Anti-Cancer Agents in Medicinal Chemistry (v.9, #1)

Pyrrolo[2,1-c][1,4]benzodiazepine as a Scaffold for the Design and Synthesis of Anti- Tumour Drugs by Laura Cipolla, Ana Araujo, Cristina Airoldi, Davide Bini (1-31).
Compounds that bind in the minor groove of DNA have found use in the experimental treatment of cancer and certain infectious diseases. Furthermore, agents which target and can recognize discrete sequences of DNA have the potential to offer selective therapies by modulating the activity of specific transcription factors or genes. For this reason, a number of sequence-selective DNA binding agents have been evaluated with a range of affinities and recognition fidelities. In this respect, the pyrrolo[2,1-c][1,4]benzodiazepines (PBDs) are of interest as they bind to guanine residues in the minor groove with a preference for Pu-G-Pu sequences. A dramatic increase in cytotoxicity and sequence selectivity has been achieved by linking two PBD units to form PBD dimers as cross-linking agents on opposite DNA strands (e.g., interstrand cross-links). SJG-136 is currently undergoing Phase I evaluation in both the United States (through the NCI) and United Kingdom (through Cancer Research United Kingdom). This review will focus on design, synthesis and structure activity relationship studies of pyrrolobenzodiazepines as anticancer therapeutics reported since 2003.

Protein kinase B (PKB or Akt) is a central component of the PI3K - PKB - mTOR signalling cascade and is firmly established as an attractive target for pharmacological intervention in cancer. A number of small molecule inhibitors with well-defined, direct molecular interactions with PKB are now known, covering a range of mechanisms from ATP- or substrate-competitive inhibition, through allosteric modulation of the kinase activity, to inhibition of the phosphatidylinositol-dependent activation process. The development of small molecule inhibitors of PKB has benefited greatly from the application of structural biology techniques, particularly for lead generation and the optimisation of compound potency and selectivity. The development of the major chemical series of PKB inhibitors will be outlined, with an emphasis on the application of structure-based design and the strategies used to optimise compound pharmacodynamics, efficacy and therapeutic window. The development of small molecules targeting PKB for anticancer therapy has reached an exciting stage, with the first selective inhibitors entering clinical trials, and several additional chemotypes demonstrating efficacy in preclinical models.

Sumoylation has been implicated in a variety of cancers, suggesting that sumoylation manipulation could be one approach for regulating tumorgenesis. Ubc9 exerts a central function for the sumoylation pathway, interacting with almost all the partners required for sumoylation. The high-resolution structure available for Ubc9 as well as the recent determination of more interacting partner complex structures makes rational drug design that target Ubc9 possible. Structure-based virtual drug screening has been used increasingly as the first step of drug design to select potential lead templates. This review analyzes all the interfaces between Ubc9 and its binding partners while also highlighting the possible targeting sites on Ubc9 best suited for virtual screening and drug design.

2-Methoxyestradiol as a Potential Cytostatic Drug in Gliomas? by E. Kirches, M. Warich-Kirches (55-65).
Gliomas of astrocytic origin show only a limited chemotherapy response. Chemoresistance is most pronounced in glioblastoma multiforme, the most common and most malignant glioma, with median survival times not much longer than one year. Failure of chemotherapy partly relies on protective mechanisms against the commonly used DNA alkylating agents, but also on the constitutive activation of the pro-survival PI3K-Akt pathway in glioma cells, which inhibits apoptosis. Therefore, new drugs with an alternative mechanism, independent of DNA alkylation, are required. The microtubule targeting drug 2-methoxyestradiol (2-ME) efficiently induces mitotic arrest, apoptosis, but also autophagic cell death in glioma cells in vitro. Moreover, it may be able to inhibit vascularization of the highly vascular gliobastomas, because the drug influences blood vessel sprouting via a HIF-1-dependent mechanism. Although high doses of i.p. injected 2-ME were recently shown to be effective in an orthothopic rat glioma model, clinical phase I/II trials revealed low oral bioavailability. One of the most exciting future perspectives will be the currently ongoing development of improved 2-ME analogs. Compounds, sulphamoylated at positions 3 and 17, combine sufficient toxicity against tumor cells with resistance against metabolic degradation and sufficient plasma levels in experimental animals. They were found to be superior in some animal models of tumor growth and vascularization, following oral application.

Tyrosine Kinase Blockers: New Hope for Successful Cancer Therapy by Dariusz Pytel, Tomasz Sliwinski, Tomasz Poplawski, Deborah Ferriola, Ireneusz Majsterek (66-76).
Tyrosine kinases (TKs) are attractive targets for cancer therapy, as quite often their abnormal signaling has been linked with tumor development and growth. Constitutive activated TKs stimulate multiple signaling pathways responsible for DNA repair, apoptosis, and cell proliferation. During the last few years, thorough analysis of the mechanism underlying tyrosine kinase's activity led to novel cancer therapy using TKs blockers. These drugs are remarkably effective in the treatment of various human tumors including head and neck, gastric, prostate and breast cancer and leukemias. The most successful example of kinase blockers is Imatinib (Imatinib mesylate, Gleevec, STI571), the inhibitor of Bcr/Abl oncoprotein, which has become a first-line therapy for chronic myelogenous leukemia. The introduction of STI571 for the treatment of leukemia in clinical oncology has had a dramatic impact on how this disease is currently managed. Others kinase inhibitors used recently in cancer therapy include Dasatinib (BMS-354825) specific for ABL non-receptor cytoplasmic kinase, Gefitinib (Iressa), Erlotinib (OSI-774, Tarceva) and Sunitinib (SU 11248, Sutent) specific for VEGF receptor kinase, AMN107 (Nilotinib) and INNO-406 (NS-187) specific for c-KIT kinase. The following TK blockers for treatment of various human tumors are in clinical development: Lapatinib (Lapatinib ditosylate, Tykerb, GW-572016), Canertinib (CI-1033), Zactima (ZD6474), Vatalanib (PTK787/ZK 222584), Sorafenib (Bay 43-9006, Nexavar), and Leflunomide (SU101, Arava). Herein, we discuss the chemistry, biological activity and clinical potential of new drugs with tyrosine kinase blockers for cancer treatment.

Fused Xanthone Derivatives as Antiproliferative Agents by Nicole Pouli, Panagiotis Marakos (77-98).
Xanthones have been isolated from several natural sources, mainly belonging in Guttiferae and Gentianaceae families as secondary plant metabolites and many of them are endowed with diverse pharmacological properties. We have focused in the study of cytotoxic fused xanthone derivatives, having in mind that some furano- and pyranoxanthone natural products are particularly interesting, in terms of cytotoxic potency and novelty in their mechanism of action and could serve as lead compounds for the development of clinically effective anticancer agents. In this review, a general classification has been attempted based on the type of ring fusion, in such a way that natural compounds as well as synthetic derivatives are discussed. The furanoxanthone psorospermin is a highly promising isolated xanthone derivative exhibiting significant cytotoxicity through a novel mechanism of action, being an irreversible topoisomerase II poison and it was selected for further development as an antineoplastic agent. An important number of pyranoxanthones have been synthesized using as lead compound the acridone alkaloid acronycine. Adducts on the double bond of these compounds provided cytotoxic derivatives possessing cell-cycle selectivity. The synthesis of pyranoxanthones bearing aminosubstituted side-chains resulted in compounds that exhibit markedly improved cytotoxicity towards leukemic and solid tumor cell lines. Azabioisosters of the aminoderivatives exhibit solid tumor selectivity whereas additional pyrazole or/and benzene ring fusion has been incorporated into the xanthone skeleton and resulted in compounds with promising activity, which retain full antiproliferative activity against P-glycoprotein-overexpressing cells. Gambogic acid, a highly effective anticancer drug candidate with low toxicity to normal tissue, together with structurally related representative analogues are also mentioned.

Radiosensitizing Potential of Epigenetic Anticancer Drugs by Harlinde De Schutter, Sandra Nuyts (99-108).
Over the last few decades, epigenetic tumor changes characterized by promoter hypermethylation and histone modifications have become a topic of intense research. Of particular interest is the potential reversibility of these processes that has led to the development of epigenetic anticancer drugs such as demethylating agents and histone deacetylase inhibitors (HDAC-I). Besides single agent clinical activity in both hematological and solid malignancies, combinations of both types of epigenetic drugs with classic chemotherapeutics have shown promising results. In addition, as demethylating agents and HDAC-I act synergistically to reverse gene silencing, treatment schedules combining both epigenetic strategies could theoretically enhance tumor response. This assumption has been validated in vitro and in vivo for several hematological and solid cancer types, and awaits further clinical investigation. Nowadays, the majority of patients with cancer are treated with radiotherapy. To optimize the results obtained with this treatment modality, efforts are being put in strategies enhancing tumor response selectively in favor of normal tissue response. The combination of epigenetic drugs with radiotherapy is particularly valuable since a drug- and dose-dependent radiosensitizing potential of several classes of HDAC-I has been proven in vitro and in vivo. The molecular mechanisms underlying this radiosensitization have not been fully clarified yet. In general, higher concentrations of HDAC-I are believed to exert cell cycle redistribution, induction of apoptosis, and downregulation of surviving signals. The radiosensitizing effect of lower, non-toxic doses of HDAC-I has been attributed to, at least in part, acetylation- induced changes leading to altered double strand break (DSB) formation and repair. Although promising so far, further research is needed before HDAC-I administered alone or in combination with demethylating agents will be implemented in the clinic to act as radiosensitizers.

Biotechnological Production of Taxol and Related Taxoids: Current State and Prospects by O. Exposito, M. Bonfill, E. Moyano, M. Onrubia, M. Mirjalili, R. Cusido, J. Palazon (109-121).
Taxol is one of the most effective anti-cancer drugs ever developed. The natural source of taxol is the inner bark of several Taxus species, but it accumulates at a very low concentration and with a prohibitively high cost of extraction. Another problem is that the use of inner bark for taxol production implies the destruction of yew trees. For all these reasons, the growing demand for taxol greatly exceeds the supply that can be sustained by isolation from its natural source and alternative sources of the drug are being sought. Although taxol has been prepared by total synthesis, the process is not commercially viable. Taxol can also be semisynthetically produced via the conversion of baccatin III or 10-deacethylbaccatinIII found in Taxus needles but the cost and difficulty of the extraction process of the semisynthetic precursors are also very high. The most promising approach for the sustainable production of taxol and related taxoids is provided by plant cell cultures at an industrial level. Taxol is currently being clinically used against different tumour processes but due to the difficulty of its extraction and formulation, as well as the growing demand for the compound, new taxol analogues with improved properties are being studied. In this revision we discuss current research in the design of new taxol-related compounds, the chemical structure/anti-cancer activity relationship and new formulations of the drug. We also consider the optimizing strategies to improve taxol and related taxoid production in cell cultures, as well as the current knowledge of taxol metabolism, all of which are illustrated with examples, some of them from our own research.