Suboptimal Engraftment Is Associated with Reduced Exposure to Fludarabine Metabolite in Children Undergoing Allogeneic Haematopoietic Stem Cell Transplantation

Introduction: Fludarabine phosphate is commonly used as part of conditioning regimens in children undergoing allogeneic haematopoietic stem cell transplantation (HSCT) for both malignant (n = 47) and non-malignant (n = 39) diseases, but its use has been associated with engraftment failure or mixed chimerism¹. The aim of this study was to examine whether suboptimal engraftment is associated with fludarabine pharmacokinetics.

Material (or patients) and methods: The clinical pharmacokinetics of fludarabine phosphate was evaluated by studying the free concentrations of the active metabolite, (9B-D-arabinosyl-2 fludarabine, F-Ara-A). Eighty-six HSCTs were studied in 84 children 0.1 to 19 years. Glomerular filtration rate (GFR) was determined by measuring the plasma clearance of diethylenetriaminepentacetic acid (DTPA). The children in this study were treated according to specific HSCT conditioning protocols which varied with respect to the total administered fludarabine dose and the number of days over which it was administered (range: 3 to 6). A weight-based dose of 1 to 2 mg/kg was used in 13 transplants where the patient weighed <11kg, whilst 73 children received surface area-based doses ranging from 25 to 50 mg/m²/dose. Fludarabine was used with busulphan in 63 transplants. Fludarabine was infused over 0.4 to 2 hrs. Free F-Ara-A concentrations were measured in 5 to 8 blood samples collected over timed intervals after the first infusion. Pharmacokinetic parameters, including area under the concentration curve (AUC) and clearance (CL) were determined using trapezoidal rule implemented by the Kinetica (4.0) software (Innaphase, USA). F-Ara-A AUC and CL were compared between patients with suboptimal engraftment (mixed chimerism or graft rejection) and the remainder using the Mann Whitney test. Pearson's correlation coefficient was used to detect significant associations between F-Ara-A exposure and the recovery of platelets and neutrophils.

Results: Mixed chimerism or graft rejection occurred following 10 (of 86) transplants and, of these, 8 were being treated for malignant diseases. First dose free F-Ara-A AUC was significantly lower for these transplants compared with the remainder: median 3.0 (2.4 - 3.8) mg/L.h versus 4.1 (3.3 - 5.2) mg/L.h, p = 0.038, but the difference in clearance was not significant: 6.3 (5.2 - 11.0) L/h versus 4.6 (2.7 - 8.1) L/h, p = 0.06. The recovery of neutrophils also correlated significantly with first dose free AUC (r = -0.22, p= 0.044) but not with clearance (r=0.193, p = 0.075). Recovery of platelets was not significantly associated with any estimate of free F-Ara-A exposure. Within the fludarabine dose range of 25 to 40 mg/m², there was a linear association with median free F-Ara-A AUC (r ²= 0.977), which was represented by the equation median F-Ara-A AUC (mg/l.h) = 0.1306 x fludarabine dose (mg/m²). There was a significant negative correlation between clearance and GFR (r = -0.31, p < 0.005) and GFR was significantly higher in patients with suboptimal engraftment: median 126 (116 - 152) versus and 111 (92 - 133) ml/min/1.73 m² (p = 0.015). Suboptimal engraftment was significantly associated with poorer overall survival using Kaplan Meier survival analysis (p = 0.002 using log rank test).

Conclusions: The data suggests that patients with good renal function are at greater risk of suboptimal engraftment since they have lower exposure to free F-Ara-A. These children may therefore require higher doses of fludarabine. We found that mg/m² fludarabine dose was linear with free F-Ara-A AUC between the 25 and 40 mg/m² dose range.

1.Lee JH, Joo YD, Kim H, Ryoo HM, Kim MK, Lee GW, et al. Randomized trial of myeloablative conditioning regimens: busulfan plus cyclophosphamide versus busulfan plus fludarabine. J Clin Oncol 2013 Feb 20; 31(6): 701-709.