Clinical Outcomes and Pharmacokinetics of Targeted Intravenous Busulfan in Children Receiving Stem Cell Transplantation for Thalassemia.

Background: High dose busulfan (BU) in combination with cyclophosphamide (BUCY) is a preferred conditioning regimen for patients with hemoglobinopathies. Recently introduced intravenous BU (IVBU) has decreased intra-and inter-individual variability of BU systemic exposure (AUC) when compared to oral dosing. There are no data available on IVBU pharmacokinetics (PK) in a large group of patients with thalassemia to date.

Purpose: to asses the IVBU PK in relation to patient and disease-related (hepatomegaly, blood transfusion, ferritin, liver iron concentration, hepatitis, liver fibrosis) variables and the relationship of BU exposure to toxicities and transplant outcomes in children and young adults given SCT for thalassemia from HLA-matched related donors.

Methods: 57 patients with thalassemia major with median age of 9 years (range, 1.6–24) received IVBU (Busilvex, Pierre Fabre Medicament, France) as a part of conditioning regimen between 2006 and 2008. BU doses were based on actual body weight: <9–16= 1.2 mg/kg (n=10); 16–23= 1.1 mg/kg (n=13); 23–34= 0.95 mg/kg (n=23) and >34=0.8 mg/kg (n=11) and were given every 6 hours for 4 days. Valproic acid (Depakin) was administered before and during BU treatment as anticonvulsant prophylaxis. Most patients had liver disease and moderate to severe iron overload. Class 1 and class 2 patients (n=24) received IVBU in combination with CY200 ± thiotepa as conditioning regimen. Class 3 patients (n=33) before conditioning with IVBU/CY160 ± thiotepa were given cytoreduction/immunosuppression with hydroxyurea, azathioprine and fludarabine between −45 and −12 days pretransplant. GVHD prophylaxis consisted of CSA+ short MTX. Blood samples were drawn just before and 2h, 4h, and 6h after BU administration following the 1^st, 5^th, 9^th, and 13^th doses for PK assessment by HPLC-MS. Dose adjustment (DA) was made at the 3^rd dose as needed, to target an AUC range of 900–1350 μol/L/min. The influence of patient and disease-related variables on IVBU PK was investigated by a population PK-based approach using the NONMEM program.

Results: PK parameters following the 1^st dose are reported in Table. Overall, 58% of patients AUC were within, 37% were below and 5% were above the target range following the 1^st dose of IVBU. Dose elevations of 5.2–54% (median, 18.8%) were made in 17 patients and dose reductions of 5–34.2% (median,9.2%) in 19 patients. Following DA 79 % of patients after the 5^th and 9^th doses and 91 % after the 13^th dose reached the target range. The inter-patient variability in IV BU clearance was moderate (CV=19%) and the intra-patient variability was low (CV=7%). Only weight or body surface area significantly explained the PK variability. Fifty three patients had sustained engraftment, 4 had primary (n=2) or secondary (n=2) graft failure. Forty eight (89%) of 54 evaluable patients were complete and 6 (11%) mixed chimeras. One patient had moderate hepatic VOD resolved with supportive care. Seventeen patients developed grade 2–4 acute GVHD. None of patients had seizure within 30 days post-transplant. Grade 1 or 2 ALT/AST increases were observed in 19,3% and 44% of patients and stomatitis/diarrhoea in 47% and 19% of patients respectively. Five patients died. There was no relationship between busulfan exposure and toxicities, engraftment time, chimerism, rejection, GVHD and survival in univariate analysis. No association was found between PK parameters and transplant outcomes in a subgroup of patients (n=19) who never needed DA. This study demonstrates that IVBU in children with thalassemia who have important organ damage due to their disease and treatment is well tolerated with no increase in organ system toxicity. IVBU exposure did not predict rejection, liver VOD or death in patients with thalassemia who received HLA-matched related SCT.