High-dose busulfan is an important component of myeloablative regimens. Variable drug exposure may occur following oral administration. Therefore, the use of intravenous busulfan has been advocated. Previous work has suggested a cumulative dosage of 16 mg/kg for haematopoietic transplantation in children less than 3 years of age, but only limited data are available in infants. Pharmacokinetics of intravanous busulfan administered at the suggested dosage were studied in 14 infants (median age 4.7 months). Busulfan plasma concentrations were measured by either GC-MS or HPLC-UV. In seven patients, the dose was decreased to target an area- under- the- curve of 600–1300 μmol min. The median total dose given was 13.8 mg/kg. All patients engrafted. Severe veno-occlusive disease occurred in one patient. Our study demonstrates that a cumulative dosage of 16 mg/kg is associated with higher exposure than expected in infants. We suggest an initial dose of 0.8 mg/kg followed by pharmacokinetically guided dose adjustment.
High-dose busulfan is widely used in conditioning regimens prior to haematopoietic stem cell transplantation (HSCT), as an alternative to total body irradiation. Although busulfan may be administered once or twice daily,1,2 the most common dosage schedule is 1 mg/kg orally every 6 h for 4 days to a total dosage of 16 mg/kg.3 Busulfan has a narrow therapeutic index and veno-occlusive disease (VOD) – one of the major dose-dependent toxicities – has a mean incidence of 20–30% and an associated mortality rate from 3 to 67%.4,5 However, low drug exposure has been associated with higher recurrence rates and graft failures.6,7 Following administration of the oral formulation, very wide inter- and intrapatient systemic exposure is observed with two- to sixfold of coefficient variability. This wide bioavailability range may be linked to erratic intestinal absorption (± emesis), variable hepatic metabolism, circadian rhythm, genetic polymorphism of α-glutathione-S-transferase, initial diagnosis, previous treatment, drug–drug interaction and/or patient age. Hepatic and renal clearance mechanisms are generally underdeveloped and inefficient in the neonate, but they may change dramatically in the months following birth. Thus, pharmacokinetically guided dosage adjustment appears mandatory, particularly in children.5,7,8,9,10,11,12
Recently, different intravenous busulfan formulations have been developed in order to minimize variations of inter- and intrapatient systemic exposure and to provide complete dose assurance.13,14 In adults, the recommended dosage was 0.8 mg/kg/dose for 16 consecutive doses.13,15,16 Wall et al17 suggested a dosage of 1.0 mg/kg/dose every 6 h for 16 doses for children less than 4 years of age, whereas Cremers et al18 recommended 0.8 mg/kg/dose. However, data in infants, that is, children under 1 year of age, are scarce, precluding a recommended dosage for that particular age group. In this retrospective analysis, we report clinical and pharmacokinetic results of intravenous busulfan, prescribed off-label, in infants undergoing SCT at our institutions
Between October 2000 and August 2002, 14 patients under 1 year of age and less than 10 kg received intravenous busulfan as part of a conditioning regimen prior to allogeneic HSCT at the Methodist Children's Hospital of South Texas (San Antonio, TX; n=8) and at Sainte Justine Hospital (Montreal, Quebec; n=6). The underlying diseases were inherited syndromes in 10 cases, haematologic malignancies in three cases and amegakaryocytosis in one case. Grafts were one or two mismatched-unrelated cord blood (n=9), sibling bone marrow donor (n=3), matched-unrelated bone marrow donor (n=1) or haplotype mismatch-related bone marrow donor (n=1) (Table 1). The median recipient age and body weight were 4.8 months (range: 0.7–12) and 5.9 kg (range: 3.5–10), respectively (Table 2).
An intravenous busulfan (Busulfex®, Orphan Medical, Minnetonka, MN, USA) dose of 1 mg/kg was administered every 6 h for 16 doses in all patients but three (nos. 1, 3, 8), in whom a dose of 0.8 mg/kg was selected. Pharmacokinetically guided adjustment was usually performed , if needed, at dose 7, to target an area under the curve (AUC) of 600–900 μM min in children undergoing HLA-matched related transplantation and 900–1300 μM min in the others, in steady state. The busulfan infusion was carried out over 2 h via a central venous catheter using a controlled-rate infusion pump to mimic the time of maximum plasma concentration (about 1–2 h) observed following oral administration. According to initial diagnosis and centre, different conditioning regimens were used: (1) busulfan–cyclophosphamide association, where busulfan was given on days −9 to −6 and cyclophosphamide 50 mg/kg once daily i.v. on days −5 to −2 (total dose 200 mg/kg) to seven patients (nos. 5, 9–14); (2) busulfan–fludarabine, with busulfan given on days –9 to –6 and fludarabine given at 30 mg/m2 once daily i.v. on days –5 to −2 (total dose 120 mg/m2) to three patients (nos. 1, 2 and 8); (3) busulfan–melphalan, with busulfan given on days –8 to –5 and melphalan given at 45 mg/m2 once daily i.v. on days –4 to –2 (total dose 135 mg/m2) to four patients (nos. 3, 4, 6 and 7). Intravenous busulfan was always administered as the first i.v. chemotherapy component of the preparative regimen; interaction with other chemotherapy agents that may interfere with PK characteristics had been avoided. All patients received intravenous anticonvulsant prophylaxis as either midazolam (1.2 mg/kg/day) or phenytoin infusion (dose adjusted to maintain therapeutic levels) from the day before to the day after the administration of busulfan. Ursodiol was administered as VOD prophylaxis. Hepatic VOD was defined and graded according to criteria described by McDonald et al.19,20 GVHD prophylaxis consisted of antithymocyte globulin (except for patient no. 14), cyclosporin A and short-course methotrexate or methylprednisolone. Patients were given standard supportive care. Patients received HSCT after 1 day of rest at completion of the preparative regimen.
Blood samples were withdrawn from central venous lines into heparinized tubes immediately before and after the administration of i.v. busulfan, and at 15, 30, 60, 120, 180 and 240 min after the end of infusion, for all patients. When possible, blood samples were also withdrawn before and after the ninth dose and at 15, 30, 120 and 240 min after the end of infusion. Plasma busulfan values were determined by GC-MS or HPLC-UV, as described elsewhere.21,22 Pharmacokinetic modelling was performed using WinNonlin Professional 3.1 software (Pharsight Corporation) and noncompartmental analysis was used to determine pharmacokinetic parameters such as the elimination rate constant, clearance and AUC. From these, the individualized maintenance dose that results in an AUC near the target was calculated.
Definition of end points
Engraftment: primary measures for haematopoietic recovery were the time to achieve an absolute neutrophil count (ANC) of 0.5 × 109/l for 3 consecutive days and time to achieve a platelet count of 20 × 109/l, independent of platelet transfusion, for 3 consecutive days. In cases of death before haematopoietic recovery, engraftment documentation was performed by pathology sampling.
Transplant-related mortality: deaths during the first 28 days after HSCT or deaths more than 28 days after HSCT in remission.
Overall survival : time to death from any cause.
Overall survival and treatment-related mortality censored at 24 months and 100 days, respectively, were estimated using the method of Kaplan and Meier.23. Analysis was performed using GraphPad Prism 3.03 software (GraphPad, San Diego, CA, USA).
Results and discussion
Using the AUC normalized for 1 mg/kg for patients nos. 1, 3 and 8 who had received 0.8 mg/kg, median and mean (±s.d.) AUC of busulfan at steady state was 1234 μmol min (range: 992–1986) and 1224±290 μmol.min (95% CI 1056–1391), respectively (Table 2). Using the real AUC for these three patients did not significantly modify the median AUC (1153 μmol.min), and only slightly changed the mean: 1175±317 μmol.min (95% CI 992–1358). One patient in 14 had an AUC greater than 1500 μm.min, which can be considered as the lower limit for life-threatening toxicity since in a study published by Dix et al24 this was a significant risk for VOD development when patients had an AUC >1500 μmol.min. In pharmacokinetic studies of oral busulfan in children older than 4 years receiving 1 mg/kg/dose, published ranges of AUC are 402–1491 with a mean of 952±285 μmol.min (ie 1652–6120 and 3918±1170 ng h/ml as reported by the authors).25 A similarly wide range was reported from infants included in another study.26 The presented range of AUC and the standard deviation, although lower, still reflect wide pharmacokinetic variability. Our data suggest that in infants, in spite of interindividual busulfan hepatic metabolism variations as seen with oral busulfan, the intravenous formulation of this drug assures delivered dose precision. In addition, its use is more convenient in infants. Nevertheless, this variability remains substantial,9 and does not obviate the need for pharmacokinetically guided adjustment.
There was no infusion-related toxicity. All patients engrafted as defined above. All surviving patients had documented full donor chimerism; patients transplanted for SCIDs, who had had a submyeloablative preparative regimen, had lymphoid donor chimerism. In all, 11 patients are alive with a median follow-up of 13.9 (0.2–28.2) months. Kaplan–Meier 24 month-overall survival and day 100-treatment-related mortality were 78.6±11 and 14.3±9.3%, respectively. This outcome compares favourably with international registry reports of paediatric SCT recipients. Patient no. 2 died at day +8 from sepsis and pulmonary haemorrhage, and marrow engraftment was identified in a post-mortem marrow sample. Patient no. 8 died at day +58 from multiple organ failure. Patient no. 13 experienced intraventricular cerebral haemorrhage followed by a compressive peri-medullar haematoma at day +60 when his platelet count was more than 50 × 109/ml. There is no evidence that this neurological toxicity was attributable to intravenous busulfan. The patient eventually died at day +231 of an unrelated cause. Patient no. 9, who received a haplo-identical graft, developed severe VOD complicated by portal hypertension and oesophageal varices. None of the other patients experienced any busulfan-related complications.
In adults transplanted for CML, a recent study showed a significantly decreased risk of death for patients having a per-dose AUC between 950 and 1500 μmol.min.27 The initial dosage prescribed in our study was 1 mg/kg to target an AUC of 600–900 μM.min for children undergoing HLA-matched related transplantation and 900–1300 μM.min for the others. According to pharmacokinetic results and targeted AUC, the dose was decreased in one of the three patients in the former group, and in six of 11 in the latter. The dose was increased to more than 1 mg/kg in only one patient. The median total dose of busulfan was 13.75 mg/kg (range: 11.5–18.25). There were no failures of engraftment. Three patients who had a dosage modification had repeated AUC determinations after the 9th dose of i.v. busulfan. The results were in the targeted range for all the three (Table 2). The mean pharmacokinetically guided adjusted dose was 0.86 mg/kg (range 0.57–1.25). These data support a dosage of 0.8 mg/kg given as a 2-h intravenous infusion every 6 h for 16 doses as the initial dosage in infants. Even though no absolute data are available, according to Hassan et al,28 phenytoin may be responsible for higher clearance, lower AUC and a shorter elimination half-life of busulfan than benzodiazepines, presumably by induction of α-GST and busulfan intestinal wall transport modifications. Here, both intravenous phenytoin and busulfan were used. Alteration of busulfan intestinal wall transport cannot be incriminated, but the induction of α-GST can still play a role in busulfan pharmacokinetics. In our cohort, only two of eight patients given phenytoin, but five of six given midazolam, needed busulfan reduction. However, the data are too limited to allow conclusion on to whether the phenytoin–busulfan interaction was significant. No correlation was observed between weight or age and AUC in this complete cohort of infants (Figure 1). There appears to be a trend for correlation between age and AUC in the female subgroup.
Our data suggest that intravenous busulfan is suitable as an alternative to oral busulfan in conditioning before haematopoietic transplantation in infants, including high-risk transplantation and obviates the difficulties of oral intake in these patients. The use of an intravenous formulation of busulfan was followed by reducing intra- and interindividual variations. Pharmacokinetically guided dose adjustment is still required, however, given the narrow therapeutic range of busulfan. An initial dosage of 0.8 mg/kg every 6 h, with 16 planned doses is suggested for future studies in infants.
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JH Dalle is a fellow of La Fondation Charles Bruneau.
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Dalle, J., Wall, D., Theoret, Y. et al. Intravenous busulfan for allogeneic hematopoietic stem cell transplantation in infants: clinical and pharmacokinetic results. Bone Marrow Transplant 32, 647–651 (2003) doi:10.1038/sj.bmt.1704209
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