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Reducing Lung Cancer Risk^*

Early Detection

^* From the Intervention Section, Department of Cell and Cancer Biology, Medicine Branch, Division of Clinical Science, National Cancer Institute, National Institutes of Health, Bethesda, MD.

Correspondence to: James Mulshine, MD, National Cancer Institute, Bldg 10, Rm 12N226, NIH Clinical Center, Bethesda, MD 20892-1906; e-mail: mulshinej{at}bprb.nci.nih.gov

	Abstract

TOP Abstract Introduction Early Detection Strategies Identification of Molecular... Molecular Markers in Lung... Role of Chemoprevention Conclusions References

 
Strategies for the early detection of lung cancer are being investigated in an attempt to improve the poor prognosis associated with the disease. Such approaches, which include the identification of biomarkers for preclinical disease, must be integrated into multimodal cancer prevention strategies. Recent investigations have identified potential markers of early disease, including heterogeneous nuclear ribonucleoprotein, although the use of multiple markers may be required to provide the sensitivity and specificity necessary for mass screening. Early detection necessitates the development of effective chemoprevention strategies for the airway-confined phase of lung cancer. Current research efforts explore the utility of direct drug delivery, such as with the use of aerosolized delivery of retinoids, to maximize delivery of the active agent to the site of early lung cancer while avoiding systemic adverse effects.

	Introduction

TOP Abstract Introduction Early Detection Strategies Identification of Molecular... Molecular Markers in Lung... Role of Chemoprevention Conclusions References

 
Smoking is responsible for 87% of lung cancers¹ ; thus, primary prevention has been a focus of lung cancer control efforts. After peaking in 1963, per capita tobacco consumption among Americans 18 years of age steadily declined by 43% through 1997.¹ However, from 1991 to 1997, tobacco consumption increased 32% among high school students. Because the risk of lung cancer remains markedly elevated for up to 15 years after smoking cessation,² these trends in smoking suggest that the prevalence of and mortality from the disease will remain high during the next 10 to 20 years.

The persistent risk of lung cancer, which remains elevated two-fold 15 years after smoking cessation, underscores the need to study the disease for a protracted time frame. By analyzing long-term trends, the effects of educational, preventive, and treatment strategies on the natural history of lung cancer can be determined. For example, Chute et al³ analyzed the survival rates among patients with small cell lung cancer (SCLC) treated during a 20-year period at the U.S. National Cancer Institute (NCI). Patients were stratified according to disease stage (limited vs extensive stage) and according to the decade of treatment (1973 to 1983 vs 1983 to 1993).³ This stratification allowed examination of differences in outcomes caused by treatment, inasmuch as cyclophosphamide-based regimens were used during the first decade and cisplatin-based regimens were used during the second decade. Survival rates were slightly increased during the second decade compared with the first decade, but the differences were not statistically significant in patients with either limited- or extensive-stage SCLC.³ These data indicate that advances in the treatment of SCLC during the past three decades have been modest, including those for patients with limited-stage disease in which the most progress in lung cancer therapeutics has been made.

The prognosis for patients with advanced non-SCLC (NSCLC) is similarly discouraging. Approximately 70% of these patients present with advanced disease, defined as either stage III or IV.⁴ Cures are achieved in < 20% of patients with stage III NSCLC and only in isolated cases for those with metastatic disease.⁴

The poor prognosis of lung cancer is related to the progression from a localized primary to a disseminated metastatic disease. With our current diagnostic technology, by the time lung cancer reaches a point at which it is clinically detectable, the disease is already in the late stages of its natural history and is only a couple of doublings away from reaching a lethal tumor burden. Lung cancer tumor burden typically exceeds 10⁹ cells at the time of diagnosis (a 1-cm³ volume).⁵ Thus, an important goal for lung cancer management is to develop improved techniques of identifying the premetastatic phases of lung cancer when the disease can be more successfully treated.

These findings have provided impetus for the development of cancer prevention strategies. Cancer prevention involves a multimodal approach to reducing disease risk and includes a number of components, such as public education, public health policies, early detection of disease, and treatment. These components are not mutually exclusive; rather, they should be considered complementary and part of an overall comprehensive strategy for managing lung cancer. For example, improved early detection methods can be used to encourage current smokers to quit. This article will review recent advances in the early detection of lung cancer and the potential applications for chemoprevention.

	Early Detection Strategies

TOP Abstract Introduction Early Detection Strategies Identification of Molecular... Molecular Markers in Lung... Role of Chemoprevention Conclusions References

 
In most cases, lung cancer is caused by exposure of the bronchial epithelium to the tobacco combustion stream, which is loaded with carcinogens. This process is initiated in the lungs many years before the development of invasive or metastatic cancer. Thus, a window of opportunity exists to diagnose the disease in the earliest stages by evaluating the cells recovered from the site of carcinogenic injury. This is the impetus to look at exfoliated bronchial epithelial cells recovered in the sputum.

The concept of early detection of lung cancer has been the subject of research for many years; however, the relatively slow progress in the application of this concept is related to the formidable problems associated with early detection. As early as 1975, Saccomanno et al⁶ developed a model of bronchial epithelial carcinogenesis in which progressive stages of increasingly aggressive cancer were defined on the basis of cytologic examinations of sputum collected from a group of uranium miners. A goal of the early detection research was to establish cytomorphologic criteria to identify early invasive cancer of the bronchial epithelium. The NCI subsequently initiated three early detection trials. Approximately 30,000 subjects were enrolled at the Memorial Sloan-Kettering Cancer Center, Mayo Clinic, and Johns Hopkins Oncology Center.⁷ The trials included men who were heavy cigarette smokers and 45 years of age. Details of the screening varied at the different centers, but participants at Johns Hopkins were randomly assigned to receive either annual chest radiograph examinations only or a dual screen with annual chest radiographs and annual sputum cytology evaluations.⁷ Although dual screening was able to detect presymptomatic, early stage carcinoma (particularly squamous cell carcinoma),⁷ it was not associated with improved overall mortality compared with chest radiograph alone.⁸ One reason for this inability to improve survival is likely related to the poor sensitivity of the sputum cell morphologic studies that were available at the time.⁹ Of early lung cancers detected in the NCI trial, < 10% were detectable only by routine sputum cell morphology. Thus, only a small proportion of cancers was identified in a curable stage. The negative findings of the NCI trial resulted in a loss of enthusiasm for lung cancer early detection. As a result, the emphasis of clinical research shifted toward the treatment of advanced lung cancer.¹⁰

Despite the negative results from the NCI-sponsored trials, the specimens obtained during the trials provided a potential means for identifying other markers of early lung cancer. Annual sputum specimens obtained from individuals screened at Johns Hopkins were archived, and the patients were monitored for 8 years.⁹ Because the clinical outcome of these patients was known, the sputum was screened for the presence of biomarkers that could indicate the presence of lung tumors in an early, preinvasive stage.

Tockman et al¹¹ compared the sputum from patients who went on to develop cancer with the sputum from patients who remained cancer free in an attempt to distinguish patterns of marker expression. Two monoclonal antibodies (MoAbs) were applied to the archived specimens, and positive staining predicted the subsequent development of lung cancer approximately 2 years before clinical recognition of the disease, with a sensitivity of 91% and a specificity of 88%.¹¹

Moving to routine screening for early lung cancer is a large fundamental change in the manner in which patients with lung cancer are managed. Given the cost and scope of that change, such a transition would have to be justified on the basis of definitive clinical trial results. Toward that end, since the completion of the first sputum-based immunocytochemical trial, we have been conducting systematic research directed toward improving the basic understanding of the underlying processes involved in lung cancer to determine how screening procedures can be applied to routine clinical practice.

	Identification of Molecular Markers

TOP Abstract Introduction Early Detection Strategies Identification of Molecular... Molecular Markers in Lung... Role of Chemoprevention Conclusions References

 
The sputum immunocytochemistry trial by Tockman et al¹¹ used two MoAbs, 703D4 and 624H12, as markers of early disease. Of the 22 known positive cases of lung cancer, 20 were detected by dual antibody analysis. Nineteen of these cases were recognized by the 703D4 antibody alone, whereas 624H12 was less sensitive, detecting eight cases of lung cancer alone.¹¹ Because of the higher sensitivity of the 703D4 antibody, much of the biochemical research has been focused on this marker.

After a 4-year effort, the antibody that interacts with 703D4 was identified.¹² The 703D4 MoAb recognizes heterogeneous nuclear ribonucleoprotein (hnRNP) A2/B1,¹² which is one of a family of hnRNPs involved in pre-mRNA processing, RNA metabolism, transcription, DNA replication, and recombination.¹³ These properties suggest that hnRNPs are integrally involved in cellular proliferation and could potentially be involved in the carcinogenesis cascade. hnRNP A2/B1 is expressed at low concentrations in all cells but is frequently overexpressed in transformed bronchial epithelium and in most NSCLC primary and metastatic cancer cells.¹²

The pulmonary distribution of hnRNP A2/B1 is diffuse, with frequent expression in the bronchi, bronchioli, and alveoli, particularly in various composite type II cells.¹⁴ This diffuse expression was somewhat unexpected but is consistent with the concept of field cancerization, which was originally proposed by Slaughter et al¹⁵ in 1953, and suggests that there is broad exposure of the upper respiratory tract to carcinogens, resulting in multiple foci of malignant transformation. If exposed to the carcinogen for long enough periods and in great enough concentrations, cells progress through a number of morphologic changes from normal epithelium to invasive cancer.¹⁵

hnRNP A2/B1 has been found to be overexpressed at critical periods of normal lung development, and the level of overexpression is similar to that found in lung cancers and preneoplastic lesions.¹⁶ The expression of hnRNP A2/B1 is high during fetal lung development, whereas the expression of this protein is restricted in mature lung tissue.¹⁶ The pattern of expression during carcinogenesis (especially for type II cell hyperplasia) is similar to that seen during fetal lung development.¹⁶ This pattern of expression is consistent both at the protein and the message level, indicating that hnRNP A2/B1 may be an oncodevelopmental protein.¹⁶ Further studies are required to elucidate the mechanism by which this molecule is involved in the transformation of tissues from a normal to a cancerous state.

	Molecular Markers in Lung Cancer Screening

TOP Abstract Introduction Early Detection Strategies Identification of Molecular... Molecular Markers in Lung... Role of Chemoprevention Conclusions References

 
The role of hnRNP A2/B1 overexpression for detecting preclinical lung cancer is the subject of two trials conducted in large patient populations that are at high risk of developing lung cancer. A case-cohort study in China included approximately 6,000 tin miners who are heavy smokers and who have an extraordinary rate of lung cancer.¹⁷ Results indicated that 703D4 MoAb detection of hnRNP A2/B1 overexpression in sputum epithelial cells is two- to three-fold more sensitive for the detection of lung cancer than standard chest radiograph and sputum cytology methods and that the method is particularly effective in identifying early disease.¹⁷ The sensitivity of MoAb detection of hnRNP was 74% compared with 21% for cytology and 42% for chest radiograph.¹⁷ However, the biomarker had a lower specificity (70%) compared with cytology (100%) and radiograph (90%).¹⁷

The other prospective validation trial of sputum immunocytochemical early detection of lung cancer is currently under way at 11 thoracic oncology centers in North America. The trial is designed to include 1,000 patients with stage I NSCLC who have undergone resection.⁹ This population was selected because these patients have a high risk of developing second primary cancers.¹⁸ Preliminary results from this study indicate that the positive predictive value of hnRNP MoAb is 67%.¹⁹ This study is ongoing through a consortium called the Lung Cancer Early Detection Working Group and involves the NCI, the Eastern Cooperative Oncology Group, and Bayer Diagnostics through a cooperative research and development agreement with the National Institutes of Health.

Efforts to improve the sensitivity of hnRNP markers are ongoing. The most exciting of these efforts involve the use of combined screening with spiral CT.²⁰ Through several ongoing screening trials, the differential utility of biomarker-based vs anatomic imaging-based screening will be defined. From preliminary data, these techniques have complementary strengths that may greatly enhance the routine detection of premetastatic cancer.

	Role of Chemoprevention

TOP Abstract Introduction Early Detection Strategies Identification of Molecular... Molecular Markers in Lung... Role of Chemoprevention Conclusions References

 
The ability to detect the premetastatic phase of lung cancer in high-risk populations provides a rationale for development of chemoprevention strategies. A number of chemoprevention approaches have been investigated and are reviewed in detail by Lippman et al.²¹ Retinoids are one group of agents that have demonstrated significant chemoprevention potential. Oral retinoids have been shown to decrease the risk of second primary tumors in patients with head and neck cancer²² and to prevent oral carcinogenesis.²³ However, in a placebo-controlled study involving smokers who had a high metaplasia index, 6 months of treatment with isotretinoin had no effect on squamous metaplasia.²⁴ A follow-up study, which evaluated the tissues from this study, found no significant effect of treatment on hnRNP A2/B1 expression.²⁵ These results contrast with those of in vitro studies in which retinoids at concentrations that mediate antiproliferative and apoptotic response effectively downregulate hnRNP.

The pharmacokinetics of oral retinoids may contribute to their lack of clinical effect. Retinoids administered orally undergo extensive first-pass metabolism, and, once absorbed, the drugs are highly bound (99.9%) to plasma proteins. These characteristics suggest that < 1% of administered drug is available for delivery to the desired site. Thus, larger retinoid doses must be administered to achieve adequate tissue concentration of active drug. Moreover, because chemoprevention involves the administration of active treatment to patients who are not symptomatic and who may never develop lung cancer, the ideal chemoprevention agent should be associated with minimal systemic toxicity. Oral delivery of retinoids is associated with a number of adverse effects, such as mucocutaneous effects, severe itching, adverse lipid profiles, elevation of liver enzymes, and skeletal changes.²⁶ To overcome these difficulties, aerosolized retinoid administration is being investigated to improve the therapeutic index of the drug. Because lung cancer is an airborne carcinogen-mediated disease, it may be more effective to administer the chemoprevention agent in the same manner. Inhalation delivery provides much higher concentrations of active drug in the critical compartments with smaller doses and without systemic exposure.²⁷ On the basis of favorable preclinical data both with regard to toxicity and efficacy, a clinical trial with aerosolized 13-cis-retinoic acid should be starting at the National Institutes of Health Clinical Center in late 1999. This work is supported by another recent experience. Intrapulmonary administration of a liposomal formulation of all-trans-retinoic acid to the lungs of mice has recently been reported.²⁸ The drug was effectively delivered to the lungs and retained for up to 96 h after a single exposure.²⁸ The aerosol-delivered drug retained its biological activity and was not associated with observable adverse effects.²⁸ These data indicate that aerosol delivery of retinoids could be an efficient and nontoxic method of administering higher doses of retinoids to the lung.

	Conclusions

TOP Abstract Introduction Early Detection Strategies Identification of Molecular... Molecular Markers in Lung... Role of Chemoprevention Conclusions References

 
Developments in molecular-based diagnostics have generated enthusiasm for population-based screening of lung cancer. Molecular markers have been identified that are predictive of future lung cancer. hnRNP A2/B1 is one such marker that has been found to have a high predictive value. To achieve a high level of sensitivity and specificity, however, it is likely that effective strategies will involve multiple markers. The value of population-based screening techniques for lung cancer depends on the development of safe and effective chemotherapeutic agents that can arrest the disease process in the preclinical, preinvasive phase. Administration of chemoprevention agents directly to the site of carcinogenesis may improve their therapeutic index. Aerosolized retinoids are one promising example of this approach. Future studies in the early detection of lung cancer will be aimed at identifying sensitive and reliable biomarkers and optimal agents and regimens for chemoprevention. Integration of these strategies in lung cancer studies is necessary to improve lung cancer mortality rates.

	Acknowledgements

 
The work described in this review has been possible only due to the help of a number of talented, long-term collaborators, including Drs. Frank Cuttitta, Melvyn Tockman, Anthony Treston, Bruce Johnson, Ilona Linnoila, Robert Dedrick, Luigi De Luca, Jack Ruckdeschel, and the Lung Cancer Early Detection Working Group investigators, as well as Dr. Michael Placke and colleagues from Battelle Pulmonary Therapeutics and Dr. Steven Weiss and colleagues from Bayer Diagnostics.

	Footnotes

 
Abbreviations: hnRNP = heterogeneous nuclear ribonucleoprotein; MoAb = monoclonal antibody; NCI = National Cancer Institute; NSCLC = non–small cell lung cancer; SCLC = small cell lung cancer

	References

TOP Abstract Introduction Early Detection Strategies Identification of Molecular... Molecular Markers in Lung... Role of Chemoprevention Conclusions References

 

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