EGFR tyrosine kinase inhibitors activate autophagy as a cytoprotective response in human lung cancer cells

Epidermal growth factor receptor tyrosine kinase inhibitors gefitinib and erlotinib have been widely used in patients with non-small-cell lung cancer. Unfortunately, the efficacy of EGFR-TKIs is limited because of natural and acquired resistance. As a novel cytoprotective mechanism for tumor cell to survive under unfavorable conditions, autophagy has been proposed to play a role in drug resistance of tumor cells. Whether autophagy can be activated by gefitinib or erlotinib and thereby impair the sensitivity of targeted therapy to lung cancer cells remains unknown. Here, we first report that gefitinib or erlotinib can induce a high level of autophagy, which was accompanied by the inhibition of the PI3K/Akt/mTOR signaling pathway. Moreover, cytotoxicity induced by gefitinib or erlotinib was greatly enhanced after autophagy inhibition by the pharmacological inhibitor chloroquine (CQ) and siRNAs targeting ATG5 and ATG7, the most important components for the formation of autophagosome. Interestingly, EGFR-TKIs can still induce cell autophagy even after EGFR expression was reduced by EGFR specific siRNAs. In conclusion, we found that autophagy can be activated by EGFR-TKIs in lung cancer cells and inhibition of autophagy augmented the growth inhibitory effect of EGFR-TKIs. Autophagy inhibition thus represents a promising approach to improve the efficacy of EGFR-TKIs in the treatment of patients with advanced non-small-cell lung cancer.     

Structural and mechanistic underpinnings of the differential drug sensitivity of EGFR mutations in non-small cell lung cancer

EGFR and other ErbB-family tyrosine kinases are overexpressed in many human tumors, and their aberrant expression and mutational activation is associated with the development, progression and aggressiveness of a number of malignancies. Thus the EGFR kinase has long been recognized as a potential drug target in oncology, and small-molecule inhibitors have been under development for more than two decades. As a result of their effectiveness in treating non-small cell lung cancers (NSCLCs) driven by somatic mutations in the EGFR kinase, gefitinib and erlotinib were the first EGFR tyrosine kinase inhibitors (TKIs) approved for clinical use. Ironically, these drugs found their target against mutant forms of the EGFR kinase, which have altered enzyme active sites, and not against the wild type (WT) kinase against which their potency and selectivity was carefully honed. Here we review recent structural and enzymological studies that explore the exquisite sensitivity of a subset of these lung cancer mutants to gefitinib and erlotinib. We discuss available structural evidence for the mechanisms of activation of the EGFR kinase by these mutants, and compare it to physiologic activation of the kinase by ligand-induced dimerization. Finally, we consider the mechanisms by which the secondary T790M “gatekeeper” mutation confers resistance to gefitinib and erlotinib.     

The Relative Expression of Mig6 and EGFR Is Associated with Resistance to EGFR Kinase Inhibitors

The sensitivity of only a few tumors to anti-epidermal growth factor receptor EGFR tyrosine kinase inhibitors (TKIs) can be explained by the presence of EGFR tyrosine kinase (TK) domain mutations. In addition, such mutations were rarely found in tumor types other than lung, such as pancreatic and head and neck cancer. In this study we sought to elucidate mechanisms of resistance to EGFR-targeted therapies in tumors that do not harbor TK sensitizing mutations in order to identify markers capable of guiding the decision to incorporate these drugs into chemotherapeutic regimens. Here we show that EGFR activity was markedly decreased during the evolution of resistance to the EGFR tyrosine kinase inhibitor (TKI) erlotinib, with a concomitant increase of mitogen-inducible gene 6 (Mig6), a negative regulator of EGFR through the upregulation of the PI3K-AKT pathway. EGFR activity, which was more accurately predicted by the ratio of Mig6/EGFR, highly correlated with erlotinib sensitivity in panels of cancer cell lines of different tissue origins. Blinded testing and analysis in a prospectively followed cohort of lung cancer patients treated with gefitinib alone demonstrated higher response rates and a marked increased in progression free survival for patients with a low Mig6/EGFR ratio (approximately 100 days, P = 0.01).     

Identification of EGFR kinase domain mutations among lung cancer patients in China： implication for targeted cancer therapy

Lung cancer is one of the leading causes of death with one of the lowest survival rates. However, a subset of lung cancer patients who are of Asian origin and carry somatic mutations in epidermal growth factor receptor or EGFR have responded remarkable well to two tyrosine kinase inhibitors, gefitinib and erlotinib. While EGFR mutation profiles havebeen reported from Japan, South Korea, and Taiwan, there is no such report from mainland of China where the largest pool of patients reside. In this report, we identified ten somatic mutations from a total of 41 lung cancer patients in China. Among them, seven mutations were found in 17 adenocarcinomas. In contrast to previous reports, eight of these mutations are deletions in exon 19 and two of these deletions are homozygous. These results suggest that a large portion of Chinese adenocarcinoma patients could benefit from gefitinib or erlotinib. This unique mutation profile provides a rationale to develop the next generation of EGFR inhibitors more suitable for the Chinese population.     

EGF receptor tyrosine kinase inhibitors diminish transforming growth factor-alpha-induced pulmonary fibrosis

Transforming growth factor-alpha (TGF-alpha) is a ligand for the EGF receptor (EGFR). EGFR activation is associated with fibroproliferative processes in human lung disease and animal models of pulmonary fibrosis. We determined the effects of EGFR tyrosine kinase inhibitors gefitinib (Iressa) and erlotinib (Tarceva) on the development and progression of TGF-alpha-induced pulmonary fibrosis. Using a doxycycline-regulatable transgenic mouse model of lung-specific TGF-alpha expression, we determined effects of treatment with gefitinib and erlotinib on changes in lung histology, total lung collagen, pulmonary mechanics, pulmonary hypertension, and expression of genes associated with synthesis of ECM and vascular remodeling. Induction in the lung of TGF-alpha caused progressive pulmonary fibrosis over an 8-wk period. Daily administration of gefitinib or erlotinib prevented development of fibrosis, reduced accumulation of total lung collagen, prevented weight loss, and prevented changes in pulmonary mechanics. Treatment of mice with gefitinib 4 wk after the induction of TGF-alpha prevented further increases in and partially reversed total collagen levels and changes in pulmonary mechanics and pulmonary hypertension. Increases in expression of genes associated with synthesis of ECM as well as decreases of genes associated with vascular remodeling were also prevented or partially reversed. Administration of gefitinib or erlotinib did not cause interstitial fibrosis or increases in lavage cell counts. Administration of small molecule EGFR tyrosine kinase inhibitors prevented further increases in and partially reversed pulmonary fibrosis induced directly by EGFR activation without inducing inflammatory cell influx or additional lung injury.     

Recent Development of the Second and Third Generation Irreversible Epidermal Growth Factor Receptor Inhibitors

Recent reports suggested that essential directions for new lung cancer, breast carcinoma therapies, as well as the roomier realm of targeted cancer therapies were provided through targeting the epidermal growth factor receptor (EGFR). Patients who carrying non‐small cell lung carcinoma (NSCLC) with activating mutations in EGFR initially respond well to the EGFR inhibitors erlotinib and gefitinib, which were located the active site of the EGFR kinase and designed to act as competitive inhibitors of combining with the ATP. However, patients who were treated with the erlotinib and gefitinib will relapse because of the emergence of drug‐resistant mutations, with T790M mutations accounting for approximately 60% of all resistance. In order to overcome drug resistance, Pharmaceutical chemistry experts recently devoted great endeavors to the development of second‐generation irreversible selective inhibitors which covalently modify Cys797 or Cys773 at the ATP binding cleft. Nevertheless, these inhibitors have not reached ideal effect of experts in patients with T790M positive mutation and apparently because of the dose‐limiting toxicities associated with inhibition of wild type EGFR. A novel class of ‘third generation’ EGFR TKIs have been developed that is sensitising and T790M mutant‐specific whilst sparing WT EGFR, representing a significant breakthrough in the treatment in NSCLC patients with acquired resistance harboring these genotypes. Herein, we provides an overview of the second and third generation inhibitors currently approved, in clinical trial and also encompasses novel structures of discovery. This review mainly focuses on drug resistance, their mechanisms of action, development of structure–activity relationships and binding modes.     

Mapping inhibitor response to the in-frame deletions,insertions and duplications of epidermal growth factor receptor (EGFR) in non-small cell lung cancer

Human epidermal growth factor receptor (EGFR) has become a well-established target for the treatment of non-small cell lung cancer (NSCLC). However, a large number of in-frame deletion, insertion and duplication mutations in the EGFR tyrosine kinase (TK) domain have been observed to alter drug response to such a kinase target. Thus, a systematic investigation of the intermolecular interactions between the clinical small-molecule agents and various EGFR in-frame mutants would help to establish a complete picture of drug response to kinase mutations in lung cancer, and to design new EGFR inhibitors with high potency and selectivity to target drug-resistant mutants. Here, we describe a combined pipeline to explore the drug response of five representative EGFR inhibitors, including three FDA-approved agents (gefitinib, erlotinib and lapatinib) and two compounds under clinical development (AEE788 and TAK-285) to a number of clinically relevant EGFR in-frame mutations, aiming at a comprehensive understanding of molecular mechanism and biological implication underlying drug resistance and sensitivity to EGFR in-frame mutations. It was found that the insertion and duplication mutations in exon 20 can generally cause drug resistance to EGFR due to the reduced size of kinase’s active pocket, while deletion mutations in exon 19 associate closely with increased inhibitor sensitivity to EGFR by establishing additional non-bonded interactions across complex interface, including hydrogen bonds, cation–π interactions and hydrophobic contacts.     

The efficacy of TKIs in treatment of human primary small cell lung cancer xenograft model in vivo

Objective: To study the treatmaient of non-small cell lung cancer, we established the HU-Prim allograft transplantation tumor model. Methods: The fresh tumor samples were transplanted in the right scapular subcutaneous layer of the severe combined immunodeficient Non-obese diabetic/severe combined immunodeficient(NOD/SCID) mice. The pathological features of the tumors were observed. Nonnecrotic tissue was inoculated subcutaneously into the right axillary. When the tumor in burdened rat grew approximately 100 mm3, according to the tumor size all the animals were divided into the following four groups, eight rats in each group: solvent control group, gefitinib group(100 mg/kg), erlotinib group(50 mg/kg), afatinib group(20 mg/kg). Aniamals were treated with drugs by intragastric(i.g.) administrated, once daily, for consecutively 14 days. Measure the tumor size 2-3 times every week. Results: Hu Prime1-NSCLC mutant sensitive xenograft model research data showed that reversible tyrosine kinase inhibitors gefitinib, erlotinib and irreversible tyrosine kinase inhibitor afatinib could effectively inhibit tumor growth in EGFR positive NSCLC allografts model. The pharmacodynamic activity of irreversible inhibitor was better than that of the reversible inhibitor. Specimens from clinical anthropogenic tumor retain characteristics of the human primary malignancy, histopathology, biological characteristics, and tumor markers, etc., which can more accurately reflect the characteristics of the tumor and the impact of interventions. Conclusion: The model is not only a good antitumor drug experimental platform, but also a new evaluation tool of individualized medication.     

Biochemical Assay-Based Selectivity Profiling of Clinically Relevant Kinase Inhibitors on Mutant Forms of EGF Receptor

Gefitinib and erlotinib are potent EGFR tyrosine kinase inhibitors (potentially) useful for the treatment of non-small-cell lung cancer (NSCLC). Clinical responses, however, in NSCLC patients have been linked to the presence of certain activating mutations of EGFR. We used an ELISA-based biochemical assay to confirm the selective inhibitory efficacy of gefitinib and erlotinib on the activated mutant receptor. Our results are in line with the clinical observations providing evidence for the predictive power of the kinase assay. Four additional compounds were also investigated: CI-1033 and EKB-569 had dramatic inhibitory effects on all EGFR forms, whereas PD153035 and AG1478 were active on wild-type and activating mutant protein. In docking simulations with wild-type EGFR, our inhibitory data are in good agreement with the binding scores. These data confirm that anilinoquinazolines are good starting structures for the next generation of selective drugs against mutant EGFR, whereas CI-1033 and EKB-569 may represent advances for patients with both wild-type and anilinoquinazoline-resistant mutant tumors.     

Biochemical assay-based selectivity profiling of clinically relevant kinase inhibitors on mutant forms of EGF receptor

Gefitinib and erlotinib are potent EGFR tyrosine kinase inhibitors (potentially) useful for the treatment of non-small-cell lung cancer (NSCLC). Clinical responses, however, in NSCLC patients have been linked to the presence of certain activating mutations of EGFR. We used an ELISA-based biochemical assay to confirm the selective inhibitory efficacy of gefitinib and erlotinib on the activated mutant receptor. Our results are in line with the clinical observations providing evidence for the predictive power of the kinase assay. Four additional compounds were also investigated: CI-1033 and EKB-569 had dramatic inhibitory effects on all EGFR forms, whereas PD153035 and AG1478 were active on wild-type and activating mutant protein. In docking simulations with wild-type EGFR, our inhibitory data are in good agreement with the binding scores. These data confirm that anilinoquinazolines are good starting structures for the next generation of selective drugs against mutant EGFR, whereas CI-1033 and EKB-569 may represent advances for patients with both wild-type and anilinoquinazoline-resistant mutant tumors.     

Gefitinib-mediated Reactive Oxygen Specie (ROS) Instigates Mitochondrial Dysfunction and Drug Resistance in Lung Cancer Cells

Therapeutic benefits offered by tyrosine kinase inhibitors (TKIs), such as gefitinib (Iressa) and erlotinib (Tarceva), are limited due to the development of resistance, which contributes to treatment failure and cancer-related mortality. The aim of this study was to elucidate mechanistic insight into cellular perturbations that accompany acquired gefitinib resistance in lung cancer cells. Several lung adenocarcinoma (LAD) cell lines were screened to characterize epidermal growth factor receptor (EGFR) expression and mutation profile. To circumvent intrinsic variations between cell lines with respect to response to drug treatments, we generated gefitinib-resistant H1650 clone by long-term, chronic culture under gefitinib selection of parental cell line. Isogenic cells were analyzed by microarray, Western blot, flow cytometry, and confocal and transmission electron microscope. We observed that although chronic gefitinib treatment provided effective action against its primary target (aberrant EGFR activity), secondary effects resulted in increased cellular reactive oxygen species (ROS). Gefitinib-mediated ROS correlated with epithelial-mesenchymal transition, as well as striking perturbation of mitochondrial morphology and function. However, gefitinib treatment in the presence of ROS scavenger provided a partial rescue of mitochondrial aberrations. Furthermore, withdrawal of gefitinib from previously resistant clones correlated with normalized expression of epithelial-mesenchymal transition genes. These findings demonstrate that chronic gefitinib treatment promotes ROS and mitochondrial dysfunction in lung cancer cells. Antioxidants may alleviate ROS-mediated resistance.     

Extracellular vesicles shed from gefitinib‐resistant nonsmall cell lung cancer regulate the tumor microenvironment

Epidermal growth factor receptor (EGFR)‐tyrosine kinase inhibitors (TKIs), including gefitinib, are the first‐line treatment of choice for nonsmall cell lung cancer patients who harbor activating EGFR mutations, however, acquired resistance to EGFR‐TKIs is inevitable. The main objective of this study was to identify informative protein signatures of extracellular vesicles (EV) derived from gefitinib‐resistant nonsmall cell lung cancer cells using proteomics analysis. Nano‐LC–MS/MS analysis identified with high confidence (false discovery rate < 0.05, fold change ≥2) 664 EV proteins enriched in PC9R cells, which are resistant to gefitinib due to EGFR T790M mutation. Computational analyses suggested components of several signal transduction mechanisms including the AKT (also PKB, protein kinase B)/mTOR (mechanistic target of rapamycin) pathway are overrepresented in EV from PC9R cells. Treatment of recipient cells with EV harvested from PC9R cells increased phosphorylation of signaling molecules, and enhanced proliferation, invasion, and drug resistance to gefitinib‐induced apoptosis. Dose‐ and time‐dependent pharmaceutical inhibition of AKT/mTOR pathway overcame drug resistance of PC9R cells and those of H1975 exhibiting EGFR T790M mutation. Our findings provide new insight into an oncogenic EV protein signature regulating tumor microenvironment, and will aid in the development of novel diagnostic strategies for prediction and assessment of gefitinib resistance.     

Biomarkers to Predict Response to Epidermal Growth Factor Receptor Inhibitors

Epidermal growth factor receptors (EGFRs) are amplified and overexpressed in many different human cancers, a phenomenon generally associated with poor prognosis. Inhibitors of the tyrosine kinase activity associated with this receptor have been approved for the treatment of chemotherapy-refractory non-small cell lung cancer, and are in clinical trials for additional tumor types. While these inhibitors, gefitinib and erlotinib, display limited response rates when assessed in a cohort that includes all patients, there are subgroups, defined by patient and tumor characteristics, that preferentially respond to these agents. We recently performed an analysis of tumors obtained form a Phase I trial of erlotinib in patients with glioblastoma multiforme (GBM), the most common malignant brain tumor in adults. We showed that patients whose tumors exhibited overexpression and amplification of EGFR responded better than patients who had normal levels of this gene and protein. We also demonstrated that the phosphorylation state of PKB/Akt was an important determinant for response, with low phospho-PKB/Akt levels predicting good response to erlotinib. We discuss these findings in the context of recent molecular analyses of the placebo-controlled Phase III trials that led to approval of EGFR inhibitors. These data underscore the importance of placebo-controlled trials to distinguish between prognostic indicators of disease progression more generally and predictive markers of response to therapy. Ultimately the goal of these studies is to allow selection of patients who will preferentially respond to EGFR inhibitors.     

Temporal resolution of autophosphorylation for normal and oncogenic forms of EGFR and differential effects of gefitinib

Epidermal growth factor receptor (EGFR) is a member of the ErbB family of receptor tyrosine kinases (RTK). EGFR overexpression or mutation in many different forms of cancers has highlighted its role as an important therapeutic target. Gefitinib, the first small molecule inhibitor of EGFR kinase function to be approved for the treatment of nonsmall cell lung cancer (NSCLC) by the FDA, demonstrates clinical activity primarily in patients with tumors that harbor somatic kinase domain mutations in EGFR. Here, we compare wild-type EGFR autophosphorylation kinetics to the L834R (also called L858R) EGFR form, one of the most common mutations in lung cancer patients. Using rapid chemical quench, time-resolved electrospray mass spectrometry (ESI-MS), and Western blot analyses, we examined the order of autophosphorylation in wild-type (WT) and L834R EGFR and the effect of gefitinib (Iressa) on the phosphorylation of individual tyrosines. These studies establish that there is a temporal order of autophosphorylation of key tyrosines involved in downstream signaling for WT EGFR and a loss of order for the oncogenic L834R mutant. These studies also reveal unique signature patterns of drug sensitivity for inhibition of tyrosine autophosphorylation by gefitinib: distinct for WT and oncogenic L834R mutant forms of EGFR. Fluorescence studies show that for WT EGFR the binding affinity for gefitinib is weaker for the phosphorylated protein while for the oncogenic mutant, L834R EGFR, the binding affinity of gefitinib is substantially enhanced and likely contributes to the efficacy observed clinically. This mechanistic information is important in understanding the molecular details underpinning clinical observations as well as to aid in the design of more potent and selective EGFR inhibitors.     

The Use of a Two-Tiered Testing Strategy for the Simultaneous Detection of Small EGFR Mutations and EGFR Amplification in Lung Cancer

Lung cancer is the leading cause of cancer-related death worldwide. Recent progress in lung cancer diagnosis and treatment has been achieved due to a better understanding the molecular mechanisms of the disease and the identification of biomarkers that allow more specific cancer treatments. One of the best known examples of personalized therapy is the use of tyrosine kinase inhibitors, such as gefitinib and erlotinib, for the successful treatment of non-small-cell lung cancer patients selected based on the specific EGFR mutations. Therefore, the reliable detection of mutations is critical for the application of appropriate therapy. In this study, we tested a two-tiered mutation detection strategy using real-time PCR assays as a well-validated high-sensitivity method and multiplex ligation-dependent probe amplification (MLPA)-based EGFRmut+ assay as a second-tier standard-sensitivity method. One additional advantage of the applied MLPA method is that it allows the simultaneous detection of EGFR mutations and copy-number alterations (i.e., amplifications) in EGFR, MET and ERBB2. Our analysis showed high concordance between these two methods. With the use of this two-tier strategy, we reliably determined the frequency of EGFR mutations and EGFR, MET and ERBB2 amplifications in over 200 lung cancer samples. Additionally, taking advantage of simultaneous copy number and small mutation analyses, we showed a very strong correlation between EGFR mutations and EGFR amplifications and a mutual exclusiveness of EGFR mutations/amplifications with MET and ERBB2 amplifications. Our results proved the reliability and usefulness of the two-tiered EGFR testing strategy.     

Trying to compose the puzzle with all the pieces: epidermal growth factor receptor tyrosine kinase inhibitors in non-small cell lung cancer

Small molecule epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) have shown promising activity in patients with non-small cell lung cancer (NSCLC). Gefitinib has been the first of these drugs to be licensed for third-line treatment of advanced NSCLC patients. More recently, erlotinib has been shown to be more effective than placebo in increasing overall survival (OAS) and has been approved for NSCLC patients after failure of chemotherapy. However, a large body of clinical and experimental evidence suggests that the benefit from these drugs is limited to a subgroup of patients. The availability of clinical or molecular criteria for predicting sensitivity to EGFR-TKIs is the most relevant issue for their correct use and for planning future research. Determination of EGFR expression is not sufficient to predict sensitivity to EGFR-TKIs. However, several clinical features (female gender, adenocarcinoma/bronchioloalveolar histotype, never-smoking status, Oriental Asian origin) are associated with major clinical responses. The identification of somatic mutations in the tyrosine kinase domain of the EGFR gene represents the most important molecular marker of sensitivity to EGFR-TKIs. These "activating" mutations can be found in a high proportion of gefitinib- or erlotinib-responding patients. However, clinical effectiveness might not be limited to patients carrying EGFR mutations, in which the objective response is probably the detectable effect of apoptosis induction in cancer cells. In fact, clinical efficacy with gefitinib or erlotinib is also observed in another subgroup of patients, in which a tumor growth delay, determined by a block in cancer cell proliferation, could induce a prolonged and clinically relevant disease stabilization.     

Contribution of EGFR and ErbB-3 Heterodimerization to the EGFR Mutation-Induced Gefitinib- and Erlotinib-Resistance in Non-Small-Cell Lung Carcinoma Treatments

EGFR mutation-induced drug resistance has become a major threat to the treatment of non-small-cell lung carcinoma. Essentially, the resistance mechanism involves modifications of the intracellular signaling pathways. In our work, we separately investigated the EGFR and ErbB-3 heterodimerization, regarded as the origin of intracellular signaling pathways. On one hand, we combined the molecular interaction in EGFR heterodimerization with that between the EGFR tyrosine kinase and its inhibitor. For 168 clinical subjects, we characterized their corresponding EGFR mutations using molecular interactions, with three potential dimerization partners (ErbB-2, IGF-1R and c-Met) of EGFR and two of its small molecule inhibitors (gefitinib and erlotinib). Based on molecular dynamics simulations and structural analysis, we modeled these mutant-partner or mutant-inhibitor interactions using binding free energy and its components. As a consequence, the mutant-partner interactions are amplified for mutants L858R and L858R_T790M, compared to the wild type EGFR. Mutant delL747_P753insS represents the largest difference between the mutant-IGF-1R interaction and the mutant-inhibitor interaction, which explains the shorter progression-free survival of an inhibitor to this mutant type. Besides, feature sets including different energy components were constructed, and efficient regression trees were applied to map these features to the progression-free survival of an inhibitor. On the other hand, we comparably examined the interactions between ErbB-3 and its partners (EGFR mutants, IGF-1R, ErbB-2 and c-Met). Compared to others, c-Met shows a remarkably-strong binding with ErbB-3, implying its significant role in regulating ErbB-3 signaling. Moreover, EGFR mutants corresponding to poor clinical outcomes, such as L858R_T790M, possess lower binding affinities with ErbB-3 than c-Met does. This may promote the communication between ErbB-3 and c-Met in these cancer cells. The analysis verified the important contribution of IGF-1R or c-Met in the drug resistance mechanism developed in lung cancer treatments, which may bring many benefits to specialized therapy design and innovative drug discovery.     

Mechanisms of resistance to HER family targeting antibodies

The epidermal growth factor (EGF) family of receptor tyrosine kinases consists of four members: EGFR (HER1/ErbB1), HER2/neu (ErbB2), HER3 (ErbB3) and HER4 (ErbB4). Receptor activation via ligand binding leads to downstream signaling that influence cell proliferation, angiogenesis, invasion and metastasis. Aberrant expression or activity of EGFR and HER2 have been strongly linked to the etiology of several human epithelial cancers including but not limited to head and neck squamous cell carcinoma (HNSCC), non-small cell lung cancer (NSCLC), colorectal cancer (CRC), and breast cancer. With this, intense efforts have been made to inhibit the activity of the EGFR and HER2 by designing antibodies against the ligand binding domains (cetuximab, panitumumab and trastuzumab) or small molecules against the tyrosine kinase domains (erlotinib, gefitinib, and lapatinib). Both approaches have shown considerable clinical promise. However, increasing evidence suggests that the majority of patients do not respond to these therapies, and those who show initial response ultimately become refractory to treatment. While mechanisms of resistance to tyrosine kinase inhibitors have been extensively studied, resistance to monoclonal antibodies is less well understood, both in the laboratory and in the clinical setting. In this review, we discuss resistance to antibody-based therapies against the EGFR and HER2, similarities between these resistance profiles, and strategies to overcome resistance to HER family targeting monoclonal antibody therapy.