622

CHEMOTHERAPY OF CANCERBy K. A. NEWTON, M.R.C.P., F.F.R.

Consultant Radiotherapist, Westminster Hospital

IntroductionThe Seventh International Canaer Congress,

recently hefd in--:London, has underlined theresearch that is being carried out on a world-widebasis, aimed at the discovery of chemical agents ofvalue in the treatment of cancer. Undoubtedly amaj6r part of this effort has fallen to the UnitedStates of America where, in a single year, over40,000 compounds have been investigated for anti-tumour activity. In the Soviet Union there is anactive and progressive school of chemotherapyand this country has also made a valuable con-tribution, particularly in the field of alkylatingagents of chemotherapeutic value.Modern cancer chemotherapy can be said to

have started during and shortly after the SecondWorld War when the first clinical trials of nitrogenmustard were conducted in the United States(Rhoads,l Goodman et al.2; and in this country byWilkinson3). Since then in the relatively shortperiod of 15 years, a very large number of com-pounds have reached the stage of clinical trial andthere are approximately 20 which now have anaccepted, place in the treatment of various types ofneoplastic disease. Following the discovery ofnitrogen mustard (HN2), a further massive stepforward was made in the chemical treatment ofcancer by the discovery of aminopterin in. 1948,by Dr. Farber4 and his group in Boston. Thisanti-metabolic substance is capable of causingprolonged remission in acute leukaemia. Thesetwo compounds, then, represent the two greatclasses of chemotherapeutic agents. Nitrogenmustard is' an example of an alkylating agent andaminopterin of an anti-metabolite.The potentialities of nitrogen mustard have led

workers to search for other forms of chemicallyactive alkylating agents, and this has resulted in thediscovery of the ethylenimines of which TEM wasthe first compound commonly used in clinicalpractice. The suphonyl oxybutanes of which theanti-leukaemic substance, myleran, is a well-known example, furnishes further proof of thesearch for new alkylating agents. In the UnitedStates much work has been done on the anti-metabolites and many new examples of these

compounds have now-reached the stage of clinicaltrial and some, sucb as 6-mercaptopurine, apotent anti-purine, have an accepted place inclinical medicine.

ClassificationAlkylating Agents

I. Mustards:HN2 and HN3CB .148 (erysan)NitrominCB I348 (leukeran and chlorambucil)CB 3025 (melphalan and sarcolysin)Degranol (BCM)Hemi-sulfur mustardsChloroquine and quinacrine mustards

2. Ethylenimines:TEMThioTEPAE 39 (ethylenimino quinone)

3., Suphonyl oxybutanes:Myleran (busulphan)Dimethyl myleranNonane

4. Epoxides:Di-epoxybutane

Anti-metabolitesI. Folic acid antagonists:

AminopterinAmethopterinDihydrotriazines

2. Glutamine antagonists:Azaserine

3. Purine antagonists:6-Mercaptopurine (purinethol)Puromycin6-Chloropurine

4. Pyrimidine antagonists:2-Thiouracil

5. Amino acid antagonists:EthionineSelenium cystineHydroxylysine

6. Probable anti-metabolites:Actinomycin C

December 1958 NEWTON: Chemotherapy of Cancer 623

Actinomycin DUrethane

General Cell Poisons:Colcemid (demecolcin)ThiocolciranThis classification, though containing the

names of a large variety of compounds, cannot beconsidered to be fully comprehensive. Thosecompounds (in heavy type) can be said to havepassed through the stage of full experimental andclinical trial and to have an established place in thetreatment of advanced neoplasia.An account will now be given of the more

important compounds, omitting many chemicalagents, which though active, possess no particularadvantage.ALKYLATING AGENTSNitrogen Mustard

During the last war a clinical trial was conductedboth in this country and in America, into com-pounds which have since become known as;HNg and HN3 (Rhoads,l Goodman et al.2 andWilkinson3). These substances are derivativesof the First World War mustard gas, and sincetheir description, HN2 in particular has been usedvery extensively in the treatment of lymphomas,leukaemias and advanced tumours. HN2 (Methyl-bis (p-chloroethyl)amine hydrochloride) is capableof producing dramatic and prolonged remission inHodgkin's disease and other lymphomas. It canbring about complete disappearance of large glandmasses, restoration of a high pyrexia to normal,diminution in size of spleen and return of thepatient to good health. It is also useful in chroniclymphatic leukaemia, where the condition haspassed to one of progressive lymphadenopathy andhepato-splenomegaly associated with anaemia.This type of case does not, as a rule, respondcompletely, and is liable to undue haematologicaldepression. Occasionally HN2 is capable ofproducing temporary objective improvement indisseminated epithelial cancer, but the frequencywith which this occurs scarcely makes this treat-ment worth while as a routine method. As it isgiven intravenously and has a variety of side-effects, it is more suitable for in-patient administra-tion and short course therapy.

Side-effects. Nausea, vomiting, pyrexia, whitecell count depression particularly lymphopaenia,depression of haemoglobin and platelets,

Dose. Usual dosage is o.1 to 0.4 mg. per kg.daily' for four to five injections, The authorprefers to give a routine graduated dose increasestarting; at 2 mg. and working up to 8 mg. perinjection on alternate days, being guided by the

individual variation in toxic and haematologicaleffects.

Leukeran (Chlorambucil)Leukeran (CB I348) (p-(di-2-chloroethyl-

amino)-phenylbutyric acid) was first synthesizedin this country at the Chester Beatty ResearchInstitute by Everett et al.5 and is an example of thedevelopment by bio-chemists of the original HN2formula. This compound is an aromatic sub-stance as opposed to the aliphatic HN2, is non-vesicant and can be taken by mouth. In the pastfew years it has become well established thatleukeran has a definite part to play, particularly inthe malignant lymphomas. Altman et al.6 de-scribe i out of 8 cases of chronic lymphaticleukaemia which had objective improvement ratedfrom good to excellent. Galton et al.7 in treatingsix cases of giant follicular lymphoma had fivewhich experienced' good benefit,' when only fourof 23 with Hodgkin's disease had a similar goodresponse, although in 14 cases there was someeffect, and no observable effect in five cases.Results in reticulum cell sarcoma have been dis-appointing, as have many of the other cytotoxicagents. Leukeran has the great advantage that itcan be used for out-patient therapy, is quitecapable of controlling and temporarily arrestingdisease progress in lymphomas and the effectivetherapeutic dose level can be considered safe.Pillers and Magalini8 have stated that there is noobservable haematological effect in a dosage of0.15 mg. per kg. per day. This is borne out bythe experience at Westminster Hospital where itis found possible to give a daily dosage of 5 mg.for six months with no observable effect on thewhite or red cell count.

Side-effects. Very occasional mild nausea andanorexia. Haemopoietic depression after pro-longed treatment with large doses.

Melphalan (CB 3025) (Sarcolysin)This substance P-Di-2-Chloroethylamino-L-

Phenylalanine synthesized simultaneously bothin this country at the Chester Beatty ResearchInstitute and in the Soviet Union, has beenreported on very favourably, particularly in theU.S.S.R. (Larionov et al.,9 Blokhin et al.10).It has been postulated that the amino acid phenylanaline grouping acts as a carrier for the transportof the mustard to the site of action (Larionovet al.9). They consider that this compound' isvery much more efficacious against solid tumoursthan other previously described nitrogen mustardderivatives.; Amongst the more significant resultsclaimed, have been the production of tumourregression in 19 out of z3 cases of seminoma of thetestis, and three out of six cases of multiple

624 POSTGRADUATE MEDICAL JOURNAL December 1958

myeloma.10 Other solid tumours in which successhas been claimed are angio-endotheliomas, Ewing'stumours and apart from these, reticulosarcomas.There has been little experience of its use in thiscountry, but Papac et al.n describe 37 cases, oneof which, a teratoma of the testis, derived con-siderable benefit following 250 mg. given in threemonths. In the United States, Holland andRegelson12 describe the use of melphalan in 16patients with malignant melanoma of which onlytwo showed a limited objective improvement.

Dosage. io mg. daily for two to three weeks, oralternatively, 4 to 6 mg. daily for eight to tenweeks. At more than 20 mg. daily some nauseaand vomiting are evident. Neutropenia andthrombocytopenia may occur after two to threeweeks at a dose of io mg. daily. Whilst thissubstance must be considered an extremely in-teresting one and a probable precursor of othersimilar amino acid mustards, its precise positionin the treatment of advanced malignancy isunknown. It seems that its use is justified in theradio-sensitive type of disseminated solid tumours.

Degranol (BCM)In Hungary, this compound (i,6-Bis(p-chloro-

ethylamino)-i, 6-Deoxy-D-Mannitol Dihydro-chloride) has been extensively used in a largenumber of patients; for example, 3I8 werereported by Sellei and Eckhardt.13 It is claimedthat this substance is more efficacious and lesstoxic than HN,, for although sickness, malaise andvomiting do occur, these reactions are not severeand occur in approximately 45 per cent. of cases.These side-effects are most evident when the doselevel is more than 0.3 mg. per kg. Isolatedsuccesses are claimed in various miscellaneoussolid tumours. Their most striking success was22 cases out of 59 cases of H6dgkin's disease whichwere stated to have remained symptomless for twoyears following a course of treatment. This sub-stance has recently become available in this countryand it would seem worth while giving it a trialparticularly in cases where intravenous therapy isadvised and as an alternative to intravenousnitrogen mustard.

Dosage. 50 mg. alternate days up to 600 or800 mg.

Side effects. Nausea, anorexia, vomiting andbone marrow depression.TEM

Rose et al.14 investigated the inhibitory actionof various ethyleneimine compounds againstanimal tumours and found that TEM (triethylene-melamine) was the post effective of the compoundstested. Subsequently this compound was testedclinically by Bayrd et a.l5 in 38 patients, the most

striking results being obtained in chronic lym-phatic leukaemia. Improvement was observed in8 out of 13 cases treated. Rundles and Bartonl6considered that TEM was the agent of choice inthe treatment of chronic lymphatic leukaemia,although a 30 per cent. remission was obtainedin Hodgkin's disease (Silverberg and Damashek17).It is now recognized that although this drug iscapable of producing good tumour regression andsymptomatic improvement in lymphomas it isliable to cause extensive bone marrow damageparticularly in lymphatic leukaemia. This bonemarrow damage does not manifest itself till some-times one to three weeks after the completion oftreatment. Kravitz et al.l1 have described kidneydamage and haematuria, polyuria and uraemiaand Bayrd et aL51 describe renal failure in theirpatients who died whilst on TEM therapy.

Dose. It is preferred to give TEM at not morethan 24 mg. on alternate days and it is possibleto give 15 to 30 mg. total.

Side effectsBone marrow damage, uraemia, alopecia and

urticaria. It would seem therefore that althoughthis agent is effective in lymphomas chronicmyeloid and chronic lymphatic leukaemias it isperhaps more toxic than other equally efficaciouscompounds. If it is used it should be under closehaematological supervision.ThioTEPAThioTEPA Triethylenethiophosphoramide is a

development of TEM to which it is relatedchemically and has been studied extensivelyclinically, particularly in the United States. It isa substance which has been subjected to widelydiffering reports, for example, Bateman, McCabe19reported that 25 out of 32 patients with advancedcarcinoma of the breast were significantly palliatedand six of I2 with ovarian carcinoma also had goodresults, whereas Ultmann et al.20 describe uniformfailure in response in breast carcinomas. Thereis no doubt that ThioTEPA is capable of pro-ducing a good remission in the lymphoma groupof patients, for example, Ultmann et al.20 describe19 of 43 patients with malignant lumphomas whoshowed definite objective improvement. Ourexperience at Westminster Hospital of intravenousThioTEPA shows that it appears to possess notherapeutic advantage over intravenous HN2.Ultmann et al. describe haematological toxicitywith depression of white cell count in 45 per cent.of patients having reasonable doses of ThioTEPA.No other toxic effects are mentioned with anyfrequency.

Dose. Average 50 mg. total intravenously overseven days.

December 1958 NEWTON: Chemotherapy of Cancer 625

Myleran (Busulphan)In 1953, Haddow and Timmis21 described the

synthesis of i :4-dimethane-sulphonyloxybutane,one of a series of sulphonic acid esters. The sub-stance itself is an .alkylating agent and thereforecapable of producing improvement in lymphomasbut its principal action is against the white cells ofthe granular series and this does direct its use asan important drug in chronic myeloid leukaemia.Galton22 described the use of this substance in i9cases of chronic myeloid leukaemia and demon-strated that it was possible to produce a remissionin this disease which was in every way comparableto that obtained with high voltage irradiation to thespleen. One case of a remission lasting 2I monthsis cited. Many other authors since then haveconfirmed the value of this compound in chronicmyeloid leukaemia. Its exact place in the treat-ment of this condition is difficult to assess but thesumming up of Haut et al.23 is a fair statement ofthe position. They suggest this agent is at leastequal to X,ray therapy to the spleen or radioactivephosphorus and is possibly superior to otherchemotherapeutic agents; whilst Galton and Till24consider that the chief use of myleran occurredwhen radiotherapy had been of limited value orhad ceased to be effective. To this can be addedthe author's opinion that is reasonable to alternatemyleran and high voltage X-rays during successiveperiods of relapse.

Dose rate. o.o6 mg. per kg. per day and ifmaintenance therapy is attempted i to 4 mg. daily.

Side-effects. Thrombocytopenia, brownish pig-mentation of the face, forearms, chest andabdomen.Two derivatives of myleran have recently been

mentioned in the literature-dimethyl myleran(Bierman et al.25) and nonane, Miller26 which wasstated to have wide anti-tumour effect in trans-planted animal tumours, but it is too early yet toassess efficacy of these compounds.Anti-MetabolitesAminopterin

Farber4 demonstrated it was possible to producedisease remissions in children with acute leukaemiaby means of the folic acid antagonist, 4-amino-pteroylglutamic acid (aminopterin). This obser-vation has inspired a great deal of work into meansof interfering and blocking the bio-synthetic path-ways which lead to the synthesis of nucleic acid andnucleo proteins. This is sometimes called arational approach to chemotherapy or combinationchemotherapy. Intensive work into the searchfor new anti-metabolites has resulted in the findingof the valuable compound, 6-mercaptopurine, ananti-purine.

Methotrexate (Amethopterin)It has been shown by much experimental work

that the 4-amirtoderivatives of folic acid such asamethopterin, prevent the conversion of PGA(Pteroylglutamic acid) into citrovorum factor.The precise site of action of these compounds hasbeen shown to be the prevention of reduction ofPGA to tetrahydro PGA Blakley.27A vast amount of experience has been accumu-

lated particularly in the Children's Hospital atBoston and at the Memorial Centre in New Yorkin the use of methotrexate in the treatment ofacute leukaemias. American workers are agreedthat the first line of attack in the acute leukaemiasof children lies in the use of methotrexate. Forsome curious reason in the acute leukaemias ofadults, methotrexate is far less efficacious than is6-mercaptopurine. In children, however, Farber28has shown what great strides have been made inthe control of the acute leukaemias. He quotes549 cases of which 50 per cent. survived ninemonths. This is to be compared with the medianduration of acute untreated cases as quoted byTivey29 who mentions one to two months. Farber28also estimates that the average length of remissionfollowing methotrexate (eight months) is longerthan average remission following 6-mercaptopurine(six months). No significant benefit has beenreported in solid tumours and other lymphomasbut in children, occasional good results haveoccurred in rhabdomyosarcomas, Hodgkin's dis-ease and neuroblastomas.

Dosage. 2.5 to 7.5 mg. daily by mouth followedby maintenance therapy of 1.25 to 2.5 mg. per day.

Side-effects. Ulceration of buccal mucosa andgastro-intestinal tract, diarrhoea and intestinalbleeding. Bone marrow depression.6-Mercaptopurine (Purinethol)

6-Mercaptopurine was first synthesized at theWellcome Research Laboratories (Hitchings andElion30) following investigation into a series ofchemical relatives of natural purine and pyrimidinebases. Its clinical use has been fully described byBurchenal et al.3 in trials on 269 patients withvarious neoplastic diseases. They decided that nobeneficial effect followed its use in metastaticcarcinomas, sarcomas, lymphosarcomas, Hodgkin'sdisease or chronic lymphatic leukaemia. It iscapable of producing good clinical remission ofdisease with temporary return of the child to goodhealth in acute leukaemia. They mention goodclinical and haematological remissions in 41 out of87 children but similar response in only 7 out of50 in adults with the same disease. The place ofmercaptopurine then lies principally in the treat-ment of acute leukaemias of childhood when itshould be used, not as the initial treatment, but

626 POSTGRADUATE MEDICAL JOURNAL December 1958for the first relapse of disease following metho-trexate therapy. It should be noted that it takes4 to 13 weeks to attain satisfactory response totreatment with 6-mercaptopurine. It is thereforeessential to continue treatment at full therapeuticleviels for an adequate trial period. 6-Mercapto-purine apart from its capabilities in the chronicphase of the disease, has been reported as pro-ducing remission in two out of six cases of chronicmyeloid leukaemia in the acute myloblastic phaseand also has been successful in producing oc-casional remissions in generalized reticulum cellsarcoma (Hyman et al.32).

Dosage. 2.5 mg. per kg. per day by mouth for4 to 13 weeks, followed by maintenance therapy of1.5 to 2.5 mg. per kg. per day.

Side-effects. Nausea, oral lesions, gastro-in-testinal disturbance and bone marrow depression.Actinomycin 'D'

Actinomycin 'D' is an antibiotic which hasbeen shown both experimentally and clinically tohave important anti-tumour effects (Farber,33Tan et al.34). Both these groups of observers haveevaluated the potentialities of' this antibioticprincipally in malignant disease of childhood.Tan mentioned definite objective response in fourout of I2 cases of Wilm's tumours, three out ofseven neuroblastomas and two out of threelymphomas. Farber also was impressed by theregression which had been brought about inWilm's tumour. Both these authors have com-mented on the curious ability of actinomycin ' D 'to potentiate radiotherapeutic effects. For ex-ample, a patient previously treated by radiotherapyand subsequently given a course of actinomycin' D' is liable to a skin flare up in the previouslyirradiated areas. There can be no. doubt thatactinomycin ' D' is an extremely interesting andpotentially valuable substance which requiresfurther investigation.

Dosage. Short-course therapy: 75 tig. per kg.daily intravenously for five days. Long-coursetherapy.: 0oig. a day given until signs of toxicityappear.

Side-effects. Oral ulceration, nausea, anorexia,vomiting, alopecia and occasional white blood celland platelet depression.Urethane (Ethylcarbonate): Urethane (ethylcarbonate) came to the fore in1946 wheni its potentialities in the treatment ofchronic myeloid leukaemia was discovered(Paterson -et al5). 'It is not now much usedexeept: i-i the treatment of multiple; myelomatosisalthough in the author's opinion it is not superiorin' this:refpect to nitrogen mustard or leukeran.

More recently sarcolysin has been reported by theRussians (Blokhinl0) as being capable of bringingabout a satisfactory clinical response in multiplemyeloma.

Dosage. I.5 to 2.5 mg. per kg. per day.Side-effects. Anorexia, nausea, stomatitis,

vomiting and drowsiness.

General Cell PoisonsColcemid

Desacetylmethylcolchicine, an alkaloid isolatedfrom colchicum autumnale, has a toxicity inanimals I/3oth that of colchicine itself. Moeschlinet al.,3 following initial experimental work inwhichit was demonstrated that colcemid had selectivelyinhibited granulocytopoesis, treated patients withchronic myeloid leukaemia. There was no doubtthat a good remission could be obtained with thiscompound. Moeschlin mentions good improve-ment in the general condition with increasinghaemoglobin and diminution in white cell countand size of spleen in 28 out of 30 patients withchronic myeloid leukaemia. In addition he main-tains that maintenance therapy for as long as three:years is perfectly safe provided the patient is keptunder observation. Furthermore, in the mylo-blastic transformation of chronic myeloidleukaemia, three out of five cases were said to haveresponded well. Myleran itself is probably notactive in this stage of the disease. It is generallyagreed that colcemid is contra-indicated in chroniclymphatic leukaemia, because of the danger ofagranulocytosis (Zbinden37). In 22 cases ofcarcinomas and sarcomas no definite therapeuticeffect was observed in any case (Moeschlin36).

Dosage. 4 mg. daily gradually increasing up to6 mg. daily after two to three weeks with a limit of8 to IO mg. daily. It is advisable to discontinuetreatment when the total white cell count hasfallen 'to 30,000. If maintenance therapy isattempted, one-third of original daily dose is given.

Side-effects. Loss of hair, rarely nausea andvomiting.Thiocolciran

Desacetylthiocolchicine, a derivative of colcemid,has been developed in France. Huguenin et al.38claimed that there was little effect on solid tumoursbut striking results were accomplished by a.combination of radiotherapy and thiocolciran.The French claim that thiocolciran acts as a radio-sensitizer is not supported by Levine.39 He:mentions that when thiocolciran is administeredintra-peritoneally to mice bearing five-day-olntumour implants and these are subsequently givedhigh voltage X-rays, there is no significant increasein the number of mice showing complete tunourregression as compared with controls. Neverthe-

December 1958 NEWTON: Chemotherapy of Cancer 627

less, thiocolciran is probably a useful agent in thatit is not unduly toxic and there are no undesirableside-effects apart from occasional pain at tumoursites following intravenous injections. At West-minister Hospital, in two cases of epithelial cancer,namely, carcinoma of the stomach and lympho-epithelioma of the naso-pharynx, definite tem-porary disease control has been achieved withintravenous thiocolciran.

Dosage. 2 mg. daily rising to maximum of20 mg. per day, then 20 mg. three times a weekwhich may be continued as long as the bloodcount is satisfactory.

Side-effects. Pain at site of tumour. Nonausea or vomiting observed. Bone marrowdepression.

Chemotherapy in Relation to SurgeryIt has been stated that chemotherapeutic sub-

stances such as ThioTEPA are capable of markedlyreducing the number of artificial pulmonarymetastases resulting from injecting melanomacells in mice (Kramer et al.40). ThioTEPA hasalso proved successful in the prevention ordiminution of the number of,pulmonary metas-tases following removal of the primary malignantmelanoma on the limbs of mice (W. Kramer andW. Schatten41). Animal work such as this hasrationalized the administration of chemothera-peutic compounds during surgical removal ofprimary malignant disease. At WestminsterHospital it is now common to give intravenousnitrogen mustard, HN2, before, during and afteran operation for excision of a primary malignantmelanoma. There would seem to be reasonablegrounds for giving similar courses of chemo-therapy when other types of primary malignanttumours are removed.

SummaryChemotherapeutic agents in present-day use

have been classified and an account given of themore important agents together with indicationfor their use, dosage and side-effects.

AcknowledgmentI would like to take the opportunity of thanking

Sir' Stanford'Cade who is'largely responsible forinitiating my interest in this subject.

REFERENCESI. RHOADS, C. P. (r946), J. Amer. med. Ass., I3I, 656.

2. GOODMAN, L. S., WINTROBE, M. M., DAMESHEK, W.,GOODMAN, M. J., GILMAN, A., and MCLENNAN,M. T. (I946), J. Amer. med. Ass., I32, 126.

3. WILKINSON, J. F., and FLETCHER (1947), Lancet, ii, 540.4. FARBER, S., DIAMOND, L. K., MERCER, R. D.,

SYLVESTER, R. F., Jr., and WOLFF, J. A. (1948), NewEngt. J. Med., 238, 787.

5. EVERETT, J. L., ROBERTS, J. R., and ROSS, W. C. J.(I953), J' ehem. Soc., 2386.

6. ALTMAN, S. J., HAUT, A., CARTWRIGHT, G. E., andWINTROBE, M. M. (1956), Cancer, 9, 512.

7. GALTON, D. A. G., ISRAELS, L. G., NABARRO, J. D. N.,and TILL, M. (i955), Brit. med. J., ii, 1172.

8. PILLERS, E. K., MAGALINI, S. I. (1958), Cancer, I, 410.9. LARIONOV, L. F., SHKODINSKAJA, E. N., TROOSHEI-

KINA, V. I., KHOKHLOV, A. S., VASINA, O. S., andNOVIKOVA, M. A. (1955), Lancet, ii, 169.

io. BLOKHIN, N., LARIONOV, L. F., PEREVODCHIKOVA,M., CHEBOTAREVA, L., and MERKULOVA, N. (1958),Ann. N.Y. Acad. Sci., 68, 1128.

ii. PAPAC, R., GALTON, D. A. G., TILL, M., andWILTSHAW, E. (1958), Ann. N.Y. Acad. Sci., 68, 1126.

12. HOLLAND, J. 'F., and REGELSON, W. (1958), Ann. N.Y.Acad. Sci., 68, 1122.

13. SELLEI, C., and ECKHARDT, S. (I958), Ann. N.Y. Acad.Sci., 68, 1164.

14. ROSE, F. L., HENDRY, J. A., and WALPOLE, A. L. (1950),Nature, 165, 993.

15. BAYRD, E. D., STICKNEY, J. M., HALL, B. E., andWATKINS, C. H. (1952), Cancer, 5; 336.

I6. RUNDLES, R. W., and BARTON, W. B.. (1952), Blood, 7,483.

17. SILVERBERG, J., and DAMESHEK (1952), J. Amer. med.Ass., 148, -Ioi5.

I8. KRAVITZ, S. C., DIAMOND, H. D., and CRAVER, L. F.(1952), Blood, 7, 729.

I9. BATEMAN, J. C., and MCCABE, M. (i955), J. Acta UnioIntern. Contra Canorum, ii, iiI.

20. ULTMANN, J., HYMAN, G., CRANDALL, C., NAUJOKSH., and GELLHORN, A. (1957), Cancer, 10, 5, 902.

21. HADDOW, A., and TIMMIS, G. (1953), Lancet, i, 207.22. GALTON, D. A. G. (I953), Lancet, i, 208.23. HAUT, A., ALTMAN, S. J., CARTWRIGHT, G. E., and

WINTROBE, M. M. (I955), Arch. intern. Med., 96, 451.24. GALTON; D. A. G., and TILL, M. (i955), Lancet, 1, 425.25. BIERMAN, H. R,, .KELLY, K. H., KNUDSON, A. G. Jr.,

MAEKAWA, T., and TIMMIS, G. M. (i958), Ann. . YAcad. Sci., 68, I211.

26. MILLER, E. (1958), Ann. N.Y. Acad. Sci., 68, 1205.27. BLAKLEY, R. L. (I954), Biochem. J., 58, 448.28. FARBER, S. (i957), The Leukaemias N. Y, 623.29. TIVEY, H. (95:4), Ann. N.Y. Acad. Sci., 60, 322.30. HITCHINGS, G. H., and ELION, G. B. (i954), Ann. N.Y.

Acad, Sci., 60, 195.31. BURCHENAL, J. H., ELLISON, R. R., MURPHY, M. L.,

KARNOFSKY, D. A., SYKES, M. P., TAN, C. T. C.MERMANN, A. C., YUCEOGLU, M., MYERS, W.- P.L.,; KRAKOFF, I., and ALBERSTADT, N. (1954),Ann. N.Y. Acad. Sci., 60, 359.

32. HYMAN, G., GELLHORN, A., and WOLFF, J. (1954),Ann. N.Y. Acad. Sci., 60, 430.

33. FARBER, S. (1958), 7th International Cancer Congress.Abstracts of Papers, p. 7.

34. TAN, C. T. C., MURPHY, M. L., and BURCHENAL, J. H.(I958),, 7th International Cancer Congress. Abstractsof Papers, p. 295.

35. PATERSON, E., HADDOW, A., THOMAS, J. A., andWATKINSON, J. M. (1946), Lancet, i, 677.

36. MOESCHLIN, S., MEYER, H., and LIGHTMAN, A. (1954),in 'Ciba Foundation Symposium on Leukaemia Research',p. 216.

37. ZBINDEN, F. (1955), Schweiz. med. Wschr., 85, 994.38. HUGtJENIN, R., TRUHAUT, R., and SARACINO, R.

(955), Bul. Ass. franc. Cancer 42, 308.39. LEVINE, M.('I951), Ann. N.Y. Acad. Sci., 51, 1365.40. KRAMER, W. M., ECK, R. V., and SMITH, R. R. (I958),

Surg. Gynec. Obstet., 106, 427.41. KRAMER, W. M., and SCHATTEN, W. E. (I958), Cancer, II,

463.