Gene Mutations in Lung Cancer: Promising Predictive Factors for the Success of Molecular Therapy

Gefitinib and erlotinib are two new treatments for advanced lung cancer. Gene mutations in the cancer may help predict which patients will respond to these treatments

L ung cancer is the leading cause of cancer death in many countries. To date, chemotherapy with cytotoxic agents has been the mainstay of treatment for advanced lung cancer. However, the activity of these agents is quite limited, and they have severe adverse effects. Recent, rapid advances in molecular biology have led to the development of many new agents that inhibit the activities of specifi c molecules related to tumor growth, invasion, or metastasis [1], and these agents have the potential to improve the outcome of lung cancer treatment. But we have not yet managed to successfully deliver these agents from the bench to the bedside.

Molecular Therapy for Lung Cancer
Gefi tinib is the fi rst "molecular-target agent" for lung cancer that inhibits the tyrosine kinase of the epidermal growth factor receptor (EGFR; also known as ERBB1). EGFR is frequently overexpressed in non-small-cell lung cancer (NSCLC), especially in squamous cell carcinoma, and its expression is related to the cancer's proliferation. Initial clinical trials of gefi tinib showed its modest clinical activity in patients who had failed previous standard chemotherapy. But subsequent randomized trials in patients with previously untreated, advanced NSCLC have not shown a clinical advantage of gefi tinib combined with standard chemotherapy over chemotherapy alone [2,3,4,5].
Erlotinib, another EGFR tyrosine kinase inhibitor for treating NSCLC, which was approved by the United States Food and Drug Administration in November 2004, has a therapeutic profi le similar to that of gefi tinib. Erlotinib has shown a survival benefi t in a phase III trial for chemo-refractory NSCLC that compared erlotinib to best supportive care [6]. (To date, there have been no trials showing a survival benefi t of EGFR inhibitors over standard chemotherapy.) Interestingly, in these trials of EGFR inhibitors, EGFR expression levels in the tumors were not correlated with the response; high response rates were seen in women, patients with adenocarcinoma, nonsmokers, and Japanese patients. Recent studies have now shed light on why certain patients are more likely to respond than others-the key lies in the presence of gene mutations.

Perspectives
The Perspectives section is for experts to discuss the clinical practice or public health implications of a published study that is freely available online.

EGFR Gene Mutations in NSCLC
In April and May 2004, two new studies published in Science and the New England Journal of Medicine helped to explain at the molecular level the low clinical activity of EGFR inhibitors [7,8]. These studies identifi ed somatic mutations in the EGFR gene, especially around the region encoding the ATP-binding pocket of the receptor's tyrosine kinase domain. These mutations increased the sensitivity of tumor cells to gefi tinib. A high incidence of mutations was detected in patients with NSCLC that had a durable clinical response to gefi tinib, and subsequent studies revealed that these mutations were also related to the response to erlotinib. Moreover, such mutations were more frequently detected in patients with adenocarcinoma, in women, in Japanese patients, and in nonsmokers [9]-results that were compatible with previous clinical data.
These fi ndings should hopefully lead to the identifi cation of subgroups of patients who are likely to benefi t substantially from such EGFR inhibitors. But another question was raised by these studies: is it only EGFR gene mutations that determine the response to EGFR inhibitors?

RAS Gene Mutations in NSCLC
The RAS proteins are low-molecularweight GTPases that are bound to the inner side of the cell membrane. They are involved in signal transduction pathways: they regulate downstream effector proteins such as Raf/MAP kinase and PI3 kinase, under the infl uence of various cell surface receptors including EGFR ( Figure  1). Mutations of the K-ras gene have been found in up to 30% of lung adenocarcinomas and have been considered a poor prognostic factor [10]. A study by Pao and colleagues published in this issue of PLoS Medicine suggests that the K-ras mutation is an important predictive factor in defi ning which patients will benefi t from receiving EGFR inhibitors [11]. In Pao and colleagues' study, the K-ras mutations were completely associated with a lack of response to EGFR inhibitors (0/14 tumors with K-ras mutations were sensitive). The EGFR mutations were signifi cantly related to response (17/17 tumors with EGFR mutations were sensitive), as observed in previous studies.
Although the number of tumors examined in this study may be too small to lead to a defi nite conclusion-and, furthermore, about half of tumors were retrospectively collected-this study is the fi rst to show that mutations of EGFR and K-ras are not related and that K-ras mutations are associated with a lack of sensitivity to EGFR inhibitors. However, since the sensitivity of the method for fi nding each mutation infl uences how often it is detected, standardization of detection methods is important. Hence, the true incidence of K-ras mutations in NSCLC (including non-adenocarcinoma) and their refractoriness to EGFR inhibitors need to be established in further studies. In fact, the reported incidence of EGFR mutations differs between institutions. In addition, although Pao and colleagues' study examined such mutations only by DNA sequence, mRNA or protein expression would show mutation status more accurately.

Therapeutic Implications
In the clinical setting, prolonged disease stabilization with no measurable reduction in tumor size is seen in about half of patients treated with EGFR inhibitors, and a signifi cant survival benefi t in this group was shown in a phase III trial [6]. The mutation status of the EGFR gene and K-ras gene among those stabilized tumors should be evaluated; it may reveal two different groups differentiated by mutation status.
It will also be important to establish whether, in tumors with an EGFR mutation that eventually acquire resistance to EGFR inhibitors, the resistance is associated with a change of EGFR status (mutation or change in expression) or is associated with a change of K-ras status. Such changes can be revealed by re-examination of tumors at the time of relapse. If resistance is K-ras dependent, the new "K-ras inhibitors" (unfortunately not yet available) may be of help for patients who have developed a K-ras mutation, as well as for patients whose tumors harbor K-ras mutations from the beginning.
Although many issues still need to be resolved step by step through prospective trials to show the benefi t of this new strategy, these mutations are promising predictive factors for the success of EGFR inhibitors. Even if the population who may benefi t from EGFR inhibitors (such as patients who are positive for an EGFR mutation and negative for a K-ras mutation) is very  small, the response rate of over 80% is encouraging, and has never before been achieved in advanced NSCLC. Finally, it is important to detect which patients derive no benefi t from EGFR inhibitors because severe adverse effects such as acute lung injury can occur in any patient treated with these drugs [12]. By combining all the factors that relate to response or resistance, patients who will benefi t from treatment can hopefully be identifi ed. Undoubtedly we have taken a great step forward in molecular therapy for lung cancer treatment.