Impact of clonal plasma cells in autografts on outcomes in high-risk multiple myeloma patients

Most patients with multiple myeloma (MM) undergoing autologous hematopoietic stem cell transplantation (autoHCT) eventually relapse, perhaps due to the presence of clonal plasma cells (CPC) in the autograft. We conducted a retrospective analysis to evaluate the impact of CPC in the autograft on the outcomes of high-risk chromosomal abnormalities (HRMM) patients undergoing autoHCT between 2008 and 2018. Patients were divided into CPC+ or CPC- in the autograft by next-generation flow cytometry (NGF). There were 75 CPC + autografts (18%) and 341 CPC- (82%). The CPC + group was less likely to achieve MRD-negative complete remission post-transplant (11% vs. 42%; p < 0.001). Median progression free survival (PFS) and overall survival (OS) were (12.8 vs. 32.1 months) and (36.4 vs. 81.2 months) in the CPC + and CPC- groups, respectively (both p < 0.001). Also in the subset of patients with MRD-negative >= VGPR prior to autoHCT, those with CPC + autografts had inferior PFS (HR 4.21, p = 0.006) and OS (HR 7.04, p = 0.002) compared to CPC-. In multivariable analysis, the degree of CPC positivity in the autograft was independently predictive of worse PFS (HR 1.50, p = 0.001) and OS (HR 1.37, p = 0.001). In conclusion, both the presence and degree of CPC in the autograft were highly predictive of inferior PFS and OS.