Current Pharmaceutical Design - Volume 9, Issue 25, 2003

Conventional cancer therapy is administered in the form of surgery, radiotherapy or chemotherapy. Immunotherapy is the latest asset to the panel of anti-cancer treatments. This approach appears favorable over the other more conventional methods for various reasons: (1) it is highly specific for cancer cells and, therefore, low toxicity should be expected; (2) it recognizes and eliminates cancer cells regardless of their phase in the cell cycle; (3) tumors that developed drug resistances would still be a suitable target for immunotherapy. (4) Immunotherapy offers the possibility of preventive immunization of high-risk patients. Due to the diverse mechanisms that result in the transformation of cells and subsequent tumor development, not all cancers respond similarly to treatment. Significant effort is currently invested in the characterization of the underlying regulatory network in individual cancers responsible for tumorigenesis. Understanding tumors better allows on one hand the identification of essential pathways that can be intercepted to kill the transformed cells more specifically. On the other hand, these insights also allow us to exclude therapeutic strategies with little chance of success when dealing with tumor escape mutants thus saving valuable time and resources. Any tumor therapy puts selective pressure on tumors thus favoring the outgrowth of therapy-resistant variants. This review summarizes current knowledge on tumor escape mechanisms and some of the efforts to overcome these mechanisms.

The biological effects of radiation affect both neoplastic and normal tissues. The nature and extent of such effects, however, depend on selected biological parameters (e.g., oxygen supply, cell cycle) and can be modified by chemical agents such as radiosensitizers, radioprotectors and chemotherapeutic agents. A precise control of the mode of action of the radiation is important in order to achieve the maximum effect on tumor tissue, while minimizing the effect on normal tissues. Most of the known and routinely used radiosensitizers are neither selective nor tumor specific. This article reviews a new selective and specific modality that increases the sensitivity of solid tumor tissue, especially of radio resistant, hypoxic tumor cells, to radiation. This modality is currently under early clinical evaluation and encompasses the application of Photofrin II, which is already used as a photosensitizer in photodynamic therapy (PDT) at predetermined times prior to irradiation.

The nonapeptide bradykinin is an important growth factor for many cancers. Certain peptide and non-peptide bradykinin antagonists show remarkable anti-cancer activities in both in vitro and in vivo cancer models, especially of lung and prostate cancers. Bradykinin antagonists stimulate apoptosis in cancers by a novel “biased agonist” mechanism: they block intracellular increase of calcium concentration but stimulate the MAP kinase pathway. This unbalanced effect stimulates caspase activation. In nude mouse xenotransplants of lung and prostate cancers the antagonists inhibit angiogenesis and activation of membrane metalloproteases (MMP 2 and 9). In the xenotransplants certain bradykinin antagonists showed higher potency than standard anti-cancer drugs, without evident toxicity to the hosts. These compounds offer great promise for development of new anti-cancer drugs.

The inherited or acquired deregulation of protein kinase activity has been implicated in the pathogenesis of many human diseases, including cancer. Therefore, the inhibition of kinases has been proposed to be a promising strategy in the context of anti-cancer treatment. Many other kinases have been selected as drug discovery targets based on the prevalence of mutations, over-expression and unscheduled activation in human cancer. Of the various protein kinases chosen, Src family kinases are amongst the most extensively studied kinase oncogenes in academia and industry. This review focuses on our current understanding of the deregulation and role of Src family kinases in human cancer and leukemia. Recent data implicate the action of c-Src in cancer metastasis, mediated by up-regulation of various protease systems (calpain, uPA) as well as disruption of E-cadherin signalling. Moreover, novel roles of various Src family members in the development of human leukemia have been found. New insights into downstream signalling mechanisms, including the activation of STAT3, PDK1 and Akt, further corroborate the importance of Src family kinases in tumorigenesis and chemoresistance. Despite our rather clear understanding of Src family kinases as pro-oncogenes no Src family kinase inhibitor has entered a clinical trial so far. This review will discuss prerequisites to be fulfilled for clinically targeting c-Src and its homologues using small molecule drugs.

Metastatic melanoma patients have a dismal prognosis with poor responsiveness to chemotherapy, radiation therapy and current immunotherapy regimens and a median survival of less than six months. Novel therapies directed at melanoma-selective molecular targets are urgently needed. Based on the frequent constitutive activation of the mitogenactivated protein kinase (MAPK) signaling pathway in malignant melanomas and the selective inhibition of MAPK signaling by anthrax lethal factor which proteolytically cleaves MAPK kinases, anthrax lethal toxin may be a useful agent for patients with metastatic melanoma. Anthrax lethal toxin consists of two proteins--protective antigen and lethal factor. These two proteins have been separately produced in good yields and in high purity. The three-dimensional structures of these proteins have been solved, and their molecular mechanisms of cell binding and action determined. Preclinical studies with anthrax lethal toxin show sensitivity of malignant melanoma cell lines in tissue culture and anti-tumor efficacy in melanoma xenograft models. Additional studies to define the maximal tolerated doses and dose-limiting toxicity of anthrax lethal toxin in rodent and primate models should pave the way for phase I studies testing the efficacy of the anthrax lethal toxin in patients with metastatic melanoma.

Vitamin A and its biologically active derivatives are involved in a complex arrangements of physiological and developmental responses in many tissue of higher vertebrates. Retinoids are natural and synthetic compounds related to retinoic acid that act through interaction with two basic types of nuclear receptors: retinoic acid receptors (RAR α, RARβ and RARγ) and retinoid X receptors (RXRα, RXRβ and RXRγ) as retinoid-inducible transcription factors. Thus, the retinoid receptors are considered to be ligand-activated, DNA-binding, trans-acting, transcription-modulating proteins involved in a general molecular mechanism responsible for transcriptional responses in target genes. They exert both beneficial and detrimental activity; they have tumor-suppressive activity but on the other hand they are teratogenic. Retinoids inhibit carcinogenesis, suppress premalignant epithelial lesions and tumor growth and invasion in a variety of tissues. Natural and synthetic retinoids have therapeutical effects due to their antiproliferative and apoptosis-inducing effects. They are known to cause redifferentiation or to prevent further dedifferentiation of various neoplastic tissues. A number of novel chemical compounds, receptor selective retinoids and rexinoids, have been synthesized up to now and tested both in vitro and in vivo, using animal models against different cancer cells. In spite of that progress, there is still an urgent call for novel synthetic retinoids and rexinoids with greater retinoid receptor selectivity, reasonable chemotherapeutic or chemopreventive effects and reduced toxicity and side effects. This article summarizes selected effects of biologically active natural or synthetic retinoids and rexinoids, acting through their cognate nuclear receptors, and their use in chemotherapy and chemoprevention of various types of cancer.

25 years after the first approval of cisplatin in the clinic against a number of cancer diseases, cisplatin and related compounds continue to be among the most efficient anticancer drugs used so far. Efforts are focused to develop novel platinum- and non-platinum-based antitumor drugs to improve clinical effectiveness, to reduce general toxicity and to broaden the spectrum of activity. In the field of non-platinum compounds exhibiting anticancer properties, ruthenium complexes are very promising, showing activity on tumors which developed resistance to cisplatin or in which cisplatin is inactive. Furthermore, general toxicity was found to be very low. The first ruthenium compound NAMI-A entered phase I clinical trials in 1999 as an antimetastatic drug, whereas the ruthenium complex KP1019 will enter phase I clinical trials in 2003 as an anticancer drug which is among others very active against colon carcinomas and their metastases. Remarkable progress is also seen in developing tumor inhibiting gallium compounds. One of them, KP46, will also enter phase I clinical trials in 2003. This article reviews briefly the achievements in the field of anticancer metal complexes focusing the discussion onto the impact of the group of Bioinorganic Chemistry at the Department of Inorganic Chemistry at the University of Vienna. The development of pH sensitive platinum prodrugs, platinum-based drug targeting strategies with low-molecular-weight carriers, kinetically inert platinum(IV) complexes, as well as tumor inhibiting non-platinum anticancer drugs based on ruthenium and gallium is covered in the following sections.