan in Thai children undergoing hematopoietic stem cell transplantation: A pilot study

Busulfan (Bu) is commonly used in myeloablative conditioning regimens for children undergoing hematopoietic stem cell transplantation. The standard target area under the concentration-time curve (AUC) of Bu is approximately 900-1500 µM min. In previous studies using five fixed doses (0.8-1.2 mg/kg) for Bu without dose adjustment, 75% patients achieved the target AUC. The aim of this pilot study was to determine the percentage of target AUC for intravenous (IV) Bu in Thai children. IV Bu was administered every 6 h over 16 doses. Blood samples were collected for pharmacokinetic (PK) analysis after the first, ninth, and thirteenth doses of Bu. Seven patients (2-14 years; median 6 years) were diagnosed with thalassemia (n = 4), acute myeloid leukemia (n = 2), and pure red cell aplasia. Three, two, and two patients received Bu at 1.1, 1.2, and 0.8 mg/kg, respectively. The AUC of Bu varied from 292-1714 µM min (median = 804). Nine (42.86%), eleven (52.38%), and one (4.76%) AUC values were within, below, and above the target, respectively. The median (range) Bu clearance was 5.93 (1.91-14.65) mL/min/kg. In this study, 42.86% AUC value achieved the target, which was lower than that in previous studies. Therapeutic drug monitoring (TDM) of Bu should be considered in Thai children receiving five fixed doses of IV Bu, and dose adjustment should be performed as necessary. Further PK studies for Bu with a larger sample size are warranted for confirming the necessity of TDM in every step dose of Bu.(Trial registration numbers; TCTR20190528003).

Optimization of Busulfan Dosing Regimen in Pediatric Patients Using a Population Pharmacokinetic Model Incorporating GST Mutations

Purpose

The aim of this study was to develop a novel busulfan dosing regimen, based on a population pharmacokinetic (PPK) model in Chinese children, and to achieve better area under the concentration-time curve (AUC) targeting.

Patients and Methods

We collected busulfan concentration-time samples from 69 children who received intravenous busulfan prior to allogeneic hematopoietic stem cell transplantation (allo-HSCT). A population pharmacokinetic model for busulfan was developed by nonlinear mixed effect modelling and was validated by an external dataset (n=14). A novel busulfan dosing regimen was developed through simulated patients, and has been verified on real patients. Limited sampling strategy (LSS) was established by Bayesian forecasting. Mean absolute prediction error (MAPE) and relative root mean Squared error (rRMSE) were calculated to evaluate predictive accuracy.

Results

A one-compartment model with first-order elimination best described the data. GSTA1 genotypes, body surface area (BSA) and aspartate aminotransferase (AST) were found to be significant covariates of Bu clearance, and BSA had significant impact of the volume of distribution. Moreover, two equations were obtained for recommended dose regimens: dose (mg)=34.14×BSA (m²)+3.75 (for GSTA1 *A/*A), Dose (mg)=30.99×BSA (m²)+3.21 (for GSTA1 *A/*B). We also presented a piecewise dosage based on BSA categories for each GSTA1 mutation. A two-point LSS, two hours and four hours after dosing, behaved well with acceptable prediction precision (rRMSE=1.026%, MAPE=6.55%).

Conclusion

We recommend a GSTA1-BSA and BSA-based dosing (Q6 h) based on a PPK model for personalizing busulfan therapy in pediatric population. Additionally, an optimal LSS (C_2h and C_4h) provides convenience for therapeutic drug monitoring (TDM) in the future.

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.

New dosing nomogram and population pharmacokinetic model for young and very young children receiving busulfan for hematopoietic stem cell transplantation conditioning

BACKGROUND

Busulfan (Bu) is the cornerstone of conditioning regimens prior to hematopoietic stem cell transplantation, widely used in both adults and children for the treatment of malignant and nonmalignant diseases. Despite an intravenous formulation, interindividual variability (IIV) remains high and optimal exposure difficult to achieve, especially in neonates and infants.

PROCEDURE

To ensure both efficacy and safety, we set up in 2005 an observational study designed for children not fully assessed during the drug registration procedure. From a large cohort of 540 patients, we developed a Bu population pharmacokinetic model based on body weight (BW) and maturation concepts to reduce IIV and optimize exposure. A new dosing nomogram was evaluated to better fit the population pharmacokinetic model.

RESULTS

Bu clearance IIV was significantly decreased from 61.3% (covariate-free model) to 28.6% when combining BW and maturation function. Median Bu area under the curve (AUC) was 1179 µmol/L × min compared to 1025 with the EMA dosing nomogram for children <9 kg. The target AUC was reached for each BW strata, significantly increasing the percentages of patients achieving reaching the targeted AUC as compared to FDA schedule.

CONCLUSION

This new model made it possible to propose a novel dosing nomogram that better considered children below 16 kg of BW and allowed better initial exposure as compared to existing dosing schedules. This nomogram, which would be easy to use to determine an optimal dosing schedule in daily practice, will need to be validated in clinical routine. Therapeutic drug monitoring remains strongly advisable for small children and those with specific diseases.

Assessment of a Model-Informed Precision Dosing Platform Use in Routine Clinical Care for Personalized Busulfan Therapy in the Pediatric Hematopoietic Cell Transplantation (HCT) Population

Introduction

Population pharmacokinetic (PK) studies demonstrate model-based dosing for busulfan that incorporates body size and age improve clinical target attainment as compared to weight-based regimens. Recently, for clinical dosing of busulfan and TDM, our institution transitioned to a cloud-based clinical decision support tool (www.insight-rx.com). The goal of this study was to assess the dose decision tool for the achievement of target exposure of busulfan in children undergoing hematopoietic cell transplantation (HCT).

Patients and Methods

Patients (N = 188) were grouped into cohorts A, B, or C based on the method for initial dose calculation and estimation of AUC: Cohort A: Initial doses were based on the conventional dosing algorithm (as outlined in the manufacturers' package insert) and non-compartmental analysis (NCA) estimation using the trapezoidal rule for estimation of AUC following TDM. Cohort B: Initial doses for busulfan were estimated by a first-generation PK model and NCA estimation of AUC following TDM. Cohort C: Initial doses were calculated by an updated, second-generation PK model available in the dose decision tool with an estimation of AUC following TDM.

Results

The percent of individuals achieving the exposure target at the time of first PK collection was higher in subjects receiving initial doses provided by the model-informed precision dosing platform (cohort C, 75%) versus subjects receiving initial doses based on either of the two other approaches (conventional guidelines/cohort A, 25%; previous population PK model and NCA parameter estimation, cohort B, 50%). Similarly, the percent of subjects achieving the targeted cumulative busulfan exposure (cAUC) in cohort C was 100% vs. 66% and 88% for cohort A and B, respectively. For cAUC, the variability in the spread of target attainment (%CV) was low at 4.1% for cohort C as compared to cohort A (14.8%) and cohort B (17.1%).

Conclusion

Achievement of goal exposure early on in treatment was improved with the updated model for busulfan and the Bayesian platform. Model-informed dosing and TDM utilizing a Bayesian-based platform provides a significant advantage over conventional guidelines for the achievement of goal cAUC exposure.

Pharmacokinetic analysis for model-supported therapeutic drug monitoring of busulfan in Japanese pediatric hematopoietic stem cell transplantation recipients

This prospective observational study analyzed the pharmacokinetics of busulfan in Japanese children and evaluated the predicting accuracy of previous pediatric PPK models of busulfan. This study enrolled five patients (aged 2-12 years, BW 14-48 kg) receiving a busulfan-based conditioning regimen for hematopoietic stem cell transplantation at our hospital between January 2017 and December 2018. All patients received a 2-hour intravenous busulfan infusion four times daily for a total of 16 doses. After the infusions, 51 plasma samples were collected with the plasma busulfan concentration measured by liquid chromatography-tandem mass spectrometry. PPK model fitting was analyzed using the (%MPE) and the (%MAPE). Limited sampling strategies for estimating busulfan AUC were also evaluated. High interpatient variability was observed in the PK parameters. The most suitable PPK model that reflected our data was McCune's two-compartment model (%MPE -8.7, %MAPE 19.3). A combination sampling method using the busulfan concentration at 2 and 6 hours after the start of the first busulfan dose was found to be able to estimate AUC_4 day . These results provide useful information on busulfan therapeutic drug monitoring in the Japanese pediatric population.

Pharmacokinetics and population pharmacokinetics in pediatric oncology

Pharmacokinetic research has become increasingly important in pediatric oncology as it can have direct clinical implications and is a crucial component in individualized medicine. Population pharmacokinetics has become a popular method especially in children, due to the potential for sparse sampling, flexible sampling times, computing of heterogeneous data, and identification of variability sources. However, population pharmacokinetic reports can be complex and difficult to interpret. The aim of this article is to provide a basic explanation of population pharmacokinetics, using clinical examples from the field of pediatric oncology, to facilitate the translation of pharmacokinetic research into the daily clinic.

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.

Population pharmacokinetic modelling of busulfan and the influence of body composition in paediatric Fanconi anaemia patients

Aims

Fanconi anaemia (FA) is a rare disorder characterized by progressive bone marrow failure that requires haematopoietic cell transplantation (HCT). Busulfan is used in conditioning regimens prior to HCT. Doses used in non‐FA patients cause life‐threatening toxicities in FA patients and data on busulfan pharmacokinetics (PK) in this population are limited. This study characterized busulfan PK in paediatric FA patients using population PK modelling and evaluated the effect of body composition on steady‐state concentrations (C_ss).

Methods

A total of 200 busulfan plasma concentrations in 29 FA patients from a recent study (http://Clinicaltrials.gov; NCT01082133) were available for population PK modelling. The effect of different body size‐scaled doses and body compositions on C_ss was investigated using population PK modelling.

Results

Fat free mass (FFM) was identified as the best size descriptor in a two‐compartment busulfan PK model in FA patients. Conventional dosing, based on an amount of busulfan per kilogram of total body mass, resulted in higher C_ss in FA patients with higher body mass index (BMI). A newly proposed FFM‐based dosing strategy would eliminate the observed trend of higher concentrations in high BMI patients, and achieve consistent C_ss across a wide BMI spectrum.

Conclusions

This is the first study to describe the population PK of busulfan in paediatric FA patients. The proposed model will facilitate PK model‐informed precision dosing. FFM‐based dosing is expected to improve the probability of achieving target C_ss, particularly in obese patients, while minimizing the risk of overdosing.

Proposed Therapeutic Range of Treosulfan in Reduced Toxicity Pediatric Allogeneic Hematopoietic Stem Cell Transplant Conditioning: Results From a Prospective Trial

Treosulfan is given off‐label in pediatric allogeneic hematopoietic stem cell transplant. This study investigated treosulfan's pharmacokinetics (PKs), efficacy, and safety in a prospective trial. Pediatric patients (n = 87) receiving treosulfan‐fludarabine conditioning were followed for at least 1 year posttransplant. PKs were described with a two‐compartment model. During follow‐up, 11 of 87 patients died and 12 of 87 patients had low engraftment (≤ 20% myeloid chimerism). For each increase in treosulfan area under the curve from zero to infinity (AUC_(0‐∞)) of 1,000 mg hour/L the hazard ratio (95% confidence interval) for mortality increase was 1.46 (1.23–1.74), and the hazard ratio for low engraftment was 0.61 (0.36–1.04). A cumulative AUC_(0‐∞) of 4,800 mg hour/L maximized the probability of success (> 20% engraftment and no mortality) at 82%. Probability of success with AUC_(0‐∞) between 80% and 125% of this target were 78% and 79%. Measuring PK at the first dose and individualizing the third dose may be required in nonmalignant disease.

Population pharmacokinetic analysis of intravenous busulfan: GSTA1 genotype is not a predictive factor of initial dose in Chinese adult patients undergoing hematopoietic stem cell transplantation

PURPOSE

This study aimed to develop a population pharmacokinetic (PPK) model to investigate the impact of GSTA1, GSTP1, and GSTM1 genotypes on busulfan pharmacokinetic (PK) variability in Chinese adult patients.

METHODS

Forty-three and 19 adult patients who underwent hematopoietic stem cell transplantation (HSCT) were enrolled for modeling group and validation group, respectively. All patients received twice-daily intravenous busulfan as part of conditioning regimen before HSCT. The PPK model was developed by nonlinear mixed-effect modeling. Covariates investigated were age, sex, actual body weight, body surface area, diagnoses, hepatic function markers, GST genotypes and conditioning regimen.

RESULTS

A total of 488 busulfan concentrations from 43 patients were obtained for the PPK model. The PK of intravenous busulfan was described by one-compartment model with first-order elimination with estimated clearance (CL) of 14.2 L/h and volume of distribution of 64.1 L. Inclusion of GSTA1 genotype as a covariate accounted for 1.1% of the inter-individual variability of busulfan CL (from 17.8% in the basic model to 16.7% in the final model). The accuracy and applicability of the final model were externally validated in the independent group. The difference of busulfan PK between Chinese patients and Caucasian patients existed because of the rarity of haplotype *B in Chinese population.

CONCLUSIONS

Although the GSTA1 genotype-based PPK model of intravenous busulfan was successfully developed and externally validated, the GSTA1 genotype was not considered to be clinically relevant to busulfan CL. We did not suggest the guidance of GSTA1 genotype on initial busulfan dose in Chinese adult patients.

Association of Antiepileptic Medications with Outcomes after Allogeneic Hematopoietic Cell Transplantation with Busulfan/Cyclophosphamide Conditioning

High-dose busulfan (BU) followed by high-dose cyclophosphamide (CY) before allogeneic hematopoietic cell transplantation (HCT) has long been used as treatment for hematologic malignancies. Administration of phenytoin or newer alternative antiepileptic medications (AEMs) prevents seizures caused by BU. Phenytoin induces enzymes that increase exposure to active CY metabolites in vivo, whereas alternative AEMs do not have this effect. Lower exposure to active CY metabolites with the use of alternative AEMs could decrease the risk of toxicity but might increase the risk of recurrent malignancy after HCT. Previous studies have not determined whether outcomes with alternative AEMs differ from those with phenytoin in patients treated with BU/CY before allogeneic HCT. We studied a cohort of 2155 patients, including 1460 treated with phenytoin and 695 treated with alternative AEMs, who received BU/CY before allogeneic HCT between 2004 and 2014. We found no differences suggesting decreased overall survival or relapse-free survival or increased risks of relapse, nonrelapse mortality, acute or chronic graft-versus-host disease, or regimen-related toxicity associated with the use of alternative AEMs compared with phenytoin. The risk of dialysis was lower in the alternative AEM group than in the phenytoin group. Alternative AEMs are safe for prevention of seizures after BU administration and can avoid the undesirable toxicities and drug interactions caused by phenytoin.

Age- and Bodyweight-dependent Allometric Exponent Model for Scaling Clearance and Maintenance Dose of Theophylline From Neonates to Adults

BACKGROUND

In population pharmacokinetic modeling, bodyweight is often incorporated as an important covariate using fixed (0.75) or single-exponent model. In recent years, several variations of allometric models have been suggested for the prediction of drug clearance across a wide age range. The objective of this study is to develop and evaluate single-exponent, bodyweight-dependent allometric exponent (BDE), age-dependent exponent (ADE), and segmented regression models for predicting clearance and maintenance dose of theophylline.

METHODS

The BDE model was described by the following equation: (Equation is included in full-text article.), where L × BW defines the BDE for clearance. The coefficient and the exponents L and M were estimated. The ADE model consisted of several empirical exponents based on age and ranged from 0.75 (children >5 years and adults) to 1.2 (premature neonates). Data for model development and validation were based on 52 subjects each.

RESULTS

All structural and statistical parameters were estimated with acceptable precision for single-exponent and BDE models (<30%); however, the BDE model was superior in describing theophylline clearance across a wide age range for the training data. The segmented regression model on log-transformed data also adequately described theophylline clearance. When models were evaluated with validation data, a single-exponent model overpredicted clearance and dosing rate in premature neonates and adults with a mean prediction error of ≥50%. For premature neonates and adults, mean clearance and dosing rate were predicted within a 30% prediction error using the BDE, ADE, and segmented models.

CONCLUSIONS

This study demonstrates that the BDE, ADE, and segmented models performed better than a single-exponent model for predicting clearance and dose of theophylline across a wide age range.

Haploidentical Hematopoietic Stem Cell Transplantation with Post-Transplant Cyclophosphamide for Primary Immunodeficiencies and Inherited Disorders in Children

Allogeneic hematopoietic stem cell transplantation (HSCT) is a potentially curative treatment for some inherited disorders, including selected primary immunodeficiencies (PIDs). In the absence of a well-matched donor, HSCT from a haploidentical family donor (HIFD) may be considered. In adult recipients high-dose post-transplant cyclophosphamide (PTCY) is increasingly used to mitigate the risks of graft failure and graft-versus-host disease (GVHD). However, data on the use of PTCY in children (and especially those with inherited disorders) are scarce. We reviewed the outcomes of 27 children transplanted with an HIFD and PTCY for a PID (n = 22) or osteopetrosis (n = 5) in a single center. The median age was 1.5 years (range, .2 to 17). HSCT with PTCY was a primary procedure (n = 21) or a rescue procedure after graft failure (n = 6). The conditioning regimen was myeloablative in most primary HSCTs and nonmyeloablative in rescue procedures. After a median follow-up of 25.6 months, 24 of 27 patients had engrafted. Twenty-one patients are alive and have been cured of the underlying disease. The 2-year overall survival rate was 77.7%. The cumulative incidences of acute GVHD grade ≥ II, chronic GVHD, and autoimmune disease were 45.8%, 24.2%, and 29.6%, respectively. There were 2 cases of grade III acute GVHD and no extensive cGVHD. The cumulative incidences of blood viral replication and life-threatening viral events were 58% and 15.6%, respectively. There was evidence of early T cell immune reconstitution. In the absence of an HLA-identical donor, HIFD HSCT with PTCY is a viable option for patients with life-threatening inherited disorders.

Accurate Prediction of Initial Busulfan Exposure Using a Test Dose With 2- and 6-Hour Blood Sampling in Adult Patients Receiving a Twice-Daily Intravenous Busulfan-Based Conditioning Regimen

This study aimed to predict the area under the curve (AUC) of the initial busulfan dose using a test dose with the sparse sampling scheme in adult patients who underwent hematopoietic cell transplant. A test dose of 0.8 mg/kg busulfan was used 2 days before twice-daily intravenous busulfan-based conditioning regimens were administered. The AUC and the clearance (CL) were calculated for both the test dose and the first dose (AUC_T , CL_T , AUC_1, and CL₁ ) by noncompartmental analysis. The sparse sampling schemes of the test dose were developed by Bayesian method based on the population pharmacokinetic model. The optimal sparse sampling schemes were determined by evaluating the mean prediction error, the root mean square error, the absolute mean prediction error, and Bland-Altman plot. The mean AUC₁ was 7.20 ± 1.48 mg • h/L, which ranged from 4.70 to 9.46 mg • h/L. The AUC₁ was below the therapeutic concentration of 7.38 mg • h/L in 45% (9 of 20) of the patients. The CL_T of 3.05 ± 0.56 mL/min/kg was not significantly different with the CL₁ of 3.03 ± 0.69 mL/min/kg (P = .901). A sampling scheme at 2 and 6 hours after the test dose was developed to predict the AUC_T (mean prediction error of 1.64%, root mean square error of 6.17%, and absolute mean prediction error of 4.94%). Additionally, the Bland-Altman plot showed that the 2-sampling scheme provided an acceptably accurate prediction of the AUC₁ . A test dose with a 2-sampling scheme was sufficient to personalize the initial busulfan dosing in hematopoietic cell transplant recipients.

Therapeutic Drug Monitoring of Busulfan for the Management of Pediatric Patients: Cross-Validation of Methods and Long-Term Performance

BACKGROUND

Busulfan (Bu) is an alkylating agent used as part of the conditioning regimen in pediatric patients before hematopoietic stem cell transplantation. Despite intravenous (IV) administration and dosing recommendations based on age and weight, reports have revealed interindividual variability in Bu pharmacokinetics and the outcomes of hematopoietic stem cell transplantation. In this context, adjusting doses to Bu's narrow therapeutic window is advised. We aimed to assess the utility of therapeutic drug monitoring (TDM) of Bu in children, the reliability of Bu quantification methods, and its stability in plasma when stored for up to 5 years.

METHODS

Eighteen patients from our TDM center (252 samples) were included. All of them received a 2-hour Bu IV infusion 4 times daily for a total of 16 doses. The first dose of Bu was age/weight-based, and the subsequent doses were adjusted from third or fifth dose onward based on the estimated first dose pharmacokinetic parameters to target steady-state concentrations (Css) of 600-900 ng/mL. The performance of our unit's high-performance liquid chromatography with tandem mass spectrometry method was assessed using a quality control (QC, 35 series) chart. International, multicenter, cross-validation test (n = 21) was conducted to validate different analytical methods. To assess Bu stability, regression analyses and Bland-Altman plots were performed on measurements at repeated time points on samples stored at -80°C for up to 5 years.

RESULTS

We observed a 4.2-fold interindividual variability in Bu Css after the first dose, with only 28% of children having a Css within the target range. During the 4 days of conditioning, 83% of children had their doses modified according to TDM recommendations. This achieved a Css within the target range in 75% of the children. Routine QC measurements were generally within the ±15% range around theoretical values, showing the optimal robustness of our center's analytical method. Two of the 21 Bu TDM centers returned inadequate results during cross-validation testing; both used a UV detection method. Storage at -80°C led to a fall in Bu content of 14.9% ± 13.4% at 2-4 years and of 20% ± 5% by 5 years (roverall = 0.92).

CONCLUSIONS

We conclude that TDM is an effective method of achieving targeted Bu levels in children. QC programs are crucial to monitoring and maintaining the quality of an analytical method.

Incorporation of GSTA1 genetic variations into a population pharmacokinetic model for IV busulfan in paediatric hematopoietic stem cell transplantation

AIMS

The aim of this study is to develop a population pharmacokinetic (PopPK) model for intravenous busulfan in children that incorporates variants of GSTA1, gene coding for the main enzyme in busulfan metabolism.

METHODS

Busulfan concentration-time data was collected from 112 children and adolescents (median 5.4 years old, range: 0.1-20) who received intravenous busulfan during the conditioning regimen prior to stem cell transplantation. Weight, sex, baseline disease (malignant vs. non-malignant), age, conditioning regimen and GSTA1 diplotypes were evaluated as covariates of pharmacokinetic parameters by using nonlinear mixed effects analysis. The ability to achieve the target AUC_24h (3600-6000 μM min^-1 ) was assessed by estimating the first dose based on the present PopPK model and by comparing the results with other available models in children.

RESULTS

A one-compartment model with first-order elimination best described the data. Allometric scaling of weight and a factor of busulfan metabolism maturation were included in the base model. GSTA1 diplotypes were found to be a significant covariate of busulfan clearance, which was 7% faster in rapid metabolizers and 12% slower in poor metabolizers, in comparison with normal ones. Busulfan doses calculated using the parameters of the proposed PopPK model were estimated to achieve the target AUC in 85.2% of the cases (95% CI 78.7-91.7%).

CONCLUSION

This is the first PopPK for busulfan that successfully incorporated GSTA1 genotype in a paediatric population. Its use may contribute to better prediction of busulfan exposure in children and adolescents since the first dose, by tailoring the dose according to the individual metabolic capacity.

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.

Multistep Unified Models Using Prior Knowledge for the Prediction of Drug Clearance in Neonates and Infants

Allometric approaches are widely used for interspecies scaling for the prediction of pharmacokinetic (PK) parameters during drug development. The concept of allometry can also be extended to predict PK parameters from adults to children. Three methods for extrapolating pediatric clearance were developed and evaluated using the clearance values of 4 drugs. The first method was established using a simple allometric (SA) model with estimated coefficient and exponent based on data ranging from children older than 2 years to adult. Then we developed a unified multistep single-exponent (MSE) and multistep body-weight-dependent exponent (MBDE) models. The major steps in these 2 new methods include generating pseudopredicted clearance for unobserved new populations such as preterm neonates, term neonates, and infants. Subsequent steps involve incorporating the pseudopredicted clearance with the actual PK data from older children and adults. All 3 models were then used to predict drug clearance in children ≤2 years old (N = 278). Drug clearance was predicted with mean absolute error of 29.6, 14.2, and 12.9 using SA, MSE, MBDE, respectively. The root mean square error was 65.9, 29.8, 24.7 for SA, MSE, MBDE, respectively. Approximately 41%, 72%, and 74% of the children's clearance data were within 0.5 to 1.5-fold of the observed values when drug clearance was extrapolated using SA, MSE, and MBDE models, respectively. The present multistep unified extrapolation approaches improved the prediction of clearance from preterm neonates to 2 years of age and may have practical use for first-in-pediatric dose selection.

Pharmacokinetics and Pharmacodynamics of Treosulfan in Patients With Thalassemia Major Undergoing Allogeneic Hematopoietic Stem Cell Transplantation

A treosulfan (Treo)-based conditioning regimen prior to hematopoietic stem cell transplantation (HSCT) has been successfully used in treating hematological malignant and nonmalignant diseases. We report Treo pharmacokinetics (PK) in patients with thalassemia major undergoing HSCT (n = 87), receiving Treo at a dose of 14 g/m² /day. Median Treo AUC and clearance (CL) was 1,326 mg*h/L and 10.8 L/h/m² , respectively. There was wide interindividual variability in Treo AUC and CL (64 and 68%) which was not explained by any of the variables tested. None of the Treo PK parameters were significantly associated with graft rejection or toxicity; however, Treo CL <7.97 L/h/m² was significantly associated with poor overall (hazard ratio (HR) 2.7, confidence interval (CI) (1.09-6.76), P = 0.032) and event-free survival (HR 2.4, CI (0.98-5.73), P = 0.055). Further studies in a larger cohort are warranted to identify the factors explaining the variation in Treo PK as well as to establish a therapeutic range of Treo for targeted dose adjustment to improve HSCT outcome.

A Nonparametric Method to Optimize Initial Drug Dosing and Attainment of a Target Exposure Interval: Concepts and Application to Busulfan in Pediatrics

The traditional approach for model-based initial dosing is based on the use of a single vector of typical population parameters for targeting a specific exposure. This approach is theoretically ill-suited for targeting a range of exposure. The objective of this work was to develop a general approach for optimal (OPT) targeting of a drug exposure interval. After methodological purposes, we applied our method to the busulfan case. We used a nonparametric population pharmacokinetic model of intravenous busulfan to estimate the individual pharmacokinetic parameters of 163 bone marrow-transplanted children. Then, an array of 151 doses of busulfan ranging from 0.5 to 2 mg/kg was simulated a priori in each patient. For each dose, 29 possible busulfan plasma concentration profiles, corresponding to the nonparametric prior, each associated with a probability, were obtained. The multiple-model-based, OPT dose was identified as the dose maximizing the a priori probability of achieving the busulfan target area under the concentration-time curve (AUC). Two AUC targets were considered: 900-1500 (conventional) or <1500 µM min^-1. Finally, the OPT dose was individually simulated in each patient. We compared the ability of this method to achieve the target exposure interval with that of three other traditional model-based methods and one based on the non-parametric approach. When targeting the busulfan conventional AUC range, the OPT dose provided better attainment than the best of the three other methods after one dose (82.2 vs. 41.7 %, p < 0.005), two doses (79.1 vs. 65.0 %, p < 0.005), and at the end of therapy (80.4 vs. 76.7 %, p < 0.42). The approach provided a balanced distribution between under- (10.4 %) and overexposure (9.2 %), while other approaches showed higher rates of underexposure (≥19 %). When targeting an AUC <1500 µM min, the OPT dose was successful in minimizing overexposure as 0 % of children showed simulated AUC >1500 µM min^-1. Our approach has been designed to optimize the targeting of an exposure interval. When applied to busulfan in children, it outperformed the traditional model-based dosing approach, with earlier and better achievement of busulfan target AUC. The approach can be applied for OPT dosing of many drugs, when the target objective is an interval.

Population pharmacokinetics analysis of intravenous busulfan in Chinese patients undergoing hematopoietic stem cell transplantation

There are several reports describing population pharmacokinetic (popPK) models of busulfan (BU). However, limited information is available in Chinese hematopoietic stem cell transplantation (HSCT) patients. The present study aimed to establish a popPK model of intravenous BU in Chinese HSCT patients for individualized drug therapy. The popPK model of BU was developed from a total of 284 concentration-time points from 53 patients. The effects of demographic and biochemical covariates were investigated by nonlinear mixed effect model (NONMEM) software. Plots, visual predictive check (VPC), bootstrap and normalized prediction distribution error (NPDE) were performed to determine the stability and the reliability of the final model. A one-compartment model with first-order elimination process was confirmed as the final structural model for BU. For a typical patient whose body surface area (BSA) is 1.7 m² , the population typical values of CL and Vd were 11.86 L/h, and 48.2 L, respectively. The result suggested BSA showed significant influence on CL and Vd (P<.001). Plots revealed the final model was performing a goodness fit. The steady rate verified by bootstrap was 100%, relative deviation was less than 4.00%, estimated value of final model was in the 95% confidence interval (CI). The VPC results showed the observed values were almost all positioned within the 5th and 95th CIs. The mean and variance of the NPDE were 0.0363 (Wilcoxon signed-rank test, 0.298) and 0.877 (Fisher variance test, 0.134; SW test of normality, 0.108), respectively. The global adjusted P value was 0.305, which indicated that the prediction of the BU popPK model was adequate. A physician-friendly Microsoft Excel-base tool was implemented using the final popPK model for designing individualized dosing regimens.

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.

GSTA1 Genetic Variants and Conditioning Regimen: Missing Key Factors in Dosing Guidelines of Busulfan in Pediatric Hematopoietic Stem Cell Transplantation

Busulfan (Bu) is a key component of conditioning regimens used before hematopoietic stem cell transplantation (SCT) in children. Different predictive methods have been used to calculate the first dose of Bu. To evaluate the necessity of further improvements, we retrospectively analyzed the currently available weight- and age-based guidelines to calculate the first doses in 101 children who underwent allogenic SCT in CHU Sainte-Justine, Montreal, after an intravenous Bu-containing conditioning regimen according to genetic and clinical factors. The measured areas under the curve (AUCs) were within target (900 to 1500 µM/min) in 38.7% of patients after the administration of the first dose calculated based on age and weight, as locally recommended. GSTA1 diplotypes linked to poor Bu metabolism (G3) and fludarabine-containing regimens were the only factors associated with AUC within target (OR, 4.7 [95% CI, 1.1 to 19.8, P = .04]; and OR, 9.9 [95% CI, 1.6 to 61.7, P = .01], respectively). From the 11 methods selected for dose calculation, the percentage of AUCs within the target varied between 16% and 74%. In some models G3 was associated with AUCs within the therapeutic and the toxic range, whereas rapid metabolizers (G1) were correlated with subtherapeutic AUCs when different methods were used. These associations were confirmed by clearance-prediction analysis, in which GSTA1 diplotypes consistently influenced the prediction errors of the methods. These findings suggest that these factors should be considered in Bu dose prediction in addition to the anthropometric data from patients. Furthermore, our data indicated that GSTA1 diplotypes was a factor that should be included in future population pharmacokinetic models, including similar conditioning regiments, to improve the prediction of Bu exposure after its initial dose.

Opening Marrow Niches in Patients Undergoing Autologous Hematopoietic Stem Cell Gene Therapy

Successful gene therapy for genetic disorders requires marrow niches to be opened to varying degrees to engraft gene-corrected hematopoietic stem cells (HSC). For example, in severe combined immunodeficiency, relatively limited chimerism is necessary for both T- and B-cell immune reconstitution, whereas for inborn errors of metabolism maximal donor chimerism is the goal. Currently, alkylating chemotherapy is used for this purpose. Significant pharmacokinetic variability exists in drug clearance in children less than 12 years old. Thus, pharmacokinetic monitoring is needed to achieve the targeted exposure goal for busulfan.

Pediatric patients undergoing hematopoietic stem cell transplantation can greatly benefit from a novel once-daily intravenous busulfan dosing nomogram

Busulfan, a bifunctional alkylating agent, has been used as a conditioning regimen prior to allogeneic hematopoietic stem cell transplantation (HSCT). The aim of this study was to derive a novel once-daily intravenous (IV) busulfan dosing nomogram for pediatric patients undergoing HSCT using a population pharmacokinetic (PK) model. A population PK analysis was performed using 2183 busulfan concentrations in 137 pediatric patients (age: 0.6-22.2 years), who received IV busulfan once-daily for 4 days before undergoing HSCT. Based on the final population PK model, an optimal once-daily IV busulfan dosing nomogram was derived. The percentage of simulated patients achieving the daily target area under the concentration-time curve (AUC) by the new nomogram was compared with that by other busulfan dosing regimens including the FDA regimen, the EMA regimen, and the empirical once-daily regimen without therapeutic drug monitoring (TDM). A one-compartment open linear PK model incorporating patient's body surface area, age, dosing day, and aspartate aminotransferase as a significant covariate adequately described the concentration-time profiles of busulfan. An optimal dosing nomogram based on the PK model performed significantly better than the other dosing regimens, resulting in >60% of patients achieving the target AUC while the percentage of patients exceeding the toxic AUC level was kept <25% during the entire treatment period. A novel once-daily busulfan dosing nomogram for pediatric patients undergoing HSCT is useful for clinicians, particularly in a setting where TDM service is not readily available or to optimize the dose on day 1.

Pharmacometabolomics for predicting variable busulfan exposure in paediatric haematopoietic stem cell transplantation patients

Owing to its narrow therapeutic range and high pharmacokinetic variability, optimal dosing for busulfan is important to minimise overexposure-related systemic toxicity and underexposure-related graft failure. Using global metabolomics, we investigated biomarkers for predicting busulfan exposure. We analysed urine samples obtained before busulfan administration from 59 paediatric patients divided into 3 groups classified by area under the busulfan concentration-time curve (AUC), i.e., low-, medium-, and high-AUC groups. In the high-AUC group, deferoxamine metabolites were detected. Phenylacetylglutamine and two acylcarnitines were significantly lower in the high-AUC group than in the low-AUC group. Deferoxamine, an iron-chelating agent that lowers serum ferritin levels, was detected in the high-AUC group, indicating that those patients had high ferritin levels. Therefore, in a retrospective study of 130 paediatric patients, we confirmed our hypothesis that busulfan clearance (dose/AUC) and serum ferritin level has a negative correlation (r = −0.205, P = 0.019). Ferritin, acylcarnitine, and phenylacetylglutamine are associated with liver damage, including free radical formation, deregulation of hepatic mitochondrial β-oxidation, and hyperammonaemia. Our findings reveal potential biomarkers predictive of busulfan exposure and suggest that liver function may affect busulfan exposure.

Population Pharmacokinetics: Some Observations in Pediatric Modeling for Drug Clearance

The objective of this study is to evaluate the predictive performance of several models to predict drug clearance in preterm and term neonates. Five models using different types of allometric and linear models were developed. Two sets of data were used to develop these models (data from preterm neonates to adults and data from preterm and term neonates). Models were also developed with (normalized to 70 kg) or without body weight normalization (body weight 1 kg). From the literature, clearance values for four drugs from neonates to adults were obtained. External data were used to evaluate the predictive performance of these models in preterm and term neonates. The results of the study indicated that (1) normalization to a standard body weight had no impact on the predictive performance of the models, (2) the model developed from preterm neonates to adults using fixed exponent 0.75 provided inaccurate estimate (overestimation) of drug clearance in neonates, (3) a far superior prediction of clearance was observed with the model when the exponents of allometry were estimated than the model using exponent 0.75, (4) linear models with the exception of the model with intercept provided comparable results to the estimated exponent model and were superior in their predictive performance to the model using exponent 0.75, and (5) when the models were developed from neonate data, the predictive performance of all models were similar. Overall, the study indicated that body weight normalization had no impact on the performance of model prediction, the exponents of allometry in pharmacostatistical models should be estimated rather than fixed, and more studies are needed to evaluate the suitability of linear models for the prediction of drug clearance in neonates.

Personalizing Busulfan-Based Conditioning: Considerations from the American Society for Blood and Marrow Transplantation Practice Guidelines Committee

The Practice Guidelines Committee of the American Society of Blood or Marrow Transplantation (ASBMT) sought to develop an evidence-based review about personalizing busulfan-based conditioning. The Committee sought to grade the relevant published studies (June 1, 2008 through March 31, 2016) according to criteria set forth by the Steering Committee for Evidence Based Reviews from ASBMT. Unfortunately, the published literature was too heterogeneous and lacked adequately powered and sufficiently controlled studies for this to be feasible. Despite this observation, the continued interest in this topic led the Practice Guidelines Committee to develop a list of most frequently asked questions (FAQs) regarding personalized busulfan dosing. This "Considerations" document is a list of these FAQs and their responses, addressing topics of practical relevance to hematopoietic cell transplantation clinicians.

Accurately Achieving Target Busulfan Exposure in Children and Adolescents With Very Limited Sampling and the BestDose Software

BACKGROUND

Busulfan dose adjustment is routinely guided by plasma concentration monitoring using 4-9 blood samples per dose adjustment, but a pharmacometric Bayesian approach could reduce this sample burden.

METHODS

The authors developed a nonparametric population model with Pmetrics. They used it to simulate optimal initial busulfan dosages, and in a blinded manner, they compared dosage adjustments using the model in the BestDose software to dosage adjustments calculated by noncompartmental estimation of area under the time-concentration curve at a national reference laboratory in a cohort of patients not included in model building.

RESULTS

Mean (range) age of the 53 model-building subjects was 7.8 years (0.2-19.0 years) and weight was 26.5 kg (5.6-78.0 kg), similar to nearly 120 validation subjects. There were 16.7 samples (6-26 samples) per subject to build the model. The BestDose cohort was also diverse: 10.2 years (0.25-18 years) and 46.4 kg (5.2-110.9 kg). Mean bias and imprecision of the 1-compartment model-predicted busulfan concentrations were 0.42% and 9.2%, and were similar in the validation cohorts. Initial dosages to achieve average concentrations of 600-900 ng/mL were 1.1 mg/kg (≤12 kg, 67% in the target range) and 1.0 mg/kg (>12 kg, 76% in the target range). Using all 9 concentrations after dose 1 in the Bayesian estimation of dose requirements, the mean (95% confidence interval) bias of BestDose calculations for the third dose was 0.2% (-2.4% to 2.9%, P = 0.85), compared with the standard noncompartmental method based on 9 concentrations. With 1 optimally timed concentration 15 minutes after the infusion (calculated with the authors' novel MMopt algorithm) bias was -9.2% (-16.7% to -1.5%, P = 0.02). With 2 concentrations at 15 minutes and 4 hours bias was only 1.9% (-0.3% to 4.2%, P = 0.08).

CONCLUSIONS

BestDose accurately calculates busulfan intravenous dosage requirements to achieve target plasma exposures in children up to 18 years of age and 110 kg using only 2 blood samples per adjustment compared with 6-9 samples for standard noncompartmental dose calculations.

Levetiracetam for the prevention of busulfan-induced seizures in pediatric hematopoietic cell transplantation recipients

Objective The objective of this study was to compare clinical outcomes in children undergoing hematopoietic cell transplantation who received levetiracetam versus those who received phenytoin for the prevention of busulfan-induced seizures. Methods This study was an IRB-approved, single-center, retrospective analysis of pediatric patients receiving intravenous busulfan for hematopoietic cell transplantation conditioning from January 2009 to July 2014. The primary study endpoint was the incidence of seizure during busulfan administration (day -8 to 0). Key transplant related-outcomes were also collected, including the incidence of graft rejection, sinusoidal obstruction syndrome, relapse, and death. Results A total of 20 patients met criteria for inclusion in the study. The population was heterogeneous with regard to the indication for hematopoietic cell transplantation, donor type, stem cell source, and conditioning regimen. Nine patients (45%) received levetiracetam and 11 (55%) received phenytoin for seizure prophylaxis. No seizures or graft rejections were observed in the study population. One relapse, one case of sinusoidal obstruction syndrome, and two deaths occurred in the levetiracetam group, while no relapses, two cases of sinusoidal obstruction syndrome, and one death occurred in the phenytoin group. Conclusion These data suggest similar safety and effectiveness between levetiracetam and phenytoin for the prevention of busulfan-induced seizures in a small, heterogeneous pediatric hematopoietic cell transplantation population.

Iron Overload Exacerbates Busulfan-Melphalan Toxicity Through a Pharmacodynamic Interaction in Mice

PURPOSE

Busulfan-melphalan high-dose chemotherapy followed by autologous stem cell transplantation is an essential consolidation treatment of high-risk neuroblastoma in children. Main treatment limitation is hepatic veno-occlusive disease, the most severe and frequent extra-hematological toxicity. This life threatening toxicity has been related to a drug interaction between busulfan and melphalan which might be increased by prior disturbance of iron homeostasis, i.e. an increased plasma ferritin level.

METHODS

We performed an experimental study of busulfan and melphalan pharmacodynamic and pharmacokinetics in iron overloaded mice.

RESULTS

Iron excess dramatically increased the toxicity of melphalan or busulfan melphalan combination in mice but it did not modify the clearance of either busulfan or melphalan. We show that prior busulfan treatment impairs the clearance of melphalan. This clearance alteration was exacerbated in iron overloaded mice demonstrating a pharmacokinetic interaction. Additionally, iron overload increased melphalan toxicity without altering its pharmacokinetics, suggesting a pharmacodynamic interaction between iron and melphalan. Based on iron homeostasis disturbance, we postulated that prior induction of ferritin, through Nrf2 activation after oxidative stress, may be associated with the alteration of melphalan metabolism.

CONCLUSION

Iron overload increases melphalan and busulfan-melphalan toxicity through a pharmacodynamic interaction and reveals a pharmacokinetic drug interaction between busulfan and melphalan.

Population pharmacokinetics of busulfan in pediatric and young adult patients undergoing hematopoietic cell transplant: a model-based dosing algorithm for personalized therapy and implementation into routine clinical use

BACKGROUND

Population pharmacokinetic (PK) studies of busulfan in children have shown that individualized model-based algorithms provide improved targeted busulfan therapy when compared with conventional dose guidelines. The adoption of population PK models into routine clinical practice has been hampered by the tendency of pharmacologists to develop complex models too impractical for clinicians to use. The authors aimed to develop a population PK model for busulfan in children that can reliably achieve therapeutic exposure (concentration at steady state) and implement a simple model-based tool for the initial dosing of busulfan in children undergoing hematopoietic cell transplantation.

PATIENTS AND METHODS

Model development was conducted using retrospective data available in 90 pediatric and young adult patients who had undergone hematopoietic cell transplantation with busulfan conditioning. Busulfan drug levels and potential covariates influencing drug exposure were analyzed using the nonlinear mixed effects modeling software, NONMEM. The final population PK model was implemented into a clinician-friendly Microsoft Excel-based tool and used to recommend initial doses of busulfan in a group of 21 pediatric patients prospectively dosed based on the population PK model.

RESULTS

Modeling of busulfan time-concentration data indicates that busulfan clearance displays nonlinearity in children, decreasing up to approximately 20% between the concentrations of 250-2000 ng/mL. Important patient-specific covariates found to significantly impact busulfan clearance were actual body weight and age. The percentage of individuals achieving a therapeutic concentration at steady state was significantly higher in subjects receiving initial doses based on the population PK model (81%) than in historical controls dosed on conventional guidelines (52%) (P = 0.02).

CONCLUSIONS

When compared with the conventional dosing guidelines, the model-based algorithm demonstrates significant improvement for providing targeted busulfan therapy in children and young adults.

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.