Current Cancer Drug Targets - Volume 2, Issue 3, 2002

IGF-IR (Insulin-like growth factor receptor 1) is a tetrameric glycoprotein composed of two α and two β subunits. The α subunit localizes extra-cellularly for ligand binding, whereas the β subunit consists of transmembrane chains and a cytoplasmic tyrosine kinase domain for enzymatic activity. IGF-IR ligands, IGF-I and IGF-II, are mitogens and survival factors for many cancer cells. Binding of ligands to the IGF-IR initiates a cascade of events leading to activation of signal transduction pathways, mainly MAPK and PI-3K pathways, to stimulate proliferation / mitogenesis, to induce neoplastic transformation, to inhibit apoptosis, and to promote angiogenesis and metastasis. It has been shown that the presence of IGF-IR was required for transformation induced by many oncogenes and over-expression or constitutive activation of IGF-IR gave rise to transformed phenotypes. Significantly, over-expression of IGF-IR was observed in multiple human cancers including carcinomas of breast, lung, colon, and prostate. Patients with IGF-IR positive cancers had a worse prognosis in some cases. Furthermore, down-regulation or functional inactivation of IGF-IR sensitized tumor cells to apoptosis and reversed tumor cell phenotype. Thus, IGF-IR appears to be a promising cancer target. Indeed, a variety of approaches aimed at targeting IGF-IR have been utilized to prove the concept, or are being developed for potential anticancer therapies. These include targeting functional IGF-IR on cell surface, targeting ligand / receptor interaction, targeting receptor expression and functions, and targeting receptor kinase activity. Cancer patients could eventually benefit from the development of these specific IGF-IR antagonists.

A major challenge in cancer chemotherapy is the selective delivery of small molecule anti-cancer agents to tumor cells. Water-soluble polymer-drug conjugates exhibit good water solubility, increased halflife, and potent anti-tumor effects. By localizing the drug at the desired site of action, macromolecular therapeutics have improved efficacy and enhanced safety at lower doses. Since small molecule drugs and macromolecular drugs enter cells by different pathways, multi-drug resistance (MDR) can be minimized. Anticancer polymer-drug conjugates can be divided into two targeting modalities: passive and active. Tumor tissues have anatomic characteristics that differ from normal tissues. Macromolecules penetrate and accumulate preferentially in tumors relative to normal tissues, leading to extended pharmacological effects. This “enhanced permeability and retention” (EPR) effect is the principal reason for current successes with macromolecular anti-cancer drugs. Both natural and synthetic polymers have been used as drug carriers, and several bioconjugates have been clinically approved or are in human clinical trials.While clinically useful anti-tumor activity has been achieved using passive macromolecular drug delivery systems, further selectivity is possible by active targeting. Attachment of targeting moieties to the polymer backbone can further exploit differences between cancer and normal cells through selective receptor-mediated endocytosis. This strategy would augment the EPR effect, thereby further improving the therapeutic index of the macromolecular drug. This review discusses the development and therapeutic potential of prototype macromolecular drugs for use in cancer chemotherapy. Specific examples are selected to illustrate the basic design principles for soluble polymeric drug delivery systems.

ß-Lapachone is an ortho naphthoquinone, originally isolated from a tree whose extract has been used medicinally for centuries. Recent investigations suggest its potential application against numerous diseases. Its lethality at micromolar (μm) concentrations against a variety of cancer cells in culture indicates its potential against tumor growth. A few experiments with positive results have been performed that apply the compound to tumors growing in animals. Particularly promising is the remarkably powerful synergistic lethality between ß-lapachone and taxol against several tumor cell lines implanted into mice; the mice did not appear to be adversely affected. Enhanced lethality of X-rays and alkylating agents to tumor cells in culture was reported when ß-lapachone was applied during the recovery period, because of inhibition of DNA lesion repair. Clinical trials are still to be initiated.The detailed mechanism of cell death induced by ß-lapachone remains for investigation. DNA topoisomerase I was the first biochemical target of ß-lapachone to be discovered, although its role in cell death is not clear. A proposed mechanism of cell death is via activation of a futile cycling of the drug by the cytoplasmic two-electron reductase NAD(P) H: quinone oxidoreductase, also known as NQO1, DTdiaphorase and Xip3. Death of NQO1 expressing cells is prevented by the NQO1 inhibitor dicoumarol, and cells with low NQO1 are resistant. At higher drug concentrations the production of reactive oxygen species (ROS) appears to be responsible. Furthermore, this process is p53- and caspase- independent. Either apoptotic or necrotic cell death can result, as reported in various studies performed under differing conditions. ß-Lapachone is one of a few novel anticancer drugs currently under active investigation, and it shows promise for chemotherapy alone and especially in combinations.

Nuclear Hormone Receptors (NR) represent one of the most promising protein families in terms of therapeutic applications. These transcription factors are naturally switched on and off by small molecule hormones presenting physico-chemical properties very similar to therapeutic chemical entities. NRs represent therefore intrinsically a very good family of protein targets for the prevention and treatment of diverse diseases, including cancer. Several known anti-cancer drugs, such as tamoxifen or flutamide, are targeting NRs, and many more are expected to reach market. The detailed knowledge of the structural mechanism underlying activation and inhibition of NRs by small molecule modulators begets important therapeutic opportunities. The crystal structure of at least nine NR ligand binding domains (LBDs) revealed at the atomic level how natural or synthetic agonists and antagonists can promote recruitment of co-activator and co-repressor proteins. Interestingly, it was recently shown that nucleotide polymorphisms located in NR LBDs could alter or even reverse the response of the receptors to small molecule ligands. Mapping these polymorphisms on the structure of the LBD can reveal why agonists or antagonists become inactive against the mutated receptor, allow atomic models for resistance to cancer therapy, and open the door to the rational design of improved anti-cancer drugs, customized for each patient.

Studies of cancer invasion / metastasis and drug resistance have in the past generally proceeded along the separate pathways of research. Recently, however, interest has been focused on the possible relationship between drug resistance and cancer invasion and metastasis. A relationship between these two phenotypes has been demonstrated by two types of observation: firstly, some tumor cells selected for resistance to drugs are more invasive / metastatic relative to non-resistant parental cells, secondly, in some cases, secondary (more metastatic) tumors are more resistant to chemotherapeutic drugs than their primary counterparts. In other instances reported in the literature, no correlation is seen between drug exposure / resistance and cancer invasion / metastasis. The possibility that treatment with some chemotherapeutic drugs may be able to promote cancer invasion and metastasis needs further investigation because of its potential clinical relevance. A better understanding of any relationship between drug resistance and cancer invasion could lead to more effective cancer treatment.