CANCER CLUES FROM COMPARATIVE MEDICINE William L. Simpson, M.D., Ph. D. Scientif ic Director of Detyoit Institute of Cancer Research Professor of Oncology a t Wayne University, Detroit, Michigan

We will start by asking a few ques- tions: Wha t causes cancer? Can it be prevented? Can it be cured? Is it hereditary? Is it transmitted by a vi- rus! Does smoking cause lung cancer? How about smog? These and a hun- dred other questions plague us day to day, because work as we will, we do nct have unequivocal answers to any of them. You will understand I know if 1 try to touch on several important areas cf research in summary fashion; I will refer to work with expuimental animals and to cancer that oc,curs spon- taneously c r naturally in species other than man. Only with respect t o prac- tical matters and legal restrictions can I recognize medicine as being divided be- tween humans and the other animals. 'There is no such division in medical or biologi.ca1 research. Most of what we learn about human disease and its con- trol, we learn from other animals.

Cancer is a universal disease. It oc- cws naturally in nearly every species in the animal kingdom and in many plants as well. I t may be present in any multicellular organism but it can never exist in a single cell. I n other words, cancer, as we know it and discuss it to- day, is a disease of an intact or whole organism. For centuries caacer was thought t o be a uniquely human disease. Ncit until animals were extensively do- mesticated and maintained as pets or in zoological gardens until they attained old age was it found that tumors often cccu.rred in many species. I t was less than a century ago that the widespread d is t r ih t ion of cancer as a cause of ill- ness and death in other mammals than man was recognized.

Attempts to produce can,cer in labora- tory animals so that the disease might be studied at will are even more recent. Ideas on the causation of cancer were heavily influenced by the German path- ologist, Virchow, whose speculation was

put fcrth just over a hundred years ago, in a series of 20 lectures he gave in 1858. Discovery that certain German coal tars were both irritants and asso- ciated with skin cancer in workers added weight to the chronic irritation hypothe- sis which Virchow had announced. Shale oils in Scotland and lubricating oils in English cotton spinning mills quickly joined the ranks as suspected irritants and thereby associated with skin cancer. It was about 1875 that these three sources of occupational can- cer were recognized.

A German pathologist, Hanau, was an early investigator. Hanau is remem- bered principally as the first person to transfer o r transplant successfully a tu- mor from one animal into another. But Hanau also tried diligently to produce cancer in experimental animals by re- peated and prolonged applications of crude coal tars t o the skin of rats and dogs. He never succeeded. Failing in his efforts, and unrecognized for his pi- oneering in the study of tumor trans- plantation, Hanau committed suicide. He never knew it was just bad luck that had made him choose two of the most resistant species to chemical carcinogenic agents. T h e dog and the rat proved their resistance many, many years after that. Had he used rabbits as did Yama- giwa and Itchikawa in 1915 or mice as did Tsutsui in 19 18, he might well have anticipated their classical reports by more than a quarter of a century. To- day, we know literally hundreds of chemicals that will cause cancer to ap- pear in experimental animals, and more are added each year. Many of these are also known or at least suspected of caus- ing cancer in man.

T h e evaluation of the potential haz- ards of chemicals t o man has been and remains a distressing problem. You will recall the clamor attending the re- cent announcement that certain coloring agents were to be banned by the Food

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and Drug Administration, and could no longer be used on citrus fruits. Other dyes were to be eliminated from lip- sticks. Even worse was the panic that gripped many persons after it was an- nounced that certain weed killers used iq cranberry bogs were feared as poten- tial carcinogenic agents. Pressed for the reasons behind these actions, Federal authcrities had to admit that them was relative evidence of carcinogenicity in one or another of the rodent species and that they had no way to measure the hazards to humans. Th. old adage that one man’s meat is another man’s poison isn’t sufficient when dealing with such dangerous agents. In scite of the limi- tations of our knowledge of the com- parative carcinogenicity of agents from one specks to another, prudence tells us that we must err on the conservative side until better ways are found to test the effects of certain chemi’cals on man. We are not really worried about the lipstick that mice might use o r the cran- berries that rats might eat, but we still have to look at the effects of these chem- icals on rodents as cause for caution in the case of their being used as food ad- ditives for humans.

T h i s approach to the discovery of cancer-producing agents is in sharp con- trast to the historical record. You will recall that Percivall Pot t suspected chim- ney tars and soots to be carcinogenic on the basis of his observation of scrota1 cancer in chimney sweeps, as far back as 1775. As noted earlier, clinical ob- servations that the occupational hazards of the German coal tars, the Scottish shale oils, and the English lubricating oils were made a century later. T h e direct test for the carcinogenicity of shale oils was not made for a half century after the clinical suspicions. Only in 1922 was the report published by Scott showing that the prolonged application of shale oils t o the skin of mice led to the development of typical squamous cell carcinomas and sarcomas. From that time to the present, it has been common practice to use some other spe- cies than man to prove the cancer-pro- ducing properties of suspected chemicals and to test for possible carcinogenicity of new compounds before they are intro- duced into human medicines, foodstuffs and the like. Based chiefly on their

carcinogenic effects on mice and rats, we have found that our increasingly com- plex chemical environment presents many hazards. T h e old coal tars, oils and soots were bad enough, but now we can make thousands of new compounds from ccal Gr petroleum, by cracking and synthetic processes. I t is no secret that these procwec also lead to many new circinogenic compounds. Continuing research into the production of cancer in animals has led to the discovery of many diverse carcinogenic agents, chemical, physical and biological.

In addition to the chemi8cal, there are the physical agents. Radiation of all types produces cancer in the experimen ~

tal animal, and in domestic animals with which you are familiar. Among the more unusual observations of the physi- cal carcinogens is that of the Oppen- heimers showing that plastic films o r even metal films introduced under the skin of rats will lead to the development of sarcoma in this species. There is no point in recounting for you the great variety of chemicals known to be car- cinogenic for one or another species. Suffice it to say that the variations in chemical structure are so great that it makes a most difficult task to find any common mechanism by which they might operate to convert a normal cell to a malignant one.

Although many people are working on the problem of mechanisms of action of carcinogenic chemicals, and a certain nnmber of provocative theories have been offered, we have to admit that we are still almost totally ignorant in this area. Theories have been based on elec- tron density in particular regions of the hydrocarbon molecule, others on the formation of cross linkages between chromatin strands as a result of radiomi- metic drugs or on the formation of new heritable structures either in the nucleus or the cytoplasm. All these have flour- ished from time to time within recent years. Unfortunately none of them seems to be broad enough to explain the effects of the great variety of chemical and physical agents now known to be carcinogenic. An alternate approach to the explana-

tion of carcinogenicity is that of sub-

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microscopic biological agents commonly referred to as viruses. From the pub- licity given this concept recently, one might think it had just been discovered. Yet historically it can be traced at least to Borrel in 1907; and those of you in veterinary medbcine will recall well that fowl leukosis was identified as attribut- able to a filterable agent by Ellerman and Bang in 1908. Three years after this, Rous and his associates at the Rockefeller Institute showed that a spontaneous sar- coma in Plymouth Rock chickens was attributable to a virus, and could be transmitted by cell free extracts. Viruses were subsequently found to be associ- ated with the transmission of a few tu- mors in lower forms, for example the kidney tumor described by Balduin Luck6 in the leopard frog. There was no substantial suggestion that a virus could be associated in any way with a mammalian tumor for a full quarter of a ‘century. The milk factor, described by the staff of the Roscoe B. Jackson Laboratory in 1935, was the first such agent and it did not attain the status of an acceptable virus for many more years.

The beginnings of a real upsurge in the tumor virus field came when Ludwig Gross reported that he had been able to transmit leukemia in his inbred mice by cell free extracts and believed that a virus was responsible. This was a radical departure from all previous reports on mammalian tumors. Other investiga- tors were for a time unable to reproduce Gross’ results. T h e acceptance of his findings was delayed for several years, then almost over night Gross’ results were confirmed, and a rash of other new viruses were found in mice.

Stewart and Eddy at the National Cancer Institute, Charlotte Friend at the Sloan-Kettering I n s t i t u t e, Steven Schwartz at the Hektoen Institute were among those describing new tumor vi- ruses. Schwartz claimed to have iso- lated a mouse tumor virus from the brain tissue of human cancer patients. The Stewart-Eddy virus provided an- other new facet, for here the agent was apparently capable of crossing the species barrier. I t produced tumors in hamsters as well as mice and was capable of pro- ducing several types of tumors in mice. Hence it acquired the name Polyoma vi-

rus. Until the Polyoma virus was iso- lated, there had been an extremely high degree of specificity ascribed to each vi- rus. One of the arguments used by those who did not accept the virus con- cept was we would have to postulate many thousands of kinds of viruses in order to have a specific virus not only for every species but for every organ or site which was involved and for every type of tumor which occurred in each of these sites.

It is difficult to know what changes in technique have been primarily respon - sible for the successes now reported in the virus field. This is a contrast to the long and dismal record of failures by early investigators who sought to find viruses in tumors. Undoubtedly the development of tissue culture methods for propagation of tumor viruses helped in providing higher concentrations of the agents than had previously been attain- able. This is the same method of grow- ing viruses in tissue culture that pro- vided the turning point for research on vaccines for the prevention of poliomy- elitis. The initial observations were at- tributed largely to Enders of Harvard Medical College.

One prerequisite for transmission of tumor viruses appeared to be a require- ment for introduction of suitable cell free suspensions at a very early point in the life of the recipient animals. Resist- ance to most oaf the murine tumor vi- ruses develops quickly, even within a day or two of birth. As studies have continued and it has been possible to build up higher concentrations of virus, it has been found that the period of in- fectivity may be prolonged for several weeks in the case of the leukemia virus. Even though these viruses are introduced at such an earIy stage, tumors may not appear for many months. This has led many investigators to question whether the virus should be considered an acute carcinogenic agent or as merely starting in motion some chain of biological or biochemical activity within a susceptible host. It might thus reproduce the same chain of events as may be initiated later in fife by a variety of chemical carcino- genic agents. The virologists, in defense of their specialty, would ordinarily pro- pose, however, that the virus is the real

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carcinogenic agent and that chemicals merely activate the viruses, which nor- mally reside in the cells of the organism in a dormant or latent: fashion.

Because viruses represent almost pure suspensions or solutions of nucleopro- teins, much attention has been directed toward discovering a mechanism of viral action. Considerable evidence has al- ready been adduced that the nucleic acid portions of the nucleoproteins are capable of transmitting infection and that this portion of the molecule, in contrast to the total protein complex, is capable of crossing species barriers. One may recall at this point the beauti- ful work by Dr. Fraenkel-Conrat, work- ing in Stanley's Laboratory at the Uni- versity of California on tobacco mosaic viruses, showing that it was possible to separate the protein from the nucleic acid portion of tobacco mosaic viruses and subsequently to recombine these t o form a complete and effective virus. Com- binations of nucleic acid from one type virus and protein from another were ~ l s o effective and thereby produced a totally new type of virus. As yet com- parable progress has not been made with the tumor virus from animal species, but this is currently one of the most active fields of investigation.

Let us now turn to the question of heredity. Cancer research has made great progress in the question of heredity in cancer, especially in mice. During the early decades of this century there were many heated arguments on the role of genetics in determining susceptibility to cancer. One of the strongest pro- ponents of the concept that cancer was a heritable disease was probably Maude Slye of the University of Chicago. Her conviction that the disease was attribut- able to a dominant gene, which could, if we wished, be bred out of existence, was unfortunately not 'correct. At- tempts to resolve this question contrib- uted immeasurably to research in an- other way, however.

I t was the challenge of mouse genetics that led Strong, Little, Murray, Heston and others t o develop the many highly inbred strains of mice, which are now mch a valuable part of our armamen- tarium for investigating every sort of bi- ological problem. T h e existence of

hundreds of well characterized strains is one of the factors that has made the mcuse the animal most used in medical research. A few years ago it was esti- mated that our 'consumption of mice cxceeded 10,000,000 per year in this country and I am certain that the num- ber has not declined. Currently more like 25,000,000 are used annually.

Discovery of an extra chromosomal factcr transmitted in the milk of mice in 1935 shook the foundations of mouse genetics mightily. Since that time re- finements in genetic techniques have made the role of inheritance much clear- er, if at the same time less important, in the development of mammary cancer. We now accept the fact that susceptibil- ity to carcinogenic agents is largely regulated by genetic factors. T h e milk factor, or Bittner virus, if you prefer, was cnly operative to produce cancer if the genetically determined susceptibility was at least moderately high, and only when a suitable endocrine environment existed; the latter can be shown to be under separate genetic control. Heston at the National Cancer Institute, and Snell at Bar Harbor have both investi- gated effects of specific genes and gene combinations on such susceptibility. It now appears that at least one set of genes associated with obesity in mice is sig- nificantly correlated with the develop- ment of cancer in the lungs and other sites in mice. Regrettably, we know lit- tle about the role of heredity in cancer in the human.

Reliable data on the existence of can- cer as a cause of death and on specific sites and types of cancer in man are dif- ficult to obtain. In only a few 'coun- tries have cancer registries been estab- lished for many years. I t is principally from these parts of the world that our best information comes. And yet if you look at your own family situation, you will know how difficult it would be for you to look back and tell any inves- tigator the specific cause of death of your own father or mother, certainly your grandmother, grandfather or aunt and uncle. In most cases you simply don't know and nobody knows this kind of information. In recent years, however, we have begun to get some important data out of limited populations where

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good genetic records were kept within individual families, notably some of the Mormon pcpulation in Utah.

Because of this difficulty in looking backward, one might think we should work on a forward looking study of cancer in man. But as you will recog- nize, it takes a long time to conduct snch a forward study in man. It is also quite rncertain how reliable comparisons are between genetic factors in a highly in- bred mouse population and a totally het- erogenous population as humans.

It seems to me that this is an area in which veterinary medicine can make a most valuable contrjbutiod Genetic records are already being maintained most carefully on several species. The establishment of registries in which the precise cause of death for a number of breeds of dogs, cats, and cattle would appear to be well within the capability of the profession. Such registries would be welcomed by cancer research workers generally, and I feel sure could be sup- ported by cancer research funds. I am sure, too, that the results of such studies would provide much closer comparisons with the genetic situation that exists in hu.mans than does the work done with mouse colonies. Working on a modest scale, some important contributions have already been made by studying genetics of cancer susceptibility in cattle. The group at M. D. Anderson Hospital and Tumor Institute in Houston show- ed that at least two genetic factors are asscciated with inheritance of suscepti- bility to so-called cattle eye cancer.

You already know that the Congress of the United States decided a few years ago to foster a major effort designed for the chemical cure of cancer. On advice of public testimony before House and Senate committees, a token appropri- ation of nearly a million dollars was made the first year for development of the program. T h e next year the alloca- ticns for this program were 3 million, then 9 million, then 15 and 24 million, u.ntil last year it reached 39 million dol- lars per year. It would seem justifiable to expect a report of some progress for that kind of investment, but a stum- bling block to progress has always been the lack of a reliable biological test for

the efficiency of a proposed therapeutic regimen.

Since the time that Paul Ehrlich suc- ceeded through the tedious process of trying 606 related compounds, there has existed a dream in the minds of many scientists that the cure for cancer could be found if we only tried enough chem- icals on enough different test systems. A further impetus to this kind of ran- dom screening came with the discovery of prontosil by the great German sci- entist, Gerhardt Domagk in the 30's. This made way, of course, for the whole battery of sulfonamides. Still further the chance observations by Fleming lead- ing to the discovery of penicillin seemed to support the notion that if we only looked long enough and hard enough we might find a cure for any disease.

The bald truth is that we still have no really satisfactory test for chemo- therapeutic agents against human cancer. The present screening methods were se- lected following a 'comprehensive evalu- ation of many diverse biologic and bio- chemical systems for their responses to 27 selected compounds known to have some effect on the growth of cancer or could be related to some of these effec- tive compounds. Thus, following the report by Gellhorn and Hirschberg the government sponsored agency known as the Caecer Chemotherapy National Service Center selected three transplant- able mouse tumors for the primary screening of chemotherapeutic drugs. Other screens were set up for the com- pounds that passed the primary screen- ing.

During intervening years still further hurdles, especially in the capacity to effect the growth of drug resistant lines of tumors have been added. At the present time the goal is to test nearly 50 thousand substances per year, a t least through the primary screening. Of these nearly 40 thousand are the prod- mcts of the growth of bacteria or other micro-organisms. These are first tested as crude filtrates. The other 10 thou- sand are pure compounds synthesized in organic c h e m i s t r y laboratories throughout the world. We operate such a synthetic laboratory at the Detroit In- stitute of Cancer Research.

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Turning again to these products of micro-biological research, nearly 10 per cent of the filtrates proved effective against the primary screening, and about 1 per cent pass the secondary screen- ing. In spite of this, there have been very few compounds yet come out of this vast program with sufficient value to be carried on into clinical medicine. Those which have found a place in clin- ical medicine had for the most part been discovered independently, or at least were known to have some effectiveness befcre they were entered into the screen- ing program. You have heard the names of these and have read of them in the popular press. Many of the com- pounds were found before the major screening activity was initiated. These include such drugs as the antifolic com- pounds, nitrogen mustard, triethylene- me 1 am i ne, triethylenephosphoramide, and triethylenethiophosphoramide.

As you know, methotrexate has been shown to control certain percentages of the rare cancer, choriocarcinoma, which occurs sometimes in humans following pregnancy. More recent introductions include 5-fluorouracil, which came from the laboratory of the University of Wis- consin. Actinomycin-D came from Ger- many and Rutgers University and more recently vincoleukoblastin from the Lil- ly Laboratories in Indianapolis. Much effort is now devoted to the develop- ment of better tests for new drugs. T o o much time and effort is now required to bring a drug from the point of initial screening to the level of evaluation in human subjects with advanced and un- treatable cancer. Qigh among the pos- sibilities I should like to recommend for consideration, the concerted effort to employ spontaneous tumors in dogs and cats for evaluation of new drugs.

Although there are differences in the most common varieties of cancer to be found in man and dogs, both are sub- ject rather frequently to progressive fa- tal neoplastic disease in which surgery and radiation are ineffective. Were such animals available for :the therapeutic trials of new drugs we might have much more reliable guides for efficiency before doing extensive toxicological studies and then going on into work with humans at the clinical level. Properly organ-

ized, with the cooperation of your pro- fession and the cancer research institutes, such a program ought to be welcomed by the owners of pets who would thus get a chance of having the disease ar- rested. Failing that, they would have had for their pets the best treatment now cffered to human victims of cancer, and might speed the day when truly useful chemicals would control cancer in man and other spacies.

I should like to pass over a couple of other areas that I think are important and areas in which progress is being made, but which do not lend themselves especially to discussion a t this time. One is the field of immunology where major efforts are being made toward the de- velopment of vaccines against cancer. The second is that of host-tumor inter- actions. This is an area that I feel has very great importance but it is so rela- tively new that there is not much to talk about as yet. It was the subject of an International conference at the Henry Ford Hospital here in Detroit, three months ago, when three days were de- voted by scientists to try and find what kind of base lines we have for studying the interactions of a tumor with the nor- mal tissues and systems of the host or- ganism.

Before concluding, I would like to discuss briefly another way in which the Federal Government may influence can- cer research and medical research gen- erally. As I indicated previously, al- most every cancer reasearch worker de- pends heavily on Government support, and those who are interested in chemo- therapy find Federal funds by all odds the principal source of research money. But the role of the Government in can- cer research and medical research will change drastically should the bills that have been introduced into the House and Senate concerning the regulation of animal experimentation ever receive ap- proval.

Many scientists who have examined the proposed legislation feel that these are thinly veiled attempts to put crip- pling limitations on the use of animals in medical research. Under these acts there would be legal definitions of what an animal is, what pain is and what

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a laboratory is. A laboratory is some- thing you would all be involved in for it is “a school, institute, organjzation, group, corporation, partnership, or per- son who uses or intends to use animals in research, tests, experiments, teaching, or in a production of materials.”

I need not review here a11 of the de- tails of the bureaucracy that would b.: set up by sach legislation. Suffice it t o say that certificates of compliance with the act would have to be obtained and that one bill would put the limitations in the following form: “No use of ani- mals shall be undertaken by any holder of a certificate of compliance with this act until a project plan has been filed with the agency of Laboratory Animal Control in such form as the commis- sioner shall prescribe, describing the na- ture and purposes of the project, and

the procedures to be employed for the respect to living animals and project plan has been approved by the com- missioner.”

If you share my conviction that such legislation would be detrimental to the cause of medical research in the United States, I trust that you will express that opinion not to me but to your Congress- man or Senator. Additional informa- tion is readily available from the Na- tional Society of Medical Research in Chicago, Illinois.

In the last forty minutes I have touched on a number of problems in cancer research. Possibly the report is one of less glowing progress than you might like, but I think it is a fairly fac- t ua l one. Progress in cancer research may not always be made rapidly, but I do think it is being made steadily.

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