In this issue of the journal, Kim et al. report their examination of the effect of the cyclooxygenase-2 (COX-2) inhibitor celecoxib on the Ki-67 proliferation index in the bronchial epithelium (5
). The authors found that a high dose of celecoxib (400 mg twice daily, the U.S. Food and Drug Administration [FDA]-approved dose for colorectal polyp reduction in patients with familial adenomatous polyposis) statistically significantly reduced Ki-67 labeling in the bronchial epithelium of current and former smokers after three months of treatment. In contrast, a lower dose (200 mg twice daily) and placebo had no effect on Ki-67. Therefore, a biological effect on the bronchial epithelium was clearly demonstrated with high-dose celecoxib treatment. The question that follows, however, is, How do these results inform the decision to proceed with a definitive efficacy phase III trial?
To be useful, Ki67 or any other marker needs to meet several requirements (2
). It should be integrally involved in the process of carcinogenesis, such that its expression correlates with the disease course. Its expression should differ between normal and at-risk epithelium, and it should be easily and reproducibly measurable in specimens likely to be obtained in clinical trials. Last, the expression of the marker should be modulated by effective interventions, and there should be minimal spontaneous fluctuations and no modulation by ineffective agents. A marker that satisfies these criteria then needs to be validated in prospective clinical trials (2
). Since different classes of agents can act through different pathways, a marker that accurately predicts cancer incidence reduction with one class of agents unfortunately may not be as predictive, or predictive at all, with a different agent class.
The value of the information obtained from Ki-67 as a modulatable endpoint in lung-cancer chemoprevention trials has been reviewed previously (6
), and Ki-67 satisfies some (but not all) of the surrogate endpoint requirements outlined in the previous paragraph. Dysregulated proliferation is a well-established hallmark of carcinogenesis (8
), is associated with a poor prognosis in lung cancer (9
), and is increased in premalignant lesions in the bronchial epithelium (10
). The interpretation of the Ki-67 labeling index is complicated, however, by the ability of smoking cessation alone to decrease it, as shown by the different baseline expression levels in current versus former smokers in the studies of Kim et al. and others (5
). The time course of this decrease is not well understood, although measurements of Ki-67 and epithelial remodeling in patients with chronic obstructive pulmonary disease (COPD) suggest that hyper-proliferation resolves over a period of years after the patient stops smoking (12
). The average number of cells expressing Ki-67 per millimeter of basement membrane among these COPD patients was 18.6 for current smokers, 6.9 for former smokers who quit within 3.5 years, and 2.8 for former smokers who had quit for more than 3.5 years. Therefore, time since smoking cessation complicates serial Ki-67 measurements within clinical trials, and there appears to be considerable variability in the rate of decline in Ki-67 expression between individuals who have quit smoking (10
It is tempting to hypothesize that individuals with increased proliferation in their bronchial mucosa are at an increased risk of lung cancer (thereby supporting the converse hypothesis that Ki-67 reduction will be mirrored by a decreased risk of lung cancer), but supporting data are lacking. First, animal studies have shown that a number of irritants not necessarily associated with carcinogenesis, such as high oxygen content, ozone, sulfur dioxide, mechanical irritation, and infection, all increase lung epithelial cell proliferation likely without increasing lung cancer risk (13
). Second, it takes many years before the risk of lung cancer is reduced after smoking cessation, not with standing the early decrease in proliferation (albeit not necessarily to “normal” levels) following smoking cessation (14
). Extended follow-up from the Lung Health Study showed that a decrease in lung cancer incidence was not demonstrable until 14.5 years after smoking cessation, well past the time when average Ki-67 expression has already significantly decreased according to other studies (16
). Therefore, decreased Ki-67 was not mirrored by an early decrease in lung cancer risk. Miller et al. also found that Ki-67 expression in nonmalignant bronchial cells is not related to lung cancer risk, although these authors did confirm the risk association with histology and smoking (16
). Of interest, these authors found a similar expression of Ki-67 in non-smokers and former smokers and did not find any evidence of a continued decrease in Ki-67 expression with increasing duration of smoking cessation, suggesting that the main decrease in Ki-67 occurs in a relatively short time frame after quitting.
Taken together, these data indicate that Ki-67 is a dynamic index under the influence of a variety of external stimuli. Kim et al. have shown that high-dose celecoxib is one of these influences. Mao et al. reported a similar significant decrease (35%) in Ki-67 in a single-arm pilot study of celecoxib in 20 current heavy smokers (17
). These celecoxib studies show statistically significantly reduced Ki-67 across a relatively large population, with the larger Kim et al. randomized controlled trial confirming and expanding the results of the smaller non-randomized Mao et al. study. However, variability due to the influence of a myriad of factors, such as infection and environmental exposures, indicates that Ki-67 expression is too subject to modulation by influences that do not impact lung carcinogenesis to be an accurate reflection of the modulation of lung cancer risk, at the level of either a broad population or an individual.