Piroxicam and other cyclooxygenase inhibitors: potential for cancer chemoprevention

Chemoprevention of intestinal adenomas in the ApcMin mouse by piroxicam: kinetics, strain effects and resistance to chemosuppression

Previous cancer chemoprevention studies have demonstrated that NSAIDs can be effective in suppressing the development of intestinal tumors. To further explore this issue, we performed cross-over chemoprevention studies using the drug piroxicam in the ApcMin mouse to evaluate the kinetics of NSAID-mediated tumor regression, the effects of genetic background and the incidence of resistance to chemoprevention. Starting at the time of weaning, C57BI/ 6J-ApcMin mice were fed either the control diet (AIN-93G) or AIN-93G plus 200 p.p.m. piroxicam. Tumor multiplicity was significantly reduced in ApcMin mice that were fed 200 p.p.m. piroxicam until 100 or 200 days of age (94.4 and 95.7% reduction in tumor number, respectively; P < 0.001 versus AIN-93G controls). When the administration of piroxicam was delayed until 100 days of age and the mice were killed at 200 days of age, tumor multiplicity was reduced by 96.2% (P < 0.001 versus controls). Alternatively, when the administration of piroxicam was suspended at 100 days of age and the mice were killed at 200 days of age, tumor multiplicity was reduced by 68.0% (P < 0.001 versus controls). Short-term drug treatment periods for ApcMin animals with established tumors revealed that the kinetics of piroxicam-induced tumor regression were rapid: >90% reduction in tumor multiplicity was observed after 1 week of treatment with 200 p.p.m. piroxicam. The distribution of residual tumors in piroxicam-treated mice suggests that tumors of the duodenum and colon were relatively resistant to chemosuppression. Treatment of interspecific hybrid ApcMin mice with 200 p.p.m. piroxicam revealed that there was a strain-related effect on chemosuppression, suggesting the existence of genetic elements which modulate NSAID chemosensitivity. Finally, whole-genome allelic loss studies showed that there were few unique chromosomal deletions in the NSAID-resistant tumors from F1 mice, implying that loss-of-function mutations secondary to Apc inactivation are not likely to account for the observed difference in chemoresistance.

Role of prostaglandins in the urinary bladder: an update

Our knowledge of prostanoids is rapidly increasing. In this review we survey the factors governing the synthesis of prostanoids by the urinary bladder, their role in the maintenance of normal bladder function, the pattern of their secretion in bladder disease and the possible use of prostanoids in the treatment of bladder pathology.

Non-steroidal anti-inflammatory drugs and prostate cancer progression

Experimental studies have suggested that the biosynthesis of arachidonic acid-derived eicosanoids such as prostaglandin E2 via the cyclo-oxygenase pathway may play a significant role in supporting cell proliferation in human prostate cancer cell lines. However, the aetiological significance of this for clinical prostate cancer has remained unclear. In particular, the potential for prostate cancer chemoprevention using nonsteroidal anti-inflammatory drugs (cyclo-oxygenase inhibitors; NSAIDs) has received little attention. The purpose of our study was to investigate associations between prostate cancer risk and use of NSAIDs. A population-based case-control study was carried out over 13 months from 1996 in metropolitan Auckland, New Zealand. A total of 317 newly diagnosed prostate cancer cases (including 192 "advanced" cases) representative of all cancer cases in the study population were identified from urology clinic referrals and histology reports. A total of 480 age-matched controls were recruited following random selection from the study population using electoral rolls as the sampling frame. After adjusting for potential confounding by socio-economic status and dietary fat consumption, there was a trend toward reduced risks of advanced prostate cancer associated with regular use of total NSAIDs (RR = 0.73; 95% CI 0.50-1.07) and total aspirin (RR = 0.71; 95% CI 0.47-1.08). However, these associations failed to reach statistical significance at the usually accepted levels. Weaker inverse associations were found for total prostate cancers, which included a number of small, low-grade tumours of less clinical significance. These findings lend support to proposed underlying aetiological hypotheses which imply a role for cyclo-oxygenase activity in prostate cancer progression.

Cancer chemopreventive activity of synthetic colorants used in foods, pharmaceuticals and cosmetic preparations

In continuation with our studies to uncover cancer chemopreventive effects of non-toxic natural colorants and other products of biologic and synthetic origin, we tested several Food and Drug Administration-approved synthetic colorants for antitumor promoting potential by the in vitro Epstein-Barr virus early antigen activation in Raji cells in response to the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). Among 29 such colorants used in foods, pharmaceuticals and cosmetics and evaluated in vitro, six of the 10 most effective had an azo group. Three structurally unrelated colorants tested in this assay were also studied in vivo for chemoprevention of 7,12-dimethylbenz[a]anthracene (DMBA)-induced TPA-promoted mouse skin carcinogenesis. The results indicate that tartrazine, indigo carmine and erythrosine are potent inhibitors of skin tumor promotion in mice treated with DMBA and TPA.

Effect of indomethacin on tumorigenicity and immunity induction in a murine model of mammary carcinoma

Indomethacin, an inhibitor of cyclo-oxygenase given orally, reduced the tumorigenicity of cancer cells in a non-immunogenic murine model of mammary adenocarcinoma (4T1). In the presence of indomethacin, a dose-dependent immune protection could be induced most effectively by immunizing mice with 1 to 3 doses of irradiated tumor cells inoculated at intervals of 7 days prior to challenge with a tumorigenic cell dose. Three immunizations given without indomethacin resulted in tumor growth in 88% of the recipients, and indomethacin treatment started 28 days prior to the challenge dose and given without immunizations led to tumor onset in 83% of mice. In contrast, tumor was documented only in 12% of mice vaccinated with 3 immunization doses and given concomitantly indomethacin. Moreover, 53% of disease-free survivors resisted a second challenge with a high tumorigenic dose. Induction of an anti-tumor immunity in indomethacin-treated mice was further studied as a therapy for tumor-bearing mice. Complete cure was induced in 50% of mice, and a significant reduction in tumor size as well as prolonged survival time were observed in the remaining animals. Immunostimulation by tumor cell vaccination given in the presence of a tolerable dose of indomethacin, therefore, may be incorporated into immunotherapy protocols to activate an anti-tumor response against residual tumor cells that escaped surgery and/or high-dose chemo/radiotherapy.

Aspirin and nonsteroidal anti-inflammatory agents and risk for colorectal adenomas

BACKGROUND & AIMS

Aspirin and nonsteroidal anti-inflammatory drugs have been reported to protect against the development of colorectal cancer. Because adenomas are precursors to most colorectal cancers, the aim of this study was to examine the relationship of these medications to the risk for colorectal adenomas in a colonoscopy-based case-control study.

METHODS

Study participants were drawn from patients who underwent colonoscopy at the University of North Carolina Hospitals. Medication use was assessed by telephone using a comprehensive list of prescription and nonprescription drugs as well as questions about dietary and lifestyle factors that might be relevant for adenoma development.

RESULTS

There were 210 patients with adenomas and 169 adenoma-free controls. After adjusting for potential confounders, regular users were half as likely to currently have adenomas compared with nonusers (adjusted odds ratio, 0.56; 95% confidence interval, 0.34-0.92). Regular users who stopped medication at least 1 year before colonoscopy were still protected (adjusted odds ratio, 0.59; 95% confidence interval, 0.21-1.67), although small numbers make this conclusion tentative. The protective effects of aspirin and the nonaspirin nonsteroidal anti-inflammatory drugs were similar.

CONCLUSIONS

The results suggest that aspirin and nonsteroidal anti-inflammatory drugs cause early disruption of the adenoma-carcinoma sequence. The challenge for the future will be to learn more about dose, duration, and mechanism of action.

Non-steroidol anti-inflammatory drug effect on crypt cell proliferation and apoptosis during initiation of rat colon carcinogenesis

Sustained use of non-steroidal anti-inflammatory drugs (NSAIDs) may prevent colorectal cancer. However, the optimal drug, period of efficacy and mechanism(s) of action are unknown. Experiments were undertaken to determine which of several NSAIDs would modulate colon crypt cell proliferation or apoptosis when given during the initiation phase of 1,2-dimethylhydrazine (DMH)-induced rat colon cancer. Colon crypts located both away from and over an aggregate of lymphoid nodules (ALN) were examined. Rats were injected with aspirin, indomethacin, nabumetone, sodium salicylate, 16,16-dimethyl prostaglandin E2 or saline for 3 days and DMH or DMH vehicle on day 4 of each week for 8 weeks, then killed 3 days after the last DMH injection. At the time of killing, DMH had significantly increased crypt cell proliferation but not apoptosis. There was significantly more cell proliferation and apoptosis in crypts over the ALN than away from the ALN. Aspirin and salicylate increased proliferation and apoptosis in crypts over the ALN. Finally, the distributional peaks of cell proliferation and apoptosis were shifted significantly closer together after DMH. Thus, DMH increases proliferation and alters the distribution of proliferating and apoptotic cells in colon crypts early in carcinogenesis. Aspirin may suppress tumour incidence via salicylate by enhancing apoptosis in carcinogen-initiated cells.

Up-regulation of cyclooxygenase-2 by product-prostaglandin E2

The development of prostate cancer has been linked to high level of dietary fat intake. Our laboratory investigates the connection between cancer cell growth and fatty acid products. Studying human prostatic carcinoma PC-3 cells, we found that prostaglandin E2 (PGE2) increased cell growth and up-regulated the gene expression of its own synthesizing enzyme, cyclooxygenase-2 (COX-2). PGE2 increased COX-2 mRNA expression dose-dependently with the highest levels of stimulation seen at the 3-hour period following PGE2 addition. The NSAID flurbiprofen (5 microM), in the presence of exogenous PGE2, inhibited the up-regulation of COX-2 mRNA and cell growth. These data suggest that the levels of local intracellular PGE2 play a major role in the growth of prostate cancer cells through an activation of COX-2 gene expression.

Chemoprevention for colorectal carcinoma

The literature on chemoprevention for colorectal carcinoma can be summarized as follows: (1) Aspirin and NSAIDs usage can decrease polyp formation and promote polyp regression and have a strong epidemiologic link to colorectal cancer prevention. (2) Fiber intake is strongly associated with a decreased incidence of colorectal carcinoma. Whether supplemental fiber can prevent colorectal neoplasia is not yet clear. (3) Calcium and vitamin D intake is inversely proportional to the risk of developing colorectal carcinoma. Prospective trials make the role of supplemental calcium as a chemoprotective agent unclear: (4) Chemoprevention is an exciting area of research. More work needs to be done to establish the precise steps necessary for neoplastic transformation of cells so that pharmaceuticals can be developed to target carcinogenesis at several levels.

Chemopreventive efficacy of piroxicam administered alone or in combination with lycopene and beta-carotene on the development of rat urinary bladder carcinoma after N-butyl-N-(4-hydroxybutyl)nitrosamine treatment

The effects of the non-steroidal anti-inflammatory drug (NSAID) piroxicam and the carotenoids lycopene and beta-carotene, alone or in combination, on the development of rat superficial urinary bladder carcinomas induced by N-butyl-N-(4-hydroxybutyl)nitrosamine (BBN) were studied. Male Fischer 344 rats, 6 weeks old, were given 0.05% BBN in the drinking water for 8 weeks followed by administration of piroxicam (0.0075% in the diet), lycopene (0.0025% in the drinking water) and/or beta-carotene (0.0025% in the drinking water) for 12 weeks, then killed for histological analysis of urinary bladder lesions. Cell proliferation potential was analyzed by immunohistochemical staining of the proliferative cell nuclear antigen (PCNA). Piroxicam alone, piroxicam+lycopene, and piroxicam +lycopene+ beta-carotene all significantly decreased the incidences and numbers of transitional cell carcinomas (TCCs), but the combination of piroxicam with carotenoids did not result in a clear improvement in the preventive potential of piroxicam. Piroxicam+ beta-carotene also caused a significant reduction and lycopene alone a slight but not significant reduction in the number of TCCs. In contrast, beta-carotene alone and lycopene+ beta-carotene were without inhibitory influence on any of the lesion categories examined, and the latter significantly increased the proportion of high-grade TCCs. Nevertheless, all of the chemopreventive agents, either alone or in combination, significantly decreased the TCC PCNA index, the effect extending to the surrounding epithelium in the piroxicam+lycopene and piroxicam+lycopene+beta-carotene groups. These results indicate that the NSAID piroxicam may be a more effective chemopreventive agent than lycopene and beta-carotene for superficial urinary bladder carcinogenesis.

Cyclo-oxygenase and colon cancer: clues to the aspirin effect?

Epidemiological and experimental studies indicate an inverse relationship between the risk of colon cancer development and intake of aspirin and other nonsteroidal anti-inflammatory drugs (NSAIDs). All NSAIDs are known inhibitors of cyclo-oxygenase, the enzyme responsible for converting arachidonic acid to prostaglandins. Prostaglandins have been implicated in the pathogenesis of colon cancer and it has been suggested that the preventive effect of NSAIDs is due to inhibition of cyclo-oxygenase activity. Cyclo-oxygenase exists in two different isoforms, cyclo-oxygenase-1 and cyclo-oxygenase-2, and data obtained during the last few years have suggested that cyclo-oxygenase-2 might be involved in both human and experimental colon carcinogenesis. The purpose of this review is to provide an update on recent studies regarding cyclo-oxygenase, in particular cyclo-oxygenase-2, in relation to colon cancer in humans and in experimental models.

Induction of cyclo-oxygenase-2 mRNA by prostaglandin E2 in human prostatic carcinoma cells

Prostaglandins are synthesized from arachidonic acid by the enzyme cyclo-oxygenase. There are two isoforms of cyclooxygenases: COX-1 (a constitutive form) and COX-2 (an inducible form). COX-2 has recently been categorized as an immediate-early gene and is associated with cellular growth and differentiation. The purpose of this study was to investigate the effects of exogenous dimethylprostaglandin E2 (dmPGE2) on prostate cancer cell growth. Results of these experiments demonstrate that administration of dmPGE2 to growing PC-3 cells significantly increased cellular proliferation (as measured by the cell number), total DNA content and endogenous PGE2 concentration. DmPGE2 also increased the steady-state mRNA levels of its own inducible synthesizing enzyme, COX-2, as well as cellular growth to levels similar to those seen with fetal calf serum and phorbol ester. The same results were observed in other human cancer cell types, such as the androgen-dependent LNCaP cells, breast cancer MDA-MB-134 cells and human colorectal carcinoma DiFi cells. In PC-3 cells, the dmPGE2 regulation of the COX-2 mRNA levels was both time dependent, with maximum stimulation seen 2 h after addition, and dose dependent on dmPGE2 concentration, with maximum stimulation seen at 5 microg ml(-1). The non-steroidal anti-inflammatory drug flurbiprofen (5 microM), in the presence of exogenous dmPGE2, inhibited the up-regulation of COX-2 mRNA and PC-3 cell growth. Taken together, these data suggest that PGE2 has a specific role in the maintenance of human cancer cell growth and that the activation of COX-2 expression depends primarily upon newly synthesized PGE2, perhaps resulting from changes in local cellular PGE2 concentrations.

Cyclooxygenase-2 expression during rat neocortical development and in Rett syndrome

Cyclooxygenase or prostaglandin endoperoxide H synthase-2 (PGHS-2) is the first enzyme in the prostanoid biosynthetic pathways and, in brain, it is regulated as an immediate-early gene (IEG). PGHS-2 mRNA and protein are rapidly induced by physiological synaptic activity, and high basal expression in cerebral cortex appears to be maintained by the natural synaptic activity. In contrast to other IEGs, PGHS-2 is a dendritic protein that is enriched in dendritic spines and is, therefore, likely to play a direct role in synaptic physiology. Consistent with a signaling function in mature dendritic spines, PGHS-2 expression is strongly regulated during normal postnatal development in the rat, with peak expression during the third and fourth weeks. Here we use immunocytochemical approaches to compare the developmental expression of PGHS-2 in rat neocortex with that of other well characterized markers of dendritic maturation. PGHS-2 immunoreactivity (ir) follows histogenetic gradients and expression in secondary or more distal dendrites postdates that of even the most delayed dendritic proteins. This developmental pattern parallels the critical period for somatosensory and visual cortex development. Accordingly, PGHS-2-ir may be a useful marker of the final activity-dependent stages of cortical development. Consistent with the potential histochemical utility, we demonstrate that the normal laminar pattern of PGHS-2-ir in human cortex is altered in patients with Rett syndrome, a form of mental retardation with known alterations of dendritic maturation. Further studies of the developmental expression of PGHS-2 in human cortical development may permit analyses of dendritic abnormalities, in syndromes associated with disturbances of activity-dependent development, as well as provide an anatomic basis for understanding the role of prostaglandin signaling in cortical development.

Regulation and significance of apoptosis in the stem cells of the gastrointestinal epithelium

In rapidly proliferating tissues the stringent control of cell proliferation and cell death by apoptosis is central to the maintenance of tissue homeostasis. In the gastrointestinal tract most work studying the control of tissue cell number has traditionally focused on the growth factor control of proliferation, and the changes that occur during carcinogenesis. However, in recent years it has become increasingly apparent that the control of apoptosis is also crucial. Apoptosis is an important mechanism for eliminating both excess normal cells and those cells which have sustained damage; therefore maintaining a tissue, i.e., stem cells with preserved DNA integrity. In this review the incidence of apoptosis in the stem cells of both the small and large intestine will be discussed in relation to the expression of a number of apoptosis regulating genes (e.g. p53, Bcl-2, bax) within these cells. The importance of apoptosis as a means of controlling stem cell number (and therefore cellular output) will be addressed, as will the mechanisms by which any alterations to this process may contribute to malignancy.

Long-term use of nonsteroidal antiinflammatory drugs and other chemopreventors and risk of subsequent colorectal neoplasia

Our objective was to study the relationship between dispensed aspirin, nonaspirin nonsteroidal antiinflammatory drugs (NSAIDs), steroidal antiinflammatory drugs (SAIDs), acetaminophen, calcium, psyllium, and multivitamin preparations and the risk for subsequent colorectal adenoma and adenocarcinoma. The design was a case-control study. The patient population was from a large municipal teaching hospital in Atlanta, Georgia. In logistic regression models, the risk of colorectal adenoma or adenocarcinoma decreased in the first two years of continuous NSAID use in a linear, time-dependent manner. The risk of colorectal neoplasia after two years of continuous NSAID use was reduced significantly (P < 0.01) as compared to nonusers. Risk reduction appeared greater for adenocarcinoma than adenoma. The use of SAIDs, calcium, multivitamins, and psyllium, as prescribed to our patient population during the mean six-year study period, conferred no measurable risk reduction. These results suggest that in prospective chemoprevention trials, a significant risk reduction can be expected after only two years of aspirin use, in doses similar to those recommended for the prevention of cardiovascular disease, or nonaspirin NSAIDs [correction of nonaspirin. NSAIDs], in doses commonly prescribed for the management of musculoskeletal pain. The results also imply that any short-term reduction in the incidence of colorectal adenoma detected in a phase II trial would underestimate the chemopreventive effect of NSAIDs on the risk of adenocarcinoma.

NSAID-induced polyp regression in familial adenomatous polyposis patients

NSAIDs inhibit prostaglandin synthesis. In 1983, Waddell et al first reported that sulindac, a NSAID (Clinoril), caused regression of rectal adenomatous polyps in several patients with familial adenomatous polyposis, an inherited form of colorectal cancer. Subsequently, NSAIDs have been used as chemopreventive agents in animal carcinogenesis models and adenoma regression had been confirmed in human trials with sulindac. This article summarizes these developments and describes possible mechanisms of colorectal neoplasia chemoprevention.

Mammary tumorigenesis and chemoprevention studies in carcinogen-treated rats

The objectives of this review are to describe the induction of mammary gland tumors by chemical carcinogens and to discuss their application to mammary cancer chemoprevention research. Special emphasis will be placed on the dimethylbenz[a]anthracene (DMBA) and methylnitrosourea (MNU) models because of the extensive information available about the pathogenesis of tumor growth associated with these two compounds. Both models have been widely used in the investigation of novel cancer chemopreventive agents. The current status of a number of different approaches will be summarized briefly here to provide an overview of research opportunities. Despite the popularity of the DMBA and MNU models in laboratory studies of mammary cancer biology and prevention, neither of these carcinogens has ever been implicated in the etiology of human breast cancer. This shortcoming has prompted a growing interest in other relevant environmental chemicals which are capable of producing mammary tumors in experimental animals. The new models have yet to be fully characterized, but they may be more appropriate than the DMBA and MNU models as paradigms for assessing cancer risk in humans and for developing suitable cancer prevention strategies.

Complete regression of villous adenomas of the colon using piroxicam, a nonsteroidal anti-inflammatory drug

A 73-year old patient with a past history of rectal carcinoma was found to have two sessile villous adenomas of the cecum. Because he refused surgical resection, a course of nonsteroidal anti-inflammatory drug therapy using piroxicam was given, and the villous adenomas regressed within three months. The cecum has remained clear at one year.

Prospective study of chemoprevention of hepatocellular carcinoma with Sho-saiko-to (TJ-9)

BACKGROUND

Most hepatocellular carcinomas (HCC) arise in patients with cirrhosis, in whom its incidence is high. The prevention of HCC in patients with cirrhosis is important.

METHODS

A prospective, randomized, nonblind controlled study was performed to evaluate the preventive effect of Sho-saiko-to (TJ-9) on HCC development. TJ-9 is a Chinese herbal medicine that contains crude extracts of seven herbs; it has antitumor effects in experimental animals. Two hundred sixty patients with cirrhosis were randomly assigned to two groups, matched for age, sex, presence of hepatitis B surface antigen, and the severity of liver damage. The patients in the trial group were given TJ-9 at a daily oral dose of 7.5 g in addition to the conventional drugs given to the control patients. The patients were prospectively monitored for 60 months and the cumulative incidence of HCC and the survival rate in the two groups were calculated.

RESULTS

The cumulative incidence curve for 5 years of the trial group was lower than that of the control group (P = 0.071). For the patients without HBs antigen, the difference was significant (P = 0.024). The survival curve for 5 years of the trial group was higher than that of the control group (P = 0.053). For the patients without HBs antigen, the difference was significant (P = 0.043).

CONCLUSIONS

TJ-9 helped to prevent the development of HCC in patients with cirrhosis, particularly in patients without HBs antigen.

The role of nonsteroidal anti-inflammatory drugs in colorectal cancer prevention

Colorectal cancer is the second leading cause of cancer death in the U.S.A. Recent research suggests that nonsteroidal anti-inflammatory drugs (NSAIDs) are effective in the prevention of colorectal neoplasia. This review summarises the results of research in animals and humans of these compounds in preventing tumours of the colorectum.

Selective preservation of protein kinase C-zeta in the chemoprevention of azoxymethane-induced colonic tumors by piroxicam

While nonsteroidal anti-inflammatory drugs have been shown to exert preventive effects against the development of colonic tumors in humans and in chemically-induced tumors in animal models, the mechanism(s) involved in this phenomenon is unclear. We have recently demonstrated that one such agent, piroxicam, when supplemented (75 ppm) in the diets of rats administered azoxymethane, reduced the incidence of rats bearing tumors. To date, the effects of piroxicam on protein kinase C, a family of serine/threonine kinases which may be intimately involved in the colonic malignant transformation process, have not been examined. It was, therefore, of interest to determine whether piroxicam altered the expression of one or more isoforms of this kinase in these tumors. The present studies demonstrate that dietary piroxicam selectively preserved the expression of protein kinase C-zeta in azoxymethane-induced tumors; suggesting that this is at least one mechanism involved in this agent's chemopreventive actions in this organ.

Inhibition of aberrant crypt growth by non-steroidal anti-inflammatory agents and differentiation agents in the rat colon

Aberrant crypts are aggregates of single to multiple colonic crypts evidencing hallmarks of dysplasia and may be the earliest detectable pathological lesions for colon cancer. The aberrant crypt assay has been developed in 2 protocols. In one, putative chemoprevention agents are tested for inhibitory effects when administered concomitantly with a carcinogen. In the other, the objective of this study, aberrant crypts were induced in F344 rats by parenteral injection of the colon carcinogen azoxymethane (AOM) and allowed to develop for 4 weeks, when an average of 90-100 aberrant crypt foci per colon were found in the methylene blue-stained colon. Then, during the second 4 weeks of the experiment, aberrant crypts were allowed to further develop to a frequency of > 150 foci per colon, a time when multi-crypt foci were observed. During this time we tested the inhibitory effects of 4 analgesic drugs and 2 differentiation agents for effects of aberrant crypt growth and development. We found the non-steroidal anti-inflammatory drugs piroxicam, aspirin and ibuprofen, but not acetaminophen, to be effective in suppressing aberrant crypt formation or the progression to foci of multiple aberrant crypts. Treatment with chemosuppressing agents 13-cis-retinoic acid (13-cRA) and 4-hydroxyphenretinamide (4-HPR), known differentiating agents, however, did suppress expansion of aberrant crypt foci, with 13-cRA being the much more potent agent.

On the inhibition of prostanoid formation by SK&F 96365, a blocker of receptor-operated calcium entry

1. The proposed blocker of receptor-operated calcium channels, SK&F 96365 was shown to inhibit formation of prostaglandin E2 in two osteoblast-like cell lines, MC3T3-E1 and UMR-106 in a dose-dependent manner at an IC50 of 3-4 microM. Inhibition was observed with various stimuli (arachidonic acid, bradykinin and calcium ionophore A23187). 2. This effect was also observed in human platelets, where SK&F 96365 dose-dependently blocked thromboxane biosynthesis and formation of 12-hydroxy-heptadecatrienoic acid after stimulation with arachidonic acid (IC50 = 4 microM). 3. The compound had no effect on 12-hydroxy-eicosatetraenoic acid production by human platelets. Additionally, linoleic acid oxidation by soybean 15-lipoxidase was not impaired by SK&F 96365. The results thus provide evidence for cyclo-oxygenase inhibition by SK&F 96365 at concentrations used to block receptor-operated calcium influx.

Aspirin and the prevention of colorectal cancer: a review of the evidence

In all but one of seven recent epidemiologic case-control and cohort studies directly examining the association between aspirin and colorectal cancer and polyps, regular aspirin use reduced the risk of these diseases by about half. Although these studies show a biologically plausible relationship between aspirin and colorectal cancer, information regarding dose and duration and risk change after discontinuation of aspirin is limited and contradictory. Additionally, selection bias, recall bias, and confounding cannot be completed discounted. The one randomized trial of aspirin and placebo showed that aspirin at a dose adequate for preventing myocardial infarction (325 mg every other day) did not reduce colorectal cancer incidence during five years of randomized treatment and follow-up. Further studies need to determine the biologic effects of aspirin, the minimum dose required, and whether other factors, such as age, illnesses, and reasons for aspirin use, modify or confound colorectal cancer development.

Chemoprevention of cancer

Chemoprevention is a strategy used to block the development of cancers in human beings. This emerging field has broad potential for influencing cancer incidence rates in defined high-risk groups and the general population. In this review, we define some of the mechanisms of carcinogenesis, describe some of the genetic markers of carcinogenesis, and list possible biomarkers that may serve as surrogate end points in chemoprevention studies. A major component of this review is a description of the agents that are currently under investigation in animal systems or in human trials. They are grouped according to the agents that block or suppress mutation, such as oltipraz, selenium, vitamin C and the flavones, or according to agents that block promotion and proliferation, such as difluoromethylornithine, tamoxifen, nonsteroidal antiinflammatory drugs, and the vitamin A derivatives. We describe the issues that are considered in the design of chemoprevention trials and in the phase I, II, and III components of these trials. The following national trials are discussed: the Breast Cancer Prevention Trial, which uses tamoxifen; the Prostate Cancer Prevention Trial, which uses finasteride; and a Lung Cancer Prevention Trial, which uses 13-cis-retinoic acid. The review ends with some insights about future studies in chemoprevention.