Current Assessment and Management of Measurable Residual Disease in Patients with Acute Lymphoblastic Leukemia in the Setting of CAR-T-Cell Therapy
Acute lymphoblastic leukemia (ALL) is a hematologic malignancy characterized by the abnormal proliferation of B-cell or T-cell lineage cells originating in the bone marrow. Despite advancements in treatment, the survival rate of patients with relapsed or refractory (r/r) ALL remains unsatisfactory, with an overall 5-year survival rate of only 10%–20%. Chimeric antigen receptor (CAR)-modified T-cell therapy has emerged as a novel immunotherapy that genetically modifies T cells to specifically identify and eliminate tumor cells expressing specific antigens. This therapy has achieved remarkable success in treating hematological malignancies, including ALL. As of June 2022, six CAR-T-cell products have been approved by the U.S. Food and Drug Administration, with tisagenlecleucel (Kymriah) and brexucabtagene autoleucel (Tecartus) approved for r/r ALL. However, long-term follow-up data from clinical trials indicate that recurrence after CAR-T-cell treatment remains a significant challenge.
Measurable/minimal residual disease (MRD) refers to a small number of post-therapy tumor cells that cannot be detected by traditional methods but can be identified through highly sensitive techniques such as molecular biology and flow cytometry (FCM). MRD monitoring plays a crucial role in evaluating treatment response, predicting recurrence, and guiding further therapeutic decisions. The integration of MRD assessment into CAR-T-cell therapy has gained increasing attention, particularly in the context of targeted therapy. This review explores the common and novel methods of MRD monitoring, emphasizes the prognostic value of MRD, and discusses MRD-directed combination therapies, including CAR-T-cell therapy and allogeneic hematopoietic stem cell transplantation (allo-HSCT).
Assessment of MRD in Patients Receiving CAR-T-Cell Therapy
The assessment of MRD in patients undergoing CAR-T-cell therapy involves several critical considerations, including detection techniques, sample requirements, detection timepoints, and data analysis. The primary methods for MRD detection include FCM, polymerase chain reaction (PCR), and next-generation sequencing (NGS). Each method has its advantages and limitations. FCM is fast and inexpensive but may be confounded by immunological phenotypic shifts. PCR offers high sensitivity and standardization but is time-consuming and requires patient-specific primers. NGS provides very high sensitivity and the potential to identify small subclones and clonal evolution but is relatively expensive and requires complex bioinformatics.
Bone marrow (BM) is the preferred sample for MRD detection in B-cell ALL (B-ALL), while peripheral blood (PB) is an alternative for T-cell ALL (T-ALL). However, BM sampling is invasive, and its uneven distribution of tumor cells can lead to false results. Liquid biopsy, which analyzes cell-free DNA (cfDNA) and other peripheral blood components, has emerged as a non-invasive alternative. Despite its promise, liquid biopsy has primarily been applied for MRD monitoring in diffuse large B-cell lymphoma (DLBCL) rather than ALL.
The optimal timepoints for MRD detection in CAR-T-cell therapy remain undefined. However, researchers recommend BM puncture every three months for six to twelve months post-therapy in patients who do not receive further treatments. Additionally, MRD detection is advised at arbitrary timepoints when disease progression is suspected. The threshold for MRD positivity is typically set at 0.01%, although this value can vary based on detection timepoints, treatment methods, and specimen types.
Recent studies have highlighted the superior sensitivity of NGS over FCM and PCR in monitoring MRD status. For example, Huang et al. found that NGS predicted tumor load dynamics and prognosis better than FCM in patients with B-ALL who achieved complete remission (CR) after CAR-T-cell therapy. Similarly, Pulsipher et al. demonstrated that NGS-MRD was more sensitive than FCM-MRD in both BM and PB samples, making it a more reliable biomarker for predicting relapse after CAR-T-cell therapy.
Prognostic Value of MRD Assessment in CAR-T-Cell Therapy
MRD status before and after CAR-T-cell therapy has significant prognostic implications. Higher disease burden before CAR-T-cell therapy, as indicated by bone marrow blasts ≥5% or extramedullary disease, is associated with a higher risk of disease progression. Conversely, MRD negativity before therapy predicts better overall survival (OS) and leukemia-free survival (LFS). Continuous MRD monitoring post-therapy is equally critical, as MRD negativity is strongly associated with longer event-free survival (EFS) and OS.
Several clinical studies have demonstrated the prognostic value of MRD status after CAR-T-cell therapy. For instance, Park et al. found that patients with MRD-negative CR had significantly longer EFS and OS compared to those with MRD-positive CR or no response. Similarly, Hay et al. reported that MRD-negative CR was associated with better EFS and OS. These findings underscore the importance of MRD monitoring in evaluating treatment efficacy and predicting patient outcomes.
MRD-Directed CAR-T-Cell Therapy: Enhancing Precision and Efficacy in ALL
The integration of MRD assessment into the decision-making process is crucial for optimizing CAR-T-cell therapy. MRD status can guide the timing of CAR-T-cell therapy, with MRD-negative patients prior to therapy experiencing better outcomes. Additionally, MRD assessment can inform the adjustment of CAR-T-cell therapy strategies, such as intensifying preparative regimens or increasing cell infusion doses in MRD-positive patients.
Combining CAR-T-cell therapy with other treatment modalities, particularly allo-HSCT, has shown promise in improving outcomes. CAR-T-cell therapy can be used to reduce tumor burden or clear MRD in patients with morphological CR before allo-HSCT. Conversely, it can serve as consolidation or maintenance therapy to reduce the risk of post-transplant recurrence in high-risk B-ALL patients. The optimal timing and indications for allo-HSCT following CAR-T-cell therapy remain under investigation, but pre-HSCT MRD status is a significant prognostic factor.
CAR-T-Cell Therapy Bridging to Allo-HSCT
Consolidative allo-HSCT following CAR-T-cell therapy is an effective treatment model for r/r B-ALL patients. However, determining which patients should receive allo-HSCT and when it should be performed remains challenging. Studies have shown that MRD negativity before HSCT predicts favorable outcomes and reduces the risk of treatment-related toxicities. For example, Zhao et al. found that MRD positivity at transplantation was associated with poor disease-free survival (DFS), OS, and higher cumulative incidence of relapse (CIR).
Achieving MRD negativity before allo-HSCT is particularly beneficial for high-risk patients. Jiang et al. demonstrated that consolidative allo-HSCT significantly prolonged EFS and relapse-free survival (RFS) in patients with high pre-infusion MRD or poor prognostic markers. However, the benefits of allo-HSCT in patients with MRD-positive CR or morphological relapse after CAR-T-cell therapy are less clear. Some studies suggest that salvage allo-HSCT can be effective, but more data is needed to confirm these findings.
CAR-T-Cell Infusion After Allo-HSCT
CAR-T-cell therapy is also being explored as a preemptive strategy post-allo-HSCT to prevent relapse. Preemptive CAR-T-cell infusion in patients with positive MRD after transplantation has shown promising results. For instance, Zhao et al. reported that preemptive donor-derived CD19-CAR-T-cell infusion achieved MRD-negative CR in all patients without acute graft-versus-host disease (GvHD). This approach resulted in a CIR of 42.8%, DFS of 65.6%, and OS of 100% with a median follow-up of 424.5 days.
New Challenges for MRD Detection in CAR-T-Cell Therapy
Relapse after CAR-T-cell therapy presents new challenges for MRD detection, particularly in cases of immune escape and clonal evolution. Relapse can be classified into two patterns: target-antigen-positive and target-antigen-negative. Target-antigen-positive relapse typically occurs early and is associated with rapid exhaustion and dysfunction of CAR-T cells. In contrast, target-antigen-negative relapse often occurs later and is characterized by the loss of target antigen expression due to mutation, alternative splicing, lineage conversion, or clonal evolution.
The detection of MRD in target-antigen-negative relapse requires modifications to traditional FCM gating strategies. Researchers have suggested extending FCM-MRD testing to other antigens expressed in early B-cell precursors (BCPs) to track B-ALL clones more sensitively. Additionally, cytoplasmic B-specific antigens like cytoplasmic CD79a can be beneficial in detecting CD19-negative B-ALL cells. NGS has also shown promise in identifying subclones and clonal evolution, making it a valuable tool for MRD monitoring in high-risk patients.
Conclusion and Future Directions
Future research should focus on optimizing MRD detection methods, exploring the relationship between MRD status and patient prognosis, and conducting prospective clinical trials for MRD-guided personalized management. Additionally, evaluating MRD as a surrogate endpoint in clinical trials could accelerate drug approval and improve treatment strategies. Other promising MRD-directed interventions, such as antibodies, immunomodulatory drugs, and targeted therapies, should also be explored to enhance the efficacy of CAR-T-cell therapy.
In conclusion, MRD monitoring plays a critical role in the field of CAR-T-cell therapy, offering valuable prognostic insights and guiding treatment decisions. The integration of MRD assessment with CAR-T-cell therapy and other treatment modalities, particularly allo-HSCT, has the potential to reduce the risk of relapse and improve patient outcomes. Despite the challenges posed by relapse with negative target antigen, advancements in MRD detection techniques, such as NGS, continue to enhance the precision and efficacy of CAR-T-cell therapy.
doi.org/10.1097/CM9.0000000000002945
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