Juliette Zwaveling

Association between Busulfan Exposure and Outcome in Children Receiving Intravenous Busulfan before Hematologic Stem Cell Transplantation

Busulfan, combined with therapeutic drug monitoring-guided dosing, is associated with higher event-free survival (EFS) rates due to fewer graft failures/relapses and lower toxicity. The optimal target area under the curve (AUC) and dosing schedule of intravenous busulfan in children undergoing hematopoietic stem cell transplantation (HSCT) remain unclear, however. We conducted a retrospective analysis of the association between busulfan exposure and clinical outcome in 102 children age 0.2 to 21 years who received busulfan 1 or 4 times daily before undergoing HSCT (46 malignant and 56 nonmalignant indications). EFS and overall survival after a median of 2 years of follow-up were 68% and 72%, respectively. EFS was optimal when the exposure of busulfan (AUC) was 78 mg x h/L (95% confidence interval=74 to 82 mg x h/L). Acute graft-versus-host disease (aGVHD) grade II-IV occurred more frequently with greater busulfan exposure. The addition of melphalan was an independent risk factor; melphalan use combined with high busulfan exposure (AUC >74 mg x h/L) was associated with high incidences of aGVHD (58%), veno-occlusive disease (66%), and mucositis grade III-IV (26%). Dosing frequency (1 or 4 times daily) was not related to any outcome. In conclusion, dose targeting of busulfan to a narrow therapeutic range was found to increase EFS in children. Adding melphalan to optimal busulfan exposure is associated with a high incidence of toxicity.

Body Weight-Dependent Pharmacokinetics of Busulfan in Paediatric Haematopoietic Stem Cell Transplantation Patients: Towards Individualized Dosing

AbstractBackground and Objectives: The wide variability in pharmacokinetics of busulfan in children is one factor influencing outcomes such as toxicity and event-free survival. A meta-analysis was conducted to describe the pharmacokinetics of busulfan in patients from 0.1 to 26 years of age, elucidate patient characteristics that explain the variability in exposure between patients and optimize dosing accordingly. Patients and Methods: Data were collected from 245 consecutive patients (from 3 to 100 kg) who underwent haematopoietic stem cell transplantation (HSCT) in four participating centres. The inter-patient, inter-occasion and residual variability in the pharmacokinetics of busulfan were estimated with a population analysis using the nonlinear mixed-effects modelling software NONMEM VI. Covariates were selected on the basis of their known or theoretical relationships with busulfan pharmacokinetics and were plotted independently against the individual pharmacokinetic parameters and the weighted residuals of the model without covariates to visualize relations. Potential covariates were formally tested in the model. Results: In a two-compartment model, body weight was the most predictive covariate for clearance, volume of distribution and inter-compartmental clearance and explained 65%, 75% and 40% of the observed variability, respectively. The relationship between body weight and clearance was characterized best using an allometric equation with a scaling exponent that changed with body weight from 1.2 in neonates to 0.55 in young adults. This implies that an increase in body weight in neonates results in a larger increase in busulfan clearance than an increase in body weight in older children or adults. Clearance on the first day was 12% higher than that of subsequent days (p < 0.001). Inter-occasion variability on clearance was 15% between the 4 days. Based on the final pharmacokinetic-model, an individualized dosing nomogram was developed. Conclusions: The model-based individual dosing nomogram is expected to result in predictive busulfan exposures in patients ranging between 3 and 65 kg and thereby to a safer and more effective conditioning regimen for HSCT in children.

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

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

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

Summary:We studied the pharmacokinetics of intravenous busulfan (Bu) in children in order to further optimize intravenous Bu dosing in relation to toxicity and survival. A total of 31 children undergoing Bu-based conditioning for allogeneic SCT were enrolled in a study. The starting dose was 1.0 mg/kg (age <4 years) and 0.8 mg/kg (age ⩾4 years), four doses per day during 4 days. Dose adjustment was allowed up to a maximum dose of 1.0 mg/kg per dose if the target area under the serum concentration–time curve (AUC) was not reached. Pharmacokinetic studies were performed after the first dose. Donor engraftment was established in 28 out of 31 patients. The average AUC after the first dose was the same in children <4 years as in children ⩾4 years. Mean clearance was higher in children <4 years than in children ⩾4 years. In 35% of all patients, total AUC was within the target AUC. The other childrens AUCs were below the target range. No relationships were found between systemic exposure to Bu and toxicity or clinical outcome. We concluded that, in accordance with previous data, within the observed AUCs no clear relationship was observed between Bu AUC and outcome with respect to toxicity, engraftment and relapse.

Association of busulfan exposure with survival and toxicity after haemopoietic cell transplantation in children and young adults: a multicentre, retrospective cohort analysis

BACKGROUND Intravenous busulfan combined with therapeutic drug monitoring to guide dosing improves outcomes after allogeneic haemopoietic cell transplantation (HCT). The best method to estimate busulfan exposure and optimum exposure in children or young adults remains unclear. We therefore assessed three approaches to estimate intravenous busulfan exposure (expressed as cumulative area under the curve [AUC]) and associated busulfan AUC with clinical outcomes in children or young adults undergoing allogeneic HCT. METHODS In this retrospective analysis, patients from 15 centres in the Netherlands, USA, Canada, Switzerland, UK, Italy, Germany, and Australia who received a busulfan-based conditioning regimen between March 18, 2001, and Feb 12, 2015, were included. Cumulative AUC was calculated by numerical integration using non-linear mixed effect modelling (AUCNONMEM), non-compartmental analysis (AUC from 0 to infinity [AUC0-∞] and to the next dose [AUC0-τ]), and by individual centres using various approaches (AUCcentre). The main outcome of interest was event-free survival. Other outcomes of interest were graft failure or relapse, or both; transplantation-related mortality; acute toxicity (veno-occlusive disease or acute graft versus-host disease [GvHD]); chronic GvHD; overall survival; and chronic-GvHD-free event-free survival. We used propensity-score-adjusted Cox proportional hazard models, Weibull models, and Fine-Gray competing risk regressions for statistical analyses. FINDINGS 790 patients were enrolled, 674 of whom were included: 274 (41%) with malignant and 400 (59%) with non-malignant disease. Median age was 4·5 years (IQR 1·4-10·7). The median busulfan AUCNONMEM was 74·4 mg × h/L (95% CI 31·1-104·6), which correlated with the standardised method AUC0-∞ (r2=0·74), but the latter correlated poorly with AUCcentre (r2=0·35). Estimated 2-year event-free survival was 69·7% (95% CI 66·2-73·0). Event-free survival at 2 years was 77·0% (95% CI 72·1-82·9) in the 257 patients with an optimum intravenous busulfan AUC of 78-101 mg × h/L compared with 66·1% (60·9-71·4) in the 235 patients at the low historical target of 58-86 mg × h/L and 49·5% (29·2-66·0) in the 44 patients with a high (>101 mg × h/L) busulfan AUC (p=0·011). Compared with the low AUC group, graft failure or relapse occurred less frequently in the optimum AUC group (hazard ratio [HR] 0·57, 95% CI 0·39-0·84; p=0·0041). Acute toxicity (HR 1·69, 1·12-2·57; p=0·013) and transplantation-related mortality (2·99, 1·82-4·92; p<0·0001) were significantly higher in the high AUC group (>101 mg × h/L) than in the low AUC group (<78 mg × h/L), independent of indication; no difference was noted between AUC groups for chronic GvHD (<78 mg × h/L vs ≥78 mg × h/L, HR 1·30, 95% CI 0·73-2·33; p=0·37). INTERPRETATION Improved clinical outcomes are likely to be achieved by targeting the busulfan AUC to 78-101 mg × h/L using a new validated pharmacokinetic model for all indications. FUNDING Research Allocation Program and the UCSF Helen Friller Family Comprehensive Cancer Center and the Mt Zion Health Fund of the University of California, San Francisco.

Personalized busulfan and treosulfan conditioning for pediatric stem cell transplantation: the role of pharmacogenetics and pharmacokinetics.

Busulfan- and treosulfan-based conditionings are the cornerstone of pediatric allogeneic hematopoietic stem cell transplantation (HSCT). Although both drugs are alkylating agents, their mechanisms of action, pharmacokinetics (PK) and toxicity profiles are different. Experience with busulfan in pediatric HSCT is broad and the knowledge on the pharmacodynamics (PD), PK and, to a lesser extent, pharmacogenetics (PG) has resulted in a more effective therapy. Treosulfan has only recently been introduced in pediatric HSCT and is considered a promising new therapy because of its beneficial toxicity profile. However, knowledge of the PK and PG of treosulfan is limited. In this review, we describe the pharmacology of both agents and discuss factors causing variability in PK in relation to therapeutic outcome in HSCT.

Effect of genetic polymorphisms in genes encoding GST isoenzymes on BU pharmacokinetics in adult patients undergoing hematopoietic SCT

BU is used in conditioning regimens before hemopoietic SCT. High BU exposure is associated with toxicity, whereas low BU exposure leads to higher rates of therapy failure. The pharmacokinetics of BU show large interpatient variability, hypothesized to be caused by variability in BU metabolism. In this report, the effect of genetic polymorphisms in three gluthatione S-transferase genes involved in BU metabolism (hGSTA1), GSTM1 (deletion–mutation) and GSTP1 (313A/G) on the pharmacokinetics of BU in Caucasian adult patients was investigated. In all, 66 adult patients received BU as part of their conditioning regimen. After the first infusion, two serum samples were collected and measured using a HPLC assay. A one-compartment population model was used to estimate individual pharmacokinetic parameters. The genetic variants of the three glutathione S-transferase (GST) genes were determined by pyrosequencing and PCR. A reduction of 14% in BU clearance was seen for the GSTA1*B allele and an increase in BU exposure was found. No relationship was found between polymorphisms in GSTM1 and GSTP1 and BU pharmacokinetics. This study shows that an increasing number of copies of GSTA1*B allele results in a significant decrease of BU clearance.

Exploratory analysis of 1936 Snps in Adme genes for association with busulfan clearance in adult hematopoietic stem cell recipients

Background Busulfan is used in preparative regimens before stem cell transplantation. There is significant interpatient variability in busulfan pharmacokinetics (PK) and exposure is related to outcome. Polymorphisms in genes encoding glutathione-S-transferases have been associated with busulfan PK but only explain a limited portion of the observed variability. Aim The aim of this study is to identify additional genetic variants associated with busulfan PK by interrogating 1936 variants in 225 genes involved in drug absorption, distribution, metabolism, and excretion (ADME). Materials and methods In an exploratory cohort (n=65), patients who received busulfan were genotyped with the DMET array. Top SNPs and haplotypes associated with busulfan clearance were validated in an independent validation cohort (n=78). Results In the exploratory cohort, seven variants were identified to be associated with busulfan clearance (P<0.001). In the validation cohort, only GSTA5 (rs4715354 and rs7746993) remained significantly associated with busulfan clearance (P=0.025). Conclusion This is the first study using an exploratory pharmacogenetic approach to explain the interindividual variability in busulfan PK. The role of glutathione-S-transferases was confirmed, but no additional genetic markers involved in drug ADME appear to be associated with busulfan PK.

Effect of genetic variants GSTA1 and CYP39A1 and age on busulfan clearance in pediatric patients undergoing hematopoietic stem cell transplantation

BACKGROUND Busulfan is used in preparative regimens prior to stem cell transplantation in pediatric patients. There is significant interpatient variability in busulfan pharmacokinetics (PK) and exposure is related to outcome. To date, only polymorphisms in genes encoding for glutathione-S-transferases were studied, but could only explain a small portion of the variability in PK. AIM To investigate the effect of seven genetic markers on busulfan clearance and the effect of ontogenesis on these genetic variants in a pediatric population. MATERIALS & METHODS In an earlier study of our group seven genetic markers in GSTA1, CYP2C19, CYP39A1, ABCB4, SLC22A4 and SLC7A8 were associated with busulfan clearance in adult patients. Eighty four pediatric patients were genotyped for these markers and genotype was associated with busulfan clearance. RESULTS & CONCLUSION GSTA1 and CYP39A1 were found to be associated with busulfan clearance. When combined, the two haplotypes explained 17% of the variability in busulfan clearance. Furthermore, the effect of GSTA1 haplotype on clearance was dependent on age.

PHC-014 Exploratory Analysis of 1,936 SNPs in 225 ADME Genes For Association with Busulfan Clearance in Adult Hematopoietic Stem Cell Recipients

Background Busulfan is used in preparative regimens prior to stem cell transplantation (SCT). There is significant inter-patient variability in busulfan pharmacokinetics (PK) and outcome is related to exposure. To date, only polymorphisms in genes encoding for glutathione-S-transferases have been studied; they could only explain a small portion of the variability in PK. Purpose To investigate the role of other genetic variants on busulfan clearance by interrogating 1,936 variants in 225 genes that are involved in drug absorption, distribution, metabolism and excretion (ADME). Materials and Methods 62 adult patients who received busulfan were genotyped using the Drug Metabolizing Enzymes and Transporters (DMET) array. Busulfan clearance was estimated with a limited sampling (t = 2.5, 4 hrs) PK model. Individual SNPs were associated with busulfan clearance. Top SNPs and haplotypes were replicated in an independent cohort (n = 78). Results In the discovery cohort 7 variants (3 SNPs and 4 haplotypes) explained 64% (adjusted R2) of variance in busulfan clearance (p < 0.001). These genetic variants, located in GSTA5, CYP2C19, CYP39A1 (2 haplotypes), ABCB4, SLC22A4 and SLC7A8, were replicated in the second cohort. One haplotype in GSTA5 (rs4715354 and rs7746993) remained statistically significant (P = 0.025) for correlation with busulfan clearance. Conclusions This is the first study using an exploratory pharmacogenetic approach in 225 genes involved in ADME to explain the inter-individual variability in busulfan clearance. The GSTA5 haplotype was significantly correlated with busulfan clearance, both in the discovery and replication cohort. No additional genetic markers involved in drug metabolism and transport appear to be associated with busulfan clearance. No conflict of interest.