Intravenous busulfan in adults prior to haematopoietic stem cell transplantation: a population pharmacokinetic study

One versus two sets of busulfan therapeutic drug monitoring in myeloablative allogeneic hematopoietic cell transplant

INTRODUCTION

Busulfan is a common component of allogeneic hematopoietic cell transplant (alloHCT) conditioning, however interpatient pharmacokinetic variability can result in enhanced toxicity or increased relapse risk. Therapeutic drug monitoring (TDM) can minimize variability, yet the optimal frequency of TDM is unknown. We compared outcomes for patients with one versus two sets of busulfan TDM during myeloablative conditioning (MAC) prior to alloHCT.

METHODS

We analyzed the impact of busulfan TDM frequency and dose adjustments, with the primary outcome being relapse-free survival (RFS). Other outcomes included the incidence of acute and chronic graft versus host disease (GVHD), oral mucositis, pulmonary toxicity, sinusoidal obstruction syndrome (SOS), the cumulative incidence of relapse (CIR), and overall survival (OS).

RESULTS

Twenty-two patients underwent one set of sampling while 53 patients underwent two sets. Similar baseline characteristics were observed between the groups. There were no significant differences observed in RFS by day +180 (77.3% vs. 79.2%, p = 1.0), CIR by day +180 (18.2% vs. 17.8%, p = 0.74), or OS (p = 0.73). The incidences of acute GVHD, chronic GVHD, SOS, and severe mucositis were also similar. In each group, 63% received busulfan dose adjustments after one set, with 52.8% receiving further dose adjustments following the second set.

CONCLUSION

We observed no significant difference in alloHCT outcomes between patients who underwent one versus two sets of busulfan TDM sampling, suggesting that a single-time TDM and dose adjustment may be adequate to maximize outcomes after MAC alloHCT.

Pharmacokinetic Modeling and Simulation with Pharmacogenetic Insights Support the Relevance of Therapeutic Drug Monitoring for Myeloablative Busulfan Dosing in Adult HSCT

Therapeutic drug monitoring (TDM) of busulfan (Bu) is well-established in pediatric hematopoietic stem cell transplantation (HSCT), but its use in adults is limited due to a lack of clear recommendations and scarcity of evidence regarding its utility. GSTA1 promoter variants are reported to affect Bu clearance in both adults and pediatric patients. This study aimed to evaluate the value of preemptive genotyping GSTA1 and body composition (obesity) in individualizing Bu dosing in adults, through pharmacokinetic (PK) modeling and simulations. A population pharmacokinetic (PopPK) model was developed and validated with data from 60 adults who underwent HSCT. Simulations assessed different dosing scenarios based on body size metrics and GSTA1 genotypes. Due to the limited number of obese patients in the cohort, the effect of obesity on Bu pharmacokinetics (PK) was evaluated in silico using a physiologically-based pharmacokinetic (PBPK) model and relevant virtual populations from Simcyp software. Patients with at least 1 GSTA1*B haplotype had 17% lower clearance on average. PopPK simulations indicated that adjusting doses based on genotype increased the probability of achieving the target exposure (3.7 to 5.5 mg.h/L) from 53% to 60 % in GSTA1*A homozygous patients, and from 50% to 61% in *B carriers. Still, Approximately 40% of patients would not achieve this therapeutic window without TDM. A 2-sample optimal design was validated for routine model-based Bu first dose AUC0-∞ estimation, and the model was implemented in the Tucuxi user-friendly TDM software. PBPK simulations confirmed body surface area-based doses of 29 to 31 mg/m2/6h as the most appropriate, regardless of obesity status. This study emphasizes the importance of individualized Bu dosing strategies in adults to achieve therapeutic targets. Preemptive genotyping alone may not have a significant clinical impact, and routine TDM may be necessary for optimal transplantation outcomes.

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.

Limited Sampling Strategy Using End of Infusion and Six-Hour Concentrations Overestimates Intravenous Busulfan Clearance Compared With Standard Six-Point Sampling in Hematopoietic Stem Cell Transplant Patients

BACKGROUND

Therapeutic drug monitoring for busulfan (Bu) is important to improve outcomes of hematopoietic stem cell transplantation. However, standard therapeutic drug monitoring requires multiple samples and is inconvenient, labor-intensive, and costly. Accordingly, a limited sampling strategy (LSS) was evaluated, using 2-point sampling at end of infusion and at 6 hours, and the area-under-the-curve and Bu clearances (CLs) were compared with the results obtained from the standard sampling strategy (SSS) using 5-6 samples.

METHOD

The analysis was based on retrospective clinical data from 202 patients receiving intravenous Bu before hematopoietic stem cell transplantation for malignant or nonmalignant conditions. Bu plasma concentrations were measured via liquid chromatography tandem-mass spectrometry, and pharmacokinetic parameters were calculated using the PKCNA package in R program.

RESULT

A total of 502 doses were analyzed by applying SSS and LSS. Using the modified Bland-Altman plot, the mean percentage difference in CL between the SSS and LSS estimates of Bu 6-hourly regimen was -41% (Limits: -53% and -30%). In the once daily regimen, the mean difference in CL between the 2 strategies on the modified Bland-Altman plot was -22% (Limits: -66% and +22%).

CONCLUSIONS

The Bu CL values estimated based on the BU concentration at end of infusion and at 6 hours postinfusion were significantly higher than the values obtained via the SSS.

Pharmacokinetics of obese adults: Not only an increase in weight

Obesity is a pathophysiological state defined by a body mass index > 30 kg/m2 and characterized by an adipose tissue accumulation leading to an important weight increased. Several pathologies named comorbidities such as cardiovascular disease, type 2 diabetes and cancer make obesity the fifth cause of death in the world. Physiological changes impact the four main phases of pharmacokinetics of some drugs and leads to an inappropriate drug-dose. For absorption, the gastrointestinal transit is accelerated, and the gastric empty time is shortened, that can reduce the solubilization and absorption of some oral drugs. The drug distribution is probably the most impacted by the obesity-related changes because the fat mass (FM) increases at the expense of the lean body weight (LBW), leading to an important increase of the volume of distribution for lipophilic drugs and a low or moderately increase of this parameter for hydrophilic drugs. This modification of the distribution may require drug-dose adjustments. By various mechanisms, the metabolism and elimination of drugs are impacted by obesity and should be considered as similar or lower than that non-obese patients. To better understand the necessary drug-dose adjustments in obese patients, a narrative review of the literature was conducted to highlight the main elements to consider in the therapeutic management of adult obese patients.

Effects of combined test dose and therapeutic drug monitoring strategy in exposure-directed busulfan

Although exposure-directed busulfan (BU) dosing can improve allogeneic hematopoietic stem cell transplantation outcomes, there is still large variability in BU exposure with test dose alone due to changes in BU clearance caused by drug interactions. We conducted a single-arm phase II trial using the combined test dose and therapeutic drug monitoring strategy (PK-guided group) and compared the outcomes with an external historical cohort receiving a fixed-dose (fixed-dose group). The first eight and second eight doses were adjusted based on the area under the blood concentration-time curve (AUC) of the test and first doses, respectively, targeting a total AUC of 82.1 mg·h/L. All patients received either BU and cyclophosphamide conditioning (BU/CY) or fludarabine (FLU)-containing conditioning. The BU clearance at the first dose decreased more in patients receiving FLU than in those receiving BU/CY; however, BU clearance also declined over time in patients who received BU/CY. The simulated total AUC (sAUC) with test dose only was significantly higher in patients who received FLU than in those who received BU/CY, but sAUC with the combined strategy was comparable. The 100-day progression-free survival was 85.5% (95% confidence interval [CI]: 71.9-92.8%), and was not inferior to that in the fixed-dose group. For the FLU-containing regimens, the PK-guided group showed decreased relapse (0.0% vs. 26.9%, p = 0.03), and favorable overall survival (75.1% vs. 57.0%, p = 0.07) at 1 year. The combined strategy effectively controlled the BU exposure close to the target levels, potentially improving efficacy, especially in patients receiving the FLU-containing regimen. Clinical evaluation of efficacy of dose-modified intravenous busulfan in allogeneic hematopoietic stem cell transplantation for hematological malignancy (#UMIN000014077, June 15th, 2014).

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.

Sitagliptin ameliorates busulfan-induced pulmonary and testicular injury in rats through antioxidant, anti-inflammatory, antifibrotic, and antiapoptotic effects

Busulfan (BUS) is an anticancer agent with serious adverse effects on various body organs, including the lung and testis. Sitagliptin was proven to have antioxidant, anti-inflammatory, antifibrotic, and antiapoptotic effects. This study aims to evaluate whether sitagliptin, a DPP4I, ameliorates BUS-induced pulmonary and testicular injury in rats. Male Wistar rats were split into control, sitagliptin (10 mg/kg), BUS (30 mg/kg), and sitagliptin + BUS groups. Weight change, lung and testis indices, serum testosterone, sperm parameters, markers of oxidative stress [malondialdehyde (MDA) and reduced glutathione (GSH)], inflammation [tumor necrosis factor-alpha (TNF-α)], and relative expression of sirtuin1 (SIRT1) and forkhead box protein type O1 (FOXO1) genes were estimated. Histopathological examination of lung and testicular tissues was done to detect architectural changes [Hematoxylin & Eosin (H&E)], fibrosis (Masson's trichrome), and apoptosis (caspase-3). Sitagliptin treatment reduced body weight loss, lung index, lung and testis MDA, serum TNF-α and sperm abnormal morphology, and increased testis index, lung and testis GSH, serum testosterone, sperm count, viability and motility. SIRT1/FOXO1 balance was restored. Also, sitagliptin attenuated fibrosis and apoptosis in lung and testicular tissues via reducing collagen deposition and caspase-3 expression. Accordingly, sitagliptin ameliorated BUS-induced pulmonary and testicular damage in rats via attenuating oxidative stress, inflammation, fibrosis, and apoptosis.

Chemotherapy and Anticancer Drugs Adjustment in Obesity: A Narrative Review

BACKGROUND

Obese individuals have higher rates of cancer incidence and cancer- related mortality. The worse chemotherapy outcomes observed in this subset of patients are multifactorial, including the altered physiology in obesity and its impact on pharmacokinetics, the possible increased risk of underdosing, and treatment-related toxicity.

AIMS

The present review aimed to discuss recent data on physiology, providing just an overall perspective and pharmacokinetic alterations in obesity concerning chemotherapy. We also reviewed the controversies of dosing adjustment strategies in adult and pediatric patients, mainly addressing the use of actual total body weight and ideal body weight.

METHODS

This narrative review tried to provide the best evidence to support antineoplastic drug dosing strategies in children, adolescents, and adults.

RESULTS

Cardiovascular, hepatic, and renal alterations of obesity can affect the distribution, metabolism, and clearance of drugs. Anticancer drugs have a narrow therapeutic range, and variations in dosing may result in either toxicity or underdosing. Obese patients are underrepresented in clinical trials that focus on determining recommendations for chemotherapy dosing and administration in clinical practice. After considering associated comorbidities, the guidelines recommend that chemotherapy should be dosed according to body surface area (BSA) calculated with actual total body weight, not an estimate or ideal weight, especially when the intention of therapy is the cure.

CONCLUSION

The actual total body weight dosing appears to be a better approach to dosing anticancer drugs in both adults and children when aiming for curative results, showing no difference in toxicity and no limitation in treatment outcomes compared to adjusted doses.

External Evaluation of Population Pharmacokinetic Models of Busulfan in Chinese Adult Hematopoietic Stem Cell Transplantation Recipients

Objective: Busulfan (BU) is a bi-functional DNA-alkylating agent used in patients undergoing hematopoietic stem cell transplantation (HSCT). Over the last decades, several population pharmacokinetic (pop PK) models of BU have been established, but external evaluation has not been performed for almost all models. The purpose of the study was to evaluate the predictive performance of published pop PK models of intravenous BU in adults using an independent dataset from Chinese HSCT patients, and to identify the best model to guide personalized dosing.

Methods: The external evaluation methods included prediction-based diagnostics, simulation-based diagnostics, and Bayesian forecasting. In prediction-based diagnostics, the relative prediction error (PE%) was calculated by comparing the population predicted concentration (PRED) with the observations. Simulation-based diagnostics included the prediction- and variability-corrected visual predictive check (pvcVPC) and the normalized prediction distribution error (NPDE). Bayesian forecasting was executed by giving prior one to four observations. The factors influencing the model predictability, including the impact of structural models, were assessed.

Results: A total of 440 concentrations (110 patients) were obtained for analysis. Based on prediction-based diagnostics and Bayesian forecasting, preferable predictive performance was observed in the model developed by Huang et al. The median PE% was -1.44% which was closest to 0, and the maximum F₂₀ of 57.27% and F₃₀ of 72.73% were achieved. Bayesian forecasting demonstrated that prior concentrations remarkably improved the prediction precision and accuracy of all models, even with only one prior concentration.

Conclusion: This is the first study to comprehensively evaluate published pop PK models of BU. The model built by Huang et al. had satisfactory predictive performance, which can be used to guide individualized dosage adjustment of BU in Chinese patients.

Gallic acid ameliorates busulfan-induced testicular toxicity and damage in mature rats

Here, we studied the protective effect of gallic acid (GAL) as a potent anti-oxidant and anti-inflammatory agent against damage caused by busulfan (BUS) in the testes of adult rats. The adult Wistar rats were assigned as control, BUS: was intraperitoneally (i.p.) treated with busulfan (15 mg/kg, day 7 and 14), GAL + BUS: was co-treated with busulfan (i.p., 15 mg/kg, day 7 and 14) and orally treated (per os) with gallic acid (60 days, 20 mg/kg) and GAL: was treated with gallic acid (per os, 60 days, 20 mg/kg). The results showed that GAL co-treatment increased the numbers of spermatogonia (Type A and B), spermatocytes (primary and secondary) and round spermatids, along with the tubular diameter, epithelial height and gonado-somatic index. In addition, BUS-induced increase in 3β-hydroxysteroid dehydrogenase and γ-glutamyl transpeptidase activities were inhibited on GAL co-treatment. Similarly, BUS-induced decrease in gluthathione concentration, catalase and superoxide dismutase activities along with increase in myeloperoxidase activity and malondialdehyde concentration were significantly normalized to control values on GAL co-treatment. Busulfan-induced elimination of tubular germ cells was completely prevented by GAL. Overall, GAL may inhibit BUS-mediated spermatogenesis arrest via decreasing inflammatory-mediated oxidative stress in a rat experimental model.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Effect of GSTA1 Variants on Busulfan-Based Conditioning Regimen Prior to Allogenic Hematopoietic Stem-Cell Transplantation in Pediatric Asians

Busulfan is widely used as a chemotherapy treatment before hematopoietic stem-cell transplantation (HSCT). However, the response of busulfan is highly variable and unpredictable, whereby the pharmacogenetic interference of glutathione S-transferase (GST) has strong evidence in Caucasians and some adult Asians but not in pediatric Asian patients. This study was aimed at investigating the associations of GST genetic polymorphisms with variations in the pharmacokinetic (PK) properties of busulfan in pediatric Asian patients. This retrospective cohort study recruited 92 pediatric patients. The polymorphism of GSTA1 was genotyped by Sanger sequencing, and GSTM1 and GSTP1 were genotyped by real-time PCR. Drug concentration and PK estimation were identified using an LC-MS/MS method and a noncompartmental model. Statistical analysis was performed by R software. Out of 92 patients, 48 (53%) were males, the mean age was 8.4 ± 5.12 years old, and the average weight was 26.52 ± 14.75 kg. The allele frequencies of GSTA1*B and of GSTM1 and GSTP1* deletions were 16.9%, 68.5%, and 21.2%, respectively. Patients with GSTA1*B had a statistically significant impact on the PK of busulfan, whereas those with GSTM1 and GSTP1 did not (p > 0.05). The carriers of GSTA1*B showed a significant difference compared to noncarriers in terms of t1/2 (for first dose: 161.9 vs. 134.3 min, p = 0.0016; for second dose: 156.1 vs. 129.8, p = 0.012), CL (88.74 vs. 124.23 mL/min, p = 0.0089), Cmax (4232.6 vs. 3675.5 ng/mL, p = 0.0021), and AUC (5310.6 vs. 4177.1 µM/min, p = 0.00033). The augmentation of AUC was around 27.1% in patients carrying the GSTA1*B variant. The GSTA1 polymorphism was significantly associated with variations of the pharmacokinetic properties of busulfan treatment in pediatric Asian patients.

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.

Pharmacokinetically guided, once-daily intravenous busulfan in combination with fludarabine for elderly AML/MDS patients as a conditioning regimen for allogeneic stem cell transplantation

The efficacy of pharmacokinetically (PK) guided, once-daily administration of busulfan (BU) was evaluated in elderly patients with acute myeloid leukemia/myelodysplastic syndrome (AML/MDS). Twenty-one patients (median age 61) received 30 mg/m² fludarabine for 6 days and BU for 4 days, starting from 3.2 mg/m² and subsequently adjusted to the target area under the curve (AUC) of 6000 µmol-min/L. The median AUC of day 1 (AUC1), AUC4, and their average were 4871.3, 6021.0, and 5368.1 µmol-min/L, respectively. Veno-occlusive disease/sinusoidal obstructive syndrome (VOD/SOS) occurred in five patients (24%) but all recovered well. Four patients (20%) had non-infectious pulmonary complications (NIPCs). Patients with high AUC1 had frequent gastrointestinal adverse events, but similar incidence of VOD/SOS and NIPCs. Two-year overall survival (OS), non-relapse mortality (NRM), and relapse rates were 44.4%, 28.6%, and 29.1%, respectively. Patients with high AUC1 had significantly high NRM (57.1% vs. 14.3%, P = 0.04) and inferior OS (14.3% vs. 60.1%, P = 0.002), while patients with high AUC4 had a significantly low relapse rate (8.3% vs. 55.6%, P = 0.02). In conclusion, once-daily BU and a PK-guided dose intensification were beneficial for reducing relapse in elderly patients with AML/MDS. However, caution should be exercised as rapid BU dose elevation may contribute to NRM.

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.

Choosing the Allometric Exponent in Covariate Model Building

BACKGROUND

Allometric scaling is often used to describe the covariate model linking total body weight (WT) to clearance (CL); however, there is no consensus on how to select its value.

OBJECTIVES

The aims of this study were to assess the influence of between-subject variability (BSV) and study design on (1) the power to correctly select the exponent from a priori choices, and (2) the power to obtain unbiased exponent estimates.

METHODS

The influence of WT distribution range (randomly sampled from the Third National Health and Nutrition Examination Survey, 1988-1994 [NHANES III] database), sample size (N = 10, 20, 50, 100, 200, 500, 1000 subjects), and BSV on CL (low 20%, normal 40%, high 60%) were assessed using stochastic simulation estimation. A priori exponent values used for the simulations were 0.67, 0.75, and 1, respectively.

RESULTS

For normal to high BSV drugs, it is almost impossible to correctly select the exponent from an a priori set of exponents, i.e. 1 vs. 0.75, 1 vs. 0.67, or 0.75 vs. 0.67 in regular studies involving < 200 adult participants. On the other hand, such regular study designs are sufficient to appropriately estimate the exponent. However, regular studies with < 100 patients risk potential bias in estimating the exponent.

CONCLUSION

Those study designs with limited sample size and narrow range of WT (e.g. < 100 adult participants) potentially risk either selection of a false value or yielding a biased estimate of the allometric exponent; however, such bias is only relevant in cases of extrapolating the value of CL outside the studied population, e.g. analysis of a study of adults that is used to extrapolate to children.

Pharmacokinetics and Pharmacodynamics of Drugs in Obese Pediatric Patients: How to Map Uncharted Clinical Territories

Clinicians are increasingly faced with challenges regarding the pharmacological treatment of obese pediatric patients. To provide guidance for these treatments, a better understanding of the impact of obesity on pharmacological processes in children is needed. Results on pharmacological studies in adults show however ambiguous patterns regarding the impact of obesity on ADME processes or on drug pharmacodynamics. Additionally, based on the limited research performed in obese pediatric patients, it becomes clear that findings from obese adults cannot be expected to always translate directly to similar findings in obese children. To improve knowledge on drug pharmacology in obese pediatric patients, studies should focus on quantifying the impact of maturation, obesity, and other relevant variables on primary pharmacological parameters and on disentangling systemic (renal and/or hepatic) and presystemic (gut and/or first-pass hepatic) clearance. For this, data is required from well-designed clinical trials that include patients with not only a wide range in age but also a range in excess body weight, upon oral and intravenous dosing. Population modelling approaches are ideally suitable for this purpose and can also be used to link the pharmacokinetics to pharmacodynamics and to derive drug dosing regimens. Generalizability of research findings can be achieved by including mechanistic aspects in the data analysis, for instance, using either extrapolation approaches in population modelling or by applying physiologically based modelling principles. It is imperative that more and smarter studies are performed in obese pediatric patients to provide safe and effective treatment for this special patient population.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Use of busulfan in conditioning for allogeneic hematopoietic stem cell transplantation in adults: a survey by the Transplant Complications Working Party of the EBMT

A survey was carried out among EBMT centers about the use of busulfan for conditioning in allogeneic stem cell transplantation. Of 109 responding centers, 106 used busulfan for conditioning, 102 in conventional myeloablative doses, and 93 in reduced doses (RIC). The route of administration was mostly intravenous, but ~10% of the centers gave the drug orally. The number of doses in i.v. administration varied and was in myeloablative conditioning mostly one (50 centers) or four (43 centers) doses a day. Seventeen of the 106 centers used pharmacokinetics for dose adjustment in myeloablative conditioning, nine in RIC. The details of pharmacokinetic monitoring varied markedly. Three quarters of the centers reported adjusting the dose based on obesity in myeloablative conditioning and about 60% in RIC. The most common method for dose calculation was ideal body weight + 0.25 × (actual body weight - ideal body weight). In conclusion, the present survey showed marked heterogeneity in the current practices of busulfan administration for conditioning. The impact of the heterogeneity is not well known. Due to this and the scarcity of support from controlled clinical studies, no clear guidelines can be presented, but some prevailing policies to be recommended were identified.

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.

Pharmacokinetic and clinical outcomes when ideal body weight is used to dose busulfan in obese hematopoietic stem cell transplant recipients

Weight-based dosing of intravenous busulfan is widely used in hematopoietic cell transplantation. However, a variety of dosing weights have been described. The objective of this retrospective study was to determine the pharmacokinetic impact of using ideal body weight as the initial dosing weight in obese as compared to non-obese transplant recipients. The secondary objectives were to describe the use of alternative dosing weights, the impact on survival, and the rates of toxicities. The mean steady-state concentration was 779.3 ng/mL (n = 82) in the non-obese cohort and 673.7 ng/mL (n = 63) in the obese cohort (p < 0.001). A smaller proportion of concentrations were below goal in the non-obese cohort (10% vs. 41%, p < 0.001). Ideal body weight and adjusted body weights with a 25 and 40% correction factor are appropriate in non-obese patients; adjusted body weights with a 25 and 40% correction factor are appropriate in obese patients. There was no difference in overall survival (p = 0.18); there was a difference in median progression-free survival (1078 vs. 500 days, p = 0.045) in the non-obese compared to obese cohorts. The use of ideal body weight to dose busulfan resulted in lower steady-state concentrations, a larger proportion of subtherapeutic concentrations, and worse progression-free survival in obese patients.

Sinusoidal obstruction syndrome/veno-occlusive disease after high-dose intravenous busulfan/melphalan conditioning therapy in high-risk Ewing Sarcoma

This mono-institutional observational study was conducted to determine incidence, severity, risk factors, and outcome of sinusoidal obstruction syndrome/veno-occlusive disease (SOS/VOD) in high-risk Ewing sarcoma (ES) patients treated with intravenous busulfan and melphalan (BU-MEL) followed by autologous stem cell transplantation (ASCT). During the past 10 years, 75 consecutive ES patients resulted evaluable for the analysis. After diagnosis of SOS/VOD, defibrotide therapy was started as soon as the medication was available. The variables analyzed as potential risk factors were: gender, patient's age at diagnosis, primary tumor site, disease stage, and prior radiation therapy (RT) given, focusing on RT liver exposure. The median age at diagnosis was 18.8 years. Five patients developed moderate to severe SOS/VOD (cumulative incidence, 6.67%). None of 32 pediatric patients (≤17 years) developed SOS/VOD (p = 0.0674). In univariate analysis, prior RT liver exposure resulted statistically significant (p = 0.0496). There was one death due to severe SOS/VOD. This study reports the largest series of high-risk ES patients treated with intravenous BU-MEL before ASCT. The incidence of SOS/VOD was lower when compared with other studies that used oral busulfan. Any prior RT liver exposure should be avoided. Earlier defibrotide treatment confirms to be effective.

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.

Therapeutic drug monitoring-guided dosing of busulfan differs from weight-based dosing in hematopoietic stem cell transplant patients

Busulfan (Bu)-based preparative regimens in hematopoietic stem cell transplantation are commonly used. Previous studies have shown that Bu at a fixed dose of 3.2mg/kg/day (FBD) given intravenously decreases variability in drug pharmacokinetics and this decreases the dependency on therapeutic drug monitoring (TDM) of Bu. We compared the Bu dose given using TDM with the FBD of 3.2mg/kg/day. Seventy-three patients with acute leukemia, myelodysplasia, chronic myeloid leukemia, thalassemia major, and sickle cell disease were included. The mean age at transplant was 15years (range 2-55years) with 57% adults. Indication for transplantation was leukemia/myelodysplastic syndrome in 46% of the patients, while the remaining 54% were transplanted for inherited blood disorders. We found that the median FBD was lower than the median TDM dose by 39mg/day with a statistically significant difference (p<0.001) even after adjusting for the weight (median total FBD of 349mg, median TDM dose of 494mg, p<0.0001). Age and underlying condition (malignant vs. nonmalignant) were the main factors affecting Bu clearance (p<0.001 and p<0.07, respectively). TDM remains an important tool for the appropriate dosing of Bu in preparative regimens of hematopoietic stem cell transplantation, especially in populations with genetic admixture.

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

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

Population pharmacokinetics and pharmacodynamics of busulfan with GSTA1 polymorphisms in patients undergoing allogeneic hematopoietic stem cell transplantation

AIM

A population pharmacokinetic (PPK) analysis was conducted to describe the influence of GSTA1 polymorphisms on intravenous busulfan in adults undergoing allogeneic hematopoietic stem cell transplantation.

PATIENTS & METHODS

A PPK model was developed from 36 patients by a one-compartment model with first-order elimination.

RESULTS

The typical value of clearance and volume of distribution were 11.0 l/h and 42.4 l, respectively. Clearance decreased by 15% and area under the concentration-time curves (AUCs) increased with GSTA1 variants compared with wild-type (both p < 0.05). Subtherapeutic AUCs were seen only in wild-type patients.

CONCLUSION

To our knowledge, this is the first PPK study to suggest that GSTA1 polymorphisms in adults are associated with busulfan PK.

Trough level monitoring of intravenous busulfan to estimate the area under the plasma drug concentration-time curve in pediatric hematopoietic stem cell transplant recipients

Optimizing systemic busulfan exposure, the area under the concentration-time curve (AUC), improves the outcomes for hematopoietic stem cell transplantation (HSCT). The AUC is conventionally calculated using six plasma concentrations (AUC(0-∞)) drawn after the first of 16 intravenous busulfan doses given as a 2-h infusion every 6 h. The aim of the present study was to develop limited sampling strategies using three or fewer busulfan concentrations to reliably calculate AUC(0-∞) in patients undergoing HSCT. We investigated the pharmacokinetics of busulfan 46 times in 29 pediatric patients receiving intravenous busulfan. Limited sampling strategies using one, two, or three plasma busulfan concentrations were developed by multiple linear regression that showed excellent agreement with AUC(0-∞). In single-point sampling strategies, the AUC(0-∞) predicted based on C(6) (trough level: busulfan plasma concentration 6 h after the start of the infusion) was significantly correlated with, and not statistically different from, actual values as follows: AUC(0-∞) = 2556.5 C6 + 320.9 (r(2) = 0.929, P < 0.0001, mean bias 0.282 %, precision 7.91 %). In contrast, the predicted AUCs derived from the other sampling single points did not meet the criteria. The trough level well correlated with actual AUC(0-∞), suggesting that this time-point is acceptable for busulfan monitoring.

Outcomes after autologous SCT in lymphoma patients grouped by weight

Obesity continues to be an increasing global health issue contributing to the complexity of chemotherapy dosing in the field of SCT. Investigation into the optimal dosing weight used to calculate chemotherapy doses in obese patients undergoing SCT is limited and inconclusive. Our single-center, retrospective study compared safety and efficacy outcomes by body mass index (BMI) for 476 adult lymphoma patients who underwent auto-SCT with a myeloablative chemotherapeutic regimen of BU, CY and etoposide dosed using adjusted body weight. Three weight groups categorized based on BMI were defined: normal/underweight ⩽24.9 kg/m(2), overweight 25-29.9 kg/m(2) and obese ⩾30 kg/m(2). Severity of mucositis, incidence of secondary malignancy, incidence of bacteremia and median hospital length of stay did not differ among the groups. The median times to absolute neutrophil count and platelet recovery were 10 days (P=0.75) and 14 days (P=0.17), respectively. Obese patients had a lower 100-day mortality compared with other weight groups, although this did not translate into an OS benefit. OS and disease relapse were similar among the groups. Our study demonstrates that use of adjusted body weight to calculate chemotherapy doses does not negatively have an impact on outcomes in obese patients undergoing auto-SCT with BU, CY and etoposide.

Dosing algorithm revisit for busulfan following IV infusion

PURPOSE

Busulfan (Bu) exposure is critical for efficacy and safety. Body weight (BW), or adjusted ideal body weight (AIBW)-based dosing (WBD) algorithm, has been used in hematopoietic stem cell transplantation (HSCT). A recently completed phase 2 study revealed that 33.6 % of the subjects were under-, or over-exposed, with this WBD algorithm. This paper was to investigate Bu dosing algorithm in an attempt to improve the suboptimal Bu exposure.

METHODS

Population PK modeling was conducted using the data from 207 patients. Dosing algorithm was developed based on derived covariate model of CL. Model-based simulation was conducted to assist test PK study design. A simplified CL estimation method was proposed based on the PK structure model for Bu.

RESULTS

A one-compartment structure model adequately described the PK profile of Bu following an IV infusion. BSA best described the inter-individual variability of CL. The proposed dosing algorithm was dose (mg) = (31.7 × BSA - 11.6) × target AUC [µM min]/1,000. With this dosing algorithm, 14.3 % patients could be under- or over-exposed. A test PK study with reduced study duration and three PK samples can provide as nearly as good an estimate of CL compared to 12 PK samples on two different occasions.

CONCLUSION

The proposed dosing algorithm can significantly improve the sub-exposure of Bu. A shortened test PK study duration with reduced PK samples can provide as near as good estimate for Bu CL. A simplified CL estimation method is valid.

Effect of age on the pharmacokinetics of busulfan in patients undergoing hematopoietic cell transplantation; an alliance study (CALGB 10503, 19808, and 100103)

PURPOSE

Older patients with acute myeloid leukemia (AML) and myelodysplastic syndrome have often been excluded from myeloablative-conditioning regimens containing busulfan because of non-disease-related morbidity and mortality. We hypothesized that busulfan clearance (BuCL) in older patients (>60 years) would be reduced compared to that in younger patients, potentially explaining observed differences in busulfan tolerability.

METHODS

AML patients in three CALGB hematopoietic cell transplantation studies were treated with a conditioning regimen using IV busulfan, dosed at 0.8 mg/kg. Plasma busulfan concentrations were determined by LC-MS and analyzed by non-compartmental methods. BuCL was normalized to actual (ABW), ideal (IBW), or corrected (CBW) body weight (kg). Differences in BuCL between age groups were examined using the Wilcoxon rank sum test.

RESULTS

One hundred and eighty-five patients were accrued; 174 provided useable pharmacokinetic data. Twenty-nine patients ≥ 60 years old (median 66; range 60-74) had a significantly higher BuCL versus those <60 years old (median 50; range 18-60): BuCL 236 versus 168 mL/min, p = 0.0002; BuCL/ABW 3.0 versus 2.1 mL/min/kg, p = 0.0001; BuCL/IBW 3.8 versus 2.6 mL/min/kg, p = 0.0035; BuCL/CBW 3.4 versus 2.6 mL/min/kg, p = 0.0005. Inter-patient variability in clearance (CV %) was up to 48 % in both age groups. Phenytoin administration, a potential confounder, did not affect BuCL, regardless of weight normalization (p > 0.34).

CONCLUSIONS

Contrary to our hypothesis, BuCL was significantly higher in older patients compared to younger patients in these studies and does not explain the previously reported increase in busulfan toxicity observed in older patients.

Predictive performance of a physiologically based pharmacokinetic model of busulfan in children

A physiologically based pharmacokinetic (PBPK) model of the DNA-alkylating agent busulfan was slightly modified and scaled from adults to children in order to predict the systemic busulfan drug exposure in children. Capitalizing on the recent major software release of PK-Sim®, we refined our PBPK model by implementing glutathione S transferase (GST) in 11 organs using the software integrated enzyme expression database. In addition, two irreversible binding processes (i.e., DNA and plasma protein binding) were applied by using Koff and KD values. The model was scaled from adults to children. Simulations were computed and compared to concentration-time data after intravenous (i.v.) busulfan administration to 36 children. Based on the results, an age-dependent enzyme activity and maturation ratio was tailored and evaluated with an external dataset consisting of 23 children. Initial adult to children scaling indicated lower clearance values for children in comparison to adults. Subsequent age-dependent maturation ratio resulted in three different age groups: Activity of busulfan-glutathione conjugate formation was 80%, 61%, and 89% in comparison to adults for children with an age of up to 2 years, > 2-6 years, and > 6-18 years, respectively. Patients of the evaluation dataset were simulated with a mean percentage error (MPE) for all patients of 3.9% with 3/23 children demonstrating a MPE of > ±30%. The PBPK model parameterization sufficiently described the observed concentration-time data of the validation dataset while showing an adequate predictive performance. This PBPK model could be helpful to determine the first dose of busulfan in children.

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

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

BACKGROUND AND OBJECTIVE

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Busulfan dosing (Q6 or Q24) with adjusted or actual body weight, does it matter?

BACKGROUND

In hematopoietic stem cell transplantation (HSCT), patients receive individualized treatment planning in conditioning regimens to prevent unwarranted toxicities while maximizing desired outcomes. The dose of a widely studied agent in this setting, busulfan, can be adjusted based on area under the curve (AUC); however, choice of actual body weight (ABW) versus adjusted body weight (DBW) weight to calculate the initial dose may be critical in attaining goal AUC.

OBJECTIVE

To determine which weight best correlates with achievement of goal AUC for patients receiving busulfan conditioning for HSCT. Secondary objectives include evaluation of AUC results with clinical outcomes such as toxicity and survival.

METHODS

An institutional review board-approved retrospective analysis was performed on 31 allogeneic HSCT recipients who received intravenous busulfan (Q6H with cyclophosphamide [Bu/Cy] or once daily with fludarabine [Flu/Bu]).

RESULTS

Eighteen patients received Flu/Bu (50% ABW, 50% DBW) and 13 received Bu/Cy (23% ABW, 77% DBW). Overall, patients dosed by DBW were more likely to undershoot goal AUC (-12.8% vs. +19.5%, p = 0.018) and require dose increases (+20% vs. -19.9%, p = 0.012) versus those dosed by ABW. Subgroup analysis confirmed these results for Bu/Cy (-23.6% vs. +2.2%, p < 0.001 for goal AUC; +36.2% vs. -4.5%, p = 0.008 for busulfan dose increase), but not Flu/Bu (-0.8% vs. +25.3%, p = 0.123 for goal AUC; +3.4% vs. -25.1%, p = 0.174 for busulfan dose increase). Time to engraftment, progression-free survival, and overall survival were not different between dosing groups (p > 0.05). No patient experienced busulfan-related toxicity.

CONCLUSIONS

Further prospective studies are warranted to elucidate which weight is most likely to achieve goal AUC and subsequent optimal patient outcomes.

Review of therapeutic drug monitoring of anticancer drugs part 1--cytotoxics

Most anticancer drugs are characterised by a steep dose-response relationship and narrow therapeutic window. Inter-individual pharmacokinetic (PK) variability is often substantial. The most relevant PK parameter for cytotoxic drugs is the area under the plasma concentration versus time curve (AUC). Thus it is somewhat surprising that therapeutic drug monitoring (TDM) is still uncommon for the majority of agents. Goals of the review were to assess the rationale for more widely used TDM of cytotoxics in oncology. There are several reasons why TDM has never been fully implemented into daily oncology practice. These include difficulties in establishing appropriate concentration target ranges, common use of combination chemotherapies for many tumour types, analytical challenges with prodrugs, intracellular compounds, the paucity of published data from pharmacological trials and 'Day1 = Day21' administration schedules. There are some specific situations for which these limitations are overcome, including high dose methotrexate, 5-fluorouracil infusion, mitotane and some high dose chemotherapy regimens. TDM in paediatric oncology represents an important challenge. Established TDM approaches includes the widely used anticancer agents carboplatin, busulfan and methotrexate, with 13-cis-retinoic acid also recently of interest. Considerable effort should be made to better define concentration-effect relationships and to utilise tools such as population PK/PD models and comparative randomised trials of classic dosing versus pharmacokinetically guided adaptive dosing. There is an important heterogeneity among clinical practices and a strong need to promote TDM guidelines among the oncological community.

Conditioning chemotherapy dose adjustment in obese patients: a review and position statement by the American Society for Blood and Marrow Transplantation practice guideline committee

Hematopoietic stem cell transplantation (HCT) is a potentially life-saving therapy for patients with malignant and nonmalignant disease states. This article reviews the current published literature on the dosing of pharmacologic agents used for HCT preparative regimens with specific focus on the obese patient population. The review found that dose adjustments for obesity have, to date, been based empirically or extrapolated from published data in the nontransplantation patient population. As a result, the Committee determined that clear standards or dosing guidelines are unable to be made for the obese population because Level I and II evidence are unavailable at this time. Instead, the Committee provides a current published literature review to serve as a platform for conditioning agent dose selection in the setting of obesity. A necessary goal should be to encourage future prospective trials in this patient population because further information is needed to enhance our knowledge of the pharmacokinetics and pharmacodynamics of conditioning agents in the setting of obesity.

Busulfan in infant to adult hematopoietic cell transplant recipients: a population pharmacokinetic model for initial and Bayesian dose personalization

PURPOSE

Personalizing intravenous busulfan doses to a target plasma concentration at steady state (Css) is an essential component of hematopoietic cell transplantation (HCT). We sought to develop a population pharmacokinetic model to predict i.v. busulfan doses over a wide age spectrum (0.1-66 years) that accounts for differences in age and body size.

EXPERIMENTAL DESIGN

A population pharmacokinetic model based on normal fat mass and maturation based on postmenstrual age was built from 12,380 busulfan concentration time points obtained after i.v. busulfan administration in 1,610 HCT recipients. Subsequently, simulation results of the initial dose necessary to achieve a target Css with this model were compared with pediatric-only models.

RESULTS

A two-compartment model with first-order elimination best fit the data. The population busulfan clearance was 12.4 L/h for an adult male with 62 kg normal fat mass (equivalent to 70 kg total body weight). Busulfan clearance, scaled to body size-specifically normal fat mass, is predicted to be 95% of the adult clearance at 2.5 years postnatal age. With a target Css of 770 ng/mL, a higher proportion of initial doses achieved the therapeutic window with this age- and size--dependent model (72%) compared with dosing recommended by the U.S. Food and Drug Administration (57%) or the European Medicines Agency (70%).

CONCLUSION

This is the first population pharmacokinetic model developed to predict initial i.v. busulfan doses and personalize to a target Css over a wide age spectrum, ranging from infants to adults.

Physiologically based pharmacokinetic modelling of Busulfan: a new approach to describe and predict the pharmacokinetics in adults

PURPOSE

A physiologically based pharmacokinetic (PBPK) model was established and evaluated describing the pharmacokinetics (PK) of the DNA-alkylating agent Busulfan in adults in order to predict the systemic Busulfan drug exposure in both plasma and toxicity-related organs.

METHODS

A generic PBPK model was tailored to describe Busulfan PK by implementing compound-specific physicochemical and metabolism data. With regard to possible influences of glutathione S transferase (GST) variations on Busulfan PK, two different PBPK model parameterizations were investigated: a first parameterization with individual GST activity (expressed as different estimated V(max) values) for each patient, and a resulting second model parameterization with a mean GST activity for all patients. Simulations were computed and compared to concentration-time data after intravenous Busulfan administration to 108 adults serving as development dataset. Subsequently, appropriateness of the PBPK model was evaluated with an external dataset not used for model development, consisting of 95 adults.

RESULTS

Both PBPK model parameterizations of Busulfan successfully described the observed plasma concentrations. For the validation dataset, calculated PK parameters were as follows: clearance 0.16 ± 0.03 L/h/kg and volume of distribution 0.65 ± 0.06 L/kg (mean ± standard deviation). Mean absolute percentage error was less than 30 % for each PK parameter. Mass balances for distribution and excretion were in good agreement with the literature data.

CONCLUSIONS

Both PBPK model parameterizations sufficiently described the observed concentration-time data while showing an adequate predictive performance. The model should be further evaluated for its ability to explain the between-subject variability in intravenous Busulfan PK parameters.

Pharmacology of dimethanesulfonate alkylating agents: busulfan and treosulfan

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

A simple dosing scheme for intravenous busulfan based on retrospective population pharmacokinetic analysis in korean patients

Busulfan is an antineoplastic agent with a narrow therapeutic window. A post-hoc population pharmacokinetic analysis of a prospective randomized trial for comparison of four-times daily versus once-daily intravenous busulfan was carried out to search for predictive factors of intravenous busulfan (iBu) pharmacokinetics (PK). In this study the population PK of iBu was characterized to provide suitable dosing recommendations. Patients were randomized to receive iBu, either as 0.8 mg/kg every 6 h or 3.2 mg/kg daily over 4 days prior to hematopoietic stem cell transplantation. In total, 295 busulfan concentrations were analyzed with NONMEM. Actual body weight and sex were significant covariates affecting the PK of iBu. Sixty patients were included in the study (all Korean; 23 women, 37 men; mean [SD] age, 36.5 [10.9] years; weight, 66.5 [11.3] kg). Population estimates for a typical patient weighing 65 kg were: clearance (CL) 7.6 l/h and volume of distribution (V_d) 32.2 l for men and 29.1 L for women. Inter-individual random variabilities of CL and V_d were 16% and 9%. Based on a CL estimate from the final PK model, a simple dosage scheme to achieve the target AUC_0-inf (defined as median AUC_0-inf with a once-daily dosage) of 26.18 mg/l·hr, was proposed: 24.79·ABW^0.5 mg q24h, where ABW represents the actual body weight in kilograms. The dosing scheme reduced the unexplained interindividual variabilities of CL and Vd of iBu with ABW being a significant covariate affecting clearance of iBU. We propose a new simple dosing scheme for iBu based only on ABW.

Impact of obesity on drug metabolism and elimination in adults and children

The prevalence of obesity in adults and children is rapidly increasing across the world. Several general (patho)physiological alterations associated with obesity have been described, but the specific impact of these alterations on drug metabolism and elimination and its consequences for drug dosing remains largely unknown. In order to broaden our knowledge of this area, we have reviewed and summarized clinical studies that reported clearance values of drugs in both obese and non-obese patients. Studies were classified according to their most important metabolic or elimination pathway. This resulted in a structured review of the impact of obesity on metabolic and elimination processes, including phase I metabolism, phase II metabolism, liver blood flow, glomerular filtration and tubular processes. This literature study shows that the influence of obesity on drug metabolism and elimination greatly differs per specific metabolic or elimination pathway. Clearance of cytochrome P450 (CYP) 3A4 substrates is lower in obese as compared with non-obese patients. In contrast, clearance of drugs primarily metabolized by uridine diphosphate glucuronosyltransferase (UGT), glomerular filtration and/or tubular-mediated mechanisms, xanthine oxidase, N-acetyltransferase or CYP2E1 appears higher in obese versus non-obese patients. Additionally, in obese patients, trends indicating higher clearance values were seen for drugs metabolized via CYP1A2, CYP2C9, CYP2C19 and CYP2D6, while studies on high-extraction-ratio drugs showed somewhat inconclusive results. Very limited information is available in obese children, which prevents a direct comparison between data obtained in obese children and obese adults. Future clinical studies, especially in children, adolescents and morbidly obese individuals, are needed to extend our knowledge in this clinically important area of adult and paediatric clinical pharmacology.