In non-M3 AML, the cytogenetic/molecular categorization of leukemic cells is the most relevant prognostic parameter
[1], and for about forty-five years, the achievement of complete remission (CR), defined by the presence of less than 5% bone marrow (BM) leukemic cells on morphological examination, has remained the primary goal of any AML treatment
[2]. However, the cure rate of AML patients in CR is still 30–50% on average, a datum that suggests BM CR is not sufficient to predict the risk of disease relapse
[3][4][5][6]. Thus, to improve this gloomy clinical outcome, various techniques aimed at identifying 10
−4–10
−6 BM or peripheral blood (PB) leukemic cells (i.e., “Measurable Residual Disease”, MRD) in AML patients in morphological CR have been developed
[2][7]. The higher sensitivity of these techniques, which include multi-parameter flow cytometry (MFC), reverse quantitative polymerase chain reaction (RT-qPCR), digital droplet PCR (ddPCR) and Next Generation Sequencing (NGS), has allowed for a deeper definition of remission status, thanks to a multimodal assessment of MRD quantification and the immuno-phenotypic and molecular tracking of leukemic cell defects
[5][8][9]. Today, MFC and RT-qPCR (whose features, along with those of conventional cytogenetics, FISH, ddPCR and NGS, are summarized in
Table 1) are considered the methods of choice for MRD detection
[7], whereas NGS is a relatively new technique with a still low but improvable sensitivity
[10]. Beyond their classical assays, MFC and RT-qPCR may identify which patients are at higher risk of relapse by evaluating the percentage of neoplastic cells at diagnosis
[11][12][13] and analyzing non-stable molecular markers. These last biomarkers, which flag the genotype, phenotype and function heterogeneity of leukemic initiating cells (LICs)
[14][15], may promptly identify unique chemo-resistant AML subpopulations not detected on clinical diagnosis but potentially responsible for disease relapse
[16][17][18]. By applying these technologies, various retrospective studies, as well as recent meta-analysis, have proved the negative value of MRD positivity (MRD+) on AML clinical outcome not only in patients submitted to conventional chemotherapy but also in those submitted to allogeneic hematopoietic stem cell transplantation (Allo-HSCT)
[19]. In contrast, these same studies and others have revealed that MRD negativity (MRD−) consistently predicts a lower relapse rate (RR) and superior long-term overall survival (OS)
[13][20][21][22][23][24][25][26] even if about 20–50% of MRD− patients still relapse. This observation reveals that MRD− as currently assessed does not always provide an absolute certainty of “cure” and strengthens the notion that the MRD threshold should be considered as a moving target that needs to be constantly reset. In addition, MRD assessment is affected by some issues: what is the optimal MRD assay? What is the right time point to perform the assay? What is the influence of MRD status on post-remission treatment strategies? Finally, does the conversion from MRD+ to MRD− during post-remission has any prognostic value? Up until now, this last point has been addressed by one study only, in which post-consolidation MRD− had a significant prognostic influence on five-years relapse-free survival (RFS) and OS independently of the time of its achievement
[27]. Despite these limits, MRD is currently employed in the clinic as a routine biomarker for clinical decision making and in clinical trials as a surrogate endpoint to accelerate the discovery and the use of innovative drugs. The development of these innovative drugs, which include small inhibitors of various signal transduction pathways altered by specific gene mutations, regulators of apoptosis, hypo-methylating agents (HMA) and immune checkpoints inhibitors (ICI), remains an area of intensive research in AML. As opposed to standard intensive chemotherapy and allo-HSCT, these new agents allow a more personalized medicine as they specifically target the phenotypic and molecular features of the disease
[28]. However, most of these new small molecule inhibitors as single agents have provided unsatisfactory results, probably because multiple “driver” mutations are required for AML development, whereas they have provided successful results when combined with intensive chemotherapy
[29]. These findings might suggest that molecular inhibitors, as well as monoclonal antibodies, should be chosen depending on specific mutations or cell surface markers and should always be employed in combination with intensive induction or salvage or maintenance chemotherapy to achieve sounding results. Other investigational drugs that have provided relevant results are those exploring the possibility of overcoming a chemo-resistant phenotype through an immune-based approach and those combining a Bcl-2 inhbitor (venetoclax) with ICI or HMA
[5]. This review is aimed at discussing the prognostic MRD power and innovative treatment strategies (excluding cell therapies) which might potentially lead to its clearance. The main studies reporting MRD analysis in AML by molecular markers are presented in
Table S1 [21][22][30][31][32][33][34][35][36][37]; those based on MFC analysis of MRD are collected in
Table S2 [8][9][13][20][25][27][37][38][39][40][41][42][43][44][45][46][47][48][49][50][51][52][53][54][55][56][57].