Download Chemoprevention of Cancer – Focusing on Clinical Trials

Jpn J Clin Oncol 2003;33(9)421–442
Review Article
Chemoprevention of Cancer – Focusing on Clinical Trials
Tadao Kakizoe
National Cancer Center, Tokyo, Japan
Received July 8, 2003; accepted August 20, 2003
Chemoprevention of cancer is reviewed from the viewpoints of action mechanisms and methodology of clinical trials in order to introduce promising agents discovered by in vitro and/or in
vivo studies to applications in humans. The clinical trial procedure essentially follows the phase
study which has been employed for chemotherapeutic drugs. Chemoprevention of bladder
cancer, prostate cancer, gastric cancer, hepatocellular carcinoma, breast cancer, head and
neck cancer, colorectal cancer and lung cancer is reviewed, mainly focusing on clinical trials.
Previous clinical trials have shown the effectiveness of the following: polyprenoic acid (acyclic
retinoid) for hepatocellular carcinoma; tamoxifen for breast cancer; retinoic acids for head and
neck tumor; and aspirin, a COX-2 inhibitor, for colorectal cancer. Despite the advantageous
effects of some of these agents, their toxic effects must also be of concern at the same time.
For example, in a chemoprevention trial of lung cancer, b-carotene was unexpectedly found to
increase the risk of lung cancer among high-risk groups. It is also noted that large-scale clinical
trials demand large research grants, which may not be affordable in Japan. Chemoprevention
is still an emerging field of oncology where researchers in both basic and clinical sciences face
great challenges.
Key words: chemoprevention – cancer – clinical trial
INTRODUCTION
The term ‘chemoprevention’ was first introduced by Sporn (1)
in 1976, in contrast to ‘chemotherapy’, when he referred to the
prevention of development of cancer both by natural forms of
vitamin A and by its synthetic analogues, combined called
retinoids. This strategy seems to be promising for reducing
cancer incidence both in well-defined high-risk groups of
people and also in the general population. Chemoprevention is
now defined as the use of specific agents to suppress or reverse
carcinogenesis and thereby to prevent the development of
cancers (2).
Primary prevention of cancer is one of the key approaches
to the control of cancer. It includes (i) avoiding exposure to
known cancer-causing agents, (ii) enhancement of host defense
mechanisms, (iii) modifying life style and (iv) chemoprevention. Thus, the National Cancer Institute (NCI) in the USA has
given priority to cancer prevention research in such diverse
areas as diet and nutrition, tobacco cessation, chemoprevention
and early detection and screening (3). The NCI also sponsors
small- and large-scale clinical trials for this purpose. The final
goal of all those programs is to achieve reductions in cancer
incidence and mortality.
As President of National Cancer Center, Japan, I completely
agree with this strategy of the NCI. In addition to the four
approaches mentioned above, however, prevention and
eradication of tumor-associated viruses, bacteria and parasites
should be emphasized. This review stresses the importance of
chemoprevention.
I. MECHANISMS OF ACTION OF
CHEMOPREVENTIVE AGENTS
Findings obtained so far from carcinogenesis experiments,
chemical/biological sciences, pathology and epidemiology
have promoted our understanding of the basic mechanisms of
chemoprevention. Cancer chemoprevention may target various
processes such as those listed in Table 1, originally proposed
by Kelloff et al. (4).
For reprints and all correspondence: Tadao Kakizoe, President, National
Cancer Center, 5–1–1, Tsukiji, Chuo-ku, Tokyo 104-0045, Japan.
E-mail: [email protected]
© 2003 Foundation for Promotion of Cancer Research
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Chemoprevention of cancer
Table 1. Possible mechanisms of action of chemopreventive agents [Kelloff et al. (4)]
Action
Candidate agent
Carcinogen-blocking activities
Inhibit carcinogen uptake
Calcium
Inhibit formation or activation of carcinogen
Arylalkyl isothiocyanates, DHEA, NSAIDs, polyphenols
Deactivate/detoxify carcinogen
Oltipraz, other GSH-enhancing agents
Prevent carcinogen binding to DNA
Oltipraz, polyphenols
Increase level or fidelity of DNA repair
NAC, protease inhibitors
Antioxidant activities
Scavenge reactive electrophiles
GSH-enhancing agents
Scavenge oxygen radicals
Polyphenols, vitamin E
Inhibit arachidonic acid metabolism
Glycyrrhetinic acid, NAC, NSAIDs, polyphenols, tamoxifen
Antiproliferation/antiprogression activities
Modulate signal transduction
Glycyrrhetinic acid, NSAIDs, polyphenols, retinoids, tamoxifen
Modulate hormonal/growth factor activity
NSAIDs, retinoids, tamoxifen
Inhibit oncogene activity
Genistein, NSAIDs, monoterpenes
Inhibit polyamine metabolism
DFMO, retinoids, tamoxifen
Induce terminal differentiation
Calcium, retinoids, vitamin D
Restore immune response
NSAIDs, selenium, vitamin E
Induce programmed cell death (apoptosis)
Butyric acid, genistein, retinoids, tamoxifen
Correct DNA methylation imbalances
Folic acid
Inhibit angiogenesis
Genistein, retinoids, tamoxifen
Inhibit basement membrane degradation
Protease inhibitors
Activate antimetastasis genes
1. CARCINOGEN-BLOCKING ACTIVITIES
INHIBITION OF CARCINOGEN UPTAKE
As an example of this mechanism, calcium is listed. It inhibits
colon tumor formation and colonic mucosal hyperproliferation
in rats. The effect was attributed to a combination of calcium
ions and excess bile and free fatty acids in the colonic lumen
(5). This experimental observation also suggests that other
chelating agents may have chemopreventive potential by
inhibiting carcinogen uptake to the body.
INHIBITION OF CARCINOGEN FORMATION/ACTIVATION
A very good example of this mechanism is inhibition of the
formation of carcinogenic nitrosamines by water-soluble
vitamin C or oil-soluble vitamin E, which block the reaction
between secondary amines and nitrite under very acidic conditions in the stomach (6).
Carcinogen formation can also be inhibited by preventing
metabolic activation of a procarcinogen. Many chemopreventive agents (e.g. allylic sulfides, arylalkyl isothiocyanates,
carbamates and flavonoids) exhibit this type of activity. Several compounds that inhibit aromatase also inhibit chemical
carcinogenesis in estrogen-sensitive tissues (7).
DEACTIVATION/DETOXICATION OF CARCINOGENS
Carcinogen deactivation and detoxication constitute a very
important mechanism of chemoprevention. Two metabolic
pathways are related. One is the phase I metabolic enzymes,
which are primarily the microsomal mixed-function oxidases.
The polar groups of chemicals (e.g. hydroxyl group) become
substrates for this conjugation. The other pathway is by the
phase II metabolic enzymes related to conjugation and
formation of glucuronides, glutathione (GSH) conjugates and
sulfates.
GSH is a prototype carcinogen scavenger. It reacts spontaneously or via catalysis by GSH S-transferases (GSTs) with
numerous activated carcinogens such as MNNG, AFB1 and
B[a]P diol epoxide and other activated polycyclic aromatic
hydrocarbons (8).
A number of promising chemopreventive agents belong to
this category. Prominent compounds among them are the
allylic sulfides, which are natural products found in onion,
garlic and other Allium genus vegetables. Oltipraz [5-(2-pyrazinyl)-4-methyl-1,2-dithiol-3-thione] is a potent GST inducer
with a wide spectrum (9). N-Acetyl-L-cysteine (NAC) is essentially a precursor of GSH (4).
Jpn J Clin Oncol 2003;33(9)
PREVENTION OF CARCINOGEN BINDING TO DNA
After activation and formation of a carcinogen in the body, a
DNA–carcinogen adduct is formed, which is another important
target process for chemoprevention. Oltipraz prevents the
formation of AFB1–DNA adducts. This reaction is attributed
to increased detoxication of AFB1 by GST and, as a consequence, formation of AFB1–DNA adducts is inhibited (4).
ENHANCEMENT OF DNA REPAIR
The DNA-damage modulating enzyme poly(ADP-ribosyl)transferase (ADPRT) is decreased by carcinogens; for
example, NAC prevents the decrease in ADPRT caused by the
carcinogen 2-acetylaminofluorence (4).
2. ANTIOXIDANT/ANTI-INFLAMMATORY ACTIVITIES
SCAVENGING OF OXYGEN RADICALS
Oxygen radicals such as singlet oxygen, peroxy radicals,
superoxide anion and hydroxyl radicals are closely associated
with carcinogenesis acting in any phase, namely initiation, promotion and progression (9). Oxygen radicals exert a mutagenic
effect by oxidizing DNA bases, and radicals also cause DNA
strand breaks and chromosome deletions/rearrangements.
Many compounds scavenging activated oxygen species
exhibit chemopreventive effects. As an example, NAC is
known to react with hydroxyl radicals (10). The reaction of
b-carotene with singlet oxygen is well known. Phenolic antioxidants are known to scavenge peroxy radicals. Particularly,
vitamin E (a-tocopherol) is known to scavenge peroxy radicals, singlet oxygen and superoxide radicals.
INHIBITION OF ARACHIDONIC ACID (AA) METABOLISM
AA is metabolized by oxidative enzymes to prostaglandins
(PGs), thromboxanes, leukotrienes and so forth. Aspirin and
non-steroidal anti-inflammatory drugs (NSAIDs) are useful
chemopreventive agents for colon cancer. Those drugs are
potent inhibitors of cyclooxygeneses, enzymes that catalyze
the synthesis of prostaglandins from AA. These drugs have
attracted much attention from the viewpoint of the relationship
between cancer and inflammation (11).
3. ANTIPROLIFERATION/ANTIPROGRESSION ACTIVITIES
MODULATION OF SIGNAL TRANSDUCTION
It is of interest to regard carcinogenesis as a disorder of signal
transduction. Hormones and growth factors that regulate
cell growth, proliferation and differentiation communicate
across cell membranes, via receptors and receptor-associated
enzymes. Second messengers which convey information
between the cell membrane and the nucleus through cytoplasm
include cyclic adenosine monophosphate, inositol (1,4,5)-trisphosphate, diacylglycerol, prostaglandins and regulatory proteins such as mitogen-activated protein (MAP) kinases. The
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various steps of signal transduction are potential target sites for
chemoprevention in restoring normal cellular controls (12).
MODULATION OF HORMONAL/GROWTH FACTOR ACTIVITY
The regulation of cell growth and proliferation may occur at
multiple levels: hormones and growth factors, membrane/cytoplastic/nuclear receptors (e.g. estrogen, progesterone, retinoid,
glucocorticoid, vitamin D and thyroid receptors). Transforming growth factor b (TGF-b) has antiproliferative activity in
both normal and neoplastic cells (13). Insulin-like growth
factor 1 (IGF-1) stimulates cell replication in various tumors.
These steps are all targets of chemoprevention.
INHIBITION OF POLYAMINE METABOLISM
Polyamines are one of the essential factors in cell proliferation,
differentiation and malignant transformation (14). A critical
step in polyamine biosynthesis is the ornithine decarboxylase
(ODC)-catalyzed formation of putrescine from ornithine.
Chemical agents that inhibit induction of ODC thus possess
chemopreventive potential. Difluoromethylornithine (DFMO)
is a specific and irreversible inhibitor of ODC.
RESTORATION OF IMMUNE RESPONSE
There are many reports regarding the importance and usefulness of antibodies to oncogene products or oncoproteins (15).
Prostaglandin E2 is known to suppress the immune response in
certain tumor cells and inhibitors of cyclooxygenase diminish
this immune suppression. Chemopreventive retinoids work as
immuno-stimulants. In fact, retinoic acid enhances cell-mediated and natural killer cell cytotoxicity. Immunostimulative
effects of selenium and a-tocopherol have been reported,
which may explain part of their action as chemopreventive
agents.
II. PROCESSES OF CLINICAL TRIALS TO
PROVE THE EFFICACY OF
CHEMOPREVENTIVE AGENTS
The development process for establishing a certain specific
chemopreventive agent against a particular cancer is basically
similar to the process of evaluating an investigative new drug
(IND) being approved by the US Food and Drug Administration (FDA). It starts from preclinical evaluation and then
continues stepwise to clinical trials. Such mechanisms and
systems are most developed in the USA, led by the NCI. The
NCI has established a system for a scientific approach to developing chemopreventive agents. Based on epidemiological and
basic laboratory research, stepwise clinical trials are conducted
and the approval of the FDA is finally obtained for marketing
and applying chemoprevention to humans (16).
1. PRECLINICAL EVALUATION
The animal models most frequently used for initial in vivo
screening are azoxymethane-induced rat colon cancer or aber-
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rant cryptic foci in the colonic mucosa. Another frequently
used model is N-methyl-N¢-nitrosourea- or 7,12-dimethylbenz[a]anthracene-induced rat breast cancers.
To choose appropriate chemopreventive agents, sufficient
information regarding organotropism is essential. On some
occasions, effects of promoting adverse tumors must be considered in organs other than the target. For example, although
naturally occurring and synthetic antioxidants inhibit liver
carcinogenesis, they promote tumor development or may even
induce cancer in other organs at the same time (17). Thus,
appropriate experimental protocols are necessary to assess
wide-spectrum organ systems to clarify whether any particular
agent might have practical application as a chemopreventive
compound. Ito and Imaida (18) developed models of multiorgan carcinogenesis systems using male and female B6C3F1
mice treated with N,N¢-dimethylnitrosamine (DEN) and Nmethyl-N¢-nitrosourea (MNU) or using F344 male and female
rats treated with DEN, MNU, N-butyl-N-4-hydroxybutylnitrosamine (BHBN), 1,2-dimethylhydrazine (DMH) and N,N-bis(2hydroxy)propylnitrosamine.
If the results from those in vivo assays are promising, then
the compounds are further processed for in-depth efficacy evaluation and preclinical toxicity and pharmacokinetic testing. In
the USA, the FDA requires the data from preclinical toxicology studies to qualify agents to be used safely in clinical trials
in human subjects. Those studies include acute and subchronic
toxicity and pharmacokinetics. If the developmental process
proceeds further, it is required to investigate the effects of the
agent on the reproductive system, their potential for carcinogenesis and drug-specific effects such as neurotoxicity and
other adverse effects. Only agents proved to have high efficiency and low toxicity at this stage in animal models are
allowed to proceed further for clinical evaluation.
In the USA, these processes are supervised by the NCI;
however, in Japan there is no national agency having such a
capacity and authority. Pharmaceutical companies are not
interested in developing chemopreventive agents mainly for
economic reasons. For appropriate expansion of research in
this field in Japan, the Ministry of Health, Labor and Welfare
should make great efforts to establish guidelines for researchers.
2. EARLY-PHASE CLINICAL TRIALS
If agents show promising results in the preclinical studies, they
are evaluated in an early-phase clinical trial to determine the
dose-related safety and pharmacokinetics. For those chemopreventive agents which are already used in humans and the
pharmacokinetics of which are well known (such as b-carotene
and vitamin A), this phase I trial is omitted. Also, further phase
I studies may be allowed to be skipped for drugs already
approved for use for other purposes in humans at doses and
durations greater than or equal to those planned for chemopreventive trials.
3. PHASE II TRIALS
Phase II chemopreventive trials are randomized, blinded,
placebo-controlled studies, requiring 50–100 subjects or
patients for each arm for the chemopreventive agent being administered for 6 months to 1 year. A primary objective of these
short-term phase II trials is the identification and validation of
intermediate surrogate biomarkers accurately predicting the
future occurrence of cancer in a relatively short period.
At the NCI, cancers of 10 different organs are currently
being targeted for chemopreventive phase II trials, namely
the colon, prostate, lung, breast, bladder, cervix, oral cavity,
esophagus, liver and skin. The high incidences and mortality
rates of these cancers are well known (19). Two examples of
such trials are (i) tamoxifen and N-(4-hydroxyphenyl)retinamide (4-HPR) for patients scheduled for breast biopsy, assessing the histopathological effects of these two agents, either
singly or in combination, and (ii) trials in China using oltipraz
against aflatoxin B1, (AFB1)-induced liver cancers, evaluating
the modulation of carcinogen–DNA adducts in serum and
urine (2). If we can use an intermediate biomarker such as
cholesterol levels to detect the progression of atherosclerosis as
a surrogate marker for myocardial infarction risk (20), it
would be very helpful to conduct chemopreventive trials. Some
examples of the intermediate biomarkers used in chemopreventive trials are dysplasia and/or intraepithelial neoplasia,
which are used in cervical and prostate cancer. Other chemopreventive trials are in progress using as biomarkers dysplastic
oral leukoplakia, Barret’s oesophagus, colorectal polyps,
superficial papillary bladder cancers, bronchial dysplastic
metaplasia and atrophic gastritis.
4. PHASE III TRIALS
If a promising agent meeting the criteria of high efficacy and
low toxicity is identified, phase III chemopreventive trials may
be conducted. Such trials take a long time, at least 5–10 years
or longer, to complete, because, since cancer incidence is usually taken as the primary endpoint, considerable resources are
needed. The NCI uses review criteria to assess the merits of a
proposed large-scale trial in relation to the extent to which the
trial proposed may reduce the cancer incidence and mortality.
On-going trials and completed trials will be discussed later in
the section for each organ system.
A wide range of findings must be taken into account before a
particular agent is recognized as preventing cancers, summarized in Table 2 according to IARC.
III. CHEMOPREVENTION OF VARIOUS
CANCERS
This section briefly reviews the chemoprevention of cancers in
various organs such as the bladder, prostate, stomach, colorectum, liver, breast and head and neck, concentrating mainly
on human trials. For bladder cancer, as a starting point of the
review, both animal experiments and clinical trials will be
discussed.
Jpn J Clin Oncol 2003;33(9)
Table 2. Outline of data presentation scheme for evaluating chemopreventive agents by IARC
1. Chemical and physical characteristics of the agent
2. Occurrence, production, application, analysis and
human use or exposure
2.1 Occurrence
2.2 Production
2.3 Use and application
2.4 Analysis
2.5 Human use or exposure
3. Metabolism, kinetics and genetic variation
3.1 Human studies
3.2 Experimental studies
4. Preventive effects
4.1 Human studies
4.2 Experimental models
4.2.1 Animal studies
4.2.2 In vitro models
4.3 Mechanisms of chemoprevention
5. Other beneficial effects
6. Carcinogenicity
6.1 Human studies
6.2 Experimental studies
7. Other toxic effects
7.1 Toxic and other adverse effects
7.1.1 Human studies
7.1.2 Experimental studies
7.2 Reproductive and developmental effects
7.2.1 Human studies
7.2.2 Experimental studies
7.3 Genetic and related effects
7.3.1 Human studies
7.3.2 Experimental studies
8. Summary of data
8.1 Chemistry, occurrence and human exposure
8.2 Metabolism and kinetic properties
8.3 Chemopreventive effects
8.3.1 Human studies
8.3.2 Experimental animal studies
8.3.3 Mechanism of action
8.4 Other beneficial effects
8.5 Carcinogenic effects
8.5.1 Human studies
8.5.2 Experimental animal studies
8.6 Toxic effects
8.6.1 Human studies
8.6.2 Experimental studies
9. Recommendations for further research
10. Evaluation
10. 1 Degree of evidence for cancer-preventive activity in
humans and in experimental animals and supporting
evidence
10.1.1 In humans
10.1.2 In experimental animals
10.2 Overall evaluation
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Chemoprevention of cancer
Figure 1. Biphasic recurrence curves estimated by hazard function (29).
1. BLADDER CANCER
Transitional cell carcinoma (TCC) of the bladder is an ideal
candidate as a model of chemoprevention strategies and trials.
About two-thirds of human bladder cancers present as superficial papillary TCC. Their recurrence rates after complete
transurethral resection (TUR) reach 66% at 5 years and 88% at
15 years, while progression to muscle-invasive tumors occurs
in 15% of patients (21,22).
This review concentrates on the oral agents that have been
shown to exhibit effects in decreasing the incidence or recurrence of bladder cancers both in animals and in humans. Intravesicular instillation of chemotherapeutic and immunogeneic
agents is the mainstay of prophylaxis against bladder cancer
but will not be touched upon in this review.
The mechanism behind the high rate of intravesical recurrences has long been debated between two theories: implantation of cancer cells versus multifocal carcinogenesis in field
cancerization (23). Multiple random biopsies during TUR for
bladder cancer or step-section analysis of cystectomized specimens often show a multifocal distribution of carcinoma in situ
(CIS)/dysplasia in the normal-appearing bladder mucosa (24).
These findings support multifocal tumor development in different stages of disease. In fact, similar neoplastic and/or preneoplastic lesions such as CIS and/or dysplasia are detected in
the renal pelvis and ureter in association with renal pelvic and
ureteral cancers. On the other hand, recent molecular analyses
of multifocal bladder cancers or metachronous bladder cancer
after nephroureterectomy for renal pelvic and/or ureteral cancer indicate that multifocal cancers in the bladder are of monoclonal origin. This suggests the possibility of implantation of
original cancer cells in the bladder (25–28). Several reports
have used different molecular analysis techniques such as Xchromosome inactivation (25), analysis of mutation pattern of
p53 (27), loss of heterozygosity (LOH) and microsatellite
shifts (28) to elucidate how multiple bladder cancers arise from
the dispersion of a single transformed cell.
By analyzing the hazard curve of recurrences after TUR,
Akaza et al. (29) demonstrated a biphasic recurrence curve
with an initial peak within 3–6 months and the second peak
between 1.5 and 2.5 years (Fig. 1). The natural course of
Figure 2. Relationship between vitamin A (retinol), retinal and retinoic acid.
bladder carcinogenesis after TUR is probably a mixture of
implantation and field carcinogenesis. In this respect, the
Akaza model coincides with the observations on clonal cells
spread in the bladder and the co-existence of CIS/dysplasia in
the normal-appearing mucosa.
Intravesical recurrence after TUR is a good target for
chemoprevention from various standpoints such as de-differentiation, anti-inflammation, anti-proliferation and anti-angiogenesis. As shown by Kelloff et al. (30), various steps of the
carcinogenesis process can be targeted for chemoprevention.
Laboratory and clinical studies on chemoprevention of bladder
carcinogenesis will therefore be reviewed. Carcinogenesis is
postulated to be a multi-step process including at least initiation and promotion. Cancer chemoprevention aims to stop the
carcinogenic process or to prolong the onset of carcinogenesis
by intervening with efficacious, non-toxic and inexpensive
agents to prevent, suppress or reverse the malignant transformation.
INHIBITION OF FORMATION OF MUTAGENS/CARCINOGENS
Phase II metabolic enzyme inducers such as N-acetyl-Lcysteine, S-allyl-L-cysteine, oltipraz, phanhexyl isothiocyanate
are possible candidates (30). Polyphenols such as ellagic acid
and other substances such as curcumin are candidate agents as
antimutagens/carcinogen blocking agents (30). However, most
of these agents have not been examined in the bladder carcinogenesis model. From the standpoint of in vivo nitrosation
(endogenous formation of volatile nitrosamines in the human
Jpn J Clin Oncol 2003;33(9)
Figure 3. Major retinoic acids being used for chemoprevention.
body), vitamins C and E are interesting as candidate agents to
inhibit nitrosation in the stomach (6). Vitamins C and E have
antioxidant effects and the suppressive effect of high doses of
vitamin C was associated with a 40% reduction of recurrences
(31).
VITAMIN A AND RELATED COMPOUNDS
Fig. 2 shows the relationship between vitamin A (retinol), retinal and retinoic acid. Vitamin A is a fat-soluble micronutrient
and occurs in nature in various forms. Vitamin A (retinol) and
dehydroretinol (retinal) can be derived from carotenoids.
Vitamin A is retinol which is orally taken as a food. Retinol is
converted to retinal by metabolic oxidation and is biologically
essential for light perception in the retina. Retinal is further
transformed into retinoic acid by strictly controlled metabolic
oxidation. The important biological function of vitamin A is
mainly through retinoic acid, which has a close relationship
with important cellular functions such as morphogenesis,
cellular proliferation and cellular differentiation. Fig. 3 shows
the major retinoic acids now being used for chemoprevention
including bladder cancer. All-trans-aromatic retinoid causes
acute promyelocytic leukemia blasts to differentiate into polymorphonuclear leukocytes (32).
A retrospective epidemiological study on 569 bladder cancer
patients and 1025 age-matched controls indicated an increased
risk of bladder cancer associated with lower levels of vitamin
A intake, infrequent milk and carrot intake and infrequent
consumption of cruciferous vegetables (33).
Another cohort study (34) showed that serum carotene and
retinol levels were significantly lower in patients with bladder
cancer than in controls. The effect of 13-cis-retinoic acid was
studied by the National Bladder Cancer Collaborative Group A
in patients with rapidly recurring bladder cancer. However,
because of toxicity, this study was stopped early without show-
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ing any benefits. Etretinate has been shown to have a significant inhibiting effect on the recurrence of superficial papillary
bladder cancers. Studer et al. (35) organized a prospective
randomized double-blind multicenter trial to test the effect of
etretinate or placebo on Ta and T1 papillary bladder cancers.
The time to first recurrence was similar in both groups. However, the mean interval to subsequent recurrence was significantly longer in the etretinate group and the frequency of TUR
per patient-year was significantly reduced. Although toxicity
such as cheilitis and dryness of the mucosa and skin was
tolerable, the long-term safety of etretinate has not been
demonstrated. With regard to the effect of retinoids to prevent
cancer, there are also numerous negative studies that make the
relationship between bladder cancer and vitamin A confusing.
In animal studies, hypervitaminosis A inhibits squamous
metaplasia and squamous carcinoma, while avitaminosis A
promotes bladder carcinogenesis by FANFT in mice (36).
Sporn and co-workers (37,38) reported inhibition by 13-cisretinoic acid of bladder cancer induced by MNU or BHBN.
Murasaki et al. (39) reported significant inhibition by all-transaromatic retinoids of BHBN-induced bladder cancer in rats.
However, all-trans-aromatic retinoids, when used in humans,
showed strong toxicities. Pruritus, conjunctivitis, arthralgia
and cheilosis were frequently observed (unpublished data).
According to mice experiments (40), all-trans-4-hydroxyphenylretinamide (4HPR) appears to be the most active and
least toxic of the retinoids and may be one of the candidate
compounds for future chemopreventive studies. In fact, 4HPR
has been reported to be effective in inhibiting prostate carcinogenesis in rats (41).
VITAMIN B6 (PYRIDOXINE)
Brown et al. (42) reported that large amounts of tryptophan
metabolites such as kynurenine, 3-hydroxykynurenine and 3hydroxyanthranilic acid were excreted in the urine of bladder
cancer patients. When these compounds were instilled into the
bladder of mice, they produced bladder cancer. Pyridoxine has
the capacity to correct this metabolic variation of tryptophan,
which was observed in ~50% of bladder cancer patients. Byar
and Blackard (43) studied 118 superficial bladder cancer
patients randomized to receive a placebo, 25 mg pyrodoxine
p.o. or intravesical instillation of thiotepa. Recurrence rates
were 60% in placebo group, 46% in pyridoxine group and 47%
in thiotepa group, indicating that pyridoxine was significantly
better than placebo and as effective as thiotepa. EORTC (44)
also organized a double-blind randomized phase III trial to test
the effect of pyridoxine on the recurrence of superficial bladder
cancers. This trial accrued 291 patients and demonstrated no
significant difference between 20 mg pyridoxine and placebo
groups in terms of the time to first recurrence and of the recurrence rate. Discrepancies between the two studies may be
explained by the mild chemopreventive effect of pyridoxine,
which requires a large number of patients for long observation
periods to confirm statistical significance.
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VITAMIN C (ASCORBIC ACID)
Vitamin C is the major water-soluble antioxidant and acts as
a free radical scavenger. Vitamin C inhibits malignant transformation in vitro, decreases chromosome damage in lymphocytes
caused by exposures to bleomycin and inhibits in vivo nitrosation in the stomach, resulting in decreased levels of serum
nitrosamines.
There is epidemiological evidence associating increased
intake of fresh fruits and vegetables, i.e. vitamin C along with
many other vitamins and micronutrients, with a reduced incidence of various cancers. For stomach cancer, this association
may be explained by the inhibition of nitrosation in vivo by
vitamin C (6). A similar explanation can be offered for the
reduction in vivo of the formation of carcinogenic N-nitroso
compounds, including 3-hydroxyanthranilic acid and nitrosamines, by vitamin C, which may be relevant to bladder
carcinogenesis (45).
VITAMIN E (a-TOCOPHEROL)
Vitamin E is a lipid-soluble vitamin among which a-tocopherol is the most potent and abundant and acts as an antioxidant
to protect unsaturated lipids in cell membranes and as a free
radical scavenger. Vitamin E has a capacity to inhibit N-nitrosation. N-Nitroso compounds have relevance to various cancers. N-Nitrosamines occur in tobacco smoke and nitrite in
cured meat. Because of these effects, vitamin E is one of the
possible agents for chemoprevention of bladder cancer. Lamm
et al. (31) studied the effect of a megadose vitamin combination in a double-blind randomized fashion.
Sixty-five patients with bladder cancer were randomized to
receive the recommended daily allowance of multiple vitamins
plus 40 000 IU vitamin A, 100 mg vitamin B6, 2000 mg, vitamin C and 400 IU vitamin E. The 5-year estimates of bladder
cancer recurrence were 91% in the recommended daily allowance group and 41% in the megavitamin group (P = 0.0014)
and the overall recurrence was 80 and 40%, respectively (P =
0.0011). Immunopotentiation may also be related to megavitamins, because a statistically significant, long-term increase in
natural killer cell activity was observed compared with baseline. In bladder cancer, a significant protective effect is conferred by a combination of high doses of vitamins A, B6, C and
E (31).
NON-STEROIDAL ANTI-INFLAMMATORY DRUGS (NSAIDS)
NSAIDS such as piroxicam, indomethacin, sulindac and acetylsalicylic acid strongly inhibited bladder carcinogenesis in
rats and mice induced by BHBN (46–48). A prominent biological activity of NSAIDS is inhibition of prostaglandin synthesis, in particular, inhibition of cyclooxygenase (COX) 1 and 2.
However, in humans NSAIDS are associated with toxicities
such as gastroduodenal ulcers, bleeding and sometimes gastric
perforation due to deep ulceration. These toxicities hampered
the long-term use of NSAIDS for chemoprevention. In
prostaglandin synthesis, COX-1 and COX-2, the two COX
isozymes, are involved. COX-1 is constitutively expressed and
contributes to physiological functions in many tissues, whereas
COX-2 is inducible and involved in inflammation and cell
proliferation. Selective COX-2 inhibitors may become more
effective and safer chemopreventive agents than classical
NSAIDS, which preferentially inhibit COX-1. One of the
COX-2 inhibitors, nimesulide, was reported to be a potent
inhibitor of BHBN-induced bladder carcinogenesis in rats
(49). We also confirmed that ~40% of bladder cancer tissues in
humans demonstrated positive histochemical staining of COX2 (unpublished data). Selective COX-2 inhibitors may be a new
type of chemopreventor of bladder cancer.
SELENIUM
A micronutrient, selenium may have a chemoprevention effect
on bladder cancer. Herzlsouer et al. (50) collected serum samples from 25 802 individuals in Washington County, MD, in
1974. In the subsequent 12-year period, 35 cases of bladder
cancer were identified. This cohort study revealed that the level
of selenium was significantly lower among cases than two
matched controls and there was a nearly linear increase in risk
of bladder cancer with decreasing serum selenium level.
DIFLUOROMETHYLORNITHINE (DFMO)
DFMO is an irreversible inhibitor of ornithine decarboxylase
and is involved in the synthesis of polyamines and consequently related to cell proliferation. DFMO has been demonstrated to inhibit BHBN-induced bladder carcinogenesis in
rats. We tested the dose dependence of DFMO and found that
0.2 and 0.1% DFMO significantly inhibited BHBN-induced
bladder carcinogenesis in rats whereas concentrations of 0.03
and 0.01% were ineffective (51). Loprinzi et al. (52) evaluated
the toxicity of DFMO in 76 patients randomized to receive
0.125, 0.25, 0.5 or 1.0 g of DFMO daily and demonstrated
good tolerance for up to 1 year. Combination treatment with
DFMO or 4HPR or both agents, in addition to piroxicam (an
NSAID), did not improve chemopreventive potential in rats.
OTHER POSSIBLE CHEMOPREVENTIVE AGENTS
Bropirimine (53), an interferon inducer taken orally, was
reported to cause regression in carcinoma in situ lesions of
bladder cancer patients. Lactobacillus casei (54) inhibited 26%
of recurrences when taken orally after TUR. Oltipraz, which
was developed as an antischistosomal agent, has been reported
to inhibit chemical carcinogenesis in various organs including
the bladder.
So far, chemoprevention of bladder cancer both in humans
and animals has been reviewed. Hereafter, for each organ
system, only chemoprevention trials conducted in humans will
mainly be reviewed, although there are a large number of
experiments using animals as reviewed here for bladder cancer.
Jpn J Clin Oncol 2003;33(9)
429
Table 3. Promising pharmaceutical and natural product classes and their targets for prostate cancer chemoprevention
[Lieberman (56)]
Antiandrogens (5AR, AR)
finasteride, dutasterlde, bicalutamide
Antioxidant nutrients (ROS, GSTs)
selenium, vitamin E, lycopene, Phytoeneea polyphenols, sulforaphane, curcumin
Arachidonic acid modulators
apoptotic cascades: NFkB, 5, 1 2-HETE. PGE-2
COX-1/COX-2 (ASA), selective COX-2 (ceiecoxib), resveratrol, R-fiurbiprofen, sulindac sulfone, lyprinol
Phytoestrogens. (ER-b, 5AR, AR, RTK)
genistein, daidzein, lignans, PC SPES
Antiestrogens (ER-a/b, aromatase)
toremifene, raloxifene, faslodex, arimidex
Differentiation agents (VDR/RXR, HDAC)
vitamin D analogs, targretin, phenylbutyrate
Gene-based vaccines (P53, Rb, E1 A/PSA, PSA PSMA)
Anti-proliferation agents (ODC, RXR/RAR)
DFMO, polyamine analogs, panretin
Receptor tyrosine kinase modulators
EGFR-Iressa, cKit-STI-571, VEGFR/PDGFR-SU-6668
Ras/falnesyl transferase modulators
FTl-276, SCH 66336, perillyl alcohol
Anti-angiogenesis modulators
angiostatin, endostatin, 2-methoxyestradiol thalidolnide, celecoxib
PPAR modulators
glitazones, DHEA analogs, retinoids, NSAIDs, 15d-PGJ2
IGF-1/IGFBP-3 modulators
tamoxifen, vitamin D, lycopene, 4HPR
Novel growth factor modulators (ET-1, IL-6)
atrasentan, celecoxib, Bowman–Birk inhibitor
Telomerase modulators
vitamin D, zinc, NGF, SERMS
5AR = 5-a-reductase, AR = androgen receptor, ROS = reactive oxygen species, GSTs = glutathione-S transferase, NFkB =
nuclear factor-kB, HETE = hydroxyeicosatetraenoic acid, PGE-2 = prostaglandin E2, COX = cyclooxygenase, ASA = acetylsalicylic acid, ER = estrogen receptor, RTK = receptor tyrosine kinase, PS SPES = prostate cancer botanical containing 8 different
herbal species, VDR = vitamin D receptor, RXR = retinoic acid ´ receptor, HDAC = histone deacetylase, Rb = retinoblastoma,
PSA = prostate specific antigen, PSMA = prostate specific membrane antigen, ODC = ornithine decarboxylase, RAR = retinoic
acid receptor, EGFR = epidermal growth factor receptor, VEGFR = vascular endothelial growth factor receptor, PDGFR = platelet-derived growth factor receptor, PPAR = peroxisome proliferator-activated receptor, DHEA = dehydroepiandrosterone,
NSAIDs = non-steroidal anti-inflammatory drugs, 15d-PGJ2 = 15-deoxyprostaglandin J2, IGF-1 = insulin-like growth factor,
IGFBP-3 = insulin-like growth factor receptor building protein-3, 4APR = N-(4-hydroxyphenyl)retinamide, ET-1 = endothelin1, IL-6 = interleukin-6, NGF = nerve growth factor, SERMs = selective estrogen receptor modulators.
2. PROSTATE CANCER
Pathological processes in the prostate during carcinogenesis
involve the expansion of multiple clones of proliferating ductal/acinal epithelia being recognized as dysplasia. Well recognized precursor lesions in the peripheral zones are a kind of
surrogate for prostate cancer, including high-grade prostatic
intraepithelial neoplasia (HGPIN) that appear to develop years
before the onset of clinical prostate cancer.
Low-grade prostatic intraepithelial neoplasia (LGPIN) and
HGPIN are closely associated with these foci of invasive cancers in the peripheral zones of the prostate. Atypical adenomatous hyperplasia may precede LGPIN and HGPIN. Atypical
small acinal proliferation and proliferative inflammatory atrophy in the peripheral zones are often associated with HGPIN.
Probably in the ductal/acinal cells, atypical adenomatous
hyperplasia, LGPIN, HGPIN and invasive carcinoma are
430
Chemoprevention of cancer
Table 4. Prostate cancer chemoprevention trials supported by the National Cancer Institute [Lieberman (56)]
Agent
Cohort (treatment period)
Sample Endpoints
size
Phase I trials
Lycopene
Pilot: tomato paste/oil in normal subjects: no control 25
(single dose)
Carotenoid pharmacokinetics, safety
Lycopene
Multidose: African American males with elevated
18
PSA and negative biopsy; placebo control (12 weeks)
Pharmacokinetics, safety, drug-effect measurements (markers of
oxidative stress)
Soy isoflavones
Single dose: normal subjects; no control
30
Pharmacokinetics, safety, MTD
Soy isoflavones
Multidose: normal subjects, stage C or D prostatecancer patients; placebo control (3 months)
30
Multidose pharmacokinetics, safety, PSA
Phase II trials
DFMO
Patients with strong family history (brothers and first 100
cousin males) of prostate cancer; placebo control (12
months)
Prostatic polyamines (spermidine:spermine ratio) histopathology, a6integrin, SNPs, in ODC, SSAT
DFMO ± Casodex
Scheduled for prostate cancer surgery or
brachytherapy: observation control (4 weeks) (2 ´ 2
design)
40
Prostatic polyamines, histopathology, nuclear polymorphism, ploidy,
proliferation biomarkers (PCNA, Ki-67), apoptosis
Flutamide
Patients with high-grade PIN; placebo control (12
months)
212
Cancer incidence, PIN grade, nuclear polymorphism, nucleolar size,
ploidy; other endpoints – PCNA, angiogenesis, apoptosis, LOH
chromosome 8, growth factor, PSA
Exisulind
Scheduled for radical prostatectomy; matched
control (4 weeks)
130
Apoptosis, PIN grade and incidence, nuclear polymorphism, nucleolar
size, ploidy; proliferation biomarkers, bcl-2, PTEN
Flutamide ±
toremifene
Scheduled for radical prostatectomy; placebo control 80
(6 weeks) (2 ´ 2 design)
PIN grade and incidence, nuclear polymorphism, nucleolar size, ploidy;
proliferation biomarkers – S-phase fraction, angiogenesis, apoptosis
Selenium ±
vitamin E
Scheduled for radical prostatectomy; placebo control 40
(3–6 weeks) (2 ´ 2 design)
Ki-67, NFkB, COX-2, p53; markers of oxidative stress apoptosis
Selenized yeast
Watchful waiting (early prostate cancer); placebo
control (45 months)
220
PSA velocity, time to progression
Soy isoflavones
(genistein)
Scheduled for prostate cancer surgery: placebo
control (3 weeks)
80
Prostate isoflavone levels; markers of oxidative stress – CX43, Rb, p53,
p2l, bcl-2, bax, cyclin Dl, CDK5, CDK6, EGFR, MIB-1, E-cadherin
Vitamin D analog
(1aD2)
Scheduled for prostate-cancer surgery: observation
control (4–6 weeks)
60
Proliferation (MIB-1), apoptosis (TUNEL), microvessel density,
nuclear morphometry, VEGF, BFGF, TGPb, PSA, PSMA, IGF-1, IGF,
BP-3
Celecoxib
Scheduled for prostate cancer surgery: placebo
control (6–8 weeks)
60
PGE-2 modulation, COX-2 mRNA, histology, oxidized DNA bases,
apoptosis, proliferation, p21, p27, angiogenesis
Soy protein
Men with rising PSA after prostate cancer surgery;
milk protein control (8 months)
130
PSA velocity, time to PSA failure
Soy protein
Prostate cancer patients at high risk for biochemical
failure postsurgery; milk protein control (2 years)
220
PSA velocity, circulating prostate-cancer cells (PSMA – RT/PCR)
Non-melamoma skin cancer low-selenium areas in
USA; placebo control (4.5 years)
1312
PSA levels, biopsy-proven, prostate cancer, long-term follow-up –
colon and lung cancer
Phase III trials
Selenized yeast
Selenized yeast
Elevated PSA, negative Bx placebo control (5 years) 700
PCA incidence, PSA velocity
L-Selenomethionine
High-grade PIN; placebo control (3 years)
466
PCA incidence; intermediate biomarkers – nuclear morphometry, Ki67, apoptosis
PCPT– finasteride
Normal males aged >55, PSA <3 ng/ml; placebo
control (7 years)
18 800
PCA incidence (mandatory exit biopsy), multiple secondary and
tertiary endpoints
PCPT-2 SELECT
selenium ±
vitamin E
Normal males aged >50 AAM; aged >55 CM PSA
<4; placebo control; 2 ´ 2 factorial (7 years);
selenium 200 mg; vitamin E 400 IU
32 400
PCA incidence, multiple secondary and tertiary endpoints
MTD = maximum tolerated dose, DFMO = difluoromethylornithine, SNP = single nucleotide polymorphism, PCNA = proliferating cell nuclear antigen, LOH =
loss of heterozygosity, BFGF = basic fibroblast growth factor, TGFB = transforming growth factor-B, RT = reverse transcriptase, PCR = polymerase chain reaction, SELECT = Selenium and Vitamin E Chemoprevention Trial, PCPT = Prostate Cancer Prevention Trial, TUNEL = terminal deoxynucleotidyl transferase
mediated dUTP biotin nick end labeling, AAM = African American males, CM = Caucasian, PCA = prostate cancer.
Jpn J Clin Oncol 2003;33(9)
scattered sequentially and/or simultaneously in the prostate of
high-risk patients.
Chemoprevention of prostate cancer can be defined as the
administration of natural or synthetic agents that inhibit the
multiple steps mentioned above in the natural history of carcinogenesis occurring in the prostate. The goal of prostate
cancer chemoprevention is to find effective, non-toxic agents
being taken orally that modulate the promotion and progression from normal epithelium to dysplasia to LGPIN and
HGPIN to locally invasive carcinoma and metastatic disease.
This idea is supported by the significant epidemiological difference concerning the higher incidence of clinical prostate
cancer observed in the Western countries than in Asian
countries, although the incidences of occult prostate cancer
are very similar between Western countries and Japan (55).
The pathological processes of prostate cancer development
include a biological spectrum of progressive genetic changes
such as loss of heterozygosity on 8p21 involving NKX3.1 and
chromosome 10, oxidized DNA bases, epigenetic phenomena
such as inactivation of GST-P1 expression and loss of p27 expression and biochemical changes such as elevated PSA and
IGF-1 occurring a long time before prostate cancer develops.
Some of the promising pharmaceutical and natural product
classes and their targets summarized by Lieberman (56) are
shown in Tables 3 and 4. Agents currently in phase II developmental, translational and efficacy trials sponsored by the NCI
include DFMO, DFMO and/or Casodex, toremifene, vitamin D
analog (Hectoral), selective COX-2 inhibitors (Celecoxib),
sulindac sulfone (Exisulinol), genistein-enriched soy isoflavones, soy protein, nutritional products (lycopene, soy isoflavones) and selenized yeast.
In the early 1990s, NCI initiated the first large-scale phase III
trial called the Prostate Cancer Prevention Trial (PCPT) (57),
using the 5a-reductase inhibitor finasteride in over 18 000
healthy men. Definitive results of this trial are expected in
2003. Since PCPT clearly demonstrated the feasibility of largescale trials for prostate cancer prevention, NCI initiated the
second major phase III prostate cancer prevention trial called
SELECT (58), which will test the preventive effects of a-tocopherol and selenium in over 32 000 healthy men at average
risk, asymptomatic men 50 years or older with normal PSA for
prostate cancer. Furthermore, selenium and flutamide are
being evaluated in prevention trials sponsored by the NCI in
high-risk subjects with HGPIN (selenium, flutamide) or
elevated PSA (selenized yeast) (59,60).
In 1998, the effect of supplemental dietary selenium on the
incidence of prostate cancer was tested (61). A total of 974
men with a history of either basal cell or squamous cell carcinoma of the skin were randomized to either a daily supplement
of 200 mg of selenium or a placebo. Patients were treated for a
mean of 4.5 years and followed for a mean of 6.5 years. Selenium treatment was associated with a significant (63%) reduction in the secondary endpoint of prostate cancer incidence
during 1983–93. There were 13 prostate cancer cases in the
selenium-treated group and 35 cases in the placebo group (RR
= 0.37, P = 0.002). Selenium showed no protective effects
431
against the primary endpoint of squamous and basal cell carcinomas of the skin.
In addition, as summarized by Lieberman (56) in Table 4,
many NCI-supported clinical trials in high-risk populations for
prostate cancer (subjects with HGPIN, subjects with elevated
PSA and negative biopsies and subjects scheduled for radical
prostatectomy) are now in progress, testing many natural and
synthetic agents such as antiandrogens, antiestrogens, vitamin
D analogs, COX-2 inhibitors and a variety of antioxidants
(selenium, lycopene, soy isoflavones).
Principles of combination of various agents are (i) using two
or more agents with different mechanisms of action, (ii) using
agents with non-overlapping toxicity and (iii) reducing the
dose of each agent.
Considering the long latency period of prostate carcinogenesis, cohorts for chemoprevention include asymptomatic males
over the age of 55 years (white males) or age 50 years (African
Americans) with normal total PSA, men with borderline
elevations of PSA, men with consistent elevations of PSA and
a negative biopsy, men with a strong family history of early
onset of prostate cancer, men with suspicious histological
changes including atypia and proliferative imflammatory atrophy, men with HGPIN, patients with low volume/low Gleason
grade localized cancer allocated for watchful waiting, patients
at high risk for recurrence following surgery or radiotherapy
and so forth.
Considering the higher incidences of prostate cancer in
Western countries and its sharply increasing trend in some
Asian countries including Japan, a new era of prostate cancer
chemoprevention has begun, although we still have very few
definite results of clinical trials.
One issue that we have to consider seriously is the economic
aspects of clinical trials. To run a phase III trial in EORTC, the
average cost per patient involved is between US$1000 and
2000. One may calculate that for a sample-size of 30 000 men,
US$20–40 million are needed, not including hospital visits,
examinations and biopsies. Economic considerations will be
the largest obstacle in the future (62).
3. GASTRIC CANCER
Epidemiological and experimental studies have elucidated the
risk factors for gastric cancer such as high salt intake, Helicobacter pylori infection, endogenous nitrosamine formation
and, less so, infrequent taking of vegetables and fruits.
Since the discovery of Helicobacter pylori in 1983 (63), the
management of upper gastrointestinal disease has changed
drastically. In the 1990s, various epidemiological and laboratory studies revealed that Helicobacter pylori, a Gram-negative
bacterium commonly detected in the stomach, increased the
risk of gastric cancer in human beings.
Evidence suggesting a link between Helicobacter pylori
infection and risk of gastric cancer has primarily come from
epidemiological and clinical studies. In a study in the UK (64),
29 cases of gastric cancer were identified from two cohort
studies, one of 20 179 men and one of 2512 men. Using nested
432
Chemoprevention of cancer
case-control analysis, the prevalence of Helicobacter pylori
infection was compared between cases and controls selected
among the cohorts. Patients with Helicobacter pylori infection
had a risk of gastric cancer 2.8 times higher than those without
infection. Parsonnet et al. (65) reported an odds ratio of 3.6 from
a nested case-control analysis of 109 patients with gastric cancer identified from a cohort of 188 992 residents of California.
Nomura et al. (66) reported an odds ratio of 6.0 from a nested
case-control analysis of 7498 Japanese men living in Hawaii.
Uemura et al. (67) prospectively studied 1526 Japanese
patients who had duodenal ulcers, gastric ulcers, gastric hyperplasia or nodular dyspepsia at the time of enrollment. Of
these, 1246 had Helicobacter pylori infection and 280 did not.
Patients underwent endoscopy with biopsy at the time of
enrollment and then between 1 and 3 years afterwards. The
mean follow-up was 7.8 years. Gastric cancers were detected
in 36 (2.9%) of the infected and none of the uninfected
patients. Those with histological findings of severe gastric
atrophy, corpus-predominant gastritis or intestinal metaplasia
are at increased risk.
Helicobacter pylori infection is very common, but only a
small proportion of people infected with the bacterium develop
gastric cancer. Such infection is therefore not sufficient, in
itself, to cause gastric cancer. One possible explanation is that
the infection induces predisposing pathological changes in the
gastric mucosa, such as atrophic gastritis and intestinal metaplasia, which in turn increase the risk of gastric cancer.
Saitoh et al. (personal communication) organized a prospective randomized trial covering all Japan to study the chemopreventive effect of Helicobacter pylori eradication by
administering a combination of antibiotics and proton-pump
inhibitor for 1 week. However, since many patients wanted to
be eradicated of the bacteria when they knew that they were
infected with Helicobacter pylori, they were forced to change
the trial design from observing the decreased incidence of
gastric cancer in the Helicobacter pylori eradicated group to
observing the effect of eradication on status of atrophic gastritis or intestinal metaplasia. This trial is still in progress and will
be opened in 2004.
There is strong epidemiological evidence that diets high in
fruits and vegetables are associated with a reduced risk in several cancers, including cancers of the esophagus and stomach.
The people of Linxian County, China, have one of the world’s
highest rates of esophageal/gastric cardia cancer and a persistently low intake of several micronutrients. Individuals living
in four Linxian communes of ages 40–69 years were recruited
in 1985 (68). Mortality and cancer incidence during March
1986 and May 1991 were ascertained for 29 584 people who
received daily vitamin and mineral supplementation throughout this period. The subjects were randomly assigned to intervention groups according to a 24 factorial experimental design:
(A) retinol and zinc, (B) riboflavin and niacin, (C) vitamin C
and molybdenum and (D) b-carotene, vitamin E and selenium.
Doses ranged from one to two times the US Recommended
Daily Allowances. A total of 2127deaths were observed during
the intervention period. Cancer was the leading cause of death,
with 32% of all deaths being due to esophageal or stomach
cancer. Significant lower total mortality occurred among those
receiving supplementation with b-carotene, vitamin E and
selenium (RR = 0.91, P < 0.03). The reduction was mainly due
to lower cancer rates (RR = 0.87), especially gastric cancer.
This was an NIH-sponsored large-scale, population-based
chemopreventive study. However, as already stated, people
living in Linxian County are known to take persistently low
levels of several micronutrients. Therefore, the situation in
Western countries and Japan may be different and it may not be
possible to apply this result to people living in these developed
countries.
(–)-Epigallocatechin gallate (EGCG) is the main constituent
of green tea polyphenols. EGCG and green tea extract in drinking water inhibited carcinogenesis of various organs including
the stomach (69). In 1986, 8552 individuals aged over 40 years
in Saitama Prefecture were surveyed on their living habits,
including daily consumption of green tea. During the 10 years
since 1986, 419 cancer patients were found among participants
in the cohort study. Their daily green tea consumption was
divided into three groups: <3 cups (120 ml/cup), 4–9 cups and
>10 cups per day. Nakachi et al. (70) calculated the average age
at cancer onset of all these patients from clinical charts. Cancer
onset for patients who had consumed >10 cups of green tea per
day was 7.3 years later among females and 3.2 years later
among males compared with patients who had consumed <3
cups per day. Hong initiated the first clinical trials of green tea
extract with humans in the USA, in a population that had not
previously been consuming green tea. In 1997, the US FDA
approved a phase I clinical trial with green tea capsules. Since
then, various trials have been in progress in the USA, and we
have to wait for the results of clinical trials of green tea extract
or EGCG or green tea itself.
We started a randomized controlled trial in order to assess
the effects of diet supplementation with b-carotene and vitamin
C on the development of gastric cancer for high-risk persons
with chronic atrophic gastritis in a district of Akita Prefecture
in north-eastern Japan, one of the regions with the highest mortality from gastric cancer in Japan. However, in response to an
NCI press report (71) released on January 18, 1996, indicating
that two b-carotene trials conducted in Finland and the USA
exhibited no benefit and potential harm owing to increasing the
risk of lung cancer from the supplement, we modified the study
protocol and halted the supplementation of b-carotene (72).
In 2003, we obtained the results of the above-mentioned trial
to examine the effect of vitamin C supplementation on serum
pepsinogen (PG) level, Helicobacter pylori infection and cytotoxin-associated gene A (Cag A) status. Among 635 subjects
diagnosed as having chronic gastritis on the bases of serum
PG levels, after excluding ineligible cases, 439 subjects were
assigned to one of four groups using a 2 ´ 2 factorial design
(0 or 15 mg/day b-carotene and 50 or 500 mg/day vitamin C).
Although b-carotene was discontinued as indicated above,
vitamin C supplementation was continued. Finally, 120 subjects in the low-dose group of vitamin C 50 mg/day and 124
subjects in the high-dose group of vitamin C 500 mg/day
Jpn J Clin Oncol 2003;33(9)
433
Figure 4. Conceptual relationship between hepatocellular carcinoma and HCV/HBV infection.
completed the 5-year supplementation. The difference in the
change of PG I/II ratio between baseline and after a 5-year
follow-up was statistically significant between the intervention
groups among those who completed the supplementation:
–0.25 for the low-dose group and –0.13 for the high-dose
group (P = 0.0046). We concluded that vitamin C supplementation may protect against progression of gastric mucosal
atrophy (73).
4. HEPATOCELLULAR CARCINOMA
Hepatocellular carcinomas (HCC) are common in Asian
countries where hepatitis B virus (HBV) and hepatitis C
virus (HCV) infection prevail. One of the well-known biological characteristics of HCC is the high rate of recurrences in the
remaining liver after complete resection of the primary tumor.
Three years after resection, the incidence of recurrences of
HCC in the remnant liver is as high as 70% (74). It is a good
target of chemoprevention to control this high rate of tumor
recurrences for high-risk patients.
The relationship between HCV and HBV infection and
sequential series of events in hepatocellular carcinogenesis is
illustrated schematically in Fig. 4.
INTERFERON-a (IFN-a)
Hoofnagle et al. (75) reported the efficacy of IFN-a in non-A,
non-B chronic active hepatitis. Since then, several randomized
trials have shown that IFN-a therapy leads to a rapid decrease
Figure 5. Cumulative incidence of hepatocellular carcinoma (77).
in serum alanine aminotransferase (ALT) activity and to a disappearance of serum HCV RNA in about one-third of patients
with chronic hepatitis C (76).
Nishiguchi et al. (77) evaluated the effects of IFN-a in cirrhotic patients with HCV infection because of their high risk
of HCC in a prospective randomized controlled trial. Ninety
patients with compensated chronic active hepatitis C with
cirrhosis were randomly allocated to receive IFN-a, 6 MU,
3´/week for 12–24 weeks, or symptomatic treatment and
then were followed up for 2–7 years. In some patients, ALT
decreased in both the treatment group and control. However,
the mean change in ALT was not significantly different
between the two groups. Hepatitis C viral RNA disappeared in
seven (16%) of the 45 IFN-a-treated patients and in none of the
45 controls. HCC was detected in two (4%) IFN-a patients and
17 (38%) controls, as illustrated in Fig. 5.
These data clearly show that IFN-a improved liver function
in chronic active hepatitis C with cirrhosis and its use was
associated with a decreased incidence of HCC.
The mechanism by which IFN-a reduces the incidence of
HCC is not known. One possibility suggested is activation of
the host immune system.
ADOPTIVE IMMUNOTHERAPY
Between 1992 and 1995, Takayama et al. (78) conducted a
randomized trial on 150 patients who had undergone curative
Figure 6. Time to first recurrence in hepatocellular carcinoma (78).
434
Chemoprevention of cancer
Figure 7. Structure of polyprenoic acid (3,7,11,15-tetramethyl-2,4,6,10,14hexadecapentaenoic acid).
resection for HCC. These patients were assigned for either
adoptive immunotherapy (n = 76) or no adjuvant treatment (n
= 74). Autologous lymphocytes activated in vitro with recombinant interleukin-2 and antibody to CD3 were infused five
times during the first 6 months. Primary endpoints were time to
first recurrence and recurrence-free survival.
Seventy-six patients received 370 (97%) of 380 scheduled
lymphocyte infusions (mean cell number per patient 7.1 ´ 1010;
CD3 and HLA-DR cells 78%). After a median follow-up of
4.4 years, adoptive immunotherapy decreased the frequency of
recurrence by 18% compared with controls [45 (57%) vs 57
(77%) patients] and reduced the risk of recurrence by 41%.
Time to first recurrence in the immunotherapy group was significantly longer than that in the control group (48 vs 33%
at 3 years, 38 vs 22% at 5 years, P = 0.008) (Fig. 6).
The immunotherapy group had significantly longer recurrence-free survival (P = 0.01) and disease-specific survival (P
= 0.04) than the control group. Overall survival did not differ
significantly between the groups (P = 0.09). Thus, adoptive
immunotherapy is a safe, feasible treatment that can lower
recurrence and improve recurrence-free outcomes after surgery
for HCC. However, it may be difficult to define this treatment
as chemoprevention of HCC, because the administration is via
the venous route and the cost of lymphocyte treatment is high.
ACYCLIC RETINOIDS
In 1981, Muto et al. (79) reported that 20-carbon polyprenoic
acid (Fig. 7) binds to cellular retinoic acid-binding protein and
has relatively low toxicity. This agent, called acyclic retinoid,
inhibits chemically induced HCC in rats (80) and spontaneous
HCC in mice and suppresses cell growth and the production
of a-fetoprotein in human HCC-derived cells (81). The same
authors, after a phase I study using acyclic retinoid, conducted
a prospective randomized trial (82) on 89 patients who were
free of disease after complete resection of HCC or the percutaneous injection of ethanol. They randomly assigned the
patients to receive either polyprenoic acid 600 mg/day or placebo for 12 months. They examined the liver by ultrasonography every 3 months after randomization. The primary endpoint
of this study was the appearance of a histologically confirmed
recurrence or new hepatoma.
Figure 8. Kaplan–Meier estimates of the proportion of patients without second
primary hepatomas with and without polyprenoic acid (82).
Treatment with polyprenoic acid significantly reduced the
incidence of recurrent or new hepatomas. After a median follow-up period of 38 months, 12 (27%) in the polyprenoic acid
group had recurrence or new hepatomas compared with 22
(49%) in the control group (P = 0.04). The most striking difference was in the groups that had second primary tumors: 7 in the
polyprenoic acid group vs 20 in the placebo group (P = 0.04)
(Fig. 8). Thus, oral polyprenoic acid prevents the second primary hepatomas after surgical resection of the original tumor
or the percutaneous injection of ethanol.
In this section, chemoprevention of HCC was reviewed only
with respect to the main randomized clinical trials that have
been conducted so far.
5. BREAST CANCER
It was Beatson (83) who revealed the relationship between the
ovary and breast cancers for the first time in 1896, based on
the observation that ovariectomy from premenopausal women
with metastatic breast cancer could cause disease regression
and extend survival in selected cases. Even at the time when
the endocrine role of the ovaries and their production of steroidal hormones were totally unknown, a close relationship was
thus established between an ovarian factor and the growth of
breast cancer.
Research conducted in the 1920s led to the discovery of
estrogens produced by the ovaries and circulating in the blood.
Soon it was established that the breast is one of the target
tissues under their endocrine control. In 1936, Lacassagne
(84) for the first time suggested the concept of chemoprevention of breast cancer: its prevention by developing a therapeutic
antagonist of estrogen action.
To assess the estrogen action, a model using ovariectomized
mice was developed by Allen and Doisy (85). Parallel to this,
research into the synthesis of non-steroidal antiestrogenic
drugs was conducted based on triphenylethylene-structured
compounds, resulting in the development of tamoxifen.
Tamoxifen prevented DMBA-induced or MNU-induced
beast cancers in rats (86–88) and also inhibited spontaneous
mammary carcinogenesis in high-incidence strains of mice
(89).
Jpn J Clin Oncol 2003;33(9)
435
Figure 9. Newer generation selective estrogen receptor modulators (SERMs).
The first clinical observation of tamoxifen’s efficacy as a
chemopreventive agent was a report by Cuzick and Baum (90),
who described a decreased incidence of contralateral breast
cancer in women taking this drug. This observation has been
confirmed by other randomized clinical trials (91). Tamoxifen
has since been extensively tested in clinical trials of adjuvant
therapy for breast cancer for 20 years.
The National Surgical Adjuvant Breast and Bowel Project
(NSABP) P-1 study (92) started in 1992 with a primary endpoint of determining whether or not the use of tamoxifen
reduced the incidence of invasive breast cancer in high-risk
women. A randomized clinical trial was carried out on 13 388
women at increased risk for breast cancer for: (i) 60 years of
age or older, (ii) 35–59 years of age with a 5-year predicted risk
for breast cancer of at least 1.66%, (iii) a history of lobular
carcinoma in situ. They were randomly assigned to receive
20 mg/day tamoxifen (n = 6681) or placebo (n = 6707) for 5
years. A multivariate logistic regression model by Gail et al.
(93) using combinations of risk factors was used to estimate
the risk of occurrence of breast cancer. Tamoxifen reduced the
risk of invasive breast cancer by 49% (two-sided P < 0.00001),
with cumulative incidence through 69 months of follow-up of
22.0 vs 43.4 per 1000 women in the tamoxifen and placebo
groups, respectively. This was a definitive result indicating
that tamoxifen is effective as a chemopreventive agent for
those who are at high risk of breast cancer. However, it was
also found that participants who received tamoxifen had a 2.53
times greater risk of developing an invasive endometrial cancer
than did women who received placebo. The increased risk was
predominantly in women 50 years of age or older. Invasive
cancers at sites other than the breast and endometrium were
equally distributed. Of particular importance were the observations that no liver cancers occurred in either group and there
was no increase in the incidence of colon, rectal, ovarian or
other genitourinary tumors.
The number of participants who had a myocardial infarction
in the tamoxifen and placebo groups was 31 and 28, respec-
tively. Fractures of the hip, radius and spine were also evaluated. A total of 955 women experienced bone fractures, 472
and 483 in the tamoxifen and placebo groups, respectively.
Pulmonary emboli were observed in almost three times as
many women in the tamoxifen group than in the placebo group
(18 vs 6; RR = 3.01).
As a second-generation selective estrogen receptor modulator (SERM), raloxifene (Fig. 9) was developed, which has the
potential for preventing breast cancer while producing the
beneficial effects of estrogen in postmenopausal women such
as decreasing the risk of osteoporosis.
Two clinical trials to test the efficacy of raloxifene for
prevention of osteoporosis identified that it could reduce the
incidence of breast cancer as a beneficial side-effect of the prevention of osteoporosis. The multiple outcomes of raloxifene
evaluation (MORE) randomized 7704 postmenopausal women
of mean age 66.5 years who had osteoporosis but had no history of breast or endometrial cancer to placebo or 60 or 120 mg
raloxifene daily. Results at 4 years showed a 72% reduction in
the risk of breast cancer (94).
The longer-term effect of raloxifene 60 mg/day on reducing
the incidence of breast cancer in postmenopausal women will
be evaluated in the Continuing Outcomes Relevant to Evista
(CORE) trial. This study will follow the incidence of breast
cancer in a subset of the MORE cohort for an additional 4
years.
The results in these trials provide the framework for the
next breast cancer prevention trial, namely the P. 2 Study of
Tamoxifen and Raloxifene (STAR) trial. This trial is comparing the current standard of care with the test drug raloxifene in
postmenopausal women with a high risk of developing breast
cancer (95).
6. HEAD AND NECK CANCER
Head and neck cancers are defined as cancers arising in the
tongue, oral cavity, pharynx, larynx, tonsils, thyroid and upper
436
Chemoprevention of cancer
Table 5. Selected completed chemoprevention trials in oral premalignancy involving retinoids and/or b-carotene and/or a-tocopherol [Bolla et al. (105)]
Investigator(s)
Year
Agent(s)
No. of
patients
Results
Hong et al. (98)
1986
13-CRA at 1–2 mg/kg/day for 3 months versus
placebo
44
67% response (13-CRA) versus 10% response (placebo) (P =
0.0002)
Stich et al. (97)
1988
b-Carotene + retinol (100 000 IU/week)
(180 mg/week) versus placebo
103
27.5% versus 14.8% versus 3.0% (P < 0.001)
Stich et al. (100)
1988
Vitamin A (200 000 IU/week) orally for 6 months
versus placebo
64
57.1% complete remission (vitamin A) versus 30% (controls)
(P < 0.01)
Han et al. (101)
1990
4HPR (40 mg/day) versus placebo
61
87% complete remission versus 17% (P < 0.01)
Lippman et al. (99)
1993
13-CRA (1.5 mg/kg/day) for 3 months followed by
13-CRA (0.5 mg/kg/day) for 9 months versus bcarotene (30 mg/day) for 9 months
70
Initial response 55% to high-dose 13-CRA: continued response
or stable disease 92% in 13-CRA maintenance versus 45% in
b-carotene (P < 0.001)
Chiesa et al. (102)
1993
4HPR (200 mg/day) for 52 weeks versus placebo
153
Failure in 6% (4HPR) versus 30% (placebo) (P < 0.05)
13-CRA = 13-cis-retinoic acid; 4HCR = 4-(hydroxycarbophenyl)retinamide; 4HPR = 4-N-(4-hydroxyphenyl)retinamide.
part of the esophagus. Statistics in 1995 indicate that there
were nearly 900 000 new cases of head and neck cancer worldwide. One of the well-known characteristics of this cancer is a
constant annual 4–7% additional risk of developing potentially
fatal second primary tumors, mostly in foci of smoking-related
carcinogenesis. This phenomenon is consistent with the multistep carcinogenesis model demonstrating that chromosomal
abnormalities occur not only in tumor cells, but also in histologically defined premalignant lesions such as oral leukoplakia
and non-malignant epithelial tissue adjacent to a tumor. In
addition, the concept of field cancerization proposed by
Slaughter et al. (23) in 1953 makes head and neck cancer a
good target for chemoprevention.
Several randomized chemoprevention trials involving retinoids and/or b-carotene are ongoing or have been completed in
the head and neck area which were summarized by Khuri et al.
(96). Retinoids including retinol, retinyl palmitate, all-transretinoic acid (ATRA), 13-cis-retinoic acid (13-CRA), etretinate and 4-N-(hydroxyphenyl)retinamide (4HPR) are the most
frequently and extensively studied agents for head and neck
cancer. Chemoprevention trials using retinoids aim at head and
neck cancer to reverse oral preneoplastic lesions or to prevent
the development of second primary tumors.
Oral preneoplastic lesions such as leukoplakia and erythroplakia have a risk of developing oral cancer from 5% to 30–
40%. As is summarized in Table 5, two placebo-controlled
trials have been conducted for b-carotene in oral leukoplakia.
Stich et al. (97) reported that combined b-carotene plus retinol,
b-carotene alone and placebo exhibited complete response
rates of 27.5, 14.8 and 3.0%, respectively. In 1986, Hong et al.
(98) reported the results of a prospective, randomized, doubleblind clinical trial of high-dose 13-CRA (1–2 mg/kg/day) in
oral leukoplakia. Clinical responses were observed in 16 (67%)
of the 24 patients in the 13-CRA group and in two (10%) of 20
patients in the placebo group (P = 0.002). Reversal of dysplasia
was also higher in the 13-CRA group (54 vs 10%, P = 0.01).
Substantial toxicity of 13-CRA and a high rate of relapse of
>50% within 2–3 months of discontinuing therapy were serious clinical problems.
Concerning the toxicity and relapse problems, Lippman et al.
(99) studied the effect of low-dose 13-CRA. Low-dose (0.5
mg/kg/day) 13-CRA maintenance was well tolerated together
with a 55% response rate by high-dose (1.5 mg/kg/day) induction therapy.
Stich et al. (100) investigated the effect of 200 000 IU/week
of vitamin A orally for 6 months compared with placebo on
tobacco–betel nut chewers in India. Tobacco–betel nut chewers
are notorious for a high rate of well-differentiated oral leukoplakia. CR was observed in 12 (57.1%) of the 21 patients
receiving vitamin A vs one (3%) of the 33 control subjects.
Using 4HPR and 4-(hydroxycarbophenyl)retinamide, Han et
al. (101) and Chiesa et al. (102) conducted a randomized trial,
obtaining a good response as indicated in Table 5.
Concerning the prevention of second primary tumors following treatment of early head and neck squamous cell carcinomas, Hong et al. (103) conducted a randomized, double-blind,
placebo-controlled trial of high-dose 13-CRA as adjuvant
therapy after curative surgery and/or radiation therapy.
A total of 103 patients were randomly assigned to receive
either high-dose 13-CRA (50–100 mg/m2/day) or placebo for
12 months. By 32 months after intervention, the incidence of
second primary tumors was 4% in 13-CRA-treated patients
and 24% in the placebo group (P = 0.005). However, the
toxicity of high-dose 13-CRA was severe and one-third of
the retinoid-treated patients required dose reduction or termination of therapy.
Pastorino et al. (104) used retinyl palmitate 300 000 IU/day
for 12 months to prevent second primary tumors in patients
having a prior history of primary stage I non-small cell lung
cancer. A reduction in the incidence of second primary tumors
and a prolongation of time to development of tobacco-related
second primary tumors were observed in the retinoid arm.
Only 13 second primary tumors were observed in patients in
the retinyl palmitate arm versus 25 in control patients.
Jpn J Clin Oncol 2003;33(9)
437
Table 6. Randomized chemoprevention trials in patients with aerodigestive tumors to prevent second primary tumors involving retinoids and/or b-carotene [Bolla
et al. (105)]
Investigator(s)
Year
Patient
population
No. of
Median follow-up Agent(s)
patients (months)
Hong et al. (103)
1990
HNSCC
103
54
13-CRA (50–100 mg/m2) for 12 months
versus placebo
Pastorino et al. (104)
1993
NSCLC
307
46
Retinyl palmitate (300 000 IU) for 12 months 8.5% SPTs versus 18.8% SPTs
versus placebo
(P = 0.05)
Bolla et al. (105)
1994
HNSCC
316
41
Etretinate (50 mg/day) for 1 month followed No difference (25% in both groups)
by 25 mg/day for 24 months versus placebo
Bolla et al. (105) administered etretinate 50 mg/day for 1
month, followed by 25 mg/day for 24 months or placebo to
patients with prior squamous cell carcinomas of the oral cavity
or oropharynx. By 41 months, there was no second primary
tumor reduction in the retinoid arm (Table 6).
To resolve the conflicting results of the two previous second
primary tumors trial, the University of Texas MD Anderson
Cancer Center launched in 1992 the largest head and neck cancer chemoprevention trial with a plan to recruit 1302 patients
with stage I or II head and neck squamous cell carcinomas
treated with surgery or radiation therapy. Patients were randomly assigned to receive either 30 mg/day of 13-CRA for 3
years or a placebo. Regarding toxicity of 13-CRA, a lower dose
than in the trial of Hong et al. (103) mentioned above was
relatively well tolerated.
Head and neck cancers are a good target for chemoprevention regarding inhibition of second primary tumors using vitamin A analogues and retinoids. Less toxic agents are expected
to be developed in the future that can be used for a longer time.
7. COLORECTAL CANCER
Fearon and Vogelstein (106) proposed a genetic model in
which colon cancer develops through the acquisition of a
number of successive genetic alterations in an orderly fashion.
This model is composed of the combination of mutational
activation of oncogenes and mutational inactivation of tumorsuppressor genes.
The initial molecular event is silencing of the adenomatous
polyposis coli (APC) gene. Mutations of the ras oncogene have
been well documented in colorectal cancer patients at the early
and intermediate adenoma stages. The genetic abnormality frequently succeeding this event in colorectal cancer formation is
the loss of 18q21. This chromosomal abnormality is usually
seen at the later stages of adenoma formation. Two tumor-suppressor genes have been mapped to this region, called deleted
in colorectal carcinogenesis (DCC) gene. Other molecular
events observed during the late stage of carcinogenesis include
17p abnormalities associated with loss of p53 function and
other mutational events. Those changes result in enhanced
genomic instability associated with a replication error positive
(RER+) phenotype. Genes associated with DNA repair include
MSH2, MLH1, PMS2, PMS1 and MSH6. Targets of chemo-
Results
14% SPTs versus 31% (P = 0.005)
prevention for colorectal cancer can be considered based on
this multistep carcinogenesis theory.
There are two enzymes, COX-1 and COX-2, as the cyclooxygenases (prostaglandin H synthase) which catalyze both
oxidative and reductive reactions in the prostaglandin synthesis
pathway. COX-1, which is a constitutive enzyme, is detectable
in the human colon of all normal subjects, whereas for COX-2,
which is an inducible enzyme, its gene expression is increased
in human colon carcinomas. It is postulated that increased
COX-2 expression plays a major role in over-expression of
prostaglandins in malignant tissues.
To determine whether the regular use of aspirin, which is an
inhibitor of COX-1 and COX-2, decreases the risk of colorectal
cancer, a prospective cohort study (107) was conducted with
47 900 male health professionals, 40–75 years of age, throughout the USA who responded to a mailed questionnaire in 1986.
The questionnaires aimed to assess the use of aspirin and other
variables including occurrence of cancer in 1986, 1988 and
1990; 251 new patients were diagnosed with colorectal cancer
during the study period. Regular users of aspirin (twice or more
per week) in 1986 had a lower risk of total colorectal cancer
(RR = 0.68) and advanced (metastatic and fatal) colorectal
cancer (RR = 0.51). These results support the contention that
regular use of aspirin is associated with a decreased risk for
colorectal cancer. Multiple population-based case-controlled
studies have been conducted and aspirin repeatedly exhibited a
50% reduction in risk of occurrence of colorectal cancer (108).
Related to prostaglandin synthesis, many human studies
using various NSAIDs have been conducted. Patients with
familial adenomatous polyposis, who are at extremely highrisk for colon cancer, are a good target for chemoprevention.
Three of these studies were small, randomized clinical trials
using sulindac as a chemopreventive agent. Combining these
trials, 45 FAP patients were treated in these studies and
administration of sulindac resulted in a statistically significant decrease in the number of polyps (109,110). A COX-2
inhibitor, celecoxib, was tested at 100 mg twice daily and
400 mg twice daily for 6 months (111). Treatment with
100 mg resulted in a 12% decrease in the number of polyps.
Celecoxib at 400 mg reduced the number of polyps by 28%
from the baseline, which was statistically significant. The
major issue regarding the beneficial effects of NSAIDs and
COX-2 inhibitors on polyp formation is that these effects are
transient and disappear soon after drug withdrawal.
438
Chemoprevention of cancer
Table 7. Agents under study or with developmental potential for colorectal cancer chemoprevention
Class
Sample agents
Proposed mechanism of action
Non-selective NSAIDs
Aspirin, sulindac, piroxicam
Inhibition of COX enzymes, suppression of prostaglandin synthesis (PGE2), modulation of
LOX, non-COX-related mechanisms, pro-apoptitic mechanisms
Selective NSAIDs (COX-2
inhibitors)
Celecoxib, rofecoxib
Inhibition of prostanoid metabolism through selective inhibition of COX-2
Micronutrients
Calcium and vitamin D
Binding of bile and fatty acids, induction of cellular differentiation
Vitamin C
Blocking agent (neutralization of carcinogens), antioxidant
Vitamin E
Antioxidant
Organoseleniums (e.g. pmethoxybenzyl selenocyanide)
Inhibition of carcinogen activation, decrease in DNA binding of carcinogens, antioxidative
properties, others
Monoterpenoids (e.g. D-limonene, Ras inhibition
perillyl alcohol)
Curcuminoids
COX inhibition, LOX modulation, antioxidation, anti-angiogenesis, modulation of NFkB
Folate
DNA synthesis, DNA methylation
Polyamine inhibitors
Difluoromethylornithine (DFMO) Affects the production of polyamine through the inhibition of ornithine decarboxylase
Hormones
Estrogens
Complex dietary interventions Fiber (especially wheat bran)
Berries (black raspherries)
Decrease production of secondary bile acids, inhibition of insulin-like growth factors
Carcinogen binding, bile-acid binding, promotion of stool bulk, decrease conversion of
primary into secondary bile acids
Multi-mechanism
Table 7 shows the agents under study for chemoprevention
of colorectal tumors including non-selective NSAIDs and
selective NSAIDs (COX-2 inhibitors).
Epidemiological data regarding the association between calcium ingestion and colorectal cancer risk are not yet solid. A
number of case-control and cohort studies have suggested that
increased intake of calcium and vitamin D may have some
protective effect. However, other studies have not clearly
supported this association (112). Calcium may bind with bile
and fatty acids in the intestinal lumen, decreasing exposure
of the large bowel epithelium to these substances.
Curcumin has strong antiproliferative effects established on
human colon cancer cell models (113). Preclinical and early
clinical pharmacological data suggest that curcumin is poorly
absorbed through the gastrointestinal tract, making it an attractive candidate for chemoprevention of gastrointestinal cancer
(114).
Hormone-replacement therapy in postmenopausal women
and its effects in colorectal cancer have been evaluated in many
studies. Postmenopausal use of hormones was associated with
a significant decrease in colorectal cancer mortality in the
Cancer Prevention Study II with an overall RR of 0.71 (115).
a-Difluoromethylornithine (DFMO) is the main class of
inhibitors of ornithine decarboxylase (ODC) activity. A shortterm phase IIa trial by Meyskens et al. (116) demonstrated that
DFMO at 0.10 g/m2/day was able to inhibit polyamine production in rectal mucosa after 1 month. They concluded that a dose
of 0.50 g/m2/day of DFMO was safe and effective and they
therefore recommended this dose level of administration for
use in combination phase IIb or single-agent phase III chemoprevention trials (117).
8. LUNG CANCER
Lung cancer is the number one cause of death due to cancer in
the USA, exceeding breast, prostate and colon cancer deaths
combined. Known risk factors for the development of lung
cancer include smoking cigarettes, asbestos exposure and radon
exposure. Five RCTs of chemoprevention have been reported
(118) for subjects with one or more of these risk factors.
The ATBC trial (119) examined the effect of a-tocopherol
and b-carotene on the incidence of lung cancer in 29 133 Finnish male smokers using a 2 ´ 2 factorial design. The selection
of a-tocopherol and b-carotene was based on epidemiological
studies linking a vegetable-rich diet with a decrease in the risk
of lung cancer.
Subjects recruited between 1985 and 1993 took supplements
of a-tocopherol 50 mg/day and b-carotene 20 mg/day for 5–8
years (mean 6.1 years). No effect of a-tocopherol on lung cancer incidence was observed. However, the RR of lung cancer
incidence in the b-carotene arms was 1.18 (95% CI, 1.03 to
1.36). That means b-carotene increased the risk of lung cancer
18%.
The CARET study (120) (The Carotene and Retinol Efficacy
Trial) was a large two-arm study that was designed to compare
the effects of a combination of b-carotene and retinol with
those of placebo on lung cancer incidence in subjects who were
at high risk for the development of lung cancer. This study was
based on the epidemiological observation indicating decreased
RR of lung cancer in subjects showing high dietary intake and
serum levels of b-carotene and vitamin A. A pilot study (121)
of 1029 high-risk subjects demonstrated the safety and tolerability of b-carotene 50 mg/day, retinol 25 000 IU/day and the
combination of the two. The CARET study enrolled a total of
18 314 subjects, including current and former smokers of both
Jpn J Clin Oncol 2003;33(9)
genders and male asbestos workers. The mean length of follow-up was 4.0 years. A planned interim analysis demonstrated
a statistically significant increased RR for the development of
lung cancer (RR 1.28; 95% CI, 1.07 to 2.00), resulting in early
termination of the active intervention arm. It has been suggested that higher concentrations of b-carotene resulting from
supplementation may have pro-oxidant effects, not antioxidant
effects, inducing DNA damage and membrane instability
(122).
The Physicians’ Health Study (123) was a randomized,
double-blind, placebo-controlled trial of aspirin, 325 mg every
other day and b-carotene, 50 mg every other day, conducted
using a 2 ´ 2 factorial design among 22 071 male physicians in
the USA. The study began in 1982 and the aspirin arms were
terminated in 1988 when a significant reduction in first myocardial infarction incidence was noted. An analysis of the
cancer incidence in patients in the b-carotene arms at the end
of the study in 1995 showed no significant difference in the
incidence of lung cancer.
The Women’s Health Study (124) recruited nearly 40 000
women for RCT who were at least 45 years of age in a 2 ´ 2 ´
2 factorial design to assess the effect of b-carotene, aspirin and
vitamin E on incidences of cancer and cardiovascular disease.
Following the publication of other studies, the b-carotene arm
was closed. Supplementation of b-carotene was continued for
2.1 years and additionally followed up for 2.0 years (median
total follow-up, 4.1 years). There was no difference in cancer
incidence or death regarding b-carotene supplementation.
Thirty cases of lung cancer were observed in the b-carotene
arms and 21 cases in the placebo arms (RR 1.42; 95% CI, 0.88
to 2.49), a result that is not statistically significant.
An Australian study (125) of 1024 asbestos workers randomly assigned subjects in equal proportions to either b-carotene 30 mg/day or retinol 25 000 IU/day from 1990 to 1995;
21% of subjects were current smokers and 52% were former
smokers. After a median intervention and follow-up period of
4.5 years, no difference in lung cancer incidence was found
(RR 0.66; 95% CI, 0.19 to 2.32). The incidence of malignant
mesothelioma was statistically significantly lower in the retinol
group than the b-carotene group (RR 0.24; 95% CI, 0.07 to
0.86).
From the disappointing experiences of the ATBC Study and
CARET, in the next generation of chemoprevention trials,
systematic application of DNA microarray and proteomics
methods will be considered to investigate influences of genetic
polymorphisms, validated surrogate endpoints, epigenetic
relationships and so forth. We have at least to bear in mind the
possibility of hazardous effects of chemopreventive agents.
439
serially as phase I, II and III studies need enormous amounts of
money. For example, a prostate cancer prevention trial using
finasteride (56) required almost 8 billion yen. As mentioned
earlier regarding the EORTC experience (62), the average cost
per patient involved in a phase III study was between US$1000
and 2000, indicating that for a sample-size of 30 000 men,
US$20–40 million are needed. (ii) Pharmaceutical companies
are not interested in developing new agents for chemoprevention because of the high R & D costs and low profit after clinical trials. (iii) The target population for chemoprevention is
healthy volunteers or a high-risk group of people for specific
cancers. This is a serious difference from clinical trials with
new chemotherapeutic agents, where patients are actually suffering from cancer and these new agents are applied as a step
of treatment. Consequently, it may not be easy to conduct a
large-scale study, particularly for a country such as Japan. The
USA is a rich, powerful country which can afford large grants
for large-scale clinical trials for chemoprevention. If we consider a clinical trial in Japan, I think, it is necessary to concentrate on extremely high-risk groups of people. Using small
numbers of participants, well-designed clinical trials can prove
the effectiveness of some agents, as was described for hepatocellular carcinoma in this review. In this respect, we have to
focus our attention on the cancers common in Japan such as
gastric cancer and liver cancer. The USA and Europe will
concentrate on large-scale chemoprevention studies on prostate cancer, breast cancer, colorectal cancer and so forth. Such
types of allocation of targets may be necessary.
In the near future, we will be able to use genetic information
on individuals regarding their cancer susceptibility on the condition that their privacy is strictly protected. In such a setting, a
chemoprevention study for a high-risk group may become
more sharply focused.
For further development of chemoprevention studies in
Japan, the amounts of grants from the government should be
expanded, infrastructures such as data management centers
should be more strengthened and the concept of preventive
coverage should be introduced to the health insurance system.
In planning future clinical trials, we have to bear in mind
the lessons we have learned already, such as the unexpected
results of the b-carotene studies (120,121). We should also be
reminded that it is necessary to evaluate multi-organ systems
for the effectiveness of any agent because such an agent may
exhibit inhibition of cancer in one organ but may promote
cancer in others. We need more basic research on chemoprevention from the viewpoint of biological mechanisms.
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