William Pao, assistant member in the Human Oncology and Pathogenesis Program
The Pao Laboratory aims to perform translational research in the area of solid tumor biology, using lung cancer as a paradigm. The overall goal is to develop molecularly-tailored treatments for patients with lung cancer.
Lung cancer is the leading cause of cancer-related death in the US and worldwide. Most cases arise in former or current smokers, but about ten percent of cases also occur in individuals who smoked less than 100 cigarettes in a lifetime ("never smokers"). Lung cancers are currently classified by histopathological techniques as either small-cell lung cancer (SCLC) or non-small cell lung cancer (NSCLC). In North America, adenocarcinoma (a type of NSCLC) is the most frequent type of histological tumor, accounting for 40 percent of all cases of lung cancer.
New "targeted" epidermal growth factor receptor (EGFR) tyrosine kinase inhibitors (TKIs) like gefitinib (Iressa) and erlotinib (Tarceva) have given us a window of opportunity to elucidate clinically relevant molecular subsets of lung adenocarcinomas. For example, clinical trials have shown that gefitinib has an overall response rate of ten percent in American and European populations, and 28 percent in Japanese patients. Retrospective analyses suggested that gefitinib is most efficacious in "never smokers" with adenocarcinoma histology. Such findings can now be largely accounted for by research from our group and others showing the relatively high incidence of mutations in the gene encoding EGFR in these respective populations and the association of EGFR mutations with increased sensitivity to both gefitinib and erlotinib.
While EGFR mutations are common in tumors from never smokers, mutations in KRAS, which encodes a signaling molecule downstream of EGFR, more commonly occur in individuals with substantial cigarette use. Moreover, EGFR and KRAS mutations appear to be mutually exclusive, suggesting that EGFR and KRAS mutations within lung epithelia are equivalent in their tumorigenic effects. We examined the status of KRAS in tumors sensitive or refractory to gefitinib or erlotinib, and found that mutations in KRAS are associated with primary resistance to these drugs. This suggests that pre-treatment mutational profiling of both EGFR and KRAS may help guide treatment decisions regarding the use of these agents.
Unfortunately, virtually all patients who initially respond to gefitinib and erlotinib eventually develop acquired resistance. We have shown that tumor cells from patients whose disease progresses after initial responses on therapy with these agents frequently harbor second-site mutations in EGFR. The predominant second mutation substitutes methionine for threonine at position 790 in EGFR, which is predicted to block binding of gefitinib and erlotinib to the ATP-binding pocket of the kinase. Interestingly, the T790M amino acid change is analogous to changes seen in other kinases targeted by a related kinase inhibitor, imatinib (Gleevec), in patients that develop acquired resistance to that drug. Using a genomic approach, we have also recently found that tumor samples from patients with acquired resistance to gefitinib or erlotinib harbor amplification of MET, which encodes another tyrosine kinase. MET amplification appears to occur independently of T790M mutations. Importantly, MET inhibitors are currently being developed in the clinic.
The Pao Laboratory is now focused on the following specific areas:
Defining further molecular subsets of lung cancers, based primarily upon mutational profiling of the oncogenome in tumor samples from the Memorial Sloan-Kettering tumor bank.
Elucidating additional mechanisms of sensitivity and resistance to EGFR inhibitors in lung cancer. Using a panel of human lung cancer cell lines that harbor EGFR mutations, we recently showed that EGFR kinase inhibition in drug-sensitive cells provokes apoptosis via the intrinsic pathway of caspase activation. The process requires induction of the pro-apoptotic BH3-only protein, BIM; erlotinib dramatically induces BIM levels in sensitive but not in resistant cell lines, and knockdown of BIM expression by RNA interference virtually eliminates drug-induced cell killing in vitro. BIM status is regulated at both transcriptional and posttranscriptional levels and is influenced by the ERK signaling cascade downstream of EGFR. Consistent with these findings, lung tumors and xenografts from mice bearing mutant EGFR-dependent lung adenocarcinomas display increased concentrations of Bim after erlotinib treatment. Moreover, an inhibitor of anti-apoptotic proteins, ABT-737, enhances erlotinib-induced cell death in vitro. Thus, in drug-sensitive EGFR mutant lung cancer cells, induction of BIM is essential for apoptosis triggered by EGFR kinase inhibitors. These data imply that the intrinsic pathway of caspase activation may influence both sensitivity and/or resistance of EGFR mutant lung tumor cells to EGFR kinase inhibition. Manipulation of the intrinsic pathway could be a therapeutic strategy to enhance further the clinical outcomes of patients with EGFR mutant lung tumors.
Identifying ways to overcome resistance to gefitinib and erlotinib. We have generated mouse lung tumor models driven by EGFR T790M mutants and which are resistant to erlotinib. We are now using these models as well as other molecular, cellular, and pharmacologic techniques to uncover agents and strategies that overcome acquired resistance to the T790M amino acid change.
All of these studies are being conducted in close collaboration with our clinical colleagues in the Thoracic Oncology Service, so that we can directly translate our basic science findings to the treatment of patients.
