Busulfan Area-under-the-Curve Finding Study within a Busulfan/Fludarabine (BuFlu) Conditioning Regimen before Allogeneic Hematopoietic Cell Transplantation (HCT).

Relapse of malignancy is frequent in patients transplanted with advanced diseases. There is a relationship between higher Bu exposure and lower risk of relapse, but also higher transplant-related mortality, and data indicated the maximum tolerated daily Bu AUC is < 6000 microMol/min when Bu is used in combination with cyclophosphamide (Cy). To reduce transplant-related mortality, Flu has been substituted for Cy in many transplant centers. We hypothesized that pharmacokinetics-targeting of IV Bu, which results in a predictable systemic exposure, would produce values of tolerable Bu AUC that are higher for BuFlu than for BuCy. Our standard HCT regimen since July 2004 consists of once daily IV BuFlu where fludarabine 40 mg/m² is given intravenously daily for four days and each infusion is followed immediately by an initial IV Bu dose of 130 mg/m². Pharmacokinetic analysis is performed after the first infusion of busulfan; the goal is to adjust the busulfan doses for days 3 and 4 to achieve an average exposure targeted to an AUC of 4500–5600 microMol/min. Outcomes for the first sixty-nine patients were analyzed. Thirty-nine of 63 (62%) evaluable patients had their Bu doses adjusted, 30 increased [median adjustment 50% (8 – 175%)], and 9 decreased [median 30% (11 – 60%)], while 24 (38%) pts had an AUC within the desired range without adjustment. Most of the patients (98%) had hematological malignancies and 73% had high CIBMTR risk, median age was 48 (22–68) years, donors were HLA-identical siblings (33), matched (23) or mismatched unrelated donors (13). With a median patient follow-up of 392 (248 – 707) days, the non-relapse mortality was 4% at 100 days and 17% at one year. The K-M estimates of survival and event-free survival are 88%±4% and 83%±5% at 100 days, and 61%±6% and 51%±6% at 1 year, respectively. Subsequently, as part of a prospective, targeted AUC-dose finding trial, 20 patients received an initial Bu dose of 170 mg/m² with subsequent doses targeted to achieve a 4-day average AUC of 5400–6600 microMol/min. Pharmacokinetic analysis was performed after the first and fourth infusion of busulfan. All patients had hematologic malignancies, 65% had high CIBMTR risk, median age was 48 (22 – 61) years, donors were HLA-A, B, C, DRB1, DQB1 matched siblings (11) or matched unrelated donors (9). Thirteen (65%) patients had their doses adjusted, six had it increased [median 37.1% (35.6 – 61.9 %)] and seven decreased [median 41.6% (18.3 – 55.2%)], while seven patients had an AUC within the desired range without adjustment. With a median follow-up of 111 (range 21–312) days, the 100-day K-M estimates of survival are 88%±8% and event-free survival 83%±9%. The complete observation of 100-day safety, relapse and survival will be presented at the meeting and data will determine further AUC escalation.