Risk stratification in myeloma by detection of circulating plasma cells prior to autologous stem cell transplantation in the novel agent era

The impact of circulating plasma cells (CPCs) prior to autologous stem cell transplantation (ASCT) for multiple myeloma has not been defined in the novel agent era. We evaluated the impact of pre-transplant CPCs, detected by six-color flow cytometry in patients undergoing early ASCT on post-transplant response, progression-free survival (PFS) and overall survival (OS). CPCs were detected in 162 out of 840 (19.3%) patients, with the median number of CPCs being 58 per 150 000 events. Ninety-nine percent of patients had received proteasome inhibitor and/or immunomodulator-based induction. The incidence of post-transplant stringent complete response (sCR) in the subgroups with and without CPCs was 15% and 38%, respectively, (P<0.001). The median PFS in the subgroups with and without CPCs was 15.1 (95% confidence interval (CI), 12.5-17.8) and 29.6 months (95% CI, 26.2-32.8), respectively, and the median OS was 41.0 months (95% CI, 32.6-58.2) and not reached (NR) (95% CI, 99.1-NR), respectively, (P<0.001 for both). On multivariate analysis for OS, factors independently predictive of mortality were the presence of CPCs (hazard ratio (HR) 2.5; 95% CI, 1.8-3.6; P<0.001) and sCR post transplant (HR 0.4; 95% CI, 0.2-0.6; P<0.001). Presence of CPCs prior to transplant has a high prognostic impact and should be prospectively validated in clinical trials.