Overcoming molecular mechanisms of resistance to first-generation epidermal growth factor receptor tyrosine kinase inhibitors

The epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) gefitinib and erlotinib have provided substantial benefits to patients with advanced non-small cell lung cancer (NSCLC). However resistance to these agents has emerged as a significant clinical issue; most patients who initially respond to treatment eventually experience relapse. The mechanisms underlying gefitinib and erlotinib resistance are multifactorial and several have been described. Clearly there is a need for novel and more effective therapies that can overcome resistance to the currently available TKIs. Several agents are in clinical development, including irreversible EGFR TKIs, inhibitors of the MET pathway, and others. In this review we discuss the various underlying mechanisms of gefitinib and erlotinib resistance and highlight the agents currently in clinical development that may have potential for overcoming this resistance.

New strategies in overcoming acquired resistance to epidermal growth factor receptor tyrosine kinase inhibitors in lung cancer

The management of non-small cell lung carcinoma (NSCLC) has been transformed by the observation that lung adenocarcinomas harboring mutations in epidermal growth factor receptor (EGFR) are uniquely sensitive to EGFR tyrosine kinase inhibitors (TKI). In these patients, acquired resistance to EGFR-TKI develops after a median of 10 to 14 months, at which time the current standard practice is to switch to conventional cytotoxic chemotherapy. Several possible mechanisms for acquired resistance have been identified, the most common being the development of an EGFR T790M gatekeeper mutation in more than 50% of cases. In this review, we discuss recent advances in the understanding of acquired TKI resistance in EGFR-mutant lung cancer and review therapeutic progress with second generation TKIs and combinations of targeted therapies.

[Sampling and search for the EGFR mutation: what's taking, when and how to test?]

Since the IPASS study, that has shown a benefit in term of progression-free survival in first line of treatment in non-small cell lung cancer (NSCLC) with activating EGFR mutations, and the restricted prescription of this treatment to the presence of these mutations, the determination of the mutational status has became necessary at the diagnosis. This research study of these mutations, present in about 10% of non-selected NSCLC, has to be preferred in case of clinical factors predictive of mutations (sex, race, tobacco status, adenocarcinoma), but don't have to be exclusive, the correlation between these clinical factors and the presence of the mutations being imperfect. Also, the research study of the EGFR mutations should be performed for patients non eligible for chemotherapy (PS>2). Molecular testing could be performed on biopsies, and also on cytology, knowing that there is a mismatch between the tumor and the metastasis for EGFR status. Samples should be prepared according to standardized protocols, avoiding fixation with Bouin. The gold-standard for molecular testing is still sequencing, but alternative targeted tests (allele-specific PCR...) seem to be more sensitive. Some immunohistochemistry tests are in development for EGFR mutations, but have to be validated on large prospective cohorts. At last, new tumoral sampling have to be performed in case of progression under gefitinib in first line, to look for acquired resistance EGFR mutations, or for other resistance molecular markers (Met amplification).

[Management of patients with resistance to EGFR-TKI]

Patients presenting with non-small cell lung cancer (NSCLC) and active EGFR mutation have a high response rate (60-70%) to EGFR tyrosine kinase inhibitors (TKI) with little immediate progression (primary resistance). However, progression on this treatment (secondary resistance) is inevitable even for those who responded initially. These two situations are distinct in terms of management. In case of primary resistance, screening for other associated molecular abnormalities (tumour heterogeneity) should be done, even resulting in a false positive in the initial screening of EGFR mutation. In case of secondary resistance, a new pathology sample should be taken insofar as is possible to determine the presence of an acquired mutation of EGFR resistance (T790M in 60% of cases) or c-Met amplification (20% of cases). The presence of a T790M mutation could respond to irreversible EGFR-TKI, while a c-Met amplification could be managed with a targeted anti-Met therapy. However, the gold standard is still cytotoxic chemotherapy at present if a clinical trial based on a targeted therapy is not possible.

The search for improved systemic therapy of non-small cell lung cancer--what are today's options?

Although stage IV non small cell lung cancer (NSCLC) remains an incurable disease and drug resistance ultimately develops, important steps forward have been made within the last few years. The number of new active agents is rapidly increasing and the area of personalized medicine has definitively arrived. Treatment choices are starting to be made upon tumour characteristics, and more effective and better tolerated agents are now available. In the molecular era we are facing many new challenges: The availability of sufficient tissue, which is often not easily accessible in a tumour arising from an internal organ, sensitivity and specificity of biomarker testing, heterogeneity of marker profiles between primary tumour and metastases or even within a single tumour mass, application and reproducibility of the correct biomarker test method and the differentiation of the predictive vs. the prognostic value of biomarkers. Unfortunately, the development of new targeted agents frequently continues to be performed in unselected patient populations, lacking the early identification of relevant predictive molecular biomarkers. Consequently, thousands of patients are enrolled into clinical trials with a low probability of success. This overview will focus on the most recent new systemic treatment options in NSCLC, including the role of pemetrexed, bevacizumab, cetuximab, maintenance therapy and tyrosine kinase inhibitors in patients with an activating epidermal growth factor receptor mutation. In addition, emerging treatment strategies will be described.