Novel Treatment Options for T790M-Mutant Non-Small Cell Lung Cancer


Neelesh Sharma, MD, PhD
Published Online: May 08, 2014

Neelesh Sharma, MD, PhD Neelesh Sharma, MD, PhD

Assistant Professor Thoracic Oncology
University Hospitals/Case Comprehensive
Cancer Center
Case Western Reserve University

Abstract

First-generation, reversible, epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) such as erlotinib and geftinib have become the standard of care and first-line treatment option for patients with non-small cell lung cancer (NSCLC) with activating EGFR mutations. Despite initial clinical benefit, overall efficacy of these agents is limited by emergence of drug-resistance mutations, including the gatekeeper T790M mutation. Substitution of methionine for threonine at position 790 of EGFR leads to increased affinity for adenosine triphosphate (ATP), causing resistance to competitive inhibition by reversible EGFR-TKIs. Second-generation, irreversible EGFR-TKIs were developed to overcome this resistance and demonstrated significant preclinical activity in mouse models of T790M-mutant lung cancer. However, these agents had limited success in clinical trials because of concurrent inhibition of wild-type EGFR and associated toxicity that prevented achieving adequate drug levels in plasma for sufficient inhibition of EGFR T790M. CO-1686 and AZD9291 are third-generation, irreversible T790M mutant-specific EGFR-TKIs with little activity on wildtype EGFR and robust activity in T790M preclinical models. In ongoing phase I clinical trials, both agents have shown promising responses in patients with EGFR-TKI-resistant tumors carrying T790M mutation.

Introduction

Developments in last decade have established a central role for epidermal growth factor receptor (EGFR) pathway in the treatment of non-small cell lung cancer (NSCLC). Initial observation that EGFR protein is highly expressed in various tumors, including lung cancer, led to the development of first-generation small molecule EGFR tyrosine kinase inhibitors (TKIs). Advances in structure-activity relationship (SAR) and molecular modeling made it easier to generate novel molecules that were complementary in shape and electrostatics to the EGFR kinase domain topography.1 A major breakthrough in this field was the discovery of 4-anilinoquinazolines (gefitinib, erlotinib, and lapatinib), which are potent and selective but reversible inhibitors of EGFR. The quinazoline ring binds in the adenine pocket with hydrogen bonds and the anilino ring binds in an adjacent, unique hydrophobic pocket, causing competitive inhibition for adenosine triphosphate (ATP) binding in the tyrosine kinase domain.2

Phase I and II clinical trials with reversible firstgeneration EGFR-TKI (erlotinib, geftinib) revealed dramatic responses in some patients with lung adenocarcinoma who were never smokers, women, or of Asian origin.3,4 Later, it was recognized that these patients had activating mutations in EGFR gene, which was predictive for higher response rates (RR) and survival with EGFR inhibitors.5-7 The frequency of EGFR mutations depends on the histology and ethnicity; it can be as high as 30% to 40% in an East Asian population with adenocarcinoma, and as low as 15% to 20% in Caucasians.8 More than 145 different types of nucleotide changes have been reported within the EGFR kinase domain, but most clinically relevant EGFR mutations occur within the first 4 exons encoding the ATP-binding pocket of the kinase domain (exons 18-21).9 Deletions in exon 19 that eliminate a common amino acid motif (LREA) and point mutation in exon 21 that involves the substitution of an arginine for a leucine at codon 858 (also known as L858R), account for approximately 85% of EGFR mutations in lung cancer. These mutants preferentially bind to EGFR-TKI as demonstrated by their higher Km for ATP and a lower Ki for erlotinib and gefitinib.10-12 In the last few years, multiple clinical trials have demonstrated that first-line treatment with EGFR-TKIs in lung adenocarcinoma patients with EGFR mutations is superior to traditional platinum-based chemotherapy in terms of objective response rate (ORR), progression-free survival (PFS), and tolerability.13-15 At present, EGFR-TKI has become the standard of care first-line treatment for this subset of NSCLC.

Despite a rapid and profound response, almost all patients receiving reversible first-generation EGFRTKI for the treatment of EGFR-mutant NSCLC eventually progress because of emergence of acquired resistance. Several mechanisms of acquired resistance have been described (Table 1), the most common (~50%) being mutation of the gatekeeper T790 residue (T790M) in exon 20.16

Clinical Pearls

  • Despite initial clinical benefit with first-generation, EGFR TKIs, such as erlotinib and geftinib in first-line treatment of NSCLC, overall efficacy of these agents is limited by emergence of drug-resistance mutations, including the gatekeeper T790M mutation.
  • Second-generation, irreversible EGFR-TKIs were developed to overcome this resistance and demonstrated significant preclinical activity in mouse models of T790M-mutant lung cancer.
  • These agents had limited success in clinical trials because of concurrent inhibition of wild-type EGFR and associated toxicity that prevented achieving adequate drug levels in plasma for sufficient inhibition of EGFR T790M.
  • CO-1686 and AZD9291 are third-generation, irreversible T790M mutant-specific EGFR-TKIs with little activity on wildtype EGFR and robust activity in T790M preclinical models.

Mechanism of Resistance

T790M mutation results in an amino acid substitution at position 790, from a threonine (T) to a methionine (M). Threonine 790 is critical for the binding of anilinoquinazoline inhibitors to the EGFR because of its key location at the entrance to a hydrophobic pocket in the ATP binding cleft. Kobayashi et al introduced T790M substitution into the sequence of the wild-type EGFR, delL747–P753insS-mutant EGFR (a frequently identified deletion mutant) and the L858R-mutant EGFR (the most common point mutation). 17 These constructs with or without T790M were transiently transfected in COS-7 and NIH-3T3 cells. All construct pairs demonstrated identical levels of expression of total EGFR and phosphorylated EGFR, suggesting that the presence of the T790M mutation did not alter the production, degradation, activation, or deactivation of the EGFR protein. In contrast, when these cells were treated with gefitinib, the constructs without T790M were completely inhibited at a concentration of 20 nM, whereas constructs carrying the T790M substitution were resistant to concentrations of gefitinib as high as 2 μM. Interestingly, CL-387,785, a specific and irreversible anilinoquinazoline EGFR inhibitor, strongly inhibited EGFR phosphorylation in constructs with T790M with a half-maximal inhibitory concentration (IC50) of 3.0 nM.

Multiple observations indicate that in addition to causing drug resistance, T790M mutations may also provide growth advantage. Somatic T790M mutations have been described at the time of diagnosis of lung cancer in the absence of previous EGFR-TKI treatment.18,19 A germ-line T790M mutation has been reported in a family with a hereditary predisposition to lung cancer, suggesting that this mutation may be involved in development of lung cancer.20 Transgenic mice with induced expression of EGFR T790M in mouse lung epithelia develop lung adenocarcinomas.21,22

T790M is analogous to T315I mutation in ABL, which causes resistance to most of the BCR-ABL1 TKIs used for the treatment of chronic myeloid leukemia (CML). It was initially thought that similar to T315I, methionine substitution for threonine introduces a bulkier amino acid side chain, resulting in steric hindrance with binding of reversible EGFR-TKIs.17 Although the T315I substitution in CML imparts resistance to all the tested TKIs, the corresponding T790M mutation is inhibited in vitro by multiple irreversible EGFR-TKIs (CL-387,785, EKB-569, and HKI-272) that closely resemble the anilinoquinazoline inhibitors. The fact that irreversible inhibitors that form a covalent bond with Cys- 797 at the edge of the ATP-binding cleft can bind and inhibit T790M does not support the steric hindrance as a mechanism of resistance.11

TABLE 1. Mechanisms of Acquired Resistance to EGFR-TKI

Mechanisms of Acquired Resistance
Acquired secondary mutations in tyrosine kinase domain (~60%, mostly T790M)
Activation of other receptor tyrosine kinases (eg, HER2 amplification, MET amplification, PIK3CA mutations, BRAF mutation, AXL activation)
FAS/NFκB activation
Epithelial-mesenchymal transition
Loss or spliced variant of BIM
Small-cell cancer transformation


Yun et al have described the structural and enzymologic effects of the T790M mutation in the context of both the wild-type and the L858R-mutant EGFR kinases.11 In direct-binding assay, T790M mutant binds geftinib in low nanomolar affinity, which is only slightly weaker than that for L858R mutant. This small difference in gefitinib binding because of T790M mutation is unlikely to explain the significant differences in sensitivity. Crystal structure of T790M mutant in combination with the inhibitor also does not support steric hindrance to inhibitor binding. Further analysis of ATP affinity of wildtype and mutant EGFR revealed a marked decrease in the Km for ATP in the drug-resistant L858R/ T790M mutant compared with the drug-sensitive L858R mutant. T790M mutation nearly restores the ATP affinity to wild-type levels in the L858R/T790M double mutant. Reversible first-generation inhibitors such as erlotinib compete with ATP for binding to the kinase active site; the enhanced ATP affinity caused by T790M mutation reduces potency of these drugs. Irreversible EGFR inhibitors can overcome T790M resistance through covalent binding because they are not in a competitive, reversible equilibrium with ATP.

Second-Generation EGFR-TKIs

A number of quinazolinebased, second-generation, irreversible EGFR inhibitors are under evaluation, such as BIBW-2992 (afatinib), HKI-272 (neratinib), PF00299804 (dacomitinib), CL387785, EKB- 569, and CI-1033.23,24 These compounds are also ATP mimetic but covalently bind Cys-797 of EGFR and are thus considered ‘irreversible’ inhibitors. The irreversible covalent binding leads to longer and more potent suppression of kinase activity than do reversible TKIs since the activity is suppressed until the synthesis of new receptors. Many of these inhibitors have shown promising preclinical activity in T790M-mutant lung cancer cell lines and mice models. In a cell-free system, afatinib is 100-fold more active against the geftinib-resistant double-mutant EGFR L858R/T790M with an IC50 of 10 nM.25Engelman et al showed that PF00299804, an irreversible pan- EGFR inhibitor, has significant activity in NSCLC cell lines harboring endogenous T790M mutations, cell lines engineered to express EGFR T790M in cis to an activating mutation and in their xenograft model.26 HKI-272 is very effective in NCI-H1975 cell line, harboring both L858R and T790M, as well as in NSCLC cell lines selected for geftinib resistance.23 Preclinical efficacy of second-generation, irreversible EGFR-TKIs has translated into only modest clinical efficacy in patients with NSCLC who are resistant to erlotinib or geftinib (Table 2).

HKI-272 (Neratinib)

HKI-272 is an irreversible pan-EGFR receptor TKI, which is the first second-generation EGFR-TKI examined in a large-scale study of patients with NSCLC who had previously benefited from an EGFR-TKI such as gefitinib or erlotinib.27,28 In this phase II trial, 137 patients with NSCLC were treated in 3 arms. Patients in arm A and B had progressed after at least 12 weeks of prior EGFR-TKIs and arm C included patients who were TKI-naïve. Patients were placed in arm A if they were EGFR-mutation positive, in arm B if they were wild-type, and in arm C if they were light smokers and had adenocarcinoma. Overall, HKI-272 had low efficacy, with 3.4% RR in arm A, 0% RR in arm B or C, and median PFS of 15.3 weeks (90% confidence interval [CI]: 14.7-15.9 weeks) for all patients. Twelve patients (7%) had T790M mutation, including 11 from arm A with prior TKI exposure and one patient who was TKI-naïve from arm C. None of the patients with T790M mutation had response. All patients initially received neratinib at 320 mg daily, which caused significant diarrhea (grade 3 in 50% of patients); therefore the dose was subsequently reduced to 240 mg with decrease in grade 3 diarrhea to 25%. Pharmacokinetic data were not available for patients on this study but based on the previous phase I study, lowering the dose for excessive diarrhea might have decreased drug bioavailability below the threshold required to inhibit T790M (based on preclinical models).23

BIBW-2992 (Afatinib)

Afatinib is an orally bioavailable, pyrimidine-based, irreversible inhibitor of EGFR, HER2, and HER4.29 It has gained FDA approval for clinical use in the firstline treatment of patients with NSCLC who have the EGFR mutation. Afatinib was evaluated in LUX-Lung 1, a randomized, placebo-controlled, phase IIb/III trial in patients with NSCLC who had progressed after at least 12 weeks of treatment with erlotinib or gefitinib.30 Patients were not required to have been tested for EGFR mutation to enter the study. However, the requirement for at least 12 weeks of treatment with EGFR inhibitors served as an enrichment strategy for patients with EGFR mutations and acquired resistance. There was no difference in the median overall survival (OS) between the two groups. Median OS was 10.8 months in the afatinib group (95% CI: 10.0-12.0) versus 12.0 months (10.2- 14.3) in the placebo group (hazard ratio [HR] = 1.08; 95% CI: 0.86-1.35; P = .74). Median PFS was longer in the afatinib group than in the placebo group (3.3 months vs 1.1 months; HR = 0.38; 95% CI: 0.31- 0.48; P <.0001). The confirmed disease control rates (DCRs; ORR plus stable disease) for ≥8 weeks for afatinib was 58.2% compared with placebo, DCR of 18.5% (P <.0001). Only 8 patients had known T790M mutation, and responses were not reported in the study. Diarrhea and rash were the most common severe adverse effects (AEs) observed in patients who received afatinib, requiring dose reduction in 21% and 15 % patients, respectively. Incidence of diarrhea in the afatinib arm was 86.9%, of which 16.9% were of grade 3 intensity, based on common terminology criteria for adverse events (CTCAEs). Any grade rash/acne was observed in 76.7% of patients receiving afatinib and grade 3 rash was reported in 14.4% of patients.

Similar to the LUX-Lung 1 trial, the LUX-Lung 4 trial evaluated efficacy of afatinib in Japanese patients with NSCLC in third-line and fourth-line settings after progression on erlotinib and/or gefitinib.31 The study enrolled 61 patients; 72.6% had an EGFR-mutant tumor, and 82% of patients fulfilled Jackman’s criteria of acquired resistance to EGFRTKIs. The results were consistent with the LUX-Lung 1 trial. In patients meeting the criteria for acquired resistance, median PFS was 4.4 months with partial response (PR) of 5.9%, and DCR of 68.6%. Two patients had acquired T790M mutations and achieved stable disease, 1 for 9 months, and the other for 1 month. Diarrhea and skin rash were the most commonly reported AEs occurring in almost all patients. Grade 3 diarrhea occurred in 37.1% of patients and grade 3 rash occurred in 27.4% of patients.

PF-00299804 (Dacomitinib)

Dacomitinib is an oral, irreversible, small-molecule inhibitor of HER1/EGFR, HER2, and HER4 TKIs with antitumor activity in both gefitinib-sensitive and gefitinib-resistant preclinical NSCLC models. A phase II trial examined efficacy and safety of dacomitinib in patients with KRAS wild-type NSCLC who progressed after 1 or 2 chemotherapy regimens and erlotinib.32 Patients with adenocarcinoma as well as squamous histology were included. Among EGFR mutation-positive patients, RR was 8% and PFS was 4.5 months. Median PFS for overall population was 12 weeks. Among 6 patients with known EGFR T790M; 3 had stable disease for more than 9, 12, and 12 weeks, respectively, and 3 had progressive disease. Diarrhea (grade 3, 12%), acneiform rash (grade 3, 6%), and fatigue (grade 3, 3%) were the most common treatment-related AEs. Patients with radiographic response to treatment also had improvement in NSCLC-related symptoms of dyspnea, cough, and pain (chest, arm/shoulder), first observed after 3 weeks on therapy. An ongoing phase II study is evaluating efficacy of dacomitinib in 2 groups of patients with NSCLC, those whose tumor has a documented T790M mutation and those without this mutation (NCT01858389).

BMS-690514

BMS-690514 is a second-generation, reversible, multikinase inhibitor with activity on EGFR, HER2, HER4, and vascular endothelial growth factor receptors (VEGFRs) 1 to 3.33 In a phase I-IIa study, BMS- 690514 was evaluated in patients who were erlotinib- naïve (cohort A) or erlotinib-resistant (cohort B) with advanced NSCLC.34 Maximum-tolerated dose (MTD) was determined to be 200 mg daily. DCR (≥4 months) and RR, respectively, were 43.3% and 3.3% (cohort A) and 22.6% and 3.2% (cohort B). Disease control was achieved in 7 of 10 (70%) patients with EGFR mutations (including T790M) and 6 of 21 (29%) patients with wild-type EGFR. Tumor shrinkage was observed in 1 of 2 patients with EGFR T790M mutation. The most frequent treatment-related AEs were diarrhea and acneiform rash.

XL-647

XL-647 is a second-generation, reversible inhibitor of multiple receptor TKIs, including EGFR, VEGFR2, HER2, and EphB4.35 In preclinical studies, XL-647 showed encouraging efficacy against tumors harboring T790M.35 However, a phase II trial in patients with NSCLC who progressed after ≥12 weeks treatment with erlotinib or gefitinib and/or those with a documented EGFR T790M, demonstrated a disappointing 3% RR and significantly worse PFS in patients with T790M mutation.36 Of 33 evaluable patients, 12 (36%) had tumors with T790M mutation. None of the patients with T790M mutation had objective response and 67% (8/12) had progression of disease as best response compared with 14% (3/21) of those without this mutation.

Afatinib and Cetuximab

Afatinib showed significant activity in NSCLC cell lines carrying T790M mutation, but no meaningful response was seen with afatinib alone in transgenic mouse models of EGFR L858R/T790M and EGFR T790M.37 Analysis of mRNA profile from the tumors in these models of lung cancer showed higher expression of EGFR ligands, amphiregulin, and epiregulin, in tumors compared with normal lung. Combined treatment with EGFR antibody, cetuximab and afatinib resulted in near complete response in T790M transgenic mouse model. Tumor lysates from the mice that received combined treatment showed marked depletion of both phospho-EGFR and total EGFR levels, consistent with the mechanism of the individual drugs. Based on these results, Janjigian et al conducted a pilot study of afatinib, 40 mg daily, and escalating-dose cohorts of biweekly cetuximab at 250 mg/m2 and 500 mg/m2 in patients with EGFRmutant lung cancer who had progressed on prior EGFR- TKIs.38 Combination of afatinib 40 mg daily and biweekly cetuximab 500 mg/m2 was recommended for further phase II evaluation. No dose-limiting toxicities (DLTs) were observed. Grade 1/2 rash and diarrhea were common AEs. Objective responses were seen in 8 of 22 evaluable patients (36%, 95% CI: 0.17-0.59) and disease control was observed in all patients. Because of these encouraging results, this study was expanded, and updated results were presented at the 2012 meeting of the European Society for Medical Oncology (ESMO).39 Among 90 patients evaluable for efficacy, DCR was 94% and PFS at 3, 6, and 9 months were 70%, 42%, and 18%, respectively. The OR rate was 40% (95% CI: 27.6-53.5), similar in both T790M+ (38%) and T790M– (47%) tumors. The median PFS was 4.7 months, and the median duration of response was 7.7 months. Most common AEs included rash (grade 1/2, 65%; grade 3, 12%) and diarrhea (grade 1/2, 63%; grade 3, 6%).

Acquired resistance develops in some xenograft models of T790M after chronic exposure to afatinib and cetuximab combination.40 None of the common secondary EGFR mutations were detected in these resistant cells. Analysis of resistant tumors by array comparative genomic hybridization (aCGH) revealed additional amplification at the EGFR locus, and immunoblotting of cell lysates showed increased EGFR protein expression. Interestingly, resistant cell lines displayed in vitro sensitivity to AZD9291, a third-generation, irreversible mutant-specific EGFR- TKI.40

Third-Generation Nonquinazoline-Based EGFR-TKIs

Discrepancy between preclinical and clinical efficacy of second-generation EGFR-TKIs could be related to inability to achieve enough drug concentration in plasma to sufficiently inhibit EGFR T790M.27-41 The T790M secondary mutation restores ATP affinity of mutant EGFR to that of wild-type EGFR. The concentration of quinazoline-based EGFR inhibitors required to inhibit EGFR T790M also inhibits wildtype EGFR, leading to AEs of diarrhea and skin rash that would limit the ability to achieve plasma concentration required for inhibition of EGFR T790M. To circumvent this problem, third-generation irreversible EGFR-TKIs have been designed with preferential activity against EGFR T790M compared with EGFR wild-type (Table 2).

TABLE 2. Second- and Third-Generation EGFR-TKI in Patients With NSCLC Who Have Acquired Resistance to Erlotinib or Geftinib

EGFR-TKI Target Study Phase Response Rate PFS
HKI-272 (Neratinib) Pan-ERBB Phase II 3.4% 15.3 weeks
BIBW-2992 (Afatinib) EGFR, HER2, and HER4 LUX-Lung 1 (phase IIb/III)
LUX-Lung 4 (phase I)
7%
5.9%
4.5 months
4.4 months
PF-00299804 (Dacomitinib) EGFR, HER2, and HER4 Phase II 8% 4.5 months
BMS-690514 EGFR
HER2, HER4 VEGFR 1-3
Phase I-IIa 3.2% -
XL647 EGFR, VEGFR2 HER2, EphB4 Phase II 3% -
Afatinib and Cetuximab EGFR, HER2, and HER4 Phase I 38% 4.7 months
CO-1686 EGFR Phase I 67% -
AZD9291 EGFR Phase I 58% -

PFS indicates progression-free survival.

WZ4002

The common anilinoquinazoline scaffold of the EGFR- TKI relies on the hydrogen bonding interactions with the gatekeeper threonine of wild-type EGFR and may not be the best fit for potent and specific inhibition of EGFR T790M. Zhou et al prepared a library of common kinase inhibitor core scaffolds that would be predicted to react with Cys 797 of EGFR, based on molecular modeling.41 This library was screened for compounds that could inhibit the growth of both gefitinib-resistant and gefitinibsensitive lines but were not toxic to cell lines with wild-type EGFR. WZ4002, a pyrimidine, was identified from this screen and was found to have 300- fold lower IC50 against EGFR T790M compared with HKI-272. Furthermore, WZ4002 was more potent in inhibiting kinase activity of recombinant L858R/ T790M protein than wild-type EGFR and was 100- fold less effective at inhibiting phosphorylation of wild-type EGFR than the quinazoline inhibitors. In T790M-containing murine models, treatment with WZ4002 resulted in significant tumor regression with little activity on wild-type EGFR, as evidenced by no significant EGFR phosphorylation in hair bulbs from mouse skin. Clinical trials with WZ4002 have not been initiated yet.

CO-1686

CO-1686 is also a pyrimidine molecule, designed with molecular modeling to covalently bind with Cys797 in the ATP-binding pocket of the EGFR kinase domain.42 In kinetic studies using recombinant proteins, CO- 1686 has 22-fold higher selectivity for T790M-mutant EGFR compared with wild-type EGFR. CO-1686 potently inhibited the kinase activity of EGFR carrying T790M mutation both in vitro and in vivo (cell line xenograft and T790M transgenic mouse models). In vitro studies with chronic exposure to CO-1686 demonstrate that acquired resistance develops over time and is associated with epithelial-mesenchymal transition (EMT).42,43 No further mutations in EGFR tyrosine kinase domains were found in cells resistant to CO-1686. Higher basal levels of phosphor-AKT were also observed in the resistant cells, and AKT inhibitors restored partial drug sensitivity when used in combination with CO-1686.43

CO-1686 is being developed by Clovis Oncology and is the most advanced third-generation EGFR-TKI in clinical development. A phase I, dose-escalation study of CO-1686 in advanced EGFR-mutant NSCLC is ongoing.44 Patients are enrolled after progression on EGFR-TKIs with 5 days washout from prior reversible EGFRI-TKI and 14 days from irreversible EGFR-TKI. All patients are required to undergo tumor tissue biopsy within 28 days before study drug dosing for central EGFR genotyping. CO-1686 is administered orally in a continuous 21-day cycle. Initially, free-base capsule formulation was used, which was later changed to hydrobromide salt of CO-1686 with improved drug bioavailability and reduced variability. The study has enrolled 56 patients, given doses ranging from 150 mg daily to 900 mg twice daily. Most common EGFR mutations were exon 19 deletion (57%) and L858R (36%); T790M mutation was positive in 39 (70%) patients. Common AEs were grade 1 and 2 nausea, vomiting, and fatigue. A DLT is hyperglycemia, which is mostly asymptomatic and easily managed by dose reduction and oral hypoglycemics. Because CO- 1686 does not cause sufficient inhibition of wild-type EGFR, typical AEs of first-generation EGFR-TKIs, such as rash and diarrhea, were not seen. Recommended phase II dose for the hydrobromide salt form of CO- 1686 was 750 mg twice daily. The PFS for patients with the T790M mutation who had CO-1686 plasma concentrations >200 ng/mL for >16 hours was 194 days compared with 72.5 days for those who achieved these concentrations for <16 hours. In evaluable patients with the T790M mutation, who were treated at 900-mg twice daily dose (free base), RR was 67%, based on Response Evaluation Criteria in Solid Tumors (RECIST) criteria. According to results from this study presented at the European Lung Cancer Conference (ELCC) in Geneva in March 2014, overall response rates (ORR) were 64% with an overall clinical benefit of 91%. At 6 months, the median PFS was not reached for patients with T790M mutations compared with a 3-month median for patients who were T790M negative.

TABLE 3. TIGER Program for Further Development of CO-1686 in EGFR-Mutant NSCLC

CO-1686 TIGER (Third-Generation Inhibitor of Mutant EGFR in Lung Cancer) program
TIGER1 Phase 2/3 randomized registration study in newly diagnosed patients (vs erlotinib)
TIGER2 Phase 2 registration study in second-line T790M+ patients directly progressing on first TKI
TIGER3 Phase 2 registration study in later-line patients, progressing on second or later TKI or subsequent chemotherapy
TIGER4 Phase 2 study in second-line or later-line patients with T790M detected with a blood/plasma assay
TIGER5 Phase 3 randomized confirmatory study in second-line or later-line patients (vs chemotherapy)
The TIGER (Third-generation Inhibitor of mutant EGFR in lung cancer) study is a global phase II/III clinical trial of CO-1686 in EGFR-mutant lung cancer (Table 3).

EGFR mutation detection in circulating DNA from plasma samples is also being explored in the ongoing trial of CO-1686.45 Matched tumor and plasma were obtained from patients with NSCLC at baseline and evaluated with cobas EGFR FFPET and cobas EGFR blood test that use allele-specific polymerase chain reaction (PCR) and detect mutations, including T790M, L858R, and exon 19 deletions. Serial blood samples were obtained and analyzed by a BEAMing (beads, emulsion, amplification, magnetics) technique that is a highly sensitive test using droplet digital PCR followed by flow cytometry for longitudinal monitoring. Overall, there was strong agreement between these 2 test results. Preliminary results demonstrate that a high proportion of EGFR mutations identified in tissue was also detected in plasma and that a decrease in plasma EGFR DNA generally correlates with response to CO-1686. Detecting EGFR mutation from plasma sample has many advantages over tissue sample. It is noninvasive and could overcome the obstacle of lack of sufficient tissue for molecular testing. Tumor heterogeneity is better represented in the plasma sample and it obviates the need for repeat biopsy for serial monitoring of EGFR-mutation status during treatment.

AZD9291

AZD9291 is a third-generation, irreversible, oral TKI with promising activity against NSCLC with activating EGFR mutations and resistant T790M mutation. AZD9291 effectively inhibits EGFR phosphorylation in H1975 cell line harboring both L858R and T790M while having much less activity against wild-type EGFR.46 In xenograft and transgenic mice models of T790M, AZD9291 showed significant tumor shrinkage and profound inhibition of downstream signaling pathway. Chronic treatment with AZD9291 in these models showed no visible tumors beyond 100 days. A phase I study is under way.

AZD9291 is being evaluated in patients with advanced NSCLC after disease progression with an EGFR- TKI (NCT01802632; sponsor AstraZeneca).47 The preliminary results, reported at European Cancer Congress 2013, showed that 27 patients were treated at 20-mg, 40-mg, and 80-mg dose levels and in an expansion cohort of T790M-positive patients. No DLTs were reported. Most common AEs were grade 1 diarrhea and rash. Two confirmed PRs were seen at the first dose level (20 mg) in patients with T790M mutation. Overall, RR was 46% (12/26) in all patients and 58% in patients with tumors carrying T790M mutation. A phase II study (AURA2) is planned to assess the safety and efficacy of AZD9291 (80 mg, orally, once daily) in patients with a confirmed diagnosis of EGFR mutation-positive and T790M mutation-positive NSCLC, who have progressed following prior therapy with an approved EGFR-TKI (NCT02094261).

Conclusion

Since the first description of EGFR-mutant lung cancer as a distinct clinical entity in 2004, the remarkable advances made in the treatment of this unique subset of lung cancer serve as a paradigm for the treatment of oncogene-addicted solid tumors. Acquired resistance to TKIs because of secondary mutations in the TKI domain is an expected and common phenomenon. Clear understanding of the resistance mechanism at the structural and molecular level has led to custom designing of mutant-specific inhibitors with little activity on the wild-type receptor. CO-1686 and AZD9291 are promising, novel, third-generation EGFR-TKIs with encouraging activity in patients with T790M-mutant lung cancer, and both of these agents have an excellent safety profile. Ongoing clinical trials are likely to establish their role, not only in the treatment of EGFR-TKI-resistant disease, but also in the front-line treatment of EGFR-mutant lung cancer. Preclinical evidence suggests that resistance to mutant-specific EGFR-TKIs is not likely to be caused by secondary mutations in TKI domain and that combination with other targeted agents will further increase efficacy.

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