Risk of Seizures in Children Receiving Busulphan-Containing Regimens for Stem Cell Transplantation

Impact of prophylactic echinocandin on the development of neurological complications in patients receiving busulfan-containing conditioning regimens for stem cell transplantation: A single-center retrospective study

BACKGROUND

Although neurotoxicity is a major adverse event associated with busulfan, little information is available regarding the association between drug interactions and neurological symptoms during busulfan-based regimens. This study evaluated the association between prophylactic echinocandins and neurological complications in patients receiving busulfan-containing conditioning regimens for stem cell transplantation.

METHODS

We retrospectively included consecutive patients who administered intravenous busulfan as a conditioning regimen at our facility between 2007 and 2022. Prophylactic echinocandin use was defined as the use of an echinocandin antifungal drug to prevent invasive fungal disease in SCT recipients. The primary outcome was the incidence of neurological complications within 7 days of busulfan initiation and was compared between the echinocandin group (patients received prophylactic echinocandin) and nonechinocandin group (patients received prophylactic antifungal drugs other than echinocandin and those without antifungal prophylaxis).

RESULTS

Among the 59 patients included in this study, the incidence of neurological complications in the echinocandin (n = 26) and nonechinocandin groups (n = 33) was 30.8% and 63.6%, respectively. We observed a negative association between prophylactic echinocandin use and the development of neurological complications after adjusting for the propensity score for receiving prophylactic echinocandins (adjusted odds ratio 0.294, 95% confidence interval 0.090 to 0.959). We observed a lower incidence of neurological complications in the echinocandin group than in the nonechinocandin group.

CONCLUSION

Our results suggested that the choice of antifungal prophylaxis is associated with busulfan neurotoxicity.

Plasma ubiquitin C-terminal hydrolase-L1 (UCH-L1) level as a blood biomarker of neurological damage after allogeneic hematopoietic cell transplantation

Several biofluid-based biomarkers for traumatic brain injury show promise for use in diagnosis and outcome prediction. In contrast, few studies have investigated biomarkers for non-traumatic brain injury. We focused on ubiquitin C-terminal hydrolase-L1 (UCH-L1), which has been proposed as a screening tool for traumatic brain injury, and investigated whether the plasma UCH-L1 level could also be a useful biomarker in patients with non-traumatic brain injury. We measured UCH-L1 in 25 patients who had experienced neurological complications after allogeneic hematopoietic cell transplantation (HCT) and 22 control patients without any complications or graft-versus-host disease. Although UCH-L1 levels before HCT did not differ significantly (P = 0.053), levels after HCT were higher in patients with neurological complications compared with the control group (P < 0.001). At a UCH-L1 cutoff value of 0.072 ng/ml, sensitivity was 68.0% and specificity was 100%. The statistical power of UCH-L1 for neurological complications seemed to be higher than that of CT and comparable to that of MRI. Thus, increased levels of UCH-L1 might reflect the presence of neurological damage even in patients with non-traumatic brain injury. Further large cohort investigations are warranted.

An innovative hematopoietic stem cell gene therapy approach benefits CLN1 disease in the mouse model

Hematopoietic stem and progenitor cells (HSPCs) can establish a long-lasting microglia-like progeny in the central nervous system of properly myeloablated hosts. We exploited this approach to treat the severe CLN1 neurodegenerative disorder, which is the most aggressive form of neuronal ceroid lipofuscinoses due to palmitoyl-protein thioesterase-1 (PPT1) deficiency. We here provide the first evidence that (i) transplantation of wild-type HSPCs exerts partial but long-lasting mitigation of CLN1 symptoms; (ii) transplantation of HSPCs over-expressing hPPT1 by lentiviral gene transfer enhances the therapeutic benefit of HSPCs transplant, with first demonstration of such a dose-effect benefit for a purely neurodegenerative condition like CLN1 disease; (iii) transplantation of hPPT1 over-expressing HSPCs by a novel intracerebroventricular (ICV) approach is sufficient to transiently ameliorate CLN1-symptoms in the absence of hematopoietic tissue engraftment of the transduced cells; and (iv) combinatorial transplantation of transduced HSPCs intravenously and ICV results in a robust therapeutic benefit, particularly on symptomatic animals. Overall, these findings provide first evidence of efficacy and feasibility of this novel approach to treat CLN1 disease and possibly other neurodegenerative conditions, paving the way for its future clinical application.

Optimal regimen of levetiracetam for prevention of busulfan-induced seizure in patients undergoing hematopoietic stem cell transplantation: A review of available evidence

OBJECTIVE

This review aimed to summarize the available data and offer a practical recommendation regarding the optimal regimen of levetiracetam (LEV) for the prevention of busulfan-induced seizure (BIS) in patients undergoing hematopoietic stem cell transplantation (HSCT).

DATA SOURCES

Published articles by searching databases (PubMed, Google Scholar, Cochrane Library, ScienceDirect) were reviewed. All types of original studies performed in pediatric and adult populations have been investigated and required data was extracted.

DATA SUMMARY

Eleven articles were eligible to be included in this review. A loading dose was not used in any of the studies. LEV had been started from 6 to 48 h before busulfan (Bu) initiation and continued up to 24 to 48 h after its termination. The dose range of LEV was 10 to 20 mg/kg/day divided every 12 h in pediatrics and 500 to 1000 mg twice daily in adults. Both oral and intravenous (IV) routes of administration were used. Except for three studies, no seizure had occurred in patients who had received LEV.

CONCLUSIONS

Considering the available evidence, LEV with the dose range from 500 to 1000 mg twice daily in adults and 10 mg/kg twice daily (20 mg/kg/day in 2 divided doses) in children orally or IV started from 6 to 24 h before Bu initiation up to 24 to 48 h after the last dose of Bu seems to prevent BIS appropriately. More prospective clinical trials with a larger population are needed to validate the optimal dosing of LEV for BIS prophylaxis in patients undergoing HSCT.

Impact of valproic acid on busulfan pharmacokinetics: In vitro assessment of potential drug-drug interaction

Busulfan (Bu) is an alkylating agent commonly used at high doses in the preparative regimens of hematopoietic stem cell transplantation (HSCT). It has been shown that such high doses of Bu are associated with generalized seizures which are usually managed by prophylactic antiepileptic drugs (AEDs) such as valproic acid (VPA). Being a strong enzyme inhibitor, VPA may inhibit Bu metabolism and thus increase its potential toxicity. Despite its clinical relevance, the potential interaction between Bu and VPA has not yet been evaluated. The aim of the present study was to assess and evaluate the potential drug-drug interaction (DDI) between Bu and VPA. This study was carried out by incubating Bu in laboratory-prepared rat liver-subcellular fractions including S9, microsomes, and cytosol, alone or in combination with VPA. The liver fractions were prepared by differential centrifugation of the liver homogenate. Analysis of Bu was employed using a fully validated LC-MS/MS method. The validation parameters were within the proposed limits of the international standards guidelines. Bu metabolic stability was assessed by incubating Bu at a concentration of 8 μg/ml in liver fractions at 37°C. There were significant reductions in Bu levels in S9 and cytosolic fractions, whereas these levels were not significantly (P ˃ 0.05) changed in microsomes. However, in presence of VPA, Bu levels in S9 fraction remained unchanged. These results indicated, for the first time, the potential metabolic interaction of Bu and VPA being in S9 only. This could be explained by inhibiting Bu cytosolic metabolism by the interaction with VPA either by sharing the same metabolic enzyme or the required co-factor. In conclusion, the present findings suggest, for the first time, a potential DDI between Bu and VPA in vitro using rat liver fractions. Further investigations are warranted in human-derived liver fractions to confirm such an interaction.

High-dose thiotepa, in conjunction with melphalan, followed by autologous hematopoietic stem cell transplantation in patients with pediatric solid tumors, including brain tumors

Among pediatric malignancies, solid tumors, particularly within the central nervous system (CNS), are common. Thiotepa, a myeloablative, high-dose chemotherapeutic (HDT) treatment administered prior to autologous hematopoietic stem cell transplantation (HSCT), can cross the blood-brain barrier and rapidly penetrate the CNS. We evaluated thiotepa HDT in conjunction with melphalan in Japanese patients with pediatric CNS/non-CNS solid tumors in a multicenter, open-label, non-comparative study. Thiotepa (200 mg/m²/day) was administered intravenously (IV) over 24 h on days -12, -11, -5, and -4 before scheduled HSCT. Melphalan (70 mg/m²/day) was administered IV over 1 h on days -11, -5, and -4. The safety analysis population comprised 41 patients, of whom 16 (39.0%) had solid tumors and 25 (61.0%) had brain tumors. The most frequently reported adverse events were diarrhea (40/41 [97.6%] patients) and febrile neutropenia (34/41 [82.9%]). No unexpected safety events were observed, and no events resulted in death or treatment discontinuation. All patients experienced bone marrow suppression and 39/41 (95.1%) achieved engraftment (neutrophil count ≥500/mm³ for 3 consecutive days after HSCT). The survival rate at day 100 post-autologous HSCT was 100%. These data confirm the safety of IV thiotepa plus melphalan HDT prior to autologous HSCT for patients with pediatric CNS/non-CNS solid tumors. Trial registration: JapicCTI-173654.

Acute Graft-Versus-Host Disease, Infections, Vascular Events and Drug Toxicities Affecting the Central Nervous System

Allogeneic hematopoietic cell transplantation (allo-HCT) is a curative therapy for patients with hematological malignancies. Acute Graft versus host diseases (GVHD) is a major immune mediated side effect of allo-HCT that can affect the central nervous system (CNS) in addition to post-allo-HCT vascular events, drug toxicity or infections. Here we summarize and discuss recent preclinical data on the CNS as a target of acute GVHD and the known mechanisms contributing to neurotoxicity with a focus on microglia and T cells. We also discuss open questions in the field and place the findings made in mouse models in a clinical context. While in mice the neurological deficits can be assessed in a controlled fashion, in patients the etiology of the CNS damage is difficult to attribute to acute GVHD versus infections, vascular events, and drug-induced toxicity. Ultimately, we discuss novel therapies for GVHD of the CNS. Our understanding of the biological mechanisms that lead to neurotoxicity after allo-HCT increased over the last decade. This review provides insights into CNS manifestations of GVHD versus other etiologies of CNS damage in mice and patients.

Epileptic Seizures After Allogeneic Hematopoietic Stem Cell Transplantation

Technique in allogeneic hematopoietic stem cell transplantation has greatly advanced over the past decades, which has led to an increase in the number of patients receiving transplantation, but the complex procedure places these transplant recipients at high risk of a large spectrum of complications including neurologic involvement. As a common manifestation of neurological disorders, epileptic seizures after transplantation have been of great concern to clinicians because it seriously affects the survival rate and living quality of those recipients. The aim of this review is to elucidate the incidence of seizures after allogeneic hematopoietic stem cell transplantation, and to further summarize in detail its etiologies, possible mechanisms, clinical manifestations, therapeutic schedule, and prognosis, hoping to improve doctors' understandings of concurrent seizures following transplantation, so they can prevent, process, and eventually improve the survival and outlook for patients in a timely manner and correctly.

Comparison of levetiracetam versus phenytoin/fosphenytoin for busulfan seizure prophylaxis at a pediatric institution

INTRODUCTION

Busulfan is a chemotherapy agent used in hematopoietic stem cell transplant (HSCT) conditioning regimens. Busulfan is associated with tonic-clonic seizures in ~10% of patients if administered without seizure prophylaxis. Historically, phenytoin was the most commonly utilized seizure prophylaxis agent; however, phenytoin is associated with CYP450 drug interactions and potentially increases the clearance of busulfan. Levetiracetam is being used more recently for busulfan seizure prophylaxis and is not associated with drug-drug interactions; however, data supporting use in pediatric patients are limited. The primary objective is to determine whether there is any difference in seizure rates or safety profile between phenytoin and levetiracetam when used for seizure prophylaxis.

METHODS

We conducted a retrospective chart review including patients who received busulfan between 2010 and 2019 were identified. The data were evaluated to compare the incidence of busulfan-induced seizures in HSCT patients receiving either phenytoin or levetiracetam and to determine the impact of drug-drug interactions on treatment outcomes/adverse events.

RESULTS

A total of 342 patients were included with a median age of six years. Overall, five patients within the phenytoin group (n = 126) (4%) and zero patients in the levetiracetam group (n = 216) experienced a seizure (P = .007). There were no differences in liver enzyme elevations, recurrence rates of primary disease, and veno-occlusive disease.

CONCLUSION

Levetiracetam is effective at preventing seizures associated with busulfan administration with no clinically significant adverse effects when compared to phenytoin.

A Review of Acute and Long-Term Neurological Complications Following Haematopoietic Stem Cell Transplant for Paediatric Acute Lymphoblastic Leukaemia

Despite advances in haematopoietic stem cell transplant (HSCT) techniques, the risk of serious side effects and complications still exists. Neurological complications, both acute and long term, are common following HSCT and contribute to significant morbidity and mortality. The aetiology of neurotoxicity includes infections and a wide variety of non-infectious causes such as drug toxicities, metabolic abnormalities, irradiation, vascular and immunologic events and the leukaemia itself. The majority of the literature on this subject is focussed on adults. The impact of the combination of neurotoxic drugs given before and during HSCT, radiotherapy and neurological complications on the developing and vulnerable paediatric and adolescent brain remains unclear. Moreover, the age-related sensitivity of the nervous system to toxic insults is still being investigated. In this article, we review current evidence regarding neurotoxicity following HSCT for acute lymphoblastic leukaemia in childhood. We focus on acute and long-term impacts. Understanding the aetiology and long-term sequelae of neurological complications in children is particularly important in the current era of immunotherapy for acute lymphoblastic leukaemia (such as chimeric antigen receptor T cells and bi-specific T-cell engager antibodies), which have well-known and common neurological side effects and may represent a future treatment modality for at least a fraction of HSCT-recipients.

Noninfectious neurologic complications of hematopoietic cell transplantation: A systematic review

OBJECTIVE/BACKGROUND

Neurological complications occur at a high frequency after hematopoietic cell transplantation (HCT). However, an absence is noted in the published literature as regards the quantification of the exact burden and the outcomes thereof. In this systematic review, we endeavored to detail if the recipients of HCT developed any noninfectious neurological events/complications.

METHODS

According to the PICO criteria, medical literature was searched. Complications that were evaluated included: stroke, peripheral neuropathy, myasthenia gravis, seizures, and posterior reversible encephalopathy syndrome. After strictly defining relevant variables and parameters, data from 173 eligible articles were then extracted accordingly, from the full text for each, for quantitative analysis; additionally, two American Society of Hematology conference abstracts were also subject to data extraction.

RESULTS

As is evident from the results of the data analysis, an increased frequency of these complications was seen in the HCT recipient population in comparison to the general population. The relative risk ranged from 1.33× to 142× depending on the complication studied.

CONCLUSION

These findings demonstrate that the recipients of HCT had a significantly higher risk of neurological complications and that their early recognition can enhance the monitoring of HCT survivors for the early developmental signs of neurological toxicity. This would facilitate timely interventions, thus ensuring a better quality of life.

Oral levetiracetam for prevention of busulfan-induced seizures in adult hematopoietic cell transplant

Background Antiseizure prophylaxis is recommended when high-dose of busulfan is given as part of the conditioning regimens in the allogenic hematopoietic stem cell transplant. Phenytoin has been widely used but its pharmacokinetics and pharmacodynamics profile makes its use complicated. Levetiracetam is a safe, effective and well tolerated antiseizure drug with good results in epileptic patients. Objective To describe our experience using oral (p.o) levetiracetam 1000 mg every 12 h (q12h) as an antiseizure prophylaxis, evaluating its preventive effects and adverse event rates after a high-dose of intravenous (i.v.) busulfan, as part of the conditioning regimen Methods Retrospective study of patients who underwent an allogenic hematopoietic stem cell transplant with a conditioning regimen based on high-dose of busulfan between January and November 2017. Results The study population comprised 36 patients, of whom 18 (50%) had acute myeloid leukemia as diagnosis. No seizures occurred in any patient. Levetiracetam was well tolerated and no serious adverse events were reported. Conclusions Our results suggest that giving levetiracetam at 1000 mg q12h p.o starting 12 h before the administration of i.v. busulfan until 48 h after the last dose, can be used as an alternative in the prevention of busulfan-induced seizures in adults.

Nanostructured liquid crystalline formulation as a remarkable new drug delivery system of anti-epileptic drugs for treating children patients

PURPOSE

Development of a new dosage-form of antiepileptic-drugs appropriated for children.

METHODS

Clonazepam (Cl) was formulated as cubosomal-gel (cub-gel) to be used as a patch reservoir through transdermal-route. Cubosomes prepared using glycerol-mono-oleate(GMO)/Pluronic-F127(PF127) mixture. An actual-statistical design was used to investigate the effect of different stabilizing agents (Ethanol and PVA) and surfactant concentration on cubosomes' particle size and entrapping-efficiency. The selected formulae were evaluated by testing particle-morphology, in vitro drug release and stability. Cub-gel was prepared using selected cubosome formulae. The optimal cub-gel subjected to in vitro dissolution, ex-vivo permeation and skin deposition studies followed by studying its pharmacological effect.

RESULTS

Using PVA or Et as stabilizers with PF127 significantly decreases the average cubosomes'PS (352 ±  2.8 and 264 ± 2.16 nm) and increases EE (58.97 ± 4.57% and 54.21 ± 3.89%). Cubosomes increase the initial release rate of Cl to ensure rapid therapeutic effect (37.39% and 46.04% in the first hour) followed by a prolonged release till 4 h. Cub-gel containing PVA showed significantly higher Cl-transdermal permeation when compared to Cl-suspension. Moreover, increases the retention-time (89.57% at 48 h) and skin-deposition up to 6-times. It also reduces the epileptic seizures and alters the behavioral parameters induced by pilocarpine.

CONCLUSIONS

Cubosomal-gel could be considered an innovative dosage-form for Cl through the transdermal route.

Seizures after transplantation

PURPOSE

To summarize information on the history, incidence, clinical manifestation, best treatment, as well as prognosis of seizures in transplant recipients.

METHODS

In October 2017, we searched the literature on PubMed in English with the search terms: "transplantation" AND "seizure", "transplantation" AND "epilepsy", "transplantation"AND "status epilepticus", "immunosuppressant" AND "seizure", "immunosuppressant" AND "epilepsy". Publications not based on new data and original research were not included in this article.

RESULTS

Seizures including generalized seizures, focal seizures and status epilepticus are a common central nervous system complication after transplantation. The incidence of seizures varied between different kinds of transplantations. The reported incidence of seizures was 7%-27% in association with solid organ transplantations and 1.6%-15.4% with hematopoietic stem cell transplantation. Most of seizures appeared in the early post-transplantation period. Patients often had a favorable prognosis, however, in some conditions, recurrent or intractable seizures may occur.

CONCLUSIONS

The underlying pathogenesis of new-onset seizures or epilepsy in recipients of transplantation needs to be further elucidated. In addition, more information is required from prospective studies and research focusing on therapeutic strategies.

Comparison of levetiracetam with phenytoin for the prevention of intravenous busulfan-induced seizures in hematopoietic cell transplantation recipients

Purpose

Busulfan is used as a conditioning regimen for hematopoietic stem cell transplantation and is known to cause seizures as a side effect. As various anticonvulsant drugs have been reported, we conducted a retrospective investigation regarding the preventive effects and adverse events associated with different anticonvulsants administered alongside intravenous busulfan (ivBu) in our institution.

Methods

We targeted 104 patients who received ivBu at our institution from May 1, 2010 to April 30, 2017. We investigated the seizure prevention rate and adverse events rate under anticonvulsant prophylaxis.

Results

There were 70 cases (67.3%) of phenytoin administration and 34 cases (32.7%) of levetiracetam administration for anticonvulsant therapy. The seizure prevention rate was 98.6% for phenytoin and 100% for levetiracetam; seizures occurred in one out of 104 patients. There were no significant differences in the seizure prevention rate depending on the type of anticonvulsant. Further, there were no differences in adverse events.

Conclusions

Anticonvulsant prophylaxis is considered necessary for safe conditioning with ivBu. Adverse events associated with the use of levetiracetam are within an acceptable range. Further, levetiracetam is considered useful as a preventive drug against seizures during ivBu administration because it is easy to administer and has ideal pharmacokinetics for supportive care.

Central Nervous System Complications in Children Receiving Chemotherapy or Hematopoietic Stem Cell Transplantation

Therapy-related neurotoxicity greatly affects possibility of survival and quality of life of pediatric patients treated for cancer. Central nervous system (CNS) involvement is heterogeneous, varying from very mild and transient symptoms to extremely severe and debilitating, or even lethal syndromes. In this review, we will discuss the broad scenario of CNS complications and toxicities occurring during the treatment of pediatric patients receiving both chemotherapies and hematopoietic stem cell transplantation. Different types of complications are reviewed ranging from therapy related to cerebrovascular with a specific focus on neuroradiologic and clinical features.

Neurologic Complications after Allogeneic Hematopoietic Stem Cell Transplantation

Neurologic complications after hematopoietic stem cell transplantation are frequently life-threatening, and their clinical management can be highly challenging. A wide spectrum of causative factors-including drug-related toxicities; infections sustained by virus, bacteria, or invasive molds; metabolic encephalopathy; cerebrovascular disorders; immune-mediated disorders; and disease recurrence-may lead to potentially lethal complications. Moreover, given that some neurologic complications are not uncommonly diagnosed post mortem, their overall incidence is likely to be underestimated. Their prompt recognition and timely treatment are of paramount importance to reduce the risk for transplantation-related death.

Non-myeloablative preconditioning with ACK2 (anti-c-kit antibody) is efficient in bone marrow transplantation for murine models of mucopolysaccharidosis type II

Mucopolysaccharidosis type II (MPS II) is a lysosomal storage disease caused by the deficient activity of iduronate 2-sulfatase (IDS), which is involved in the lysosomal catabolism of the glycosaminoglycans (GAGs) dermatan and heparan sulfate. Such a deficiency leads to the accumulation of undegraded GAGs in some organs. Although enzyme replacement therapy is available as a treatment of MPS II, there are some limitations, such as the requirement of weekly administration for whole life. To avoid such limitations, hematopoietic cell transplantation (HSCT) is a possible alternative. In fact, some report suggested positive effects of HSCT for MPS II. However, HSCT has also some limitations. Strong conditioning regimens can cause severe side effects. For overcome this obstacle, we studied the efficacy of ACK2, an antibody that blocks KIT, followed by low-dose irradiation as a preconditioning regimen for HSCT using a murine model of MPS II. This protocol achieves 58.7±4.92% donor chimerism at 16weeks after transplantation in the peripheral blood of recipient mice. GAG levels were significantly reduced in liver, spleen, heart and intestine. These results indicated that ACK2-based preconditioning might be one of the choices for MPS II patients who receive HSCT.

Central Nervous System Complications of Hematopoietic Stem Cell Transplant

Hematopoietic stem cell transplantation (SCT) is now commonly used to treat several hematologic and nonhematologic diseases. Central nervous system (CNS) complications post-transplantation occur commonly in the first year and result in increased mortality from infectious, toxic, metabolic, or vascular causes. Infections secondary to aspergillus, toxoplasma and viruses cause many of the complications. Drug-related toxicities arising from conditioning regimens and graft-versus-host disease prophylaxis, as well as intraparenchymal hemorrhage, are not uncommon and can result in increased morbidity. Secondary CNS cancers have a higher incidence 5 or more years after allogeneic SCT.

Single Daily Busulfan Dosing for Infants with Nonmalignant Diseases Undergoing Reduced-Intensity Conditioning for Allogeneic Hematopoietic Progenitor Cell Transplantation

Busulfan (Bu) is widely used in conditioning regimens for infants undergoing allogeneic hematopoietic progenitor cell transplantation (HPCT), but the best approach to administer Bu in this population is still unknown. Here, we report a single-center experience of the use of a test dose to guide dose adjustment of intravenous (i.v.) Bu therapy in infants. Between 2004 and 2013, 33 infants younger than 1 year with nonmalignant conditions received allogeneic peripheral blood or cord blood HPCT after a reduced-intensity conditioning (RIC) regimen consisting of fludarabine, antithymocyte globulin, and 2 single daily doses of i.v. Bu. Pharmacokinetic results of a test dose of i.v. Bu (.8 mg/kg) were used to determine the dose of 2 single daily i.v. Bu regimen doses, adjusted to target an area under the curve (AUC) of 4000 μMol*minute per day in a first cohort (n = 12) and 5000 μMol*minute in a second cohort (n = 21). The mean Bu clearance in our infant patients was found to be 3.67 ± 1.03 mL/minute/kg, and the test dose clearance was highly predictive of the regimen dose clearance. The mean AUC achieved after the first single daily regimen dose was 3951 ± 1239 in the AUC 4000 cohort and 4884 ± 766 for the AUC 5000 cohort. No patient in either cohort developed hepatic sinusoidal obstructive syndrome or seizures attributable to Bu. Primary graft failure occurred in 4 patients and secondary graft failure occurred in 3, predominantly in the AUC 4000 cohort (6 of 7). Among the engrafted patients (n = 28), 16 achieved full donor chimerism and 9 patients attained stable mixed chimerism. Overall survival of patients at 6 years after transplantation was 59.5% for the AUC 4000 cohort and 85.4% for the AUC 5000 cohort, with primary graft failure in the first cohort being a major contributor to morbidity. Logistic regression analysis showed that the risk of graft failure increased significantly if cord blood hematopoietic progenitor cells were used or if total Bu exposure was below 4000 μMol*minute per day for 2 days. The difference in clinical outcomes between the 2 cohorts supports the conclusion that targeting a higher Bu AUC of 5000 μMol*minute per day for 2 days improves donor engraftment in infants with nonmalignant conditions undergoing RIC HPCT without increasing toxicity. Measuring i.v. Bu pharmokinetics using a test dose allows timely adjustment of single daily regimen doses and optimization of total Bu exposure, resulting in an effective and safe regimen for these infants.