Summary:We retrospectively analyzed the relationship between busulfan average steady-state plasma concentration (C SS ) and graft rejection in 53 children receiving busulfan/cyclophosphamide (BU/CY) preparative regimens prior to hematopoietic stem cell transplantation (HSCT). Patients received a total oral busulfan dose of 11 to 28 mg/kg followed by a total cyclophosphamide dose of 120 to 335 mg/kg in preparation for allogeneic grafts (HLA-matched or HLA partially matched sibling, parent or unrelated donor). Graft rejection occurred in eight (15%) patients. Busulfan C SS (P = 0.0024) was the only statistically significant predictor of rejection on univariate logistic regression analysis, with the risk of rejection decreasing with an increase in busulfan C SS . Severe (grade 3 or 4) regimen-related toxicity (RRT) occurred in four patients. Ten patients (19%) had a busulfan C SS higher than 900 ng/ml, one of whom had severe RRT. Higher and variable doses of cyclophosphamide may explain the lack of a relationship between busulfan C SS and RRT in children. It may be possible to improve the outcome of HSCT in pediatric patients receiving the BU/CY regimen through optimization of busulfan C SS and better definition of the contribution of activated cyclophosphamide metabolites to toxicity. phamide was later decreased to 120 mg/kg for adults to diminish regimen-related toxicity. 7 Achieving consistent engraftment has been a greater challenge in children and the dose of cyclophosphamide in general has remained at 200 mg/kg. 8 It is now well established that children treated with highdose busulfan, based on either a mg/kg or a mg/m 2 basis, achieve lower plasma concentrations, measured as area under the plasma-concentration time curve (AUC) or steady-state concentration (C ss , which is AUC/dosing interval) than adults. 9-15 Children below the age of 4 years are most at risk of having low busulfan plasma concentrations. The lower plasma levels of busulfan are apparently due to an enhanced ability to metabolize the drug through an upregulation of busulfan-glutathione conjugase (glutathione Stransferase A1-1). 16,17 We 15 and others 18 have observed a relationship between low busulfan C ss and the incidence of graft rejection in patients receiving BU/CY. In a population of 15 adults (Ͼ18 years ) and 28 children (0.6-16 years) receiving BU/CY, there was a difference in the busulfan C ss needed to retain HLA-matched sibling grafts (Ͼ200 ng/ml) in comparison to that needed for partially matched related or matched unrelated donor grafts (Ͼ600 ng/ml). 15 Bolinger et al 18 did not observe a difference in threshold busulfan C ss between the different graft sources in 31 children receiving BU/CY Ϯ ATG, although few patients received grafts from partially matched related or matched unrelated donors (n = 6). 18 However, a busulfan C ss Ͻ600 ng/ml was correlated with rejection. 18 Further definition of the relationship between busulfan C ss and graft rejection in children should lead to the development of more effective conditioning r...

Summary:Autologous recovery is a major problem with busulfan as a marrow ablative agent in conditioning children for allogeneic BMT. Data suggest the average concentration of busulfan at steady state (Bu Css) is critical for successful engraftment. We prospectively evaluated busulfan pharmacokinetics in 31 children (age 0.6-18 years) with AML (n = 9), and non-malignant diseases (n = 22) receiving HLA-closely matched (sibling, parent, unrelated) donor grafts. Blood samples were obtained following dose 1 and 13 of a standard 16 dose, 4-day regimen. The busulfan dose varied from 14 to 20 mg/kg. Patients received cyclophosphamide 200-240 mg/kg; 22/31 received 80-90 mg/kg of ATG. Eight patients failed to engraft (26%). ATG did not appear to influence engraftment (P = 0.38). Bu Css levels Ͻ600 ng/ml correlated with autologous recovery/mixed chimerism (P = 0.018). There were no graft failures in patients with a Bu Css Ͼ600 ng/ml. A correlation between Bu Css levels and regimen-related toxicity (RRT) was not identified for grade 2 or higher toxicities, only 1/31 had a Bu Css Ͼ900 ng/ml. Our data support the use of pharmacokinetic monitoring of busulfan. Bone Marrow Transplantation (2000) 25, 925-930. Keywords: busulfan; children; bone marrow transplantation; cyclophosphamide; toxicity Busulfan is a potent myelotoxin that complements the lymphocyte toxicity of cyclophosphamide in preparative regimens for hematopoietic stem cell transplantation. 1-3 The standard dose is 1 mg/kg administered orally every 6 h over 4 days. However, this regimen results in highly variable concentrations in plasma, particularly when young children are compared to older children or adults. 4-7 At a given dose, normalized to either body weight or surface area, young children exhibit average busulfan steady-state concentrations (Css) or an area under the curve (AUC) substantially less than do older children or adults. 7-9 The Css corresponds to the AUC over a dosing interval divided by the 6 h between doses. The AUC over a dosing interval is equal to the AUC from the time the first dose is ingested to infinity if no subsequent doses are taken. A Css of 900 ng/ml is equal to an AUC of 1350 m × min.Few studies of the relationship between busulfan Css (Bu Css) or AUC and outcome of the busulfan/ cyclophosphamide preparative regimen have focused on children. Adults with CML have been shown to be at increased risk of relapse if Bu Css is low and exhibit less busulfan-related toxicity compared to patients with other diseases receiving similar treatment. 10,11 Excessively low levels of busulfan have been related to graft rejection, while high levels have been related to severe toxicities. 12,13 Of particular interest in children is the issue of graft rejection. Since cyclophosphamide is directly toxic to mature lymphocytes and busulfan is not, graft rejection has been thought to be due to inadequate dosing of cyclophosphamide. 1,14 Thus, the relatively high incidence of graft rejection in children receiving a busulfan/cyclophosphamide preparative regime...