In preclinical animal models, both a variety of NSAIDs and the COX-2 inhibitor celecoxib have been uniformly effective against carcinogen-induced colon and urinary bladder cancers (18
). For a variety of reasons, including the fact that COX-2 is presumably a major chemopreventive target and that it might have fewer gastric effects, celecoxib progressed more rapidly than other NSAIDs to large scale clinical trials. However, when examined in a large placebo controlled trial at doses beyond those normally recommended, celecoxib increased the incidence of cardiovascular (CV) events starting approximately 18 months after the initiation of treatment (23
). These results, and the even more striking results with rofecoxib, raised the possibility that most NSAIDs might increase CV accidents. At this time, it is felt that the non-specific NSAID naproxen might have the least CV effects of any of the NSAIDs. Results of studies with naproxen have been mixed, with certain studies showing limited increases in CV risk relative to placebo and certain studies showing CV protection (24
). Nevertheless, it has consistently caused fewer CV effects than any of the other NSAIDs and may even be cardio-protective (26
). In addition, a recent American Gastroenterological Association panel has recommended the use of naproxen for persons of moderate CV risk based on the initial estimation that this agent might have fewer CV problems than other NSAIDs (25. This was our rationale for evaluating the efficacy of naproxen. Interestingly, this agent has infrequently been employed in preclinical chemoprevention studies. The second major toxicity associated with the use of NSAIDs is the development of gastric lesions in susceptible individuals. In order to address this problem, we examined NO-naproxen since there is both preclinical and clinical data showing that the NO-NSAIDs have lower gastric toxicity than the parent NSAIDs (9
One of the major concerns regarding doses employed in animal studies is their comparability to human doses. Although one might attempt to compare serum levels in animals and humans, this is extremely difficult when the agent is administered by diet. A reasonable approach is to employ general scaling factors. The doses of naproxen employed in the rat studies were between 128 and 400 ppm in the diet; which is equivalent to 12 and 36 mg/kg body weight. Using a six-fold scaling factor to extrapolate rat doses to human doses, this would be equivalent to human doses of 2 and 6 mg/kg. Such doses would translate to human doses of 160 and 480 mg in a human weighing 80 kg. These doses are comparable with a single naproxen dosing in humans of approximately 225 and 500 mg, which would correspond to 1 or 2 standard doses of 225 mg/day. It is important to note that in the studies described here, we found that doses less that the standard human equivalent dose were still highly effective in preventing both colon and urinary bladder cancers. These doses can be compared with the classical doses of celecoxib used in animal studies (1000 to 1500 ppm); which are approximately 5 to 8x higher than the recommended human dose using similar scalings. The possibility that celecoxib is effective at significantly less than the standard human dose (while interesting), would require detailed dose reduction studies to confirm.
The reduction in ACF per colon by naproxen and NO-naproxen are comparable to that normally seen with a variety of NSAIDs. In our studies, the parent compound naproxen inhibited total ACFs slightly more than NO-naproxen, but both were moderately (40–60% inhibition) effective. When measuring only larger multicrypt foci, the effect is more pronounced (50–70% inhibition); again the naproxen was slightly more effective than the NO-naproxen. These two NSAID compounds were effective in the colon because the inflammatory process is proposed to drive the cancer process in this organ. The ACF has been recognized as an early preneoplastic lesion of colon cancer (28
). It is generally observed that agents that inhibit colonic ACF formation will similarly exhibit chemopreventive activity against colon cancer (30
). Previous studies have shown that reduction of colon crypt multiplicity of four or more ACFs/focus has been a consistent predictor of colon tumor inhibition (31
In the OH-BBN urinary bladder model, both naproxen and NO-naproxen were highly active in blocking the development of urinary bladder cancers. Again in bladder cancer the inflammatory process is known to promoted cancer development. Clinical trials with NSAIDs are ongoing. Most importantly, these agents were still highly effective even when the administration was initiated after microscopic cancers already existed. This implies that most of the preventive activity seen in early intervention studies may be actually due to efficacy late in the study. Furthermore, these results show the promise that these agents may be preventive in a late or adjuvant clinical setting. This is particularly important in view of the high recurrence rate for human bladder cancer.
The mammary findings are not surprising, and replicate that typically seen for NSAIDs in the MNU-induced rat mammary cancer model; i.e., limited to moderate activity for most NSAIDs and COX-2 inhibitors (33
). The data show modest non-significant reductions in mammary cancer incidence and multiplicity. Since the cancer process in the mammary gland is not driven to a large extent by the inflammatory process, such agents would likely have little effect in human breast cancer prevention. This study also demonstrates that one does not see a striking effect due to any potential release of NO in the animals.
Overall, these data provide strong evidence for the continued development of naproxen and NO-naproxen; especially as chemopreventive agents for colon (where a variety of NSAIDs have proven clinically effective) and urinary bladder cancer. The lack of effects of these agents on mammary cancers is consistent with previous data from our laboratories regarding NSAIDs. Identifying NSAIDs that have minimal CV or GI toxicity will allow these compounds to be used clinically in the prevention of colon, urinary bladder, and skin cancer.