The emerging role of measurable residual disease detection in AML in morphologic remission

Literature review and expert opinion on the treatment of high-risk acute myeloid leukemia in patients who are eligible for intensive chemotherapy

In patients with Acute Myeloid Leukemia (AML), the assessment of disease risk plays a central role in the era of personalized medicine. Indeed, integrating baseline clinical and biological features on a case-by-case basis is not only essential to select which treatment would likely result in a higher probability of achieving complete remission, but also to dynamically customize any subsequent therapeutic intervention. For young high-risk patients with low comorbidities burden and in good general conditions (also called "fit" patients), intensive chemotherapy followed by allogeneic stem cell transplantation still represents the backbone of any therapeutic program. However, with the approval of novel promising agents in both the induction/consolidation and the maintenance setting, the algorithms for the management of AML patients considered eligible for intensive chemotherapy are in constant evolution. In this view, we selected burning issues regarding the identification and management of high-risk AML, aiming to provide practical advice to facilitate their daily clinical management in patients considered eligible for intensive chemotherapy.

Minimal Residual Disease in Acute Myeloid Leukemia: Old and New Concepts

Minimal residual disease (MRD) is of major importance in onco-hematology, particularly in acute myeloid leukemia (AML). MRD measures the amount of leukemia cells remaining in a patient after treatment, and is an essential tool for disease monitoring, relapse prognosis, and guiding treatment decisions. Patients with a negative MRD tend to have superior disease-free and overall survival rates. Considerable effort has been made to standardize MRD practices. A variety of techniques, including flow cytometry and molecular methods, are used to assess MRD, each with distinct strengths and weaknesses. MRD is recognized not only as a predictive biomarker, but also as a prognostic tool and marker of treatment efficacy. Expected advances in MRD assessment encompass molecular techniques such as NGS and digital PCR, as well as optimization strategies such as unsupervised flow cytometry analysis and leukemic stem cell monitoring. At present, there is no perfect method for measuring MRD, and significant advances are expected in the future to fully integrate MRD assessment into the management of AML patients.

Real-world outcomes and prognostic factors among patients with acute myeloid leukemia treated with allogeneic hematopoietic stem cell transplantation

Acute myeloid leukemia (AML) is a malignant lymphohematopoietic tumor that ranks among the most frequent indications for allogeneic hematopoietic stem cell transplantation (allo-HSCT). This article aims to provide a comprehensive analysis of the application of allo-HSCT for AML and identify prognostic factors to enhance future treatment effect. This retrospective study collected data from 323 patients diagnosed with AML at Peking University First Hospital who underwent allo-HSCT between September 2003 and July 2022. The annual number of transplantations has steadily increased. Our center has observed a rise in the proportion of cytogenetic high-risk and measurable residual disease (MRD) positive patients since 2013, as well as an increase in the number of haploidentical transplantations. The overall leukocyte engraftment time has decreased over the past 20 years. Furthermore, both overall survival (OS) and disease-free survival (DFS) have significantly improved, while non-relapse mortality (NRM) has significantly decreased since 2013. Multivariate analysis identified transplantation before 2013, patients in complete remission (CR) 2 or non-CR, and recipients older than 50 years as risk factors for NRM, while patients in non-CR and patients with positive MRD are risk factors for recurrence. These findings offer insights into AML treatment outcomes in China, highlighting changes in transplantation practices and the need to reduce post-transplant relapse. Effective interventions, such as MRD monitoring and risk stratification schemes, are crucial for further enhancing transplant outcomes.

Flow cytometric measurable residual disease in adult acute myeloid leukemia: a preliminary report from Eastern India

Presence of measurable residual disease (MRD) in acute myeloid leukemia (AML) is considered to be an independent predictor of relapse and poorer survival outcomes. MRD can be measured by flow cytometric, quantitative PCR, and NGS-based assays at varying sensitivities. There is scant Indian data on different aspects of MFC-MRD in AML including analysis strategies as well as molecular spectrum, clinical correlation, etc. This retrospective observational study included all newly diagnosed patients of acute myeloid leukemia in whom complete baseline diagnostic workup was available including flow cytometry and cytogenetic and molecular studies. Among patients with cytogenetic abnormalities (n = 25), no statistically significant correlation was observed between flow cytometric MRD positivity and presence of ≥ 3 mutations as well as relapsed disease. However, in AML patients with normal karyotype (n = 32), MRD positivity correlated strongly with relapsed status (p = 0.02), although no significant correlation was found with respect to FLT3 mutation, IDH mutation, NPM1 mutation, or complex genotype. Interestingly, 90.5% of MRD-positive patients belonged to ELN (2017) intermediate to high-risk category unlike only 9.5% in the good risk category (p = 0.0002). Median relapse-free survival was 8.5 months with a follow-up range of 3-24 months. On the basis of the observations of the present study, it can be clearly inferred that MRD status affects relapse status in the normal karyotype subgroup and can delineate patients who require stem cell transplantation in addition to molecular signatures.

Safety and efficacy of a modified busulfan/cyclophosphamide conditioning regimen incorporating cladribine for autologous hematopoietic stem cell transplantation in acute myeloid leukemia

This is a small phase I study examining the safety and efficacy of a cladribine (CLAD)-containing conditioning regimen prior to autologous hematopoietic stem cell transplantion (auto-HSCT) for patients with acute myeloid leukemia (AML). All patients, aged 15-54 years (median 32 years), had favorable/intermediate risk AML (n = 20) or acute promyelocytic leukemia (APL; n = 2) and no evidence of minimal residual disease (MRD) prior to transplantation. Fourteen of the 22 patients received the conditioning regimen as follows: busulfan (Bu) + cyclophosphamide (Cy) + CLAD + cytarabine (Ara-c) or idarubicin. The conditioning regimen of 8 patients was without Cy nor idarubicin to reducing adverse cardiac reaction: the regimen of Bu + CLAD+ Ara-c for 6 patients; and the regimen of Bu + melphalan + CLAD + Ara-c for the other 2 patients. All 22 AML patients received peripheral blood stem cell transplantation. The number of infused mononuclear cells and CD34⁺ cells was 10.00 (2.88-20.97) × 10⁸/kg and 1.89 (1.52-10.44) × 10⁶/kg, respectively. Hematopoietic reconstitution was achieved in all patients, with a median time of 13 (10-34) days for neutrophils and 28 (14-113) days for platelets. Two patients suffered from pulmonary infection, 4 patients suffered from septicemia during the neutropenic stage, and the others suffered from infection or gastrointestinal reaction without exceeding grade 3 after conditioning, which were all alleviated by anti-infection and other supportive treatment. None of the patients died of transplantation-related complications. At a median follow-up of 29.5 (ranging from 4.0 to 60.0) months, three patients relapsed after auto-HSCT at a median time of 6 (ranging from 0.5 to 10.0) months. One patient died due to relapse at 18 months after auto-HSCT. The remaining 21 patients were all alive, including 19 patients with negative MRD. The other 2 patients achieved negative MRD after allogeneic HSCT or chemotherapy. The estimated 2-year survival, relapse, and Leukemia-free survival rates were 94.1 ± 5.7%, 14.7 ± 7.9% and 85.3 ± 7.9%, respectively. A CLAD-combination conditioning regimen is efficient and safe for auto-HSCT, indicating an effective approach for AML treatment.

Measurable residual disease by flow cytometry in acute myeloid leukemia is prognostic, independent of genomic profiling

Measurable residual disease (MRD) assessment provides a potent indicator of the efficacy of anti-leukemic therapy. It is unknown, however, whether integrating MRD with molecular profiling better identifies patients at risk of relapse. To investigate the clinical relevance of MRD in relation to a molecular-based prognostic schema, we measured MRD by flow cytometry in 189 AML patients enrolled in ECOG-ACRIN E1900 trial (NCT00049517) in morphologic complete remission (CR) (28.8 % of the original cohort) representing 44.4 % of CR patients. MRD positivity was defined as ≥ 0.1 % of leukemic bone marrow cells. Risk classification was based on standard cytogenetics, fluorescence-in-situ-hybridization, somatic gene analysis, and sparse whole genome sequencing for copy number ascertainment. At 84.6 months median follow-up of patients still alive at the time of analysis (range 47.0-120 months), multivariate analysis demonstrated that MRD status at CR (p = 0.001) and integrated molecular risk (p = 0.0004) independently predicted overall survival (OS). Among risk classes, MRD status significantly affected OS only in the favorable risk group (p = 0.002). Expression of CD25 (α-chain of the interleukin-2 receptor) by leukemic myeloblasts at diagnosis negatively affected OS independent of post-treatment MRD levels. These data suggest that integrating MRD with genetic profiling and pre-treatment CD25 expression may improve prognostication in AML.

A Focus on Intermediate-Risk Acute Myeloid Leukemia: Sub-Classification Updates and Therapeutic Challenges

Acute myeloid leukemia (AML) represents a heterogeneous group of hematopoietic neoplasms deriving from the abnormal proliferation of myeloid progenitors in the bone marrow. Patients with AML may have highly variable outcomes, which are generally dictated by individual clinical and genomic characteristics. As such, the European LeukemiaNet 2017 and 2022 guidelines categorize newly diagnosed AML into favorable-, intermediate-, and adverse-risk groups, based on their molecular and cytogenetic profiles. Nevertheless, the intermediate-risk category remains poorly defined, as many patients fall into this group as a result of their exclusion from the other two. Moreover, further genomic data with potential prognostic and therapeutic influences continue to emerge, though they are yet to be integrated into the diagnostic and prognostic models of AML. This review highlights the latest therapeutic advances and challenges that warrant refining the prognostic classification of intermediate-risk AML.

Pretransplantation minimal residual disease monitoring by multiparameter flow cytometry predicts outcomes of AML patients receiving allogeneic hematopoietic stem cell transplantation

BACKGROUND AND PURPOSE

Is minimal residual disease (MRD) monitoring by multiparameter flow cytometry (MFC) prognostic for acute myeloid leukemia (AML) patients before allogeneic hemopoietic stem cell transplantation (allo-HSCT)? And if so, what level of MRD eradication can be used to help guide the timing of HSCT? Can haplo-HSCT improve the prognosis of AML patients with MRD positive? To figure out these questions, we initiated this retrospective study.

METHODS

96 AML patients were included retrospectively and divided into 5 groups, according to pre-transplantation MRD levels (from 5 × 10^-2 to <1 × 10^-4), to analyze the overall survival (OS), disease-free survival (DFS) and cumulative incidence of relapse (CIR). Secondly, we compared the prognosis of MRD-negative (MRD^neg) and MRD-positive (MRD^pos) AML patients (cutoff value = 1 × 10^-3) who underwent allo-HSCT, and further analyzed the prognosis of MRD^pos patients after received different transplantation modalities.

RESULTS

It is found that the 2-year OS and DFS of MRD negative group were better than the MRD positive group, and that the deeper the eradication of MRD before transplantation, the better the prognosis of patients. The CIR in patients received HLA-identical transplantation, was higher in the MRD^pos than in the MRD^neg. Haploid transplantation reduced the CIR disparity between MRD^pos and MRD^neg group. Subsequently, in AML patients who remain MRD positive before HSCT, we show that haplo-HSCT offered a better prognosis than HLA-identical transplantation (MSDT and MUDT).

CONCLUSION

It is suggested that achieving MFC-MRD <10^-3 (10^-4 or even better) before allo-HSCT could reduce the relapse of AML and improve OS and DFS significantly, while haplo-HSCT may be preferred for patients not achieving MRD negativity.

Cytogenetic abnormalities in NPM1-mutated acute myeloid leukemia

NPM1^mut acute myeloid leukemia (AML) has been identified as a distinct entity of myeloid neoplasms according to the 2017 European LeukemiaNet (ELN) guidelines. It confers a favorable prognosis regardless of cytogenetic abnormalities. We evaluated 418 newly diagnosed AML patients to test the validity of this hypothesis. Seventy-four patients with NPM1^mut AML showed a good response to induction and a relatively favorable prognosis. Abnormal karyotypes were observed in 15 patients. Chromosomal abnormalities were significantly associated with a worse prognosis in NPM1^mut AML patients (5-year overall survival (OS): 38.9 ± 12.9%, p = .037; event-free survival (EFS): 33.3 ± 12.2%, p = .043, respectively). Four patients with abnormal karyotypes who underwent allogeneic hematopoietic stem cell transplantation (alloHSCT) during CR1 had longer survival than those who received chemotherapy only. Multivariable analysis revealed abnormal karyotypes independently predicted OS and EFS among NPM1^mut AML patients. In summary, cytogenetic abnormalities are strong prognostic indicators in NPM1^mut AML. Therefore, they should be classified accordingly, and alloHSCT should be performed on selected patients during CR1.

Longitudinal minimal residual disease assessment in multiple myeloma patients in complete remission – results from the NMSG flow-MRD substudy within the EMN02/HO95 MM trial

Background

Multiple myeloma remains an incurable disease with multiple relapses due to residual myeloma cells in the bone marrow of patients after therapy. Presence of small number of cancer cells in the body after cancer treatment, called minimal residual disease, has been shown to be prognostic for progression-free and overall survival. However, for multiple myeloma, it is unclear whether patients attaining minimal residual disease negativity may be candidates for treatment discontinuation. We investigated, if longitudinal flow cytometry-based monitoring of minimal residual disease (flow-MRD) may predict disease progression earlier and with higher sensitivity compared to biochemical assessments.

Methods

Patients from the Nordic countries with newly diagnosed multiple myeloma enrolled in the European-Myeloma-Network-02/Hovon-95 (EMN02/HO95) trial and undergoing bone marrow aspiration confirmation of complete response, were eligible for this Nordic Myeloma Study Group (NMSG) substudy. Longitdudinal flow-MRD assessment of bone marrow samples was performed to identify and enumerate residual malignant plasma cells until observed clinical progression.

Results

Minimal residual disease dynamics were compared to biochemically assessed changes in serum free light chain and M-component. Among 20 patients, reaching complete response or stringent complete response during the observation period, and with ≥3 sequential flow-MRD assessments analysed over time, increasing levels of minimal residual disease in the bone marrow were observed in six cases, preceding biochemically assessed disease and clinical progression by 5.5 months and 12.6 months (mean values), respectively. Mean malignant plasma cells doubling time for the six patients was 1.8 months (95% CI, 1.4–2.3 months). Minimal malignant plasma cells detection limit was 4 × 10–5.

Conclusions

Flow-MRD is a sensitive method for longitudinal monitoring of minimal residual disease dynamics in multiple myeloma patients in complete response. Increasing minimal residual disease levels precedes biochemically assessed changes and is an early indicator of subsequent clinical progression.

Trial registration

NCT01208766

Supplementary Information

The online version contains supplementary material available at 10.1186/s12885-022-09184-1.

Comparison of flow cytometry and next-generation sequencing in minimal residual disease monitoring of acute myeloid leukemia: One institute's practical clinical experience

INTRODUCTION

Monitoring patients with acute myeloid leukemia can be implemented through various techniques such as multiparameter flow cytometry, real-time quantitative polymerase chain reaction, and next-generation sequencing. However, there is scarce studies when comparing the data of next-generation sequencing and flow cytometry for monitoring disease progression, particularly how they might supplement one another when used in tandem.

METHODS

We investigated 107 patients via retrospective analysis using follow-up MFC and NGS data with a total of 717 MFC and 247 NGS studies to compare these methods in monitoring minimal/measurable residual disease.

RESULTS

197 instances were MFC⁺ /NGS⁺ , 3 were MFC^- /NGS^- , 44 were MFC^- /NGS⁺ , and 3 are MFC⁺ /NGS^- . The majority of the MFC^- /NGS⁺ cases occurred within 6 months during the post-treatment phase (64%). Among 44 MFC^- /NGS⁺ instances, 13 had similar NGS profiles to their original day 0 diagnosis. The remaining cases showed preleukemic clonal hematopoiesis mutations, "likely pathogenic mutations," or "variants of uncertain significance."

CONCLUSION

Our findings show that flow cytometry has its advantages with comparable sensitivity in detecting minimal/measurable residual disease. Next-generation sequencing could be used in an increased and more regular capacity in conjunction with flow cytometry to achieve a more comprehensive surveillance of these patients, resulting in improved outcomes.

Measurable residual disease as a biomarker in acute myeloid leukemia: theoretical and practical considerations

Several methodologies that rely on the detection of immunophenotypic or molecular abnormalities of the neoplastic cells are now available to quantify measurable ("minimal") residual disease (MRD) in acute myeloid leukemia (AML). Although the perfect MRD test does not (yet) exist, the strong association between MRD and adverse patient outcomes has provided the impetus to use measures of MRD as biomarker in the routine care of AML patients and during clinical trials. MRD test results may inform the selection of postremission therapy in some patients but evidence supporting the use of MRD as predictive biomarker is still limited. Several retrospective studies have shown that conversion from undetectable to detectable MRD or increasing MRD over time is associated with overt disease recurrence, and MRD testing may therefore be valuable as a monitoring biomarker for early detection of relapse. Interpreting serial MRD data is complex, with open questions regarding the optimal timing and frequency of testing, as well as the identification of test-specific thresholds to define relapse. Importantly, it is unknown whether intervening at the time of MRD detection, rather than at overt disease recurrence, improves outcomes. Finally, using MRD as a surrogate efficacy-response biomarker to accelerate drug development/approval has already been accepted by regulatory authorities in other diseases and is of great interest as a potential strategy in AML. While the prognostic value of MRD in AML is well established, data from prospective clinical trials confirming that treatment effects on MRD directly relate to clinical outcomes are needed to further establish the role of MRD as a surrogate endpoint in AML.

The impact of FLT3 mutation clearance and treatment response after gilteritinib therapy on overall survival in patients with FLT3 mutation-positive relapsed/refractory acute myeloid leukemia

The FLT3 inhibitor gilteritinib has clinical activity in patients with FLT3‐mutated (FLT3^mut+) relapsed/refractory (R/R) acute myeloid leukemia (AML). The impact of FLT3 mutation clearance and the achievement of composite complete remission (CRc) and complete remission/complete remission with partial hematologic recovery (CR/CRh) on overall survival (OS) in patients with FLT3^mut+ R/R AML treated with single‐agent gilteritinib in a phase 1/2 trial were evaluated. Using next‐generation sequencing, a FLT3‐ITD variant allele frequency of ≤10⁻⁴ was used to define FLT3‐ITD clearance in patients with no morphologic leukemia (ie, CRc). A total of 108 patients with FLT3‐ITD‐positive (FLT3‐ITD+) R/R AML were analyzed; 95 of these patients had received ≥80‐mg/day gilteritinib. Ten of the 95 patients had FLT3‐ITD clearance; eight of these 10 patients achieved CRc and were considered negative for measurable residual disease. There was a trend toward longer OS in patients who attained CRc with FLT3‐ITD clearance (131.4 weeks) versus those who achieved CRc and did not have FLT3‐ITD clearance (n = 41; 43.3 weeks; HR = 0.416; p = 0.066). Among patients treated with ≥80‐mg/day gilteritinib who achieved CR/CRh (n = 24), seven had FLT3‐ITD clearance. Among patients who received 120‐mg/day gilteritinib, those who achieved CR/CRh had a longer median OS (70.6 weeks) and higher 52‐week survival probability (66.7%) than patients who did not achieve CR/CRh (n = 71; median OS, 41.7 weeks; 52‐week survival probability, 20.2%). Overall, these data suggest that gilteritinib can induce deep molecular responses in patients with FLT3‐ITD+ R/R AML, and in the setting of CRc or CR/CRh, these responses may be associated with prolonged survival.

Pharmacologic Therapies to Prevent Relapse of Acute Myeloid Leukemia After Allogeneic Hematopoietic Stem Cell Transplantation

Relapse is the main cause of mortality in patients with acute myeloid leukemia (AML) after allogeneic hematopoietic stem cell transplantation (allo-HSCT). Adverse cytogenetic or molecular risk factors, as well as refractory disease or persistent measurable residual disease (MRD) at the time of transplantation are associated with an increased risk of recurrence. Salvage therapy for AML relapse after allo-HSCT is often limited to chemotherapy, donor lymphocyte infusions and/or second transplants and is rarely successful. Effective post-transplant preventive intervention in high risk AML may be crucial. The most frequent and promising approach is the use of post-transplant maintenance with hypomethylating agents or with FLT3 tyrosine kinase inhibitors when the target is present. Moreover, IDH1/IDH2 inhibitors and BCL-2 inhibitors in combination with other strategies are promising approaches in the maintenance setting. Here we summarize the current knowledge about the preemptive and prophylactic use of pharmacologic agents after allo-HSCT to prevent relapse of AML.

Applicability and reproducibility of acute myeloid leukaemia stem cell assessment in a multi-centre setting

Leukaemic stem cells (LSC) have been experimentally defined as the leukaemia‐propagating population and are thought to be the cellular reservoir of relapse in acute myeloid leukaemia (AML). Therefore, LSC measurements are warranted to facilitate accurate risk stratification. Previously, we published the composition of a one‐tube flow cytometric assay, characterised by the presence of 13 important membrane markers for LSC detection. Here we present the validation experiments of the assay in several large AML research centres, both in Europe and the United States. Variability within instruments and sample processing showed high correlations between different instruments (R_pearson > 0·91, P < 0·001). Multi‐centre testing introduced variation in reported LSC percentages but was found to be below the clinical relevant threshold. Clear gating protocols resulted in all laboratories being able to perform LSC assessment of the validation set. Participating centres were nearly unanimously able to distinguish LSC^high (>0·03% LSC) from LSC^low (<0·03% LSC) despite inter‐laboratory variation in reported LSC percentages. This study proves that the LSC assay is highly reproducible. These results together with the high prognostic impact of LSC load at diagnosis in AML patients render the one‐tube LSC assessment a good marker for future risk classification.

Different liposome patterns to detection of acute leukemia based on electrochemical cell sensor

Leukemia is the worst type of malignancy in children which its proper diagnosis can be used in the treatment. We design the turn-off sensor by using the different electrochemical patterns of liposomes to the detection of acute lymphoblastic leukemia cells. Our design is first sandwiched by lectin liposome which increases the electrochemical exchange on the electrode. With the addition of Molt-4 cells, the bonding connection between the n-glycan and lectin can also increase the electrochemical exchange with the high detection cells. Subsequently, the addition of boronic acid liposomes decreases the resistance due to covering glycosylation bond and the sensor is turn-off. But stable and specific binding with the sialic acid causes the higher detection of Molt-4 cells. The electrochemical measurements are performed between the potentials at -0.4 V and +0.4 V with 1 mM [Fe(CN)₆] ^-3/-4. So, for the first time, we designed a cells sensor based on the different patterns of liposomes to screening the N-glycan cells, which can be used in the point of care tests with higher sensitivity.

From Bench to Bedside and Beyond: Therapeutic Scenario in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a heterogeneous group of clonal disorders characterized by abnormal proliferation of undifferentiated myeloid progenitors, impaired hematopoiesis, and variable response to therapy. To date, only about 30% of adult patients with AML become long-term survivors and relapse and/or disease refractoriness are the major cause of treatment failure. Thus, this is an urgent unmet clinical need and new drugs are envisaged in order to ameliorate disease survival outcomes. Here, we review the latest therapeutic approaches (investigational and approved agents) for AML treatment. A specific focus will be given to molecularly targeted therapies for AML as a representation of possible agents for precision medicine. We will discuss experimental and preclinical data for FLT3, IDH1, BCL-2, Hedgehog pathway inhibitors, and epitherapy.

Is There Still a Role for Autologous Stem Cell Transplantation for the Treatment of Acute Myeloid Leukemia?

After intensive induction chemotherapy and complete remission achievement, patients with acute myeloid leukemia (AML) are candidates to receive either high-dose cytarabine-based regimens, or autologous (ASCT) or allogeneic (allo-SCT) hematopoietic stem cell transplantations as consolidation treatment. Pretreatment risk classification represents a determinant key of type and intensity of post-remission therapy. Current evidence indicates that allo-SCT represents the treatment of choice for high and intermediate risk patients if clinically eligible, and its use is favored by increasing availability of unrelated or haploidentical donors. On the contrary, the adoption of ASCT is progressively declining, although numerous studies indicate that in favorable risk AML the relapse rate is lower after ASCT than chemotherapy. In addition, the burden of supportive therapy and hospitalization favors ASCT. In this review, we summarize current indications (if any) to ASCT on the basis of molecular genetics at diagnosis and minimal residual disease evaluation after induction/consolidation phase. Finally, we critically discuss the role of ASCT in older patients with AML and acute promyelocytic leukemia.

Minimal residual disease and stem cell transplantation outcomes

Risk classification and tailoring of treatment are essential for improving outcome for patients with acute myeloid leukemia or high-risk myelodysplastic syndrome. Both patient and leukemia-specific characteristics assessed using morphology, cytogenetics, molecular biology, and multicolor flow cytometry are relevant at diagnosis and during induction, consolidation, and maintenance phases of the treatment. In particular, minimal residual disease (MRD) during therapy has potential as a prognostic factor of outcome, determination of response to therapy, and direction of targeted therapy. MRD can be determined by cell surface markers using multicolor flow cytometry, whereas leukemia-specific translocations and mutations are measured using polymerase chain reaction-based techniques and recently using next-generation sequencing. All these methods of MRD detection have their (dis)advantages, and all need to be standardized, prospectively validated, and improved to be used for uniform clinical decision making and a potential surrogate end point for clinical trials testing novel treatment strategies. Important issues to be solved are time point of MRD measurement and threshold for MRD positivity. MRD is used for stem cell transplantation (SCT) selection in the large subgroup of patients with an intermediate risk profile. Patients who are MRD positive will benefit from allo-SCT. However, MRD-negative patients have a better chance of survival after SCT. Therefore, it is debated whether MRD-positive patients should be extensively treated to become MRD negative before SCT. Either way, accurate monitoring of potential residual or upcoming disease is mandatory. Tailoring therapy according to MRD monitoring may be the most successful way to provide appropriate specifically targeted, personalized treatment.

Haploidentical transplantation for acute myeloid leukemia patients with minimal/measurable residual disease at transplantation

There have been conflicting results regarding the impact of minimal/measurable disease at transplant on acute myeloid leukemia (AML) outcomes after haploidentical transplantation (haplo-SCT). We assessed the impact of pre-transplant disease status on post-transplant outcomes of 143 patients treated with haplo-SCT using fludarabine-melphalan (FM) conditioning and post-transplant cyclophosphamide (PTCy). With a median follow-up of 29 months, the two-year PFS for all patients was 41%. Compared to patients in complete remission (CR) at transplant, those with active disease (n = 29) and CR with incomplete count recovery (CRi) (n = 39) had worse PFS. They had hazard ratios (HR) of 3.5 (95% CI: 2.05-6.1; P < .001) and 2.3 (95% CI: 1.3-3.9; P = .002), respectively. Among patients who were in CR at transplant, there were no differences in PFS between those who had minimal residual disease (MRD) positive (n = 24), and MRD negative (n = 41) (HR 1.85, 95%CI: 0.9-4.0; P = .1). In multivariable analysis for patients in CR, only age was predictive for outcomes, while MRD status at transplant did not influence the treatment outcomes. Our findings suggest that haplo-SCT with FM conditioning regimen and PTCy-based GVHD prophylaxis has a protective effect, and may potentially abrogate the inferior outcomes of MRD positivity for patients with AML. Patients with positive MRD may benefit from proceeding urgently to a haplo-SCT, as this does not appear to negatively impact transplant outcomes.

The Role of Measurable Residual Disease (MRD) in Hematopoietic Stem Cell Transplantation for Hematological Malignancies Focusing on Acute Leukemia

The significance of measurable residual disease (MRD) in hematopoietic stem cell transplantation (HSCT) is well recognized in different hematological malignancies, but the evidence indicate that pre-transplant MRD status is of particular importance in acute lymphoblastic leukemia (ALL) and acute myeloid leukemia (AML). In ALL, inadequate response at the level of MRD is a commonly accepted risk factor for relapse and thus an indication for allogeneic HSCT. Similarly, growing evidence from the literature strongly suggest that MRD detected by multiparameter flow cytometry or molecular techniques should be also used for risk stratification in AML at the time of HSCT. Despite the well-defined association of MRD and outcomes of HSCT in acute leukemias, there are still many open issues such as the role of additional pre-transplant consolidation for MRD eradication, the ability of HSCT to overcome negative influence of MRD positivity on survival, the impact of conditioning regimen intensity on MRD clearance post HSCT, and transplantation outcomes or the selection of optimal donor with regards to MRD status. In addition, the role of MRD assessment in guiding post-transplant maintenance treatment should also be addressed in prospective trials. These open issues mostly awaiting further clinical studies will be discussed in our current review.

MRD in AML: The Role of New Techniques

In the context of precision medicine, assessment of minimal residual disease (MRD) has been used in acute myeloid leukemia (AML) to direct individual treatment programs, including allogeneic stem cell transplantation in patients at high-risk of relapse. One of the limits of this approach has been in the past the paucity of AML markers suitable for MRD assessment. Recently, the number of biomarkers has increased, due to the identification of highly specific leukemia-associated immunophenotypes by multicolor flow-cytometry, and of rare mutated gene sequences by digital droplet PCR, or next-generation sequencing (NGS). In addition, NGS allowed unraveling of clonal heterogeneity, present in AML at initial diagnosis or developing during treatment, which influences reliability of specific biomarkers, that may be unstable during the disease course. The technological advances have increased the application of MRD-based strategies to a significantly higher number of AML patients, and the information deriving from MRD assessment has been used to design individual post-remission protocols and pre-emptive treatments in patients with sub-clinical relapse. This led to the definition of MRD-negative complete remission as outcome definition in the recently published European Leukemianet MRD guidelines. In this review, we summarized the principles of modern technologies and their clinical applications for MRD detection in AML patients, according to the specific leukemic markers.