An improved limited sampling method for individualised busulphan dosing in bone marrow transplantation in children

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.

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

BACKGROUND

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

PROCEDURE

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

RESULTS

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

CONCLUSION

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

Impact of GSTA1 Polymorphisms on Busulfan Oral Clearance in Adult Patients Undergoing Hematopoietic Stem Cell Transplantation

Background: Busulfan pharmacokinetics exhibit large inter-subject variability. Our objective was to evaluate the influence of glutathione S-transferase A1 (GSTA1) gene variants on busulfan oral clearance (CLo) in a population of patients undergoing hematopoietic stem cell transplantation. Methods: This is a quasi-experimental retrospective study in adult patients (n = 87 included in the final analyses) receiving oral busulfan. Pharmacokinetics data (area under the plasma concentration-time curve (AUC) determined from 10 blood samples) were retrieved from patients’ files and GSTA1 *A and *B allele polymorphisms determined from banked DNA samples. Three different limited sampling methods (LSM) using four blood samples were also compared. Results: Carriers of GSTA1*B exhibited lower busulfan CLo than patients with an *A/*A genotype (p < 0.002): Busulfan CLo was 166 ± 31, 187 ± 37 vs. 207 ± 47 mL/min for GSTA1*B/*B,*A/*B and *A/*A genotypes, respectively. Similar results were obtained with the tested LSMs. Using the standard AUC method, distribution of patients above the therapeutic range after the first dose was 29% for GSTA1*A/*A, 50% for *A/*B, and 65% for *B/*B. The LSMs correctly identified ≥91% of patients with an AUC above the therapeutic range. The misclassified patients had a mean difference less than 5% in their AUCs. Conclusion: Patients carrying GSTA1 loss of function *B allele were at increased risk of overdosing on their initial busulfan oral dose. Genetic polymorphisms associated with GSTA1 explain a significant part of busulfan CLo variability which could be captured by LSM strategies.

Evaluation of Limited Sampling Methods for Oral Busulfan Pharmacokinetic Monitoring in Adult Patients Undergoing Hematopoietic Stem Cell Transplantation

BACKGROUND

Monitoring busulfan area under the plasma concentration-time curve (AUC) to establish the dose regimen for stem cell transplantation desirable to achieve efficacy while avoiding toxicity.

OBJECTIVE

Our objective was to compare AUCs calculated by 18 limited sampling methods (LSMs) from 2 to 5 samples to reference AUCs determined from 10 samples in a retrospective study of 103 adult patients receiving oral busulfan. LSMs using 2 or 3 samples were ineffective.

METHODS

Four LSMs using 4 or 5 blood samples that accurately characterized busulfan AUC were identified. The best 2 methods were obtained with sampling at 0.5, 1, 2, 4, and 6 hours and after 1, 1.5, 2, 4, and 6 hours postdose. For these LSMs, the incidence of 20% difference between AUCs from LSMs and reference AUCs was less than 1.3%.

CONCLUSIONS

Effective and safe determination of AUC for oral busulfan can be made with strategies using only 4 or 5 concentration timepoints.

Busulfan dosing algorithm and sampling strategy in stem cell transplantation patients

AIM

The aim of this investigation was to develop a model-based dosing algorithm for busulfan and identify an optimal sampling scheme for use in routine clinical practice.

METHODS

Clinical data from an ongoing study (n = 29) in stem cell transplantation patients were used for the purposes our analysis. A one compartment model was selected as basis for sampling optimization and subsequent evaluation of a suitable dosing algorithm. Internal and external model validation procedures were performed prior to the optimization steps using ED-optimality criteria. Using systemic exposure as parameter of interest, dosing algorithms were considered for individual patients with the scope of minimizing the deviation from target range as determined by AUC(0,6 h).

RESULTS

Busulfan exposure after oral administration was best predicted after the inclusion of adjusted ideal body weight and alanine transferase as covariates on clearance. Population parameter estimates were 3.98 h(-1), 48.8 l and 12.3 l h(-1) for the absorption rate constant, volume of distribution and oral clearance, respectively. Inter-occasion variability was used to describe the differences between test dose and treatment. Based on simulation scenarios, a dosing algorithm was identified, which ensures target exposure values are attained after a test dose. Moreover, our findings show that a sparse sampling scheme with five samples per patient is sufficient to characterize the pharmacokinetics of busulfan in individual patients.

CONCLUSION

The use of the proposed dosing algorithm in conjunction with a sparse sampling scheme may contribute to considerable improvement in the safety and efficacy profile of patients undergoing treatment for stem cell transplantation.

Comparison of algorithms for oral busulphan area under the concentration-time curve limited sampling estimate

BACKGROUND AND OBJECTIVES

Therapeutic drug monitoring (TDM) of the first dose of busulphan during conditioning prior to allogeneic stem cell transplantation provides the possibility of improving the clinical outcome via dose adjustment of subsequent doses. The plasma area under the concentration-time curve (AUC) for busulphan is generally accepted as the parameter that gives the best exposure estimate; however, the sampling frequency needed for reliable AUC calculation remains controversial. The aim of the present investigation was to develop and evaluate a limited sampling model for oral busulphan.

METHODS

We have compared models using three to four samples with standard WinNonlin(®) adaptive compartment modeling based on eight samples as reference. The evaluated study population included both adult and pediatric patients, but the linear model was devised using analysis of only pediatric patient plasma concentrations. The present model was developed using data from 23 patients with a mean age of 38 years (range 13-59 years) and was evaluated in 20 pediatric patients with a mean age of 6 years (range 0.1-13 years) as well as 23 adult patients (mean age 43 years; range 18-67 years).

RESULTS

In 23 patients, the mean AUC from a curve fitting model (Purves method) and a single compartment model had an intraclass correlation coefficient (ICC) of 0.947. From a log-log plot of AUC values it was evident that using this estimate of the AUC would affect dose adjustment decisions for very few of the patients. Applying the linear model using three samples resulted in an ICC of 0.932, mostly due to worse performance in the adult population.

CONCLUSIONS

The present results support the use of limited sampling in clinical TDM for oral busulphan provided adequate algorithms and sampling times are used. Moreover, they also demonstrate the caution that is needed when transferring a pharmacokinetic model from a pediatric population to an adult population.

Limited sampling strategies to estimate the area under the concentration-time curve. Biases and a proposed more accurate method

BACKGROUND

Over 100 limited sampling strategies (LSSs) have been proposed to reduce the number of blood samples necessary to estimate the area under the concentration-time curve (AUC). The conditions under which these strategies succeed or fail remain to be clarified.

OBJECTIVES

We investigated the accuracy of existing LSSs both theoretically and numerically by Monte Carlo simulation. We also proposed two new methods for more accurate AUC estimations.

METHODS

We evaluated the following existing methods theoretically: i) nonlinear curve fitting algorithm (NLF), ii) the trapezium rule with exponential curve approximation (TZE), and iii) multiple linear regression (MLR). Taking busulfan (BU) as a test drug, we generated a set of theoretical concentration-time curves based on the identified distribution of pharmacokinetic parameters of BU and re-evaluated the existing LSSs using these virtual validation profiles. Based on the evaluation results, we improved the TZE so that unrealistic parameter values were not used. We also proposed a new estimation method in which the most likely curve was selected from a set of pre-generated theoretical concentration-time curves.

RESULTS

Our evaluation, based on clinical profiles and a virtual validation set, revealed: i) NLF sometimes overestimated the absorption rate constant Ka, ii) TZE overestimated AUC over 280% when Ka is small, and iii) MLR underestimated AUC over 30% when the elimination rate constant Ke is small. These results were consistent with our mathematical evaluations for these methods. In contrast, our two new methods had little bias and good precision.

CONCLUSIONS

Our investigation revealed that existing LSSs induce different but specific biases in the estimation of AUC. Our two new LSSs, a modified TZE and one using model concentration-time curves, provided accurate and precise estimations of AUC.

A systematic review of limited sampling strategies for platinum agents used in cancer chemotherapy

Despite evidence in the literature suggesting that a strong correlation exists between the pharmacokinetic parameters and pharmacodynamic effect of anticancer agents, many of these agents are still dosed by body surface area. Therapeutic drug monitoring with the aim of pharmacokinetic-guided dosing would not only maintain target concentrations associated with efficacy but may potentially minimise the likelihood of dose-related systemic toxicities. The pharmacokinetic parameter that displays the best correlation with the pharmacodynamics of anticancer drugs is the area under the plasma concentration-time curve (AUC). However, accurate determination of the AUC requires numerous blood samples over an extended interval, which is not feasible in clinical practice. Therefore, limited sampling strategies (LSSs) have been proposed as a means to accurately and precisely estimate pharmacokinetic parameters with a minimal number of blood samples. LSSs have been developed for many drugs, particularly ciclosporin and other immunosuppressants, as well as for certain anticancer drugs. This systematic review evaluates LSSs developed for the platinum compounds and categorises 18 pertinent citations according to criteria adapted from the US Preventive Services Task Force. Thirteen citations (four level I, six level II-1, three level II-2) pertained to LSSs for carboplatin, four citations (one level II-1, one level II-2, two level III) to cisplatin LSSs, and one citation (level II-2) to nedaplatin. Based on the current evidence, it appears that LSSs may be useful for pharmacokinetic-guided dosage adjustments of carboplatin in both adults and children with cancer. Although some validation studies suggest that LSSs can be extended to different cancer populations or different chemotherapy regimens, other studies dispute this finding. Although the use of LSSs to predict the pharmacokinetic parameters of cisplatin and nedaplatin appear promising, the quality of evidence from published studies does not support routine implementation at this time.LSSs represent one approach in which clinicians can make specific dosage adjustments for individual patients to optimise outcomes. However, the limitations of these strategies must also be taken into consideration. There is also a need for prospective studies to demonstrate that application of LSSs for platinum agents ultimately improves patient response and decreases systemic toxicities.

Influence of Underlying Disease on Busulfan Disposition in Pediatric Bone Marrow Transplant Recipients: A Nonparametric Population Pharmacokinetic Study

Busulfan is an alkylating agent used in a conditioning regimen prior to bone marrow transplantation. Busulfan has a narrow therapeutic index, giving rise to major liver toxicity (veno-occlusive disease), and a wide interpatient and intrapatient pharmacokinetic variability. This report presents the results of a population pharmacokinetic analysis leading to models based on underlying diseases requiring bone marrow transplantation. One hundred children received oral busulfan-based conditioning regimens between March 1998 and February 2006. Busulfan pharmacokinetic parameter estimates (Ka, first order absorption rate constant; Vs, volume of distribution related to the body weight; and Cl/F, apparent clearance) were estimated by using the nonparametric adaptative grid (NPAG) algorithm in patients divided into four groups according to initial diagnosis: metabolic diseases, hemoglobinopathies, hematological malignancies, and immune deficiencies. Ka and Vs did no differ significantly in the four subgroups. Cl/F and areas under the plasma concentration curve were significantly different in the four groups. Cl/F was significantly higher in the hemoglobinopathies group (P = 0.002), with a mean value of 7.78 L . h, whereas the immune deficiencies group was characterized by the lowest Cl/F (3.59 L . h). Interindividual variability was shown by high interindividual parameter percent coefficients of variation (CV%) but, nevertheless, with less diversity in the population parameter distributions for Vs in the three subgroups-metabolic diseases, hemoglobinopathies, and malignant diseases-and in Cl/F for patients with hemoglobinopathies. The fit was good for busulfan concentration predictions based on Bayesian individual posterior values, with little bias and good precision. In comparison with the overall population, the only model of subgroup presenting a greater precision was patients with hemoglobinopathies (P = 0.002). Use of these more specific models of a given disease may well result in more accurate individualization of busulfan dose regimens, especially in very sparse blood sampling situations.

Development of a Pharmacokinetic and Bayesian Optimal Sampling Model for Individualization of Oral Busulfan in Hematopoietic Stem Cell Transplantation

Therapeutic drug monitoring is used to minimize toxicity and maximize the therapeutic efficacy of busulfan, which shows high intra- and interpatient pharmacokinetic variability and erratic oral absorption. This study was designed to develop a pharmacokinetic model that could accommodate the erratic oral absorption of busulfan and to use this model to develop an optimal sparse pharmacokinetic sampling strategy to improve the precision and efficiency of therapeutic drug monitoring. Twenty-one pharmacokinetic profiles were collected from 12 patients receiving oral busulfan before hematopoietic stem cell transplantation. Each pharmacokinetic profile was defined by 5 to 9 plasma concentrations. Candidate pharmacokinetic models were initially fit to the data by maximum likelihood, with model discrimination by Akaike's Information Criterion. Maximum likelihood results were used to derive Bayesian previous parameter estimates, and D-optimal design was used to determine optimal sparse sampling strategies. Each candidate sampling strategy was tested in each patient by comparing the resultant Css obtained from the sparse strategy to the actual Css derived from each patient's full pharmacokinetic dataset. The final model was a 1-compartment model, with oral busulfan absorbed in 1 to 3 phases, and fit the data well. All limited sampling models tested were unbiased in their results, and a 4-sample scheme proved to adequately characterize busulfan pharmacokinetics, and should allow for a reduced sampling frequency for therapeutic drug monitoring.

Quantification of Busulfan in Saliva and Plasma in Haematopoietic Stem Cell Transplantation in Children

BACKGROUND AND OBJECTIVE

Busulfan pharmacokinetic studies suggest that an individual dosing strategy may be necessary to optimise systemic exposure in order to decrease toxicity and improve outcome in haematopoietic stem cell transplantation. Therapeutic and toxic effects of the busulfan/cyclophosphamide regimen have been related to the area under the busulfan plasma concentration-time curve. Because of practical limitations in obtaining blood from children, saliva was evaluated as an alternative matrix for therapeutic drug monitoring, offering the advantages of a non-invasive, rapid and easy sampling procedure. Another objective was to evaluate an easy and robust liquid chromatography- tandem mass spectrometry method for plasma and saliva busulfan determination.

METHODS

An online extraction cartridge with column-switching technique, analytical liquid chromatography over a Chromolith RP 18 e column, and tandem mass spectrometry were used to quantify busulfan concentrations in matched plasma and saliva samples. The study population consisted of ten patients, aged 1.3-19 years (median age 11.8 years, seven females, three males), undergoing haematopoietic stem cell transplantation. All patients received busulfan 0.8-1.3 mg/kg orally every 6 hours for a total of 16 doses, followed by two doses of cyclophosphamide (60 mg/kg/day).

RESULTS

The lowest limit of detection was 2 microg/L and the lower limit of quantification was 10 microg/L. Only 100 microL of plasma/saliva was needed. The mean recoveries (SD) of busulfan were 97.2% (2.7) in plasma and 100.4% (1.3) in saliva. Intra- and inter-assay imprecision was 2-3% and 2-4% for plasma, and 1-2% and 2-4% for saliva (concentration range 30-1,500 microg/L). The bias was <4% for both plasma and saliva. The correlation between the busulfan concentration in plasma and saliva was highly significant (r=0.958; p<0.0001; saliva/plasma ratio=1.09+/-0.04; n=69 sample pairs). The apparent plasma clearance was slightly higher than the apparent saliva clearance (202+/-31 mL/h/kg vs 189+/-28 mL/h/kg; p=0.001). The mean elimination half-life was found to be 2.31+/-0.46 hours for plasma and 2.30+/-0.36 hours for saliva; these were not significantly different (p=0.83).

CONCLUSION

The present study demonstrated that busulfan analysis in saliva could be a valuable and reliable alternative to plasma analysis.

Tandem Mass Spectrometry Method for the Quantification of Serum Busulfan

Busulfan, an alkylating agent, is most commonly used as a component of bone marrow transplantation preoperative regimens. Significant interpatient and intrapatient variations in pharmacokinetics require individualizing the dosage based on area under the time-versus-concentration curve. Timely result reporting is critical to dose adjustment to reduce morbidity and mortality associated with the regimen. The authors developed a rapid, accurate, and sensitive method for the quantification of serum busulfan using direct inject tandem mass spectrometry. Plasma samples (50 microL) are extracted in 1 mL of methanol containing 1,6-bis-(methanesulfonyloxy)hexane as an internal standard. The supernatant is dried under nitrogen (40 degrees C, 30 minutes) and then dissolved in 200 microL methanol and transferred into a clean glass vial suitable for LC/MS/MS analysis. The sample is delivered using an HPLC pump that delivers 0.2 mL of methanol per minute, and 20 microL of sample is injected into a turbo ion spray-equipped tandem mass spectrometer. Total analysis time is 5 minutes. The Q1/Q3 transition for busulfan (BU) is monitored at 269/55 and 297.1/55.1 for the internal standard. The assay is linear to 10 micromol/L and sensitive to at least 0.5 micromol/L. The interassay reproducibility at 1, 2.2, and 4.4 micromol/L were 4.2%, 5.6%, and 6.3%, respectively. Within-run precision using 3 different control samples was 3.9%, 3.9%, and 6.9%. Mean recovery of 4 different BU concentrations spiked into 10 different BU free plasma samples was 98%. Correlation with an established HPLC-UV method revealed a slope of 0.98, an intercept of 0.1, and r = 0.95 (n = 48). No significant interfering substances or ion suppression was identified. This method is a significant improvement over the existing HPLC-UV method for BU determination. The method is highly accurate, reproducible, and requires less specimen, sample preparation, and analysis time.

Pharmacokinetic variability of anticancer agents

The translation of advances in cancer biology to drug discovery can be complicated by pharmacokinetic variation between individuals and within individuals, and this can result in unpredictable toxicity and variable antineoplastic effects. Previously unrecognized variables (such as genetic polymorphisms) are now known to have a significant impact on drug disposition. How can the pharmacokinetic variability of anticancer agents be reduced? This will require the understanding of correlations between pharmacokinetics and treatment outcomes, the identification of relevant patient parameters, mathematical modelling of individual and population pharmacokinetics, and the development of algorithms that will tailor doses to the individual patient.

Development of a rapid and specific assay for detection of busulfan in human plasma by high-performance liquid chromatography/electrospray ionization tandem mass spectrometry

A sensitive and specific assay for detection of busulfan in human plasma was developed. The assay is based on rapid isolation of busulfan by liquid-liquid extraction with ethyl acetate, and detection by high-performance liquid chromatography with electrospray ionization and tandem mass spectrometry. 1,6-Bis(methanesulfonyloxy)hexane, a synthesized analogue of busulfan, was used as the internal standard (IS). The acquisition was performed in the multiple reaction monitoring mode; busulfan and the IS were detected with no interferences from plasma matrix. The method was linear over the range 5-2500 ng mL(-1), with r2 > 0.99 and a run time of only 3.5 min. The intra- and inter-assay precisions were in the ranges 2.1-11.9% and 3.2-10.1%, respectively, and the intra- and inter-assay accuracies were 92.2-107.6% and 94.7-104.1%, respectively. The absolute recoveries were 82.0% (20 ng mL(-1)), 90.6% (1000 ng mL(-1)) and 80.0% (2000 ng mL(-1)) for busulfan, and 89.1% for the IS (1000 ng mL(-1)). The limits of detection and quantification were 2 and 5 ng mL(-1), respectively. The validated method was successfully applied to analyze plasma samples obtained from six adults receiving doses of 1 mg kg(-1) in a conditioning regimen prior to bone marrow transplantation. A marked intra-patient variation in busulfan concentrations during the steady state was observed, which limits the application of pharmacokinetic modeling and suggests that continuous therapeutic monitoring is necessary for adequate individualized dosing. In this regard, the present assay brings important advantages relative to other methods described in the literature, i.e., it is highly specific and simple to perform, with a rapid chromatographic run time (3.5 min), and the whole procedure can be completed in 4-5 h, which would permit dose corrections after the third dose allowing earlier and better dosing adjustments towards the target level of busulfan.

Individualised Cancer Chemotherapy: Strategies and Performance of Prospective Studies on Therapeutic Drug Monitoring with Dose Adaptation

Therapeutic drug monitoring (TDM) is increasingly used in clinical practice for the optimisation of drug treatment. Although pharmacokinetic variability is an established factor involved in the variation of therapeutic outcome of many chemotherapeutic agents, the use of TDM in the field of oncology has been limited thus far. An important reason for this is that a therapeutic index for most anticancer agents has not been established; however, in the last 20 years, relationships between plasma drug concentrations and clinical outcome have been defined for various chemotherapeutic agents. Several attempts have been made to use these relationships for optimising the administration of chemotherapeutics by applying pharmacokinetically guided dosage. These prospective studies, individualising chemotherapy dose during therapy based on measured drug concentrations, are discussed in this review. We focus on the way a target value is defined, the methodologies used for dose adaptation and the way the performance of the dose-adaptation approach is evaluated. Furthermore, attention is paid to the results of the studies and the applicability of the strategies in clinical practice. It can be concluded that TDM may contribute to improving cancer chemotherapy in terms of patient outcome and survival and should therefore be further investigated.

I.V. busulfan in pediatrics: a novel dosing to improve safety/efficacy for hematopoietic progenitor cell transplantation recipients

A retrospective population pharmacokinetic (PPK) analysis was performed in 24 pediatric patients (PEDS) (0.45-16.7 years old) receiving i.v. busulfan/cyclophosphamide (i.v. Bu/Cy 4) regimen prior to allogeneic hematopoietic stem cell transplantation. I.V. Bu doses were given as a 2-hour infusion every 6 h over 4 days. Initial dosing of i.v. Bu was 1 mg/kg for children < or =4 years old and 0.8 mg/kg for patients >4 years old. Bu plasma concentrations at doses 1, 9 and 13 were analyzed through a multivariate NONMEM analysis. A close log-linear relationship between body weight (BW) and i.v. Bu clearance was demonstrated with no further age-dependency or gender effect. The interpatient coefficient of variation (CV) in Bu clearance significantly decreased from 56% (covariate-free model) to 19% (BW covariate model) and reproducible i.v. Bu exposure between doses was illustrated (intraindividual CV=9%). Based on the PPK model, a novel Bu dosing regimen (ie: doses in mg/kg adjusted to discrete weight categories) for a better AUC targeting was developed by simulation on 1000 patients. Age-based dosing was demonstrated not to be clinically relevant with i.v. Bu. Use of the new BW-based dosing appears to be more appropriate for the PEDS.

Pharmacokinetics and individualized dose adjustment of intravenous busulfan in children with advanced hematologic malignancies undergoing allogeneic stem cell transplantation

We investigated the pharmacokinetics (PK) of a recently approved intravenous busulfan (IVBU) formulation as a part of the preparative regimen in 20 children with advanced hematologic malignancies undergoing allogeneic hematopoietic stem cell transplantation. Seventeen patients received a thiotepa, IVBU, and cyclophosphamide-based regimen, and 3 patients received an IVBU and cyclophosphamide-based regimen. All patients received IVBU 0.8 mg/kg for the first 2 doses; thereafter, the IVBU dose was modified, if required, to achieve a final area under the concentration-time curve (AUC) at steady state of 1150 micromol/L/min per dose (range, 1000-1300 micromol/L/min per dose; SD +/-13%) based on the first-dose PK determination. PK studies were repeated on subsequent doses to verify the final AUC. Initial mean IVBU clearance and half-life were 3.96 mL/min/kg and 1.98 hours, respectively. Sixteen (80%) of the 20 patients received dose adjustments: 14 patients required dose escalations, and 2 required dose reductions. Overall, thirteen (72%) of 18 available sample sets at final follow-up PK analysis showed the IVBU exposure to be within the targeted range. IVBU PK was linear, and interpatient variability was much lower than that observed with oral busulfan. IVBU was well tolerated, and no case of hepatic veno-occlusive disease was encountered. Mild and transient hyperbilirubinemia was observed in 7 patients. Thirteen of the 20 patients were alive at a median follow-up of 651 days (range, 386-1555 days). We conclude that a standardized IVBU dose of 0.8 mg/kg in children does not always result in an AUC within the reference range defined in this study. Therapeutic drug monitoring with dose adjustment based on first-dose PK can optimize the systemic busulfan exposure for children undergoing allogeneic hematopoietic stem cell transplantation.

Myeloablative conditioning regimens for AML allografts: 30 years later

During the last three decades, several myeloablative conditioning regimens have been used for AML allografts. In this review, we systematically examine the data from studies reporting on myeloablative conditioning regimens for AML allografts. High-dose busulfan combined with cyclophosphamide (BuCy) and cyclophosphamide in combination with total body irradiation (CyTBI) are the two most commonly used conditioning regimens for AML allografts. From the available data, there are no significant differences in survival with these two regimens. A small benefit of decreased relapse rate with CyTBI is counterbalanced by a nonsignificant increase in treatment-related mortality. The incidence of veno-occlusive disease is significantly higher in patients treated with BuCy. Therapeutic monitoring of busulfan was not reported in any of the studies comparing the regimens. Either of the regimens can be used for AML allografts, and the choice may ultimately depend on local availability and expertise. Further improvements may be possible from modifications of the standard regimens. Data from these latter studies seem to be encouraging, but are not based on comparative randomized trials.

On-line derivatization utilizing solid-phase microextraction (SPME) for determination of busulphan in plasma using gas chromatography-mass spectrometry (GC-MS)

Busulphan (Bu) is an alkylating agent used in preparative regimen before stem cell transplantation (SCT). Bu has a narrow therapeutic window, and underdosing or overdosing may have a fatal outcome for the patient. Therapeutic drug monitoring (TDM) combined with dose adjustment is currently used to optimize and individualize therapy with Bu. However, this approach is limited to centers with laboratory facilities. An automated and easy method for measurement of Bu plasma concentrations may facilitate TDM for Bu and thus improve the clinical outcome. A solid-phase microextraction (SPME) on line with gas chromatography (GC) and mass-spectrometric detection to quantify Bu in human plasma samples was developed using in-vial derivatization. Bu was mixed with reagent in a 2-mL vial and shaken for 15 minutes at 80 degrees C; subsequently, the SPME fiber was immersed into the vial for 15 minutes. The fiber was washed in water for 10 seconds before injection. Several parameters influencing the extraction and recovery were studied, such as absorption and desorption times, the effects of the temperature on the reaction, and the shaking time on the derivatization yield. Carbowax-divinylbenzene, polyacrylate, and polydimethylsiloxane fibers were tested. The carbowax-divinylbenzene fiber resulted in the highest recovery in plasma samples. The validation of the method showed a high chromatographic selectivity and a good sensitivity (LOQ = 20 ng/mL). Coefficient of variation for SPME was less than 15%. The results showed good correlation between Bu concentrations and response within the range of 40 to 2500 ng/mL (R2 = 0.999). The accuracy ranged from 94% to 106%. This is well in line with the international criteria for validation. The present method was applied to patient plasma. The obtained results were comparable with the results obtained from GC with electron capture detection. The authors conclude that this method has shortened the analysis time considerably and is fully automated, which benefits TDM of Bu in SCT patients.

The effect of metronidazole on busulfan pharmacokinetics in patients undergoing hematopoietic stem cell transplantation

Busulfan (Bu) is an important component of some myeloablative regimens prior to stem cell transplantation (SCT). Over the last few years it has been shown that other drugs administered concomitantly can influence Bu pharmacokinetics. In the present study, we compared Bu concentrations (trough levels) in three groups of patients. Group A (n=5) received metronidazole as graft-versus-host disease prophylaxis during Bu treatment. Group B (n=9) received Bu only for 2 days followed by 2 days of Bu and metronidazole. Group C (n=10) was a control group that received Bu without metronidazole. The mean Bu levels for Group A receiving metronidazole during conditioning was significantly (P<0.001) higher (948+/-280 ng/ml), compared to those observed in the control group (507+/-75 ng/ml). In Group B, the administration of metronidazole resulted in a significant (P<0.001) increase in Bu levels (807+/-90 ng/ml) during the last 2 days, compared to 452+/-68 ng/ml during the first 2 days. In Group A, one patient died with multiorgan failure, three experienced veno-occlusive disease (VOD) and one developed hemorrhagic cystitis. Elevated liver transaminases (AST, ALT) and bilirubin were detected in all Group A patients. In Group B, six patients had elevated liver function tests but no VOD was observed. We conclude that metronidazole should not be administered simultaneously with Bu to avoid the high plasma levels of Bu, which may lead to severe toxicity and/or treatment related mortality.

A phase II trial of liposomal busulphan as an intravenous myeloablative agent prior to stem cell transplantation: 500 mg/m(2) as a optimal total dose for conditioning

We conducted a phase I/II trial, to evaluate the efficacy and safety of an intravenous liposomal formulation of busulphan (LBu) as a myeloablative agent for stem cell transplantation (SCT). The liposomal busulphan was administered as a 3 h infusion twice daily over 4 consecutive days. Six adults received 1.6-2 mg/kg/dose and 18 children received 1.8-3 mg/kg/dose. Pharmacokinetic parameters were studied after the first and the last dose of busulphan. No significant difference in clearance, AUC, elimination half-lives or distribution volume between the first and the last dose was found in either groups. A significantly (P < 0.005) higher clearance was observed in children after the first and the last dose (3.61 and 3.79 ml/min/kg, respectively) compared to adults (2.40 and 2.33 ml/min/kg, respectively). The elimination half-lives after the first and the last dose were significantly (P < 0.005) shorter in children (2.59 and 2.72 h, respectively) compared to adults (3.35 and 3.61 h, respectively). Clearance correlated significantly with age. However, no significant correlation with age was observed when clearance was adjusted to the body surface area. Two cases of VOD following a total dose of 24 mg/kg were observed. Six patients experienced mucositis. No other organ toxicity was observed. We conclude that intravenous liposomal busulphan pharmacokinetics is age dependent. A dosage schedule based on body surface area should be used especially in young children to reduce the age-dependent difference in kinetics. An intravenous liposomal dose of busulphan of 500 mg/m(2) is suggested to reach a similar systemic exposure and myeloablative effect in both children and adults. Moreover, the novel liposomal form of busulphan showed a favorable toxicity profile and seems safe as a part of the high-dose therapy prior to SCT.

The effect of modulation of glutathione cellular content on busulphan-induced cytotoxicity on hematopoietic cells in vitro and in vivo

Busulphan is used in conditioning regimens prior to SCT. A relationship between exposure to busulphan, expressed as an area under the plasma concentration time curve (AUC), and effect and/or adverse effects, such as veno-occlusive disease (VOD), was reported. Exhaustion of glutathione (GSH) contributes to VOD and modulation of intracellular levels of GSH influences bulsulphan-induced toxicity in hepatocytes. Thus, increase of GSH might serve as prophylaxis against VOD. However, it should not interfere with the myeloablative effects of busulphan. We investigated the relationship between exposure to busulphan, and its in vitro toxicity to CD34(+) hematopoietic progenitors from volunteers using clonogenic assays. Busulphan inhibited colony formation by CD34(+) cells in an AUC-dependent manner. Myeloid progenitors were more sensitive than erythroid progenitors, expressed as 100% inhibition of colony formation (68.7 +/- 7.5 microg.h/ml and 140.3 +/- 35.7, respectively). The observed exposure corresponds to the total AUC obtained in patients treated with busulphan (1 mg/kg/day) for 4 days. Secondly, we studied the effect of modulation of GSH cellular levels on busulphan-induced toxicity in vitro in CD34(+) cells from volunteers, and in vivo in bone marrow cells from Balb/c mice. The intracellular concentration of GSH was increased or decreased by treatment with N-acetylcysteine or buthionine sulfoximine, respectively. Neither in vitro nor in vivo treatment with GSH modulators affected the hematological toxicity of busulphan. Thus, N-acetylcysteine would not interfere with the myeloablative effect of busulphan and therefore it is a potential candidate for VOD prophylaxis during busulphan-based conditioning regimens.

Allogeneic bone marrow transplantation in children with chronic myelogenous leukemia

PURPOSE

To determine the outcome of children undergoing allogeneic bone marrow transplantation for chronic myelogenous leukemia (CML) at the authors' institution.

PATIENTS AND METHODS

Between 1985 and 1999, 18 allogeneic bone marrow transplantations were performed in 17 patients with CML at the Hospital for Sick Children in Toronto. Median age at diagnosis was 9.5 years (range 3-17). Fourteen patients had disease in the first chronic phase, one had disease in the second chronic phase, and two had disease in the accelerated phase. Preparative regimens varied, with radiation-based protocols used in eight patients. Thirteen donors were related (11 matched, 2 mismatched); four were unrelated (2 matched, 2 mismatched). Patients received T-cell-replete bone marrow a median of 7.5 months (range 2.2-22) from diagnosis. A median of 3.0 x 10(8)/kg nucleated cells was infused (range 1.6-6.7). Graft-versus-host disease (GVHD) prophylaxis consisted of cyclosporine and methotrexate in 13 children. cyclosporine in three, and methotrexate in one.

RESULTS

Primary graft failure occurred in one patient. Grade 2 acute GVHD or more developed in 11 of the 17 children (64%; grade 2 in 4, grade 3 in 7). Chronic GVHD occurred in 6 of the 16 patients at risk (37.5%; 5 extensive, 1 localized). No patient experienced overt or cytogenetic relapse. There were two deaths (12%): one from acute GVHD and cytomegalovirus pneumonia and the other from chronic GVHD. Probability of 5-year event-free survival was 87 +/- 9%.

CONCLUSIONS

These results strongly support the practice of allotransplantation in children with CML, even in the setting of advanced disease and histoincompatibility. Efforts should be aimed at reducing the transplantation-related death rate.

Target dose adjustment of busulfan in pediatric patients undergoing bone marrow transplantation

Published data suggest that the average concentration of busulfan at steady state (Bu Css) is critical for successful engraftment in children receiving busulfan as a conditioning agent for bone marrow transplantation (BMT). We previously found in children that a Bu Css <600 ng/ml correlated with autologous recovery/mixed chimerism; there was no correlation between Bu Css and regimen-related toxicity (RRT). In a cohort continuous with the previous trial, we prospectively evaluated targeted busulfan concentrations in 32 pediatric patients (age 0.6-18.5 years) with AML (n = 6), CML (n = 6) and non-malignant disorders (n = 20) receiving HLA-closely matched donor grafts. In this trial, individual busulfan pharmacokinetics were performed prior to admission. Busulfan doses were then adjusted to achieve a Bu Css target range of 600-900 ng/ml +/- 10% depending on donor source and disease. A repeat study was done following dose 1 of the conditioning regimen. Thirty of thirty-two (94%) patients achieved target concentrations. Total busulfan doses ranged from 10.9 to 29 mg/kg. Thirty of thirty-two patients (94%) have durably engrafted. Grade 3/4 RRT occurred in seven patients (21%). Targeting Bu Css ranges of 600-900 ng/ml significantly improved our rate of successful engraftment from 74% to 94% (P = 0.043). These results indicate that targeted busulfan dosing optimizes allogeneic engraftment in children.

Quantification of busulfan in plasma by liquid chromatography-ion spray mass spectrometry. Application to pharmacokinetic studies in children

Optimisation of busulfan dosage in patients undergoing bone marrow transplantation is recommended in order to reduce toxic effects associated with high drug exposure. A new method was developed coupling liquid chromatography with mass spectrometry (LC-MS) and was validated for the determination of busulfan concentrations in plasma. Recovery was 86.7%, the limit of detection was 2.5 ng/ml and linearity ranged from 5 to 2500 ng/ml. The correlation between the busulfan concentrations measured by our previously published HPLC-UV method and the new HPLC-MS method was highly significant (P<0.0001). Sample volume was reduced and the method was rapid, sensitive and less expensive than the methods previously used in our laboratory. This method was used to determine the pharmacokinetic parameters of busulfan after the first administration of 1 mg/kg orally, in 13 children receiving the drug as part of the preparative regimen for bone marrow transplantation. Our results were similar to previously reported data. They showed that the apparent oral clearance of busulfan was 0.299+/-0.08 l/h/kg, and that it was significantly higher (P=0.02) in patients below the age of 5 years than in older children.

Evaluation of existing limited sampling models for busulfan kinetics in children with beta thalassaemia major undergoing bone marrow transplantation

Busulfan pharmacokinetic parameters are useful in predicting the outcome of allogeneic bone marrow transplantation (BMT). Standard pharmacokinetic measurements require multiple blood samples. Various limited sampling models (LSM) have been proposed for reducing the sample number required for these measurements, essentially for patients with malignant disorders undergoing BMT. This study was undertaken to evaluate the existing LSM for busulfan pharmacokinetics to find out the most suitable method for patients with thalassaemia major undergoing BMT. Busulfan levels in plasma samples were analysed by HPLC. The AUC calculated by non-compartmental analysis using the program 'TOPFIT' was compared with previously published LSMs. Our seven sample pharmacokinetic data for AUC calculation was compared with the published LSMs. The three sample models suggested by Chattergoon et al and Schuler et al showed significant agreement with AUC TOPFIT (R(2) = 0.98 and 0.94, respectively) in our clinical context. Other models resulted in significant over or under representation of observed values (Vassal's model R(2) = 0.61; Chattergoon's two sample model R(2) = 0.84; four sample model R(2) = 0.83; Schuler's two sample model R(2) = 0.79). By these data the three sample LSM proposed by Chattergoon et al and Schuler et al are suitable for calculation of the AUC in patients with thalassaemia major undergoing BMT conditioned with oral busulfan.

Improved clinical outcome of paediatric bone marrow recipients using a test dose and Bayesian pharmacokinetic individualization of busulfan dosage regimens

In order to control busulfan pharmacokinetic variability and toxicity, a specific monitoring protocol was instituted in our bone marrow transplant BMT paediatric patients including a test dose, daily Bayesian forecasting of busulfan plasma levels, and Bayesian individualization of busulfan dosage regimens. Twenty-nine children received BMT after a busulfan-based conditioning regimen. Individual pharmacokinetic parameters were obtained following a 0.5 mg*kg test dose and were used for daily individualization of dosage regimens during the subsequent 4-day course of treatment. Doses were adjusted to reach a target mean AUC per 6 h between 4 and 6 microg.h.ml(+1). Plasma busulfan assays were performed by liquid chromatography. Pharmacokinetic analysis used the USC*PACK software. The performance of the test dose to predict AUC during the busulfan regimen was evaluated. Incidence of toxicity, chimerism and relapse, overall Kaplan-Meier survival, and VOD-free survival were compared after matching our patients (group A) with patients conditioned by using standard doses of busulfan (group B). Busulfan doses were decreased in 69% of patients compared to conventional doses. Expected AUC was significantly correlated with observed AUC and predictability of the test dose was 101.9 +/- 17.9%. Incidence of VOD in group A was 3.4% vs 24.1% in group B, while the incidence of stomatitis was similar. Engraftment was successful in all patients in group A. The rate of full engraftment at 3 months post-BMT was higher in group A (P = 0.012). Long-term overall survival did not differ between the two groups, in contrast to the 90-day survival. VOD-free survival was higher in group A (P = 0.026). Pharmacokinetic monitoring and individualization of busulfan dosage regimen are useful in improving clinical outcome and reducing early mortality in paediatric bone marrow transplant recipients.

Population pharmacokinetic analysis resulting in a tool for dose individualization of busulphan in bone marrow transplantation recipients

The aims of the present study were (1) to investigate and quantify the pharmacokinetics, including inter-occasion variability and covariate relationships, of busulphan in BMT patients and (2) to develop a user-friendly initial dosing and therapeutic drug monitoring (TDM) strategy for the treatment of those patients with busulphan. The pharmacokinetics of busulphan was studied in 64 adults and 12 children who received busulphan (1 mg/kg) four times daily for 4 days. A one-compartment model with first order absorption and a lag time was sufficient in describing the concentration-time profile. Oral clearance (CL/F) was found to be correlated to weight (+1.2%/kg), ALT (-13%/microcat/l) and concomitant phenytoin treatment (+21%). CL/F and the volume of distribution (V/F) were estimated to 9.23 l/h and 39.3 l, respectively, in a typical individual. Inter-occasion variability (9.4%) in CL/F was estimated to be less than inter-individual variability (28%), a prerequisite for the value of TDM. Bayesian CL/F estimates based on three samples were in good accordance with those based on all samples. The final population model was implemented into the program Excel. The resulting flexible and easy to use dosing program might be used for both initial and, requiring only three plasma samples, maintenance dose individualization of busulphan therapy.

Pharmacokinetics of liposomal busulphan in man

High doses of busulphan are used in conditioning regimens before stem cell transplantation. Great inter-patient variations in pharmacokinetics and a correlation between toxicity and high systemic exposure of busulphan have been shown in several studies. Some authors have suggested therapeutic drug monitoring and intravenous busulphan aiming to reduce the conditioning-related toxicity. Liposomal busulphan (LBu) might be an alternative to intravenous administration of high-dose busulphan in conditioning. In the present study, we investigated the pharmacokinetics of LBu in man. Seventeen consecutive patients were enrolled in the study. LBu as a single low dose (2 to 8 mg) was given to 12 patients (six adults and six children). Five patients received two high doses of LBu which replaced the first and the last doses of the conditioning regimen. The high dose of LBu was raised from 0.4 to 0.9 mg/kg. A significant linear correlation (r2 = 0.928) was found between the dose of LBu and the area under the plasma concentration-time curve (AUC) (P < 0.001). AUC corrected for 1 mg/kg was 5491 +/- 912 ng.h/ml and 5955 +/- 627 ng.h/ml (low dose of LBu in children and adults, respectively) and 6167 +/- 1385 ng.h/ml and 6933 +/- 656 ng.h/ml (ie the first and the last high doses of LBu, respectively). No significant correlation was found between clearance and age or apparent volume of distribution and age (r2 = 0.146 and r2 = 0.046, respectively). No toxicity related to the liposomal formulation of busulphan was observed. We conclude that LBu is suitable for conditioning before stem cell transplantation.

Retrospective appraisal of busulfan dose adjustment in children

This report describes and critically appraises our experience with busulfan dose adjustment in children undergoing bone marrow transplant between April 1997 and March 1999. All children received an initial busulfan dose of 40 mg/m2 p.o. or by nasogastric tube. Whole blood samples were obtained 1, 1.5 and 6 h later and analyzed for busulfan content by gas chromatography with electron capture detection. The area under the whole blood busulfan concentration vs time curve (AUC) and an individualized dose which would achieve an AUC of 1300 microM/min were calculated. Mean and median busulfan doses were calculated using actual, ideal and effective body weight and stratified according to age. The relationship between the busulfan concentration at hour 6 and AUC was determined using linear regression. Thirty-nine courses of busulfan were evaluated in 38 patients. A change from the initial busulfan dose was required to achieve the target AUC in 34 courses (87%). Most children >1 to 5 years old required dose increments while most children >5 years old required dose reductions. Obesity did not significantly affect busulfan dose requirements. Busulfan concentrations at 6 h only weakly predicted the AUC achieved (r2 = 0.496; P = 0.001). Based on these findings, we recommend that the initial busulfan dose be assigned according to patient age and actual body weight. We also recommend that busulfan AUC be calculated for children using a four-sample (1, 1.5, 4 and 6 h) limited sampling technique.

Individualizing high-dose oral busulfan: prospective dose adjustment in a pediatric population undergoing allogeneic stem cell transplantation for advanced hematologic malignancies

We investigated whether adjusting the oral busulfan (BU) dosage on the basis of early pharmacokinetic data to achieve a targeted drug exposure could reduce transplant-related complications in children with advanced hematologic malignancies. Twenty-five children received a preparative regimen consisting of thiotepa (250 mg/m2 i.v. daily for 3 days), BU (40 mg/m2 per dose p.o. every 6 h for 12 doses), and cyclophosphamide (60 mg/kg i.v. daily for 2 days) and then underwent allogeneic stem cell transplantation. Busulfan clearance and area under concentration time-curve (AUC) were determined after the first dose using a one-compartment pharmacokinetic (PK) model with first-order absorption. The initial PK analysis was successfully completed after the first BU dose in 21 patients (84%). A final AUC of 1000-1500 microM x min/dose was targeted and subsequent doses were modified as necessary to achieve this value. Fourteen of the 25 patients (56%) required dose adjustment. Follow-up PK analysis was completed in 21 patients and 16 of these achieved the targeted BU exposure for the course of therapy. Interpatient variability in BU clearance was high (up to five-fold). The most frequent regimen-related toxicities were cutaneous and gastrointestinal (stomatitis and diarrhea). Only one patient developed hepatic veno-occlusive disease. Our study demonstrates the feasibility of adjusting the oral BU dose in individual pediatric patients. Although toxicity associated with BU seemed to be reduced, this conclusion is tempered by the fact that the overall regimen-related toxicity (RRT) remains substantial and reflected the effects of all agents used in the preparative regimen.

Busulfan pharmacokinetics do not predict relapse in acute myeloid leukemia

The purpose of this study was to evaluate the influence of busulfan (BU) pharmacokinetics on survival, grades II-IV acute graft-versus-host disease (GVHD), non-relapse mortality (NRM) and relapse in a group composed of 45 children (<18 years) and seven adults with acute myeloid leukemia (AML) in first complete remission and undergoing hematopoietic stem cell transplantation (SCT). Fifty-two patients underwent autologous (n = 25) or allogeneic (n = 27) SCT. The median age was 8.9 years (range 0.6-53 years). Conditioning therapy consisted of BU and cyclophosphamide. Improved disease-free survival was found in those patients with a steady-state concentration of BU (CssBU) below the median (<578 mg/ml, P = 0.05), and the same trend was noted for overall survival (P = 0.07). This was secondary to a higher incidence of NRM in the group of patients with CssBU above the median (P = 0.06). There was no significant correlation with CssBU and relapse (P = 0.31). No association between CssBU and GVHD was found in allogeneic patients (P = 0.30). Relapse was evaluated among the subgroups of age (< or >10 years) and transplant type (allogeneic or autologous) with no statistically significant association observed among these factors. Multiple regression analysis for relapse revealed no significant correlation with CssBU above or below the median, age, or transplant type. In this study, CssBU below the median did not correlate with an inferior outcome for patients with AML. Pharmacokinetic dosing of BU may be important for prevention of NRM but does not appear to influence the risk of relapse in this largely pediatric population with AML.

The clinical pharmacology of alkylating agents in high-dose chemotherapy

Alkylating agents are widely used in high-dose chemotherapy regimens in combination with hematological support. Knowledge about the pharmacokinetics and pharmacodynamics of these agents administered in high doses is critical for the safe and efficient use of these regimens. The aim of this review is to summarize the clinical pharmacology of the alkylating agents (including the platinum compounds) in high-dose chemotherapy. Differences between conventional and high doses will be discussed.

Plasma concentration monitoring of busulfan: does it improve clinical outcome?

High dosage busulfan (1 mg/kg orally every 6 hours x 16 doses) is frequently used in preparative regimens for haemopoietic stem cell transplantation (HSCT). Busulfan is well absorbed after oral administration, exhibits low protein binding and is metabolised through conjugation with glutathione to form a thiophenium ion. At a given dose, there is considerable variability in the systemic exposure of busulfan, typically expressed as area under the plasma concentration-time curve (AUC) or average concentration at steady state (Css). Relative to that in adolescents and adults, patients less than 4 years of age have an increased apparent oral clearance (CL/F) of busulfan and a higher conjugation rate of busulfan with glutathione in the enterocyte. Several investigators have identified relationships between busulfan Css and outcome in patients undergoing HSCT. Busulfan concentration-response relationships are regimen-, age- and disease-dependent. The busulfan/cyclophosphamide (BU/CY) regimen is the only regimen for which substantial concentration-outcome data exist. Generally, the risk of hepatic veno-occlusive disease is increased with busulfan Css > 900 microg/L. The impact of busulfan Css on veno-occlusive disease may be influenced by the age of the patient and the dose of cyclophosphamide. Lower rates of relapse in chronic myelogenous leukaemia occur in patients with a busulfan Css > 917 microg/L without an increased risk of toxicity. Busulfan Css is also related to the engraftment rate in children, and escalating busulfan doses to achieve a target Css > 600 microg/L improves graft retention. Therapeutic drug monitoring of busulfan should be performed to maximise the likelihood of engraftment and minimise the risk of toxicity and relapse in HSCT patients receiving the BU/CY preparative regimen.

The role of busulfan in bone marrow transplantation

High-dose busulfan is an important component in many conditioning protocols for hematopoietic stem cell transplantation (HSCT) or bone marrow transplantation (BMT) in both adults and children. During the past 12y several studies have reported the wide inter-individual variability in busulfan disposition. Age, disease status, hepatic function, circadian rhythmicity, drug interactions and bioavailability, were identified as factors contributing to the high inter-individual variability found in busulfan disposition. Traditionally, a standard busulfan dose of 4mg/kg/d for four days is used in most BMT/HSCT protocols. Many investigations have pointed out the pharmacodynamic relationship between a high busulfan systemic exposure and the occurrence of BMT related toxicity including hepatic veno-occlusive disease (VOD), interstitial pneumonia and alopecia in adult patients. However, studies in young patients have shown a high rate of graft failure and subsequently relapse which most probably is due to the low systemic exposure despite the standard dose schedule. In children and infants VOD was not observed with the standard doses. Increasing interest for the drug and new modification strategies for children led to higher rate of VOD and CNS toxicity when busulfan was administered according to the body surface area. More pharmacodynamic studies are required to establish the relation between the systemic exposure to busulfan and the therapeutic efficacy, especially in young children undergoing BMT or HSCT. In the present time an accurate and effective busulfan plasma level monitoring combined with dose adjustment based on the known pharmacological parameters may improve the clinical outcome for patients undergoing BMT.