The
EML4-ALK translocation defines a new molecular subset of NSCLC with distinct clinical and pathologic features. Previous studies have reported a low frequency of
EML4-ALK that has ranged from 1.5% to 6.7% in unselected populations.
7,11,13–17 These studies have involved predominantly Asian patients with surgically resected disease. In this study, we show that, in a select subpopulation of predominantly white patients, the majority of whom had metastatic NSCLC, the frequency of
EML4-ALK is significantly higher than that reported for unselected patients. Among the 141 patients screened, we identified 19 with
EML4-ALK and 31 with
EGFR mutation, which corresponded to frequencies of 13% and 22%, respectively. Within the group of never/light smokers in this study, the frequencies of
EML4-ALK and
EGFR were 22% and 32%, respectively; among never/light smokers without
EGFR mutation, the frequency of
EML4-ALK was 33%. These findings suggest that, in patients with NSCLC who have clinical characteristics associated with
EGFR mutation but who have negative
EGFR testing, as many as one in three patients may harbor
EML4-ALK.
Previous reports that describe the frequency of
EML4-ALK in NSCLC have been inconsistent in terms of clinical features that define this molecular subset. For example, in the first report of
EML4-ALK in NSCLC,
ALK rearrangement was detected in five patients, two of whom were noted to have a smoking history.
7 In subsequent studies,
EML4-ALK has been variably detected in both smokers and nonsmokers,
8,11,15,16 which suggests a lack of association between smoking history and presence of
EML4-ALK. Here, 19 of 85 patients classified as never/light cigarette smokers were positive for
EML4-ALK, whereas all 56 patients with a smoking history (> 10 pack years) were negative. This result suggests that
EML4-ALK is, in fact, strongly associated with never/light smoking history. This association was likely obscured in other studies because of small sample sizes and, possibly, differences in ethnic background.
Although
EML4-ALK patients share several clinical features with patients who have
EGFR mutant, including never/light smoking history and adenocarcinoma histology, this study demonstrates that
EML4-ALK is associated with at least three distinct features. First, compared with
EGFR or WT/WT patients,
EML4-ALK patients are more likely to be men. As female sex was used as one of the clinical selection criteria for genetic screening, our study tested almost twice as many women as men. However, we found that a significantly greater percentage of men than women were positive for
EML4-ALK (23%
v 9%). The sex difference observed in this study cannot be explained by differences in smoking history, as 60% of men and 60% of women were never/light smokers. Second, compared with
EGFR or WT/WT patients,
EML4-ALK patients are significantly younger. The difference in median age between
EML4-ALK patients and either
EGFR or WT/WT patients exceeded 10 years. Of note, the median age of our
EGFR cohort was similar to that reported in other studies.
22–24 Among the 19 patients with
EML4-ALK, four were younger than 40 years old. One recent study of
EML4-ALK in Asian patients with NSCLC noted a nonstatistically significant trend toward younger median age.
17 Interestingly, several other cancers known to harbor
ALK rearrangements, such as anaplastic large cell lymphomas, neuroblastomas, and inflammatory myofibroblastic tumors, are also associated with younger age and are, in fact, most common in children and young adults. Third,
EML4-ALK–positive tumors appear histologically distinct from
EGFR mutant and WT/WT tumors. The diagnostic and clinical implications of this finding will be discussed in a separate report (Rodig et al, manuscript submitted for publication), but this observation suggests that
EML4-ALK may represent a unique pathologic subtype of nonsmoking-related NSCLC.
In the clinic, the distinction between
EML4-ALK and
EGFR mutant tumors has important therapeutic implications. Whereas
EGFR mutation confers sensitivity to EGFR TKIs,
EML4-ALK is strongly associated with resistance. Among the 19 patients in this study with any response to erlotinib or gefitinib, 16 (84%) harbored an activating
EGFR mutation, whereas none harbored
EML4-ALK. Conversely, among the 34 patients refractory to EGFR TKIs, 10 (29%) were positive for
EML4-ALK. These findings are consistent with preclinical studies, which showed that the
EML4-ALK–containing NSCLC cell line H3122 is resistant to erlotinib.
11 These findings are also reminiscent of the resistance to EGFR TKIs conferred by activating mutations in
KRAS.
25 However, whereas
KRAS mutations are more commonly found in smokers,
26 both
EML4-ALK and
EGFR mutations are found in a similar population of never/light smokers. As a result, in the absence of genetic testing,
EML4-ALK patients are likely to be treated like patients with
EGFR mutation. Indeed, in this study, five of 15
EML4-ALK patients with metastatic NSCLC received erlotinib in the first-line setting. These results illustrate the importance of pretreatment genetic testing to guide clinical treatment recommendations, especially with regard to EGFR TKIs.
Overall, the clinical response of
EML4-ALK patients more closely resembles that of WT/WT patients rather than patients with
EGFR mutation. Both
EML4-ALK and WT/WT patients are unlikely to respond to EGFR TKIs and have lower rates of response to platinum-based chemotherapy than patients with
EGFR mutation. This difference does not appear to be related to imbalances among the cohorts in terms of type of platinum or inclusion of bevacizumab. The higher response rate associated with
EGFR mutation is consistent with previous studies, including the recently presented IPASS study (IRESSA Pan Asia Study), in which clinically selected patients with metastatic NSCLC were randomly assigned in the first-line setting to either carboplatin/paclitaxel or gefitinib. Among patients treated with chemotherapy, the objective response rates were 47.3% in
EGFR mutation–positive patients and 23.5% in
EGFR mutation–negative patients.
6 This study suggests that
EML4-ALK, in contrast to
EGFR mutation, is not associated with enhanced chemosensitivity.
Previous studies have not examined the outcome of patients with NSCLC who harbor
EML4-ALK. Here, we evaluated outcome by determining TTP and OS among patients with metastatic disease. This analysis was limited by the retrospective design of the study, by the relatively short duration of follow-up, and by the small number of events in the mutant cohorts. Nevertheless,
EML4-ALK patients had a longer median survival compared with WT/WT patients, though the power was too low to detect a significant difference. Patients who harbored
EGFR mutations showed a statistically significant improvement in survival compared with WT/WT patients (
P = .018), which is consistent with previous reports that demonstrated a more favorable outcome among patients with
EGFR mutation.
27 This survival analysis is additionally complicated by baseline differences in demographic features, particularly age and smoking history, between
EML4-ALK–positive and –negative patients, as well as by differences in the number and types of therapies received. In addition, to date, seven of 17
EML4-ALK patients with metastatic disease have participated in a phase 1 study of PF-02341066, a dual
MET/ALK TKI.
28 The clinical activity of this novel agent has not yet been reported, but its use in a significant proportion of
EML4-ALK patients may have influenced the outcome of this cohort.
In conclusion,
EML4-ALK defines a new molecular subset of NSCLC with distinct clinical and pathologic features. The patients most likely to harbor
EML4-ALK are young, never/light smokers with adenocarcinoma. As some of these features are also associated with
EGFR mutation, it is essential to screen such patients by mutation testing and not to rely solely on the presence of clinical predictors. We recommend screening first for
EGFR mutation, because
EGFR mutations are more common than
EML4-ALK rearrangements and because, importantly, EGFR TKIs are now used as first-line agents in advanced, mutation-positive disease. In the absence of an
EGFR mutation, patients then should be screened for
EML4-ALK. Preclinical studies have shown that
EML4-ALK confers sensitivity to
ALK inhibitors,
10,11,29 and studies suggest that patients with this chromosomal translocation may derive clinical benefit from specific
ALK inhibition. This hypothesis currently is being tested in the clinic and if confirmed will validate
ALK as a therapeutic target in NSCLC.