Maximally Tolerated Busulfan Systemic Exposure in Combination with Fludarabine as Conditioning before Allogeneic Hematopoietic Cell Transplantation

Systemic exposure to high-dose busulfan has been correlated with efficacy and toxicity after hematopoietic cell transplantation for malignancy. We used the area under the concentration-time curve (AUC) to prospectively determine the maximally tolerated systemic exposure to i.v. busulfan when given once daily after fludarabine administered at 40 mg/m2 for 4 days. Three target AUC levels were planned: 6,000, 7,500, and 9,000 μM-min. Included were patients 16 to 65 years old, with a hematologic malignancy, an HLA A, B, or C, DRB1 8/8 or 7/8 matched donor, Karnofsky performance status ≥70%, and adequate organ function. For level 1 patients, i.v. busulfan doses 1 and 2 were 170 mg/m2/day, then doses 3 and 4 were adjusted based on first-dose pharmacokinetic modeling to achieve an average daily AUC of 6,000 μM-min. Doses 1 and 2 for the subsequent cohorts were based on the level 1 data: 180 mg/m2/day for AUC 7,500 μM-min (level 2) and 220 mg/m2/day for AUC 9,000 μM-min (level 3), with pharmacokinetic targeting for doses 3 and 4. Pharmacokinetic analysis after the last dose showed that 88% of the patients had been exposed to a mean AUC within 10% of the target. Forty patients were treated at level 1, 29 patients at level 2, and three patients at level 3. DLT was veno-occlusive disease of the liver, which occurred in none of 40 patients (0%) at level 1, two of 29 patients (7%) at level 2, and three of three patients (100%) at level 3. Dermatitis (P < .01) and pulmonary toxicity (P = .01) were also increased at higher AUC levels. Level 2 (7,500 μM-min × 4 days) was the maximally tolerated AUC. Within the confines of the trial’s small sample size, there was no suggestion that escalating busulfan AUC from 6,000 to 7,500 μM-min × 4 days increased nonrelapse mortality. Assessment of the higher busulfan AUC on relapse prevention requires trials in patients with a homogeneous risk of relapse.