PF598 STEM CELL YIELD AND TRANSPLANTATION IN TRANSPLANT‐ELIGIBLE NEWLY DIAGNOSED MULTIPLE MYELOMA PATIENTS RECEIVING DARATUMUMAB + BORTEZOMIB/THALIDOMIDE/DEXAMETHASONE (D‐VTD): PHASE 3 CASSIOPEIA STUDY

Background: High‐dose therapy (HDT) followed by autologous stem‐cell transplantation (ASCT) is the standard of care in transplant‐eligible newly diagnosed multiple myeloma (NDMM). In the phase 3 CASSIOPEIA study, daratumumab + standard‐of‐care regimen VTd (D‐VTd) significantly improved stringent complete response (sCR), complete response or better (≥CR), and minimal residual disease (MRD)‐negative rates and reduced the risk of progression or death versus VTd in NDMM patients who were eligible for transplant. Aims: Here, we assessed stem cell yield and transplantation results among patients receiving D‐VTd versus VTd induction prior to HDT and ASCT in Part 1 of the CASSIOPEIA trial. Methods: In Part 1, transplant‐eligible NDMM patients ages 18–65 years were randomized 1:1 to 4 pre‐transplant induction and 2 post‐transplant consolidation cycles of D‐VTd or VTd alone. After induction, patients underwent stem cell mobilization with cyclophosphamide 3 g/m 2 (recommended dose) and granulocyte colony‐stimulating factor (G‐CSF). Peripheral blood stem cells were harvested based on response to mobilization. Plerixafor was administered if stem cell collection failed at first attempt and in accordance with institutional practice. Melphalan 200 mg/m 2 IV was given as HDT prior to ASCT. Results: A total of 1,085 patients were randomized to D‐VTd (n = 543) or VTd (n = 542). Among patients who completed mobilization (D‐VTd, 506; VTd, 492), more patients in the D‐VTd arm received plerixafor during mobilization than in the VTd arm (21.7% vs 7.9%). Patients underwent a median (range) of 2 (1–6) versus 1 (1–4) days of apheresis for D‐VTd versus VTd. The median number of CD34 + cells collected was lower in patients receiving D‐VTd versus VTd (6.3 × 10 6 /kg vs 8.9 × 10 6 /kg). Nevertheless, a similar percentage of intention‐to‐treat patients receiving D‐VTd versus VTd underwent ASCT (90.1% vs 89.3%). The median number of CD34 + cells transplanted for D‐VTd versus VTd was 3.3 × 10 6 /kg versus 4.3 × 10 6 /kg. Hematopoietic reconstitution rates were high and similar in transplanted patients receiving D‐VTd versus VTd (99.8% vs 99.6%). For D‐VTd versus VTd, a median (range) of 13.0 (6–54) versus 13.0 (4–43) days was required to achieve sustained absolute neutrophil counts >500 cells/mm 3 , and a median (range) of 14.0 (2–56) versus 12.0 (1–47) days was required to achieve sustained platelets >20,000 cells/mm 3 without transfusion. Summary/Conclusion: Stem cell mobilization and collection was feasible with D‐VTd induction. Adding daratumumab to VTd allowed successful transplantation in patients with NDMM who were transplant eligible.