Vitamin E
The term “vitamin E” is used to refer to a group of naturally occurring substances that have vitamin E activity including a-, β-, δ-, and y-tocopherols (that have saturated side chains) and tocotrienols (that have unsaturated side chains). These agents have some degree of difference in biopotency with the naturally occurring d-a-tocopherol approximately 30% more potent than the synthetic forms — d- or d, l-a-tocopherol alone, acetate, or succinate. The principal food sources of vitamin E are vegetable and seed oils as well as alfalfa and lettuce.
A primary function of vitamin E is as an antioxidant, interacting with free radicals (e.g., singlet oxygen, superoxide anion, organic peroxide radicals, hydrogen peroxide, and others) that are generated as a normal part of cellular metabolism. These free radicals can interact with cellular structures, primarily membranes, and lead to cellular damage, generally through lipid peroxidation. Over time, the oxidative stress in humans increases and, similarly, with aging, the endogenous antioxidant capability (from glutathione, vitamins A, C, and E, superoxide dismutase, catalase, and glutathione peroxidase) decreases.’ In addition to the effects of free radicals on vascular and inflammatory diseases, the initiation and promotion of cancer have been attributed to these agents.
A variety of lines of evidence suggest that vitamin E can modulate prostate cancer risk. As noted above, an increase in antioxidant levels may reduce cumulative cellular damage from free radicals and thereby reduce the likelihood of the first steps in carcinogenesis. Evidence also suggests that elevated vitamin E levels are associated with enhanced immune function, perhaps leading to improved antineoplastic immune surveillance.In addition, changes in oxidative stress in cells have been demonstrated to alter the production of various transcription factors, altering gene expression.’ Kelloff’s exhaustive review lists several other potential actions including protection of cytochrome P-450 metabolism, induction of differentiation, gap junction intercellular communication, inhibition of proliferation, arachidonic acid metabolism, nitrosamine formation, and ornithine decarboxylase activity.
There is growing evidence suggesting that vitamin E may have efficacy in the prevention of, specifically, carcinoma of the prostate. Perez Ripoll studied the effect of doxorubicin, vitamin E, and the combination in the human prostatic cancer cell line DU 145. Vitamin E enhanced the cytotoxic effect of doxorubicin but also inhibited tumor growth when used alone. In another study using the Nb rat prostate cancer model, vitamin E not only enhanced the efficacy of doxorubicin but had a similar effect with cyclophos-phamide and methotrexate. In the same Nb rat prostate cancer model, Nesbit found that the doxorubicin-vitamin E combination had a lower final tumor volume than control animals. In a series of experiments using the CRL-1740 prostate cancer cell line, vitamin E was shown to have a potent suppressive effect on tumor growth at relatively low concentrations (0.1 mM), and at higher concentrations (> 0.25 mM), no cells survived to day 24. In this study, deoxyribonucleic acid (DNA) analysis demonstrated extensive apoptosis due to vitamin E supplementation.
Epidemiologic studies have generally observed that lower rates of vitamin E intake have been associated with higher prostate cancer risk. Plasma vitamin E levels were measured in 2974 men from Basel, Switzerland, between 1971 and 1973, and mortality and cancer diagnoses were summarized in 1990. While overall levels of vitamin E did not correlate with prostate cancer risk, within the group of men who were smokers, the relative risk of prostate cancer diagnosis was 3.2 times higher in those with low levels of vitamin E.
Selenium
Selenium is an essential trace element required for the normal function of glutathione peroxidase, a major intracellular antioxidant. Selenium is found in varying concentrations in soil and is incorporated into forage crops and thereafter into animals. Plants convert the inorganic selenium in the soil to organic selenium via a sulfur assimilatory pathway. Human intake is thereby from grains, vegetables including onions and garlic, and animal meat. While in the United States most agricultural areas have acceptable amounts of selenium in the soil, in various regions of the world including New Zealand and China, some areas are notable for very low levels of selenium. Previously, until selenium was incorporated into fertilizers, levels of selenium intake were very low in Finland. This may have confounded earlier studies in this population. In the United States, the recommended daily allowance for men is 70 pg. In areas with extremely low dietary selenium intake, various diseases have been reported. Keshan disease, a cardiomyopathy with diffuse necrosis of the myocardial cells, has been identified in China in areas with low selenium intake. Kashin-Beck disease, a disease associated with cartilage necrosis, compromised epiphyseal growth, short stature, and joint abnormalities, has also been found in selenium-deficient areas. While studies have demonstrated that daily intake of 200 pg is not associated with any toxicity, selenium can be associated with significant side effects with intake levels only slightly above these. Indeed, the maximum tolerated dose is 819 pg and the mean lethal dose (LD50) in rodents is 26 mg per kg. Turner and Finch have recently summarized the literature pertaining to selenium toxicity. They noted that examples of selenium poisoning have been reported on farms, commercial piggeries, and more recently in wildlife in a California sanctuary in which selenium-contaminated water led to poisoning of wildlife.
A variety of animal studies have suggested that selenium may prevent or reduce the development of a variety of neoplasms. Reddy and colleagues studied the ability of organoselenium compounds (p-methoxybenzyl seleno-cyanate and l,4-phenylenebis[methylene]selenocyanate) to impact on colon carcinogenesis in rats treated with a carcinogen and fed either a low-fat or high-fat diet. It has been observed that the impact of selenium is greatest when given during the early stages of cancer progression or during the period of carcinogen administration in some tumor models. Using the human prostate cancer cell line DU 145, Webber and colleagues found a 50% reduction in growth related to selenium supplementation. These effects were seen in concentrations analogous to those achievable in serum associated with selenium supplementation. The direct association between dietary selenium and rodent mammary and skin cancers has been best demonstrated through supplementation with onions or garlic grown in selenium-enriched soils or from eggs from hens given a selenium-enriched diet thereby reducing chemical-induced cancers. Perhaps the most intriguing study was conducted in a mammary carcinogenesis model in rodents in which garlic grown in selenium-enriched soil was used as a preventive agent. The authors found increases in glutathione peroxidase levels as well as concomitant reductions in cancer rates with increasing selenium intake.
Population studies have demonstrated that overall cancer rates are higher in populations with low selenium intake. No phase III trials of selenium have been conducted to assess the impact on prostate cancer incidence or mortality. However, there have been several observations made from other clinical trials that may have considerable import. Yoshizawa and colleagues conducted a case-control analysis of 181 men in the Health Professionals Follow-up Study who developed advanced prostate cancer compared to controls from within the study. All men in the study had contributed toenail clippings in 1987, and with these specimens, selenium levels were analyzed. Higher selenium levels in this study were associated with a reduced risk of advanced prostate cancer (the odds ratio comparing highest to lowest quintile of intake being 0.49). Hardell and colleagues conducted a case-control study of selenium levels in plasma and glutathione peroxidase in erythrocytes in 164 patients with prostate cancer and 152 controls with benign prostatic hyperplasia, all from a urology clinic. The authors found that among men who were not taking selenium supplementation, selenium levels in those men with cancer were 0.99 pmol per L compared with 1.08 pmol per L in controls (p = .0007). A similar study was conducted in the Hypertension Detection and Follow-up Program. In this study of 10,940 men and women aged 30 to 69 years with diastolic blood pressure > 90 mm Hg, baseline venous blood samples were obtained. During a 5-year period, 111 patients were diagnosed with cancer and two controls were identified for each case. Case-control analysis demonstrated significantly lower selenium serum levels in cancer cases than in controls. This difference was greatest for gastrointestinal and prostatic tumors. (Selenium levels in prostate cancer cases and controls averaged 0.128 and 0.139 pg/mL, respectively.) Supporting the antioxidant hypothesis of the mechanism of action of selenium, serum levels of retinol and vitamin E were strongly associated with this effect; the relative risk of cancer in the lowest tertile of selenium was 2.4 and 3.9 for the lowest tertiles of vitamin E and retinol, respectively. Another large case-control study was conducted within the Nurses’ Health Study, a prospective analysis begun in 1976 of 121,700 female registered nurses aged 30 to 55 years living in the United States. The authors studied toenail selenium levels and found that levels declined with age and were lower in smokers. At odds with other similar studies, the authors did not find an association between selenium and overall cancer levels. This study added to the observations that a greater association between selenium and cancer risk may occur in men compared to women.
Perhaps the most intriguing observation regarding selenium and prostate cancer was made by Clark and colleagues. With the hypothesis that selenium intake would reduce the incidence of carcinoma of the skin, a group of 1312 patients at seven dermatology clinics, primarily in areas of the United States with low selenium intake, were randomized to receive either 200 pg of selenium daily or placebo. Fortunately, the authors also collected information on other cancers as melanoma rates were identical and there was no difference in squamous or basal cell carcinoma in both the study arms. However, when the authors analyzed all carcinomas, there were 104 in the placebo arm compared to 59 in the selenium arm of the study (RR = 0.55). Additionally, 35 prostate cancers were detected in the placebo arm compared with only 13 in the selenium arm of the study (RR = 0.37). The authors found no toxicity from selenium supplementation. Also of interest was a significant reduction in total cancer mortality (29 versus 57 in the placebo arm, RR = 0.5).
The mechanism of action of selenium, if it is a chemopreventive agent in humans, is unknown. Like oc-tocopherol, selenium may function as an antioxidant, thereby preventing cumulative cellular damage from reactive oxygen species. Indeed, there are data that selenium and a-tocopherol may have synergistic effects and that in the presence of low intake of either, supplementation with the other may reduce the carcinogenic effect. A second postulated action is enhancement of immune response. This effect has evidence in the observation in a number of animal species that selenium deficiency is associated with decreased cell-mediated immune function. Evidence is also available that at higher than usual concentrations, selenium may protect against the action of a number of carcinogens. Selenium supplementation has also been demonstrated to inhibit tumor growth and to stimulate apoptosis in cell culture.
A variety of methods exist to measure selenium intake. It is possible to extrapolate selenium deficiency in certain areas of the world, where low selenium soil concentrations exist and there is a low intake of animal protein; however, in the United States, with such diverse food sources, only by measurement of individual patient levels can selenium intake be determined. Serum selenium concentrations will provide estimates of current intake but a more relevant measure comes from toenail levels, which provide an approximation of the intake over the past year.
While certainly there is a body of literature suggesting that selenium may indeed reduce cancer risk, a number of confounding factors mitigate this conclusion. First is the observation that most human sources of selenium are organic — principally selenomethionine and selenocysteine — while many of the animal and other studies have used inorganic sources such as sodium selenite, often using levels of supplementation many times greater than that seen in human populations. A second problem is the very small number of cases of prostate cancer summarized in previous studies. Until Yoshizawa and colleagues’ study of 181 men with advanced prostate cancer, four previous studies provided data on a total of only 123 men with prostate cancer. Another concern is that while selenium levels differed significantly between cases and controls in Hardell’s study, there were no differences observed in levels of erythrocyte glutathione peroxidase. This brings into question whether serum selenium levels actually correlate with intracellular antioxidant activity. Thus, saturation of glutathione peroxidase levels occurs at relatively low selenium intakes, calling into question whether supplementation actually changes antioxidant concentrations in cells. An internal inconsistency is rarely mentioned in reviews of selenium activity. Yan and Spallholz have demonstrated that selenium can interact with sulfur-containing compounds (thiols) to yield substances that lead to the formation of superoxide and hydrogen peroxide free radicals. They speculate that this may be the mechanism whereby apoptosis can be promoted by selenium but is in direct conflict with one of the proposed methods of action: as an antioxidant. In addition, a very large trial from Linxian, China, found no reduction in prostate cancer incidence or mortality. An additional problem in case-control studies analyzing cancer cases is evidence suggesting that selenium levels in serum can be lowered by the presence of cancer. In advanced tumors or tumors with a long prediagnostic phase (e.g., prostate cancer), this can be a major confounding factor in such studies even if toenail assays for selenium are used. Dietary assessments of selenium can also be unreliable due to the wide variability of soil selenium levels. Animal study conclusions may also be confounded by different mechanisms of selenium metabolism among species. Finally, merely the observation that millions of Chinese have profoundly selenium-deficient diets and yet no dramatic increases in cancer rates give pause to those who attribute dramatic differences in cancer rates merely to selenium intake. Additionally, the doses used for chemoprevention (50 to 200 pg/d) are quite close to those at which toxic effects are seen. Side effects most commonly encountered are dermatitis, hair loss, and abnormal nails; growth failure, hepatic dysfunction, and anemia can also be seen.
Anti-inflammatory Agents
A wide range of anti-inflammatory agents have been demonstrated to have chemopreventive effects in a number of tumor systems. Generally referred to as nonsteroidal anti-inflammatory drugs, the mechanism of action leading to this chemopreventive effect is unknown, yet most frequently attributed to its inhibition of a key enzyme in the metabolism of arachidonate-cyclooxygenase. Two isoforms of cyclooxygenase have been identified: COX-1, which is constituitively expressed in many tissues, and COX-2, an inducible and more recently discovered enzyme. Many nonsteroidal anti-inflammatory drugs and aspirin inhibit both COX-1 and COX-2; thus, due to the COX-1 effect, this results in a number of undesirable side effects including gastritis and risk of neurologic sequelae (hemorrhagic stroke). It is for this reason that the advent of COX-2 inhibitors have opened a new avenue for the prevention of a variety of tumors.
Inhibitors of prostaglandin (or prostanoid) synthesis have been observed to have chemopreventive effects in a number of preclinical and clinical trials. In experimental animal models, indomethacin, aspirin, and piroxicam have been demonstrated to reduce fibrosarcoma and colon cancer growth. Similar effects have been observed in animal models of prostate cancer. Viljoen and colleagues found that aspirin substantially reduced the growth of the human prostate cancer line DU 145. In the Nb rat prostatic ade-nocarcinoma model, Drago and Murray found that aspirin and indomethacin significantly reduced the number of metastases in animals. Providing further evidence that arachidonic acid metabolism is the etiology of this effect, Ghosh and Myers demonstrated that arachidonic acid replacement dramatically increased the growth of human prostate cancer cell lines. Using a variety of agents to alter metabolism at several levels (ibuprofen for cyclooxygenase, SKF-525A for cytochrome P-450, baicalein and BHPP for 12Tipoxygenase, AA861 and MK886 for 5Tipoxygenase), the authors found that the most significant degree of inhibition occurred with 5-lipoxygenase inhibition. Of interest, other evidence suggests that nonsteroidal anti-inflammatory drugs may actually increase apoptosis in rectal mucosa and in colon cancer cell lines. The observations provide evidence of a number of possible effects of these agents.
In a substantial number of observational studies as well as in clinical trials, aspirin and sulindac have been demonstrated to significantly reduce the incidence of colon cancer and adenomatous polyps. Interestingly, in patients with familial adenomatous polyposis or Gardner’s syndrome, polyp counts have usually returned to pretreatment levels after stopping NSAID treatment. At this time, there have been no observations from large-scale clinical trials of prostate cancer to suggest that COX-2 inhibitors may be effective in preventing this disease. Almost certainly, case-control studies from large NSAID trials as well as observations from case-control analyses of large trials such as the Prostate Cancer Prevention Trial should shortly provide evidence as to the effect of these agents in carcinoma of the prostate.
Isoflavonoids/Lignans
The observation that native Japanese have very low rates of clinical prostate cancer despite similar background rates of latent disease and that rates of clinical disease increase among Japanese who immigrate to the United States and develop the local dietary habits has led to the speculation that dietary factors prevent disease in native Japanese. While animal products are a major source of protein in the United States, soybean products represent a major component of Japanese and other Asian diets. In Taiwan, the average consumption of soy protein is 35 g per day. Major components of soy products are isoflavonoids including daidzein, daidzen, genistein, o-desmethylangolensin, and equol, with daidzein and genistein being the most common. Of these agents, perhaps the most interesting and promising is genistein. Daily intake of genistein in Japan is 1.5 to 4.1 mg compared to < 1 mg in the United Kingdom and little more in the United States. One major putative method of action of these isoflavones has been suggested to be estrogenic or antiestrogenic through increased sex-hormone-binding globulin, modulation of pituitary response to gonadotropins, or inhibition of aromatase and 17-beta-hydroxysteroid oxidoreductase.
The initial recognition of the estrogenic activity of genistein was in the investigation of “clover disease” — decreased fertility in sheep allowed to graze on clover. The estrogenic activity has since been observed in mice, rats, and guinea pigs, and there are some similar observations through epidemiologic studies in humans. Kelloff has suggested that the composite of observations in humans is that the effect of genistein depends on the existing hormonal environment — if endogenous estrogen levels are low, genistein functions as a weak estrogen. If endogenous estrogen levels are high, it acts as an antiestrogen.
As prostate carcinogenesis is intimately dependent upon androgens, it stands to reason that one of the putative explanations for the chemopreventive activity of isoflavones is through its estrogenic activity. Genistein has been reported to inhibit 5 a-reductase and P-450 aromatase. Similarly, one of the more important enzymes in estrogen metabolism, 17-beta-hydroxysteroid oxidoreductase, has been demonstrated in prostate cancer cells to be inhibited by exposure to flavonoids.
A number of preclinical studies have added to the body of evidence that isoflavonoids may be potential preventive agents for carcinoma of the prostate. Barnes and colleagues studied the effect of varying concentrations of genistein on the growth of LNCaP and DU 144 human prostate cancer cells. These authors found dramatic reductions of tumor growth even at levels of < 5pg per mL. Using the Lobund-Wistar rat model of spontaneous and metastasizing adenocarcinoma in the prostate-seminal vesicle complex, Pollard and Luckert found that not only was the incidence of tumors reduced by high isoflavone dietary supplements but the disease-free period was pro-
longed. Genistein has also been demonstrated to induce apoptosis in prostate cancer cells. This observation has been replicated in a second study of LNCaP cells.
A number of phase II clinical trials are currently ongoing to investigate the possible roles of isoflavones in the prevention of prostate cancer. Studies are primarily focusing on small groups of patients at a higher risk of disease using intermediate endpoints or evaluating the modulation of various biomarkers. At this time, a large-scale dietary intervention trial using a phase III design is not currently planned.
Hormonal Prevention
It has been said that only two criteria are universally required for the development of prostate cancer in man: aging and an intact hormonal axis. While the former is generally inescapable, evidence is accumulating that modulation of the latter may reduce the risk of prostate cancer development.
A number of observations support the concept that cumulative androgen exposure contributes to the risk of prostate cancer development. The first of these stems from observations of the results of androgen deficiency syndromes. Perhaps the most notable of these syndromes is associated with a single gene and, therefore, single amino acid defect in 5 a-reductase. In the prostate, the type II isoform of 5 a-reductase converts testosterone (T) into dihydrotestosterone. At the androgen receptor, dihydrotestosterone is as much as 4 to 5 times more potent as T. The syndrome of 5 a-reductase deficiency, first described by Imperato-McGinley in 1979 provided the first evidence that androgen modulation may reduce the risk of prostate cancer development. In a relatively isolated area of the Dominican Republic, a group of children were identified at birth with pseudovaginal hypospadias. At puberty, masculinization of the genitalia developed as well as male gender identity and a deep voice despite a persistent female escutcheon and an absence of body hair. Further study of the affected subjects during adulthood found a pancake-shaped rudimentary prostate and an undetectable prostate-specific antigen level in serum. Biopsy of these glands found no evidence of prostatic epithelium. These observations ultimately led to the development and eventual clinical availability of the first 5 a-reductase inhibitor, finasteride, for the treatment of benign prostatic hyperplasia. In the syndrome of absence of 5 a-reductase, not only is there an absence of 5 a-reductase and very low levels of dihydrotestosterone during adulthood but these two changes to the prostatic hormonal milieu are also operational during embryogenesis and puberty, two periods when androgens have a major action on the prostate.
A series of important observations have been made regarding the impact of androgen deprivation on normal and neoplastic prostatic epithelium. Androgen deprivation has a well-established apoptotic effect on normal and
neoplastic epithelium. This action probably accounts for the reduction in prostate gland volume in patients treated with finasteride. In patients with extant carcinoma of the prostate, androgen deprivation leads to a considerable fall in serum prostate-specific antigen levels, dramatic improvements in symptoms, a decrease in prostatic volume, and major improvements in survival in men who have an undetectable level of prostate-specific antigen as a nadir value after androgen deprivation.
A variety of epidemiologic observations further support the notion that cumulative androgen exposure increases prostate cancer risk. It is well established that native Japanese have one of the lowest prostate cancer rates in the world. By comparison, Caucasians in the United States have an intermediate rate, with African Americans having one of the highest rates in the world. The prostate cancer risk link with androgen levels arises from a series of observations:
1. Some large population studies have found serum androgen levels to be highest in African American men.
2. The activity of 5 a-reductase (and, thereby, the levels of the most active androgen, dihydrotestosterone) is highest in African Americans, intermediate in Caucasians, and lowest in Japanese.
3. A small study of pregnant women found serum androgen levels to be highest in African American women. Conceivably, such higher in utero androgen levels may be responsible for a higher “gonadostat” set point, leading to higher cumulative androgen levels and thereby a higher prostate cancer risk in African American men.
4. Substantial interindividual variability was recently found in the CYP3A4 gene, a gene responsible for the oxidation of testosterone to several metabolic products. In a group of 230 Caucasian men with prostate cancer, an altered 5′ regulatory element was found in 46% of men with locally advanced (T3 to T4) prostate cancer compared to only 5% of men with Tl tumors. One hypothesis from this investigation may be that men with an altered ability to metabolize intraprostatic testosterone may be at a higher risk of prostate cancer and that reduction of intraprostatic androgens may reduce prostate cancer risk.
5. Androgenic stimulation of the prostate is associated with cellular proliferation. Evidence from a number of other organ sites suggests that methods to decrease cellular proliferation may reduce the clinical development of cancer.
6. Conditions associated with chronically low levels of androgens (e.g., cirrhosis) are associated with a low risk of prostate cancer.
Potential Interventions for Hormonal Prevention Historically, traditional hormonal therapy has not been an attractive option for the prevention of prostate cancer in a population of healthy men at risk of the disease. These traditional hormonal manipulations include bilateral simple orchiectomy, luteinizing hormone-releasing hormone agonist, and antiandrogens. These options are associated with considerable toxicity including impotence, decreased libido, muscle wasting, mood changes, hot flashes, gynecomastia, and osteoporosis. Even with antiandrogens, perhaps the alternative with the fewest side effects, over 50% of patients can expect side effects of some degree.
It is because of the toxicity of traditional hormonal therapy that the discovery of finasteride, the first 5 a-reductase inhibitor, heralded tremendous interest in a new opportunity to prevent prostate cancer. Finasteride competitively inhibits 5 a-reductase and principally affects the type II isoform, which is most active in the prostate gland. (By contrast, the type I isoenzyme is found primarily in the liver and scalp.) Approved by the U.S. Food and Drug Administration for the treatment of bladder outlet symptoms associated with benign prostatic hyperplasia, finasteride has a number of clinical effects: (1) serum prostate-specific antigen concentrations fall by 50%; (2) dihydrotestosterone concentrations fall by 70 to 80%; (3) peak urinary flow rate increases by approximately 20%; (4) urinary symptom scores improve by 23%; and (5) prostate volume decreases by 20%. Most germane to the concept of chemoprevention of prostate cancer, side effects seen with finasteride therapy are relatively uncommon. The three side effects that have been seen more commonly in finasteride-treated patients than in controls in randomized, double-blind trials have been decreased libido, impotence, and ejaculatory disturbances. In the Veterans Administration trial, these three side effects were seen in 5, 9, and 2% of patients receiving finasteride compared to 1, 5, and 1%, respectively, of patients receiving placebo. Of interest, two other studies had somewhat different findings. The PROSPECT Canadian benign prostatic hyperplasia trial which randomized 613 men to either finasteride or placebo, found these three side effects in 10, 7.7, and 15.8% of patients receiving finasteride compared to 6.3,1.7, and 6.3% in those receiving placebo. In the PLESS study of 3040 men randomized to either finasteride or placebo, the rates of these side effects in the first year of study were 6.4, 8.1, and 3.7% in patients receiving finasteride compared to 3.4, 3.7, and 0.8% in those receiving placebo. In the years 2 to 4, the rates dropped to 2.6, 5.1, and 1.5% in finasteride patients compared with 2.6, 5.1, and 0.5% in those receiving placebo. Thus, the composite of these very large studies suggests that while sexual toxicity will occur in some patients, the rate will be quite low and tends to disappear over time.
A number of observations in the preclinical realm provide insight as to the potential application of hormonal therapy in general and finasteride in particular in the prevention of prostate cancer. Tsukamoto and colleagues have explored the impact of 5 a-reductase inhibitors on the development of carcinoma of the prostate in the F344 rat prostate carcinoma model in two separate studies. In this model, prostate cancer is induced through a combination of a carcinogen (3,2′-dimethyl-4-aminobiphenyl [DMAB]) and exogenous testosterone proprionate. Animals were grouped into controls, high- and low-dose finasteride (15 and 5 mg/kg), and three doses of bicalutamide (15, 30, and 60 mg/kg) with all doses administered three times weekly. Despite relatively low doses of finasteride (5 mg/kg for rats while some studies have used as high as 25 to 160 mg/kg), the rates of all prostatic tumors were reduced from 68.6% in control animals to 40 and 50% in high-dose and low-dose finasteride-treated animals, respectively, and 45, 50, and 20% in animals receiving increasing doses of bicalutamide.
A variety of other conclusions have been reached by other investigators when analyzing the effects of 5 a-reductase inhibitors in animal prostate cancer models. Many of the conclusions are of questionable value for several reasons. (1) The metabolism of 5 a-reductase inhibitors in animal models is extremely variable, and rapid metabolism in rodents may lead to only transient effects. (2) Many studies have not designated which type of tumor line was used, an item of special importance when dealing with the R-3327 Dunning tumor. While prostate cancer has been reported to have relatively low levels of 5 a-reductase, in humans an almost two-log differential in 5 a-reductase activity has been found in various sublines of the R-3327 tumor. Thus, it is not surprising that in the H-tumor (with high intratumor 5 a-reductase levels), Lamb found little activity of the 5 a-reductase inhibitor SKF 105857 while in the G-tumor (with 5 a-reductase levels more like that found in human prostate cancer), inhibition was witnessed that was similar to the effect of castration. Nevertheless, several studies using additional 5 a-reductase inhibitors have demonstrated growth inhibition of various prostate cancer cells.
An observation that has been made on several occasions in animal models of prostate cancer is that 5 a-reductase inhibitors can affect both androgen-sensitive and -insensitive tumors. For example, Bologna found that finasteride inhibited both PC-3 and DU 145 human prostate cancer cells with inhibition being dose dependent. The explanation for this may lie in the observation of Wang that finasteride may actually regulate prostate-specific antigen gene expression at the transcriptional level, thereby contributing to the fall in prostate-specific antigen in patients treated with this agent. This observation, combined with the concept that prostate-specific antigen itself may exert a trophic influence in prostate cancer cells (e.g., LNCaP), may go far to explain its potential effect on prostate cancer development and progression.
Clinical trials with finasteride provide, as stated above, considerable evidence regarding the safety profile of the drug. However, the data regarding the potential of this agent to reduce the risk of prostate cancer are conflicting. In the PLESS trial of benign prostatic hyperplasia treatment, no significant difference in the rate of prostate cancer detection was observed. (Although 2.4% of patients with a baseline prostate-specific antigen < 4.0 ng per mL treated with finasteride compared to 2.8% treated with placebo were found on biopsy to have prostate cancer, this difference was not statistically significant.) In a very small study of 57 patients with elevated prostate-specific antigen and a negative prostate biopsy who were randomized to finasteride or no treatment, 30% of finasteride-treated patients ultimately developed prostate cancer compared with 4% of those receiving no treatment. In this study, the majority of positive biopsies occurred in men with a previous diagnosis of prostatic intraepithelial neoplasia, a condition for which a 30 to 50% positive-rebiopsy rate would be expected. Thus, while the 30% positive biopsy rate was not unexpected in men treated with finasteride, the aberrant 4% positive biopsy rate in controls was quite unusual.
On the basis of compelling evidence that cumulative androgen exposure is associated with prostate cancer risk and that methods to reduce this androgenic stimulus should reduce prostate cancer risk, in 1992, the Board of Scientific Counselors of the Division of Cancer Prevention and Control of the National Cancer Institute approved the concept of a randomized, placebo-controlled trial of finasteride for prostate cancer prevention. The Prostate Cancer Prevention Trial opened for participant enrollment in October 1993 and over the subsequent 3 years randomized 18,884 men to either finasteride or placebo. The trial design is illustrated in Figure 48-6. Eligible participants were over age 55 years and had a normal digital rectal examination and a prostate-specific antigen < 3.0 ng per mL. The study endpoint is cumulative prostate cancer incidence over the course of the 7-year study with a prostate biopsy in all men at the end of 7 years. It is anticipated that results of the study will be available in late 2004.
Vitamin D
Vitamin D is an essential vitamin, obtained both from diet and from sunlight. Figure 48-7 displays the pathways of vitamin D metabolism. As can be seen, vitamin D is synthesized from 7-dehydrocholesterol in skin exposed to sunlight. Thereafter, passage through the liver converts vitamin D to 25 (OH) vitamin D and thereafter in the kidney to 1,25 (OH)2 vitamin D. A number of studies have suggested that vitamin D can have substantial chemopreventive effects in cancer cells. The mechanism of action of vitamin D is unknown but Kelloff has reviewed available literature that lists potential modes of action as inhibition of proliferation; modulation of signal transduction, modulation of oncogene expression; inhibition of ODC induction, lipid peroxidation, and angiogenesis; and induction of differentiation, transforming growth factor expression, and apoptosis.
Several pieces of evidence suggest that indeed vitamin D may have a preventive role in the development of prostate cancer. Hancette and Schwartz surveyed geographic distribution of ultraviolet radiation and prostate cancer mortality in 3073 counties in the United States. They found a strong north-south trend of prostate cancer mortality; death rates from prostate cancer were the highest in the most northern latitudes, with vitamin D levels being a potential cause of the effect. It is well established that the majority of vitamin D is obtained through exposure to the sun. As skin pigments can reduce the amount of ultraviolet radiation that reaches the skin layers containing 7-dehydrocholesterol, the degree of pigmentation is inversely related to vitamin D levels. This observation may explain the fact that African American men in the United States have the highest incidence of prostate cancer.
Two observations from the Health Professionals Follow-up Study provide clinical support for this hypothesis. The authors studied the impact of calcium and fructose intake on the risk of prostate cancer to include a subset analysis of advanced and metastatic prostate cancers. As calcium leads to inhibition of the parathyroid hormone and thus a reduction in vitamin D production, it would stand to reason that increased calcium intake might increase prostate cancer risk. Conversely, as fructose intake reduces serum phosphate levels and as hypophosphatemia leads to increased production of vitamin D production, increased fructose intake should reduce prostate cancer risk. These were exactly the findings of the authors with high calcium intake associated with a relative risk of 2.97 while high fructose intake was associated with a relative risk of 0.51.
Several strategies for the chemoprevention of prostate cancer emerge from these observations. The first and simplest is a recommendation for increased intake of fruits, generally excellent sources of fructose. As fruits are also generally high in fiber, a number of other benefits may accrue. Observations from cell culture studies suggest that the amount of vitamin D3 required to cause cellular differentiation is associated with significant toxicity, generally through significant elevation in serum calcium levels. For this reason, a major effort has been made in the development of vitamin D analogues — agents that have activity at the vitamin D receptor yet do not have a similar effect on serum calcium. Finally, traditional dietary sources of vitamin D such as fish liver and milk can be maintained.
Retinoids
Retinoids are a class of compounds that are both natural and synthetic derivatives of vitamin A and function as regulators of cell growth and activity through action at the retinoid receptor. Retinol (vitamin A) is metabolized through nicotinamide-adenine dinucleotide (phosphate) (NAD [P])-linked dehydrogenase to all-trans-retinoic acid (RA); RA can be acted upon either by photoisomerization or enzymatic isomerization to 13-ris-retinoic acid (cRA) or via cytochrome P-450-dependent monooxygenase to trans-A hydroxy-retinoic acid. A quite promising agent, 9-cis-retinoic acid (9cRA), is a stereo- and photoisomer of RA. Carotenoids are a group of naturally occurring compounds that are found in vegetables and fruits that are metabolized to vitamin A and thereby to other retinoids. Included among this family are beta-carotene, crocetin, cryptoxanthin, lutein, astaxanthin, zeaxanthin, lycopene, and canthaxanthine. All these compounds are thought to affect cancer promotion by inhibiting cellular proliferation, leading to cellular differentiation, affecting apopto-sis, induction of transforming growth factor-beta expression, inhibition of arachidonic acid metabolism, antioxidant activity, and several other actions. Retinoid receptors are classified into two types: RAR and RXR, with RXR perhaps the most active in the potential chemopreventive role. Epidemiologic studies have repeatedly noted a reduction in cancer risk among populations with a high intake of foods containing retinoids.
In preclinical studies, RA has been found to induce differentiation of the promyelocytic cell line HL-60. Similar differentiation effects have been seen in murine teratocarcinoma cell lines. As stated above, RA being a powerful inducer of transforming growth factor-beta production, it is notable that postandrogen-deprivation apoptosis is dramatically inhanced by increased levels of transforming growth factor-beta messenger ribonucleic acid (mRNA). Studies of animal tumor models have found that retinoids reduce mammary tumors, buccal pouch tumors, and 7, 12-dimethylbenz [a] anthracene (DMBA)-induced rat salivary gland tumors.
In the realm of prostate cancer, Pasquali et al. have studied levels of retinol and RA in normal tissue, benign prostatic hyperplasia, and prostate cancer. While retinol levels for these three tissues were 60, 146, and 77, respectively, interestingly, RA levels were 5, 8, and at or near the lower limit of detection (1 ng/g wet weight), respectively, suggesting a potential role of retinoids in either chemoprevention or therapy of carcinoma of the prostate. Studying three prostate cancer cell lines (PC-3, DU 145, and LNCaP), de Vos and colleagues found a variety of RAR and RXR heterodimers to have growth inhibitory activity for all cell lines. Liarizole, an imidazole derivative, has a variety of activities, one of which is postulated to be an increase in intracellular retinoid accumulation through inhibition of the catabolism of RA. In both androgen-dependent and -independent Dunning rat prostate tumors, liarozole inhibited cell growth. Finally, Pollard and Luckert have demonstrated that 4-hydroxyphenyl retinamide (4-HPR) retinoid, currently employed in a variety of clinical trials, not only substantially reduces the development of primary prostate tumors in the Lobund-Wistar rat model but also reduces the rate of metastasis in PA-III cells.
Despite the promise of these mechanisms of action and the epidemiologic observations suggesting the potential efficacy of retinoids in cancer chemoprevention, several observations call into question the efficacy of these agents. A recent study of vitamin A in the prevention of local recurrences and second primaries in patients with head and neck cancer found an increased risk of recurrence in those receiving vitamin A. In the realm of prostate cancer, two case-control studies, one in Hawaii and one in Utah, found higher vitamin A or carotenoid intake among cases than controls. In an analysis of the Physician’s Health Study (22,071 male physicians randomized in 2×2 fashion to beta-carotene or placebo and aspirin or placebo), after an average follow-up of 12 years Hennekens and colleagues found no significant effect on cancer risk. Of greater concern was the finding of an excess number of lung cancers in the ATBC trial of beta-carotene in Finnish smokers. Finally, Pienta and colleagues studied the effect of 4-HPR in men at a high risk of prostate cancer (elevated prostate-specific antigen) . With a mean prostate-specific antigen at study entry of 8.6 ng per mL, the study was actually terminated prematurely when 40% of patients were found to have a positive prostate biopsy.
Although epidemiologic and other evidences suggest that retinoids ought to have preventive activity in prostate cancer, there is little clinical evidence at this time of this effect, and more troubling is the evidence of an excess number of tumors in treated patients. For this reason, it is doubtful that there will be any large-scale trials using these types of agents until a better understanding of the mechanism of action and more selective agents are available. It is also reasonable for physicians to counsel patients as to the potential disadvantages of over-the-counter retinoid supplementation, especially among smokers.
Low-Fat Diet
While micronutrients, such as vitamins E and D, and selenium, have been suggested to be potential chemopreventive agents for prostate cancer, another major component of diet — fat — may play a major role. Such an association is not novel as high-fat diets have been associated with cancers of the colon, rectum, and breast. Overall population patterns support the association of high-fat (especially fat from animal sources) intake and prostate cancer. In general, Asian countries such as Japan and China have very low amounts of animal fat intake and have parallel low rates of clinical prostate cancer and prostate cancer mortality. Conversely, Western European and North American countries have some of the highest rates of animal fat consumption as well as rates of prostate cancer mortality.
Multiple observational studies support the notion that a low-fat diet is associated with a lower risk of prostate cancer. A population-based case-control study from Utah of 358 cases with prostate cancer and 679 controls matched by age and location of residence found dietary fat to be significantly associated with prostate cancer risk, most notably in older patients and those with aggressive tumors. The odds ratio was 2.9 for total fat and 2.2 for saturated fat. In a study of similar design (452 cases and 899 age-matched controls) from Hawaii, Kolonel and associates found a substantially increased risk of prostate cancer in men with the highest quartile of fat intake. The odds ratio was 1.7 for this group. A smaller study from Madrid, Spain came to a similar conclusion with an odds ratio of 2.56 for high animal fat diets. Giovannucci reported the results from the Health Professions Follow-up Study finding a high-fat diet to be associated with a greater risk of advanced prostate cancer. A more recent study from Canada analyzed a group of 427 men with prostate cancer, segregating patients into either local or advanced cases. While statistical significance was not reached, a positive trend to the development of advanced prostate cancer and total fat was noted while a negative trend was noted for total vegetable fat intake. Cases in the highest quartile of saturated fat had a significant odds ratio of 2.15 (95% CI, 1.14 to 4.04). Zhou and Blackburn reviewed all previously reported human studies of dietary fat and prostate cancer in 1997. Of the descriptive studies reviewed, 8 of 16 were positive and 8 showed no association. Of the 14 case-control studies surveyed, the results were similar: 8 of 12 reporting on total fat found a positive association, and 4 of 12 found no association; 4 of 6 reporting on total animal fat intake found a positive association, and 2 of 6 found no association; 4 of 5 reporting on saturated fat intake found a positive association, and 1 of 5 found no association; 2 studies reporting on monounsaturated fat intake found a positive association in both. Negative associations were found in 3 of 4 studies reporting on polyunsaturated fat intake. Most important is the lack of any epidemiologic observation of a negative association between prostate cancer and animal fat. Most recently, an international study of 59 countries comparing dietary patterns with prostate cancer mortality found an inverse relationship between prostate cancer mortality and intake of cereals, nuts and oilseeds, and fish, and a direct relationship with animal meat/fat intake.
Various mechanisms of action have been proposed to explain the association of prostate cancer and dietary fat.
Probably the best known is the association of animal fat and androgens. It has been observed in women that estradiol and estrone levels fall with dietary fat reduction; a similar effect has been seen in healthy men fed a low-fat diet, in whom excretion of urinary androgens increase with a high-fat diet. Investigators have hypothesized that the cumulative increased exposure to androgens by the prostate then leads to increased cellular proliferation and, eventually, an increased risk of cancer. Of interest, in a small study, African American women were noted to have higher androgen levels during the first trimester of pregnancy than their Caucasian counterparts. Ross and Henderson have merged this and other observations to develop a global hypothesis to explain the diet-prostate cancer link. In utero, a high-fat diet may lead to a “low-gonadostat set point,” leading to higher testosterone levels later in life. Such higher levels lead to earlier puberty (or menarche in girls), thus making the prostate in affected individuals “older” than chronologic age. Finally, a lifetime of increased serum testosterone levels leads to a state of relative hyperproliferation and decreased apoptosis in the prostate, unmasking or promoting other initiating events, thereby leading to a higher risk of prostate cancer.
Few studies in preclinical models have analyzed the impact of dietary fat on the growth of rodent and human prostate cancer lines. Wang and colleagues, using the human prostate cancer line LNCaP, studied the effect of various fractions of total calories of dietary fat (from 2.3 to 40.5% kcal% fat) on prostate cancer growth. As expected, tumor growth rates were highest in the 40.5% group and lowest in the 2.3% group. Serum prostate-specific antigen levels were similarly highest in the high-fat diet group and lowest in the low-fat diet group. In the Lobund-Wistar model of spontaneous prostate cancer development,* prostate cancer rates are reduced significantly with a reduction of dietary fat. Finally, a most intriguing observation was made by Kondo and associates in the ACI/Seg model of spontaneous prostate cancer. In this study, a high-fat (20% corn oil) or a low-fat (5% corn oil) diet was fed to mother rats and thereafter to their male offspring. At 100 weeks of age, adenocarcinoma of the prostate was found in 20% of the high-fat group compared with none of the low-fat group. Further supporting this conclusion, the rate of atypical hyperplasia was reduced from 73.3 to 20%, respectively. This observation has a close clinical parallel in several epidemiologic studies in that environmental factors (e.g., diet) have the most significant impact early in life rather than late.
Several parallel studies provide some commentary on the relationship of diet and fat. As diet and cholesterol are generally directly related, one might expect a higher risk of prostate cancer among men with higher cholesterol levels. Such has not been noted in two separate studies. Similarly, in a 1997 meta-analysis of 16 randomized trials of cholesterol lowering with statin drugs, no effect on cancer mortality was observed. Conversely, in two large studies, in populations of patients with a low rate of cardiovascular disease, prostate and overall cancer rates were reduced (perhaps again due to the relationship of dietary fat in both disease processes).
At the present time, there are no ongoing prospective studies on the impact of diet on overall prostate cancer incidence or mortality. Certainly, such studies cannot be blinded to the participant. However, the publication of a recent 2-year trial analyzing the impact of dietary fat on the radiographic appearance of the breast (patients were randomized to a low-fat diet or regular prestudy diet) illustrates that such studies are possible. Whether such studies are necessary may be open to debate as the bulk of evidence suggests that many additional benefits would accrue from such a dietary change (e.g., reduced breast and colon cancer, reduced cardiovascular disease). Perhaps the challenge for the cancer prevention community is to better “market” the message of the benefits of a reduced-fat (especially reduced animal-fat) diet and the potential salutary effect on the risk of various neoplasms including prostate cancer.
Miscellaneous Agents
A host of other agents have been suggested to have a possible preventive role in prostate cancer. Included among these are green tea, HMG-CoA inhibitors (cholesterol biosynthesis blocking agents), zinc, and DFM0. At this time, none of these agents have been sufficiently tested in prostate cancer to allow an objective evaluation. It is reasonable, however, to move these agents into preclinical trials or into models of cancer prevention (e.g., short-duration treatment prior to radical prostatectomy, treatment of patients with prostatic intraepithelial neoplasia, or treatment of men at high risk of prostate cancer [elevated prostate-specific antigen and negative biopsy]).
