MRD and Plasma Cell Dynamics after CAR T-cell Therapy in Myeloma

In this issue of the journal, Paiva and colleagues (1) evaluated the impact of complete response (CR) and minimal residual disease (MRD) dynamics longitudinally in patients with relapsed/refractory multiple myeloma treated with autologous B-cell maturation antigen (BCMA)–targeted chimeric antigen receptor (CAR) T therapy, and found that deep serological response and MRD remission correlate with prolonged progression-free survival (PFS). Importantly, the study showed that MRD negativity appears faster in the bone marrow [using next-generation sequencing (NGS) or multicolor flow cytometry] than CR in peripheral blood (using serum protein assays) after highly effective BCMA-directed CAR T-cell therapy, which is similar to what has been reported for highly effective 4-drug combination therapies (2). To overcome this limitation, Paiva and colleagues discuss on the need for future MRD guidelines to explore new definitions where MRD-negativity in the bone marrow is independent of monoclonal (M) proteins or serum free light chains in peripheral blood. They stress that this is particularly relevant for clinical trials using early MRD-negativity as an endpoint (2).

The study by Paiva and colleagues is based on 125 patients with relapsed/refractory multiple myeloma who were treated with autologous BCMA-targeted CAR T therapy idecabtagene vicleucel (ide-cel) on the KarMMa phase II trial (1). This prespecified analysis includes several types of comparisons. For example, the authors assessed the relationship between NGS and multicolor flow cytometry for the purpose of tracking low levels of residual tumor cells after the completion of planned therapy. The authors found that the main discordance between the two MRD assay platforms was primarily due to an issue of hemodiluted samples (1). However, overall, the two assay platforms were concordant (94%, 88%, 90%, and 83% at months 1, 3, 6, and 12 after ide-cel therapy, respectively) when performed in accord with high quality standards (i.e., excluding hemodiluted samples). Recently, in a large study from Memorial Sloan Kettering Cancer Center (MSKCC), including 438 samples from 251 patients, the use of NGS targeting the IGH and IGK genes for clonal characterization and monitoring was compared with multicolor flowcytometry for MRD detection in multiple myeloma (3). In that single-center study, the concordance rate was 93% between the two assay platforms and discordant cases were observed only in patients with a low level of disease approaching detection limits for both assays (3). Based on their own results, the MSKCC authors concluded that, in the standard-of-care setting, the choice of the MRD test in multiple myeloma may depend on practical, rather than test performance, considerations. Given that the study by Paiva and colleagues is a multicenter study, the quality of the samples varied more (i.e., higher rate of hemodilution) than the single-center study by MSKCC. Furthermore, given that the KarMMa phase II trial is part of the registrational package for ide-cel; for regulatory purposes, the FDA-cleared NGS-based MRD assay “ClonoSeq” by Adaptive Biotechnologies was used.

The authors also investigated the role of sustained (vs. nonsustained) MRD negativity, and, consistent with prior publications (4, 5), they found sustained negativity to be associated with longer PFS. These observations support the notion that the duration and sensitivity of MRD responses are more important than the methodology used to assess MRD as long as the assay used is validated at ≤10⁻⁵ sensitivity per current International Myeloma Working Group (IMWG) criteria. Lastly, the authors assessed the role of using more stringent criteria to determine MRD negativity than the current level of detection, which is set to 1 tumor cell in 100,000 evaluated bone marrow cells (10⁻⁵). Specifically, the authors found that 10⁻⁶ (vs. 10⁻⁵) was associated with a longer PFS, which is similar to prior studies evaluating the sensitivity of MRD assays in relation to clinical outcomes in multiple myeloma (6). Therefore, the authors concluded that MRD shall be analyzed with the highest possible sensitivity available, and publications need to clearly state which level of MRD negativity was used (i.e., 10⁻⁵ vs. 10⁻⁶).

For a subset of patients where positron emission tomography/computed tomography (PET/CT) was performed, the authors assessed the data to better define the role of PET/CT, refine the depth of response to therapy, and rule out the presence of extramedullary disease (7). Unfortunately, PET/CT was not performed systematically in all patients and at all time points. Based on available data, the presence of extramedullary disease at months 1, 3, 6, and 12 after infusion with ide-cel was associated with inferior PFS in patients achieving MRD negativity based on bone marrow–based assays.

Interestingly, the authors found that the reappearance of normal plasma cells (PC) in the bone marrow after anti-BCMA CAR T therapy predicted inferior PFS in patients who had achieved MRD negativity (1). Based on their findings, they speculate that there may be dynamic responses after CAR T therapy, whereby after initial peak expansion and subsequent tumor depletion (i.e., MRD negativity), CAR T cells may no longer persist or lose function, which leads to the reappearance of normal PC before conversion from MRD negativity to MRD positivity. The study did not include any functional studies to test this hypothesis. Because all cases with the reappearance of normal PC at month 12 had detectable ide-cel levels by quantitative polymerase chain reaction (qPCR), they further hypothesize that loss of function is the most likely explanation. Emerging data from patients with diffuse large B-cell lymphoma (DLBCL) treated with CD19-directed CAR-19 T cells show that tumor-intrinsic genomic modifications are key factors in the complex interplay of factors underlying CAR-19 efficacy as well as resistance. In that DLBCL study, predictors of CAR-19 resistance included complex genomic structural variants, APOBEC mutational signatures, and genomic damage from reactive oxygen species detectable prior to CAR-19 treatment (8). Furthermore, in patients who failed CAR T-cell therapy, the recurrent 3p21.31 chromosomal deletion containing the RHOA tumor suppressor gene was strongly enriched. Contrary to expectations, low-level expression or monoallelic loss of CD19 were not associated with poor treatment effect, suggesting CAR-19 therapy success and resistance are related to several different mechanisms (8). Currently, we lack detailed knowledge on the potential role of tumor-intrinsic genomic alterations in multiple myeloma in relation to BCMA-targeted CAR T-cell efficacy and resistance. Future studies are needed to better clarify mechanisms of resistance to CAR T-cell therapy in patients with multiple myeloma. Lastly, on a practical and clinical note, based on their results, the authors suggest that monitoring normal cell types expressing target antigens (e.g., BCMA) and tumor burden could provide complementary information.

A clinically important observation of the study by Paiva and colleagues was the discordance between CR and MRD negativity (1). Specifically, the MRD negativity rates at months 1, 3, and 6 were higher than the CR rates. The MRD negativity rates peaked at 3 months, whereas the CR rates peaked at 6 months. The observation of MRD negativity appearing faster in the bone marrow (using NGS or multicolor flow cytometry) than CR in peripheral blood (using serum protein electrophoresis, serum immunofixation, and serum-free light chain assays) after highly effective BCMA-directed CAR T-cell therapy is similar to what has been reported for highly effective 4-drug combination therapies (2). The authors address an important topic in their Discussion where they suggest that because of the long half-life of immunoglobulins, early MRD negativity may better reflect the dynamics of CAR T peak expansion and depth of initial tumor clearance. They state that this is a possible limitation of current response criteria that binds MRD negativity to the achievement of CR (9). As a logical extension, the authors propose that future guidelines need to explore new MRD-negative definitions that are independent of the M protein to overcome this issue, particularly in clinical trials using early deep treatment response (i.e., MRD) as an endpoint (e.g., after modern combination therapies and CAR T-cell therapy; ref. 2).

Finally, the authors discuss how new and more sensitive flow cytometry protocols empowered to detect MRD in peripheral blood potentially could help in decreasing the number of bone marrow aspirates (1). They also address that such approaches could inform on the reappearance of BCMA-positive normal PCs that circulate in peripheral blood. For the past few years, several publications have shown that there are more sensitive approaches to detect abnormal proteins in peripheral blood. For example, mass spectrometry is a highly sensitive method that can be used to detect the reappearance of M proteins at later time points (10). This approach could, for example, be valuable if early rescue intervention before clinical progression is being considered; however, there is currently a lack of comprehensive data addressing this topic. Future studies are needed to test these hypotheses and to define strategies on how MRD assessments should be performed in relapsed/refractory multiple myeloma patients treated with CAR T cells. Figure 1 summarizes select current and future approaches—and relative test sensitivity—to determine multiple myeloma tumor burden.