Outpatient Autologous Stem Cell Transplantation in Patients with Multiple Myeloma Decreases Antibiotic Exposure: A Retrospective, Single-Center Cohort Study

Background Traditionally, autologous stem cell transplantation (ASCT) for multiple myeloma (MM) has required lengthy inpatient (inpt) admission to monitor transplant-related toxicities, resulting in significant use of hospital resources. Several groups have previously reported the feasibility of outpatient (opt) ASCT in MM. Here, we report our single-center experience with opt ASCT in MM and the impact of ASCT setting on antibiotic exposure, hospital utilization, and transplant outcomes. Methods We conducted a retrospective study of patients with MM (2019-2023) undergoing high-dose (140-200 mg/m2) melphalan-based ASCT. Patient demographics, prior treatments, and transplant-related complications and outcomes were extracted. Univariate linear regressions were used to determine association between transplant settings, hospital utilization, and antibiotic exposure. Results A total of 98 patients who underwent ASCT for MM were evaluated; 50% of whom underwent opt ASCT. Baseline characteristics (Table 1) were similar between opt and inpt groups except for Karnofsky Performance scale (KPS) where significantly fewer patients in the opt group had KPS of 70-80% compared to those in the inpt group (20.4 vs 42.9%; p=0.03). Overall, 58.2% of patients were male and median age at ASCT was 62. Most patients underwent consolidative ASCT in the first-line and 58.2% achieved at least a very-good partial response (VGPR) prior to ASCT. Patients with high-risk cytogenetics, del(17p), t(4;14), t(14;16), represented 9.2% of the whole cohort.Transplant related outcomes (Table 2) were similar between inpt and opt ASCT, with no significant difference in time to engraftment for neutrophils (11.5 vs 12 days) and platelets (16.8 vs 17 days). No transplant-related mortalities between day 0 and +90 were observed.Most patients (59%) in the opt group did not require ED evaluation or hospitalization from day -1 to day +90 of ASCT. Mean length of stay for those in the opt who did require hospitalization (41%) was significantly shorter compared to the inpt group, 5.5 vs 16.8 days (p<0.001), respectively. A non-significantly, greater proportion of patients in the inpt compared to opt group developed neutropenic fever between days -1 and +30 (53% vs 40.8%, p=0.302). Antibiotic exposure (days) was significantly reduced in the opt compared to the inpt group at 6.70 vs 9.87 days (p=0.03), respectively. Three cases of C. difficile infection occurred, all in the inpt group. Conclusion Overall, our results are consistent with prior reports of safety and feasibility of opt ASCT for patients with MM. Patients at our institution undergoing opt ASCT had significantly reduced hospital utilization and IV antibiotic exposure, without evidence of increased transplant-related mortality. Additionally, there was a trend towards lower incidence of neutropenic fever and C. difficile infections.