Three critical aspects govern successful Phase II cancer chemoprevention trials--well-characterized agents, suitable cohorts, and reliable intermediate biomarkers for measuring efficacy. 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. The cohort should be suitable for measuring the chemopreventive activity of the agent and the intermediate biomarkers chosen. Also, many cohorts proposed for Phase II trials are patients with previous cancers or premalignant lesions. For such patients, the trials should be conducted within the context of standard treatment to avoid unusual risks. 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 decreased 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, as they may appear 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 relying solely on biomarkers representing 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, but are nonspecific.(ABSTRACT TRUNCATED AT 250 WORDS)

Anticarcinogenic Agents↗

PubMed1994

Development of surrogate endpoint biomarkers for clinical trials of cancer chemopreventive agents: relationships to fundamental properties of preinvasive (intraepithelial) neoplasia.

The tissue changes offering the greatest immediate potential for development as surrogate endpoint biomarkers (SEBs) to be used in Phase II trials of cancer chemopreventive agents are those derived from the microscopic tissue changes pathologists use to make the diagnosis of preinvasive (intraepithelial) neoplasia. These changes comprise four categories: proliferative index, ploidy, nuclear morphometry (size, shape, texture, and pleomorphism), and nucleolar morphometry (number, size, shape, position, and pleomorphism). Computer-assisted image analysis (CIA) permits dozens of additional morphometric parameters to be developed. Other categories of candidate SEBs are: DNA and chromosomal structural changes associated with genomic instability, activation of oncogenes and inactivation of tumor suppressor genes, structural changes in differentiated molecules, and aberrations of growth factor/receptor structure and function. Self-perpetuating DNA breakage with secondary mutator mutations in genomic stability genes is a major mechanism by which the genomic instability characteristic of neoplasia occurs, and from which stem other basic neoplastic properties, including clonal evolution, along multiple pathways of genetic variation that are stochastically determined, continuously increasing proliferation, rate and extent of phenotypic heterogeneity. SEBs resulting from genomic instability include homogeneously staining regions, double minute chromosomes, micronuclei, dicentrics, gene amplification, loss of heterozygosity, and alterations in chromosome number. Newly developed assays for detecting genomic instability include comparative genomic hybridization using fluorescence in situ hybridization on > 20 micron-thick sections monitored by confocal laser scanning microscopy, assays for microsatellite instability, and restriction landmark genomic scanning. These assays offer promise for detecting the earliest molecular changes of neoplasia in normal-appearing epithelium prior to the onset of the dysplastic phase of intraepithelial neoplasia.

Anticarcinogenic Agents↗

PubMed1993

Development of chemopreventive agents for lung and upper aerodigestive tract cancers.

The lung and upper aerodigestive tract (oral cavity, larynx, pharynx, upper esophagus) will harbor the greatest proportion (approximately 20%) of estimated new cancer cases in 1992. The estimated mortality rate is even higher (32%), which is reflected in a 5-year survival rate of only 7% and 12% for esophageal and lung cancer, respectively. Tobacco use appears to remain the major cause of aerodigestive cancers despite efforts at primary prevention--cessation of exposure. Another strategy to decrease this public health problem is secondary prevention or chemoprevention. Cancer chemoprevention is defined as intervention with chemical agents before invasion to halt or slow the carcinogenic process; potential agents may include minor dietary constituents and pharmaceuticals. The main objective of the Division of Cancer Prevention and Control (DCPC), National Cancer Institute, is to develop promising chemopreventive drugs for use in humans. The testing of cancer chemopreventives for efficacy in the clinic differs from that of cancer treatment drugs. Chemopreventive drug trials involve healthy target populations, and the endpoints are reduced cancer incidence or mortality, or increased latency, with no to minimal toxicity. The lung and upper aerodigestive tract represent a unique opportunity for intervention in this setting. Even with cessation of tobacco exposure, the risk of cancer in the entire epithelium remains high for years due to the "field cancerization" effect. Some of the first chemopreventive trials made use of this system due to the availability of a study population with a tissue at demonstrably high risk for malignant progression. Much of the evidence for chemopreventive efficacy is in the oral cavity because of the well-defined epithelial neoplastic progression, the existence of well-established preclinical models, and relative ease of tissue monitoring and sampling. In one of the first randomized trials, Hong and co-workers demonstrated that 13-cis-retinoic acid prevents the appearance of second primary tumors in patients previously treated for squamous cell carcinomas of the oral cavity and upper respiratory tract. Even using a high risk population, chemoprevention trials involve large sample sizes, lengthy duration and follow-up, and high cost. To circumvent these problems, the use of intermediate biomarkers as surrogate endpoints is being explored. Intermediate biomarkers are defined as biological alterations in tissue (histological, genetic, biochemical, proliferative, differentiation-related) occurring prior to cancer development. In the oral cavity, studies using modulation of a histological intermediate biomarker, dysplastic leukoplakia, as the endpoint have demonstrated response to a retinoid.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1993

Intraepithelial neoplasia, surrogate endpoint biomarkers, and cancer chemoprevention.

Neoplasia is a progression of molecular, cellular, and tissue changes starting with a critical cell mutation and advancing by clonal evolution, involving further multiple mutations and expanding mutated clones. This process is characterized by five general stages: latency, focal growth of normal-appearing but disorganized cells, abnormal-appearing cells (dysplasia), microinvasion, and finally, metastasis. The two driving forces of neoplastic progression in an epithelium are mutagenesis and mitogenesis. These forces frequently occur concurrently, produced by exposure of the epithelium to environmental and endogenous mutagens and mitogens. The major strategy of chemoprevention is to block the effects of both mutagens and mitogens during the early stages of predysplasia and dysplasia. Surrogate endpoint biomarkers (SEBs) are tissue, cellular, and molecular changes that correlate with the later development of cancer. Because of the savings in cost, labor, and time, SEBs are urgently needed to replace the use of cancer incidence reduction as the endpoint for chemopreventive agent clinical trials. The advent of computer-assisted cytometry allows each of the seven basic criteria of dysplasia to be individually assayed as an SEB. Since the dysplastic changes that characterize intraepithelial neoplasia are embodied in the causal pathway to invasive neoplasia, they are already validated as predictors of cancer incidence. More attention should be paid to the quality control of SEB assays, including control of variation in cell composition of tissue samples, assay protocol, instrumentation used, and observer performance. The dose-response relationship between a known chemopreventive agent and the SEB should also be evaluated. The Division of Cancer Prevention and Control, National Cancer Institute, has begun a program to test chemopreventive agents in short-term Phase II clinical trials using dysplasia-based SEBs. The SEBs are assayed, when possible, by computerized cytometry. Trials are being conducted for oral leukoplakia, cutaneous actinic keratosis, superficial bladder cancer, pulmonary metaplasia/dysplasia, cervical dysplasia (CIN III), and adenomatous colonic polyps.

Animals↗

PubMed1993

Development of breast cancer chemopreventive drugs.

Breast cancer is the second highest cause of cancer mortality (19%) estimated for U.S. women in 1993 and accounts for the highest proportion of new cancer cases (32%) in this population. The rate of documented cases increased during the early 1970s and again in 1980-87, probably due to early mammographic detection. Increased knowledge of personal risk may also have been a consideration; however, 60% of women diagnosed with breast cancer have no known risk factor(s), such as family history, early age at menarche, late age at menopause, nulliparity, late age at first live birth, socioeconomic status, contraceptive use, postmenopausal estrogen replacement, or high fat intake. To prevent cancer, one strategy undertaken by the NCI is cancer chemoprevention, or intervention with chemical agents at the precancer stage to halt or slow the carcinogenic process. An objective of the NCI, DCPC is to develop promising cancer chemopreventive chemical agents as drugs for human use. Briefly, the process begins with identification of potential agents (e.g., pharmaceuticals, natural products, minor dietary constituents) from surveillance and analysis of the literature and from in vitro prescreen assays. Data on both efficacy (i.e., biological activities that either directly or indirectly indicate inhibition of carcinogenesis) and toxicity are gathered these sources. Various criteria are used to select and prioritize agents for entry into the NCI, DCPC preclinical testing program. The program begins with battery of in vitro efficacy screens using both animal and human cells to select agents for further testing; agents positive in these assays are considered for further testing. In the assay used for breast cancer chemoprevention, 7,12-dimethylbenz(a)anthracene (DMBA)-induced mouse mammary organ culture, 64 chemicals have inhibited formation of hyperplastic alveolar-like nodules. A panel of organ-specific animal screening assays are then used to assess efficacy in vivo. Two assays relevant for breast cancer chemoprevention are inhibition of N-methyl-N-nitrosourea- and DMBA-induced rat mammary gland carcinogenesis. Of 89 agents tested, 29 have inhibited cancer incidence, multiplicity, or both in at least one of the mammary assays; 21 agents are currently on test. Highly promising agents are then placed in traditional preclinical toxicity tests performed in two species. Finally, the most promising and least toxic agents enter clinical trials. Phase I clinical trials are designed to investigate human dose-related safety and pharmacokinetics of the drug. Phase II trials are small scale, placebo-controlled studies designed to determine chemopreventive efficacy and optimal dosing regimens.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗

PubMed1993

Intraepithelial and postinvasive neoplasia as a stochastic continuum of clonal evolution, and its relationship to mechanisms of chemopreventive drug action.

The progression of intraepithelial and postinvasive neoplasia depends on the occurrence of clonal evolution, defined as the continuous development of mutations and selective clonal expansions in the neoplastic cell population. The two continuously repeating events of clonal evolution, mutation and clonal expansion, occur at unpredictable times and locations. Therefore the neoplastic process is best characterized as a stochastic, i.e., probabilistic, continuum. The rate of intraepithelial neoplastic progression is continuously driven by the dosage level of exposure to mutagens and mitogens. For example, in chronic smokers the length of time before development of lung cancer depends on the number of cigarettes smoked per day. A commonly held misconception is that human carcinogenesis develops after an initial short period of mutation followed by a long period of stimulated proliferation (the multistage model). This incorrect idea derives from the sequential nature of the consecutive two- or three-step operational protocols imposed on experimental animal models by the experimenter. In reality, human carcinogenesis develops as the result of simultaneous and continuous exposure to mutagens and mitogens over the entire period of tumor development. A recent example is the finding that the intraepithelial neoplasia of colorectal adenomas continuously progresses through serial waves of mutation and clonal expansion. The rational design of chemopreventive agents should be based on blocking the two parameters which continuously drive neoplasia: mutagenesis and mitogenesis. In addition to blocking exposure, chemopreventive agents may act at many points during activation and DNA adduction of mutagens, or during stimulation of the proliferation signal pathway by mitogens. Based on the chemopreventive strategy of blocking mutagenesis and mitogenesis, chemopreventive agents are classed as either antimutagenic or antimitogenic. A third class, the antioxidants, are both antimutagenic and antimitogenic, and operate by the common mechanism of breaking free radical chain reactions initiated by reactive oxygen species. In the program of the Chemoprevention Investigational Studies Branch, Division of Cancer Prevention and Control, National Cancer Institute, preclinical development of antimutagens, antimitogens, and antioxidants is well under way, and some of these agents are highlighted here.

Antioxidants↗