Epidermal growth factor receptor tyrosine kinase inhibitors: current status and future perspectives in the development of novel irreversible inhibitors for the treatment of mutant non-small cell lung cancer

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.

Combined use of anti-ErbB monoclonal antibodies and erlotinib enhances antibody-dependent cellular cytotoxicity of wild-type erlotinib-sensitive NSCLC cell lines

Background

The epidermal growth factor receptor (EGFR) is an established target for anti-cancer treatment in different tumour types. Two different strategies have been explored to inhibit this pivotal molecule in epithelial cancer development: small molecules TKIs and monoclonal antibodies. ErbB/HER-targeting by monoclonal antibodies such as cetuximab and trastuzumab or tyrosine-kinase inhibitors as gefitinib or erlotinib has been proven effective in the treatment of advanced NSCLC.

Results

In this study we explored the potential of combining either erlotinib with cetuximab or trastuzumab to improve the efficacy of EGFR targeted therapy in EGFR wild-type NSCLC cell lines. Erlotinib treatment was observed to increase EGFR and/or HER2 expression at the plasma membrane level only in NSCLC cell lines sensitive to the drug inducing protein stabilization. The combined treatment had marginal effect on cell proliferation but markedly increased antibody-dependent, NK mediated, cytotoxicity in vitro. Moreover, in the Calu-3 xenograft model, the combination significantly inhibited tumour growth when compared with erlotinib and cetuximab alone.

Conclusion

Our results indicate that erlotinib increases surface expression of EGFR and/or HER2 only in EGFR-TKI sensitive NSCLC cell lines and, in turns, leads to increased susceptibility to ADCC both in vitro and in a xenograft models. The combination of erlotinib with monoclonal antibodies represents a potential strategy to improve the treatment of wild-type EGFR NSCLC patients sensitive to erlotinib.

Metabolism of the EGFR tyrosin kinase inhibitor gefitinib by cytochrome P450 1A1 enzyme in EGFR-wild type non small cell lung cancer cell lines

Background

Gefitinib is a tyrosine kinase inhibitor (TKI) of the epidermal growth factor receptor (EGFR) especially effective in tumors with activating EGFR gene mutations while EGFR wild-type non small cell lung cancer (NSCLC) patients at present do not benefit from this treatment.

The primary site of gefitinib metabolism is the liver, nevertheless tumor cell metabolism can significantly affect treatment effectiveness.

Results

In this study, we investigated the intracellular metabolism of gefitinib in a panel of EGFR wild-type gefitinib-sensitive and -resistant NSCLC cell lines, assessing the role of cytochrome P450 1A1 (CYP1A1) inhibition on gefitinib efficacy. Our results indicate that there is a significant difference in drug metabolism between gefitinib-sensitive and -resistant cell lines. Unexpectedly, only sensitive cells metabolized gefitinib, producing metabolites which were detected both inside and outside the cells. As a consequence of gefitinib metabolism, the intracellular level of gefitinib was markedly reduced after 12-24 h of treatment. Consistent with this observation, RT-PCR analysis and EROD assay showed that mRNA and activity of CYP1A1 were present at significant levels and were induced by gefitinib only in sensitive cells. Gefitinib metabolism was elevated in crowded cells, stimulated by exposure to cigarette smoke extract and prevented by hypoxic condition. It is worth noting that the metabolism of gefitinib in the sensitive cells is a consequence and not the cause of drug responsiveness, indeed treatment with a CYP1A1 inhibitor increased the efficacy of the drug because it prevented the fall in intracellular gefitinib level and significantly enhanced the inhibition of EGFR autophosphorylation, MAPK and PI3K/AKT/mTOR signalling pathways and cell proliferation.

Conclusion

Our findings suggest that gefitinib metabolism in lung cancer cells, elicited by CYP1A1 activity, might represent an early assessment of gefitinib responsiveness in NSCLC cells lacking activating mutations. On the other hand, in metabolizing cells, the inhibition of CYP1A1 might lead to increased local exposure to the active drug and thus increase gefitinib potency.

Synergistic activity of letrozole and sorafenib on breast cancer cells

Estrogens induce breast tumor cell proliferation by directly regulating gene expression via the estrogen receptor (ER) transcriptional activity and by affecting growth factor signaling pathways such as mitogen-activated protein kinase (MAPK) and AKT/mammalian target of rapamycin Complex1 (mTORC1) cascades. In this study we demonstrated the preclinical therapeutic efficacy of combining the aromatase inhibitor letrozole with the multi-kinase inhibitor sorafenib in aromatase-expressing breast cancer cell lines. Treatment with letrozole reduced testosterone-driven cell proliferation, by inhibiting the synthesis of estrogens. Sorafenib inhibited cell proliferation in a concentration-dependent manner; this effect was not dependent on sorafenib-mediated inhibition of Raf1, but involved the down-regulation of mTORC1 and its targets p70S6K and 4E-binding protein 1 (4E-BP1). At concentrations of 5-10 μM the growth-inhibitory effect of sorafenib was associated with the induction of apoptosis, as indicated by release of cytochrome c and Apoptosis-Inducing Factor into the cytosol, activation of caspase-9 and caspase-7, and PARP-1 cleavage. Combination of letrozole and sorafenib produced a synergistic inhibition of cell proliferation associated with an enhanced accumulation of cells in the G(0)/G(1) phase of the cell cycle and with a down-regulation of the cell cycle regulatory proteins c-myc, cyclin D1, and phospho-Rb. In addition, longer experiments (12 weeks) demonstrated that sorafenib may be effective in preventing the acquisition of resistance towards letrozole. Together, these results indicate that combination of letrozole and sorafenib might constitute a promising approach to the treatment of hormone-dependent breast cancer.

Everolimus restores gefitinib sensitivity in resistant non-small cell lung cancer cell lines

The epidermal growth factor receptor (EGFR) is a validated target for therapy in non-small cell lung cancer (NSCLC). Most patients, however, either do not benefit or develop resistance to specific inhibitors of the EGFR tyrosine kinase activity, such as gefitinib or erlotinib. The mammalian target of rapamycin (mTOR) is a key intracellular kinase integrating proliferation and survival pathways and has been associated with resistance to EGFR tyrosine kinase inhibitors. In this study, we assessed the effects of combining the mTOR inhibitor everolimus (RAD001) with gefitinib on a panel of NSCLC cell lines characterized by gefitinib resistance and able to maintain S6K phosphorylation after gefitinib treatment. Everolimus plus gefitinib induced a significant decrease in the activation of MAPK and mTOR signaling pathways downstream of EGFR and resulted in a growth-inhibitory effect rather than in an enhancement of cell death. A synergistic effect was observed in those cell lines characterized by high proliferative index and low doubling time. These data suggest that treatment with everolimus and gefitinib might be of value in the treatment of selected NSCLC patients that exhibit high tumor proliferative activity.

Molecular damage and induction of proinflammatory cytokines in human endothelial cells exposed to Shiga toxin 1, Shiga toxin 2, and alpha-sarcin

Treatment of human endothelial cells with Shiga toxin 1 and 2 leads to the upregulation of genes encoding proinflammatory molecules involved in the pathogenesis of hemolytic-uremic syndrome. The paradoxical effect of inhibitors of mRNA translation, such as Shiga toxins, that at the same time induce protein expression was investigated by studying the relationship between their enzymatic activity (abstraction of adenine from nucleic acids) and the induction of interleukin-8 and granulocyte-macrophage colony-stimulating factor in human endothelial cells. As a positive control, the fungal toxin alpha-sarcin, acting on the same rRNA sequence targeted by Shiga toxins with a different mechanism (RNase activity), was used. The three toxins caused ribosomal lesions that, in turn, induced the activation of p38 stress kinase with kinetics that paralleled the inhibition of translation. Alpha-sarcin was devoid of activity on DNA. Shiga toxin 2 targeted nuclear DNA with more rapid kinetics than did Shiga toxin 1. Since the fungal ribotoxin was fully effective in the induction of proinflammatory proteins, we conclude that damage to ribosomes is indispensable and sufficient to activate protein expression via induction of the stress-kinase cascade. However, gene upregulation events induced by Shiga toxin 2 were much more efficient than those triggered by Shiga toxin 1, although the two toxins impaired translation to the same extent and had overlapping time courses of stress kinase activation. Regulations independent of the ribotoxic stress were assumed to operate in intoxicated cells. We hypothesized that the two bacterial toxins recognize different DNA sequences inducing different regulating effects on gene expression.

Effect of inducible FHIT and p53 expression in the Calu-1 lung cancer cell line

Loss of FHIT expression and p53 mutations are critical events in the early stages of lung carcinogenesis. The restoration of Fhit function in FHIT-negative cancer cells has been reported to cause tumour suppression by inhibition of cell proliferation and/or activation of apoptotic pathways. However, the studies designed to elucidate the biological role of Fhit and its potential interaction with p53 have produced conflicting results. We investigated here the effects of the simultaneous restoration of FHIT and p53 in Calu-1 cells by using a hormone-inducible gene expression system. We demonstrate that the restoration of FHIT expression reinforces the anti-proliferative effect associated with the simultaneous replacement of p53. Indeed, a more pronounced inhibition of cell proliferation associated with an earlier and higher induction of p21(waf1) mRNA and protein expression was observed in Fhit/p53-expressing cells compared with cells expressing p53 alone. This effect was not due to Fhit-mediated up-regulation of p53 expression; in fact p53 protein was expressed at the same level in both FHIT-positive and FHIT-negative cell clones. Consistent with this result, Fhit did not affect the expression of MDM2, a protein known to interact directly with p53 and target p53 for proteolytic degradation, thus down-regulating its activity.

Hyperosmotic stress response: comparison with other cellular stresses

Cellular responses induced by stress are essential for the survival of cells under adverse conditions. These responses, resulting in cell adaptation to the stress, are accomplished by a variety of processes at the molecular level. After an alteration in homeostatic conditions, intracellular signalling processes link the sensing mechanism to adaptive or compensatory changes in gene expression. The ability of cells to adapt to hyperosmotic stress involves early responses in which ions move across cell membranes and late responses characterized by increased synthesis of either membrane transporters essential for uptake of organic osmolytes or of enzymes involved in their synthesis. The goal of these responses is to return the cell to its normal size and maintain cellular homeostasis. The enhanced synthesis of molecular chaperones, such as heat shock proteins, is another important component of the adaptive process that contributes to cell survival. Some responses are common to different stresses, whereas others are specific. In the first part of the review, we illustrate the characteristic and specific features of adaptive response to hypertonicity; we then describe similarities to and differences from other cellular stresses, such as genotoxic agents, nutrient starvation and heat shock.

5-Benzylidene-hydantoins as new EGFR inhibitors with antiproliferative activity

A series of 1,5-disubstituted hydantoins, whose structure was designed to interact at the ATP-binding site of EGFR, was synthesized and evaluated for inhibition of EGFR kinase activity and antiproliferative action. Some of these compounds, characterized by a 1-phenethyl and a 5-(E)-benzylidene substituent, inhibited EGFR autophosphorylation and polyGAT phosphorylation, and also inhibited the growth and proliferation of human A431 cells, which overexpress EGFR. These compounds can therefore be regarded as examples of a new scaffold for tyrosine kinase inhibitors.

Creatine as a compatible osmolyte in muscle cells exposed to hypertonic stress

Exposure of C2C12 muscle cells to hypertonic stress induced an increase in cell content of creatine transporter mRNA and of creatine transport activity, which peaked after about 24 h incubation at 0.45 osmol (kg H(2)O)(-1). This induction of transport activity was prevented by addition of either cycloheximide, to inhibit protein synthesis, or of actinomycin D, to inhibit RNA synthesis. Creatine uptake by these cells is largely Na(+) dependent and kinetic analysis revealed that its increase under hypertonic conditions resulted from an increase in V(max) of the Na(+)-dependent component, with no significant change in the K(m) value of about 75 mumol l(-1). Quantitative real-time PCR revealed a more than threefold increase in the expression of creatine transporter mRNA in cells exposed to hypertonicity. Creatine supplementation significantly enhanced survival of C2C12 cells incubated under hypertonic conditions and its effect was similar to that obtained with the well known compatible osmolytes, betaine, taurine and myo-inositol. This effect seemed not to be linked to the energy status of the C2C12 cells because hypertonic incubation caused a decrease in their ATP content, with or without the addition of creatine at 20 mmol l(-1) to the medium. This induction of creatine transport activity by hypertonicity is not confined to muscle cells: a similar induction was shown in porcine endothelial cells.

Inhibition by suramin of protein synthesis in vitro. Ribosomes as the target of the drug

Suramin, a drug widely used both as a therapeutic agent and in research, inhibits translation in eukaryotic cell-free systems from rabbit reticulocyte lysate (IC(50)=142-241 microM). Suramin affects both initiation (block of 43S pre-initiation complex formation) and elongation (impairment of poly(U) translation). The drug induces an increase in the pools of ribosomal subunits and the formation of high molecular weight ribosomal complexes, thus causing the disappearance of polysomes. Ribosomes isolated from suramin-treated translating mixtures are inactivated. [(3)H]Suramin binds to ribosomes and to isolated 60S and 40S ribosomal subunits (116, 106 and 3 binding sites, respectively) showing higher affinity for the small subunit (K(d)=2 microM).

Increased Levels of Inducible HSP70 in Cells Exposed to Electromagnetic Fields

Because reports in the literature on the effects of electromagnetic fields (EMFs) on expression of the 70-kDa heat-shock protein (HSP70) are somewhat contradictory, we studied the influence of low-frequency EMFs on the accumulation of inducible HSP70 in several cell models. Some of the cell types tested showed increased levels of HSP70 protein when exposed for 24 h to 50 Hz, 680 microT EMFs. In endothelial cells, EMFs alone induced only a poor and transient activation of the heat-shock transcription factor 1 (HSF1); however, neither the level of HSP70 mRNA nor the synthesis of HSP70 appeared to be altered significantly. Accordingly, transfection experiments involving HSP70 promoter showed that gene transcription was not affected. We also noted a marked reduction in proteasome activities in cell extracts exposed to EMFs. Interestingly, the heat-shock-induced levels of HSP70 mRNA and protein were increased by a concomitant weak stressor like EMFs. Taken together, our results indicate that in EMF-exposed endothelial cells, HSP70 gene transcription and translation are unaffected; however, EMFs alone promoted accumulation of the inducible HSP70 protein, probably by increasing its stability, and it enhanced accumulation and translation of the heat-induced HSP70 mRNA when applied in concert with heat shock.

Shiga toxin 1 and ricin inhibit the repair of H2O2-induced DNA single strand breaks in cultured mammalian cells

A growing body of evidence suggests that ribosome-inactivating proteins (RIPs) remove adenine moieties not only from rRNA, but also from DNA--an effect leading to DNA damage in cultured cells. We herein report that two distinct RIPs of bacterial (shiga toxin 1, Stx1) and plant (ricin) origin, inhibit the repair of the DNA lesions generated by hydrogen peroxide in cultured human cells. This effect is unrelated either to inhibition of protein synthesis or to depletion of cellular antioxidant defenses and is likely to derive from direct interactions with cellular DNA repair machinery. Therefore, the genotoxicity of these toxins on mammalian cells seems to be a complex phenomenon resulting from the balance between direct (DNA damaging activity), indirect (DNA repair inhibition) effects and the eventual presence of other DNA damaging species. In particular, with regard to Stx1, it could be hypothesized that Stx-producing bacteria increase the risk of transformation of surrounding, inflamed tissues in the course of human infections.

Dose-dependent effect of FHIT-inducible expression in Calu-1 lung cancer cell line

Abnormalities in the expression of the tumour suppressor fragile histidine triad (FHIT) gene have been reported in a variety of human tumours, including lung cancer and restoration of its expression in cancer cell lines resulted in the inhibition of proliferation and apoptosis induction. Most of the studies that have assigned a proapoptotic role to the FHIT gene were performed in adenoviral-FHIT-transduced cancer cells expressing high levels of the Fhit protein. The present work was the first study designed to investigate the effects of FHIT gene replacement in a human FHIT-negative non-small-cell lung cancer (NSCLC) cell line (Calu-1) by using a hormone-inducible expression system that allows tight modulation of the transgene expression. Through this approach, we demonstrated that a prolonged induction was required to accumulate the Fhit protein at levels adequate to promote a significant decrease of cell proliferation. Analysis of cell-cycle phase distribution showed an accumulation of cells in the G0/G1 phase and a concomitant decrease in the S phase. Moreover, an upregulation of p21waf1 transcript was found, which could account for the alteration of the cycling properties of the cells. The growth-inhibitory effects observed were not associated with apoptosis appearance, and although in these conditions the Fhit protein content was higher than in normal bronchial human epithelial cells (NHBE), it was still significantly lower than the level capable of inducing apoptosis in Calu-1 cells after adenoviral-mediated FHIT gene transfer. These results indicate that the tumour suppressor properties of Fhit are strictly related to its expression level and show that the Fhit protein has a dose-dependent antiproliferative effect on the Fhit-negative Calu-1 lung cancer cell line.