Influence of pre-transplant minimal residual disease on prognosis after Allo-SCT for patients with acute lymphoblastic leukemia: systematic review and meta-analysis

Acute lymphoblastic leukemia (ALL) is a hematologic malignancy of bone marrow featured by the overproduction of immature lymphoblasts [1]. It represents 75–80% of childhood acute leukemias and 20% of all leukemias in adults, with approximately 6000 cases diagnosed every year in the United States [1, 2]. Despite evolving treatment protocols, the relapse rate is approximately 15–20% in ALL, and the cure rate is much lower after relapse [3]. These relapses are due to the persistence of residual malignant cells, namely minimal residual disease (MRD), that cannot be detected by the morphological examination of the bone marrow [4]. Great efforts have been made to standardize MRD quantification using real-time polymerase chain reaction (PCR) of immunoglobulin and T-cell receptor (TCR) gene rearrangements, real-time PCR-based detection of fusion gene transcripts [e.g., breakpoint cluster region-Abelson (BCR-ABL)] or breakpoints, and flow cytometric immunophenotyping [5, 6]. MRD allows a more precise assessment of treatment efficacy and reduction of leukemic burden [7]. It has important prognostic and therapeutic implications for adults and children with ALL [8, 9]. The UKALL 2003 trial suggested that MRD risk stratification was helpful in adjusting the treatment intensity [10].

Allogeneic stem cell transplantation (allo-SCT) is the preferred treatment for adults with relapsed disease and children with high-risk relapses [11, 12]. The SCT mortality due to relapse is 30–40% in adults and children. The treatment-related mortality is also 30–40% in adults but lowers in children [13, 14]. A body of evidence indicated a direct correlation of the likelihood of relapse after transplant with the MRD status before transplantation [15, 16]. However, this significant association was not observed in some studies [17‐20]. Also, patients’ age, detection methods, and adjustment of clinical covariates largely varied among different studies [16]. Additionally, the impact of MRD on overall survival (OS) and nonrelapse mortality (NRM) remained uncertain. Therefore, this systematic review and meta-analysis was conducted to explore the impact of MRD prior to transplantation on the prognosis for patients with ALL.

The prognostic value of pre-transplant MRD was demonstrated in several ways. First, the collected data revealed that patients with positive MRD before allo-SCT had a higher cumulative incidence of relapse in the follow-up. In accordance, the RFS was much shorter for the MRD (+) patients. The EFS was a composited outcome defined as the time from allo-SCT to the first occurrence of relapse or death. Positive MRD before allo-SCT was also an independent negative predictor of EFS. Furthermore, patients with positive pre-transplant MRD were proved to have a significant dismal OS. To the best of my knowledge, this meta-analysis was the first to appraise the role of MRD assessment in the pre-transplant setting in patients with ALL.

The prognostic power of pre-transplant MRD, as well as the sources of heterogeneity, was examined by subgroup analyses. A careful statistical process was conducted with caution that several clinical covariates might interact with the role of MRD and potentially cause bias. Thus, the adjusted estimates were preferred. The most relevant studies paid attention to the well-known confounding factors of disease status (CR1 or CR2), age, sex, and genetic mutations. Notably, the pooled HRs from the adjusted multivariate analysis or crude analysis from KM curves uniformly demonstrated a significant association between pre-transplant MRD and the outcomes of relapse, EFS, and OS. Only for RFS, the crude analysis by three studies failed to show the significant correlation. The included studies using PCR assay mostly amplified the rearranged immunoglobulin and TCR gene segment in the leukemic clone. Only few studies tested the fusion gene of BCR-ABL1. The FC-based assay, by examining the immunophenotypes, was advantageous in terms of rapid process and readily available results. Subgroup analyses demonstrated that the statistical significance for relapse, RFS, EFS, and OS was estimated using either PCR or FC method. The leukemogenic events were different for pediatric and adult patients with ALL [43]. A higher prevalence of unfavorable genetic subtypes, such as the BCR-ABL1 fusion protein, was observed among older patients [2]. The latest National Comprehensive Cancer Network guideline stated that the OS decreased substantially with increased age for patients with ALL [1]. Interestingly, MRD was not found to be a significant predictor of dismal OS for adult patients with ALL. However, MRD prior to transplantation was a constant negative predictor among the outcomes of relapse, RFS, and EFS, regardless of discrepancies in patients’ ages. Compared with prospective studies, retrospective studies relied on data recall or information from previous medical records. However, a significant difference was not observed by analyzing retrospective or prospective studies alone.

Of note, when assessing the outcome of NRM, the MRD status prior to transplantation did not have a significant role. Myeloablative conditioning for MRD-positive patients versus reduced intensity conditioning for patients with undetected MRD might affect the NRM outcomes. A small number of studies might limit the statistical power. Additionally, some other factors might outweigh MRD in predicting NRM. Previous evidence suggested that younger adults had reduced post-transplant mortality. Myeloablation might be not feasible in patients older than 35 years because a higher toxicity was more commonly seen in these recipients [26, 44]. Thus, MRD might only be a subordinate factor for this outcome.

This meta-analysis had several strengths. It included 21 studies with a total of 2323 patients around the world. Comprehensive prognostic outcomes, including relapse, RFS, EFS, OS, and NRM, were evaluated. The prognostic value of MRD was appraised according to different assaying modalities, populations, and study designs. Both adjusted and crude data were presented. Largely, the association between MRD and outcomes remained stable among subgroups. It was confirmed that the detection of MRD was of considerable importance in identifying patients with poor outcome after allo-SCT. MRD was advocated to be a useful molecular biomarker for accurate triage of patients’ =pre-transplantation and preemptive escalation of post-transplant interventions [15].

This study also had several shortcomings. Several studies collected data retrospectively. Some studies had small sample sizes, which might have reduced the statistical power. Patients’ disease status of NRM might have biased this relationship. The inclusion criteria were heterogeneous, and patients were treated using varied chemotherapy protocols. The timing and duration of follow-up were inconsistent. Also, the definitions of assay-specific thresholds and the lack of one universal detection method or testing target were heterogeneous among the included studies. No consensus was reached regarding the standardization of MRD measurement. Furthermore, it failed to give strong justification for providing a quantitative assessment of the influence of pre-transplant MRD. A multitude of confounding factors, such as the use of tyrosine kinase inhibitors, the pre-transplant remission type (CR1 or CR2), donor source, and graft-versus-leukemia, were not sufficiently adjusted in many studies when analyzing the impact of MRD. In fact, these factors were even inconsistent within an individual study [25, 40]. Even for studies that reported adjusted HRs, the degree of adjustment largely varied. The subgroup findings should be considered as exploratory, and thus would need to be tested in original studies. Finally, this study was conducted with summary statistics rather than with individual data, which might have ignored the impact of some covariates on the outcomes at the patient level. The availability of data from individual patients could resolve this problem and increase the power of meta-analysis.

Future studies should aim to decide how best to use the prognostic information of MRD. Several ways can be considered to improve the outcomes for MRD (+) patients at transplantation. Pre-transplantation treatment with non-cross-resistant agents might be helpful in decreasing the residual malignant clone [14, 40]. Preemptive immunotherapy or chemotherapy might be beneficial during the post-transplantation stage [45, 46]. Lankester et al. preliminarily revealed that alloimmune intervention after allo-SCT was feasible in reducing residual leukemic cells [17]. Further, a randomized trial should be performed on patients with ALL in complete remission who had positive MRD and received either allo-SCT or additional novel chemotherapy before transplantation.

In conclusion, this meta-analysis provided evidence that positive MRD prior to allo-SCT was associated with higher relapse and poor survival in patients with ALL. Allo-SCT appeared to be insufficient for some patients with positive MRD at transplantation. The findings of this study suggested the rationale for future studies to prevent relapse and improve survival for this group of high-risk patients.