Chemical Target: Cancer C h e m i c a l s vs. c a n c e r cells — u p p e r h a n d , but the s e a r c h t h e r a p e u t i c a g e n t s is b e i n g p o t e n t drugs m a y b e in t h e JLROGRESS
IN
THE
CHEMICAL
WAR
against cancer has not been marked by any spectacular break-throughs yet. But a close look at t h e research of many scattered laboratories and hospitals shows that many reconnaissance missions have r e t u r n e d successfully from the enemy's territory and that important inroads have been made into his Unes. T w o examples of t h e success attained in fighting malignancies: • A few chemicals have been found that will definitely retard the growth, or even completely destroy, animal cancers in mice and rats. • Leukemia in children can b e temporarily alleviated by a combination treatment with folic acid antagonists 5138
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1955
the d i s e a s e still has t h e for p r o m i s i n g c h e m o stepped up a n d more offing
and 6-mercaptopurine, either alone o r with azaserine, u n d e r expert medical care; children w h o without t r e a t m e n t would have lived b u t a few months now have a better than 50°/o chance t o survive for a year or longer. T h e s e achievements in themselves do not add u p to a cure for human cancer, b u t they suggest that control of the disease may sooner or later be p o s sible. Indicative of t h e widespread s c o p e of research in cancer c h e m o t h e r a p y was t h e establishment of the Cancer Chemotherapy National Committee (C&EN, J u n e 2 0 , page 2 6 0 6 ) , h e a d e d by Sidney Farber of Children's C a n cer Research Foundation i n Boston, and
h e a d q u a r t e r e d a t t h e Cancer Chemotherapy National Service Center in Bethesda, M d . Aim of C C N C : to provide a clearinghouse that will serve to tie together and accelerate t h e research of t h e several hundred laboratories in this country that are searching for a chemical t o stop t h e growth of cancer cells. A rough estimate tags expenditures for cancer chemotherapy research this year at a b o u t $15 million, in industrial, government, and independent research labs. Of this, about $5 million is being s p e n t by the National Cancer Institute, $ 1.5 million is coming from t h e American Cancer Society, and $453,000 is b e i n g earmarked for chemotherapy b y t h e Damon Runyon Memorial F u n d . Congressional appropriations have j u m p e d from $1 million in fiscal 1954 to $ 5 million this year. Government or private research grants support some
303 projects. Since the W a r Aithough a few small-scale programs for screening chemicals for cancer therapy w e r e set i n motion prior to
World W a r II, notably by Hans Lettre in G e r m a n y , little of clinical importance was accomplished. One of the first systematic programs w a s started in 1944 b y M u r r a y Shear at the National Cancer Institute. Shortly afterward, C. P . Rhoads at Sloan-Kettering Institute, F a r b e r at Children's Hospital, a n d Alexander H a d d o w in England began similar programs for evaluating chemical c o m p o u n d s . Radioactive isotopes were being used prior to t h e war to treat certain cancers. In t h e early forties estrogenic and androgenic hormones were used with some success for treating prostatic and breast cancers. These materials attack b o t h normal a n d malignant cells in specific tissues of t h e body. W i t h t h e end of the war, progress in chemotherapy was stepped u p on m a n y fronts: • Nitrogen mustard, developed as a potential w a r gas by military research, was found to be of use in treating lymphosarcomas; extensive dinical trials b e g a n in 1945. • Folic acid antagonists—its 4-amino analogs—were used b y Farber in 1948 to retard temporarily the progression of a c u t e leukemia in children, after it was observed that folic acid was needed for cell g r o w t h in bone marrow and lymph nodes. • Triethylenemelamine (TEM) was developed in 1950 as a nitrogen mustard t y p e of material that could b e given orally for treatment of leukemias, Hodgkin's disease, and other types of cancer. • 6-Mercaptopurine ( 6 - M P ) , jointly developed b y Sloan-Kettering and Burroughs Wellcome, more recently has been found useful against leukemia, especially w h e n used in conjunction with folic acid antagonists and azaserine. In t h e considerable strides m a d e since the w a r lies basis for hope that even m o r e useful drugs will be forthcoming.
Chemicals Through a Screen O n e b i g problem in t h e search for a chemical for cancer t h e r a p y is develo p m e n t of a suitable m e t h o d for picking specific beneficial materials from a vast iiiiHiDsr ox available compounds. T h e job requires the expert cooperation of chemists, biologists, pharmacologists, a n d clinicians. "All the techniques d e veloped so far leave much to b e d e sired," says K. M. Endicott, chief of the Cancer Chemotherapy National
Service Center, "but they h a v e turned u p many valuable leads." A common p r o c e d u r e is t o transplant into mice or rats any one of a number of possible types of t u m o r s . T h e animals then are given a maximum dose of test material, a n d growth of their t u m o r is compared with that of a tumor on a control animal. Since success or failure of the test is the basis for judging whether a material is to be discarded or given further evaluation, test conditions must b e carefully controlled. Use of animal tumors for screening has some definite limitations. There are m a n y different types of cancer in both animals and man; some comp o u n d s which have proved effective in controlling certain tumors in animals have been ineffective for other animal cancers or for h u m a n treatment. Other compounds may b e effective in man but are inactive in animals. Screening devices, therefore, do not provide entirely accurate indications of a material's effectiveness in man. And they tend to consume b o t h time a n d scarce test materials in considerable quantities. T h e transplanting of h u m a n tumors into mice, rats, a n d hamsters is a recent development in t h e use of animals for screening. Several types can be g r o w n successfully in this way. Do such tumors, after having grown in animals, still have t h e characteristics of h u m a n tumors? W o r k at Sloan-Kettering indicates that h u m a n cancers, even after having grown on a nonhuman host, retain their original microscopic appearance. E v e n more significant, transplanted back into the original patient, they will continue t o grow. Unfortunately, animals which a r e to be implanted with h u m a n tumors must first be preconditioned b y treatment with cortisone or radiation; such treatm e n t lowers the resistance of the animals and limits the amount of toxic drugs which they can be given. Another promising way of using hum a n tumors for screening is to inoculate eggs with h u m a n cancer cells. Sloan-Kettering^ D a v i d Karnofsky finds that chicks h a t c h e d from such eggs develop tumors w h i c h seem t o be identical with those of man—and no preconditioning with cortisone or radiation is necessary. Tissue cultures provide another means of growing cancer cells for use in screening. T h e most commonly used culture m e d i u m s have been natural extracts or serums. Because these are not easy to define chemically or to
duplicate, attempts arc under w a y t o develop synthetic culture m e d i u m s made u p of amino acids, vitamins, carbohydrates, and minerals. Harry E a g l e at the National Institutes of H e a l t h h a s made u p media of this type w h i c h contain only a small amount of serum from a natural source. H e has been a b l e t o grow human cancer cells in t h e m . Moreover, he has been able to e s t a b lish precisely the growth requirements. Tissue cultures offer some i m p o r t a n t advantages for screening: they p r o vide a fairly well defined mediurri for human cells to grow in, and t h e y p e r mit tests with quantities of materials that are too small for use in a n i m a l screens. Nevertheless, many researchers have been disappointed with tissue culture methods of screening, finding a lack of correlation w i t h r e sults from tests using tumors g r o w i n g in animals. Culture cells, since they are not grown in the environment of an intact animal, are not influenced b y t h e body's complex processes of m e t a b o lism, growth regulation, change, a n d excretion. I n a living animal, these processes might destroy the anticancer agent or remove it from the c a n c e r site.
Hunting for a Cancer Killer W h a t types of chemicals are b e i n g tested for chemotherapeutic effects on cancer cells? In the broadest sense, practically any materials showing physiological activity or related to * * reactive with compounds r e q u i r e d b y human cells are getting attention. Substances of natural origin, b e c a u s e of successful pharmaceutical a p p l i c a tions of antibiotics, are of interest to many investigators. Other materials that have been tested include such biologically active materials as herbicides, bactericides, and antimalarials. Antimetabolites include a broad r a n g e of compounds that interfere with n u c l e o protein metabolism or might tie u p essential amino acids. Antivitamins, enzyme poisons, and antimitotic a g e n t s have all been considered. T h e range of test materials i s suggested by the screening p r o g r a m conducted at Sloan-Kettering: Some 14,500 chemicals and about 17,000 materials of natural origin have been tested since ^"h— end of the war. Only a h a n d ful has proved of value. Sheer size of t h e screening p r o g r a m is one reason for formation of t h e C a n cer Chemotherapy National C o m m i t tee. A big item in the committee's program will be a nation-wide screenNOV.
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5139
Typical Chemicals for Cancer Chemotherapy Chemotherapy
JFcprssaazIs
Applications
Pol/functional alkylating agents Methylbis( β-chloroethyl) amine HC1 { H N 2 , nitrogen mus tard, Mustargen)
[ |
Triethylenemelamine (TEM)
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Chronic myelocytic leukemia, chronic lymphatic leukemia;.
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X
Hodgfcinrs disease, lympho ma fraync**» |**TTtr CâUCCr CSTaOer
c
of ovary or breast
^ X ;χ—c
Triethylenephosphoramide ( Τ Έ Ρ Α )
Chronic myelocytic leukemia, HodgkhTs disease, lympnosarcoma, cancer of lung οι ovary
c—χ;
Leukemias, Hodgkin's disease, long cancer
j CH-
1,4-Dimethylsulf onyloxybutane (Myleran)
^CHÏ
χ
Chronic myelocytic leukemia
ce^woicn-wsGiCH*
Lymphatic leukemia
7- {p-[di ( 0-chloroethyl ) ]amino} phenylbutyric acid (CB-1S18) Cl—•CHaCHa/
Antimetabolites ^Amino-N^-niethylpteroylghitamic acid ( Amethopterin, Methotrexate )
X
Acute leukemia
X
H 2 X*
V
^.
1
>,
CM3 ff y—CE-—X—^~^—COXH—C—COOH €Ή
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4-Aminopteroylglutamic acid ( Aminopterin )
X
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x
| Acute leukemia
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6-Mercaptopurine (6-MP)
! Leukemias J
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MC
Azaserine
χ*
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! 5140
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NOV.
2 8,
V 19 55
Acute leukemia
EL
iN CH--O0—Ο—CH2J—C—COOH '
s
ί
Hormones
Chemotherapy Applications
Formula
Methyltestosterone
Breast cancer
Diethylstilbestrol
Breast cancer, prostatic cancer
Acute leukemia, lymphatic leukemia, multiple myeloma, breast cancer, cancer of lung or prostate
CHsOH
cortisone
I °.
H3
S Jv-—OH
Radioisotopes Iodine
Cancer of myeloma
thyroid,
multiple
Phosphorus
Chronic myelocytic leukemia, chronic lymphatic leukemia, multiple myeloma, polycythemia vera
Gold
Cancer of lung, ovary, breast, or prostate
Miscellaneous Ethyl carbamate than )
Chronic myelocytic leukemia, chronic lymphatic leukemia, multiple myeloma
( Ure-
ing operation. T h e committee, and the Cancer Chemotherapy National Service Center, plan to act as a clearinghouse to which chemists in college and industrial laboratories lacking adequate facilities for screening can supply new chemicals. T h e committee will have these screened by contracting with independent laboratories. T h e committee also expects to place orders for interesting chemicals in the large quantities necessary for a complete pharmacological and clinical evaluation. In addition, it hopes to serve as a centralized information center and to set u p a catalog of biological activity adapted for machine searching and cov-
ering the results from tests on all thousands of compounds that screened. CCNC, in its assault o n cancer, is ceiving the backing and cooperation
the are reof:
• Atomic Energy Commission—which has facilities to carry o n the synthesis and evaluation of radioactive drugs. • Veterans Administration—which runs many hospitals a n d operates a large-scale program for -veterans' medical care. • Federal Drug Administrationwhich will carry out pharmacological and toxicological tests on promising drugs.
• National Cancer Institute—which receives federal funds for research grants a n d contracts, and has its own long-standing program in fundamental cancer research and clinical cancer chemotherapy. • American Cancer Society—which will supply private funds to back CCNC's operation as well as continuing its own research grants program. • Damon Runyon Memorial Fund— which also will supply money for grants to further cancer chemotherapy research. To provide direction for its program, the committee has set up advisory NOV. 2 8, 1955
C&EN
5141
panels for clinical studies, pharmacology, screening, and chemistry, as well as an industry subcommittee to enlist the help of t h e pharmaceutical and chemical industries. Specific compounds t h a t may solve the cancer problem may come from some totally unexpected source. Nitrogen mustards, for instance, were an outgrowth of wartime research on poison gas; triethylenemelamine was first developed as a cross-linking agent for cellulose materials. Therefore, CCNC plans to screen a wide variety
lar in structure to the normal intermediates of nucleic acid synthesis, chemicals can b e incorporated into t h e partially synthesized nucleic acid t h a t will block further build-up of t h e molecule or of its purine precursors. 6-MP, for instance, may act by replacing t h e natural purine, adenine, in partially formed nucleic acids. Nucleic acid synthesis probably is not the result of one single p a t h w a y of reactions; various types of antimetabolites will inhibit t h e synthesis at various stages in the buildup process—for
H o w t© Fool €a Cancer Cell
A-
^cN CH
T/
HO
I:
")CH
H W H Because of its structural similarity with adenine (left), a required building block for nucleic acid, 6-mercaptopurine can be used by the cancer cell in the biosynthesis of nucleic acids. The mercapto group on the purine molecule inhibits further growth after it enters the molecule of compounds from many sources. Progress in understanding the chemical nature of cancer is still not far enough advanced that any likely material can be discarded without at least a preliminary test. As data on the physiological activity of chemicals are amassed, however, the range from which possible chemotherapeutic agents will be selected should narrow. Even now the beneficial effects of such polyfunctional alkylating agents as the nitrogen mustards or of antimetabolic agents such as the folic acid antagonists and certain purine derivatives are so well established t h a t they are getting special emphasis. Antimetabolites Hold Promise Antimetabolites are assuming a growing importance in many researchers' thinking. These materials are aimed directly at blocking biosynthesis or function of nucleic acids a n d their components, which are essential to the division and growth of cells, -whether normal or malignant. Nucleic acids are made u p of purine and pyrimidine segments, sugars, a n d phosphoric acid; the cell can form them by utilizing simple precursors, such as carbon dioxide, ammonia, water, a n d glycine, or it can incorporate preformed pviiines and pyrimidines. By using compounds sufficiently simi5142
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1955
these two reasons, a combination of blocking agents may prove effective in inhibiting nucleic acid formation. Since folic acid apparently is required by all cells, antifolic acids are injurious in proportion to t h e rate of cell growth. Similarly, nitrogen mustard alkylating agents are toxic to cells in proportion to growth rate. Neither class of agents is specific for malignant cells, so each is most effective against certain cancers—leukemias and lymphatic cancers are examples—in which t h e growth rate of the cells is high relative to that of many normal tissues. For these reasons, they, like all other known anticancer agents, are quite toxic and can be administered only with expert medical care and in amounts so small as not to kill all t h e cancer cells. In addition, surviving cancer cells build u p a resistance to the drugs, much like the resistance microorganisms develop to antibiotics, so that beneficial effects are transitory. A Rational A p p r o a c h Recent observations indicate, however, that desoxyribosenucleic acid ( D N A ) , a vital nucleic acid found in cell genes, may differ in composition, precursor requirements, or mode of formation among different types of cells. For instance, the rate of uptake of some D N A precursors is not the
same for cancer as it is for normal cells. This is the basis for what Rhoads of Sloan-Kettering calls "rational" chemotherapy. If chemical differences, however slight, exist between D N A i n cancer and in normal cells, it might be possible to tailor-make antimetabolites which will selectively inhibit t h e growth of specific malignant cells. 6-Mercaptopurine is one of t h e first successful anticancer agents to result from this line of thinking. Preparation of related compounds is under way. in t h e h o p e that one finally will b e uncovered t h a t is nontoxic to normal cells but will destroy all the cancer cells with w h i c h it comes into contact. Present knowledge of nucleic acid synthesis and of t h e structural arrangement of nucleic acid components is still not great enough to suggest how well these hopes will be fulfilled. T h e experiments of the past decade have led to no chemical cure for cancer. For most cancers, current chemotherapy cannot substitute for surgery or x-ray therapy. But w h e r e the disease is t o o far advanced for t h e s e to be applicable, or where cancer cells h a v e become widely disseminated through the f>ody, chemical agents can provide temporary relief, make the patient more comfortable, and perhaps in some cases lengthen life. Many investigators are not particularly hopeful that a cure for cancer will ever be found, in the sense that penicillin cures many infectious diseases. W h a t they regard as more probable is a chemical control that will prolong useful life by checking the growth and spread of malignant cells, roughly as insulin controls diabetes. Other cancer researchers go so far as to say that any applied research aimed at discovering a specific therapeutic agent for cancer is premature, and t h a t more work should be expended instead on basic research into the biochemistry of cancer. However, the search for an anticancer chemical has in itself turned u p m u c h fundamental information. There is no denying, moreover, t h e possibility that a curative agent m a y h e found. As one scientist put it, "There is a parallel between our present stage in the development of cancer chemotherapy and the situation in t h e search for a pneumonia drug just prior t o t h e discovery of the sulfa compounds." "If a n y t h i n g / ' adds another, "knowledge of the biochemistry of cancer is even more advanced now."
