Safety and Efficacy of Outpatient Myeloablative Busulfan Conditioning in Allogeneic Stem Cell Transplantation

INTRODUCTION: Busulfan-based regimens are the backbone of myeloablative conditioning (MAC) in allogeneic hematopoietic stem cell transplant (allo-HSCT), particularly for patients (pts) with Acute Myelogenous Leukemia (AML). However, literature on busulfan conditioning administered in the outpatient (OP) setting is limited. Here, we investigate the safety and efficacy of OP busulfan as compared to standard inpatient (IP) administration.

METHODS: Pts > 18 years of age who received pharmacokinetically dose adjusted IV busulfan (130 mg/m2 x 4 days) conditioning prior to HSCT between January 2019 and April 2023 at our institution were included in this retrospective analysis. Pts who received non-myeloablative conditioning or who underwent a haploidentical HSCT were excluded. All pts undergoing HSCT were classified as having low-, intermediate-, or high-risk disease by the Disease Risk Index. Pts with AML were classified as having favorable, intermediate, or adverse disease by 2022 ELN classification. CIBMTR guidelines were used to determine neutrophil and platelet engraftment. Oral mucositis was graded per the World Health Organization scale. Clinical grading of acute graft-vs-host disease (AGVHD) was assigned per standard criteria. Adverse events (AEs) were reported in accordance with the CTCAE, v.5. Categorical variables were compared using the Fisher’s Exact test. Continuous variables were compared using the Wilcoxon Rank-Sum test. Overall survival (OS) was defined as time from HSCT to time of death from all-cause mortality. Relapse-free survival (RFS) was defined as time from HSCT to time of relapse or non-relapse mortality. The Kaplan-Meier method was used to estimate OS and RFS. Survival differences between cohorts were compared using the Log-rank test. All hypothesis testing was two-sided and conducted at a significance level of 0.05.

RESULTS: In total, 54 pts met eligibility criteria; 43 IP and 11 OP. The OP cohort was younger than the IP cohort (35 years vs 47 years median age; p=0.0284). All other patient, disease, and transplant characteristics were similar between cohorts. The majority of pts were male (IP: 56%, OP: 55%) and White/Caucasian (IP: 100%, OP: 91%). AML (IP: 88%, OP: 82%) was the most common indication for HSCT. Most pts were in CR1 (IP: 74%, OP: 89%) and had intermediate risk disease by both DRI (IP: 84%, OP: 64%) and 2022 ELN classification (IP: 62%, OP: 67%). PBSCT (IP: 88%, OP: 91%) and 10/10 MUD (IP: 67%, OP: 45%) were the most common types of HSCT. The majority of pts received a conditioning regimen of Flu/Bu4 (IP: 58%, OP: 55%). The use of PTCy was comparable between groups (p=0.6593). Of the 11 pts who received OP busulfan, 1 had to complete their conditioning IP after experiencing a seizure in the setting of prior meningitis. Between the 2 cohorts, there was no significant difference in the incidence of AEs > grade 2 (p=1), grade 2 AEs (p=0.8695), grade 3 AEs (p=0.7592), grade 4 AEs (p=0.2487), and grade 5 AEs (p=0.6457) within 30 days of conditioning. Incidences of transplant-related GI toxicities, including nausea (p=1), vomiting (p=0.4979), diarrhea/enteritis (p=1), and mucositis (oral: p=0.6102, GI: p=0.2821) were similar. There was no difference in time to neutrophil (p=0.1606) or platelet engraftment (p=0.9742). Length of hospitalization was shorter in the OP vs IP cohort (18 vs 24 days; p<0.0001). During hospitalization, there was no difference in the incidences of neutropenic fever (p=1), infection (p=0.5104), ICU transfer (p=1), or engraftment syndrome (p=0.7076). Number of infections per patient during hospitalization were comparable (p=0.3764). D30 chimerism was similar IP vs OP. Within 180 days of HSCT, there was no difference in the incidence of AGVHD (p=0.5152) or Grade II-IV AGVHD (p=1). Differences in 2-year OS (LR=0.2200) and 4-year RFS (LR=0.3144) were not significant.

CONCLUSION: In pts undergoing MAC allo-HSCT with IV busulfan, OP administration shortens the length of hospitalization without delaying engraftment or increasing the risk of toxicity, neutropenic fever, infection, or ICU admission. Additionally, D30 chimerism, rates of engraftment syndrome and AGVHD, and survival were not affected. This makes OP busulfan an attractive option for transplant centers and their pts.