Current Pharmaceutical Design - Volume 19, Issue 5, 2013

Receptor tyrosine kinases play important roles in animal development and their deregulation has been linked to several pathologies, including cancer or diabetes. In cancer, the ERBB/HER family of receptors has been shown to participate in the pathophysiology of breast, gastric, colorectal, lung and head and neck tumors. Activation of HER receptors occurs by receptor-receptor interactions facilitated by ligand binding, overexpression or molecular alterations of the HER receptors. The best example is the known role of HER2 in the tumorigenesis of a proportion of breast tumors. In this review, we will describe the biological bases that govern HER receptor activation, and this will represent the bases for the explanation of how to target HER receptors in cancer. We will discuss the current therapeutic options to target HER receptors, which are based on anti-receptor antibodies or small molecule kinase inhibitors. We will also describe current clinical applications and future developments of agents which target HER receptors. Finally, we will mention mechanism of resistance to anti-HER therapies, and will describe options to overcome such resistances.

Conventional chemotherapeutic regimens have reached an efficacy plateau against most solid tumors and deal with significant toxicity. Recently, the goal of oncologic research to improve outcome and reduce treatment-related side-effects has led to the development of novel anticancer treatments targeting specific proteins or genes involved in cancer growth and progression. In particular, the tyrosine- kinase inhibitors (TKIs) gefitinib and erlotinib targeting the epidermal growth factor receptor (EGFR) have been approved for the treatment of non-small-cell lung cancer (NSCLC). Their clinical activity has been related to different clinical and biological parameters, such as the presence of activating mutations in the kinase domain of the target. Disappointingly, their clinical efficacy is limited by the development of resistance which is caused in more than 50% of the cases by the emergence of a secondary point-mutation (T790M) in the ATP-binding cleft of EGFR. Several novel EGFR inhibitors, able to covalently bind the target and prolong its inactivation, have been developed with the aim to overcome such resistance and are evaluated in ongoing clinical studies. However, not all clinical outcomes, including tolerability, are explained, and the identification/validation of novel biomarkers of sensitivity or resistance to such agents is a viable area of research to improve their clinical use. This review summarizes the current knowledge on the functional role of activating mutations of EGFR, pivotal primary/acquired resistance mechanisms as well as clinical data of small molecule EGFR-TKIs, and discusses the future of such therapeutic approach in NSCLC.

Background. We evaluated the mutation status of c-Met in small cell lung cancer (SCLC) and neuroendocrine tumors (NET), for which relatively limited therapeutic targets have been explored. Materials and Methods. c-Met was re-sequenced using cell lines and clinical samples. For in vitro studies, DNA constructs containing a juxtamembrane domain (JMD) and tyrosine kinase domain (TKD) were generated. Detected mutations were introduced into the construct and effects on c-Met phosphorylation and interaction with tyrosine kinase inhibitor drugs BMS777607 and SU11274 were assessed. Results. 97 specimens were analyzed: 13 SCLC and 2 pulmonary carcinoid cell lines, 46 SCLC and 36 NET clinical specimens. Mutations were only detected in the JMD. No mutations were detected in the TKD. Found mutations consisted of the previously reported R988C and T1010I mutations. One novel JMD mutation, P996S, was detected in a SCLC specimen. The mutation rate in SCLC cell lines was 25% (31% including a derivative cell line), and 6.5% in clinical specimens. The mutation rate in NET was 8.3%. In vitro, there were no differences between wild type, R988C or T1010I mutants regarding c-Met phosphorylation at Y1003, located in the JMD, and at Y1234/1235, located in the TKD. BMS777607 and SU11274 were shown to inhibit phosphorylation of c-Met in wild type and R988C and T1010I mutants in a similar fashion. Conclusions. In SCLC and neuroendocrine tumors MET mutations are relatively rare. Detected mutations were located in the juxtamembrane domain and were of no functional relevance as they did not influence c-Met phosphorylation, regardless of TKI treatment.

Colorectal cancer is one of the most common cancers worldwide and one of the leading causes of cancer-related death in the Western world. Tumor progression towards metastasis affects a large number of patients with colorectal cancer and seriously affects their clinical outcome. Therefore, considerable effort has been made towards the development of therapeutic strategies that can decrease or prevent colorectal cancer metastasis. Standard treatment of metastatic colorectal cancer with chemotherapy has been improved in the last 10 years by the addition of new targeted agents. The currently used antibodies bevacizumab, cetuximab and panitumumab target the VEGF and EGFR signaling pathways, which are crucial for tumor progression and metastasis. These antibodies have shown relevant efficacy in both first- and second-line treatment of metastatic colorectal cancer. Additionally, other signaling pathways, including the Wnt and HGF/Met pathways, have a well-established role in colorectal cancer progression and metastasis and constitute, therefore, promising targets for new therapeutic approaches. Several new drugs targeting these pathways, including different antibodies and small-molecule tyrosine kinase inhibitors, are currently being developed and tested in clinical trials. In this review, we summarize the new developments in this field, focusing on the inhibitors that show more promising results for use in colorectal cancer patients.

The REarranged during Transfection (RET) proto-oncogene and its activated signalling pathways have been shown to play an important role in cancer. RET genetic alterations including germline, somatic mutations and gene rearrangements have been demonstrated in several solid tumours, and numerous clinical trials using multikinase inhibitors containing RET as a target have shown significant activity against RET. Sorafenib and sunitinib have been approved for the treatment of renal, hepatocellular, gastrointestinal and pancreatic neuoendocrine carcinomas. Vandetanib has recently been approved for the treatment of unresectable locally advanced or metastatic medullary thyroid carcinomas. Novel genomic rearrangements and RET signalling interactions are now being studied in a variety of tumours and will provide the basis for new therapeutic strategies. Combination or sequential targeted therapies that are based on solid preclinical data regarding the inhibition of RET-mediated parallel or different -signalling pathways will likely be more effective.

Mitogen-activated protein kinases (MAPK) are involved in a complex network which regulates a variety of cellular processes including proliferation, survival and death. The molecular characterization of the pathway has shown aberrant activation in several human tumors, due to the deregulation of receptor tyrosine kinases or to mutations of pathway components. Progress in understanding the MAPK network has led to the development of target-specific agents in clinical trials. The relevance of MAPK in response and resistance to antitumor agents has been recognized, although the outcome of MAPK activation can vary depending on the molecular background of tumor cells and on the type of activated kinase. The canonical cascade of MAPK, i.e., depending on the Extracellular Signal-Regulated Kinases (ERK), can act in protective signalling pathways, thereby limiting DNA damage. Since tumor cell survival can be sustained by ERK and cross talk of ERK with other pathways, modulation of sensitivity to antitumor agents by targeting the ERK cascade appears to be an amenable approach. Indeed, ERK play a role in resistance to both cytotoxic and target-specific agents. Preclinical studies support the relevance of drug combination approaches to enhance the efficacy of antitumor treatments. Combinations of pharmacological inhibitors of the ERK cascade and conventional or target-specific antitumor agents may be helpful in an attempt to overcome drug resistance. A deeper understanding of the genetic alterations of tumor cells and of tumor heterogeneity as well as of drug resistance mechanisms is expected to contribute to the rational design of MAPK-mediated drug combinations that will lead to reversal of drug resistance.

The PI3K/Akt/mTOR signaling pathway plays a key role in diverse physiologic processes. It is also central to many aspects of the malignant process. Genetic phenomena that lead to constitutive pathway activation are common in human cancer; the most relevant are mutations affecting the catalytic subunit of PI3K and loss of function of the PTEN tumor suppressor. These factors have made this important cascade attractive as a potential target for cancer therapeutics. A host of inhibitors are now in various stages of development that target key nodes within the PI3K pathway. To date, however, the efficacy of these agents has fallen short of expectation, with at least one possible explanation being the presence of feedback loops and cross-talk that exists within and between PI3K and other signaling pathways. Accordingly, enthusiasm is again high as strategies employing therapeutic combinations are gaining pace, with encouraging results documented in both preclinical studies and emerging clinical trials. Here, we review the agents that have reached evaluation in early phase clinical studies of human subjects with cancer, and discuss the rationale for and use of novel drug combinations.

Despite their individual key roles in promoting cancer progression and treatment resistance, our knowledge about the impact of tumor hypoxia on the activity of the epidermal growth factor receptor (EGFR) pathway in cancer and vice versa remains limited. Preclinical and clinical studies support an important link between hypoxia and upregulation of EGFR in cancers that do not display genetic alterations of the receptor. Subsequent EGFR signaling stimulates hypoxia-inducible factor (HIF) signaling and thus augments induction of proteins that promote cellular survival in a hostile microenvironment. Considering the effects of EGFR-targeting agents under reduced oxygen conditions, it is now accepted that, together with their demonstrated antiproliferative and proapoptotic effects, the antiangiogenic activity of these drugs also contributes to their overall antitumor activity in vivo. Treatment of human tumor cells with EGFR inhibitors leads to decreased HIF-1α and VEGF secretion by tumor cells, resulting in vascular normalization, improved blood flow and thus improved oxygenation. These findings may have major implications with respect to the efficacy of both radiotherapy and subsequent chemotherapy when combined with EGFR inhibitors. A major challenge remains to assess which sequence of these drugs with radiation or chemotherapy is optimal. Moreover, recent data suggest that the lack of clinical responses to EGFR-directed therapy may be circumvented by supplementation of the anti-EGFR therapy with additional approaches targeting HIF-1α or VEGF. Further studies thus are warranted to define the precise mechanistic and therapeutic implications of the hypoxic response relative to the EGFR signaling pathway in cancer.

Single-agent therapy with molecularly targeted agents has shown limited success in tumor growth control, mainly because escape or resistance mechanisms are activated once a signalling molecule is inhibited. Rational combinations of target-specific agents could counteract this response providing a useful strategy in cancer treatment. In this regard, the EGFR and mTOR inhibitors have been used together to generate a synergistic effect and maximize the efficacy of each individual agent. Overall, the in vivo and in vitro evidences support the utilization of combinations targeting EGFR and mTOR, for malignancies characterized by deregulated EGFR/PI3K/Akt/ mTOR signalling cascade; whereas the clinical experience points out that the assessment of the therapeutic value of such combination awaits further investigations.

Combination of drugs with different targets is a logical approach to overcome multilevel cross-stimulation among key pathways in NSCLC progression such as EGFR, K-Ras and VEGFR. The sorafenib-erlotinib combination showed clinical activity and acceptable safety. Therefore, we evaluated mechanisms underlying sorafenib-erlotinib interaction in seven NSCLC cell lines selected for their heterogeneous pattern of EGFR and Raf-kinase-inhibitor protein (RKIP) expression, and EGFR/K-Ras mutations. Pharmacologic interaction was studied using MTT/SRB assays and the combination index (CI) method, while effects on EGFR, Erk1/2 and Akt phosphorylation, cell cycle and apoptosis were studied with western-blot, ELISA, and flow cytometry. Intracellular drug concentrations were measured with LC-MS/MS, whereas kinase activity profiles were generated on tyrosine kinase peptide substrate arrays. Synergism was detected in all cell lines, with CIs<0.6 in K-Ras mutated A549, SW1573 and H460, as well as in H1975 (EGFR-T790M) cells. Sorafenib slowed cell cycle progression and induced apoptosis, which was significantly increased in the combination. Moreover, sorafenib reduced Akt/ERK phosphorylation in erlotinib-resistant cells, associated with significant RKIP up-regulation. No direct drug interaction was detected by LC-MS/MS measurement, while lysates from A549 and H1975 cells exposed to erlotinib+sorafenib showed a significant inhibition in the phosphorylation of 16 overlapping peptides, including sites from RAF, VEGFR2, PDGFR, CDK2 and SRC, suggesting new markers to identify NSCLC patients who are likely to respond to this treatment. In conclusion, several mechanisms, including apoptosis-induction, modulation of expression/phosphorylation of RKIP and crucial kinases contribute to erlotinib-sorafenib synergistic interaction and should be evaluated in future trials for the rational development of this combination in NSCLC.

Pancreatic-ductal-adenocarcinoma (PDAC) is amongst the most lethal malignancies, mainly because of its metastatic spread and multifactorial chemoresistance. Since c-Met is a marker of pancreatic-cancer-stem-cells (CSC), playing a key role in metastasis and chemoresistance, this study evaluated the therapeutic potential of the novel c-Met/ALK inhibitor crizotinib against PDAC cells, including the Capan-1-gemcitabine-resistant cells (Capan-1-R). Crizotinib inhibited PDAC cell-growth with IC50 of 1.5 μM in Capan-1-R, and synergistically enhanced the antiproliferative and proapoptotic activity of gemcitabine, as detected by sulforhodamine-B-assay, flow cytometry and combination-index method. Capan-1-R had higher expression of the CSC markers CD44+/CD133+/CD326+, but their combined expression was significantly reduced by crizotinib, as detected by quantitative-RT-PCR and FACS-analysis. Similarly, Capan-1-R cells had significantly higher protein-expression of c-Met (≈2-fold), and increased migratory activity, which was reduced by crizotinib (e.g., >50% reduction of cell-migration in Capan-1-R after 8-hour exposure, compared to untreated-cells), in association with reduced vimentin expression. Capan-1-R had also significantly higher mRNA expression of the gemcitabine catabolism-enzyme CDA, potentially explaining the higher CDA activity and statistically significant lower levels of gemcitabine-nucleotides in Capan-1-R compared to Capan-1, as detected by Liquid-chromatography-massspectrometry. Conversely, crizotinib significantly reduced CDA expression in both Capan-1 and Capan-1-R cells. In aggregate, these data show the ability of crizotinib to specifically target CSC-like-subpopulations, interfere with cell-proliferation, induce apoptosis, reduce migration and synergistically interact with gemcitabine, supporting further studies on this novel therapeutic approach for PDAC.

Glioblastoma multiforme (GMB) is the most malignant and common type of all astrocytic tumors. Current standard of care entails maximum surgical resection of the tumor, followed by radiotherapy and chemotherapy, usually by the alkylating agent Temozolomide (TMZ). Despite this aggressive combination therapy, the survival rate of GBM patients is still low. Deregulation of the phosphatidylinositol 3-kinase (PI3K) / Akt pathway is a frequent occurrence in GBM. Activation of the PI3K-Akt pathway results in disturbance of control of cell growth and cell survival, which contributes to a competitive growth advantage, metastatic competence as well as to therapy resistance. The PI3K-Akt pathway is therefore an attractive therapeutic target in GBM, because it serves as a convergence point for malignant processes and intervention might overcome resistance to chemotherapy and radiation. The present review shows the importance of Akt in GBM and its role in the DNA damage response. Furthermore, an overview is given of specific inhibitors of Akt which are currently being tested in preclinical and in early phase clinical studies.

Irinotecan is a major drug for treatment of metastatic colorectal cancer and a promising agent for other applications like gastric cancer. Its clinical activity is currently limited by both intrinsic (natural) and acquired drug resistance. A better understanding of the underlying resistance mechanisms is needed to develop novel therapeutic strategies. Exposure of tumor cells to irinotecan or its active metabolite SN-38 is accompanied by EGFR activation, either by stimulation of EGFR autophosphorylation or by Src-mediated phosphorylation. Accordingly, combinations of irinotecan and EGFR inhibitors have been associated with supra-additive activity. We now show that acquired resistance to SN-38 is accompanied by increased expression of EGFR, HER2, HER3 and Src proteins in two colorectal cancer cell models as well as by Src activation. One SN-38 resistant model (HT-29) showed increased sensitivity to erlotinib, an EGFR inhibitor, and afatinib, a dual EGFR/HER2 inhibitor, while the other SN-38 resistant model (HCT-116) showed increased resistance to erlotinib but unchanged or increased sensitivity to afatinib. Unexpectedly, both models showed increased or unaltered resistance to the Src inhibitor dasatinib. Therefore, tyrosine kinase upregulation is not necessarily accompanied by increased sensitivity to targeted agents. Taken together, our findings demonstrate that prolonged exposure to topoisomerase I inhibitors is accompanied by upregulation of different signal transduction pathways which can alter tumor sensitivity to molecular targeted agents. These results suggest that chemotherapy exposure may lead to creation of novel targets which could be exploited therapeutically.