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DO RODENT STUDIES PREDICT HUMAN CANCERS? by Aaron Wildavsky Why does it matter if animal cancer studies are worthwhile or worthless or someplace in between? The answer to this question is that regulation of exposure to chemicals, including the intermittent exposures to trace elements to which the general pubic is subject, are largely based on interpretations of animal-cancer bioassays. If these tests are reasonably accurate in predicting the probability, sites, and severity of human cancers, then regulation of chemicals suspected of causing cancer (carcinogens) is on firm ground. But if these animal cancer tests are weak or worse, so that one cannot reasonably predict human cancers from them, then regulation rests on quicksand. . Whether or not rodent tests predict human cancers, animal studies have many other important uses. Research into cancer mechanisms or problems of the immune system, for instance, may be furthered by introducing novel genes into small animals, such as transgenic mice, to discover better how life systems work.1 There is no doubt that models based on research with animals have increased our understanding of metastasis, which is so important in the spread of cancer.2 None of the many invaluable uses of animal cancer tests, however, tells us whether they can come close enough often enough to be a valid source of evidence in predicting human cancer. I

The Right Ouestions We would like to know how much damage to human populations is caused by different types and quantities of exposure to various substances. That estimate requires answering subquestions about conditions. Routes of exposure may differ in that some come from breathing, others from eating, still others, like x- rays, through the skin, from natural sources and medical uses. Quantities may differ from a little to a lot to immense. Timing differs from exposure all at once or over periods of time. If a precise answer to the question of adverse effects cannot be given, we might be satisfied with knowing that there is a great deal of harm or moderate or very little or probably none.3 How reliable are these tests? If the tests were repeated on the same species, would we get nearly the same results? If they were repeated on different animal species, would we come up with similar results? If chemicals are carcinogenic in several animal species, it is more likely that they are carcinogenic to mammals in general, including human beings, than if they cause cancer in a single species. It is also important to distinguish rates and sites of cancer by age, because cancer is largely though not entirely a disease of old age, and by sex, because men and women are affected differently. Dioxin in large and continuous doses appears unfriendly to mammals but it is the dose that matters.

3 Regulatory agencies assume that chemicals carcinogenic at some dose in any animal are also carcinogenic to human beings. We want to find out if that assumption is true. As precisely as possible, we wish to answer Freedman and Zeisel's question, "Are chemicals that have been shown to be carcinogenic through experimental animals also carcinogenic to humans?"4 The reason for their inclusion of the modifier "experimental animals" has to do with the particular conditions under which animals are tested_ Therefore they also ask, "Do experimental animals (rodents, in particular) and humans have similar susceptibility to the carcinogenic effect of chemicals, or are rodents incomparably more susceptible than humans?"5 The answer to the first question is "sometimes" rodent cancers are cancers in humans too, but we do not know when_ The same is true of one type of rodent to another. The answer to the second question is "yes, mostly, but not always." If we know that a single dose LD50 for dioxin ranges from 2500 ug/kg in guinea pigs to 5000 in hamsters, a difference of 2500, does that give us confidence about rodent to human transfers? Suppose that we arrive at several different answers: one is that there is a ten percent probability that a substance causing cancer in a mouse or rate at a given dose will do the same in a human being. From one point of view, nine times out of ten the extrapolation from mouse to man would be wrong. From another point of view, why take chances with human health if the probability of getting a cancer is that high? Were we to find, however, that when we get the answers from the tests we would know within a N M N ~ a 6t ~ 0 N

factor of several hundred times to several hundred thousand times whether rodents predict to humans, that might not be a reasonable approximation. The question is not only whether we can get an answer but what kind of answer we will get. The Process of Animal Cancer Testing Around 1915 or 1916 scientists learned that they could induce cancer in animals by treating them with certain chemicals. The methods of giving animals cancer vary greatly. "Chemicals have been introduced into experimental animals by every orifice (orally, nasally, urethrally, vaginally, rectally), by various types of injections (intramuscular, intraperitoneal, intravenous, subcutaneous), by skin painting, by surgery, and by other methods.6 Approximately 30 percent of the rodents get some form of cancer absent exposure to chemicals, though all 30 percent to do not die of it. This is one reason why a control group is essential. Because a chemical's effects at high doses may not show up at low doses, it is necessary to further subdivide the animals into different dose groups. Given that sex plays an important role in cancer, a further subdivision is between male and female. Usually there are three dose groups (0, half or one-tenth the MTD, MTD) and two species. There are at least twelve groups of animals. By convention and by statistical necessity, there are usually fifty animals, most often rodents, in each group. Though only a few facts about the process of animal cancer testing have been given, we are already in a position to understand

5 three of its most basic aspects--its short time compared to human epidemiological studies, its high cost, and its essentially statistical character. A great advantage of rodent testing is that these animals live only about two years. Therefore one doesn't have to wait too long to get results. One can also test any chemical, including new chemicals, for which epidemiological evidence may not be available. But the task is not easy or cheap. It is costly to keep these animals under controlled conditions for up to two years. The painstaking work of examining animals for tumors requires pathologists. When each animal dies or, as the too-kindly parlance states, is sacrificed, several pathologists must carefully examine around forty sites around and about animal organs and tissues to search for tumors, some of which are so small they can be discerned only with high- powered microscopes. That is why there is a team of pathologists who first work separately and then meet to resolve differences before their findings are accepted for further evaluation.7 These pathologists consider, whether the tumors or other abnormalities are actually induced by the chemical, an opinion based on what they know about the normal incidence of tumors and their experience. They ask themselves not only whether the incidence of tumors is higher but whether they are of a different size or shape or color or contain any other signs that might show them to be similar to or different from naturally occurring lesions.8 In order to understand better whether the proper dose was administered, the animals have to be weighed to discern whether they have lost appropriate amounts of weight and examined to see

whether the dose is either so large as to threaten their lives from causes other than cancer or so small as to make its effects unnoticeable. Now we are in a position to understand why rodent-cancer tests are so expensive. When one multiplies the time these tests take, roughly three years, times the cost of keeping twelve groups of animals in controlled conditions, and adds the cost of killing and dissecting them as well as the cost of preparing and examining forty slides per animal, and of reconciling differences, the substantial costs do not appear out of Iine.g It is possible for a government regulator to conclude that the tests are inadequate or that the substance being tested is actually a carcinogen. But it is not possible under the rules to say that the substance is not, insofar as is known, a carcinogen. Instead, the closest government scientists are allowed to come is to say that "the compound has not been shown to be carcinogenic."1o What, we may ask, is the meaning of classifying a substance as a suspected carcinogen? It is worth attending closely to Chu and his colleagues' discussion: If malignant tumors or a combination of malignant and benign tumors are produced, then the compound is considered carcinogenic to the animals. If the significant result is only the production of benign tumors, then the compound may pose a potential health hazard and is termed a suspected carcinogen or a carcinogen, depending on the nature of the benign tumor. For example, 2,4-dinitrotoluence . . . was considered a

7 suspected carcinogen since it induced only benign tumors (fibromas of the skin and subcutaneous tissue in male Fischer 344 rats and fibroadenomas of the mammary gland in females)_ Ideally, a distinction should be made between truly benign tumors, which never progress to malignancy, and tumors that are in a benign state according to histopathologic criteria at the time of diagnosis. Scientific judgments in this area are limited by inability to predict the biological behavior of a lesion on the basis of morphological criteria, but it appears that there are few, if any, truly benign tumors in rodents.1 1 The two sentences are telling. The "it appears" reflects a judgment that any tumor might turn bad; the "if this were true" suggests that this assumption lacks credibility. Is it in the interests of public safety to treat all tumors, however benign in appearance, as if they might turn malignant, because we do not know they won't? Or is this "might turn malignant" a way of prejudicing the outcome so the - chemicals will be found to induce cancers whether they do or don't? In the EPA's 1976 "Interim Procedures and Guidelines for Health Assessments of Suspected Carcinogens," EPA Administrator Russell Train acknowledged that animal tests could not prove that a chemical would be carcinogenic in people, but that a substance would be considered a "presumptive cancer risk" if it "causes a statistically significant excess incidence of benign or malignant tumors in humans or animals."12 If benign is bad, what could be good? Calculating Toxicity by the LDSp Test N N O N i ~ i O ~

In the field of pesticide regulation, lethality is calculated through the assignment of an LD50, the lethal dose for one-half of the test animals during the test period_ The relevant number for aspirin would be 730mg/kg, signifying that 50 percent of the test animals died when exposed to 730 mg of aspirin per kilogram of their body weight.13 The larger the LD50, the more of a substance it takes to produce a toxic effect, the less harmful the chemical. Among species most commonly used to carry out the LD50 test are fish, birds, rabbits, mice, and rats, although occasionally monkeys and dogs are used. Generally, about 60 animals of a particular species and a specific dosing method are used. The application is made by inserting a tube down the throat of the animal, by forcing injection of vapors, or by application to the skin.14 The usual test lasts about two weeks, during which the animals either die or, at the end, are killed off. The usual symptoms are bleeding from the mouth or eyes, convulsions, diarrhea, and what are exquisitely termed . "unusuat vocalizations." Rather than tolerate early death, according to the British Toxicological Society, "There is pressure on the toxicologists to allow the study to continue, even when the animals are in distress since their premature killing may alter the end-point of the study, and so possibly affect the classification of the material being tested."15 Needless to say, animal rights advocates are not happy with this method. Whether one believes that the LD50 test involves "a ritual mass execution of animals"16 or that "the main information they give is an indication of the size of dose required to commit suicide,"17 or even whether most experts consider "the

9 modern toxicological routine procedure a wasteful endeavor in which scientific inventiveness and commonsense have been replaced by a thoughtless completion of standard protocols,"t$ there is ample scientific doubt about the value of the LD50 test for the purpose of predicting human effects.19 The basic difficulty is that enormous differences between different (even from closely related) species are reported, ranging from 5 to 75 times, which renders findings suspect_ZO If LD50 tests are useful in providing evidence to save human life from suffering, there would still have to be a debate over whether the animal suffering entailed can be justified. If, however, the observations are too unreliable to be useful, no such question arises. Toxicity The best explanation for laymen I have heard of the difference with which we have been concerned--between very large and very small exposures to different kinds of species--comes from reporter Richard Harris of National Public Radio, together with a number of cancer researchers and government officials. Their dialogue is instructive: Penelope Fenner Crisp (Environmental Protection Agency): We're coming to discover that there are more differences between species than we had expected or, frankly, hoped that existed.

1 Harris: It turns out that a great many chemicals that can cause cancer in one species don't seem to do anything at all in another species. Here's an analogy. (Excerpt from music from a CD) Harris: The difference between rodents and people can be as dramatic as the difference between this CD and an LP. You could drop this CD, get it dusty, even scratch it, and you wouldn't necessarily hurt it. (Sound of record being scratched by a stereo needle and music played from an LP) Harris: By try the same thing with a record, and you can just hear the damage. To be sure, some things will damage either a CD or a record album--say, a hot windowsill. Likewise, John Doull from the University of Kansas says some chemicals do cause cancer in all sorts of animals ... Now nobody's suggesting that these chemicals are harmless, but in some cases scientists believe that the standards may be vastly overstating the health risks. Again, this comes down to a necessary but flawed shortcut the EPA uses to size up a chemical. Scientists give a huge dose of chemicals to rats and then estimate the effects of that chemical at lower doses. By