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

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

BACKGROUND

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

METHODS

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

RESULTS

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

CONCLUSIONS

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

Covariate pharmacokinetic model building in oncology and its potential clinical relevance

When modeling pharmacokinetic (PK) data, identifying covariates is important in explaining interindividual variability, and thus increasing the predictive value of the model. Nonlinear mixed-effects modeling with stepwise covariate modeling is frequently used to build structural covariate models, and the most commonly used software-NONMEM-provides estimations for the fixed-effect parameters (e.g., drug clearance), interindividual and residual unidentified random effects. The aim of covariate modeling is not only to find covariates that significantly influence the population PK parameters, but also to provide dosing recommendations for a certain drug under different conditions, e.g., organ dysfunction, combination chemotherapy. A true covariate is usually seen as one that carries unique information on a structural model parameter. Covariate models have improved our understanding of the pharmacology of many anticancer drugs, including busulfan or melphalan that are part of high-dose pretransplant treatments, the antifolate methotrexate whose elimination is strongly dependent on GFR and comedication, the taxanes and tyrosine kinase inhibitors, the latter being subject of cytochrome p450 3A4 (CYP3A4) associated metabolism. The purpose of this review article is to provide a tool to help understand population covariate analysis and their potential implications for the clinic. Accordingly, several population covariate models are listed, and their clinical relevance is discussed. The target audience of this article are clinical oncologists with a special interest in clinical and mathematical pharmacology.

Weight-based strategy of dose administration in children using intravenous busulfan: clinical and pharmacokinetic results

BACKGROUND

A prospective clinical trial was performed in order to validate the pharmacokinetic (PK) and clinical benefits of a new dosing schedule of intravenous busulfan (IV Bu) in children.

PROCEDURE

IV Bu was administered as a 2-hr infusion every 6 hr for 4 days. Five dose levels were given according to body-weight strata.

RESULTS

The 67 children aged from 4 months to 17.2 years were followed up over 50 months after autologous or allogeneic stem-cell transplantation. Reduced PK variability was seen after IV Bu administration enabling efficient targeting with 78% of patients within the 900-1,500 µM · min therapeutic window and reproducible exposures across administrations. No neurological complications occurred. The low incidence of hepatic veno-occlusive disease (VOD) recorded was not correlated with high area under the curve (AUC). Only stomatitis was correlated with high AUC in the autologous group. The 4-year overall survival was 59% in the autologous group and 82% in the allogeneic group.

CONCLUSION

The new dosing schedule using IV Bu provides adequate therapeutic targeting from the first administration, with low toxicity and good disease control in high-risk children. The choice of this formulation of Bu should be considered because of its low morbidity and good outcome.

Optimal sampling strategy development methodology using maximum a posteriori Bayesian estimation

Maximum a posteriori Bayesian (MAPB) pharmacokinetic parameter estimation is an accurate and flexible method of estimating individual pharmacokinetic parameters using individual blood concentrations and prior information. In the past decade, many studies have developed optimal sampling strategies to estimate pharmacokinetic parameters as accurately as possible using either multiple regression analysis or MAPB estimation. This has been done for many drugs, especially immunosuppressants and anticancer agents. Methods of development for optimal sampling strategies (OSS) are diverse and heterogeneous. This review provides a comprehensive overview of OSS development methodology using MAPB pharmacokinetic parameter estimation, determines the transferability of published OSSs, and compares sampling strategies determined by MAPB estimation and multiple regression analysis. OSS development has the following components: 1) prior distributions; 2) reference value determination; 3) optimal sampling time identification; and 4) validation of the OSS. Published OSSs often lack all data necessary for the OSS to be clinically transferable. MAPB estimation is similar to multiple regression analysis in terms of predictive performance but superior in flexibility.

Population pharmacokinetics of busulfan in children: increased evidence for body surface area and allometric body weight dosing of busulfan in children

PURPOSE

To evaluate the best method for dosing busulfan in children, we retrospectively analyzed two different data sets from three different dosing regimens by means of population pharmacokinetics using NONMEM.

EXPERIMENTAL DESIGN

The development data set consisted of plasma samples from 94 children, in the age range of 0.4 to 18.8 years, receiving either oral or intravenous busulfan. The external model evaluation data set comprised 24 children, in the age range of 0.1 to 18.9 years, who belonged to the once-daily intravenous busulfan dosing regimen. A one-compartment model with first-order absorption using body surface area (BSA) or allometric body weight (BW) as covariate on clearance (CL) and BW as covariate on volume of distribution (V) were used to describe the results sufficiently. In addition to interindividual variability on all pharmacokinetic parameters, interoccasion variability was included for CL and V.

RESULTS

CL values in the present study did not reflect the shape of the CL versus weight curve reported in previous investigations. By external model evaluation, we were able to confirm these findings. Furthermore, bioavailability was calculated to be between 93% and 99% for the development data set. On the basis of the final models, we simulated two dosing schemes according to allometric BW and BSA showing that we estimated to include about 30% more patients into the proposed therapeutic area under the curve (AUC) range of 900 to 1,500 μM*min and could, furthermore, achieve a reduction in the AUC variability when dosed according to the labeled European Medicines Agency (EMA) dosing recommendation.

CONCLUSION

We recommend a BSA or an allometric BW dosing regimen for individualizing busulfan therapy in children to reduce variability in busulfan exposure and to improve safety and efficacy of busulfan treatment.

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

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

Intravenous compared with oral busulfan as preparation for allogeneic hematopoietic progenitor cell transplantation for AML and MDS

BU and CY is a common conditioning regimen for allogeneic hematopoietic progenitor cell transplantation (HPCT). I.v. BU is increasingly used in place of the oral formulation for conditioning. We compared the outcomes of 135 consecutively treated AML and myelodysplastic syndrome patients who underwent allogeneic HPCT at our institution with BUCY2 using oral (n=93) or i.v. (n=42) BU, without dose adjustment. The i.v. BU patients had a lower incidence of any severity of oral mucositis (3 versus 55%, P=0.002) and severe mucositis (3 versus 24%, P=0.005). Other post transplant outcomes were comparable between the groups. In all 26 i.v. BU and 33 oral BU patients are alive; however, the median follow-up was significantly longer for the oral BU group. One- and two-year non-relapse mortality for the i.v. BU patients was 21% for both, and for the oral BU group was 23% and 29%, respectively. One- and two-year relapse mortality for the i.v. BU patients was 21% for both, and for the oral BU group was 24% and 29%, respectively. Substituting i.v. for oral BU reduces variability in drug exposure and potentially improves toxicity as suggested by our finding of significantly less oral mucositis and decreased severity with i.v. BU.

Accurate targeting of daily intravenous busulfan with 8-hour blood sampling in hospitalized adult hematopoietic cell transplant recipients

Daily intravenous (i.v.) busulfan is increasingly being used in hematopoietic cell transplantation (HCT) conditioning regimens. Intravenous busulfan doses administered at the traditional frequency of every 6 hours can be targeted ((T)Bu) to a patient-specific concentration at steady state (C(ss)) using therapeutic drug monitoring (TDM). In this report, we describe our experiences with TDM of daily i.v. busulfan in an adult population, with the specific aims of (1) evaluating covariates associated with busulfan clearance, and (2) assessing the feasibility of TDM for outpatient administration of daily (T)Bu with pharmacokinetic sampling over 6 hours. A retrospective pharmacokinetic analysis was conducted in 87 adults receiving daily (T)Bu as part of cyclophosphamide followed by (T)BU (CY/(T)BU), fludarabine monophosphate (fludarabine) followed by (T)BU, or (T)BU concurrent with fludarabine conditioning. The desired C(ss) was achieved in 85% of patients receiving daily i.v. busulfan. Busulfan clearance was not associated with sex or age, but was associated with the day of dosing and conditioning regimen (P = .0016). In patients receiving CY/(T)BU, no differences in clearance were found between dosing days (P > .36); however, clearance decreased significantly in patients receiving fludarabine-based regimens (P = .0016). Busulfan clearance and C(ss) estimates from pharmacokinetic sampling over 8, 11, or 24 hours were comparable (P > .4). However, pharmacokinetic modeling of individual patient concentration-time data over 6 hours could not reliably estimate busulfan clearance or C(ss).

Phase-2 trial of an intensified conditioning regimen for allogeneic hematopoietic cell transplant for poor-risk leukemia

Patients with poor-risk leukemia have a high relapse rate despite allogeneic transplant. We report on the phase II trial of an intensified allogeneic transplant regimen whose aim was tolerable toxicity and durable remission. Study patients (n=30) had unfavorable first remission cytogenetics, progression from myelodysplasia or active disease due to induction failure or relapse. Conditioning was intravenous busulfan, targeted to a first-dose plasma area under the curve (AUC) of 700–900 µM·min, VP-16 at 30 mg/kg of adjusted ideal body weight and fractionated total body irradiation (FTBI) at 1200 cGy in ten fractions. Graft-versus-host disease (GVHD) prophylaxis was cyclosporine A and mycophenolate mofetil. Regimen-related toxicities (Bearman) included grade 3 mucositis in 29 patients (97%) and grade 4 in one, grade 2–3 sinusoidal obstructive syndrome in 2 patients (7%), and grade 2–3 skin toxicity in 8 patients (27%). The 30- and 100-day transplant-related mortalities were 0% and 7% respectively. The median follow-up was 83.7 months (60.7–96.4) for surviving patients. The 5-yr overall and disease-free survival was 40% for all patients. Cumulative 5-yr relapse incidence was 23% and transplant-related mortality was 37%. We have shown promising overall survival and relapse incidence in these poor-risk patients, who typically have few curative options.

Influence of glutathione S-transferase A1, P1, M1, T1 polymorphisms on oral busulfan pharmacokinetics in children with congenital hemoglobinopathies undergoing hematopoietic stem cell transplantation

BACKGROUND

Busulfan (BU), often used in high dose for myeloablation before hematopoietic stem cell transplantation (HSCT), has been implicated in certain HSCT toxicities, including the occurrence of hepatic veno-occlusive disease (HVOD). In addition to weight and age, gene polymorphisms in specific members of the glutathione-transferase (GST) gene family (A1, P1, M1, and T1), involved in BU metabolism, may play a role in the wide inter-patient variability in systemic BU concentrations.

PROCEDURE

The present study integrated clinical data regarding the occurrence of HVOD, graft versus host disease (GVHD), BU pharmacokinetics and GSTA1, GSTP1, GSTM1, and GSTT1 genotypes of 18 children who received BU in their pre-HSCT conditioning regimen. The children were all treated for congenital hemoglobinopathies and were all of Arab Moslem descent.

RESULTS

The data demonstrate an association between GSTA1 and GSTP1 genotypes and BU-maximal concentration (C(max)) (P = 0.01, P = 0.02, respectively), area under the concentration-time curve (AUC) (P = 0.02, P = 0.01, respectively) and oral BU clearance/kg body weight (P < 0.02, P = 0.08, respectively). GSTM1-null individuals demonstrated lower BU-AUC/Kg compared to GSTM1-positive individuals. In addition, an association between GVHD and GSTM1-null genotype was found.

CONCLUSIONS

GSTA1, GSTP1, and GSTM1 genotyping prior to HSCT in children with congenital hemoglobinopathies may allow better prediction of oral BU kinetics and the need for BU dose adjustment, as well as prediction of transplant related toxicity such as GVHD, thereby improving clinical outcome.

Effect of obesity on the pharmacokinetics of drugs in humans

The prevalence of obesity has dramatically increased in recent years and now includes a significant proportion of the world's children, adolescents and adults. Obesity is linked to a number of co-morbidities, the most prominent being type 2 diabetes mellitus. While many agents are available to treat these conditions, the current knowledge regarding their disposition in the obese remains limited. Over the years, both direct and indirect methodologies have been utilized to assess body composition. Commonly used direct measures include underwater weighing, skinfold measurement, bioelectrical impedance analysis and dual-energy x-ray absorptiometry. Unfortunately, these methods are not readily available to the majority of clinicians. As a result, a number of indirect measures to assess body composition have been developed. Indirect measures rely on patient attributes such as height, bodyweight and sex. These size metrics are often utilized clinically and include body mass index (BMI), body surface area (BSA), ideal bodyweight (IBW), percent IBW, adjusted bodyweight, lean bodyweight (LBW) and predicted normal weight (PNWT). An understanding of how the volume of distribution (V(d)) of a drug changes in the obese is critical, as this parameter determines loading-dose selection. The V(d) of a drug is dependent upon its physiochemical properties, the degree of plasma protein binding and tissue blood flow. Obesity does not appear to have an impact on drug binding to albumin; however, data regarding alpha(1)-acid glycoprotein binding have been contradictory. A reduction in tissue blood flow and alterations in cardiac structure and function have been noted in obese individuals. At the present time, a universal size descriptor to describe the V(d) of all drugs in obese and lean individuals does not exist. Drug clearance (CL) is the primary determinant to consider when designing a maintenance dose regimen. CL is largely controlled by hepatic and renal physiology. In the obese, increases in cytochrome P450 2E1 activity and phase II conjugation activity have been observed. The effects of obesity on renal tubular secretion, tubular reabsorption, and glomerular filtration have not been fully elucidated. As with the V(d), a single, well validated size metric to characterize drug CL in the obese does not currently exist. Therefore, clinicians should apply a weight-normalized maintenance dose, using a size descriptor that corrects for differences in absolute CL between obese and non-obese individuals. The elimination half-life (t((1/2))) of a drug depends on both the V(d) and CL. Since the V(d) and CL are biologically independent entities, changes in the t((1/2)) of a drug in obese individuals can reflect changes in the V(d), the CL, or both. This review also examines recent publications that investigated the disposition of several classes of drugs in the obese--antibacterials, anticoagulants, antidiabetics, anticancer agents and neuromuscular blockers. In conclusion, pharmacokinetic data in obese patients do not exist for the majority of drugs. In situations where such information is available, clinicians should design treatment regimens that account for any significant differences in the CL and V(d) in the obese.

Development of a population pharmacokinetics-based sampling schedule to target daily intravenous busulfan for outpatient clinic administration

Therapeutic drug monitoring of daily intravenous (IV) busulfan currently requires hospital admission. Population pharmacokinetic modeling and determination of an optimal pharmacokinetic sampling schedule over 6 hours could allow for personalizing these busulfan doses in the outpatient clinic. A retrospective evaluation of daily IV busulfan pharmacokinetics was conducted in 37 adults. SPK and NONMEM software were used to estimate the population pharmacokinetic parameters. Subsequent to model building, the area under the concentration-time curve (AUC) was computed using NONMEM. A 1-compartment model best fit the data. The optimal 6-hour outpatient sampling schedule was constructed using a simulation approach that sought to minimize scaled mean squared error for the clearance and volume parameters for each simulated individual. The best sampling times were 2.75, 3, 3.25, 5.5, 5.75, and 6 hours from the start of a 3-hour infusion. With these sampling times, the maximum a posteriori (MAP) Bayesian estimation was superior to maximum likelihood estimation with more samples. An individual patient's busulfan AUC and pharmacokinetic parameters may be accurately estimated with an outpatient sampling schedule that is used in conjunction with MAP Bayesian estimation, with a parameter prior based on population pharmacokinetic modeling. Prospective validation of this approach is needed.

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

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

Busulfan in hematopoietic stem cell transplant setting

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

Integration of modelling and simulation into the development of intravenous busulfan in paediatrics: an industrial experience

Busulfan (Bu) is commonly used in preparative conditioning regimen prior to bone marrow transplantation in infants (< 1 year old), children and adolescents (up to 17 years old). The clinical development of an intravenous form of busulfan (Busilvex) was based on pharmacokinetic (PK) modeling and simulation techniques. A retrospective population PK analysis was initially performed from a first study in 24 pediatric patients (0.45-16.7 years old) and a log-linear relationship between body weight and Busilvex clearance was demonstrated with no age-dependency. For an optimal area under the curve (AUC) targeting, a new Bu dosing regimen [i.e. 5 dose levels (0.80 to 1.20 mg/kg) adjusted to 5 discrete weight categories] was developed and assessed through population PK-based simulations. The benefit from this new dosing strategy was validated in a second trial including 55 children (0.30-17.2 years old). This prospective trial confirmed the previous simulations: an efficient therapeutic targeting whatever the patient's age or body weight. Over 80% of the children were within the desired plasma exposure window, and the initial PK model was validated on the confirmatory dataset.

Once daily i.v. busulfan and fludarabine (i.v. Bu-Flu) compares favorably with i.v. busulfan and cyclophosphamide (i.v. BuCy2) as pretransplant conditioning therapy in AML/MDS

We postulated that fludarabine (Flu) instead of cyclophosphamide (Cy) combined with i.v. busulfan (Bu) as preconditioning for allogeneic hematopoietic stem cell transplantation (HSCT) would improve safety and retain antileukemic efficacy. Sixty-seven patients received BuCy2, and subsequently, 148 patients received Bu-Flu. We used a Bayesian method to compare outcomes between these nonrandomized patients. The groups had comparable pretreatment characteristics, except that Bu-Flu patients were older (46 versus 39 years, P < .01), more often had unrelated donors (47.3% versus 20.9%, P < .0003), and had shorter median follow-up (39.7 versus 74.6 months). To account for improved supportive care and other unidentified factors that may affect outcome ("period" effects), 78 acute myelogenous leukemia (AML) patients receiving Melphalan-Flu (MF), treated in parallel during this time (1997-2004) were used to estimate the period effect. The MF patients' outcomes worsened during this period. Therefore, the period effect is unlikely to explain the greatly improved outcome with Bu-Flu. Patients transplanted with Bu-Flu in the first complete remission (CR1) had a 3-year overall survival and event-free-survival (EFS) of 78% and 74%, respectively, whereas CR1 patients younger than age 41 had a 3-year EFS of 83%. These results support replacing BuCy +/- ATG with Bu-Flu +/- rabbit-antithymocyte globulin (ATG), and warrant a prospective comparison between allogeneic HSCT and conventional induction/consolidation chemotherapy for AML in CR1.

High busulfan exposure is associated with worse outcomes in a daily i.v. busulfan and fludarabine allogeneic transplant regimen

Low plasma busulfan (Bu) area under the concentration-time curve (AUC) is associated with graft failure and relapsed leukemias, and high AUC with toxicities when Bu is used orally or i.v. 4 times daily combined with cyclophosphamide in myeloablative hematopoietic stem cell transplantation (SCT) conditioning regimens. We report Bu AUC and its association with clinical outcomes in 130 patients with hematologic malignancies given a once-daily i.v. Bu (3.2 mg/kg days -5 to -2) and fludarabine (Flu, 50 mg/m(2) days -6 to -2) regimen. Total-body irradiation (TBI) 200 cGy x 2 was added for 51 patients with acute leukemias. Plasma AUC varied 3.6-fold (2184-7794 microM.min, median 4699 microM.min). Patients with an AUC >6000 microM.min had lower overall survival (OS) than those with AUC < or =6000 microM.min at 12 months (38% versus 74%) and 36 months (23% versus 68%, P < .001). This effect was apparent in patients with standard-risk and high-risk disease, and persisted when potential confounders were considered (hazard ratio 3.2, 95% confidence interval 1.7-6.3). Nonrelapse mortality (NRM) at 100 days (6% versus 19%) and progression free survival (PFS; 58% versus 16%) at 3 years were better with AUC < or =6000 microM.min. These data support a role for therapeutic dose monitoring and dose adjustment with daily i.v. busulfan.

Successful unmanipulated peripheral blood progenitor cell transplantation from an HLA haploidentical 2-locus-mismatched mother in a thalassemic patient with primary graft failure after transplantation of bone marrow and cord blood from unrelated donors

We report on a boy with beta-thalassemia major who developed early graft failure after double-unit unrelated cord blood transplantation, who subsequently received a myeloablative preconditioning regimen using non-T-cell-depleted PBSCT from his HLA-haploidentical 2-loci-mismatched mother. Neutrophil recovery with full donor chimerism was observed at post-transplantation day +11. Furthermore, GVHD was easy to control. The patient was transfusion-independent with complete donor chimerism eight months post-transplant. The result indicated that fetomaternal microchimerism may be an important attribute of a successful transplant. We suggested that a third allo-HSCT may be taken into consideration for patients with transfusion-dependent thalassemia who experience graft failure, even after two previous transplants.

Pharmacokinetic disposition and clinical outcomes in infants and children receiving intravenous busulfan for allogeneic hematopoietic stem cell transplantation

We conducted a retrospective pharmacokinetic analysis of i.v. busulfan in children undergoing hematopoietic stem cell transplantation (HSCT) and describe its relation to transplantation outcomes. Forty-five children (median age, 3 yr) underwent HSCT at The Hospital for Sick Children from April 2003 through January 2006 and received i.v. busulfan every 6 h as part of their conditioning regimen. Initial busulfan doses were based on actual patient weight: <9 kg, 0.95 mg/kg per dose; 9-16 kg, 1.2 mg/kg per dose; 16-23 kg, 1.1 mg/kg per dose; 24-34 kg, 0.95 mg/kg per dose; >34 kg, 0.8 mg/kg per dose. Plasma busulfan concentrations were obtained after the first dose. The fourth and subsequent busulfan doses were adjusted to achieve an area under the concentration versus time curve (AUC) of 900-1500 microM.min. Development of hepatic venous occlusive disease (HVOD; modified Baltimore criteria) and engraftment (absolute neutrophil count >or=0.5 x 10(9)/L) were evaluated. Busulfan pharmacokinetic parameters were calculated using 1-compartment methods. Mean busulfan pharmacokinetic parameters were maximum concentration (C(max); 4.7 +/- 0.75 microM), volume of distribution at steady state (0.68 +/- 0.17 L/kg), elimination rate constant (0.0051 +/- 0.0010 min(-1)), total body clearance (3.5 +/- 1.23 mL/[min.kg]), and AUC (1271 +/- 280 microM.min). Mean volume of distribution at steady state was larger in children <1 yr of age (0.77 +/- 0.24 vs 0.64 +/- 0.11 L/kg; P = .040) and children <4 yr of age (0.73 +/- 0.18 vs 0.60 +/- 0.11 L/kg; P = .001) than in older children. Compared with older children, mean weight-adjusted total body clearance was higher in children <4 yr of age (3.8 +/- 1.40 versus 3.0 +/- 0.76 mL/[min.kg]). HVOD was diagnosed in 8 children (18%), including 4 children <1 yr of age. Children who developed HVOD achieved a lower C(max) than did those without HVOD (4.2 +/- 0.68 versus 4.8 +/- 0.73 microM; P = .035). Other than C(max), no association was observed between busulfan disposition and development of HVOD in children for whom i.v. busulfan doses were adjusted to achieve a target AUC. The influence of factors other than busulfan disposition on transplantation outcomes, such as genetic polymorphisms, should be evaluated.

Prospective validation of a novel IV busulfan fixed dosing for paediatric patients to improve therapeutic AUC targeting without drug monitoring

INTRODUCTION

Oral busulfan clearance is age-dependent and children experience a wide variability in plasma exposure. BSA- or age-based dosing is used with therapeutic drug monitoring (TDM) to reduce this variability.

PURPOSE

A new intravenous (IV) dosing of busulfan (Bu) based on body weight, designed to improve AUC targeting without TDM and dose-adjustment, was prospectively evaluated.

METHOD

Bu was administered as a 2 h IV infusion every 6 h over 4 days (16 administrations). Five dose levels were defined on body weight as follows: 1.0 mg/kg for <9 kg; 1.2 mg/kg for 9 to <16 kg; 1.1 mg/kg for 16-23 kg; 0.95 mg/kg for >23-34 kg; 0.80 mg/kg for >34 kg. Bu treatment was followed by Cyclophosphamide or Melphalan prior to allogeneic or autologous transplantation in 55 children aged 0.3-17.2 years (median 5.6 years).

RESULTS

No difference in AUC values was observed between weight strata (mean +/- SD 1248 +/- 205 micromol.min), whereas a significant difference in Bu clearance was demonstrated. This new dosing enabled to achieve a mean exposure comparable to that in adults. At dose 1, 91% of patients achieved the targeted AUC range (900-1500 micromol.min) while no patients were underexposed. At doses 9 and 13, over 75% of patients remained within that target whilst most of the others were slightly above. Successful engraftment was achieved in all patients. In conclusion, from infants to adults this new dosing enabled, without TDM and dose adjustment, to successfully target a therapeutic AUC window.

Elevated plasma ferritin and busulfan pharmacodynamics during high-dose chemotherapy regimens in children with malignant solid tumors

Hepatic veno-occlusive disease (HVOD) is a frequent complication during hematopoietic stem-cell transplantation (HSCT). A strong relationship has been demonstrated between busulfan exposure and HVOD for busulfan-cyclophosphamide and allogeneic HSCT in adults. Busulfan disposition after the first intake was studied in 77 children treated for solid malignancies with high-dose busulfan-containing regimens and autologous HSCT. Busulfan was combined with cyclophosphamide and melphalan (n=30), melphalan (n=27), and thiotepa (n=20). No relationship was observed between busulfan exposure and HVOD. In contrast, plasma ferritin at baseline was higher in patients with HVOD (750 ng/ml (20-3,110)) compared with those without HVOD (189 ng/ml (8-3,967), P=0.012). Multivariate analysis showed that a ferritin level exceeding 300 ng/ml was the only risk factor for HVOD with an odds ratio of 4.0 (confidence interval 95% (1.5-11.2), P=0.0071). A high ferritin level at baseline was explained by the diagnosis of neuroblastoma, related treatments and transfusions.