Efficacy and safety of gemtuzumab ozogamicin in patients with CD33-positive acute myeloid leukaemia in first relapse

Transforming toxins into treatments: the revolutionary role of α-amanitin in cancer therapy

Amanita phalloides is the primary species responsible for fatal mushroom poisoning, as its main toxin, α-amanitin, irreversibly and potently inhibits eukaryotic RNA polymerase II (RNAP II), leading to cell death. There is no specific antidote for α-amanitin, which hinders its clinical application. However, with the advancement of precision medicine in oncology, including the development of antibody-drug conjugates (ADCs), the potential value of various toxic small molecules has been explored. These ADCs ingeniously combine the targeting precision of antibodies with the cytotoxicity of small-molecule payloads to precisely kill tumor cells. We searched PubMed for studies in this area using these MeSH terms "Amanitins, Alpha-Amanitin, Therapeutic use, Immunotherapy, Immunoconjugates, Antibodies" and did not limit the time interval. Recent studies have conducted preclinical experiments on ADCs based on α-amanitin, showing promising therapeutic effects and good tolerance in primates. The current challenges include the not fully understood toxicological mechanism of α-amanitin and the lack of clinical studies to evaluate the therapeutic efficacy of ADCs developed based on α-amanitin. In this article, we will discuss the role and therapeutic efficacy of α-amanitin as an effective payload in ADCs for the treatment of various cancers, providing background information for the research and application strategies of current and future drugs.

Conditional activation of an anti-IgM antibody-drug conjugate for precise B cell lymphoma targeting

Cancerous B cells are almost indistinguishable from their non-malignant counterparts regarding their surface antigen expression. Accordingly, the challenge to be faced consists in elimination of the malignant B cell population while maintaining a functional adaptive immune system. Here, we present an IgM-specific antibody-drug conjugate masked by fusion of the epitope-bearing IgM constant domain. Antibody masking impaired interaction with soluble pentameric as well as cell surface-expressed IgM molecules rendering the antibody cytotoxically inactive. Binding capacity of the anti-IgM antibody drug conjugate was restored upon conditional protease-mediated demasking which consequently enabled target-dependent antibody internalization and subsequent induction of apoptosis in malignant B cells. This easily adaptable approach potentially provides a novel mechanism of clonal B cell lymphoma eradication to the arsenal available for non-Hodgkin's lymphoma treatment.

Mechanisms of ADC Toxicity and Strategies to Increase ADC Tolerability

Anti-cancer antibody-drug conjugates (ADCs) aim to expand the therapeutic index of traditional chemotherapy by employing the targeting specificity of monoclonal antibodies (mAbs) to increase the efficiency of the delivery of potent cytotoxic agents to malignant cells. In the past three years, the number of ADCs approved by the Food and Drug Administration (FDA) has tripled. Although several ADCs have demonstrated sufficient efficacy and safety to warrant FDA approval, the clinical use of all ADCs leads to substantial toxicity in treated patients, and many ADCs have failed during clinical development due to their unacceptable toxicity profiles. Analysis of the clinical data has demonstrated that dose-limiting toxicities (DLTs) are often shared by different ADCs that deliver the same cytotoxic payload, independent of the antigen that is targeted and/or the type of cancer that is treated. DLTs are commonly associated with cells and tissues that do not express the targeted antigen (i.e., off-target toxicity), and often limit ADC dosage to levels below those required for optimal anti-cancer effects. In this manuscript, we review the fundamental mechanisms contributing to ADC toxicity, we summarize common ADC treatment-related adverse events, and we discuss several approaches to mitigating ADC toxicity.

Leucine-rich alpha-2-glycoprotein 1 (LRG1) as a novel ADC target

Leucine-rich alpha-2-glycoprotein 1 (LRG1) is present abundantly in the microenvironment of many tumours where it contributes to vascular dysfunction, which impedes the delivery of therapeutics. In this work we demonstrate that LRG1 is predominantly a non-internalising protein. We report the development of a novel antibody-drug conjugate (ADC) comprising the anti-LRG1 hinge-stabilised IgG4 monoclonal antibody Magacizumab coupled to the anti-mitotic payload monomethyl auristatin E (MMAE) via a cleavable dipeptide linker using the site-selective disulfide rebridging dibromopyridazinedione (diBrPD) scaffold. It is demonstrated that this ADC retains binding post-modification, is stable in serum and effective in in vitro cell studies. We show that the extracellular LRG1-targeting ADC provides an increase in survival in vivo when compared against antibody alone and similar anti-tumour activity when compared against standard chemotherapy, but without undesired side-effects. LRG1 targeting through this ADC presents a novel and effective proof-of-concept en route to improving the efficacy of cancer therapeutics.

High-Affinity Human Anti-c-Met IgG Conjugated to Oxaliplatin as Targeted Chemotherapy for Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is one of the most mortality-causing solid cancers globally and the second largest cause of death among malignancies. Oxaliplatin, a platinum-based drug, has been widely utilized in the treatment of malignancies such as colorectal cancer and hepatocellular carcinoma, yet its usage is limited because of severe side effects of cytotoxicity to normal tissues. c-Met, a receptor tyrosine kinase, is expressed aberrantly on the surface of HCC. The purpose of this study was to synthesise a humanized antibody against c-Met (anti-c-Met IgG) and conjugate it to oxaliplatin to develop a novel antibody-drug conjugate (ADC). Anti-c-Met IgG was detected to be loaded with ~4.35 moles oxaliplatin per mole of antibody. ELISA and FCM confirmed that ADC retained a high and selective binding affinity for c-Met protein and c-Met-positive HepG2 cells. In vitro, the cytotoxicity tests and biological function assay indicated that ADC showed much higher cytotoxicity and functioning in c-Met-positive HepG2 cells, compared with shMet-HepG2 cells expressing lower levels of c-Met. Furthermore, compared with free oxaliplatin, ADC significantly improved cytotoxicity to c-Met-positive tumours and avoided off-target cell toxicity in vivo. In conclusion, by targeting c-Met-expressing hepatoma cells, ADC can provide a platform to reduce drug toxicity and improve drug efficacy in vitro and in vivo.

The expansion of targetable biomarkers for CAR T cell therapy

Biomarkers are an integral part of cancer management due to their use in risk assessment, screening, differential diagnosis, prognosis, prediction of response to treatment, and monitoring progress of disease. Recently, with the advent of Chimeric Antigen Receptor (CAR) T cell therapy, a new category of targetable biomarkers has emerged. These biomarkers are associated with the surface of malignant cells and serve as targets for directing cytotoxic T cells. The first biomarker target used for CAR T cell therapy was CD19, a B cell marker expressed highly on malignant B cells. With the success of CD19, the last decade has shown an explosion of new targetable biomarkers on a range of human malignancies. These surface targets have made it possible to provide directed, specific therapy that reduces healthy tissue destruction and preserves the patient's immune system during treatment. As of May 2018, there are over 100 clinical trials underway that target over 25 different surface biomarkers in almost every human tissue. This expansion has led to not only promising results in terms of patient outcome, but has also led to an exponential growth in the investigation of new biomarkers that could potentially be utilized in CAR T cell therapy for treating patients. In this review, we discuss the biomarkers currently under investigation and point out several promising biomarkers in the preclinical stage of development that may be useful as targets.

CD99 is a therapeutic target on disease stem cells in myeloid malignancies

Acute myeloid leukemia (AML) and the myelodysplastic syndromes (MDS) are initiated and sustained by self-renewing malignant stem cells; thus, eradication of AML and MDS stem cells is required for cure. We identified CD99 as a cell surface protein frequently overexpressed on AML and MDS stem cells. Expression of CD99 allows for prospective separation of leukemic stem cells (LSCs) from functionally normal hematopoietic stem cells in AML, and high CD99 expression on AML blasts enriches for functional LSCs as demonstrated by limiting dilution xenotransplant studies. Monoclonal antibodies (mAbs) targeting CD99 induce the death of AML and MDS cells in a SARC family kinase-dependent manner in the absence of immune effector cells or complement, and anti-CD99 mAbs exhibit antileukemic activity in AML xenografts. These data establish CD99 as a marker of AML and MDS stem cells, as well as a promising therapeutic target in these disorders.

Dose-related efficacy and toxicity of gemtuzumab ozogamicin in pediatric acute myeloid leukemia

Gemtuzumab ozogamicin, an anti-tumour antibiotic linked to an anti-CD33 antibody (Mylotarg®), has been well studied in AML in adults but to a lesser extent in children. No review has yet been published on the dose-related toxicity and efficacy of gemtuzumab ozogamicin in pediatric AML patients. Here we looked at 14 studies then scatterplots and linear regressions were used to estimate the relationship between the dose of gemtuzumab and its toxicity and efficacy. A non-significant increase in bilirubin level and in incidence of veno-occlusive disease was seen with higher doses of gemtuzumab ozogamicin when used as single-agent. In terms of efficacy, even a low dose of 3 mg/m(2) of gemtuzumab ozogamicin can have antileukemic effect, but available data do not allow conclusions on its dose-dependency. Data indicate that higher doses of gemtuzumab ozogamicin account for more adverse events. The data do not show that a high dose is required for anti-leukemic efficacy of gemtuzumab ozogamicin. This study also indicates that there seems to be a role for gemtuzumab ozogamicin in the treatment of pediatric AML and further studies are required to assess its optimal dose, schedule and balance between efficacy and side-effects.

Relapsed and refractory pediatric acute myeloid leukemia: current and emerging treatments

Survival rates for children with acute myeloid leukemia (AML) exceed 60 % when modern, intensified chemotherapeutic regimens and enhanced supportive care measures are employed. Despite well-recognized improvements in outcomes, primary refractory or relapsed pediatric AML yields significant morbidity and mortality, and improved understanding of this obstinate population along with refined treatment protocols are urgently needed. Although a significant number of patients with refractory or relapsed disease will achieve remission, long-term survival rates remain poor, and efforts to identify therapies which will improve OS are under continuous investigation. The current fundamental goal of such investigation is the achievement of as complete a remission as possible without dose-limiting toxicities, and the progression to hematopoietic stem cell transplantation thereafter. In this review the scope of the problem of relapsed and refractory AML as well as current and emerging chemotherapy options will be discussed.

CD33 in Alzheimer's disease

The amyloid-beta peptide (Aβ) cascade hypothesis posits that Aβ accumulation is the fundamental initiator of Alzheimer's disease (AD), and mounting evidence suggests that impaired Aβ clearance rather than its overproduction is the major pathogenic event for AD. Recent genetic studies have identified cluster of differentiation 33 (CD33) as a strong genetic locus linked to AD. As a type I transmembrane protein, CD33 belongs to the sialic acid-binding immunoglobulin-like lectins, mediating the cell-cell interaction and inhibiting normal functions of immune cells. In the brain, CD33 is mainly expressed on microglial cells. The level of CD33 was found to be increased in the AD brain, which positively correlated with amyloid plaque burden and disease severity. More importantly, CD33 led to the impairment of microglia-mediated clearance of Aβ, which resulted in the formation of amyloid plaques in the brain. In this article, we review the recent epidemiological findings of CD33 that related with AD and discuss the levels and pathogenic roles of CD33 in this disease. Based on the contributing effects of CD33 in AD pathogenesis, targeting CD33 may provide new opportunities for AD therapeutic strategies.

Risks and untoward toxicities of antibody-based immunoconjugates

Antibody-based immunoconjugates are specifically targeted monoclonal antibodies that deliver a cytotoxic payload to their target. The cytotoxic agents can be highly potent drugs, radionuclides or toxins. Such molecules, referred to as antibody-drug conjugates, radioimmunoconjugates and immunotoxins, respectively, represent a promising approach for enhancing the efficacy of unconjugated (naked) antibodies for improved therapeutic results. Though tremendous progress has been achieved over the last few decades, the safety of these molecules still remains a matter of concern and a careful design is required for achieving a relatively safe toxicity profile along with therapeutic effectiveness. This review focuses on the toxicities arising from the use of these potent agents.

Effective elimination of acute myeloid leukemic cells by recombinant bispecific antibody derivatives directed against CD33 and CD16

Single-chain Fv triplebodies (sctb), consisting of a single polypeptide chain with 3 single-chain antibody variable fragments connected in tandem, were generated as antileukemic agents. A CD19-specific sctb of this format has previously been shown to be superior to a bispecific single-chain Fv antibody fragment (bsscFv) for the elimination of leukemic B-lineage cells, but corresponding targeted agents for the treatment of acute myeloid leukemia are still lacking. For this purpose, both a bsscFv and a sctb specific for CD33 and the trigger molecule CD16 (FcgammaRIII) were produced. The sctb displayed 3.5-fold greater avidity for CD33 than the bsscFv 33xds16, whereas both had close to equal affinity for CD16. In antibody-dependent cellular cytotoxicity (ADCC) reactions with human mononuclear cells as effectors, both the bsscFv 33xds16 and the sctb induced lysis of tumor cells with half maximum effective concentrations (EC50) in the low picomolar range. It is interesting to note that the sctb promoted equal lysis of human leukemia-derived cell lines at 10 to 200-fold lower concentrations than the bsscFv. Both molecules mediated ADCC of primary patient cells. In conclusion, both the bsscFv 33xds16 and the sctb 33xds16x33 eliminated acute myeloid leukemia cells in ADCC reactions, but the novel sctb format showed significantly greater specific activity.

High dose cytarabine plus gemtuzumab ozogamicin for patients with relapsed or refractory acute myeloid leukemia: Cancer and Leukemia Group B study 19902

Gemtuzumab ozogamicin (GO), an anti-CD33 immunoconjugate, was combined with high dose cytarabine (HiDAC; cytarabine 3g/m(2) over 3h daily for 5 days) for adults with relapsed or refractory AML. HiDAC plus GO 9mg/m(2) on day 7 and 4.5mg/m(2) on day 14 was not tolerated, but HiDAC followed by GO 9mg/m(2) on day 7 was safe: 12/37 (32%) patients with relapsed AML achieved complete remission. Median overall survival was 8.9 months. No grade 4 hepatic veno-occlusive disease was observed. This regimen merits further study, both in this setting and as a remission consolidation therapy.

A recombinant trispecific single-chain Fv derivative directed against CD123 and CD33 mediates effective elimination of acute myeloid leukaemia cells by dual targeting

Two trivalent constructs consisting of single-chain Fv antibody fragments (scFvs) specific for the interleukin-3 receptor alpha chain (CD123), CD33 and the Fcgamma-receptor III (CD16) were designed and characterized for the elimination of acute myeloid leukaemia (AML) cells. The dual targeting single-chain Fv triplebody (sctb) [123 x ds16 x 33] and the mono targeting sctb [123 x ds16 x 123] both specifically bound their respective target antigens and were stable in human serum at 37 degrees C for at least 5 d. Both constructs induced potent antibody-dependent cellular cytotoxicity (ADCC) of two different AML-derived CD33- and CD123 double-positive cell lines in the low picomolar range using isolated mononuclear cells (MNCs) as effector cells. In these experiments the dual targeting molecule produced significantly stronger lysis than the mono targeting agent. In addition, the sctbs showed a high potency in mediating ADCC of primary leukaemia cells isolated from peripheral blood or bone marrow of seven AML patients. Hence, these novel molecules displayed potent anti-leukaemic effects against AML cells in vitro and represent attractive candidates for further preclinical development.

Sequence of administration and methylation of SOCS3 may govern response to gemtuzumab ozogamicin in combination with conventional chemotherapy in patients with refractory or relapsed acute myelogenous leukemia (AML)

In older patients suffering from acute myelogenous leukemia (AML), aggressive chemotherapy is accompanied with high treatment-related morbidity and mortality. Gemtuzumab ozogamicin (GO), a humanized monoclonal anti-CD33 antibody, represents a well tolerated treatment option, but optimal treatment schedules are still unknown. Additionally, Suppressor of cytokine signaling 3 (SOCS3) inhibits the CD33-induced block on cytokine-induced proliferation. Consequently, a variable response of AML cells to anti-CD33-targeted therapy may be caused by modulation of SOCS3 expression. Twenty-four patients with refractory or relapsed CD33-positive AML received GO as a single agent before or after conventional chemotherapy. The methylation status of the SOCS3 CpG island was assessed by methylation-specific polymerase chain reaction. Response (RR) and overall survival (OS) were significantly higher in 16 patients receiving chemotherapy before GO (RR 81%, OS 14.8 months) compared to three patients who received GO single agent therapy (RR 33%, OS 7.2 months) or 16 with GO before chemotherapy (RR 0% OS 2.2 months, P = 0.01 for RR and P < 0.001 for OS). Methylation of the SOCS3 CpG island was found in 8/24 patients. There was a trend towards a higher RR and longer OS in patients with SOCS3 hypermethylation (RR 86%, OS 25.1 months) compared to unmethylated SOCS3 (RR 56%, OS 10.3 months, P = 0.09). Administration of GO a few days after chemotherapy seems to provide better response and survival compared to administration of GO directly before chemotherapy. The potential role of SOCS3 hypermethylation as a biomarker should be further investigated in patients undergoing GO containing therapies.

Novel conjugates of single-chain Fv antibody fragments specific for stem cell antigen CD123 mediate potent death of acute myeloid leukaemia cells

Four new single-chain Fv antibody fragments (scFvs) specific for the human leucocyte surface antigen CD123 (interleukin-3 receptor alpha) were generated to achieve preferential targeting of leukaemia stem cells (LSCs) in acute myeloid leukaemia (AML). The scFvs were isolated from a phage display library generated with spleen RNA from mice, immunized with a fusion protein consisting of the extracellular domain of CD123 and the Fc domain of a human immunoglobulin G1. The scFvs displayed CD123-specific binding on tumour cells (binding constants (K(D)) 4.5-101 nmol/l). The scFv with the highest affinity was used to design two cell death-inducing molecules. First, an immunotoxin, a fusion protein with truncated Pseudomonas Exotoxin A, induced potent apoptosis of AML-derived MOLM-13 and SKNO-1 cells at nanomolar concentrations. Second, the fusion to another scFv, specific for the low affinity Fcgamma-receptor III (CD16), created a bispecific single chain Fv (bsscFv). This bsscFv [123 x ds16] mediated potent lysis of AML-derived MOLM-13, THP-1 and SKNO-1 cells in antibody-dependent cellular cytotoxicity (ADCC) reactions at picomolar concentrations. The recruitment of CD16-positive effector cells for the lysis of AML cells via CD123 represents a novel combination with attractive prospects for future clinical testing.

Potent antibody drug conjugates for cancer therapy

Significant progress has been made in the past few years in the area of antibody drug conjugates (ADCs) for the selective delivery of cytotoxic drugs to tumors. Early work in this field incorporated clinically approved drugs and mouse monoclonal antibodies (mAbs), which had modest activities, and were generally immunogenic. The results of these studies prompted investigation that led to the identity of several key parameters that influenced activity and tolerability. These included the antigen target, the use of non-immunogenic mAb carriers, the incorporation of highly potent drugs and novel conditionally stable linker technologies, and the specific methods used to attach drugs to mAbs. As a result of these investigations, new agents with pronounced clinical activities have been developed. These include SGN-35, an ADC directed against the CD30-positive malignancies such as Hodgkin's disease and anaplastic large cell lymphoma, and trastuzumab-DM1 which has shown activity in metastatic breast carcinoma. This review details many of the technological advancements, and provides examples of promising ADCs that are currently in clinical trials.

Basic and clinical immunology of Siglecs

Siglecs are cell-surface proteins found primarily on hematopoietic cells. By definition, they are members of the immunoglobulin gene super-family and bind sialic acid. Most contain cytoplasmic tyrosine motifs implicated in cell signaling. This review will first summarize characteristics common and unique to Siglecs, followed by a discussion of each human Siglec in numerical order, mentioning in turn its closest murine ortholog or paralog. Each section will describe its pattern of cellular expression, latest known immune functions, ligands, and signaling pathways, with the focus being predominantly on CD33-related Siglecs. Potential clinical and therapeutic implications of each Siglec will also be covered.

Antibody therapy for acute myeloid leukemia

Due to the high rate of relapse in younger patients and the overall poor outcome in older patients, novel therapies are needed for the treatment of acute myeloid leukemia (AML). Monoclonal antibodies have become an important treatment modality in cancer therapy. Gemtuzumab ozogamicin (GO), an anti-CD33 immunoconjugate, was approved by the US Food and Drug Administration (FDA) for the treatment of elderly patients with relapsed AML who are not candidates for standard chemotherapy. Single-agent GO and combinations with standard chemotherapeutics have been explored extensively in this disease. Hepatotoxicity and delayed myelosuppression have been dose-limiting. Its toxicity profile is reduced with decreased doses of GO and even by administering only a single infusion. In patients with acute promyelocytic leukemia (APL), the addition of GO can produce molecular remissions and is well tolerated. Targeted immunotherapy with GO for treatment of AML has produced remissions. In order to reduce toxicity and improve efficacy, its optimal dose and schedule and pairing with other standard chemotherapeutic agents need to be defined better in large clinical trials.

Impaired CD163-mediated hemoglobin-scavenging and severe toxic symptoms in patients treated with gemtuzumab ozogamicin

We describe a novel syndrome of severe toxic symptoms during intravascular hemolysis due to impaired hemoglobin scavenging in 2 children with acute myeloid leukemia undergoing CD33-directed therapy with the immunotoxin gemtuzumab ozogamicin (GO). A simultaneous high plasma hemoglobin, haptoglobin, and low bilirubin after septicemia-induced intravascular hemolysis indicated abrogated clearance of haptoglobin-hemoglobin complexes. This was further supported by low levels of plasma soluble CD163 and a concordant low number of CD163-expressing monocytes. We show that CD163 positive monocytes and macrophages from liver, spleen, and bone marrow coexpress CD33, thus suggesting that the GO-induced cellular cytotoxicity of CD33 positive cells eradicates a significant part of the CD163 positive monocytes and macrophages. The risk of severe toxic symptoms from plasma hemoglobin should be considered after CD33-targeted chemotherapy when the disease is complicated by a pathologic intravascular hemolysis. Furthermore, the cases provide further circumstantial evidence of a key role of (CD163-expressing) monocytes/macrophages in plasma hemoglobin clearance in vivo.

Novel therapeutic agents in acute myeloid leukemia

Acute myeloid leukemia (AML) is an intrinsically resistant disease. Prognosis is poor for the majority of AML patients, based on age and/or adverse biologic features. Standard therapy for AML is highly toxic and poorly tolerated, particularly by the group of older patients for whom few useful therapies exist. Allogeneic hematopoietic stem cell transplantation is an important option for patients with high-risk AML during first remission, as well as for any patient in second or subsequent remission. Use of reduced intensity conditioning transplantations has made allogeneic stem cell transplantation available for a wider group of individuals, but the impact of this novel procedure on the natural history of AML is unknown. The major thrust of novel therapeutics in AML is development of so-called targeted therapies, which are based on exploitation of newly understood pathophysiological events critical for leukemogenesis. Such events include unbridled proliferation, failure to differentiate, stromal cell-mediated survival factors, and failure to undergo normal programmed cell death. Therapies developed to deal with these problems include inhibitors of ras physiology and activated tyrosine kinases, such as fms-like tyrosine kinase 3; histone deacetylase inhibitors, and DNA-hypomethylating agents, which promote transcription of silenced genes; angiogenesis inhibitors; and anti-bcl-2 agents, respectively. Challenges in therapeutic development include the likelihood that only a subset of AML patients will respond to any of these therapies, based on the patient's intrinsic pathophysiology as well as the fact that many of these agents will only work in conjunction with chemotherapy or other viable antileukemic therapies.

Technology Insight: cytotoxic drug immunoconjugates for cancer therapy

The successful introduction of genetically engineered human and chimeric immunoglobulin proteins has established monoclonal antibodies (mAbs) as a validated approach for treating malignancies. The unique properties of mAb therapies including their high affinity and specificity, and the differential expression of target antigen in tumor cells versus normal cells make them attractive agents for cancer immunotherapy. The field of immunoconjugate development attempts to combine the specificity of mAb therapies with cytotoxic and radionuclide molecules, thereby combining the best characteristics of these two different modalities. Two radiolabeled mAbs, (90)Y-ibritumomab tiuxetan and (131)I-tositumomab, and one drug conjugate, gemtuzumab ozogamicin have been approved for the treatment of malignancies. Other conjugates carrying toxic payloads of calicheamicin, geldanamycin, maytansinoids and taxoids as well as peptide exotoxins are undergoing preclinical and clinical development. Nevertheless, several obstacles have limited robust antitumor activity and broad application of imunoconjugates, including the optimization of three structural components of the immunoconjugate (i.e. mAb and target specificity, chemical linker design, and the cytotoxin), as well as issues common to mAb therapy such as heterogeneous antigen expression, which can limit uniform antibody delivery. This Review examines optimal design, the lessons learned from clinical immunoconjugate development, and the promising agents in early preclinical/clinical development for the treatment of cancer.

Leukemia-targeting ligands isolated from phage-display peptide libraries

Ligands specifically binding to leukemia cells may be used for drug targeting, resulting in more effective treatment with less side effects. Little is known about receptors specifically expressed on acute myeloid leukemia (AML) cells or ligands thereof. We selected random phage display peptide libraries on Kasumi-1 AML cells. A peptide with the sequence CPLDIDFYC was enriched. Phage displaying this peptide strongly bound to Kasumi-1 and SKNO-1 cells and binding could be inhibited by the cognate peptide. Both, Kasumi-1 and SKNO-1 cells carry the chromosomal translocation t(8;21), leading to aberrant expression of the fusion protein AML1/ETO. CPLDIDFYC also strongly and specifically bound primary AML1/ETO-positive AML blasts as well as U-937 cells with forced AML1/ETO expression, suggesting that the CPLDIDFYC receptor may be upregulated upon AML1/ETO expression. Gene expression profiling comparing a panel of CPLDIDFYC-binding and CPLDIDFYC-nonbinding cell lines identified a set of potential receptors for the CPLDIDFYC peptide. Further analysis suggested that alpha4beta1 integrin (VLA-4) is the CPLDIDFYC receptor. Finally, we showed that the CPLDIDFYC-phage is internalized upon receptor binding, suggesting that the CPLDIDFYC-receptor-ligand interaction may be exploitable for targeting drugs or gene therapy vectors to leukemia cells carrying the suitable receptor.

A CD33-specific single-chain immunotoxin mediates potent apoptosis of cultured human myeloid leukaemia cells

A novel single-chain immunotoxin was constructed by combining a CD33-specific single chain Fv (scFv) antibody fragment with an engineered variant of Pseudomonas exotoxin A (ETA). The variant toxin carries the KDEL peptide at its C-terminus, a cellular peptide mediating improved retrograde transport to the endoplasmic reticulum. The purified recombinant fusion protein induced potent apoptosis of the human myeloid cell lines U937, HL-60 and THP-1. Up to 98% of U937 cells were eliminated after treatment for 72 h with a single dose of 500 ng/ml (c. 7 nmol/l). Killing was antigen-specific and occurred by apoptosis. A control protein, consisting of a CD19-specific scFv antibody fragment fused to the ETA-KDEL toxin, failed to induce death of the CD19-negative cell lines U937, HL-60 and THP-1. The CD33-ETA toxin also mediated apoptosis of fresh patient-derived acute myeloid leukaemia cells from bone marrow and peripheral blood. The pronounced antigen-restricted cytotoxicity of the novel fusion protein makes it a candidate for further evaluation of its therapeutic potential.

Designing antibodies for oncology

The past five years have witnessed the emergence of monoclonal antibodies as important therapeutics for cancer treatment. Lower toxicity for antibodies versus small molecules, the potential for increased efficacy by conjugation to radioisotopes and cellular toxins, or the ability to exploit immune cell functions have led to clinical performances on par or superior to conventional drug therapies. This review outlines the various immunoglobulin design strategies currently available, techniques used to reduce Ig antigenicity and toxicity, and points to consider during the manufacture of antibodies for use in clinical oncology.

The Evolution of Antibodies into Versatile Tumor-Targeting Agents

In recent years, monoclonal antibodies have become important weapons in the arsenal of anticancer drugs, and in select cases are now the drugs of choice due to their favorable toxicity profiles. Originally developed to confer passive immunity against tumor-specific antigens, clinical uses of monoclonal antibodies are expanding to include growth factor sequestration, signal transduction modulation, and tumor-specific drug delivery. In this review, we shall present the origins of antibody therapeutics within the field of immunotherapy and their evolution into effective anticancer agents, then discuss their multiple mechanisms of action, the basis of their tumor selectivity, and their therapeutic properties compared with traditional therapies. Antibodies are complex molecules whose efficacy and toxicity depend on the antigen, the antibody, any conjugated groups, and even the patient. Finally, we shall present new technologies being developed to increase the efficacy and selectivity of antibody-based therapeutics. Interestingly, many of the new approaches straddle the middle ground between immunotherapy and the traditional modalities of chemotherapy and radiotherapy, and can be seen as ways of combining the selectivity of the former with the efficacy of the latter.

Caspase-mediated cell death in hematological malignancies: theoretical considerations, methods of assessment, and clinical implications

Apoptosis, the caspase-mediated cell death, plays an important role in the etiology, pathogenesis and therapy of a variety of diseases. Abnormalities of apoptosis regulation, resulting in either its inhibition or enhancement, play a key role in the development of various malignant hematological disorders. Several routine and new therapeutic strategies in Oncohematology are based on apoptosis modulation. Cytotoxic effects of most antineoplastic drugs are based on induction of apoptosis. The accurate estimate of incidence of apoptosis, therefore, is of importance in Oncohematology. In this review we provide an overview of the methods designed to measure the incidence of apoptosis, including the recently developed assays that are based on detection of caspases activation. We also review recent findings on the role of caspase-mediated cell death in hematological malignancies and discuss their clinical implications, including new therapeutical strategies that evolve from these findings.

Detection of minimal residual disease in hematologic malignancies by real-time quantitative PCR: principles, approaches, and laboratory aspects

Detection of minimal residual disease (MRD) has prognostic value in many hematologic malignancies, including acute lymphoblastic leukemia, acute myeloid leukemia, chronic myeloid leukemia, non-Hodgkin's lymphoma, and multiple myeloma. Quantitative MRD data can be obtained with real-time quantitative PCR (RQ-PCR) analysis of immunoglobulin and T-cell receptor gene rearrangements, breakpoint fusion regions of chromosome aberrations, fusion-gene transcripts, aberrant genes, or aberrantly expressed genes, their application being dependent on the type of disease. RQ-PCR analysis can be performed with SYBR Green I, hydrolysis (TaqMan) probes, or hybridization (LightCycler) probes, as detection system in several RQ-PCR instruments. Dependent on the type of MRD-PCR target, different types of oligonucleotides can be used for specific detection, such as an allele-specific oligonucleotide (ASO) probe, an ASO forward primer, an ASO reverse primer, or germline probe and primers. To assess the quantity and quality of the RNA/DNA, one or more control genes must be included. Finally, the interpretation of RQ-PCR MRD data needs standardized criteria and reporting of MRD data needs international uniformity. Several European networks have now been established and common guidelines for data analysis and for reporting of MRD data are being developed. These networks also include standardization of technology as well as regular quality control rounds, both being essential for the introduction of RQ-PCR-based MRD detection in multicenter clinical treatment protocols.

An extremely facile aza-Bergman rearrangement of sterically unencumbered acyclic 3-aza-3-ene-1,5-diynes

The factors that affect the kinetics of the aza-Bergman cyclization of aza-enediynes (C,N-dialkynyl imines) have not previously been elucidated. Here we report our kinetic studies of the aza-Bergman reactions of a series of 6-triisopropylsilyl and 6-unsubstituted 1-phenyl-4-aryl-3-aza-3-ene-1,4-diynes in which the aryl group is phenyl, o-(methoxy)phenyl, or p-(methoxy)phenyl. These aza-enediynes are prepared as single isomers in modest yield from the corresponding 1-aryl-3-(triisopropylsilyl)propynone oximes. These aza-enediynes undergo aza-Bergman reaction followed by a rapid retro-aza-Bergman cyclization to afford beta-alkynyl acrylonitrile products. In no case are products corresponding to trapping the intermediate 2,5-didehydropyridine diradical isolated. While the rate of aza-Bergman cyclization is not greatly affected by the nature of the 4-aryl substituent, the rate is very dependent on the nature of the 6-substituent. 1-Phenyl-4-aryl-3-aza-3-ene-1,5-diynes that lack a 6-substituent undergo aza-Bergman cyclization spontaneously at 20 degrees C with first-order half-lives of 36-78 min. The effect of solvent on the kinetics of aza-Bergman cyclization of 1,4-diphenyl-3-aza-3-ene-1,5-diyne was investigated. The rate of this cyclization is solvent dependent, proceeding more rapidly in less polar solvents.

The combination of chemotherapy and systemic immunotherapy and the concept of cure in murine leukemia and lymphoma

While important advances have been made in our understanding of the molecular and biochemical processes that lead to the malignant transformation of myeloid and lymphoid cells, no major breakthroughs leading to long-term survival of patients of acute myeloid leukemia (AML) and lymphoma have been achieved during the last decade. Treatment failure, particularly in AML, is mostly related to the problem of resistance to multiple chemotherapeutic drugs and the morbidity and mortality associated with intensive chemotherapy. Thus, a significant challenge that remains is to develop novel therapeutic strategies that would ideally be able to address these issues. Novel immune approaches to cancer immunotherapy, while promising for specificity and long-term protection as single therapies, have not typically proven potent enough to generate long-lasting therapeutic responses. Recent evidence suggests that combining immunotherapy with chemotherapy can lead to increased effectiveness of chemotherapy without making the treatment intolerable to patients. This review focuses on the role of T cell costimulation in tumor immunosurveillance and on the therapeutic efficacy of a combination regimen consisting of chemotherapy and immunotherapy with recombinant B7.2-IgG fusion protein in preclinical AML and lymphoma models.