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Some thoughts on resistance to cancer chemotherapy
Cancer Twatment
Revitwr
(1984)
Some thoughts Stephen
11 (Supplement
A), 3-7
on resistance
to cancer chemotherapy
K. Carter
Anti-Cancer Research, Pharmaceutical .New York, NY 10154, U.S.A.
Research and Development Division,
Bristol-Myers
Company,
Despite its successes cancer chemotherapy fails to cure the great majority of patients treated (2). This failure is due to the overgrowth of resistant cells. The final common pathway of all chemotherapy strategies is to overcome the resistance of tumor cells to eradication at dose levels which are tolerable by the host. The concept of resistance is as old as cytotoxic chemotherapy itself. The views on resistance mechanisms, however, have been changing over the years as it has become obvious that the older hypotheses were not resulting in new successful treatment strategies. In the early days of chemotherapy the major postulated mechanism of resistance was viewed to be kinetic. Drugs were viewed as being optimally effective against actively proliferating cells. Drugs were classified according to their preferential killing within the cell cycle based on cell culture kinetic studies. The most resistant cell was viewed to be the nonproliferating cell and such cells were labelled as Go to separate them from cells that were moving through the actively proliferating phases of the cell cycle (M --) G, --) S -+ G, + M). Solid tumors were viewed as being more resistant than hematological malignancies because they had a large percentage of cells in the Go phase and reciprocally their growth fraction was low and their doubling time long. New drugs were sought which would be able to kill Go cells. This search was hampered by the fact that the in viva screening systems in common use had very high growth fractions and short doubling times in comparison to human solid tumors. A second view ofresistance which began to evolve relatively early in the history of cancer chemotherapy was the biochemical view. A significant number of the early effective cytotoxic drugs were anti-metabolites. The early chemotherapists developed elegant biochemical paradigms for the mechanisms of action of drugs such as methotrexate, 6-mercaptopurine and 5-Fluorouracil. These paradigms enabled elegant hypothesis of biochemical resistance to be developed which to a great degree were testable, and seemingly validated, in cell culture studies. This early love of postulated biochemical patterns for mechanisms of action and resistance can still be found today in the chemotherapy literature. The biochemical view of drugs action led to the concept of ‘rational’ drug design which was set up in opposition to the empirically based approach of 0305-7372/84/l
lA0003+
05 $03.00/O
0
1984 Academic
Press Inc.
(London)
Limited
4
S. K.
CARTER
random screening in proliferation oriented animal models. The chemotherapy literature of the 1950-70 era contains a massive data base on rationally designed thiopurines, fluorinated pyrimidines, anti-folates and other types of anti-metabolites. Many were placed into clinical trial based on biochemically oriented tests but none were found to be more successful than the first anti-metabolites which were empirically discovered. Only 5-Fluorouracil can be viewed as a success of the rational biochemical approach but the subsequent failures of 5-FUDR and FsTDR demonstrated that the situation contained additional complexities beyond biochemical action concepts. The third concept of resistance which came into prominence was that of pharmacology. The drug could not kill the tumor cell unless it reached the tumor cell at an adequate concentration for an appropriate period of time. It was recognized that cancer drugs, like all drugs, went through some or all of the classic steps of absorption, activation, inactivation, degradation and excretion. Extensive plasma pharmacokinetic studies were, and still are, performed elucidating the half life, disappearance curves and excretion patterns of intact drug and all presumed important metabolites. The concept of the pharmacologic sanctuary of the CNS led to the successful development of therapeutic approaches to CNS leukemia. Despite the obvious and basic importance of pharmacology in understanding drug action the practical impact of pharmacology on the clinical use of cytotoxic chemotherapy has been relatively little in comparison to other diseases. Nearly all chemotherapy schedules bave been developed empirically and at best pharmacology has developed some post-hoc rationalizations for why one empirically discovered schedule has been superior to others. The main problem for pharmacology studies in cancer is that the critical aspect is the tissue concentration over time of the active moiety of the drug. This cannot be easily or routinely studied and may not be correlated with the classical plasma pharmacokinetic data which are routinely elucidated. By the early 1970s the view of cancer chemotherapy resistance was an integration of the proliferative, biochemical and pharmacologic concepts all placed within the broad conceptual context of the cell kill hypothesis. This integration explained all of the successes of cancer chemotherapy and seemed, in the strategies of adjuvant chemotherapy, supportive care and toxicity blocking, to offer a possible solution to the tremendous resistance of most solid tumors. DeVita (3) describes the principles of cancer chemotherapy from 1960 to 1975 as enlightened empiricism as opposed to the total empiricism of the years preceding 1960. The period of 1960 to 1975, in cancer chemotherapy, was dominated by the cell kill hypothesis as elucidated by Skipper and colleagues (8). The essential bottom line for this concept was that ifonly the dose could be increased to a high enough level, in relation to the number ofcancer cells existing, then the fractional cell kill would lead to total eradication of the tumor mass and cure. This model supports the use of high dose intermittent drug schedules which have dominated medical oncology throughout this age of enlightened empiricism. The strategies of toxicity blocking, supportive care, e.g. bone marrow transplantation and regional drug delivery, have all been predicated on increasing the concentration of drug that can be brought to bear on the tumor mass so as to increase the fractional cell kill. The strategy of adjuvant chemotherapy has worked from the opposite end. This strategy has attempted to bring standard doses to bear on smaller tumor masses (micrometastatic disease) which were presumed to be kinetically more sensitive. This model has been essentially a proliferative model which has assumed that the dormant or Go cell is not a major problem ifthe tumor mass is small enough. DeVita (3) has pointed out that by 1975 a barrier became evident to the oncology
RESISTANCE
TO
CHEMOTHERAPY
5
community. First, many of the more common cancers, although they sometimes responded dramatically to drugs, were not curable by chemotherapy. The second was that ‘adjuvant therapy was not as effective as prevalent therapy predicted.’ The failures were perceived as related somewhat to tumor mass. Surgical curability is clearly related to tumor mass. This correlation of mass to surgical curability relates predominately to the propensity for metastatic relapse outside the operated field. The larger the primary tumor mass at diagnosis the greater the probability that micrometastatic disease exists. The new paradigm to explain chemotherapy’s successes and failures focuses on resistance. It draws strong analogy between cancer chemotherapy and the successful use of antibiotics for the treatment of infectious disease. It starts with the observations of Luria and Delbruck in 1943 (7). Their observations indicated that bacteria have an inherent capacity to mutate toward resistance to agents they have never seen. The size of a resistant population in sub-cultures of similar size differed depending upon when this mutation occurred. This type of resistance was a permanent genetic change. In 1979, Goldie and Coldman (6) re-examined the data of Luria and Delbruck and developed a mathematical model which related curability to the time of appearance of a singly or doubly resistant cell line. Their model assumes a mutation rate approximating the natural mutation frequency. The model predicts that there is a variation in size of the resistant fraction in tumors of the same size and type, depending upon the value of the mutation rate and the point at which a mutation develops. This model also predicts, as does the cell kill hypothesis, that smaller tumor masses will be more curable by drugs than larger masses, but for different reasons. Now a small mass is more curable because there is a smaller probability that mutation to resistance of two or more drugs has occurred. This model still supports the administration of the highest dose possible ofcancer drugs, but recognizes that if resistance has developed increasing the dose will not necessarily increase cell kill. It is still an essentially proliferative model which shifts the emphasis from tumor cell kinetics to tumor cell resistance but still ignores the problem of tumor cell dormancy. The basic assumption of the Gold&Coldman model is that tumors are curable by chemotherapy if no permanently resistant cell lines are present. Curability diminishes rapidly with the appearance of a singly resistant line if only one effective therapy is available, or with the appearance of a double resistant line if two equally effective therapies are available. This model also assumes the following: (1) Tumor cell kill is a logarithmic function; (2) The spontaneous mutation toward resistance occurs at about the natural frequency of 10m5 or 10e6; (3) Mutation to resistance is a stepwise function from sensitivity to resistance to the first treatment to resistance to the second treatment with the appearance of a doubly resistant line; (4) Similar growth characteristics in multiple metastatic sites ofthe same tumor in the same or different patients: (5) Equivalent log cell kill for each treatment program. The Gold&Coldman model explains to some degree why adjuvant chemotherapy has been so disappointing to date in relation to the experimental predictions of the cell kill hypothesis. If the primary tumor has grown to the age and size which allows for the establishment of metastatic disease then the possibility of mutation to resistance is great.
6
S. K.
CARTER
Even though the residual micrometastatic mass after surgery is relatively small, its potential for resistance may be quite high. Given the experimental data (5) indicating the metastases are more genetically unstable than the primary tumors from which they are derived, this likelihood is quite high. If resistance has developed chemotherapy will not be able to eradicate micrometastasis despite that relatively small number of cells to be attacked. The desperate medical oncologist, in the face of this conceptual barrier, is now forced to hope that starting adjuvant chemotherapy a few weeks earlier will beat the devil of spontaneous mutation to resistance. DeVita (3) has outlined a series of implications for adjuvant chemotherapy design based on the Goldie-Coldman model. They are the following: (1) Combination chemotherapy will be superior to single agent chemotherapy; (2) The intensity of adjuvant chemotherapy should be as great as it is in clinically evident metastatic disease; (3) Drugs that produce partial responses in patients with clinically evident disease should not necessarily be expected to produce better results (cures) in the adjuvant setting; (4) Duration of treatment may not need to be as lengthy as has been previously thought; (5) Pre-operative chemotherapy may provide important clues to the usefulness of a drug program. And alternate explanation for the resistance of microscopic residual cancer to adjuvant cancer chemotherapy could arise if the residual cancer cells were mitotically quiescent ( 1). In this state they would be refractory to cell kill by the usual cytotoxic agents. The concept of mitotically quiescent but metabolically active cells can explain the clinically observed phenomenon of long disease-free intervals after primary tumor ablation which are followed by relapses in tumors such as breast cancer and malignant melanoma. Mechanisms which could explain this mitotic quiescence include: (1) Lack of, or blocking of, essential (2) Cells undergoing a process akin cessation of division.
tumor growth factors; to differentiation which
is associated
with
reversible
Alexander (1) has postulated that the reason why the cells in clinically evident disease do not become quiescent, when those in microscopic disease do, could be that the tumor itself releases a diffusible product which either prevents differentiation or produces a growth factor without which the tumor does not proliferate. As a result, the tumor cells within small lesions would either differentiate or not divide, because the tumor produced an inhibitor of differentiation or the local concentration of growth factor was too low. In essence, this concept postulates that tumor cell dormancy is the major cause of the failure of adjuvant chemotherapy rather than mutation to intrinsic resistance. A major difference between these two concepts, which are not necessarily incompatible, relates to the perceived relevance of host factors. The Gold&Coldman model ignores the possible importance of host factors and in essence assumes no role for them. The tumor dormancy concept relates the kinetic or mitotic activity of tumor cells to host factors. This is not only intuitively more attractive than a purely proliferative-resistance model, but offers another potential avenue of drug development and clinical research, which might link cytotoxic therapy with approaches deemed ‘non-cytotoxic’, which are often set up in opposition to each other.
RESISTANCE
TO
CHEMOTHERAPY
References 1. Alexander, P. (1982) 2nd Gordon Hamilton Fairley lecture. Need for new approaches to the treatment of patients in clinical remission with special reference to acute myeloid leukemia. BY. 3. Cancer 46: 15 I-159. 2. Carter, S. K., Bakoswki, M. & Hellmann, K. (1982) Chemotherapy of Cancer. New York: John Wiley and Sons. 3. DeVita, V. T. Jr (1983) The James Lewing lecture. The relationship between tumor mass and resistance to chemotherapy. Implications for surgical adjuvant treatment of cancer. Cancer 51: 1209-1220. 4. Fidler, I. J. & Kripke, M. L. (1977) Metastasis results from pre-existing variant cells within a malignant tumor. S&ace 197: 893-895. 5. Fidler, I. J. (1978) Tumor heterogeneity and the biology of cancer invasion and metastases. Cancer Res. 38: 2651-2660. 6. Goldie, J. H. & Coldman, A. J. (1979) A mathematical model for relating the drug sensitivity oftumors to their spontaneous mutation rate. Cancer Treat. Rep. 63: (1 l-12) 172. 7. Luria, S. E. & Delbruck, M. (1943) Mutations of bacteria from virus sensitivity to virus resistance. Genetics 28: 491. 8. Skipper, H. E., Schabel, F. M. & Wilcox, W. S. (1964) Experimental evaluation of potential anti-cancer of experimental leukemia. Cancer agents XII: on the criteria and kinetics associated with “curability” Chmother. Rep. 35: l-l 11.
Revitwr
(1984)
Some thoughts Stephen
11 (Supplement
A), 3-7
on resistance
to cancer chemotherapy
K. Carter
Anti-Cancer Research, Pharmaceutical .New York, NY 10154, U.S.A.
Research and Development Division,
Bristol-Myers
Company,
Despite its successes cancer chemotherapy fails to cure the great majority of patients treated (2). This failure is due to the overgrowth of resistant cells. The final common pathway of all chemotherapy strategies is to overcome the resistance of tumor cells to eradication at dose levels which are tolerable by the host. The concept of resistance is as old as cytotoxic chemotherapy itself. The views on resistance mechanisms, however, have been changing over the years as it has become obvious that the older hypotheses were not resulting in new successful treatment strategies. In the early days of chemotherapy the major postulated mechanism of resistance was viewed to be kinetic. Drugs were viewed as being optimally effective against actively proliferating cells. Drugs were classified according to their preferential killing within the cell cycle based on cell culture kinetic studies. The most resistant cell was viewed to be the nonproliferating cell and such cells were labelled as Go to separate them from cells that were moving through the actively proliferating phases of the cell cycle (M --) G, --) S -+ G, + M). Solid tumors were viewed as being more resistant than hematological malignancies because they had a large percentage of cells in the Go phase and reciprocally their growth fraction was low and their doubling time long. New drugs were sought which would be able to kill Go cells. This search was hampered by the fact that the in viva screening systems in common use had very high growth fractions and short doubling times in comparison to human solid tumors. A second view ofresistance which began to evolve relatively early in the history of cancer chemotherapy was the biochemical view. A significant number of the early effective cytotoxic drugs were anti-metabolites. The early chemotherapists developed elegant biochemical paradigms for the mechanisms of action of drugs such as methotrexate, 6-mercaptopurine and 5-Fluorouracil. These paradigms enabled elegant hypothesis of biochemical resistance to be developed which to a great degree were testable, and seemingly validated, in cell culture studies. This early love of postulated biochemical patterns for mechanisms of action and resistance can still be found today in the chemotherapy literature. The biochemical view of drugs action led to the concept of ‘rational’ drug design which was set up in opposition to the empirically based approach of 0305-7372/84/l
lA0003+
05 $03.00/O
0
1984 Academic
Press Inc.
(London)
Limited
4
S. K.
CARTER
random screening in proliferation oriented animal models. The chemotherapy literature of the 1950-70 era contains a massive data base on rationally designed thiopurines, fluorinated pyrimidines, anti-folates and other types of anti-metabolites. Many were placed into clinical trial based on biochemically oriented tests but none were found to be more successful than the first anti-metabolites which were empirically discovered. Only 5-Fluorouracil can be viewed as a success of the rational biochemical approach but the subsequent failures of 5-FUDR and FsTDR demonstrated that the situation contained additional complexities beyond biochemical action concepts. The third concept of resistance which came into prominence was that of pharmacology. The drug could not kill the tumor cell unless it reached the tumor cell at an adequate concentration for an appropriate period of time. It was recognized that cancer drugs, like all drugs, went through some or all of the classic steps of absorption, activation, inactivation, degradation and excretion. Extensive plasma pharmacokinetic studies were, and still are, performed elucidating the half life, disappearance curves and excretion patterns of intact drug and all presumed important metabolites. The concept of the pharmacologic sanctuary of the CNS led to the successful development of therapeutic approaches to CNS leukemia. Despite the obvious and basic importance of pharmacology in understanding drug action the practical impact of pharmacology on the clinical use of cytotoxic chemotherapy has been relatively little in comparison to other diseases. Nearly all chemotherapy schedules bave been developed empirically and at best pharmacology has developed some post-hoc rationalizations for why one empirically discovered schedule has been superior to others. The main problem for pharmacology studies in cancer is that the critical aspect is the tissue concentration over time of the active moiety of the drug. This cannot be easily or routinely studied and may not be correlated with the classical plasma pharmacokinetic data which are routinely elucidated. By the early 1970s the view of cancer chemotherapy resistance was an integration of the proliferative, biochemical and pharmacologic concepts all placed within the broad conceptual context of the cell kill hypothesis. This integration explained all of the successes of cancer chemotherapy and seemed, in the strategies of adjuvant chemotherapy, supportive care and toxicity blocking, to offer a possible solution to the tremendous resistance of most solid tumors. DeVita (3) describes the principles of cancer chemotherapy from 1960 to 1975 as enlightened empiricism as opposed to the total empiricism of the years preceding 1960. The period of 1960 to 1975, in cancer chemotherapy, was dominated by the cell kill hypothesis as elucidated by Skipper and colleagues (8). The essential bottom line for this concept was that ifonly the dose could be increased to a high enough level, in relation to the number ofcancer cells existing, then the fractional cell kill would lead to total eradication of the tumor mass and cure. This model supports the use of high dose intermittent drug schedules which have dominated medical oncology throughout this age of enlightened empiricism. The strategies of toxicity blocking, supportive care, e.g. bone marrow transplantation and regional drug delivery, have all been predicated on increasing the concentration of drug that can be brought to bear on the tumor mass so as to increase the fractional cell kill. The strategy of adjuvant chemotherapy has worked from the opposite end. This strategy has attempted to bring standard doses to bear on smaller tumor masses (micrometastatic disease) which were presumed to be kinetically more sensitive. This model has been essentially a proliferative model which has assumed that the dormant or Go cell is not a major problem ifthe tumor mass is small enough. DeVita (3) has pointed out that by 1975 a barrier became evident to the oncology
RESISTANCE
TO
CHEMOTHERAPY
5
community. First, many of the more common cancers, although they sometimes responded dramatically to drugs, were not curable by chemotherapy. The second was that ‘adjuvant therapy was not as effective as prevalent therapy predicted.’ The failures were perceived as related somewhat to tumor mass. Surgical curability is clearly related to tumor mass. This correlation of mass to surgical curability relates predominately to the propensity for metastatic relapse outside the operated field. The larger the primary tumor mass at diagnosis the greater the probability that micrometastatic disease exists. The new paradigm to explain chemotherapy’s successes and failures focuses on resistance. It draws strong analogy between cancer chemotherapy and the successful use of antibiotics for the treatment of infectious disease. It starts with the observations of Luria and Delbruck in 1943 (7). Their observations indicated that bacteria have an inherent capacity to mutate toward resistance to agents they have never seen. The size of a resistant population in sub-cultures of similar size differed depending upon when this mutation occurred. This type of resistance was a permanent genetic change. In 1979, Goldie and Coldman (6) re-examined the data of Luria and Delbruck and developed a mathematical model which related curability to the time of appearance of a singly or doubly resistant cell line. Their model assumes a mutation rate approximating the natural mutation frequency. The model predicts that there is a variation in size of the resistant fraction in tumors of the same size and type, depending upon the value of the mutation rate and the point at which a mutation develops. This model also predicts, as does the cell kill hypothesis, that smaller tumor masses will be more curable by drugs than larger masses, but for different reasons. Now a small mass is more curable because there is a smaller probability that mutation to resistance of two or more drugs has occurred. This model still supports the administration of the highest dose possible ofcancer drugs, but recognizes that if resistance has developed increasing the dose will not necessarily increase cell kill. It is still an essentially proliferative model which shifts the emphasis from tumor cell kinetics to tumor cell resistance but still ignores the problem of tumor cell dormancy. The basic assumption of the Gold&Coldman model is that tumors are curable by chemotherapy if no permanently resistant cell lines are present. Curability diminishes rapidly with the appearance of a singly resistant line if only one effective therapy is available, or with the appearance of a double resistant line if two equally effective therapies are available. This model also assumes the following: (1) Tumor cell kill is a logarithmic function; (2) The spontaneous mutation toward resistance occurs at about the natural frequency of 10m5 or 10e6; (3) Mutation to resistance is a stepwise function from sensitivity to resistance to the first treatment to resistance to the second treatment with the appearance of a doubly resistant line; (4) Similar growth characteristics in multiple metastatic sites ofthe same tumor in the same or different patients: (5) Equivalent log cell kill for each treatment program. The Gold&Coldman model explains to some degree why adjuvant chemotherapy has been so disappointing to date in relation to the experimental predictions of the cell kill hypothesis. If the primary tumor has grown to the age and size which allows for the establishment of metastatic disease then the possibility of mutation to resistance is great.
6
S. K.
CARTER
Even though the residual micrometastatic mass after surgery is relatively small, its potential for resistance may be quite high. Given the experimental data (5) indicating the metastases are more genetically unstable than the primary tumors from which they are derived, this likelihood is quite high. If resistance has developed chemotherapy will not be able to eradicate micrometastasis despite that relatively small number of cells to be attacked. The desperate medical oncologist, in the face of this conceptual barrier, is now forced to hope that starting adjuvant chemotherapy a few weeks earlier will beat the devil of spontaneous mutation to resistance. DeVita (3) has outlined a series of implications for adjuvant chemotherapy design based on the Goldie-Coldman model. They are the following: (1) Combination chemotherapy will be superior to single agent chemotherapy; (2) The intensity of adjuvant chemotherapy should be as great as it is in clinically evident metastatic disease; (3) Drugs that produce partial responses in patients with clinically evident disease should not necessarily be expected to produce better results (cures) in the adjuvant setting; (4) Duration of treatment may not need to be as lengthy as has been previously thought; (5) Pre-operative chemotherapy may provide important clues to the usefulness of a drug program. And alternate explanation for the resistance of microscopic residual cancer to adjuvant cancer chemotherapy could arise if the residual cancer cells were mitotically quiescent ( 1). In this state they would be refractory to cell kill by the usual cytotoxic agents. The concept of mitotically quiescent but metabolically active cells can explain the clinically observed phenomenon of long disease-free intervals after primary tumor ablation which are followed by relapses in tumors such as breast cancer and malignant melanoma. Mechanisms which could explain this mitotic quiescence include: (1) Lack of, or blocking of, essential (2) Cells undergoing a process akin cessation of division.
tumor growth factors; to differentiation which
is associated
with
reversible
Alexander (1) has postulated that the reason why the cells in clinically evident disease do not become quiescent, when those in microscopic disease do, could be that the tumor itself releases a diffusible product which either prevents differentiation or produces a growth factor without which the tumor does not proliferate. As a result, the tumor cells within small lesions would either differentiate or not divide, because the tumor produced an inhibitor of differentiation or the local concentration of growth factor was too low. In essence, this concept postulates that tumor cell dormancy is the major cause of the failure of adjuvant chemotherapy rather than mutation to intrinsic resistance. A major difference between these two concepts, which are not necessarily incompatible, relates to the perceived relevance of host factors. The Gold&Coldman model ignores the possible importance of host factors and in essence assumes no role for them. The tumor dormancy concept relates the kinetic or mitotic activity of tumor cells to host factors. This is not only intuitively more attractive than a purely proliferative-resistance model, but offers another potential avenue of drug development and clinical research, which might link cytotoxic therapy with approaches deemed ‘non-cytotoxic’, which are often set up in opposition to each other.
RESISTANCE
TO
CHEMOTHERAPY
References 1. Alexander, P. (1982) 2nd Gordon Hamilton Fairley lecture. Need for new approaches to the treatment of patients in clinical remission with special reference to acute myeloid leukemia. BY. 3. Cancer 46: 15 I-159. 2. Carter, S. K., Bakoswki, M. & Hellmann, K. (1982) Chemotherapy of Cancer. New York: John Wiley and Sons. 3. DeVita, V. T. Jr (1983) The James Lewing lecture. The relationship between tumor mass and resistance to chemotherapy. Implications for surgical adjuvant treatment of cancer. Cancer 51: 1209-1220. 4. Fidler, I. J. & Kripke, M. L. (1977) Metastasis results from pre-existing variant cells within a malignant tumor. S&ace 197: 893-895. 5. Fidler, I. J. (1978) Tumor heterogeneity and the biology of cancer invasion and metastases. Cancer Res. 38: 2651-2660. 6. Goldie, J. H. & Coldman, A. J. (1979) A mathematical model for relating the drug sensitivity oftumors to their spontaneous mutation rate. Cancer Treat. Rep. 63: (1 l-12) 172. 7. Luria, S. E. & Delbruck, M. (1943) Mutations of bacteria from virus sensitivity to virus resistance. Genetics 28: 491. 8. Skipper, H. E., Schabel, F. M. & Wilcox, W. S. (1964) Experimental evaluation of potential anti-cancer of experimental leukemia. Cancer agents XII: on the criteria and kinetics associated with “curability” Chmother. Rep. 35: l-l 11.