(P-BT-13) Flow Cytometry Assessment of Residual Disease and Engraftment Outcome in Multiple Myeloma Patients Treated with Hematopoietic Progenitor Cell Products

Hematopoietic stem cell (HSC) transplant is one of the first-line treatments for patients with multiple myeloma (MM). Cellular therapy labs are responsible for processing hematopoietic progenitor cell (HPC) products after collection. Although not required, some laboratories perform flow cytometry on HPC products to evaluate the persistence of residual disease (monotypic/clonal cells) in the product prior to transplant. In this study, we evaluate the role of flow cytometry in ensuring the quality of HPC-A (apheresis) products prior to the transplant and results of engraftment in patients with MM.

Study

Design/Methods:

Patients with MM who underwent autologous HSC transplant from October 2022-January 2024 were included in this retrospective study. Flow cytometry results (residual disease), demographic data, and engraftment results were recorded. Flow cytometry results from HPC products, pre-collection/pre-transplant samples, and post-transplant samples were categorized as 0%, or >0% residual disease. Engraftment was defined as 3 consecutive days with an absolute neutrophil count (ANC) >500 × 10⁶/L and platelet count >20 × 10⁹/L independent from transfusion. Statistical comparison of engraftment days by residual disease category was done by non-parametric Wilcoxon Rank Sums test with SAS version 9.4.

Results/Findings:

A total of 352 patients (53.3 % male) with a mean age of 60 years were studied. Of those, 273 (77.7%) underwent HSC transplant (54.6% male with mean age of 60 years). All transplanted patients reached bone marrow engraftment. Engraftment data was available for 184 patients. Mean time to ANC engraftment was 11.8 days (range 6-34 days), and mean time to platelet engraftment was 18.7 days (range 10-117). Six (1.7%) HPC products had positive flow cytometry results (all < 1% residual disease); of these, 3 were infused. Time to ANC engraftment was not significantly different for HPC products with or without residual disease (mean 11.0 ± 1.0 vs. 11.8 ± 2.0, p=0.47). Time to platelet engraftment varied slightly between the 2 groups: mean 16.0 ± 1.0 vs. 17.0 ± 1.0 days, p=0.051. When examining marrow residual disease pre-collection/pre-transplant, there were no differences in ANC engraftment (p=0.71) or platelet engraftment (p=0.83) for residual disease of 0%, or >0%. When examining marrow residual disease post-transplant (average 94 days post-transplant), there were no differences in ANC engraftment (p=0.99) or platelet engraftment (p=0.53) for residual disease of 0% or >0%.

Conclusions:

These findings show the rarity (less than 2%) of the presence of monotypic/clonal cells in HPC products in MM patients undergoing HSC transplant. Additionally, there were no significant differences between HPC product residual disease flow cytometry results and engraftment times. As such, the utility of flow cytometry assessment of HPC-A products is unclear.