New agents in acute myeloid leukemia and other myeloid disorders

Combination of Tissue Microarray Profiling and Multiplexed IHC Approaches to Investigate Transport Mechanism of Nucleoside Analog Drug Resistance

Nucleoside analogs (NAs) are an established class of anticancer agents being used clinically for the treatment of diverse cancers, either as monotherapy or in combination with other established anticancer or pharmacological agents. To date, nearly a dozen anticancer NAs are approved by the FDA, and several novel NAs are being tested in preclinical and clinical trials for future applications. However, improper delivery of NAs into tumor cells because of alterations in expression of one or more drug carrier proteins (e.g., solute carrier (SLC) transporters) within tumor cells or cells surrounding the tumor microenvironment stands as one of the primary reasons for therapeutic drug resistance. The combination of tissue microarray (TMA) and multiplexed immunohistochemistry (IHC) is an advanced, high-throughput approach over conventional IHC that enables researchers to effectively investigate alterations to numerous such chemosensitivity determinants simultaneously in hundreds of tumor tissues derived from patients. In this chapter, taking an example of a TMA from pancreatic cancer patients treated with gemcitabine (a NA chemotherapeutic agent), we describe the step-by-step procedure of performing multiplexed IHC, imaging of TMA slides, and quantification of expression of some relevant markers in these tissue sections as optimized in our laboratory and discuss considerations while designing and carrying out this experiment.

Co-operation of ABT-199 and gemcitabine in impeding DNA damage repair and inducing cell apoptosis for synergistic therapy of T-cell acute lymphoblastic leukemia

T-cell acute lymphoblastic leukemia (T-ALL) is a high-risk subtype of acute lymphoblastic leukemia with limited therapeutic options available. Here, we evaluated the therapeutic potential of the combination of the Bcl-2 antagonist ABT-199 and cytotoxic agent gemcitabine in T-ALL cell lines. Our results showed that the combination of ABT-199 and gemcitabine exhibited synergistic cytotoxicity and induced significant apoptosis in human T-ALL cell lines (Jurkat and Molt4). The augmented apoptosis induced by combination treatment was accompanied by the greater extent of mitochondrial depolarization and enhanced DNA damage. Importantly, single agent induced DNA damage alone but did not inhibit RAD51/BRCA1-mediated repair for DNA double-strand breaks. In contrast, the combination of ABT-199 and gemcitabine disrupted RAD51/BRCA1-dependent DNA repair and remarkably activated caspase-3 and PARP to trigger apoptosis. Moreover, ABT-199 exerted an antagonistic action towards Bcl-2 and Bcl-xL, but to a certain extent moderately increased Mcl-1 level that could be compromised by gemcitabine. In conclusion, our study showed that the combination of ABT-199 and gemcitabine exhibited synergistic cytotoxicity in T-ALL cells by cooperatively targeting DNA damage repair pathway and Bcl-2 family proteins.

A high-throughput screen indicates gemcitabine and JAK inhibitors may be useful for treating pediatric AML

Improvement in survival has been achieved for children and adolescents with AML but is largely attributed to enhanced supportive care as opposed to the development of better treatment regimens. High risk subtypes continue to have poor outcomes with event free survival rates <40% despite the use of high intensity chemotherapy in combination with hematopoietic stem cell transplant. Here we combine high-throughput screening, intracellular accumulation assays, and in vivo efficacy studies to identify therapeutic strategies for pediatric AML. We report therapeutics not currently used to treat AML, gemcitabine and cabazitaxel, have broad anti-leukemic activity across subtypes and are more effective relative to the AML standard of care, cytarabine, both in vitro and in vivo. JAK inhibitors are selective for acute megakaryoblastic leukemia and significantly prolong survival in multiple preclinical models. Our approach provides advances in the development of treatment strategies for pediatric AML.

Core-binding factor acute myeloid leukemia: Heterogeneity, monitoring, and therapy

Core binding factor acute myelogenous leukemia (CBF AML) constitutes 15% of adult AML and carries an overall good prognosis. CBF AML encodes two recurrent cytogentic abnormalities referred to as t(8;21) and inv (16). The two CBF AML entities are usually grouped together but there is a considerable clinical, pathologic and molecular heterogeneity within this group of diseases. Recent and ongoing studies are addressing the molecular heterogeneity, minimal residual disease and targeted therapies to improve the outcome of CBF AML. In this article, we present a comprehensive review about CBF AML with emphasis on molecular heterogeneity and new therapeutic options.

Bone Marrow and Peripheral Blood AML Cells Are Highly Sensitive to CNDAC, the Active Form of Sapacitabine

Achieving improvements in survival and reducing relapse remains a challenge in acute myelogenous leukemia (AML) patients. This study evaluated the in vitro efficacy of the active form of novel agent sapacitabine, CNDAC, compared to current chemotherapeutic drugs Ara-C and mitoxantrone using two AML cell lines, HL-60 (promyelocytic) and THP-1 (monocytic), as well as bone marrow (BM) and peripheral blood (PB) cells collected from AML patients. Cell lines were exposed to compound for 3-6 days and primary cells for 4 days. The viability of primary cells was additionally evaluated 3, 7, and 31 days after removal of tested compound to determine the durability of the response. Our studies indicate that CNDAC and mitoxantrone have a greater impact on viability than ara-C in primary AML cells and AML cell lines. CNDAC is more effective at reducing viability and inducing apoptosis than ara-C at equivalent concentrations in the THP-1 cell line, which is defined as displaying resistance to ara-C. As sapacitabine has shown in vivo activity at clinically achievable doses, future studies are warranted to assess the potential for combining it with ara-C and/or mitoxantrone, with an emphasis on cells and patients insensitive to ara-C treatment.

A potential role of ruxolitinib in leukemia

INTRODUCTION

An increased understanding of cellular signaling pathways, like the JAK?STAT pathway, and the identification of the JAK2 V617F mutation in the classic Philadelphia chromosome-negative myeloproliferative neoplasms (MPNs), has generated great interest in the development of targeted JAK2 inhibitors. In a recently completed Phase I?II study, ruxolitinib, a selective orally available JAK1 and JAK2 inhibitor, has shown efficacy in patients with advanced myelofibrosis. Constitutive activation of the JAK?STAT pathway has also been implicated in other hematological malignancies suggesting a potential role of JAK kinase inhibitors in these malignancies.

AREAS COVERED

This article reviews the chemistry, pharmacodynamics, pharmacokinetics, clinical efficacy, safety and tolerability of ruxolitinib. The literature for this article was retrieved from PubMed database searches using the keywords ?ruxolitinib?, ?INCB 018424?, ?JAK2 inhibitors? and ?leukemia?.

EXPERT OPINION

The JAK?STAT signaling pathway plays a vital role in leukemogenesis. Ruxolitinib, a potent JAK1 and JAK2 inhibitor, known to decrease spleen size and alleviate constitutional symptoms in myelofibrosis, represents a potentially promising agent for the treatment of leukemias by inhibiting the JAK?STAT signaling. Further studies of ruxolitinib, in patients with acute and chronic leukemias, are now needed to establish the clinical usefulness of this promising drug.

A phase 1-2 study of a farnesyltransferase inhibitor, tipifarnib, combined with idarubicin and cytarabine for patients with newly diagnosed acute myeloid leukemia and high-risk myelodysplastic syndrome

BACKGROUND

The authors conducted a phase 1/2 study of tipifarnib in combination with idarubicin and cytarabine (IA) in 95 patients with previously untreated acute myeloid leukemia (AML) or high-risk myelodysplastic syndrome.

METHODS

Induction consisted of idarubicin 12 mg/m(2) a day on days 1-3, cytarabine 1.5 g/m(2) intravenously continuously daily on days 1-4 (days 1-3 if age ≥60 years), and tipifarnib, with the first cohort (n = 6) receiving 200 mg orally twice a day and all others receiving 300 mg twice a day for 21 days every 28 days. Consolidation consisted of 5 courses of idarubicin 8 mg/m(2) a day on days 1-2, cytarabine 0.75 g/m(2) a day on days 1-3, and tipifarnib 300 mg twice a day for 14 days every 4-6 weeks. Maintenance with tipifarnib 300 mg twice a day for 21 days every 4-6 weeks was continued for 6 months.

RESULTS

With a median follow-up of 33 months, 61 patients achieved complete remission (CR) (64%), and 9 achieved complete remission with incomplete platelet recovery (CRp) (9%). The median duration of CR was not reached. Median overall survival was 17 months. The most common grade 3 adverse events were gastrointestinal toxicities, liver dysfunction, and skin rash. Compared with historical IA, IA and tipifarnib showed a better CR duration (P = .04) and a trend toward a higher CR rate in patients with chromosome 5/7 abnormalities.

CONCLUSIONS

The combination of IA and tipifarnib is safe and active. Further studies exploring different dosages and schedules are warranted, particularly in patients with poor-risk AML.

Clofarabine combinations as acute myeloid leukemia salvage therapy

BACKGROUND

Outcome of patients with relapsed acute myeloid leukemia (AML) remains unsatisfactory. Clofarabine is a nucleoside analog with activity in adult AML. Combinations with cytarabine in AML are feasible and effective. Idarubicin is another active AML drug, which has not yet been tested with clofarabine.

METHODS

The authors therefore designed a phase I study of clofarabine +/- cytarabine, plus idarubicin. Patients with primary refractory or first-relapse AML were assigned to either clofarabine plus idarubicin (CI) if previously exposed to cytarabine with a response lasting <12 months, or clofarabine and idarubicin plus cytarabine (CIA) for responses > or = 12 months, or if never exposed to cytarabine. A standard "3 + 3" phase 1 design was followed to define maximum tolerated dose (MTD). Forty-four patients were treated (23 CI; 21 CIA).

RESULTS

Dose-limiting toxicities were hyperbilirubinemia and hepatic transaminase elevations for CI-treated patients in addition to mucositis and diarrhea for CIA-treated patients. MTD for CI was clofarabine 22.5 mg/m(2) intravenously daily x 5 and idarubicin 10 mg/m(2) intravenously daily x 3. MTD for CIA was clofarabine 22.5 mg/m(2) intravenously x 5, idarubicin 6 mg/m(2) intravenously x 3, and cytarabine 0.75 g/m(2) intravenously x 5 days.

CONCLUSIONS

A phase 2 randomized trial is in process to compare activity between treatment arms.

Therapeutic advances in leukemia and myelodysplastic syndrome over the past 40 years

Major therapeutic progress has been accomplished in leukemia and myelodysplastic syndrome (MDS) over the past 40 years, which may not be fully appreciated by the larger medical community. The objective of this review was to briefly highlight the treatment breakthroughs in leukemia and MDS. Therapeutic progress happened through better understanding of disease pathophysiologies and rational development of targeted agents, like imatinib mesylate in chronic myeloid leukemia (CML), and through astute, empirical discoveries in the clinic, like all-trans retinoic acid and arsenic trioxide in acute promyelocytic leukemia (APL) and chlorodeoxyadenosine in hairy cell leukemia (HCL). Today, the 5- to 10-year survival rates in patients with APL and HCL exceed 80%. In patients with CML, imatinib therapy has been associated with estimated 5- to 7-year survival rates from 85% to 90%. In patients with adult acute lymphocytic leukemia, modern intensive regimens have improved the 5-year survival rates from 20% up to 40%. In patients with chronic lymphocytic leukemia, chemoimmunotherapy recently produced high rates of quality responses and improved long-term outcome. In younger patients with acute myeloid leukemia (AML), the 5-year survival rates range from 40% to 50%, although elderly AML remains a therapeutic challenge. In patients with MDS, it was recently demonstrated that epigenetic therapy with hypomethylating agents improved survival. Much therapeutic progress has been witnessed in leukemia and MDS, and much more is expected to occur soon.

Drug-resistant AML cells and primary AML specimens are killed by 111In-anti-CD33 monoclonal antibodies modified with nuclear localizing peptide sequences

Multidrug resistance (MDR) is a major challenge to the successful treatment of acute myeloid leukemia (AML). Our purpose was to determine whether (111)In-HuM195 anti-CD33 antibodies modified with peptides harboring nuclear localizing sequences (NLS) could kill drug-resistant AML cell lines and primary AML patient specimens expressing MDR transporters through the emission of Auger electrons.

METHODS

HuM195, M195, and irrelevant mouse IgG (mIgG) were conjugated to 10+/-3 NLS peptides and then labeled with (111)In by diethylenetriaminepentaacetic acid substitution to a specific activity of 1-8 MBq/microg. The binding affinity of HuM195 and M195 was determined for HL-60 and mitoxantrone-resistant HL-60-MX-1 cells. Nuclear localization of (111)In-NLS-HuM195, (111)In-NLS-M195, (111)In-HuM195, and (111)In-M195 was measured by subcellular fractionation. The antiproliferative effects of (111)In-NLS-HuM195, (111)In-NLS-M195, (111)In-HuM195, and (111)In-M195 (2.5-250 kBq/well) on HL-60 and HL-60-MX-1 were studied using the WST-1 assay. Clonogenic survival of HL-60 and HL-60-MX-1 leukemic cells and 10 primary AML specimens with MDR phenotype (assessed by flow cytometry) was determined after exposure for 3 h at 37 degrees C to 2.5-250 mBq/cell of (111)In-NLS-HuM195, (111)In-HuM195, or (111)In-NLS-mIgG. Clonogenic survival versus the amount of radioactivity incubated with the cells (mBq/cell) was plotted, and the mean lethal amount of radioactivity and the lower asymptote of the curve (plateau) were determined.

RESULTS

The (111)In-labeled anti-CD33 monoclonal antibodies HuM195 and M195 modified with NLS were efficiently routed to the nucleus of HL-60 cells and their mitoxantrone-resistant clone after CD33-mediated internalization. The following are the principal findings of our study: (111)In-NLS-HuM195 was more effective at killing HL-60 and HL-60-MX-1 cells than was (111)In-HuM195, a strong correlation between the specific activity of the (111)In-labeled radioimmunoconjugates and their cytotoxicity toward AML cells existed, and leukemic cells from patients were killed by (111)In-NLS-M195 or (111)In-M195, but the cytotoxic response among specimens was heterogeneous.

CONCLUSION

NLS conjugation enhanced the nuclear uptake and cytotoxicity of (111)In-HuM195 and (111)In-M195 toward drug-resistant AML cell lines as well as patient specimens expressing a diversity of MDR phenotypes, including Pgp-170, BCRP1, or MRP1 transporters. Targeted Auger electron radioimmunotherapy using (111)In-labeled anti-CD33 monoclonal antibodies modified with NLS may be able to overcome MDR and provide a means of treating chemotherapy-resistant myeloid leukemias in patients.

Bone Marrow Transplantation

Hematopoietic transplantation is increasingly being used as treatment for a variety of severe diseases. Data from International Registries indicate that more than 25,000 transplants are performed every year in Europe, and a similar number in the United States (US) (Copelan 2006; Gratwohl et al. 2007). The objectives of this procedure are: (1) to replace hematopoiesis affected by a severe and irreversible disorder, (2) to rescue the patient from intense marrow toxicity induced by high-dose chemotherapy and/or radiation, and (3) to use a fraction of cells contained in the graft as anti-tumor immunotherapy. Of note, one or more of these objectives may be pursued in a particular situation; for example, in a patient with acute leukemia, transplantation aims to replace the neoplastic hematopoiesis by administering high-dose cytotoxic therapy and taking advantage of the graft-versus-leukemia effect of donor T-lymphocytes from the graft. In contrast, in aplastic anemia the only goal of the procedure is to restore an adequate hematopoiesis.

Progress in the treatment of acute myeloid leukemia

Significant progress in understanding the mechanisms leading to the development of acute myeloid leukemia (AML) has led to the identification of numerous molecular abnormalities that may be responsible for leukemogenesis. Over the same period, large trials have established standard regimens combining cytotoxic agents for the treatment of patients with AML. Current research is attempting to better stratify patients by identifying risk factors responsible for resistance, and to discern ways for incorporating newer agents with specific and targeted activity into our standard regimens, Herein the recent developments in the diagnosis and treatment of AML are reviewed.

CD33 expression and P-glycoprotein-mediated drug efflux inversely correlate and predict clinical outcome in patients with acute myeloid leukemia treated with gemtuzumab ozogamicin monotherapy

Gemtuzumab ozogamicin (GO) contains an anti-CD33 antibody to facilitate uptake of a toxic calicheamicin-gamma(1) derivative. While recent in vitro data demonstrated a quantitative relationship between CD33 expression and GO cytotoxicity, previous correlative studies failed to identify a significant association between CD33 expression and clinical outcome. Studying patients undergoing GO monotherapy for relapsed acute myeloid leukemia (AML), we now find that AML blasts of responders have a significantly higher mean CD33 level and lower P-glycoprotein (Pgp) activity compared with nonresponders. CD33 expression and Pgp activity are inversely correlated. While both variables are associated with outcome, Pgp remains significantly associated with outcome even after adjusting for CD33, whereas CD33 does not show such an association after adjusting for Pgp. The inverse relationship between CD33 and Pgp suggests a maturation-stage-dependent expression of both proteins, and offers the rationale for using cell differentiation-promoting agents to enhance GO-induced cytotoxicity.

Immunoepigenetics: the unseen side of cancer immunoediting

Cancer immunosurveillance representing, till recently, the explanatory framework relating cancer and the immune system, does not convincingly explain tumor escape. At the beginning of the decade, a new theory emerged, namely the immunoediting theory, and it comprehensively defines the role of the immune system in carcinogenesis. The core of this theory embraces the concept that the immune system on the one hand protects the body from cancer and on the other it shapes the immunogenicity of these cancers, thus presents a persuasive rationalization of the resistance of tumors against the immune response. With the immune system playing, in this context, such a pivotal role in shaping the tumor immune profile and in subsequent oncogenesis, it seems rather paradoxical to accept the immunocompetent host's immune system as a constant moiety. While DNA mutations of immune genes create a rather polymorphic condition, their frequency is much lower than that of other genetic events. Of these, epigenetic alterations give rise to new epialleles, which can reach up to 100% per locus. Bearing in mind that cancer is characterized by a tremendous amount of epigenetic aberrations, in both gene and global level, it is reasonable to postulate that, for the same unknown causes, analogous aberrations could affect the immune genes. Should this be the case, the relation between oncogenesis and the immune system appears much more dynamic and complex. Such an immunoepigenetic approach to carcinogenesis could improve our understanding of a series of common cancer-related aspects, such as environmental risk factors, effectiveness of demethylating agents, failure of current immunotherapies, etc. Moreover, this immunoepigenetic paradigm will take the current perception of the immune system and cancer interrelation further and beyond, constituting that the immunoresistant cancer cell phenotype is not shaped by the immune system acting as a steady and rigid evolutionary pressure, but rather as an extremely dynamic variable.

Trends in the treatment of acute myeloid leukaemia in the elderly

INTRODUCTION

Acute myeloid leukaemia (AML) is the most common type of leukaemia among adults in the US. However, data on longitudinal treatment patterns and outcomes associated with AML and its relapse are sparse, particularly among the elderly. This study documents changes in treatment patterns and outcomes among elderly AML patients over the past decade.

METHODS

Using the linked Surveillance, Epidemiology, and End Results (SEER)-Medicare database, we retrospectively evaluated trends in mortality, treatment patterns, healthcare resource utilisation and Medicare payments associated with AML and its relapse among Medicare beneficiaries > or =65 years of age who were initially diagnosed with AML in a SEER registry between 1991 and 1999. Chemotherapy was ascertained from examination of inpatient and outpatient bills. AML relapse and retreatment were identified using a validated algorithm. Costs of care were based on total Medicare payments.

RESULTS

A total of 3439 elderly patients with AML were identified. Median survival across all study patients was 2.4 months (mean +/- SD 5.6 +/- 6.8 months), with medians of 3.9, 2.2 and 1.4 months for patients 65-74 years of age, 75-84 years of age and > or =85 years of age, respectively. Fewer than 7% of patients were alive at 2 years, and there was very little variation during the decade of our analysis. Costs and overall healthcare utilisation patterns also changed very little, with the exception of those relating to hospice use and chemotherapy. Hospice use more than doubled during the decade (from 12% to 29% among patients diagnosed in 1991 and 1999, respectively; p < 0.0001), mostly among the oldest patients. Administration of chemotherapy also increased from 29% of patients diagnosed in 1991 to 38% of patients diagnosed in 1999 (p = 0.014), with the increase being seen mostly among younger patients and those treated in teaching hospitals. Average total costs (+/- SD) were US$51,888 +/- $54,825 and declined by age as a result of lower survival. A total of 192 patients (16% of treated patients) relapsed and received retreatment with chemotherapy. These patients survived a median 18 months, with a median duration of remission of 8 months, and average total costs three times higher than the overall sample.

CONCLUSIONS

The high early mortality and costs associated with AML have not changed significantly over the past decade. However, treatment patterns appear to be changing, with increasing use of chemotherapy and hospice care. The ongoing introduction of new treatments for AML in the elderly is likely to further impact treatment patterns, and may change the economic burden of the disease. Our findings can be used as a baseline against which the benefits of new therapies can be compared.

Phase II study of gemcitabine in children with relapsed acute lymphoblastic leukemia or acute myelogenous leukemia (ADVL0022): a Children's Oncology Group Report

BACKGROUND

To determine the response rate and toxicity to gemcitabine administered as 10 mg/m2/min x 360 min weekly for 3 weeks in children with relapsed acute lymphoblastic leukemia (ALL) or acute myelogenous leukemia (AML). Gemcitabine is a deoxycytidine analog that inhibits DNA synthesis and repair and has a broad spectrum of antitumor activity.

PROCEDURE

From April 2001 to April 2003, 23 male and 9 female eligible patients were recruited for the Children's Oncology Group (COG) phase II study of Gemcitabine (ADVL0022).

RESULTS

One of 20 evaluable patients with ALL and none of 10 evaluable patients with AML had complete responses to gemcitabine; there were no partial responses. Grade 3 or 4 hematologic toxicity and liver toxicity were common during therapy. Only one patient was alive 1 year after entry. The estimated 1-year overall survival probability for the 32 patients was 4% (SE = 3%).

CONCLUSIONS

Gemcitabine at the dose and schedule in this trial was not effective for children with relapsed AML or ALL.

Adult acute myeloid leukemia

Acute myeloid leukemia (AML) is a group of several different diseases, the treatment and outcome of which depend on several factors, including leukemia karyotype, patient age, and comorbid conditions. Despite advances in understanding the molecular biology of AML, its treatment remains challenging. Standard regimens using cytarabine and anthracyclines for induction followed by some form of postremission therapy produce response rates of 60% to 70%, with less than 20% of all patients achieving long-term disease-free survival. New therapies are emerging based on the definition of specific cytogenetic-molecular abnormalities. Such targeted therapies offer the promise of better antileukemic activity in adult AML.

Combination therapy with DNA methyltransferase inhibitors in hematologic malignancies

A variety of epigenetic changes contribute to transcriptional dysregulation in myelodysplastic syndromes (MDSs) and acute myeloid leukemia (AML). DNA methyltransferase (DNMT) inhibitors--azacitidine and decitabine--have significant activity in the treatment of MDS. Despite marked activity in myeloid malignancy, monotherapy with DNMT inhibitors is limited by low complete and partial response rates (7-20%) and median response durations of 15 months. As with classical cytotoxic therapy, the targeting of biologic pathways and mechanisms may best be accomplished using a combination of agents offering complementary mechanisms and synergistic pharmacodynamic interactions. The goal of this approach is to improve response rates, quality, and duration, and to minimize adverse events. There are a number of new therapies under development for the management of MDS and AML. This review article touches on some of the more promising combination regimens in various phases of investigation. The treatment of MDS and AML is undergoing rapid evolution. Cytogenetic complete remission and prolonged survival represent important goals. Incremental improvements in disease state and quality-of-life issues are also important for patients. Given the overall failure of cytotoxic chemotherapy in the achievement of cures in MDS and MDS-related AML, the application of less toxic, biologically directed agents may represent a more promising approach to treatment. Combination therapies with DNMT inhibitors using optimal dosing regimens to focus on methylation reversal with lower doses over longer periods of time, rather than direct cytotoxic effects, are beginning to suggest promising results in MDS and 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.

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.

Thromsbospondin-1 binds to the heavy chain of elastase activated coagulation factor V (FVaHNE) and enhances thrombin generation on the surface of a promyelocytic cell line

INTRODUCTION

Thrombospondin 1 (TSP1) has the ability to bind to HL-60 cells and to reversibly inhibit human neutrophil elastase (HNE). Human factor V (FV) can be cleaved by HNE thereby providing FV with cofactor activity (FVa(HNE)). Experiments were performed to evaluate the ability of HNE expressed on the surface of HL-60 cells to generate FVa(HNE) to support thrombin generation, and to determine the effect of TSP1 on this reaction.

RESULTS

Western blot analysis showed TSP1 forming a complex with FVa(HNE) within a region corresponding to the heavy chain of FV. Enzymatic reactions were performed to determine the role of TSP1-HNE-FVa(HNE) on the surface of HL-60 cells, namely the assembly of the prothrombinase complex. Thrombin generation was measured by the chromogenic substrate S2238. Exposure of factor V to HL-60 cells prior to the addition of prothrombin and activated factor X provided FV with cofactor activity. HL-60 cells were found capable of synthesizing factor V with cofactor activity, but HL-60 cells failed to synthesize and/or to provide factor X with enzymatic activity. The ability of HL-60 cells to synthesize FV and TSP1 was demonstrated. The addition of exogenous TSP1 enhanced both the rate and amount of thrombin generated on the HL-60 cell surface.

CONCLUSION

Despite the ability of TSP1 to reversibly inhibit HNE in a purified system, TSP1 expression favors the reactions leading to thrombin generation on the HL-60 cell surface. These observations are relevant to clinical conditions where there is a prothrombotic state such as malignant tumors.

CD33-directed therapy with gemtuzumab ozogamicin in acute myeloid leukemia: progress in understanding cytotoxicity and potential mechanisms of drug resistance

CD33 is expressed on the malignant blast cells in most cases of acute myeloid leukemia (AML) but not on normal hematopoietic pluripotent stem cells. Antibody-based therapies for AML have, therefore, focused on CD33 as a suitable tumor-associated target antigen. The most promising results have been obtained with gemtuzumab ozogamicin (GO, Mylotarg), a humanized IgG(4) anti-CD33 monoclonal antibody joined to a calicheamicin-gamma(1) derivative. Engagement of CD33 by GO results in immunoconjugate internalization and hydrolytic release of the toxic calicheamicin moiety, which, in turn, causes DNA damage and cell death. Since 2000, when GO was approved for clinical use, treatment trials and pilot studies have revealed potential expanded applications along with additional limitations. At the same time, correlative biological and in vitro functional studies have further characterized CD33 expression patterns in AML, the significance of CD33-antibody interactions, pathways involved in GO-induced cytotoxicity and potential drug resistance mechanisms. This review summarizes the recent data addressing mechanisms of GO action and discusses their relevance with regard to clinical applications and the limitations of using experimental model systems to mimic in vivo conditions. As the first drug conjugate approved for clinical use, GO serves as an important paradigm for other immunoconjugates against internalizing tumor antigens.

Self-renewal and solid tumor stem cells

Solid tumors arise in organs that contain stem cell populations. The tumors in these tissues consist of heterogeneous populations of cancer cells that differ markedly in their ability to proliferate and form new tumors. In both breast cancers and central nervous system tumors, cancer cells differ in their ability to form tumors. While the majority of the cancer cells have a limited ability to divide, a population of cancer stem cells that has the exclusive ability to extensively proliferate and form new tumors can be identified based on marker expression. Growing evidence suggests that pathways that regulate the self-renewal of normal stem cells are deregulated in cancer stem cells resulting in the continuous expansion of self-renewing cancer cells and tumor formation. This suggests that agents that target the defective self-renewal pathways in cancer cells might lead to improved outcomes in the treatment of these diseases.

Accelerated and blastic phases of chronic myelogenous leukemia

Chronic myelogenous leukemia (CML) may have a biphasic or triphasic course, whereby patients who were initially diagnosed in the chronic phase (CP) develop more aggressive disease, frequently pass through an intermediate or accelerated phase (AP), and finally evolve into an acute leukemia like blastic phase (BP). A slowing in the rate of development of AP or BP has accompanied successive improvements in therapy for patients who have CP CML. Variable diagnostic criteria for AP and BP are used in the literature, rendering comparisons difficult. The management of patients in AP or BP consistently has been less effective than the management of those inCP for all modalities of therapy. This article reviews the current diagnostic criteria, therapeutic strategies, outcomes, and investigational therapies for AP and BP CML.