Current Cancer Drug Targets - Volume 12, Issue 4, 2012

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Angiogenesis is a key factor in the carcinogenesis process. In oncological practice, angiogenesis inhibition, mainly through the blockade of the VEGF family and its receptors, has been robustly demonstrated to produce clinical benefits and, in specific disease subsets such as colorectal cancer, to extend the overall survival of treated patients. VEGF is a multifunctional growth factor that mediates its functions through cognate receptors on endothelial cells and it has been discovered for its capability to induce macromolecule hyperpermeability in veins and venules. Several approaches have been taken to target angiogenesis in cancer: drugs that target one or more soluble ligands of the VEGF family, drugs that selectively inhibit one or more receptors of the VEGF receptor family, and drugs that inhibit VEGF receptor(s) among other, non VEGF-related targets. At present, two compounds have shown significant clinical activity, bevacizumab, Avastin® and aflibercept, Zaltrap®, and only one of these (bevacizumab) has so far been registered for use in clinical practice. In the present review, we explore and summarize the main features of the angiogenetic process, concerning in particular a common and potentially lethal disease as colorectal cancer. We overview the molecular pathways that characterize angiogenesis, focusing on VEGF family, the current applications and limitations of its blockade in oncology, and the hypothetical future perspectives of anti-angiogenic therapy.

Personalized medicine emphasizes the practice of considering individual patient characteristics as opposed to that centered on standards derived from epidemiological studies which, by definition, do not take into account the variability of individuals within a given population. When applied to oncology, personalized medicine is an even more complex concept because it extends the variability beyond the individual patient to the individual tumor. Indeed, the great genotypic and phenotypic variability (both in primary and metastatic sites of cancer) the development of targeted therapies, and the growing availability of biological assays complicate the scenario of personalized medicine in the oncological field. In this paper we review the results of anti-epidermal growth factor receptor (EGFR) monoclonal antibody (mAb) therapy in metastatic colorectal cancer (mCRC) in the context of tumor biology, delineating the future prospects of patient-tailored medicine in this area. In particular, we deal with EGFR inhibition by Cetuximab, a chimeric mouse human IgG1 mAb, and panitumumab, a fully human IgG2 mAb. We discuss the clinical impact of anti-EGFR mAbs on wild-type (WT) KRAS mCRC, also taking into account the feasibility of novel multi-marker approaches to treatment decision-making, aimed at increasing the predictive power of pre-therapy biomarkers. Experimental topics and fields of ongoing research, such as targeting microRNAs (miRNAs) with novel anticancer drugs and epigenetics in CRC are also addressed.

Over the past two decades, progresses in colorectal cancer treatment have significantly improved patient survival and quality of life. However, unresectable metastatic colorectal cancer remains virtually incurable, making the search for new effective therapeutics mandatory. An important limitation to the development of new agents has been the difficulty to exploit mutated tumor suppressors or “undruggable” oncogenes as a target. Recently, evidence that mutations in tumor suppressors, such as BRCA1/2, make cancer cells highly susceptible to inhibitors of a compensatory DNA repair pathway [poly-(ADP-ribose) polymerase 1 (PARP1)] has broadened the range of possible therapeutic targets by extending it to gene products that are in a “synthetic lethal” relationship with oncogenes and tumor suppressors. Inhibition of such targets blocks specific buffer-mechanisms that are required for survival in the presence of defined oncogenic mutations, but not in their absence. As a consequence, selective elimination of mutation-bearing cells results. This approach has led to identify compounds that are highly active in the presence of different types of mutated tumor suppressors and oncogenes, including DNA repair genes, RAS, and Myc. In addition, ongoing studies promise to identify new mechanisms which, when pharmacologically interfered with, will selectively eradicate mutated cancer cells. Here, we revise and discuss these new aspects of cancer biology and highlight their potential applications in colorectal cancer treatment.

This review article is part of a special Current Cancer Drug Targets issue devoted to colorectal cancer and molecularly targeted treatments. In our paper we made an attempt to connect more basic aspects with preclinical, pharmacological / therapeutic and clinical aspects. Reconstruction of a Molecular Interaction Map (MIM) comprising an important part of the G0 – G1 – S cell cycle transition, was a major component of our review. Such a MIM serves also as a convenient / organized database of a large set of important molecular events. The frequency of mutated / altered signaling-proteins indicates the importance of this signaling-network region. We have considered problems at different scale levels. Our MIM works at a biochemical-interaction level. We have also touched the multi-cellular dynamics of normal and aberrant colon crypts. Until recently, dynamic simulations at a biochemical or multi-cellular scale level were considered as a sort of esoteric approach. We tried to convince the reader, also on the basis of a rapidly growing literature, mostly published in high quality journals, that suspicion towards simulations should dissipate, as the limitations and advantages of their application are better appreciated, opening the door to their permanent adoption in everyday research. What is really required is a more interdisciplinary mentality and an interdisciplinary approach. The prize is a level of understanding going beyond mere intuition.

Although several drugs have been designed in the last few years to target specific key pathways and functions in colorectal cancer (CRC), the backbone of CRC treatment is still made up of compounds which rely on DNA damage to accomplish their role. DNA damage response (DDR) and checkpoint pathways are intertwined signaling networks that arrest cell cycle, recognize and repair genetic mistakes which arise during DNA replication and transcription, as well as through the exposure to chemical and physical agents that interact with nucleic acids. The good but highly variable activity of DNA damaging agents in the treatment of CRC suggests that intrinsic alterations in DDR pathways and cell cycle checkpoints may contribute differentially to the way cancer cells react to DNA damage. In the present review, our aim is to depict the recent advances in understanding the molecular basis of the activity of DNA damaging agents used for the treatment of CRC. We focus on the known and potential drug targets that are part of these complex and intertwined pathways. We describe the potential role of the checkpoints in CRC, and how their pharmacological manipulation could lead to chemopotentiation or synergism with currently used drugs. Novel therapeutic agents playing a role in DDR and checkpoint inhibition are assessed. We discuss the possible rationale for combining PARP inhibition with DNA damaging agents, and we address the link between DDR and EGFR pathways in CRC.

Thal has antiangiogenic and immunomodulatory activity. Clinical research provided clear evidence that Thal belongs to the most active drugs for the treatment of multiple myeloma e.g. leading to decrease of monoclonal protein of at least 50 % in 30 % of patients with relapsed or refractory multiple myeloma. Randomized trials that were designed based on a large body of evidence from phase II trials determined that Thal significantly increases total response rate, progression-free and in some studies overall survival in combination regimens (dexamethason and or chemotherapy) for relapsed as well as newly diagnosed patients and was therefore approved for first-line treatment of Multiple Myeloma. Strict guidelines apply due to the teratogenic effects of Thal and to monitor and prevent other potential adverse events as neuropathy and thrombosis has been recognized by leading organizations as part of the treatment concept for patients with relapsed or refractory disease. The success of Thal has sparked the development of Thal analogues with Lenalidomide (Len) the most advanced compound which was approved for relapsed multiple myeloma. As Len has a lower incidence of polyneuropathy, constipation and somnolence compared to Thalidomid but at least equal if not higher efficacy Len is meanwhile used more frequently in clinical routine and has advantages in combination therapies with Bortezomib. Additional randomized studies will now define the status of Thal and Len for maintenance therapy and their optimal integration in multi-agent treatment regimen.

Recent results of phase II trials which used dasatinib or nilotinib as single agent, or phase III trials comparing second-generation tyrosine kinase inhibitors to imatinib, showed greater potency of these two inhibitors in newly diagnosed chronic myeloid leukemia (CML) patients in chronic phase (CP). In the present review we detail and summarize clinical results of both agents as first-line therapeutic strategy, and also discuss on critical points emerged from the last follow-up of trials comparing new generation tyrosine kinase inhibitors with imatinib. In terms of safety, dasatinib and nilotinib have shown favorable toxicity profile, with peculiar and distinct pattern of adverse events. Based on these results, USA FDA approved both drugs as first-line treatment in newly diagnosed CML: now several therapeutic strategies are available to treat patients at onset of disease. Longer follow-up is however needed to prove the advantages of faster and deeper responses in terms of disease progression compared to imatinib.

Refractoriness to the pharmacological treatment of cancer is dependent on the expression levels of genes involved in mechanisms of chemoresistance and on the existence of genetic variants that may affect their function. Thus, changes in genes encoding solute carriers may account for considerable inter-individual variability in drug uptake and the lack of sensitivity to the substrates of these transporters. Moreover, changes in proteins involved in drug export can affect their subcellular localization and transport ability and hence may also modify the bioavailability of antitumor agents. Regarding pro-drug activation or drug inactivation, genetic variants are responsible for changes in the activity of drugmetabolizing enzymes, which affect drug clearance and may determine the lack of response to anticancer chemotherapy. The presence of genetic variants may also decrease the sensitivity to pharmacological agents acting through molecular targets or signaling pathways. Recent investigations suggest that changes in genes involved in DNA repair may affect the response to platinum-based drugs. Since most anticancer agents activate cell death pathways, the evasion of apoptosis plays an important role in chemoresistance. Several genetic variants affecting death-receptor pathways, the mitochondrial pathway, downstream caspases and their natural modulators, and the p53 pathway, whose elements are mutated in more than half of tumors, and survival pathways, have been reported. The present review summarizes the available data regarding the role of genetic variants in the different mechanisms of chemoresistance and discusses their potential impact in clinical practice and in the development of tools to predict and overcome chemoresistance.

Pancreatic ductal adenocarcinoma (PDAC) is quite resistant to conventional treatments, and gemcitabine, the standard chemotherapeutic agent, offers only a small benefit. Development and progression of PDAC are complex processes involving dysregulation of multiple signal transduction pathways arising from not only genetic but also epigenetic alterations. This makes the epigenetic approach to the treatment of PDAC of great interest. Histone deacetylases, a family of enzymes that, by removal of acetyl groups from a variety of histone and nonhistone proteins, play an important role in the epigenetic regulation of gene expression, are frequently dysregulated in PDAC. In particular, overexpression of class I histone deacetylases has been related to higher tumor grade, poor prognosis and development of chemoresistance. Histone deacetylase inhibitors (HDACIs), a novel class of agents endowed with pleiotropic antitumor effects, appear promising either for their preferential toxicity towards transformed as compared to normal cells and their ability to synergistically enhance the anticancer activity of radiotherapy and many chemotherapeutic agents. Many HDACIs have been shown to increase the antiproliferative and proapoptotic effects of gemcitabine, 5-fluorouracil and bortezomib, a new proteasome inhibitor, in vitro and in vivo PDAC xenograft models. MGCD0103, romidepsin, panobinostat, vorinostat and valproic acid, are currently being tested in association with radiotherapy or chemotherapy (gemcitabine, fluoropyrimidines, proteasome inhibitors) in phase I-II clinical trials in patients with locally advanced or metastatic PDAC.