Euroflow-Based Ultrasensitive NGF for the Evaluation of Bone Marrow Minimal Residual Disease in Multiple Myeloma

Background:

Euroflow next-generation flow cytometry sensitively assesses minimal residual disease (MRD) in multiple myeloma (MM). It is widely adopted in labs worldwide for tracking disease progression, therapy efficacy, and decision-making in MM trials. Euroflow MRD significantly enhances MM patient outcomes, alongside advancements in anti-cancer approaches like BiTEs, CAR-T cells, and ADCs, resulting in increased overall survival rates. However, blood-based methods do not always align with bone marrow PC analysis, requiring heightened sensitivity to detect residual malignant cells post-therapy. The recent BloodFlow method, achieves remarkable sensitivity up to 10^-8 by enriching CD138⁺ PCs from 50 mL peripheral blood. In this study, we compare conventional Euroflow BM-MRD with the ultrasensitive method using CD138 enrichment of BM cells before analysis.

Methods:

BM-MRD followed Euroflow group guidelines. For ultrasensitive assessment, CD138⁺ plasma cells were isolated using CD138 MicroBeads. Data analysis was performed with Infinicyt software. Automatic gate and identification tool was used for conventional MRD, while data was manually analyzed for ultrasensitive MRD assessment.

Results:

We compared conventional and ultrasensitive MRD in the bone marrow of 70 MM patients. All positive MRDs using conventional NGF remained positive with ultrasensitive MRD. Notably, among 40 patients with negative conventional NGF-MRD, 5 cases turned positive after ultrasensitive MRD, by detecting aPCs from 0,0003 to 0,002% in all nucleated cells in the bone marrow. Patients 1, 2, and 3, diagnosed between 2020 and 2022, were in CR during assessment. Patients 4 and 5, diagnosed in 2016 and 2014 respectively, underwent multiple therapies, two autologous hematopoietic cell transplants, and patient 5 also had an allogeneic hematopoietic cell transplant. Ultrasensitive MRD for patients 4 and 5 was done after anti-BCMA CAR T cell therapy. The CD138 isolation approach allowed evaluating BM-MRD at detection levels of 10^-8 cells. Further studies using ultrasensitive MRD will prospectively evaluate the predictive value of the assay in patients with NDMM, especially with high and ultra-high-risk disease.

Conclusions:

This study demonstrates the significant advantages of direct CD138 enrichment over bulk lysis for detecting residual malignant PCs in MM. Ultrasensitive MRD offers superior sensitivity compared to conventional MRD, a critical factor in the era of more effective therapies leading to deeper responses. Currently, we are prospectively assessing the clinical benefits in a larger patient cohort.