Current Medicinal Chemistry - Anti-Cancer Agents - Volume 5, Issue 2, 2005

Advances in molecular biology have identified tumor markers that not only predict prognosis and therapeutic response but may also function as potential therapeutic targets. Activated growth factor receptors induce breast cancer cells to proliferate, invade, and metastasize in experimental models. Overexpression of growth factor receptors has been associated with a poor clinical outcome in breast cancer patients. Biological therapy with monoclonal antibody directed against growth factor receptor pathways became important targeted therapy in breast cancer and is being pursued on various fronts. The anti-HER2 antibody trastuzumab is approved in the metastatic setting and is now trying to find the place in the adjuvant setting. Phase II and III studies with antibodies directed against VEGF and EGFR are also ongoing.

Anticancer enzyme / prodrug approaches to therapy are designed to activate prodrugs specifically at tumor loci, to achieve antitumor responses with minimal toxicity. The equivocal success of these approaches thus far has led to searches for more efficient combinations. This mini-review evaluates and compares characteristics of seven selected enzyme / prodrug combinations, and discusses goals for future development of effective combinations.

The inhibition of angiogenesis is an emerging therapeutic strategy for cancer treatment. In contrast to conventional therapies, anti-angiogenic therapies primarily target tumor-associated endothelial cells which serve as a lifeline for tumor growth, progression and metastasis. By blocking the supply of essential nutrients and the removal of metabolites, anti-angiogenic therapies aim to delay both primary and metastastic tumor growth while overcoming the inherent cytotoxicities of classical chemotherapies. Indeed, tumor-related angiogenesis is a multi-step process initiated by a cascade of proangiogenic factors secreted from both the tumor and host tissues. These intricate processes involve a close interaction of tumor and associated endothelial cells as well as an intimate communication between proliferating endothelial cells, stromal cells and extracellular matrix components. Inhibition of these proangiogenic mechanisms has become a major challenge for the development of anti-cancer treatment modalities. In this regard, anti-angiogenic therapies embody a potentially powerful adjunct to traditional cancer therapies. In this review, we provide an overview of traditional anti-cancer drugs and discuss the fundamentals of anti-angiogenic therapies. While presenting the salient features of the anti-angiogenic agents targeting the individual phases of angiogenesis, we highlight the potential for specific agent development as novel anti-angiogenic therapeutics. Finally, we present and summarize emerging angiogenesis inhibitors.

Tumor suppressor genes can promote p53-mediated apoptosis. Apoptosis is an important protective mechanism for normal cell growth. Aberrant regulation of p53 expression is linked to cancer development. The loss of function of p53 often results in tumorigenicity. It is reported that the high incidence of tumors in p53-deficient animals is highly attributed to p53-induced apoptosis. Malignancies that retain the wild-type p53 gene are associated with the biologic activity of p53 function. Most cancer cells show defects in p53 or inhibition in the associated pathways. A lot of effort has been focused on reactivating mutant p53, or recombinant technique to incorporate p53 in cells. Regulation of p53 has been described at both transcription and translation level. Activation of the p53 pathway appears to be an effective approach in inhibiting tumor development. In the present study, we have reviewed the recent developments of specific compounds that can regulate p53 expression and its function in cell growth and development. Integral to this is the function of other proteins that affect p53 activity.

cisplatin (cis-Diamminedichloroplatinum(II)) is now clinically used as one of the most effective anticancer drugs in the treatment of a variety of human solid tumors, such as genitourinary. Unfortunately, its usefulness is limited due to development of resistance in tumor cells and its significant side effects. Thus, a continuing effort is being made to develop analogs to overcome the above shortcomings. However, direct structural analogs of cisplatin have not shown greatly improved clinical efficacy in comparison with the parent drug. The explanation for this finding is that all cis- [PtX2(amine)2] compounds have shown similar DNA-binding modes, thereby resulting in similar biological consequences. One approach is to look beyond structure-activity on the basis of cisplatin analogs antitumor agents, by identifying novel materials that can be utilized as building blocks. These may have DNA binding modes quite different from that of cisplatin. The introduction of such aromatic N-containing ligands as pyridine, imidazole and 1,10- phenanthroline, and their derivatives (whose donor properties are somewhat similar to the purine and pyrimidine bases) to antitumor agents is drawing attention. Many platinum and non-platinum metal complexes such as palladium, ruthenium, rhodium, copper, and lanthanum, with these aromatic N-containing ligands, have shown very promising antitumor properties in vitro and in vivo in cisplatin-resistant model systems or against cisplatin-insensitive cell lines. For example, one Ru(III) compound, [ImH][trans-Cl4(Me2SO)(Im)Ru(III)] (Im = imidazole, NAMI-A) successfully entered phase I clinical trials. In this review, medicinal chemistry, DNA binding modes, and the development status of these metal complexes are discussed.

Azadirachta indica, commonly known as neem, has attracted worldwide prominence in recent years, owing to its wide range of medicinal properties. Neem has been extensively used in Ayurveda, Unani and Homoeopathic medicine and has become a cynosure of modern medicine. Neem elaborates a vast array of biologically active compounds that are chemically diverse and structurally complex. More than 140 compounds have been isolated from different parts of neem. All parts of the neem tree- leaves, flowers, seeds, fruits, roots and bark have been used traditionally for the treatment of inflammation, infections, fever, skin diseases and dental disorders. The medicinal utilities have been described especially for neem leaf. Neem leaf and its constituents have been demonstrated to exhibit immunomodulatory, anti-inflammatory, antihyperglycaemic, antiulcer, antimalarial, antifungal, antibacterial, antiviral, antioxidant, antimutagenic and anticarcinogenic properties. This review summarises the wide range of pharmacological activities of neem leaf.

Gene therapy is part of a growing field in molecular medicine, which will gain importance in the treatment of human diseases. Until now, almost two thirds of all clinical trials performed in gene therapy are directed against Cancer As solid tumors exceeding a certain size rely on blood supply, the administration of particulate gene delivery vectors via the bloodstream is a promising concept. Tumor cells and the tumor vasculature both offer specific molecular targets, which can be utilized for the site directed delivery of therapeutic genes. Passive targeting of macromolecular drugs including gene delivery vectors to tumors can be achieved by the so called enhanced permeability and retention (EPR) effect. The specificity can be markedly enhanced when tumor targeting ligands are used. Viral vectors, which usually do not have a natural tropism for tumor tissue, were generated to carry tumor targeting molecules on their surface. Synthetic gene delivery vectors, based on cationic lipids or cationic polymers were biochemically modified to incorporate ligands specific for tumor cells or tumor vasculature. For systemic application, these delivery systems have to fulfill certain conditions. The delivery vector should not induce any immunogenic and inflammatory responses. Several studies were conducted to reduce the immunogenicity of viral vectors; surface modification of non-viral gene delivery systems reduced their non-specific interaction with blood components. On the genetic level, tumor specific promoters add additional layers of specificity restricting the transgene expression to the tumor tissue. This review will cover the systemic application of particulate gene transfer vectors targeted to tumors and will give an overview of therapeutic concepts for cancer gene therapy.

Aporphinoids form an important group of plant secondary metabolites. Some of these compounds are used for a long time in traditional medicine for the treatment of various diseases, from benign syndromes to more severe illnesses. More than 500 aporphine alkaloids have been isolated from various plant families and many of these compounds display potent cytotoxic activities which may be exploited for the design of anticancer agents. Here we review the origin, biosynthesis, structure and cytotoxic properties of the prominent members of this class of compounds. Simple aporphinoids (boldine, dicentrine) as well as oxo-, pro- and dehydro-aporphines, and dimeric forms such as thalicarpine, are discussed here. Their mechanisms of action are not well known but DNA-manipulating enzymes such as polymerases and topoisomerases are among the most frequently cited targets for these benzylisoquinoline compounds. This review presents an updated view of the cytotoxic properties of the aporphinoids and their potential contribution to the development of anticancer agents.