Activating mutations in the epidermal growth factor receptor (EGFR) gene have come to define a distinct population of patients with non-small cell lung cancer (NSCLC). Cancers that harbor these EGFR mutations have been demonstrated to possess profound sensitivity to EGFR tyrosine kinase inhibitors (TKIs),[1-3] giving them a unique biology and natural history.[4, 5] Numerous studies have now demonstrated that patients with advanced NSCLC harboring specific EGFR-activating mutations (exon 19 deletions or exon 21 L858R) should receive first-line treatment with EGFR TKIs. These agents exhibit minimal toxicity and are broadly active, with only 3% to 10% of patients exhibiting refractory disease with frank progression while receiving TKIs.[6-8]
The initial responses achieved with either standard first-generation EGFR kinase inhibitors (gefitinib, erlotinib) or recently approved alternative agents (icotinib, afatinib) are temporary and marred by the inevitable emergence of acquired treatment resistance.[6, 7, 9, 10] The management of acquired resistance has thus become the central challenge in the treatment of patients with EGFR-mutant advanced NSCLC. Herein we reviewed current knowledge regarding the definition of acquired resistance, mechanisms of resistance, and the optimal management thereof.
Defining Resistance to EGFR Kinase Inhibitors
The development of acquired resistance to EGFR kinase inhibitors is both predictable and unavoidable. Importantly, acquired resistance is distinct in both mechanism and management from primary treatment resistance. The latter refers to a heterogeneous population of cancers lacking TKI sensitivity due to the absence of an EGFR mutation, a distinct biology (eg, the presence of another oncogenic driver mutation), or due to the baseline presence of a secondary mutation lending resistance (eg, EGFR mutation plus EGFR T790M); primary resistance is outside of the scope of this review but has been reviewed recently elsewhere. In contrast, acquired resistance refers specifically to resistance that develops after initial EGFR TKI sensitivity. Although a clinical definition of resistance was previously proposed that included nongenotyped patients with progressive disease after initial EGFR TKI response, the widespread adoption of EGFR genotyping has resulted in acquired resistance now loosely referring to EGFR-mutant lung cancers with disease progression occurring during therapy with an EGFR kinase inhibitor after an initial period of response or stable disease.
Acquired resistance to EGFR kinase inhibitors is believed to be initiated by the emergence of clones possessing genomic alterations conferring a survival advantage under the selective pressure of the TKI. The point at which resistant clones emerge compared with the initiation of TKI therapy remains controversial, particularly given technical challenges in detecting low-prevalence resistance mutations before TKI therapy. To the best of our knowledge, no relationship has ever been demonstrated between the detection of a pretreatment resistance mutation in a minor population of cells and the ultimate acquired resistance mechanism. However, once resistant clones emerge they eventually grow to predominate and lead to clinically apparent disease progression (Fig. 1).[15, 16]
One proposed criteria for defining resistance to EGFR kinase inhibitors has used radiographic disease progression as determined by Response Evaluation Criteria In Solid Tumors (RECIST) criteria. The use of these criteria to define disease progression and thus acquired resistance has obvious use in the conduct of clinical trials. However, caution must be applied when applying these criteria to treatment decision-making in patients with EGFR-mutant NSCLC with emerging acquired resistance. Patients with robust initial responses to EGFR TKIs may have minimal remaining disease, such that clinically insignificant changes on imaging may meet RECIST criteria for disease progression despite indolent growth and a lack of symptoms. Put differently, the determination of resistance based on radiographic disease progression does not necessarily suggest clinically significant disease progression and treatment failure. The key clinical challenge is thus determining the point at which the degree of acquired resistance as manifested by radiographic progression has reached a threshold that warrants changing treatment. This important question is currently being investigated by the ASPIRATION trial, which is prospectively studying continued single-agent erlotinib beyond RECIST disease progression (NCT01310036).
Mechanisms of Acquired Resistance to EGFR Kinase Inhibitors
Several molecular mechanisms have been elucidated that are capable of triggering acquired resistance to EGFR TKIs. For the purposes of this review, we will broadly group these mechanisms into 3 categories based on the degree to which each mechanism is potentially actionable or affects clinical management.
It is important to note that pharmacokinetic failure of EGFR kinase inhibitors constitutes a separate mechanism of apparent resistance that is not well encompassed in this schema. This phenomenon has been described secondary to drug-drug interactions and smoking-related effects on EGFR TKI metabolism, in which a patient with disease progression may be able to respond to an increased EGFR kinase inhibitor dose.[18, 19] Isolated central nervous system (CNS) disease progression due to poor drug penetration into the cerebrospinal fluid represents another type of pharmacokinetic failure, in which the blood-brain barrier limits drug penetration to subtherapeutic doses, allowing regrowth of EGFR-mutant disease (Fig. 1).
Clinically actionable resistance mechanisms
The EGFR T790M mutation is the most common mechanism of acquired resistance, and is found in 49% to 63% of rebiopsies performed after resistance develops to EGFR TKIs.[20-22] The T790M mutation alters the affinity of EGFR for ATP, dramatically reducing the ability of first-generation and second-generation TKIs to compete for binding.[23, 24] The presence of the T790M resistance mutation thus confers a survival advantage to tumor cells when subjected to the selective pressure of EGFR kinase inhibitors. However, the growth kinetics of T790M-positive tumor cells are inferior to T790M-negative EGFR-mutant tumor cells in the absence of EGFR TKI.[15, 16] This may explain, in part, the phenomenon of both tumor flare noted at the time of cessation of EGFR TKIs, as sensitive clones overgrow the resistant clones, as well as subsequent re-response of these sensitive clones to retreatment with the same TKI (Fig. 1).[25, 26]
Clinically, T790M-mediated acquired resistance often exhibits a distinctive indolent pattern of disease progression,[13, 15, 16] and in some series has been found to be associated with a favorable prognosis compared with T790M-negative resistance.[15, 16] In what to our knowledge is one of the largest rebiopsy series published to date, the presence of T790M was associated with a lower incidence of new metastatic sites, higher performance status, and longer survival. Beyond its role as a prognostic marker, the T790M mutation also has an emerging role as a predictive biomarker given that early data regarding novel third-generation EGFR kinase inhibitors have suggested high response rate (RRs) in patients with T790M-positive lung cancers (Table 1).[20-22, 24, 27-42]
|Tier 1: Clinically Actionable Resistance Mechanisms|
|Mechanism||Prevalence||Potential Therapy||Efficacy Data|
|EGFR T790M||49%-63%[20-22]||CO-168627||66% RR in 9 T790M+ patients at highest dose levela|
|AZD929128||43% RR in 35 patients from all dose levels - 50% RR in 18 T790M+ patients - 20% RR in 5 T790M- patients|
|Afatinib plus cetuximab||30% RR and median 4.7-mo PFS in 96 patients at maximum tolerated dose - 32% RR in 53 T790M+ patients - 28% RR in 39 T790M- patients|
|Small cell transformation||3%-14%[21, 22]||Platinum-etoposide||60% RR in 5 patients|
|Tier 2: Clinical Investigations Ongoing|
|Mechanism||Prevalence||Potential Therapy||Ongoing Clinical Trials|
|MET amplification[31, 32]||5%-11%[20-22]||Cabozantinib plus erlotinib||Phase 2 (NCT01866410)|
|LY2875358 ± erlotinib||Phase 2 (NCT01900652)|
|INC280 plus gefitinib||Phase 1b/2 (NCT01610336)|
|HER2 amplification||12%-13%[22, 33]||High-dose intermittent afatinib||Phase 1b (NCT01647711)|
|Dacomitinib||Phase 3 vs placebo (NCT1000025), with a preplanned subgroup analysis in EGFR-mutant cancers|
|Intermittent dacomitinib||Phase 2 (NCT01858389)|
|PIK3CA mutation||0%-5%[21, 22]||BKM120 plus gefitinibBKM120 plus erlotinib||Phase 1 (NCT01570296)Phase 2 (NCT01487265)|
|ERK amplification||NA||Selumetinib plus gefitinib||Phase 1b/2 (NCT02025114)|
|BRAF V600E||1%||Combinations of BRAF and EGFR inhibitors are in development in patients with colorectal cancer|
|Tier 3: Primarily preclinical data|
|CRKL amplification, AXL overexpression, elevated HGF[38-42]|
Small cell transformation is another discrete resistance mechanism found in a subset of cases of acquired resistance in which neuroendocrine histological features are observed with the original EGFR mutation maintained. The clinical course of transformed disease has been difficult to study due to its rarity (3%-14%), but anecdotally can be associated with aggressive behavior (Fig. 1). One report found 3 of 5 patients with this type of transformed disease responded to standard chemotherapy with the combination of platinum and etoposide.
Potentially actionable resistance mechanisms
The second genomic mechanism discovered to mediate acquired resistance to EGFR kinase inhibitors was amplification of the MET gene and associated overexpression of the MET kinase.[31, 32] MET amplification bypasses reliance on the EGFR signaling pathway by alternatively activating the phosphoinositide 3-kinase (PI3K)/AKT pathway via ErbB3 signaling. The prevalence of MET amplification in recent clinical series has ranged between 5% and 11%,[20-22] which is lower than the 20% prevalence noted in smaller early reports.[31, 32] Several MET inhibitors have been developed and are now in clinical trials as both single agents and in combination with erlotinib (Table 1).[20-22, 24, 27-42]
Two other highly targetable oncogenes, human epidermal growth factor receptor 2 (HER2) and BRAF, have also been identified as mediating acquired resistance in a small subset of cases.[33, 36] Amplification of HER2 has previously been postulated as a mechanism of acquired resistance, and was recently identified by fluorescence in situ hybridization in 3 patients in a rebiopsy series of 24 patients. Mutations in BRAF have been demonstrated to confer acquired resistance in preclinical models and have also been identified in a small number of patients (2 of 195 patients) in a recent rebiopsy study. Although these resistance mechanisms may be too rare for dedicated clinical trials, currently trials of the pan-HER kinase inhibitors afatinib and dacomitinib are ongoing for acquired resistance (Table 1),[20-22, 24, 27-42] and synergy between EGFR and BRAF kinase inhibitors is an area of active investigation among patients with colorectal cancer. PIK3CA (phosphatidylinositol-4,5-bisphosphate 3-kinase, catalytic subunit alpha) mutations have similarly been demonstrated to confer gefitinib resistance in vitro and were identified in a small number of patients (2 of 37 patients) in 1 rebiopsy series21; this has not been identified in subsequent rebiopsy studies and the role of these mutations in acquired resistance remains controversial.[20, 22, 34] Hoping to identify some synergy, early-phase clinical trials of PI3K inhibitors combined with erlotinib have moved forward in patients with acquired resistance. Reactivation of ERK signaling through ERK amplification has also been identified in preclinical studies as a resistance mechanism to third-generation EGFR kinase inhibitors targeting T790M and a trial combining selumetinib and gefitinib currently is underway.
Additional resistance mechanisms with targeted agents in preclinical development
Several potential mechanisms for acquired resistance have been identified in primarily preclinical studies and small rebiopsy series including CRKL amplification, AXL kinase overexpression, and increased levels of hepatocyte growth factor.[38-41] These preclinical targets, although promising, are beyond the scope of the current article and have recently been reviewed elsewhere.
Managing Acquired Resistance to EGFR Kinase Inhibitors
Much has been learned regarding the management of acquired resistance since the early studies of TKIs in patients with EGFR-mutant lung cancer.[44, 45] Before EGFR kinase inhibitors, the only systemic therapies for the treatment of patients with advanced NSCLC were cytotoxic chemotherapies, drugs with modest efficacy and significant toxicity that often necessitated discontinuing treatment at the first sign of disease progression. In contrast, EGFR TKIs are given daily by mouth and have remarkable efficacy for EGFR-mutant lung cancer with modest toxicity. These differences have altered the basic balance between continued treatment versus treatment cessation in patients with acquired resistance.
The first question posited at the time of apparent disease progression must be whether a patient's progression is clinically significant enough to warrant initiating a new line of therapy (Fig. 2). Indolent, asymptomatic progression of existing metastatic disease, without the involvement of new organ systems, is often observed when acquired resistance initially develops. Continuing treatment with a TKI beyond disease progression in this context has been suggested to delay the need for chemotherapy. What is more, stopping treatment prematurely may carry a risk of disease flare, particularly if immediate treatment with systemic chemotherapy is not planned. Disease flare causing hospitalization or death was noted in 23% of patients who withdrew from treatment with EGFR TKIs for subsequent clinical trial enrolment in one series.
The next important issue to consider in patients exhibiting disease progression is whether their disease progression is localized, and potentially controlled with palliative local therapy (Fig. 2). Isolated CNS progression on TKI therapy may, in some cases, be due to limited drug penetration into the CNS due to the blood-brain barrier. Palliative radiotherapy (RT) followed by retreatment with TKI has the potential to regain control of the disease; in one series of 51 patients, an additional median progression-free survival (PFS) of 6.2 months was reported. If systemic control and prevention of flare is a concern, it is also possible to continue EGFR TKI during whole-brain RT (WBRT), which was demonstrated to be safe in a phase 2 study of erlotinib plus WBRT. The Radiation Therapy Oncology Group 0320 trial examined the combination of erlotinib, WBRT, and stereotactic RT in patients with NSCLC with limited brain metastases. This study demonstrated an increased incidence of grade 3 to 5 toxicity (CTCAE) among those patients receiving erlotinib and stereotactic RT and underscored the risk of combining these strategies. Alternative strategies using pulsed high-dose EGFR kinase inhibitors to overcome the blood-brain barrier are currently investigational with variable rates of clinical benefit described in several series.[50, 51]
Acquired treatment resistance may also manifest as limited oligometastatic disease progression outside of the CNS. Locally ablative therapies such as palliative RT can certainly be beneficial in the context of symptomatic oligometastatic disease. However, whether the treatment of oligometastatic disease with local ablation changes the natural history of acquired resistance or improves long-term outcomes remains unclear. Uncontrolled studies have described favorable outcomes in selected patients with acquired resistance receiving aggressive surgery or RT to focal sites of disease,[47, 52] but evidence from properly controlled randomized studies will be needed before the broad adoption of this strategy.
One question to consider in a patient with progressive disease while receiving EGFR TKI is whether rebiopsy may be valuable (Fig. 2). Several large studies have shown rebiopsy to be feasible to characterize the molecular mechanisms of resistance.[20-22] Rebiopsy can provide prognostic information given the favorable prognosis observed in 2 series with T790M-mediated acquired resistance.[15, 16] Furthermore, rebiopsy is particularly valuable when considering a clinical trial: if pathology shows small cell transformation, immediate chemotherapy makes more sense, whereas the detection of specific resistance mutations outlined previously could steer a patient toward a trial of a specific targeted therapy (Table 1).[20-22, 24, 27-42] The importance of enrolling patients onto clinical trials for acquired resistance cannot be overemphasized, given that this is a setting in which to our knowledge no positive phase 3 trial has ever been performed and the standard of care remains loosely defined.
For patients with systemic disease progression and no clinical trial available, cytotoxic chemotherapy is an appropriate second-line therapy after failure of EGFR TKI given the demonstrated efficacy of platinum-based chemotherapy in patients with NSCLC. However, to the best of our knowledge, published data are lacking regarding the activity of cytotoxic chemotherapy after TKI failure. Two retrospective studies described RRs of 15% and 18% to chemotherapy using a variety of regimens.[53, 54] However, a recently presented Japanese study reported a more favorable RR of 40% (95% confidence interval, 22%-58%) to carboplatin, paclitaxel, and bevacizumab in 30 patients with acquired resistance. More data regarding this topic are needed. Many clinicians add cytotoxic chemotherapy to continued EGFR TKI for patients with acquired resistance based on preclinical work describing synergy, believed to occur through controlling the percentage of cells that remain TKI-sensitive. Indeed, to our knowledge, the only published prospective trial of chemotherapy for acquired resistance is a single-arm phase 2 trial of pemetrexed plus continued TKI with a RR of 26% and a median PFS of 7 months reported. A retrospective study of 78 patients receiving chemotherapy for acquired resistance found an improved RR for those continuing TKI with chemotherapy (41% vs 18%), but there was no difference noted with regard to PFS. The potential synergy of continuing EGFR inhibition when chemotherapy is initiated for acquired resistance is the subject of the ongoing IMPRESS trial of the combination of cisplatin and pemetrexed with or without gefitinib (NCT01544179).
Targeted Therapies For Acquired Resistance To EGFR Kinase Inhibitors
Targeted therapies for EGFR TKI resistance have been an area of active investigation for nearly a decade; the earliest studies, many of which were negative, have been reviewed elsewhere recently. Herein we will focus on therapies that currently are commercially available or under active investigation (Table 1).[20-22, 24, 27-42]
Second-generation, irreversible, EGFR kinase inhibitors have been investigated as therapies for acquired resistance based on preclinical data suggesting increased activity against models with EGFR T790M. To the best of our knowledge, afatinib is the most studied drug in this class, with its use in acquired resistance having been evaluated in the phase 2/3 LUX-Lung 1 trial. This study was negative for its primary endpoint of overall survival (10.8 months vs 12 months; P = .74), but demonstrated a statistically significant improvement in PFS (3.3 months vs 1.1 months; P > .0001) and RR (7% vs 0.5%; P = .007). Because afatinib inhibits wild-type EGFR more potently than T790M (Fig. 3),[27, 58] it has been hypothesized that EGFR-related toxicity impairs the clinical delivery of this drug at levels sufficient to block T790M-mediated signaling. For this reason, a study of intermittent high-dose afatinib currently is underway. Dacomitinib is another irreversible pan-HER inhibitor that has demonstrated promising activity in early studies in resistant disease. It is interesting to note that both afatinib and dacomitinib have activity against the HER2 kinase and therefore could be active against resistance mediated by HER2 amplification. The National Cancer Institute of Canada Br.26 trial randomized 720 patients who had developed disease progression while receiving standard therapy to either dacomitinib or placebo, including a preplanned EGFR-mutant subgroup with acquired TKI resistance; the trial has reportedly failed to meet its primary survival endpoint although the outcome in the EGFR-mutant subgroup remains to be reported.
Third-generation EGFR kinase inhibitors comprise a particularly promising class of investigational drugs for acquired resistance. These agents are structurally distinct from first-generation and second-generation inhibitors and were designed specifically to inhibit EGFR T790M while sparing wild-type EGFR (Fig. 3).[62, 63] Several agents of this class have entered phase 1 clinical trials, with the most studied being CO-1686 and AZD9291, both of which have demonstrated impressive responses in patients with TKI-resistant disease. The CO-1686 study has reported on 9 patients with T790M from the highest dosing cohort (900 mg twice daily), and 6 achieved a partial response. The AZD9291 study reported on 34 patients across all dosing cohorts (20-80 mg daily) and 15 achieved a partial response, including 9 of 18 patients who were positive for T790M and only 1 of 5 patients who were negative for T790M. No significant reports of rash or diarrhea have been noted, which is consistent with a lack of inhibition of wild-type EGFR. Further studies of the relative effectiveness of these agents among both T790M-positive and T790M-negative patients will serve to elucidate the potential role of T790M as a predictive marker for this new class of agents.
Another approach used to maximize inhibition of EGFR signaling in acquired resistance has been the combination of an EGFR kinase inhibitor with an EGFR-targeted antibody, a synergy that was initially discovered through studies of genetically engineered mouse models. Although a phase 2 trial combining erlotinib and cetuximab revealed minimal activity, preliminary results of a phase Ib trial of afatinib and cetuximab in 60 patients with acquired resistance revealed a 30% response rate and a median PFS of 4.7 months, with grade 3 rash and diarrhea (CTCAE) noted in 18% and 7% of patients, respectively (Table 1).[20-22, 24, 27-42] Responses were seen regardless of T790M status. Preclinical work demonstrated that the combination of an EGFR antibody and a covalent kinase inhibitor markedly reduced both EGFR signaling as well as total levels of EGFR protein. Additional clinical trials with this regimen are currently in development.
Several other combinations currently are under investigation for acquired resistance, most commonly adding a new targeted agent to a reversible EGFR TKI (Table 1).[20-22, 24, 27-42] Several studies are evaluating MET inhibitors (either antibodies or kinase inhibitors) added to an EGFR TKI, although to the best of our knowledge at the current time none is limited to MET-mediated resistance. The heat shock protein 90 inhibitor AUY922 is being studied in acquired resistance with response rates of approximately 18% (12 of 66 patients) with the use of AUY922 alone and 16% (4 of 25 patients) when combined with erlotinib.[66, 67] A phase 2 study currently is ongoing randomizing patients with TKI-resistant disease to AUY922 versus single-agent chemotherapy (NCT01646125). The combination of an EGFR TKI with the PI3K inhibitor BKM120 is also currently under investigation, with toxicities including hyperglycemia, rash, and diarrhea being reported. In general, these combination studies have included all patients with acquired resistance (a molecularly heterogeneous population) rather than focusing on a biomarker-enriched subset of patients with EGFR-mutant lung cancers who are more likely to respond.
Lastly, the use of programmed cell death 1 (PD-1) and PD-1 ligand (PD-L1) inhibitors for acquired resistance must be considered given the promising early data in patients with advanced NSCLC. Recent preclinical data have suggested that EGFR mutation-positive lung cancer may preferentially use PD-1/PD-L1–mediated mechanisms to evade immune surveillance. To investigate whether there might be synergy in EGFR-mutant lung cancers, several phase 1 studies are currently or will soon be combining EGFR TKI with PD-1 or PDL-1 antibodies (NCT01454102, NCT02039674, and NCT02013219).
The coming year is expected to bring greater availability of third-generation EGFR TKIs as clinical trial options for patients with acquired resistance. This exciting therapeutic development raises an important drug development question: when planning a phase 3 trial for patients with acquired resistance, what standard therapy should be given to the control arm? To the best of our knowledge, a single phase 3 trial has been performed for acquired resistance, the third-line/fourth-line LUX-Lung1 trial, and its primary endpoint was negative. Some might consider treating patients who develop disease progression while receiving first-line TKIs as “second-line NSCLC” and support a minimalist standard therapy such as single-agent pemetrexed. Others might consider these patients to be platinum-naive and support an aggressive standard therapy containing cisplatin or bevacizumab. Still others, believing in erlotinib continuation after disease progression, might support erlotinib with chemotherapy as standard. This lack of a clear standard is certain to add complexity to drug development in this space.
A second important question raised by the emergence of EGFR T790M inhibitors is how to incorporate rebiopsy and T790M testing into the standard management of patients with acquired resistance. Although technically feasible at academic centers,[20-22] rebiopsy for molecular analysis creates technical and resource allocation challenges, particularly at community centers. Exciting technologies are emerging that may allow for the noninvasive genotyping of cell-free plasma DNA, and have been proposed as a potential replacements for either initial EGFR genotyping or the subsequent identification of the T790M resistance mutation.[71, 72] Such noninvasive T790M genotyping has the potential to be implemented more broadly than current strategies requiring biopsy tissue.