EDITORIAL

Predicting Success in Cancer Prevention Trials

Jason S. Vourlekis, Eva Szabo

Affiliation of authors: J. S. Vourlekis, E. Szabo, Lung and Upper Aerodigestive Cancer Research Group, Division of Cancer Prevention, National Cancer Institute, National Institutes of Health, Department of Health and Human Services, Bethesda, MD.

Correspondence to: Eva Szabo, M.D., Lung and Upper Aerodigestive Cancer Research Group, Division of Cancer Prevention, 6130 Executive Blvd., Rm. 2132, Bethesda, MD 20892 (e-mail: szaboe{at}mail.nih.gov).

The tremendous public health burden resulting from more than 200 000 new diagnoses and 165 000 deaths each year that are attributable to tobacco-related cancers arising in the lung and upper aerodigestive tract requires new treatment and prevention strategies (1). One such approach is chemoprevention, which relies on interventions during early phases of carcinogenesis to reduce the incidence of invasive cancer and, ultimately, to reduce cancer-related morbidity and mortality. Definitive studies of investigational chemopreventive agents typically are carried out in phase III clinical trials, where cancer incidence or mortality serves as the primary end point. However, the high cost and lengthy duration of phase III studies limit their application to only the most promising agents. How can we improve our ability to perform clinically meaningful cancer prevention studies in a more cost-effective and time-efficient manner? This issue of the Journal reports two strategies that address this question—the use of molecular intermediate end points in phase II trials (2) and the use of genotypic analysis to identify the most appropriate cohorts for intervention (3).

Phase II cancer prevention trials present a unique opportunity to establish safety and preliminary efficacy of specific treatment regimens in a smaller number of participants over a shorter time period than is required for definitive efficacy phase III trials. Given the lack of measurable cancer in participants in phase II trials, the development of intermediate end points that are appropriate surrogates for cancer incidence is critical to the success of such studies. At a minimum, markers that would be appropriate intermediate end points should 1) be integrally involved in the process of carcinogenesis so that modulation of expression correlates closely with disease course, 2) have different expression in normal versus preinvasive or at-risk epithelium, 3) be easily and reproducibly measured from biologic specimens obtained during clinical trials, and 4) be able to be modulated by a successful chemopreventive intervention with minimal spontaneous fluctuation and remission (4,5). Before integration into routine use, promising markers require validation in prospective clinical trials.

The complex issues that are encountered during the marker development process, such as which markers deserve testing and what degree of marker modulation after intervention is sufficient to justify further study, are exemplified in the study by Kurie et al. (2) in this issue of the Journal. The investigators report that 9-cis-retinoic acid (9-cis-RA), but not 13-cis-retinoic acid (13-cis-RA or isotretinoin), co-administered with {alpha}-tocopherol (AT) increased the expression of retinoic acid receptor-{beta} (RAR-{beta}) in the bronchial epithelium of former smokers. The magnitude of the effect was small, a finding likely caused, in part, by the high baseline expression of RAR-{beta} in this population. Neither 9-cis-RA nor 13-cis-RA plus AT resulted in substantial regression of bronchial histologic abnormalities, although abnormalities were present in too few participants to provide definitive data. The lack of activity of 13-cis-RA was surprising, given that previous studies showed that 13-cis-RA increased the expression of RAR-{beta} in current smokers (6,7). This lack of activity was attributed to inherent biologic differences between smokers and former smokers, a contention supported by in vitro data showing decreased activity of trans-retinoic acid in nicotine-treated lung cancer cells (8). The concomitant use of AT also may have contributed to the lack of biomarker modulation.

Retinoids have been an intense focus of cancer prevention research for more than 30 years. 13-cis-RA reduces the incidence of second primary tumors in head and neck cancer survivors (9) and induces the regression of preinvasive oral lesions (10). This latter effect is associated with the 13-cis-RA-induced increased expression of RAR-{beta} (11). In lung cancer cell lines, transfection of RAR-{beta} decreases the rate of cellular proliferation and decreases tumorigenicity after inoculation into mice (12). These effects depend on the induction of RAR-{beta}, suggesting that increased expression of the RAR-{beta} gene is important in mediating the antiproliferative effects of retinoids (13–15). Animal studies also suggest a role for RAR-{beta} as a tumor suppressor gene because antagonism of RAR-{beta}2 gene expression with antisense mRNA leads to spontaneous lung tumor formation in mice (16). In humans, both protein and gene expression of RAR-{beta} are decreased or absent in approximately 60% of non-small-cell lung cancers (NSCLCs), and protein expression progressively decreases in bronchial epithelium during the transition from normal to dysplasia to carcinoma (17–19). Loss of heterozygosity of the short (p) arm of chromosome 3, which contains the gene locus for RAR-{beta}, is present in 40%–50% of NSCLCs and is likely an important mechanism for altered RAR-{beta} gene expression (18,20). Methylation of the RAR-{beta} promoter may be another relevant mechanism (21).

What does the study by Kurie et al. (2) add to our present knowledge? The validation of potential intermediate end points remains a difficult but critical component of the cancer prevention research agenda and ultimately must be tied to cancer incidence and mortality. Although Kurie et al. and other researchers have been able to show that retinoid therapy increases bronchial expression of RAR-{beta} in both current smokers (6,7,19) and former smokers (2), low-dose 13-cis-RA was ineffective in preventing second primary lung tumors in early-stage NSCLC survivors, and no benefit was seen in either former smokers or active smokers after subgroup analysis (22). Whether higher doses of 13-cis-RA alone would result in a more favorable chemopreventive effect is unknown, but the likely toxicity precludes this strategy. Given the differential effects of 9-cis-RA and 13-cis-RA in Kurie et al. (2), it is possible that a chemopreventive trial with 9-cis-RA may have a different outcome. However, the key questions of how well the restoration of RAR-{beta} expression correlates with reduced potential to progress to overt cancer and what level of modulation of RAR-{beta} is sufficient to achieve a biologic and, ultimately, a clinically meaningful response remain unanswered. In the absence of data clearly defining the relationship between biomarker modulation and cancer development, it is unclear what the next step should be after a phase II study that uses a molecular biomarker as its primary end point. Unfortunately, the transition from phase II clinical trials with biomarker end points to phase III trials with cancer incidence end points remains a daunting challenge for the entire field of cancer prevention.

A second article in this issue of the Journal, by Izzo et al. (3), addresses the important concept that a chemopreventive agent may have different effects on individuals because of underlying genetic differences. Izzo et al. report that the presence of the A allele at nucleotide 870 of the cyclin D1 gene is associated with an attenuated response to chemopreventive intervention with the combination of 13-cis-RA, AT, and interferon-{alpha} and is also associated with shortened progression-free survival. These observed effects may be caused by a reduced potency of the chemopreventive agents in carriers of the A allele. Cyclin D1 is a key regulator of cell proliferation and facilitates cell transition from the G1 phase to the S phase in the cell cycle. Intracellular levels of cyclin D1 are regulated by ubiquitin-dependent proteolysis, a process that is potentiated by retinoic acid (23,24). The A allele preferentially encodes an alternate protein that is less susceptible to ubiquitin-mediated proteolysis and, ultimately, to the effects of retinoids (25). The implication of these data in the context of cancer prevention is that interventions may need to be tailored to an individual based on his or her unique genotype and to the particular biologic target. This study (3) underscores the importance of choosing an appropriate cohort for testing chemopreventive interventions, because heterogeneous cohorts can mask the effectiveness of various interventions.

Progress in cancer prevention is dependent on a better understanding of the events surrounding carcinogenesis and on improvements in early-stage clinical trial design and performance. Success in cancer prevention requires, at a minimum, the identification of appropriate targets for intervention, the definition of appropriate primary end points that are sufficiently predictive of reduced cancer incidence, and the selection of appropriate high-risk patient cohorts that are most likely to respond to the interventions. If any of these factors is not addressed appropriately during trial design, the effectiveness of a particular intervention may be missed or overestimated. The studies by Kurie et al. and Izzo et al. therefore represent important steps in refining early-phase cancer prevention trial methodologies that we hope will eventually lead to real reductions in cancer incidence.

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