Current Cancer Drug Targets - Volume 11, Issue 1, 2011

Almost 50 years ago, David Hungerford and Peter Nowell first described the Philadelphia chromosome as characteristic cytogenetic abnormality in Chronic Myeloid Leukemia (CML, reviewed in [1]). In the following decades, the advent of molecular biology allowed the exact characterization of the molecular pathogenesis of the disease, which served as basic knowledge for subsequent development of tyrosine kinase inhibitors (TKI) directly inhibiting the constitutive activity of the BCR-ABL tyrosine kinase. Due to their outstanding clinical activity, these molecularly designed drugs represent a paradigm for targeted therapies in modern hemato-oncology. Thus, it appeared at the first half of the last century, that the milestones achieved in the understanding of the pathogenesis and therapy of CML would outcompete the clinical progress in Philapdelphia-negative myeloproliferative neoplasia (MPN). The latter have first been systematically categorized as a disease group by Wiliam Dameshek in the last century by identification of common clinical characteristics such as hemorrhage, thrombosis and leukemic transformation, which are hallmarks of polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF, reviewed in [2]). Accordingly, the gain-of-function mutation JAK2V617F has been described as the most prevalent genetic abnormality in Ph-negative MPN [3-5]. However, recent systematic genetic screens including analyses of SNPs and copynumber variations revealed a much more complex picture of the molecular pathobiology of Ph-negative MPN than initially assumed [6, 7]. In analogy to the development of TKIs in CML, the high abundance of the JAK2 mutation in Ph-negative MPN also provided a rational target for novel treatment strategies using JAK2 inhibiting compounds. To the much of our surprise, despite marked clinical activity of JAK2 inhibitors in Ph-negative MPN in terms of improving clinical symptoms, their anticlonal activity is obviously limited [8] when compared to the extremely high anti-clonal activity of TKI therapy in CML [9]. This observation led to a reverse translation from bed-to-benchside, with the ultimate goal to more accurately explore the functional relevance of the JAK2 mutation for the disease phenotype. In CML, we are currently on a path towards cure by further exploring potential targeting strategies to eradicate the CML-repopulating stem cell population, which is despite the high anticlonal activity of TKIs in CML, not eliminated in most patients so far. Thus, extensive research is currently ongoing to further characterize the molecular basis for a better understanding of stem cell TKI resistance thereby providing the key for future innovative curative treatment approaches. In contrast, in Ph-negative MPN, the repopulating stem cell population remains to be identified. This review series covers the current state of the art knowledge of the complex molecular pathogenesis of Ph-positive and negative MPN. This serves as a basis for the understanding of current treatment concepts, which are discussed in detail throughout this MPN special edition. Moreover the potential of immunotherapies for disease eradication in MPN as well as the appealing stem cell concept raising the problem of definite disease eradication are discussed in light of latest scientific data. The gust editors cordially thank all experts for their contribution to this MPN special edition.

CML is characterized by the presence of the Philadelphia chromosome, which is the product of a reciprocal translocation between chromosomes 9 and 22 that results in the formation of BCR-ABL1. Apart from its diagnostic importance in CML patients BCR-ABL1 it is a potent oncogene. The natural evolution of CML is to progress into accelerated phase and blast crisis after a rather indolent chronic phase. Clinical experience shows that long term remissions can be achieved at a high rate at least in chronic phase by specific inhibition of BCR-ABL1. This underlines the importance of BCR-ABL1 at this stage of the disease. However, in accelerated phase and blast crisis the effect of these substances is of inferior importance as relapses are the rule rather than the exception. Treatment failure in advanced disease is frequent in patients without detectable resistance mechanisms such as BCR-ABL1-mutations, which suggests that the previously BCR-ABL1 dependent pathways probably become autonomous. Such pathways include signal transduction as well as DNA damage surveillance and repair. Especially the latter appear to be crucial for disease progression by causing genetic instability, accumulation of mutations and additional chromosomal alterations leading to the loss of tumor suppressors. How is BCR-ABL1 organized on the genetic level, is there a genetic precursor lesion as discussed for Philadelphianegative myeloproliferative diseases, what is its role in pathogenesis and progression of CML and what is its role in the CML-stem cell? These questions will be discussed in this review.

Clonal hematopoiesis triggered by somatic mutations plays a central role in the pathogenesis of Philadelphia chromosome negative chronic myeloproliferative neoplasms (MPNs). After the discovery of JAK2 and MPL mutations, continual technological advances have led to the identification of increasing numbers of genetic defects in MPN patients, most of them chromosomal aberrations such as deletions and acquired uniparental disomies. Although efforts to map the genetic lesions to single genes resulted in the discovery of defects in the TET2 and CBL genes, most of the target genes comprised in the chromosomal lesions still remain to be identified. In this review, the different genetic defects found in MPN and their relationships to each other and to disease pathogenesis are critically evaluated. At present, most of the evidence points to a random acquisition of phenotypic and non phenotypic mutations contributing to clonal heterogeneity in MPN. The origin of genetic instability in context of hereditary factors and the common JAK2 haplotype predisposing for the disease are discussed. Furthermore, we address how the choice of therapeutic approaches could be influenced by the genetic complexity.

The elucidation of the triggering molecular mechanism of chronic myeloid leukemia gave rise to the development of imatinib, a tyrosine kinase inhibitor and a prototype of target-oriented drugs. Imatinib led to impressing response and survival rates and now represents the standard therapy of CML. However, a significant proportion of patients do not tolerate or fail to respond to imatinib treatment. Alternative therapies can be offered to those patients. The particular challenge of CML patient management is to recognize an impending imatinib failure by adequate surveillance and to know about therapeutic options to prevent progression of the disease to accelerated phase or blast crisis since these are more difficult to control. Targeted therapy with second-generation tyrosine kinase inhibitors should be used in synopsis with mutational analysis and the patients' history. In this review we present current knowledge of diagnosis, monitoring and therapy strategies of patients with CML.

Since William Dameshek has described the concept of “myeloproliferative disorders (MPD)” by identifying common clinical characteristics (i.e. hemorrhage, thrombosis and leukemic transformation) of polycythemia vera (PV), essential thrombocythemia (ET), and primary myelofibrosis (PMF), the advent of molecular biology has provided substantial molecular insight into the pathobiology of myeloproliferative neoplasia (MPN). Recently, the description of the gain-of-function mutation of JAK2 (JAK2V617F) has been identified in classical Philadelphia (Ph)-negative MPN, thus providing a rational target for novel innovative treatment strategies. In addition, molecular characterization of atypical Phnegative MPN (e.g. the KITD816V mutation in mastocytosis and PDGF-receptor rearrangements in hypereosinophilic syndromes/chronic eosinophilic leukemia) complement the molecular knowledge of this heterogeneous disease family. Currently, clinical studies testing various JAK2-inhibitors in PV, ET as well as in primary and secondary myelofibrosis (MF) are under way. Interestingly, first data indicate that despite marked clinical activity in terms of spleen size reduction and improvement of constitutional symptoms, these inhibitors might not sufficiently reduce disease burden. Thus, alternative and well established treatment strategies, such as inhibition of thrombocyte aggregation by low dose aspirin, cytotoxics (e.g. hydroxyurea), immuno- and stroma-modifying therapy with interferon, tyrosine kinase inhibitors and, in selected cases, allogeneic stem cell transplantation are still important treatment options for patients suffering from MPN, which will be discussed in detail in this review.

The concept of leukemic stem cells (LSC) is increasingly employed to explain the biology of various myeloid neoplasms and to screen for essential targets, with the hope to improve drug therapy through elimination of disease initiating cells. Although the stem cell hypothesis may apply to all neoplasms, leukemia-initiating cells have so far only been characterized in some detail in advanced acute (AML) and chronic myeloid leukemia (CML). An intriguing observation is that although expressing various targets, LSC often remain unresponsive against most drugs, presumably because of ‘intrinsic’ resistance. Moreover, LSC represent heterogeneous populations of cells, grow in separate subclones, and acquire numerous defects, which points to substantial genetic instability and stem cell plasticity. The situation is complicated by the fact that stem cell evolution is a step-wise process with variable latency periods, so that many LSC-derived subclones remain small (undetectable) at diagnosis, but later, during therapy, may expand to a dominant clone and clinically overt relapsing disease. Finally the interaction between LSC and the microenvironment may contribute to stem cell function and LSC resistance. Taking all these considerations into account, the application of broadly acting targeted drugs and of drug combinations has been proposed in order to better suppress or even eliminate LSC in AML and CML. The current article provides a summary of our knowledge on LSC in various myeloid neoplasms with special reference to novel arising treatment concepts.

The four major entities that form the group of myeloproliferative neoplasms (MPN) are BCR-ABL positive chronic myeloid leukaemia (CML), chronic idiopathic myelofibrosis (CIMF), essential thrombocythemia (ET) and polycythemia vera (PV). All four are clonal diseases of the haematopoietic stem or precursor cell, they are of a chronic nature and potentially aggravate to myelofibrosis or transform into acute leukaemia. Several strategies are pursued in the treatment of MPN. On the one hand, targeted therapies such as tyrosine kinase inhibitors (imatinib, dasatinib, nilotinib) and JAK2-inhibitors are adopted in MPN as well as rather unspecific treatment with interferon-alpha and with the newer group of immunomodulatory drugs (IMIDs). On the other hand, cellular immunotherapeutical options as allogeneic haematopoietic stem cell transplantation (HSCT) and donor lymphocyte infusion (DLI) are exerted in patients with MPN. Evidence resulting from graft-versus-leukaemia (GvL) effect was the key to develop more specific immunotherapies for patients with haematologic malignancies. In this context, CML is a model for immunotherapeutic approaches, and therefore, vaccination trials using peptides derived from leukaemia-associated antigens (LAA) to stimulate specific T cells are currently under investigation. But also in BCR-ABL-negative MPN, antigens have been identified and immunomodulatory treatment strategies have been performed. All of the current immunotherapeutical options in patients with MPN will be discussed throughout this review.

The topic of this review covers a very important branch of cancer research, cancer vaccination. The growing knowledge in tumor immunology has evolved rapidly, starting from unspecific generic stimulation of the immune system to more specific approaches based on the availability of tumor antigens. The review covers molecular and cell biology, and pharmaceutical technology of cancer vaccines. Particularly, it is aimed at highlighting the results of cancer vaccines from phase II and III clinical trials, an issue that is of relevance to better understand how cancer vaccines can successfully complement antitumor therapy, including conventional chemotherapy and the recently developed target-based drugs.

Therapeutic vaccines continue to be one of the most active fields in cancer research. However, despite clear evidence of antitumor effect in laboratory animals, and despite the ability of current vaccine candidates to elicit tumor specific antibodies and T-cells in humans, objective responses in the clinical trials are rare. The role of therapeutic vaccines in advanced cancer patients, if any, would be to decrease the rate of disease progression and to increase survival and quality of life. Due to the redundant regulatory loops contracting the immune response to antigens that cannot be eliminated, such a role would require chronic vaccination, which is at first sight at odds with the classic experience of vaccinology. During the last decade our team has been developing a therapeutic vaccine for advanced lung cancer, which consists of human recombinant Epidermal Growth Factor (EGF) chemically conjugated to a carrier protein from Neisseria meningitides. Several clinical trials have been carried out, showing increase in anti-EGF antibody titers, decrease in plasma EGF concentration and survival advantage in vaccinated patients. In the present paper we review data from 58 patients who were vaccinated monthly for more than one or two years. Long term vaccination was feasible and safe, and there was no evidence of cumulative toxicity. Patients kept high anti-EGF antibody titers during all the time of vaccination, without evidence of immune response exhaustion. Continued vaccination increased the probability to get a high antibody response, which has been previously shown to be, in turn, associated with a better survival. Observations done in this series of patients suggest that long term therapeutic vaccination is a feasible strategy, worth to be further explored with the aim of transforming advanced cancer into a chronic disease.

Purpose: Oncolytic viral therapy is a newly developed modality to treat tumors. Many clinical trials worldwide have examined the efficacy of locally injected oncolytic viruses. However, systemic intravascular injections are limited by the humoral immune response, which dramatically decreases the level of infection. To overcome this limitation, we encapsulated the oncolytic virus in liposomes. Methods: The infectious properties of the herpes simplex virus type 1 (HSV-1) mutant, hrR3, with or without liposomes in the presence of neutralizing antibodies were evaluated using replication and cytotoxicity assays in vitro. To evaluate the efficacy of intravascular virus therapy with liposomes in the presence of neutralizing antibodies, immunized mice bearing multiple liver metastases were intraportally or peritoneally administered hrR3 or hrR3 complexed with liposomes. Results: Anti-HSV antibodies attenuated the infectiousness and cytotoxicity of hrR3, whereas hrR3/liposome complexes were not attenuated by these anti-HSV antibodies. Although the survival rate of non-immunized mice treated with hrR3 alone was similar to that of mice treated with the hrR3/liposome complexes, the survival rates of immunized mice treated with hrR3 alone were significantly reduced compared to mice treated with the hrR3/liposome complexes. Conclusions: This systemic intravascular delivery of hrR3/liposome complexes in the presence of pre-existing neutralizing antibodies is effective to treat multiple liver metastases.

Background and aim: Translational data suggest that nucleoside transporters, in particular human equilibrative nucleoside transporter 1 (hENT1), play an important role in predicting clinical outcome after gemcitabine chemotherapy for several types of cancer. The aim of this study was to retrospectively determine patients' outcome according to the expression of hENT1 in tumoral cells of patients receiving gemcitabine-based therapy. Materials and Methods: The immunohistochemistry analysis was performed on samples from thirty-one patients with unresectable biliary tract cancer (BTC) consecutively treated with first line gemcitabine-based regimens. Results: Positive hENT1 staining patients were 21 (67.7%); negative hENT1 staining patients were 10 (32.3%). Statistical analysis revealed no association between baseline characteristics, toxicities and tumor response to gemcitabine and hENT1 levels. In the univariate analysis, HENT1 expression was significantly correlated with time to progression (TTP) (p=0.0394; HR 2.902, 95%CI 1.053-7.996). The median TTP was 6.33 versus 2.83 months, respectively in patients with positive versus negative hENT1 staining. Moreover, patients with positive hENT1 expression showed a longer median overall survival when compared with patients with low hENT1 expression (14 versus 7 months, respectively), but this difference did not reach the statistical significance (p=0.128). Conclusions: Therefore, hENT1 may be a relevant predictive marker of benefit from gemcitabine-based therapies in patients with advanced BTC.