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

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

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

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

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

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

The Correlation Between Busulfan Exposure and Clinical Outcomes in Chinese Pediatric Patients: A Population Pharmacokinetic Study

Aims: The aims of the study were to 1) establish a population pharmacokinetic (Pop-PK) model for busulfan in Chinese pediatric patients undergoing hematopoietic stem cell transplantation (HSCT) and then estimate busulfan exposure and 2) explore the association between busulfan exposure and clinical outcomes.

Methods: A total of 128 patients with 467 busulfan concentrations were obtained for Pop-PK modeling using nonlinear mixed effect model (NONMEM) software. Sixty-three patients who received the 16-dose busulfan conditioning regimen were enrolled to explore the correlations between clinical outcomes and the busulfan area under the concentration–time curve (AUC) using the Cox proportional hazards regression model, Kaplan–Meier method and logistic regression.

Results: The typical values for clearance (CL) and distribution volume (V) of busulfan were 7.71 L h⁻¹ and 42.4 L, respectively. The allometric normal fat mass (NFM) and maturation function (Fmat) can be used to describe the variability in CL, and the fat-free mass (FFM) can be used to describe the variability in V. Patients with AUCs of 950–1,600 µM × min had 83.7% (95% CI: 73.3–95.5) event-free survival (EFS) compared with 55.0% (95% CI: 37.0–81.8) for patients with low or high exposure (p = 0.024). The logistic regression analysis results showed no association between transplant-related toxicities and the busulfan AUC (p > 0.05).

Conclusions: The variability in busulfan CL was related to the NFM and Fmat, while busulfan V was related to the FFM. Preliminary analysis results suggested that a busulfan AUC of 950–1,600 µM × min was associated with better EFS in children receiving the 16-dose busulfan regimen.

Clinical outcomes of individualized busulfan-dosing in hematopoietic stem cell transplantation in Chinese children undergoing with therapeutic drug monitoring

To identify relationships between busulfan (Bu) exposure and outcomes of a cohort pediatric patients receiving hematopoietic stem cell transplantation (HSCT), along with a targeted busulfan-based conditioning regimen. We retrospectively evaluated targeted busulfan concentrations in 53 pediatric patients (age 0.4-16 years) who received busulfan 4 times daily according to recommended weight-based doses in a single-center analysis between 2018 and 2020. In this trial, individual busulfan pharmacokinetics were performed following dose 5 of the conditioning regimen. Twenty four of 53 patients (45.3%) studies did not require dose adjustments. Equal number of patients (24/53) required one dose adjustments while two-dose adjustment applied for 5 of 53 (9.4%). Twenty-one percent of the patients exhibited ll-lV aGVHD. The incidence of veno-occlusive disease (VOD) was in 3.8% of the 53 patients, while incidence of hemorrhagic cystitis (II-III) reached to 9.7%. Engraftment was successful in 98% of the 53 patients with relapse in 2% of cases. The probability of overall survival and disease-free survival at day 100 was 96% and 94%, respectively. In conclusion, therapeutic drug monitoring (TDM) and individualization of Bu dosage are essential to improve the efficacy and safety of busulfan-based regimen in Chinese pediatric HSCT recipients.

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

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

Two-point blood sampling is sufficient and necessary to estimate the area under the concentration-time curve for intravenous busulfan in infants and young children

BACKGROUND

Therapeutic drug monitoring for busulfan is important to prevent adverse events and improve outcomes in stem cell transplantation. We investigated intravenous busulfan pharmacokinetics and evaluated the utility of limited sampling strategy (LSS) as a simple method to estimate the area under the concentration-time curve (AUC).

PROCEDURE

The study comprised 87 busulfan measurements in 54 children who received intravenous busulfan between August 2015 and May 2020. AUCs were calculated from three to five blood sampling points in each patient, and the correlation between AUC and plasma concentrations (ng/mL) at 1, 2, 3, 4, and 6 h after initiating busulfan infusion (C₁ , C₂ , C₃ , C₄ , and C₆ , respectively).

RESULTS

By one-point sampling strategy, the most relevant predicted AUC was based on C₆ (r²  = 0.789; precision, 11.0%) in all patients. The predicted AUC based on C₆ was acceptable (r²  = 0.937; precision, 5.9%) for adolescent patients weighing >23 kg, but the correlation was poor in infants and young children weighing ≤ 23 kg (r²  = 0.782; precision, 11.4%). By two-point sampling strategy, the predicted AUC based on C₃ and C₆ showed the most relevant concentrations (r²  = 0.943; precision, 6.4%), even in infants and young children, whereas the predicted AUC based on C₃ and C₆ was acceptable (r²  = 0.963; precision, 5.7%).

CONCLUSIONS

The AUC of busulfan can be predicted based on C₆ in adolescent patients. However, there was substantial interindividual variation in busulfan pharmacokinetics in infants and young children, in whom two-point LSS was necessary for accurate AUC prediction.

Treosulfan-based conditioning for inborn errors of immunity

Inborn errors of immunity (IEI) are inherited disorders that lead to defects in the development and/or function of the immune system. The number of disorders that can be treated by haematopoietic stem-cell transplantation (HSCT) has increased rapidly with the advent of next-generation sequencing. The methods used to transplant children with IEI have improved dramatically over the last 20 years. The introduction of reduced-toxicity conditioning is an important factor in the improved outcome of HSCT. Treosulfan has myeloablative and immunosuppressive properties, enabling engraftment with less toxicity than traditionally used doses of busulfan. It is firmly incorporated into the conditioning guidelines of the Inborn Errors Working Party of the European Society for Blood and Marrow Transplantation. Unlike busulfan, pharmacokinetically guided dosing of treosulfan is not part of routine practice, but data are emerging which indicate that further improvements in outcome may be possible, particularly in infants who have a decreased clearance of treosulfan. It is likely that individualized dosing, not just of treosulfan, but of all agents used in conditioning regimens, will be developed and implemented in the future. This will lead to a reduction in unwanted variability in drug exposure, leading to more predictable and adjustable exposure, and improved outcome of HSCT, with fewer late adverse effects and improved quality of life. Such conditioning regimens can be used as the basis to study the need for additional agents in certain disorders which are difficult to engraft or require high levels of donor chimerism, the dosing of individual cellular components within grafts, and effects of adjuvant cellular or immunotherapy post-transplant. This review documents the establishment of treosulfan worldwide, as a safe and effective agent for conditioning children with IEI prior to HSCT.

Stem cell transplantation for children with hemophagocytic lymphohistiocytosis: results from the HLH-2004 study

We report the largest prospective study thus far on hematopoietic stem cell transplantation (HSCT) in hemophagocytic lymphohistiocytosis (HLH), a life-threatening hyperinflammatory syndrome comprising familial/genetic HLH (FHL) and secondary HLH. Although all patients with HLH typically need intensive anti-inflammatory therapy, patients with FHL also need HSCT to be cured. In the international HLH-2004 study, 187 children aged <18 years fulfilling the study inclusion criteria (5 of 8 diagnostic criteria, affected sibling, or molecular diagnosis in FHL-causative genes) underwent 209 transplants (2004-2012), defined as indicated in patients with familial/genetic, relapsing, or severe/persistent disease. Five-year overall survival (OS) post-HSCT was 66% (95% confidence interval [CI], 59-72); event-free survival (EFS) was 60% (95% CI, 52-67). Five-year OS was 81% (95% CI, 65-90) for children with a complete response and 59% (95% CI, 48-69) for those with a partial response (hazard ratio [HR], 2.12; 95% CI, 1.06-4.27; P = .035). For children with verified FHL (family history/genetically verified, n = 134), 5-year OS was 71% (95% CI, 62-78) and EFS was 62% (95% CI, 54-70); 5-year OS for children without verified FHL (n = 53) was significantly lower (52%; 95% CI, 38-65) (P = .040; HR, 1.69; 95% CI, 1.03-2.77); they were also significantly older. Notably, 20 (38%) of 53 patients without verified FHL had natural killer cell activity reported as normal at diagnosis, after 2 months, or at HSCT, suggestive of secondary HLH; and in addition 14 (26%) of these 53 children had no evidence of biallelic mutations despite having 3 or 4 FHL genes analyzed (natural killer cell activity not analyzed after 2 months or at HSCT). We conclude that post-HSCT survival in FHL remains suboptimal, and that the FHL diagnosis should be carefully investigated before HSCT. Pretransplant complete remission is beneficial but not mandatory to achieve post-HSCT survival. This trial was registered at www.clinicaltrials.gov as #NCT00426101.

Targeted busulfan-based reduced-intensity conditioning and HLA-matched HSCT cure hemophagocytic lymphohistiocytosis

Reduced-intensity/reduced-toxicity conditioning and allogeneic T-cell replete hematopoietic stem cell transplantation are curative in patients with hemophagocytic lymphohistiocytosis (HLH). Unstable donor chimerism (DC) and relapses are clinical challenges . We examined the effect of a reduced-intensity conditioning regimen based on targeted busulfan to enhance myeloid DC in HLH. The European Society for Bone and Marrow Transplantation-approved reduced-intensity conditioning protocol comprised targeted submyeloablative IV busulfan, IV fludarabine, and serotherapy comprising IV alemtuzumab (0.5-0.8 mg/kg) for unrelated-donor and IV rabbit anti-T-cell globulin for related-donor transplants. We assessed toxicity, engraftment, graft-versus-host disease (GHVD), DC in blood cell subtypes, and overall survival/event-free survival. Twenty-five patients from 7 centers were treated (median age, 0.68 year). The median total dose and cumulative area under the curve of busulfan was 13.1 mg/kg (6.4-26.4) and 63.1 mg/L × h (48-77), respectively. Bone marrow, peripheral blood stem cell, or cord blood transplants from HLA-matched related (n = 7) or unrelated (n = 18) donors were administered. Donor cells engrafted in all patients (median: neutrophils d+20/platelets d+28). At last follow-up (median, 36 months; range, 8-111 months), the median DC of CD15+ neutrophils, CD3+ T cells, and CD16+56+ natural killer cells was 99.5% (10-100), 97% (30-100), and 97.5% (30-100), respectively. Eight patients (32%) developed sinusoidal obstruction syndrome, resolving after defibrotide treatment. The 3-year overall survival and event-free survival rates were both 100%. None of the patients developed acute grade III to IV GHVD. Limited chronic GVHD was encountered in 4%. This regimen achieves excellent results with stable DC in patients with HLH.

Favorable outcomes of hematopoietic stem cell transplantation in children and adolescents with Diamond-Blackfan anemia

Diamond-Blackfan anemia (DBA) is a congenital pure red cell aplasia associated with congenital abnormalities and cancer predisposition. Allogeneic hematopoietic stem cell transplantation (HSCT) can correct the hematological phenotype and is indicated in transfusion-dependent patients. In 70 children reported to the German DBA and French HSCT registries, HSCT was performed from 1985 to 2017. Median age at HSCT was 5.5 years (range, 0.9-17.3 years). Two-thirds of patients (64%) were transplanted from a matched sibling donor (MSD), and most procedures were performed after the year 1999 (73%). Primary engraftment was achieved in all patients. One patient developed secondary graft failure. Cumulative incidence of acute graft-versus-host disease (GVHD) was 24% for °II-IV (95% confidence interval [CI], 16% to 37%) and 7% for °III-IV (95% CI, 3% to 17%); cumulative incidence of chronic GVHD was 11% (95% CI, 5% to 22%). The probability of chronic GVHD-free survival (cGFS) was 87% (95% CI, 79% to 95%) and significantly improved over time (<2000: 68% [95% CI, 47% to 89%] vs ≥2000: 94% [95% CI, 87% to 100%], P < .01). cGFS was comparable following HSCT from a MSD and an unrelated donor (UD). Of note, no severe chronic GVHD or deaths were reported following MSD-HSCT after 1999. The difference of cGFS in children transplanted <10 years of age compared with older patients did not reach statistical significance (<10 years: 90% [95% CI, 81% to 99%] vs 10-18 years 78% [95% CI, 58% to 98%]). In summary, these data indicate that HSCT is efficient and safe in young DBA patients and should be considered if a MSD or matched UD is available. HSCT for transfusion dependency only must be critically discussed in older patients.

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

Purpose

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

Patients and Methods

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

Results

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

Conclusion

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

Review of the Pharmacokinetics and Pharmacodynamics of Intravenous Busulfan in Paediatric Patients

We aimed to review the pharmacokinetics (PK) of intravenous busulfan in paediatric patients, identify covariate factors influencing exposure, investigate evidence of changes in PK behaviour over time, and correlate exposure with efficacy and toxicity outcomes. A literature review was undertaken of original research published between 2007 and 2019, investigating the PK and pharmacodynamics (PD) of intravenous busulfan in patients ≤ 18 years of age. The review identified 41 publications characterising the PK, and 45 publications describing the PD, of busulfan. Median typical clearance (CL) was 0.22 L/h/kg and median typical volume of distribution was 0.69 L/kg. Patient weight, age, glutathione-S-transferase A1 (GSTA1) genotype and busulfan dosing day/time were the most commonly identified factors affecting CL. Of nine studies investigating changes in CL, seven reported reduced CL over the 4-day course of treatment. Exposure monitoring methods and therapeutic targets were heterogeneous across studies. Relationships between busulfan exposure and patient outcomes were observed in five studies. One study observed a cumulative area under the concentration-time curve over all days of treatment of between 78 and 101 mg/L·h, and two studies observed an average concentration at first dose of < 600 ng/mL improved overall survival, transplant-related mortality, or relapse. One study observed increased sinusoidal obstructive syndrome with maximum busulfan concentration > 1.88 ng/mL. Patient weight, age and GSTA1 genotype are important covariates to consider when individualising busulfan therapy. Reduced busulfan CL over time may need to be accounted for, particularly in patients not receiving phenytoin co-therapy. Standardised monitoring of busulfan exposure over the entire course of treatment and further investigation of the role of busulfan metabolites and pharmacogenomics is warranted.

Risk factors for mixed chimerism in children with hemophagocytic lymphohistiocytosis after reduced toxicity conditioning

BACKGROUND

Reduced toxicity conditioning for hematopoietic stem cell transplantation of patients with hemophagocyticlymphohistiocytosis (HLH) results in favorable survival, however at the expense of relevant rates of mixed chimerism. Factors predisposing to mixed chimerism remain to be determined.

PROCEDURE

Patients with primary HLH transplanted 2009-2016 after treosulfan- or melphalan-based conditioning regimens were analyzed in a retrospective multicenter study for survival, engraftment, chimerism, and adverse events. Mixed chimerism was considered substantial if < 25% donor chimerism occurred and/or if secondary cell therapy was administered. Donor type, graft source, type of alkylating agent, type of serotherapy, and remission status were analyzed as potential risk factors in a multivariable logistic regression model.

RESULTS

Among 60 patients, engraftment was achieved in 95%, and the five-year estimated overall survival rate was 75%. Prevalence of any recipient chimerism was 48%. Substantial recipient chimerism was recorded in 32% of patients. Secondary post-HSCT cell therapy was administered in 30% of patients. A human leukocyte antigen (HLA)-mismatched donor (< 10/10) was the only significant risk factor for the occurrence of substantial recipient chimerism (P = 0.01; odds ratio, 5.8; CI 95%, 1.5-26.3).

CONCLUSION

The use of an HLA-matched donor is the most important factor to avoid substantial recipient chimerism following treosulfan -or melphalan-based conditioning in primary HLH.

Targeted-dose of busulfan: Higher risk of sinusoidal obstructive syndrome observed with systemic exposure dose above 5000 µMol⸱min. A historically controlled clinical trial

Busulfan is given in the conditioning regimens preceding hematopoietic stem cell transplantation (HSCT), and plasma levels can be monitored. A targeted, individualized systemic exposure (SE) dose can be achieved by calculating the area under the plasma concentration versus time curve (AUC). The objective of this study was to determine a cutoff value for safety for the AUC for busulfan plasma levels in patients undergoing HSCT. A total of 149 consecutive HSCT patients were studied. After an oral test dose of busulfan, we set target doses of 4000, 5000, or 6000 µMol⸱min/day, and analyzed the AUC of oral or intravenous Bu. These patients were compared with 53 historical control subjects who had received myeloablative conditioning regimen without busulfan pharmacokinetic monitoring. Using a test dose and the administration route had no impact on the sinusoidal obstructive syndrome (SOS) incidence, transplant-related mortality or 1-year overall survival. However, patients receiving busulfan at doses set up at AUC > 5000 had an increased risk to develop SOS after HSCT (hazard ratio 3.39, p = 0.034, 95% CI 1.09-10.52). Adjusting the busulfan dose according to SE levels target dose during conditioning is associated with lower rates of oral severe mucositis and SOS. A cutoff of 5000 µMol⸱min is safe and does not impair survival.

A rapid HPLC-MS/MS method for determining busulfan in hemolytic samples from children with hematopoietic stem cell transplantation

A rapid and sensitive method for the quantitative detection of busulfan (BU) in children's hemolytic samples by HPLC-tandem mass spectrometry (MS/MS) was established. In this study, the sample preparation procedure involved a one-step protein precipitation with acetonitrile (ACN) solution, and the HPLC-MS/MS method used Hypersil GOLD C₁₈ . The mobile phase consisted of 10 mM ammonium acetate solution (containing 0.1% formic acid) and ACN with a flow rate of 0.4 mL/min. Multiple reaction monitoring modes were used for quantitative analysis and the ion pairs of BU and BU-d8 were m/z 263.9 → 150.9 and 272.0 → 159.0, respectively. BU had a good linearity in the range of 0.01-10 μg mL^-1 . The intra- and inter-day relative error was between -7.21% and 8.26%, and the coefficient of variation was less than 12.64%. The average extraction recovery rate in plasma samples was 99.76% ± 6.53%, and the matrix in normal plasma and hemolyzed plasma had no significant effect on the detection results. Normal and hemolytic samples could maintain good stability at 4, 25 and -40°C. As a result, this method is particularly suitable for determining BU in hemolytic samples from children with hematopoietic stem cell transplantation (HSCT), and this study provides the methodological basis for further research on the pharmacokinetics of BU in children with HSCT.

Busulfan systemic exposure and its relationship with efficacy and safety in hematopoietic stem cell transplantation in children: a meta-analysis

Background

Busulfan (Bu) is a key component of several conditioning regimens used before hematopoietic stem cell transplantation (HSCT). However, the optimum systemic exposure (expressed as the area under the concentration-time curve [AUC]) of Bu for clinical outcome in children is controversial.

Methods

Research on pertinent literature was carried out at PubMed, EMBASE, Web of science, the Cochrane Library and ClinicalTrials.gov. Observational studies were included, which compared clinical outcomes above and below the area under the concentration-time curve (AUC) cut-off value, which we set as 800, 900, 1000, 1125, 1350, and 1500 μM × min. The primary efficacy outcome was notable in the rate of graft failure. In the safety outcomes, incidents of veno-occlusive disease (VOD) were recorded, as well as other adverse events.

Results

Thirteen studies involving 548 pediatric patients (aged 0.3–18 years) were included. Pooled results showed that, compared with the mean Bu AUC (i.e., the average value of AUC measured multiple times for each patient) of > 900 μM × min, the mean AUC value of < 900 μM × min significantly increased the incidence of graft failure (RR = 3.666, 95% CI: 1.419, 9.467). The incidence of VOD was significantly decreased with the mean AUC < 1350 μM × min (RR = 0.370, 95% CI: 0.205–0.666) and < 1500 μM × min (RR = 0.409, 95% CI: 0182–0.920).

Conclusions

In children, Bu mean AUC above the cut-off value of 900 μM × min (after every 6-h dosing) was associated with decreased rates of graft failure, while the cut-off value of 1350 μM × min were associated with increased risk of VOD, particularly for the patients without VOD prophylaxis therapy. Further well-designed prospective and multi centric randomized controlled trials with larger sample size are necessary before putting our result into clinical practices.

Cellular Therapies in Chronic Granulomatous Disease

Allogeneic hematopoietic stem cell transplantation (HSCT) has become the main curative treatment in patients with chronic granulomatous disease (CGD). CGD is caused by inherited defects of the phagolysomal NADPH-oxidase, leading to a lifelong propensity for invasive infections and granulomatous inflammation. After successful allogeneic HSCT, chronic infections and inflammation resolve and quality-of-life improves. Favorable long-term outcome after HSCT is dependent on the prevention of primary and secondary graft failure (GF), including falling myeloid donor chimerism (DC) below 10 %, and chronic graft-vs.-host-disease (cGVHD). The risk of GF and GvHD increases with the use of HLA-incompatible donors and this may outweigh the benefits of HSCT, mainly in patients with severe co-morbidities and in asymptomatic patients with residual NADPH-oxidase function. Seventeen scientific papers have reported on a total of 386 CGD-patients treated by HSCT with HLA-matched family/sibling (MFD/MSD), 9/10-/10/10-matched-unrelated volunteer (MUD) and cord blood donors. The median OS/EFS-rate of these 17 studies was 91 and 82%, respectively. The median rates of GF, cGVHD and de-novo autoimmune diseases were 14, 10, and 12%, respectively. Results after MFD/MSD and 10/10-MUD-transplants were rather similar, but outcome in adults with significant co-morbidities and after transplants with 9/10 HLA-MUD were less successful, mainly due to increased GF and chronic GVHD. Transplantation protocols using T-cell depleted haploidentical donors with post-transplant cyclophosphamide or TCR-alpha/beta depletion have recently reported promising results. Autologous gene-therapy after lentiviral transduction of HSC achieved OS/EFS-rates of 78/67%, respectively. Careful retrospective and prospective studies are mandatory to ascertain the most effective cellular therapies in patients with CGD.

Conditioning Regimens for Hematopoietic Cell Transplantation in Primary Immunodeficiency

PURPOSE OF REVIEW

Hematopoietic cell transplantation (HCT) is an established curative treatment for children with primary immunodeficiencies. This article reviews the latest developments in conditioning regimens for primary immunodeficiency (PID). It focuses on data regarding transplant outcomes according to newer reduced toxicity conditioning regimens used in HCT for PID.

RECENT FINDINGS

Conventional myeloablative conditioning regimens are associated with significant acute toxicities, transplant-related mortality, and late effects such as infertility. Reduced toxicity conditioning regimens have had significant positive impacts on HCT outcome, and there are now well-established strategies in children with PID. Treosulfan has emerged as a promising preparative agent. Use of a peripheral stem cell source has been shown to be associated with better donor chimerism in patients receiving reduced toxicity conditioning. Minimal conditioning regimens using monoclonal antibodies are in clinical trials with promising results thus far. Reduced toxicity conditioning has emerged as standard of care for PID and has resulted in improved transplant survival for patients with significant comorbidities.

Reduced Risk of Sinusoidal Obstruction Syndrome of the Liver after Busulfan-Cyclophosphamide Conditioning Prior to Allogeneic Hematopoietic Stem Cell Transplantation

The aim of this study is to evaluate the incidence of sinusoidal obstruction syndrome (SOS) of the liver and the clinical outcome after hematopoietic stem cell transplantation (HSCT) based on several modifications in our protocols. We retrospectively investigated 372 patients undergoing myeloablative conditioning with oral busulfan (Bu) and cyclophosphamide before allogeneic HSCT during 1990–2015. Patients' supportive care was changed in order to reduce the regimen‐related toxicities. Norethisterone use was terminated in 1998, therapeutic drug monitoring of Bu was initiated in 2000, and the use of liver supportive drugs, such as ursodeoxycholic acid and N‐acetyl‐L‐cysteine, were started in 2002 and 2009, respectively. In total, 26 patients (7.0%) developed SOS at a median of 19 days after transplantation. Of these 26 patients, 20 died at a median of 119 days after HSCT and 102 days after the diagnosis of SOS. The incidence of SOS decreased over time in accordance with the improvements in supportive care. The highest incidence of SOS was during 1995–1999 (16.2%) compared with 2.3% during 2010–2015. Overall survival for patients with SOS was 62%, 46%, and 27% at 100 days, 1 year, and 5 years after HSCT, respectively, compared with 92%, 77%, and 66% for those who did not develop SOS (P < 0.001). In conclusion, the incidence of SOS and related deaths were significantly decreased over the last years. Our institution pursues massive preventative and personalized measures for SOS. This strategy may also be applicable in other conditioning protocols in order to reduce the incidence of SOS and, hence, improve the clinical outcome.

Lentiviral haemopoietic stem/progenitor cell gene therapy for treatment of Wiskott-Aldrich syndrome: interim results of a non-randomised, open-label, phase 1/2 clinical study

Background

Wiskott-Aldrich syndrome is a rare, life-threatening, X-linked primary immunodeficiency characterised by microthrombocytopenia, infections, eczema, autoimmunity, and malignant disease. Lentiviral vector-mediated haemopoietic stem/progenitor cell (HSPC) gene therapy is a potentially curative treatment that represents an alternative to allogeneic HSPC transplantation. Here, we report safety and efficacy data from an interim analysis of patients with severe Wiskott-Aldrich syndrome who received lentiviral vector-derived gene therapy.

Methods

We did a non-randomised, open-label, phase 1/2 clinical study in paediatric patients with severe Wiskott-Aldrich syndrome, defined by either WAS gene mutation or absent Wiskott-Aldrich syndrome protein (WASP) expression or a Zhu clinical score of 3 or higher. We included patients who had no HLA-identical sibling donor available or, for children younger than 5 years of age, no suitable 10/10 matched unrelated donor or 6/6 unrelated cord blood donor. After treatment with rituximab and a reduced-intensity conditioning regimen of busulfan and fludarabine, patients received one intravenous infusion of autologous CD34+ cells genetically modified with a lentiviral vector encoding for human WAS cDNA. The primary safety endpoints were safety of the conditioning regimen and safety of lentiviral gene transfer into HSPCs. The primary efficacy endpoints were overall survival, sustained engraftment of genetically corrected HSPCs, expression of vector-derived WASP, improved T-cell function, antigen-specific responses to vaccinations, and improved platelet count and mean platelet volume normalisation. This interim analysis was done when the first six patients treated had completed at least 3 years of follow-up. The planned analyses are presented for the intention-to-treat population. This trial is registered with ClinicalTrials.gov (number NCT01515462) and EudraCT (number 2009-017346-32).

Findings

Between April 20, 2010, and Feb 26, 2015, nine patients (all male) were enrolled of whom one was excluded after screening; the age range of the eight treated children was 1·1–12·4 years. At the time of the interim analysis (data cutoff April 29, 2016), median follow-up was 3·6 years (range 0·5–5·6). Overall survival was 100%. Engraftment of genetically corrected HSPCs was successful and sustained in all patients. The fraction of WASP-positive lymphocytes increased from a median of 3·9% (range 1·8–35·6) before gene therapy to 66·7% (55·7–98·6) at 12 months after gene therapy, whereas WASP-positive platelets increased from 19·1% (range 4·1–31·0) to 76·6% (53·1–98·4). Improvement of immune function was shown by normalisation of in-vitro T-cell function and successful discontinuation of immunoglobulin supplementation in seven patients with follow-up longer than 1 year, followed by positive antigen-specific response to vaccination. Severe infections fell from 2·38 (95% CI 1·44–3·72) per patient-year of observation (PYO) in the year before gene therapy to 0·31 (0·04–1·11) per PYO in the second year after gene therapy and 0·17 (0·00–0·93) per PYO in the third year after gene therapy. Before gene therapy, platelet counts were lower than 20 × 10⁹ per L in seven of eight patients. At the last follow-up visit, the platelet count had increased to 20–50 × 10⁹ per L in one patient, 50–100 × 10⁹ per L in five patients, and more than 100 × 10⁹ per L in two patients, which resulted in independence from platelet transfusions and absence of severe bleeding events. 27 serious adverse events in six patients occurred after gene therapy, 23 (85%) of which were infectious (pyrexia [five events in three patients], device-related infections, including one case of sepsis [four events in three patients], and gastroenteritis, including one case due to rotavirus [three events in two patients]); these occurred mainly in the first 6 months of follow-up. No adverse reactions to the investigational drug product and no abnormal clonal proliferation or leukaemia were reported after gene therapy.

Interpretation

Data from this study show that gene therapy provides a valuable treatment option for patients with severe Wiskott-Aldrich syndrome, particularly for those who do not have a suitable HSPC donor available.

Funding

Italian Telethon Foundation, GlaxoSmithKline, and Orchard Therapeutics.

Effect of glutathione S-transferase genetic polymorphisms on busulfan pharmacokinetics and veno-occlusive disease in hematopoietic stem cell transplantation: A meta-analysis

This meta-analysis was conducted to derive an integrated conclusion about the influence of glutathione S-transferase (GST) genetic polymorphisms on busulfan pharmacokinetic (PK) parameters and veno-occlusive disease (VOD). Studies which analysed the effect of GST genetic polymorphisms on area under the curve (AUC), clearance (CL) or VOD were searched for and selected. A pooled analysis was conducted using Comprehensive Meta-Analysis programme. Nineteen studies were included in this meta-analysis. GSTA1*B and GSTM1 null genotypes significantly decreased CL_IV of busulfan (standardized difference in means (SDM) = -1.103; P = 0.019 and SDM = -0.418; P = 0.002, respectively). GSTA1*B significantly increased AUC_IV of busulfan (SDM = 0.832; P = 0.046), whereas GSTM1 did not (SDM = 0.155; P = 0.478). The PK parameters of oral busulfan did not differ according to GST genotype. GSTA1, GSTM1 and GSTP1 were not significantly associated with VOD occurrence. GSTA1 and GSTM1 genotypes affected CL_IV of busulfan, but only GSTA1 affected AUC_IV . There was no significant difference in the PK parameters of oral busulfan (CL_PO and AUC_PO ) and VOD when only GST genotypes were considered.

Monitoring of Busulphan Concentrations in Children Undergone Hematopoietic Stem Cell Transplantation: Unicentric Experience over 10 years

BACKGROUND AND OBJECTIVES

The aim of this report is to describe the experience in the management of busulphan-based conditioning regimen administered before hematopoietic stem cell transplantation (HSCT) in children.

METHODS

We report the values of the first dose AUC (area under the concentration-time curve, normal target between 3600 and 4800 ng·h/mL) in children treated with oral and intravenous busulphan, and we analyze the impact of some clinical variables in this cohort of patients.

RESULTS

82 children treated with busulphan before HSCT were eligible for the study: 57 received oral busulphan with a mean AUC of 3586 ng·h/mL, while 25 received intravenous busulphan with a mean AUC of 4158 ng·h/mL. Dose adjustment was based on first dose AUC. The dose was increased in 36 children (43.9%) and decreased in 26 patients (31.7%). Age at HSCT (P = 0.015), cumulative dose of busulphan as mg/m² (P < 0.001), busulphan dose prescribed as mg/Kg (P = 0.001), intravenous busulphan administration (P < 0.001), type of stem source cells (P = 0.016), and type of HSCT (P = 0.03) were associated with AUC levels. No statistically significant differences were found between transplant-related toxicity, acute and chronic graft versus host disease, engraftment, and AUC levels.

CONCLUSIONS

We concluded that older age at HSCT, intravenous administration of busulphan, cumulative, and prescribed dose of busulphan are associated with higher AUC levels. The absence of significant correlations between toxic events, graft failure, and AUC suggests the efficacy of busulphan concentrations monitoring in our patients.

Diagnosis and severity criteria for sinusoidal obstruction syndrome/veno-occlusive disease in pediatric patients: a new classification from the European society for blood and marrow transplantation

The advances in hematopoietic cell transplantation (HCT) over the last decade have led to a transplant-related mortality below 15%. Hepatic sinusoidal obstruction syndrome/veno-occlusive disease (SOS/VOD) is a life-threatening complication of HCT that belongs to a group of diseases increasingly identified as transplant-related, systemic endothelial diseases. In most cases, SOS/VOD resolves within weeks; however, severe SOS/VOD results in multi-organ dysfunction/failure with a mortality rate >80%. A timely diagnosis of SOS/VOD is of critical importance, given the availability of therapeutic options with favorable tolerability. Current diagnostic criteria are used for adults and children. However, over the last decade it has become clear that SOS/VOD is significantly different between the age groups in terms of incidence, genetic predisposition, clinical presentation, prevention, treatment and outcome. Improved understanding of SOS/VOD and the availability of effective treatment questions the use of the Baltimore and Seattle criteria for diagnosing SOS/VOD in children. The aim of this position paper is to propose new diagnostic and severity criteria for SOS/VOD in children on behalf of the European Society for Blood and Marrow Transplantation.

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

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

Long term outcomes of severe combined immunodeficiency: therapy implications

INTRODUCTION

Newborn screening has led to a better understanding of the prevalence of Severe Combined Immunodeficiency (SCID) overall and in terms of specific genotypes. Survival has improved following hematopoietic stem cell transplantation (HCT) with the best outcomes seen following use of a matched sibling donor. However, questions remain regarding the optimal alternative donor source, appropriate use of conditioning and the impact of these decisions on immune reconstitution and other late morbidities. Areas covered: The currently available literature reporting late effects after HCT for SCID and use of alternative therapies including enzyme replacement, alternative donors and gene therapy are reviewed. A literature search was performed on Pubmed and ClinicalTrials.gov using key words 'Severe Combined Immunodeficiency', 'SCID', 'hematopoietic stem cell transplant', 'conditioning', 'gene therapy', 'SCID newborn screening', 'TREC' and 'late effects'. Expert commentary: Newborn screening has dramatically changed the clinical presentation of newborn SCID. While the majority of patients with SCID survive HCT, data regarding late effects in these patients is limited and additional studies focused on genotype specific late effects are needed. Prospective studies aimed at minimizing the use of alkylating agents and reducing late effects beyond survival are needed. Gene therapy is being developed and will likely become a more commonly used treatment that will require separate consideration of survival and late effects.

Defibrotide for the treatment of hepatic veno-occlusive disease/sinusoidal obstruction syndrome with multiorgan failure

Hepatic veno-occlusive disease, also called sinusoidal obstruction syndrome (VOD/SOS), is a potentially life-threatening and unpredictable complication of hematopoietic stem cell transplantation (HSCT). Characterized by a prothrombotic-hypofibrinolytic state, VOD/SOS typically presents with hyperbilirubinemia, ascites, weight gain and painful hepatomegaly; VOD/SOS with multiorgan failure may be associated with >80% mortality. Treatment has been mainly supportive. However, defibrotide is now approved in the USA for treatment of hepatic VOD/SOS with renal or pulmonary dysfunction following HSCT and in the European Union for treatment of severe hepatic VOD/SOS post-HSCT. In vitro evidence suggests defibrotide may restore thrombotic-fibrinolytic balance at the endothelial level and protect endothelial cells. Defibrotide has demonstrated significant reduction in VOD/SOS-related mortality and resolved VOD/SOS-related symptoms, with a manageable safety profile.

Allogeneic Reduced-Intensity Hematopoietic Stem Cell Transplantation for Chronic Granulomatous Disease: a Single-Center Prospective Trial

PURPOSE

The purpose of this study was to evaluate engraftment and adverse events with a conditioning and prophylactic regimen intended to achieve high rates of engraftment with minimal graft-versus-host disease (GVHD) in allogeneic transplantation for chronic granulomatous disease in a single center.

METHODS

Forty patients, 37 male, with chronic granulomatous disease were transplanted. Transplant products were matched sibling peripheral blood stem cells (PBSCs) in four and matched unrelated donor (MUD) bone marrow in three, and one patient received mismatched unrelated PBSCs. Thirty-two patients received MUD PBSCs. All patients received a conditioning regimen of busulfan/alemtuzumab (with low-dose total body irradiation for MUD recipients) with sirolimus graft-versus-host disease prophylaxis.

RESULTS

Engraftment occured in 38/40 recipients (95%). Acute or chronic GVHD occurred in 18 (45%) and 5 (12.5%), respectively, with 6 episodes of grades III-IV and/or steroid refractory GVHD. Overall survival was 33/40 (82.5%) and event-free survival was 30/40 (80%). Successful engraftment was associated with myeloid and NK cell, but not CD3+ chimerism. Myeloid engraftment was greater than 70% in 30/32 recipients at mean follow-up of 3.4 years. Evidence of persistent immunodeficiency was not seen in successful transplants. Attempts to rescue failed or poorly functioning grafts were associated with unacceptable morbidity and mortality.

CONCLUSIONS

A reduced-intensity allogeneic transplant protocol based on alemtuzumab and busulfan with sirolimus GVHD prophylaxis produced high rates of successful engraftment and minimal regimen-related toxicity. Prolonged clinical follow-up has confirmed its efficacy in ameliorating CGD-related disease. Outcomes were not acceptable with donor cell infusion rescue of cause with poor graft function.

Current Knowledge and Priorities for Future Research in Late Effects after Hematopoietic Stem Cell Transplantation (HCT) for Severe Combined Immunodeficiency Patients: A Consensus Statement from the Second Pediatric Blood and Marrow Transplant Consortium International Conference on Late Effects after Pediatric HCT

Severe combined immunodeficiency (SCID) is 1 of the most common indications for pediatric hematopoietic cell transplantation (HCT) in patients with primary immunodeficiency. Historically, SCID was diagnosed in infants who presented with opportunistic infections within the first year of life. With newborn screening (NBS) for SCID in most of the United States, the majority of infants with SCID are now diagnosed and treated in the first 3.5 months of life; however, in the rest of the world, the lack of NBS means that most infants with SCID still present with infections. The average survival for SCID patients who have undergone transplantation currently is >70% at 3 years after transplantation, although this can vary significantly based on multiple factors, including age and infection status at the time of transplantation, type of donor source utilized, manipulation of graft before transplantation, graft-versus-host disease prophylaxis, type of conditioning (if any) utilized, and underlying genotype of SCID. In at least 1 study of SCID patients who received no conditioning, long-term survival was 77% at 8.7 years (range out to 26 years) after transplantation. Although a majority of patients with SCID will engraft T cells without any conditioning therapy, depending on genotype, donor source, HLA match, and presence of circulating maternal cells, a sizable percentage of these will fail to achieve full immune reconstitution. Without conditioning, T cell reconstitution typically occurs, although not always fully, whereas B cell engraftment does not, leaving some molecular types of SCID patients with intrinsically defective B cells, in most cases, dependent on regular infusions of immunoglobulin. Because of this, many centers have used conditioning with alkylating agents including busulfan or melphalan known to open marrow niches in attempts to achieve B cell reconstitution. Thus, it is imperative that we understand the potential late effects of these agents in this patient population. There are also nonimmunologic risks associated with HCT for SCID that appear to be dependent upon the genotype of the patient. In this report, we have evaluated the published data on late effects and attempted to summarize the known risks associated with conditioning and alternative donor sources. These data, while informative, are also a clear demonstration that there is still much to be learned from the SCID population in terms of their post-HCT outcomes. This paper will summarize current findings and recommend further research in areas considered high priority. Specific guidelines regarding a recommended approach to long-term follow-up, including laboratory and clinical monitoring, will be forthcoming in a subsequent paper.

Targeted busulfan and fludarabine-based conditioning for bone marrow transplantation in chronic granulomatous disease

Chronic granulomatous disease (CGD) is a primary immunodeficiency disease caused by impaired phagocytic function. Hematopoietic stem cell transplantation (HSCT) is a definitive cure for CGD; however, the use of HSCT is limited because of associated problems, including transplantation-related mortality and engraftment failure. We report a case of a patient with CGD who underwent successful HSCT following a targeted busulfan and fludarabine reduced-toxicity myeloablative conditioning. Intravenous busulfan was administered once daily for 4 consecutive days (days -8 to -5), and the target area under the curve was 75,000 µg·hr/L. Fludarabine (40 mg/m²) was administered once daily for 6 consecutive days from days -8 to -3. Antithymocyte globulin (2.5 mg/kg/day) was administered from days -4 to -2. The patient underwent successful engraftment and did not have any severe toxicity related to the transplantation. Conditioning with a targeted busulfan and fludarabine regimen could provide a better outcome for HSCT in CGD, with close regulation of the busulfan dose.

Glutathione Transferase Gene Variants Influence Busulfan Pharmacokinetics and Outcome After Myeloablative Conditioning

Supplemental Digital Content is Available in the Text.

Background:

Busulfan (Bu) and cyclophosphamide (Cy) are frequently included in conditioning regimens before hematopoietic stem cell transplantation (HSCT). Both drugs are detoxified by glutathione transferases (GST), and GST gene variants may explain some of the interindividual variability in pharmacokinetics and drug toxicity.

Methods:

The study investigated adult patients (n = 114) receiving oral Bu pre-HSCT. Bu doses were adjusted to obtain an average steady-state concentration (C_ss) of 900 mcg/L.

Results:

Median first dose Bu C_ss was 1000 mcg/L (600–1780 mcg/L). Patients carrying 1 and 2 GSTA1*B (rs3957357) alleles demonstrated median 12% and 16% higher Bu C_ss (P ≤ 0.05). Bu exposure (average C_ss; odds ratio = 1.009, 95% confidence interval = 1.002–1.017, P = 0.013) and GSTM1 gene copy number (odds ratio = 17.1, 95% confidence interval = 1.46–201, P = 0.024) were significant predictors of mortality ≤30 days. The mortality was 25% versus 2% among carriers of 2 versus no GSTM1 copies (P = 0.021). Mortality ≤3 months was associated with higher first dose Bu exposure (1090 versus 980 mcg/L, P = 0.021). GSTM1 expression and high Bu exposure may increase Cy toxicity by reducing intracellular glutathione.

Conclusions:

GST genotyping before HSCT may allow better prediction of Bu pharmacokinetics and drug toxicity, and thereby improve outcome after BuCy conditioning.

Hematopoietic stem cell transplantation for homozygous β-thalassemia and β-thalassemia/hemoglobin E patients from haploidentical donors

Thalassemia-free survival after allogeneic stem cell transplantation (SCT) is about 80-90% with either matched-related or -unrelated donors. We explored the use of a mismatched-related ('haplo- ') donor. All patients received two courses of pretransplant immunosuppressive therapy (PTIS) with fludarabine (Flu) and dexamethasone (Dxm). After two courses of PTIS, a conditioning regimen of rabbit antithymocyte globulin, Flu and IV busulfan (Bu) was given followed by T-cell-replete peripheral blood progenitor cells. GvHD prophylaxis consisted of cyclophosphamide (Cy) on days SCT +3 and +4 (post-Cy), and on day SCT +5 tacrolimus or sirolimus was started together with a short course of mycophenolate mofetil. Thirty-one patients underwent haplo-SCT. Their median age was 10 years (range, 2-20 years). Twenty-nine patients engrafted with 100% donor chimerism. Two patients suffered primary graft failure. Median time to neutrophil engraftment was 14 days (range, 11-18 days). Five patients developed mild to moderate, reversible veno-occlusive disease, while nine patients developed acute GvHD grade II. Only five patients developed limited-chronic GvHD. Projected overall and event-free survival rates at 2 years are 95% and 94%, respectively. The median follow up time is 12 months (range, 7-33 months).

Influence of glutathione S-transferase gene polymorphisms on busulfan pharmacokinetics and outcome of hematopoietic stem-cell transplantation in thalassemia pediatric patients

Hematopoietic stem-cell transplantation (HSCT) is currently the only curative therapeutic option for the treatment of thalassemia. In spite of the high cure rate, HSCT can lead to life-threatening adverse events in some patients. Busulfan (Bu) is a key component of the conditioning regimen prior to HSCT. Inter-individual differences in Bu pharmacokinetics (PK) are hypothesized to influence Bu efficacy and toxicity. Since Bu is mainly metabolized by glutathione S-transferase (GST), we investigated the relationship of GSTA1 and GSTM1 genotypes with first-dose PK and HSCT outcomes in 44 children with thalassemia intermedia and thalassemia major. All children received a myeloablative conditioning regimen with IV Bu. Association analysis revealed a relationship between GSTA169C>T (or haplotype *A/*B) and first Bu dose PK that was dependent on sex and Pesaro risk classification (PRC). Among female patients and patients with PRC I-II, homozygous individuals for the GSTA1T-69 allele defining haplotype *B, had higher Bu exposure and lower clearance (P⩽0.01). Association with HSCT outcomes showed that patients with the GSTM1 null genotypes had higher occurrence of regimen-related toxicity (P=0.01). These results suggest that GST genotypes could be useful to tailor the first Bu dose accordingly to improve HSCT outcome.

Should busulfan therapeutic range be narrowed in pediatrics? Experience from a large cohort of hematopoietic stem cell transplant children

Busulfan, the corner stone of hematopoietic stem cell transplantation regimens, has a narrow therapeutic window. Therapeutic drug monitoring (TDM)-guided dosing to reach the conventional area under the concentration-time curve (AUC) target range of 900-1500 μmol min/L is associated with better outcomes. We report our experience with busulfan TDM in a large cohort of children. The aims were to investigate the relevance of using a more restricted therapeutic range and investigate the association between busulfan therapeutic range and clinical outcome. This study includes 138 children receiving 16 doses of intravenous busulfan, with the first dose assigned based on weight and doses adjusted to a local AUC target range of 980-1250 μmol min/L. Busulfan TDM combined with model-based dose adjustment was associated with an increased probability of AUC target attainment, for both target range: 90.8% versus 74.8% for the conventional target range and 66.2% versus 43.9% for the local target range (P<0.001). The median follow-up was 56.2 months. Event-free survival was 88.5%, overall survival was 91.5% and veno-occlusive disease occurred in 18.3% of patients. No difference was observed for clinical outcomes depending on the selected target range. Pharmacokinetic monitoring and individualization of busulfan dosage regimen are useful in improving target attainment, but using a restricted target range has no impact on clinical outcomes.

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.