Current Pharmaceutical Biotechnology - Volume 7, Issue 5, 2006

Significant advances in the molecular biology and therapy of hematological malignancies have been made over the last decade. The therapeutic approaches to the hematological malignancies such as acute and chronic leukemias, multiple myeloma, and malignant lymphoma are basically chemotherapy to eradicate the malignant cells. However, severe side effects and complications due to anti-cancer drugs are major problems in the clinical settings. Moreover, relapsed cases are usually refractory to chemotherapy and have poor prognosis. These clinical problems suggest that the current therapeutic strategies for the hematological malignancies have limitations and novel effective therapeutic approaches with less toxicity are needed. In the 1980s, the use of all-trans retinoic acid (ATRA) for differentiation-inducing therapy in acute promyelocytic leukemia (APL) has altered the therapeutic strategies of leukemia and dramatically improved the outcome. In addition, high remission rates in patients with chronic myeloid leukemia (CML) who receive imatinib mesylate indicate that molecular-targeted therapy for hematological malignancies is effective and safe. At the same time, progress in cellular biology has resulted in an increased understanding of the molecular genetics of the diseases and a characterization of the molecular targets for drug development. However, as for the other molecular-targeted agents, it is difficult to find a great deal of differences in the clinical outcome of patients who received standard therapies over the last decades. Therefore, investigators have actively sought out new agents with the ability to stimulate cellular differentiation and induce apoptosis in various cancer cells. Cellular proliferation, differentiation, and apoptosis are regulated by a number of extra- and intracellular molecules. These molecules mediate gene transcription either directly or indirectly by activating various signaling pathways. Molecular lesions of genes encoding for transcriptional factors are common oncogenic events in hematological malignancies. The complexicity of the transcriptional process offers a large number of substrates for designing therapeutic agents. However, the success of ATRA in the treatment of APL indicates that the targeted therapy for transcriptional factors can be highly effective and safe. As a consequence, the advances of molecular basis in hematological malignancies have spawned the development of effective agents such as monoclonal antibodies, specific enzyme inhibitors, and inhibitors of transcriptional factors. This special issue attempts to provide a practical introduction to the attractive and developing field of molecular-targeted therapy in hematological malignancies.

Differential methylation of CpG islands is a regulatory mechanism for promoter activity of different classes of genes, including tissue-specific genes. These CpG islands are targets for transformation-associated, aberrant hypermethylation activity during leukemogenesis. Therefore the pharmacological reversion of this methylator phenotype (e.g. by reactivation of tumor suppressor gene expression) is an important rationale for development of inhibitors of DNA methyltransferase activity. In vitro , inhibition of methylation using azanucleosides results in modest differentiation of transformed myeloid cell lines. In vivo, low doses of these agents induce DNA demethylation of malignant myeloid cells. Indeed, the first drug specifically approved for the treatment of myelodysplastic syndrome (MDS) was the azanucleoside 5- azacytidine (Vidaza®). The most potent DNA demethylating agent available, 5-aza-2' deoxycytidine (Decitabine, Dacogen ®) also has recently been approved by the U.S.A. FDA for treatment of MDS of all subtypes. About 30% of MDS patients with an abnormal karyotype have normalization of their karyotype after receiving the drug. This activity is especially relevant in patients with high-risk karyotypic abnormalities (complex karyotype and / or abnormalities of chromosome 7) compared to patients with intermediate-risk karyotype. Both drugs offer a novel, non-intensive therapeutic approach, particularly in the older patient population who due to comorbidities and / or reduced performance status are ineligible for aggressive chemotherapies. Target genes being particularly prone to demethylation by these drugs in the aberrant cells (e.g. p15/INK4b) are under active investigation. Future translational and clinical studies will be aimed at improving the response rate and duration of response to non-intensive treatment with demethylating agents, by studying rational drug combinations e.g. with inhibitors of histone deacetylase activity.

Recent advances in genetic and molecular biology have provided greater insight into the biology of acute myeloid leukemia (AML). These investigations have shown that AML is a heterogeneous disease of biologically different entities. Current therapeutic approaches to AML are based on chemotherapy, but the side effects of the drugs used and various complications, including infections and bleeding, are sometimes fatal. In addition, responses to therapy and longterm outcome differ depending on the subentity in question. Therefore, it is essential to develop new therapeutic strategies such as biology adapted treatment based on the individual molecular pathogenesis of AML. Natural compounds appear to be safer than the current chemotherapeutic drugs, and we have therefore sought new potential agents among various natural compounds with the ability to induce the apoptosis of myeloid leukemic cells. Recently, we found that a highly toxic reactive oxygen species (ROS) generated via the hydrogen peroxide/myeloperoxidase [H2O2/MPO/halide] system by natural compounds induces apoptosis in MPO-positive leukemic cells. This result is of great interest in establishing novel therapeutic approaches to AML mediated through ROS-induced apoptosis of leukemic cells.

Myeloid malignancies are frequently associated with translocations and mutations of tyrosine kinase genes. Fusion genes involving ABL, ARG, PDGFRs, JAK2, SYK, TRKC, and FGFRs, and gain-of-function mutations of FLT3, KIT and JAK2 have been detected at various rates in myeloproliferative disease and acute myeloid leukemia. Furthermore, abnormal overexpression of tyrosine kinases such as FLT3 has also been reported. These gene products are constitutively activated and potentially transform hematopoietic cells by augmentation of proliferation and enhanced viability. Since the fusion or mutation of tyrosine kinase is a primary and central event in chronic myeloproliferative diseases, targeting the kinase activity has been thought to be an ideal intervention to treat these diseases. The clinical success of imatinib for chronic myeloid leukemia has made this idea a reality, and has accelerated the development of new tyrosine kinase inhibitors (TKIs). Challenging studies with TKIs have also been reported for acute myeloid leukemia. This review will focus on recent trials of TKIs against oncogenic tyrosine kinases (ABL, PDGFRs, FLT3 and KIT) in myeloid malignancies.

The IMiDs™ represent a new proprietary class of thalidomide analogues that possess greater potency and less toxicity than the parent compound. As a group, these agents share the pharmacologic property of modulating cellular response to ligand activation, the precise biologic effect of which is cell lineage and stimulant-dependent. Lenalidomide (CC-5013; Revlimid™), a second generation IMiD, has shown significant erythropoietic activity in patients with lower risk MDS that have failed or are not candidates for recombinant erythropoietin treatment. Unlike cytokine therapy, lenalidomide suppresses select MDS clones and enhances erythropoietin receptor signaling to restore erythropoiesis. Activity is greatest in patients with interstitial deletions involving chromosome 5q31.1. A multicenter phase II study reported a 76% overall transfusion response rate in transfusion-dependent patients with deletion 5q, with 67% achieving transfusion independence after a median interval of 4.6 weeks of treatment. Cytogenetic responses were observed in 73% of patients with complete cytogenetic remission in 45% patients. Both transfusion response and cytogenetic response frequency were independent of karyotype complexity, raising excitement that this new treatment strategy might favorably alter the natural history of disease in higher risk patients with deletion 5q. Lenalidomide was approved by the U.S. Food and Drug Administration on December 27, 2005, for the treatment of IPSS Low and intermediate-1 risk MDS patients with del(5q) abnormality. A phase III Intergroup trial (ECOG 2905) will test the capacity to potentiate erythropoietin response by comparing response to lenalidomide monotherapy to the combination of darbepoetin and lenalidomide in non-deletion 5q MDS patients.

Currently, patients with acute myeloid leukemia (AML) are treated with cytotoxic chemotherapy and hematopoietic stem cell transplantation (HSCT). With this approach, the majority of patients still die of their disease because of both treatment-related mortality and relapse. Recently, monoclonal antibodies and immunoconjugates have been developed which potentially may increase the efficacy of treatment and decrease morbidity and mortality by specifically targeting the malignant cell. Unconjugated monoclonal antibodies have shown only moderate activity. A second, more effective, approach involves antibody conjugation with radioactive particles or chemotherapeutic agents, such as, immunotoxins, targeted delivery of cell killing. The antigens CD33, CD45, and CD66, are three antigens to which monoclonal antibodies have been directed. Most experience has been with gemtuzumab ozogamicin (Mylotarg) which is an immunoconjugate of an anti-CD33 antibody chemically linked to a potent cytotoxic agent, calicheamicin. Gemtuzumab ozogamicin appears to be particularly active in patients with acute promyelocytic leukemia, possibly related to the high expression of the CD33 antigen on the cell surface. Although gemtuzumab ozogamicin has activity as a single agent, the most promising result may be seen when this agent is combined with conventional cytotoxic chemotherapy. Preliminary studies have suggested a high complete remission rate and randomized clinical trials are underway. A unique potential toxicity has been identified, namely venoocclusive disease or sinusoidal obstructive syndrome which may be problematic among patients who subsequently undergo HSCT. An additional strategy includes radiolabeled monoclonal antibodies to intensify the conditioning regimen prior to HSCT. The most promising results have been obtained with radiolabeled anti-CD45 antibodies.

Imatinib mesylate, Abl tyrosine kinase inhibitor, has improved the treatment of Bcr-Abl-positive leukemia such as chronic myeloid leukemia (CML) and Philadelphia chromosome positive acute lymphoblastic leukemia (Ph+ALL). However, resistance is often reported in patients with advanced-stage disease. Several novel tyrosine kinase inhibitors, which have been developed to override imatinib resistance mechanisms such as overexpression of Bcr-Abl and point mutations within the Abl kinase domain, are currently competing. Inhibitors of Abl tyrosine kinase are divided into two main groups, namely, ATP-competitive and ATP non-competitive inhibitors. Moreover, ATP-competitive inhibitors are fall into two subclasses, i.e. the Src/Abl inhibitors, and 2-phenylaminopyrimidin-based compounds. Dasatinib (formerly BMS-354825), AP23464, SKI-606 and PD166326 are classified as Src/Abl inhibitors while AMN107 and NS-187 (INNO-406) belong to the latter subclass of inhibitors. Among these agents, clinical studies on dasatinib and AMN107 had started earlier than the others and favorable results are accumulating. Clinical studies of other compounds including NS-187 (INNO-406) will be performed in rapid succession. Because of its strong affinity, most ATP competitive inhibitors may be effective against imatinib-resistant patients. However, to date, an ATP-competitive inhibitor that can inhibit the phosphorylation of T315I Bcr-Abl has not yet been developed. To address this problem, ATP non-competitive inhibitors such as ON012380, Aurora kinase inhibitor VX-680 and p38 MAP kinase inhibitor BIRB-796 have been developed. It may be necessary for the improvement of CML and Ph+ALL treatment to be taken into consideration of the combination therapy with novel ATP-competitive inhibitors and these agents.

Multiple Myeloma (MM) remains an incurable plasma cell malignancy in the bone marrow (BM) despite conventional therapies as well as high-dose therapies with stem cell support. Therefore novel biologically-based therapeutic approaches are required. Recently, intensive laboratory and preclinical studies have identified and validated therapeutic molecular targets in MM. In particular, recognition of the biologic significance of the BM microenvironment in MM pathogenesis and as a potential target for novel therapeutics has derived several promising approaches. Novel FDA approved agents including thalidomide/thalomid®, its immunomodulatory derivatives lenalidomide/Revlimid®, and proteasome inhibitor bortezomib/Velcade® are directed at molecular targets not only in MM cells but also in its BM milieu, and have already achieved promising results in clinical studies. Here we discuss the mechanisms of action of these novel drugs and their clinical application, alone or combined with conventional or novel drugs.