The Use of Rodent Tumors in Experimental Cancer Therapy Conclusions and Recommendations from an International Workshop1'2

[CANCER RESEARCH 45, 6541-6545, December 1985] Meeting Report The Use of Rodent Tumors in Experimental Cancer Therapy Conclusions and Recommendations from an International Workshop1'2 This workshop was convened to address the question of how of a specific tumor cannot be predicted with any certainty. Virus- best to use animal models of solid tumors in cancer therapy induced rodent tumors almost always express common virus- research. The discussion among the 58 participants focused on coded antigens and are usually strongly immunogenic, but hosts the following topics: appropriate solid tumor models; xenografts neonatally infected with virus may be immunologically tolerant to of human tumors; assay systems such as local tumor control, virus-coded antigens. Chemically induced rodent tumors tend to clonogenic assays following tumor excision, and tumor regrowth express individually distinct antigens. Depending partially upon delay; and applications of such models in chemotherapy, radio the carcinogen and the target organ, the degree of immunoge therapy, and combined modalities studies. nicity is variable, but a large proportion of chemically induced A major goal of the meeting was to produce a set of guiding tumors are immunogenic. Spontaneous rodent tumors are usu principles for experiments on animal tumors. This report is largely ally nonimmunogenic, although a minority may be weakly to a statement of those guiding principles, with suggestions and moderately immunogenic (1, 2). comments concerning investigations using animal models in Because no tumor can be expected to be nonimmunogenic if cancer therapy research.3 The authors believe that this discus it is placed in an allogeneic environment, tumors should as a sion of "tricks of the trade" will be useful to research workers general rule only be transplanted syngeneically. Also it should be who are unfamiliar with the various procedures, but it should be noted that passage of cells through serum-containing medium recognized that they are based on current information and that may alter immunogenicity because of adsorbed serum proteins it is axiomatic in science that the current knowledge base is which cannot easily be washed off (3, 4). Immunogenicity may continuously being modified and reinterpreted. induce a rejection response and may thus invalidate the results of in vivo experiments, particularly those involving tumor re Tumor/Host Systems sponse to cytotoxic treatments. Therefore evaluation of the immunogenicity of the tumor-host system being used is always Model Systems in Cancer Research. An animal tumor model necessary, notwithstanding the etiology of the tumor, and should should be selected according to the question being asked. There be undertaken at appropriate intervals. Tumor immunogenicity is no single good model for cancer therapy research, and no may be ascertained using challenge-protection tests, after suit single system can be expected to be applicable to all problems. able preimmunization, as described elsewhere.23 Spontaneous tumors, early syngeneic passages of spontaneous Host Animals. To maintain syngeneic conditions between tumors, xenografted human tumors, or established transplant- tumor and host, as far as it is possible to do so, the animals able tumor lines can each be appropriate for investigating specific used as hosts must be acceptably homozygous and homoge questions. Because no single tumor system is a perfect model, neous at all times. It is desirable that every inbred subline be institutions and/or laboratories should endeavor to have available established by at least 40 generations of brother-sister inbreed more than a single tumor system for investigations of questions ing (5) and then be perpetuated by full sibling matings. Genetic related to cancer therapy. purity should be tested by second set skin grafts. Sublines of Tumor Immunogenicity. Evidence of immunogenicity of hu inbred animals can be expected to drift genetically and hence man tumors is derived from clinical anecdotes or from in vitro will vary with respect to their compatibility with each other and experiments and must be regarded in most instances as of with established tumor cell lines. Publications should give details unproven relevance to tumor rejection in vivo. In most human of sublines according to accepted convention (6-8). Genetic drift tumors, there is no unequivocal evidence that host immunity in both animals and tumors may be limited by freezing both plays a part in response to therapy. This supports the choice of embryos and tumor cells, respectively. nonimmunogenic models for experimental therapy studies (1). An exception to the general rule that tumor-host systems In laboratory animals, although the immunogenicity of tumor should be syngeneic is the use of human tumors transplanted types tends to be correlated with etiology, the immunogenicity xenogeneically into laboratory animals. Xenografts are attractive as model systems because of their clinical origins. There is a Received 4/4/85; revised 8/15/85; accepted 9/10/85. modest amount of data suggesting that xenografts to some 1This international workshop on "The Use of Rodent Tumors in Experimental extent maintain the therapeutic sensitivity of the source human Cancer Therapy" was held on October 21-24, 1984, in Schloss Reisensburg, tumor (9), despite the fact that changes in tumor cell kinetics Federal Republic of Germany. It was supported by USPHS Grant CA 38523, and in parenchyma-stroma relationships occur during xenograft- awarded by the National Cancer Institute, Department of Health and Human Services, and by grants from the Deutsche Forschungsgemeinschaft, Bad Godes- ing. Because xenografts can be grown only in immunodeficient berg, Federal Republic of Germany, and the Sir Samuel Scott of Yews Trust, animals, such systems are prone to artifacts arising from residual London, United Kingdom. 2The complete proceedings of the workshop will be published separately in immunity or from immunological responsiveness which returns book form by Pergamon Press in 1986. to the host animals. Very careful and controlled experimentation 3 Several of the basic procedures and related information have been summarized is required if these problems are to be avoided. in a set of appendices to this report. They will be distributed by R. F. Kallman upon request. Xenografts may be used profitably to study inherent properties CANCER RESEARCH VOL. 45 DECEMBER 1985 6541 Downloaded from cancerres.aacrjournals.org on September 26, 2021. © 1985 American Association for Cancer Research. MEETING REPORT of human tumor cells, such as drug metabolism, cellular sensitiv for investigating treatments that cannot kill sufficient numbers of ity to drugs and radiation, and intracellular repair. It must be cells to achieve cure. This is particularly a problem in investiga tions of cytotoxic drugs. Radiation "top up" (14) is one useful remembered that the vasculature and connective tissue of a human tumor xenograft in a mouse are murine in origin; therefore means of obtaining tumor control curves when the tumor is not some properties of xenografts that depend strongly on the controlled by drug alone, but interaction between the drug and stroma may deviate considerably from those of the source tumor radiation can generate serious problems in the analysis of such in the patient. An example of this could be the degree of hypoxia. experiments. Thus if this method is to be used, interaction of Tumors. To maintain the stability of an established tumor line, this kind should be carefully evaluated. samples should be preserved in the frozen state, e.g., in liquid Because of the long time (normally 120-180 days) that is nitrogen. To avoid drift in tumor characteristics during serial required to assess whether or not a tumor has been controlled, I.e., "cured," this assay method is very suitable for assessing transplantation, tumors should be renewed from the frozen stock at appropriate intervals; i.e., tumors should not be used for more effects of extended or fractionated treatment schedules. than about 20 serial passages in syngeneic hosts. Procedures Tumor Growth Delay Assay. The main advantage of the for freezing and thawing tumors and tumor cells are given in Ref. tumor growth delay assay (15) is that it is suitable for use with 10, and a suitable method whereby live in vivo and in vitro almost any experimental tumor provided that its size can be passage are alternated with frozen storage has been described measured. It can be used to test the effectiveness of a wide (11). range of doses, although it is usually not sensitive enough to Al tumor systems should be characterized before being used detect small effects, e.g., when less than about 50% of cells are for extensive studies, both to assist interpretation of data and to killed. Because the effect on growth occurs over such a wide facilitate identification of drift. Characterization of any tumor dose range, there is usually little need for doing extensive pilot model should include: (a) growth curve in vivo, and possibly in studies; this assay facilitates the examination of dose-dependent vitro, if applicable; (b) sensitivity to at least one reference agent, dose modification, e.g., as seen in relative biological effective e.g., radiation or drug; (c) plating efficiency, in vivo cloning ness experiments. Growth delay is more economical than local efficiency, or TDso,4 whichever is applicable; (d) degree of im- control in that assays usually can be done with fewer animals munogenicity; (e) morphology. In addition, cell kinetic parameters and in much less time. Also it can often be used with tumors and karyotype may be useful. It is desirable to check some or all that metastasize and that cannot be assayed by the local control of these parameters whenever a tumor is reestablished from method. frozen stock, because of the possibility that only a subpopulation Growth delay results from more than simply the killing of of cells may have survived preferentially the freezing and thawing clonogenic tumor cells. For this reason, it is usually not possible procedure. to derive accurate surviving fractions from growth delay dose- response curves.

Load more