(CANCER RESEARCH 29, 2300-2307, December 1969]

The Mechanism of Induction of Complete Remission in AcuteMyeloblastic Leukemia in Man

J. S. Hart, S. Shirakawa, J. Trujillo, and E. Frei, III

M. D. Anderson Hospital and Tumor Institute, Texas Medical Center, Houston, Texas 77025

Complete remission of neoplastia disease in man is usuallydefined as the complete disappearance of clinical, radio-graphic, and laboratory evidence of the disease; the abatementof symptoms; and the return of the patient to a normalperformance status (Chart 1). A partial remission is defined asa greater than 50% reduction in the size of radiographicallyevident or palpable tumors. This is considered in terms of thenumber of neoplastic cells on the right-hand side of Chart 1.One hundred to one thousand grams of tumor would beclinically evident, and these masses consist of IO11 to IO12

tumor cells (6). Thus, a 50% reduction in volume shouldrepresent a corresponding decrease in neoplastic cell numbers(one-half log in Chart 1), whereas a complete remission wouldconsist of one or more orders of magnitude or log reduction.Drs. Goldin, Skipper, and Bruce have shown that, for thechemotherapy of tumors, a given amount of treatment causesa constant fractional tumor cell reduction (7, 12). Thus, ittakes as much treatment to reduce the tumor number from

TUMOR SIZE(product of 2 diameters)

50%-

PARTIALREMISSION

COMPLETE | ;REMISSION * 0%-"-

PARTIALREMISSION

TUMOR CELL NUMBER(log scale)

10" -|-

lO'O . -

10" - -

102 -

10" -•-

Chart 1. Extent of remissions when considered with respect to tumorsize and reduction of surviving tumor cells.

IO1' to IO10 as it does for any other log increment on thescale in Chart 1. The tumor in a sense is an "exponentialiceberg," and even though a complete regression is rendered,

only the top of this exponential iceberg is eliminated, and onestill has a long way to go if cell eradication and cure (theultimate goal) are to be achieved. Thus, the production of acomplete remission is merely the first step and represents onlyan opportunity to affect the disease in a major way. Oncecomplete remission is effected, the real business is toeffectively apply treatment during remission, to maximallyreduce and, hopefully, to eradicate the tumor cells. Theseconcepts are consistent with the fact that with two possibleexceptions, it is only those diseases wherein complete remission has been achieved in the majority of patients and technicsof treatment during remission have been applied that prolonged survival and, for a few tumor categories, "cure" has

occurred.For many categories of solid tumors in man, unfortunately,

the best that can be achieved is a partial remission rate of lessthan 50%. While it is important not to underestimate thesubjective and objective benefit that such treatment provides,the very limited effect of such treatment, in a fundamentalsense, must be recognized. An example of the dilemma thatsuch treatment presents is illustrated in Table 1. Drs. Luce andThurmond have found the new agent, imidazole carboxamidetriazeno, capable of producing objective tumor regression in24% of 67 patients with metastatic melanoma (8). Only 3 ofthe 16 responding patients had complete regression of tumormasses. With a variety of other antitumor agents, the remissionrate in melanoma ranges from 6 to 15%, and completeremissions are rare.

Since a substantial complete remission rate has not occurred,the first step to major control of the disease has not beenachieved. Different dose schedules, combinations, andcongener studies with the imidazole derivative are under studyin an effort to achieve a substantial complete remission rate.Frequently, analysis of subcategories of a neoplastic diseasewill disclose favorably prognostic factors. Those obtained forthe imidazole^nelanoma experience are shown in Table 2. A39% overall and 10% complete response rate occurs in womenwith melanoma; this is significantly better than the 13%response rate that occurs in men. This has a number ofimplications which are under exploration.

Having emphasized the importance of complete remission asa first step in major disease control, it is essential to point outthe exceptions. In Table 3, the cytogenetic findings in the

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Remission in Acute Leukemia

Table1DrugImidazole

carboximide/triazenoAlkylating

agentsAntimetabolites0Naturalproducts'"AzotomycinMethylpregnenetrioneMitomycinHydroxyureaNo.

ofpatients67177516021442744Complete33001200RemissionPartial1325332345Total(%)16

(24)28(14)3(6)3(6)3(15)5(11)4(15)5(11)ReferenceLuceandThurmanArch.Intern. Med.,MayArch.Intern. Med.,MayArch.Intern. Med.,MayCCRCSept.'68CCR,Dec.'66CCR,Oct.'68Collected'67'67'67

Metastatic melanoma response to chemotherapy"Fluorouracil, fluorodeoxyuridine, Methotrexate, mercaptopurine.¿'Vinblastine,vincristine, colchicinic acid derivatives.CCCR, Cancer Chemotherapy Reports.

Table 2 Table 3

SexofpatientsMaleFemaleTotalNo.ofpatients392867Complete033ResponsePartial5813Total(%)5(13)11(39)16

(24)

Effect of sex on melanoma response to imidazole.

bone marrow of patients with various types of leukemia areconsidered. A substantial proportion of patients with activelymphocytic leukemia have abnormal aneuploid lines (14).When the patient enters remission, the leukemic cellsmorphologically, and as evidenced by cytogenetic studies,disappear from the marrow (9). The majority of patients withchronic myelocytic leukemia are Ph1 chromosome positive.

Though complete remissions, as evidenced clinically andhematologically, can be achieved in the majority of patients,the Ph1 chromosome persists in the bone marrow (2). Thus,

complete remission in chronic myelocytic leukemia apparentlyinvolves a shift in the maturity of the neoplastic cells, but asubstantial reduction of such cells does not occur. This is inaccord with the fact that survival has not been prolonged inpatients with chronic myelocytic leukemia and contrasts withthe data on acute lymphocytic leukemia where a highcomplete remission rate and marked prolongation of survivalhave been achieved (2,6).

One of our major therapeutic efforts during the past severalyears has been to increase the complete remission rate inpatients with acute myelocytic leukemia. Thus, it wasimportant to establish whether remission in acute myelocyticleukemia is a valid and meaningful one as it is in the case ofacute lymphocytic leukemia or whether it is a "pseudoremis-sion" such as occurs in chronic myelocytic leukemia. Drs. Hart

and Trujillo have studied this phenomenon in patients withacute myelocytic leukemia at the University of Texas M. D.Anderson Hospital (Tables 4, 5) (personal communication).

Of 43 patients with active disease, 35% had aneuploid lines.Of these 15, six entered complete remission (Table 4). In 5 ofthese patients, the aneuploid lines disappeared, and in the oneexception there was a probable Ph1 chromosome suggesting a

relationship to chronic myelocytic leukemia. One such patientis illustrated in Table 5. The chemotherapy produced adiminution in the number of leukemic cells in the bone

DiseaseAcutelymphocyticleukemiaChronicmyelocyticleukemiaAcutemyelocyticleukemiaCytogeneticActivedisease50-70%ofpatientshaveaneuploidline(s)80%

ofpatientshave

Ph1Chromosome50-70%

ofpatientshaveaneuploidline(s)Complete

remissionAneuploidlinedisappearsPh1

persistsin

morphologically normalmarrowcellsAneuploidlineusuallydisappearsRelapse

orblastcrisisSamelinereturnsPh

persistsandadditionalabnormalitymayappearSame

linereturnsabnormalities

in leukemia.

marrow to less than 10%, i.e., a complete remission. Paripasu,there occurred a diminution in the aneuploid Unes with areturn of the normal cell lines during remission. The abnormalline returned again in quantity as the patient relapsed. Theabove cytogenetic data reassure us that a complete remissionin acute myelocytic leukemia has meaning similar to that ofacute lymphocytic leukemia and can, in fact, be interpreted asa valid first step in the control of the disease.

During the past 6 years, the complete remission rate foracute myelocytic leukemia has increased from 10% with6-mercaptopurine to figures approaching 40% for some of therecent programs. Factors contributing to this improvement aremore effective dose scheduling, combination chemotherapy,new agents such as daunomycin and arabinosylcytosine, andsupportive care.

The remainder of this paper will focus on the dynamics ofremission induction in acute myelocytic leukemia. In additionto providing basic information concerning the disease, suchinformation should supply therapeutic leads. Acutemyelocytic leukemia is primarily a disease of the bonemarrow. Reduction of leukemic cells and return of normalcells in this organ are the essence of remission induction.Accordingly, a study was initiated one year ago whereinparameters were followed in the bone marrow at weekly and,

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J. S. Hart, S. Shirakawa, J. Tru/illo, and E. Frei, III

Table 4 Table 6

No. of Patients43]No.with aneuploid cell lines15(35%)No.

entered remission6AneuploidlinesdisappearedNo.

relapsedSameaneuploid line returned2Effect

of treatment on cytogenetic abnormality in patients with acutemyelocyteleukemia."One

patient with persistent abnormality had Phchromosomein

some instances, twice weekly intervals. These includeaquantitativedetermination of marrow cellularity (ChalkleyAbsolute

marrowleukemicDiseaseAge

(yr.)(AMLonly)ResponseVariableAMLALL15-4041-60

>60AMI

\CRINRALL

\CRiNRcell

infiltrationMedian557453

6563555874

81(%)67%

range28-7270-8240-73

36-8050-6530-7238-7570-82

76 81

counts of marrow sections) and differential morphology. Withthese two values, one can determine the absolute degree ofinfiltration of leukemic cells and differential normal cellsrather than just the relative infiltration usually obtained. Inaddition, DNA synthesis was measured by in vitro tritiatedthymidine labeling followed by scintillation counting and, inselected bone marrows, by autoradiography

The relationship of several variables to the absolute marrowleukemic infiltration is presented in Table 6. The marrow wasmore extensively infiltrated in acute lymphocytic leukemiathan in acute myelocytic leukemia. However, the marrowleukemic infiltration did not differ for the different age groupsnor did it affect responsiveness to chemotherapy.

The effect of combination chemotherapy with 6-mer-captopurine and 6-methylmercaptopurine riboside on 9patients who entered complete remission and 7 who did notare presented in (Chart 2) (1, 11, 13). The marrow cellularityprior to treatment was 100% and the percent of leukemic cellswas 60%; thus the absolute leukemic cellularity was 60%. Witheffective treatment, the absolute leukemic cellularity decreaseslinearly on this semilog plot with a T/2 of 15.5 days. If thisrate of leukemic cell destruction were sustained down through11 or 12 orders of magnitude ("logs") it would take 17

months of treatment to approach cell eradication. In thosepatients who did not respond, the median marrow cellularityremained 100%, and the percent of leukemic cell infiltrationdecreased only slightly.

This composite obscures the fact that the decrease incellularity and percent of leukemic cells are not alwaysparallel. In about 25% of the patients, one will change morerapidly and occur as much as two months before the other. In

Disease variables and pretreatment in absolute marrow leukemic cellinfiltration. AML, acute myelocytic leukemia; ALL, acute lymphocyticleukemia; CR, complete remission; NR, no response.

these circumstances where the percent of leukemic cellinfiltration does not change but the marrow cellularitydecreases (about 10% of all patients), treatment might bediscontinued because of its supposed ineffectiveness if onewere not following the marrow cellularity.

Complete Remission (9 Potients) No Remission (7 Patients)

0 10 20 30 40DAYS

10-• Treatment Courses «ith MP t MMPR- Cellulari t y (%)

--LejkemicCells/Nucleated Cells 1%)—Absolute LeuKemicCellularity

0 10 20 30 40 50DAYS

Chart 2. Effect of treatment on acute myelocytic leukemia marrowcellularity. MP, 6-mercaptopurine; MMPR, 6-methylmercaptopurineriboside.

Table5Date

(in1968)Leukemiccellsin

marrow(%)No.ofmetaphasesanalyzed46XXC

- G - D + E+46XXG- D+Normal

(46XX)Nonspecificabnormalities1-15571374112-229141222-1227442-26520204-12403644-155251785-133211915-265227-2213302738-12302312119-126888

Patient C.A.R. whose aneuploid cell lines disappeared.

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Remission of Acute Leukemia

For the majority of patients responding to the varioustreatment programs, the decrease in absolute leukemiccellularity was linear enough so that the T/2 could becalculated. The relationship of this T/2 to several clinicalvariables was studied.

The range of half-times and the relationship to type ofleukemia and pretreatment marrow infiltration is presented inChart 3. The T/2 for a few patients was as short as 5 days, themajority of values falling between 10 and 20 days, and 30% ofpatients had a T/2 of greater than 20 days. There is nocorrelation with the degree of pretreatment marrowinfiltration. It was anticipated that patients with high degreesof marrow infiltration might have a larger pool ofnonproliferating cells and thus a longer T/2, but that did notprove to be the case.

100-

sg 80-160-

iu O

St

40-

20-

•. o"

10 20 4030DAYS

DECREASE MARROW LEUKEMIC CELL

INFILTRATION

Chart 3. Decrease in marrow leukemic cell infiltrate with treatment.ALL, acute lymphocytic leukemia; AML, acute myelocytic leukemia.

The median rate of decrease of marrow leukemic cells wasslower in acute myelocytic leukemia than in acutelymphocytic leukemia. The different treatment programs didnot result in substantially different half-times (Table 7).

The effect of the rate of decrease in marrow leukemic cellson the duration of remission was analyzed (Chart 4). It mightbe anticipated that the faster the leukemic cells disappearedfrom the marrow with treatment the more "sensitive" were

such cells and the longer would be the duration of remission.All treatment programs were continued during remission.

While many of the patients are still in remission, there is noreadily evident correlation between this rate of decrease andthe duration of remission. Perhaps those cell kinetic factorswhich make cells disappear quickly with treatment also makethem ascend rapidly when treatment is no longer effective.

son

l S 40-

30-

«I

5 °" í 20-»~

If^ 5 10H ©

123456789 IO 12

DURATION OF REMISSION, mo.

Chart 4. Effect of rate of decrease of marrow leukemic cells onduration of remission. ALL, acute lymphocytic leukemia; MP,6-mercaptopurine; MMPR, 6-methylmercaptopurine riboside; ARA-C,arabinosylcytosine: CYCLO; cyclophosphamide; POMP, prednisone,vincristine, Methotrexate, and 6-mercaptopurine.

Chart 5 illustrates the changes in tritiated thymidine uptakeby marrow cells and the absolute leukemic cell marrowinfiltration in five patients treated with arabinosylcytosine incombination with cyclophosphamide (3, 5, 10). The medianvalues are plotted. The marrow examinations were performedat intervals varying from 4 days to 2 weeks. In this group ofpatients there is some evidence that the decrease in marrowleukemic cells following a course of treatment is not sustainedthrough two weeks but that some degree of recovery doesoccur. With each course of treatment, the DNA synthesis rateper cell decreases 5- to 10-fold, with the low level reachedduring or at the end of the 5-day course of treatment. Within 4to 6 days, there is partial (approximately 50%) recovery, andby 2 weeks recovery is usually complete. Thus, while theadverse effect on DNA synthesis is marked, it is short-lived,and recovery occurs rapidly. After 2 to 3 courses of treatment,when complete remission is in process, normal bone marrowelements increase and contribute a substantial portion of theDNA sythesis (see below).

Table 7

DiseaseAML

ALLTotal ALT/2

in day^s(median)17

1216TreatmentPOMP

MP + MMPRara-C

ara-C + cycloT/2

in days(median)14

15.51512

Effect of disease and treatment on rate of marrow leukemic cell decrease duringremission induction. POMP, prednisone, vincristine, Methotrexate, and6-mercaptopurine; MP, 6-mercaptopurine; MMPR, 6-methylmercaptopurine riboside;ara-C, arabinosylcytosine; cyclo, cyclophosphamide; AML, acute myelocytic leukemia;ALL, acute lymphocytic leukemia; AL, acute leukemia.

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J. S. Hart, S. Shirakawa, J. Trujillo, and E. Frei, III

I00n

10

ONA Synlhesis Per IO6

Morro« Cells x 1000

Courses of Treatment

150 200 160

Morro« Infiltration «ittiLeukemic cells

(absolute %l160

0 10 20 30 40 50DAYS OF TREATMENT

Ara-C t Cyclophosphomide in mg/sq m/dayx4

Chart 5. Remission induction in 5 patients with acute myelocyticleukemia who entered complete remission.

Charts 6 and 7 show interesting variations of this pattern. InPatient A. L. (Chart 6) there was no change in the absoluteleukemic marrow cellularity for the first three months oftreatment with arabinosylcytosine and cyclophosphamide.Since the patient was not responding and was on the maximalsupportive care program, the doses were progressivelyincreased to three times the conventional dose levels. At higherdoses, the patient rapidly entered complete remission and hasremained there for 8 months. While DNA synthesis wasdetermined relatively infrequently during the early part oftreatment, it was not affected in a major way until the higherdose levels were reached. This is an example of the importanceof dose and duration of treatment on response.

Another variation is illustrated in Chart 7. This patient had avery low rate of DNA synthesis prior to treatment. DNAsynthesis progressively increased during treatment in spite of

10- ,A

Courses of60 90

' DNA Synthesis Per IO6

Morro« Cells i 1000

Morrow Infiltration with

Leukemic Cells (absolute %]

Treatment120 120 120 120

I II II IIIIIIIug

O 20 40 60 80 100 120

DAYS OF TREATMENT

Aro-C + Cyclophosphamide in ma/sq m/day x 4

Chart 6. Remission induction in patient (A. L.) with acutemyelocytic leukemia.

lOO-i

50-

10-

Morrow Infiltration withLeukemic cells (absolute %)

0 10 ZO 100 120 140DAYS OF TREATMENT

Ara-O Cyclophosphomide In mg/sq m/day x 4

Chart 7. Remission induction in patient (C. A. L.) with acutemyelocytic leukemia.

the fact that the leukemia cell infiltration in the marrow didnot change during the first 40-60 days. The effect of treatmenton DNA synthesis was relatively minor until late in remissioninduction. We tentatively interpret this as representingrecruitment of cells into the proliferating pool during the earlystage of treatment.

Since the bone marrow in patients with acute myelocyticleukemia is relatively heterogeneous with respect to cellularcomponents and changes during remission induction, it wasimportant to determine the DNA synthesis rates for bothnormal and neoplastic cells capable of replication. Thus, inaddition to scintillation counting, autoradiograms wereperformed, and the percent of labeled cells for the leukemiccells and normal cells were determined separately (Chart 8). Inthis patient, 60% of the marrow cells prior to treatment wereleukemic, and this decreased to less than 5% after two coursesof treatment. The overall rate of DNA synthesis decreasedwith the first two courses of treatment and remained down forsome two weeks following the second course of treatment. Itthen increased to higher than pretreatment levels, decreasedmarkedly with the third course of chemotherapy, butrecovered quickly and rose to high levels on the 70th day. Thedecrease in DNA synthesis following the first two courses oftreatment resulted almost exclusively from change in thenumber and percent labeling index of the leukemic cells. Thus,for the first course of treatment, the percent of labeledleukemic cells decreased from sixteen to six. Two weeks afterthe first course of treatment, the labeling index was back up to24%. With the second course it decreased to 5%. Only 3% ofthe marrow cells prior to treatment were nucleated red cellswith a labeling index of 21%. These cells disappeared from themarrow through the first two courses of treatment, but they

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Remission in Acute Leukemia

dpa,per IO6

cell100,000

50,000

TREATMENT100 _.

SO-

IO,000

5,000

•DNASynthesis Per IO6 Marrow Cells0 X leulcemlc Cell« In Marrow

•7. Nucleated Red Cells MarrowQLabellng Index, 1.e,7.Labeled Cells

1,000

Chart 8. Remission induction in patient (R. E.) with acute myelocytic leukemia. DNA synthesis and labeling index studies.Treatment consisted of a 5-day course of arabinosylcytosine (Ara-C) and cyclophosphamide (Cyclo.) administration, eachdrug at 120 mg/sq m/day.

increased to 40% of the marrow nucleated cells, of which 21%were labeled, prior to the third course of treatment. At thistime, close to one-half of the marrow radioactivity was due tonucleated red blood cell activity. The decrease following thethird course of chemotherapy was due both to a decrease inred cell and leukemic cell proliferative activity, but the markedincrease in DNA synthesis by the 70th day was a function ofmarked increase in erythroid activity in the marrow (40% ofthe nucleated cells) with a labeling index of 73%. At this time,the leukemic cells had decreased to less than 5% of the marrowcells. Most of the remaining elements in the marrow wererelatively mature cells of the granulocytic series. The percentof cells which were myelocytes and their labeling index waslow. The results in this patient were similar to results in otherpatients and indicated that the change in DNA synthesis forthe first two or three courses of remission-induction treatmentis due primarily to a change in the proliferative activity of theleukemic cells. As the patient approaches remission, there isincreasing contribution from normal bone marrow elements,and, particularly early in remission, much of the radioactivityis due to labeled nucleated red blood cells.

DNA synthesis of marrow cells during remission induction inpatients with acute myelocytic leukemia treated with 5-daycourses of combined arabinosylcytosine and cyclophosphamide at two-week intervals is presented in Table 8. Thecourses of treatment are given across the top of the figure, and

the DNA synthesis rate is expressed as counts per minute perIO6 marrow cells X 1000 at the start of a course of treatment

(S) and 7 days after the start of each course of treatment (N).The median values for DNA synthesis prior to treatment was50% higher in those patients who entered complete remissionas compared to those patients who did not. A 3- to 10-folddecrease in the median rates of DNA synthesis occurredfollowing each course of treatment. Recovery was in progressbut not usually complete by the time of the subsequentcourses of treatment. During the early stages of treatmentthere were no consistent differences between the respondersand nonresponders. However, in the majority of respondingpatients, an increase in DNA synthesis occurred which wasoften of marked degree and preceded the development of bonemarrow remission by 2 to 4 weeks. As in Chart 7, when thiswas studied autoradiographically, it was found to be dueprimarily to an increase in erythropoiesis. Such an increase inDNA synthesis did not occur in two of the patients who hadrelatively short remissions, suggesting that the proliferativethrust of the normal marrow did not normalize.

It should be emphasized that this is the report of an ongoingstudy concerning cell dynamic changes during remissioninduction in patients with acute myelocytic leukemia. Thedecrease in rate of DNA synthesis following treatment and itsrecovery by the 14th day following a 5-day course oftreatment were similar for responding and nonresponding

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J. S. Hart, S. Shirakawa, J. Trujillo, and E. Frei, III

Table 8

Course oftreatment12Patient's

initials1SComplete

remissionAl12Cal5Car2Da10Sm53Gon4Ko9Wo

4Median

8No

responsePa5Da3Th3St39La

7Median

52N0.50.2615220.30.80.20.60.5S1614352135151512975N80.113230.320.6180.913S10212633515232154N9112110.50.72214

5 6 7 8 910SNSNSNSNSNSNSN3

,, 16 3.5 0.4 0.8 0.2 5 0.8 26 2.537301•150.2 l 220.752 50 12 26 1 219641

140.3 2 5 2 1 V Shortremissions3

4 1 3\2215

0.7231119

5b6

DNA synthesis of marrow cells during remission induction in acute myelocytic leukemia. Bars indicate the onset of remission. All numbers inthe table are calculated as cpm/10 X 1000 marrow cells. S, start; N, nadir.

°Eachcourse of arabinosylcytosine and cytosine lasted for 5 days and was repeated every 2 weeks.''Died of sepsis with improving marrow.

patients. However, a decreasing proportion of cells recover inthose patients entering remission, and this rate of decreasetends to be exponential with a half-time of around 15 days. Itwould appear that the magnitude and duration of depressionof DNA synthesis is relatively constant for a given type oftreatment in acute myelocytic leukemia regardless of whetherthe patient enters remission. How, then, does one explain thedifference between responders and nonresponders? There isevidence, in experimental studies, that cells with reasonablysimilar kinetic behavior may differ markedly in their ability torecover from a given insult to DNA biosynthesis. Thus,continuously replicating cells, such as bone marrow andgastrointestinal stem cells, are much less able to recover frominhibition of DNA biosynthesis by hydroxyurea or arabinosylcytosine than cells induced into rapid replication, such asthose of the regenerating liver or isoproteranol stimulatedsalivary glands (4). Clearly, cellular events in addition toinhibition of DNA biosynthesis distinguish responding fromnonresponding patients.

Conclusions

1. Complete remission represents a major initial step in thecontrol of neoplasia. However, in cy tokinetic terms (first-orderkinetic effect of chemotherapy and neoplastic cell numbers) itrepresents as little as 10% of the therapeutic objective ofneoplastic cell eradication.

2. Partial remission in a minority of patients with a giventype of neoplasm is a very limited achievement. If a substantialcomplete remission rate cannot be achieved by various

manipulations of the therapeutic regimen, it is questionablewhether such treatment should be pursued.

3. The significance of a complete remission within a givendisease category should be studied. "Pseudoremissions" occur

in chronic myelocytic leukemia and do not favorably affectsurvival. Remissions in tumors of mixed cellular componentssuch as Hodgkin's disease and teratomas deserve study. It is

possible that tumor regression results from a selective decreasein normal or less malignant cellular elements. Completeremissions in patients with acute myelocytic leukemia, asdetermined from cytogenetic studies, are similar to remissionsin acute lymphocytic leukemia in that at least a 90% reductionin neoplastic cells occurs. This contrasts with the "pseudo-remissions" of chronic myelocytic leukemia.

4. Cell differential by smear and cellularity by sectionshould both be determined in order to adequately evaluateleukemic and normal cellular infiltration of the bone marrow.In 25% of the patients, these values may not varyconcordantly with respect to leukemic infiltration, andemployment of the bone marrow smear alone may bemisleading.

5. Pretreatment absolute bone marrow infiltration wasgreater in acute lymphocytic leukemia than in acutemyelocytyic leukemia, was not affected by age, and did notdiffer with respect to subsequent responsiveness to chemotherapy.

6. With treatment that effected a complete remission, thedecrease in absolute bone marrow leukemic cell infiltrationtended to be linear on a semilog plot, with a half-time ofapproximately 15 days and a range of 5 to 50 days.

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Remission in Acute Leukemia

l. The rate of decrease of marrow leukemic cells wassomewhat slower in acute myelocytic leukemia than in acutelymphocytic leukemia, did not differ for the various treatmentprograms, and did not correlate with the duration ofsubsequent remission.

8. DNA synthesis of the bone marrow was studied by invitro incubation with tritiated thymidine followed byscintillation counting and autoradiography. During the earlycourses of remission-induction treatment, the change in DNAsynthesis with treatment was primarily due to changes inleukemic cell DNA synthesis. A 3-to 10-fold decrease in DNAsynthesis with 5-day courses of arabinosylcytosine andcyclophosphamide occurred, followed by recovery whichapproached completion by the 14th day. After 3 to 4 coursesof treatment at 2-week intervals in those patients enteringcomplete remission, the leukemic cells had decreasedsubstantially, and most of the radioactivity during earlyremission induction was due to erythropoiesis.

9. The pretreatment rate of leukemic cell DNA synthesis wassomewhat greater in patients destined to respond than innonresponders. However, the magnitude and duration of DNAsynthesis depression during early treatment was comparable inresponding and nonresponding patients. After 4 to 5 coursesor more of treatment, responding patients developed a markedincrease in DNA synthesis incident to the proliferative thrustof normal bone marrow cellular elements.

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