Busulfan, cyclophosphamide and fractionated total body irradiation for allogeneic marrow transplantation in advanced acute and chronic myelogenous leukemia: phase I dose escalation of busulfan based on targeted plasma levels.

A previous phase I study determined that the maximum tolerated dose (MTD) of busulfan (BU) that could be given with a fixed dose of cyclophosphamide (CY) of 50 mg/kg and total body irradiation (TBI) dose of 12.0 Gy was 7 mg/kg. A phase II study was carried out in patients with advanced myeloid malignancies receiving allogeneic transplants without improvement in outcome as compared to historical controls. In that study, steady-state concentration (Css) of BU in 13 patients receiving a fixed dose of BU varied from 209 to 735 ng/ml. In an attempt to decrease the variability of the Css of BU, a study of targeting specific plasma concentrations was performed. In this study, BU dose was adjusted up or down based on first dose pharmacokinetics. The first dose level evaluated was 7.5 mg/kg with a target BU plasma level of 460 ng/ml. Six patients were entered at this level and the median BU plasma concentration achieved was 410 (range 390-533). One of six patients developed grade 3-4 regimen-related toxicities (RRT). Dose level II was a target of 559 ng/ml with a starting oral dose of BU of 9.6 mg/kg. Twelve patients were entered at this level and median plasma BU level was 548 (range 427-689). Three of 12 (25%) patients developed grade 3-4 RRTs and this was considered to be the MTD. The actuarial probability of grade II-IV acute GVHD was 0.70. Eight of 21 evaluable patients (38%) developed chronic GVHD. Of 18 patients who died, seven died of relapse at a median of 160 days (range 65-353) and 11 (61%) died of causes other than relapse at a median of 152 days (range 18-570). The actuarial probabilities of DFS, relapse and relapse-free mortality at 2 years in all patients were 0.13, 0.50 and 0.75, respectively. This study showed that targeted BU plasma levels within 10% of target can reliably be achieved with a bias of -2.07% and mean absolute error of 7.47%. Overall, targeting made a -31.8% to 100% in plasma BU Css as compared to expected BU Css based on first dose pharmacokinetics if targeting were not performed in this study. Thus targeting avoided much of the variability in BU concentrations seen in other studies. When compared with our previous phase II experience in same group of patients receiving same regimen, dose escalation of BU based on targeted plasma levels did not improve the outcome.