Current Drug Targets - Volume 15, Issue 14, 2014

Development of molecular targeting agents, starting with imatinib for chronic myeloid leukemia or gefitinib for non-small cell lung cancer (NSCLC), has recently progressed at a rapid rate. Epidermal growth factor receptor tyrosine kinase inhibitors (EGFR-TKIs) have already been developed to the 2nd and 3rd generation, and novel drug development targeted towards Met activation, which is an EGFR-TKI resistance mechanism, is ongoing. Although the era of new anticancer agents is moving towards an era of molecular targeting agents, the methods used for drug development are not different than before. In addition to the importance of pharmacokinetics (PK) and pharmacodynamics (PD) for drug development, emerging evidence is also demonstrating the significance of pharmacogenomics, since certain types of gene alteration may greatly affect drug metabolism, excretion, and notably, clinical efficacy. It is desirable to determine optimal doses of anticancer drugs by taking into account these factors that could potentially influence PK/PD. The following article reviews the clinical development of EGFR/Met inhibitors for NSCLC and the clinical pharmacology of these drugs.

Epidermal Growth Factor Receptor (EGFR) tyrosine kinase inhibitors (TKIs) have changed the paradigm of treatment in non-small cell lung cancer (NSCLC). The molecular biology study of EGFR has led to clinical trials that select patients more accurately, regarding the presence of EGFR activating mutations. Nonetheless, a lack of response or a temporary condition of the response has been detected in patients on EGFR TKIs. This has urged to study potential resistance mechanisms underneath. The most important ones are the presence of secondary mutations in EGFR, such as T790M, or the overexpression of mesenchymal-epithelial transition factor (MET) that may explain why patients who initially respond to EGFR TKIs, may ultimately become refractory. Several approaches have been taken and new drugs both targeting EGFR resistance-mutation or MET are currently being developed. Here we review and update the EGFR biological pathway as well as the clinical data leading to approval of the EGFR TKIs currently in the market. New compounds under investigation targeting resistance mutations or dually targeting EGFR and other relevant receptors are also reviewed and discussed.

The advent of the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) represented the most important innovation in NSCLC treatment over the last years. However, despite a great initial activity, secondary mutations in the same target, or different alterations in other molecular pathways, inevitably occur, leading to the emergence of acquired resistance, in median within the first year of treatment. In this scenario, the mesenchymal–epidermal transition (cMET) tyrosine kinase receptor and its natural ligand, the hepatocyte growth factor (HGF), seem to play an important role. Indeed either the overexpression or the amplification of cMET, as well as the overexpression of the HGF, have been reported in a substantial subgroup of NSCLC patients resistant to EGFR-TKIs. Several cMET-inhibitors have been developed as potential therapeutic candidates, and are currently under investigation in clinical trials. These compounds include both monoclonal antibodies and TKIs, and most of them have been investigated as dual combinations including an anti-EGFR TKI, to improve the efficacy of the available treatments, and ultimately overcome acquired resistance to EGFR-inhibitors.

Pancreatic Ductal Adenocarcinoma (PDAC) is among the most lethal solid tumors with grim prognosis. This dismal outcome can partially be explained by the resistance to currently available chemotherapy regimens or the failure of most anticancer agents, which prompted the development of new and effective therapeutic-approaches, such as inhibitors of the epidermal growth factor receptor (EGFR). Some of these EGFR inhibitors (e.g., erlotinib) are approved for lungcancer, however available data are inconclusive for treatment of pancreatic cancer patients with EGFR-targeted-therapies. Here we describe the critical role of EGFR pathway in pancreatic-cancer, strategies to enhance the effectiveness of EGFRinhibitors as well as the preclinical/clinical studies with particular emphasis on recent findings with monoclonal antibodies and tyrosine-inhibitors. Several combinations of EGFR inhibitors with other agents illustrate inhibition of tumor-induced angiogenesis and cell growth. Moreover, combination of erlotinib with gemcitabine showed statistically significance in overall-survival, compared to gemcitabine-alone. However high cost, little survival gain and increased risk of toxicities have limited its efficacy. Considering the multiple genetic mutations and the crosstalk of signaling pathways, (1) development of multiple targeted-therapies; (2) identification of predictive-biomarkers; and (3) those patients who are most likely benefit from therapy, could provide valuable direction for the clinical development of EGFR inhibitors. Moreover further preclinical/clinical studies are warranted to identify determinants of the activity of EGFR-inhibitors and mechanisms leading to resistance to EGFR inhibitors, through the analysis of genetic and environmental alterations affecting EGFR and parallel pro-cancer pathways. These studies will be critical to improve the efficacy and selectivity of current anticancer strategies targeting EGFR in pancreatic cancer.

Aberrant activation of receptor-tyrosine kinase c-Met/HGF pathway is shown to be associated with cell proliferation, invasion, metastasis and poor-prognosis in several tumor types, including upper gastrointestinal-malignancies. The interaction of c-Met with multiple signalling-pathways involved in tumorigenic-properties and invasive-phenotype has gained substantial attention, suggesting its role as an intriguing-target for cancer-therapy. In recent years, there have been considerable efforts in the development of effective c-Met inhibitors with potential clinical-applications and one of them, crizotinib (dual c-Met/ALK inhibitor), has recently been approved for lung-cancers with ALKrearrangement. However several important questions remain to be answered on the molecular mechanisms underlying the antitumor effects of crizotinib, as well as on its possible role in the treatment of different tumor types, including uppergastrointestinal- cancers. The aim of this review is to give an overview on critical role of the c-Met/HGF pathway in cancer, and the preclinical/clinical studies on c-Met inhibitors. There are accumulating evidences on therapeutic potential of c-Met inhibitors for the treatment of other malignancies, such as gastric and pancreatic cancers. However, further investigations are needed to identify determinants of the activity of c-Met inhibitors, through the analysis of genetic/ environmental alterations affecting c-Met and parallel pro-cancer pathways; mechanisms result in developing resistance to anti-c-Met agents; and selection of patients that might benefit from therapy. These studies will be essential to improve the selectivity/efficacy of future anticancer strategies of c-Met targeted-therapies in the treatment of upper gastrointestinal- cancers.

The active metabolite (JM118) of the oral platinum analog satraplatin (JM216) was investigated for potential synergism with erlotinib, an epidermal growth factor receptor (EGFR) inhibitor. JM118 sensitivity of 7 cancer cell lines (ovarian: 2008, A2780; colon: Lovo92, WiDr; lung: A549, SW1573; epidermoid: A431), was enhanced most pronounced when JM118 preceded erlotinib, which was associated with increased formation of DNA-platinum adducts. The combination increased G2/M phase accumulation and enhanced apoptosis. JM118 increased the phosphorylation of the cell cycle proteins CDK2 and CHK1 after 24 hr exposure. JM118/erlotinib enhanced Erk and Akt phosphorylation after 2 hr. JM118 significantly decreased the phosphorylation of PTEN, VEGFR, EPHA1, ERBB4, FGF-R, andSTAT3 by 20 (PTEN) to >90% (STAT3). Conclusion: Erlotinib enhanced the effects of JM118, even in cells with mutations in Ras. The mechanism of synergy involved a combination of effects on platinum-DNA adduct formation, cell cycle distribution and signaling.

While multidrug resistance (MDR) in cancer is well established, little is known about the cellular pathways regulating the expression and trafficking of the MDR efflux transporter like BCRP (ABCG2). Here we evaluated the role of signalling downstream of EGFR on BCRP expression and sub-cellular localization using lung cancer cells harboring BCRP but expressing various EGFR and Ras activating mutations; A549 (K-Ras-G12S), H292 wild-type EGFR and Ras, and H1650 (EGFR-DelE747-A750). Immunocytochemistry and immunofluorescence studies demonstrated that BCRP was predominantly intracellular but its expression was found also on the plasma membrane in A549 and H1650 cells with activated Ras and EGFR. Remarkably, EGFR inhibition by erlotinib at IC50 concentrations induced a differential timedependent alteration in BCRP gene and protein expression. In H1650 cells, erlotinib enhanced both the total and plasma membrane degradation of BCRP by ubiquitination within 6-24 hours, whereas BCRP expression regained the original basal levels after 48 hours. In erlotinib treated H292 cells, BCRP levels decreased at 24 hours until 72 hours, whereas in A549 cells erlotinib initially reduced BCRP expression but then induced its accumulation on the plasma membrane at 72 hours. We further found that the PI3K/Akt inhibitor LY294002 down-regulated BCRP expression, hence showing that the Akt pathway is involved in the regulation of BCRP expression but not in its localization in these lung cancer cell lines. Finally, the selective BCRP transport inhibitor Ko143 did not increase erlotinib sensitivity, but did decrease the transport activity of BCRP in A549 and H1650 cells as it induced the accumulation of its transport substrate topotecan. In conclusion, our results suggest that the EGFR and Akt pathways are involved in regulation of BCRP expression, trafficking and drug transport activity. These findings warrant future studies on the pharmacologic modulation of these pathways to enhance the efficacy of anticancer combinations of erlotinib with drugs that are BCRP transport substrates.

Malignant pleural mesothelioma (MPM) is a lethal disease with scarce therapeutic options, and preclinical studies on new targeted-agents are warranted. Because previous studies reported high c-Met expression and alterations in the microtubules network in most MPM samples, we evaluated the activity of tivantinib, which has been recently suggested to affect microtubule polymerization in addition to inhibiting c-Met. In four MPM cell lines tivantinib inhibited both c-Met activity and microtubule polymerization, resulting in inhibition of cell-growth with IC50s ranging between 0.3 µM (MSTO-211H) and 2.4 µM (H2052). Furthermore tivantinib synergistically enhanced the antiproliferative and proapoptotic activity of pemetrexed, as detected by sulforhodamine-B-assay and flow cytometry. The synergistic interaction was associated with reduction of thymidylate synthase expression and inhibition of migratory activity. In aggregate, these data show the ability of tivantinib to specifically target key pathways in MPM cells and synergistically interact with pemetrexed, supporting further studies on this therapeutic approach.

Signal transducer and activator of transcription 3 (STAT3) is activated in many cancer types and can regulate pathways involving tumorigenesis, cell proliferation, cell survival and angiogenesis. Upstream cytokine signaling through multiple trans-membrane receptors can enhance the activation of STAT3 and promote tumor progression. Importantly, STAT3 activation can also be induced via the Janus-activated kinase 1/2 (JAK1/2) and Src family kinases. Target-specific drug therapies have been developed to inhibit many of the upstream receptor and non-receptor activators of STAT3 and are now approved for clinical use. Recently, resistance to standard-of-care therapies has been linked to constitutive or unabated STAT3 activation, suggesting that combination therapy with STAT3 inhibitors may be of clinical benefit. Furthermore, STAT3 activity has also been shown to regulate self-renewal of cancer stem cells that are often refractory to chemotherapy treatment. This review will focus on STAT3 mediated resistance to cancer therapy and discuss strategies to overcome this resistance.