Pharmacokinetic-Guided Busulfan Based Myeloablative Versus Fixed Dose Reduced Intensity Conditioning Regimen in Patients Older Than 55 Years Undergoing Allogeneic Stem Cell Transplantation for High Risk Hematological Malignancies

BACKGROUND

Reduced intensity conditioning (RIC) combining fludarabine, busulfan and ATG is commonly used in patients with advanced age and/or comorbid conditions, thus deemed unfit for standard myeloablative conditioning (MAC) regimens (i.e. BuCy or CyTBI12). Although non-relapse mortality (NRM) is dramatically reduced with RIC, relapse remains a major concern. In younger patients, myeloablative doses of busulfan in combination with fludarabine (i.e. myeloablative reduced toxicity conditioning [MA-RTC] regimen) produce similar antitumor effect compared with standard MAC while keeping low NRM. The feasibility of such an approach for older patients is not known. We hypothesized that a pharmacokinetics (PK) guided strategy could improve the safety of busulfan administration, allowing the use of myeloablative doses in older patients.

STUDY DESIGN AND METHODS

We compared the outcome of patients older than 55 years with hematological malignancies included in a prospective monocentric single arm trial (BxPK: NCT02483325) evaluating the feasibility of PK-guided busulfan doses in myeloablative conditioning regimen (BxPK group: fludarabine 150 mg/m² + IV busulfan targeted AUC 5300 x 4 days + ATG 5 mg/kg) before matched sibling (MSD) or unrelated (URD) donor AlloSCT, with a retrospectively enrolled control cohort of patients with same inclusion criteria, treated during the same period but receiving fixed busulfan dose RIC regimen including fludarabine (150 mg/m²), IV busulfan (260 mg/m²) and ATG (5 mg/kg) [Bx2 group]. The primary objective was 2-year progression-free survival (PFS). Hazard ratio of BxPK vs. Bx2 were adjusted by age, disease risk index (DRI), donor type and hematopoietic cell transplantation comorbidity index (HCT-CI).

RESULTS

We analyzed 83 consecutive patients (BxPK: n=27; Bx2: n=56) without significant difference in baseline characteristics between both conditioning groups. In the BxPK and Bx2 groups: median age was 65 (range: 57-70) and 63 (range: 55-70) years, respectively (p = 0.552); 10/10 URDs were used for 15 (56%) and 37 (66%) patients, respectively (p = 0.493); HCT-CI was ≥ 3 in 18 (67%) and 28 (50%) patients , respectively (p = 0.232); and DRI was high in 5 (19%) and 5 (9%) patients, respectively (p = 0.369).

Based on PK data from day-6 busulfan administration, 22 (81%) patients had dose adjustment (increased and decreased dose in 12 and 10 patients, respectively) in the BxPK group. Median busulfan AUC on day-2 (post adjustment) was 5287 µmol.min (range: 3326-7556).

Four patients had AUC > 6000 µmol.min on day-2.

Grade 3-4 mucositis (BxPK vs Bx2: 52% vs. 11%, p < 0.001) and nausea (BxPK vs Bx2: 11% vs 0%, p = 0.050) were more frequently observed in the BxPk group. No difference was observed between BxPK and Bx2 patients in grade 3-4 liver (BxPK vs Bx2: 15% vs 11%, p = 0.810), renal (BxPK vs Bx2: 0% vs 1.8%, p = 1) and gastrointestinal tract (BxPK vs Bx2: 11% vs 0%, p = 0.450) toxicities.

There was a trend to higher cumulative incidence of grades 2-4 acute graft-versus-host disease (GVHD) at 100 days in the BxPK group (BxPK vs Bx2: 41% vs 29%, p = 0.058) while no difference was observed in 2-year chronic GVHD (BxPK vs Bx2: 41% vs. 36%, p = 0.547). Those trends were confirmed by multivariate analyses (acute GVHD: HR 2.22, 95Cl [1.10-4.48], p = 0.026; chronic GVHD: HR 1.30, 95Cl [0.60-2.80], p=0.502)

With a median follow up of 33 months (range: 9-60), 2-year NRM was 19% and 18% (p=0.960) in the BxPK and Bx2 group, respectively. Multivariate analysis confirmed the absence of significant increase in the risk of NRM in the BxPK group (HR 1.11, 95CI [0.36-3.45], p = 0.859). The 2-year cumulative incidence of relapse was 19% and 31% (p=0.269) in the BxPK and Bx2 group, respectively (HR 0.55, 95IC [0.20-1.56], p = 0.264). No significant difference in 2-year PFS (BxPK vs Bx2: 62% vs. 51%, p = 0.391) and OS (BxPK vs Bx2: 70% vs. 61%, p = 0.667) was observed.

CONCLUSION

In patients older than 55 years who are usually candidates for RIC regimen, the use of PK-guided busulfan myeloablative dose allows delivering higher antitumor intensity without increasing NRM. Beyond the feasibility, we observed low incidence of relapse (19%) and promising OS (70%) suggesting that conditioning intensification may lead to long term disease control. These trends need to be confirmed in a larger comparative study for validating the optimal conditioning intensity in older and/or unfit patients.