Multiple myeloma (MM) is an incurable hematologic malignancy, although the development of proteasome inhibitors (PIs), immunomodulatory drugs (IMiDs), autologous stem cell transplantation (ASCT), and monoclonal antibody (MoAb) drugs has prolonged survival in patients [1]. Previously, rwesearchers re reviewed the literature regarding the total therapy strategy, including PIs, IMiDs, anti-CD38 MoAb, and ASCT, which not only induce a therapeutic effect on the myeloma cells but also improve the bone marrow microenvironment, including the enhancement of anti-myeloma immunological activity and the suppression of inhibitory anti-myeloma immunological effects ^[2][3][2,3]. Furthermore, it has recently been suggested that minimal residual disease (MRD) negativity is a surrogate marker for prolonged survival ^[4][5][4,5]. In fact, several studies have shown that myeloma cells detected as MRD may develop drug resistance and affect the surrounding immune environment.

The Monoclonal Antibody-Based Sequential Therapy for Deep Remission in Multiple Myeloma (MASTER) trial, a phase II study, demonstrated that a four-drug combination of daratumumab (DARA), carfilzomib (CFZ), lenalidomide (LEN), and dexamethasone (DEX) (D-KRd) followed by ASCT and subsequent D-KRd consolidation therapy is effective for patients with newly diagnosed MM (NDMM). Once a patient entered an MRD surveillance (MRD-SURE) phase, after bone marrow samples tested MRD-negative twice consecutively, the treatment was discontinued [6]. In the MASTER study, MRD-SURE was achieved in most patients, and the survival outcome was excellent in patients with zero or one high-risk chromosomal aberration (HRCA). However, patients with two or more HRCAs, who were considered to have ultra-high-risk chromosomal aberrations (UHRCAs), not only had a low MRD negativity rate but also achieved MRD-SURE relapse in some cases. That is, the MASTER trial results suggest that other strategies are necessary for patients with UHRCAs, even if the total therapeutic strategy concerning the bone marrow microenvironment was performed and MRD negativity was achieved [6].

2. MRD in Autografts Might Predict Clinical Outcome

In the MASTER trial, the MRD negativity rate after D-KRd induction therapy was lower in patients with UHRCAs than in the other CA groups, indicating that the myeloma cells are more frequently contaminated in the former than in the latter [6]. In two clinical studies, the MRD status of the autograft correlated with the survival time after ASCT ^[7][8][49,50]. Therefore, rwesearchers  discuss the role of MRD eradication in autografts in achieving persistent MRD negativity, particularly in patients with UHRCAs. In the MASTER trial, consecutive MRD assessments after D-KRd induction, ASCT, and four or eight cycles of D-KRd consolidation were performed using bone marrow samples. Before PIs and IMiDs were available, the presence of MRD in autografts was not associated with subsequent survival ^[9][51]. However, rwesearchers  considered that the contamination of myeloma cells not affecting the clinical outcomes in several previous trials because the therapeutic efficacy of conventional cytotoxic induction treatments, such as vincristine, doxorubicin, and dexamethasone, was inferior to the current induction therapy using novel agents ^{[9][10][11][12]}[51,52,53,54]. In contrast, a single-center, retrospective analysis from Japan reported that the presence of MRD in autografts detected by next-generation sequencing (NGS) or real-time PCR predicts shorter OS and PFS after ASCT, and the lower levels of MRD in autografts are associated with longer OS and PFS ^[7][49]. In a retrospective study of patients with NDMM who underwent ASCT, those who achieved MRD negativity before ASCT had prolonged PFS compared with those who achieved MRD negativity after ASCT, suggesting that the achievement of earlier MRD negativity might be associated with a good response to induction therapy and that the incidence of MM cell contamination in the autograft might be lower in patients with MRD negativity before ASCT, leading to improved prognosis after ASCT ^[13][55]. Recently, two retrospective analyses employed next-generation flow cytometry (NGF) to reveal that MRD-negativity in autografts could predict long PFS and OS after ASCT ^[14][15][56,57]. Additionally, according to a retrospective analysis of MDACC, MRD-negativity was identified in patients treated with VRD induction therapy and without del17p and 1q21gain. MRD-negativity in autografts could predict long PFS and OS independent of induction therapy regimens ^[14][56]. While MRD-negativity in autografts could predict long PFS, the PFS in HRCA was short compared with non-HRCA, even in the patients with MRD-negativity in autografts ^[15][57]. Notably, the autograft MRD status could not be determined based on bone marrow samples, and the association of MRD status between autograft and peripheral blood samples was not analyzed in these two studies. Thus, the achievement of MRD negativity in autografts might be essential for longer OS and PFS, although the associated clinical significance has not yet been elucidated in large-scale prospective clinical trials in the era of novel agents. Myeloma is generally distributed throughout the bone marrow. Therefore, residual myeloma cells may be present in untested bone marrow sites or extramedullary lesions, even when the tested bone marrow samples are MRD-negative ^{[2][16][17][18][19]}[2,58,59,60,61]. Thus, a negative MRD status in bone marrow samples after induction therapy does not necessarily correspond to negative autografts, as MRD-positive autografts indicate the presence of circulating tumor cells (CTCs) in the peripheral blood ^[7][20][49,62]. Therefore, treatment strategies to eliminate MRD in the autograft include in vivo purging, which involves attacking CTCs using chemotherapy before mobilization, or ex vivo purging, which involves the positive selection of CD34-positive cells in the autograft. In several trials, ex vivo purging suppressed the contamination of myeloma cells in the autograft but did not improve survival time ^[11][21][53,63]. Hence, it is unlikely that a survival benefit from ex vivo purging will be achieved in the era of novel agents, considering that novel methods of ex vivo purging have not, to our knowledge, been studied on a large scale. Moreover, there is no evidence that a MoAb has been approved for in vivo purging following PI and IMiD approval. The incidence of myeloma cell contamination in autografts is higher in patients with HRCAs, such as del(13q), even before PIs and IMiDs were available ^[22][64]. This was demonstrated by the FORTE trial, in which the MRD-positive rate before maintenance therapy in double-hit patients was lower than that in patients with a single HRCA regardless of the treatment group ^[23][65]. Thus, patients with UHRCAs may be more likely to have myeloma cells in their autografts than patients without UHRCAs. Accordingly, rwesearchers c consider that it is essential to reduce the tumor burden with intensive induction therapy to obtain MRD-negative autografts, especially in patients with UHRCAs. Moreover, given that a significant association has been reported between MRD in autografts and peripheral blood, following induction therapy, MRD assessment of the peripheral blood should be performed to monitor for CTCs and prevent myeloma cell contamination in autografts ^[8][50]. Indeed, some patients with negative MRD status in autografts test positive in the bone marrow ^[24][66]. As a result, MRD assessment of autograft specimens may be the most reliable method to confirm the MRD negativity of an autograft because the MRD positivity rate in autografts might be low even in patients with positive bone marrow samples ^[25][67].

3. Analyzing MRD Status: Optimal Sample and Device for UHRCA

According to the International Myeloma Working Group (IMWG) criteria, the CR criteria are focused on three biomarkers: levels of monoclonal (M) protein (products from myeloma cells), distribution of myeloma, and presence of myeloma cells in the bone marrow [4]. Although MRD assessment is currently focused primarily on myeloma cells in the bone marrow, it may be more useful to consider their presence throughout the entire body, particularly after achieving MRD negativity in the bone marrow ^[4][19][26][4,61,68]. This is important because the site of bone marrow aspiration is not indicative of myeloma in the body, given that myeloma has a partial, not diffuse, distribution ^[27][28][69,70]. Moreover, EMD can be observed at relapse after MRD negativity ^[29][71], suggesting that myeloma cells can be independent of the microenvironment or escape harmful microenvironments even if the myeloma cell burden is reduced below the cutoff level of MRD negativity as detected by NGS or NGF. Although the technology for MRD detection has developed over time, MRD negativity cannot be considered a sign of eradication of all myeloma cells ^[30][31][32][72,73,74]. Thus, intensive treatment should be continued for myeloma cells in patients with UHRCAs as they tend to relapse aggressively owing to changes in the beneficial microenvironment for myeloma cells with 1q21 CA and the incidence of EMD in patients with 1q21 amp and del(17p) ^{[33][34][35][36]}[23,26,32,33]. In the MASTER trial, the MRD-negativity rate as the best response was similar among SRCA, HRCA, and UHRCA; meanwhile, the PFS in the UHRCA group was shorter than those in the other groups even when MRD-SURE was achieved ^[32][74]. However, MRD assessment was performed in bone marrow samples using NGF, and patients were neither tested for myeloma disease distribution using positron emission tomography/computerized tomography (PET/CT) or magnetic resonance imaging (MRI) ^[27][37][38][69,75,76] nor for M-protein levels using mass spectrometry ^{[39][40][41][42]}[77,78,79,80]. The IMWG criteria suggest combining MRD measurements using bone marrow samples with imaging MRD measurements, indicating the importance of MRD imaging to confirm the presence of extramedullary lesions [4]. Considering that myeloma cells in patients with UHRCAs frequently exhibit genomic instability and are prone to EMD complications, MRD should be analyzed using various strategies to confirm the achievement of true MRD negativity.