Minimal residual disease in acute leukaemia: preclinical studies in a relevant rat model (BNML)

Current assessment and management of measurable residual disease in patients with acute lymphoblastic leukemia in the setting of CAR-T-cell therapy

ABSTRACT

Chimeric antigen receptor (CAR)-modified T-cell therapy has achieved remarkable success in the treatment of acute lymphoblastic leukemia (ALL). Measurable/minimal residual disease (MRD) monitoring plays a significant role in the prognostication and management of patients undergoing CAR-T-cell therapy. Common MRD detection methods include flow cytometry (FCM), polymerase chain reaction (PCR), and next-generation sequencing (NGS), and each method has advantages and limitations. It has been well documented that MRD positivity predicts a poor prognosis and even disease relapse. Thus, how to perform prognostic evaluations, stratify risk based on MRD status, and apply MRD monitoring to guide individual therapeutic decisions have important implications in clinical practice. This review assesses the common and novel MRD assessment methods. In addition, we emphasize the critical role of MRD as a prognostic biomarker and summarize the latest studies regarding MRD-directed combination therapy with CAR-T-cell therapy and allogeneic hematopoietic stem cell transplantation (allo-HSCT), as well as other therapeutic strategies to improve treatment effect. Furthermore, this review discusses current challenges and strategies for MRD detection in the setting of disease relapse after targeted therapy.

Measurable residual disease testing in acute myeloid leukaemia

There is considerable interest in developing techniques to detect and/or quantify remaining leukaemia cells termed measurable or, less precisely, minimal residual disease (MRD) in persons with acute myeloid leukaemia (AML) in complete remission defined by cytomorphological criteria. An important reason for AML MRD-testing is the possibility of estimating the likelihood (and timing) of leukaemia relapse. A perfect MRD-test would precisely quantify leukaemia cells biologically able and likely to cause leukaemia relapse within a defined interval. AML is genetically diverse and there is currently no uniform approach to detecting such cells. Several technologies focused on immune phenotype or cytogenetic and/or molecular abnormalities have been developed, each with advantages and disadvantages. Many studies report a positive MRD-test at diverse time points during AML therapy identifies persons with a higher risk of leukaemia relapse compared with those with a negative MRD-test even after adjusting for other prognostic and predictive variables. No MRD-test in AML has perfect sensitivity and specificity for relapse prediction at the cohort- or subject levels and there are substantial rates of false-positive and -negative tests. Despite these limitations, correlations between MRD-test results and relapse risk have generated interest in MRD-test result-directed therapy interventions. However, convincing proof that a specific intervention will reduce relapse risk in persons with a positive MRD-test is lacking and needs testing in randomized trials. Routine clinical use of MRD-testing requires further refinements and standardization/harmonization of assay platforms and results reporting. Such data are needed to determine whether results of MRD-testing can be used as a surrogate end point in AML therapy trials. This could make drug-testing more efficient and accelerate regulatory approvals. Although MRD-testing in AML has advanced substantially, much remains to be done.

Physiologically based toxicokinetic modeling of secondary acute myelolytic leukemia

Benzene, designated as environmental and occupational carcinogen and hematotoxin, has been associated with secondary leukemia. To develop a toxicokinetic model of AML, benzene can be used as leukemogenic agent. The aim of the present study was to optimize the dose, period and time of cumulative benzene exposure of Swiss Albino mice and to analyze survival rate; alteration in cell cycle regulation and other clinical manifestations in mice exposed to benzene vapour at a dose 300 ppm × 6 h/day × 5 days/week for 2 weeks, i.e., 9000(a)ppm cumulative dose. Analyzing physiological parameters like plasma enzyme profile, complete hematology (Hb %, RBC indices and WBC differentials), hematopoietic cells morphology, expression of cell cycle regulatory proteins, tissue histology and analysis of DNA fragmentation, optimum conditions were established. Down regulation of p53 and p21 and up regulation of CDK2, CDK4, CDK6, cyclin D1 and E in this exposed group were marked as the optimum conditions of cellular deregulation for the development of secondary AML. Elevated level of Plasma AST/ALT with corresponding changes in liver histology showing extended sinusoids within the hepatocytic cell cords in optimally exposed animals also confirmed the toxicokinetic relation of benzene with leukemia. It can be concluded from the above observations that the 9000(a)ppm exposed animals can serve as the induced laboratory model of secondary acute myeloid leukemia.

Animal models of acute myelogenous leukaemia - development, application and future perspectives

From the early inception of the transplant models through to contemporary genetic and xenograft models, evolution of murine leukaemic model systems have been critical to our general comprehension and treatment of cancer, and, more specifically, disease states such as acute myelogenous leukaemia (AML). However, even with modern advances in therapeutics and molecular diagnostics, the majority of AML patients die from their disease. Thus, in the absence of definitive in vitro models which precisely recapitulate the in vivo setting of human AMLs and failure of significant numbers of new drugs late in clinical trials, it is essential that murine AML models are developed to exploit more specific, targeted therapeutics. While various model systems are described and discussed in the literature from initial transplant models such as BNML and spontaneous murine leukaemia virus models, to the more definitive genetic and clinically significant NOD/SCID xenograft models, there exists no single compendium which directly assesses, reviews or compares the relevance of these models. Thus, the function of this article is to provide clinicians and experimentalists a chronological, comprehensive appraisal of all AML model systems, critical discussion on the elucidation of their roles in our understanding of AML and consideration to their efficacy in the development of AML chemotherapeutics.

Multidimensional flow cytometry for detection of minimal residual disease in acute myeloid leukemia

The term minimal residual disease (MRD) describes the situation in which, after chemotherapy for acute leukemia (AL), a morphologically normal bone marrow (BM) can still harbor a relevant amount of residual malignant cells. Several techniques are now amenable to investigate MRD, and all together they have designated a new era in which a re-definition of the current criteria of complete remission (CR) is required. Depending upon the measured level of MRD we can distinguish a variety of clinical situations ranging from a potentially cured disease to short-term remission. In the context of this spectrum of conditions there would be room for different therapeutic strategies ranging from no further therapy to pre-emptive therapy to treat early relapses (immunologic and/or molecular relapses). This review will focus on the state of art of MRD detection in acute myeloid leukemia (AML) using multidimensional flow cytometry (MFC), and will cover the laboratory and clinical aspects of this approach.

Minimal residual disease in acute myeloid leukaemia

Relapse remains the main cause of treatment failure in acute myeloid leukaemia (AML). Studies to date suggest that monitoring of minimal residual disease (MRD) in AML is useful in identifying patients at high risk of relapse from those in durable remission. This chapter describes the methodological advances in the detection of MRD and, in particular, focuses on the development of highly sensitive RT-PCR techniques, including real-time, for quantifying MRD. Preliminary results on the clinical utility of MRD monitoring in AML with t(8;21) and inv(16) are promising and provide the basis for further evaluation by quantitative real-time analysis in prospective clinical trials. For AML without a specific fusion transcript, the WT1 gene is an alternative molecular target. The clinical value of quantitative MRD monitoring in AML, however, will need to be confirmed in future studies.

Amifostine (WR2721) for dose escalation in marrow-ablative treatment of leukaemia

The in vivo effect of the radiochemoprotectant Amifostine on the therapeutic efficacy of marrow ablative treatment with cyclophosphamide (CP) and total body irradiation (TBI) followed by bone marrow transplantation (BMT) was studied in normal rats as well as in the Brown Norway rat acute myelocytic leukaemia (BNML) model. In normal rats, when the dose of TBI was escalated and the CP dose was kept constant, pretreatment with Amifostine yielded a positive dose modification factor of 1.26. No significant improvement was found after Amifostine pretreatment when the TBI dose was kept constant and CP dose escalated. When leukaemic rats received CP as the only antileukaemia treatment, Amifostine pretreatment did not lead to a reduction in the antileukaemic efficacy of CP, although protection against treatment-related mortality was observed. In the CP only groups, 9 out of 40 animals died of treatment-related toxicity, compared with none of the 40 animals in the Amifostine pretreatment groups. When applying the maximum tolerated treatment of CP and TBI in various combinations to leukaemic rats, 25 out of 36 rats died from treatment-related toxicity, whilst pretreatment with Amifostine reduced this to 11 out of 36, (P = 0.002). Of those animals which survived the CP + TBI conditioning treatment, 10 out of 25 in the Amifostine pretreatment group were cured, versus 8/11 in the CP + TBI only control group (P = 0.146). In conclusion, incorporation of Amifostine as a radiochemoprotectant in a marrow-ablative conditioning regimen allows the use of escalated doses of chemoradiotherapy without reducing the antileukaemic efficacy.

Genetic changes: relevance for diagnosis and detection of minimal residual disease in acute lymphoblastic leukaemia

Cure can now be achieved in a proportion of patients with ALL. However, relapse and eventual treatment failure occur in many cases receiving identical treatment, presumably as a result of failure to eradicate MRD. While for many years marrow morphology has been the standard by which leukaemic remission has been assessed, more sensitive techniques have been developed for detection of MRD including immunophenotypic analysis, and as discussed in this chapter, methods which detect leukemia-associated clonal genetic changes at the karyotypic and genomic levels. Table 10 lists the applicability and sensitivity of various markers used in MRD analysis in ALL. It is apparent that of the karyotypic and molecular approaches described, only PCR-based strategies for detection of either leukaemia-specific translocations or clonal Ag receptor rearrangements are reliably applicable to a high proportion of both B- and T-ALL at sufficiently high sensitivity. Initial clinical studies of patients undergoing therapy for ALL using a variety of PCR-based methods suggest that in some cases a persistent or increasing level of residual disease may be predictive for clinical relapse, although a number of technical factors and the phenomena of oligo-clonality and clonal evolution may limit the usefulness of this analysis in a few instances. From current available data it appears that in order to define the potential predictive value of PCR detection of MRD a large number of patients will need to be prospectively assessed over several years at multiple time points during and after therapy, preferably using more than one semi-quantitative PCR approach. In addition to reliable prediction of clinical relapse allowing appropriate individual treatment modification, progress in the molecular detection of MRD in ALL is also likely to be of benefit in the assessment of the efficacy of autograft purging and the evaluation of new therapeutic strategies such as the use of biological response modifiers to eliminate a low tumour burden.

Immune ablation with stem-cell rescue: a possible cure for systemic lupus erythematosus?

The impressive prolongation of survival has been the most important progress made in clinical systemic lupus erythematosus (SLE). Quality of life has also greatly improved, including pregnancy. However, persisting disease and therapy-related morbidity outcomes justify new approaches, different from the usual long-term palliative immunosuppression. Haematopoietic stem cells (HSCs) from healthy histocompatible mice are capable of curing murine SLE after eradication of the original HSCs with total body irradiation. Syngeneic and even autologous HSCs are also capable of curing induced experimental autoimmune diseases such as adjuvant arthritis and experimental allergic encephalomyelitis. In man allogeneic bone-marrow transplantation (BMT) is becoming progressively safer, but cannot yet be offered to SLE patients. However, syngeneic transplants from twins non-concordant for the disease would be justified. Conditioning with high-dose cyclophosphamide followed by autologous HSC rescue, from the marrow and/or from the peripheral blood, may already be regarded as a powerful immunosuppressive procedure for selected cases of SLE and other severe autoimmune diseases. Autologous transplant procedures are not saddled with the immunologic problems of allo-BMT. Although eradication of SLE may not be achieved by auto-BMT, minimal residual immunologic disease can be suppressed or controlled, and long-term self-maintained remissions may be expected.

The graft versus leukemia (GVL) effect after allogeneic bone marrow transplantation for chronic myelogenous leukemia (CML)

Immune mechanisms superimposed to the myeloablative conditioning regimens exert an additional powerful effect in eradicating leukemia and in achieving immunological control of minimal residual disease. The impact of GVHD-independent GVL has been evaluated to be absent, or near absent, in ALL, about 30% in AML and about 40% in CML. While until little time ago most of the evidence in favor of an immune antileukemia mechanism exerted by allo BMT in CML was indirect, based on the lack of GVL, there is now solid evidence of a positive type, based on the antileukemia effect of donor lymphocyte infusions in patients having relapsed after transplant. There are three lines of indirect clinical evidence for GVL in CML: they include the classical linkage between GVHD and reduced relapse rate, increased relapse rate after identical twin allografts, and increased relapse risk after effective GVHD prophylaxis, with T lymphocyte depletion in the foreground. The eradicating effects of donor lymphocyte infusions in relapsed patients are the ultimate demonstration that allogeneic immune competent cells are capable of recognizing and destroying the Ph-positive clone. However the frequency of irreversible aplasia indicates that donor lymphocytes act in the same way on residual host hematopoiesis, so that a second graft, without repeat conditioning, should be programmed for such cases.