Beyond the Usual Suspects
Authors: Peter J. Mazzone
Journal: Journal of Thoracic Oncology (2012)

All aspects of lung cancer care will benefit from the development of accurate molecular biomarkers. Those that can predict the risk of developing lung cancer will help in the selection of subjects for lung cancer screening. Those that can identify the presence of early-stage lung cancer will help in the evaluation of indeterminate lung nodules. Those capable of prognosticating outcome will assist in planning the aggressiveness of treatment. Those capable of predicting the response to systemic therapies will help to individualize their selection. Over the past decade, molecular biomarker discovery has been dominated by studies assessing the genome, transcriptome, and proteome. Alterations of individual molecules, or patterns of multiple alterations, have been presented as potential biomarkers. To date, with the exception of epidermal growth factor receptor mutation testing1Rosell R Moran T Queralt C Spanish Lung Cancer Group et al.Screening for epidermal growth factor receptor mutations in lung cancer.N Engl J Med. 2009; 361: 958-967Crossref PubMed Scopus (2041) Google Scholar and echinoderm microtubule-associated protein-like 4 anaplastic lymphoma kinase identification,2Kwak EL Bang YJ Camidge DR et al.Anaplastic lymphoma kinase inhibition in non-small-cell lung cancer.N Engl J Med. 2010; 363: 1693-1703Crossref PubMed Scopus (3812) Google Scholar none of the molecular biomarkers have been developed to the point of technical and clinical validation, and/or proven clinical utility. Metabolomics is a source of potential molecular biomarkers, which has been relatively under-represented in the literature. Metabolomics refers to an assessment of the products of cellular metabolism. In a systems biology approach, the metabolome can be considered the product of the interaction between the genome, transcriptome, and proteome with an individual's physiologic status. Traditionally, metabolomics involves the study of nonvolatile endogenous small molecule metabolites such as amino acids, carbohydrates, and lipids, in tissue, blood, or urine. Nonvolatile metabolite profiling can be performed as an overview of the metabolome, or as a targeted look at a metabolic pathway, or a specific metabolite. To date, published studies of nonvolatile metabolite profiling in lung cancer have been small, pilot studies. Differences in metabolite signatures between tumor and adjacent control tissue,3Duarte IF Rocha CM Barros AS et al.Can nuclear magnetic resonance (NMR) spectroscopy reveal different metabolic signatures for lung tumours?.Virchows Arch. 2010; 457: 715-725Crossref PubMed Scopus (31) Google Scholar as well as serum and urine of lung cancer patients compared with healthy controls,4Jordan KW Adkins CB Su L et al.Comparison of squamous cell carcinoma and adenocarcinoma of the lung by metabolomic analysis of tissue-serum pairs.Lung Cancer. 2010; 68: 44-50Abstract Full Text Full Text PDF PubMed Scopus (64) Google Scholar, 5An Z Chen Y Zhang R et al.Integrated ionization approach for RRLC-MS/MS-based metabonomics: finding potential biomarkers for lung cancer.J Proteome Res. 2010; 9: 4071-4081Crossref PubMed Scopus (90) Google Scholar, 6Yang Q Shi X Wang Y et al.Urinary metabonomic study of lung cancer by a fully automatic hyphenated hydrophilic interaction/RPLC-MS system.J Sep Sci. 2010; 33: 1495-1503Crossref PubMed Scopus (74) Google Scholar have been reported. In addition to nonvolatile metabolites, volatile organic compounds (VOCs) can be produced or degraded as part of cellular metabolism and the handling of oxidative stresses. They can be identified in the headspace gas of tissues,7Sponring A Filipiak W Ager C et al.Analysis of volatile organic compounds (VOCs) in the headspace of NCI-H1666 lung cancer cells.Cancer Biomark. 2010; 7: 153-161PubMed Google Scholar blood,8Deng C Zhang X Li N Investigation of volatile biomarkers in lung cancer blood using solid-phase microextraction and capillary gas chromatography-mass spectrometry.J Chromatogr B Analyt Technol Biomed Life Sci. 2004; 808: 269-277Crossref PubMed Scopus (178) Google Scholar urine,9Hanai Y Shimono K Matsumura K et al.Urinary volatile compounds as biomarkers for lung cancer.Biosci Biotechnol Biochem. 2012; 76: 679-684Crossref PubMed Scopus (77) Google Scholar and in the exhaled breath.10Mazzone PJ Exhaled breath volatile organic compound biomarkers in lung cancer.J Breath Res. 2012; 6: 027106Crossref Scopus (47) Google Scholar They can be evaluated for differences in a specific compound, class of compounds, or as a general overview of the spectrum of VOCs present in the biospecimen. VOC profiles in lung cancer have been analyzed with gas chromatography mass spectrometry (GC-MS), other spectrometry technologies, and cross-responsive chemical sensor matrices. GC-MS is capable of identifying selected VOCs and their concentrations, whereas the signal from the sensor technologies reflects the broad mixture of VOCs within the specimen without identifying the specific components of the mixture. These studies have been relatively small and highly variable in their methods, but have shown promising accuracy. In this issue of the Journal Thoracic Oncology, Pelid et al. report on the development of a breath VOC biomarker identified by a cross-responsive sensor matrix using a combination of a gold nanoparticle sensor and a single-walled carbon nanotube sensor, as well as evaluation of selected breath samples with GC-MS. They sampled alveolar breath from 53 lung cancer subjects with early- and late-stage disease as well as 19 control subjects with benign lung nodules. The groups were well matched for smoking histories, but differed in age and size of the primary nodule/mass. The sensor signal distinguished between the two groups with an accuracy of 88%. The influence of clinical variables on the model's accuracy was not studied, and the model was not validated in a separate cohort of subjects. The sensor signal was also capable of accurately characterizing lung cancer histology and stage. This study adds to the growing literature in this area, suggesting that breath VOC biomarkers have the potential to be developed into clinically useful diagnostic tests. In addition, the results support the premise that a metabolic-based biomarker can be used to help characterize the nature of the cancer. Future breath VOC biomarker development will benefit from advances in many areas. Novel gas analysis technologies will help to define more accurately the unique components of the breath VOC mixture, allowing study of their origin, improvement in our knowledge of lung cancer metabolics, and refinement of point-of-care sensor systems. Standardization of breath sampling techniques will lead to greater reproducibility of the results of breath testing. Lung cancer phenotyping will promote the development of breath VOC signatures of more refined populations. In time, the promise shown in early studies could translate into technically and clinically validated, inexpensive, noninvasive molecular biomarkers that are accurate enough to improve patient care.