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(Chest. 2003;123:60S-71S.)
© 2003 American College of Chest Physicians

Lung Cancer Prevention*

The Guidelines

Konstantin H. Dragnev, MD; Diane Stover, MD, FCCP and Ethan Dmitrovsky, MD

* From the Norris Cotton Cancer Center and Dartmouth Medical School (Drs. Dragnev and Dmitrovsky), Lebanon, NH; and the Pulmonary Section (Dr. Stover), Department of Medicine, Memorial Sloan-Kettering Cancer Center, New York, NY.

Correspondence to: Konstantin H. Dragnev, MD, Hematology/Oncology Section, Department of Medicine, Dartmouth-Hitchcock Medical Center, Lebanon, NH 03756; e-mail: dragnev{at}dartmouth.edu


   Abstract
TOP
 Abstract
Introduction
 Recommendations
 Lung Cancer Chemoprevention...
 Innovative Delivery Approaches
 Conclusions
 Summary of Recommendations
 References
 
Lung carcinogenesis is a chronic and multi-step process resulting in malignant lung tumors. This progression from normal to neoplastic pulmonary cells or tissues could be arrested or reversed through pharmacologic treatments, which are known as cancer chemoprevention. These therapeutic interventions should reduce or avoid the clinical consequences of lung cancer by treating early neoplastic lesions before the development of clinically evident signs or symptoms of malignancy. Preclinical, clinical, and epidemiologic findings relating to different classes of candidate chemopreventive agents provide strong support for lung cancer prevention as an attractive therapeutic strategy. Smoking prevention and smoking cessation represent an essential approach to reduce the societal impact of tobacco carcinogenesis. However, even if all the goals of the national antismoking efforts were met, there still would be a large population of former smokers who would be at increased risk for lung cancers. Lung cancer also can occur in those persons who never have smoked. This article focuses on what is now known about pharmacologic strategies for lung cancer prevention. Randomized clinical trials using ß-carotene, retinol, isotretinoin or N-acetyl-cysteine did not show benefit for primary and tertiary lung cancer prevention. There is also evidence that the use of ß-carotene and isotretinoin for lung cancer chemoprevention in high-risk individuals may increase the risk for lung cancer, especially in individuals who continue to smoke. There is a need for relevant in vitro models to identify pathways that activate chemopreventive effects in the lung. An improved understanding of cancer prevention mechanisms should aid in the design of clinical trials and in the validation of candidate chemopreventive targets as well as the discovery of new targets. Until such studies are completed, no agent or combination of agents should be used for lung cancer prevention outside of a clinical trial.

Key Words: chemoprevention • epidermal growth factor receptor • G1 cyclin • lung cancer • retinoids


   Introduction
TOP
 Abstract
 Introduction
Recommendations
 Lung Cancer Chemoprevention...
 Innovative Delivery Approaches
 Conclusions
 Summary of Recommendations
 References
 
Lung carcinogenesis is a chronic process involving multiple genetic, cellular, and tissue alterations. This results from mutagenic damage to growth-regulating genes and their products that ultimately leads to the development of invasive or metastatic cancer.1 The transformation steps from normal through preneoplasia to overt malignancy occur as a consequence of the following: (1) initiation, in which DNA damage occurs; (2) promotion, in which genetic and epigenetic changes confer additional genomic damage; and (3) progression to locally invasive or metastatic disease. Carcinogen exposure forms "fields" of altered cells long before invasive malignant disease is detected clinically, as was first hypothesized by Slaughter and coworkers in 1953.2 The concept of field cancerization provides a basis for understanding the clonality of preneoplastic cells. Some of these genetically altered cells acquire a malignant phenotype, while others do not.

There are many interventions that might be considered as strategies for reducing lung-specific cancer risks including smoking prevention and cessation, lifestyle as well as dietary or nutritional changes, effective screening of identified high-risk individuals, among others. Of these strategies, only smoking prevention and cessation has been shown to reduce lung cancer risk. Although the focus of this article is on the chemoprevention of lung cancer, it is primary prevention (ie, smoking prevention) that should be a major focus within our society including local communities, schools from kindergarten through college, and among persons in the medical profession. Strategies that have been the most successful in preventing children from starting to smoke include all-grade inclusive school programs that emphasize a "life skills training approach," the use of brief recurring antismoking messages that point out the positive aspects of being nicotine-free, and the enforcement of high excise taxes on tobacco products. For current smokers, there is strong evidence that brief recurring physician advice significantly increases long-term smoking abstinence rates. Clinician-based approaches should always include the routine identification of tobacco users, which in turn increases the rate of clinician intervention with patients who smoke.


   Recommendations
TOP
 Abstract
 Introduction
 Recommendations
Lung Cancer Chemoprevention...
 Innovative Delivery Approaches
 Conclusions
 Summary of Recommendations
 References
 

  1. For all individuals, smoking prevention should be strongly encouraged to decrease the risk of lung cancer. Level of evidence, good; benefit, substantial; grade of recommendation, A
  2. For all individuals, school-based and community- based interventions that are aimed at reducing tobacco exposure should be recommended, including a "life skills training" approach that is aimed at reducing tobacco, alcohol, and illicit drug use, campaigns with brief recurring antismoking messages, high tobacco excise taxes, and restrictions on smoking in the workplace. Level of evidence, good; benefit, substantial; grade of recommendation, A
  3. Smokers should be identified as smoking cessation reduces the risk of lung cancer. Level of evidence, good; benefit, substantial; grade of recommendation, A
  4. Current smokers should be advised to quit smoking, and, when appropriate, clinicians should prescribe and monitor pharmacotherapy. Individuals who smoke and want to quit also should have access to psychosocial treatment and behavioral modification therapies as indicated. There is sufficient-to-strong evidence that indicates these practices will help to increase long-term smoking abstinence rates. Level of evidence, good; benefit, substantial; grade of recommendation, A


   Lung Cancer Chemoprevention Agents
TOP
 Abstract
 Introduction
 Recommendations
 Lung Cancer Chemoprevention...
Innovative Delivery Approaches
 Conclusions
 Summary of Recommendations
 References
 
Complex cytogenetic or molecular genetic abnormalities can occur in lung cancers or in nonmalignant lung tissues that are isolated from patients with lung cancer.3 4 5 6 7 It has been hypothesized7 that multiple changes are required for the progression of lung carcinogenesis. The gain and loss of specific growth regulatory species or DNA loci often are found in lung carcinogenesis.8 9 10 11 12 Alterations involve dominant genetic events that occur through the activation of oncogenes and recessive genetic changes that occur through the deletion or inactivation of tumor suppressor genes.13 Some studies12 14 reveal that loss of heterozygosity (LOH) on chromosomes 3p, 9p, or 17p often is detected not only in lung cancers, but also in preneoplastic lesions and even in adjacent histologically normal lung tissues. Bronchial biopsy specimens from former smokers also exhibit genetic damage. LOH at chromosomes 9p and 17p is reported in former smokers. LOH at chromosome 3p is also frequent but might be a reversible alteration.4 These oncogenic changes could be required for the development or maintenance of lung cancer and might represent attractive targets for lung cancer prevention. Conceivably, these would be useful as surrogate markers with which to monitor clinical response to lung cancer chemoprevention agents.

Squamous cell lung carcinomas often arise in association with areas of metaplasia, dysplasia, or carcinoma in situ. Because of this association, Saccomanno and coworkers3 proposed in 1974 a model that emphasized the progressive changes of squamous cell metaplasia (ie, hyperplasia), metaplasia with atypia (ie, dysplasia) that was mild, moderate, or marked, or carcinoma in situ that preceded the development of invasive squamous cell carcinoma. It is now possible to highlight specific genetic or chromosomal alterations and changes in gene expression that are associated with these preneoplastic changes in the lung, as is summarized in Figure 1 . Related models exist for the development of lung adenocarcinoma. It is not yet known which individual or cassette of carcinogenic changes is rate-limiting in the maintenance or progression of preneoplastic lesions in the lung. These could be distinct for each carcinogenic agent or lung cancer histopathologic subtype.



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  		Figure 1.. Steps involved in squamous cell lung carcinogenesis. This figure depicts the multiple steps involved in the development of squamous cell cancers of the lung. Focal atypia can arise in preneoplastic lesions. Some preneoplastic lesions are viewed as reversible (solid arrow), while others may not be reversible (arrow without a solid line). The outlined box has highlighted preneoplastic targets for lung cancer chemoprevention. Also displayed are some representative alterations (genetic, chromosomal, or gene expression) that are frequent in bronchial preneoplasia. The sites where these occur during lung carcinogenesis are shown by their relative positions. The text provides a detailed discussion of these alterations.

 
The chronic and multistep nature of lung carcinogenesis makes attractive a chemopreventive approach that would arrest or reverse one or more of these carcinogenic changes. The cancer chemoprevention concept that was first proposed by Sporn et al15 stresses interventions at early stages of carcinogenesis, even before malignancies become clinically evident. If chemoprevention were effective in the lung, this would avoid many clinical consequences of lung cancer and would reduce the need for the treatment of disseminated lung cancers that are often resistant to combination chemotherapy. Clinical validation for the cancer prevention concept was provided by a randomized trial using the selective estrogen receptor modulator (SERM) tamoxifen in women who are at high risk for breast cancer development.16 In women who were randomized to receive tamoxifen as compared to control subjects, there was a highly statistically significant reduction in the risk of invasive and noninvasive breast cancers.16 These findings provide an impetus to identify other pharmacologic agents that would reduce the risk of primary or second lung cancers.

Despite the fact that the best way to prevent most lung cancers is not to smoke, there remains an alarming number of current smokers and former smokers worldwide. Chemoprevention offers an appealing therapeutic approach assuming that certain nontoxic agents can be identified. An improved understanding of the basic biology of lung carcinogenesis remains essential for the development of effective lung cancer chemoprevention. The defined steps in lung carcinogenesis could be targeted by antiproliferative, differentiation-inducing, proapoptotic, or antiangiogenic agents.17 This article focuses on those chemopreventive agents that exert their pharmacologic actions through specific pathways that can be targeted in the lung.

Pharmacologic strategies for chemoprevention should target multiple steps in carcinogenesis.18 Effective chemopreventive agents could block DNA damage occurring as an initiating step in carcinogenesis or could suppress the growth or progression of preneoplastic cells that already have acquired genomic DNA damage. Agents also might act at the promotion or progression steps of carcinogenesis. Telomerase activation, cell-stromal interactions, as well as new blood vessel formation (ie, neoangiogenesis) likely have important roles in the development of invasive lung cancers. An empirical approach to cancer chemoprevention has been replaced by targeted therapeutic strategies that emphasize mechanistic actions of candidate preventive agents in clinical chemopreventive trials. These strategies build on the basic scientific understanding of the relevant pathways that are involved in cancer chemoprevention. Many candidate lung cancer therapeutic or preventive agents exist with diverse structures and mechanisms of action.17 18 19 Several of these agents are already available for testing in lung cancer prevention trials. A partial list of these agents appears in Table 1 . In addition to these agents, other small-molecule agonists and antagonists are undergoing preclinical testing and will soon be available for testing in lung cancer chemoprevention trials.


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  		Table 1.. A Partial List of Candidate Lung Cancer Chemoprevention Agents*

 
In vitro carcinogen-induced or engineered animal models should be useful to validate whether a therapeutic target is required for the maintenance or progression of a preneoplastic or neoplastic lung lesion.20 The presence of the differential overexpression of growth regulatory species in preneoplastic or malignant lung tissues vs normal lung tissues provides one basis for targeting these species in lung carcinogenesis.21 22 23 24 Clinical pharmacologic data already exist for some chemopreventive agents in the advanced-disease setting. From these clinical data, therapeutic activity and toxicity profiles would already be available. Findings could be extrapolated to the use of the same agent in lung cancer prevention. An attractive target for clinical chemoprevention would have pharmacologic data from preclinical testing in in vitro and animal models. Potential targets include receptor and nonreceptor kinases, growth factor receptors or their ligands, and enzymes (especially as part of signal transduction pathways), among other targets. For clinical chemoprevention in the lung or at other sites, several desirable features exist for the selection of an optimal agent for study. These include having a validated target, based on appropriate preclinical and clinical pharmacologic data for this target, completed phase I safety and dose-response data, objective responses in phase I and II trials, and convenient schedules and routes of drug administration. In the next sections, several candidate lung cancer chemopreventive agents will be discussed.

Retinoids
Antiproliferative, differentiation-inducing, as well as proapoptotic agents could target carcinogenic changes. The retinoids are a class of prevention agents that could exert many of these potential clinical chemopreventive effects. Retinoids are natural and synthetic derivatives of vitamin A that have diverse chemical structures, pharmacologic properties, nuclear receptor affinities, and associated toxicity profiles.17 18 A strong rationale for use of the retinoids in lung cancer prevention stems from results obtained from experimental animal models, epidemiologic studies, and clinical trials.25 Wolbach and Howe26 first identified in 1925 vitamin A- dependent pathways that are required for epithelial cell homeostasis. They discovered that vitamin A deficiency in rodents caused squamous metaplasia in the trachea as well as at other epithelial sites. This was reversed by the correction of the vitamin A deficiency. These metaplastic changes were similar to those that arose in smokers, implicating a role for vitamin-A dependent signals in suppressing lung carcinogenesis. Further evidence for an association between vitamin A and cancer incidence came from epidemiologic data establishing an inverse relationship between vitamin A levels and the incidence of cancer at specific epithelial sites.25 These and other findings provided a basis for the use of retinoids in lung cancer prevention.

Additional support for a retinoid role in cancer chemoprevention derived from clinical trials that were conducted using retinoids that resulted in the successful treatment of certain premalignant conditions such as oral leukoplakia,27 cervical dysplasia,28 and xeroderma pigmentosum.29 Other clinical trials revealed retinoid activity in reducing some second primary cancers. These include trials that demonstrated a reduction in second aerodigestive tract cancers in patients having prior head and neck carcinomas,30 lung carcinomas,31 or hepatocellular carcinomas.32 In contrast to these promising trials, a randomized Intergroup trial that was conducted with subjects who had been treated with 13-cis-retinoic acid following resection of stage I lung cancers did not show clinical benefit in smokers, although a reduction in second cancers was observed in subjects who never had smoked.33 This indicated the potential for negative clinical actions when chemopreventive agents are administered in active smokers. Likewise, this underscores the need to understand how retinoids may be clinically useful in chemoprevention for former smokers or never-smokers.

Epidemiologic evidence for an inverse relationship between serum vitamin A or ß-carotene levels and specific cancer incidences led to cancer prevention trials using ß-carotene. Chemopreventive treatment with ß-carotene had appeal because of its role as a vitamin A precursor with antioxidant activity and its reduced clinical toxicity compared to 13-cis-retinoic acid. Treatment with ß-carotene also was hypothesized to restore physiologic carotenoid levels. However, randomized trials using ß-carotene for primary lung cancer prevention in high-risk individuals did not result in a reduction of lung cancers and appeared harmful in this setting, especially when subjects continued to smoke or to consume ethanol.34 35 36 These trials are discussed in detail in another chapter. Yet, it is important to emphasize here that some of the lessons learned from these trials included the differences between physiologic levels and pharmacologic treatments with chemopreventive agents, the potential for negative interactions in smokers who are treated with these agents, and the clinical need to combine lung cancer prevention agents with smoking cessation.

It is also important to note that nonclassic retinoids such as retinoid X receptor (RXR) agonists (ie, rexinoids) have yet to be studied in lung cancer prevention. A rationale for their use comes from a study reporting clinical activity of bexarotene when used with combination chemotherapy for the treatment of patients with advanced-stage lung cancer.37 Rexinoids and retinoids activate distinct nuclear receptors but can engage a similar chemopreventive pathway in the lung, as discussed below.38 39 Other nonclassic retinoids exist that promote apoptosis, inhibit AP1, or signal other biological effects, as reviewed.17 18 These pharmacologic agents also could become available for testing in lung cancer chemoprevention trials.

Recommendations

Mechanisms of Retinoid Actions
The retinoids exert their biological effects through nuclear retinoid receptors that are associated with inhibitory corepressors or stimulatory coactivators.40 41 42 These ligand-dependent interactions result in the activation of target genes that signal retinoid growth-suppressive effects or differentiation effects. There are three retinoic acid nuclear receptors (RAR{alpha}, RARß, and RAR{gamma}) as well as three retinoid X nuclear receptors (RXR{alpha}, RXRß, and RXR{gamma}), along with multiple isoforms.43 These share homology with other members of the steroid receptor superfamily of nuclear receptors, which include the glucocorticoid receptor, vitamin D receptor, and estrogen receptor, among others. Orphan nuclear receptors exist, and their physiologic ligands remain to be discovered. Retinoid receptor-dependent pathways have been identified through the use of homologous recombination in defined cell contexts44 45 46 and through the use of retinoid receptor-specific agonists.47 48 The expression of specific retinoid receptors is linked to retinoid responses in certain preneoplastic and malignant diseases. For example, the induction of RARß expression is associated with clinical responses in oral leukoplakia.49

Retinoids bind to the ligand binding domain of specific retinoid nuclear receptors. These nuclear receptors also contain DNA-binding domains that recognize defined DNA response elements in the genome.40 41 Following these ligand-receptor and receptor-DNA interactions, the transcription of direct target genes that signal retinoid biological effects are activated or repressed. Retinoid nuclear receptors can undergo heterodimerization or homodimerization during retinoid receptor signaling.50 51 52 These receptors associate with coregulator proteins that are known as inhibitory corepressors or stimulating coactivators.42 Protein-protein interactions between retinoid receptors and their coregulators provide an important level of regulation to the retinoid-signaling pathway since these can affect the basal transcriptional machinery53 through chromatin remodeling that is related to changes in the state of acetylation. Coregulators represent additional pharmacologic targets for lung cancer prevention.

Other retinoid binding proteins exist, including the cytosolic retinoic acid binding protein (CRABP) and the cytosolic retinol binding protein (CRBP). These regulate the intracellular binding of retinoids and appear to contribute to retinoid metabolism and signaling pathways. These cytosolic receptors may serve as intracellular storage sites for the retinoids or may facilitate retinoid transport from the cytoplasm into the nucleus. Alternatively, these proteins could sequester retinoids in the cytoplasm and perhaps reduce extracellular stores, as reviewed.54 Cytosolic retinoid receptors might contribute to clinical retinoid resistance, as in patients with acute promyelocytic leukemia, although pharmacologic mechanisms also could confer this resistance.55 56

Pharmacologic agonists and antagonists have been engineered to affect specific components of the retinoid signaling pathway. For instance, all- trans-retinoic acid (RA) is an agonist for the RAR but not the RXR pathway, while 9-cis-retinoic acid is bifunctional, activating the RAR and RXR pathways.57 58 Bexarotene is an RXR-selective agonist that has been approved for clinical use by the US Food and Drug Administration. Some retinoids target the AP-1 transcription factor,59 while others, such as N-(4-hydroxyphenyl)retinamide (fenretinide 4HPR), act through receptor-independent mechanisms that can generate reactive oxygen species60 and preferentially signal apoptosis even in cells that harbor retinoid receptor defects that confer RA resistance.61 4HPR triggers apoptosis even in RA-resistant tumor cells that also have acquired chemotherapy resistance.61 Clinical benefits have been reported with 4HPR in the prevention of second breast cancers.62 Whether 4HPR would have chemopreventive effects in the lung remains to be determined.

To date, lung cancer chemoprevention clinical trials have emphasized the use of classic retinoids that activate the RAR pathway. Since RARß repression is frequent in several epithelial cancers including lung cancers, this could affect clinical chemopreventive responses to classic retinoids.49 63 The mechanisms that are responsible for RARß repression are under active study. Preclinical evidence points to a role for methylation-induced silencing of this nuclear receptor, among other possible mechanisms. Perhaps demethylation agents that target RARß sequences could be used in conjunction with an active retinoid to overcome RARß repression and to engage clinical chemopreventive effects. Clinical cancer chemoprevention trials in the lung should consider the use of retinoids that do not activate the classic retinoid signaling pathway, thus bypassing RARß repression. While transcriptional mechanisms linked to retinoid actions have been under intensive study, posttranscriptional mechanisms also are recognized as being important. These likely contribute to retinoid chemopreventive effects.38 A posttranslational mechanism linked to retinoid chemoprevention was identified through studies conducted in an in vitro model, as discussed below.38 39

Retinoid Lung Cancer Chemoprevention Mechanism
The optimal retinoids to be used in lung cancer chemoprevention trials need to be determined. If clinical outcome is the only end point used for clinical lung cancer prevention activity, then progress in this field will not be rapid. One approach with which to assess the activities of candidate chemoprevention agents is to examine the effects in relevant preclinical models of lung carcinogenesis. These models include in vitro cellular models,20 experimental animal models of carcinogen-induced lung tumors,64 as well as genetically engineered mice that develop lung cancer.65 The mechanistic insights derived from these models should aid in the conduct of clinical chemoprevention trials. This chapter focuses on the use of cellular models of lung cancer prevention.

Epithelial cell transformation can be prevented in vitro by retinoid treatment.20 The BEAS-2B immortalized human bronchial epithelial cell line was adapted to investigate tobacco-related carcinogenic transformation mechanisms. These cells were transformed after independent exposure to the following carcinogens: cigarette smoke condensate; or N- nitrosamine-4-(methylnitrosamino)-1-(3 pyridyl)-1-butanone (NNK).20 RA treatment prevented cigarette smoke condensate-dependent or NNK-dependent carcinogenic transformation of these bronchial epithelial cells. This chemopreventive activity in human bronchial epithelial cells was linked to the triggering of G1 cell cycle arrest, concomitant growth suppression, and a decline in the expression of G1 cyclin proteins.20 38 39 The signaling of this G1 arrest involved a posttranslational mechanism. Inhibitors of the proteasome-dependent degradation pathway blocked retinoid repression of G1 cyclin expression.38 39 It was hypothesized that the delay at G1 signaled by retinoid treatment permitted the repair of genomic DNA damage caused by carcinogens. This may be a common retinoid mechanism that signaled growth suppression in that it was also activated during tumor cell differentiation.66

When normal, immortalized, or carcinogen-transformed human bronchial epithelial cells were treated with receptor-selective retinoids, RARß-dependent and RXR-dependent agonists signaled growth suppression, while RAR{alpha} and RAR{gamma} agonists did not.39 Carotenoids were unable to repress cyclin D1 protein expression or to activate this proteasome-dependent degradation pathway.39 RARß and RXR agonists, unlike other examined retinoid receptor-selective agonists, induced the proteasome-dependent proteolysis pathway that previously was shown to be activated by RA treatment. Whether this chemopreventive mechanism is activated in vivo during a retinoid clinical trial is the subject of current work. Independent evidence for an important role for retinoid induction of a ubiquitin-dependent degradation pathway was found from microarray analyses.67 Retinoid target genes were studied during leukemic cell differentiation. One cluster of species that was induced by RA treatment was that of the proteasomal degradation pathway. These and other findings66 were consistent with the view that the promotion of ubiquitination by retinoid treatment is involved directly in the triggering of G1 arrest and the progression of a tumor cell differentiation program.

One prediction from these bronchial epithelial cell chemopreventive studies was that either cyclin D1 or cyclin E would be expressed aberrantly in bronchial preneoplasia. Bronchial preneoplastic tissues were examined for G1 cyclin expression. It was hypothesized that the expression of these species would be deregulated early during lung carcinogenesis. The aberrant expression of cyclin D1 or cyclin E was found to be frequent in bronchial preneoplasia and in associated squamous cell carcinomas.8 The aberrant expression of these cyclins was more frequent than that detected for either p53 or the retinoblastoma gene product (Rb).8 These findings implicated the deregulated expression of G1 cyclins as candidate biomarkers for lung cancer prevention trials. Perhaps treatment with an effective lung cancer chemoprevention agent would repress aberrant G1 cyclin expression in lung tissues that were at high risk for malignant conversion. Future clinical trials will determine whether this is the case.

Cyclooxygenase-2 Inhibitors
Inducible cyclooxygenase (COX)-2 is involved in the synthesis of prostaglandins from arachidonic acid and often is activated during inflammation (Fig 2 ). Evidence exists for COX-2 as a therapeutic target for cancer chemoprevention, as has been reviewed previously.17 18 Notably, celecoxib, a selective COX-2 inhibitor, decreased the number of polyps in familial polyposis coli (FAP), a cancer predisposition syndrome.68 Several lines of evidence have provided a rationale for the use of a COX-2 inhibitor in lung carcinogenesis.17 18 COX-2 regulated synthesis of prostaglandins that promoted tumorigenesis and COX-2 inhibition reduced NNK-mediated lung adenomas in the A/J mouse. Preclinical studies indicated that COX-2 overexpression inhibited apoptosis, and COX-2 also was linked to the regulation of angiogenesis. The differential expression of COX-2 was observed in lung cancers, and this had a negative prognostic impact in stage I lung cancers.69 Epidemiologic evidence also was consistent with a role for COX-2 inhibition in lung cancer therapy or prevention.17 18 Several COX-2 inhibitors are already available for cancer chemoprevention trials, as shown in Figure 3 . COX-2 inhibitors have targeted inducible COX-2, which results in pharmacologic selectivity that overcomes clinical toxicities from the inhibition of constitutively expressed COX-1. Diverse biological effects could result from this inhibition of prostaglandin synthesis. Some prostaglandins exert anticarcinogenic actions. Others could have an opposite effect. Future work should identify those regulated prostaglandins that directly mediate chemopreventive effects. These could represent other pharmacologic targets for lung cancer prevention.



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  		Figure 2.. Arachidonic acid generates biologically active prostaglandins that can play an important role in cancer chemoprevention. Two enzymes are involved in this biosynthesis, constitutive COX-1 and inducible COX-2. Celecoxib and rofecoxib preferentially inhibit COX-2, and this inhibition is associated with cancer chemopreventive activity.

 


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  		Figure 3.. The structure of candidate lung cancer chemoprevention agents. The depicted pharmacologic agents include several that are already available for clinical chemopreventive trials. These include COX-2 inhibitors (ie, celecoxib and rofecoxib), inhibitors of the tyrosine kinase associated with the EGFR, such as ZD1839 (Iressa; AstraZeneca; Wilmington, DE) and OSI-774 (erlotinib, Tarceva; Genentech; South San Francisco, CA), as well as the RXR agonist bexarotene (Targretin; Ligand Pharmaceuticals; San Diego, CA).

 
Epidermal Growth Factor Receptor Inhibition
Preclinical and clinical data provided a strong rationale for targeting the epidermal growth factor receptor (EGFR) pathway in lung cancer prevention, as shown in Figure 4 . Differential overexpression of the EGFR and several of its ligands was reported in malignant and preneoplastic compared to normal lung tissues.70 71 In vitro studies71 demonstrated promotion of mitogenesis of pulmonary cells following treatment with the epidermal growth factor. In xenograft models, pharmacologic inhibitors of the EGFR (blocking antibodies or inhibitors of the EGFR-associated tyrosine kinase) were shown to oppose this mitogenic stimulation pathway.70 Direct evidence for EGFR as a target for lung cancer prevention comes from finding that the exposure of human bronchial epithelial cells to the carcinogen NNK increased EGFR expression.71 Notably, treatment with the chemopreventive agent RA prevented this induction.71 Other evidence for an important role for the EGFR in lung cancer prevention comes from clinical findings of activity of EGFR-specific inhibitors in the treatment of advanced-stage cancers including chemotherapy-refractory lung cancer cases.70 72 73 The favorable toxicity profile observed in the advanced-disease setting with these agents indicated that targeting the EGFR should be considered for lung cancer chemoprevention trials.



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  		Figure 4.. Schematic representation of the cellular EGFR pathway. Potential therapeutic targets of this pathway are depicted. The EGFR is located at the cellular surface, where it is activated by interactions with its ligands (•) secreted in an autocrine or paracrine manner. This leads to receptor autophosphorylation and to mitogenic signals as well as other signals that are transmitted to the nucleus through second messenger pathways. In the nucleus, these effects often result in an induction of AP-1 components (shown here as {uparrow}Fos and {uparrow}Jun). These ligand-receptor interactions can be pharmacologically targeted by using anti-EGFR antibodies or small molecule inhibitors of the EGFR-associated tyrosine kinase ({blacktriangleup}).

 
Other Agents
Other candidate cancer chemoprevention agents include those acting through specific nuclear receptors such as the vitamin D receptor (ie, deltanoids), peroxisome proliferator-activated receptor-{gamma}, or other nuclear receptors. One promising class of pharmacologic agents for study is the synthetic triterpenoids, which are derivatives of natural products, the cyclosqualenoids.74 Triterpenoids have exhibited potent differentiation-inducing, antiproliferative and anti-inflammatory activities. Pertinent to their potential role in cancer chemoprevention, one of the synthesized triterpenoids, CDDO, suppressed the induction of the inflammatory enzymes inducible nitric oxide synthase (iNOS) and COX-2.74 Whether these findings will be extended to the setting of chemoprevention of lung tumors or other tumors is the subject of ongoing work. Other candidate lung cancer chemoprevention agents are displayed in Table 1 .

Combination Therapy
Clinical cancer chemoprevention trials have features that are distinct from other therapeutic trials. To exert the desired clinical effects, chemoprevention agents would be administered on a long-term basis and should have few, if any, associated clinical toxicities. For individuals who are at increased risk for lung cancer, primary cancer prevention with chemopreventive agents, when coupled with smoking cessation and other interventions, would be an attractive strategy to reduce cancer risk. Even in individuals who were at high risk for a primary lung cancer, a chemoprevention agent would not be clinically adopted when side effects frequently occur. In contrast, subjects who already have undergone resection for a primary lung cancer may accept some side effects of chemoprevention agents, if this would reduce the risk of a second cancer.

One way to limit the clinical toxicities of cancer chemopreventive agents would be through combination therapy. Agents targeting different chemopreventive pathways could each be administered at dosages lower than when those agents are used as single agents. This should reduce the clinical toxicities of each agent. For each chemopreventive agent that is used in a combination regimen, a valid target would be required based on preclinical and clinical activities. A combination regimen also must be associated with a tolerable toxicity profile as well as a safe and convenient schedule of long-term administration. Ideally, synergistic or additive effects should be observed in in vitro and animal models. Animal model testing could establish that a combination regimen is safe. If available, the clinical evidence for drug synergy in the treatment of patients with advanced-stage lung cancer might provide a basis for the use of a regimen in lung cancer chemoprevention. An example of combination chemopreventive regimens to consider would involve the use of a rexinoid with an EGFR inhibitor.71 Alternatively, a retinoid might be used in conjunction with an agent that would modify chromatin structure, such as a histone deacetylase inhibitor.75 76 Other potential combination regimens exist.

Recommendation


   Innovative Delivery Approaches
TOP
 Abstract
 Introduction
 Recommendations
 Lung Cancer Chemoprevention...
 Innovative Delivery Approaches
Conclusions
 Summary of Recommendations
 References
 
Chemopreventive treatments typically have involved oral administration of a pharmacologic agent. Yet, aerosolized delivery of chemopreventive agents is a promising alternative delivery strategy.77 Since pulmonary tissues are directly targeted, this delivery approach should reduce or prevent systemic toxicities. Proof-of-principle animal studies78 79 80 highlighted the use of aerosolized retinoids or steroids in lung cancer chemoprevention. These preclinical studies have supported a similar strategy in clinical trials. Aerosolized delivery could target small or large airways. Drug formulations need to be optimized to deliver to the desired airways. Technical advances that will become clinically available include the use of programmable delivery systems. Other potential approaches for some chemopreventive agents might include intranasal delivery or sustained release from intramuscular injections.

Clinical Considerations: Biomarkers and Surrogate End Points
Cancer chemoprevention trials are of a large size and require long clinical follow-up. For this reason, biomarkers or surrogate end points have been proposed to assess chemopreventive responses even before clinical outcomes become known. These biomarkers could be indicative of carcinogen-induced changes in affected pulmonary cells or tissues. Examples in the lung include genomic instability that leads to aneuploidy or LOH, cell cycle alterations that affect cellular proliferation, as well as changes in transcription that perhaps are due to methylation-dependent or acetylation-dependent changes in the genome, among other alterations. Examples of genetic alterations that occur in lung carcinogenesis include those affecting oncogenes (eg, ras, myc, EGFR, and others) or tumor suppressor genes (eg, p53 and Rb). Conceivably, these not only represent targets for cancer chemoprevention but also may serve as surrogate end points for the response to cancer chemopreventive agents. If validated biomarkers are identified, then clinical trials could be designed around changes in these markers rather than evaluating lung cancer mortality as in major trials to date. Trials designed around surrogate end points should be smaller in their sample sizes and shorter in their duration than trials designed around mortality.

Lung cancer chemoprevention trials that have similar eligibility criteria and study designs would permit comparisons to be made between studies conducted at different centers. It would be helpful to have improved models with which to assess lung cancer risk. A possible inclusion criterion for lung cancer chemoprevention trials might include the presence of persistent dysplasia found in repeat biopsy or cytology specimens. Subjects in randomized, placebo-controlled chemoprevention trials should be stratified for smoking history. Perhaps chemopreventive agents would exert different actions in current, former, and never-smokers. Every effort should be made to encourage smoking cessation, but this may not always occur in the study population. Lung cancer chemoprevention trials should carefully monitor for adverse events, especially for potential adverse events in current smokers. It is desirable to enroll former smokers and never-smokers in lung cancer chemoprevention trials. There is some concern about the inclusion of current smokers in these trials, given the potential for negative interactions with chemopreventive agents. Evidence for the activity of pharmacologic agents in patients with advanced-stage lung cancers would provide a rationale for their use in lung cancer chemopreventive trials.

Proof of principle lung cancer chemoprevention trials are warranted. These would validate the desired therapeutic effects on a target. Short-term mechanistic trials could be undertaken in which pharmacologic effects on a target would be assessed by comparing changes in posttreatment biopsy specimens compared to those obtained before treatment. The biochemical effects of the agent on the target could be assessed in harvested tissues. Pharmacokinetic studies should be performed with candidate chemopreventive agents. These trials might yield early objective tumor responses that would offer another reason for the use of a pharmacologic agent in lung cancer chemoprevention. A validated agent could be used in a combination chemopreventive regimen.

Pathologic changes after chemopreventive agent treatment would be reasonable end points to consider, especially when persistent dysplastic alterations occur in initial bronchial biopsy specimens, bronchial washings, or sputum specimens. Fluorescence bronchoscopy81 as an adjunct to white-light bronchoscopy may be useful to monitor responses in lung cancer chemoprevention trials. Candidate genetic, biochemical, or cellular markers in the lung include changes in genomic DNA methylation or acetylation, the presence of LOH, as well as apoptosis, angiogenesis, inflammation, or changes in cellular proliferation in the affected tissues.


   Conclusions
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Cancer chemoprevention is an attractive approach with which to reduce lung cancers by treating early steps in lung carcinogenesis. There is a convergence of basic scientific and clinical findings in lung cancer chemoprevention. Smoking cessation and prevention are essential to reduce the societal impact of tobacco carcinogenesis. Pharmacologic interventions also can be used to reverse or arrest the progression of lung carcinogenesis. While promising pharmacologic evidence exists for lung cancer chemoprevention, the optimal agent has not yet been identified that is active in primary or secondary lung cancer chemoprevention. For this reason, additional clinical trials are needed that emphasize a mechanistic approach in which mechanisms identified in vitro can be validated in vivo. Given the long-term nature of the interventions for cancer chemoprevention, pharmacologic agents should be administered with few, if any, associated clinical toxicities. Combination chemopreventive regimens offer a strategy to reduce the number of associated clinical toxicities since each agent could be prescribed at dosages lower than when administered as single agents. One way to develop future lung cancer chemoprevention trials is to explore chemopreventive activities in relevant preclinical models. These models should help to identify important pathways that are responsible for chemopreventive effects and help to validate known therapeutic targets or highlight novel therapeutic targets for lung cancer prevention. A potential example of this is the pharmacologic effect on G1 cyclins found in human bronchial epithelial cells.38 39

Biomarkers or surrogate end points should prove useful in identifying chemopreventive targets as well as in highlighting changes that place the affected pulmonary cells or tissues at high risk for the development of lung cancer. Changes in these markers represent candidate surrogate end points for clinical cancer chemoprevention trials. In the near term, as the clinical lung cancer chemoprevention field advances, it will be important to understand mechanistically how chemopreventive agents act and when they should be prescribed. An improved understanding of the pharmacology of cancer chemoprevention agents should aid in the design and conduct of lung cancer chemoprevention trials.


   Summary of Recommendations
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  1. For all individuals, smoking prevention should be strongly encouraged to decrease the risk of lung cancer. Level of evidence, good; benefit, substantial; grade of recommendation, A
  2. For all individuals, school-based and community-based interventions that are aimed at reducing tobacco exposure should be recommended, including a "life skills training" approach that is aimed at reducing tobacco, alcohol, and illicit drug use, campaigns with brief recurring antismoking messages, high tobacco excise taxes, and restrictions on smoking in the workplace. Level of evidence, good; benefit, substantial; grade of recommendation, A
  3. Smokers should be identified as smoking cessation reduces the risk of lung cancer. Level of evidence, good; benefit, substantial; grade of recommendation, A
  4. Current smokers should be advised to quit smoking, and, when appropriate, clinicians should prescribe and monitor pharmacotherapy. Individuals who smoke and want to quit also should have access to psychosocial treatment and behavioral modification therapies as indicated. There is sufficient to strong evidence that indicates these practices will help to increase long-term smoking abstinence rates. Level of evidence, good; benefit, substantial; grade of recommendation, A
  5. Individuals who are at risk for lung cancer and were treated with ß-carotene, retinol, isotretinoin, or N-acetyl-cysteine for lung cancer prevention did not experience clinical benefits. There is also evidence that the use of ß-carotene or isotretinoin for lung cancer chemoprevention in high-risk individuals may increase the risk for lung cancer, especially in individuals who continue to smoke. These agents should not be used outside of a clinical trial for primary, secondary, or tertiary lung cancer prevention. Level of evidence, good; benefit, harmful; grade of recommendation, D
  6. For individuals who are at risk for lung cancer and for patients with a history of lung cancer, there are not yet sufficient data to recommend the use of any agent either alone or in combination for primary, secondary, or tertiary lung cancer chemoprevention outside of a clinical trial. Level of evidence, insufficient; benefit, lacking data; grade of recommendation, I


   Footnotes
 
Abbreviations: COX = cyclooxygenase; EGFR = epidermal growth factor receptor; 4HPR = fenretinide; LOH = loss of heterozygosity; NNK = N-nitrosamine-4-(methylnitrosamino)-1-(3 pyridyl)-1-butanone; RA = all-trans retinoic acid; Rb = retinoblastoma gene product; RXR = retinoid X receptor

This work was supported in part by grants RO1-CA87546 (ED) and RO1-CA62275 (ED) from the National Institutes of Health, and by grant RPG-90-019-10-DDC (ED) from the American Cancer Society. Dr. Dragnev was supported in part by the American Society of Clinical Oncology (ASCO) Young Investigator Award. This work was supported in part by the Oracle Giving Fund and by the Samuel Waxman Cancer Research Foundation.


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