Optimizing the Conditioning Regimen for Hematopoietic Cell Transplant in Myelofibrosis: Long-Term Results of a Prospective Phase II Clinical Trial

Optimal conditioning regimens for older patients with myelofibrosis undergoing allogeneic hematopoietic cell transplant are not known. Likewise, the role of dose intensity is not clear. We conducted a nonrandomized, prospective, phase II trial using low-dose, later escalated to high-dose (myeloablative conditioning), busulfan with fludarabine (Bu-Flu) in myelofibrosis patients up to age 74 years. The first 15 patients received i.v. busulfan 130 mg/m²/day on days -3 and -2 ("low dose"); 31 patients received high-dose conditioning, either 100 mg/m²/day (days -5 to -2; n = 4) or pharmacokinetic-guided area under the curve of 4000 μmol/min (days -5 to -2; n = 27). The primary endpoint was day 100 nonrelapse mortality (NRM). Median age was 58 years (interquartile range [IQR], 53-63). Dynamic international prognostic scoring system-plus was intermediate (n = 28) or high (n = 18). Donors were related (n = 19) or unrelated (n = 27). Cumulative incidence of NRM was 9.7% (95% confidence interval [CI], 0-20.3) at day 100 and at 3 years in the high-dose group and 0% in the low-dose group at day 100, which increased to 20% (95% CI, 0-41.9) at 3 years. With a median follow-up of 5.1 years (IQR, 3.8-6), 3-year relapse was 32.3% (95% CI, 15.4-49.1) in high dose versus 53.3% (95% CI, 26.6-80.1) in low dose. Event-free survival was 58% (95% CI, 43-78) versus 27% (95% CI, 12-62), and overall survival was 74% (95% CI, 60-91) versus 60% (95% CI, 40-91). In multivariate analysis, high-dose busulfan had a trend toward lower relapse (hazard ratio, .44; 95% CI, .18-1.07; P = .07), with no impact on NRM. Intensifying the Bu-Flu regimen using pharmacokinetic-monitoring appears to be promising in reducing relapse without increasing NRM.