Current Cancer Drug Targets - Volume 7, Issue 8, 2007

The tubulin/microtubule system is an integral component of the cytoskeleton. Microtubules are highly dynamic structures that play a critical role in orchestrating the separation and segregation of chromosomes during mitosis. This makes microtubules highly valued as anticancer drug targets. Tubulin-binding agents (also known as anti-microtubule, microtubule-binding, microtubule-targeting drugs) are derived from natural sources and include a large number of agents with diverse chemical structures. What all tubulin-binding agents share in common is their ability to disrupt microtubule dynamics, induce mitotic arrest and cell death. The best known of these agents are the vinca alkaloids and taxanes, which at high doses cause microtubule destabilisation and microtubule stabilisation, respectively. Improvements in our understanding of the mechanisms of action and resistant to tubulin-binding agents has continued to drive a strong interest in identifying and developing new derivatives of tubulin-binding agents. Well over 200 of these agents are under various stages of development. Despite the fact that the first tubulin-binding agents, were clinically introduced in the 1960's, there are still many aspects of these agents that are not understood and in recent years, previously unknown cellular effects and protein interactions have begun to be discovered. It has been known for some time that tubulin-binding agents disrupt microtubule dynamics, but the differences between the different classes of tubulin-binding agents and their effects on microtubule dynamics and mitosis have been difficult to pinpoint. Jordan and Kamath give a valuable overview of microtubule dynamics, endogenous regulators of microtubule function and the effect of different classes of tubulin-binding agents on microtubule dynamics. The role of microtubule binding proteins in the regulation of microtubule dynamics and drug response have been studied, yet limited attention has been given to the role of tubulin folding pathways as a means to regulate microtubule dynamics. Beghin, Galmarini and Dumontet, review current understanding regarding tubulin-folding pathways, their relation to disease, and discuss their potential influence on microtubule dynamics and their value as new drug targets. Despite the clinical success of tubulin-binding agents, drug resistance can emerge as a significant clinical problem. There has been much interest in recent years associated with alterations in the cellular target of these agents, the tubulin/microtubule system. The mechanisms mediating resistance in the clinic are not well understood. Ferlini et al. review resistance mechanisms and discuss the benefits of targeting TUBB3 (also known as class III β-tubulin or βIII-tubulin), a β-tubulin isotype commonly increased in drug resistant epithelial cancers. Tubulin-binding agents are known to be potent inducers of apoptosis. The link between microtubule disruption and apoptosis induction are not clear. Esteve, Carre and Braguer give a comprehensive overview of the apoptotic processes known to be activated following treatment of cancer cells with tubulin-binding agents. The tubulin/microtubule system remains one the most important drug targets for the treatment of cancer. This special issue provides current and new insight into the mechanisms of action of these agents, resistance and new drug targets within the tubulin/microtubule repertoire of microtubule dynamic regulators. Improved insight into the mechanisms of action and resistance to these agents will lead to improved targeting of existing agents and the development of more effective agents to treat resistant disease.

As microtubules are essential in many cell functions, they have been used as a target of a variety of anticancer drugs that are grouped as stabilizing (taxanes) and destabilizing (vinca-alkaloids, colchicinoids) microtubule agents. It appears clearly now that the dynamic behaviour more than modifications of microtubule mass are altered by antitubulin agents in the range of serum concentrations obtained after administration in humans. While the role of microtubule associated proteins in the regulation of microtubule dynamics has been extensively studied, there is a growing body of data suggesting that tubulin folding could also play an important role in microtubule dynamics. We review the current knowledge regarding tubulin folding pathways, their relation to disease, and their possible influence on microtubule dynamics.

Vinca alkaloids and taxanes represent the mainstay of medical treatment of hematological and solid tumors. Unfortunately, a major clinical problem with these agents is drug resistance. Although a plethora of mechanisms of drug resistance have been described, only a few of them have been validated in clinical trials. Among these, the one involving the protein TUBB3 seems to represent a promising target for studying drug resistance. In fact, it seems that this protein is a factor promoting cell survival and represents an endogenous element of an inherent drug-resistance program built into cells to counteract the activity of microtubule-interacting drugs. Its pivotal role has been ascertained in clinical trials in lung, breast, and ovarian cancer, three diseases that can be successfully treated with microtubule-interacting drugs. Although TUBB3 is probably not a unique factor in drug resistance, the hope is that direct targeting of this protein will increase the response to microtubule-interacting drugs, thereby overcoming an important element in the growth of drug resistance.

Microtubule-Targeting Agents (MTAs) constitute a class of drugs largely used in cancer treatment. Among them, both taxanes and Vinca-alkaloids are known to inhibit cancer cell proliferation by inducing cell cycle arrest and subsequent apoptosis. These agents modify the cytoskeleton by affecting the tubulin/microtubule system. In cancer cells, both classes suppress microtubule dynamics through inhibition of microtubule dynamic instability and treadmilling, and commonly induce diverse signals responsible for cell death initiation and execution via the mitochondrial intrinsic pathway. However, links between microtubule network disturbance and the involvement of mitochondria in apoptosis are not obvious, and one may think that they could be independent. Nevertheless, several intracellular proteins could connect microtubules and the apoptotic machinery. The aim of the present review is to provide elements that could answer to the question : is microtubule disruption dispensable for MTA-induced apoptosis? The first section is focused on the mechanisms responsible for the MTA-mediated apoptosis. Then, links between cell cycle regulators and apoptosis are underlined since MTA induce cell cycle arrest by inhibiting microtubules. In the third part, the potential involvement of microtubule-sequestered and/or -transported proteins in apoptotic signalisation is discussed. Lastly, the possible role of the tubulin/microtubule system in direct effects of MTAs on mitochondria is summarized. Thus, it becomes clear that microtubule network and apoptosis are deeply linked in MTA effectiveness, through a cascade of cellular events. It could lead to identification of new biomarkers of MTA effectiveness, that could improve combinatorial therapy with MTAs and provide crucial arms to circumvent resistance of cancer cells.

The importance of microtubules in mitosis makes them a superb target for a group of highly successful, chemically diverse anticancer drugs. Knowledge of the mechanistic differences among the many drugs of this class is vital to understanding their tissue and cell specificity, the development of resistance, the design of novel improved drugs, optimal scheduling of treatment, and potential synergistic combinations. This overview covers microtubule assembly dynamics, the exquisite regulation of microtubule dynamics in cells by endogenous regulators, the important role of microtubule dynamics in mitosis, the diversity and number of microtubule-targeted drugs undergoing clinical development, the antimitotic mechanisms of microtubule-targeted drugs with emphasis on suppression of microtubule dynamics by vinblastine and taxol, the role of drug uptake and retention in the efficacy of microtubule-targeted drugs, and the anti-angiogenic and vascular-disrupting mechanisms of microtubule targeted drugs. In view of the success of this class of drugs, it has been argued that microtubules represent the single best cancer target identified to date, and it seems likely that drugs in this class will continue to remain an important chemotherapeutic class of drugs even as more selective chemotherapeutic approaches are developed.

Current cancer chemotherapeutic drugs have limited efficacy due to the fact that tumour cells are a rapidly changing target characterised by genomic instability. Unlike tumour cells, activated endothelial cells (ECs) required for angiogenesis, a process indisputably crucial to tumour growth and metastasis, were originally considered to be ideal therapeutic targets free of drug resistance. Additionally, unlike preclinical studies in mice using inhibitors targeting the powerful EC mitogen- vascular endothelial growth factor (VEGF)- overall survival benefit with anti-VEGF therapy used as monotherapy has yet to be demonstrated in phase III clinical trials. In contrast, VEGF-specific antibodies combined with current chemotherapy have resulted in improved outcomes in certain previously untreated cancers. This has led some researchers to hypothesize that combined treatments targeting other angiogenic molecules besides VEGF, and/or targeting not only ECs but other angiogenic non-EC types, may offer alternative but effective therapeutic options for eradicating malignant tumours. A rational approach to effective anti-angiogenic combination therapy will, however, require further understanding of the molecular and cellular mechanisms which undergird tumour vascularisation. Recent studies involving judicious use of powerful new genetic approaches have provided unprecedented insights into how different molecular and cellular mechanisms cooperate to build, branch and mature the growing vessel network so pivotal to tumour growth and survival. This review covers our current understanding of how the various key players - the tumour cells, stromal cells, endothelial cells and pericytes, and bone-marrowderived haematopoietic and putative endothelial progenitors interact via their cell-derived pro- or anti-angiogenic factors to regulate tumour angiogenesis.

A trend in investigating the use of several nutritional compounds for cancer chemoprevention has revealed that phytochemicals demonstrated anti-cancer properties by inhibiting signal transduction pathways essential for cancer cell proliferation, tumor growth, invasion and metastasis. Emerging evidence suggests that the anti-proliferative and anti-oxidant effects of some of these dietary agents could be utilized to both potentiate the response of cancer cells to radiotherapy and reduce radiation-induced toxicity to normal surrounding tissues. Using pre-clinical orthotopic models of prostate cancer, studies on the combination of soy isoflavones with tumor irradiation demonstrate a synergistic anti-cancer effect between these two modalities and emphasize the potential and safety of dietary factors to improve conventional radiotherapy for a better control of tumor growth and metastasis. The goal of this review is to focus on the role of soy isoflavones as potent radiosensitizers for prostate cancer and other malignancies. We will discuss molecular pathways regulated by soy isoflavones that inhibit survival pathways activated by radiation and ultimately drive the cells to cell death both in vitro and in vivo in pre-clinical models.

With several successful anticancer drugs on the market and numerous compounds in clinical developments, antimitotic agents represent an important category of anticancer agents. However, clinical utility of the tubulin-binding agents is somewhat limited due to multiple drug resistance (MDR), poor pharmacokinetics and therapeutic index. There is ongoing need for the modulators of other intracellular targets that result in the same anti-mitotic effect without adverse effects of “traditional” tubulin binders. This review describes progress made to-date in development of novel and emerging biotargets affecting the mitotic events, and their small-molecule modulators.

The avoidance of apoptosis is one of the hallmarks of cancer cells. In addition, failure to induce apoptosis by anticancer agents, either due to limitations of the drug or the tumour cell evading apoptosis, is a reason for chemotherapeutic failure. Two general pathways for apoptotic cell death have been characterised, the extrinsic and intrinsic pathways which merge in the final common pathway. X-linked inhibitor of apoptosis protein (XIAP) is an anti-apoptotic protein in the final common pathway that inhibits caspases and suppresses apoptosis. XIAP is over-expressed in many cancer cell lines and cancer tissues. High XIAP expression has been correlated with resistance to chemotherapy and radiotherapy and to poor clinical outcome by some investigators. Manipulation of apoptosis is an attractive therapeutic concept. Much effort has been spent on inhibiting the anti-apoptotic protein, B cell lymphoma gene 2 (Bcl-2) which is part of the intrinsic pathway. Now attention is turning to inhibition of XIAP as a cancer drug target. It has been argued that it is more effective to block the final common pathway rather than just the intrinsic arm. Inhibition of XIAP can be with either antisense oligonucleotides (ASO) or small molecule inhibitors. In vitro, XIAP antagonists produce XIAP knockdown and apoptosis which is associated with sensitisation of tumour cells to radiotherapy and cytotoxic drugs. In vivo, XIAP antagonists have antitumour effects and sensitise tumours to the effects of chemotherapy. This review will summarise the preclinical data for both ASO and small molecule inhibition of XIAP and discuss emerging Phase I data. Future strategies for manipulation of XIAP and the clinical development of XIAP inhibitors will be discussed.

Attempts to control cancer involve a variety of means, including the use of suppressing, blocking and transforming agents. Suppressing agents prevent the formation of new cancers from pro-carcinogens, blocking agents prevent carcinogenic compounds from reaching critical initiation sites, while transformation agents act to facilitate the metabolism of carcinogenic components into less toxic materials or to prevent their biological actions. Flavonoids can act as all three types of agent. Epidemiological and animal studies suggest that flavonoids have a protective effect against cardiovascular diseases and some types of cancer. Although flavonoids have been studied for about 50 years, the cellular mechanisms involved in their biological action are still not completely understood. In recent years, experimental studies have provided growing evidence supporting the beneficial action of flavonoids on multiple cancer-related biological pathways (carcinogen bio-activation, cell-signaling, cell cycle regulation, angiogenesis and inflammation). Within the last decade, reports on flavonoid activity have largely associated with enzyme inhibition and anti-proliferative activity. Many of these studies have pointed to a structural-functional relationship, in that the antioxidant, enzyme-inhibition or antiproliferative activities of flavonoids are dependent on particular structural motifs. Our own studies have shown that structural factors would explain the antioxidant, antiproliferative and antimetastasic properties of some citrus flavonoids. In this paper we discuss the relation between each structural factor and the anticancer activity of Citrus flavonoids.