Regulatory Forum Opinion Piece: Carcinogen Risk Assessment: The Move from Screens to Science.

Throughout the last 50 years, the paradigm for carcinogenicity assessment has depended on lifetime bioassays in rodents. Since 1997, the International Conference on Harmonisation (ICH) S1B has permitted the use of a 2-year rodent bioassay (usually in the rat) and an alternative, genetically modified mouse model to support cancer risk assessment of pharmaceuticals. Since its introduction, it has become apparent that many of the stated advantages of the 6-month Tg mouse bioassay have, in actual fact, not been realized, and the concern exists that an albeit imperfect, 2-year mouse bioassay has been replaced by a similarly imperfect 6-month equivalent. This essay argues strongly that model systems, using cancer as the end point, should be discontinued, and that the recent initiatives, from the Organization for Economic Cooperation and Development and Institute of Peace and Conflict Studies, on "mode of action," "adverse outcome pathways," and "human relevance framework" should be embraced as being risk assessments based upon the available science. The recent suggested revisions to the ICH S1 guidelines, utilizing carcinogenicity assessment documents, go some way to developing a science-based risk assessment that does not depend almost entirely on a single, imperfect, cancer-based end point in nonrelevant animal species.

History of chronic toxicity and animal carcinogenicity studies for pharmaceuticals.

During the 20th century, as drug products were being developed to treat both known and emerging human diseases and conditions, determining the safety of these new chemicals became of increasing importance and necessity. For a time, the safety of use in human populations was of question, let alone whether the drug product was truly effective. As such, US and international regulatory agencies have played a major role in establishing standardized testing to evaluate the safety and efficacy of drug products. Pharmacologic and toxicologic evaluation of a new drug in animals is an important part of the pharmaceutical development process prior to its first-time use in humans, as well as its potential chronic use in affected populations. Just as both science and technology have evolved over the past century and further, so have the guidelines that have been put forth to adequately and efficiently evaluate the toxicity of new drugs and their subsequent safety in humans. This review summarizes the historical highlights of the conduct of drug safety evaluations in animals, particularly with regard to chronic toxicity and carcinogenicity assessments, and how we have progressed to our current standards and protocols to ensure safe use of drug products in human populations.

Rodent cell transformation assays-a brief historical perspective.

In vitro cell transformation is a process characterized by a series of progressive distinctive events that often emulate manifestations occurring in vivo and which are associated with neoplasia. Attendant cellular and sub-cellular alterations include, among others: cellular immortality, phenotypic changes, aneuploidy, genetic variability, cellular disarray, anchorage-independent growth, and tumorigenicity in vivo. Early chemically induced neoplastic transformation studies involved the use of normal diploid (Syrian) hamster embryo (SHE) cells and monitored the formation of morphologically altered colonies. Later investigations employed primarily two established mouse cell lines, i.e. the BALB/c 3T3 A31 cell line and the C3H 10T 1/2 cell line, and monitored the induction of morphologically aberrant foci. In either case, such transformed cellular clusters (colonies and foci) could induce tumors upon inoculation in vivo. Some subsequent noteworthy advancements using these systems included pH adjustments, metabolic supplementation, amplification of expression of formerly latent transformed foci, concurrent detection of mutagenesis and transformation, and use of a Bhas 42 cell line (v-Ha-ras transfected BALB/c 3T3 cells) to detect both tumor initiators and promoters. Over time, such transformation assay systems have been found useful in academic, industry and regulatory laboratories, generally for research purposes, but also occasionally as screening tools for potential chemical carcinogens. Nevertheless, to date, use of these assays for decision-making purposes in the regulatory arena remains elusive and will require comprehensive validation to gain universal acceptance.

Historical perspective on the use of animal bioassays to predict carcinogenicity: evolution in design and recognition of utility.

The animal testing protocols used today to evaluate the carcinogenicity of chemicals are very different from those used in the earlier part of the 20th century. To explore how cancer bioassays have changed over time, we surveyed the literature discussing test design and interpretation from the 1930s to the present. We also analyzed compendia of bioassays published by the US Public Health Service (US PHS) from 1938 to 1978, and evaluated the data to understand the evolution of testing methodology (e.g., animals used, test duration) and the types of chemicals being studied. The cancer bioassay evolved in several stages. At the beginning of the 20th century, animal bioassays were primarily used to re-create known human diseases, whereas in the 1940s to 1960s, animal bioassays were largely used to evaluate the safety of chemicals in foods, drugs, and cosmetics. Beginning in the late 1960s and 1970s, chemicals primarily associated with occupational or environmental exposures were also evaluated. Testing strategies now emphasize a suite of tests including multiple in vitro tests and both short-term and long-term animal tests. The objectives of testing are broader, too, with test goals encompassing information regarding mode of action and other parameters aimed at evaluating potential species differences (e.g., in toxicokinetics) and their relevance for evaluating human risks. It is important to consider this evolution when evaluating the testing methodology and scientific conclusions in earlier eras. As toxicology continues to develop, testing methods will continue to change in concert with increased knowledge and understanding.

Isolation of plasmid pKM101 in the Stocker laboratory.

pKM101 is a mutagenesis-enhancing resistance transfer plasmid (R plasmid) that was introduced into several tester strains used in the Salmonella/microsome mutation assay (Ames test). Plasmid pKM101 has contributed substantially to the effectiveness of the Ames assay, which is used on a world-wide basis to detect mutagens and is required by many government regulatory agencies for approval to market new drugs and other chemical agents. Widely used since 1975, the Ames test is still regarded as one of the most sensitive genetic toxicity assays and a useful short-term test for predicting carcinogenicity in animals. Plasmid pKM101, which is a deletion derivative of plasmid R46 (also referred to as R-Brighton after its origin of isolation in Brighton, England), has also been used to elucidate molecular mechanisms of mutagenesis. It was isolated in the laboratory of Professor Bruce A.D. Stocker at Stanford University as part of my doctoral research with 20 R plasmids. Professor Stocker's phenomenal insight into the genetics of Salmonella typhimurium and plasmid behavior was a major factor that led to the isolation of pKM101. This paper includes a tribute to Bruce Stocker, together with a summary of my research with mutagenesis-enhancing R plasmids and a brief discussion of the molecular mechanisms involved in pKM101 plasmid-mediated bacterial mutagenesis.

Remembering Professor Cesare Maltoni.

Professor Cesare Maltoni, a renowned leader in the research of the hazards of carcinogens in the workplace, died on January 22, 2001 at the age of 70. Born in Faenza (Ravenna), Italy on November 17, 1930, he received his M.D. degree from the University of Bologna in 1954-1955. He was Director of the Institute of Oncology of Bologna (1964 to 1997), Director of the Bologna Centre for the Prevention and Detection of Tumours and Oncological Research (1966 to 1989), and Scientific Director, European Foundation of Oncology and Environmental Sciences "B. Ramazzini" from 1993 until he died. Maltoni conducted long-term carcinogenic studies on some 200 agents. He was the first to demonstrate that vinyl chloride is a carcinogen that produces, among other tumors, angiosarcoma of the liver. He was the first to show that benzene is a powerful multipotential carcinogen. Maltoni authored more than 700 original scientific publications, books, and proceedings. He was editor and coeditor of many journals. Among his many awards were the Stokinger Award, American Conference of Governmental Industrial Hygienists (ACGIH), Kansas City, 1995; International Award "B. Ramazzini" of the Collegium Ramazzini, Washington, 1995; International I.J. Selikoff Memorial Award, Washington, 1995; and the Sigillum Magnum of the University of Bologna, 1997. Many fellow Ramazzinians and his coworkers wish to be considered part of the group of, as he once wrote, his "always family friends" and to remember happy moments when we were together. A man of great stature and many contributions, Cesare Maltoni will never be forgotten.

Carcinogenicity and mutagenicity testing, then and now.

Cancer is a dread disease worldwide. Mortality of individuals suffering from cancer is high, despite the current improved methods of precocious detection, surgery and therapy. Prevention of cancer is the recognized goal of many activities in cancer research. This aim was recognized early to involve the bioassay of environmental chemicals or mixtures. The first such study involved application of coal tar to the ear of rabbits, and later on to the skin of mice. Subsequently, laboratory rats were introduced, and hamsters were utilized as a substitute for the unwieldy tests in rabbits. Investigators also became concerned with the mechanisms of carcinogenesis, and more definitive approaches to carcinogen bioassay in laboratory animals, as possible indicators of cancer risk in humans. These tests were expensive and lengthy, and did not serve the important purpose of accurately measuring risk of cancer to humans. Once it was realized that DNA and the genetic apparatus might be a key target, rapid bioassays in bacterial and mammalian cell systems were introduced successfully. Thus, batteries of tests are now available to detect effectively human cancer risks, and provide novel approaches to determine the underlying mechanisms, as a sound basis for cancer prevention.

A brief history of the use of laboratory animals for the prediction of carcinogenic risk for man with a note on needs for the future.

This review discusses developments during the last 60 years in the field of carcinogenicity testing based on the use of laboratory animals. Improvements that have occurred in the quality of animals and in the way in which tests are conducted are considered, along with the importance of distinguishing between fatal and incidental tumours. Still to be faced is a need to control calorie intake in the course of carcinogenicity testing. A necessity for a better understanding of how disturbances of physiological and/or hormonal status can predispose to tumour development and for more comparative metabolism studies is stressed. Recognition of the fact that thresholds exist for carcinogenesis by non-genotoxic compounds poses a need for avoiding unrealistically high levels of exposure and for more and better information on how different species metabolise test agents.

Regulating the cancer-inducing potential of non-steroidal anti-inflammatory drugs: some lessons from the 1970s and 1980s.

This article systematically examines government regulation of medicines in the U.K. and the U.S. with specific reference to carcinogenic risk assessment. By taking four non-steroidal anti-inflammatory drugs (NSAIDs) as case studies, it is argued that there have been inconsistencies between regulatory practice and the scientific standards supposed to have been upheld by drug regulatory agencies. Moreover, those inconsistencies are shown to form a trend over time which suggests an erosion and neglect of regulatory rigour during the 1980s. This takes the form of awarding the benefit of the many scientific doubts in carcinogenicity testing to manufacturers rather than to patients.