Mechanistic considerations in chemopreventive drug development

Studies on biomarkers in cancer etiology and prevention: a summary and challenge of 20 years of interdisciplinary research

Sensitive, specific methods have been developed that allow quantitative measurements of the metabolites of carcinogen metabolites and of DNA and protein adducts in humans exposed occupationally, environmentally and endogenously to genotoxic agents. The interrelationship between exposure to carcinogens, host risk factors and the responses of biomarkers has been examined in cross-sectional, ecological and case-control studies which provided new insights into the causes of cancer and the mechanisms of carcinogenesis. The identification of hitherto unknown DNA-reactive chemicals formed in the human body from dietary precursors and of carcinogenic components of complex mixtures has increased the possibility of establishing causal relationships in etiology. The identification of individuals and subgroups heavily exposed to carcinogens has led to the development of measures for avoiding or decreasing exposure to carcinogenic risk factors. New, ultrasensitive methods for measuring DNA adducts allow the quantification and structural elucidation of specific DNA damage in humans arising from oxidative stress and lipid peroxidation (LPO), which have been found to be the driving forces in several human malignancies. Background DNA damage in "unexposed" individuals has been shown unequivocally to be due to LPO products, and a significant interindividual variation in adduct levels has been shown in individuals with comparable exposure to carcinogens. Thus, pharmacogenetic variants with higher susceptibility to carcinogenic insults, due to genetic polymorphism in xenobiotic-metabolizing enzymes, have been characterized by a combination of genotyping and measurements of macromolecular adducts. Dosimetry has been used in human studies to evaluate the efficacy of interventions with chemopreventive agents like ascorbic acid, dietary phenols and green tea. Advances in the application of selected biomarkers in human studies are reviewed and illustrated by examples from the author's research conducted during the past two decades.

Progress in cancer chemoprevention

More than 40 promising agents and agent combinations are being evaluated clinically as chemopreventive drugs for major cancer targets. A few have been in vanguard, large-scale intervention trials--for example, the studies of tamoxifen and fenretinide in breast, 13-cis-retinoic acid in head and neck, vitamin E and selenium in prostate, and calcium in colon. These and other agents are currently in phase II chemoprevention trials to establish the scope of their chemopreventive efficacy and to develop intermediate biomarkers as surrogate end points for cancer incidence in future studies. In this group are fenretinide, 2-difluoromethylornithine, and oltipraz. Nonsteroidal anti-inflammatories (NSAID) are also in this group because of their colon cancer chemopreventive effects in clinical intervention, epidemiological, and animal studies. New agents are continually considered for development as chemopreventive drugs. Preventive strategies with antiandrogens are evolving for prostate cancer. Anti-inflammatories that selectively inhibit inducible cyclooxygenase (COX)-2 are being investigated in colon as alternatives to the NSAID, which inhibit both COX-1 and COX-2 and derive their toxicity from COX-1 inhibition. Newer retinoids with reduced toxicity, increased efficacy, or both (e.g., 9-cis-retinoic acid) are being investigated. Promising chemopreventive drugs are also being developed from dietary substances (e.g., green and black tea polyphenols, soy isoflavones, curcumin, phenethyl isothiocyanate, sulforaphane, lycopene, indole-3-carbinol, perillyl alcohol). Basic and translational research necessary to progress in chemopreventive agent development includes, for example, (1) molecular and genomic biomarkers that can be used for risk assessment and as surrogate end points in clinical studies, (2) animal carcinogenesis models that mimic human disease (including transgenic and gene knockout mice), and (3) novel agent treatment regimens (e.g., local delivery to cancer targets, agent combinations, and pharmacodynamically guided dosing).

Chemoprevention: general perspective

Chemoprevention is the use of natural or synthetic compounds to block, reverse, or prevent the development of invasive cancers. Cellular carcinogenesis forms the biologic basis for the identification of chemopreventives, assessment of their activity, and ultimately the success or failure of a chemopreventive. Chemopreventive agents undergo multistep evaluations to assess efficacy that are similar in concept but vastly different in practice to standard ablative oncologic therapeutics. In vitro assessments of potential anticarcinogenesis efficacy include measurements of an agent's antioxidant activity, induction of phase II metabolizing enzymes and effects upon cellular proliferation and apoptotic control pathways. In vivo efficacy is assessed primarily in rodent models of carcinogenesis that are specific for a given organ target. The role of genetically modified animal models in the in vivo assessment of chemoprevention agents remains unclear. Clinical assessment of chemopreventive agent efficacy consists of a multistep process of identification of an optimal chemopreventive agent (phase 1), demonstration of efficacy in humans through the modulation of reversal of a tissue, biochemical, and molecular surrogates for neoplastic transformation and invasion (phase 2) and cancer risk reduction in large cohort trials (phase 3). Opportunities and future needs include the development of reliable, predictive in vivo models of carcinogenesis, careful exploration of the preventive pharmacology of therapeutic agents being used for non-cancer prevention indications, and the incorporation of genetic risk cohorts to define cancer chemopreventive efficacy.

Progress in cancer chemoprevention: development of diet-derived chemopreventive agents

Because of their safety and the fact that they are not perceived as "medicine," food-derived products are highly interesting for development as chemopreventive agents that may find widespread, long-term use in populations at normal risk. Numerous diet-derived agents are included among the >40 promising agents and agent combinations that are being evaluated clinically as chemopreventive agents for major cancer targets including breast, prostate, colon and lung. Examples include green and black tea polyphenols, soy isoflavones, Bowman-Birk soy protease inhibitor, curcumin, phenethyl isothiocyanate, sulforaphane, lycopene, indole-3-carbinol, perillyl alcohol, vitamin D, vitamin E, selenium and calcium. Many food-derived agents are extracts, containing multiple compounds or classes of compounds. For developing such agents, the National Cancer Institute (NCI) has advocated codevelopment of a single or a few putative active compounds that are contained in the food-derived agent. The active compounds provide mechanistic and pharmacologic data that may be used to characterize the chemopreventive potential of the extract, and these compounds may find use as chemopreventives in higher risk subjects (patients with precancers or previous cancers). Other critical aspects to developing the food-derived products are careful analysis and definition of the extract to ensure reproducibility (e.g., growth conditions, chromatographic characteristics or composition), and basic science studies to confirm epidemiologic findings associating the food product with cancer prevention.

Cancer chemoprevention: progress and promise

Cancer chemoprevention is the use of agents to inhibit, delay or reverse carcinogenesis. The focus of chemoprevention research in the next millennium will include defining the genotypic and phenotypic (functional and histological) changes during carcinogenesis, the cancer risk conferred by these changes, their modulation in preclinical experimentation and randomised clinical trials by chemopreventive drugs, dietary agents and regimens and treatments resulting from early detection. The key elements of this research effort will be basic and translational risk evaluation programmes; chemopreventive and dietary agent drug discovery and development; development of transgenic animal models; required safety and pharmacology studies; well-designed phase I, II and III chemoprevention studies; and much expanded early detection programmes. The large number of chemoprevention research programmes now ongoing ensures that the promise of chemoprevention will continue to be realised in the next decade.

Cancer chemoprevention: progress and promise

Cancer chemoprevention is the use of agents to inhibit, delay or reverse carcinogenesis. The focus of chemoprevention research in the next millennium will include defining the genotypic and phenotypic (functional and histological) changes during carcinogenesis, the cancer risk conferred by these changes, their modulation in preclinical experimentation and randomised clinical trials by chemopreventive drugs, dietary agents and regimens and treatments resulting from early detection. The key elements of this research effort will be basic and translational risk evaluation programmes; chemopreventive and dietary agent drug discovery and development; development of transgenic animal models; required safety and pharmacology studies; well-designed phase I, II and III chemoprevention studies; and much expanded early detection programmes. The large number of chemoprevention research programmes now ongoing ensures that the promise of chemoprevention will continue to be realised in the next decade.

Space radiation concerns for manned exploration

Spaceflight exposes astronaut crews to natural ionizing radiation. To date, exposures in manned spaceflight have been well below the career limits recommended to NASA by the National Council of Radiation Protection and Measurements (NCRP). This will not be the case for long-duration exploratory class missions. Additionally. International Space Station (ISS) crews will receive higher doses than earlier flight crews. Uncertainties in our understanding of long-term bioeffects, as well as updated analyses of the Hiroshima. Nagasaki and Chernobyl tumorigenesis data, have prompted the NCRP to recommend further reductions by 30-50% for career dose limit guidelines. Intelligent spacecraft design and material selection can provide a shielding strategy capable of maintaining crew exposures within recommended guidelines. Current studies on newer radioprotectant compounds may find combinations of agents which further diminish the risk of radiation-induced bioeffects to the crew.

Preclinical drug development paradigms for chemopreventives

Preclinical screening studies and animal efficacy testing models currently are used by the National Cancer Institute's chemoprevention drug discovery program to assess and identify chemical agents and natural products that may have the potential to prevent human cancer. Identification of potential cancer preventing agents begins by subjecting each compound to a sequential series of short-term, in vitro prescreens of mechanistic, biochemical assays to provide quantitative data to help establish an early indication of chemopreventive efficacy and to assist in prioritizing agents for further evaluation in longer-term, in vitro transformation bioassays and whole animal models. Promising chemical agents or combinations of agents that work through different inhibitory mechanisms subsequently are tested in well-established, chemically induced, animal tumor models, which include models of the lung, bladder, mammaries, prostate, and skin. These preclinical bioassays afford a strategic framework for evaluating agents according to defined criteria, and not only provide evidence of agent efficacy, but also serve to generate valuable dose-response, toxicity, and pharmacokinetic data required prior to phase I clinical safety testing. Based on preclinical efficacy and toxicity screening studies, only the most successful agents considered to have potential as human chemopreventives progress into clinical chemoprevention trials.

Clinical models of chemoprevention for colon cancer

Colon cancer is a common malignancy in the westernized world and is incurable in its advanced stages. This article summarizes the currently available information on colorectal cancer chemoprevention. A brief outline of the incidence and etiologic factors is followed by a discussion of the evidence on which chemopreventive strategies for colon cancer are modeled. This includes a description of the development of surrogate endpoint biomarkers and experimental models to study colorectal cancer chemopreventives, a review of the promising colorectal cancer chemopreventives, and a discussion of the issues to be addressed in the design of future chemoprevention trials. The article concludes with an emphasis on the development and validation of biomarkers and selection of high-risk cohorts using genetic and epidemiologic tools as the main goals of future colon cancer chemoprevention trials before large-scale, risk-reduction trials are conducted.

Models for early chemoprevention trials in breast cancer

Several models are being explored for use in the phase I and phase II evaluation of breast cancer chemoprevention agents. The short-term DCIS/small invasive cancer model is probably best used in late phase I trials in conjunction with agents likely to have activity in the progression phase of neoplastic development in addition to activity in earlier phases. The core biopsy or FNA hyperplasia models may be best used with drugs that are likely to have activity primarily in the promotion phase of neoplastic development and that are suitable for longer duration trials lasting several months to years. Morphology currently is the key surrogate endpoint biomarker for assessing efficacy in phase II trials. Other biomarkers that may undergo modulation will have to be validated, in that modulation will have to be shown to be directly related to decreased cancer risk in subsequent phase III trials. Only then can they be considered as validated surrogate endpoint biomarkers and used as stand-alone efficacy markers in phase II trials. Despite accrual challenges and technologic hurdles, interest in phase I and phase II chemoprevention trials is high.

High grade prostatic intraepithelial neoplasia in military working dogs with and without prostate cancer

BACKGROUND

High grade prostatic intraepithelial neoplasia (PIN) is the most likely precursor of human prostate cancer and is commonly found in men undergoing prostatic needle biopsy for suspected cancer. Recent work has demonstrated that pet dogs, like humans, develop PIN spontaneously and in association with prostate cancer. Pet dogs are the most domesticated animal, sharing the habitat and oftentimes the diet of their owners. If PIN and prostate cancer are strongly related to environmental factors, then the prevalence of these findings might differ in a population of dogs such as military working dogs which is not exposed to the habitat and diet of humans. In this study, we determined the prevalence of PIN in prostates of aged military working dogs with and without prostatic adenocarcinoma.

METHODS

Cases were selected from the military working dog slide and tissue archive at the Armed Forces Institute of Pathology, Washington, DC. The most recent 329 necropsies (1991 to 1996) were examined histologically by multiple reviewers; of these, 199 dogs (60%) were found to have evaluable prostatic tissue. In addition, the most recent 50 necropsies (1958 to 1996) with the diagnosis of prostatic cancer were examined, of which 25 cases (50%) were found to have evaluable prostatic adenocarcinoma. In most cases, a single large transverse section of prostatic tissue was available for review. Medical records for each dog were reviewed independently, and age, clinical history, indications for euthanasia, and other health problems were recorded.

RESULTS

High grade PIN was identified in 3% of dogs (6 of 199 dogs) without prostate cancer. A total of 50.8% of dogs in this study group (101 of 199 dogs) were known to be sexually intact, 26.7% of dogs (53 of 199 dogs) were castrated, and the status of the remaining 22.6% of dogs (45 of 199 dogs) was unknown. High grade PIN was present in 18 of 25 dogs (72%) with prostatic adenocarcinoma. Of these cases, 11 dogs (44%) were castrated, 4 dogs (16%) were intact, and the status of 10 dogs (40%) dogs was unknown. Gleason scores ranged from 6 to 10, with a mean of 8.4 and a median of 8.

CONCLUSIONS

High grade PIN is present in a small but substantial number (3%) of military working dogs. Of military working dogs with prostatic adenocarcinoma, 72% had high grade PIN. The true prevalence in each of these cohorts is likely to be higher given the sampling variation inherent in evaluating a single random histologic section. Aged male dogs seem to have substantial clinical utility as an animal model for prostatic carcinogenesis. We recommend that serial sectioning and total embedding of the prostate should be used to more thoroughly characterize premalignant and malignant diseases in aged military working dogs. This method will provide important data to determine whether a model of spontaneous PIN in elderly dogs may have clinical utility in developing strategies directed toward preventing and treating prostate.

Correlation between plasma transforming growth factor-beta 1 and second primary breast cancer in a chemoprevention trial

The relationship between plasma transforming growth factor-beta 1 (TGF-beta 1) and second primary breast cancer was explored in a prevention trial of the synthetic retinoid fenretinide (N-(4-hydroxyphenyl)retinamide; 4-HPR). Plasma concentrations of TGF-beta 1 were measured by radioimmunoassay in plasma obtained at randomisation and after approximately 1 year of intervention in 28 women treated with 4-HPR and 27 untreated controls with stage I breast cancer. Baseline and 1 year growth factor concentrations were not significantly different between treated and control groups. After a median follow-up of 65 months, an increase in TGF-beta 1 over 1 year was the only significant, independent predictor of a shorter survival free from secondary primary breast cancer. Moreover, the change in TGF-beta 1 levels had a tendency to influence the treatment effect on second breast cancer incidence. Our data suggest that the role of plasma TGF-beta 1 as a surrogate endpoint of breast carcinogenesis should be assessed further.

Unusual central nervous system toxicity in a phase I study of N1N11 diethylnorspermine in patients with advanced malignancy

The objectives of this study were to determine the dose limiting toxicity (DLT) and other major toxicities, the maximum tolerated dose (MTD) and the human pharmacokinetics of N1N11 diethylnorspermine (DENSPM), a new polyamine analog which in experimental systems inhibits the biosynthesis of intracellular polyamines and promotes their degradation by inducing the enzyme spermine/spermidine N-acetyl transferase. These objectives were incompletely achieved because of the occurrence of an unusual syndrome of acute central nervous system toxicity which forms the basis of the present report. Fifteen patients with advanced solid tumors were entered into a phase I study of DENSPM given by a 1 h i.v. infusion every 12 h for 5 days (10 doses). The starting dose was 25 mg/m2/day (12.5 mg/m2/dose) with escalation by a modified Fibonacci search. Doses of 25 and 50 mg/m2/day were tolerated with only minor side effects of facial flushing, nausea, headache and dizziness (all grade I). At doses of 83 and 125 mg/m2/day, a symptom complex of headache, nausea and vomiting, unilateral weakness, dysphagia, dysarthria, numbness, paresthesias, and ataxia, was seen in 3 patients, one after 2 courses of 83 and 2 after 1 course of 125 mg/m2/day. This syndrome occurred after drug administration was complete and the patients had returned home. Lesser CNS toxicity was seen in 2 other patients at lower daily doses. Preliminary pharmacokinetics of DESPM measured in plasma by HPLC in 8 patients showed linearity with dose and a rapid plasma decay with a t1/2 of 0.12 h. We conclude that great caution is warranted in administering DENSPM on this schedule at doses of > or = 83 mg/m2/day.

Cancer biomarkers in the field of tea

Suitable intermediate end-point biomarkers are needed in order to reduce the time and expense associated with cancer chemoprevention studies. Because of the multi-step nature of cancer intermediate biomarkers reflect a continuum of events associated with different stages of disease development, and range from genetic damage linked to cancer initiation, to tumor growth and expansion during disease progression. With tea, experimental evidence points to potential protection at several stages of carcinogenesis including endogenous carcinogen formation, carcinogen activation and detoxification, DNA damage and destabilisation, cell proliferation, neoplastic growth and metastatic spread. If used effectively in tea research, biomarkers should contribute to a better understanding of the possible extent of cancer protection afforded by tea, as well as the putative mechanisms involved and the principal components of tea with prophylactic potential.

DNA adducts and chronic degenerative disease. Pathogenetic relevance and implications in preventive medicine

Chronic degenerative diseases are the leading causes of death in developed countries. Their control is exceedingly difficult due to their multiplicity and diversity, the interconnection with a network of multiple risk factors and protective factors, the long latency and multistep pathogenesis, and the multifocal localization. Adducts to nuclear DNA are biomarkers evaluating the biologically effective dose, reflecting an enhanced risk of developing a mutation-related disease more realistically than the external exposure dose. The localization and accumulation of these promutagenic lesions in different organs are the composite result of several factors, including (a) toxicokinetics (first-pass effect); (b) local and distant metabolism; (c) efficiency and fidelity of DNA repair; and (d) cell proliferation rate. The last factor will affect not only the dilution of DNA adducts but also the possible evolution towards either destructive processes, such as emphysema or cardiomyopathies, or proliferative processes, such as benign or malignant tumors at various sites. They also include heart tumors affecting fetal myocytes after transplacental exposure to DNA-binding agents, blood vessel tumors, and atherosclerotic plaques. In this article, particular emphasis is given to molecular alterations in the heart, which is the preferential target for the formation of DNA adducts in smokers, and in human aorta, where an extensive molecular epidemiology project is documenting the systematic presence of adducts to the nuclear DNA of smooth muscle cells from atherosclerotic lesions, and their significant correlation with known atherogenic risk factors. Exocyclic DNA adducts resulting from lipid peroxidation, and age-related indigenous adducts (I-compounds) may also originate from endogenous sources, chronic infections and infestations, and inflammatory processes. Type II I-compounds are bulky DNA lesions resulting from oxidative stress, whereas type II-compounds are presumably normal DNA modifications, which display positive correlations with median life span and are decreased in cancer and other pathological conditions. Profiles of type II-compounds strongly depend on diet and are related to the antidegenerative effects of caloric/ dietary restriction. Even broader is the possible meaning of adducts to mitochondrial DNA, which have been detected in rodents exposed to genotoxic agents and complex mixtures, as well as in untreated rodents, in larger amounts when compared to the nuclear DNA of the same cells. Mutations in mitochondrial DNA increase the number of oxidative phosphorylation-defective cells, especially in energy-requiring postmitotic tissues such as brain, heart and skeletal muscle, thereby playing an important role in aging and a variety of chronic degenerative diseases. A decreased formation of DNA adducts is an indicator of reduced risk of developing the associated disease. Therefore, these molecular dosimeters can be used as biomarkers in the prevention of chronic degenerative diseases, pursued either by avoiding exposure to adduct-forming agents or by using chemopreventive agents. Interventions addressed to the human organism by means of dietary measures or pharmacological agents have encountered a broad consensus in the area of cardiovascular diseases, and are deserving a growing interest also in cancer prevention. The efficacy of chemopreventive agents can be assessed by evaluating inhibition of nuclear DNA or mitochondrial DNA adduct formation in vitro, in animal models, and in phase II clinical trials in high-risk individuals.

Cell proliferation indexes: a biomarker in solid tumors

Clinical studies have shown that carcinogenesis is linked to the development of proliferative abnormalities. Proliferative activity has been found to have prognostic significance in a variety of human tumors. Because proliferative abnormalities can precede the occurrence of morphological abnormalities, their measurement could also serve as useful biomarkers for chemoprevention trials. The variety of techniques for measuring cell proliferation in routine sections include mitosis counting, AgNORs, DNA precursor uptake (bromodeoxyuridine), and immunohistochemical detection of cell cycle proteins (PCNA, Ki-67/MIB-1). It is essential that the virtues and limitations of these methods be examined to ensure collection of meaningful clinical data.

The use of biomarker expression to characterize neoplastic processes

Biomarkers have been used by pathologists to aid the diagnosis of tumors for almost three decades. Their use has resulted in the re-evaluation and reclassification of several types of tumors. Currently, biomarkers are required to differentiate certain specific tumors with similar histologic patterns. Additional uses of biomarkers in the characterization of neoplastic processes are discussed including their use is prognosis, detecting early neoplastic processes, identifying tumor recurrence, measuring the effectiveness of various therapies (surrogate end point biomarkers), and identifying targets for novel therapies including immunotherapy and gene therapy. We propose that these newer uses of biomarkers will be just as important to pathology in the future as the uses of biomarkers in diagnosis have been over the past two decades.

Biomarkers--the new frontier in the pathology of invasive and preinvasive neoplasias

The papers in this symposium were presented at the 1996 Annual Meeting of the Biological Stain Commission. The authors hope that the discussion of biomarkers in the papers included in this issue and the literature reviewed in them will be useful to diagnostic pathologists as well as to investigators studying neoplastic processes in general.

Chemoprevention of colorectal cancer

This review summarizes the principles of cancer chemoprevention and discusses the evidence from epidemiologic and experimental studies and preclinical and clinical trials of potential colorectal chemopreventive agents. The putative mechanisms of action of the drugs in chemoprevention and their potential to reduce the incidence and mortality rate of colorectal neoplasms are discussed. The future of colorectal chemoprevention will depend on important new insights into molecular carcinogenesis of colorectal cancer, application of molecular markers as surrogate endpoints, and ultimately on therapeutic targets of prevention in clinical trials.

Effects of fixation and tissue processing on immunohistochemical demonstration of specific antigens

Identification of biomarkers in archival tissues using immunochemistry is becoming increasingly important for determining the diagnosis and prognosis of tumors, for characterizing preinvasive neoplastic changes in glandular tissues such as prostate, for evaluating the response of tumors and preinvasive neoplastic changes to certain therapies (i.e., as a surrogate intermediate end point), for selecting patients who are candidates for specific therapies (e.g., immunotherapy) and for retrospective studies. For detecting specific biomarkers it is important to understand the limitations imposed by the fixation methods and processing of the tissues. This study was designed to determine the effects of fixation on the detection in archival paraffin blocks of selected antigens postulated to be important in tumor biology. We evaluated the antigens TGF alpha, p185erbB-2, broad spectrum keratins, p53, and TAG-72 (B72.3). Fixatives evaluated included standard preparations of neutral buffered formalin, acid formalin, zinc formalin, alcoholic formalin, ethanol, methanol, and Bouin's fixative. We found that in general neutral buffered formalin is the poorest fixative for maintaining antigen recognition by immunohistochemistry and that no single fixative was best for all antigens. The dehydrating (coagulant) fixatives (e.g., ethanol and methanol) preserved immunorecognition of p53 and broad spectrum keratins best while the slow cross-linking fixatives (e.g., unbuffered zinc formalin) were best for demonstrating TGF alpha and p185erbB-2. Fixatives other than neutral buffered formalin produced equivalent recognition of the epitope of TAG-72 by B72.3. In formalin fixed archival tissues, only a portion of the antigen signal can be detected by routine immunohistologic methods.

New agents for cancer chemoprevention

Clinical chemoprevention trials of more than 30 agents and agent combinations are now in progress or being planned. The most advanced agents are well known and are in large Phase III chemoprevention intervention trials or epidemiological studies. These drugs include several retinoids [e.g., retinol, retinyl palmitate, all-trans-retinoic acid, and 13-cis-retinoic acid], calcium, Beta carotene, vitamin E, tamoxifen, and finasteride. Other newer agents are currently being evaluated in or being considered for Phase II and early Phase III chemoprevention trials. Prominent in this group are all-trans-N-(4-hydroxy phenyl)retinamide (4-HPR) (alone and in combination with tamoxifen), 2-difluoromethylornithine (DFMO), nonsteroidal antiinflammatory drugs (aspirin, piroxicam, sulindac), oltipraz, and dehydroepiandrostenedione (DHEA). A third group is new agents showing chemopreventive activity in animal models, epidemiological studies, or in pilot clinical intervention studies. They are now in preclinical toxicology testing or Phase I safety and pharmacokinetics trials preparatory to chemoprevention efficacy trials. These agents include S-allyl-l-cysteine, curcumin, DHEA analog 8354 (fluasterone), genistein, ibuprofen, indole-3-carbinol, perillyl alcohol, phenethyl isothiocyanate, 9-cis-retinoic acid, sulindac sulfone, tea extracts, ursodiol, vitamin D analogs, and p-xylyl selenocyanate. A new generation of agents and agent combinations will soon enter clinical chemoprevention studies based primarily on promising chemopreventive activity in animal models and in mechanistic studies. Among these agents are more efficacious analogs of known chemopreventive drugs including novel carotenoids (e.g., alpha-carotene and lutein). Also included are safer analogs which retain the chemopreventive efficacy of the parent drug such as vitamin D3 analogs. Other agents of high interest are aromatase inhibitors (e.g., (+)-vorozole), and protease inhibitors (e.g., Bowman-Birk soybean trypsin inhibitor). Combinations are also being considered, such as vitamin E with l-selenomethionine. Analysis of signal transduction pathways is beginning to yield classes of potentially active and selective chemopreventive drugs. Examples are ras isoprenylation and epidermal growth factor receptor inhibitors.

Chemoprevention trials and surrogate end point biomarkers in the cervix

Cervical cancer is the second most common malignancy in women worldwide and remains a significant health problem for women, especially minority and underserved women. Despite an understanding of the epidemiologic risks, the screening Papanicolaou smear, and morbid and costly treatment, overall survival remains 40%. New strategies, based on the clinical and molecular aspects of cervical carcinogenesis, are desperately needed. Chemoprevention refers to the use of chemical agents to prevent or delay the development of cancer in healthy populations. Chemoprevention studies have several unique features that distinguish them from classic chemotherapeutic trials; these features touch on several disciplines and weave knowledge of the biology of carcinogenesis into the trial design. In the design of chemoprevention trials, four factors are important: high risk cohorts must be identified; suitable medications must be selected; study designs should include Phases I, II, and III; and studies should include the use of surrogate end point biomarkers. Surrogate end point biomarkers are sought because the cancer develops over a long period of time, and studies of chemopreventives would require a huge number of subjects followed for many years. Surrogate end point biomarkers serve as alternative end points for examination of the efficacy of chemopreventives in tissue. High risk cohorts include women with cervical intraepithelial neoplasia (CIN) or squamous intraepithelial lesions (SIL). Nutritional studies have helped define micronutrients of interest (folate, carotenoids, vitamin C, vitamin E). Other medications of interest include retinoids (4-hydroxyphenylretinamide [4-HPR], retinyl acetate gel, topical all-trans-retinoic acid), polyamine synthesis inhibitors (alpha-difluoromethylornithine [DFMO]), and nonsteroidal anti-inflammatory drugs (ibuprofen). Phase I chemoprevention studies of the cervix have tested retinyl acetate gel and all-trans-retinoic acid. Phase II trials of all-trans-retinoic acid, beta-carotene, and folic acid have been and are being carried out, whereas Phase III trials of all-trans-retinoic acid have been completed and have shown significant regression of CIN 2 but not CIN 3. Phase I studies of DFMO and Phase II studies of DFMO and 4-HPR are underway. Surrogate end point biomarkers under study include (1) quantitative cytology and histopathology; (2) human papillomavirus type testing; (3) biologic measures of proliferation, regulation, differentiation, and genomic instability; and 4) fluorescence spectroscopic emission. Clinical trials with biologic end points will contribute to our understanding of the neoplastic process and hence aid us in developing new preventive and therapeutic strategies.

Chemo- and dietary prevention of colorectal cancer

Because of the substantial morbidity and mortality associated with colorectal cancer, and the limitations and costs of treating this disease, prevention remains a desirable (if elusive) goal. In this paper, we discuss both chemo- and dietary prevention strategies for colorectal cancer, recognising the overlap and cross-fertilisation between these two approaches. Chemopreventive compounds are drugs and are developed for clinical use like other pharmaceuticals. A formal sequential multi-phase programme for development of chemopreventive agents has been instituted by the National Cancer Institute, U.S.A. This involves both preclinical efficacy and clinical studies. Such studies increasingly employ preneoplastic intermediate markers (such as proliferation measures) as well as neoplastic adenomas as endpoints. Promising chemopreventive agents include calcium, aspirin and other non-steroidal anti-inflammatory drugs, vitamins (such as vitamin E and folate), 2-dimethylfluorornithine (DFMO), oltipraz and ursodeoxycholic acid. Several lines of evidence implicate diet in colorectal carcinogenesis. Key hypotheses in diet and colorectal cancer (which are amenable to prevention, research and action), in addition to those pertaining to the micronutrient chemopreventives, include dietary fat and fibre, food mutagens, red meat, and overall low-fat, high-fibre, high fruit and vegetable dietary patterns and cuisines. Several adenomatous polyp recurrence studies with fibre supplement, macronutrient or dietary pattern interventions have been undertaken internationally. We review early findings from this new generation of studies, and anticipate the future results from these investigations and the ambitious Women's Health initiative in the U.S.A. Results from these studies may convert the promise of colorectal cancer prevention into reality.

Strategies for phase II cancer chemoprevention trials: cervix, endometrium, and ovary

Well-designed and conducted Phase II clinical trials are very important to cancer chemoprevention drug development. Three critical aspects govern the design and conduct of these trials--well-characterized agents, suitable cohorts, and reliable biomarkers for measuring efficacy that can serve as surrogate endpoints for cancer incidence. Requirements for the agent are experimental or epidemiological data showing chemopreventive efficacy, safety on chronic administration, and a mechanistic rationale for the chemopreventive activity observed. Agents that meet these criteria for chemoprevention of cervical cancer include antiproliferative drugs (e.g., 2-difluoromethylornithine), retinoids, folic acid, antioxidant vitamins and other agents that prevent cellular oxidative damage. Because of the significant cervical cancer risk associated with human papilloma virus (HPV) infection, agents that interfere with the activity of HPV products may also prove to be effective chemopreventives. In endometrium, unopposed estrogen exposure has been associated with cancer incidence. Thus, pure antiestrogens and progestins may be chemopreventive in this tissue. Ovarian cancer risk is correlated to ovulation frequency; therefore, oral contraceptives are potentially chemopreventive in the ovary. Recent clinical observations also suggest that retinoids, particularly all-trans-N-4-hydroxyphenylretinamide, may be chemopreventive in this tissue. The cohort should be suitable for measuring the chemopreventive activity of the agent and the intermediate biomarkers chosen. In the cervix, patients with cervical intraepithelial neoplasia (CIN) and in endometrium, patients with atypical hyperplasia, fit these criteria. Defining a cohort for a Phase II trial in the ovary is more difficult. This tissue is less accessible for biopsy; consequently, the presence of precancerous lesions is more difficult to confirm. The criteria for biomarkers are that they fit expected biological mechanisms (i.e., differential expression in normal and high-risk tissue, on or closely linked to the causal pathway for the cancer, modulated by chemopreventive agents, and short latency compared with cancer), may be assayed reliably and quantitatively, measured easily, and correlate to decrease cancer incidence. They must occur in sufficient incidence to allow their biological and statistical evaluation relevant to cancer. Since carcinogenesis is a multipath process, single biomarkers are difficult to validate as surrogate endpoints, perhaps appearing on only one or a few of the many possible causal pathways. Panels of biomarkers, particularly those representing the range of carcinogenesis pathways, may prove more useful as surrogate endpoints. It is important to avoid solely on biomarkers that do not describe cancer but represent isolated events that may or may not be on the causal pathway or otherwise associated with carcinogenesis. These include markers of normal cellular processes that may be increased or expressed during carcinogenesis. Chemoprevention trials should be designed to evaluate fully the two or three biomarkers that appear to be the best models of the cancer. Additional biomarkers should be considered only if they can be analyzed efficiently and the sample size allows more important biomarkers to be evaluated completely. Two types of biomarkers that stand out regarding their high correlation to cancer and their ability to be quantified are measures of intraepithelial neoplasia and indicators of cellular proliferation. Measurements made by computer-assisted image analysis that are potentially useful as surrogate endpoint biomarkers include nuclear polymorphism comprising nuclear size, shape (roundness), and texture (DNA distribution patterns); nucleolar size and number of nucleoli/nuclei; DNA ploidy, and proliferation biomarkers such as S-phase fraction and PCNA...