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* From the Experimental Therapeutics Program, Vanderbilt-Ingram Comprehensive Cancer Center, Nashville, TN.
Correspondence to: David P. Carbon, MD, PhD, Experimental Therapeutics Program, Vanderbilt-Ingram Comprehensive Cancer Center, 648 Preston Rsch Bldg, Nashville, TN 37232-6838
| Introduction |
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It is a particular pleasure for me to both attend and summarize this particular conference, as it is organized and run by pulmonologists, and I am a medical oncologist. The fact that this conference focuses on lung cancer signifies an era of increasing awareness and collaboration between pulmonary medicine and the oncology specialties that is desperately important if we are to make real progress in fighting this disease. This has not always been the case.
There are many obstacles to overcome in order to make progress in reducing the morbidity and mortality attributable to lung cancer, and only some of them are scientific. Others are sociological or political. For example, there has been the perception that patients with lung cancer are to blame for their disease, so it is not important to try to cure or prevent it. However, over half of patients with lung cancer diagnosed today are never-smokers or ex-smokers, and I strongly feel that even smokers do not "deserve" lung cancer. The opinion that there are no effective therapies is also being chipped away with the advent of new drugs such as Iressa (gefitnib) and Alimta (pemetrexed), and a positive adjuvant chemotherapy trial at this years American Society of Clinical Oncology. The lack of a powerful advocacy group has also led to a relative underfunding for lung cancer research, with no Department of Defense grants, and only approximately one tenth the funding of breast cancer per cancer death. The fact that many states use their multimillion-dollar tobacco settlements to balance their budgets and not to combat tobacco-related disease is also a travesty of justice.
My particular enthusiasm for this years conference relates to the fact that there has also been a historical lack of interest in lung cancer by some pulmonologists, the specialty usually responsible for its diagnosis. The quality and progress reflected by this conference is evidence that these historical barriers to progress are being overcome. I will thus attempt to summarize the highlights of this meeting from the perspective of a researcher and a lung cancer medical oncologist.
| Conference Themes |
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A secondary theme for this conference for me was not just the increasing recognition of the complexity of lung cancers, preneoplasia, and populations at risk, but of the availability of emerging technologies capable of grappling with that complexity. Not all lung cancers are the same; every individuals genetic background and individual environmental exposures factor into their personal risk of acquiring lung cancer. Beyond recognizing these facts that have long been clinically suspected, we now have data on the mechanisms of this heterogeneity, and most importantly the tools to begin to quantify and understand what is going on.
| Inherited Risk Modifiers |
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Both Dr. Schwartz and Jonathan Samet reviewed the data on susceptibility genes in lung cancer. The genes analyzed to date are primarily those associated with variability in the handling of environmental carcinogens, including phase I, phase II, and DNA repair genes. Single genes tended to be associated with risks of 1.3 to 2.0, but the combination of CYP1A1 with GSTM1 had an overall risk of 3.0 to 5.8. Dr. Schwartz reported on a population study in nonsmoking and early-onset disease based on the Detroit Surveillance, Epidemiology, and End Results registry. This study involved nonsmokers aged 40 to 84 years, and has identified 325 cases and 141 biological samples. Also identified were those with early onset disease appearing at age < 40 years, with 641 cases and 372 samples obtained. There were 181 cases of early onset disease in nonsmokers. The strongest risk identified to date was for CYP1B1 with an odds ratio of 5.5 (CI, 3.4 to 9.1) in nonsmokers. This gene is involved in metabolism of polycyclic aromatic hydrocarbons and estrogen metabolism. It is perhaps counterintuitive, however, that a phase I enzyme association should be found to strongest in nonsmokers, as it is thought to be important in handling the carcinogens associated with cigarette smoking. One possible interpretation is that high doses of carcinogens from smoke overwhelm inherited variability in the phase I enzymes, and that these are manifest only with lower-level exposures that occur in nonsmokers.
It was pointed out by Dr. Samet that since these studies all adjust for tobacco exposure, it is also possible that genetic susceptibility to nicotine addiction would not be detected in these studies. There are data, in fact, suggesting a linkage between dopamine receptor polymorphisms and tobacco use. These sorts of associations should be further explored.
If the observed associations with these genes hold up in larger studies, they could have individual clinical predictive significance, but it is clear that risk associations in smokers may involve whole pathways of genes rather than single genes due to the complexity of carcinogens in tobacco smoke and their various metabolic pathways. In the future, use of bioprobes for entire pathways or comprehensive single nucleotide polymorphism analysis of large numbers of genetic polymorphisms may be useful for more accurately estimating individual risks.
| Mechanisms of Genetic Alteration |
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The changing proportion of adenocarcinoma among all non-small cell lung cancers is an unexplained, but potentially important observation that could reveal underlying mechanisms and candidates for cancer control strategies. Hampering the study of this phenomenon is the lack of good animal models for nonadenocarcinoma models for lung cancer. Possibilities include the increased use of filtered cigarettes with a concomitant decrease in average particle size and site of airway deposition. Also raised was the introduction of "ammonia technology" in the 1960s to produce a product with a stronger nicotine effect. This may have caused a transition from benzpyrene to nitrosamine predominance in smoke, and thus altered the mutation profiles as well as perhaps the biological consequences of this damage. The data for temporal changes in genetic mutations corresponding to the observed changes in clinical phenotypes is lacking and in need of further investigation.
Provocative data relevant to the identification of the dominant carcinogen in tobacco smoke were presented by Hanspeter Witschi in a smoking A strain mouse model.23 These animals get lung tumors when exposed to chronic tobacco smoke. Filtering the tobacco smoke that dramatically reduces both BAP and nitrosamine content curiously has no effect on the number or type of tumors in this model, which are all adenocarcinomas in either case. In fact, when pure NNK or BAP are used, many specific chemopreventive agents (such as PITC or d-limonene) reduced the number of tumors observed, but these agents were unexpectedly completely ineffective at reducing the number of tumors resulting from cigarette smoke. These data imply that BAP and NNK may not account for all (or even the majority) of the carcinogenic activity in cigarette smoke, increasing the complexity and complicating the analysis of laboratory carcinogenesis studies. An interesting and unexplained observation in this model is the fact that the tumor number is increased when smoking exposure is followed by a period of clean air exposure. This effect was very clear in that mice killed after continuous smoking exposure had fewer tumors than mice killed after being allowed several months of clean air after smoke exposure. It is thus possible that smoke both induces the necessary changes and restrains the manifestation of tumors in this model.
Thomas Slaga discussed the use of skin cancer as a model for lung tumorigenesis, and reviewed the data strongly linking obesity with cancer. It was pointed out, however, that in the largest study of the linkage between obesity and cancer published to date in recent issue of the New England Journal of Medicine,4 there was no association with lung cancer. Again, this may be related to the fact that smoking is such a strong risk factor that it overwhelms more modest ones. He also described the potent chemoprotective effect of glucocorticoids in several models of carcinogenesis, including the smoking mouse model. Confirmation of this in human epidemiologic studies is complicated by the fact that the population of patients with COPD is both at highest risk and most likely to use inhaled steroids for symptom control.
Curt Harris discussed the central importance of the p53 pathway in oncogenesis and emphasized the importance of "molecular archeology"that identification of somatic alterations of specific genes in tumors reveals important biochemical pathways and mechanisms, and that the types of mutations that occur in these genes can shed light on the mutational mechanisms involved. One example is the fact that lung cancers typically have a predominance of G to T transversions compared to the G to A transitions commonly observed in colon cancer, thought to be related to the differences in environmental etiology.5 He highlighted a recent study6 showing that in lung tumors in which the MGMT gene (involved in the repair of methyl guanine) is silenced by methylation, an increased number of G to A mutations is observed. The biology underlying the clear differences in the codon location of mutations in p53 is less well understood, but may be primarily "chemical"differences in the local sequence affect benzo-a-pyrene adduct formation vs CpG methylation.
Both Drs. Gazdar and Herman described nonmutation-based mechanisms tumors utilize for the silencing of tumor-suppressor genes involving promoter methylation. This is clearly now a validated mechanism confirmed by several groups. Data were presented that multiple genes important in oncogenesis including p16, MGMT, and RARß were frequently methylated and thus transcriptionally silenced. The underlying alterations that cause these changes in methylation are as yet unclear and may ultimately be better intervention targets than global demethylation strategies.
| Detecting Early Cancer and Premalignant States |
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However, while these criteria identify a population at risk, the individual predictive value of light microscopic changes is not ideal. Jeremy George, Middlesex, UK, presented intriguing data on this point.7 He presented a surveillance study of "premalignancy" observed by light microscopy of bronchoscopic biopsy specimens. He found that over time some of the severe dysplasias regressed, while some progressed. Cancers developed in some of these sites of severe dysplasia, but not others. Most interesting, several cancers developed in sites within the lung with a normal appearance by white light and Laser Induced Fluorescent Endoscopy bronchoscopy, and also normal by light microscopy. He concluded that the airways in these patients are extremely unstable. This is also evidence that the canonical sequence of metaplasia dysplasia carcinoma in situ invasive cancer is not as linear as initially thought. It is clear that light microscopically observable preneoplastic alterations of bronchial epithelial cells precede only a relatively small fraction of clinical cancers. It is in fact difficult to find adjacent dysplasia in resection specimens. These abnormalities also sometimes regress spontaneously. In addition, while these lesions are highly associated with squamous cell carcinoma (a decreasing subset of non-small cell lung cancer), they are less so for adenocarcinoma or small cell lung cancer, the latter disease also highly smoking related. I feel that smoking causes both dysplasia and cancer, but that these are "true, true, and only partially related." The most informative investigation would be to study underlying genetic alterations in bronchial epithelial cells independent of light microscopic changes (perhaps, however, focusing on patient populations identified as being at high risk by virtue of the presence of these light microscopic changes). Thus, there should be an increasing emphasis on molecular methods for lung cancer early detection or risk evaluation "unbiased" by light microscopy.
| Molecular Screening |
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Along these lines, I presented work from my laboratory that identified patterns of proteins detected by mass spectrometry associated with lung cancer and its subtypes, as well as biological behaviors such as nodal involvement and survival. The concept of thinking in multigene modules was also emphasized by Naftali Kaminski, who presented complementary DNA microarray data from the University of Pittsburgh using advanced statistical methods and a training-testing design. He introduced the concept of "in silico microdissection," in which patterns associated with various cell types (such as stromal, hematopoietic, or vascular cells) could be teased out of array data from bulk tumor samples. These analyses could be useful for dissecting important tumor-host interactions.
Dr. Kaminski also presented intriguing data that gene expression patterns of peripheral blood mononuclear cells of smokers with cancer could be distinguished from smokers without cancer on array analysis. He discussed the proteomics approaches to serum detection pioneered by Petricoin et al, and contrasted the array approach. The richness of the data obtained is much greater with the complementary DNA array approach, but Dr. Kaminski hypothesized that ultimately clinically useful screening tools will likely be based on protein markers.
| Radiographic Screening |
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| Chemoprevention |
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-Tocopherol, Beta-Carotene Cancer Prevention Study11 and the Beta-Carotene and Retinol Efficacy Trial,12 involving tens of thousands of subjects, and they both found increases in the incidence of lung cancer in the arms administered beta-carotene, with an approximate 1.3-fold increased risk, with about 59 cases vs 46 cases per 10,000 person-years. The pilot trials in progress were also reviewed, including those using celecoxib, 5-LOX inhibitors, and other novel targets. In his summary, Dr. Omenn emphasized several points that bear repeating: (1) the importance of pushing new agents into clinical trials because the potential benefit is great; (2) lung cancers are heterogeneous, and agents may affect one subset and not another; (3) pathways found to be effective in chemopreventing other cancers should be tested in lung cancer (eg, nonsteroidal anti-inflammatory drugs with colon and signal transduction targets in breast); (4) consideration of targeting inflammatory or stromal components; (5) continued development of surrogate markers and end points; (6) recognition that all agents have multiple effects, positive and negative; and (7) consider the use of proven therapy drugs in chemoprevention (eg, Iressa). The role of cyclooxygenase-2 (COX-2) in lung cancer was another emphasis at this meeting, with many investigators presenting data from University of Texas Southwestern, University of Colorado, University of California Los Angeles (UCLA) and Vanderbilt. While studies are underway evaluating the effect of nonsteroidal anti-inflammatory drug use on lung cancer incidence and at least one celecoxib chemoprevention trial is open, the theme presented at this meeting was the complexity and sometimes-opposite effects of COX-2 products in lung cancer. Dr. Gazdar pointed out that while COX-2 is overexpressed in many non-small cell lung cancers (especially adenocarcinomas), it has a lower expression in small cell lung cancer than normal lung, raising the possibility that inhibitors will have differential effects on these two basic histologies. COX-2 inhibitors may also have differential effects on adenocarcinoma vs squamous cell carcinoma. The COX-2 system, as many others presented at this conference, is also complicated by the multitude of metabolites downstream of cyclooxygenase. Data were presented that while prostacyclins and prostaglandins are both COX-2 products, prostacyclins seem to inhibit cancer while prostaglandins13 and thromboxanes seem to promote cancer. Mark Geraci and Patrick Nana-Sinkam at the University of Colorado found that a surfactant protein C promoter-driven prostacyclin synthase transgenic mouse is resistant to carcinogen-induced lung cancer14 and that human lung cancers are high in prostaglandin E2 but low in prostacyclin, the opposite of that observed in normal lung. It thus may make more sense to target more distal pathways such as using a combination of a prostaglandin antagonist with a prostacyclin agonist. Iloprost is a prostacyclin agonist being tested in a randomized chemoprevention trial at University of Colorado based on these observations, and enrolls smokers with sputum atypia. The end points for this trial are dysplasia index and Ki67-staining surrogate markers.
| Novel Targets |
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| Conclusions |
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| Appendix |
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Conference Sponsors
Boehringer Ingelheim Pharmaceuticals, Inc.
GlaxoSmithKline
Ortho-Biotech
Pfizer Inc./Boehringer Ingelheim Pharmaceuticals, Inc.
Conference Contributors
Affymetrix
Amgen Inc.
Berlex Laboratories, Inc.
Dey Laboratories, Inc.
Elan Pharmaceuticals, Inc.
Eli Lilly and Company
Genentech
Inspire Pharmaceuticals, Inc.
InterMune, Inc.
James Ehrlich/Colorado Heart Imaging
Ligand Pharmaceuticals
Ortho-McNeil Pharmaceutical, Inc.
OSI Pharmaceuticals, Inc.
Perceptronix Medical Inc.
Pfizer Inc.
Roche Laboratories Inc.
Schering Laboratories
Sepracor Inc.
Xillix Technologies Corp.
The Francis Families Foundation
FUTURE CONFERENCES
47th Annual Meeting 2004
Cellular and Molecular Pathobiology of Pulmonary Hypertension
48th Annual Meeting 2005
Lung Immunology
49th Annual Meeting 2006
COPD
| Footnotes |
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| References |
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