Treatment of relapsed or refractory acute myeloid leukemia with humanized anti-CD33 monoclonal antibody HuM195

Natural compounds targeting nuclear receptors for effective cancer therapy

Human nuclear receptors (NRs) are a family of forty-eight transcription factors that modulate gene expression both spatially and temporally. Numerous biochemical, physiological, and pathological processes including cell survival, proliferation, differentiation, metabolism, immune modulation, development, reproduction, and aging are extensively orchestrated by different NRs. The involvement of dysregulated NRs and NR-mediated signaling pathways in driving cancer cell hallmarks has been thoroughly investigated. Targeting NRs has been one of the major focuses of drug development strategies for cancer interventions. Interestingly, rapid progress in molecular biology and drug screening reveals that the naturally occurring compounds are promising modern oncology drugs which are free of potentially inevitable repercussions that are associated with synthetic compounds. Therefore, the purpose of this review is to draw our attention to the potential therapeutic effects of various classes of natural compounds that target NRs such as phytochemicals, dietary components, venom constituents, royal jelly-derived compounds, and microbial derivatives in the establishment of novel and safe medications for cancer treatment. This review also emphasizes molecular mechanisms and signaling pathways that are leveraged to promote the anti-cancer effects of these natural compounds. We have also critically reviewed and assessed the advantages and limitations of current preclinical and clinical studies on this subject for cancer prophylaxis. This might subsequently pave the way for new paradigms in the discovery of drugs that target specific cancer types.

Association between Immunophenotypic Parameters and Molecular Alterations in Acute Myeloid Leukemia

Acute myeloid leukemia (AML) is a hematologic malignancy that occurs due to alterations such as genetic mutations, chromosomal translocations, or changes in molecular levels. These alterations can accumulate in stem cells and hematopoietic progenitors, leading to the development of AML, which has a prevalence of 80% of acute leukemias in the adult population. Recurrent cytogenetic abnormalities, in addition to mediating leukemogenesis onset, participate in its evolution and can be used as established diagnostic and prognostic markers. Most of these mutations confer resistance to the traditionally used treatments and, therefore, the aberrant protein products are also considered therapeutic targets. The surface antigens of a cell are characterized through immunophenotyping, which has the ability to identify and differentiate the degrees of maturation and the lineage of the target cell, whether benign or malignant. With this, we seek to establish a relationship according to the molecular aberrations and immunophenotypic alterations that cells with AML present.

Immune therapy: a new therapy for acute myeloid leukemia

Although complete remission could be achieved in about 60%-70% of acute myeloid leukemia (AML) patients after conventional chemotherapy, relapse and the state of being refractory to treatment remain the main cause of death. In addition, there is a great need for less intensive regimens for all medically frail patients (both due to age/comorbidity and treatment-related). Immune therapy anticipates improved prognosis and reduced toxicities, which may offer novel therapeutic rationales. However, one of the major difficulties in developing immune therapies against AML is that the target antigens are also significantly expressed on healthy hematopoietic stem cells; B-cell malignancies are different because CD20/CD19/healthy B-cells are readily replaceable. Only the anti-CD33 antibody-drug conjugate gemtuzumab-ozogamicin is approved by the FDA for AML. Thus, drug development remains extremely active, although it is still in its infancy. This review summarizes the clinical results of immune therapeutic agents for AML, such as antibody-based drugs, chimeric antigen receptor therapy, checkpoint inhibitors, and vaccines.

Treatment of Patients with Acute Myeloid Leukemia with the Targeted Alpha-Particle Nano-Generator Actinium-225-Lintuzumab

PURPOSE

The anti-CD33 antibody lintuzumab has modest activity against acute myeloid leukemia (AML). To increase its potency, lintuzumab was conjugated to actinium-225 (225Ac), a radionuclide yielding 4 α-particles. This first-in-human, phase I trial was conducted to determine the safety, pharmacology, and biological activity of 225Ac-lintuzumab.

PATIENTS AND METHODS

Eighteen patients (median age, 64 years; range, 45-80) with relapsed or refractory AML received a single infusion of 225Ac-lintuzumab at activities of 18.5-148 kBq/kg.

RESULTS

The maximum tolerated dose was 111 kBq/kg. Dose-limiting toxicities included myelosuppression lasting > 35 days in one patient receiving 148 kBq/kg and death from sepsis in two patients treated with 111 and 148 kBq/kg. Myelosuppression was the most common toxicity. Significant extramedullary toxicities were limited to transient grade 3 liver function abnormalities. Pharmacokinetics were determined by gamma counting serial whole blood, plasma, and urine samples at energy windows for the 225Ac daughters, francium-221 and bismuth-213. Two-phase elimination kinetics were seen with mean plasma t1/2-α and t1/2-β of 1.9 and 38 hours, respectively. Peripheral blood blasts were eliminated in 10 of 16 evaluable patients (63%) but only at doses of {greater than or equal to} 37 kBq/kg. Bone marrow blasts were reduced in 10 of 15 evaluable patients (67%), including 3 patients with marrow blasts {less than or equal to} 5% and 1 patient with a morphologic leukemia-free state.

CONCLUSIONS

Therapy for AML with the targeted α-particle generator 225Ac-lintuzumab was feasible with an acceptable safety profile. Elimination of circulating blasts or reductions in marrow blasts were observed across all dose levels.

Approaches for the detection and analysis of anti-drug antibodies to biopharmaceuticals: A review

Antibody-based therapeutic agents and other biopharmaceuticals are now used in the treatment of many diseases. However, when these biopharmaceuticals are administrated to patients, an immune reaction may occur that can reduce the drug's efficacy and lead to adverse side effects. The immunogenicity of biopharmaceuticals can be evaluated by detecting and measuring antibodies that have been produced against these drugs, or anti-drug antibodies (ADAs). Methods for ADA detection and analysis can be important during the selection of a therapeutic approach based on such drugs and is crucial when developing and testing new biopharmaceuticals. This review examines approaches that have been used for ADA detection, measurement, and characterization. Many of these approaches are based on immunoassays and antigen binding tests, including homogeneous mobility shift assays. Other techniques that have been used for the analysis of ADAs are capillary electrophoresis, reporter gene assays, surface plasmon resonance spectroscopy, and liquid chromatography-mass spectrometry. The general principles of each approach will be discussed, along with their recent applications with regards to ADA analysis. This article is protected by copyright. All rights reserved.

CD33-Targeted Therapies: Beating the Disease or Beaten to Death?

CD33 is a transmembrane protein that is found on cells of myeloid lineage. It is also intensely expressed on acute myeloid leukemia (AML) progenitor cells but not on normal stem cells. It internalizes on binding and dimerization, making it a specific and ideal target for AML therapeutics and drug delivery. Several targeted therapies have been tested and many are still currently in development. Gemtuzumab ozogamicin was the first and only CD33-directed antibody-drug conjugate to be US Food and Drug Administration approved for AML. Other targeted agents have not achieved such success. Promising new strategies include cellular therapy mechanisms and linker molecules. This is an exciting target that requires a considerable amount of precision to yield clinical benefit.

Reinforcing the primary immunotherapy modulators against acute leukemia; monoclonal antibodies in AML

Recent therapeutic advances in cancer treatment recruit immune system potentiation against malignant cells. Numerous ongoing clinical trials on immunotherapy methods, either monotherapy or combination therapy, are investigating the impeding factors on the way of acute myeloid leukemia (AML) treatment. Due to the genetic diversity in AML progenitors, combining various strategies is more likely to be useful for improving patient outcomes. This review describes the details of applying monoclonal antibodies against AML, focusing on CD33, CD123, FLT3, CD45 and CD66 targeting. Furthermore, it clarifies the importance of immunotoxins, bispecific antibodies, chimeric antigen receptor (CAR)-T cells and T cell receptor-modified cells as reinforcing agents for monoclonal antibodies.

CAR-T cells beyond CD19, UnCAR-Ted territory

CAR-T cells have made dramatic inroads in targeting CD19-positive B-cell malignancies. This review focuses on application of CAR-T cells in hematologic malignancies beyond targeting CD19, with specific attention to Hodgkin's lymphoma and acute myeloid leukemia.

Radiopharmaceuticals for Relapsed or Refractory Leukemias

Radiopharmaceuticals, meaning drugs that hold a radionuclide intended for use in cancer patients for treatment of their disease or for palliation of their disease-related symptoms, have gained new interest for clinical development in adult patients with relapsed or refractory leukemia. About one-third of adult patients outlive their leukemia, with the remainder unable to attain complete remission status following the first phase of treatment due to refractory bone marrow or blood residual microscopic disease. The National Cancer Institute (NCI) Cancer Therapy Evaluation Program conducted 49 phase 1-1b trials in adult patients with leukemia between 1986 and 2017 in an effort to discover tolerated and effective therapeutic drug combinations intended to improve remission and mortality rates. None of these trials involved radiopharmaceuticals. In this article, the NCI perspective on the challenges encountered in and on the future potential of radiopharmaceuticals alone or in combination for adult patients with relapsed or refractory leukemia is discussed. An effort is underway already to build-up the NCI's clinical trial enterprise infrastructure for radiopharmaceutical clinical development.

Investigational CD33-targeted therapeutics for acute myeloid leukemia

INTRODUCTION

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

AREAS COVERED

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

EXPERT OPINION

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

The progress and current status of immunotherapy in acute myeloid leukemia

Recently, there has been remarkable progress in basic and preclinical studies of acute myeloid leukemia (AML). The improved outcomes of AML can largely be attributed to advances in supportive care and hematopoietic cell transplantation as opposed to conventional chemotherapy. However, as the 5-year survival rate remains low due to a high incidence of relapse, novel and effective treatments are urgently needed. Increasing attention is focusing on identifying suitable immunotherapeutic strategies for AML. Here, we describe the immunological features, mechanisms of immune escape, and recent progress in immunotherapy for AML. Problems encountered in the clinic will also be discussed. Although current outcomes may be limited, ongoing preclinical or clinical efforts are aimed at improving immunotherapy modalities and designing novel therapies, such as vaccines, monoclonal antibody therapy, chimeric antibody receptor-engineered T cells (CAR-T), TCR-engineered T cells (TCR-T), and checkpoint inhibitors, which may provide promising and effective therapies with higher specificity and efficacy for AML.

Antibody Based Therapies in Acute Leukemia

Despite great progress in the curative treatment of acute leukemia, outcomes for those with relapsed and/or chemotherapy-refractory disease remain poor. Current intensive cytotoxic therapies can be associated with significant morbidity and novel therapies are needed to improve outcomes. Immunotherapy based approaches provide an alternative mechanism of action in the treatment of acute leukemia. Due to cell surface antigen expression, leukemia in particular is amenable to targeted therapies, such as antibody-based therapy. Based on the potential for non-overlapping toxicity, the possibility of synergistic action with standard chemotherapy, and by providing a novel method to overcome chemotherapy resistance, antibody-based therapies have shown potential for benefit. Modifications to standard monoclonal antibodies, including drug conjugation and linkage to T-cells, may further enhance efficacy of antibody-based therapies. Identifying the ideal timing for incorporation of antibody-based therapies, within standard regimens, may lead to improvement in overall outcomes. This article will provide an overview of antibody-based therapies in clinical development for the treatment of acute leukemia in children and adults, with a particular focus on the current strategies and future developments.

EpCAM Inhibition Sensitizes Chemoresistant Leukemia to Immune Surveillance

The lack of effective tumor-associated antigens restricts the development of targeted therapies against myeloid leukemia. In this study, we compared gene expression patterns of acute myeloid leukemia (AML) and normal bone marrow samples and found that epithelial cell adhesion molecule (EpCAM) is frequently overexpressed in patients with AML, with EpCAM⁺ leukemic cells exhibiting enhanced chemoresistance and oncogenesis. The chemotherapeutic resistance of EpCAM-positive leukemic cells is a consequence of increased WNT5B signaling. Furthermore, we generated EpCAM antibodies that enabled phagocytosis or cytotoxicity of AML cells by macrophage or natural killer cells, respectively. Finally, EpCAM antibody treatment depleted AML in subcutaneous, disseminated, and intramedullary engrafted mice. In summary, EpCAM exhibits promise as a novel target for the treatment of leukemia. Cancer Res; 77(2); 482-93. ©2016 AACR.

Driving CAR T-cells forward

The engineered expression of chimeric antigen receptors (CARs) on the surface of T cells enables the redirection of T-cell specificity. Early clinical trials using CAR T cells for the treatment of patients with cancer showed modest results, but the impressive outcomes of several trials of CD19-targeted CAR T cells in the treatment of patients with B-cell malignancies have generated an increased enthusiasm for this approach. Important lessons have been derived from clinical trials of CD19-specific CAR T cells, and ongoing clinical trials are testing CAR designs directed at novel targets involved in haematological and solid malignancies. In this Review, we discuss these trials and present strategies that can increase the antitumour efficacy and safety of CAR T-cell therapy. Given the fast-moving nature of this field, we only discuss studies with direct translational application currently or soon-to-be tested in the clinical setting.

Immunotherapy in acute myeloid leukemia

Despite the remarkable progress made in some leukemias such as CML and CLL, cytotoxic treatment for AML remains essentially unchanged over the last 4 decades. Several lines of evidence, including the graft versus leukemia effect associated with allogeneic hematopoietic stem cell transplantation (HSCT), suggest that immunotherapy is an active modality in AML. Given the lack of progress for chemotherapy in this disease, many novel immunologic treatment approaches have been explored. The goals of non-transplant-based immune approaches have largely consisted of the stimulation or restoration of endogenous immune responses or the targeting of specific tumor antigens by immune cells. These strategies have been associated with less toxicity than allogeneic HSCT but typically have inferior efficacy. Allogeneic HSCT exploits major and minor histocompatibility differences between the donor and recipient in order to recognize and eradicate malignancy. With the recognition that the immune system itself provides a basis for treating AML, immunotherapy continues to be an attractive modality to exploit in the treatment of this disease.

Cancer stem cells in basic science and in translational oncology: can we translate into clinical application?

Since their description and identification in leukemias and solid tumors, cancer stem cells (CSC) have been the subject of intensive research in translational oncology. Indeed, recent advances have led to the identification of CSC markers, CSC targets, and the preclinical and clinical evaluation of the CSC-eradicating (curative) potential of various drugs. However, although diverse CSC markers and targets have been identified, several questions remain, such as the origin and evolution of CSC, mechanisms underlying resistance of CSC against various targeted drugs, and the biochemical basis and function of stroma cell-CSC interactions in the so-called ‘stem cell niche.’ Additional aspects that have to be taken into account when considering CSC elimination as primary treatment-goal are the genomic plasticity and extensive subclone formation of CSC. Notably, various cell fractions with different combinations of molecular aberrations and varying proliferative potential may display CSC function in a given neoplasm, and the related molecular complexity of the genome in CSC subsets is considered to contribute essentially to disease evolution and acquired drug resistance. In the current article, we discuss new developments in the field of CSC research and whether these new concepts can be exploited in clinical practice in the future.

Antigen-specific immunotherapies for acute myeloid leukemia

Antigen-specific immunotherapies have emerged as important components of curative treatment algorithms for many cancers. In acute myeloid leukemia (AML), success has been less obvious. Nonetheless, among the few drugs shown to improve survival in recent randomized trials is the CD33 antibody-drug conjugate gemtuzumab ozogamicin. Significant antileukemic activity is also well documented for radioimmunoconjugates targeting CD33, CD45, or CD66. These therapeutics can intensify conditioning before hematopoietic cell transplantation, but their effect on patient outcomes needs clarification. Emerging data now suggest clinical antileukemic activity of several novel antibodies and perhaps some adoptive T-cell immunotherapies and vaccines. In parallel, numerous other agents targeting a wider variety of antigens are currently being explored. However, the antigenic heterogeneity characteristic of AML is a considerable limitation for all these therapeutics, and many important questions related to the ideal target antigen(s), disease situation in which to use these therapies, most suitable patient populations, exact treatment modalities, and details of supportive care needs remain open. Addressing such questions in upcoming studies will be required to ensure that antigen-directed therapies become an effective tool in AML, a disease for which outcomes with standard "3 + 7"-based chemotherapy have remained unsatisfactory in many patients.

Target Therapy in Hematological Malignances: New Monoclonal Antibodies

Apart from radio- and chemotherapy, monoclonal antibodies (MoAbs) represent a new, more selective tool in the treatment of hematological malignancies. MoAbs bind with the specific antigens of the tumors. This interaction is a basis for targeted therapies which exhibit few side effects and significant antitumor activity. This review provides an overview of the functional characteristics of MoAbs, with some examples of their clinical application. The promising results in the treatment of hematological malignancies have led to the more frequent usage of MoAbs in the therapy. Development of MoAbs is a subject of extensive research. They are a promising method of cancer treatment in the future.

Targeting human C-type lectin-like molecule-1 (CLL1) with a bispecific antibody for immunotherapy of acute myeloid leukemia

Acute myeloid leukemia (AML), which is the most common acute adult leukemia and the second most common pediatric leukemia, still has a poor prognosis. Human C-type lectin-like molecule-1 (CLL1) is a recently identified myeloid lineage restricted cell surface marker, which is overexpressed in over 90% of AML patient myeloid blasts and in leukemic stem cells. Here, we describe the synthesis of a novel bispecific antibody, αCLL1-αCD3, using the genetically encoded unnatural amino acid, p-acetylphenylalanine. The resulting αCLL1-αCD3 recruits cytotoxic T cells to CLL1 positive cells, and demonstrates potent and selective cytotoxicity against several human AML cell lines and primary AML patient derived cells in vitro. Moreover, αCLL1-αCD3 treatment completely eliminates established tumors in an U937 AML cell line xenograft model. These results validate the clinical potential of CLL1 as an AML-specific antigen for the generation of a novel immunotherapeutic for AML.

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.

Immunotherapy for pediatric leukemia

Substantial progress has been made in the treatment of leukemia in childhood. Despite this, leukemia remains a leading cause of pediatric cancer-related mortality and the prognosis is guarded for individuals with relapsed or refractory disease. Standard therapies are associated with a wide array of acute and long-term toxicities and further treatment intensification may not be tolerable or beneficial. The curative potential of allogeneic stem cell transplantation is due in part to the graft-versus-leukemia effect, which provides evidence for the therapeutic capacity of immune-based therapies. In recent years there have been significant advances in the development and application of immunotherapy in the treatment of leukemias, including the demonstration of activity in chemotherapy-resistant cases. This review summarizes immunotherapeutic approaches in the treatment of pediatric leukemia including current results and future directions.

Antibody-based therapy of acute myeloid leukemia with gemtuzumab ozogamicin

Antibodies have created high expectations for effective yet tolerated therapeutics in acute myeloid leukemia (AML). Hitherto the most exploited target is CD33, a myeloid differentiation antigen found on AML blasts in most patients and, perhaps, leukemic stem cells in some. Treatment efforts have focused on conjugated antibodies, particularly gemtuzumab ozogamicin (GO), an anti-CD33 antibody carrying a toxic calicheamicin-g 1 derivative that, after intracellular hydrolytic release, induces DNA strand breaks, apoptosis, and cell death. Serving as paradigm for this strategy, GO was the first anti-cancer immunoconjugate to obtain regulatory approval in the U.S. While efficacious as monotherapy in acute promyelocytic leukemia (APL), GO alone induces remissions in less than 25-35% of non-APL AML patients. However, emerging data from well controlled trials now indicate that GO improves survival for many non-APL AML patients, supporting the conclusion that CD33 is a clinically relevant target for some disease subsets. It is thus unfortunate that GO has become unavailable in many parts of the world, and the drug's usefulness should be reconsidered and selected patients granted access to this immunoconjugate.

Anti-CD33-antibodies labelled with the alpha-emitter Bismuth-213 kill CD33-positive acute myeloid leukaemia cells specifically by activation of caspases and break radio- and chemoresistance by inhibition of the anti-apoptotic proteins X-linked inhibitor of apoptosis protein and B-cell lymphoma-extra large

AIM

The emerging interest in radioimmunotherapies employing alpha-emitters for cancer treatment like high risk-leukaemia leads to the question of how these radionuclides exhibit their cytotoxicity. To clarify the molecular mechanisms of cell death induction, we investigated the molecular effects of the alpha-emitter Bismuth-213 (Bi-213) bound to a monoclonal anti-CD33-antibody ([Bi-213]anti-CD33) on the cell cycle and on apoptosis induction in sensitive as well as in beta- and gamma-radiation-resistant CD33-positive acute myeloid leukaemia (AML) cells.

METHODS

The cytotoxic potential of the radioimmunoconjugate [Bi-213]anti-CD33 was analysed in the CD33-expressing human AML cell line HL-60 and in radiation- and chemoresistant HL-60-derived cell lines. Cell cycle and apoptosis induction analyses were performed via flow cytometry. Activation of apoptosis pathways was determined by immunodetection.

RESULTS

[Bi-213]anti-CD33 induced apoptotic cell death in CD33-positive AML cells specifically. Molecular analyses revealed that the intrinsic mitochondrial pathway of apoptosis was activated resulting in caspase-9 activation. In the apoptotic executioner cascade caspase-3 was activated and its substrate poly (ADP-ribose) polymerase (PARP) was cleaved. Notably, [Bi-213]anti-CD33 overcame radio- and chemoresistance by reversing deficient activation of apoptosis pathways in resistant CD33-positive AML cells and by the downregulation of inhibitors of apoptosis B-cell lymphoma-extra large (Bcl-xL) and X-linked inhibitor of apoptosis protein (XIAP) involved in leukaemia resistance.

CONCLUSION

[Bi-213]anti-CD33 exhibits its cytotoxic effects specifically in CD33-expressing AML cells via induction of the intrinsic, mitochondrial pathway of apoptosis. The abrogation of chemo- and radioresistances and the reactivation of apoptotic pathways seem to be promising for the treatment of patients with so far untreatable resistant AML and underline the importance of this emerging therapeutic approach of targeted alpha-therapies.

Randomized phase IIb study of low-dose cytarabine and lintuzumab versus low-dose cytarabine and placebo in older adults with untreated acute myeloid leukemia

Improving outcomes in older adults with acute myeloid leukemia remains a formidable challenge. Lintuzumab (SGN-33; HuM195) is a humanized monoclonal antibody directed against CD33, which is expressed on the majority of myeloblasts in acute myeloid leukemia. The primary objective of this randomized, double-blinded, placebo-controlled trial was to determine whether addition of lintuzumab to low-dose cytarabine would increase overall survival in adults aged 60 years and over with untreated acute myeloid leukemia. Randomization was stratified by age, previous hematologic disorder, and performance status. All patients received cytarabine (20 mg subcutaneously twice daily) on Days 1-10 of each 28-day cycle. Patients received lintuzumab (600 mg) or placebo intravenously once weekly in Cycle 1 and once every other week in Cycles 2-12. A total of 211 patients (107 lintuzumab, 104 placebo) were randomized. Median age was 70 years (range 60-90). Survival was not significantly prolonged with lintuzumab treatment (hazard ratio 0.96; 95% confidence interval (CI) 0.72-1.28; P=0.7585). Median survival was similar between treatment arms (4.7 months lintuzumab vs. 5.1 months placebo) and in the subgroup of patients with high-risk cytogenetics (4.5 months). Infusion-related reactions, predominantly Grades 1-2, occurred more commonly in the lintuzumab arm (51% vs. 7% placebo); no other clinically significant difference in safety was noted. These results confirm that lintuzumab in combination with low-dose cytarabine did not prolong survival and that low-dose cytarabine remains a valid comparator for trials of non-intensive therapies in older patients with acute myeloid leukemia, regardless of cytogenetic profile.

What happened to anti-CD33 therapy for acute myeloid leukemia?

CD33, a 67-kDa glycoprotein expressed on the majority of myeloid leukemia cells as well as on normal myeloid and monocytic precursors, has been an attractive target for monoclonal antibody (mAb)-based therapy of acute myeloid leukemia (AML). Lintuzumab, an unconjugated, humanized anti-CD33 mAb, has modest single-agent activity against AML but failed to improve patient outcomes in two randomized trials when combined with conventional chemotherapy. Gemtuzumab ozogamicin, an anti-CD33 mAb conjugated to the antitumor antibiotic calicheamicin, improved survival in a subset of AML patients when combined with standard chemotherapy, but safety concerns led to US marketing withdrawal. The activity of these agents confirms that CD33 remains a viable therapeutic target for AML. Strategies to improve the results of mAb-based therapies for AML include antibody engineering to enhance effector function, use of alternative drugs and chemical linkers to develop safer and more effective drug conjugates, and radioimmunotherapeutic approaches.

Ab therapy of AML: native anti-CD33 Ab and drug conjugates

MAb have become an important treatment modality in cancer therapy.Genetically engineered chimeric and humanized Ab have demonstrated activity against a variety of tumors. While the humanized anti-CD33MAb lintuzumab has only modest single-agent activity against overt AML, it can eliminate minimal residual disease detectable by reverse transcription-PCR in acute promyelocytic leukemia. Targeted chemotherapy with the anti−CD33−calicheamicin construct gemtuzumab ozogamicin has produced remissions in patients with relapsed AML and appears promising when used in combination with standard chemotherapy in the treatment of newly diagnosed AML.

Sequential cytarabine and alpha-particle immunotherapy with bismuth-213-lintuzumab (HuM195) for acute myeloid leukemia

PURPOSE

Lintuzumab (HuM195), a humanized anti-CD33 antibody, targets myeloid leukemia cells and has modest single-agent activity against acute myeloid leukemia (AML). To increase the potency of the antibody without the nonspecific cytotoxicity associated with β-emitters, the α-particle-emitting radionuclide bismuth-213 ((213)Bi) was conjugated to lintuzumab. This phase I/II trial was conducted to determine the maximum tolerated dose (MTD) and antileukemic effects of (213)Bi-lintuzumab, the first targeted α-emitter, after partially cytoreductive chemotherapy.

EXPERIMENTAL DESIGN

Thirty-one patients with newly diagnosed (n = 13) or relapsed/refractory (n = 18) AML (median age, 67 years; range, 37-80) were treated with cytarabine (200 mg/m(2)/d) for 5 days followed by (213)Bi-lintuzumab (18.5-46.25 MBq/kg).

RESULTS

The MTD of (213)Bi-lintuzumab was 37 MB/kg; myelosuppression lasting >35 days was dose limiting. Extramedullary toxicities were primarily limited to grade ≤2 events, including infusion-related reactions. Transient grade 3/4 liver function abnormalities were seen in five patients (16%). Treatment-related deaths occurred in 2 of 21 (10%) patients who received the MTD. Significant reductions in marrow blasts were seen at all dose levels. The median response duration was 6 months (range, 2-12). Biodistribution and pharmacokinetic studies suggested that saturation of available CD33 sites by (213)Bi-lintuzumab was achieved after partial cytoreduction with cytarabine.

CONCLUSIONS

Sequential administration of cytarabine and (213)Bi-lintuzumab is tolerable and can produce remissions in patients with AML.

Application of immunotherapy in pediatric leukemia

Despite great progress in the curative treatment of leukemia in pediatrics, current therapies are associated with multiple toxicities and the prognosis after relapse is guarded. Novel approaches are needed to overcome resistance to standard therapy and decrease adverse effects. The efficacy of allogeneic stem cell transplantation and the demonstration of a graft-versus-leukemia effect suggest that immune-based therapies can be effective in the treatment of childhood leukemia. Efforts to apply new immunotherapy approaches to the treatment of leukemia in pediatrics have recently begun. The optimal reagents, methods, and regimens have yet to be fully defined. Ongoing clinical trials offer promise in that regard.

Complete remissions observed in acute myeloid leukemia following prolonged exposure to lintuzumab: a phase 1 trial

A multi-institutional, phase 1 dose-escalation trial of lintuzumab (humanized anti-CD33 antibody; SGN-33, HuM195) was performed in patients with CD33-positive myeloid malignancies. In this study, higher doses than previously tested and prolonged duration of treatment for responding patients were evaluated. Over the dose range of 1.5-8 mg/kg/week, lintuzumab was well tolerated, and a maximum tolerated dose was not defined. The most common adverse event was transient chills with the initial lintuzumab infusion (39%). Responses were observed in 7 of 17 patients with acute myeloid leukemia: morphologic complete remission (n = 4), partial remission (n = 2), and morphologic leukemia-free state (n = 1). Of 14 patients with myelodysplastic syndrome or myeloproliferative diseases, 1 patient had major hematologic improvement and 9 patients had stable disease. In contrast to aggressive conventional chemotherapy, lintuzumab was administered in an ambulatory clinic setting with acceptable toxicity.

Neoplastic stem cells: current concepts and clinical perspectives

Neoplastic stem cells have initially been characterized in myeloid leukemias where NOD/SCID mouse-repopulating progenitors supposedly reside within a CD34+/Lin- subset of the malignant clone. These progenitors are considered to be self-renewing cells responsible for the in vivo long-term growth of neoplastic cells in leukemic patients. Therefore, these cells represent an attractive target of therapy. In some lymphoid leukemias, NOD/SCID mouse-repopulating cells were also reported to reside within the CD34+/Lin- subfraction of the clone. More recently, several attempts have been made to transfer the cancer stem cell concept to solid tumors and other non-hematopoietic neoplasms. In several of these tumors, the cell surface antigens AC133 (CD133) and CD44 are considered to indicate the potential of a cell to initiate permanent tumor formation in vivo. However, several questions concerning the phenotype, self-renewal capacity, stroma-dependence, and other properties of cancer- or leukemia-initiating cells remain to be solved. The current article provides a summary of our current knowledge on neoplastic (cancer) stem cells, with special emphasis on clinical implications and therapeutic options as well as a discussion about conceptual and technical limitations.

Current and emerging therapies for acute myeloid leukemia

BACKGROUND

Acute myeloid leukemia (AML) is a clonal disease characterized by the proliferation and accumulation of myeloid progenitor cells in the bone marrow, which ultimately leads to hematopoietic failure. The incidence of AML increases with age, and older patients typically have worse treatment outcomes than do younger patients.

OBJECTIVE

This review is focused on current and emerging treatment strategies for nonpromyelocytic AML in patients aged <60 years.

METHODS

A literature review was conducted of the PubMed database for articles published in English. Publications from 1990 through March 2009 were scrutinized, and the search was updated on August 26, 2009. The search terms used were: acute myeloid leukemia in conjunction with treatment, chemotherapy, stem cell transplantation, and immunotherapy. Clinical trials including adults with AML aged > or =19 years were selected for analysis. Conference proceedings from the previous 5 years of The American Society of Hematology, The European Hematology Association, and The American Society for Blood and Marrow Transplantation were searched manually. Additional relevant publications were obtained by reviewing the references from the chosen articles.

RESULTS

Cytarabine (AraC) is the cornerstone of induction therapy and consolidation therapy for AML. A standard form of induction therapy consists of AraC (100-200 mg/m(2)), administered by a continuous infusion for 7 days, combined with an anthracycline, administered intravenously for 3 days. Consolidation therapy comprises treatment with additional courses of intensive chemotherapy after the patient has achieved a complete remission (CR), usually with higher doses of the same drugs as were used during the induction period. High-dose AraC (2-3 g/m(2)) is now a standard consolidation therapy for patients aged <60 years. Despite substantial progress in the treatment of newly diagnosed AML, 20% to 40% of patients do not achieve remission with the standard induction chemotherapy, and 50% to 70% of first CR patients are expected to relapse within 3 years. The optimum strategy at the time of relapse, or for patients with the resistant disease, remains uncertain. Allogeneic stem cell transplantation has been established as the most effective form of antileukemic therapy in patients with AML in first or subsequent remission. New drugs are being evaluated in clinical studies, including immunotoxins, monoclonal antibodies, nucleoside analogues, hypomethylating agents, farnesyltransferase inhibitors, alkylating agents, FMS-like tyrosine kinase 3 inhibitors, and multidrug-resistant modulators. However, determining the success of these treatment strategies ultimately requires well-designed clinical trials, based on stratification of the patient risk, knowledge of the individual disease, and the drug's performance status.

CONCLUSIONS

Combinations of AraC and anthracyclines are still the mainstay of induction therapy, and use of high-dose AraC is now a standard consolidation therapy in AML patients aged <60 years. Although several new agents have shown promise in treating AML, it is unlikely that these agents will be curative when administered as monotherapy; it is more likely that they will be used in combination with other new agents or with conventional therapy.

Immunotherapy of childhood cancer: from biologic understanding to clinical application

PURPOSE OF REVIEW

Most children with cancer can be cured with combination regimens of chemotherapy, radiation, surgery, or all. However, standard therapies are toxic to normal tissues, cancer cells commonly develop resistance to chemotherapy, and relapsed malignancy is a leading cause of mortality in pediatrics. Elucidation of the principles of the normal immune response and tumor biology, coupled with technological developments, have led to important advances in the field of cancer immunotherapy. This review summarizes the biologic basis of cancer immunotherapy and highlights recent examples of progress in the application of novel humoral and cellular immunotherapies to children and adolescents with malignancy.

RECENT FINDINGS

Clinical trials of immunotherapy for pediatric cancer have recently been initiated. To date, most immune-based therapies have been well tolerated and some have shown clinically significant activity against specific refractory high-risk malignancies.

SUMMARY

Recent clinical trial results provide proof-of-principle that cancer immunotherapy has the capacity to overcome chemotherapy resistance without the usual toxicities associated with cytotoxic regimens. Immunotherapy holds promise in the treatment of children and adolescents with cancer and has the potential to improve both survival and quality of life.

Anti-leukemic activity of lintuzumab (SGN-33) in preclinical models of acute myeloid leukemia

Despite therapeutic advances, the long-term survival rates for acute myeloid leukemia (AML) are estimated to be 10% or less, pointing to the need for better treatment options. AML cells express the myeloid marker CD33, making it amenable to CD33-targeted therapy. Thus, the in vitro and in vivo anti-tumor activities of lintuzumab (SGN-33), a humanized monoclonal anti-CD33 antibody undergoing clinical evaluation, were investigated. In vitro assays were used to assess the ability of lintuzumab to mediate effector functions and to decrease the production of growth factors from AML cells. SCID mice models of disseminated AML with the multi-drug resistance (MDR)-negative HL60 and the MDR(+), HEL9217 and TF1-alpha, cell lines were developed and applied to examine the in vivo antitumor activity. In vitro, lintuzumab significantly reduced the production of TNFalpha-induced pro-inflammatory cytokines and chemokines by AML cells. Lintuzumab promoted tumor cell killing through antibody-dependent cellular cytotoxicity (ADCC) and phagocytosis (ADCP) activities against MDR(-) and MDR(+) AML cell lines and primary AML patient samples. At doses from 3 to 30 mg/kg, lintuzumab significantly enhanced survival and reduced tumor burden in vivo, regardless of MDR status. Survival of the mice was dependent upon the activity of resident macrophages and neutrophils. The results suggest that lintuzumab may exert its therapeutic effects by modulating the cytokine milieu in the tumor microenvironment and through effector mediated cell killing. Given that lintuzumab induced meaningful responses in a phase 1 clinical trial, the preclinical antitumor activities defined in this study may underlie its observed therapeutic efficacy in AML patients.

Low dose interleukin-2 following intensification therapy with high dose cytarabine for acute myelogenous leukemia in first complete remission

The most important problem in the therapy of patients with acute myeloid leukemia (AML) is relapse after intensive therapy. We sought to determine if interleukin-2 (low-dose with intermittent boluses) administration could be feasibly administered after standard therapy to potentiate anti-tumor immunity in a fashion analogous to the post-allogeneic stem cell transplant "graft-vs-leukemic" effect. Adults with de novo AML received daunorubicin and cytosine arabinoside induction therapy. Patients achieving complete remission received high dose ara-C (HIDAC) for three courses followed by low dose rIL-2 (Amgen), administered by continuous infusion (450,000 U/m(2)/day) for 10 weeks with intermittent boluses (500,000/U/m(2) over 2 hr) given in weekly intervals starting on Week 4. Of the 32 enrolled patients, 27 achieved CR; 8/11 who received rIL-2 completed therapy. 6/11 are long term survivors (median follow-up, 139 months). rIL-2 was well tolerated and associated with a 5-fold increase in circulating NK-lymphocytes and a 3-fold increase in circulating T-cells. Mononuclear cells from patients receiving rIL-2 exhibited enhanced cytolytic activity in vitro against cryopreserved autologous leukemia cells. This study supports further investigation of immunotherapy in the post-intensive chemotherapy setting in the management of patients with AML.

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.

Integration of monoclonal antibodies and immunoconjugates into the treatment of acute myeloid leukemia

PURPOSE OF REVIEW

This review addresses use of monoclonal antibodies and immunoconjugates to treat acute myeloid leukemia.

RECENT FINDINGS

Monoclonal antibodies used in acute myeloid leukemia have been directed against the antigens CD33, CD45, and CD66. Unconjugated monoclonal antibodies such as lintuzumab have modest activity against overt acute myeloid leukemia but can eliminate minimal residual disease in acute promyelocytic leukemia. Most experience with immunoconjugates is with gemtuzumab ozogamicin, an anti-CD33 monoclonal antibody linked to the potent antitumor antibiotic calicheamicin. Gemtuzumab ozogamicin has shown activity both singly, particularly in acute promyelocytic leukemia, and combined with conventional cytotoxic chemotherapy. Radiolabeled monoclonal antibodies against CD45 and CD66 have also been used to intensify the conditioning regimen before stem cell transplantation. The most promising results were obtained with radiolabeled anti-CD45 antibodies. Antibodies reactive with CD66 have been used to deliver targeted radiation to hematopoietic tissues in patients with advanced myeloid malignancies.

SUMMARY

Both unlabeled monoclonal antibodies and immunoconjugates appear to have a limited role if used as single agents to treat acute myeloid leukemia. These agents hold promise as potentially useful additions to conventional therapy, but the optimal dosing and timing remain to be defined.

An Affinity Capture Elution (ACE) assay for detection of anti-drug antibody to monoclonal antibody therapeutics in the presence of high levels of drug

Monoclonal antibody therapeutics typically have relatively long half-lives and can be dosed at high levels. Although formation of anti-drug antibodies (ADA) is relatively rare, detection of these antibodies can be very difficult in the presence of high circulating levels of drug. Typically these ADA are detected by bridging ELISAs which can be very sensitive to even low levels of drug. We describe an ELISA method based on affinity capture of ADA on solid-phase drug followed by removal of excess free drug, release and transfer of bound ADA and subsequent detection using biotinylated drug. The assay is both sensitive and highly tolerant to free drug with detection of 500 ng/ml of ADA readily achieved in the presence of 500 mug/ml of drug.

Monoclonal antibody therapy of APL

Acute promyelocytic leukemia (APL) is a rare subtype of acute myeloid leukemia (AML) for which a number of targeted therapies have been developed. The "targets" have included both genotypic and phenotypic features of the disease. The application of monoclonal antibodies (MAbs) to this disease to date have been limited to a relatively small number of studies where this therapy has been used to supplement effective approaches to the disease. The preliminary results have been promising, and further development of this modality as an effective adjunct to existing treatment regimens will most certainly occur in the near future.

Monoclonal antibodies and immunoconjugates in acute myeloid leukemia

The use of monoclonal antibodies for patients with acute myeloid leukemia is based on targeting cell-surface antigens preferentially expressed on leukemic blasts while sparing normal cells and tissues. The majority of studies performed to date have used antibodies reactive with the CD33 antigen. Phase II studies have demonstrated antileukemic responses with all agents, although less so with unlabeled antibodies. The most promising results have been obtained in the treatment of minimal residual disease in patients with acute promyelocytc leukemia. Antibody-targeted chemotherapy with gemtuzumab ozogamicin has also shown significant activity in patients with relapsed acute myeloid leukemia. Radioimmunotherapy with beta-particle emitters may be most effective for the treatment of bulky disease or as part of a conditioning regimen for hematopoietic stem-cell transplantation, whereas radioimmunotherapy with alpha-particle emitters may be better suited to the treatment of small-volume or minimal residual leukemia. Whether or not monoclonal antibody therapy will improve disease outcome compared with conventional treatment regimens remains to be demonstrated by well-designed clinical trials.

Immunotherapy for acute myeloid leukemia

Immunotherapeutic strategies have become part of standard cancer treatment. Chimeric and humanized antibodies have demonstrated activity against a variety of tumors. Although the humanized anti-CD33 antibody HuM195 has only modest activity against overt acute myeloid leukemia (AML), it can eliminate minimal residual disease in acute promyelocytic leukemia. High-dose radioimmunotherapy with b-particle-emitting isotopes targeting CD33, CD45, and CD66 can potentially allow intensification of antileukemic therapy before hematopoietic stem cell transplantation. Conversely, a-particle immunotherapy with isotopes such as bismuth-213 or actinium-225 offers the possibility of selective tumor cell kill while sparing surrounding normal tissues. Targeted chemotherapy with the anti-CD33- calicheamicin construct gemtuzumab ozogamicin has produced remissions in relapsed AML and appears promising when used in combination with standard chemotherapy for newly diagnosed AML. T-cell recognition of peptide antigens presented on the cell surface in combination with major histocompatibility complex antigen provides another potentially promising approach for the treatment of AML.

Recent advances in targeted therapy of human myelogenous leukaemia

Despite major advances in the diagnosis and treatment of myelogenous leukaemia during the past few decades, this group of diseases remains a serious medical concern with > 15,000 new cases each year and a mortality rate of approximately 10,000 in the US alone. Current available conventional therapies, including chemotherapy and bone marrow transplantation, often cause severe side effects owing mainly to the lack of specificity of the treatment. In the past years, significant progress has been made towards understanding the pathogenesis of myelogenous leukaemia from the molecular standpoint. To this end, a growing number of approaches are being exploited for the identification and validation of new therapeutic targets suitable for more potent and specific or 'targeted' intervention. In this review, the authors focus their discussion on the four most promising myelogenous leukaemia-associated molecular targets currently being pursued by major pharmaceutical and biotechnology companies, fms-like tyrosine kinase 3 (FLT3), CD33, farnesyl transferase and BCR-Abl, with emphasis on recent progress on the clinical development of therapeutic agents, including both kinase inhibitors and monoclonal antibodies, to these targets.

Effect of double antigen bridging immunoassay format on antigen coating concentration dependence and implications for designing immunogenicity assays for monoclonal antibodies

The double antigen bridging immunoassay has been used extensively for detection of immunogenicity responses to therapeutic monoclonal antibodies. We have analyzed parameters affecting performance of this type of immunoassay including microtiter plate antigen coating concentration, enzyme-labeled antigen conjugate dilution and assay format (one-step versus two-step). We present results demonstrating that the format of the assay has a significant impact on the optimal parameters to maximize assay performance. A one-step assay format achieves maximal sensitivity across a broad range of coating concentrations and at a lower concentration of conjugate than that in a two-step format. In contrast, a two-step format requires very low coating concentrations and higher conjugate concentrations to achieve maximal sensitivity and suffers from significantly reduced sensitivity at higher coating concentrations. Together, these findings indicate that a one-step assay format can greatly reduce the effect of coating concentration variation on assay performance.

Phase III Randomized Multicenter Study of a Humanized Anti-CD33 Monoclonal Antibody, Lintuzumab, in Combination With Chemotherapy, Versus Chemotherapy Alone in Patients With Refractory or First-Relapsed Acute Myeloid Leukemia

Purpose

Lintuzumab (HuM195) is an unconjugated humanized murine monoclonal antibody directed against the cell surface myelomonocytic differentiation antigen CD33. In this study, the efficacy of lintuzumab in combination with induction chemotherapy was compared with chemotherapy alone in adults with first relapsed or primary refractory acute myeloid leukemia (AML).

Patients and Methods

Patients with relapsed or primary resistant AML (duration of first response, zero to 12 months) were randomly assigned to receive either mitoxantrone 8 mg/m2, etoposide 80 mg/m2, and cytarabine 1 g/m2 daily for 6 days (MEC) in combination with lintuzumab 12 mg/m2, or MEC alone. Overall response, defined as the rate of complete remission (CR) and CR with incomplete platelet recovery (CRp), was the primary end point of the study, with additional analyses of survival time and toxicity.

Results

A total of 191 patients were randomly assigned from November 1999 to April 2001. The percent CR plus CRp with MEC plus lintuzumab was 36% v 28% in patients treated with MEC alone (P = .28). The overall median survival was 156 days and was not different in the two arms of the study. Apart from mild antibody infusion–related toxicities (fever, chills, and hypotension), no differences in chemotherapy-related adverse effects, including hepatic and cardiac dysfunction, were observed with the addition of lintuzumab to induction chemotherapy.

Conclusion

The addition of lintuzumab to salvage induction chemotherapy was safe, but did not result in a statistically significant improvement in response rate or survival in patients with refractory/relapsed AML.

Influence of CD33 expression levels and ITIM-dependent internalization on gemtuzumab ozogamicin–induced cytotoxicity

Gemtuzumab ozogamicin (GO; Mylotarg), a novel immunoconjugate used for treatment of acute myeloid leukemia (AML), contains the humanized anti-CD33 antibody (hP67.6) as a carrier to facilitate cellular uptake of the toxic calicheamicin-γ1 derivative. By use of lentivirus-mediated gene transfer to manipulate CD33 expression in myeloid cell lines that normally lack CD33 (murine 32D cells) or have very low levels of CD33 (human OCI-AML3 and KG-1a cells), we here show a quantitative relationship between CD33 expression and GO-induced cytotoxicity. The CD33 cytoplasmic immunoreceptor tyrosine-based inhibitory motifs (ITIMs) control internalization of antibody bound to CD33. Disruption of the ITIMs by introduction of point mutations not only prevented effective internalization of antibody-bound CD33 but also significantly reduced GO-induced cytotoxicity. Together, our data imply a pivotal role of both the number of CD33 molecules expressed on the cell surface and the amount of internalization of CD33 following antibody binding for GO-induced cytotoxicity and suggest novel therapeutic approaches for improvement of clinical outcome of patients treated with GO.

Anti-CD33 monoclonal antibodies enhance the cytotoxic effects of cytosine arabinoside and idarubicin on acute myeloid leukemia cells through similarities in their signaling pathways

OBJECTIVE

Chemotherapy agents (CA) such as cytosine arabinoside (ara-C), idarubicin (IDA), and etoposide (VP-16) are widely used in the treatment of acute myeloid leukemia (AML) However, their effects on signaling pathways leading to cytotoxicity have only been described recently. Ligation of the leukemia-associated antigen CD33 by anti-CD33 monoclonal antibody (mAb) also results in signaling events that induce a downregulation of cell growth. We examined the possibility that anti-CD33 mAb and CA might cooperate in mediation of growth inhibition in primary AML samples and AML cell lines.

MATERIALS AND METHODS

We investigated two AML cells lines and 14 primary AML samples for their proliferative response ((3)H-thymidine incorporation), colony formation, and biochemical (Western blot analysis) to anti-CD33 mAb treatment combined with chemotherapy agents.

RESULTS

CD33 ligation induced a significant increase in ara-C- or IDA- but not VP-16-or Bryostatin-mediated inhibition of proliferation and colony formation. Ara-C and IDA induced SHP-1 and SHP-2 protein tyrosine phosphatase (PTPs) phosphorylation and Lyn/SHP-1 complex formation, while VP-16 and Bryostatin did not. CD33 ligation, however, mediated phosphorylation of these PTPs and Syk/SHP-1 complex formations. Combined treatment of AML cells by ara-C or IDA with anti-CD33 mAb resulted in higher levels of SHP-1 phosphorylation. Reduction in SHP-1 by short interfering RNA abrogated these effects.

CONCLUSION

These data suggest that combined incubation of leukemia cells with anti-CD33 mAb and ara-C or IDA, but not VP-16 or Bryostatin, independently triggers similar events in the downstream signaling cascade, and therefore leads to additive antiproliferative effects and enhanced cytotoxicity.