CD33-specific chimeric antigen receptor T cells exhibit potent preclinical activity against human acute myeloid leukemia

A novel luciferase-based assay for the detection of Chimeric Antigen Receptors

Chimeric Antigen Receptor-T (CAR-T) cell immunotherapy has produced dramatic responses in hematologic malignancies. One of the challenges in the field is the lack of a simple assay for the detection of CARs on the surface of immune effector cells. In this study, we describe a novel luciferase-based assay, termed Topanga Assay, for the detection of CAR expression. The assay utilizes a recombinant fusion protein, called Topanga reagent, generated by joining the extra-cellular domain of a CAR-target in frame with one of the marine luciferases or their engineered derivatives. The assay involves incubation of CAR expressing cells with the Topanga reagent, a few washes and measurement of luminescence. The assay can detect CARs comprising either immunoglobulin- or non-immunoglobulin-based antigen binding domains. We further demonstrate that addition of epitope tags to the Topanga reagent not only allows its convenient one step purification but also extends its use for detection of CAR cells using flow cytometry. However, crude supernatant containing the secreted Topanga reagent can be directly used in both luminescence and flow-cytometry based assays without prior protein purification. Our results demonstrate that the Topanga assay is a highly sensitive, specific, convenient, economical and versatile assay for the detection of CARs.

Chimeric Antigen Receptor (CAR) T Cell Therapy in Acute Myeloid Leukemia (AML)

Despite high response rates after initial chemotherapy in patients with acute myeloid leukemia (AML), relapses occur frequently, resulting in a five-year-survival by <30% of the patients. Hitherto, allogeneic hemotopoietic stem cell transplantation (allo-HSCT) is the best curative treatment option in intermediate and high risk AML. It is the proof-of-concept for T cell-based immunotherapies in AML based on the graft-versus-leukemia (GvL)-effect, but it also bears the risk of graft-versus-host disease. CD19-targeting therapies employing chimeric antigen receptor (CAR) T cells are a breakthrough in cancer therapy. A similar approach for myeloid malignancies is highly desirable. This article gives an overview on the state-of-the art of preclinical and clinical studies on suitable target antigens for CAR T cell therapy in AML patients.

CAR-Expressing Natural Killer Cells for Cancer Retargeting

Since the approval in 2017 and the outstanding success of Kymriah® and Yescarta®, the number of clinical trials investigating the safety and efficacy of chimeric antigen receptor-modified autologous T cells has been constantly rising. Currently, more than 200 clinical trials are listed on clinicaltrial.gov. In contrast to CAR-T cells, natural killer (NK) cells can be used from allogeneic donors as an "off the shelf product" and provide alternative candidates for cancer retargeting. This review summarises preclinical results of CAR-engineered NK cells using both primary human NK cells and the cell line NK-92, and provides an overview about the first clinical CAR-NK cell studies targeting haematological malignancies and solid tumours, respectively.

[CAR-T cells: Lymphocytes that express a chimeric antigen receptor]

CAR-T cells are genetically modified human lymphocytes and gene therapy medicinal products. They are developed to treat cancers that express a membrane antigen targeted by the CAR. The FDA approved the two first-in-class medicinal products in 2017 and EMA in August 2018; both are autologous CAR-T cells targeting CD19 that is expressed at the surface of normal B-cells throughout their differentiation, and on B-cell lymphoid malignancies. Clinical efficacy was demonstrated for B-cell acute lymphoblastic leukemias, non-Hodgkin's lymphoma and chronic lymphocytic leukemia, although the marketing authorizations are less liberal in terms of indications. Manufacturing of these personalized treatments necessitates that a novel organization and supply chain be set in place, to ensure product preservation, patient safety and compliance with complex regulatory requirements. Side effects are commensurate with clinical efficacy and can be life-threatening: proper management imposes tight coordination between various specialists, particularly between hematologists and intensive care practitioners. High pricing for these treatments is part of a long-term trend for increasing costs of innovations in hematology and oncology; it questions the ability of healthcare systems to sustain their reimbursement.

Generation of CAR-T Cells for Cancer Immunotherapy

T cells engineered with chimeric antigen receptors (CARs) are emerging as powerful cancer immunotherapies. Remarkable efficacies have been demonstrated in treating B-cell malignancies with CAR-T cells, leading to the FDA's first approval of gene therapy. Currently, numerous clinical trials for hematological malignancies and solid tumors are underway worldwide. Production of CAR-T cells with proper qualities is essential for CAR-T success in vivo. Here we detail optimized protocols for the generation of CAR-T cells for preclinical studies using lentiviral gene transfer, expansion of CAR-T cells in culture, detection of CAR expression, and evaluation of CAR-T cellular cytotoxicity in vitro.

Generation of Suicide Gene-Modified Chimeric Antigen Receptor-Redirected T-Cells for Cancer Immunotherapy

Chimeric antigen receptor (CAR)-redirected T-cells are a powerful tool for the treatment of several type of cancers; however, they can cause several adverse effects including cytokine release syndrome, off-target effects resulting in potentially fatal organ damage or even death. Particularly, for CAR T-cells redirected toward acute myeloid leukemia (AML) antigens myelosuppression can be a challenge. The previously validated inducible Caspase9 (iC9) suicide gene system is one of the approaches to control the infused cells in vivo through its activation with a nontherapeutic chemical inducer of dimerizer (CID). We performed a preclinical validation using a model of CD33⁺ AML, and generated iC9 CAR T-cells co-expressing a CAR targeting the AML-associated antigen CD33 and a selectable marker (ΔCD19). ΔCD19 selected (sel.) iC9-CAR.CD33 T-cells were effective in controlling leukemia growth in vitro, and could be partially eliminated (76%) using a chemical inducer of dimerization that activates iC9. Moreover, to completely eliminate residual cells, a second targeted agent was added. Future plans with these methods are to investigate the utility of iC9-CAR.CD33 T-cells as part of the conditioning therapy for an allogeneic hematopoietic stem cell transplant. Additional strategies that we are currently validating include (1) the modulation of the suicide gene activation, using different concentrations of the inducing agent(s), to be able to eliminate CAR T-cells modified by a regulatable gene, ideally aiming at preserving a proportion of the infused cells (and their antitumor activity) for mild to moderate toxicities, or (2) the co-expression of an inhibitory CAR aiming at sparing normal cells co-expressing an antigen not shared with the tumor.

Gemtuzumab ozogamicin for treatment of newly diagnosed CD33-positive acute myeloid leukemia

In September 2017, the US FDA announced re-approval of gemtuzumab ozogamicin (GO), a CD33-targeting immunoconjugate, for treatment of newly diagnosed and relapsed/refractory acute myeloid leukemia (AML). This is a very significant step toward defining new treatment regimens in AML, as the treatment has essentially stayed unchanged with the '7 + 3 induction regimen' (7 days cytarabine and 3 days of anthracycline) since 1973. GO is the first antibody-drug conjugate to receive FDA approval for treating cancer. This review article discusses the challenges faced and lessons learned during the journey of GO for AML treatment. Selected trials that have made significant contribution in our understanding of the most efficacious and safe use of GO for treating AML patients as well as factors influencing GO response are highlighted in this article.

A Unique Human Immunoglobulin Heavy Chain Variable Domain-Only CD33 CAR for the Treatment of Acute Myeloid Leukemia

Acute myeloid leukemia (AML) remains a challenging pediatric and adult disease. Given the elevated expression of the CD33 antigen on leukemic blasts, therapeutic approaches to AML now feature the approved antibody drug conjugate (Mylotarg, GO) and investigational CART cell approaches incorporating CD33-binding domains derived from humanized scFvs. We designed a functional chimeric antigen receptor utilizing a human targeting sequence, derived from a heavy chain variable domain, termed CAR33VH. Lentiviral-based expression vectors which encoded CAR constructs incorporating the novel binding domain (CAR33VH), or the My96 scFv control binder (My96CAR) in frame with a CD8 hinge and transmembrane domain, a 4-1BB costimulatory domain and a CD3 zeta activation domain, were transduced into primary human CD4⁺ and CD8⁺ T cells, and CAR expression was confirmed by flow cytometry. CAR33VH, similar to My96CAR, demonstrated robust and specific cytotoxicity in short-term and long-term co-incubation killing assays against CD33⁺ AML lines. In overnight cytokine release assays in which CAR T cells were challenged with the CD33⁺ tumor cells HL-60, MOLM-14 and KG-1a, CAR33VH elicited IFN-gamma, TNF-alpha and IL-2. This was seen with CD33⁺ cell lines, but not when CAR T were cultured alone. Studies with a CD33⁻ cell line engineered to stably express the full length CD33 variant 1, or the naturally occurring CD33 splice variant 2, revealed that both CAR33VH and My96CAR, target the V domain of CD33, suggesting a similar therapeutic profile. Colony-formation assays utilizing peripheral blood CD34⁺ hematopoietic stem cells treated with CAR33VH, My96CAR, or with an untransduced T cell control, yielded similar numbers of BFU-E erythroid and CFU-GM myeloid colonies, suggesting a lack of CAR-related overt toxicity. In an in vivo AML model, NSG mice engrafted with MOLM-14 cells stably expressing firefly luciferase, both CAR33VH and CARMy96 efficiently eliminated tumors. In conclusion, we demonstrate for the first time the feasibility and efficacy of employing human variable domain-only binder derived from a phage display library in an anti-AML CAR design. CAR33VH, comprised of a human heavy-chain variable fragment-only antigen binding domain, was efficient in tumor killing in vitro and in vivo, and showed comparable functionality to the scFv-based My96CAR.

Acute myeloid leukemia and NK cells: two warriors confront each other

Acute myeloid leukemia (AML) is a heterogeneous disease whose therapies currently show elevated toxicity and a high rate of relapse. Recently, the burgeoning of new anti-tumor therapeutic strategies aimed at enhancing the immune response has pushed natural killer cells (NKs) into the spotlight. These cells are powerful warriors that can bring about the lysis of tumor cells through their cytotoxic ability. However, tumor cells have developed strategies to evade recognition mediated by NKs. Here, we review the mechanisms triggered by AML cells and discuss the emerging immunotherapeutic strategies that potentiate the anti-tumor functions of NKs.

The functional verification of EGFR-CAR T-cells targeted to hypopharyngeal squamous cell carcinoma

BACKGROUND

The aim of this study was to validate the antitumor function of EGFR-chimeric antigen T-cells (CART) targeted to FaDu cells, a hypopharyngeal squamous cell carcinoma cell line, and to provide a preclinical basis for the application of CART cell technology in hypopharyngeal squamous cell carcinoma.

METHODS

Detection of cytokine secretions of EGFR-CAR T and CART-controls in the presence of target cells and nontarget cells as an indicator of CART cell activation. Detection of the cytotoxic effects of EGFR-CAR T on specific tumors in the presence of target cells was evaluated by LDH release and CART cell proliferation.

RESULTS

The results showed that cytokine secretion increased significantly after EGFR-CAR T-cells were incubated with target cells, and EGFR-CAR T-cells has higher cytotoxic effect on target cells than the CART-control group. The target cell lysis rate was 52.66%. The proliferation of EGFR-CAR T-cells in the presence of target cells was not distinctly observed.

CONCLUSION

In this study, we validated the antitumor function of EGFR-CAR T-cells targeted to the FaDu cell line and provided the foundation for application of the CART technique in the treatment of hypopharyngeal carcinoma.

Characterization of clinical grade CD19 chimeric antigen receptor T cells produced using automated CliniMACS Prodigy system

BACKGROUND

Chimeric antigen receptor (CAR) T-cell therapy is highly effective for treating acute lymphoblastic leukemia and non-Hodgkin's lymphoma with high rate complete responses. However, the broad clinical application of CAR T-cell therapy has been challenging, largely due to the lack of widespread ability to produce and high cost of CAR T-cell products using traditional methods of production. Automated cell processing in a closed system has emerged as a potential method to increase the feasibility of producing CAR T cells locally at academic centers due to its minimal reliance on experienced labor, thereby making the process less expensive and more consistent than traditional methods of production.

METHOD

In this study, we describe the successful production of clinical grade CD19 CAR T cells using the Miltenyi CliniMACS Prodigy Automated Cell Processor at University of Colorado Anschutz Medical Campus in a rapid manner with a high frequent CD19 CAR expression.

RESULTS

The final CAR T-cell product is highly active, low in immune suppression, and absent in exhaustion. Full panel cytokine assays also showed elevated production of Th1 cytokines upon IL-2 stimulation when specifically killing CD19+ target cells.

CONCLUSION

These results demonstrate the feasibility of producing CAR T cells locally in a university hospital setting using automated cell processor for future clinical applications.

CD7 CAR T Cells for the Therapy of Acute Myeloid Leukemia

Chimeric antigen receptor (CAR) T cell therapy for the treatment of acute myeloid leukemia (AML) has the risk of toxicity to normal myeloid cells. CD7 is expressed by the leukemic blasts and malignant progenitor cells of approximately 30% of AML patients but is absent on normal myeloid and erythroid cells. Since CD7 expression by malignant blasts is also linked with chemoresistance and poor outcomes, targeting this antigen may be beneficial for this subset of AML patients. Here, we show that expression of a CD7-directed CAR in CD7 gene-edited (CD7^KO) T cells effectively eliminates CD7⁺ AML cell lines, primary CD7⁺ AML, and colony-forming cells but spares myeloid and erythroid progenitor cells and their progeny. In a xenograft model, CD7 CAR T cells protect mice against systemic leukemia, prolonging survival. Our results support the feasibility of using CD7^KO CD7 CAR T cells for the non-myeloablative treatment of CD7⁺ AML.

Chimeric Antigen Receptor-Engineered T Cell Therapy in Acute Myeloid Leukaemia

Acute myeloid leukaemia (AML) is a disease with a very poor outcome and remains an area of significant unmet need, necessitating novel therapeutic strategies. The progress made in the field of immunotherapy, in particular chimeric antigen receptor (CAR)-engineered T cells, has given rise to many hopes for pathologies such as B cell acute lymphoblastic leukaemia and B cell lymphoma, and many studies have attempted to translate these successes to AML. This review summarises the recent advances in, and defines an ideal target for, CAR T cell therapy in AML.

Building upon the success of CART19: chimeric antigen receptor T cells for hematologic malignancies

Chimeric antigen receptor T cell (CART) therapy has dramatically changed the therapeutic prospects for B cell malignancies. Over the last decade CD19-redirected CART have demonstrated the ability to induce deep, long-lasting remissions and possibly cure patients with relapsing B cell neoplasms. Such impressive results with CART19 fostered efforts to expand this technology to other incurable malignancies that naturally do not express CD19, such as acute myeloid leukemia (AML), Hodgkin lymphoma (HL) and multiple myeloma (MM). However, to reach this goal, several hurdles have to be overcome, in particular: (i) the apparent lack of suitable targets as effective as CD19; (ii) the immunosuppressive tumor microenvironment; (iii) intra-tumoral heterogeneity and antigen-negative relapses. Therefore, new strategies that allow safer and more potent CART platforms are under development and may provide grounds for new exciting breakthroughs in the field.

New and emerging therapies for acute myeloid leukaemia

The treatment of acute myeloid leukemia (AML) has remained relatively unchanged for the past 3-4 decades with generally poor outcomes, especially in elderly populations unfit for intensive therapy. Recent advancements, however, have identified several cytogenetic and molecular markers that have not only improved prognostication but have also led to the development of several new targeted therapies for specific subpopulations. In 2017, the US Food and Drug Administration approved four new treatments with indications for fms like tyrosine kinase 3 (FLT3)-mutated AML (midostaurin), newly diagnosed or relapsed/refractory CD33+AML (gemtuzumab ozogamicin), newly diagnosed therapy-related AML or AML with myelodysplasia-related changes (CPX-351) and relapsed/refractory AML with an isocitrate dehydrogenase (IDH)2 mutation (enasidenib). These newly approved therapies have demonstrated improved response in their target populations in several pivotal clinical trials with some also demonstrating improved overall survival. Additional novel therapies in development for AML include agents that target B cell lymphoma 2, FLT3, IDH1, the ubiquitination pathway, as well as cell therapy using engineered T cells with chimeric antigen receptors. This review provides a summary of the four newly approved therapies for AML, as well as several promising therapies currently in development.

Post-remission strategies for the prevention of relapse following allogeneic hematopoietic cell transplantation for high-risk acute myeloid leukemia: expert review from the Acute Leukemia Working Party of the European Society for Blood and Marrow Transplantation

Acute myeloid leukemia (AML) is an aggressive hematopoietic malignancy generally associated with poor prognosis. Allogeneic hematopoietic cell transplantation (alloHCT) continues to be the most potent anti-leukemia treatment for adult patients with intermediate and high-risk AML. However, disease relapse after alloHCT remains unacceptably high and is the primary cause of treatment failure and mortality following alloHCT. It is therefore that post-transplant early cellular or pharmacologic maintenance or preemptive strategies to enhance the graft-versus-leukemia effect or to eradicate persistent minimal residual disease have been of renewed interest, particularly with the availability of more sensitive technologies to measure residual AML. Although preliminary studies have demonstrated improved outcomes with the use of post-alloHCT remission therapies, prospective randomized trials are required to determine their clinical efficacy and role in the treatment of AML. On behalf of the Acute Leukemia Working Party of the European Society for Blood and Marrow Transplantation, we summarize the available evidence on the use and efficacy of available pharmacologic post-remission therapies, including hypomethylating agents, deacetylase inhibitors, and tyrosine kinase inhibitors, as well as cellular therapies, including preemptive and prophylactic donor lymphocyte infusions for the prevention of relapse of AML.

A Novel Anti-LILRB4 CAR-T Cell for the Treatment of Monocytic AML

To effectively improve treatment for acute myeloid leukemia (AML), new molecular targets and therapeutic approaches need to be identified. Chimeric antigen receptor (CAR)-modified T cells targeting tumor-associated antigens have shown promise in the treatment of some malignancies. However, CAR-T cell development for AML has been limited by lack of an antigen with high specificity for AML cells that is not present on normal hematopoietic stem cells, and thus will not result in myelotoxicity. Here we demonstrate that leukocyte immunoglobulin-like receptor-B4 (LILRB4) is a tumor-associated antigen highly expressed on monocytic AML cells. We generated a novel anti-LILRB4 CAR-T cell that displays high antigen affinity and specificity. These CAR-T cells display efficient effector function in vitro and in vivo against LILRB4⁺ AML cells. Furthermore, we demonstrate anti-LILRB4 CAR-T cells are not toxic to normal CD34⁺ umbilical cord blood cells in colony-forming unit assays, nor in a humanized hematopoietic-reconstituted mouse model. Our data demonstrate that anti-LILRB4 CAR-T cells specifically target monocytic AML cells with no toxicity to normal hematopoietic progenitors. This work thus offers a new treatment strategy to improve outcomes for monocytic AML, with the potential for elimination of leukemic disease while minimizing the risk for on-target off-tumor toxicity.

Steering CAR T cells to distinguish friend from foe

CD19-specific chimeric antigen receptor (CAR)⁺ T cells have demonstrated clinical efficacy and long-lasting remissions, concomitant with tolerable normal B-cell aplasia. However, many tumor-associated antigens (TAAs) are expressed on normal tissues, the destruction of which would lead to intolerable toxicity. Thus, there is a need to engineer CAR⁺ T cells with improved safety profiles to restrict toxicity against TAA-expressing normal tissues. Bioengineering approaches include: (i) targeting CAR⁺ T cells to the tumor site, (ii) limiting CAR⁺ T-cell persistence, and (iii) restricting CAR activation. We review and evaluate strategies to engineer CAR⁺ T cells to reduce the potential of on-target, off-tissue toxicity.

Dual-targeting triplebody 33-16-123 (SPM-2) mediates effective redirected lysis of primary blasts from patients with a broad range of AML subtypes in combination with natural killer cells

A number of agents designed for immunotherapy of Acute Myeloid Leukemia (AML) are in preclinical and early clinical development. Most of them target a single antigen on the surface of AML cells. Here we describe the development and key biological properties of a tri-specific agent, the dual-targeting triplebody SPM-2, with binding sites for target antigens CD33 and CD123, and for CD16 to engage NK cells as cytolytic effectors. Primary blasts of nearly all AML patients carry at least one of these target antigens and the pair is particularly promising for the elimination of blasts and leukemia stem cells (LSCs) from a majority of AML patients by dual-targeting agents. The cytolytic activity of NK cells mediated by SPM-2 was analyzed in vitro for primary leukemic cells from 29 patients with a broad range of AML-subtypes. Blasts from all 29 patients, including patients with genomic alterations associated with an unfavorable genetic subtype, were lysed at nanomolar concentrations of SPM-2. Maximum susceptibility was observed for cells with a combined density of CD33 and CD123 above 10,000 copies/cell. Cell populations enriched for AML-LSCs (CD34pos and CD34pos CD38neg cells) from 2 AML patients carried an increased combined antigen density and were lysed at correspondingly lower concentrations of SPM-2 than unsorted blasts. These initial findings raise the expectation that SPM-2 may also be capable of eliminating AML-LSCs and thus of prolonging survival. In the future, patients with a broad range of AML subtypes may benefit from treatment with SPM-2.

Coexpression profile of leukemic stem cell markers for combinatorial targeted therapy in AML

Targeted immunotherapy in acute myeloid leukemia (AML) is challenged by the lack of AML-specific target antigens and clonal heterogeneity, leading to unwanted on-target off-leukemia toxicity and risk of relapse from minor clones. We hypothesize that combinatorial targeting of AML cells can enhance therapeutic efficacy without increasing toxicity. To identify target antigen combinations specific for AML and leukemic stem cells, we generated a detailed protein expression profile based on flow cytometry of primary AML (n = 356) and normal bone marrow samples (n = 34), and a recently reported integrated normal tissue proteomic data set. We analyzed antigen expression levels of CD33, CD123, CLL1, TIM3, CD244 and CD7 on AML bulk and leukemic stem cells at initial diagnosis (n = 302) and relapse (n = 54). CD33, CD123, CLL1, TIM3 and CD244 were ubiquitously expressed on AML bulk cells at initial diagnosis and relapse, irrespective of genetic characteristics. For each analyzed target, we found additional expression in different populations of normal hematopoiesis. Analyzing the coexpression of our six targets in all dual combinations (n = 15), we found CD33/TIM3 and CLL1/TIM3 to be highly positive in AML compared with normal hematopoiesis and non-hematopoietic tissues. Our findings indicate that combinatorial targeting of CD33/TIM3 or CLL1/TIM3 may enhance therapeutic efficacy without aggravating toxicity in immunotherapy of AML.

Novel Approaches to Acute Myeloid Leukemia Immunotherapy

Acute myeloid leukemia (AML) is a rapidly progressive, poor-prognosis malignancy arising from hematopoietic stem/progenitor cells. The long history of successful use of allogeneic hematopoietic cell transplantation (alloHCT) in AML indicates that this disease is immunoresponsive, leading to optimism that novel immunotherapies such as bispecific antibodies, chimeric antigen receptor T cells, and immune checkpoint inhibitors will generate meaningful disease control. However, emerging data on the immunoevasive tactics employed by AML blasts at diagnosis and at relapse indicate that optimism must be tempered by an understanding of this essential paradox. Furthermore, AML has a low mutational burden, thus presenting few neoantigens for attack by autologous T cells, even after attempted reversal of inhibitory receptor/ligand interactions. In this review, we outline the known AML targets, explore immune evasion mechanisms, and describe recent data and current clinical trials of single and combination immunotherapies. Clin Cancer Res; 24(22); 5502-15. ©2018 AACR.

Defining success with cellular therapeutics: the current landscape for clinical end point and toxicity analysis

Cellular therapies play a major and expanding role in the treatment of hematologic diseases. For each of these therapies, a narrow therapeutic window exists, where efficacy is maximized and toxicities minimized. This review focuses on one of the most established cellular therapies, hematopoietic stem cell transplant, and one of the newest cellular therapies, chimeric antigen receptor-T cells. In this review, I will discuss the current state of the field for clinical end point analysis with each of these therapeutics, including their critical toxicities, and focus on the major elements of success for each of these complex treatments for hematologic disease.

Genetic Inactivation of CD33 in Hematopoietic Stem Cells to Enable CAR T Cell Immunotherapy for Acute Myeloid Leukemia

The absence of cancer-restricted surface markers is a major impediment to antigen-specific immunotherapy using chimeric antigen receptor (CAR) T cells. For example, targeting the canonical myeloid marker CD33 in acute myeloid leukemia (AML) results in toxicity from destruction of normal myeloid cells. We hypothesized that a leukemia-specific antigen could be created by deleting CD33 from normal hematopoietic stem and progenitor cells (HSPCs), thereby generating a hematopoietic system resistant to CD33-targeted therapy and enabling specific targeting of AML with CAR T cells. We generated CD33-deficient human HSPCs and demonstrated normal engraftment and differentiation in immunodeficient mice. Autologous CD33 KO HSPC transplantation in rhesus macaques demonstrated long-term multilineage engraftment of gene-edited cells with normal myeloid function. CD33-deficient cells were impervious to CD33-targeting CAR T cells, allowing for efficient elimination of leukemia without myelotoxicity. These studies illuminate a novel approach to antigen-specific immunotherapy by genetically engineering the host to avoid on-target, off-tumor toxicity.

Acute myeloid leukemia chimeric antigen receptor T-cell immunotherapy: how far up the road have we traveled?

Chemotherapy resistance and relapse remain significant sources of mortality for children and adults with acute myeloid leukemia (AML). Further intensification of conventional cytotoxic chemotherapy is likely not feasible due to the severity of acute and long-term side effects upon normal tissues commonly induced by these drugs. Successful development and implementation of new precision medicine treatment approaches for patients with AML, which may improve leukemia remission and diminish toxicity, is thus a major priority. Tumor antigen-redirected chimeric antigen receptor (CAR) T-cell immunotherapies have induced remarkable responses in patients with relapsed or chemorefractory B-lymphoblastic leukemia, and similar strategies are now under early clinical study in adults with relapsed/refractory AML. However, potential on target/off tumor toxicity of AML CAR T-cell immunotherapies, notably aplasia of normal myeloid cells, may limit broader implementation of such approaches. Careful selection of optimal target antigens, consideration of toxicity mitigation strategies, and development of methodologies to circumvent potential CAR T-cell resistance are essential for successful implementation of cellular immunotherapies for patients with high-risk AML.

Targeting FLT3 in acute myeloid leukemia using ligand-based chimeric antigen receptor-engineered T cells

Background

Chimeric antigen receptor-engineered T (CAR-T) cells have extraordinary effect in treating lymphoblastic leukemia. However, treatment of acute myeloid leukemia (AML) using CAR-T cells remains limited to date. Leukemogenesis always relates with the abnormalities of cytogenetics, and nearly one third of AML patients have activating mutations in Fms-like tyrosine kinase 3 (FLT3) which reminded poor prognosis. Considering the FLT3 expressed in AML patients’ blast cells, it may be a new candidate target for CAR-T therapy to treat FLT3⁺ AML, especially patients harboring FLT3-ITD mutation.

Methods

The FLT3L CAR-T using FLT3 ligand as recognizing domain was constructed. The specific cytotoxicity against FLT3⁺ leukemia cell lines, primary AML cells, and normal hematopoietic progenitor stem cells (HPSCs) in vitro were evaluated. In addition, FLT3⁺ AML mouse model was used to assess the effect of FLT3L CAR-T therapy in vivo.

Results

FLT3L CAR-T cells could specifically kill FLT3⁺ leukemia cell lines and AML patients’ bone marrow mononuclear cells in vitro (with or without FLT3 mutation) and have more potent cytotoxicity to FLT3-ITD cells. In a human FLT3⁺ AML xenograft mouse model, FLT3L CAR-T cells could significantly prolong the survival of mice. Furthermore, it was found that FLT3L CAR-T cells could activate the FLT3/ERK signaling pathway of FLT3⁺ leukemia cells with wild-type FLT3; meanwhile, it had no inhibitory effects on the colony formation of CD34⁺ stem cells derived from normal human umbilical cord blood.

Conclusions

The ligand-based FLT3L CAR-T cells could be a promising strategy for FLT3⁺ AML treatment, especially those carried FLT3 mutation.

Electronic supplementary material

The online version of this article (10.1186/s13045-018-0603-7) contains supplementary material, which is available to authorized users.

PI3K orchestration of the in vivo persistence of chimeric antigen receptor-modified T cells

In vivo persistence of chimeric antigen receptor (CAR)-modified T cells correlates with therapeutic efficacy, yet CAR-specific factors that support persistence are not well resolved. Using a CD33-specific CAR in an acute myeloid leukemia (AML) model, we show how CAR expression alters T cell differentiation in a ligand independent manner. Ex vivo expanded CAR-T cells demonstrated decreased naïve and stem memory populations and increased effector subsets relative to vector-transduced control cells. This was associated with reduced in vivo persistence. Decreased persistence was not due to specificity or tumor presence, but to pre-transfer tonic signaling through the CAR CD3ζ ITAMs. We identified activation of the PI3K pathway in CD33 CAR-T cells as responsible. Treatment with a PI3K inhibitor modulated the differentiation program of CAR-T cells, preserved a less differentiated state without affecting T cell expansion, and improved in vivo persistence and reduced tumor burden. These results resolve mechanisms by which tonic signaling of CAR-T cells modulates their fate, and identifies a novel pharmacologic approach to enhance the durability of CAR-T cells for immunotherapy.

Is Disease-Specific Immunotherapy a Potential Reality for MDS?

Myelodysplastic syndromes (MDSs) and related myeloid neoplasms represent heterogeneous diseases with overall poor outcomes related to lack of efficacious agents. Immunotherapy has revolutionized the treatment paradigm in solid malignancies and select hematologic malignancies with durable remissions in treatment-refractory populations. The initial clinical studies using immunotherapy in MDS are under way, although significant efforts are required in which to better understand their role in this disease. Likely, combinations of therapies will be required to improve outcomes in this patient population. Furthermore, identification of novel targets for antibody or T-cell modulation strategies is a matter of intensive investigation and ideally will provide treatment options in the near future for our patients.

Antineoplastic effects of CPPTL via the ROS/JNK pathway in acute myeloid leukemia

Drug resistance and human leukocyte antigen (HLA) matching limit conventional treatment of acute myeloid leukemia (AML). Although several small molecule drugs are clinically used, single drug administration is not sufficient to cure AML, which has a high molecular diversity. Metabolic homeostasis plays a key role in determining cellular fate. Appropriate levels of reactive oxygen species (ROS) maintain the redox system balance, and excessive amounts of ROS cause oxidative damage, thus providing a strategy to eliminate cancer cells. CPPTL is a novel analogue of parthenolide that exhibited significant cytotoxicity to AML cells in vitro and induced apoptosis in a dose-dependent manner. Additionally, CPPTL's prodrug DMA-CPPTL decreased the burden of AML engraftment and prolonged survival in a mouse model administered human primary AML cells in vivo. CPPTL induced apoptosis of AML cells by stimulating ROS production, and accumulation of ROS then activated the JNK pathway, thereby promoting mitochondrial damage. These results demonstrated that CPPTL effectively eradicated AML cells in vitro and in vivo and suggested that CPPTL may be a novel candidate for auxiliary AML therapy.

Immunotherapy with CAR-Modified T Cells: Toxicities and Overcoming Strategies

T cells modified via chimeric antigen receptors (CARs) have emerged as a promising treatment modality. Unparalleled clinical efficacy recently demonstrated in refractory B-cell malignancy has brought this new form of adoptive immunotherapy to the center stage. Nonetheless, its current success has also highlighted its potential treatment-related toxicities. The adverse events observed in the clinical trials are described in this review, after which, some innovative strategies developed to overcome these unwanted toxicities are outlined, including suicide genes, targeted activation, and other novel strategies.

Development of Anti-Human Mesothelin-Targeted Chimeric Antigen Receptor Messenger RNA-Transfected Peripheral Blood Lymphocytes for Ovarian Cancer Therapy

CD19-targeted chimeric antigen receptor (CAR) engineered T/natural killer (NK)-cell therapies can result in durable clinical responses in B-cell malignancies. However, CAR-based immunotherapies have been much less successful in solid cancers, in part due to "on-target off-tumor" toxicity related to expression of target tumor antigens on normal tissue. Based on preliminary observations of safety and clinical activity in proof-of-concept clinical trials, tumor antigen-specific messenger RNA (mRNA) CAR transfection into selected, activated, and expanded T/NK cells may permit prospective control of "on-target off-tumor" toxicity. To develop a commercial product for solid tumors, mesothelin was selected as an antigen target based on its association with poor prognosis and overexpression in multiple solid cancers. It was hypothesized that selecting, activating, and expanding cells ex vivo prior to mRNA CAR transfection would not be necessary, thus simplifying the complexity and cost of manufacturing. Now, the development of anti-human mesothelin mRNA CAR transfected peripheral blood lymphocytes (CARMA-hMeso) is reported, demonstrating the manufacture and cryopreservation of multiple cell aliquots for repeat administrations from a single human leukapheresis. A rapid, automated, closed system for cGMP-compliant transfection of mRNA CAR in up to 20 × 10⁹ peripheral blood lymphocytes was developed. Here we show that CARMA-hMeso cells recognize and lyse tumor cells in a mesothelin-specific manner. Expression of CAR was detectable over approximately 7 days in vitro, with a progressive decline of CAR expression that appears to correlate with in vitro cell expansion. In a murine ovarian cancer model, a single intraperitoneal injection of CARMA-hMeso resulted in the dose-dependent inhibition of tumor growth and improved survival of mice. Furthermore, repeat weekly intraperitoneal administrations of the optimal CARMA-hMeso dose further prolonged disease control and survival. No significant off-target toxicities were observed. These data support further investigation of CARMA-hMeso as a potential treatment for ovarian cancer and other mesothelin-expressing cancers.

Advances in immunotherapy for acute myeloid leukemia

Evasion of the host immune system is a key mechanism to promote malignant progression. Therapeutically targeting immune pathways has radically changed the treatment paradigm for solid and lymphoid tumors but has yet to be approved for myeloid malignancies. Here, we summarize the most recent advances in immunotherapy for acute myeloid leukemia. Topics reviewed here include adoptive cellular approaches (chimeric antigen receptor-T cells, natural killer and other immune cells), checkpoint inhibitors (anti-PD-1/PD-L1, anti-CTLA-4 and TIM-3) and vaccines (WT-1, HLA-A2 and hTERT). Emphasis is placed on agents with clear evidence of tumor-specific immune responses and/or clinical activity in early-phase trials. Despite concerns regarding heterogeneous antigen expression and cytokine release syndrome, immunotherapy remains a highly promising strategy for acute myeloid leukemia, particularly transplant-ineligible patients and minimal residual disease states.

Human c-SRC kinase (CSK) overexpression makes T cells dummy

Adoptive cell therapy with T-cell receptor (TCR)-engineered T cells represents a powerful method to redirect the immune system against tumours. However, although TCR recognition is restricted to a specific peptide-MHC (pMHC) complex, increasing numbers of reports have shown cross-reactivity and off-target effects with severe consequences for the patients. This demands further development of strategies to validate TCR safety prior to clinical use. We reasoned that the desired TCR signalling depends on correct pMHC recognition on the outside and a restricted clustering on the inside of the cell. Since the majority of the adverse events are due to TCR recognition of the wrong target, we tested if blocking the signalling would affect the binding. By over-expressing the c-SRC kinase (CSK), a negative regulator of LCK, in redirected T cells, we showed that peripheral blood T cells inhibited anti-CD3/anti-CD28-induced phosphorylation of ERK, whereas TCR proximal signalling was not affected. Similarly, overexpression of CSK together with a therapeutic TCR prevented pMHC-induced ERK phosphorylation. Downstream effector functions were also almost completely blocked, including pMHC-induced IL-2 release, degranulation and, most importantly, target cell killing. The lack of effector functions contrasted with the unaffected TCR expression, pMHC recognition, and membrane exchange activity (trogocytosis). Therefore, co-expression of CSK with a therapeutic TCR did not compromise target recognition and binding, but rendered T cells incapable of executing their effector functions. Consequently, we named these redirected T cells "dummy T cells" and propose to use them for safety validation of new TCRs prior to therapy.

Retargeting of T lymphocytes to PSCA- or PSMA positive prostate cancer cells using the novel modular chimeric antigen receptor platform technology "UniCAR"

New treatment options especially of solid tumors including for metastasized prostate cancer (PCa) are urgently needed. Recent treatments of leukemias with chimeric antigen receptors (CARs) underline their impressive therapeutic potential. However CARs currently applied in the clinics cannot be repeatedly turned on and off potentially leading to severe life threatening side effects. To overcome these problems, we recently described a modular CAR technology termed UniCAR: UniCAR T cells are inert but can be turned on by application of one or multiple target modules (TMs). Here we present preclinical data summarizing the retargeting of UniCAR T cells to PCa cells using TMs directed to prostate stem cell- (PSCA) or/and prostate specific membrane antigen (PSMA). In the presence of the respective TM(s), we see a highly efficient target-specific and target-dependent activation of UniCAR T cells, secretion of pro-inflammatory cytokines, and PCa cell lysis both in vitro and experimental mice.

Investigational CD33-targeted therapeutics for acute myeloid leukemia

INTRODUCTION

There is long-standing interest in drugs targeting the myeloid differentiation antigen CD33 in acute myeloid leukemia (AML). Positive results from randomized trials with the antibody-drug conjugate (ADC) gemtuzumab ozogamicin (GO) validate this approach. Partly stimulated by the success of GO, several CD33-targeted therapeutics are currently in early phase testing.

AREAS COVERED

CD33-targeted therapeutics in clinical development include Fc-engineered unconjugated antibodies (BI 836858 [mAb 33.1]), ADCs (SGN-CD33A [vadastuximab talirine], IMGN779), radioimmunoconjugates (²²⁵Ac-lintuzumab), bi- and trispecific antibodies (AMG 330, AMG 673, AMV564, 161533 TriKE fusion protein), and chimeric antigen receptor (CAR)-modified immune effector cells. Besides limited data on ²²⁵Ac-lintuzumab showing modest single-agent activity, clinical data are so far primarily available for SGN-CD33A. SGN-CD33A has single-agent activity and has shown encouraging results when combined with an azanucleoside or standard chemotherapeutics. However, concerns about toxicity to the liver and normal hematopoietic cells - the latter leading to early termination of a phase 3 trial - have derailed the development of SGN-CD33A, and its future is uncertain.

EXPERT OPINION

Early results from a new generation of CD33-targeted therapeutics are anticipated in the next 2-3 years. Undoubtedly, re-approval of GO in 2017 has changed the landscape and rendered clinical development for these agents more challenging.

Targeting Leukemia Stem Cells in the Bone Marrow Niche

Abstract: The bone marrow (BM) niche encompasses multiple cells of mesenchymal and hematopoietic origin and represents a unique microenvironment that is poised to maintain hematopoietic stem cells. In addition to its role as a primary lymphoid organ through the support of lymphoid development, the BM hosts various mature lymphoid cell types, including naïve T cells, memory T cells and plasma cells, as well as mature myeloid elements such as monocyte/macrophages and neutrophils, all of which are crucially important to control leukemia initiation and progression. The BM niche provides an attractive milieu for tumor cell colonization given its ability to provide signals which accelerate tumor cell proliferation and facilitate tumor cell survival. Cancer stem cells (CSCs) share phenotypic and functional features with normal counterparts from the tissue of origin of the tumor and can self-renew, differentiate and initiate tumor formation. CSCs possess a distinct immunological profile compared with the bulk population of tumor cells and have evolved complex strategies to suppress immune responses through multiple mechanisms, including the release of soluble factors and the over-expression of molecules implicated in cancer immune evasion. This chapter discusses the latest advancements in understanding of the immunological BM niche and highlights current and future immunotherapeutic strategies to target leukemia CSCs and overcome therapeutic resistance in the clinic.

Engineering chimeric antigen receptor-T cells for cancer treatment

Intratumor heterogeneity of tumor clones and an immunosuppressive microenvironment in cancer ecosystems contribute to inherent difficulties for tumor treatment. Recently, chimeric antigen receptor (CAR) T-cell therapy has been successfully applied in the treatment of B-cell malignancies, underscoring its great potential in antitumor therapy. However, functional challenges of CAR-T cell therapy, especially in solid tumors, remain. Here, we describe cancer-immunity phenotypes from a clonal-stromal-immune perspective and elucidate mechanisms of T-cell exhaustion that contribute to tumor immune evasion. Then we assess the functional challenges of CAR-T cell therapy, including cell trafficking and infiltration, targeted-recognition and killing of tumor cells, T-cell proliferation and persistence, immunosuppressive microenvironment and self-control regulation. Finally, we delineate tumor precision informatics and advancements in engineered CAR-T cells to counteract inherent challenges of the CAR-T cell therapy, either alone or in combination with traditional therapeutics, and highlight the therapeutic potential of this approach in future tumor precision treatment.

CAR T-cells targeting FLT3 have potent activity against FLT3-ITD+ AML and act synergistically with the FLT3-inhibitor crenolanib

FMS-like tyrosine kinase 3 (FLT3) is a transmembrane protein expressed on normal hematopoietic stem and progenitor cells (HSC) and retained on malignant blasts in acute myeloid leukemia (AML). We engineered CD8⁺ and CD4⁺ T-cells expressing a FLT3-specific chimeric antigen receptor (CAR) and demonstrate they confer potent reactivity against AML cell lines and primary AML blasts that express either wild-type FLT3 or FLT3 with internal tandem duplication (FLT3-ITD). We also show that treatment with the FLT3-inhibitor crenolanib leads to increased surface expression of FLT3 specifically on FLT3-ITD⁺ AML cells and consecutively, enhanced recognition by FLT3-CAR T-cells in vitro and in vivo. As anticipated, we found that FLT3-CAR T-cells recognize normal HSCs in vitro and in vivo, and disrupt normal hematopoiesis in colony-formation assays, suggesting that adoptive therapy with FLT3-CAR T-cells will require subsequent CAR T-cell depletion and allogeneic HSC transplantation to reconstitute the hematopoietic system. Collectively, our data establish FLT3 as a novel CAR target in AML with particular relevance in high-risk FLT3-ITD⁺ AML. Further, our data provide the first proof-of-concept that CAR T-cell immunotherapy and small molecule inhibition can be used synergistically, as exemplified by our data showing superior antileukemia efficacy of FLT3-CAR T-cells in combination with crenolanib.

New developments in immunotherapy for pediatric leukemia

PURPOSE OF REVIEW

Immunotherapy for the treatment of cancer has advanced at a tremendous pace over the last decade. In this review, we provide an overview of recent progress in immunotherapy for the treatment of leukemia, focusing on antibody-drug conjugates (ADC), bi-specific T-cell engagers (BiTE), and chimeric antigen receptor (CAR) T cells.

RECENT FINDINGS

Ongoing clinical trials of CAR T cells directed against CD19 have produced complete remission rates as high as 93%, prompting global multicenter phase 2 trials and the first FDA approval of a CAR T-cell therapy. Insights into cytokine release syndrome, a toxicity of CAR T-cell therapy, and the cause for relapse after CAR T-cell therapy are evolving. The bispecific antibody blinatumomab and the ADCs inotuzumab and gemtuzumab have also recently received FDA approval for ALL and AML, respectively, moving these agents into a more prominent role in the relapse setting.

SUMMARY

The use of immunotherapy for leukemia has been successful in creating durable remissions for multiply relapsed and refractory patients who previously had little chance of cure. The ongoing clinical and preclinical work continues to advance our understanding of these immune-based therapies, and will shape the next generation of clinical trials.

Precision therapy for acute myeloid leukemia

Acute myeloid leukemia (AML) is a molecularly and clinically heterogeneous disease. Despite advances in understanding the pathogenesis of AML, the standard therapy remained nearly unchanged over the past three decades. With the poor survival for older patients and high relapse rate, multiple studies are ongoing to address this important issue. Novel therapies for AML, including the refinements of conventional cytotoxic chemotherapies and genetic and epigenetic targeted drugs, as well as immunotherapies, have been developed in recent years. Here, we present a mechanism-based review of some promising new drugs with clinical efficacy, focus on targeted drugs that are most potential to pave the road to success, and put forward the major challenges in promoting the precision therapy for AML.

Retargeting of UniCAR T cells with an in vivo synthesized target module directed against CD19 positive tumor cells

Recent treatments of leukemias with T cells expressing chimeric antigen receptors (CARs) underline their impressive therapeutic potential but also their risk of severe side effects including cytokine release storms and tumor lysis syndrome. In case of cross-reactivities, CAR T cells may also attack healthy tissues. To overcome these limitations, we previously established a switchable CAR platform technology termed UniCAR. UniCARs are not directed against typical tumor-associated antigens (TAAs) but instead against a unique peptide epitope: Fusion of this peptide epitope to a recombinant antibody domain results in a target module (TM). TMs can cross-link UniCAR T cells with tumor cells and thereby lead to their destruction. So far, we constructed TMs with a short half-life. The fast turnover of such a TM allows to rapidly interrupt the treatment in case severe side effects occur. After elimination of most of the tumor cells, however, longer lasting TMs which have not to be applied via continous infusion would be more convenient for the patient. Here we describe and characterize a TM for retargeting UniCAR T cells to CD19 positive tumor cells. Moreover, we show that the TM can efficiently be produced in vivo from producer cells housed in a sponge-like biomimetic cryogel and, thereby, serving as an in vivo TM factory for an extended retargeting of UniCAR T cells to CD19 positive leukemic cells.