Differential Effects of Tyrosine Kinase Inhibitors on Normal and Oncogenic EGFR Signaling and Downstream Effectors

The role of CBL family ubiquitin ligases in cancer progression and therapeutic strategies

The CBL (Casitas B-lineage lymphoma) family, as a class of ubiquitin ligases, can regulate signal transduction and activate receptor tyrosine kinases through various tyrosine kinase-dependent pathways. There are three members of the family: c-CBL, CBL-b, and CBL-c. Numerous studies have demonstrated the important role of CBL in various cellular pathways, particularly those involved in the occurrence and progression of cancer, hematopoietic development, and regulation of T cell receptors. Therefore, the purpose of this review is to comprehensively summarize the function and regulatory role of CBL family proteins in different human tumors, as well as the progress of drug research targeting CBL family, so as to provide a broader clinical measurement strategy for the treatment of tumors.

A review on the role of epidermal growth factor signaling in the development, progression and treatment of cervical cancer

The sub-committee constituted by the Indian Council of Medical Research (ICMR) for the management of cervical cancer (CC) detailed in the consensus document (2016) reported CC as a significant cause of morbidity and mortality in women. The incidence of an increase in CC and associated mortality in women is a major cause of cancer. To date, human papilloma viral (HPV) infection accounts for more than 99% of CC. However, there are individuals infected with HPV do not develop CC. There is a greater correlation between HPV infection and upregulation of the epidermal growth factor receptor (EGFR) signaling cascade during the initiation, sustenance, and progression of CC. Therefore, EGFR is often targeted to treat CC using tyrosine kinase inhibitors (TKIs) and monoclonal antibodies (mAB). The current review analyzed the existing clinical/pre-clinical studies and the significance of EGFR abundance using the Kaplan-Meier (KM) survival plot analysis for disease-free survival (DFS) and overall survival (OS). We performed a series of bioinformatics analyses to screen the crucial role of the EGFR gene in CC. Further, different transcription factors that are dysregulated due to EGFR abundance and their relevance were determined using computational tools in this review. Endogenous microRNAs (miRNA) that undergo changes due to alterations in EGFR during CC were identified using computational database and consolidated the information obtained with the published in the area of miRNA and EGFR with special reference to the initiation, sustenance and progression of CC. The current review aims to consolidate contemporary approaches for targeting CC using EGFR and highlight the current role of miRNA and genes that are differently regulated during CC involving EGFR mutations. Potential resistance to the available EGFR therapies such as TKIs and mABs and the need for better therapies are also extensively reviewed for the development of newer therapeutic molecules with better efficacy.

Cxcl10 chemokine induces migration of ING4-deficient breast cancer cells via a novel crosstalk mechanism between the Cxcr3 and Egfr receptors

The chemokine Cxcl10 has been associated with poor prognosis in breast cancer, but the mechanism is not well understood. Our previous study have shown that CXCL10 was repressed by the ING4 tumor suppressor, suggesting a potential inverse functional relationship. We thus investigated a role for Cxcl10 in the context of ING4 deficiencies in breast cancer. We first analyzed public gene expression datasets and found that patients with CXCL10-high/ING4-low expressing tumors had significantly reduced disease-free survival in breast cancer. In vitro, Cxcl10 induced migration of ING4-deleted breast cancer cells, but not of ING4-intact cells. Using inhibitors, we found that Cxcl10-induced migration of ING4-deleted cells required Cxcr3, Egfr, and the Gβγ subunits downstream of Cxcr3, but not Gαi. Immunofluorescent imaging showed that Cxcl10 induced early transient colocalization between Cxcr3 and Egfr in both ING4-intact and ING4-deleted cells, which recurred only in ING4-deleted cells. A peptide agent that binds to the internal juxtamembrane domain of Egfr inhibited Cxcr3/Egfr colocalization and cell migration. Taken together, these results presented a novel mechanism of Cxcl10 that elicits migration of ING4-deleted cells, in part by inducing a physical or proximal association between Cxcr3 and Egfr and signaling downstream via Gβγ. These results further indicated that ING4 plays a critical role in the regulation of Cxcl10 signaling that enables breast cancer progression.

Small Molecule Kinase Inhibitor Drugs (1995-2021): Medical Indication, Pharmacology, and Synthesis

The central role of dysregulated kinase activity in the etiology of progressive disorders, including cancer, has fostered incremental efforts on drug discovery programs over the past 40 years. As a result, kinase inhibitors are today one of the most important classes of drugs. The FDA approved 73 small molecule kinase inhibitor drugs until September 2021, and additional inhibitors were approved by other regulatory agencies during that time. To complement the published literature on clinical kinase inhibitors, we have prepared a review that recaps this large data set into an accessible format for the medicinal chemistry community. Along with the therapeutic and pharmacological properties of each kinase inhibitor approved across the world until 2020, we provide the synthesis routes originally used during the discovery phase, many of which were only available in patent applications. In the last section, we also provide an update on kinase inhibitor drugs approved in 2021.

Canine parvovirus induces G1/S cell cycle arrest that involves EGFR Tyr1086 phosphorylation

Canine parvovirus (CPV) has been used in cancer control as a drug delivery vehicle or anti-tumor reagent due to its multiple natural advantages. However, potential host cell cycle arrest induced by virus infection may impose a big challenge to CPV associated cancer control as it could prevent host cancer cells from undergoing cell lysis and foster them regain viability once the virotherapy was ceased. To explore CPV-induced cell cycle arrest and the underlying mechanism toward improved virotherapeutic design, we focus on epidermal growth factor receptor (EGFR), a cellular receptor interacting with TfR that mediates CPV-host interactions, and alterations on its tyrosine phosphorylation sites in response to CPV infection. We found that CPV could trigger host G1/S cell cycle arrest via the EGFR (Y1086)/p27 and EGFR (Y1068)/STAT3/cyclin D1 axes, and EGFR inhibitor could not reverse this process. Our results contribute to our understandings on the mechanism of CPV-induced host cellular response and can be used in the onco-therapeutic design utilizing CPV by preventing host cancer cells from entering cell cycle arrest.

Assessing the activation/inhibition of tyrosine kinase-related pathways with a newly developed platform

The phosphorylation of cellular proteins plays a crucial role in the transduction of various signals from outside the cell into the nucleus. The signals are transduced by phosphorylation chain reactions within multiple pathways; however, determining which pathways are responsible for each defined signal has proven challenging. To estimate the activity of each pathway, we developed a phosphorylation array platform comprising a protein array with 1200 proteins belonging to 376 signalling pathways and an analytical method to estimate pathway activity based on the phosphorylation levels of proteins. The performance of our system was assessed by reconstructing kinase-substrate relationships, as well as by estimating pathway activity upon epidermal growth factor (EGF) stimulation and the pharmacological inhibition of epidermal growth factor receptor (EGFR). As a result, kinase-substrate relationships were reliably reconstructed based on the precise measurement of phosphorylation levels of constituent proteins on the array. Furthermore, the pathway activities associated with EGF stimulation and EGFR inhibition were successfully traced through the related pathways from the outer membrane to the nucleus along a time course. Thus, our phosphorylation array system can effectively assess the activity of specific signalling pathways that are perturbed by extracellular stimuli, such as various drugs.

Nanodelivery Systems Targeting Epidermal Growth Factor Receptors for Glioma Management

A paradigm shift in treating the most aggressive and malignant form of glioma is continuously evolving; however, these strategies do not provide a better life and survival index. Currently, neurosurgical debulking, radiotherapy, and chemotherapy are the treatment options available for glioma, but these are non-specific in action. Patients invariably develop resistance to these therapies, leading to recurrence and death. Receptor Tyrosine Kinases (RTKs) are among the most common cell surface proteins in glioma and play a significant role in malignant progression; thus, these are currently being explored as therapeutic targets. RTKs belong to the family of cell surface receptors that are activated by ligands which in turn activates two major downstream signaling pathways via Rapidly Accelerating Sarcoma/mitogen activated protein kinase/extracellular-signal-regulated kinase (Ras/MAPK/ERK) and phosphatidylinositol 3-kinase/a serine/threonine protein kinase/mammalian target of rapamycin (PI3K/AKT/mTOR). These pathways are critically involved in regulating cell proliferation, invasion, metabolism, autophagy, and apoptosis. Dysregulation in these pathways results in uncontrolled glioma cell proliferation, invasion, angiogenesis, and cancer progression. Thus, RTK pathways are considered a potential target in glioma management. This review summarizes the possible risk factors involved in the growth of glioblastoma (GBM). The role of RTKs inhibitors (TKIs) and the intracellular signaling pathways involved, small molecules under clinical trials, and the updates were discussed. We have also compiled information on the outcomes from the various endothelial growth factor receptor (EGFR)-TKIs-based nanoformulations from the preclinical and clinical points of view. Aided by an extensive literature search, we propose the challenges and potential opportunities for future research on EGFR-TKIs-based nanodelivery systems.

PLCγ1‑dependent invasion and migration of cells expressing NSCLC‑associated EGFR mutants

The increased tyrosine kinase activity of non‑small cell lung cancer (NSCLC)‑associated epidermal growth factor receptor (EGFR) mutants results in deregulated pathways that contribute to malignant cell survival, tumor progression and metastasis. Previous studies investigating lung cancer‑associated EGFR have focused on the prognostic implications of receptor kinase mutations in patients with NSCLC; however, the role of EGFR mutations in tumor cell invasion and migration remains undetermined. The present study was designed to investigate the role of NSCLC‑associated mutant EGFR‑driven signaling pathways in cell proliferation and invasion. Non‑endogenous EGFR‑expressing 293 cells stably expressing EGFR mutants that are sensitive or resistant to Food and Drug Administration (FDA)‑approved EGFR‑targeted tyrosine kinase inhibitors (TKIs) were used in the present study. The experiments demonstrated an increased phosphorylation of phospholipase (PLC)γ1, c‑Cbl, signal transducer and activator of transcription (Stat), extracellular regulated kinase (Erk)1/2, Akt, Shc and Gab1 proteins in cells expressing a mutant form, rather than the wild‑type receptor. As PLCγ1 is a known regulator of metastatic development, mutant receptor‑mediated PLCγ1 activation was further evaluated. To examine the effects of EGFR and PLCγ1 phosphorylation, the metastatic potential of cells expressing mutants was investigated using wound healing, Transwell cell migration and invasion assays. The inhibition of receptor phosphorylation with the 1st, 2nd and 3rd generation TKIs, gefitinib, afatinib, osimertinib, respectively, reduced PLCγ1 phosphorylation, and reduced the invasive and migratory potential of 293 cells, confirming PLCγ1 as one of the probable downstream effectors of mutant EGFR signaling. However, the PLC inhibitor, U73122, inhibited cell migration and invasion without affecting EGFR signaling and PLCγ1 phosphorylation. Notably, U73122 reduced Akt and Erk1/2 phosphorylation within 25 min of its application; however, 100% cell viability was recorded even after 48 h. Upon further investigation, proliferative signaling pathways remained active at 48 h, in accordance with cell viability. Therefore, the present study concludes that mutant receptor‑mediated PLCγ1 activation may play a significant role in the migration and invasion of NSCLC tumors; however, its regulatory role in tumor cell proliferation warrants further investigation and validation in lung tumor cell lines harboring EGFR mutations.

Annexin A6 improves anti-migratory and anti-invasive properties of tyrosine kinase inhibitors in EGFR overexpressing human squamous epithelial cells

Annexin A6 (AnxA6), a member of the calcium (Ca²⁺ ) and membrane binding annexins, is known to stabilize and establish the formation of multifactorial signaling complexes. At the plasma membrane, AnxA6 is a scaffold for protein kinase Cα (PKCα) and GTPase-activating protein p120GAP to promote downregulation of epidermal growth factor receptor (EGFR) and Ras/mitogen-activated protein kinase (MAPK) signaling. In human squamous A431 epithelial carcinoma cells, which overexpress EGFR, but lack endogenous AnxA6, restoration of AnxA6 expression (A431-A6) promotes PKCα-mediated threonine 654 (T654)-EGFR phosphorylation, which inhibits EGFR tyrosine kinase activity. This is associated with reduced A431-A6 cell growth, but also decreased migration and invasion in wound healing, matrigel, and organotypic matrices. Here, we show that A431-A6 cells display reduced EGFR activity in vivo, with xenograft analysis identifying increased pT654-EGFR levels, but reduced tyrosine EGFR phosphorylation compared to controls. In contrast, PKCα depletion in A431-A6 tumors is associated with strongly reduced pT654 EGFR levels, yet increased EGFR tyrosine phosphorylation and MAPK activity. Moreover, tyrosine kinase inhibitors (TKIs; gefitinib, erlotinib) more effectively inhibit cell viability, clonogenic growth, and wound healing of A431-A6 cells compared to controls. Likewise, the ability of AnxA6 to inhibit A431 motility and invasiveness strongly improves TKI efficacy in matrigel invasion assays. This correlates with a greatly reduced invasion of the surrounding matrix of TKI-treated A431-A6 when cultured in 3D spheroids. Altogether, these findings implicate that elevated AnxA6 scaffold levels contribute to improve TKI-mediated inhibition of growth and migration, but also invasive properties in EGFR overexpressing human squamous epithelial carcinoma.

Peroxiredoxin V (PrdxV) negatively regulates EGFR/Stat3-mediated fibrogenesis via a Cys48-dependent interaction between PrdxV and Stat3

Activation of the epidermal growth factor receptor (EGFR)/signal transducer and activator of transcription 3 (Stat3) signaling pathway has been reported to be associated with renal fibrosis. We have recently demonstrated that peroxiredoxin V (PrdxV) acted as an antifibrotic effector by inhibiting the activity of Stat3 in TGF-β-treated NRK49F cells. However, the underlying mechanism of PrdxV remains poorly understood. To investigate molecular mechanism of PrdxV, we used a transgenic mouse model expressing PrdxV siRNA (PrdxVsi mice) and performed unilateral ureteral obstruction (UUO) for 7 days. 209/MDCT cells were transiently transfected with HA-tagged WT PrdxV and C48S PrdxV. Transgenic PrdxVsi mice displayed an exacerbated epithelial-to-mesenchymal transition (EMT) as well as an increase in oxidative stress induced by UUO. In the UUO kidney of the PrdxVsi mouse, knockdown of PrdxV increased Tyr1068-specific EGFR and Stat3 phosphorylation, whereas overexpression of WT PrdxV in 209/MDCT cells showed the opposite results. Immunoprecipitation revealed the specific interaction between WT PrdxV and Stat3 in the absence or presence of TGF-β stimulation, whereas no PrdxV-EGFR or C48S PrdxV-Stat3 interactions were detected under any conditions. In conclusion, PrdxV is an antifibrotic effector that sustains renal physiology. Direct interaction between PrdxV and Stat3 through Cys48 is a major molecular mechanism.

Gelatin nonwovens-based epithelial morphogenesis involves a signaling axis comprising EGF-receptor, MAP kinases ERK 1/2, and β1 integrin

In biomaterials research, biomechanics which support tissue regeneration steadily gains of importance. Hence, we have previously shown that gelatin-based electrospun nonwoven mats (NWMs) with a distinct modulus of elasticity (3.2 kPa) promotes epithelial morphogenesis. Since molecular mechanisms of this morphogenesis are still unknown, the present study aims at identifying molecules, involved herein. Epithelia established on the NMWs showed persistence of the activated state of the epidermal growth factor receptor (EGF-R), phosphorylated at the src-specific tyrosine 845 (EGF-R_T845 ) throughout the observation period of 10 days. To elucidate whether the observed morphogenesis mechanistically involves EGF-R signaling, we inhibited EGF-R, by employing the EGF-R_T845 specific inhibitor Gefitinib (IRESSA®). Gefitinib administration yielded a reduced expression of the β1 integrin subunit, a well-known cell-matrix interaction receptor, concomitant with downregulation of p42/44 ERK1/2 MAP-kinase activity. To elucidate whether the observed downregulation of β1 is EGF-R_T845 -dependent or emerging from ERK1/2 signaling, we exposed epithelia, grown on the NWMs, with the ERK1/2-directed inhibitor U0126. In the absence of Gefitinib, inhibition of p42/44 MAP-kinase activity resulted in decreased β1 integrin protein levels, thus indicating that β1 expression is dependent on ERK1/2 and not EGF-R_T845 . Our results showed the first time that an EGF-R-β1 integrin-signaling axis, including ERK1/2, promotes NWM-elasticity-based epithelial morphogenesis. © 2018 Wiley Periodicals, Inc. J Biomed Mater Res Part A: 107A: 663-677, 2019.

Protein functional effector sncRNAs (pfeRNAs) in lung cancer

PIWI-interacting RNA Likes (piR-Ls) were recently reported to regulate functions of their target phospho-Proteins (p-Proteins) in somatic lung cells. However, the mechanism underlying this functionality remains unclear. piR-Ls interact with their targets through direct binding but do not follow base-pairing rules, known to have important roles at levels of transcription, RNA processing and translation for small non-coding RNA (sncRNA). These observations imply a fundamentally different type of sncRNA with behavior that causes a molecular response in their target p-Proteins. Furthermore, the interaction of piR-Ls with their targets regulates the functional efficacy of target p-Proteins. In addition, except for writers (kinase) and erasers (phosphatase), the functional efficacy of p-Proteins on their readers still remains unknown. It is reasonable to consider the existence of protein functional effector sncRNAs (pfeRNAs), which were identified by deep sequencing the immunoprecipitation products of antibodies targeting phosphorylated residues in proteins, as well as by functional analysis. pfeRNAs harbor unique features in size distribution, 3' terminal modification, shared core sequences, and functional manner, and could be new players in lung physiological and pathological conditions.

Regulation of Ste20-like kinase, SLK, activity: Dimerization and activation segment phosphorylation

The Ste20-like kinase, SLK, has diverse cellular functions. SLK mediates organ development, cell cycle progression, cytoskeletal remodeling, cytokinesis, and cell survival. Expression and activity of SLK are enhanced in renal ischemia-reperfusion injury, and overexpression of SLK was shown to induce apoptosis in cultured glomerular epithelial cells (GECs) and renal tubular cells, as well as GEC/podocyte injury in vivo. The SLK protein consists of a N-terminal catalytic domain and an extensive C-terminal domain, which contains coiled-coils. The present study addresses the regulation of SLK activity. Controlled dimerization of the SLK catalytic domain enhanced autophosphorylation of SLK at T183 and S189, which are located in the activation segment. The full-length ectopically- and endogenously-expressed SLK was also autophosphorylated at T183 and S189. Using ezrin as a model SLK substrate (to address exogenous kinase activity), we demonstrate that dimerized SLK 1–373 or full-length SLK can effectively induce activation-specific phosphorylation of ezrin. Mutations in SLK, including T183A, S189A or T193A reduced T183 or S189 autophosphorylation, and showed a greater reduction in ezrin phosphorylation. Mutations in the coiled-coil region of full-length SLK that impair dimerization, in particular I848G, significantly reduced ezrin phosphorylation and tended to reduce autophosphorylation of SLK at T183. In experimental membranous nephropathy in rats, proteinuria and GEC/podocyte injury were associated with increased glomerular SLK activity and ezrin phosphorylation. In conclusion, dimerization via coiled-coils and phosphorylation of T183, S189 and T193 play key roles in the activation and signaling of SLK, and provide targets for novel therapeutic approaches.

Impact of epidermal growth factor receptor gene expression level on clinical outcomes in epidermal growth factor receptor mutant lung adenocarcinoma patients taking first-line epidermal growth factor receptor-tyrosine kinase inhibitors

Epidermal growth factor receptor (EGFR)-tyrosine kinase inhibitors (TKIs) are first-choice treatments for advanced non-small-cell lung cancer patients harboring EGFR mutations. Although EGFR mutations are strongly predictive of patients' outcomes and their response to treatment with EGFR-TKIs, early failure of first-line therapy with EGFR-TKIs in patients with EGFR mutations is not rare. Besides several clinical factors influencing EGFR-TKI efficacies studied earlier such as the Eastern Cooperative Oncology Group performance status or uncommon mutation, we would like to see whether semi-quantify EGFR mutation gene expression calculated by 2^-ΔΔct was a prognostic factor in EGFR-mutant non-small cell lung cancer patients receiving first-line EGFR-TKIs. This retrospective study reviews 926 lung cancer patients diagnosed from January 2011 to October 2013 at the Kaohsiung Chang Gung Memorial Hospital in Taiwan. Of 224 EGFR-mutant adenocarcinoma patients, 148 patients who had 2^-ΔΔct data were included. The best cutoff values of 2^-ΔΔct for in-frame deletions in exon 19 (19 deletion) and a position 858 substituted from leucine (L) to an arginine (R) in exon 21 (L858R) were determined using receiver operating characteristic curves. Patients were divided into high and low 2^-ΔΔct expression based on the above cutoff level. The best cutoff point of 2^-ΔΔct value of 19 deletion and L858R was 31.1 and 104.7, respectively. In all, 92 (62.1%) patients showed high 2^-ΔΔct expression and 56 patients (37.9%) low 2^-ΔΔct expression. The mean age was 65.6 years. Progression-free survival of 19 deletion mutant patients with low versus high expression level was 17.07 versus 12.04 months (P = 0.004), respectively. Progression-free survival of L858 mutant patients was 13.75 and 7.96 months (P = 0.008), respectively. EGFR-mutant lung adenocarcinoma patients with lower EGFR gene expression had longer progression-free survival duration without interfering overall survival.

A phosphorylation-wide sncRNA screen reveals Protein Functional Effector sncRNAs (pfeRNAs) in human lung somatic cells

We recently reported that PIWI-interacting RNAs likes (piR-Ls) could regulate functions of the interacting phosphorylated proteins (p-Proteins). In addition, except for writers and erasers, functional efficacy of p-Proteins on their readers still remains unknown. We, therefore, reasoned there was a type of sncRNAs which could regulate functional efficacy of p-Proteins. Here, we profiled sncRNAs interacting with phosphorylated -Ser, -Thr and -Tyr residues in 3 HBE and 4 lung SCC cell lines, investigated effects and mechanisms of phosphorylated-residue-interacting sncRNAs. Our results demonstrated sncRNAs regulating functional efficacy of p-Proteins and we thus referred them as Protein Functional Effector sncRNAs (pfeRNAs). pfeRNAs were distributed among 26 to 50 nucleotides, shared some core sequences and showed distinctive expression patterns between HBE and SCC cells. Core sequences 417 (CS417), showing consistent upregulation in all 4 SCC cells, bound directly to p-Nucleolin (NCL), which was dependent on the key elements CGCG of CS417 and p-Ser619 of NCL. The CS417/p-NCL interaction was critical for functional efficacy of p-NCL in basic activities of lung normal and cancer cells. Thus, we revealed a novel type of pfeRNAs controlling functional efficacy of p-Proteins in lung somatic cells.

Analysis of a single-codon E746 deletion in exon 19 of the epidermal growth factor receptor

PURPOSE

Epidermal growth factor receptor (EGFR) gene mutations are the most established genomic biomarkers for the efficacy of EGFR tyrosine kinase inhibitors (EGFR-TKIs). The most frequent deletion in exon 19 is delE746_750, followed by del747_753insS and del747_750insP. Since investigations of delE746 have not been reported previously, it is unclear if delE746 conveys sensitivity to TKI effect of TKI on EGFR delE746. The objective was to characterize delE746 of the EGFR gene and to explore the effects of TKIs on the delE746.

METHODS

We assessed the ability of gefitinib to inhibit phosphorylation of clonal L929 cell lines expressing EGFR with delE746. 3-D structures of the EGFR proteins were also used to investigate the interaction with gefitinib.

RESULTS

The delE746 mutant EGFR-expressing cells exhibited gefitinib-sensitive autophosphorylation, which altered the structure of the EGFR and increased the instances of docking during docking simulations of gefitinib with the EGFR-TK. This mutant revealed that it exhibited molecular conformation alterations, and more frequent binding with gefitinib compared to wild-type EGFR. We administered EGFR-TKI, gefitinib to a Japanese woman with lung cancer that contained delE746. The patient achieved partial response after a 5 month of treatment with gefitinib.

CONCLUSION

Our study revealed biological, structural, and probably clinical features of the delE746 form of EGFR.

Experience with erlotinib in the treatment of non-small cell lung cancer

Non-small cell lung cancer (NSCLC) remains the leading cause of cancer-related deaths. In the last decade, the epidermal growth factor receptor (EGFR) signalling pathway has emerged as one of the most important molecular aberrations, representing an attractive therapeutic target in NSCLC. Drugs interfering with the tyrosine kinase domain of the EGFR (EGFR TKIs), such as erlotinib and gefitinib, have demonstrated efficacy in patients with advanced NSCLC irrespective of therapy line and particularly in patients harbouring activating mutations in the EGFR gene (EGFR(mut+)). Results of large phase III randomized trials clearly established that EGFR TKIs are superior to chemotherapy as frontline treatment in patients with EGFR(mut+), whereas in the EGFR wild-type (EGFR(WT)) or EGFR unknown population, platinum-based chemotherapy remains the standard of care, with no consistent benefit produced by the addition of EGFR TKI. In pretreated NSCLC, EGFR TKIs are considered more effective than standard monotherapy with cytotoxics in the presence of classical EGFR mutations, whereas in the EGFR(WT) population, a similar efficacy to docetaxel or pemetrexed in terms of survival has been demonstrated. Unfortunately, patients who initially responded to EGFR TKIs invariably develop acquired resistance. For such patients there is an urgent need for more effective strategies able to delay or possibly overcome resistance. In the present review we analysed the available data on erlotinib in the treatment of advanced NSCLC.