Introduction: The RAS/RAF/MAP kinase–ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) (MAPK) and the PI3K/AKT/mammalian target of rapamycin (mTOR) (PI3K) pathways are frequently deregulated in human cancer as a result of genetic alterations in their components or upstream activation of cell-surface receptors. These signalling cascades are regulated by complex feedback and cross-talk mechanisms. Areas covered: In this review the key components of the MAPK and AKT pathways and their molecular alterations are described. The complex interactions between these signalling cascades are also analysed. Expert opinion: The observation that the MAPK and the PI3K pathways are often deregulated in human cancer makes the components of these signalling cascades interesting targets for therapeutic intervention. Recently, the presence of compensatory loops that activate one pathway following the blockade of the other signalling cascade has been demonstrated. Therefore, the blockade of both pathways with combinations of signalling inhibitors might result in a more efficient anti-tumor effect as compared with a single agent. In addition, the MAPK and PI3K pathways are activated by mutations that coexist or can be mutually exclusive. In this regard, a large-scale characterization of the cancer genome might offer personalized cancer genomic information, which may improve the anti-tumor efficacy of signalling inhibitors.

The ErbB receptors and their cognate ligands that belong to the epidermal growth factor (EGF) family of peptides are involved in the pathogenesis of different types of carcinomas. In fact, the ErbB receptors and the EGF-like growth factors are frequently expressed in human tumors. These proteins form a complex system that regulates the proliferation and the survival of cancer cells. Therefore, ErbB receptors and their ligands might represent suitable targets for novel therapeutic approaches in human carcinomas. In this regard, different target-based agents that are directed against the ErbB receptors have been developed in the past two decades. One of these compounds, the humanized anti-ErbB-2 monoclonal antibody trastuzumab has been approved for the treatment of patients with metastatic breast cancer. The anti-EGF receptor (EGFR) antibody C225, as well as EGFR tyrosine kinase inhibitors ZD1839 and OSI-774 are currently in phase III clinical development. Several other ErbB tyrosine kinase inhibitors are in phase I/II studies. These compounds have generally been shown to have an acceptable toxicity profile and promising anti-tumor activity in heavily pretreated patients. The mechanisms of action of these compounds, as well as the potential therapeutic strategies to improve their efficacy are discussed in this review with particular regard to the combinations of anti-ErbB agents with cytotoxic drugs, or combinations of different ErbB-targeting agents.

Tamoxifen has been the mainstay of hormonal therapy in both early and advanced breast cancer patients for approximately three decades. The availability of novel compounds such as aromatase inhibitors (AIs) and fulvestrant, with different mechanism of action, is changing the scenario of endocrine treatment of postmenopausal breast cancer patients. In this review article, we have summarized the current knowledge of the mechanisms of resistance to endocrine therapy, in order to derive information that might be useful for therapeutic intervention. We propose that resistance to endocrine therapy is a progressive, step-wise phenomenon induced by the selective pressure of hormonal agents, which leads breast cancer cells from an estrogen-dependent, responsive to endocrine manipulation phenotype to a non-responsive phenotype, and eventually to an estrogen-independent phenotype. In particular, evidence suggests for each ‘action’ introduced to block estrogen stimulation of breast cancer cells (i.e. treatment with anti-estrogen), there are one or more corresponding ‘reactions’ that tumor cells can use to escape our attempts to block their growth: estrogen hypersensitivity associated with increased transcriptional activity of estrogen receptor α (ERα) and/or increased non-genomic activity of ERα, estrogen supersensitivity, increased growth factor signaling, suppression of ERα expression and finally estrogen independence. Activation of growth factor signaling is involved in each step of this phenomenon, and might ultimately substitute estrogen in sustaining the growth and the survival of breast cancer cells. In this respect, results of pre-clinical and clinical studies with AIs, fulvestrant and signaling inhibitors sustain this hypothesis. More importantly, the knowledge of the mechanisms involved in the resistance of breast cancer cells to endocrine therapy offers potential for novel therapeutic strategies.