Current Cancer Drug Targets - Volume 2, Issue 1, 2002

The maintenance of specialized nucleoprotein structures at the ends of human chromosomes called telomeres is essential for chromosome stability, and plays a fundamental role in the regulation of cellular lifespan. Without new synthesis of telome-res, chromosome ends shorten with progressive cell division, eventually triggering either replicative senescence or apoptosis when telomere length becomes critically short. The regulation of telomerase activity in human cells plays a significant role in the development of cancer. Telomerase is tightly repressed in the vast majority of normal human somatic cells but becomes activated during cell immortalization and in cancers. Recent work has demonstrated that inhibiting or targeting telomerase shows promise as a novel anti-neoplastic strategy however, the biology of telomeres and telomerase predict that such approaches will differ in important ways from traditional cytotoxic drug therapies. Understanding telomerase biology may eventually lead to several types of clinically effective, telomerase-based therapies for neoplastic disease.

Patients with widespread cancer respond initially to combination chemotherapy, immunotherapy, and / or radiotherapy, but most relapse with chemoresistant disease. Novel methods of killing resistant neoplastic stem cells are needed. One such approach is therapy with targeted toxins composed of tumor cell selective ligands covalently linked to group I peptide toxins (group II and III peptide toxins act on the cell surface). The targeted toxin is delivered to the cell by a tumor selective ligand. Once bound, the ligandreceptor complex is internalized. The catalytic domain escapes to the cytosol. The toxin then enzymatically modifies a critical cell function (protein synthesis, p21 Rho activity, protein kinase signaling, cyclic AMP signaling or others). The irreversibly damaged cells fails to divide and, eventually, undergoes lysis or programmed cell death. Targeted peptide toxins used to date in the treatment of chemotherapy refractory cancers include ricin toxin, Pseudomonas exotoxin, pokeweed antiviral protein, saporin, gelonin and diphtheria toxin. In this review, we have focused on the applications of genetically engineered diphtheria toxin for cancer therapy.

Acquired drug resistance continues to be one of the major obstacles hindering the successful treatment of many forms of cancer. Compounds utilized as antagonists of these cytoprotective mechanisms have, for the most part, proven to be ineffective at overcoming clinical resistance to cytotoxic drugs. Recently, the tumor cell microenvironment has been found to have a significant bearing on the survival of tumor cells following exposure to a wide variety of anti-neoplastic agents, prior to the acquisition of known drug resistance mechanisms. Specifically, interactions between cell surface integrins and extracellular matrix components have been shown to be responsible for this phenomenon of innate drug resistance, which we have termed Cell Adhesion Mediated Drug Resistance, or CAM-DR. Following its discovery using a multiple myeloma cell line model, evidence for CAM-DR has been found in a multitude of other human tumor cell types. In contrast to many other drug resistance mechanisms, integrin-mediated cell signaling is capable of protecting against death induced by an extremely wide variety of structurally and functionally diverse agents from traditional DNA damaging agents to the promising novel kinase inhibitor STI-571. This review examines the role of integrins in regard to their ability to protect tumor cells from drug- and radiation-induced apoptosis through numerous intracellular mechanisms. Current and future antagonists of specific integrin heterodimers may have the potential to sensitize tumor cells when used in combination with standard chemotherapy regimens. Specific signal transduction pathways initiated by integrin ligation will also be discussed as potential bridge points for inhibiting cell survival during cytotoxic drug exposure.

Wild-type Wilms' tumor gene WT1 is expressed at high levels not only in most of acute myelocytic, acute lymphocytic, and chronic myelocytic leukemia, but also in various types of solid tumors including lung cancer. We tested the ability of the gene product (WT1) to serve as a target antigen for tumor -specific immunotherapy both in human in vitro system and mouse in vivo system. In the latter, we can evaluate the efficacy and the side effects of WT1 vaccination in vivo. In the human in vitro system, two WT1 peptides that contain HLA-A2.1 binding anchor motifs were determined to bind to HLA-A2.1 molecules. Peripheral blood mononuclear cells (PBMC) from an HLA-A2.1-psitive donor were repeatedly stimulated in vitro with TAP-deficient T2 cells pulsed with each of these two peptides, and CD8-positive cytotoxic T lymphocytes (CTLs) that specifically lyse WT1-expressing, HLA-A2.1-positive tumor cells were induced. Other groups also have succeeded in generating CTLs which specifically lyse WT1-expressing leukemia cells, and which do not inhibit colony-formation of normal hematopoietic cells that express WT1 at physiological levels. In the mouse in vivo system, immunization of C57BL / 6 mice with one WT1 peptide with relatively high binding affinity for H-2D b molecules, which contain H-2D b binding anchor motifs, induced CTLs, which specifically lysed WT1-expressing tumor cells in an H-2D b -restricted manner. Furthermore, mice immunized with the WT1 peptide (peptide vaccination) or WT1 cDNA (DNA vaccination) rejected challenges by WT1-expressing tumor cells and survived with no signs of auto-aggression to WT1-expressing normal organs by the induced CTLs. The WT1 protein has been identified as a novel tumor antigen and recent investigations provide a rationale for developing WT1-based adoptive T cell therapy and vaccination against various kinds of malignant neoplasms.

The sulfonamides constitute an important class of drugs, with several types of pharmacological agents possessing antibacterial, anti-carbonic anhydrase, diuretic, hypoglycemic and antithyroid activity among others. A host of structurally novel sulfonamide derivatives have recently been reported to show substantial antitumor activity in vitro and / or in vivo. Although they have a common chemical motif of aromatic / heterocyclic sulfonamide, there are a variety of mechanisms of their antitumor action, such as carbonic anhydrase inhibition, cell cycle arrest in the G1 phase, disruption of microtubule assembly, functional suppression of the transcriptional activator NF-Y, and angiogenesis (matrix metalloproteinase, MMP) inhibition among others. Some of these compounds selected via elaborate preclinical screenings or obtained based on computer-aided drug design, are currently being evaluated in clinical trials. This review summarizes recent classes of sulfonamides and related sulfonyl derivatives disclosed ultimately as effective tumor cell growth inhibitors, or for the treatment of different types of cancer.

For many years, the vitamin A metabolite retinoic acid (RA) has been known to have profound effects on development, cellular proliferation and differentiation, and tumor growth and invasion. The wide-ranging effects of RA on cellular proliferation and migration have made it a useful chemotherapeutic agent in the treatment of many types of cancer. In the last fifteen years, with the discovery of nuclear receptors for RA, the molecular basis for the effects of this molecule has become apparent. Retinoic acid receptors (RAR) are members of a superfamily of ligand dependent transcription factors that interact with an increasingly large array of coactivators and repressors to regulate target gene expression through binding to cognate recognition sequences in the promoters of these genes. Alterations in RAR expression and function have been demonstrated in many types of cancer. The translocation of RARα with PML or PLZF genes in acute promyelocytic leukemia is a paradigm of the role of RARs in cancer biology. In addition, the development of receptor selective synthetic retinoids has greatly expanded our knowledge of RAR function in tumor cells and provided additional treatment options for cancer patients. This review will examine the development of receptor selective retinoids, their uses to date, and future potential.