Myeloablative Conditioning Using Timed Sequential Busulfan in Older Patients with Acute Myeloid Leukemia: Long Term Results of a Prospective Phase II Clinical Trial

Abstract

Background Myeloablative conditioning (MA) reduces relapse as compared to reduced intensity (RIC), [Scott BL JCO 2017]. But it is avoided in older patients and those with comorbidities due to high non-relapse mortality (NRM), whose outcomes remain dismal. A CIBMTR study of AML patients ≥60 years (100% CR1) showed NRM of 32-34%, relapse rate of 33-37%, progression free survival (PFS) of 31-34% and overall survival (OS) of 34-36% at 2 years. [McClune. JCO 2010] A prospective trial in AML (≥60 years, 100% CR1) showed better 2-year PFS (42%) and OS (48%). [Devine. JCO 2015] Herein, we report the results of MA HCT using timed-sequential (TS) busulfan (Bu) in 71 AML patients up to age 73. Methods Patients received intravenous Bu in TS manner targeted to achieve an area under the curve of 16,000-20,000 μmol.min, with fludarabine (flu) 40 mg/m2 intravenously for four days. Results Median age was 64 years (range, 29-73). Donor was HLA matched unrelated (n=45, 63%) or related (n=26, 27%). Most received peripheral blood graft (n=46, 65%). Only 26% (n=18) had MRD-negative CR1/2 at HCT; others had MRD-positive CR1/2 (13%, n=9), primary induction failure (PIF; 45%, n=32) or relapsed disease (16%, n=11). Nearly half (47%) had the European LeukemiaNet (ELN) adverse risk disease. More than half (55%) had HCT-CI ≥ 3. Median follow-up was 40 months (range, 21-63). The cumulative incidences of grade II-IV and III-IV acute GVHD at day 100 were 39% (95% CI, 28-51%) and 10% (95% CI, 3-17%), respectively and extensive chronic GVHD at 1 year was 20% (95% CI, 10-29%). NRM was 6% (95% CI, 0-11%) at day 100 and 24% (95% CI, 14-34%) at 2 years. At 2 years, relapse was 28% (95% CI, 3-53%) in the CR/MRD-negative group vs 48% (95% CI, 34-62%) in others; progression-free survival (PFS) was 78% (95% CI, 61-99.6%) vs 25% (95% CI, 16-40%) and overall survival (OS) was 78% (95% CI, 61-99.6%) vs 33% (95% CI, 22-48%), respectively. [Fig 1a] Two year OS was was 67% (95% CI 51-87%) in the CR group, irrespective of MRD [Fig 1b]. In multivariate analysis (MVA) adjusted for covariates higher dose Bu vs lower dose [HR 0.39, 95% CI 0.18-0.85, p=0.02] was the only significant predictor of lower relapse risk. In MVA adjusted for age, Bu dose and ELN risk, not being in CR/MRD-negative state was the only predictor of significantly inferior PFS [HR 3.2, 95% CI, 1.3-8.2, p=0.01]. For OS, MVA adjusted for age, Bu dose and ELN risk, HCT-CI ≥3 [HR 1.98, 95% CI, 1-3.9, p=0.05] and not being in CR/MRD-negative state [HR, 2.7, 95% CI 0.98-7.2, p=0.05] were associated with higher mortality. Conclusion Our outcomes of MA HCT in an extremely high risk older population are promising especially in CR patients, whose outcomes approach that of younger patients. This was achieved by delivering Bu in TS manner which minimized toxicity and potentially increased efficacy. This approach warrants further investigation.