Current Drug Targets - Volume 12, Issue 3, 2011

In the last years natural products have been re-discovered as valuable and effective pharmacological agents. In particular they were shown to exert positive effects in the therapy of acute and chronic diseases, like cancer and microbial syndromes, for which the use of chemotherapeutics or antibiotics failed due to increasing resistance. To this concern, many examples of natural products were reported in the recent and current literature. The aim of this special issue is to collect review articles from several research groups to get further insights in the up to date knowledge in the field of novel agents obtained from natural kingdom triggering specific valuable biological targets. This could be of great help in discovering the underlying mechanism of action of the therapeutic effects of pharmacologically active natural compounds. Eight groups answered to my proposal of contributing to this special issue and submitted review articles covering several topics, like enzymatic inhibition, surveys of the current knowledge of selected natural products, and studies on new materials and analytical techniques to investigate the mechanism of action of natural products in biological systems. Such an issue would be of great interest for several categiories of scientists operating in natural product chemistry, phytochemistry, pharmacology, molecular biology, pharmacognosy, and others. It was a pleasure for me to work with all authors and reviewers who exhibited enormous patience and professionalism during the preparation and review process of this special issue and to when I'm sending my special thanks.

Inflammation is a natural response of living organisms to the presence of internal and external substances which are recognized by the host as being “non-self” or “foreign invader”. It is also a cascade leading to the healing of damaged tissue. Uncontrolled inflammation often results in chronic diseases such as arthritis, autoimmune disorder, cancer, dementia, diabetic, neurodegeneration and vascular disease. The list keeps growing due to the increasing number of identified molecular markers that are associated with inflammatory genes or transcription factors. Among various transcription factors, nuclear factor kappa B (NF-κB) is the master switch for proinflammatory genes and transactivates arachidonic acid pathway enzymes when activated. Through evolution, plants have developed vast classes of compounds to fight inflammation. Most of them belong to the chemical group of alkaloids, coumarins, flavonoids, polyphenols and terpenoids. This review article presents and discusses results obtained from literature search on recent findings in plantderived compounds, which exhibit anti-inflammatory activity in rheumatoid arthritis, allergy and asthma via the suppression of the arachidonic acid pathway. IC50s of the compounds obtained from the literature are thus tabulated into six groups of inhibitors based on the enzyme target of phospholipase A2, cycloxygenase 1 and 2, 5-, 12- and 15- lipoxygenase. Modulation of Th1/Th2 cytokines and histamine/mucus release by some of these enzyme inhibitors are also briefly discussed.

Over the past years, there was an explosion in the knowledge of the protein target and molecular mechanism associated with various disease types and in the new research of drugs of natural origin. The key idea is to evaluate bioactive natural products interacting with protein domains of different genetic origin but structurally preserved to develop libraries of compounds biologically validated and selected from an evolutionistic point of view. Compared with synthetic compounds, natural products have a major number of unused scaffolds and are not comparable to the libraries of synthetic compounds, and could represent a promising starting point for the discovery of new bioactive compounds. Many natural products are reported to interact with proteins involved in serious diseases, such as inflammation and cancer. Recently various chemical classes of plant secondary metabolites have emerged as potential therapeutic compounds in several inflammatory diseases. Owing to the findings that triterpenoids, a common class of plant secondary metabolites, have anti-inflammatory and anti-cancer effects on humans, the interest in their potential application in human health and disease is increasing. The present review describes anti-inflammatory triterpenes derivatives from plant and fungi reported during the last two decades in order to provide an account of this field of investigation, sorting compounds according to their targets, phospholipase A2 (PLA2), cycloxygenase (COX), and lipoxygenase (LOX). The attempt is also being made to enumerate the possible leads for further synthetic and drug discovery program development.

Periodontitis is a common chronic inflammatory disorder of bacterial origin, which affects the tooth-supporting tissues. A wide range of evidences suggests that Porphyromonas gingivalis plays a key role in the initiation and progression of chronic periodontitis. This Gram-negative anaerobic bacterium produces several types of proteolytic enzymes, including gingipains, collagenases, and a dipeptidyl aminopeptidase IV. Although these enzymes have physiological functions for P. gingivalis, they have been suggested to play multiple roles in the pathogenic process of periodontitis. Indeed, P. gingivalis proteases hydrolyze a variety of serum and tissue proteins thus contributing to neutralize the immune defense system and to cause tissue destruction. Considering the key roles that P. gingivalis proteases may play in the pathogenesis of periodontitis, inhibitors of these enzymes are considered potentially new therapeutics agents. In recent years, several groups have identified natural plant-derived inhibitors effective on P. gingivalis proteases. More specifically, polyphenols isolated from cranberry and green tea were found to inhibit several proteases produced by P. gingivalis. This paper will discuss the pathological roles of P. gingivalis proteases and review the scientific literature for bioactive plant-derived compounds endowed with a capacity to inhibit these enzymes.

Curcumin (diferuloylmethane), an orange-yellow component of turmeric or curry powder, is a polyphenol natural product isolated from the rhizome of the plant Curcuma longa. For centuries, curcumin has been used in some medicinal preparation or used as a food-coloring agent. In recent years, extensive in vitro and in vivo studies suggested curcumin has anticancer, antiviral, antiarthritic, anti-amyloid, antioxidant, and anti-inflammatory properties. The underlying mechanisms of these effects are diverse and appear to involve the regulation of various molecular targets, including transcription factors (such as nuclear factor-κB), growth factors (such as vascular endothelial cell growth factor), inflammatory cytokines (such as tumor necrosis factor, interleukin 1 and interleukin 6), protein kinases (such as mammalian target of rapamycin, mitogen-activated protein kinases, and Akt) and other enzymes (such as cyclooxygenase 2 and 5 lipoxygenase). Thus, due to its efficacy and regulation of multiple targets, as well as its safety for human use, curcumin has received considerable interest as a potential therapeutic agent for the prevention and/or treatment of various malignant diseases, arthritis, allergies, Alzheimer's disease, and other inflammatory illnesses. This review summarizes various in vitro and in vivo pharmacological aspects of curcumin as well as the underlying action mechanisms. The recently identified molecular targets and signaling pathways modulated by curcumin are also discussed here.

The persistence of latent HIV-infected cellular reservoirs represents the major hurdle to virus eradication in patients treated with highly active antiretroviral therapy. The molecular mechanisms by which integrated HIV-1 is repressed during latency have been partially identified in different models of HIV-1 latency, and the involvement of multiple processes has been demonstrated. Therefore, several molecular targets amenable to pharmacological manipulation have emerged to antagonize HIV-1 latency in the viral reservoirs. In this context, it has been suggested that successful depletion of such latent reservoirs will require a combination of therapeutic agents that can specifically and efficiently act on cells harbouring latent HIV-1 provirus. HIV-1 reactivation therapy is a potential therapeutic option to purge the viral reservoirs. The goal of this therapy is to enhance the transcriptional activity of the latent HIV-1 without inducing the polyclonal activation of non-infected cells. In this sense natural or semisynthetic protein kinase C agonists lacking tumour-promoter activities clearly fulfil this criterion, thereby opening new research avenues to purge HIV-1 reservoirs. In this review article, we have succinctly summarized the known effects of “natural products”, focusing on phorboids like prostratin and ingenols, macrolides like bryostatin 1, and macrocyclic polyesters like ingols and jatrophanes. A comprehensive view on the molecular mechanisms underlying the principle of HIV-1 reactivation from latency is provided, discussing the combination of “natural products” with other experimental or conventional therapeutics.

Nanoparticles are sphere-like biocompatible materials made of inert silica, metal or crystals of a few nanometers in size. They are emerging as a novel class of therapeutics for cancer treatment. Being more selective and specific toward their targets, nanoparticles have the ability to enhance the anticancer effects and to simultaneously reduce systemic toxicity compared with conventional therapeutics. Furthermore, they offer the potential to overcome drug resistance leading to higher intracellular drug accumulation. Nowadays, nanotechnologies are applied to molecular diagnostics and incorporated in cutting-edge molecular diagnostic methods, such as DNA and protein microarray biochips. Nanotechnologies enable diagnosis at the single-cell and single-molecule levels. Recent progress in cancer nanotechnology raises exciting opportunities for specific drug delivery. The purpose of this review is to give an overview about different types of nanoparticles and to summarize the latest results regarding their diagnostic and therapeutic applications in the clinic with more focus on cancer treatment. Furthermore, we discuss opportunities, limitations, and challenges faced by therapeutic nanoparticles.

Approximately, 63% of marketed drugs derive from natural products or their semi synthetic derivatives. Compounds from living organisms often exert a biological activity, triggering several targets, which may be useful for the improvement of novel pharmaceuticals. These natural products can be extracted from plants, marine organisms, or microorganism fermentation broths. In the vast array of bioactive secondary metabolites known up to now, anthraquinones are among the most investigated natural products, in particular for what concerns their mechanism of action. This review focuses on the analytical aspects of anthraquinones, from their separation to recent and new highthroughputs techniques for the simultaneous determination of these analytes in biological matrices that can greatly contribute to sharply depict the targets of these secondary metabolites as well as on an updated survey of their biological activities.

Auraptene is the most abundant prenyloxycoumarin that occurs in nature. It has been isolated from plants belonging to many genus of the Rutaceae family, comprising several edible fruits and vegetables. Although known for a long time, only in the last decade auraptene was seen to exert valuable pharmacological properties as orally active cancer chemopreventive, anti-bacterial, anti-protozoal, anti-fungal, anti-inflammatory, and anti-oxidant agent. The aim of this review is to examine in detail the in so far reported effects and what is known about the mechanism of action underlying the observed pharmacological activities of this secondary metabolite by means of a survey of the current literature in which auraptene has been reported as an inhibitor of key biological targets like metalloproteinases, glycoprotein P, PPARs, and several others.

About 50 years ago interferon (IFN) was discovered by Isaacs and Lindenmann [1] and described as a cytokine, which was able to interfere with virus replication. The IFN-receptor was identified and shortly after the JAK/STAT-signal transduction pathway being described in several recent reviews [2-6]. Early in this development it was apparent that one of the mechanisms of action of IFN-alpha involved stimulation of immune cells [7, 8]. In the following years, all the other properties of IFN were unravelled, including its antiproliferative, immunomodulatory and antiangiogenic effects, which rapidly elicited great interest in the potential use of IFN in the treatment of several malignancies. The production and purification of human leukocyte IFNs [9] were soon followed by the first clinical study in the late 1970s on the efficacy of IFN-alpha in multiple myeloma (MM) [10]. A few years later IFN-alpha2 was cloned, allowing large amounts of IFNs to be produced for experimental research and clinical trials. Several studies in multiple myeloma and in other hematological malignancies followed including hairy-cell leukemia (HCL), chronic myelogenous leukemia (CML) and the Philadelphia-negative chronic myeloproliferative neoplasms (CMPNs) (essential thrombocythemia (ET), polycythemia vera (PV), primary myelofibrosis (PMF)), hypereosinophilic syndromes and systemic mastocytosis (SM). In the following years the outstanding breakthroughs in the treatment of HCL and CML with IFNalpha2 were confirmed in large clinical trials. Many patients with HCL achieved long-lasting complete remissions with normalization of peripheral blood values and the bone marrow in concert with a marked improvement in their immune defence towards infections. Likewise, IFN-alpha2 became a historical milestone in the medical treatment of CML, since IFN-alpha2 proved to be the first agent able to induce complete and sustained cytogenetic remissions with disappearance of Philadelphiachromosome and - in addition- in some patients even the induction of major molecular remissions with a significant and sustained reduction of the BCR-ABL transcript in a subset of patients. Accordingly, IFN-alpha2 was considered the best medical treatment of CML during the past two decades until the targeted treatment with the tyrosine kinase inhibitor imatinib mesylate substituted IFN-alpha2 about 10 years ago. In this theme issue the history, the present and the future for IFN-alpha2 in the treatment of hematological malignancies is described and discussed by experts with many years of clinical experience within the field. Simonsson et al. update on the current knowledge of IFN-alpha2 in the treatment of CML, emphasizing that the mechanisms of action in patients with CML are likely multifactorial. IFN-alpha2 can restore the adhesion of CML primitive progenitor cells to marrow stroma, downregulate the expression of the BCR-ABL1 gene and activate several transcriptional factors that regulate cell proliferation, maturation, and apoptosis. Furthermore, IFN-alpha has very potent immune enhancing capacity, inducing recognition and elimination of CML cells from the immune system [11, 12]. A novel mechanism on hematopoietic stem cells (HSC) has recently been proposed, implying induction of cell cycling in quiescent HSC and early progenitors. Chronic administration of IFN-alpha has been shown to deplete HSC underscoring that “dormant” cancer stem cells may be susceptible to manipulation via an IFN-alpha induced “wake up call” and subsequent proliferation hereby being “visible” targets for the immune system or targeted treatment [13, 14]. The authors describe the potential advantages of combination therapy with imatinib and IFN-alpha in CML, which might be far more efficacious than single agent therapy based upon the fact that the mode of action and biological effects of imatinib and IFN-alpha are quite different. This knowledge has been translated and confirmed in two large randomized studies showing improved outcome if pegylated IFN is added to the treatment with imatinib. Already in 1985 Linkesch et al. from Austria described that IFN-alpha2 was able to control myeloproliferation in myeloproliferative diseases with severe thrombocytosis [15, 16] - only a few years after the first description of the efficacy of human leukocyte IFN to control thrombocytosis in CML [17]. Several studies during the last 25 years have subsequently confirmed that IFN-alpha2 is also able to inhibit myeloproliferation in the Ph-negative CMPNs (ET, PV and PMF) with a reduction or alleviation of the need of phlebotomies in PV, disappearance of pruritus, normalization of leucocyte and platelet counts and a reduction in spleen size [18-25]. Despite all these studies IFN-alpha2 has not been the first drug of choice in the treatment of patients with CMPNs, for many reasons but mainly because of a relatively high drop-out rate (about 20-40 %) due to side effects. With the identification of the JAK2-V617F-mutation in 2005 and reports on the potential of IFN-alpha2 to induce major molecular remissions in patients, the interest in treatment of PV and related neoplasms with IFN-alpha2 has been revived. Accordingly, several studies have shown that long-term treatment with IFN-alpha2 in a subset of patients is accompanied by “complete” molecular remissions [21-25], which may be sustained even after discontinuation of IFN-alpha2 for up to two years [24]. These observations show that immune therapy with IFN-alpha2 is able to induce “minimal residual disease” (“operational cure”?) in subgroups of patients with CMPNs. These aspects are described and discussed in a comprehensive review by Hasselbalch et al. The authors conclude, that induction of “minimal residual disease” by IFN-alpha2 in PV is yet another milestone in the IFN-alpha2-story on the treatment of hematological malignancies, opening a new horizon with potential cure - at least “operational” - if immune therapy with IFN-alpha is initiated at the time of diagnosis, when the “tumor burden” and accordingly, the extent of genome instability and resistance is the least and consequently the possibility of a favorable outcome of IFN-alpha2 is optimal. Such a therapeutic strategy - possibly in the initial disease phase in combination with hydroxyurea or in the future JAK2-inhibitors or histone deacetylase inhibitors, may ultimately inhibit disease progression to the myelofibrotic “burn-out phase of the disease” with massive splenomegaly, bone marrow failure or leukemic transformation - events which are all driven by increasing genetic instability and evolution of resistant subclones during the course of the disease. The challenges in the coming years among others are combined clinical and experimental studies to identify molecular and immunological characteristics of those patients being “good IFN-alpha2 responders” versus “bad IFNalpha2- responders”. By this approach, novel information on the mechanisms of action of IFN-alpha and also on molecular markers identifying those patients who may benefit from IFN-alpha2 treatment may be achieved. In these studies, it is also relevant to investigate whether “bad responders” or patients not tolerating recombinant IFN-alpha2 (IFN-alpha2a - Pegasys - or IFN-alpha2b -PegIntron) may have a better outcome when being treated with human leukocyte-IFN (Multiferon). This agent contains a mixture of several IFN-alpha-subtypes with additive/synergistic immunomodulating properties, which theoretically might further enhance the effects of IFN-alpha in our patients. In particular, it might be efficient in those patients developing neutralizing antibodies during treatment with recombinant IFNs. The role of IFN-alpha2 in the treatment of the hypereosinophilic syndrome and systemic mastocytosis is excellently described and covered by Bjerrum, who concludes, that IFN-alpha definitely has a major place in the treatment of some, but not all patients with hypereosinophilic diseases. It is underscored that controlled and preferably randomized trials of the efficacy of IFN-alpha2 in this patient group are warranted. In addition, Bjerrum concludes, that IFN-alpha -being used for about 25 years in SM - still has a prominent position in first line treatment, but further research in mast cell biology may unravel novel treatment principles, most likely to be used in combination with IFN-alpha2, since the complete response rate for IFN-alpha2 as monotherapy is relatively low. Long-term treatment may be required to induce molecular remissions as assessed by qPCR for the V816 mutation, which is present in the large majority of patients with SM, in whom the “tumor burden” in the future may be monitored by this methodology.......

The Philadelphia-negative chronic myeloproliferative neoplasms encompass essential thrombocythemia (ET), polycythemia vera (PV) and primary myelofibrosis (PMF). A major break-through in the understanding of the pathogenesis of these neoplasms occurred in 2005 by the discovery of the JAK2 V617F mutation in the large majority of patients with PV and in half of those with ET and PMF. A number of studies have shown that the “tumor burden” may be monitored at the molecular level in JAK2-positive patients using highly sensitive real-time quantitative PCR. During the last 25 years several studies have shown that interferon-alpha (IFN-alpha) induces complete hematological remissions in a large proportion of the patients. However, its use in clinical practice has unfortunately been limited due to side effects with high drop-out rates in most studies. Recently, IFN-alpha2 has been shown to induce deep molecular remissions and also normalization of the bone marrow in PV, which may be sustained even after discontinuation of IFN-alpha2 therapy. Accordingly, in the coming years we are most likely facing a new era of increasing interest for using IFN-alpha2 in the treatment of patients with PV, ET and the hyperproliferative phase of PMF. This paper reviews the history of IFN - in principle IFN-alpha2 - and its present status in the treatment of PV and related diseases. The role of IFN-alpha2 as immune therapy in the future treatment of CMPNs is highlighted and the rationale for the concept of minimal residual disease and potentially cure after long-term immune therapy with IFN-alpha2 is discussed and foreseen as an achievable goal in the future.

Treatment of chronic myeloid leukemia (CML) with interferon-alpha (IFN-α) was introduced in the early 1980s. Several clinical trials showed a survival advantage for patients treated with IFN-α compared to conventional chemotherapy. Some patients achieved longstanding complete cytogenetic remissions (i.e. >2 log tumor mass reduction). IFN-α was then recommended as first line medical treatment until 2001. The mechanism of this anti-leukemic effect is not clear, although IFN-α has many effects of potential relevance on stem cells and immunology. There is no evidence of benefit for high dose (in practice a maximally tolerated dose) compared with lower dose IFN-α. When IFN-α is combined with other drugs, we advice lower dose IFN to minimize toxicity and increase treatment adherence and duration. IFN-α combined with Ara-C moderately improves treatment outcome. Imatinib, a tyrosine kinase inhibitor, is now first line treatment for CML, but two large randomized studies show improved outcome when pegylated IFN-α is added to the treatment with imatinib. One explanation for this might be that IFN-α, contrary to imatinib, stimulates the quiescent stem cells to proliferate and thereby potentially increases sensitivity to imatinib. Although imatinib and other tyrosine kinase inhibitors are very efficient, they are rarely curative. IFN-α could be included in curatively aimed combination treatment protocols and thus still be an important element in CML treatment.

Most of the primary conditions with eosinophilia have now been characterized by clonality in 2008 by the WHO classification, which thereby provide a basis for separation of patients who may benefit a targeted therapy, i.e. by tyrosine kinase inhibition - and who may not. Treatment with interferon-α was introduced some 20 years ago and still has a role in subsets of patients, which is evident from this review of casuistic reports of treatment. However, controlled, randomized, prospective, clinical trials in multi-center studies are needed to clarify dosages, monitoring, prognosis and perhaps combination therapies with interferon-α, i.e. antibodies or other immune suppressants, in the rare patients with primary eosinophilia,

Patients with systemic mastocytosis are rare, constitute a heterogenous clinical entity and some may not require treatment until long after diagnosis. However, most patients who present with disabling symptoms, organ involvement and fulfill B or C findings as outlined in the 2008 WHO classification need treatment. This review on interferon treatment in systemic mastocytosis documents an effect of this biological agent in some patients with mastocytosis. However, the place of interferon-α, as mono- or combination therapy, in the treatment algorithm may only be definitely established in multicenter studies, which are based on the principles of controlled clinical trials.

Interferons are soluble proteins produced naturally by cells in response to viruses. It has both anti-proliferative and immunomodulating properties and is one of the first examples of a biological response modifier use to treat the hematological malignancy multiple myeloma. Interferon has been used in this clinical practice for over thirty years. However, despite considerable efforts, numerous clinical trials and two large meta-analysis, its exact role in the management of multiple myeloma still remains unclear. Its role in the treatment of multiple myeloma has been as a single induction agent, a co-induction agent with other chemotherapy regimens, and as maintenance therapy after conventional chemotherapy or complete remission after autologous or allogeneic transplantation. Interferon as a single induction agent or co-induction agent with other chemotherapy agents appears only to have minimal benefit in myeloma. Its role as maintenance therapy in the plateau phase of myeloma also remains uncertain. More recently, the use of interferon must now compete with the “new drugs” - thalidomide, lenalidomide and bortezomib in myeloma treatment. Will there be a future role of interferon in the treatment of multiple myeloma or will interferon be resigned to the history books remains to be seen.