A Bump in the Road: How the Hostile AML Microenvironment Affects CAR T Cell Therapy

Chimeric antigen receptor T-cell therapy in childhood acute myeloid leukemia: how far are we from a clinical application?

Recurrent and/or refractory (R/R) pediatric acute myeloid leukemia (AML) remains a recalcitrant disease with poor outcomes. Cell therapy with genetically modified immune effector cells holds the promise to improve outcomes for R/R AML since it relies on cytotoxic mechanisms that are distinct from chemotherapeutic agents. While T cells expressing chimeric antigen receptors (CAR T cells) showed significant anti-AML activity in preclinical models, early phase clinical studies have demonstrated limited activity, irrespective of the targeted AML antigen. Lack of efficacy is most likely multifactorial, including: (i) a limited array of AML-specific targets and target antigen heterogeneity; (ii) the aggressive nature of R/R AML and heavy pretreatment of patients; (iii) T-cell product manufacturing, and (iv) limited expansion and persistence of the CAR T cells, which is in part driven by the immunosuppressive AML microenvironment. Here we review the results of early phase clinical studies with AML-specific CAR T cells, and avenues investigators are exploring to improve their effector function.

Targeting the innate immune system in pediatric and adult AML

While the introduction of T cell-based immunotherapies has improved outcomes in many cancer types, the development of immunotherapies for both adult and pediatric AML has been relatively slow and limited. In addition to the need to identify suitable target antigens, a better understanding of the immunosuppressive tumor microenvironment is necessary for the design of novel immunotherapy approaches. To date, most immune characterization studies in AML have focused on T cells, while innate immune lineages such as monocytes, granulocytes and natural killer (NK) cells, received less attention. In solid cancers, studies have shown that innate immune cells, such as macrophages, myeloid-derived suppressor cells and neutrophils are highly plastic and may differentiate into immunosuppressive cells depending on signals received in their microenvironment, while NK cells appear to be functionally impaired. Hence, an in-depth characterization of the innate immune compartment in the TME is urgently needed to guide the development of immunotherapeutic interventions for AML. In this review, we summarize the current knowledge on the innate immune compartment in AML, and we discuss how targeting its components may enhance T cell-based- and other immunotherapeutic approaches.

INSPIRED Symposium Part 5: Expanding the Use of CAR T Cells in Children and Young Adults

Chimeric antigen receptor (CAR) T cell therapy has demonstrated remarkable efficacy in relapsed/refractory (r/r) B cell malignancies, including in pediatric patients with acute lymphoblastic leukemia (ALL). Expanding this success to other hematologic and solid malignancies is an area of active research and, although challenges remain, novel solutions have led to significant progress over the past decade. Ongoing clinical trials for CAR T cell therapy for T cell malignancies and acute myeloid leukemia (AML) have highlighted challenges, including antigen specificity with off-tumor toxicity and persistence concerns. In T cell malignancies, notable challenges include CAR T cell fratricide and prolonged T cell aplasia, which are being addressed with strategies such as gene editing and suicide switch technologies. In AML, antigen identification remains a significant barrier, due to shared antigens across healthy hematopoietic progenitor cells and myeloid blasts. Strategies to limit persistence and circumvent the immunosuppressive tumor microenvironment (TME) created by AML are also being explored. CAR T cell therapies for central nervous system and solid tumors have several challenges, including tumor antigen heterogeneity, immunosuppressive and hypoxic TME, and potential for off-target toxicity. Numerous CAR T cell products have been designed to overcome these challenges, including "armored" CARs and CAR/T cell receptor (TCR) hybrids. Strategies to enhance CAR T cell delivery, augment CAR T cell performance in the TME, and ensure the safety of these products have shown promising results. In this manuscript, we will review the available evidence for CAR T cell use in T cell malignancies, AML, central nervous system (CNS), and non-CNS solid tumor malignancies, and recommend areas for future research.

Chimeric antigen receptor (CAR) modified T Cells in acute myeloid leukemia: limitations and expectations

Acute myeloid leukemia (AML) is an aggressive hematologic malignancy with a poor prognosis despite the advent of novel therapies. Consequently, a major need exists for new therapeutic options, particularly for patients with relapsed/refractory (R/R) AML. In recent years, it has been possible to individualize the treatment of a subgroup of patients, particularly with the emergence of multiple targeted therapies. Nonetheless, a considerable number of patients remain without therapeutic options, and overall prognosis remains poor because of a high rate of disease relapse. In this sense, cellular therapies, especially chimeric antigen receptor (CAR)-T cell therapy, have dramatically shifted the therapeutic options for other hematologic malignancies, such as diffuse large B cell lymphoma and acute lymphoblastic leukemia. In contrast, effectively treating AML with CAR-based immunotherapy poses major biological and clinical challenges, most of them derived from the unmet need to identify target antigens with expression restricted to the AML blast without compromising the viability of the normal hematopoietic stem cell counterpart. Although those limitations have hampered CAR-T cell therapy translation to the clinic, there are several clinical trials where target antigens, such as CD123, CLL-1 or CD33 are being used to treat AML patients showing promising results. Moreover, there are continuing efforts to enhance the specificity and efficacy of CAR-T cell therapy in AML. These endeavors encompass the exploration of novel avenues, including the development of dual CAR-T cells and next-generation CAR-T cells, as well as the utilization of gene editing tools to mitigate off-tumor toxicities. In this review, we will summarize the ongoing clinical studies and the early clinical results reported with CAR-T cells in AML, as well as highlight CAR-T cell limitations and the most recent approaches to overcome these barriers. We will also discuss how and when CAR-T cells should be used in the context of AML.

Current challenges and therapeutic advances of CAR-T cell therapy for solid tumors

The application of chimeric antigen receptor (CAR) T cells in the management of hematological malignancies has emerged as a noteworthy therapeutic breakthrough. Nevertheless, the utilization and effectiveness of CAR-T cell therapy in solid tumors are still limited primarily because of the absence of tumor-specific target antigen, the existence of immunosuppressive tumor microenvironment, restricted T cell invasion and proliferation, and the occurrence of severe toxicity. This review explored the history of CAR-T and its latest advancements in the management of solid tumors. According to recent studies, optimizing the design of CAR-T cells, implementing logic-gated CAR-T cells and refining the delivery methods of therapeutic agents can all enhance the efficacy of CAR-T cell therapy. Furthermore, combination therapy shows promise as a way to improve the effectiveness of CAR-T cell therapy. At present, numerous clinical trials involving CAR-T cells for solid tumors are actively in progress. In conclusion, CAR-T cell therapy has both potential and challenges when it comes to treating solid tumors. As CAR-T cell therapy continues to evolve, further innovations will be devised to surmount the challenges associated with this treatment modality, ultimately leading to enhanced therapeutic response for patients suffered solid tumors.

INSPIRED Symposium Part 4B: Chimeric Antigen Receptor T Cell Correlative Studies-Established Findings and Future Priorities

Chimeric antigen receptor (CAR) T cell therapy has revolutionized the treatment of B cell malignancies, with multiple CAR T cell products approved for numerous indications by regulatory agencies worldwide. However, significant work remains to be done to enhance these treatments. In March 2023, a group of experts in CAR T cell therapy assembled at the National Institutes of Health in Bethesda, Maryland at the Insights in Pediatric CAR T Cell Immunotherapy: Recent Advances and Future Directions (INSPIRED) Symposium to identify key areas for research for the coming years. In session 4B, correlative studies to be incorporated into future clinical trials and real-world settings were discussed. Active areas of research identified included (1) optimizing CAR T cell product manufacturing; (2) ensuring adequate lymphodepletion prior to CAR T cell administration; (3) overcoming immunoregulatory cells and tumor stroma present in the tumor microenvironment, particularly in solid tumors; (4) understanding tumor intrinsic properties that lead to CAR T cell immunotherapy resistance; and (5) uncovering biomarkers predictive of treatment resistance, treatment durability, or immune-related adverse events. Here we review the results of previously published clinical trials and real-world studies to summarize what is currently known about each of these topics. We then outline priorities for future research that we believe will be important for improving our understanding of CAR T cell therapy and ultimately leading to better outcomes for patients.

Mesenchymal stromal cells and CAR-T cells in regenerative medicine: The homing procedure and their effective parameters

Mesenchymal stromal cells (MSCs) and chimeric antigen receptor (CAR)-T cells are two core elements in cell therapy procedures. MSCs have significant immunomodulatory effects that alleviate inflammation in the tissue regeneration process, while administration of specific chemokines and adhesive molecules would primarily facilitate CAR-T cell trafficking into solid tumors. Multiple parameters affect cell homing, including the recipient's age, the number of cell passages, proper cell culture, and the delivery method. In addition, several chemokines are involved in the tumor microenvironment, affecting the homing procedure. This review discusses parameters that improve the efficiency of cell homing and significant cell therapy challenges. Emerging comprehensive mechanistic strategies such as non-systemic and systemic homing that revealed a significant role in cell therapy remodeling were also reviewed. Finally, the primary implications for the development of combination therapies that incorporate both MSCs and CAR-T cells for cancer treatment were discussed.

Establishment and validation of a prognostic immune-related lncRNA risk model for acute myeloid leukemia

Background

Acute myeloid leukemia (AML) is a cancer arising in the bone marrow and is the most common type of adult leukemia. AML has a poor prognosis, and currently, its prognosis evaluation does not include immune status assessment. This study established an immune-related long non-coding RNA (lncRNA) prognostic risk model for AML based on immune lncRNAs screening.

Methods

To construct training and validation cohorts, The Cancer Genome Atlas (TCGA) and Gene Expression Omnibus (GEO) public databases were accessed to obtain gene expression profiles and clinical data. The correlation between lncRNAs and immunity genes was analyzed using the "limma" package, and the immune-related lncRNAs were obtained. Through least absolute shrinkage and selection operator regression, a prognostic model was established with immune-related lncRNAs. Using the median risk score, patients were divided into high- and low-risk groups. The Kaplan-Meier method was used for survival analysis, whereas the accuracy of the risk model was evaluated using time-dependent receiver operating characteristic curves, risk score distribution, survival status, and risk heat maps. We utilized univariate and multivariate Cox regression to examine the association between risk score and clinical variables and AML survival and prognosis.

Results

In the immune-related lncRNA prognostic risk model, the prognosis was better for low-risk than for high-risk patients, indicating risk score of this model as an independent indicator of prognosis. The area under the curve value for 1-, 3-, and 5-year survival of TCGA patients was 0.817, 0.859, and 0.909, respectively, whereas that of GEO patients (of dataset GPL96-GSE37642) was 0.603, 0.652, and 0.624, respectively. Gene set enrichment analysis revealed the enrichment of multiple pathways, such as antigen processing, B-cell receptor signaling pathway, natural killer cell-mediated cytotoxicity, and chemokines, in high-risk patients.

Conclusions

In this study, immune-related lncRNA prognostic risk models effectively predicted AML survival and provided potential treatment targets.

Alliance between titans: combination strategies of CAR-T cell therapy and oncolytic virus for the treatment of hematological malignancies

There have been several clinical studies using chimeric antigen receptor (CAR)-T cell therapy for different hematological malignancies. It has transformed the therapy landscape for hematologic malignancies dramatically. Nonetheless, in acute myeloid leukemia (AML) and T cell malignancies, it still has a dismal prognosis. Even in the most promising locations, recurrence with CAR-T treatment remains a big concern. Oncolytic viruses (OVs) can directly lyse tumor cells or cause immune responses, and they can be manipulated to create therapeutic proteins, increasing anticancer efficacy. Oncolytic viruses have been proven in a rising number of studies to be beneficial in hematological malignancies. There are limitations that cannot be avoided by using either treatment alone, and the combination of CAR-T cell therapy and oncolytic virus therapy may complement the disadvantages of individual application, enhance the advantages of their respective treatment methods and improve the treatment effect. The alternatives for combining two therapies in hematological malignancies are discussed in this article.

The Black Hole: CAR T Cell Therapy in AML

Despite exhaustive studies, researchers have made little progress in the field of adoptive cellular therapies for relapsed/refractory acute myeloid leukemia (AML), unlike the notable uptake for B cell malignancies. Various single antigen-targeting chimeric antigen receptor (CAR) T cell Phase I trials have been established worldwide and have recruited approximately 100 patients. The high heterogeneity at the genetic and molecular levels within and between AML patients resembles a black hole: a great gravitational field that sucks in everything. One must consider the fact that only around 30% of patients show a response; there are, however, consequential off-tumor effects. It is obvious that a new point of view is needed to achieve more promising results. This review first introduces the unique therapeutic challenges of not only CAR T cells but also other adoptive cellular therapies in AML. Next, recent single-cell sequencing data for AML to assess somatically acquired alterations at the DNA, epigenetic, RNA, and protein levels are discussed to give a perspective on cellular heterogeneity, intercellular hierarchies, and the cellular ecosystem. Finally, promising novel strategies are summarized, including more sophisticated next-generation CAR T, TCR-T, and CAR NK therapies; the approaches with which to tailor the microenvironment and target neoantigens; and allogeneic approaches.

CYAD-01, an autologous NKG2D-based CAR T-cell therapy, in relapsed or refractory acute myeloid leukaemia and myelodysplastic syndromes or multiple myeloma (THINK): haematological cohorts of the dose escalation segment of a phase 1 trial

BACKGROUND

CYAD-01 is an autologous chimeric antigen receptor (CAR) T-cell product based on the natural killer (NK) group 2D (NKG2D) receptor, which binds eight ligands that are overexpressed in a wide range of haematological malignancies but are largely absent on non-neoplastic cells. Initial clinical evaluation of a single infusion of CYAD-01 at a low dose in patients with relapsed or refractory acute myeloid leukaemia, myelodysplastic syndromes, and multiple myeloma supported the feasibility of the approach and prompted further evaluation of CYAD-01. The aim of the present study was to determine the safety and recommended phase 2 dosing of CYAD-01 administered without preconditioning or bridging chemotherapy.

METHODS

The multicentre THINK study was an open-label, dose-escalation, phase 1 study for patients with relapsed or refractory acute myeloid leukaemia, myelodysplastic syndromes, or multiple myeloma, after at least one previous line of therapy. Patients were recruited from five hospitals in the USA and Belgium. The dose-escalation segment evaluated three dose levels: 3 × 10⁸ (dose level one), 1 × 10⁹ (dose level two), and 3 × 10⁹ (dose level three) cells per infusion with a 3 + 3 Fibonacci study design using a schedule of three infusions at 2-week intervals followed by potential consolidation treatment consisting of three additional infusions. The occurrence of dose-limiting toxicities post-CYAD-01 infusion was assessed as the primary endpoint in the total treated patient population. The trial was registered with ClinicalTrials.gov, NCT03018405, and EudraCT, 2016-003312-12, and has been completed.

FINDINGS

Between Feb 6, 2017, and Oct 9, 2018, 25 patients were registered in the haematological dose-escalation segment. Seven patients had manufacturing failure for insufficient yield and two had screening failure. 16 patients were treated with CYAD-01 (three with multiple myeloma and three with acute myeloid leukaemia at dose level one; three with acute myeloid leukaemia at dose level two; and six with acute myeloid leukaemia and one with myelodysplastic syndromes at dose level three). Median follow-up was 118 days (IQR 46-180). Seven patients (44%) had grade 3 or 4 treatment-related adverse events. In total, five patients (31%) had grade 3 or 4 cytokine release syndrome across all dose levels. One dose-limiting toxicity of cytokine release syndrome was reported at dose level three. No treatment-related deaths occurred, and the maximum tolerated dose was not reached. Three (25%) of 12 evaluable patients with relapsed or refractory acute myeloid leukaemia or myelodysplastic syndromes had an objective response. Among responders, two patients with acute myeloid leukaemia proceeded to allogeneic haematopoietic stem-cell transplantation (HSCT) after CYAD-01 treatment, with durable ongoing remissions (5 and 61 months).

INTERPRETATION

Treatment with a multiple CYAD-01 infusion schedule without preconditioning is well tolerated and shows anti-leukaemic activity, although without durability outside of patients bridged to allogeneic HSCT. These phase 1 data support the proof-of-concept of targeting NKG2D ligands by CAR T-cell therapy. Further clinical studies with NKG2D-based CAR T-cells are warranted, potentially via combinatorial antigen targeted approaches, to improve anti-tumour activity.

FUNDING

Celyad Oncology.

4-1BB-Based CAR T Cells Effectively Reverse Exhaustion and Enhance the Anti-Tumor Immune Response through Autocrine PD-L1 scFv Antibody

Exhaustion of chimeric antigen receptor (CAR) T cells is one of the limitations for CAR T efficacy in solid tumors and for tumor recurrence after initial CAR T treatment. Tumor treatment with a combination of programmed cell death receptor-1 (PD-1)/programmed cell death ligand-1 (PD-L1) blockage and CD28-based CAR T cells has been intensively studied. However, it remains largely unclear whether autocrine single-chain variable fragments (scFv) PD-L1 antibody can improve 4-1BB-based CAR T cell anti-tumor activity and revert CAR T cell exhaustion. Here, we studied T cells engineered with autocrine PD-L1 scFv and 4-1BB-containing CAR. The antitumor activity and exhaustion of CAR T cells were investigated in vitro and in a xenograft cancer model using NCG mice. CAR T cells with autocrine PD-L1 scFv antibody demonstrate enhanced anti-tumor activity in solid tumors and hematologic malignancies by blocking the PD-1/PD-L1 signaling. Importantly, we found that CAR T exhaustion was largely diminished by autocrine PD-L1 scFv antibody in vivo. As such, 4-1BB CAR T with autocrine PD-L1 scFv antibody combined the power of CAR T cells and the immune checkpoint inhibitor, thereby increasing the anti-tumor immune function and CAR T persistence, providing a cell therapy solution for a better clinical outcome.

CD123 a Therapeutic Target for Acute Myeloid Leukemia and Blastic Plasmocytoid Dendritic Neoplasm

In spite of consistent progress at the level of basic research and of clinical treatment, acute myeloid leukemia (AML) still represents an unmet clinical need for adult and pediatric patients. To improve the outcomes of these patients, it is necessary to identify new therapeutic targets. IL3RA (CD123, alpha subunit of the interleukin 3 receptor) is a cell membrane protein overexpressed in several hematologic malignancies, including AML blastic plasmocytoid dendritic cell neoplasms (BPDCN). Given the higher expression of CD123 on leukemic cells compared to normal hematopoietic cells and its low/absent expression on normal hematopoietic stem cells, it appears as a suitable and attractive target for therapy. Various drugs targeting CD123 have been developed and evaluated at clinical level: interleukin-3 conjugated with diphtheria toxin; naked neutralizing anti-CD123 antibodies; drug-antibody conjugates; bispecific antibodies targeting both CD123 and CD3; and chimeric antigen receptor (CAR) T cells engineered to target CD123. Some of these agents have shown promising results at the clinical level, including tagraxofusp (CD123 conjugated with diphtheria toxin) for the treatment of BPDCN and IMGN632 (anti-CD123 drug-conjugate), and flotetuzumab (bispecific anti-CD123 and anti-CD3 monoclonal antibody) for the treatment of AML. However, the therapeutic efficacy of CD123-targeting treatments is still unsatisfactory and must be improved through new therapeutic strategies and combined treatments with other antileukemic drugs.

Emerging frontiers in immuno- and gene therapy for cancer

BACKGROUND AIMS

The field of cell and gene therapy in oncology has moved rapidly since 2017 when the first cell and gene therapies, Kymriah followed by Yescarta, were approved by the Food and Drug Administration in the United States, followed by multiple other countries. Since those approvals, several new products have gone on to receive approval for additional indications. Meanwhile, efforts have been made to target different cancers, improve the logistics of delivery and reduce the cost associated with novel cell and gene therapies. Here, we highlight various cell and gene therapy-related technologies and advances that provide insight into how these new technologies will speed the translation of these therapies into the clinic.

CONCLUSIONS

In this review, we provide a broad overview of the current state of cell and gene therapy-based approaches for cancer treatment - discussing various effector cell types and their sources, recent advances in both CAR and non-CAR genetic modifications, and highlighting a few promising approaches for increasing in vivo efficacy and persistence of therapeutic drug products.

Blasts in context: the impact of the immune environment on acute myeloid leukemia prognosis and treatment

Acute myeloid leukemia (AML) is a cancer that originates from the bone marrow (BM). Under physiological conditions, the bone marrow supports the homeostasis of immune cells and hosts memory lymphoid cells. In this review, we summarize our present understanding of the role of the immune microenvironment on healthy bone marrow and on the development of AML, with a focus on T cells and other lymphoid cells. The types and function of different immune cells involved in the AML microenvironment as well as their putative role in the onset of disease and response to treatment are presented. We also describe how the immune context predicts the response to immunotherapy in AML and how these therapies modulate the immune status of the bone marrow. Finally, we focus on allogeneic stem cell transplantation and summarize the current understanding of the immune environment in the post-transplant bone marrow, the factors associated with immune escape and relevant strategies to prevent and treat relapse.

Outcomes with chimeric antigen receptor t-cell therapy in relapsed or refractory acute myeloid leukemia: a systematic review and meta-analysis

Background

We conducted a systematic review and meta-analysis to evaluate outcomes following chimeric antigen receptor T cell (CAR-T) therapy in relapsed/refractory acute myeloid leukemia (RR-AML).

Methods

We performed a literature search on PubMed, Cochrane Library, and Clinicaltrials.gov. After screening 677 manuscripts, 13 studies were included. Data was extracted following PRISMA guidelines. Pooled analysis was done using the meta-package by Schwarzer et al. Proportions with 95% confidence intervals (CI) were computed.

Results

We analyzed 57 patients from 10 clinical trials and 3 case reports. The pooled complete and overall response rates were 49.5% (95% CI 0.18-0.81, I² =65%) and 65.2% (95% CI 0.36-0.91, I² =57%). The pooled incidence of cytokine release syndrome, immune-effector cell associated neurotoxicity syndrome, and graft-versus-host disease was estimated as 54.4% (95% CI 0.17-0.90, I² =77%), 3.9% (95% CI 0.00-0.19, I² =22%), and 1.6% (95%CI 0.00-0.21, I² =33%), respectively.

Conclusion

CAR-T therapy has demonstrated modest efficacy in RR-AML. Major challenges include heterogeneous disease biology, lack of a unique targetable antigen, and immune exhaustion.

TP53-Mutated Myelodysplastic Syndrome and Acute Myeloid Leukemia: Biology, Current Therapy, and Future Directions

TP53-mutated myelodysplastic syndrome (MDS) and acute myeloid leukemia (AML) form a distinct group of myeloid disorders with dismal outcomes. TP53-mutated MDS and AML have lower response rates to either induction chemotherapy, hypomethylating agent-based regimens, or venetoclax-based therapies compared with non-TP53-mutated counterparts and a poor median overall survival of 5 to 10 months. Recent advances have identified novel pathogenic mechanisms in TP53-mutated myeloid malignancies, which have the potential to improve treatment strategies in this distinct clinical subgroup. In this review, we discuss recent insights into the biology of TP53-mutated MDS/AML, current treatments, and emerging therapies, including immunotherapeutic and nonimmune-based approaches for this entity.

SIGNIFICANCE

Emerging data on the impact of cytogenetic aberrations, TP53 allelic burden, immunobiology, and tumor microenvironment of TP53-mutated MDS and AML are further unraveling the complexity of this disease. An improved understanding of the functional consequences of TP53 mutations and immune dysregulation in TP53-mutated AML/MDS coupled with dismal outcomes has resulted in a shift from the use of cytotoxic and hypomethylating agent-based therapies to novel immune and nonimmune strategies for the treatment of this entity. It is hoped that these novel, rationally designed combinations will improve outcomes in this area of significant unmet need.

Combination strategies to optimize the efficacy of chimeric antigen receptor T cell therapy in haematological malignancies

Chimeric antigen receptor (CAR) T cell therapy has revolutionized the therapeutic landscape of haematological malignancies. However, resistance and relapse remain prominent limitations, and they are related to the limited persistence and efficacy of CAR T cells, downregulation or loss of tumour antigens, intrinsic resistance of tumours to death signalling, and immune suppressive microenvironment. Rational combined modality treatments are regarded as a promising strategy to further unlock the antitumor potential of CAR T cell therapy, which can be applied before CAR T cell infusion as a conditioning regimen or in ex vivo culture settings as well as concomitant with or after CAR T cell infusion. In this review, we summarize the combinatorial strategies, including chemotherapy, radiotherapy, haematopoietic stem cell transplantation, targeted therapies and other immunotherapies, in an effort to further enhance the effectiveness of this impressive therapy and benefit more patients.

Chimeric Antigen Receptor T-cell Therapy: Current Status and Clinical Outcomes in Pediatric Hematologic Malignancies

Chimeric antigen receptor T-cell (CART) therapy has transformed the treatment paradigm for pediatric patients with relapsed/refractory B-cell acute lymphoblastic leukemia (B-ALL), with complete remission rates in key pivotal CD19-CART trials ranging from 65% to 90%. Alongside this new therapy, new toxicity profiles and treatment limitations have emerged, necessitating toxicity consensus grading systems, cooperative group trials, and novel management approaches. This review highlights the results of key clinical trials of CART for pediatric hematologic malignancies, discusses the most common toxicities seen to date, and elucidates challenges, opportunities, and areas of active research to optimize this therapy.

CAR-T Cell Therapy in Hematological Malignancies: Current Opportunities and Challenges

Chimeric antigen receptor T (CAR-T) cell therapy represents a major breakthrough in cancer treatment, and it has achieved unprecedented success in hematological malignancies, especially in relapsed/refractory (R/R) B cell malignancies. At present, CD19 and BCMA are the most common targets in CAR-T cell therapy, and numerous novel therapeutic targets are being explored. However, the adverse events related to CAR-T cell therapy might be serious or even life-threatening, such as cytokine release syndrome (CRS), CAR-T-cell-related encephalopathy syndrome (CRES), infections, cytopenia, and CRS-related coagulopathy. In addition, due to antigen escape, the limited CAR-T cell persistence, and immunosuppressive tumor microenvironment, a considerable proportion of patients relapse after CAR-T cell therapy. Thus, in this review, we focus on the progress and challenges of CAR-T cell therapy in hematological malignancies, such as attractive therapeutic targets, CAR-T related toxicities, and resistance to CAR-T cell therapy, and provide some practical recommendations.

Immune-Based Therapeutic Interventions for Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is an aggressive, clonally heterogeneous, myeloid malignancy, with a 5-year overall survival of approximately 27%. It constitutes the most common acute leukemia in adults, with an incidence of 3-5 cases per 100,000 in the United States. Despite great advances in understanding the molecular mechanisms underpinning leukemogenesis, the past several decades had seen little change to the backbone of therapy, comprised of an anthracycline-based induction regimen for those who are fit enough to receive it, followed by risk-stratified post-remission therapy with consolidation cytarabine or allogeneic stem cell transplantation (allo-SCT). Allo-SCT is the most fundamental form of immunotherapy in which donor cytotoxic T and NK cells recognize and eradicate residual AML in the graft-versus-leukemia (GvL) effect. Building on that, several alternative or synergistic approaches to exploit both self and foreign immunity against AML have been developed. Checkpoint inhibitors, for example, CTLA-4 inhibitors, PD-1 inhibitors, and PD-L1 inhibitors block proteins found on T cells or cancer cells that stop the immune system from attacking the cancer cells. They have been used with limited success in both the AML relapsed/refractory (R/R) and post SCT settings. AML tumor mutational burden is low compared to solid tumors and thus, it is less likely to generate neoantigens and respond to antibody-mediated checkpoint blockade that has shown unprecedented results in solid tumors. Therefore, alternative therapeutic strategies that work independently of the T cell receptor (TCR) specificity have been developed. They include bispecific antibodies, which recruit T cells through CD3 engagement, and in AML have shown an overall response rate ranging between 14 and 30% in early phase trials. Chimeric Antigen Receptor (CAR) T cell therapy is a type of treatment in which T cells are genetically engineered to produce a recombinant receptor that redirects the specificity and function of T lymphocytes. However, lack of cell surface targets exclusively expressed on AML cells including Leukemic Stem Cells (LSCs) combined with clonal heterogeneity represents the biggest challenge in developing CAR therapy for AML. Antibody-Drug Conjugates (ADC) constitute the only FDA-approved immunotherapy to treat AML with Gemtuzumab Ozogamicin, a CD33-specific ADC used in CEBPα-mutated AML. The identification of additional cell surface targets is critical for the development of other ADC's potentially useful in the induction and maintenance regimens, given the ease at which these reagents can be generated and managed. Here, we will review those immune-based therapeutic interventions and highlight active areas of research investigations toward fulfillment of the great promise of immunotherapy to AML.

Transgenic Expression of IL15 Retains CD123-Redirected T Cells in a Less Differentiated State Resulting in Improved Anti-AML Activity in Autologous AML PDX Models

Immunotherapy with T-cells expressing bispecific T-cell engagers (ENG T-cells) is a promising approach to improve the outcomes for patients with recurrent/refractory acute myeloid leukemia (AML). However, similar to T-cells expressing chimeric antigen receptors (CARs), their antitumor activity is limited in the setting of chronic antigen stimulation. We therefore set out to explore whether transgenic expression of IL15 improves the effector function of ENG T-cells targeting CD123-positive AML. T-cells expressing CD123-specific ENG (CD123-ENG) ± IL15 were generated by retroviral transduction from peripheral blood T cells from healthy donors or patients with AML. In this study, we characterized in detail the phenotype and effector functions of ENG T-cell populations in vitro and in vivo. IL15-expressing CD123-ENG (CD123-ENG.IL15) T-cells retained their antigen-specificity and effector function in the setting of chronic antigen exposure for more 30 days of coculture with AML blasts in contrast to CD123-ENG T-cells, whose effector function rapidly eroded. Furthermore, CD123-ENG.IL15 T-cells remained in a less differentiated state as judged by a high frequency of naïve/memory stem T-cell-like cells (CD45RA+CCR7+/CD45RO−CD62L+ cells) without evidence of T-cell exhaustion. Single cell cytokine profiling using IsoPlexis revealed enhanced T-cell polyfunctionality of CD123-ENG.IL15 T-cells as judged by effector cytokine production, including, granzyme B, IFN-γ, MIP-1α, perforin, TNF-α, and TNF-β. In vivo, CD123-ENG.IL15 T-cells exhibited superior antigen-specific anti-AML activity and T-cell persistence in both peripheral blood and tissues (BM, spleens, and livers), resulting in a significant survival advantage in one AML xenograft model and two autologous AML PDX models. In conclusion, we demonstrate here that the expansion, persistence, and anti-AML activity of CD123-ENG T-cells can be significantly improved by transgenic expression of IL15, which promotes a naïve/TSCM-like phenotype. However, we also highlight that targeting a single tumor antigen (CD123) can lead to immune escape, reinforcing the need to develop approaches to target multiple antigens. Likewise, our study demonstrates that it is feasible to evaluate autologous T cells in AML PDX models, which will be critical for future preclinical evaluations of next generation AML-redirected T-cell therapies.

Tumor Microenvironment in Acute Myeloid Leukemia: Adjusting Niches

Acute myeloid leukemias (AML) comprise a wide array of different entities, which have in common a rapid expansion of myeloid blast cells leading to displacement of normal hematopoietic cells and also disruption of the microenvironment in the bone marrow niches. Based on an insight into the complex cellular interactions in the bone marrow niches in non-neoplastic conditions in general, this review delineates the complex relationship between leukemic cells and reactive cells of the tumor microenvironment (TME) in AML. A special focus is directed on niche cells and various T-cell subsets as these also provide a potential therapeutic rationale considering e.g. immunomodulation. The TME of AML on the one hand plays a vital role for sustaining and promoting leukemogenesis but - on the other hand - it also has adverse effects on abnormal blasts developing into overt leukemia hindering their proliferation and potentially removing such cells. Thus, leukemic cells need to and develop strategies in order to manipulate the TME. Interference with those strategies might be of particular therapeutic potential since mechanisms of resistance related to tumor cell plasticity do not apply to it.

Leukemia’s Next Top Model? Syngeneic Models to Advance Adoptive Cellular Therapy

In recent years, there has been an emphasis on harnessing the immune system for therapeutic interventions. Adoptive cell therapies (ACT) have emerged as an effective option for B-cell derived hematological malignancies. Despite remarkable successes with ACT, immune dysregulation and the leukemia microenvironment can critically alter clinical responses. Therefore, preclinical modeling can contribute to the advancement of ACT for leukemias. Human xenografts, the current mainstay of ACT in vivo models, cannot evaluate the impact of the immunosuppressive leukemia microenvironment on adoptively transferred cells. Syngeneic mouse models utilize murine tumor models and implant them into immunocompetent mice. This provides an alternative model, reducing the need for complicated breeding strategies while maintaining a matched immune system, stromal compartment, and leukemia burden. Syngeneic models that evaluate ACT have analyzed the complexity of cytotoxic T lymphocytes, T cell receptor transgenics, and chimeric antigen receptors. This review examines the immunosuppressive features of the leukemia microenvironment, discusses how preclinical modeling helps predict ACT associated toxicities and dysfunction, and explores publications that have employed syngeneic modeling in ACT studies for the improvement of therapy for leukemias.

Challenges and Advances in Chimeric Antigen Receptor Therapy for Acute Myeloid Leukemia

The advent of chimeric antigen receptor (CAR) T-cell therapy has led to dramatic remission rates in multiple relapsed/refractory hematologic malignancies. While CAR T-cell therapy has been particularly successful as a treatment for B-cell malignancies, effectively treating acute myeloid leukemia (AML) with CARs has posed a larger challenge. AML not only creates an immunosuppressive tumor microenvironment that dampens CAR T-cell responses, but it also lacks many unique tumor-associated antigens, making leukemic-specific targeting difficult. One advantage of CAR T-cell therapy compared to alternative treatment options is the ability to provide prolonged antigen-specific immune effector and surveillance functions. Since many AML CAR targets under investigation including CD33, CD117, and CD123 are also expressed on hematopoietic stem cells, CAR T-cell therapy can lead to severe and potentially lethal myeloablation. Novel strategies to combat these issues include creation of bispecific CARs, CAR T-cell "safety switches", TCR-like CARs, NK CARs, and universal CARs, but all vary in their ability to provide a sustained remission, and consolidation with an allogeneic hematopoietic cell transplantation (allo-HCT) will be necessary in most cases This review highlights the delicate balance between effectively eliminating AML blasts and leukemic stem cells, while preserving the ability for bone marrow to regenerate. The impact of CAR therapy on treatment landscape of AML and changing scope of allo-HCT is discussed. Continued advances in AML CAR therapy would be of great benefit to a disease that still has high morbidity and mortality.

Current Limitations and Perspectives of Chimeric Antigen Receptor-T-Cells in Acute Myeloid Leukemia

Simple Summary

Acute myeloid leukemia (AML) is the most frequent type of acute leukemia in adults. Allogeneic hematopoietic cell transplantation (allo-HCT) has been the only potentially curative treatment for the majority of patients. The ability of chimeric antigen receptor (CAR)-modified T-cell therapy directed against the CD19 antigen to induce durable remissions in patients with acute lymphoblastic leukemia (ALL) has provided optimism that this novel treatment paradigm can be extrapolated to AML. In this review, we provide an overview of candidate target antigens for CAR-T-cells in AML, an update on recent progress in preclinical and clinical development of investigational CAR-T-cell products, and discuss challenges for the clinical implementation of CAR-T-cell therapy in AML.

Abstract

Adoptive transfer of gene-engineered chimeric antigen receptor (CAR)-T-cells has emerged as a powerful immunotherapy for combating hematologic cancers. Several target antigens that are prevalently expressed on AML cells have undergone evaluation in preclinical CAR-T-cell testing. Attributes of an ‘ideal’ target antigen for CAR-T-cell therapy in AML include high-level expression on leukemic blasts and leukemic stem cells (LSCs), and absence on healthy tissues, normal hematopoietic stem and progenitor cells (HSPCs). In contrast to other blood cancer types, where CAR-T therapies are being similarly studied, only a rather small number of AML patients has received CAR-T-cell treatment in clinical trials, resulting in limited clinical experience for this therapeutic approach in AML. For curative AML treatment, abrogation of bulk blasts and LSCs is mandatory with the need for hematopoietic recovery after CAR-T administration. Herein, we provide a critical review of the current pipeline of candidate target antigens and corresponding CAR-T-cell products in AML, assess challenges for clinical translation and implementation in routine clinical practice, as well as perspectives for overcoming them.

Resistance of B-Cell Lymphomas to CAR T-Cell Therapy Is Associated With Genomic Tumor Changes Which Can Result in Transdifferentiation

Despite the impressive efficacy of chimeric antigen receptor (CAR) T-cell therapy (CART) in B-cell non-Hodgkin lymphomas, durable responses are uncommon. The histopathologic and molecular features associated with treatment failure are still largely unknown. Therefore, we have analyzed 19 sequential tumor samples from 9 patients, prior anti-CD19 CART (pre-CART) and at relapse (post-CART), using immunohistochemistry, fluorescence in situ hybridization, array comparative genomic hybridization, next-generation DNA and RNA sequencing, and genome-scale DNA methylation. The initial diagnosis was diffuse large B-cell lymphoma (n=6), double-hit high-grade B-cell lymphoma (n=1), and Burkitt lymphoma (n=2). Histopathologic features were mostly retained at relapse in 7/9 patients, except the frequent loss of 1 or several B-cell markers. The remaining 2 cases (1 diffuse large B-cell lymphoma and 1 Burkitt lymphoma) displayed a dramatic phenotypic shift in post-CART tumors, with the drastic downfall of B-cell markers and emergence of T-cell or histiocytic markers, despite the persistence of identical clonal immunoglobulin gene rearrangements. The post-CART tumor with aberrant T-cell phenotype showed reduced mRNA expression of most B-cell genes with increased methylation of their promoter. Fluorescence in situ hybridization and comparative genomic hybridization showed global stability of chromosomal alterations in all paired samples, including 17p/TP53 deletions. New pathogenic variants acquired in post-CART samples included mutations triggering the PI3K pathway (PIK3R1, PIK3R2, PIK3C2G) or associated with tumor aggressiveness (KRAS, INPP4B, SF3B1, SYNE1, TBL1XR1). These results indicate that CART-resistant B-cell non-Hodgkin lymphomas display genetic remodeling, which may result in profound dysregulation of B-cell differentiation. Acquired mutations in the PI3K and KRAS pathways suggest that some targeted therapies could be useful to overcome CART resistance.

Employing Synthetic T-cell Biology to Target AML without On-Target/Off-Cancer Toxicity

Ideal targets for chimeric antigen receptor T-cell therapy for acute myeloid leukemia (AML) remain elusive. In this issue of Blood Cancer Discovery, Richards and colleagues explore CD93 as a potential AML target antigen, and devise an approach to mitigate "on-target/off-cancer toxicity."See related article by Richards et al., p. 648.

Efficacy and safety of Wilms' tumor 1 helper peptide OCV-501 in elderly patients with acute myeloid leukemia: a multicenter, randomized, double-blind, placebo-controlled phase 2 trial

PURPOSE

Complete remission (CR) of acute myeloid leukemia (AML) in elderly patients has a short duration, and there is no suitable post-remission therapy. We explored the role of the Wilms' tumor 1 helper peptide OCV-501 to prevent recurrence after remission.

METHODS

This placebo-controlled phase 2 study was designed to evaluate accurately the efficacy and immunogenicity of OCV-501 in elderly AML patients. Elderly AML patients who achieved first CR were randomly allocated to receive either OCV-501 (N = 69) or placebo (N = 65) once a week for eight weeks and then every two weeks until week 104. The primary endpoint was disease-free survival (DFS).

RESULTS

Nineteen (27.5%) patients in the OCV-501 group and 23 (35.4%) patients in the placebo group completed the study without relapse. The median DFS in the OCV-501 and placebo groups was 12.1 and 8.4 months, respectively (p = 0.7671, hazard ratio [95% confidence interval]: 0.933 [0.590, 1.477]). The major drug adverse reactions were injection-site reactions. Although treatment with OCV-501 did not prolong DFS for elderly AML patients, post hoc analysis found that immune responders to OCV-501 whose specific IgG was > 10,000 ng/mL (N = 16) and whose WT1-specific interferon-γ response was > 10 pg/mL (N = 26) had significantly longer overall survival compared with placebo.

CONCLUSIONS

The placebo-controlled design of this study and quantitative immunological monitoring provides new insight into the relationship between peptide-induced immune responses and survival, suggesting future perspectives for cancer immunotherapy.

Molecular Targeted Therapy and Immunotherapy for Myelodysplastic Syndrome

Myelodysplastic syndrome (MDS) is a heterogeneous, clonal hematological disorder characterized by ineffective hematopoiesis, cytopenia, morphologic dysplasia, and predisposition to acute myeloid leukemia (AML). Stem cell genomic instability, microenvironmental aberrations, and somatic mutations contribute to leukemic transformation. The hypomethylating agents (HMAs), azacitidine and decitabine are the standard of care for patients with higher-risk MDS. Although these agents induce responses in up to 40-60% of patients, primary or secondary drug resistance is relatively common. To improve the treatment outcome, combinational therapies comprising HMA with targeted therapy or immunotherapy are being evaluated and are under continuous development. This review provides a comprehensive update of the molecular pathogenesis and immune-dysregulations involved in MDS, mechanisms of resistance to HMA, and strategies to overcome HMA resistance.

Can the New and Old Drugs Exert an Immunomodulatory Effect in Acute Myeloid Leukemia?

Simple Summary

The advent of novel immunotherapeutic strategies has revealed the importance of immune dysregulation and of a tolerogenic microenvironment for acute myeloid leukemia (AML) fitness. We reviewed the “off-target” effects on the immune system of different drugs used in the treatment of AML to explore the advantages of this unexpected interaction.

Abstract

Acute myeloid leukemia (AML) is considered an immune-suppressive neoplasm capable of evading immune surveillance through cellular and environmental players. Increasing knowledge of the immune system (IS) status at diagnosis seems to suggest ever more attention of the crosstalk between the leukemic clone and its immunologic counterpart. During the last years, the advent of novel immunotherapeutic strategies has revealed the importance of immune dysregulation and suppression for leukemia fitness. Considering all these premises, we reviewed the “off-target” effects on the IS of different drugs used in the treatment of AML, focusing on the main advantages of this interaction. The data reported support the idea that a successful therapeutic strategy should consider tailored approaches for performing leukemia eradication by both direct blasts killing and the engagement of the IS.

Myelodysplastic syndrome and immunotherapy novel to next in-line treatments

Patients with Myelodysplastic syndromes (MDS) have few therapy options for sustainable responses in the frontline setting, and even less after hypomethylating agent (HMA) failure in relapsed and refractory setting. The only potential cure is an allogeneic hematopoietic stem cell transplant which is an unrealistic option for the majority of MDS patients. Immunotherapy with checkpoint inhibition, CAR-T cells, and vaccine therapy few have shown promise in a variety cancer and have now been tested in patients with MDS. Most trials have focused on AML patients and included small numbers of MDS patients. Until now, a dedicated review of immunotherapy outcomes in MDS patients has been lacking. Thus, herein we review outcomes of MDS patients after immunotherapies on a variety of clinical trials reported to date.

Orchestration of myeloid-derived suppressor cells in the tumor microenvironment by ubiquitous cellular protein TCTP released by tumor cells

One of most challenging issues in tumor immunology is a better understanding of the dynamics in the accumulation of myeloid-derived suppressor cells (MDSCs) in the tumor microenvironment (TIME), as this would lead to the development of new cancer therapeutics. Here, we show that translationally controlled tumor protein (TCTP) released by dying tumor cells is an immunomodulator crucial to full-blown MDSC accumulation in the TIME. We provide evidence that extracellular TCTP mediates recruitment of the polymorphonuclear MDSC (PMN-MDSC) population in the TIME via activation of Toll-like receptor-2. As further proof of principle, we show that inhibition of TCTP suppresses PMN-MDSC accumulation and tumor growth. In human cancers, we find an elevation of TCTP and an inverse correlation of TCTP gene dosage with antitumor immune signatures and clinical prognosis. This study reveals the hitherto poorly understood mechanism of the MDSC dynamics in the TIME, offering a new rationale for cancer immunotherapy.

Driving CAR T Stem Cell Targeting in Acute Myeloid Leukemia: The Roads to Success

Current treatment outcome for acute myeloid leukemia (AML) patients is unsatisfactory and characterized by high rates of relapse and poor overall survival. Increasing evidence points to a crucial role of leukemic stem cells (LSC) and the bone marrow (BM) leukemic niche, in which they reside, in AML evolution and chemoresistance. Thus, future strategies aiming at improving AML therapeutic protocols are likely to be directed against LSC and their niche. Chimeric antigen receptor (CAR) T-cells have been extremely successful in the treatment of relapsed/refractory acute lymphoblastic leukemia and B-cell non-Hodgkin lymphoma and comparable results in AML are highly desirable. At present, we are at the dawn of CAR T-cell application in AML, with several preclinical studies and few early phase clinical trials. However, the lack of leukemia-specific targets and the genetic and phenotypic heterogeneity of the disease combined with the leukemia-induced remodeling of the BM microenvironment are limiting CAR T-cell exploitation in AML. Here, we reviewed AML-LSC and AML-BM niche features in the context of their therapeutic targeting using CAR T-cells. We summarized recent progress in CAR T-cell application to the treatment of AML, and we discussed the remaining therapeutic challenges and promising novel strategies to overcome them.

Current and Future Role of Medical Imaging in Guiding the Management of Patients With Relapsed and Refractory Non-Hodgkin Lymphoma Treated With CAR T-Cell Therapy

Chimeric antigen receptor (CAR) T-cells are a novel immunotherapy available for patients with refractory/relapsed non-Hodgkin lymphoma. In this indication, clinical trials have demonstrated that CAR T-cells achieve high rates of response, complete response, and long-term response (up to 80%, 60%, and 40%, respectively). Nonetheless, the majority of patients ultimately relapsed. This review provides an overview about the current and future role of medical imaging in guiding the management of non-Hodgkin lymphoma patients treated with CAR T-cells. It discusses the value of predictive and prognostic biomarkers to better stratify the risk of relapse, and provide a patient-tailored therapeutic strategy. At baseline, high tumor volume (assessed on CT-scan or on [18F]-FDG PET/CT) is a prognostic factor associated with treatment failure. Response assessment has not been studied extensively yet. Available data suggests that current response assessment developed on CT-scan or on [18F]-FDG PET/CT for cytotoxic systemic therapies remains relevant to estimate lymphoma response to CAR T-cell therapy. Nonetheless, atypical patterns of response and progression have been observed and should be further analyzed. The potential advantages as well as limitations of artificial intelligence and radiomics as tools providing high throughput quantitative imaging features is described.

Cell interactions in the bone marrow microenvironment affecting myeloid malignancies

The bone marrow is a complex tissue in which heterogeneous populations of stromal cells interact with hematopoietic cells to dynamically respond to organismal needs in defense, hemostasis, and oxygen delivery. Physiologic challenges modify stromal/hematopoietic cell interactions to generate changes in blood cell production. When either stroma or hematopoietic cells are impaired, the system distorts. The distortions associated with myeloid malignancy are reviewed here and may provide opportunities for therapeutic intervention.

Acute Myeloid Leukemia: Is It T Time?

Acute myeloid leukemia (AML) is a heterogeneous disease driven by impaired differentiation of hematopoietic primitive cells toward myeloid lineages (monocytes, granulocytes, red blood cells, platelets), leading to expansion and accumulation of "stem" and/or "progenitor"-like or differentiated leukemic cells in the bone marrow and blood. AML progression alters the bone marrow microenvironment and inhibits hematopoiesis' proper functioning, causing sustained cytopenia and immunodeficiency. This review describes how the AML microenvironment influences lymphoid lineages, particularly T lymphocytes that originate from the thymus and orchestrate adaptive immune response. We focus on the elderly population, which is mainly affected by this pathology. We discuss how a permissive AML microenvironment can alter and even worsen the thymic function, T cells' peripheral homeostasis, phenotype, and functions. Based on the recent findings on the mechanisms supporting that AML induces quantitative and qualitative changes in T cells, we suggest and summarize current immunotherapeutic strategies and challenges to overcome these anomalies to improve the anti-leukemic immune response and the clinical outcome of patients.

Therapeutic cancer vaccine therapy for acute myeloid leukemia

Antitumor function of the immune system has been harnessed to eradicate tumor cells as cancer therapy. Therapeutic cancer vaccines aim to help immune cells recognize tumor cells, which are difficult to target owing to immune escape. Many attempts at vaccine designs have been conducted throughout the last decades. In addition, as the advanced understanding of immunosuppressive mechanisms mediated by tumor cells, combining cancer vaccines with other immune therapies seems to be more efficient for cancer treatment. Acute myeloid leukemia (AML) is the most common acute leukemia in adults with poor prognosis. Evidence has shown T-cell-mediated immune responses in AML, which encourages the utility of immune therapies in AML. This review discusses cancer vaccines in AML from vaccine design as well as recent progress in vaccination combination with other immune therapies.

CAR-T Cell Therapy for Acute Myeloid Leukemia: Preclinical Rationale, Current Clinical Progress, and Barriers to Success

Chimeric antigen receptor (CAR)-T cell therapy has shown impressive results in chemorefractory B cell malignancies, raising the possibilities of using this immunotherapeutic modality for other devastating hematologic malignancies, such as acute myeloid leukemia (AML). AML is an aggressive hematologic malignancy which, like B cell malignancies, poses several challenges for clinical translation of successful immunotherapy. The antigenic heterogeneity of AML results in a list of potential targets that CAR-T cells could be directed towards, each with advantages and disadvantages. In this review, we provide an up-to-date report of outcomes and adverse effects from published and presented clinical trials of CAR-T cell therapy for AML and provide the preclinical rationale underlying these studies and antigen selection. Comparison across trials is difficult, yet themes emerge with respect to appropriate antigen selection and association of adverse effects with outcomes. We highlight currently active clinical trials and the potential improvements and caveats with these novel approaches. Key hurdles to the successful introduction of CAR-T cell therapy for the treatment of AML include the effect of antigenic heterogeneity and trade-offs between therapy specificity and sensitivity; on-target off-tumor toxicities; the AML tumor microenvironment; and practical considerations for future trials that should be addressed to enable successful CAR-T cell therapy for AML.

Immunosuppressive Cell Subsets and Factors in Myeloid Leukemias

Simple Summary

Effector immune system cells have the ability to kill tumor cells. However, as a cancer (such as leukemia) develops, it inhibits and evades the effector immune response. Such a state of immunosuppression can be driven by several factors – receptors, soluble cytokines, as well as by suppressive immune cells. In this review, we describe factors and cells that constitute immunosuppressive microenvironment of myeloid leukemias. We characterize factors of direct leukemic origin, such as inhibitory receptors, enzymes and extracellular vesicles. Furthermore, we describe suppressive immune cells, such as myeloid derived suppressor cells and regulatory T cells. Finally, we sum up changes in these drivers of immune evasion in myeloid leukemias during therapy.

Abstract

Both chronic myeloid leukemia and acute myeloid leukemia evade the immune response during their development and disease progression. As myeloid leukemia cells modify their bone marrow microenvironment, they lead to dysfunction of cytotoxic cells, such as CD8+ T cells or NK cells, simultaneously promoting development of immunosuppressive regulatory T cells and suppressive myeloid cells. This facilitates disease progression, spreading of leukemic blasts outside the bone marrow niche and therapy resistance. The following review focuses on main immunosuppressive features of myeloid leukemias. Firstly, factors derived directly from leukemic cells – inhibitory receptors, soluble factors and extracellular vesicles, are described. Further, we outline function, properties and origin of main immunosuppressive cells - regulatory T cells, myeloid derived suppressor cells and macrophages. Finally, we analyze interplay between recovery of effector immunity and therapeutic modalities, such as tyrosine kinase inhibitors and chemotherapy.

Single-cell map of diverse immune phenotypes in the acute myeloid leukemia microenvironment

BACKGROUND

Knowledge of immune cell phenotypes, function, and developmental trajectory in acute myeloid leukemia (AML) microenvironment is essential for understanding mechanisms of evading immune surveillance and immunotherapy response of targeting special microenvironment components.

METHODS

Using a single-cell RNA sequencing (scRNA-seq) dataset, we analyzed the immune cell phenotypes, function, and developmental trajectory of bone marrow (BM) samples from 16 AML patients and 4 healthy donors, but not AML blasts.

RESULTS

We observed a significant difference between normal and AML BM immune cells. Here, we defined the diversity of dendritic cells (DC) and macrophages in different AML patients. We also identified several unique immune cell types including T helper cell 17 (TH17)-like intermediate population, cytotoxic CD4+ T subset, T cell: erythrocyte complexes, activated regulatory T cells (Treg), and CD8+ memory-like subset. Emerging AML cells remodels the BM immune microenvironment powerfully, leads to immunosuppression by accumulating exhausted/dysfunctional immune effectors, expending immune-activated types, and promoting the formation of suppressive subsets.

CONCLUSION

Our results provide a comprehensive AML BM immune cell census, which can help to select pinpoint targeted drug and predict efficacy of immunotherapy.

Deciphering molecular mechanisms underlying chemoresistance in relapsed AML patients: towards precision medicine overcoming drug resistance

Background

Acute myeloid leukemia (AML) remains a devastating disease with a 5-year survival rate of less than 30%. AML treatment has undergone significant changes in recent years, incorporating novel targeted therapies along with improvements in allogeneic bone marrow transplantation techniques. However, the standard of care remains cytarabine and anthracyclines, and the primary hindrance towards curative treatment is the frequent emergence of intrinsic and acquired anticancer drug resistance. In this respect, patients presenting with chemoresistant AML face dismal prognosis even with most advanced therapies. Herein, we aimed to explore the potential implementation of the characterization of chemoresistance mechanisms in individual AML patients towards efficacious personalized medicine.

Methods

Towards the identification of tailored treatments for individual patients, we herein present the cases of relapsed AML patients, and compare them to patients displaying durable remissions following the same chemotherapeutic induction treatment. We quantified the expression levels of specific genes mediating drug transport and metabolism, nucleotide biosynthesis, and apoptosis, in order to decipher the molecular mechanisms underlying intrinsic and/or acquired chemoresistance modalities in relapsed patients. This was achieved by real-time PCR using patient cDNA, and could be readily implemented in the clinical setting.

Results

This analysis revealed pre-existing differences in gene expression levels between the relapsed patients and patients with lasting remissions, as well as drug-induced alterations at different relapse stages compared to diagnosis. Each of the relapsed patients displayed unique chemoresistance mechanisms following similar treatment protocols, which could have been missed in a large study aimed at identifying common drug resistance determinants.

Conclusions

Our findings emphasize the need for standardized evaluation of key drug transport and metabolism genes as an integral component of routine AML management, thereby allowing for the selection of treatments of choice for individual patients. This approach could facilitate the design of efficacious personalized treatment regimens, thereby reducing relapse rates of therapy refractory disease.

Acute Myeloid Leukemia: From Biology to Clinical Practices Through Development and Pre-Clinical Therapeutics

Recent studies have provided several insights into acute myeloid leukemia. Studies based on molecular biology have identified eight functional mutations involved in leukemogenesis, including driver and passenger mutations. Insight into Leukemia stem cells (LSCs) and assessment of cell surface markers have enabled characterization of LSCs from hematopoietic stem and progenitor cells. Clonal evolution has been described as having an effect similar to that of microenvironment alterations. Such biological findings have enabled the development of new targeted drugs, including drug inhibitors and monoclonal antibodies with blockage functions. Some recently approved targeted drugs have resulted in new therapeutic strategies that enhance standard intensive chemotherapy regimens as well as supportive care regimens. Besides the progress made in adoptive immunotherapy, since allogenic hematopoietic stem cell transplantation enabled the development of new T-cell transfer therapies, such as chimeric antigen receptor T-cell and transgenic TCR T-cell engineering, new promising strategies that are investigated.

Chimeric Antigen Receptor (CAR)-Modified Immune Effector Cell Therapy for Acute Myeloid Leukemia (AML)

Despite the availability of an increasing number of targeted therapeutics and wider use of allogeneic hematopoietic stem cell transplantation, many patients with acute myeloid leukemia (AML) ultimately succumb to this disease. Given their remarkable efficacy in B-acute lymphoblastic leukemia and other CD19-expressing B cell malignancies, there is hope adoptive cellular transfer, particularly chimeric antigen receptor (CAR)-modified immune effector cell (IEC) therapies, may afford a novel, potent immune-based approach for the treatment of AML that complements or replaces existing ones and improves cure rates. However, it is unclear how best to translate the success of these therapies from B cell malignancies, where use of highly potent immunotherapies is facilitated by identified target antigens with near ubiquitous expression on malignant cells and non-fatal consequences from "on-target, off-tumor cell" toxicities. Herein, we review the current status of CAR-modified IEC therapies for AML, with considerations regarding suitable, relatively leukemia-restricted target antigens, expected toxicities, and interactions of the engineered cells with a profoundly immunosuppressive tumor microenvironment that restricts their therapeutic efficacy. With these challenges in mind, we will discuss possible strategies to improve the cells' potency as well as their therapeutic window for optimal clinical use in AML.

Chimeric Antigen Receptor T Cell Exhaustion during Treatment for Hematological Malignancies

Immunotherapy, especially based on chimeric antigen receptor (CAR) T cells, has achieved prominent success in the treatment of hematological malignancies. However, approximately 30-50% of patients will have disease relapse following remission after receiving CD19-targeting CAR-T cells, with failure of maintaining a long-term effect. Mechanisms underlying CAR-T therapy inefficiency consist of loss or modulation of target antigen and CAR-T cell poor persistence which mostly results from T cell exhaustion. The unique features and restoration strategies of exhausted T cells (Tex) have been well described in solid tumors. However, the overview associated with CAR-T cell exhaustion is relatively rare in hematological malignancies. In this review, we summarize the characteristics, cellular, and molecular mechanisms of Tex cells as well as approaches to reverse CAR-T cell exhaustion in hematological malignancies, providing novel strategies for immunotherapies.

Pediatric PK/PD Phase I Trial of Pexidartinib in Relapsed and Refractory Leukemias and Solid Tumors Including Neurofibromatosis Type I-Related Plexiform Neurofibromas

PURPOSE

Simultaneously targeting the tumor and tumor microenvironment may hold promise in treating children with refractory solid tumors. Pexidartinib, an oral inhibitor of tyrosine kinases including colony stimulating factor 1 receptor (CSF-1R), KIT, and FLT3, is FDA approved in adults with tenosynovial giant cell tumor. A phase I trial was conducted in pediatric and young adult patients with refractory leukemias or solid tumors including neurofibromatosis type 1-related plexiform neurofibromas.

PATIENTS AND METHODS

A rolling six design with dose levels (DL) of 400 mg/m², 600 mg/m², and 800 mg/m² once daily for 28-day cycles (C) was used. Response was assessed at regular intervals. Pharmacokinetics and population pharmacokinetics were analyzed during C1.

RESULTS

Twelve patients (4 per DL, 9 evaluable) enrolled on the dose-escalation phase and 4 patients enrolled in the expansion cohort: median (lower, upper quartile) age 16 (14, 16.5) years. No dose-limiting toxicities were observed. Pharmacokinetics appeared linear over three DLs. Pharmacokinetic modeling and simulation determined a weight-based recommended phase II dose (RP2D). Two patients had stable disease and 1 patient with peritoneal mesothelioma (C49+) had a sustained partial response (67% RECIST reduction). Pharmacodynamic markers included a rise in plasma macrophage CSF (MCSF) levels and a decrease in absolute monocyte count.

CONCLUSIONS

Pexidartinib in pediatric patients was well tolerated at all DL tested, achieved target inhibition, and resulted in a weight-based RPD2 dose.

Modulating TNFα activity allows transgenic IL15-Expressing CLL-1 CAR T cells to safely eliminate acute myeloid leukemia

Background

C-type lectin-like molecule 1 (CLL-1) is highly expressed in acute myeloid leukemia (AML) but is absent in primitive hematopoietic progenitors, making it an attractive target for a chimeric antigen receptor (CAR) T-cell therapy. Here, we optimized our CLL-1 CAR for anti-leukemic activity in mouse xenograft models of aggressive AML.

Methods

First, we optimized the CLL-1 CAR using different spacer, transmembrane and costimulatory sequences. We used a second retroviral vector to coexpress transgenic IL15. We measured the effects of each construct on T cell phenotype and sequential (recursive) co culture assays with tumor cell targets to determine the durability of the anti tumor activity by flow cytometry. We administered CAR T cells to mice engrafted with patient derived xenografts (PDX) and AML cell line and determined anti tumor activity by bioluminescence imaging and weekly bleeding, measured serum cytokines by multiplex analysis. After euthanasia, we examined formalin-fixed/paraffin embedded sections. Unpaired two-tailed Student’s t-tests were used and values of p<0.05 were considered significant. Survival was calculated using Mantel-Cox log-rank test.

Results

In vitro, CLL-1 CAR T cells with interleukin-15 (IL15) were less terminally differentiated (p<0.0001) and had superior expansion compared with CD28z-CD8 CAR T cells without IL15 (p<0.001). In both AML PDX and AML cell line animal models, CLL-1 CAR T coexpressing transgenic IL15 initially expanded better than CD28z-CD8 CAR T without IL15 (p<0.0001), but produced severe acute toxicity associated with high level production of human tumor necrosis factor α (TNFα), IL15 and IL2. Histopathology showed marked inflammatory changes with tissue damage in lung and liver. This acute toxicity could be managed by two strategies, individually or in combination. The excessive TNF alpha secretion could be blocked with anti-TNF alpha antibody, while excessive T cell expansion could be arrested by activation of an inducible caspase nine safety switch by administration of dimerizing drug. Both strategies successfully prolonged tumor-free survival.

Conclusion

Combinatorial treatment with a TNFα blocking antibody and subsequent activation of the caspase-9 control switch increased the expansion, survival and antileukemic potency of CLL-1 CAR T-cells expressing transgenic IL15 while avoiding the toxicities associated with excessive cytokine production and long-term accumulation of activated T-cells.

Targeted Cellular Micropharmacies: Cells Engineered for Localized Drug Delivery

The recent emergence of engineered cellular therapies, such as Chimeric antigen receptor (CAR) CAR T and T cell receptor (TCR) engineered T cells, has shown great promise in the treatment of various cancers. These agents aggregate and expand exponentially at the tumor site, resulting in potent immune activation and tumor clearance. Moreover, the ability to elaborate these cells with therapeutic agents, such as antibodies, enzymes, and immunostimulatory molecules, presents an unprecedented opportunity to specifically modulate the tumor microenvironment through cell-mediated drug delivery. This unique pharmacology, combined with significant advances in synthetic biology and cell engineering, has established a new paradigm for cells as vectors for drug delivery. Targeted cellular micropharmacies (TCMs) are a revolutionary new class of living drugs, which we envision will play an important role in cancer medicine and beyond. Here, we review important advances and considerations underway in developing this promising advancement in biological therapeutics.