Biomarkers That Currently Affect Clinical Practice: EGFR, ALK, MET, KRAS

New drugs such as pemetrexed, the epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors, and the Alk inhibitor crizotinib have recently enabled progress in the management of advanced non-small-cell lung cancer (NSCLC). More drugs, especially Met inhibitors, will follow. However, the benefits of these agents are not uniform across the spectrum of NSCLC, and optimizing their utility requires some degree of subgrouping of NSCLC by the presence or absence of certain biomarkers. The biomarkers of current or imminent value are EGFR and KRAS mutational status, ALK rearrangements, and MET immunohistochemistry. As a predictor of benefit for anti-EGFR monoclonal antibodies, EGFR immunohistochemistry is also of potential interest. Some of the foregoing biomarkers (EGFR, ALK, MET) are direct drivers of the malignant phenotype. As such, they are, quite rationally, the direct targets of inhibitory drugs. However, KRAS, while definitely a driver, has resisted attempts at direct pharmacologic manipulation, and its main value might lie in its role as part of an efficient testing algorithm, because KRAS mutations appear to exclude EGFR and ALK mutations. The indirect value of KRAS in determining sensitivity to other targeted agents or to pemetrexed remains controversial. The other biomarkers (EGFR, ALK, MET) may also have indirect value as predictors of sensitivity to chemotherapy in general, to pemetrexed specifically, and to radiotherapy and molecularly targeted agents. These biomarkers have all enabled the codevelopment of new drugs with companion diagnostics, and they illustrate the paradigm that will govern progress in oncology in the immediate future. However, in NSCLC, the acquisition of sufficient biopsy material remains a stubborn obstacle to the evolution of novel targeted therapies.