Current Cancer Drug Targets - Volume 10, Issue 5, 2010

Targeted cancer therapies are drugs or other substances that block the growth and spread of cancer by interfering with specific molecules involved in cellular pathways which regulate tumor growth, cell proliferation or apoptosis, thus interfering with pathways that are genetically altered in tumor cells. For these drugs it is recognized that the molecular profile of the target cells predicts response. Nevertheless, recent data focus on the striking genetic disparity between primary tumors, circulating tumor cells (CTCs) and metastases, thus questioning whether the analysis of primary tumors may be really always considered as a surrogate for the genetics of systemic disease. Exemplificative is breast cancer, where it has been demonstrated that patients with HER2 negative primary tumors may develop HER2-positive CTCs during disease progression. These observed discrepancies may be the cause of tumor cell persistence and therapeutic failure in the adjuvant setting. Tumor heterogeneity is defined as the presence of intercellular differences, either from clonal origin or present within subpopulations of tumor cells; it represents a phenomenon of critical importance in the natural history of individual neoplasms. One of the main consequences of tumor heterogeneity is that locally growing and systemically spreading cancer cells show crucial genetic divergence. Thus, the predictive power for selection of targeted therapies will inevitably be imperfect and the analysis of the individual disease, in particular of the systemically spread cancer cells may be useful also for new drug combinations. The search for therapies that are specific for cancer cells has focused on differences in gene expression between normal and cancer cells. However, such differences within heterogeneous cancer cell populations must also be considered. Cellular antigens or signalling pathways expressed by cancer cells may not be optimal therapeutic targets if immature cancer stem cells that do not express the targets are also present. It is therefore essential that the development of new anticancer treatments focuses on the pathogenesis and biology of the diseases being treated in addition to the drugs and their specific targets. The collection of reviews here presented focuses on the current clinical evidence supporting the use of targeted therapies in cancer, on the molecular mechanisms involved in the development of drug resistance, and on the strategies available to overcome it. In addition, the efforts to identify new targets of minimal residual disease in melanoma are reviewed, together with the mechanisms of drug resistance frequently observed in this tumor type. Furthermore, due to the genetic discordance often described between primary tumors and metastasis and to the recent molecular characterization of circulating tumor cells, we wonder whether, in cancer treatment, we are maybe missing the right target.

Targeted therapies affecting specific molecular target, expressed preferentially by neoplastic cells, block cancer growth. Current targets are represented by cell-surface trans-membrane proteins, intracellular proteins, and by growth factors. Today a targeted therapy exists for most commonly diagnosed types of human cancer often combined with chemotherapy or sometimes as monotherapy option. The epidermal growth factor receptors (EGFR) and vascular endothelial growth factors (VEGF) are known as the two main control key intracellular pathways, governing fundamental processes in cancer cells. The concept of using anti-EGFR and anti-VEGF strategies, as cancer treatment, has been recently developed and exploited extensively. We review targeted drugs currently available for routine treatment of lung, breast, colorectal and renal cell cancers, summarizing the history of identification and molecular characterization of targets or signaling pathways responsible for abnormal cell growth. We also focus on new targeted strategies, still under investigation, able to affect simultaneously tightly interconnected biological pathways or directed against new molecular targets.

Over the last ten years, several new and therapeutically relevant cancer drugs targeting tyrosine kinase signaling pathways have been developed. Tyrosine kinase inhibitors (TKIs) are a pharmaceutical class of small molecules, orally available, well-tolerated, worldwide approved drugs for the treatment of several neoplasms, including lung, breast, kidney and pancreatic cancer as well as gastro-intestinal stromal tumors and chronic myeloid leukemia. This comprehensive review focuses on the most relevant members of the first and the second generation TKIs designed to interact with receptor and nonreceptor TKs. Attention is mainly focused on molecular mechanisms in in vitro and in vivo models related to the clinical activity of the drugs and to the development of resistance to treatment, still the major challenge in cancer research and care.

Mammalian target of rapamycin (mTOR) is a key protein kinase controlling signal transduction from various growth factors and upstream proteins to the level of mRNA translation and ribosome biogenesis, with pivotal regulatory effects on cell cycle progression, cellular proliferation and growth, autophagy and angiogenesis. The mTOR pathway, and its upstream regulators in the PI3K/PTEN/AKT cascade, are altered in a variety of experimental and human malignancies. This has led to the prediction that mTOR inhibitors may be used as anticancer agents. With the recent approval of two mTOR-targeted drugs (temsirolimus and everolimus) for the treatment of renal cell carcinoma and mantle cell lymphoma, this paradigm has been effectively translated into the clinical setting. In this review, we discuss mTOR biology and regulation, the mode of action of mTOR inhibitors as anti-cancer agents, and current clinical evidence supporting the use of rapamycin-like mTOR inhibitors in cancer treatment.

The cancer stem cell (CSC) hypothesis provides an attractive model of tumor development and progression, holding that solid tumors are hierarchically organized and sustained by a minority of the tumor cell population with stem cell properties, such as self-renewal, tumorigenicity and multilineage differentiation capacity. Therapeutic resistance, underlying tumor recurrence and the lack of curative treatments in metastatic disease, raise the question if conventional anticancer therapies target the right cells. Indeed, these treatments might miss CSCs, which represent a more chemoresistant and radioresistant subpopulation within cancer. Recently, a direct link between the epithelial-mesenchymal transition process and the gain of stem cell competence were demonstrated in cultured breast cells. In particular, it was shown that the induction of EMT program not only allows cancer cells to disseminate from the primary tumor, but also promotes their self-renewal capability. Furthermore, the expression of stemness and EMT markers in CTCs were associated with resistance to conventional anti-cancer therapies and treatment failure, highlighting the urgency of improving tools for detecting and eliminating minimal residual disease.

What clinical oncologists learned about the metastatic process, is that it is the main cause of cancer-related deaths. What scientists learned about the metastatic disease, is that it is due to a highly selective process, which involves a minority of tumor cells that are able to survive within the bloodstream, and to initiate a new growth in distant sites. These cells “in transit” are known as circulating tumor cells (CTCs). Although their nature is not fully understood, what is widely accepted, is that they are drug resistant, and that their presence may represent the main reason for treatment failure. Despite this body of evidence, the pharmacological approach against cancer, with both chemotherapic and biological drugs, is still targeted on the primary tumor, raising the question as to whether we are missing the target. Targeting circulating tumor cells, may represent a new promising approach to individualize anticancer therapy.

Approximately 70% of breast cancers express the estrogen receptor (ER) and endocrine therapy is the most important component of systemic therapy for hormone-responsive breast cancer. Unfortunately, endocrine-resistant ER-positive disease represents up to one-quarter of all breast cancers and a number of different mechanisms have been implicated in endocrine resistance, either intrinsic, occurring de novo at the initial exposure to endocrine therapies or acquired, occurring after an initial response to therapy. In the present work a number of molecular mechanisms accounting for intrinsic and acquired resistance to hormonal therapies have been reviewed and the most promising strategies to overcome endocrine resistance have been highlighted.

In 1991, Reverse Transcriptase-Polymerase Chain Reaction (RT-PCR) was introduced to assess the expression of Tyrosinase in the peripheral blood of melanoma patients, in order to identify the presence of Circulating Melanoma Cells. To date, hundreds of studies, some of which are reviewed here, were performed to assess the clinical value of tyrosinase expression alone, and/or, in addition to other molecular markers. Unfortunately no consensus on the utility of tyrosinase detection exists. In this paper, we underline the presence of too many variables that may interfere with the detection of circulating melanoma cells: from withdrawal and RNA extraction, to Reverse Transcriptase-Polymerase Chain Reaction and the assays used for the analysis of amplification products.

DNA methylation and histone acetylation are two most studied epigenetic markers. Aberrant methylation of gene promoter regions and histone tail lysine residue modification through acetylation and methylation play a key role in malignant disorders. Two DNA methyltransferase inhibitors, azacitidine and decitabine, have been licensed for clinical therapy for patients with myelodysplastic syndrome. New hypomethylating agents, zebularine and isothiocyanates, are in various stages of development for cancer therapy. In this review we summarize recent clinical developments on novel hypomethylating agents and new regimens from clinical trials for epigenetic therapy of cancer.

Purpose: The close association of lymphatic and blood vessels and their coordinated development in vivo suggest that there are parallel mechanisms regulating hemangiogenesis and lymphangiogenesis. Here, we hypothesize that inhibition of the Src tyrosine kinase, apart from anti-hemangiogenic effects, results in a suppression of lymphangiogenesis. Experimental design: The ability of the Src kinase inhibitor PP2 to block Src in isolated lymphatic endothelial cells (LECs) was analyzed by Western Blot. The effects of PP2 on LEC proliferation, migration, and sprouting were assessed by MTT, Boyden chamber, and spheroid assays, respectively. The level of VEGF-C secreted by L3.6pl pancreatic carcinoma cells was measured by ELISA. For in vivo assessment of lymphangiogenesis, Src kinase inhibitor AZM475271 was used in mouse corneal micropocket and lymphangioma models. Results: VEGF-C stimulation of isolated LECs led to an increased phosphorylation of Src kinase that was abrogated by PP2. Treatment with PP2 inhibited spheroid sprouting of LECs at even lower concentrations than suggested by the proliferation assay. Src inhibition significantly reduced the level of VEGF-C in L3.6pl supernatant. Treatment with PP2 also resulted in a significant reduction in the migratory activity of LECs. In vivo, Src inhibition reduced de novo formation of lymphangiomas and corneal neovascularization. Conclusions: Inhibition of Src kinase shows strong anti-lymphangiogenic activity in vitro and in vivo. Together with antiangiogenic effects mediated by Src inhibition, this strategy may be attractive in the treatment of lymphatic and hematogeneous metastasis of cancer.