摘要
Lung cancer is a common disease with more than 1.6 million new cases diagnosed worldwide in 2008. Treatments for patients with advanced disease are rarely curative, and responses to therapy are often followed by relapse, which highlights the large unmet need for novel therapies. Recent advances in cancer treatment have focused on personalized therapy, whereby patients are treated with agents that best target the molecular drivers of their disease. Thus, a better understanding of the pathways that drive cancer or drug resistance is of critical importance. One such example is the phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway, which is activated in many lung cancer patients and represents a target for therapy. PI3K/AKT/mTOR pathway activation has also been observed in tumors resistant to agents targeting upstream receptor tyrosine kinases. Agents that target this pathway have the potential to shut down survival pathways, and are being explored both in the setting of pathway-activating mutations and for their ability to restore sensitivity to upstream signaling targeted agents. Here, we examine the frequency of mutations activating the PI3K/AKT/mTOR pathway, review the novel agents being explored to target this pathway, and explore the potential role of the inhibition of this pathway in the clinical development of these agents. Lung cancer is a common disease with more than 1.6 million new cases diagnosed worldwide in 2008. Treatments for patients with advanced disease are rarely curative, and responses to therapy are often followed by relapse, which highlights the large unmet need for novel therapies. Recent advances in cancer treatment have focused on personalized therapy, whereby patients are treated with agents that best target the molecular drivers of their disease. Thus, a better understanding of the pathways that drive cancer or drug resistance is of critical importance. One such example is the phosphatidylinositol 3-kinase (PI3K)/AKT/mammalian target of rapamycin (mTOR) pathway, which is activated in many lung cancer patients and represents a target for therapy. PI3K/AKT/mTOR pathway activation has also been observed in tumors resistant to agents targeting upstream receptor tyrosine kinases. Agents that target this pathway have the potential to shut down survival pathways, and are being explored both in the setting of pathway-activating mutations and for their ability to restore sensitivity to upstream signaling targeted agents. Here, we examine the frequency of mutations activating the PI3K/AKT/mTOR pathway, review the novel agents being explored to target this pathway, and explore the potential role of the inhibition of this pathway in the clinical development of these agents. Cancer treatment is in the midst of a revolution toward personalized therapy. As the mechanisms that drive cancer are deciphered, agents targeting the affected pathways are developed. These discoveries and advances allow patients to be treated with agents that best target the molecular elements that drive their disease, potentially offering higher response rates and improvements in survival. Historical examples include the antihuman epidermal growth factor receptor 2 (HER2) agents in HER2-overexpressing breast cancer1Tokuda Y Suzuki Y Saito Y Umemura S The role of trastuzumab in the management of HER2-positive metastatic breast cancer: an updated review.Breast Cancer. 2009; 16: 295-300Crossref PubMed Scopus (12) Google Scholar and epidermal growth factor receptor (EGFR)- and anaplastic lymphoma kinase (ALK)-targeted agents in non–small-cell lung cancer (NSCLC).2Ray M Salgia R Vokes EE The role of EGFR inhibition in the treatment of non-small cell lung cancer.Oncologist. 2009; 14: 1116-1130Crossref PubMed Scopus (46) Google Scholar,3Dienstmann R Martinez P Felip E Personalizing therapy with targeted agents in non-small cell lung cancer.Oncotarget. 2011; 2: 165-177Crossref PubMed Scopus (49) Google Scholar Lung cancer, presenting as metastatic disease in the majority of cases, is responsible for 87,750 deaths (29% of cancer deaths) in men and 72,590 deaths (26% of cancer deaths) in women yearly in the United States.4Siegel R Naishadham D Jemal A Cancer statistics, 2012.CA Cancer J Clin. 2012; 62: 10-29Crossref PubMed Scopus (10424) Google Scholar Despite modest improvement in survival with the addition of bevacizumab to platinum-based therapy the treatment of metastatic lung cancer is an area of unmet need.5Sandler A Gray R Perry MC et al.Paclitaxel-carboplatin alone or with bevacizumab for non-small-cell lung cancer.N Engl J Med. 2006; 355: 2542-2550Crossref PubMed Scopus (5232) Google Scholar Recent advances in molecular profiling have identified the mutations and aberrations that drive many NSCLCs, thereby enabling a personalized medicine approach to treatment. Many of these mutations affect the phosphatidylinositol 3-kinase/AKT/mammalian target of rapamycin (PI3K/AKT/mTOR) pathway—a key regulator of many cellular processes, including cell survival, proliferation, and differentiation.6Cully M You H Levine AJ Mak TW Beyond PTEN mutations: the PI3K pathway as an integrator of multiple inputs during tumorigenesis.Nat Rev Cancer. 2006; 6: 184-192Crossref PubMed Scopus (1038) Google Scholar This review focuses on the mutations associated with PI3K/AKT/mTOR pathway activation in NSCLC, the novel agents that target these mutations, and the clinical development opportunities for PI3K/AKT/mTOR pathway inhibition. Signaling in the PI3K/AKT/mTOR pathway, which has been extensively reviewed previously7Engelman JA Targeting PI3K signalling in cancer: opportunities, challenges and limitations.Nat Rev Cancer. 2009; 9: 550-562Crossref PubMed Scopus (1969) Google Scholar, 8Liu P Cheng H Roberts TM Zhao JJ Targeting the phosphoinositide 3-kinase pathway in cancer.Nat Rev Drug Discov. 2009; 8: 627-644Crossref PubMed Scopus (2054) Google Scholar, 9Courtney KD Corcoran RB Engelman JA The PI3K pathway as drug target in human cancer.J Clin Oncol. 2010; 28: 1075-1083Crossref PubMed Scopus (1028) Google Scholar (Fig. 1), is initiated by the binding of extracellular growth factors to transmembrane receptor tyrosine kinases (RTKs), including EGFR, HER2, insulin-like growth factor receptor, vascular endothelial growth factor receptor, and platelet-derived growth factor receptor. Upon ligand binding, the RTKs become activated and recruit PI3K to the plasma membrane. There are three classes of PI3Ks, and the class IA PI3K is the most frequently implicated in human cancer. Class IA PI3Ks are heterodimeric and consist of a p110 catalytic subunit and a p85 regulatory subunit, which inhibit p110 in the absence of RTK activation by ligand. There are three p110 isoforms for class IA PI3Ks: p110α, p110β, and p110δ, which are encoded by PIK3CA, PIK3CB, and PIK3CD, respectively. Upon ligand binding and RTK phosphorylation, p85 binds the RTK and frees the catalytically active p110 subunit, enabling its localization to the plasma membrane. Here, it catalyzes the phosphorylation of phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 3,4,5-triphosphate (PIP3). PIP3 localizes AKT to the plasma membrane to allow phosphorylation by 3-phosphoinositide-dependent kinase 1. A second phosphorylation by the mTOR complex 2 (mTORC2) provides full activation of AKT. Once phosphorylated, AKT dissociates from the plasma membrane and is capable of phosphorylating 100 and more of its targets in the nucleus and cytoplasm. These targets include tuberous sclerosis 2 (which leads to mTOR complex 1 [mTORC1] activation), murine double minute 2, forkhead box O, and Bcl-2-associated death promoter. Together, activation of these targets leads to increased cell growth, metabolism, survival, and proliferation. Multiple components of the PI3K/AKT/mTOR pathway are involved in the control of oncogenic growth, and multiple loops of negative regulation exist. For example, Src homology 2 domain-containing inositol 5’-phosphatase and the tumor-suppressor phosphatase and tensin homolog (PTEN) dephosphorylate PIP3 to the inactive PIP2 and thereby attenuate AKT activation. Further conversion of PIP2 to PIP is mediated by the tumor-suppressor inositol polyphosphate-4-phosphatase, type II.10Agoulnik IU Hodgson MC Bowden WA Ittmann MM INPP4B: the new kid on the PI3K block.Oncotarget. 2011; 2: 321-328Crossref PubMed Scopus (94) Google Scholar Another negative feedback loop is mediated by S6 kinase, a target of mTORC1, which can inhibit the adaptor molecule insulin receptor substrate 1, which thereby prevents the activation of PI3K by insulin growth factor 1.11O’Reilly KE Rojo F She QB et al.mTOR inhibition induces upstream receptor tyrosine kinase signaling and activates Akt.Cancer Res. 2006; 66: 1500-1508Crossref PubMed Scopus (2155) Google Scholar The PI3K/AKT/mTOR pathway interacts with other parallel signaling cascades, such as the Rat sarcoma (RAS)/rapidly accelerated fibrosarcoma (RAF)/mitogen-activated protein kinase or Erk kinase (MEK) pathway. For example, RAS, the major upstream component of this pathway, can also directly activate class IA PI3K (Fig. 1). Given its central role, it is predictable that disruption of the PI3K/AKT/mTOR pathway affects tightly regulated cellular processes. Unsurprisingly, it is one of the most commonly deregulated pathways in cancer.8Liu P Cheng H Roberts TM Zhao JJ Targeting the phosphoinositide 3-kinase pathway in cancer.Nat Rev Drug Discov. 2009; 8: 627-644Crossref PubMed Scopus (2054) Google Scholar,12Paez J Sellers WR PI3K/PTEN/AKT pathway. A critical mediator of oncogenic signaling.Cancer Treat Res. 2003; 115: 145-167Crossref PubMed Google Scholar,13Sansal I Sellers WR The biology and clinical relevance of the PTEN tumor suppressor pathway.J Clin Oncol. 2004; 22: 2954-2963Crossref PubMed Scopus (817) Google Scholar On the basis of AKT phosphorylation, the PI3K pathway is active in 50% to 70% of NSCLCs,14Balsara BR Pei J Mitsuuchi Y et al.Frequent activation of AKT in non-small cell lung carcinomas and preneoplastic bronchial lesions.Carcinogenesis. 2004; 25: 2053-2059Crossref PubMed Scopus (253) Google Scholar, 15Cappuzzo F Ligorio C Jänne PA et al.Prospective study of gefitinib in epidermal growth factor receptor fluorescence in situ hybridization-positive/phospho-Akt-positive or never smoker patients with advanced non-small-cell lung cancer: the ONCOBELL trial.J Clin Oncol. 2007; 25: 2248-2255Crossref PubMed Scopus (195) Google Scholar, 16Tsurutani J Fukuoka J Tsurutani H et al.Evaluation of two phosphorylation sites improves the prognostic significance of Akt activation in non-small-cell lung cancer tumors.J Clin Oncol. 2006; 24: 306-314Crossref PubMed Scopus (152) Google Scholar and pathway activation confers a poor prognosis.17Tang JM He QY Guo RX Chang XJ Phosphorylated Akt overexpression and loss of PTEN expression in non-small cell lung cancer confers poor prognosis.Lung Cancer. 2006; 51: 181-191Abstract Full Text Full Text PDF PubMed Scopus (322) Google Scholar,18David O Jett J LeBeau H et al.Phospho-Akt overexpression in non-small cell lung cancer confers significant stage-independent survival disadvantage.Clin Cancer Res. 2004; 10: 6865-6871Crossref PubMed Scopus (175) Google Scholar Activation could result from mutations or alterations throughout this or parallel pathways (e.g., KRAS and EGFR), leading to increased cell survival, proliferation, and differentiation. For example, mutations in RAS, or those that cause constitutive activation of PI3K, AKT, such as the AKT1 E17K mutation,19Malanga D Scrima M De Marco C et al.Activating E17K mutation in the gene encoding the protein kinase AKT1 in a subset of squamous cell carcinoma of the lung.Cell Cycle. 2008; 7: 665-669Crossref PubMed Scopus (144) Google Scholar or upstream RTKs, can drive cancers in a ligand-independent fashion.20Yuan TL Cantley LC PI3K pathway alterations in cancer: variations on a theme.Oncogene. 2008; 27: 5497-5510Crossref PubMed Scopus (1472) Google Scholar Mutations that inactivate the negative regulator PTEN have also been observed.21Forgacs E Biesterveld EJ Sekido Y et al.Mutation analysis of the PTEN/MMAC1 gene in lung cancer.Oncogene. 1998; 17: 1557-1565Crossref PubMed Scopus (183) Google Scholar More common, however, is the observation of reduced or absent PTEN expression in NSCLC. Indeed, the high frequency of PTEN loss (~45%), coupled with an additional 29% of tumors showing reduced PTEN protein expression, suggests a role of PTEN inactivation in NSCLC tumorigenesis.17Tang JM He QY Guo RX Chang XJ Phosphorylated Akt overexpression and loss of PTEN expression in non-small cell lung cancer confers poor prognosis.Lung Cancer. 2006; 51: 181-191Abstract Full Text Full Text PDF PubMed Scopus (322) Google Scholar,22Marsit CJ Zheng S Aldape K et al.PTEN expression in non-small-cell lung cancer: evaluating its relation to tumor characteristics, allelic loss, and epigenetic alteration.Hum Pathol. 2005; 36: 768-776Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar Although loss of heterozygosity at microsatellites in chromosome 10q23 surrounding and intragenic to the PTEN locus and hypermethylation of the PTEN gene are commonly observed (19% and 26%, respectively), they are not predictors of protein expression.22Marsit CJ Zheng S Aldape K et al.PTEN expression in non-small-cell lung cancer: evaluating its relation to tumor characteristics, allelic loss, and epigenetic alteration.Hum Pathol. 2005; 36: 768-776Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar Research continues to elucidate potential transcriptional and translational explanations. Not all of the large number of mutations that can affect the PI3K/AKT/mTOR pathway have been observed in NSCLC. As shown in Table 1, the frequency of each mutation can also vary by histology.23Herbst RS Heymach JV Lippman SM Lung cancer.N Engl J Med. 2008; 359: 1367-1380Crossref PubMed Scopus (2074) Google Scholar KRAS and EGFR kinase domain mutations are rare in squamous cell carcinoma (SCC); however, KRAS mutations occur in 20% to 30% of adenocarcinomas,24Ding L Getz G Wheeler DA et al.Somatic mutations affect key pathways in lung adenocarcinoma.Nature. 2008; 455: 1069-1075Crossref PubMed Scopus (2169) Google Scholar and EGFR mutations in 10% to 20%, with a higher incidence in patients of Asian origin. Conversely, EGFR expression and PIK3CA amplifications are more common in SCC than in adenocarcinoma. The presence of these mutations may predict the outcome of targeted therapy, and several of the molecular alterations implicated in the PI3K/AKT/mTOR pathway may also provide prognostic information. For example, EGFR-activating mutations indicate a favorable prognosis,25Eberhard DA Johnson BE Amler LC et al.Mutations in the epidermal growth factor receptor and in KRAS are predictive and prognostic indicators in patients with non-small-cell lung cancer treated with chemotherapy alone and in combination with erlotinib.J Clin Oncol. 2005; 23: 5900-5909Crossref PubMed Scopus (1342) Google Scholar,26O’Byrne KJ Gatzemeier U Bondarenko I et al.Molecular biomarkers in non-small-cell lung cancer: a retrospective analysis of data from the phase 3 FLEX study.Lancet Oncol. 2011; 12: 795-805Abstract Full Text Full Text PDF PubMed Scopus (182) Google Scholar whereas increases in mesenchymal epithelial transition factor (MET) copy number27Okuda K Sasaki H Yukiue H Yano M Fujii Y Met gene copy number predicts the prognosis for completely resected non-small cell lung cancer.Cancer Sci. 2008; 99: 2280-2285Crossref PubMed Scopus (165) Google Scholar,28Cappuzzo F Jänne PA Skokan M et al.MET increased gene copy number and primary resistance to gefitinib therapy in non-small-cell lung cancer patients.Ann Oncol. 2009; 20: 298-304Crossref PubMed Scopus (271) Google Scholar and a loss of PTEN expression indicate a poor prognosis.17Tang JM He QY Guo RX Chang XJ Phosphorylated Akt overexpression and loss of PTEN expression in non-small cell lung cancer confers poor prognosis.Lung Cancer. 2006; 51: 181-191Abstract Full Text Full Text PDF PubMed Scopus (322) Google ScholarTABLE 1Frequency of Mutations Affecting the PI3K/AKT/mTOR PathwayNonsquamousSquamous (%)Adenocarcinoma (%)Large Cell (%)Total (%)ReferenceReceptor tyrosine kinasesEGFRMutation512–203–915–20 (up to 60 Asians)25,26,111–115Increased numbercopyUNK41UNK29–4026,43,111,112,116,117ALKMutation5172115Rearrangement13–7UNK2–736–38,115,118–120(EML4-ALK)METMutation12UNK2–1423,27,115,121Amplification2120UNK2–21 (higher frequency in patients with acquired resistance to TKIs)27,28,43,44,122–124RAS/RAF/MEKpathway componentsKRASMutation6–912–53188–2125,26,111,115,125PI3K/AKT/mTORpathway componentsPTENMutation6134–5115,126Loss/reductionLoss/ reduction: 70Loss/ reduction: 77Loss/ reduction: 62Loss: 24–44; reduction: 29–46 Promoter hypermethylation at PTEN: 26, at PTENP1: 6622,26,45PIK3CAMutation2–7232–5115,126–130Amplification33–706–193812–20126,130–134LKB1Mutation2733–54439–3353,135–138AKT1Mutation1<1UNK1–219,115,139,140(E17K)UNK, unknown; EGFR, epidermal growth factor receptor; EML4, echinoderm microtubule-associated protein-like 4; MET, mesenchymal epithelial transition factor; RAS, rat sarcoma; MEK, mitogen-activated protein kinase or Erk kinase; PI3K, phosphatidylinositol 3-kinase; mTOR, mammalian target of rapamycin; PTEN, phosphatase and tensin homolog; LKB1, liver kinase B1. Open table in a new tab UNK, unknown; EGFR, epidermal growth factor receptor; EML4, echinoderm microtubule-associated protein-like 4; MET, mesenchymal epithelial transition factor; RAS, rat sarcoma; MEK, mitogen-activated protein kinase or Erk kinase; PI3K, phosphatidylinositol 3-kinase; mTOR, mammalian target of rapamycin; PTEN, phosphatase and tensin homolog; LKB1, liver kinase B1. The ability to target the molecular drivers of PI3K/AKT/mTOR activation using pathway-specific inhibitors is currently being explored in NSCLC. These studies are being supported by preclinical observations. There are at least three settings in which these agents may be useful: (a) reversal of resistance to upstream RTK-targeted therapy, (b) PI3K/AKT/mTOR pathway mutations, and (c) parallel pathway activation. The RTKs represent an entry point to PI3K/AKT/mTOR pathway activation. As seen in Table 1, mutation or amplification of RTKs has been observed in NSCLC, which leads to ligand-independent signaling. Therefore, agents that target the PI3K/AKT/mTOR pathway could be used to suppress these signals. For example, EGFR is overexpressed in up to 60% of patients with NSCLC, but not all of these patients respond to the EGFR-tyrosine kinase inhibitors (TKIs) erlotinib and gefitinib, and eventually all patients who experience tumor response will have disease progression—often because of downstream PI3K/AKT/mTOR pathway activation.29Engelman JA Zejnullahu K Mitsudomi T et al.MET amplification leads to gefitinib resistance in lung cancer by activating ERBB3 signaling.Science. 2007; 316: 1039-1043Crossref PubMed Scopus (3902) Google Scholar,30Guix M Faber AC Wang SE et al.Acquired resistance to EGFR tyrosine kinase inhibitors in cancer cells is mediated by loss of IGF-binding proteins.J Clin Invest. 2008; 118: 2609-2619PubMed Google Scholar Studies have shown, however, that the combination of PI3K inhibitors with EGFR-TKIs or monoclonal antibodies can inhibit viability in cell lines resistant to EGFR-TKIs or monoclonal antibodies.31Kim Y Ko J Cui ZY et al.PTEN mutation associated with EGFR TKI de novo resistance.AACR Meet Abstr. 2010; 2010: 3608Google Scholar, 32Donev IS Wang W Yamada T et al.Transient PI3K inhibition induces apoptosis and overcomes HGF-mediated resistance to EGFR-TKIs in EGFR mutant lung cancer.Clin Cancer Res. 2011; 17: 2260-2269Crossref PubMed Scopus (91) Google Scholar, 33Kim SM Kim JS Kim JH et al.Acquired resistance to cetuximab is mediated by increased PTEN instability and leads cross-resistance to gefitinib in HCC827 NSCLC cells.Cancer Lett. 2010; 296: 150-159Abstract Full Text Full Text PDF PubMed Scopus (44) Google Scholar, 34Janmaat ML Rodriguez JA Gallegos-Ruiz M Kruyt FA Giaccone G Enhanced cytotoxicity induced by gefitinib and specific inhibitors of the Ras or phosphatidyl inositol-3 kinase pathways in non-small cell lung cancer cells.Int J Cancer. 2006; 118: 209-214Crossref PubMed Scopus (137) Google Scholar, 35Ihle NT Paine-Murrieta G Berggren MI et al.The phosphatidylinositol-3-kinase inhibitor PX-866 overcomes resistance to the epidermal growth factor receptor inhibitor gefitinib in A-549 human non-small cell lung cancer xenografts.Mol Cancer Ther. 2005; 4: 1349-1357Crossref PubMed Scopus (153) Google Scholar These data support the prospect of using PI3K/AKT/mTOR pathway inhibitors to restore sensitivity to EGFR-TKIs in resistant tumors. Furthermore, molecular profiling of EGFR-TKI–resistant tumors may help identify those patients who would benefit most from PI3K/AKT/mTOR inhibition. Fusions of the RTK ALK to echinoderm microtubule-associated protein-like 4 lead to constitutive activation of the kinase in NSCLC.36Soda M Choi YL Enomoto M et al.Identification of the transforming EML4-ALK fusion gene in non-small-cell lung cancer.Nature. 2007; 448: 561-566Crossref PubMed Scopus (4304) Google Scholar, 37Choi YL Takeuchi K Soda M et al.Identification of novel isoforms of the EML4-ALK transforming gene in non-small cell lung cancer.Cancer Res. 2008; 68: 4971-4976Crossref PubMed Scopus (379) Google Scholar, 38Koivunen JP Mermel C Zejnullahu K et al.EML4-ALK fusion gene and efficacy of an ALK kinase inhibitor in lung cancer.Clin Cancer Res. 2008; 14: 4275-4283Crossref PubMed Scopus (833) Google Scholar These patients can be treated with the recently approved ALK/MET inhibitor crizotinib; however, a number of crizotinib-resistant mutants have already been identified.39Heuckmann JM Holzel M Sos ML et al.ALK mutations conferring differential resistance to structurally diverse ALK inhibitors.Clin Cancer Res. 2011; 17: 7394-7401Crossref PubMed Scopus (157) Google Scholar,40Zhang S Wang F Keats J et al.Crizotinib-resistant mutants of EML4-ALK identified through an accelerated mutagenesis screen.Chem Biol Drug Des. 2011; 78: 999-1005Crossref PubMed Scopus (121) Google Scholar Of note, resistance to MET inhibition by crizotinib can lead to activation of the PI3K/AKT/mTOR pathway, as demonstrated in a gastric carcinoma cell line.41Qi J McTigue MA Rogers A et al.Multiple mutations and bypass mechanisms can contribute to development of acquired resistance to MET inhibitors.Cancer Res. 2011; 71: 1081-1091Crossref PubMed Scopus (175) Google Scholar Whether inhibition of the PI3K/AKT/mTOR pathway would restore sensitivity to these crizotinib-resistant tumors is yet to be shown. Amplification of MET, which encodes the hepatocyte growth factor receptor, has been observed in NSCLC.42Onozato R Kosaka T Kuwano H Sekido Y Yatabe Y Mitsudomi T Activation of MET by gene amplification or by splice mutations deleting the juxtamembrane domain in primary resected lung cancers.J Thorac Oncol. 2009; 4: 5-11Abstract Full Text Full Text PDF PubMed Scopus (244) Google Scholar, 43Beau-Faller M Ruppert AM Voegeli AC et al.MET gene copy number in non-small cell lung cancer: molecular analysis in a targeted tyrosine kinase inhibitor naïve cohort.J Thorac Oncol. 2008; 3: 331-339Abstract Full Text Full Text PDF PubMed Scopus (172) Google Scholar, 44Cappuzzo F Marchetti A Skokan M et al.Increased MET gene copy number negatively affects survival of surgically resected non-small-cell lung cancer patients.J Clin Oncol. 2009; 27: 1667-1674Crossref PubMed Scopus (467) Google Scholar MET amplification leads to erythroblastic leukemia viral oncogene homolog 3 phosphorylation and subsequent PI3K/AKT/mTOR activation. Thus, EGFR-TKI–resistant tumors with MET amplification achieve erythroblastic leukemia viral oncogene homolog 3 phosphorylation despite the inhibition of EGFR.29Engelman JA Zejnullahu K Mitsudomi T et al.MET amplification leads to gefitinib resistance in lung cancer by activating ERBB3 signaling.Science. 2007; 316: 1039-1043Crossref PubMed Scopus (3902) Google Scholar In preclinical models, inhibition of MET can restore sensitivity to the EGFR-TKI. For example, sensitivity to EGFR-TKIs has been restored using the dual PI3Kα/mTOR inhibitor PI-103, providing the proof of concept that dual inhibition of EGFR and the PI3K/AKT/mTOR pathway can overcome resistance to EGFR-TKIs in NSCLC.32Donev IS Wang W Yamada T et al.Transient PI3K inhibition induces apoptosis and overcomes HGF-mediated resistance to EGFR-TKIs in EGFR mutant lung cancer.Clin Cancer Res. 2011; 17: 2260-2269Crossref PubMed Scopus (91) Google Scholar Supporting the central role of the PI3K/AKT/mTOR pathway, mutations that cause aberrant pathway activation are commonly observed in cancers.8Liu P Cheng H Roberts TM Zhao JJ Targeting the phosphoinositide 3-kinase pathway in cancer.Nat Rev Drug Discov. 2009; 8: 627-644Crossref PubMed Scopus (2054) Google Scholar,12Paez J Sellers WR PI3K/PTEN/AKT pathway. A critical mediator of oncogenic signaling.Cancer Treat Res. 2003; 115: 145-167Crossref PubMed Google Scholar,13Sansal I Sellers WR The biology and clinical relevance of the PTEN tumor suppressor pathway.J Clin Oncol. 2004; 22: 2954-2963Crossref PubMed Scopus (817) Google Scholar For this reason, there has been great interest in understanding these mutations and in the effect of pathway-specific inhibitors on tumors with these mutations. The loss or reduction of tumor-suppressor PTEN expression is observed in 30% to 70% of NSCLC.22Marsit CJ Zheng S Aldape K et al.PTEN expression in non-small-cell lung cancer: evaluating its relation to tumor characteristics, allelic loss, and epigenetic alteration.Hum Pathol. 2005; 36: 768-776Abstract Full Text Full Text PDF PubMed Scopus (220) Google Scholar,45Soria JC Lee HY Lee JI et al.Lack of PTEN expression in non-small cell lung cancer could be related to promoter methylation.Clin Cancer Res. 2002; 8: 1178-1184PubMed Google Scholar As a negative regulator, the absence of PTEN leads to unrestricted PI3K/AKT/mTOR pathway activation, suggesting a role for pathway-specific inhibitors in the treatment of tumors with reduced or absent PTEN expression. For example, the dual PI3K/mTOR inhibitor PF-04691502 demonstrated antitumor activity in PTEN-null EGFR-TKI–resistant NSCLC tumor xenografts.46Yuan J Mehta PP Yin MJ et al.PF-04691502, a potent and selective oral inhibitor of PI3K and mTOR kinases with antitumor activity.Mol Cancer Ther. 2011; 10: 2189-2199Crossref PubMed Scopus (132) Google Scholar This targeted approach is also being explored in some ongoing studies that are testing PI3K/AKT/mTOR pathway inhibitors in patients with a PTEN mutation or low or null PTEN expression. Mutations and amplifications of PIK3CA, the gene encoding the main catalytic subunit of PI3K, can cause ligand-independent activation of the PI3K/AKT/mTOR pathway, increasing the oncogenic potential of the mutant cell(s).47Kang S Bader AG Vogt PK Phosphatidylinositol 3-kinase mutations identified in human cancer are oncogenic.Proc Natl Acad Sci USA. 2005; 102: 802-807Crossref PubMed Scopus (691) Google Scholar Furthermore, the presence of PIK3CA mutations in advanced NSCLC tumors has also been found to portend a shorter median time to progression and decreased median overall survival.48Ludovini V Bianconi F Pistola L et al.Phosphoinositide-3-kinase catalytic alpha and KRAS mutations are important predictors of resistance to therapy with epidermal growth factor receptor tyrosine kinase inhibitors in patients with advanced non-small cell lung cancer.J Thorac Oncol. 2011; 6: 707-715Crossref PubMed Scopus (146) Google Scholar In EGFR-TKI–resistant NSCLC xenografts harboring a mutated PIK3CA gene, PF-04691502 administration induced antitumor activity.46Yuan J Mehta PP Yin MJ et al.PF-04691502, a potent and selective oral inhibitor of PI3K and mTOR kinases with antitumor activity.Mol Cancer Ther. 2011; 10: 2189-2199Crossref PubMed Scopus (132) Google Scholar A second dual PI3K/mTOR inhibitor led to marked tumor regression in a lung adenocarcinoma tumor model driven by a mutated PIK3CA gene.49Engelman JA Chen L Tan X et al.Effective use of PI3K and MEK inhibitors to treat mutant Kras G12D and PIK3CA H1047R murine lung cancers.Nat Med. 2008; 14: 1351-1356Crossref PubMed Scopus (1142) Google Scholar Combined, these results suggest a role for PI3K/AKT/mTOR inhibitors in the treatment of PIK3CA-mutated tumors—a hypothesis being tested in ongoing clinical studies (see below). The liver kinase B1 (LKB1) kinase is involved in metabolism homeostasis and suppresses cell growth activation of 5′ adenosine monophosphate-activated protein kinase (AMPK) kinases when nutrient levels are depleted.50Shaw RJ LKB1 and AMP-activated protein kinase control of mTOR signalling and growth.Acta Physiol (Oxf). 2009; 196: 65-80Crossref PubMed Scopus (495) Google Scholar LKB1 mutant cells lack AMPK activity, are refractory to mTOR inhibition a glucose depletion, and are less resistant to cell death upon glucose deprivation than are wild-type cells.51Carretero J Medina PP Blanco R et al.Dysfunctional AMPK activity, signalling through mTOR and survival in response to energetic stress in LKB1-deficient lung cancer.Oncogene. 2007; 26: 1616-1625Crossref PubMed Scopus (119) Google Scholar Interestingly, KRAS mutations are commonly associated with LKB1 mutations, suggesting a role for RAS/RAF/MEK signaling in the control of mTOR activation.52Mahoney CL Choudhury B Davies H et al.LKB1/KRAS mutant lung cancers constitute a genetic subset of NSCLC with increased sensitivity to MAPK and mTOR signalling inhibition.Br J Cancer. 2009; 100: 370-375Crossref PubMed Scopus (99) Google Scholar,53Matsumoto S Iwakawa R Takahashi K et al