Successes and limitations of targeted cancer therapy in lung cancer.

Human cancers usually evolve through multistep processes. These processes are driven by the accumulation of abundant genetic and epigenetic abnormalities. However, some lung cancers depend on a single activated oncogene by somatic mutation, termed 'driver oncogenic mutations', for their proliferation and survival. EGFR(epidermal growth factor receptor) mutations and ALK(anaplastic lymphoma kinase) rearrangement are typical examples of such driver oncogenic mutations found in lung adenocarcinomas. EGFR-tyrosine kinase inhibitors (TKIs) or ALK-TKIs significantly improved treatment outcomes compared with conventional cytotoxic chemotherapy in patients with lung cancers harboring EGFR mutations or ALK rearrangement, respectively. Therefore, treatment strategies for lung cancers have dramatically changed from a 'general and empiric' to a 'personalized and evidence-based' approach according to the driver oncogenic mutation. Several novel driver oncogenic mutations, which are candidates as novel targets, such as ERBB2, BRAF, ROS1, and RET, have been discovered. Despite these successes, several limitations have arisen. One example is that some lung cancers do not respond to treatments targeting driver oncogenic mutations, as exemplified in KRAS-mutated lung cancers. Another is resistance to molecular-targeted drugs. Such resistance includes de novo resistance and acquired resistance. A number of molecular mechanisms underlying such resistance have been reported. These mechanisms can be roughly divided into three categories: alteration of the targeted oncogenes themselves by secondary mutations or amplification, activation of an alternative oncogenic signaling track, and conversion of cellular characteristics. Overcoming resistance is a current area of urgent clinical research.