REVIEW

Tobacco Smoking, Harm Reduction, and Biomarkers

Peter G. Shields

Correspondence to: Peter G. Shields, M.D., Cancer Genetics and Epidemiology, Lombardi Cancer Center, Georgetown University Medical Center, The Research Bldg., W315, 3970 Reservoir Rd. NW, Washington, DC 20057 (e-mail: pgs2{at}georgetown.edu).


    ABSTRACT
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 Use of Biomarkers to...
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 Genetic Susceptibility for...
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The only known way to reduce cancer risk in smokers is complete cessation, but many smokers are unable or unwilling to quit. Consequently, tobacco companies are now marketing products that purport to reduce carcinogen exposure, with the implication that such products provide a safer way to smoke. Moreover, researchers are exploring ways to reduce the amount of cigarette smoke carcinogens to which the smokers are exposed. Although these methods are, in theory beneficial, it is possible that the perceived availability of "safe" ways to smoke will cause some former smokers to resume smoking and some current smokers to delay quitting. Thus, the extent of exposure reduction and the impact on public health of these methods need to be considered carefully. However, risk reduction and its relation to exposure are not simple to estimate. The way people smoke and the way they respond to carcinogen exposure are both highly variable, as evidenced by the previous history of smokers who switched to light, or low-tar cigarettes. This can actually increase risk in some smokers. The evaluation of exposure reduction will therefore need to be multidisciplinary and include in vitro cell culture studies, animal studies, human clinical studies, and epidemiologic studies. Biomarkers will be critical for rapidly evaluating the effects of new strategies or products to reduce exposure to tobacco smoke carcinogens. No single biomarker will likely satisfy our assessment needs, and so a panel of biomarkers should be used that includes biomarkers of exposure, biologically effective dose, and potential harm. In addition, usefulness of new products will need to be tested in people of different susceptibilities (i.e., who vary in behavior, sex, age, genetics, and prior tobacco use). Even if the new products are shown to be effective at reducing lung carcinogens, they should not be used alone but rather be incorporated into a comprehensive tobacco control program.



    INTRODUCTION
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 Introduction
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 External Exposure Assessment
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Comprehensive tobacco control programs focus on preventing initiation and fostering cessation. However, there are many smokers for whom these outcomes are not attainable. For example, many smokers desire to quit but cannot. Consequently, a comprehensive tobacco control program should also include methods to reduce risks in those individuals who continue to smoke. Methods to reduce risk are feasible, according to a recent Institute of Medicine (IOM) report (1), by reducing carcinogen exposures in smokers to the lowest possible levels. Exposure reduction methods may include fostering a reduction in the number of cigarettes smoked per day by concurrent use of nicotine replacement therapy or through the use of cigarette-like products that deliver nicotine but have lower amounts of tobacco toxins. Essentially every tobacco company has a product that claims to reduce tobacco smoke exposure.

Nevertheless, the only known method to reduce risk through exposure reduction is complete cessation; other plausible methods have not yet been shown to reduce risk. If these newer approaches can indeed reduce risk (i.e., if they result in harm reduction), then one can envision a large public health benefit, albeit not as beneficial as the complete cessation of smoking in the population. It must be recognized, however, that harm reduction may not necessarily follow from exposure reduction. We do not yet know how much exposure reduction is needed for a measurable reduction in risk. We also do not know what procarcinogenic events might be reversible, so that some continued exposure to tobacco while using harm reduction methods will allow for reversibility and will not foster carcinogenesis. Moreover, although there are likely to be public health benefits if actual harm reduction is achieved in persons who use these methods, there are important concerns that need to be balanced against the benefits: 1) some smokers may delay quitting because they believe that they have a method that reduces harm; 2) former smokers may resume smoking because they perceive the new method as a safe way to resume their nicotine addiction; and 3) nonsmokers may initiate the habit because they also perceive a safe way to adopt the use of tobacco-containing products. Consequently, these "safer" methodologies could substantially increase the numbers of persons exposed to tobacco smoke carcinogens. If this occurs, tobacco-related disease may increase in our society, even if some reduction is achieved for individual smokers.

The scientific challenge is to identify which potential exposure reduction products (PERPs), if any, could lead to actual risk reduction and subsequently be endorsed by the public health community and government regulators. Although the concept of risk reduction is attractive, current enthusiasm is tempered because of prior experience with PERPs. Specifically, filtered and low-tar and low-nicotine cigarettes were initially introduced by the tobacco companies as an alternative to quitting and were embraced by the public health community as a safer alternative to higher tar cigarettes. However, most of the public health community did not anticipate the developing paradox, whereby smokers of lower nicotine cigarettes would actually smoke more cigarettes per day and inhale more deeply to obtain the same amount of nicotine. This compensation has increased individual risk in persons who substantially compensated (2). Moreover, the trend of increasing occurrence of adenocarcinomas of the lung has been attributed to increased smoking intensity with low-nicotine cigarettes, especially deeper inhalation of tobacco smoke and, above all, to the substantial increase in tobacco-specific nitrosamines in the smoke of low-yield cigarettes (3,4). A recent report by the National Cancer Institute came to this same conclusion (5). Thus, because of evidence that low-tar and low-nicotine cigarettes provided no benefit, and might even have increased risk, the recent IOM report (1) concluded that proof of harm reduction could only come from epidemiologic studies. However, the latency necessary to obtain such useful data, and the likely changing nature of future PERPs as technologies are improved, are problematic, because lessons will be learned only after a large number of people have developed new cancers. Thus, to rapidly understand if there is a benefit to the use of PERPs, we will need to rely on validated biomarkers of exposure and effect that can demonstrate exposure reduction that predicts sufficient harm reduction. [For more detailed information about PERPs and harm reduction, see the IOM report "Clearing the Smoke, Assessing the Science Base for Tobacco Harm Reduction" (1). That report considers noncancer diseases and includes a detailed discussion of tobacco-related cancer, tobacco products, addiction, regulatory issues, and health policy, in addition to the information provided herein.]


    USE OF BIOMARKERS TO ASSESS PERPS AND QUALITY CONTROL ISSUES
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A number of PERPs are currently being considered. Through the use of nicotine replacement therapy, or medications used to foster smoking cessation (e.g., bupropion), smokers may be able to reduce the number of cigarettes they smoke per day, how deeply they inhale, and how many puffs they take. New products are coming from tobacco companies that heat rather than burn tobacco, which might lead to reduced carcinogen delivery by more than 70% (6) (e.g., Eclipse by R.J. Reynolds Tobacco Company and Accord by Philip Morris U.S.A.; the former heats tobacco by lighting a central element, and the latter heats tobacco electrically). Other approaches involve changing the nature of the tobacco by, for example, using different curing processes, so that it delivers less of some targeted tobacco carcinogens, such as tobacco-specific nitrosamines or polycyclic aromatic hydrocarbons (e.g., StarCure by Star Scientific, Inc., and Omni by Vector Tobacco, Ltd.).

Biomarkers reviewed herein are defined as any assay that provides some measurement in a human tissue (e.g., the level of a tobacco constituent, an effect on cellular function, or DNA damage) in exhaled air, sputum, saliva, blood, skin, urine, internal organ, or body part). Exposure is defined as the interaction between tobacco smoke or tobacco-related constituents and the body's microenvironment (e.g., at the cellular level). Tobacco yield is defined as the amount of tobacco constituents delivered from a cigarette, measured at the tip of the cigarette through the use of a smoking machine; it does not necessarily reflect the actual exposure to the individual smoker.

There are four types of assays that can be considered when evaluating a potential harm reduction product. As described in the IOM report (1), the first is external exposure measurements (i.e., yield; not a biomarker); the others are biomarkers of internal exposure, biomarkers estimating the biologically effective dose (7), and biomarkers of potential harm (Table 1Go). The latter two biomarkers report the effects of exposure at the cellular level and can provide information about the effects of individual chemical components of complex tobacco smoke and the effects of an individual chemical component that enters the body from tobacco smoke and other sources (i.e., tobacco, diet, air) (8). Given the large number of available potential biomarkers, and the numerous ones under development with new technologies, a framework is needed to weigh the importance of a biomarker. The relationship of biomarkers to each other, to exposure, and to disease is shown in Fig. 1Go. Therefore, it is important to consider a biomarker's quality, validity, appropriateness, and usefulness. Table 2Go provides such a framework.


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Table 1. Measures to assess products that claim to be capable of potentially reducing harm from cigarette smoking
 


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Fig. 1. The spectrum of biomarkers shows their relationship to each other, to exposure, and to disease. Although it is optimal to obtain biomarker measurements in the target tissue, it is not always feasible, and it might be preferable to use a surrogate if that surrogate represents the effect in multiple target tissues. Adapted with permission from the IOM report, "Clearing the Smoke" (1). Dashed lines indicate hypothetical relationship; solid lines indicate mechanistic relationship.

 

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Table 2. Assessing the worth of a biomarker in evaluating cigarette smoke exposure
 
Because many existing biomarkers have the potential to be useful in the assessment of smoking risk and harm reduction, and new technologies are bringing forth others, it is critical that any such assays undergo rigorous development and validation. Without this, any data obtained with such biomarkers would be of uncertain value. A biomarker assay must be tested for specificity and sensitivity at the level of human exposure and at lower levels so that risks following exposure reduction can be inferred. Ideally, biomarkers of harm should also be validated as risk markers in former smokers, because if there is no difference in results for former smokers compared with current smokers, the assay is probably not robust enough to be useful for smaller decrements in risk. When developing new assays, it is often helpful to have complementary assays that can measure the same effect or a surrogate for the effect.

Studies of any biomarker should include a determination of the replicability (i.e., coefficient of variation), interlaboratory variability, intra-individual variation, and inter-individual variation. Laboratories should have quality control and quality assurance procedures. In the United States, laboratories that provide clinical or other test results to a subject are required to be certified for complex testing by the Health Care Finance Administration under the Certified Laboratory Improvement Amendment (CLIA). The required methods and certification are also useful for research laboratories in that they will enhance the reliability of the laboratory. The way a biomarker is validated reflects its usefulness. Given the difficulties of performing assays at the level of human exposure and in the presence of many confounding variables, we sometimes can validate biomarkers by using higher exposures in in vitro models. However, such models can give different results from those in humans [e.g., immunoassays (9)]. Some assay methodologies are prone to artifacts that may affect the assay results [e.g., introduction of oxidative damage (10)]. In some cases, measurements of in vivo formation of a compound or metabolite can be confounded if there is exposure to the actual biomarker from exogenous sources [e.g., ingestion of 3-alkyladenine (11)].


    EXTERNAL EXPOSURE ASSESSMENT
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The best validated method for assessing human exposure and predicting risk of lung cancer is to ask subjects about their tobacco smoking history (1221). Lifetime exposures can be estimated by calculating pack-years smoked (average packs smoked per day over a lifetime multiplied by number of years smoked) or cumulative tar exposure (14). Nevertheless, it is not easy to assess how someone inhales or how the body responds to the exposure. Thus, a smoking history approximates the level of exposure but, as will be described below, is less reliable in assessing exposure reduction. Tar and nicotine yields, measured by the Federal Trade Commission (FTC) method, are reported on tobacco packages and in advertising. This measure of tar yield has been in use since 1967, when it was adopted by the FTC, although it had been devised in 1936 for analytical comparisons by the American Tobacco Company. The methodology consists of a smoking machine that is intended to estimate the delivered dose to a person and to allow for a comparison of one cigarette with another. A cigarette is inserted into the smoking machine and lit. Then, in what has been considered a simulation of the way people smoke cigarettes, the machine creates puffs by using negative pressure from a syringe (35 mL over 2 seconds, every 60 seconds) until the cigarette is "smoked" to a fixed length. Particulates pulled into the machines are collected on a filter and weighed. Nicotine, which is not in the particulates, is assayed separately. Tar is measured as total particulate matter less nicotine, other alkaloids, and water.

There are substantial problems with the FTC method, however, because it does not represent the way most people smoke and how much they inhale, which are governed by the amount of absorbed nicotine and other factors. For example, the machine cannot account for changes in smoking behavior for persons who switch from higher tar and nicotine cigarettes to lower tar and nicotine cigarettes, where most people compensate by inhaling more efficiently (deeper inhalation, longer duration of inhalation, and more puffs per cigarette) to maintain baseline nicotine levels in the body (4,22,23). In fact, when the method was first used, there were no filtered cigarettes, and mainstream smoke yielded 3.4–3.8 mg of nicotine. In 1990, most consumers used filtered cigarettes, and a typical cigarette yielded 0.85–0.9 mg of nicotine. A modified FTC protocol that simulates more inhalation actually yields more tar and specific carcinogens (e.g., tobacco-specific nitrosamines and benzo[a]pyrene) from smoke. In other words, smokers who compensate by inhaling more of their low-tar and -nicotine cigarettes actually have a greater per cigarette exposure for nicotine, tar, and tobacco smoke carcinogens, compared with smokers of cigarettes with higher FTC yields of tar and nicotine (4,22,23). Although yields from the FTC method might allow for a comparison from one cigarette to another, there is a wide overlap of predicted to actual yields among types of cigarettes (i.e., low, medium, and high yields). Smokers of low-nicotine cigarettes might have higher nicotine blood levels than smokers of brands with higher FTC yields (2325). In addition, the major difference between tar yield from one cigarette to another is due to ventilation (mixing the smoke with air by porous cigarette paper) and not to tobacco type. Thus, many persons alter the intended filter performance by covering ventilation holes in the filters with their lips or fingers, which then increases yields. As cigarette-like products with different designs are developed and marketed, it should not be assumed that the FTC method will allow for a comparison of these new products with conventional cigarettes. The assessment of PERPs, therefore, needs to be done with human exposure, not smoking machines.

An external exposure assessment also can include how someone smokes a cigarette, referred to in the literature as smoking topography (e.g., puff volume, number of puffs per cigarette, puff duration, total inhalation time, and interpuff interval) (2630). Topography is measured by smoking a cigarette through a tube of similar diameter to the cigarette, fitted with an airflow transducer (31,32). Smoking topography studies have shown that smokers will smoke differently to self-titrate their blood nicotine levels when switching from high- to low-nicotine cigarettes (28,31,3337).


    BIOMARKER ASSESSMENT
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Biomarkers of exposure include any assay from a body fluid (including exhaled air) or tissue that measures a constituent or constituent metabolite of tobacco smoke. It is not a measurement of how the constituents interact with body functions or macromolecules to cause harm. Exhaled carbon monoxide (CO) is among the most useful biomarkers of exposure because it does not undergo metabolic activation. Its short residence time makes it a marker of recent exposure, although when used as an estimate of smoking topography along with smoking history, the prediction of lung cancer risk is improved (38). Limitations of CO measurements are that it is not specific for cigarette smoking, because there are nontobacco sources of exposure such as vehicle exhaust and that levels might be affected by physical activity, sex, and the presence of lung disease. Another commonly used biomarker of exposure is blood nicotine level, but because nicotine has a short half-life in the blood, level of cotinine, a metabolite of nicotine with a longer half-life, are more commonly used (3941). These biomarkers of exposure, however, are affected by interindividual differences in nicotine metabolism. Specifically, people have different metabolic rates for nicotine conversion to cotinine by cytochrome P450 (CYP) 2A6 and for the rate of oxidation and glucuronidation of cotinine to trans-3-hydroxy-cotinine and glucuronide conjugates, respectively.

Smoking topography is best assessed by measuring nicotine or CO levels before and after smoking a cigarette. In persons who are using nicotine replacement therapy as a PERP, an alternative assay to assess tobacco use is one that measures levels of other tobacco alkaloids, such as anatabine or anabasine, in the urine (42). Technologies exist for directly measuring internal exposure to tobacco smoke constituents in target organs through biopsies, such as polycyclic aromatic hydrocarbons in the lung (43) and for measuring carcinogen metabolites, such as those from tobacco-specific nitrosamines, in the urine (4446). Tobacco smoke is a complex mixture of chemicals, metals, and fibers. Measuring exposures of this complex mixture includes determining the mutagenicity in smokers' urine (4749). Levels have been reported to decrease with some cigarette-like products that heat, rather than burn, tobacco (6).

The biologically effective dose (7) represents the net effect of tobacco smoke constituents (single or complex) on a cellular macromolecule (e.g., protein or DNA) following metabolic activation to reactive intermediates, decreased rate of detoxification, decreased repair capacity, and decreased rates of cell death. A variety of assays are available to determine carcinogen-macromolecular adducts in human tissues (10,5053), each of which has strengths and limitations. A common way to assess the biologically effective dose is to measure carcinogen–DNA adducts, which are formed when carcinogen metabolites are alkylated to nucleotides. DNA adduct levels reflect exposure and are an integrated phenotype of an individual's capacity for metabolic activation, detoxification, DNA repair, cell cycle control, and programmed cell death. Some of these DNA adduct lesions are promutagenic. There are strong data to show that increased carcinogen–DNA adduct formation is associated with increased cancer risk (53). Among the best evidence is the finding from the Physicians' Health Study that higher adduct levels in the blood prospectively predict increased lung cancer risk (5457). Case–control studies provide a similar association for both lung and bladder cancer risk (5861). In humans, smoking history is associated with higher adduct levels in the lung (6264) and blood (60,63,65), and these levels can vary depending on a person's inherited metabolic abilities (6673). There is also evidence to suggest that adduct levels in the blood are predictive of adduct levels in target organs, such as the lungs, so that these measures in blood are valid (7476). Methods for protein–carcinogen adduct detection (59) have shown that levels of hemoglobin adducts are higher in smokers than in nonsmokers (77) or in smokers of different types of tobacco (blond versus black) (78). A decline of adducts follows short- and long-term smoking cessation, so that these assays might be quantitatively reliable enough to show changes from using different PERPs (79,80). Adducts are present in the lungs of ex-smokers (81), but it is not known whether this persistence is from the formation of new adducts from prior deposition of long-lived tobacco particulates, exposure to environmental tobacco smoke, or other exposures, such as diet or air pollution (82). However, it has been reported that levels are lower in smokers of filter cigarettes (59). Chemical specificity is helpful in assessing harm-reduction products when the adducts can come only from tobacco constituents (e.g., tobacco-specific nitrosamines or 4-aminobiphenyl in the absence of occupational exposure), whereas adduct assays that determine levels from endogenous sources (e.g., oxidative damage, methylation) are more difficult to use and interpret.

Biomarkers of potential harm can range from nonfunctional effects on cells that serve as surrogate markers for actual harm to preclinical and clinical disease. Although these assays are less specific than target tissue assays for cigarette smoking, a concurrent reduction of a surrogate marker while using a PERP would provide strong evidence that there is a reduction in both exposure and harm, depending on the assay. The challenge, however, is to understand if the quantitative reduction actually predicts a measurable decrease in disease incidence. Among the most promising biomarker assays of effect for assessing harm reduction claims are those that are genetics-based, reflecting DNA damage or alterations of genetic function (mutations, gross chromosomal changes, or DNA hypermethylation of promoter regions). Chromosomal damage can be measured by classical cytogenetics (8385), micronuclei formation (86), Comet assay (87,88), or fluorescence in situ hybridization (83,89,90). Polymerase chain reaction methods that assess loss of heterozygosity in lung cells have shown that abnormalities for chromosomes 3p14, 9p21, and 17p13 are more frequently observed in smokers than in former smokers (91). It has recently become possible to measure background mutations in cancer-related genes in noncancerous tissues (9193). An emerging area of interest in tumor suppressor gene silencing is the study of hypermethylation of genes in tumors. Lesions in the p16 gene in sputum of smokers have been associated with smoking (94), and so these types of assays are feasible for the evaluation of a PERP. The role of mitochondrial DNA lesions is receiving greater attention regarding cancer risk (95) than it had previously, and these lesions, when associated with smoking, might be useful for assessing harm reduction (96). Biomarkers of pathobiologic effect include morphologic markers of preneoplastic lesions (e.g., dysplasia) or altered phenotypic expression of normal cellular functions (e.g., overexpression of the proto-oncogene Erb-B2).


    GENETIC SUSCEPTIBILITY FOR SMOKING BEHAVIOR AND SMOKING RISK
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It is clear that interindividual variations in carcinogen metabolism and DNA repair affect cancer risk. Thus, individuals are likely to vary in the reduction of risk related to their use of a PERP. Furthermore, given the difference in prevalence of some genetic traits among different ethnicities and races, PERPs might be more or less successful, depending on the group. Also, there may be gender and age differences in the success of a PERP. It can be anticipated that PERPs will affect people differently, depending on age, for example, because DNA repair capacity decreases with age in humans (97,98).

Both phenotypic and genotypic methods can be used to assess smoking-related cancer risk. Phenotypic assays include the assessment of carcinogen metabolism [e.g., urinary metabolites of caffeine for CYP1A1 and N-acetyltransferase 2 (NAT2) activity (99)], of induction of aryl hydrocarbon hydroxylase (100,101), of estrogen metabolic ratios (102), and of DNA repair through the mutagen sensitivity assay (103107), among others. It should be noted that cigarette smoking induces higher than normal levels of some repair enzymes (108110), so caution must be used for some phenotypic assays. There has been extensive study of genetic polymorphisms in smoking-related cancer risk (111,112). Examples include NAT2 (113115), glutathione S-transferase M1 (GSTM1) (113,114,116120), and CYP1A1 genes (121,122), glutathione S-transferase Pi (67), and others (118,123,124). These genetic polymorphisms and others are believed to affect biomarker levels, such as DNA adducts (66,67,72,125), and also the risk of p53 gene mutations in tumors (126129). Genetic polymorphisms for DNA repair enzymes exist (130). Studies that indicate an effect of these genetic variants on tobacco-related cancer risk (131136) are now being completed.

There has been increasing interest in the predictors of tobacco addiction and how people smoke. Smoking behavior (number of cigarettes per day and smoking topography) determines how people dose themselves, and hence this behavior has implications for harm reduction. Clearly, how someone uses a PERP could affect their reduction in disease risk, and there is evidence to suggest that smoking behavior has a genetic component. The evidence includes twin studies for both smoking initiation and smoking persistence (137139) and studies of genetics involving dopamine reward mechanisms (140143). Because illnesses such as depression affect smoking (144), they might also affect the success of a PERP.


    PERSPECTIVES
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Today, the only proven method to reduce tobacco-related cancer risk is to stop smoking. Alternatives are needed for persons who cannot or will not quit. However, these alternatives should not be adopted without consideration of the general impact on public health (i.e., whether former smokers will resume tobacco use or smokers will delay quitting because of perceived benefits of the alternatives). The recent report by the IOM (1) has concluded that harm reduction is feasible, but the evaluation of actual reduction will require a substantial research effort. PERPs and other inferred harm reduction methods are currently being studied, test marketed, and actively promoted by the tobacco industry. Although explicit claims of health benefits are not allowed, tobacco company advertising for cigarette-like products clearly implies health benefits, which might entice smokers to use products that are of unproven benefit or possibly detrimental. Although the benefit of PERPs is currently unproven, it appears that the tobacco industry wants people to believe that they have nothing to lose by switching to them because of at least theoretical benefit. This is not an informed choice, however, as shown by our experience with "light" cigarettes and the FTC method for estimating tar yields. Also, we cannot assume that all people will use PERPs similarly or that exposure reduction at some level will result in harm reduction and reduced disease risk. It should be noted that former smokers, although having less cancer risk than current smokers, remain at substantial risk above never smokers (145,146). Thus, it is important to realize that the best benefit that a PERP could offer will still not be as good as complete cessation: any continued tobacco exposure is likely to result in greater disease risk compared with the risk to former smokers.

The assessment of harm and harm reduction should be made through direct human experience that represents the way the products will be used by the general population. Only long-term epidemiologic studies can provide sufficient confidence for the benefit of a PERP and the attendant reduced cancer incidence. These studies should be supported by in vitro studies, laboratory animal studies, and human experiments. However, because of cancer latencies, epidemiologic studies will provide data too slowly, could possibly delay the use of a PERP that will actually lead to harm reduction, or delay the finding that the PERP either has no effect or results in an increased cancer risk. Also, epidemiologic studies will be hindered because no single PERP will be used continuously for long periods of time; the design and technologies of PERPs will change rapidly, and research methods will identify other new ways of exposure reduction. Therefore, until epidemiologic studies can provide support for the use of PERPs, we need to rely on biomarkers in clinical studies (e.g., smoking cessation and the use of PERPs by volunteers) and short-term epidemiologic studies that will provide sufficient data to ensure that the use of a PERP will result in reduced harm and disease incidence. There are unique opportunities within epidemiologic studies to validate biomarkers for use in assessing harm reduction strategies. Specifically, cohorts of former smokers should be established, because these individuals represent the best possible achievable reduction in harm from smoking.

No single biomarker is available that can be used alone to predict the success of a PERP. Thus, much research is needed to identify which biomarkers are sufficiently validated. Depending on the biomarker, different ways of validation may be warranted. Obviously, it is critical to ensure that the marker is measuring what it is thought to measure. This would typically involve authenticated standards and controls. All factors, including reliability, sensitivity, specificity, and reproducibility, must be addressed. In general, it is preferred to rely on assays that can corroborate other types of assays measuring the same endpoints and, if different laboratories are used, can show similar results in an interlaboratory comparison. The relationship of genotypes to phenotypes, or phenotypes to more complex phenotypes, is especially helpful. When many laboratories are conducting the same assay and there is consensus that the assay is predictive of a particular outcome, then it may be worthwhile to conduct an interlaboratory comparison with authenticated standards. Issues that typically arise include sample collection and handling, initial processing, agreement on what standards are acceptable, and data analysis and reporting. In the United States, laboratories that provide results to subjects must be CLIA (Certified Laboratory Improvement Amendment)-approved by the Health Care Financing Administration. CLIA, however, does not guarantee that a laboratory is performing the assay correctly if there is not an organization that provides performance standards. Although research laboratories are not required to have CLIA certification if results are not provided to subjects or if the results do not influence their care (i.e., random assignment based on biomarker results), the requirement to have quality control and quality assurance standards, defined by the laboratory, is a worthwhile goal.

A panel of biomarkers, including biomarkers of internal exposure, biologically effective dose, and potential harm, would be most useful for the assessment of harm reduction. This panel should be studied in the context of genetic susceptibilities for cancer risk and smoking behavior. The usefulness of a PERP will need to be tested in different populations, sexes, age groups, and in persons with different smoking histories. Any assumption that a surrogate assay reflects outcomes in target organs needs to be validated. Also, biomarkers that are specific for individual carcinogens and those that consider tobacco smoke as a complex mixture are complementary and both should be used. These markers should be used in laboratories with appropriate quality control and quality assurance procedures.

The use of biomarkers as indicators of cancer risk may possibly overestimate the number of persons who actually develop disease, because not all early changes in morphology or function progress to disease. Therefore, the implication of a potential benefit of a PERP could also be overestimated, but this limitation in the scientific methodology for identifying sufficiently specific biomarkers of risk requires acceptance at the current time.

A greater challenge will be to assess PERPs in the context of many tobacco-related diseases, with attention on whether the reduction of one disease might be compensated for by an increase in another disease. For example, would methods that reduce lung carcinogens cause an increase in bladder carcinogens or those that promote cardiovascular disease? Clearly, multidisciplinary efforts will be needed. The use and labeling of a PERP as a product that has less risk than conventional cigarettes may be acceptable once there is sufficient evidence to support a conclusion of reduced risk, which should be evaluated and regulated by the Food and Drug Administration. These regulated products should then be incorporated into existing tobacco control programs and not seen as an alternative to them. There may be extraordinary opportunities through the use of PERPs, but there may also be extraordinary risks.


    NOTES
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Editor's note: The author has been retained as an expert witness by a plaintiff in a tobacco-related litigation case against a tobacco company.

I thank Drs. Neil Benowitz, Christopher Loffredo, Rogene Henderson, and the rest of the IOM committee members who were authors of the report "Assessing the Science Base for Tobacco Harm Reduction" for their instructive and insightful comments, and Dr. Loffredo for his critical review of this manuscript.


    REFERENCES
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Manuscript received January 30, 2002; revised July 3, 2002; accepted July 19, 2002.


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