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Lowe Center for Thoracic Oncology, Department of Medical Oncology, Dana-Farber Cancer Institute and Department of Medicine, Brigham and Women's Hospital, and Harvard Medical School, Boston, Massachusetts
Requests for reprints: Bruce E. Johnson, Department of Medical Oncology, Dana-Farber Cancer Institute, 44 Binney Street, Boston, MA 02115. Phone: 617-632-4790; Fax: 617-632-5786; E-mail: BEJohnson{at}Partners.org.
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| Background |
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10% of patients of European background and 30% of patients from Japan had clinical responses when treated with the EGFR inhibitors, gefitinib and erlotinib (57). The clinical characteristics that are associated with responses to gefitinib and erlotinib were defined in these studies (57). Clinical responses to these agents were more common in women than men, in patients from Japan than from Europe and the U.S., patients with adenocarcinoma than other histologic subtypes, and patients who had never smoked cigarettes. Studies of another solid tumor, gastrointestinal stromal tumor, showed that activating mutations of the C-KIT gene were associated with clinical responses to the small molecule inhibitor of the tyrosine kinase domain of the C-KIT gene, imatinib (8). This led multiple groups to identify patients with NSCLC and dramatic clinical responses following treatment with gefitinib and erlotinib for sequencing of the tyrosine kinase domain of EGFR in their tumor.
| Therapeutic Impact of EGFR Mutations |
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The in vitro studies of NSCLC cell lines and host cell lines transfected with different mutations of the EGFR have provided important insights into how the mutations activate the tyrosine kinase domain of EGFR and the dramatic impact gefitinib and erlotinib have on EGFR signaling (1, 2, 9, 12, 13). EGFR is constitutively activated as assessed by phosphorylation of the different tyrosine residues on EGFR, the downstream pathways are activated, and gefitinib and erlotinib inhibit both this signaling and the growth of the NSCLC cells (Fig. 1). Investigators have identified six cell lines with deletions and mutations of EGFR that are sensitive to treatment with gefitinib and erlotinib (2, 1215). These include PC-9, HCC-827, NCI-H1650, NCI-H1975, NCI-H3255, and DFCILU-011. Four NSCLC cell lines have deletions of exon 19 (PC-9, H1650, HCC827, and DFCILU-011) and two have point mutations of exon 21 (NCI-H1975 and NCI-H3255). The IC50 of these six NSCLC lines range from 20 to 200 nmol/L of gefitinib when assessed in dye conversion assays. Three cell lines with wild-type EGFR have intermediate sensitivities (NCI-1819, NCI-H1666, and Calu-3) with IC50's between 1 and 5 µmol/L (2, 13, 15). Most other NSCLC cell lines with wild-type EGFR require concentrations of gefitinib >10 µmol/L to inhibit their growth, >10-fold the achievable plasma concentrations in patients (2, 4, 13).
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The ability of the mutant EGFR to mediate changes in growth and signaling have been verified by transfection experiments into multiple cell systems. The mutant and wild-type EGFR have been transfected into different host cells including NMuMg, COS-7, Chinese hamster ovary cells, and HEK293 cells to determine differences in biochemical signaling and growth (1, 2, 12, 14, 15). The growth and downstream signaling inhibition of the host cell lines transfected with mutant EGFR are consistently more sensitive to treatment with gefitinib and erlotinib than cells with wild-type receptors. The wild-type EGFR receptor typically sends a downstream signal that ultimately stimulates the growth of the NSCLC cells that are dependent on the receptor and gefitinib or erlotinib can modestly inhibit this relatively weak signal (Fig. 2). The mutated EGFR receptor is constitutively activated with a prominent downstream signal that can be dramatically inhibited by gefitinib and erlotinib.
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The types of EGFR mutations in patients treated with gefitinib and erlotinib have been similar around the world (1, 9, 11, 1619). The mutations involve multiple overlapping deletion mutations of exon 19 in 45% of patients, missense mutations in exon 21 in 40% of patients (predominately L858R), and missense or insertion mutations in exons 18 to 21 in the other 15% of patients. There is no clear difference in the mutational pattern in the tumors from patients of European background versus those from East Asia. There is not yet enough information to make firm conclusions about the efficacy of gefitinib and erlotinib treatment in patients with NSCLC as a function of their different types of mutations of EGFR.
The EGFR sequence has been characterized in large numbers of patients with early stage NSCLC that have not been treated with gefitinib or erlotinib. There have been three large studies sequencing >1,600 resected NSCLCs from patients in the U.S., Europe, and East Asia that provide potential explanations about differences between the response rates of 5% to 10% in patients from the U.S. and Europe versus the response rates of 20% to 30% in patients from East Asia (2022). The mutations involve deletion mutations of exon 19 in
50% of patients, missense mutations in exon 21 in 40%, and mutations in exons 18 and 20 in the other 10%. The clinical characteristics of patients with mutations of EGFR closely parallel the subsets of patients who are more likely to respond to treatment with gefitinib and erlotinib. The patients with NSCLC from the U.S., Europe, and Australia have an EGFR somatic mutation frequency of
10% compared with a mutation rate of 30% in patients from Japan and Taiwan. There is also a close association between adenocarcinoma and mutations in the EGFR. The three studies showed that 263 of 888 patients with adenocarcinomas (30%) had mutations detected, whereas 7 of 764 patients with other types of NSCLC (1%) had a somatic mutation of EGFR (2022). The somatic mutations of EGFR were also two to three times more likely in women than men and three to five times more likely in nonsmokers than those who were current or former smokers. The findings clearly mirror the increased response rates in these same patient populations when treated with gefitinib and erlotinib.
Despite the observation that patients with NSCLC and somatic mutations of EGFR treated with gefitinib live longer than patients whose NSCLC have wild-type EGFR with similar treatment, the EGFR status does not seem to have a dramatic impact on the outcome of early stage patients. The patients with early stage lung cancer who undergo surgical resection of their NSCLC with somatic mutations of the EGFR have a similar survival compared with those with wild-type receptor (22). The Kaplan-Meier survival curves of the patients with resected NSCLC showed that >80% of the patients with somatic mutations of EGFR (91) and those with wild-type EGFR (145) were alive at 3 years.
| Acquired Resistance to EGFR Inhibitors |
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| Meaning and Implications of Mutations of the EGFR |
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80% of the patients. This may provide a rationale for selecting patients with specific histologies (adenocarcinoma) and smoking status as candidates for clinical trials with EGFR inhibitors, a new concept in the treatment of NSCLC. The retrospective collection of information shows that patients with NSCLC and EGFR mutations have a clinical response rate of
80%, can be treated for at least a year, and have at least a 2-year median survival after the start of treatment. Prospective clinical trials for patients with NSCLC are ongoing to prospectively assess the impact of giving gefitinib or erlotinib to previously untreated patients with NSCLC and mutations in the EGFR. If these retrospective observations are confirmed in prospective trials, patients with NSCLC who have a mutation identified in their EGFR will likely be offered an EGFR inhibitor as their initial therapy for NSCLC. The prospect of being able to treat a subset of patients (10-30% of the population of NSCLC) with gefitinib or erlotinib for a median of a year or more with the expectation of a median survival of
2 years has the potential to dramatically improve the outcome for this subgroup of patients. Clinical trials will also be needed to determine if patients with NSCLC and mutations of the EGFR need to be treated with erlotinib or gefitinib alone or if there is an advantage to adding chemotherapy to their regimen. The discovery of the association between EGFR mutations and the response to gefitinib and erlotinib has transformed NSCLC from one disease treated with conventional combination chemotherapy to subsets of NSCLC identified by genomic studies. Patients with NSCLC and EGFR mutations can be effectively treated with gefitinib and erlotinib. Although many aspects of EGFR mutations have been characterized, several unknown questions remain (Table 1). The information on the association between EGFR mutations and response to gefitinib and erlotinib has now been available for more than a year. The answers to the remaining important questions will come from additional well-conducted ongoing and planned preclinical and clinical studies of patients and NSCLC cell lines with wild-type and mutated EGFR treated with EGFR inhibitors.
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| Acknowledgments |
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Received 4/13/05. Revised 6/ 9/05. Accepted 6/16/05.
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