Modelling the process of drug resistance

Lung Cancer 10 Suppl. l(1994) S91-S96

Modelling the process of drug resistance James H. Goldie Dirision

of Medical

Oncology, British Columbia British Columbia

CancerAgency,

600 W 10th ALP, Vancouter.

V5Z 4E6, Canada

Abstract Modelling the process of drug resistance can provide insight into such issues as the difference between intrinsic and acquired drug resistance. Other important biological questions such as whether resistance arises through selective or inducible events can also be addressed by reference to appropriate models. If drug resistant cells are produced by some type of inducible up regulation step then this has a number of qualifications for cancer chemotherapy. Selection theories of drug resistance imply that certain sequences of drugs will be superior to others something which can be tested by experiment. Key words: Acquired resistance; Intrinsic Selected Resistance; Drug sequence

resistance;

P-glycoprotein;

Induced

resistance;

1. Introduction

The past few years have seen a great increase in the amount of information available regarding the mechanisms of drug resistance in cancer cells [9]. It is

widely accepted that some type of phenotypic change frequently occurs in individual cancer cells altering their sensitivity to anti-cancer drugs. Strategies employing biochemical modulation of the phenotypic resistant state are now being actively investigated in many centers with the expectation that this will produce improvements in therapeutic results utilizing currently available antitumour agents. The processes that lead to cancer cells expressing phenotypic differences continue to elicit discussion and debate, and a brief description of some of the principal processes that may be involved will be the subject of this present review. 0169-5002/94/$07.00 0 1994 Elsevier Science Ireland Ltd. All rights reserved. 0169~5002(93)00269-F

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2. Intrinsic versus acquired resistance The term acquired resistance is usually applied to the situation in which an unambiguous change in drug sensitivity occurs during the course of drug treatment. This may be seen while the actual treatment is ongoing or may manifest itself some period of time after treatment has been discontinued and the recurring tumour is rechallenged with the original chemotherapy. If significant pharmacokinetic changes can be excluded then it seems reasonable to assume that some type of phenotypic change has occurred in the tumour cell population. On the basis of previous studies we have argued that the most likely cause of this phenotypic alteration are random genetic changes (i.e. mutations) which over time will produce an ever greater heterogeneity of cell types within the tumour [4]. The possibility that other processes contribute to this phenotypic heterogeneity must still be considered and will be dealt with in a later part of this manuscript. The term intrinsicresistance is used to describe situations in which the malignancy appears completely refractory to drug treatment right from the beginning without there being any obvious process of drug induced selection. Many solid tumours appear to fall in to this category and it is important to know whether this phenomenon is fundamentally different from the process of acquired drug resistance. When defining intrinsic resistance it is apparent that this has to be described in relationship to a specific drug or drugs. Many types of cancer show a characteristic variability and sensitivity to a broad range of antineoplastic agents, being highly sensitive to some, moderately sensitive to a few and being generally poorly responsive to numbers of other drugs. Malignancies of the haematologic and lymphoid systems appear to show the broadest spectrum of sensitivity, followed by a number of paediatric tumours, germ cell tumours and some examples of adult tumours of neuroendocrine origin. The common epithelial derived tumours of the digestive tract as well as tumours derived from kidney, certain connective tissue tumours and melanocyte derived tumours show both lower degrees of sensitivity to antitumour agents as well as a much narrower spectrum of sensitivity. It becomes an important question to ascertain whether this global difference in sensitivity is related to a fundamental qualitative biological difference or whether it is simply the same process as seen in the sensitive tumours but with quantitative differences in capacity to express drug resistance phenotypy. It is probably not accidental that many of the tissues that produce highly drug refractory tumours function normally to provide protective barriers against toxic substances derived from the external environment. If a normal tissue constitutively expresses a number of general mechanisms associated with detoxification processes then it is not surprising that malignancies derived from these cells will start off with a considerable built in degree of drug resistance. Perhaps the most useful illustration of this process would be the expression of P-glycoprotein in normal cells and tumours [7,11]. The multi-drug resistant phenotype associated with P-glycoprotein is one of the broad categories of drug resistance (or detoxification mechanisms) that have been identified. Gastrointestinal and renal cells express significant amounts of P-glycoprotein normally and one

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would expect that the tumours derived from these cells would also have a considerable capacity for P-glycoprotein expression. Gastrointestinal epithelium appears less likely to display intrinsic resistance to platinum type agents and to antimetabolites such as Sfluorouracil, hence these drugs do have some utility in the management of these tumours. Although the quantitative degree of response that one can achieve in the management of gastrointestinal cancer is much less than that of malignancies of the lymphoid system one can see the same sequence of events of response followed by drug resistant recurrence. It is therefore certainly possible that the state of intrinsic resistance is partly based on the constitutive expression in the tumour cells of properties that were present in the original normal cell from which the tumour was derived. The question of intrinsic resistance may be in part a semantic one and does also suggest that tumours derived from epithelial and integumentary structures are likely to be well equipped with general drug resistance mechanisms. 3. Models of acquired resistance 3.1. Induction LWSUSselection The basic model of acquired drug resistance that we have described previously is based on the classic Luria-Delbruck model of resistance as seen in bacterial populations [gl. This basic drug resistance model fits the behaviour of a number of transplanted rodent tumours quite well and to a first approximation appears to fit the behaviour of certain very sensitive classes of human malignancy also (e.g. germ cell tumours, non-Hodgkin’s lymphoma etc) [5]. The basic model also provides a rationale for the observation that likelihood of curability is inversely related to tumour burden and that combinations of drugs will have significantly greater potential for generating cures than will single agent treatment. The somatic mutation model can also be used to develop arguments suggesting that certain sequences of antitumour agents are going to be superior to others. There are some problems with the simple model when one attempts to extend it to the broad range of human malignancy. For one thing, the bacterial and transplanted mouse leukemia models predict a very high sensitive to resistance cell ratio in virtually all circumstances. Whereas cures may still be difficult to achieve one would expect to see a high proportion of complete or nearly complete remissions of disease. This frequently is not the case, particularly with many solid tumours. As well, the rapidity with which clinical resistance develops in many clinical situations seems to suggest that some kind of direct induction process is occurring [l]. Moreover, an initial sequence of unsuccessful drug treatment resulting in a drug resistant tumour appears to generate a neoplastic cell population that expresses a very broad range of drug resistance. There are, however, some problems if we try to explain acquired drug resistance purely on the basis of some type of induction process. The simplest type of induction model would argue that the cell population is essentially homogenous in terms of its drug sensitivity prior to the time at which drug exposure occurs.

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Following drug treatment a certain proportion of the tumour cells are killed and a certain small proportion of surviving cells are converted in some way to a resistant state which is heritable. At least some of the inferences that can be drawn from a simple induction model appear contradicted by observation. A pure induction model would suggest that tumours exist in a pristine state with respect to drug resistance prior to exposure to a cytotoxic agent. In fact, in many instances molecular markers for drug resistance can be identified in tumour cells prior to treatment with chemotherapy [9,12]. It also implies that tumours of the same size, regardless of previous history, should respond identically to exposure to the same antineoplastic agent. An induction model also implies little variation in response from one individual to another, whereas the variance in response actually seen if anything appears even greater than that predicted by the Luria-Delbruck model. The apparent rapidity with which drug resistance can occur in many clinical situations can at least superficially resemble some type of process of up regulation where the tumour cells can quickly change the phenotypic expression of some of the drug resistance mechanisms. P-glycoprotein is probably present to at least a small degree in virtually all normal cells of the body but there does not appear to be any significant up regulation of P-glycoprotein expression in the face of repeated exposure to chemotherapeutic agents. It is possible that tumour cells may be able to up or down regulate their P-glycoprotein levels as a consequence of genetic changes associated with the conversion to the neoplastic state. Typically cellular processes that are associated with up regulation by some external agent (androgens, opiates, etc) show rapid changes in phenotypic expression which generally affects the entire target cell population. Moreover, in normal cell systems phenotype expression is down regulated when the specific ligand is removed. We would need therefore to postulate that the tumour systems behave almost in the opposite way to that of normal cell systems that are undergoing regulation by some external substance. That is, that the phenotypic change induced is permanent and that the capacity to undergo this regulation varies from individual to individual in an unpredictable fashion. It should also be remembered that even if the capacity to display up or down regulation in response to a cytotoxic agent is present, the fundamental capacity to do so may be a property that is produced through some type of mutational event. This would simply push back the genetic variability in the process by one step but it would mean that it was fundamentally no different from that of other more typical mutations which yield either an enhanced, diminished or altered gene product. To some extent the question may be moot because in both circumstances we are dealing with over expression of a drug resistance marker and this would require the same strategy of biochemical modulation in order to circumvent the effect of the induced mechanism. 4. Drug sequence and non-cross resistance We have previously indicated that depending on the properties of the drugs and the tumour system that is being treated, certain sequences of drugs will be superior

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to others in yielding probabilities of cure [6]. The sequence that has attracted the most interest has been that of utilizing alternations between putative non-cross resistant agents at every treatment cycle. This has been evaluated in a number of types of tumours with some clinical trials yielding moderately positive effects, but not on a highly consistent basis [3,10]. One of the requirements for a symmetrical strategy such as alternation at every cycle to be effective is that the basic drug and tumour kinetic parameters of interest must likewise be nearly quantitatively symmetrical. If the mutation rates to resistance to one treatment arm are significantly different than that of resistance to the other, or if some of the resistant populations of cells have differing doubling times, then one to one alternation at every cycle will cease to be the optimal treatment sequence. Depending upon the value of these parameters there will be a whole set of asymmetric strategies that will be superior to one to one alternation

El. This makes the problem of exploiting alternating chemotherapy significantly more difficult than was perhaps first recognized. Moreover, establishing degrees of non-cross resistance among agents or whole combination protocols may be substantially more complex than was at one time appreciated. Already we know of a number of broad classes of multi-drug resistance phenomena that can yield cross resistance to a wide variety of antineoplastic agents. It seems certain that more of these general drug resistant mechanisms will be discovered over time. This has the effect of diminishing the true variety of anticancer drugs at our disposal. This could be a significant factor in why, despite the availability of 30 or 40 active anticancer agents, tumour cells can readily express resistance to the whole range of them. It also suggests one reason why adding additional agents to multiple drug protocols may have much less additive effect than what might be predicted from the individual agent’s activity. 5. Conclusions

Drug resistance appears to be the major problem to be overcome in the chemotherapy of many types of neoplasm. Although the processes are undoubtedly mechanistically very complex, we still feel that the best basic model of the phenomenon is clonal heterogeneity arising through random genetic changes over time. The identification of general mechanisms of drug resistance in some ways simplifies the problem of circumventing the drug resistant state by biochemical modulation. Inactivation of a few of these general mechanisms should restore sensitivity to a great number of diverse agents, substantially increasing the therapeutic potential of the existing drug armamentarium. 6. References 1 Burt RK, Thorgeirsson SS. Coinduction of MDR-1 multi-drug resistance and cytochrome T-450 genes in rat liver by xenobiotics. J Nat1 Cancer Inst 1988; 80: 1383-6. 2 Day R. Treatment sequencing, asymmetry and uncertainty: protocol strategies for combination chemotherapy. Cancer Res 1986; 46: 3876-85.

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J.H. Goldie/Lung Cancer 10 Suppl. 1 (I 994) S91-S% Fukuoka M, Furuse K, Saijo N. et al. Randomized trial of cyclophosphamide, doxorubicin and vincristine versus cisplatin and etoposide versus alternation of these regimens in small cell lung cancer. J Nat1 Cancer Inst 1991; 83: 855-61. Goldie JH, Coldman AJ. The genetic origin of drug resistance and neoplasms: implications for systemic therapy. Cancer Res 1984; 44: 3843-53. Goldie JH, Coldman AJ. The somatic mutation theory of drug resistance: The Goldie-Coldman hypothesis revisited. Print Pratt Oncol PPO Updates 1989; 3(5): 1-12. Goldie JH, Coldman AJ, Gudauskas GA. Rationale for the use of alternating non-cross resistant chemotherapy. Cancer Treat Rep 1982; 65: 439-49. Heber M, Reed C, Kavallaris M, et al. Resistance to drugs associated with the multi-drug resistant phenotype following selection with high concentration methotrexate. J Nat1 Cancer Inst 1989; 81: 1250-4. Luria SE, Delbruck M. Mutation of bacteria from virus sensitivity to virus resistance. Genetics 1943; 28: 491-511. Moscow JA, Cowan RI-I. Multi-drug resistance. J Nat1 Cancer Inst 1988; 80: 14-20. Roth VG, Johnson DH, Einhom LA. Randomized study of cyclophosphamide, doxorubicin and vincristine versus etoposide and cisplatin versus etoposide and cisplatin versus alternation of these two regimens in extensive small cell lung cancer: a phase III trial of Southeastern Cancer Study Group. J Clin Oncol 1992; 10: 282-91. Rothenberg M, Ling V. Multi-drug resistance: molecular biology and clinical relevance. J Nat1 Cancer Inst 1989; 81: 907-10. Thorner PS, Haddad G, Ling V. Immunohistochemical detection of P-glycoprotein: prognostic corolation in soil tissue sarcoma of childhood. J Clin Oncol 1990; 8: 689-704.