Diphtheria toxin fused to human interleukin-3 is toxic to blasts from patients with myeloid leukemias

Discovery and investigation of the truncation of the (GGGGS)n linker and its effect on the productivity of bispecific antibodies expressed in mammalian cells

Protein engineering is a powerful tool for designing or modifying therapeutic proteins for enhanced efficacy, increased safety, reduced immunogenicity, and improved delivery. Fusion proteins are an important group of therapeutic compounds that often require an ideal linker to combine diverse domains to fulfill the desired function. GGGGS [(G4S)n] linkers are commonly used during the engineering of proteins because of their flexibility and resistance to proteases. However, unexpected truncation was observed in the linker of a bispecific antibody, which presented challenges in terms of production and quality. In this work, a bispecific antibody containing 5*G4S was investigated, and the truncation position of the linkers was confirmed. Our investigation revealed that codon optimization, which can overcome the negative influence of a high repetition rate and high GC content in the (G4S)n linker, may reduce the truncation rate from 5-10% to 1-5%. Moreover, the probability of truncation when a shortened 3* or 4*G4S linker was used was much lower than that when a 5*G4S linker was used in mammalian cells. In the case of expressing a bispecific antibody, the bioactivity and purity of the product containing a shorter G4S linker were further investigated and are discussed.

The role of chemokines and interleukins in acute lymphoblastic leukemia: a systematic review

Acute lymphoblastic leukemia (ALL) is the most common childhood hematological malignancy, but it also affects adult patients with worse prognosis and outcomes. Leukemic cells benefit from protective mechanisms, which are mediated by intercellular signaling molecules - cytokines. Through these signals, cytokines modulate the biology of leukemic cells and their surroundings, enhancing the proliferation, survival, and chemoresistance of the disease. This ultimately leads to disease progression, refractoriness, and relapse, decreasing the chances of curability and overall survival of the patients. Targeting and modulating these pathological processes without affecting the healthy physiology is desirable, offering more possibilities for the treatment of ALL patients, which still remains unsatisfactory in certain cases. In this review, we comprehensively analyze the existing literature and ongoing trials regarding the role of chemokines and interleukins in the biology of ALL. Focusing on the functional pathways, genetic background, and critical checkpoints, we constructed a summary of molecules that are promising for prognostic stratification and mainly therapeutic use. Targeted therapy, including chemokine and interleukin pathways, is a new and promising approach to the treatment of cancer. With the expansion of our knowledge, we are able to uncover a spectrum of new potential checkpoints in order to modulate the disease biology. Several cytokine-related targets are advancing toward clinical application, offering the hope of higher disease response rates to treatment.

Pediatric acute myeloid leukemia: Insight into genetic landscape and novel targeted approaches

Acute myeloid leukemia (AML) is a very heterogeneous hematological malignancy that accounts for approximately 20% of all pediatric leukemia cases. The outcome of pediatric AML has improved over the last decades, with overall survival rates reaching up to 70%. Still, AML is among the leading types of pediatric cancers by its high mortality rate. Modulation of standard therapy, like chemotherapy intensification, hematopoietic stem cell transplantation and optimized supportive care, could only get this far, but for the significant improvement of the outcome in pediatric AML, development of novel targeted therapy approaches is necessary. In recent years the advances in genomic techniques have greatly expanded our knowledge of the AML biology, revealing molecular landscape and complexity of the disease, which in turn have led to the identification of novel therapeutic targets. This review provides a brief overview of the genetic landscape of pediatric AML, and how it's used for precise molecular characterization and risk stratification of the patients, and also for the development of effective targeted therapy. Furthermore, this review presents recent advances in molecular targeted therapy and immunotherapy with an emphasis on the therapeutic approaches with significant clinical benefits for pediatric AML.

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

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

CD123 Expression Is Associated With High-Risk Disease Characteristics in Childhood Acute Myeloid Leukemia: A Report From the Children's Oncology Group

PURPOSE

Increased CD123 surface expression has been associated with high-risk disease characteristics in adult acute myeloid leukemia (AML), but has not been well-characterized in childhood AML. In this study, we defined CD123 expression and associated clinical characteristics in a uniformly treated cohort of pediatric patients with newly diagnosed AML enrolled on the Children's Oncology Group AAML1031 phase III trial (NCT01371981).

MATERIALS AND METHODS

AML blasts within diagnostic bone marrow specimens (n = 1,040) were prospectively analyzed for CD123 protein expression by multidimensional flow cytometry immunophenotyping at a central clinical laboratory. Patients were stratified as low-risk or high-risk on the basis of (1) leukemia-associated cytogenetic and molecular alterations and (2) end-of-induction measurable residual disease levels.

RESULTS

The study population was divided into CD123 expression-based quartiles (n = 260 each) for analysis. Those with highest CD123 expression (quartile 4 [Q4]) had higher prevalence of high-risk KMT2A rearrangements and FLT3-ITD mutations (P < .001 for both) and lower prevalence of low-risk t(8;21), inv(16), and CEBPA mutations (P < .001 for all). Patients in lower CD123 expression quartiles (Q1-3) had similar relapse risk, event-free survival, and overall survival. Conversely, Q4 patients had a significantly higher relapse risk (53% v 39%, P < .001), lower event-free survival (49% v 69%, P < .001), and lower overall survival (32% v 50%, P < .001) in comparison with Q1-3 patients. CD123 maintained independent significance for outcomes when all known contemporary high-risk cytogenetic and molecular markers were incorporated into multivariable Cox regression analysis.

CONCLUSION

CD123 is strongly associated with disease-relevant cytogenetic and molecular alterations in childhood AML. CD123 is a critical biomarker and promising immunotherapeutic target for children with relapsed or refractory AML, given its prevalent expression and enrichment in patients with high-risk genetic alterations and inferior clinical outcomes with conventional therapy.

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.

CD123 as a Biomarker in Hematolymphoid Malignancies: Principles of Detection and Targeted Therapies

Simple Summary

CD123 is overexpressed in multiple hematologic malignancies. Advances in CD123-targeted therapies over the past decade have positioned this molecule as an integral biomarker in current practice. This review provides an overview of CD123 biology and in-depth discussion of clinical laboratory techniques used to determine CD123 expression in various hematolymphoid neoplasms. In addition, we describe various pharmacologic strategies and agents that are available or under evaluation for targeting CD123.

Abstract

CD123, the α chain of the interleukin 3 receptor, is a cytokine receptor that is overexpressed in multiple hematolymphoid neoplasms, including acute myeloid leukemia, blastic plasmacytoid dendritic cell neoplasm, acute lymphoblastic leukemia, hairy cell leukemia, and systemic mastocytosis. Importantly, CD123 expression is upregulated in leukemic stem cells relative to non-neoplastic hematopoietic stem cells, which makes it a useful diagnostic and therapeutic biomarker in hematologic malignancies. Varying levels of evidence have shown that CD123-targeted therapy represents a promising therapeutic approach in several cancers. Tagraxofusp, an anti-CD123 antibody conjugated to a diphtheria toxin, has been approved for use in patients with blastic plasmacytoid dendritic cell neoplasm. Multiple clinical trials are investigating the use of various CD123-targeting agents, including chimeric antigen receptor-modified T cells (expressing CD123, monoclonal antibodies, combined CD3-CD123 dual-affinity retargeting antibody therapy, recombinant fusion proteins, and CD123-engager T cells. In this review, we provide an overview of laboratory techniques used to evaluate and monitor CD123 expression, describe the strengths and limitations of detecting this biomarker in guiding therapy decisions, and provide an overview of the pharmacologic principles and strategies used in CD123-targeted therapies.

Approval of tagraxofusp-erzs for blastic plasmacytoid dendritic cell neoplasm

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare and clinically challenging hematologic malignancy with dismal outcomes. With a median age of ∼70 years, the majority of patients with BPDCN have experienced historically suboptimal responses with intensive chemotherapy regimens. The major scientific breakthrough in this field was the recognition of overexpression of a surface receptor, CD123/interleukin 3 (IL-3) receptor α, in all patients. Importantly, a novel therapeutic agent consisting of a truncated diphtheria toxin (DT) payload fused to recombinant human IL-3 was being developed, one that targeted CD123, initially known as DT-IL-3 (later known as SL401; tagraxofusp; tagraxofusp-erzs [Elzonris]). The identification of this agent, and subsequent clinical trials specifically dedicated to patients with BPDCN (including a pilot study, followed by a larger phase 1/2 multicenter study [90% overall response rate [ORR] in frontline and 67% ORR in relapsed/refractory setting]), in part led to approval of tagraxofusp-erzs on 21 December 2018. Tagraxofusp-erzs was the first agent approved for BPDCN (for patients ages 2 years and older), and importantly, established this drug as the first CD123-targeted agent ever approved. The most notable toxicity of tagraxofusp-erzs is occurrence of the capillary leak syndrome, which occurs frequently at all grades, and has also been observed to be life-threatening, appropriately leading to a US Food and Drug Administration "black box" warning in the package insert. The preclinical and clinical aspects of drug development of tagraxofusp-erzs as monotherapy leading to drug approval are reviewed herein, with discussion of future directions of this novel agent, including consideration for rational combinations in BPDCN and beyond.

Updates in Novel Therapies for Blastic Plasmacytoid Dendritic Cell Neoplasm (BPDCN)

PURPOSE OF REVIEW

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare, clinically aggressive hematologic malignancy that has heterogeneous presentation and can involve the skin, lymph nodes, and bone marrow. Recent advancements in our patho-biologic understanding of the disease have led to the development of new targeted therapies for BPDCN. In this review, we aimed to describe some of the novel treatments that are being put forward for the management of BPDCN.

RECENT FINDINGS

Tagraxofusp is the first CD123-targeted therapy approved as the first ever targeted treatment of BPDCN in patients aged 2 years and older. This agent was approved based on a pivotal clinical trial that showed that it was associated with high rates of clinical responses in both treatment-naïve and treatment-experienced patients. The most serious adverse event was occurrence of the capillary leak syndrome. Other targeted therapies are actively being investigated in clinical trials. These include other CD123-targeted approaches, as well as active investigation in targets beyond CD123, such as the BCL-2 inhibitor, venetoclax. BPDCN is a rare hematologic clonal disorder with historically poor outcomes. Newer targeted therapies have been recently introduced, with promising results and novel toxicities that are important to recognize and understand. Stem cell transplantation after achievement of complete remission remains the mainstay of therapy among younger/fit, eligible patients, regardless of treatment modality used.

Blastic plasmacytoid dendritic cell neoplasm: diagnosis, manifestations, and treatment

PURPOSE OF REVIEW

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare and aggressive hematologic malignancy with historically poor outcomes. It typically manifests as asymptomatic skin lesions and cytopenias, which result from bone marrow involvement. Less commonly, it will present in lymph nodes or visceral organs as well. Although rare, BPDCN has been discussed more frequently in recent years as new drugs have been developed that could be effective at treating this disease.

RECENT FINDINGS

Until recently, treatment for BPDCN commonly included intensive chemotherapy regimens, which are generally reserved for management of acute myeloid leukemia or acute lymphoblastic leukemia. However, in 2018 tagraxofusp (SL-401) was approved as the only treatment specifically indicated for BPDCN. Additional clinical trials are ongoing evaluating the efficacy of newer agents, which could potentially further improve the long-term outcomes for patients with BPDCN.

SUMMARY

This manuscript reviews the diagnosis, manifestations and treatment of BPDCN.

Treatment of Acute Myeloid Leukemia in the Era of Genomics-Achievements and Persisting Challenges

Acute myeloid leukemia (AML) represents a malignant disorder of the hematopoietic system that is mainly characterized by rapid proliferation, dysregulated apoptosis, and impaired differentiation of leukemic blasts. For several decades, the diagnostic approach in AML was largely based on histologic characteristics with little impact on the treatment decision-making process. This perspective has drastically changed within the past years due to the advent of novel molecular technologies, such as whole genome next-generation sequencing (NGS), and the resulting knowledge gain in AML biology and pathogenesis. After more than four decades of intensive chemotherapy as a "one-size-fits-all" concept, several targeted agents have recently been approved for the treatment of AML, either as single agents or as part of combined treatment regimens. Several other compounds, directed against regulators of apoptotic, epigenetic, or microenvironmental pathways, as well as modulators of the immune system, are currently in development and being investigated in clinical trials. The constant progress in AML research has started to produce improved survival rates and fueled hopes that a once rapidly fatal disease can be transformed into a chronic condition. In this review, the authors provide a summary of recent advances in the development of targeted AML therapies and discuss persistent challenges.

Targeted Diphtheria Toxin-Based Therapy: A Review Article

Cancer remains one of the leading causes of death worldwide. Conventional therapeutic strategies usually offer limited specificity, resulting in severe side effects and toxicity to normal tissues. Targeted cancer therapy, on the other hand, can improve the therapeutic potential of anti-cancer agents and decrease unwanted side effects. Targeted applications of cytolethal bacterial toxins have been found to be especially useful for the specific eradication of cancer cells. Targeting is either mediated by peptides or by protein-targeting moieties, such as antibodies, antibody fragments, cell-penetrating peptides (CPPs), growth factors, or cytokines. Together with a toxin domain, these molecules are more commonly referred to as immunotoxins. Targeting can also be achieved through gene delivery and cell-specific expression of a toxin. Of the available cytolethal toxins, diphtheria toxin (DT) is one of the most frequently used for these strategies. Of the many DT-based therapeutic strategies investigated to date, two immunotoxins, Ontak^TM and Tagraxofusp^TM, have gained FDA approval for clinical application. Despite some success with immunotoxins, suicide-gene therapy strategies, whereby controlled tumor-specific expression of DT is used for the eradication of malignant cells, are gaining prominence. The first part of this review focuses on DT-based immunotoxins, and it then discusses recent developments in tumor-specific expression of DT.

Recent developments in the treatment of blastic plasmacytoid dendritic cell neoplasm

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a clinically aggressive hematologic malignancy derived from precursors of dendritic cells and involves most frequently the skin, bone marrow and lymph nodes. Diagnosis depends upon identification of specific tumor markers including CD4, CD56 and CD123. Historically, the median survival has been less than 2 years in most reported series. While for many years, conventional chemotherapy followed by stem cell transplantation was the standard of care, recently tagraxofusp, a cytotoxin directed against CD123, received United States Food and Drug Administration approval specifically for patients with BPDCN. In this review, we will discuss the markers used for diagnosis of BPDCN and focus on the new targeted treatments available. Specifically in BPDCN, tagraxofusp was highly effective with a safety profile found to be acceptable overall, with the noted occurrence of capillary leak syndrome. Future directions in therapy approaches for patients with BPDCN will include the development of other CD123-targeted agents, agents targeting beyond CD123 and investigation of rational combination approaches of CD123-directed therapy with other therapies.

CD123 as a Therapeutic Target in the Treatment of Hematological Malignancies

The interleukin-3 receptor alpha chain (IL-3Rα), more commonly referred to as CD123, is widely overexpressed in various hematological malignancies, including acute myeloid leukemia (AML), B-cell acute lymphoblastic leukemia, hairy cell leukemia, Hodgkin lymphoma and particularly, blastic plasmacytoid dendritic neoplasm (BPDCN). Importantly, CD123 is expressed at both the level of leukemic stem cells (LSCs) and more differentiated leukemic blasts, which makes CD123 an attractive therapeutic target. Various agents have been developed as drugs able to target CD123 on malignant leukemic cells and on the normal counterpart. Tagraxofusp (SL401, Stemline Therapeutics), a recombinant protein composed of a truncated diphtheria toxin payload fused to IL-3, was approved for use in patients with BPDCN in December of 2018 and showed some clinical activity in AML. Different monoclonal antibodies directed against CD123 are under evaluation as antileukemic drugs, showing promising results either for the treatment of AML minimal residual disease or of relapsing/refractory AML or BPDCN. Finally, recent studies are exploring T cell expressing CD123 chimeric antigen receptor-modified T-cells (CAR T) as a new immunotherapy for the treatment of refractory/relapsing AML and BPDCN. In December of 2018, MB-102 CD123 CAR T developed by Mustang Bio Inc. received the Orphan Drug Designation for the treatment of BPDCN. In conclusion, these recent studies strongly support CD123 as an important therapeutic target for the treatment of BPDCN, while a possible in the treatment of AML and other hematological malignancies will have to be evaluated by in the ongoing clinical studies.

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.

A CD123-targeting antibody-drug conjugate, IMGN632, designed to eradicate AML while sparing normal bone marrow cells

The outlook for patients with refractory/relapsed acute myeloid leukemia (AML) remains poor, with conventional chemotherapeutic treatments often associated with unacceptable toxicities, including severe infections due to profound myelosuppression. Thus there exists an urgent need for more effective agents to treat AML that confer high therapeutic indices and favorable tolerability profiles. Because of its high expression on leukemic blast and stem cells compared with normal hematopoietic stem cells and progenitors, CD123 has emerged as a rational candidate for molecularly targeted therapeutic approaches in this disease. Here we describe the development and preclinical characterization of a CD123-targeting antibody-drug conjugate (ADC), IMGN632, that comprises a novel humanized anti-CD123 antibody G4723A linked to a recently reported DNA mono-alkylating payload of the indolinobenzodiazepine pseudodimer (IGN) class of cytotoxic compounds. The activity of IMGN632 was compared with X-ADC, the ADC utilizing the G4723A antibody linked to a DNA crosslinking IGN payload. With low picomolar potency, both ADCs reduced viability in AML cell lines and patient-derived samples in culture, irrespective of their multidrug resistance or disease status. However, X-ADC exposure was >40-fold more cytotoxic to the normal myeloid progenitors than IMGN632. Of particular note, IMGN632 demonstrated potent activity in all AML samples at concentrations well below levels that impacted normal bone marrow progenitors, suggesting the potential for efficacy in AML patients in the absence of or with limited myelosuppression. Furthermore, IMGN632 demonstrated robust antitumor efficacy in multiple AML xenograft models. Overall, these findings identify IMGN632 as a promising candidate for evaluation as a novel therapy in AML.

[The study of expression and prognostic value of CD123 in acute myeloid leukemia bone marrow blasts]

Objective: To study the expression of CD123 in bone marrow (BM) blasts of acute myeloid leukemia (AML) patients to explore the relationship between CD123 expression and therapeutic response and prognosis. Methods: This study retrospectively analyzed expression and distribution of CD123 in BM blasts in 137 cases of newly diagnosed AML (excluded M(3)) , CD123 detected by flow cytometry≥20% was defined as positive, including 84 CD123(+) AML and 53 CD123(-) AML, efficacy and prognosis were compared between the two groups. Results: ① Among 137 patients, 84 were in group CD123(+) (61.3%) , and 53 in group CD123(-) (38.7%) . All 137 patients were classified into risk groups based on cytogenetic and molecular biology abnormalities. No significant differences were seen between the three risk groups with regard to their CD123 levels (χ(2)=0.861, P=0.650) . Compared with CD123(-) group, the CD123(+) group had higher WBC[47.7 (1.0-264.0) vs 22.4 (0.7-211.0) , z=-2.592, P=0.010]. ② The rates of first complete remission (CR1) and recurrence of CD123(+) group were 54.8% (46/84) and 50.8% (32/63) , respectively; and CD123(-) group were 73.6% (39/53) and 41.7% (20/48) , respectively. There was significant difference of CR1 between the two groups (χ(2)=5.121, P=0.027) , whereas no significant difference of the recurrence rate (χ(2)=0.911, P=0.340) . ③ The median dutations of OS between CD123(+) group and CD123(-) group were 20.0 (95%CI 13.1-26.9) months vs 44.0 (95%CI 23.6-47.3) months, respectively (χ(2)=5.874, P=0.015) ; The median durations of DFS were 7.8 (95%CI 1.4-14.1) months vs 18.6 (95%CI 0-39.7) months, respectively, no differences were observed between the two groups (χ(2)=2.939, P=0.086) . ④ CD123 retained an adverse prognosis value on DFS and OS within the intermediate group and patients ≤ 50 years older. Conclusions: CD123 widely expressed in AML patients, which was an independent risk factor for CR1 and OS, which implicating its important role in evaluating the induction chemotherapy response and prognosis of AML.

Clinical Activity and Tolerability of SL-401 (Tagraxofusp): Recombinant Diphtheria Toxin and Interleukin-3 in Hematologic Malignancies

Overcoming the leukemia stem cell resistance to intensive chemotherapy has been an area of extensive research over the last two decades. Advances and greater understanding of the molecular biology of leukemia stem cells are in rapid progress. Targeted therapies are currently being used in clinical practice with reasonable response rates, but a cure is being achieved in only a small percentage of patients, most likely due to tumor mutational heterogeneity. A genetically engineered diphtheria toxin fused with interleukin-3 (SL-401 or tagraxofusp) has shown robust activity in blastic plasmacytoid dendritic cell neoplasm and promising response rates in different myeloid malignancies, including eradication of minimal residual disease. Multiple clinical trials are being conducted using this drug and the preliminary results are encouraging. This article reviews the clinical trials for SL-401, its mechanism of action, clinical activity, and the adverse event profile.

Improved in vitro and in vivo activity against CD303-expressing targets of the chimeric 122A2 antibody selected for specific glycosylation pattern

Plasmacytoid dendritic cells (pDCs) play a central role for both innate and adaptive antiviral responses, as they direct immune responses through their unique ability to produce substantial concentrations of type I interferon (IFNs) upon viral encounter while also activating multiple immune cells, including macrophages, DCs, B, natural killer and T cells. Recent evidence clearly indicates that pDCs also play a crucial role in some cancers and several auto-immune diseases. Although treatments are currently available to patients with such pathologies, many are not fully efficient. We are proposing here, as a new targeted-based therapy, a novel chimeric monoclonal antibody (mAb) that mediates a strong cellular cytotoxicity directed against a specific human pDC marker, CD303. This antibody, ch122A2 mAb, is characterized by low fucose content in its human IgG1 constant (Fc) region, which induces strong in vitro and in vivo activity against human pDCs. We demonstrated that this effect relates in part to its specific Fc region glycosylation pattern, which increased affinity for CD16/FcγRIIIa. Importantly, ch122A2 mAb induces the down-modulation of CpG-induced IFN-α secretion by pDCs. Additionally, ch122A2 mAb shows in vitro high pDC depletion mediated by antibody-dependent cell-mediated cytotoxicity and antibody-dependent cellular phagocytosis. Remarkably, in vivo ch122A2 mAb efficacy is also demonstrated in humanized mice, resulting in significant pDC depletion in bloodstream and secondary lymphoid organs such as spleen. Together, our data indicates that ch122A2 mAb could represent a promising cytotoxic mAb candidate for pathologies in which decreasing type I IFNs or pDCs depleting may improve patient prognosis.

Novel antibody-cytokine fusion proteins featuring granulocyte-colony stimulating factor, interleukin-3 and interleukin-4 as payloads

Neutrophils can strongly influence disease activity in cancer and in chronic inflammation. Here, we report for the first time the construction and characterization of antibody-fusion proteins featuring granulocyte-colony stimulating factor and interleukin-3 as payloads capable of enhancing neutrophil activity and a novel antibody-interleukin-4 fusion protein with neutrophil inhibitory potential. We used the F8 antibody specific to the alternatively-spliced extra domain A (EDA) of fibronectin as a targeting agent, since the cognate antigen is strongly upregulated in diseases characterized by angiogenesis. The fusion proteins GCSF-F8, F8-IL3 and F8-IL4-F8, were cloned, expressed, and their targeting ability assessed, exhibiting preferential tumor uptake with tumor:blood ratios at 24 h after injection of 3.3, 18.2 and 27.3, respectively. In F9 tumor bearing-mice GCSF-F8 and F8-IL3 did not provide a therapeutic benefit, while F8-IL4-F8 showed a potent tumor growth retardation. In the collagen-induced model of arthritis, GCSF-F8 and F8-IL3 induced a worsening of the disease, while F8-IL4-F8 slowed arthritis progression but, surprisingly, exhibited substantial toxicity when used in combination with dexamethasone. Collectively, the results indicate that the novel fusion proteins could be expressed and efficiently delivered to the site of disease. However, they were not superior to other antibody-cytokine fusions previously described by our laboratory.

Optimization of the Expression of DT386-BR2 Fusion Protein in Escherichia coli using Response Surface Methodology

Background:

The aim of this study was to determine the best condition for the production of DT386-BR2 fusion protein, an immunotoxin consisting of catalytic and translocation domains of diphtheria toxin fused to BR2, a cancer specific cell penetrating peptide, for targeted eradication of cancer cells, in terms of the host, cultivation condition, and culture medium.

Materials and Methods:

Recombinant pET28a vector containing the codons optimized for the expression of the DT386-BR2 gene was transformed to different strains of Escherichia coli (E. coli BL21 DE3, E. coli Rosetta DE3 and E. coli Rosetta-gami 2 DE3), followed by the induction of expression using 1 mM IPTG. Then, the strain with the highest ability to produce recombinant protein was selected and used to determine the best expression condition using response surface methodology (RSM). Finally, the best culture medium was selected.

Results:

Densitometry analysis of sodium dodecyl sulfate-polyacrylamide gel electrophoresis of the expressed fusion protein showed that E. coli Rosetta DE3 produced the highest amounts of the recombinant fusion protein when quantified by 1 mg/ml bovine serum albumin (178.07 μg/ml). Results of RSM also showed the best condition for the production of the recombinant fusion protein was induction with 1 mM IPTG for 2 h at 37°C. Finally, it was established that terrific broth could produce higher amounts of the fusion protein when compared to other culture media.

Conclusion:

In this study, we expressed the recombinant DT386-BR2 fusion protein in large amounts by optimizing the expression host, cultivation condition, and culture medium. This fusion protein will be subjected to purification and evaluation of its cytotoxic effects in future studies.

Targeted therapies in the treatment of adult acute myeloid leukemias: current status and future perspectives

The rapid advancement of next-generation sequencing techniques and the identification of molecular driver events responsible for leukemia development are opening the door to new pharmacologic-targeted agents to tailor treatment of acute myeloid leukemia (AML) in individual patients. However, the use of targeted therapies in AML has met with only modest success. Molecular studies have identified AML subsets characterized by driver mutational events, such as NPM1, FLT3-ITD and IDH1-2 mutations, and have provided preclinical evidence that the targeting of these mutant molecules could represent a valuable therapeutic strategy. Recent studies have provided the first pieces of evidence that FLT3 targeting in FLT3-mutant AMLs, IDH1/2 inhibition in IDH-mutant AMLs and targeting membrane molecules preferentially expressed on leukemic progenitor/stem cells, such as CD33 and CD123, represent a clinically valuable strategy.

A critical review of treatment modalities for blastic plasmacytoid dendritic cell neoplasm

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a clinically aggressive tumor derived from the precursors of plasmacytoid dendritic cells. It is a rare disease presenting across all ages with either skin or both skin and bone marrow involvement often conferring a poor prognosis. Though localized radiation has been used before, acute leukemia based regimens, remains the treatment of choice for induction of remission. Hematopoietic stem cell transplant, either autologous or allogeneic, is further required for attaining sustained remissions. Recently, a number of targeted therapies and newer drugs have been used as the molecular and genetic understanding of the disease have improved.

Blastic plasmacytoid dendritic cell neoplasm: diagnostic criteria and therapeutical approaches

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare haematological malignancy derived from the precursors of plamacytoid dendritic cells, with an aggressive clinical course and high frequency of cutaneous and bone marrow involvement. Neoplastic cells express CD4, CD43 (also termed SPN), CD45RA and CD56 (also termed NCAM1), as well as the plasmacytoid dendritic cell-associated antigens CD123 (also termed IL3RA), BDCA-2 (also termed CD303, CLEC4E) TCL1 and CTLA1 (also termed GZMB). The median survival is only a few months as the tumour exhibits a progressive course despite initial response to chemotherapy. The best modality of treatment remains to be defined. Generally, patients receive acute leukaemia-like induction, according to acute myeloid leukaemia (AML)-type or acute lymphoid leukaemia (ALL)-type regimens. The frequent neuromeningeal involvement indicates systematic pre-emptive intrathecal chemotherapy in addition to intensive chemotherapy. Allogeneic haematopoietic stem cell transplantation (HSCT), particularly when performed in first remission, may improve the survival. Preliminary data suggest a potential role for immunomodulatory agents and novel targeted drugs. Herein epidemiology, clinical manifestations, diagnosis and management of BPDCN will be presented. In detail, this review focuses on the therapeutic aspects of BPDCN, proposing a treatment algorithm for the management of the disease, including induction chemotherapy, allogeneic HSCT and intrathecal prophylaxis at different steps of treatment, according to compliance, biological and clinical characteristics of patients.

The future of antiviral immunotoxins

There is a constant need for new therapeutic interventions in a wide range of infectious diseases. Over the past few years, the immunotoxins have entered the stage as promising antiviral treatments. Immunotoxins have been extensively explored in cancer treatment and have achieved FDA approval in several cases. Indeed, the design of new anticancer immunotoxins is a rapidly developing field. However, at present, several immunotoxins have been developed targeting a variety of different viruses with high specificity and efficacy. Rather than blocking a viral or cellular pathway needed for virus replication and dissemination, immunotoxins exert their effect by killing and eradicating the pool of infected cells. By targeting a virus-encoded target molecule, it is possible to obtain superior selectivity and drastically limit the side effects, which is an immunotoxin-related challenge that has hindered the success of immunotoxins in cancer treatment. Therefore, it seems beneficial to use immunotoxins for the treatment of virus infections. One recent example showed that targeting of virus-encoded 7 transmembrane (7TM) receptors by immunotoxins could be a future strategy for designing ultraspecific antiviral treatment, ensuring efficient internalization and hence efficient eradication of the pool of infected cells, both in vitro and in vivo. In this review, we provide an overview of the mechanisms of action of immunotoxins and highlight the advantages of immunotoxins as future anti-viral therapies.

T cells expressing CD123 chimeric antigen receptors for treatment of acute myeloid leukemia

PURPOSE OF REVIEW

The purpose of this article is to discuss the rationale of targeting CD123 using chimeric antigen receptor (CAR) T cells for the treatment of leukemia.

RECENT FINDINGS

CD123 is a leukemia-associated antigen that expresses at high levels in leukemic stem cells and leukemic blasts and low level in normal hematopoietic stem/progenitor cells. Immune-based therapies targeting CD123 are being developed. Preclinical data suggest that CD123 CAR T cells exhibit potent antileukemic activity and various impacts on normal hematopoiesis.

SUMMARY

CD123 is an attractive surface target for novel antileukemic therapies. CD123 CAR T-cell-based immunotherapy is a promising treatment for patients with relapsed or refractory acute myeloid leukemia.

Targeting LSCs through membrane antigens selectively or preferentially expressed on these cells

Studies of xenotransplantation of bone marrow and blood cells of AML patients have supported the existence of rare leukemic stem cells, able to initiate and maintain the leukemic process and bearing the typical leukemic abnormalities. LSCs possess self-renewal capacity and are responsible for the growth of the more differentiated leukemic progeny in the bone marrow and in the blood. These cells are more resistant than bulk leukemic cells to anti-leukemic drugs, thus survive to treatment and are, at a large extent, responsible for leukemia relapse. During the last two decades, considerable progresses have been made in the understanding of the peculiar cellular and molecular properties of LSCs. In this context, particularly relevant was the discovery of several membrane markers, selectively or preferentially expressed on LSCs. These membrane markers offer now unique opportunities to identify LSCs and to distinguish them from normal HSCs, to monitor the response of the various anti-leukemic treatments at the level of the LSC compartment, to identify relevant therapeutic targets. Concerning this last point, the most promising therapeutic targets are CD33 and CD123.

The novel structure make LDM effectively remove CD123+ AML stem cells in combination with interleukin 3

CD123 became a therapeutic target for acute myelocytic leukemia(AML) because of its overexpression only on AML stem cells. It is α subunit of interleukin-3 (multi-CSF, IL3) receptor. Lidamycin(LDM) is a novel antibiotic composed of an apoprotein (LDP) and a chromophore (AE). We cloned, expressed and isolated IL3LDP fusion protein first then assembled with AE in vitro. We found that131/132 amino acids of IL3 were the key factors for IL3 fusion protein stability and I131L/F132L mutation effectively improved the IL3 fusion protein stability. The toxicity of IL3LDM to CD123+ tumor cells was 2-10 times compared to LDM alone and 10000 times compared to ADR. Meanwhile, IL3LDM impaired the colony-forming ability of CD123+ stem-like cells but not to CD123 negative normal cord blood cells. Three drug delivery methods in vivo were adopted: prophylactic treatment and single/multiple-dosing administration. The tumor-free survival extended to 120 d and cancer cell invasion significantly decreased after IL3LDM continuous multiple treated. Moreover, IL3LDM had been shown to modulate apoptosis by arrested cell cycle in G2/M phase. Therefore, IL3LDM is expected to be a new drug for leukemia target therapy.

Blastic plasmacytoid dendritic cell neoplasm: update on molecular biology, diagnosis, and therapy

BACKGROUND

Blastic plasmacytoid dendritic cell neoplasm (BPDCN) is a rare hematological malignancy with an aggressive clinical course. Most patients with BPDCN have skin lesions and simultaneous involvement of the peripheral blood, bone marrow, and lymph nodes.

METHODS

A search of PubMed and Medline was conducted for English-written articles relating to BPDCN, CD4(+)CD56(+) hematodermic neoplasm, and blastic natural killer cell lymphoma. Data regarding diagnosis, prognosis, and treatment were analyzed.

RESULTS

BPDCN is derived from precursor plasmacytoid dendritic cells. The diagnosis of BPDCN is based on the characteristic cytology and immunophenotype of malignant cells coexpressing CD4, CD56, CD123, blood dendritic cell antigens 2 and 4, and CD2AP markers. Multiple chromosomal abnormalities and gene mutations previously reported in patients with myeloid and selected lymphoid neoplasms were identified in approximately 60% of patients with BPDCN. Prospectively controlled studies to guide treatment decisions are lacking. The overall response rate with aggressive acute lymphoblastic leukemia-type induction regimens was as high as 90%, but the durability of response was short. Median survival rates ranged between 12 and 16 months. Patients with relapsed disease may respond to L-asparaginase-containing regimens. Allogeneic hematopoietic stem cell transplantation, particularly when performed during the first remission, may produce durable remissions in selected adults.

CONCLUSIONS

BPDCN is a rare aggressive disease that typically affects elderly patients. The most commonly affected nonhematopoietic organ is the skin. Although BPDCN is initially sensitive to conventional chemotherapy regimens, this response is relatively short and long-term prognosis is poor. In the near future, novel targeted therapies may improve outcomes for patients with BPDCN.

AntiCD3Fv fused to human interleukin-3 deletion variant redirected T cells against human acute myeloid leukemic stem cells

Background

Leukemic stem cells (LSCs) are frequently seen as a cause of treatment failure and relapse in patients with acute myeloid leukemia (AML). Thus, successful new therapeutic strategies for the treatment of AML should aim at eradicating LSCs. The identification of targets on the cell surface of LSCs is getting more and more attention. Among these, CD123, also known as the interleukin-3 (IL3)-receptor α chain, has been identified as a potential immunotherapeutic target due to its overexpression on LSCs in AML as well as on AML blasts, rather than normal hematopoietic stem cells.

Methods

We constructed a CD123-targeted fusion protein antiCD3Fv-⊿IL3, with one binding site for T cell antigen receptor (TCRCD3) and the other for CD123, by recombinant gene-engineering technology. Cysteine residues were introduced into the V domains of the antiCD3Fv segment to enhance its stability by locking the two chains of Fv together with disulfide covalent bonds. The stability and cytotoxicity of the two fusion proteins were detected in vitro and in vivo.

Results

Both fusion proteins were produced and purified from Escherichia coli 16C9 cells with excellent yields in fully active forms. High-binding capability was observed between these two fusion proteins and human IL3R, leading to the specific lysis of CD123-expressing cell lines KG1a; also, mononuclear cells from primary AML patients were inhibited in a colony forming assay in vitro, presumably by redirecting T lymphocytes in vitro. In addition, they displayed an antileukemic activity against KG1a xenografts in non-obese diabetic/severe combined immunodeficient (NOD/SCID) mice, especially disulfide-stabilized (ds)-antiCD3Fv-⊿IL3 for its improved stability.

Conclusions

These results suggest that both fusion proteins display the antileukemic activity against CD123-expressing cell lines as well as leukemic progenitors in vitro and in vivo, especially ds-antiCD3Fv-⊿IL3. They could be the promising candidates for future immunotherapy of AML.

Electronic supplementary material

The online version of this article (doi:10.1186/s13045-015-0109-5) contains supplementary material, which is available to authorized users.

In vivo and in vitro sensitivity of blastic plasmacytoid dendritic cell neoplasm to SL-401, an interleukin-3 receptor targeted biologic agent

Blastic plasmacytoid dendritic cell neoplasm is an aggressive malignancy derived from plasmacytoid dendritic cells. There is currently no accepted standard of care for treating this neoplasm, and therapeutic strategies have never been prospectively evaluated. Since blastic plasmacytoid dendritic cell neoplasm cells express high levels of interleukin-3 receptor α chain (IL3-Rα or CD123), antitumor effects of the interleukin-3 receptor-targeted drug SL-401 against blastic plasmacytoid dendritic cell neoplasm were evaluated in vitro and in vivo. The cytotoxicity of SL-401 was assessed in patient-derived blastic plasmacytoid dendritic cell neoplasm cell lines (CAL-1 and GEN2.2) and in primary blastic plasmacytoid dendritic cell neoplasm cells isolated from 12 patients using flow cytometry and an in vitro cytotoxicity assay. The cytotoxic effects of SL-401 were compared to those of several relevant cytotoxic agents. SL-401 exhibited a robust cytotoxicity against blastic plasmacytoid dendritic cell neoplasm cells in a dose-dependent manner. Additionally, the cytotoxic effects of SL-401 were observed at substantially lower concentrations than those achieved in clinical trials to date. Survival of mice inoculated with a blastic plasmacytoid dendritic cell neoplasm cell line and treated with a single cycle of SL-401 was significantly longer than that of untreated controls (median survival, 58 versus 17 days, P<0.001). These findings indicate that blastic plasmacytoid dendritic cell neoplasm cells are highly sensitive to SL-401, and support further evaluation of SL-401 in patients suffering from blastic plasmacytoid dendritic cell neoplasm.

SL-401 and SL-501, targeted therapeutics directed at the interleukin-3 receptor, inhibit the growth of leukaemic cells and stem cells in advanced phase chronic myeloid leukaemia

While imatinib and other tyrosine kinase inhibitors (TKIs) are highly efficacious in the treatment of chronic myeloid leukaemia (CML), some patients become refractory to these therapies. After confirming that interleukin-3 receptor (IL3R, CD123) is highly expressed on CD34(+) /CD38(-) BCR-ABL1(+) CML stem cells, we investigated whether targeting IL3R with diphtheria toxin (DT)-IL3 fusion proteins SL-401 (DT388 -IL3) and SL-501 (DT388 -IL3[K116W]) could eradicate these stem cells. SL-401 and SL-501 inhibited cell growth and induced apoptosis in the KBM5 cell line and its TKI-resistant KBM5-STI subline. Combinations of imatinib with these agents increased apoptosis in KBM5 and in primary CML cells. In six primary CML samples, including CML cells harbouring the ABL1 T315I mutation, SL-401 and SL-501 decreased the absolute numbers of viable CD34(+) /CD38(-) /CD123(+) CML progenitor cells by inducing apoptosis. IL3-targeting agents reduced clonogenic growth and diminished the fraction of primitive long-term culture-initiating cells in samples from patients with advanced phase CML that were resistant to TKIs or harboured an ABL1 mutation. Survival was also extended in a mouse model of primary TKI-resistant CML blast crisis. These data suggest that the DT-IL3 fusion proteins, SL-401 and SL-501, deplete CML stem cells and may increase the effectiveness of current CML treatment, which principally targets tumour bulk.

CD 123 is a membrane biomarker and a therapeutic target in hematologic malignancies

Recent studies indicate that abnormalities of the alpha-chain of the interleukin-3 receptor (IL-3RA or CD123) are frequently observed in some leukemic disorders and may contribute to the proliferative advantage of leukemic cells. This review analyzes the studies indicating that CD123 is overexpressed in various hematologic malignancies, including a part of acute myeloid and B-lymphoid leukemias, blastic plasmocytoid dendritic neoplasms (BPDCN) and hairy cell leukemia.Given the low/absent CD123 expression on normal hematopoietic stem cells, attempts have been made at preclinical first, and then at clinical level to target this receptor. Since the IL-3R is a membrane receptor there are two relatively simple means to target this molecule, either using its natural ligand or neutralizing monoclonal antibodies. Recent reports using a fusion molecule composed by human IL-3 coupled to a truncated diphteria toxin have shown promising antitumor activity in BPDCN and AML patients.

Treatment of hematologic malignancies with immunotoxins and antibody-drug conjugates

To enable antibodies to function as cytotoxic anticancer agents, they are modified either via attachment to protein toxins or highly potent, low-molecular-weight drugs. Such molecules, termed immunotoxins and antibody-drug conjugates, respectively, represent a second revolution in antibody-mediated cancer therapy. Thus, highly toxic compounds are delivered to the interior of cancer cells based on antibody specificity for cell-surface target antigens.

Pentapeptide commonality between Corynebacterium diphtheriae toxin and the Homo sapiens proteome

Cross-reactivity may affect diagnostic tests and cause harmful autoimmune reactions following immunotherapy. To predict potential cross-reactivity and search for safe immunotherapeutic approaches, we analyzed sequence identity between microbial antigens and the human proteome. Using diphtheria toxin (DT) as a model, we examined its patterns of identity with human proteins at the pentapeptide level. DT shares 503 pentapeptides with the human proteome, while only 31 pentapeptides are unique to the toxin. DT pentapeptide identity involves multiple/repeated matches in human proteins (a total of 4966 occurrences). Human proteins containing bacterial peptide matches include antigens linked to fundamental cellular functions, such as cell cycle control, proliferation, development and differentiation. The data presented in this article offer a rational basis for designing peptide-based vaccines that specifically target DT and thus eliminate the potential risk of cross-reactivity with human proteins. More generally, this study proposes a methodological approach for avoiding cross-reactivity in immune reactions.

A novel fusion protein diphtheria toxin-stem cell factor (DT-SCF)-purification and characterization

Fusion toxins are an emerging class of targeted therapeutics for the treatment of cancer. Diphtheria toxin-stem cell factor (DT-SCF) is one such novel fusion toxin designed to target malignancies expressing c-kit. Since, c-kit overexpression has been reported on many types of cancers, it appeared to be a reasonably good molecule to target. In the present study, we report construction, expression, purification, and characterization of DT-SCF. DT-SCF gene coding for 1-387 amino acids of diphtheria toxin, His-Ala linker, 2-141 amino acids of SCF was cloned into expression vector with C terminal His tag. The induced DT-SCF protein was exclusively expressed in insoluble fraction. Purification of DT-SCF was achieved by inclusion body isolation and metal affinity chromatography under denaturing and reducing conditions. Purified DT-SCF was renatured partially on-column by gradually reducing denaturant concentration followed by complete refolding through rapid dilution technique. Cell viability assay provided the evidence that DT-SCF is a potent cytotoxic agent selective to cells expressing c-kit. The novelty of this study lies in employing SCF as a ligand in construction of fusion toxin to target wide range of malignancies expressing c-kit. Efficacy of DT-SCF fusion toxin was demonstrated over a range of malignancies such as chronic myeloid leukemia (K562), acute lymphoblastic leukemia (MOLT4), pancreatic carcinoma (PANC-1), and cervical carcinoma (HeLa 229). This is the first study reporting specificity and efficacy of DT-SCF against tumor cells expressing c-kit. There was significant correlation (P = 0.007) between c-kit expression on cells and their sensitivity to DT-SCF fusion toxin.

Characterization of variant diphtheria toxin-interleukin-3 fusion protein, DTIL3K116W, for phase I clinical trials

We have produced clinical grade of DTIL3K116W, a variant diphtheria toxin-interleukin-3 fusion protein, for treatment of acute myeloid leukemia. The product was filter sterilized, aseptically vialed, and stored at -80 degrees C. It was characterized by Coomassie-stained SDS-PAGE, endotoxin assay, cytotoxicity assay, sterility, mass spectroscopy, receptor binding affinity, ADP-ribosylation, inhibition of normal human CFU-GM, disulfide bond analysis, immunoblots, stability, size exclusion chromatography-HPLC, sequencing, and immunohistochemistry. Vialed product was sterile in 0.25 M NaCl/5 mM Tris, pH 7.9, and had a protein concentration of 1.08 mg/ml. Purity by SDS-PAGE was >99%. Aggregates by HPLC were <1%. Endotoxin levels were 0.296EU/mg. Peptide mapping and mass spectroscopy confirmed its composition and molecular weight. The vialed drug kept reactivity with anti-IL3 and DT antibodies. Potency study revealed a 48-h EC(50) of 0.5 pM on TF1/H-ras cell. Its binding properties were confirmed by competitive experiments showing IC(50) of 1.4 nM. ADP-ribosylation activity was equivalent to DTGM-CSF. Drug did not react with tested frozen human tissue sections by immunohistochemistry. There was no evidence of loss of solubility, proteolysis aggregation, or loss of potency over 6 months at -80 degrees C. Further, the drug was stable at 4 and 25 degrees C in the plastic syringe and administration tubing for 48 h.

Recombinant immunotoxins containing truncated bacterial toxins for the treatment of hematologic malignancies

Immunotoxins are molecules that contain a protein toxin and a ligand that is either an antibody or a growth factor. The ligand binds to a target cell antigen, and the target cell internalizes the immunotoxin, allowing the toxin to migrate to the cytoplasm where it can kill the cell. In the case of recombinant immunotoxins, the ligand and toxin are encoded in DNA that is then expressed in bacteria, and the purified immunotoxin contains the ligand and toxin fused together. Among the most active recombinant immunotoxins clinically tested are those that are targeted to hematologic malignancies. One agent, containing human interleukin-2 and truncated diphtheria toxin (denileukin diftitox), has been approved for use in cutaneous T-cell lymphoma, and has shown activity in other hematologic malignancies, including leukemias and lymphomas. Diphtheria toxin has also been targeted by other ligands, including granulocyte-macrophage colony-stimulating factor and interleukin-3, to target myelogenous leukemia cells. Single-chain antibodies containing variable heavy and light antibody domains have been fused to truncated Pseudomonas exotoxin to target lymphomas and lymphocytic leukemias. Recombinant immunotoxins anti-Tac(Fv)-PE38 (LMB-2), targeting CD25, and RFB4(dsFv)-PE38 (BL22, CAT-3888), targeting CD22, have each been tested in patients. Major responses have been observed after failure of standard chemotherapy. The most successful application of recombinant immunotoxins today is in hairy cell leukemia, where BL22 has induced complete remissions in most patients who were previously treated with optimal chemotherapy.

Recombinant immunotoxins for the treatment of chemoresistant hematologic malignancies

Recombinant immunotoxins are proteins composed of fragments of monoclonal antibodies fused to truncated protein toxins. No agents of this class are approved yet for medical use, although a related molecule, denileukin diftitox, composed of interleukin-2 fused to truncated diphtheria toxin, is approved for relapsed/refractory cutaneous T-cell lymphoma. Recombinant immunotoxins which have been tested in patients with chemotherapy-pretreated hematologic malignancies include LMB-2 (anti-CD25), BL22 (CAT-3888, anti-CD22) and HA22 (CAT-8015, anti-CD22), each containing an Fv fragment fused to truncated Pseudomonas exotoxin. Major responses were observed with LMB-2 in adult T-cell leukemia, chronic lymphocytic leukemia (CLL), cutaneous T-cell lymphoma, Hodgkin's disease, and hairy cell leukemia (HCL). BL22 resulted in a high complete remission rate in patients with HCL, particularly those without excessive tumor burden. HA22, an improved version of BL22 with higher affinity to CD22, is now undergoing phase I testing in HCL, CLL, non-Hodgkin's lymphoma, and pediatric acute lymphoblastic leukemia.

Human Red Blood Cells: Rheological Aspects, Uptake, and Release of Cytotoxic Drugs

The shape of a normal human red blood cell (RBC) is well known: under resting conditions it is that of a biconcave discocyte. However, RBCs can easily undergo transformation to other shapes with stomatocytes and echinocytes as extremes. Various anticancer agents, generally reactive and labile substances, e.g., oxazaphosphorines and fluoropyrimidines, can induce severe deformation of shape. Shape changes in erythrocytes can induce rheological disturbances, which occasionally have pathophysiological consequences. It is difficult to estimate the impact of shape changes on the in vivo behavior of agents of biological interest. However, it has been demonstrated for various anticancer agents that erythrocytes fulfill an important role in their uptake, transport, and release. Moreover, some anticancer agents are capable of influencing important transporters such as MRP and GLUT-1. Monitoring of erythrocyte concentrations of certain cytotoxic agents is therefore of interest as the data generated can have a predictive outcome for therapeutic efficacy. This is true for cyclophosphamide, ifosfamide, lometrexol, and 6-mercaptopurine, as well as MRP and GLUT-1 mediated agents.

New molecular concepts and targets in acute myeloid leukemia

PURPOSE OF REVIEW

Most patients with acute myeloid leukemia treated with chemotherapy relapse. It is increasingly recognized that the cause of chemoresistance and relapse resides within the leukemia stem cell population. Successful eradication of leukemia stem cells would require a comprehensive profile of both the acquired molecular lesions and intrinsic features of leukemia stem cells. This review describes recent work identifying molecular markers that may lead to development of novel therapeutics, ultimately aiming to eradicate leukemia stem cells in acute myeloid leukemia.

RECENT FINDINGS

In recent years, novel specific cell surface antigens have allowed identification of leukemia stem cells and permitted their distinction from normal hematopoietic stem cells. Novel concepts of leukemia stem cells and niche interaction have elucidated the mechanisms that control leukemia stem cell survival and chemoresistance. Recent detection of genetic aberrations affecting regulators of HOX gene expression and chromatin modifying enzymes, such as CDX2 and hDOT1L, respectively, elucidates new key players in stem cell self-renewal and leukemic transformation.

SUMMARY

The discovery of novel markers and survival pathways for leukemia stem cells has increased our potential to specifically target and eliminate the leukemic stem cell compartment, which is likely to improve clinical outcomes in acute myeloid leukemia.

Intracerebral infusion of an EGFR-targeted toxin in recurrent malignant brain tumors

The purpose of this study is to determine the maximum tolerated dose (MTD), dose-limiting toxicity (DLT), and intracerebral distribution of a recombinant toxin (TP-38) targeting the epidermal growth factor receptor in patients with recurrent malignant brain tumors using the intracerebral infusion technique of convection-enhanced delivery (CED). Twenty patients were enrolled and stratified for dose escalation by the presence of residual tumor from 25 to 100 ng/ml in a 40-ml infusion volume. In the last eight patients, coinfusion of (123)I-albumin was performed to monitor distribution within the brain. The MTD was not reached in this study. Dose escalation was stopped at 100 ng/ml due to inconsistent drug delivery as evidenced by imaging the coinfused (123)I-albumin. Two DLTs were seen, and both were neurologic. Median survival after TP-38 was 28 weeks (95% confidence interval, 26.5-102.8). Of 15 patients treated with residual disease, two (13.3%) demonstrated radiographic responses, including one patient with glioblastoma multiforme who had a nearly complete response and remains alive >260 weeks after therapy. Coinfusion of (123)I-albumin demonstrated that high concentrations of the infusate could be delivered >4 cm from the catheter tip. However, only 3 of 16 (19%) catheters produced intraparenchymal infusate distribution, while the majority leaked infusate into the cerebrospinal fluid spaces. Intracerebral CED of TP-38 was well tolerated and produced some durable radiographic responses at doses <or=100 ng/ml. CED has significant potential for enhancing delivery of therapeutic macromolecules throughout the human brain. However, the potential efficacy of drugs delivered by this technique may be severely constrained by ineffective infusion in many patients.

Phase I clinical study of diphtheria toxin-interleukin 3 fusion protein in patients with acute myeloid leukemia and myelodysplasia

DT(388)IL3 fusion protein containing the catalytic and translocation domains of diphtheria toxin fused to human interleukin 3 was administered in an inter-patient dose escalation trial by 15 min i.v. infusions every other day for up to 6 doses to patients with chemo-refractory acute myeloid leukemia (AML) and myelodysplasia (MDS). The maximal tolerated dose was >12.5 microg/kg/dose. Transient grade 3 transaminasemia and grade 2 fevers, chills, hypoalbuminemia, and hypotension occurred. Peak DT(388)IL3 levels correlated with dose and day of administration but not antibody titer. Anti-DT(388)IL3 antibodies developed in most patients between day 15 and 30. Of 40 evaluable AML patients, 1 had a CR (8 months) and 1 had PR (3 months). Of 5 MDS patients, 1 had a PR (4 months). Because of the prolonged infusion schedule, many patients failed to receive six doses. DT(388)IL3 produces remissions in patients with relapsed/refractory AML and MDS with minimal toxicities, and alternate schedules of administration are needed to enhance the response rate.

Targeting survival cascades induced by activation of Ras/Raf/MEK/ERK, PI3K/PTEN/Akt/mTOR and Jak/STAT pathways for effective leukemia therapy

The Raf/MEK/ERK, PI3K/PTEN/Akt/mTOR and Jak/STAT pathways are frequently activated in leukemia and other hematopoietic disorders by upstream mutations in cytokine receptors, aberrant chromosomal translocations as well as other genetic mechanisms. The Jak2 kinase is frequently mutated in many myeloproliferative disorders. Effective targeting of these pathways may result in suppression of cell growth and death of leukemic cells. Furthermore it may be possible to combine various chemotherapeutic and antibody-based therapies with low molecular weight, cell membrane-permeable inhibitors which target the Raf/MEK/ERK, PI3K/PTEN/Akt/mTOR and Jak/STAT pathways to ultimately suppress the survival pathways, induce apoptosis and inhibit leukemic growth. In this review, we summarize how suppression of these pathways may inhibit key survival networks important in leukemogenesis and leukemia therapy as well as the treatment of other hematopoietic disorders. Targeting of these and additional cascades may also improve the therapy of chronic myelogenous leukemia, which are resistant to BCR-ABL inhibitors. Furthermore, we discuss how targeting of the leukemia microenvironment and the leukemia stem cell are emerging fields and challenges in targeted therapies.

MDS: a stem cell disorder--but what exactly is wrong with the primitive hematopoietic cells in this disease?

Despite the various abnormalities identified in the immune system or the bone marrow microenvironment in patients with myelodysplastic syndrome (MDS), most of the investigation of this disorder has centered on the hematopoietic stem/progenitor compartment. It is generally written that MDS is a stem cell disorder, and there is certainly evidence supporting this view. However, whether it occurs in a cell with only myeloid multipotentiality (i.e., that involves megakaryocytic, erythroid and granulocytic/monocytic lineages) or occurs in a true stem cell is open to debate. The absence of an assay for human stem cells necessitates the use of surrogate markers for such cells, such as gene expression profiles, or the identification of specific genetic or epigenetic abnormalities that are found in multiple lineages. Clearly, the common cytogenetic and genetic abnormalities found in MDS are most indicative of a clonal myeloid disease similar to AML, rather than a lymphoid disease, and the often tri-lineage ineffective hematopoiesis and dysplasia are generally not found within the lymphoid compartment. Recent studies, using modern molecular detection techniques, have identified new recurring molecular lesions in these disorders but have not really unraveled its pathogenesis.

New immunotoxins targeting CD123, a stem cell antigen on acute myeloid leukemia cells

The specific alpha subunit of the interleukin-3 receptor (IL-3Ralpha, CD123) is strongly expressed in various leukemic blasts and leukemic stem cells and seems to be an excellent target for the therapy of leukemias. In this study, immunotoxins were developed to target CD123 only, which bypasses the dependence on other subunits to form intact IL-3R. Three anti-CD123 hybridomas (26292, 32701, and 32716) were selected on the basis of their affinity for CD123. Total RNAs were extracted from the 3 anti-CD123 hybridomas and used to clone the fragment of variable region (Fvs). The Fvs were assembled into single chain Fvs and fused to a 38-kd fragment of Pseudomonas exotoxin A to make recombinant immunotoxins. 26292(Fv)-PE38 was found to have the highest cytotoxic activity on the CD123 expressing leukemia cell line TF-1. It bound the cells with a kd of 3.5 nM. Another immunotoxin, 32716(Fv)-PE38, belonging to a different epitope group, had a similar binding ability but was less active, demonstrating the role of epitope selection in immunotoxin action. The cytotoxic activity of 26292(Fv)-PE38 was increased from 200 to about 40 ng/mL by mutating the REDLK sequence at the C terminus to KDEL. 26292(Fv)-PE38-KDEL was specifically cytotoxic to several CD123 expressing cell lines (TF-1, Molm-13, and Molm-14) with good CD123 expression but not to ML-1 or U937 with low or absent expression. In conclusion, 26292(Fv)-PE38-KDEL shows good cytotoxic activity against CD123 expressing cell lines, and merits further development for the possible treatment of acute myeloid leukemia and other CD123 expressing malignancies.

Immunotoxin treatment of cancer

Immunotoxins are proteins used to treat cancer that are composed of an antibody fragment linked to a toxin. The immunotoxin binds to a surface antigen on a cancer cell, enters the cell by endocytosis, and kills it. The most potent immunotoxins are made from bacterial and plant toxins. Refinements over many years have produced recombinant immunotoxins; these therapeutic proteins are made using protein engineering. Individual immunotoxins are designed to treat specific cancers. To date, most success has been achieved treating hematologic tumors. Obstacles to successful treatment of solid tumors include poor penetration into tumor masses and the immune response to the toxin component of the immunotoxin, which limits the number of cycles that can be given. Strategies to overcome these limitations are being pursued.

Expression of the target receptor CD33 in CD34+/CD38-/CD123+ AML stem cells

BACKGROUND

CD33 (Siglec-3) is becoming increasingly important as a target of antibody-mediated therapy in acute myeloid leukaemia (AML). In normal myelopoiesis, expression of CD33 is restricted to advanced stages of differentiation, whereas primitive stem cells do not express CD33. In the present study, we asked whether leukaemic stem cells in patients with AML express CD33.

MATERIALS AND METHODS

A multicolour-staining technique was applied in 11 patients with AML, and leukaemic progenitors defined as CD34(+)/CD38(-)/CD123(+) cells. AML stem cells were purified by cell sorting and were examined for expression of CD33 mRNA by reverse transcriptase-polymerase chain reaction (RT-PCR).

RESULTS

In all patients in whom the majority of myeloblasts expressed CD33 (n = 8), AML progenitors reacted with the CD33 antibody P67.6. Repopulation experiments utilizing irradiated NOD/SCID mice confirmed that AML stem cells in these patients reside within the CD33(+) subpopulation of the leukaemic clone. Moreover, highly purified AML stem cells (> 98% purity) from patients with CD33(+) AML were found to express CD33 mRNA in RT-PCR analyses. CD33 was neither detectable on CD34(+)/CD38(-) cells in normal bone marrow nor on leukaemic stem cells in patients with CD33-negative AML.

CONCLUSIONS

Leukaemic stem cells in patients with CD33(+) AML express CD33. This observation is in favour of novel treatment concepts employing CD33-targeting antibodies in AML.