Current Pharmaceutical Design - Volume 10, Issue 11, 2004

Activating mutations of FLT3 (FMS-Like Tyrosine kinase-3) are the most common molecular abnormality in acute myeloid leukemia (AML). Their presence is associated with a worse prognosis, and the recognition of this has led to the development of several new small molecule FLT3 tyrosine kinase inhibitors. In this review, we summarize these developments and compare and contrast these novel agents both with regards to the assays used to characterize them as well as to their clinical potential.

Acute graft-versus-host disease (GVHD) and chronic GVHD remain the major barriers to successful hematopoietic cell transplantation. The induction of GVHD may be divided into three phases- (I) recipient conditioning, (II) donor T cell activation, and (III) effector cells mediating GVHD. Cytokines have been shown to be extremely important in the initiation and propagation of GVHD. Of note, IL-2 and TNF-α lead to cellular activation as well as local tissue damage. There has been a major development in the last few years of monoclonal antibodies that target cytokines. Drugs that target the IL-2 receptor (daclizumab and basiliximab) are now commonly used to prevent renal transplant rejection. Furthermore, drugs that target TNF-α (infliximab and etanercept) are used in rheumatoid arthritis and Crohn's disease but are also being tested for a number of other autoimmune diseases. These agents are very selective immunosuppressants that have different mechanisms of action than the calcineurin inhibitors and therefore are potentially promising for treatment or prevention of GVHD. The authors present up-to-date data regarding the use and development of anti-cytokine therapy for GVHD. The most effective approach to GVHD prevention will likely be a combination regimen where the three phases of the GVHD cascade are disrupted. Once GVHD has occurred, all three phases of the cascade are activated. Developments of combination therapy for treatment of both acute and chronic GVHD will likely yield better results than monotherapy. The numerous new treatment modalities presented should improve the outlook for acute and chronic GVHD.

Harnessing the power of the immune system to eliminate infection and cancer is a long-standing goal in clinical immunology. The development of a robust immune response to foreign antigen relies, in part, on communication between cellular components of the immune system. The proteins involved in governing the magnitude and longevity of an immune response are collectively called cytokines. Cytokines act directly on immune cells to induce proliferation, differentiation and tolerance, and signaling errors can lead to autoimmune disease or immune deficiency. Identification of the molecules involved in these signaling processes has allowed investigators to manipulate immune cells for therapeutic effect, both in vivo and ex vivo. While in vivo immune modulation using cytokines has produced some exciting results, the toxicity involved with systemic administration has limited their broad use in the clinic. Therefore, research has been focused on the ex vivo manipulation of immune cells for adoptive transfer to treat cancer and infection. This review will focus on the ex vivo manipulation of immune cells with particular emphasis on stimulating cytotoxic T cell responses. Adoptive transfer of ex vivo generated cell types may then be used to treat malignant and viral disease.

Angiogenesis is defined as the formation of new capillaries from prexisting blood vessels and plays an important role in the progression of solid tumors and hematologic malignancies. Markers of angiogenesis correlate with clinical characteristics in leukemia and non-Hodgkin´s-lymphoma, serving as predictors of poor prognosis. Antiangiogenic effects of chemotherapeutics as well as of novel drugs such as farnesyltransferase inhibitors and tyrosine kinase inhibitors such as Gleevec(r) might contribute to their therapeutic potential. Thalidomide which has antiangiogenic effects and direct cytotoxic effects was found to be effective in multiple myeloma and is considered as an established treatment modality for patients with refractory or relapsed multiple myeloma. Thalidomide has a significant therapeutic effect in myelodysplastic syndrome (MDS) by improving cytopenia and achieving independence of transfusion therapy in a subset of patients. Preliminary data indicate activity of specific VEGF receptor tyrosine kinase (RTK) inhibitors in multiple myeloma (MM) and acute myeloid leukemia (AML). The positive correlation between increased levels of angiogenic cytokines and clinical response to VEGF-RTK inhibitors and thalidomide indicates the relevance of detecting angiogenesis markers to identify best candidate patients for specific approaches. Including antiangiogenic drugs into treatment protocols for hematologic malignancies is an important task for future clinical studies.

Recombinant protein technology produces drugs for human therapy in unprecedented quantity and quality. Research is now focusing on the relationship between pharmacokinetic and pharmacodynamic properties of molecules, with the aim of engineering proteins that possess enhanced therapeutic characteristics in contrast to being used as simple replacements for the natural equivalent. The addition of a polyethylene glycol (PEG) moiety to filgrastim (rmetHu-G-CSF, Neupogen®) resulted in the development of pegfilgrastim. Pegfilgrastim is a long-acting form of filgrastim that requires only once-per-cycle administration for the management of chemotherapy-induced neutropenia. The covalent attachment of PEG to the Nterminal amine group of the parent molecule was attained using site-directed reductive alkylation. Pegylation increases the size of filgrastim so that it becomes too large for renal clearance. Consequently, neutrophil-mediated clearance predominates in elimination of the drug. This extends the median serum half-life of pegfilgrastim to 42 hours, compared with between 3.5 and 3.8 hours for Filgrastim, though in fact the half-life is variable, depending on the absolute neutrophil count, which in turn reflects of the ability of pegfilgrastim to sustain production of those same cells. The clearance of the molecule is thus dominated by a self-regulating mechanism. Pegfilgrastim retains the same biological activity as filgrastim, and binds to the same G-CSF receptor, stimulating the proliferation, differentiation and activation of neutrophils. Once-per-chemotherapy cycle administration of pegfilgrastim reduces the duration of severe neutropenia as effectively as daily treatment with filgrastim. In clinical trials, patients receiving pegfilgrastim also had a lower observed incidence of febrile neutropenia than patients receiving filgrastim.

SDF-1 and CXCR4 are an important chemokine ligand / receptor pair, which play a crucial role in numerous biological processes including hematopoiesis, cardiogenesis, vasculogenesis, neuronal development and immune cell trafficking. They have also been implicated in various pathological conditions such as cancer, infection with the human immunodeficiency virus (HIV) and various inflammatory conditions. Numerous pharmacological agents exist that can modulate SDF-1 / CXCR4-induced responses both in vitro and in vivo. The usefulness of these agents in affecting the outcome of pathological conditions influenced by the SDF-1 / CXCR4 axis is currently being investigated. Whilst some of these compounds have been shown to be safe and well tolerated in phase 1 clinical trials, the full repercussions of SDF- 1 / CXCR4 inhibition or stimulation on normal physiological functions are yet to be appreciated. Inhibition of the SDF- 1 / CXCR4 axis may have positive effects in regulating tumour metastasis and growth, however, this may also negate immunological responses through dysregulated lymphocyte trafficking and contribute to disruption of hematopoiesis. As with any therapy, the usefulness of this type of intervention will require a balance between its positive effect on the disease outcome and deleterious effects on normal physiological functions. A greater understanding of the role of SDF-1 and CXCR4 in the body will allow greater manipulation of this important biological axis to affect disease outcome. Greater knowledge of the SDF-1 interaction with its receptor and the structural elements within CXCR4 mediating the different signalling events, resulting in SDF-1-induced responses, will also enhance future drug design.

Various growth factors such as the platelet derived growth factor (PDGF), insulin-like growth factor (IGF), epidermal growth factor (EGF), fibroblast growth factor (FGF), and vascular endothelial growth factor (VEGF) transduce their mitogenic signals through the activation of tyrosine kinase receptors (RTKs). Since enhanced RTK activity has been associated with the development of proliferative diseases such as atherosclerosis and cancer, there has been an increased interest recently in the development of small molecule RTK inhibitors for the prevention / treatment of the aforementioned diseases. Many cell culture and animal studies have shown that catechins, the main compounds of the green tea leaves, are potent natural inhibitors of several RTKs. In the present article we review the various molecular and cellular mechanisms through which catechins inhibit the growth factor-RTK-mediated signal transduction pathways and exert their antiproliferative / apoptotic effects.

Designing biologically active chemicals and managing their risks requires a holistic perspective on the chemical-biological interactions that form the basis of selective toxicity. The balance of therapeutic and adverse outcomes for new drugs and pesticides is managed by shaping the probabilities for transport, metabolism, and molecular initiating events. For chemicals activated as well as detoxified by metabolism, selective toxicity may be considered in terms of relative probabilities, which shift dramatically across species as well as within a population, depending on many factors. The complexity in toxicology that results from metabolism has been troublesome in QSAR research because the parent structure is less relevant to predicting ultimate effects and finding reference species / conditions for metabolic rates seems hopeless. Even the complexity of comparative xenobiotic metabolism itself seems paradoxical in light of the evidence of highly conserved catabolic processes across species. Clearly, predicting the role of metabolism in selective toxicity and adverse health outcomes requires a probabilistic framework for deterministic models as well as the many factors shaping the metabolic probability distributions under specific conditions. This paper presents a tissue metabolism simulator (TIMES), which uses a heuristic algorithm to generate plausible metabolic maps from a comprehensive library of biotransformations and abiotic reactions and estimates for system-specific transformation probabilities. The transformation probabilities can be calibrated to specific reference conditions using transformation rate information from systematic testing. In the absence of rate data, a combinatorial algorithm is used to translate known metabolic maps taken from reference systems into best-fit transformation probabilities. Finally, toxicity test data itself can be used to shape the transformation probabilities for toxicity pathways in which the metabolic activation is the rate-limiting process leading to a toxic effect. The conceptual approach for metabolic simulation will be presented along with practical uses in forecasting plausible activated metabolites.