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Murine Models for Evaluating Antiretroviral Therapy1 [CANCER RESEARCH (SUPPL.) 50. 56l8s-5627s, September 1, I990| Murine Models for Evaluating Antiretroviral Therapy1 Ruth M. Ruprecht,2 Lisa D. Bernard, Ting-Chao Chou, Miguel A. Gama Sosa, Fatemeh Fazely, John Koch, Prem L. Sharma, and Steve Mullaney Division of Cancer Pharmacology. Dana-Farber Cancer Institute ¡R.M. R., L. D. B., M. A. G. S.. F. F.. J. K.. P. L. S., S. M.J; Departments of Medicine [R. M. K.J. Pathology ¡M.A. G. S.J, and Biological Chemistry and Molecular Pharmacology [F. F., J. K., P. L. S.J. Harvard Medical School, Boston, Massachusetts 02115, and Memorial Sloan-Kettering Cancer Center ¡T-C.C.], New York, New York 10021 Abstract Prior to using a given animal model system to test a candidate anti-AIDS agent, the following questions need to be addressed: The pandemic of the acquired immunodeficiency syndrome (AIDS), (a) Is the potential anti-AIDS drug an antiviral agent or an caused by the human immunodeficiency virus type 1 (IIIV-I ). requires immunomodulator that may be able to restore certain immune rapid development of effective therapy and prevention. Analysis of can didate anti-HIV-1 drugs in animals is problematic since no ideal animal functions? Thus, is a candidate anti-AIDS drug likely to be model for IIIV-I infection and disease exists. For many reasons, including active during the phase of asymptomatic viremia or only during small size, availability of inbred strains, immunological reagents, and overt disease? (¿>)Does a candidate antiviral agent inhibit a lymphokines, murine systems have been used for in vivo analysis of retroviral function shared by all retroviruses such as gag, pol, antiretroviral agents. Here we review currently available murine models protease, integrase, RNase H, or env, or is the novel compound involving IIIV-I in transgenic mice and in chimeric mice reconstituted targeted against a regulatory gene product typical for lentivi- with human cells, as well as murine systems using retroviruses of the ruses (e.g., tat, rev, nef, or others)? (c) Does the metabolism of subfamily Oncovirinae rather than Lentivirinae. We report our results a test drug in animal cells resemble that in human cells? (d) Is on various antiretroviral treatment strategies, including chemoprophy- the quantity of test drug available limited? Prior to in vivo laxis after acute retroviral exposure, therapy of chronic viremia, quanti tative analysis of combination therapy, and therapy during pregnancy testing, the biocontainment requirements for the planned ani and in the neonatal period aimed at preventing viremia in the offspring. mal experiments as well as projected costs need to be consid Due to our highly effective postexposure treatment protocols with 3'- ered. azido-3'-deoxythymidine(zidovudine) combined with recombinant human A well-defined experimental end point must be used for drug interferon-aA/D, retrovirus-inoculated mice developed immunity to the studies in animal model systems. Examples of such measurable virus to which they were exposed, which will allow us to determine the parameters may include prevention of primary infection via nature of protective antiretroviral immunity in inbred mice. postexposure prophylaxis, a decrease of virus load in chroni cally viremic animals, a delay in the development of overt Introduction disease, or prolonged overall survival. Therapy aimed at preventing infection after acute virus ex Major efforts have been undertaken by academic institutions posure addresses a different clinical problem than therapy of as well as by the pharmaceutical industry to find effective chronic viremia or virus-induced disease. In the first case, cure therapy against HIV-1,3 the accepted cause of AIDS (1-3). may be possible if the therapeutic intervention is started While at the present time only AZT (zidovudine) has been promptly after virus exposure and if integration of the HIV-1 approved by the Food and Drug Administration for treating provirus in host chromosomal DNA, an irreversible event, can AIDS and related conditions, several other drugs are being be blocked (Fig. 1). Such a therapeutic approach must involve evaluated in clinical trials, and many more agents have proven drugs capable of inhibiting early steps in the viral replication anti-HIV-1 activity in vitro. A practical small animal model cycle. However, other agents may also play a role in postexpo system for HIV-1 viremia and disease, in which potential anti sure chemoprophylaxis. For instance, biological response mod viral compounds could be tested for toxicity and efficacy, would ifiers capable of boosting host immune defense mechanisms facilitate the selection of drugs for clinical trials. The lack of may lead to selective killing of infected cells expressing virus- such models has led researchers involved with preclinical drug specific antigens on their cell surface and, thus, may act syner- development to use various surrogate model systems. gistically with antiviral drugs. In most known retroviral systems, a phase of asymptomatic Therapy of chronically viremic individuals can only be sup- viremia lasting from a few weeks to several years precedes the pressive, however, unless a treatment modality is discovered development of overt disease. Thus, animal/retrovirus systems that can destroy all cells harboring latent virus. As long as such may represent models for viremia, models for disease, or models curative therapy is lacking, antiviral treatment in chronic vire for both viremia and disease (4). Therapeutic modalities that mia needs to be administered indefinitely. Long-term tolerance may be evaluated in the different model systems are outlined in and drug safety thus become important topics. Combination Table 1. therapy may eventually play a major role in the therapy of 1Presented at the "XlVth Symposium of the International Association for chronic viremia not only by improving the therapeutic window, Comparative Research on Leukemia and Related Diseases," October 8-12. 1989, but also by preventing selection of drug-resistant virus strains. Vail, CO. Supported by NIH Contract NO1-AI-72664, NIH Grants UO1-AI-24845 and Using inbred laboratory mice for drug development would be RO1AI29797-01, and a Faculty Research Award from the American Cancer advantageous because of well-studied genetics, immunology, Society to R. M. R., and NIH Grant U01-AI-26056 to T-C. C. hematopoiesis, and metabolism, and the availability of many 2To whom requests for reprints should be addressed, at Dana-Farber Cancer Institute, 44 Binney Street, Boston. MA 02115. immunological reagents. Furthermore, breeding can be accom 3The abbreviations used are: HIV-1, human immunodeficiency virus type 1; plished easily because of the short generation time, and the RLV, Rauscher murine leukemia virus; MuLV, murine leukemia virus; AIDS, costs for housing laboratory mice are considerably lower than acquired immunodeficiency syndrome: SCID mice, mice homozygous for the seid locus and suffering from severe combined immunodeficiency: AZT, 3'-azido-3'- for other experimental animals. Because of the small size of deoxythymidine; LTR, long terminal repeat; CAT, chloramphenicol acetyl trans- mice, drug studies can be carried out with relatively small ferase; PBL, peripheral blood leukocytes; NK. natural killer; LAK, lymphokine- activated killer; CNS, central nervous system; MoMuLV. Moloney murine leu amounts of investigational compounds that may be limited in kemia virus; HTLV-I, human T-lymphotropic virus type I. supply, especially in natural product analysis. 5618s Downloaded from cancerres.aacrjournals.org on September 26, 2021. © 1990 American Association for Cancer Research. Ml'RINE MODELS FOR ANTIRETROVIRAL THERAPY Unfortunately, cultured murine cells and mice cannot be can overcome the early block of HIV-1 infection of murine infected by HIV-1. Since it is known that infection by this virus cells. critically depends on human CD4 cell surface receptors, murine cells have been transfected with DNA encoding the human CD4 Murine Systems Involving HIV-1 receptor sequences. Even though the transformed murine cells synthesized human CD4 receptor molecules faithfully, they were nevertheless refractory to HIV-1 infection (5). Apparently, Transgenic Mice one or several post-receptor binding steps independent of CD4 Transgenic mouse systems involving HIV-1 are summarized interaction are required for successful viral entry. However, in Table 2. Transgenic mice have been created carrying either cultured murine cells are able to produce infectious HIV-1 intact (7) or partial (8-10) HIV-1 proviral genomes. progeny following transfection of a molecular HIV-1 clone (6), Activity of the HIV-1 LTR in Transgenic Mice. Leonard et demonstrating that direct proviral DNA insertion techniques al. (8) generated four lines of transgenic mice containing the reporter gene CAT under the transcriptional control of the Table 1 Animal/retrovirus models for testing therapy HIV-1 LTR. In all lines, a distinct pattern of tissue-specific Therapeutic modality CAT expression was observed; CAT activity was detected in Model system for to be tested the eye, heart, spleen, thymus, and tail. In lymphocytes as well Viremia Acute virus exposure Prevention with postexposure chemo- as in circulating monocytes, CAT expression could be stimu prophylaxis lated by cocultivation with mitogens or various cytokines, re Maternal transmission Transplacental prophylactic therapy spectively. Among cells derived from the monocyte-macrophage of the fetus, suppression of mater nal virus production lineage,
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