Current Medicinal Chemistry - Anti-Cancer Agents - Volume 5, Issue 1, 2005

The genomic era has shifted anticancer drug development from its traditional mode concentrated on natural product cytotoxic agents to mechanism-based drug design focused on signal transduction pathways. Yet traditional cytotoxic chemotherapies continue to have an important role in the armamentarium. This is particularly true when one considers that important elements of solid tumor physiology - acidosis and hypoxia - have rarely been incorporated into algorithms for anticancer drug development. It is now well established that a majority of solid tumors exist in an acidic and hypoxic microenvironment that promotes resistance to radiation and chemotherapies apart from any drug-induced target mutations or efflux protein pumps. The acidic extracellular environment leads to a pH gradient unique to tumor cells. This gradient will favor uptake and retention of small molecule drugs that are weak acids. The converse is true for weak bases. The camptothecin class of topoisomerase I inhibitors is one example of a natural product cytotoxic that can exploit the tumor pH gradient. Screening of compounds based on selective activity at acidic pH (pH modulation), rather than potency, reveals analogs that are over ten times more active under the acidic conditions prevalent in vivo. Thus, knowledge of the tumor metabolic phenotype gained at the beginning of the 20th century can lead to more effective anticancer drugs in the new millennium.

Cisplatin is one of the most potent and widely used anti-cancer agents in the treatment of various solid tumors. However, the development of resistance to cisplatin is a major obstacle in clinical treatment. Several mechanisms are thought to be involved in cisplatin resistance, including decreased intracellular drug accumulation, increased levels of cellular thiols, increased nucleotide excision-repair activity and decreased mismatch-repair activity. In general, the molecules responsible for each mechanism are upregulated in cisplatin-resistant cells; this indicates that the transcription factors activated in response to cisplatin might play crucial roles in drug resistance. It is known that the tumor-suppressor proteins p53 and p73, and the oncoprotein c-Myc, which function as transcription factors, influence cellular sensitivity to cisplatin. So far, we have identified several transcription factors involved in cisplatin resistance, including Y-box binding protein-1 (YB-1), CCAAT-binding transcription factor 2 (CTF2), activating transcription factor 4 (ATF4), zinc-finger factor 143 (ZNF143) and mitochondrial transcription factor A (mtTFA). Two of these-YB-1 and ZNF143-lack the high-mobility group (HMG) domain and can bind preferentially to cisplatin-modified DNA in addition to HMG domain proteins or DNA repair proteins, indicating that these transcription factors may also participate in DNA repair. In this review, we summarize the mechanisms of cisplatin resistance and focus on transcription factors involved in the genomic response to cisplatin.

Coumarins, an old class of compounds, are naturally occurring benzopyrene derivatives. A lot of coumarins have been identified from natural sources, especially green plants. The pharmacological and biochemical properties and therapeutic applications of simple coumarins depend upon the pattern of substitution. Coumarins have attracted intense interest in recent years because of their diverse pharmacological properties. Among these properties, their cytotoxic effects were most extensively examined. In this review, their broad range of effects on the tumors as shown by various in vitro and in vivo experiments and clinical studies are discussed. Hence, these cytotoxic coumarins represent an exploitable source of new anticancer agents, which might also help addressing side-toxicity and resistance phenomena. These natural compounds have served as valuable leads for further design and synthesis of more active analogues. In this review, plant derived coumarins and their synthetic analogues were systematically evaluated based on their plant origin, structureactivity relationship and anticancer efficacy. Owing the their diverse effects and inconclusive results from different in vitro studies, the mechanism of their action is not yet fully understood and correlation of effects with chemical structures is not conclusive at the moment. It is the objective of this review to summarize experimental data for different coumarins, used as cytotoxic agents, because promising data have been reported for a series of these agents. Yet, the results from different coumarins with various tumor lines are contradictory in part. We therefore conclude that there is still a long way to go until we know which cytotoxic agent will clinically be suitable for what tumor entity for treatment. Their ability to bind metal ions represents an additional means of modulating their pharmacological responses.

The normalization of plasma alanine aminotransferase (ALT) has been proved to be a strategy for preventing the development of hepatocellular carcinoma (HCC) in hepatitis C virus (HCV)-infection. Glycyrrhizin, a plant medicine, normalizes plasma ALT and prevents HCC. However, glycyrrhizin is administered intravenously and thereby chemical which is effective on oral administration is required. Coumarin compounds are active components of herbs used for the treatment of various diseases. The ability of coumarin compounds to lower plasma ALT were examined using mice concanavalin A-induced hepatitis and mice anti-Fas antibody-induced hepatitis. Furanocoumarins pd-Ia, pd-II and pd-III lower plasma ALT, but they are large molecules that are hardly absorbed on oral administration. Furocoumarin effectively lowers plasma ALT, but the safety range between the effective and toxic dosages is narrow. In contrast, osthole, a simple coumarin, causes strong reduction of plasma ALT and also inhibits caspase-3 activation. Furthermore, this chemical is quite safe upon large dose administration. In the structure of osthole, the methoxy group at position-7 and the 3-methyl-2- butenyl group at position-8 were elucidated to be essential for the beneficial effect of this chemical. We conclude that osthole will become a leading chemical for synthesizing a compound which prevents HCC on oral administration.

Cancer is a serious disease with a complex pathogenesis, which threats human life greatly. Currently, great efforts have been put to the identification of novel anticancer targets and the discovery of anticancer drugs following the progress of chemogenomics, which will be reviewed briefly in this article. Furthermore, during the past 5 years, the global effort of sequencing human genome has provided us with an enormous number of potential targets associated with cancer therapy. As a result, the New Drug Discovery (NDD) is undergoing a transition “from gene to drug”. Accordingly, the targets for anticancer drugs studies now are focused on some biological macromolecular targets associated with cancer and several interactive mechanisms involved in the growth and metastasis of cancer cells as well as tumor angiogenesis, such as Matrix Metalloproteinases (MMPs), Aminopeptidase N (APN), Tyrosine Kinase (TK), Farnesyltransferase (FTase) and cell Signal Transduction Pathway and so forth. Among these targets the MMP-2, -9 and APN are the most extensively studied enzymes in our laboratory. The peptidomimetics Matrix Metalloproteinase Inhibitors (MMPIs) and APN inhibitors (APNIs) with the molecular scaffold of pyrrolidine, 3-amino-2-hydroxy-4-phenyl butyric acid (AHPA) and glutamylide, which have been designed and synthesized in our laboratory, will be described in the review, among which the pyrrolidine scaffold is patented with the IC50 ranging from 1nM to 300nM against MMP-2, and MMP-9.

Chemical compounds that interfere with microtubules such as the vinca alkaloids and taxanes are important chemotherapeutic agents for the treatment of cancer. As our knowledge of microtubule-targeting drugs increases, we realize that the mechanism underlying the anti-cancer activity of these agents may mainly lie in their inhibitory effects on spindle microtubule dynamics, rather than in their effects on microtubule polymer mass. There is increasing evidence showing that even minor alteration of microtubule dynamics can engage the spindle checkpoint, arresting cell cycle progression at mitosis and eventually leading to apoptotic cell death. The effectiveness of microtubule-targeting drugs for cancer therapy has been impaired by various side effects, notably neurological and hematological toxicities. Drug resistance is another notorious factor that thwarts the effectiveness of these agents, as with many other cancer chemotherapeutics. Several new microtubule-targeting agents have shown potent activity against the proliferation of various cancer cells, including cells that display resistance to the existing microtubule-targeting drugs. Continued investigation of the mechanisms of action of microtubule-targeting drugs, development and discovery of new drugs, and exploring new treatment strategies that reduce side effects and circumvent drug resistance may provide more effective therapeutic options for cancer patients.

The treatment of advanced non-small-cell lung cancer (NSCLC is based on the combination of platin and one of the following agents: taxanes, gemcitabine, vinorelbine or irinotecan. There are no significant differences in efficacy among these combinations suggesting that the maximum efficacy has been reached. In this review, we will consider the mechanisms of chemoresistance of the five groups of cytotoxic drugs commonly used in the treatment of advanced NSCLC as well as the clinical studies which have assessed the value of chemoresistance markers. Breast Cancer Related Protein (BRCP) expression has been related to irinotecan and cisplatin (CDDP) resistance. DNA repair capacity influences response to CDDP and ERCC1 gene stands out as a predictive marker of CDDP sensitivity. Preliminary studies indicate that high tubulin III and stathmin mRNA levels correlate with response to paclitaxel and vinorelbine and that high expression of class III tubulin by tumor cells assessed immunohistochemically in patients receiving a taxane-based regimen is associated with a poor response to chemotherapy, and a shorter progression-free survival. High expression levels of ribonucleotide reductase has also been related to response to gemcitabine. Uridine diphosphate glucuronosyltransferase isoform 1A1 (UGT1A1) genotype has been reported to be associated with time to progression and survival in patients treated with irinotecan. These data suggest that pharmacogenomic strategies may be used for developing customized chemotherapy in prospective studies. Adjuvant chemotherapy which had recently shown its usefulness in limited lung cancer represents another area of investigation for pharmacogenomic studies.