Efficacy of stem cell boost (SCB) for chimeric antigen receptor-T cell therapy (CAR-T)-related hematologic toxicity in patients with relapsed/refractory multiple myeloma (RRMM)-real world experience from the US multiple myeloma immunotherapy consortium.

Prolonged cytopenias occurred in 15.4% of CAR-T recipients, of whom 39 received stem cell boost (SCB).

• Of 590 total CAR-T patients reviewed, 91 patients (15.4%) developed prolonged cytopenias.
• 39 of the 91 patients with prolonged cytopenias received SCB.
• The study period covered commercial CAR-T infusions between 6/2021 and 3/2024.
• Controls were selected from the 590 CAR-T recipients using ANC and platelet thresholds representing the 75th percentile of cytopenia severity in the SCB cohort.

Nearly all SCB patients achieved hematologic recovery, with a median time to recovery of 24 days.

• 97.4% (all but one patient) in the SCB group achieved hematologic recovery.
• Median time to hematologic recovery was 24 days (range: 9–87 days).
• Hematologic recovery was assessed using CIBMTR engraftment criteria.
• Median CD34+ cell dose administered was 2.9 million/kg (range: 1.8–23.6 million/kg).
• SCB was administered at a median of 53 days post-CAR-T (range: 24–265 days).

SCB patients had significantly higher hemoglobin and platelet counts compared to non-SCB controls at day 90 post-CAR-T infusion.

• Median hemoglobin at day 90 was 10.6 g/dL in SCB patients vs. 8.7 g/dL in nSCB patients (p = 0.002).
• Median platelet count at day 90 was 135 K/L in SCB patients vs. 35 K/L in nSCB patients (p < 0.001).
• Cell counts in the nSCB group were analyzed cross-sectionally at day 60 and 90 post-CAR-T and compared to the SCB cohort using Kruskal-Wallis tests.

No new toxicities attributable to SCB were observed.

• The study explicitly states that no new toxicities attributable to SCB were observed in any of the 39 SCB recipients.
• This finding supports the safety profile of autologous SCB as a strategy for managing CAR-T-related prolonged cytopenias.

SCB patients had numerically longer progression-free survival and overall survival compared to matched non-SCB controls.

• Median follow-up was 12.6 months in the SCB cohort and 11.6 months in the nSCB arm.
• Median PFS was 11.0 months in the SCB group vs. 8.2 months in the nSCB group.
• Median OS was not reached in the SCB group vs. 12.3 months in the nSCB group.
• PFS and OS were evaluated using Kaplan-Meier methods.

The study used a retrospective multi-institutional design comparing SCB to supportive care alone in patients with prolonged cytopenias after CAR-T infusion for relapsed/refractory multiple myeloma.

• Data were collected from the US Multiple Myeloma Immunotherapy Consortium.
• Patients were included if they received SCB within 1 year following commercial CAR-T infusion.
• The non-SCB control group was identified from the same 590-patient CAR-T recipient pool using cytopenia severity matching.
• The study period was June 2021 through March 2024.