A potent tetravalent T-cell-engaging bispecific antibody against CD33 in acute myeloid leukemia

PHE1-based IgG-like antibody platform provides a novel strategy for enhanced T-cell immunotherapy

Introduction

Bispecific antibodies (BsAbs) can simultaneously target two epitopes of different antigenic targets, bringing possibilities for diversity in antibody drug design and are promising tools for the treatment of cancers and other diseases. T-cell engaging bsAb is an important application of the bispecific antibody, which could promote T cell-mediated tumor cell killing by targeting tumor-associated antigen (TAA) and CD3 at the same time.

Methods

This study comprised antibodies purification, Elisa assay for antigen binding, cytotoxicity assays, T cell activation by flow cytometry in vitro and xenogenic tumor model in vivo.

Results

We present a novel bsAb platform named PHE-Ig technique to promote cognate heavy chain (HC)-light chain (LC) pairing by replacing the CH1/CL regions of different monoclonal antibodies (mAbs) with the natural A and B chains of PHE1 fragment of Integrin β2 based on the knob-in-hole (KIH) technology. We had also verified that PHE-Ig technology can be effectively used as a platform to synthesize different desired bsAbs for T-cell immunotherapy. Especially, BCMA×CD3 PHE-Ig bsAbs exhibited robust anti-multiple myeloma (MM) activity in vitro and in vivo.

Discussion

Moreover, PHE1 domain was further shortened with D14G and R41S mutations, named PHE-S, and the PHE-S-based BCMA×CD3 bsAbs also showed anti BCMA+ tumor effect in vitro and in vivo, bringing more possibilities for the development and optimization of different bsAbs. To sum up, PHE1-based IgG-like antibody platform for bsAb construction provides a novel strategy for enhanced T-cell immunotherapy.

Control of acute myeloid leukemia and generation of immune memory in vivo using AMV564, a bivalent bispecific CD33 x CD3 T cell engager

Off-the-shelf immunotherapeutics that suppress tumor growth and provide durable protection against relapse could enhance cancer treatment. We report preclinical studies on a CD33 x CD3 bivalent bispecific diabody, AMV564, that not only suppresses tumor growth, but also facilitates memory responses in a mouse model of acute myelogenous leukemia (AML). Mechanistically, a single 5-day treatment with AMV564 seems to reduce tumor burden by redirection of T cells, providing a time window for allogeneic or other T cells that innately recognize tumor antigens to become activated and proliferate. When the concentration of bispecific becomes negligible, the effector: target ratio has also shifted, and these activated T cells mediate long-term tumor control. To test the efficacy of AMV564 in vivo, we generated a CD33+ MOLM13CG bioluminescent human cell line and optimized conditions needed to control these cells for 62 days in vivo in NSG mice. Of note, not only did MOLM13CG become undetectable by bioluminescence imaging in response to infusion of human T cells plus AMV564, but also NSG mice that had cleared the tumor also resisted rechallenge with MOLM13CG in spite of no additional AMV564 treatment. In these mice, we identified effector and effector memory human CD4+ and CD8+ T cells in the peripheral blood immediately prior to rechallenge that expanded significantly during the subsequent 18 days. In addition to the anti-tumor effects of AMV564 on the clearance of MOLM13CG cells in vivo, similar effects were seen when primary CD33+ human AML cells were engrafted in NSG mice even when the human T cells made up only 2% of the peripheral blood cells and AML cells made up 98%. These studies suggest that AMV564 is a novel and effective bispecific diabody for the targeting of CD33+ AML that may provide long-term survival advantages in the clinic.

Heterodimerization of T cell engaging bispecific antibodies to enhance specificity against pancreatic ductal adenocarcinoma

BACKGROUND

EGFR and/or HER2 expression in pancreatic cancers is correlated with poor prognoses. We generated homodimeric (EGFRxEGFR or HER2xHER2) and heterodimeric (EGFRxHER2) T cell-engaging bispecific antibodies (T-BsAbs) to direct polyclonal T cells to these antigens on pancreatic tumors.

METHODS

EGFR and HER2 T-BsAbs were constructed using the 2 + 2 IgG-[L]-scFv T-BsAbs format bearing two anti-CD3 scFvs attached to the light chains of an IgG to engage T cells while retaining bivalent binding to tumor antigens with both Fab arms. A Fab arm exchange strategy was used to generate EGFRxHER2 heterodimeric T-BsAb carrying one Fab specific for EGFR and one for HER2. EGFR and HER2 T-BsAbs were also heterodimerized with a CD33 control T-BsAb to generate 'tumor-monovalent' EGFRxCD33 and HER2xCD33 T-BsAbs. T-BsAb avidity for tumor cells was studied by flow cytometry, cytotoxicity by T-cell mediated 51Chromium release, and in vivo efficacy against cell line-derived xenografts (CDX) or patient-derived xenografts (PDX). Tumor infiltration by T cells transduced with luciferase reporter was quantified by bioluminescence.

RESULTS

The EGFRxEGFR, HER2xHER2, and EGFRxHER2 T-BsAbs demonstrated high avidity and T cell-mediated cytotoxicity against human pancreatic ductal adenocarcinoma (PDAC) cell lines in vitro with EC50s in the picomolar range (0.17pM to 18pM). They were highly efficient in driving human polyclonal T cells into subcutaneous PDAC xenografts and mediated potent T cell-mediated anti-tumor effects. Both EGFRxCD33 and HER2xCD33 tumor-monovalent T-BsAbs displayed substantially reduced avidity by SPR when compared to homodimeric EGFRxEGFR or HER2xHER2 T-BsAbs (∼150-fold and ∼6000-fold respectively), tumor binding by FACS (8.0-fold and 63.6-fold), and T-cell mediated cytotoxicity (7.7-fold and 47.2-fold), while showing no efficacy against CDX or PDX. However, if either EGFR or HER2 was removed from SW1990 by CRISPR-mediated knockout, the in vivo efficacy of heterodimeric EGFRxHER2 T-BsAb was lost.

CONCLUSION

EGFR and HER2 were useful targets for driving T cell infiltration and tumor ablation. Two arm Fab binding to either one or both targets was critical for robust anti-tumor effect in vivo. By engaging both targets, EGFRxHER2 heterodimeric T-BsAb exhibited potent anti-tumor effects if CDX or PDX were EGFR+HER2+ double-positive with the potential to spare single-positive normal tissue.

Binding domain on CD22 molecules contributing to the biological activity of T cell-engaging bispecific antibodies

CD22, as the B-cell malignancies antigen, has been targeted for immunotherapies through CAR-T cells, antibody-drug conjugates (ADCs) and immunotoxins via interaction of antibodies with binding domains on the receptor. We hypothesized that avidity and binding domain of antibody to target cells may have significant impact on the biological function in tumor immunotherapy, and T cell-engaging bispecific antibody (TCB) targeting CD22 could be used in the therapy of hematologic malignancies. So, to address the question, we utilized the information of six previously reported CD22 mAbs to generate CD22-TCBs with different avidity to different domains on CD22 protein. We found that the avidity of CD22-TCBs to protein was not consistent with the avidity to target cells, indicating that TCBs had different binding mode to the protein and cells. In vitro results indicated that CD22-TCBs mediated cytotoxicity depended on the avidity of antibodies to target cells rather than to protein. Moreover, distal binding domain of the antigen contributed to the avidity and biological activity of IgG-[L]-scfv-like CD22-TCBs. The T cells' proliferation, activation, cytotoxicity as well as cytokine release were compared, and G5/44 BsAb was selected for further in vivo assessment in anti-tumor activity. In vivo results demonstrated that CD22-TCB (G5/44 BsAb) significantly inhibited the tumors growth in mice. All these data suggested that CD22-TCBs could be developed as a promising candidate for B-cell malignancies therapy through optimizing the design with avidity and binding domain to CD22 target in consideration.

Tracking Bispecific Antibody-Induced T Cell Trafficking Using Luciferase-Transduced Human T Cells

T cell-engaging bispecific antibodies (T-BsAbs) are in various stages of preclinical development and clinical testing for solid tumors. Factors such as valency, spatial arrangement, interdomain distance, and Fc mutations affect the anti-tumor efficacy of these therapies, commonly by influencing the homing of T cells to tumors, which remains a major challenge. Here, we describe a method to transduce activated human T cells with luciferase, allowing in vivo tracking of T cells during T-BsAb therapy studies. The ability of T-BsAbs to redirect T cells to tumors can be quantitatively evaluated at multiple time points during treatment, allowing researchers to correlate the anti-tumor efficacy of T-BsAbs and other interventions with the persistence of T cells in tumors. This method alleviates the need to sacrifice animals during treatment to histologically assess T cell infiltration and can be repeated at multiple time points to determine the kinetics of T cell trafficking during and after treatment.

A high throughput bispecific antibody discovery pipeline

Bispecific antibodies (BsAbs) represent an emerging class of immunotherapy, but inefficiency in the current discovery has limited their broad clinical availability. Here we report a high throughput, agnostic, single-cell-based functional screening pipeline, comprising molecular and cell engineering for efficient generation of BsAb library cells, followed by functional interrogation at the single-cell level to identify and sort positive clones and downstream sequence identification and functionality characterization. Using a CD19xCD3 bispecific T cell engager (BiTE) as a model, we demonstrate that our single-cell platform possesses a high throughput screening efficiency of up to one and a half million variant library cells per run and can isolate rare functional clones at a low abundance of 0.008%. Using a complex CD19xCD3 BiTE-expressing cell library with approximately 22,300 unique variants comprising combinatorially varied scFvs, connecting linkers and VL/VH orientations, we have identified 98 unique clones, including extremely rare ones (~ 0.001% abundance). We also discovered BiTEs that exhibit novel properties and insights to design variable preferences for functionality. We expect our single-cell platform to not only increase the discovery efficiency of new immunotherapeutics, but also enable identifying generalizable design principles based on an in-depth understanding of the inter-relationships between sequence, structure, and function.

Identification of immunotherapy and radioimmunotherapy targets on desmoplastic small round cell tumors

Background

Development of successful antibody-based immunotherapeutic and radioimmunotherapeutic strategies rely on the identification of cell surface tumor-associated antigens (TAA) with restricted expression on normal tissues. Desmoplastic small round cell tumor (DSRCT) is a rare and generally neglected malignancy that primarily affects adolescent and young adult males. New therapies capable of treating disseminated disease are needed for DSRCT, which is often widespread at diagnosis.

Methods

We used immunohistochemistry (IHC) on fresh frozen surgical specimens and patient-derived xenograft (PDX) tumors and flow cytometry on DSRCT cell lines to evaluate expression of TAAs in these tumors. In vitro cytotoxicity assays were used to evaluate the efficacy of T cell-engaging bispecific antibodies (T-BsAbs) directed at these targets. In vivo, we used an intraperitoneal xenograft mouse model of DSRCT to test T-BsAbs against several TAAs.

Results

In DSRCT specimens we found widespread expression of B7-H3, EGFR, GD2, HER2, mesothelin, and polysialic acid, clinical targets for which specific antibody therapeutics are available. The expression of B7-H3, EGFR, HER2, and mesothelin was confirmed on the cell surface of DSRCT cell lines. In vitro cytotoxicity assays confirmed the efficacy of T cell-engaging bispecific antibodies (T-BsAbs) directed at these targets against DSRCT cells. Remarkably, a HER2xCD3 T-BsAb was capable of completely shrinking established tumors in an intraperitoneal mouse model of DSRCT.

Conclusions

We propose that these TAAs should be further investigated in preclinical models as targets for immunotherapy and radioimmunotherapy with the hope of providing a rationale to extend these therapies to patients with advanced DSRCT.

Shortened Hinge Design of Fab x sdAb-Fc Bispecific Antibodies Enhances Redirected T-Cell Killing of Tumor Cells

T cell engager (TCE) antibodies have emerged as promising cancer therapeutics that link cytotoxic T-cells to tumor cells by simultaneously binding to CD3E on T-cells and to a tumor-associated antigen (TAA) expressed by tumor cells. We previously reported a novel bispecific format, the IgG-like Fab x sdAb-Fc (also known as half-IG_VH-h-CH2-CH3), combining a conventional antigen-binding fragment (Fab) with a single domain antibody (sdAb). Here, we evaluated this Fab x sdAb-Fc format as a T-cell redirecting bispecific antibody (TbsAbs) by targeting mEGFR on tumor cells and mCD3E on T cells. We focused our attention specifically on the hinge design of the sdAb arm of the bispecific antibody. Our data show that a TbsAb with a shorter hinge of 23 amino acids (TbsAb.short) showed a significantly better T cell redirected tumor cell elimination than the TbsAb with a longer, classical antibody hinge of 39 amino acids (TbsAb.long). Moreover, the TbsAb.short form mediated better T cell-tumor cell aggregation and increased CD69 and CD25 expression levels on T cells more than the TbsAb.long form. Taken together, our results indicate that already minor changes in the hinge design of TbsAbs can have significant impact on the anti-tumor activity of TbsAbs and may provide a new means to improve their potency.

Antibody variable region engineering for improving cancer immunotherapy

The efficacy and specificity of conventional monoclonal antibody (mAb) drugs in the clinic require further improvement. Currently, the development and application of novel antibody formats for improving cancer immunotherapy have attracted much attention. Variable region-retaining antibody fragments, such as antigen-binding fragment (Fab), single-chain variable fragment (scFv), bispecific antibody, and bi/trispecific cell engagers, are engineered with humanization, multivalent antibody construction, affinity optimization and antibody masking for targeting tumor cells and killer cells to improve antibody-based therapy potency, efficacy and specificity. In this review, we summarize the application of antibody variable region engineering and discuss the future direction of antibody engineering for improving cancer therapies.

CD33 ‐targeting extracellular vesicles deliver antisense oligonucleotides against FLT3‐ITD and miR ‐125b for specific treatment of acute myeloid leukaemia

Introduction

Acute Myeloid Leukaemia (AML) is the most common blood cancer in adults. Although 2 out of 3 AML patients go into total remission after chemotherapies and targeted therapies, the disease recurs in 60%–65% of younger adult patients within 3 years after diagnosis with a dramatically decreased survival rate. Therapeutic oligonucleotides are promising treatments under development for AML as they can be designed to silence oncogenes with high specificity and flexibility. However, there are not many well validated approaches for safely and efficiently delivering oligonucleotide drugs. This issue could be resolved by utilizing a new generation of delivery vehicles such as extracellular vesicles (EVs).

Methods

In this study, we harness red blood cell‐derived EVs (RBCEVs) and engineer them via exogenous drug loading and surface functionalization to develop an efficient drug delivery system for AML. Particularly, EVs are designed to target CD33, a common surface marker with elevated expression in AML cells via the conjugation of a CD33‐binding monoclonal antibody onto the EV surface.

Results

The conjugation of RBCEVs with the CD33‐binding antibody significantly increases the uptake of RBCEVs by CD33‐positive AML cells, but not by CD33‐negative cells. We also load CD33‐targeting RBCEVs with antisense oligonucleotides (ASOs) targeting FLT3‐ITD or miR‐125b, 2 common oncogenes in AML, and demonstrate that the engineered EVs improve leukaemia suppression in in vitro and in vivo models of AML.

Conclusion

Targeted RBCEVs represent an innovative, efficient, and versatile delivery platform for therapeutic ASOs and can expedite the clinical translation of oligonucleotide drugs for AML treatments by overcoming current obstacles in oligonucleotide delivery.

Relapsed acute myeloid leukemia in children and adolescents: current treatment options and future strategies

Pediatric acute myeloid leukemia (AML) develops from clonal expansion of hematopoietic precursor cells and is characterized by morphologic and cytomolecular heterogeneity. Although the past 40 years have seen significant improvements in overall survival, the prevailing treatment challenges in pediatric AML are the prevention of relapse and the management of relapsed disease. Approximately 25% of children and adolescents with AML suffer disease relapse and face a poor prognosis. Our greater understanding of the genomic, epigenomic, metabolomic, and immunologic pathophysiology of relapsed AML allows for better therapeutic strategies that are being developed for pediatric clinical trials. The development of biologically rational agents is critical as conventional chemotherapeutic salvage regimens are not effective for all patients and pose risk of organ toxicity in heavily pretreated patients. Another major barrier to improvement in outcomes for relapsed pediatric AML is the historic lack of availability and participation in clinical trials. There are ongoing efforts to launch multinational clinical trials of emerging therapies. The purpose of this review is to summarize currently available and newly developed therapies for relapsed pediatric AML.

Bispecific Antibody-Based Immune-Cell Engagers and Their Emerging Therapeutic Targets in Cancer Immunotherapy

Cancer is the second leading cause of death worldwide after cardiovascular diseases. Harnessing the power of immune cells is a promising strategy to improve the antitumor effect of cancer immunotherapy. Recent progress in recombinant DNA technology and antibody engineering has ushered in a new era of bispecific antibody (bsAb)-based immune-cell engagers (ICEs), including T- and natural-killer-cell engagers. Since the first approval of blinatumomab by the United States Food and Drug Administration (US FDA), various bsAb-based ICEs have been developed for the effective treatment of patients with cancer. Simultaneously, several potential therapeutic targets of bsAb-based ICEs have been identified in various cancers. Therefore, this review focused on not only highlighting the action mechanism, design and structure, and status of bsAb-based ICEs in clinical development and their approval by the US FDA for human malignancy treatment, but also on summarizing the currently known and emerging therapeutic targets in cancer. This review provides insights into practical considerations for developing next-generation ICEs.

Characterization of a novel bispecific antibody targeting tissue factor-positive tumors with T cell engagement

T cell engaging bispecific antibody (TCB) is an effective immunotherapy for cancer treatment. Through co-targeting CD3 and tumor-associated antigen (TAA), TCB can redirect CD3⁺ T cells to eliminate tumor cells regardless of the specificity of T cell receptor. Tissue factor (TF) is a TAA that involved in tumor progression. Here, we designed and characterized a novel TCB targeting TF (TF-TCB) for the treatment of TF-positive tumors. In vitro, robust T cell activation, tumor cell lysis and T cell proliferation were induced by TF-TCB. The tumor cell lysis activity was dependent upon both CD3 and TF binding moieties of the TF-TCB, and was related to TF expression level of tumor cells. In vivo, in both tumor cell/human peripheral blood mononuclear cells (PBMC) co-grafting model and established tumor models with poor T cell infiltration, tumor growth was strongly inhibited by TF-TCB. T cell infiltration into tumors was induced during the treatment. Furthermore, efficacy of TF-TCB was further improved by combination with immune checkpoint inhibitors. For the first time, our results validated the feasibility of using TF as a target for TCB and highlighted the potential for TF-TCB to demonstrate efficacy in solid tumor treatment.

Overcoming tumor heterogeneity by ex vivo arming of T cells using multiple bispecific antibodies

BACKGROUND

Tumorous heterogeneity is a hallmark of tumor evolution and cancer progression, being a longstanding challenge to targeted immunotherapy. Ex vivo armed T cells (EATs) using IgG-(L)-scFv bispecific antibodies (BsAbs) are potent tumor-specific cytotoxic effectors. To improve the anti-tumor efficacy of EATs against heterogeneous solid tumors, we explored multi-antigen targeting approaches.

METHODS

Ex vivo expanded T cells were armed with BsAbs built on the IgG-(L)-scFv platform, where an anti-CD3 (huOKT3) scFv was attached to the carboxyl end of both light chains of a tumor specific IgG. Multispecificity was created by combining monospecific EATs, combining BsAbs on the same T cell, or combining specificities on the same antibody. Three multi-antigens targeting EAT strategies were tested: (1) pooled-EATs (EATs each with unique specificity administered simultaneously) or alternate-EATs (EATs each with unique specificity administered in an alternating schedule), (2) dual-EATs or multi-EATs (T cells simultaneously armed with ≥2 BsAbs), and (3) TriAb-EATs (T cells armed with BsAb specific for two targets besides CD3 (TriAb)). The properties and efficiencies of these three strategies were evaluated by flow cytometry, in vitro cytotoxicity, cytokine release assays, and in vivo studies performed in BALB-Rag2 ^-/-IL-2R-γc-KO (BRG) mice xenografted with cancer cell line (CDX) or patient-derived tumor (PDX).

RESULTS

Multi-EATs retained target antigen specificity and anti-tumor potency. Cytokine release with multi-EATs in the presence of tumor cells was substantially less than when multiple BsAbs were mixed with unarmed T cells. When tested against CDXs or PDXs, dual-EATs or multi-EATs effectively suppressed tumor growth without clinical toxicities. Most importantly, dual-EATs or multi-EATs were highly efficient in preventing clonal escape while mono-EATs or TriAb- EATs were not as effective.

CONCLUSIONS

Multi-EATs have the potential to increase potency, reduce toxicity, and overcome tumor heterogeneity without excessive cytokine release. Arming T cells with multiple BsAbs deserves further exploration to prevent or to treat cancer resistance.

Novel potent anti-STEAP1 bispecific antibody to redirect T cells for cancer immunotherapy

BACKGROUND

The prognosis for metastatic Ewing sarcoma family of tumors (EFT) is still poor despite high-dose chemotherapy and radiation treatment. Immunotherapies hold promise, but cancer antigen-targeting immunotherapies have largely failed to induce effective T cell receptor-mediated antitumor response. However, T cell-engaging bispecific antibodies (T-BsAbs) have yet to be adequately explored.

METHODS

Rehumanized STEAP1-IgG was used to build T-BsAb (named BC261) using the 2+2 IgG-[L]-scFv platform carrying the anti-CD3 huOKT3 scFv as the second specificity. Its binding epitope mapping, species cross-reactivity, tumor cell line staining, and in vitro cytotoxicity were investigated thoroughly. Its potency in driving tumor-infiltrating lymphocytes (TILs) was quantified using bioluminescence, correlated with in vivo antitumor response against cell line-derived or patient-derived xenografts (CDXs or PDXs) and compared with anti-STEAP1 T-BsAbs built on representative antibody platforms.

RESULTS

BC261 binding epitope was mapped to its second extracellular domain of STEAP1 shared among canine and primate orthologs. BC261 induced potent cytotoxicity against panels of EFT, prostate cancer, and canine osteosarcoma cell lines despite their low antigen density. BC261 drove significantly more TILs into tumors (30-fold) and exerted superior antitumor effects compared with the other standard BsAb platforms. The antitumor efficacy of BC261 was consistent against EFT and prostate cancer CDXs and PDXs.

CONCLUSIONS

BC261 was highly efficient in driving T cell infiltration and tumor ablation. Either as stand-alone therapeutics or for ex vivo armed T cells, this novel anti-STEAP1 T-BsAb BC261 has therapeutic potential.

Structural design of tetravalent T-cell engaging bispecific antibodies: improve developability by engineering disulfide bonds

Since the advances in protein engineering and manufacture, over the last 30 years, antibody-based immunotherapeutic has become a powerful strategy to treat diseases. The T-cell engaging bispecific antibody (BsAb) by combining the Fab binding domain of tumor antigens and Fab or single-chain variable fragments (scFvs) binding domain of CD3 molecules, could redirect cytotoxic T cells to kill tumor cells. The IgG-scFv format of BsAb is a dual bivalent and asymmetrical design, which adds the benefit of potent cytotoxicity and less complicated for manufacture but limits the stability and production. Here, we engineered a series of interchain disulfide bonds in the Fab region of IgG-svFv BsAbs and evaluated its biophysical and biological properties. We found that simultaneously replaced the position of VH44-VL100 and CH1126-CL121 residues with cysteine, to form two additional disulfide bonds, could markedly increase monomeric BsAb formation and yield. The thermostability and stability against aggregation and degradation also performed better than BsAbs without extra disulfide bonds introduction. Besides, the affinity of engineered BsAbs was maintained, and the h8B-BsAb antibody had a slight enhancement in an inhibitory effect on target cells.

Interdomain spacing and spatial configuration drive the potency of IgG-[L]-scFv T cell bispecific antibodies

T cell-bispecific antibodies (BsAbs) couple cytotoxic T lymphocytes to tumor cells, inducing their destruction. Although there are more than 60 classes of BsAbs in development, the relative importance of parameters such as interdomain spacing or spatial configuration is largely unknown. Here, we dissected a symmetric dual bivalent BsAb platform (IgG-[L]-scFv: antitumor IgG with anti-CD3 scFv fused to the light chains) to explore the importance of valency and spatial configuration for BsAb-induced T cell cytotoxicity. Our results revealed that placing tumor and T cell binding domains on the same side of a BsAb (cis-configuration) elicited substantially stronger antitumor activity, in vitro and in vivo, compared to positioning them on opposite sides (trans-configuration). Moreover, using two cis-modules in the same BsAb further improved cytotoxicity (up to 2000-fold). In addition, separating antigen-binding components with a single Ig domain (C_L) markedly enhanced cytokine release and in vivo tumor responses compared to smaller (G₄S₁) or larger (C_H1-C_H2-C_H3) spacers. These findings provide guidelines for improving BsAb function and highlight the importance of spatial configuration and dual bivalency as development parameters.

BiTEs, DARTS, BiKEs and TriKEs-Are Antibody Based Therapies Changing the Future Treatment of AML?

Nearly four decades after their conceptualization, antibody-based therapies are slowly being added to the treatment landscape of acute myeloid leukemia (AML). While the antibody-drug conjugate gemtuzumab ozogamicin is the only antibody-based therapy that has been approved for AML treatment thus far, several bispecific antibodies have been developed and shown early encouraging results. Bispecific antibodies comprise a wide variety of constructs that share the common concept of simultaneous binding of a surface target on malignant cells and most commonly CD3 on T cells leading to an endogenous, HLA-independent, immune response against malignant cells. However, the use of bispecific antibodies in AML has been limited by the absence of highly specific leukemia-associated antigens leading to on-target, off-leukemia side effects as well as reduced efficacy due to antigen escape. Herein, we discuss the history and evolution of bispecific T cell engagers as well as various adaptations such as dual affinity retargeting antibodies, bi- and tri-specific killer engager antibodies. Common side effects including cytokine release syndrome and management thereof are highlighted. Lastly, we expound on the future direction and integration of such antibody-based therapies with other immunotherapies (programmed cell death-1 inhibitors and chimeric antigen receptor T cells).

Potent ex vivo armed T cells using recombinant bispecific antibodies for adoptive immunotherapy with reduced cytokine release

BACKGROUND

T cell-based immunotherapies using chimeric antigen receptors (CAR) or bispecific antibodies (BsAb) have produced impressive responses in hematological malignancies. However, major hurdles remained, including cytokine release syndrome, neurotoxicity, on-target off-tumor effects, reliance on autologous T cells, and failure in most solid tumors. BsAb armed T cells offer a safe alternative.

METHODS

We generated ex vivo armed T cells (EATs) using IgG-[L]-scFv-platformed BsAb, where the anti-CD3 (huOKT3) scFv was attached to the light chain of a tumor-binding IgG. BsAb density on EAT, in vitro cytotoxicity, cytokine release, in vivo trafficking into tumors, and their antitumor activities were evaluated in multiple cancer cell lines and patient-derived xenograft mouse models. The efficacy of EATs after cryopreservation was studied, and gamma delta (γδ) T cells were investigated as unrelated alternative effector T cells.

RESULTS

The antitumor potency of BsAb armed T cells was substantially improved using the IgG-[L]-scFv BsAb platform. When compared with separate BsAb and T cell injection, EATs released less TNF-α, and infiltrated tumors faster, while achieving robust antitumor responses. The in vivo potency of EAT therapy depended on BsAb dose for arming, EAT cell number per injection, total number of EAT doses, and treatment schedule intensity. The antitumor efficacy of EATs was preserved following cryopreservation, and EATs using γδ T cells were safe and as effective as αβ T cell-EATs.

CONCLUSIONS

EATs exerted potent antitumor activities against a broad spectrum of human cancer targets with remarkable safety. The antitumor potency of EATs depended on BsAb dose, cell number and total dose, and schedule. EATs were equally effective after cryopreservation, and the feasibility of third-party γδ-EATs offered an alternative for autologous T cell sources.

T cell engaging bispecific antibodies targeting CD33 IgV and IgC domains for the treatment of acute myeloid leukemia

BACKGROUND

Acute myeloid leukemia (AML) remains one of the most challenging hematological malignancies. Despite progress in therapeutics, majority of patients succumb to this neoplasm. CD33 is a proven therapeutic target, given its expression on most AML cells. Almost all anti-CD33 antibodies target the membrane distal immunoglobulin V (IgV) domain of the CD33 extracellular domain.

METHODS

In this manuscript, we present data on three bispecific antibodies (BsAbs) against the CD33 IgV and membrane proximal immunoglobulin C (IgC) domains. We use in vitro binding and cytotoxicity assays to show the effect of these BsAbs on AML cell lines. We also use immunodeficient mice-bearing leukemias from cell lines and patient-derived xenografts to show the effect of these BsAbs in vivo.

RESULTS

In vitro, the IgV-targeting BsAb had higher binding to AML cell lines using flow cytometry and delivered more potent cytotoxicity in T-cell-dependent cytotoxicity assays; importantly, the IgC domain-targeting outperformed the IgV domain-targeting BsAb in medullary and extramedullary leukemia animal models.

CONCLUSIONS

These data support further clinical development of this BsAb for first-in-human phase I clinical trial.

Development of a Tetravalent T-Cell Engaging Bispecific Antibody Against Glypican-3 for Hepatocellular Carcinoma

Cancer therapies benefit from accelerated development of biotechnology, and many immunotherapeutic strategies spring up including vaccines, the immune checkpoint blockade, chimeric antigen receptor T cells, and bispecific antibodies (BsAbs). Glypican-3 (GPC3) is a member of the heparan sulfate proteoglycan family of proteins and is highly expressed in hepatocellular carcinoma (HCC) cell membranes. Here, the authors describe a new tetravalent BsAb h8B-BsAb targeting GPC3 and CD3 antigens and studied its antitumor activities against HCC. h8B-BsAb was designed based on immunoglobulin G with a fragment variable fused to the light chain, whose biophysical stabilities including degradation resistance and thermostability were improved by introducing disulfide bonds. In vitro activity of h8B-BsAb showed potent T-cell recruitment and activation for HCC cell lysis by the presence of peripheral blood mononuclear cells, but no specific killing in GPC3-negative cells. In HCC xenograft mouse studies, h8B-BsAb induced robust regression of tumors. In summary, we engineered a highly stable and efficacious BsAb as a potential candidate for HCC treatment.

GD2 or HER2 targeting T cell engaging bispecific antibodies to treat osteosarcoma

BACKGROUND

The cure rate for metastatic osteosarcoma has not substantially improved over the past decades. Clinical trials of anti-HER2 trastuzumab or anti-GD2 dinutuximab for metastatic or refractory osteosarcoma were not successful, and neither was immune checkpoint inhibitors (ICIs).

METHODS

We tested various target antigen expressions on osteosarcoma cell lines using flow cytometry and analyzed in vitro T cell engaging BsAb (T-BsAb)-dependent T cell-mediated cytotoxicity using 4-h ⁵¹Cr release assay. We tested in vivo anti-tumor activities of T-BsAb targeting GD2 or HER2 in established osteosarcoma cell line or patient-derived xenograft (PDX) mouse models carried out in BALB-Rag2^-/-IL-2R-γc-KO (BRG) mice. We also generated ex vivo BsAb-armed T cells (EATs) and studied their tumor-suppressive effect against osteosarcoma xenografts. In order to improve the anti-tumor response, ICIs, anti-human PD-1 (pembrolizumab) or anti-human PD-L1 (atezolizumab) antibodies were tested their synergy with GD2- or HER2-BsAb against osteosarcoma.

RESULTS

GD2 and HER2 were chosen from a panel of surface markers on osteosarcoma cell lines and PDXs. Anti-GD2 BsAb or anti-HER2 BsAb exerted potent anti-tumor effect against osteosarcoma tumors in vitro and in vivo. T cells armed with anti-GD2-BsAb (GD2-EATs) or anti-HER2-BsAb (HER2-EATs) showed significant anti-tumor activities as well. Anti-PD-L1 combination treatment enhanced BsAb-armed T cell function in vivo and improved tumor control and survival of the mice, when given sequentially and continuously.

CONCLUSION

Anti-GD2 and anti-HER2 BsAbs were effective in controlling osteosarcoma. These data support the clinical investigation of GD2 and HER2 targeted T-BsAb treatment in combination with immune checkpoint inhibitors, particularly anti-PD-L1, in patients with osteosarcoma to improve their treatment outcome.

Overcoming leukemia heterogeneity by combining T cell engaging bispecific antibodies

Background

Leukemia represents about 5% of all human cancers. Despite advances in therapeutics, a substantial number of patients succumb to the disease. Several subtypes of leukemia are inherently more resistant to treatment despite intensive chemotherapy or targeted therapy.

Methods

Here we describe the generation of T cell engaging (CD3) bispecific antibodies (BsAbs) built on humanized IgG frameworks using the IgG(L)-scFv format against two targets expressed on acute lymphoblastic leukemia (ALL) and on acute myeloid leukemia (AML).

Results

Each BsAb mediated potent anti-leukemia effect against ALL (CD19) and AML (CD33) in vitro and in xenograft models. Importantly, the CD19-specific BsAb (BC250) was effective against hematogenous spread preventing metastases to liver and kidney in mice bearing ALL and Burkitt’s lymphoma xenografts. BC250 was more potent than the The Food and Drug Administration (FDA)-approved BsAb blinatumomab against ALL xenografts in vivo as measured by tumor bioluminescence and mouse survival. Furthermore, the combination of the CD19 and CD33 BsAbs in two xenograft models of mixed phenotype acute leukemia (biphenotypic and bilineal leukemia) was far superior than monotherapy with either of the BsAbs alone.

Conclusions

Selective combinations of these leukemia-specific BsAb offer the potential to overcome tumor heterogeneity or clonal escape in the modern era of antibody-based T cell-driven immunotherapy.

Cell-based and antibody-mediated immunotherapies directed against leukemic stem cells in acute myeloid leukemia: Perspectives and open issues

Despite new insights in molecular features of leukemic cells and the availability of novel treatment approaches and drugs, acute myeloid leukemia (AML) remains a major clinical challenge. In fact, many patients with AML relapse after standard therapy and eventually die from progressive disease. The basic concept of leukemic stem cells (LSC) has been coined with the goal to decipher clonal architectures in various leukemia-models and to develop curative drug therapies by eliminating LSC. Indeed, during the past few years, various immunotherapies have been tested in AML, and several of these therapies follow the strategy to eliminate relevant leukemic subclones by introducing LSC-targeting antibodies or LSC-targeting immune cells. These therapies include, among others, new generations of LSC-eliminating antibody-constructs, checkpoint-targeting antibodies, bi-specific antibodies, and CAR-T or CAR-NK cell-based strategies. However, responses are often limited and/or transient which may be due to LSC resistance. Indeed, AML LSC exhibit multiple forms of resistance against various drugs and immunotherapies. An additional problems are treatment-induced myelotoxicity and other side effects. The current article provides a short overview of immunological targets expressed on LSC in AML. Moreover, cell-based therapies and immunotherapies tested in AML are discussed. Finally, the article provides an overview about LSC resistance and strategies to overcome resistance.

Bispecific antibodies in acute lymphoblastic leukemia therapy

INTRODUCTION

Blinatumomab, first in a class of bispecific T-cell engagers, revolutionized treatment paradigm of B-cell precursor relapsed/refractory or minimal residual disease positive acute lymphoblastic leukemia (ALL) in adults and children, inducing deep remissions in a proportion of patients. However, significant numbers of patients do not respond or eventually relapse. Strategies for improvement of treatment outcomes are required.

AREAS COVERED

This review discusses the main structural and functional features of blinatumomab, and its place in the treatment of ALL. Furthermore, prospects to increase the efficacy of blinatumomab are addressed. The developments in the field of bispecific antibodies and their possible implications for treatment of ALL are reviewed.

EXPERT OPINION

Better understanding the mechanisms of response and resistance to blinatumomab might help us to identify the group of patients benefiting most from treatment and to spare potentially toxic subsequent treatment strategies. Data emerging from ongoing clinical trials might change the treatment landscape of ALL and beyond. Early use of blinatumomab in frontline protocols with more advantageous treatment sequences and in combination with other targeted therapies might reduce the failure rates. Exponentially increasing number of novel treatment options and their possible combinations might complicate treatment decision-making without data from randomized trials.

Cancer Stem Cells-Origins and Biomarkers: Perspectives for Targeted Personalized Therapies

The use of biomarkers in diagnosis, therapy and prognosis has gained increasing interest over the last decades. In particular, the analysis of biomarkers in cancer patients within the pre- and post-therapeutic period is required to identify several types of cells, which carry a risk for a disease progression and subsequent post-therapeutic relapse. Cancer stem cells (CSCs) are a subpopulation of tumor cells that can drive tumor initiation and can cause relapses. At the time point of tumor initiation, CSCs originate from either differentiated cells or adult tissue resident stem cells. Due to their importance, several biomarkers that characterize CSCs have been identified and correlated to diagnosis, therapy and prognosis. However, CSCs have been shown to display a high plasticity, which changes their phenotypic and functional appearance. Such changes are induced by chemo- and radiotherapeutics as well as senescent tumor cells, which cause alterations in the tumor microenvironment. Induction of senescence causes tumor shrinkage by modulating an anti-tumorigenic environment in which tumor cells undergo growth arrest and immune cells are attracted. Besides these positive effects after therapy, senescence can also have negative effects displayed post-therapeutically. These unfavorable effects can directly promote cancer stemness by increasing CSC plasticity phenotypes, by activating stemness pathways in non-CSCs, as well as by promoting senescence escape and subsequent activation of stemness pathways. At the end, all these effects can lead to tumor relapse and metastasis. This review provides an overview of the most frequently used CSC markers and their implementation as biomarkers by focussing on deadliest solid (lung, stomach, liver, breast and colorectal cancers) and hematological (acute myeloid leukemia, chronic myeloid leukemia) cancers. Furthermore, it gives examples on how the CSC markers might be influenced by therapeutics, such as chemo- and radiotherapy, and the tumor microenvironment. It points out, that it is crucial to identify and monitor residual CSCs, senescent tumor cells, and the pro-tumorigenic senescence-associated secretory phenotype in a therapy follow-up using specific biomarkers. As a future perspective, a targeted immune-mediated strategy using chimeric antigen receptor based approaches for the removal of remaining chemotherapy-resistant cells as well as CSCs in a personalized therapeutic approach are discussed.

Harnessing T Cells to Target Pediatric Acute Myeloid Leukemia: CARs, BiTEs, and Beyond

Outcomes for pediatric patients with acute myeloid leukemia (AML) remain poor, highlighting the need for improved targeted therapies. Building on the success of CD19-directed immune therapy for acute lymphocytic leukemia (ALL), efforts are ongoing to develop similar strategies for AML. Identifying target antigens for AML is challenging because of the high expression overlap in hematopoietic cells and normal tissues. Despite this, CD123 and CD33 antigen targeted therapies, among others, have emerged as promising candidates. In this review we focus on AML-specific T cell engaging bispecific antibodies and chimeric antigen receptor (CAR) T cells. We review antigens being explored for T cell-based immunotherapy in AML, describe the landscape of clinical trials upcoming for bispecific antibodies and CAR T cells, and highlight strategies to overcome additional challenges facing translation of T cell-based immunotherapy for AML.

Biology drives the discovery of bispecific antibodies as innovative therapeutics

A bispecific antibody (bsAb) is able to bind two different targets or two distinct epitopes on the same target. Broadly speaking, bsAbs can include any single molecule entity containing dual specificities with at least one being antigen-binding antibody domain. Besides additive effect or synergistic effect, the most fascinating applications of bsAbs are to enable novel and often therapeutically important concepts otherwise impossible by using monoclonal antibodies alone or their combination. This so-called obligate bsAbs could open up completely new avenue for developing novel therapeutics. With evolving understanding of structural architecture of various natural or engineered antigen-binding immunoglobulin domains and the connection of different domains of an immunoglobulin molecule, and with greatly improved understanding of molecular mechanisms of many biological processes, the landscape of therapeutic bsAbs has significantly changed in recent years. As of September 2019, over 110 bsAbs are under active clinical development, and near 180 in preclinical development. In this review article, we introduce a system that classifies bsAb formats into 30 categories based on their antigen-binding domains and the presence or absence of Fc domain. We further review the biology applications of approximately 290 bsAbs currently in preclinical and clinical development, with the attempt to illustrate the principle of selecting a bispecific format to meet biology needs and selecting a bispecific molecule as a clinical development candidate by 6 critical criteria. Given the novel mechanisms of many bsAbs, the potential unknown safety risk and risk/benefit should be evaluated carefully during preclinical and clinical development stages. Nevertheless we are optimistic that next decade will witness clinical success of bsAbs or multispecific antibodies employing some novel mechanisms of action and deliver the promise as next wave of antibody-based therapeutics.

Cancer biomarkers for targeted therapy

Tumor-associated antigens (TAA) or cancer biomarkers are major targets for cancer therapies. Antibody- based agents targeting the cancer biomarkers include monoclonal antibodies (MoAbs), radiolabeled MoAbs, bispecific T cell engagers, and antibody-drug conjugates. Antibodies targeting CD19, CD20, CD22, CD30, CD33, CD38, CD79B and SLAMF7 are in clinical applications for hematological malignancies. CD123, CLL-1, B cell maturation antigen, and CD138 are targets for cancer immunotherapeutic agents, including the chimeric antigen receptor - engineered T cells. Immune checkpoint inhibitors (ICIs) against PD-1, PD-L1, and CTLA-4 have led to the revolution of cancer immunotherapy. More ICIs targeting IDO, LAG3, TIM-3, TIGIT, SIGLECs, VISTA and CD47 are being explored. Small molecule inhibitors (SMIs) against tyrosine kinase oncoproteins such as BCR-ABL, JAK2, Bruton tyrosine kinase, FLT3, EGFR, ALK, HER2, VEGFR, FGFR, MEK, and MET have fundamentally changed the landscape of cancer therapy. SMIs against BCL-2, IDHs, BRAF, PI3 kinase, mTOR, PARP, and CDKs have become the mainstay in the treatment of a variety of cancer types. To reduce and avoid off-tumor toxicities, cancer-specific TAAs such as CD33 are being manufactured through systems biology approach. Search for novel biomarkers and new designs as well as delivery methods of targeted agents are fueling the next wave of advances in cancer therapy.

Silencing Fc Domains in T cell-Engaging Bispecific Antibodies Improves T-cell Trafficking and Antitumor Potency

Bispecific antibodies (BsAb) that engage T cells bind to tumor cells via a tumor-associated antigen and to T cells through surface CD3. BsAbs have promising antitumor properties in vivo Here, we describe the effects of Fc silencing on BsAb-driven T-cell trafficking to solid tumors. We used BsAbs specific for disialoganglioside GD2 or oncoprotein ErbB2 (HER2) and built on the IgG(L)-scFv platform with or without Fc silencing. We studied the kinetics of T-cell infiltration from blood into solid tumor masses when driven by these BsAbs. We also investigated the therapeutic efficacy of these BsAbs in two mouse models: immunodeficient mice xenografted with patient-derived GD2⁺ neuroblastoma or HER2⁺ breast cancer, and human CD3ε transgenic mice implanted with a GD2⁺ murine tumor. BsAbs built with intact Fc domain were unable to drive T cells to tumor, thereby failing to achieve an antitumor effect in mice. T cells became sequestered in lungs by myeloid cells or depleted in circulation. In contrast, when Fc function was silenced by N297A ± K322A mutations, T cells were able to infiltrate into subcutaneous solid tumors, a prerequisite for successful therapy outcome.

Immunotherapy-Based Targeting and Elimination of Leukemic Stem Cells in AML and CML

The concept of leukemic stem cells (LSC) has been developed with the idea to explain the clonal hierarchies and architectures in leukemia, and the more or less curative anti-neoplastic effects of various targeted drugs. It is now widely accepted that curative therapies must have the potential to eliminate or completely suppress LSC, as only these cells can restore and propagate the malignancy for unlimited time periods. Since LSC represent a minor cell fraction in the leukemic clone, little is known about their properties and target expression profiles. Over the past few years, several cell-specific immunotherapy concepts have been developed, including new generations of cell-targeting antibodies, antibody-toxin conjugates, bispecific antibodies, and CAR-T cell-based strategies. Whereas such concepts have been translated and may improve outcomes of therapy in certain lymphoid neoplasms and a few other malignancies, only little is known about immunological targets that are clinically relevant and can be employed to establish such therapies in myeloid neoplasms. In the current article, we provide an overview of the immunologically relevant molecular targets expressed on LSC in patients with acute myeloid leukemia (AML) and chronic myeloid leukemia (CML). In addition, we discuss the current status of antibody-based therapies in these malignancies, their mode of action, and successful examples from the field.

Treatment of hematological malignancies with T cell redirected bispecific antibodies: current status and future needs

INTRODUCTION

Enthusiasm for developing therapeutic bispecific antibodies (BsAbs) for cancer applications has become intense in the past decade facilitated by advances in molecular biology, hybridoma technology, and protein engineering. The central strategy in BsAb engineering is to combine the specificities directed at effector cells, and at a tumor target associated antigen (TAA) into a single construct.

AREAS COVERED

This article highlights the clinical use of BsAbs to target effector cells to multiple myeloma (MM), non-Hodgkin lymphoma (NHL), acute myeloid leukemia (AML), and acute lymphoblastic leukemia (ALL). We discuss the successes, challenges, and future strategies. Secondary literature search was performed using Pubmed, clinicaltrials.gov and non-proprietary internet search engines.

EXPERT OPINION

The use of BsAb constructs to target hematologic malignancies has achieved limited success to date. There continues to be a high level of enthusiasm for developing and applying new constructs to overcome the challenges in engineering and clinical application for hematologic malignancies.

Bispecific T-cell engagers: Towards understanding variables influencing the in vitro potency and tumor selectivity and their modulation to enhance their efficacy and safety

Bispecific molecules redirecting the cytotoxicity of T-cells are a growing class of therapeutics with numerous molecules being tested in clinical trials. However, it has been a long way since the proof of concept studies in the mid 1980's. In the process we have learnt about the impact of different variables related to the bispecific molecule and the target antigen on the potency of this type of drugs. This work reviews the insights gained and how that knowledge has been used to design more potent bispecific T-cell engagers. The more recent advancement of antibodies with this modality into safety studies in non-human primates and as well as in clinical studies has revealed potential toxicity liabilities for the mode of action. Modifications in existing antibody formats and new experimental molecules designed to mitigate these problems are discussed.

Expanding the Boundaries of Biotherapeutics with Bispecific Antibodies

Bispecific antibodies have moved from being an academic curiosity with therapeutic promise to reality, with two molecules being currently commercialized (Hemlibra® and Blincyto®) and many more in clinical trials. The success of bispecific antibodies is mainly due to the continuously growing number of mechanisms of actions (MOA) they enable that are not accessible to monoclonal antibodies. One of the earliest MOA of bispecific antibodies and currently the one with the largest number of clinical trials is the redirecting of the cytotoxic activity of T-cells for oncology applications, now extending its use in infective diseases. The use of bispecific antibodies for crossing the blood-brain barrier is another important application because of its potential to advance the therapeutic options for neurological diseases. Another noteworthy application due to its growing trend is enabling a more tissue-specific delivery or activity of antibodies. The different molecular solutions to the initial hurdles that limited the development of bispecific antibodies have led to the current diverse set of bispecific or multispecific antibody formats that can be grouped into three main categories: IgG-like formats, antibody fragment-based formats, or appended IgG formats. The expanded applications of bispecific antibodies come at the price of additional challenges for clinical development. The rising complexity in their structure may increase the risk of immunogenicity and the multiple antigen specificity complicates the selection of relevant species for safety assessment.