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For Presentation to a Seminar Sponsored by the International Center for Scientific Ecology,
May 10, 1993, at Paris, France

TITLE:

Is the Concept of a Linear Relationship Between Dose and Effect Still a Valid Model for Assessing Risks Related to Low Doses of Carcinogens - the DDT Example.

William Hazeltine, Ph.D., B.C.E., Oroville, California, USA

It is gratifying to think that a Scientific Organization would want to look at the old and new data on carcinogenicity of DDT, 20 years after the United States Environmental Protection Agency (EPA) Administrator announced that DDT was a carcinogen. The Administrator made this decision and overturned his hearing Officer's findings concerning the risks of allowing DDT's continued use in the United States.

Background

You have to understand that I am an Entomologist-Ecologist by training, who used DDT for crop protection, and for control of public health pests. I tried to understand the details of risk from such use, because I was the applicator on many occasions, and I was often exposed to higher doses that most others in the area being treated. In addition, I have continually practiced an approach to pest control which begins with a threshold determination, to decide when and if control is necessary.

In many areas, this is called Integrated Pest Management. I began to look at DDT in far greater detail in the early 1960s, when Rachel Carson's book "Silent Spring" appeared, particularly because her narratives were not consistent with my own experience. If I had been mistaken, I wanted to be the first to know.

At the time, I was working on control methods for an aquatic midge in Clear Lake, California, and Ms. Carson had told about the earlier control efforts for this midge in Clear Lake, in infinite (and less than accurate) detail. A few years later, I was listening to a founder of the Environmental Defense Fund (EDF) who told our State Legislature that "anyone who used or recommended the use of DDT did not know his business".

As part of the EDF presentation, which included an unequivocal statement that DDT causes cancer, I obtained the book of photocopied papers which the EDF witness had said were the “very zest of the Literature on DDT". Thirty years later, I am still looking for convincing evidence to show that I was wrong in allowing the use of this pesticide for food production and health protection.

In the scientific field dealing with pesticides, there is a need for more people to read the literature in detail, because often times, there exists a temptation, even among scientists, to use "adversary science" in place of objective science. There is also a tendency to rely on the abstract or summary, in place of reading the whole article.

Adversary science is the art of telling only the facts which are favorable to one's own belief, and hoping that the opposition (or the media) will not be able to cite the data which was omitted. Right now, the U.S. Supreme Court is trying to decide what acceptable scientific evidence is. The problem has come into focus because we are allowing lawyers and judges to replace the older and more appropriate method of scientific debate. A real scientist will be the first to try to refute his own hypothesis. Part of my commitment to look for the "facts" about DDT, and having seen apparent abuses of scientific integrity', led me to volunteer to help the lawyers for the U.S. Department of Agriculture and the manufacturer of DDT in the U. S., when they examined the witnesses from the Environmentalist's side, in the 6 month, long public hearing on Cancellation of the uses of DDT.

As some EDF members had bragged, they would be happy to submit to "the crucible of cross-examination." This was art excellent learning experience, and it provided me with the exhibits and sworn transcripts of the interesting parts of the proceedings. The issue of carcinogenicity of DDT was one of the topics covered, and it is still under debate.

Separating the Issues

The specific topic of this seminar, "Is there a linear relationship between dose (and time of exposure) and the development of cancers in animals?" requires some ground rules, before the question can be answered for the chemical DDT. These separate topics need consideration.

  1. Is each isomer and metabolite considered separately or are all of them considered as one chemical? The literature on testing seems to be based both on, purified and "Technical" DDT, neither of which are expected to contain much if any of the metabolite DDE. Good protocol should specify the use of a purified single chemical of known molecular structure.

  2. Are the doses limited to those which do not damage the homeostasis of the test animals? DDT is considered to be a relatively cumulative chemical in vertebrate animals, and lipid residue data for all test animals at sacrifice, correlated with presence and type of cancers could be a better measure of dose response than constant feeding at given doses in food or garage of neonatal animals.

  3. With DDT, it is imperative to decide whether the dose-response data should be consistent for all test animal species or hybrid classes, or only for one or more kind or species of test animal, or for larger taxonomic groups of test animals.

  4. The time of exposure and the latent period before autopsy is important, as well as the value of multi-generational studies.

  5. Is the interest in the cancers only in the test animals, or is the interest to provide predictions of risk for humans, or both?

  6. Should tumors be counted as malignancies? As I read the Background Statement for this Symposium, it seems clear that the discussion should try to compare, or at least be aware of the conditions imposed in the U.S. by the Delaney Clause (no cancers in any test animals under good test conditions) as well as to the "no-effect dose level coupled with a good safety factor" system for setting allowable residue tolerances in raw agricultural commodities.

A corollary requires a consideration of whether or not there is a true "no-effect" level for a carcinogen; this could involve use of epidemiological considerations dealing with large areas and human or other animal populations exposed to higher or lower doses of DDT for prolonged periods of time. Extrapolation From Laboratory Animals to Humans During the DDT hearing testimony of Dr Marvin Schneiderman, a statictician with the National Cancer Institute, it suddenly occurred to me that DDT, as a 25 or 50% "Tracking Powder" was very effective in controlling mice and bats, but it did not control cats.

I understood the difference to be that rat livers were like human livers in their response to DDT, but mouse livers could essentially not be induced with DDT. Dr. Schneiderman testified, as I recall, that the protocol for laboratory cancer research required that two species of test animals be used, and that both of these animals must not have a known physiological system difference, if the data was to be used to extrapolate the dose response information to humans.

The following exchange took place between Dr. Schneiderman and a lawyer at the Public Hearing (Jan. 12, 1972, page 7142):

Question: If the purposes of these studies, Doctor, is to some day draw some extrapolation to man, would you consider it a drawback that the metabolism is different in mice and man? And if, as you, say, it has been. demonstrated, then clearly one should not use an animal that clearly has a different metabolism than man as an experimental animal?

Later in his testimony, Dr Schneiderman was asked if transplantation had any relevance to carcinogenicity studies, because there was concern about how to accurately determine: what was a cancer. His answer was: “One of the criteria for malignancy is the transplantability of a tumor. If a piece of tissue does not transplant and grow up into, another tumor, it's assumed that it may not be malignant.”

Dr Schneiderman admitted that transplanting was not done in the Bionetics study which he called a "minor defect" in that study. Between mid February and November of 1972, I exchanged letters with Schneiderman. The first references to species differences in liver inducibility were (I) J.W, Gillett, 1970. in Biological Impact of Pesticides in the Environment. Oregon State Univ., Corvallis Oregon, which deals with. Japanese Quail and (2) Hart and Fonts, 1965. Arch. f. Exp. Path. and Pharmak. 249:486-500, which considers sleep time chances with barbiturates when test animals are dosed with DDT and other chlorinated hydrocarbon pesticides.

My last letter, which was not answered, included a copy of a paper by J. R. Fonts, 1972 in Environmental Health Perspectives, October 1972, 55-66, that contains the following:

Thus, at doses and schedules of doses and at times after last dose of the inducer that clearly established hepatic microsomal enzyme induction by DDT or benzopyrene in rats and other species including monkeys, the mouse did not show such induction or showed it only marginally. (references omitted:, emphasis in text) This article gives some evidence for variation in inducibility for rats and mice, but the differences in tolerated DDT doses between mice and rats in the major papers usually relied upon for determining carcinogenesis, clearly points to physiological differences, with the mouse being aberrant, compared to rats and humans in its response to DDT treatment.

This article also has a long list of references concerning inducibility. A later paper by L. Tomatis et al., 1973 (lnt. J. Cancer 12: 1-20) reports that tumors were found in Rat livers following doses of DDT. The only reference to rats was a paper by Fitzhugh and Nelson, 1947 (J. Pharm. & Exp. Therap. 89:19-30). No other organs besides the liver were involved, and only tumors were found, according to Tomatis.

In 1986, EPA published 'Guidelines for Carcinogen Risk Assessment'. (1986 vol 51, of the O.S. Federal Register, pages 33.992-34003 and reprinted in EPA/ 625/3-90/017, Sept. 1989). Some of the pertinent statements are:

“The strength of the evidence supporting a potential human carcinogenicity judgment is developed in a weight-of-evidence stratification scheme. (p33994)” The weight of evidence that an agent is potentially carcinogenic for humans increases:

(3) with the occurrence of clear-cut dose response relationships as well as a high level statistical significance of the increased tumor incidence in treated compared to control groups;

(4) when there is a dose-related shortening of the time-to-tumor occurrence or time to death with. tumor; and

(5) when there is a dose-related increase in the proportion of tumors that are malignant. (p33 995) ---

A statistically significant excess of tumors of all types in the aggregate, in the absence of a statistically significant increase of any individual tumor type, should be regarded as minimal evidence of carcinogenic action, unless there are persuasive reasons to the contrary (p33995). In the absence of appropriate human studies, data from species that respond most like humans should be used, if information to this effect exists. (p33997).

Major Studies The "Bionetics" Study (Innes et al. 1969. J Nat. Cancer Ins t 42:1101-14) was an ambitious project which was intended to screen 120 chemicals on mice at doses which were close to that which would cause toxic symptoms. For DDT, the dose was 46.4 mg/Kg, by stomach tube from day 7 through day 28 of age, and the dose in the food was then 140 ppm for the remainder of an 18 month test period. The mice used were two essentially new crosses, using two different male stocks and the same mother stock for both crosses.

The test began with 18 mice of each gender and of each cross (72 mice per chemical). All mice tested for each chemical were given the same dose of chemical, apparently for the purpose of answering only the single question of could the administered chemical induce tumors or cannot?

Dr. Falk, one the Co-authors of the lnnes et al. paper presented a speech, which, was later printed as an article (Nat'l. Pest Control Operators News, October 1971 p4-6 and 29-31). In this presentation, Dr Falk provided comments about the test methods and the implications of the test results. He also commented about the way the test mice were bred and the fact that the suitability of these crosses for cancer testing was unknown. There apparently was no history of cancer susceptibility of these crosses.

Dr Falk told about various parts of the test, then he said: “We came up with positive results, for instance, a significant number of Hepatomas in mice on 140 ppm DDT in the diet. But as we decrease the DDT level to that which we [humans] are exposed to now, most likely we would have to conclude that the hepatoma incidence would be 0 %.”(emphasis added) One paragraph of the Falk article is long but important to our understanding. It says, referring to the Bionetics Study:

“The lack of malignancy on the liver and lung tumors produced a controversy which while quite artificial was, nevertheless, quite heated. The Delaney amendment compels the Food and Drug Administration to ban any chemical for use on -or in- food if it can produce cancer at any dose level and by any mode of administration. It specifically states that the tumor has to be a cancer. So the question arose, did these tumors have a tendency, to become cancers?

Three types of tumors were observed in these studies. They were either hepatomas, and by definition a hepatom is a benign, tumor, i.e. it does not invade surrounding tissue or metastasize to other organs, in other words it grows confined to the organ of origin. With time the tumor may become invasive, in which case its cells attack the surrounding tissues, but it may as yet not spread to distant tissue like lung or lymph nodes. At that stage, it would be malignant.

This behavior is quite characteristic for mouse tumors. They do not quite behave like human cancers. Whereas a hepatoma is a benign tumor, it is generally conceded that given another six months, it would most likely have become malignant and killed its host. But you can appreciate that the FDA has a difficult decision to make because at the moment of sacrifice at 18 months, most of the tumors were benign.

Lung tumors, the second type of tumors, similarly were benign tumors which probably would have turned malignant with time. The only neoplasm that was recognized as a malignant tumor was the reticulum cell sarcoma (a type of lymphoid tumor.) (Emphasis added) The data published for the Bionetics Study (Innes et al., cited above) shows "lymphomas" as occurring at a 0.05 confidence level for all mice tested, with females of each hybrid type having "lymphomas" at the 0.01 confidence level.

These levels of confidence do not suggest clear and convincing evidence of carcinogenicity, unless one wants to enter into the speculation or "educated guess" type of logic which Dr Falk referred to.

Dr Falk then goes on to explain that if the tests were allowed to go to 2 years, there would have been natural mortality, with cannibalism or decay "when no one was looking." In early 1974, following the cancellation, in the U.S. of most uses of DDT about 2 years earlier, an Appropriations Subcommittee of the U.S. Congress asked EPA "To Initiate a Complete and Thorough Review Based on Scientific Evidence of the Decision Banning The Use of DDT".

Testimony by' Leonard Axelrod, Ph.D., the EPA person who was doing the review is recorded in the transcript of hearing for April 4, 1974. It reads in part:

“There is at the present time, no evidence that DDT is carcinogenic (or tumorogenic) in any animal species when administered at levels less than two orders of magnitude higher than the maximum dose attainable by plant manufacturers and workers over a lifetime of exposure.”

6. There is at present no evidence from experimentation that DDT is either teratogenic or mutagenic.

7. There is no evidence from human monitoring studies over a 5-year period that DDT as occurring in the environment has caused an increase of any known physiological dyscrasia. A copy of a later draft of this Report, dated June 5, 19741 was obtained from a student who was allowed to work at EPA in Washington, D.C. during the time this study was in process.

In the part dealing with "Carcinogenesis and Tumoragenesis" (pp 21-22), Dr Axelrod lists the published reports available since January 1972 which he had thoroughly reviewed. The list and conclusions with Dr. Axelrod's hand written corrections, suggest a reasonable review of the pertinent literature available at that time. Discussion and evaluation of the papers used by the environmental "experts" in the record of the public hearing are included by reference by Dr. Axelrod, in this draft.

One of the conclusions in the 6/5/74 draft appears to be based on the studies referred to as the Lyon and the Hilan Studies (Turusov et ai.1973 (J. Nat'l. Cancer Inst. 11:983-97) and Terracini et ai.1973 (Int'l. J. Cancer 11:747-64) respectively. It is important enough to be quoted here: “If all tumors were considered carcinomas then an extrapolation on a linear-dose-response basis would correspond to one or two additional cases of liver cancer in 200 million humans” --- If mice have a predictive value for human risk of cancers, which appears to be in serious disagreement with the EPA Guidelines (Protocols?), this data would suggest a 1 to 100 to 1 to 200 million added risk value, or 100 to 200 times what our EPA has proposed as a de minims cancer risk level for residues of pesticides in food, The conclusion of Dr Axelrod in June of 1974 was changed in December 2, 1974 Working Paper. In it he says: “One can conclude that available evidence indicates that the likelihood that DDT is a human carcinogen is very low.”

Note: Dr Axelrod died suddenly, shortly after the preparation of the 12/2/74 draft. The final Report by EPA is dated July 1975, and it does not read anything like the Axelrod writings. The final Report appears to have been sanitized, possibly to sustain the EPA Administrator's earlier conclusion that DDT was carcinogenic. Dr. Samuel Epstein, a witness called by the EPA and the EDF at the Public Hearings on January 14, 1972 has a different perspective of human risk than Dr. Axelrod.

Epstein testified, on page 7370:

Q.: Based on your experience and upon your survey and analysis of the literature, would you give us your professional opinion, if you have one, either the presence of DDT in the human environment represents a significant carcinogenic hazard to man?

A. “I can only answer that qualitatively, and the answer is yes,”.

Q. In your professional opinion, can a man be safely exposed to any level of DDT? The answer is no. During his: testimony, Dr. Epstein who said he was on several advisory committees to the group which, produced the document known as "the Mrak Report", introduced 4 pages of a paper referred to as 'the unpublished Fitzhugh Report,' which Dr. Epstein characterized as one of only 4 studies up to that time, "which provide definitive evidence of carcinogeniclty."

This manuscript (O.G, Fitzhugh et al. undated, titled "A Summary of a Carcinogenic Study of DDT in Mice-- Unpublished data from the Bureau of Science, Food and Drug Administration), along with 2 attached memoranda (dated 1/30/69 and 9/15169) were only made available after this witness was excused, and no longer available for testimony . The total document shows some possible reasons why this study was not published, and does not invoke a reeling of confidence in the predictive value of this study.

Page 33 of Attachment number 2 contains information, to show: Unusually high mortality in some male mice during the second year of the 2 year study resulting in relatively few animals at risk when they were most likely to exhibit tumors. Report of a tumor of the testis in a female mouse. Report of an ovarian tumor in an animal reported lost from the animal quarters earlier. A problem with the DDT dose occurred in the 67th week, where the 100 PPM dose was increased to 300 PPM, resulting in increased mortality. (page 4 of Attachment 2, dated September 15., 1969) Note: The Report shows 318 of 100 male, and 22 of 100 female mice died between weeks 62 and 66. The unpublished Fitzhugh Study used only one dose of DDT, so no dose-response information can be drawn from it.

Dr. Epstein characterizes the results on the research reported by Hayes et al. 1971(Arch, Env. Health 22:1-9), Laws et al. 1967 (Arch. Env. Health 15:766, and Laws, 1971 (Manuscript submitted to EPA May 13, 1970) as "inappropriate", yet the first two of these papers report on work done in humans, and the third showed' apparent inhibition of the "take" of transplanted cancer cells in mice, when the mice had been treated with DDT. (see discussion under Benefits)

There are tests reported where DDT was administered to dogs (Lehman, 1952. Q. Bull. Assoc. F&D Officials of US.16:7, and later Ottoboni 1977 (Arch. Environm. Cont. & Tox. 6:8-101), llansters (Agthe et al. 1970, Proc. Soc, Exp. Biol. & Med. 134:113) and Rhesus Monkeys (Durham et al. 1963, (Arch. Int. Pharmacodyn. 141:111).

All of these tests, except for Ottoboni's work are questionable because of the conditions of the tests. Ottoboni tested purebred Beagle dogs continuously dosed for 3 generations at 10 ppm DDT in their feed, with sacrifice at age 2 I/3 years. No adverse effects due to the treatment were found: She reported (personal communication): that there were observable beneficial effects in the treated dogs in this multigenerational study; she also said that the technicians began to refer to the untreated control dogs as "the DDT deficient animals."

Appended to this paper is a copy of some tables presented by Carrol Well who was with the Mellon Institute in Pittsburg, Pennsylvania. These tables were part of a presentation to the Entomological Society of America annual meeting in. 1975, to show some of the problems associated with trying to interpret the kind of data that can come from animal assays for carcinogenicity in mice. In the Table marked "A", if by chance the male group number 5 had been the control group, all the other untreated groups would have been called positive. "B" shows the potential effect of diet type and; "C" shows the difference in tumors when mice are allowed to eat all they want, compared to some limitation on the amount of food they receive.

The study reported by R. Kimbrough et al., 1964 (J Nat'l. Cancer Inst., 33:215-25) confirms that Rats also can show adverse effects (leukemia) due to use of a purified diet, and not due to the DDT used in their tests. Conclusions About Cancer Studies The most reasonable conclusion I can draw, based on the review of the references I have looked at and the other materials available to me concerning DDT and cancers in test animals, is that there is not enough data to get any kind of clear picture about dose/response for cancers with DDT in mice, if such a relationship exists.

Furthermore, there will probably be few if any new studies because DDT is essentially gone from the market-place in the U.S., which is the source of most of the funding for massive cancer research programs. If one accepts the idea that mouse livers are essentially non-inducible with DDT, then there seems to be two alternative and perhaps connected hypotheses to work with. One is that the mouse studies are really looking at animals whose physiological systems are overwhelmed by a foreign chemical and that adverse results occur because of this disruption of homeostasis in the test animals, and not due to a site or system specific carcinogen.

The second hypothesis is that there is no dose/response correlation for cancers or hepatomas, when DDT is administered. Both of these hypotheses neglect the question of whether or not DDT is carcinogenic in the animals tested so far, and the most reasonable answer to this question is that the evidence available to date does not support the idea that DDT is a carcinogen.

Two studies with humans have been reported, and both are negative for definitive adverse effects. These were the studies which Dr. Epstein dismissed as inappropriate. Hayes et al, 1971 (cited earlier) fed DDT volunteers doses of DDT at rates of up to 35 mg. per day for 21.5 months, and some of the people were observed for 5 years from the initial feeding period. The dose was calculated at 535 times the average daily intake in the U.S., and no adverse effects were observed.

Laws et al. 1967, (cited earlier) followed 35 men with 11 to 19 years of high exposure at a facility where DDT was manufactured, and these men showed no adverse effects attributed to exposure to DDT. Based on storage and excretion levels, the average daily intake was estimated at 3 to 18 mg per man per day.

This compared to .04 mg per day for the general population. Fat residues for these workers was 39 to 128 times the level in the general population. Even though the evidence is negative, it seems to be of more value in assessing the risks of DDT exposure to humans than all of the mouse data accumulated over the years. Even with the use of DDT beginning with World War II, and extending up to 1972, any evidence of increased human liver cancer rates appears to be absent. This may be analogous to a hypothetical cancer researcher saying "We must study reality to see if it works out in theory.

" Beneficial Effects of DDT in Animal Tests

It is interesting to look at the tests which do not cause some harm, but rather show some measurable results which seem to be beneficial to the test animals. Once in a while, there may be a surprise and the expected results may not occur. We all need to be open to letting the data tell us what it has to say.

M. A. Ottoboni, 1984 ('The dose makes the poison', Vincente Books, Berkeley, California, 222 pp.) provides: an understandable discussion of the types of chemical carcinogens. The list of types include:

Primary Carcinogens - They directly start the cancer process. (example-Radiomimetic drugs )

Procarcinogens - Not carcinogenic, but may be converted to a carcinogen (example-Benzopyrene)

Cocarcinogens (=promoters) - Enhance the carcinogenic action of another chemical. They may change the rate of metabolism of carcinogens, alter biochemical pathways or interfere with the repair mechanisms that would otherwise reduce the effect of the carcinogen.

Secondary Carcinogens - Not carcinogens, but they indirectly cause tissue damage. (example-Oxalic acid may produce bladder stones which, over time can irritate the bladder and cause cancer from the irritation) She also points out that chemical carcinogens are dose related and quite site specific. Furthermore, low doses and high or "heroic" doses may not act the same way on test animals. She says at page 1 the use of heroic doses is accepted without question by regulatory agencies despite its acknowledged pitfalls, while the public is generally unaware that such pitfalls exist.

The pitfalls derive from the fact that the biochemical fate of very small doses of a chemical is usually not the same as that for large doses. The differences that chemicals display between their acute and chronic toxicities is testimony to the fact. Small doses of a chemical may follow metabolic pathways that does not convert it into a carcinogen, but with increasing doses the pathway becomes saturated and the excess chemical is diverted to a new pathway that does convert it to a carcinogen. Or, small doses of the chemical may be prevented from exerting carcinogenic activity by combination with a biochemical normally present in the body.

If the supply of the biochemical is expended by the large doses, the excess chemical is then free to exert its carcinogenic effect. In considering the beneficial effects, such as the apparent prevention of cancers, there may be a whole group of physiological activities working in concert. There are some studies which I want to call to your attention, particularly because they seem to refute the idea that DDT in test animals will result in adverse effects.

K.C. Salinskas and A.B. Okey, 1975 (J. Nat, Cancer Inst.55:653-57) predosed Sprague-Dawley Rats with diets containing either 100 ppm of DDT and or 250 ppm of Malathion, for 14 days. Beginning on day 50 following the conditioning, these rats were dosed; daily by stomach tube for 21 consecutive days with 0.714 rag. of dimeythylbenzanthracene (DMBA), a known carcinogen. These animals were sacrificed 23 days following the start of the DMBA treatment. The DDT treatment reduced the incidence of rats with leukemia and mammary tumors, and no deaths occurred with the DDT treated rats until the time of autopsy.

The significance of the absence of tumors in rats treated with DDT was greater than .001. The authors speculate that the DDT caused increase hepatic enzyme activity, causing increased metabolism and excretion of the DMBA

E.M. Walker et ai.1970 (Indust. Med. 39(7):60) administered DDT in dimethyl sulfoxide (DMSO) 4 to 6 days l.P. to mice which had Ehrlich ascites carcinomas. The note I saw was short, and the reason for mentioning this reported inhibition of tumor development was to comment on the use of DMSO as a carrier. The same solvent was used with DDT in part of the Bionetics Study, which was looking for, but not finding evidence of Teratogenicity in mice when DDT in DMSO was administered sub-cutaneously.

When I received the document which contained the report of this study, I wrote to the National Institute of Health and asked about this use of DMSO. The NIH response was that DMSO alone was tolerated without teratogenesis in the types of mice under test. The concern I have is that in those pesticides where there was some reported adverse effects, the coupling effect of DMSO could cause greater than simple additive effect. In live animal tests, the cell walls might be crossed faster with a solvent such as DMSO, because of its alleged capacity to penetrate cells and its solvent which could cart another dissolved chemical through cell walls, that otherwise might not have been crossed by the chemical without the use of the solvent.

Using a test where cancer was induced in mice by subdural implantation of methyl-cholanthrene crystals, E.R. Laws (in a manuscript submitted to EPA on May 13, I1970), reported that cells from this cancer could be transplanted by injection of a saline suspension of cells into other mice. He reported that visual tumors developed in I0 to 18 days, with subsequent relentless growth and eventual death of the mice as the real consequence.

DDT administered at 5.5 mg/kg/day gave preliminary evidence of prolonging the life of the treated and challenged animals. There was some suggestion that DDT had an adverse effect on the Soi Potassium stimulated phase the two, and an alternative suggestion was that DDT might have a "general subliminal toxic effect on the whole animal which could make it a less suitable host for the transplanted tumor.

A paper by R.P.H. Thompson et al.,1969. (Lancet II (7161):4-6, July 5, 1969) explains how DDT was administered to a 17 year old boy for control of unconjugated juvenile jaundice. This treatment replaced phenobarbitone which had been used earlier. The mechanism suggested for the successful therapy was the induction of liver microsomal enzymes, which resulted in educed plasma bilirubin levels.

The treatment resulted in, producing an elevated plasma DDT level, and for 7 months after the end of DDT therapy, the bilirubin level had remained low. The authors reported no side effects were noticed, there was no proteinuria, and other liver function tests and routine haematological tests remained normal. A paper by Hazeltine in 1971 (Clinical Toxicol. 4:55-61), looked at the literature for evidence about DDT residues in people living in agricultural areas, and the possible impact this chemical and other agricultural chemicals might have on juvenile jaundice.

The conclusions I drew were that DDT appeared to: be present in significant (therapeutic) levels in the body fat of people living in this area that the observed infants who were breast fed and had a reduced incidence of jaundice, could be explained by the effects of this residue. There were other interesting parts to this literature review, such as the competitive or counteracting effect on liver enzyme induction by DDT, attributed to exposure to the pesticide Malathion.

The competitive or antidotal effect of DDT and barbiturates was the basis for hospital emergency room treatment of attempted barbiturate suicide cases with injected DDT. Richard Rappolt Sr.,MD. personal communication) Dr. Rappolt treated at least two patients with DDT dissolved in peanut oil; both patients were reported ambulatory and went home the next day. This therapy evolved from the understanding of veterinary, practitioners who use barbiturates to antidote animals that have organochlorine pesticide poisoning.

Apparently the competitive antidotal action explains the speed of effect, which is too fast for enzyme induction and metabolism of the excess chemical. It was also interesting and sad to see Dr. Rappolt dismissed from the hospital emergency room where he practiced for reasons that appeared political. The published newspaper story explained that he was dismissed for experimenting on people with DDT; his reply was that a good emergency room physician must be ready for all kinds of unusual situations.

Finally, while not directly dealing with cancer testing, but still bearing on the rat liver enzyme induction issue, two papers should be mentioned. These are P.R. Datta 1970, and Datta and Nelson, 1970 (Industrial Med. 39:190-94 and 195-98). These papers report on a study in which carbon |4 DDT and its metabolites were applied to perfused rat liver and kidney slices, with the rate and metabolite production recorded. The experiment was well quantified by accounting for the total radioactivity.

Each step of the metabolic process was confirmed by synthesizing the molecules and applying each one to confirm, the entire process. Keep in mind that this was an in-vitro study and there was no fat sink to preferentially absorb (selectively partition) the highly lipid soluble molecules, as would occur in a living rat. These workers showed that the metabolism proceeded from DDT to DDA, a water soluble metabolic product. The rats used in this test were preconditioned with DDT for three days prior to the use of their tissues. These authors reported that the first roughly two-thirds of the metabolism occurred; in the liver, and the last third in the kidney. The rates of metabolism were found to be quite rapid in this isolated tissue system.

The alleged persistent DDT metabolite in nature (DDE) was degraded to DDA, with 14.5% of the DDE converted to DDA in 24 hours in untreated, unconditioned animals, and 23.8% found as DDA after 12 hours in rats which had been conditioned for 3 days with DDT.

Summary

In response to the format proposed for this Seminar, and looking for significant correlations between forecasts and the facts available today, it appears that the suggestions and predictions made in the 1970s about DDT as a human carcinogen were only strawmen which have failed to occur. The data from a number of studies showing some alleged adverse effects in mice fed at maximum tolerated doses of DDT for lifetimes (more or less) is suggestive but extremely difficult to interpret.

At the same time, there is evidence to refute the hypothesis that DDT is a human carcinogen. These conflicts appear to occur for a number of reasons. Some of these reasons are:

  1. The choice of inappropriate test species and protocols to evaluate DDT as a carcinogen, has led to a diversity of conflicting results, high doses of DDT have seemed to produce random physiological effects, but these conflicting data have had the tendency of making the whole process of dose-response evaluations of the data for cancers remote, if not impossible.

  2. There is no clear data for the doses correlated with cancer production in mice treated with large doses of DDT, in the literature have reviewed for this seminar. This suggests that DDT is not causally related to the adverse conditions seen in the test mice, so other explanations should be sought to explain the abnormal pathology which is described.

  3. People who work in "pure research" do not seem to be interested in getting into the political arena, where their research findings may be expected to be made to fit into some social framework, or to support some cause.

  4. Extrapolations of data that may provide some meaningful insights may also involve some risk of being wrong. Therefore such extrapolations are usually avoided by "pure research" workers, and let to the “apocalyptic", who do not seem to care much about accuracy or integrity.

  5. Positive data on anticarcinogenic and therapeutic effects of are generally neglected, when looking at dose-response data. Most researchers are looking for harmful effects. Society does not reward findings of no harm.

  6. Tere is a strong tendency to neglect the unprovable such as the evidence on improved vigor and increased life span for the human population, which has been exposed to low levels of DDT for many years.

  7. We need to examine the idea that a potential or weak organic chemical carcinogen, can also act in a way analogous to a therapeutic agent at non-carcinogenic doses, to show beneficial actions within a dose-response relationship. If there are both beneficial effects and low dose, and harmful effects such as carcinogenicity at high dose, from the same chemical molecule than it could be appropriate to label a carcinogen as beneficial. Selenium, the inorganic element may to provide a model for this kind of thinking.

  8. In any carcinogenic testing experiment, the most important question concerns what is the highest dose which the test animal can tolerate without loss of normal physiological body system functions. Doses which cause loss of homoeostasis are not expected to show reproducible dose-response effects at any particular target site. This high dose loss of homeostasis is the most reasonable way to interpret the alleged DDT mouse cancer test data reviewed for this report.

References

  1. B.L. Cohen, Critical Rev. in Environ. Control. 22:243-364; 1992.
  2. B.L. Cohen, Int. Jour. of Epidemiol. 19:680-684; 1990.
  3. H. Morgenstern, Am. J. Pub. Hlth. 72:1336-1344; 1983.
  4. S. Greenland and H. Morgenstern, Int. Jour. of Epidemiol. 18:269-274; 1989.
  5. S. Greenland and J. Robins, Am. Jour. of Epidemiol. (in press) 21:422-424; 1992. (in press)
  6. B.L. Cohen, Int. Jour. of Epidemiol.
  7. B.L. Cohen, Am. Jour. of Epidemiol. 15 References
  8. BEIR (National Acad. of Sciences Com. on Biological Effects of Ionizing Radiation). Health Risks of Radon.... National Academy Press, 1988 (BEIR-IV).
  9. B.L. Cohen and G.A. Colditz, Environmental Research. (in press)
  10. U.S. Public Health Service, Smoking and Health: a national status report. DHHS Publication 87-8396; 1990.
  11. U.S. Public Health Service, Morbidity and Mortality Weekly Reports 36, 581-584; 1987.
  12. Tobacco Institute, The Tax Burden on Tobacco; 1988.
  13. B.L. Cohen, Health Physics 60, 631-642; 1991.



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