Tg.AC Genetically Altered Mouse: Assay Working Group Overview of Available Data

Abstract

In a Government/Industry/Academic partnership to evaluate alternative approaches to carcinogenicity testing, 21 pharmaceutical agents representing a variety of chemical and pharmacological classes and possessing known human and or rodent carcinogenic potential were selected for study in several rodent models. The studies from this partnership project, coordinated by the International Life Sciences Institute, provide additional data to better understand the models' limitations and sensitivity in identifying carcinogens. The results of these alternative model studies were reviewed by members of Assay Working Groups (AWG) composed of scientists from government and industry with expertise in toxicology, genetics, statistics, and pathology. The Tg.AC genetically manipulated mouse was one of the models selected for this project based on previous studies indicating that Tg.AC mice seem to respond to topical application of either mutagenic or nonmutagenic carcinogens with papilloma formation at the site of application. This communication describes the results and AWG interpretations of studies conducted on 14 chemicals administered by the topical and oral (gavage and/or diet) routes to Tg.AC genetically manipulated mice. Cyclosporin A, an immunosuppresant human carcinogen, ethinyl estradiol and diethylstilbestrol (human hormone carcinogens) and clofibrate, an hepatocarcinogenic peroxisome proliferator in rodents, were considered clearly positive in the topical studies. In the oral studies, ethinyl estradiol and diethylstilbestrol were negative, cyclosporin was considered equivocal, and results were not available for the clofibrate study. Of the 3 genotoxic human carcinogens (phenacetin, melphalan, and cyclophosphamide), phenacetin was negative by both the topical and oral routes. Melphalan and cyclophosphamide are, respectively, direct and indirect DNA alkylating agents and topical administration of both caused equivocal responses. With the exception of clofibrate, Tg.AC mice did not exhibit tumor responses to the rodent carcinogens that were putative human noncarcinogens, (di(2-ethylhexyl) phthalate, methapyraline HCl, phenobarbital Na, reserpine, sulfamethoxazole or WY-14643, or the nongenotoxic, noncarcinogen, sulfisoxazole) regardless of route of administration. Based on the observed responses in these studies, it was concluded by the AWG that the Tg.AC model was not overly sensitive and possesses utility as an adjunct to the battery of toxicity studies used to establish human carcinogenic risk.

The role of transgenic mouse models in carcinogen identification.

“…The rasH2 mouse is hemizygous for the human c-Ha-ras transgene under control of its endogenous promoter and enhancer sequences. It was developed by Saitoh et al (1990) in CB6F1 mice to evaluate the association of chemically induced transgene expression and tumor induction (Katsuki et al 1991;Yamamoto et al 1996Yamamoto et al , 1998a +d; +g (Blanchard et al 1998;+g (Yamamoto et al 1998b (Storer et al 2001) ±d; +g (Eastin et al 2001) ±g; +g; (Weisburger 1977) +g (Usui et al 2001;Yamamoto et al 1998b) Melphalan 148-82-3 1 Known +; +; +; +ip +; + +ip (Eastin et al 1998;+g (Eastin et al 1998;±ip (Yamamoto et al 1998b) (Weisburger 1977) Storer et al 2001) 2001) Cyclosporin A 79217-60-0 1 Known NT -; --g; +f; +f (Eastin et al 1998;±f (Eastin et al 1998; ±g (Maronpot et al 2000;Storer et al 2001) 2001) Usui et al 2001;Yamamoto et al 1998a) Diethylstilbestrol 56-53-1 1 Known NT -; NT -sc; +f (Eastin et al 1998;-g (Eastin et al 1998 (Yamamoto et al 1998b) (NCI/NTP 1978a) Abbreviations: -, negative; +, positive; ±, equivocal; d, dermal; f, food; g, gavage; I, inhalation; ip, intraperitoneal; LET, linear energy transfer; NT, not tested or no published record; sc, subcutaneous; wb, whole body. Individual results were found in the cited references or in the IARC (2002) or the NTP databases (NTP 2002).…”

Genetically Modified Animals in Pharmacology and Toxicology Research: Review

“…Dolayısıyla mevcut rodent modellerinin, insanların heterojen yapısı ile tür farklılıkları göz önüne alındığında, insanlarda oluşması beklenen tümör vakalarını tam olarak yansıtamadığı bildirilmiştir. [69][70][71][72] Oldukça karmaşık sonuçların elde edildiği bu modellerde ILSI/HESI'ya göre hepsinin yabani tip rodentlerde yapılan 2-yıllık kanser testlerine alternatif olabileceği sonucuna varılmıştır. p53 +/− knock-out farelerin bu amaçla kullanılabileceği daha önce ABD Ulusal Toksikoloji Programı [National Toxicology Program (NTP)] tarafından da desteklenmiştir.…”

Current Status of Carcinogenicity Assessment of Peroxisome Proliferator-Activated Receptor Agonists by the US FDA and a Mode-of-Action Approach to the Carcinogenic Potential

“…The ICH 1B guideline 15 defines several transgenic mouse assays including the p53+/-deficient model 24 , Tg.AC model 25 , TgHras2 model 26 , and Xpa deficient model 27 as alternative methods for additional in vivo testing of carcinogenicity. Although the FDA has recommended the p53+/-deficient mouse model as an alternative for 2-year carcinogenicity study, the p53+/-deficient mice model is not be applicable to carcinogenicity assessment of PPAR agonists, since non-genotoxic compounds are undetectable in this model and PPAR agonists generally yield negative results in the standard genotoxicity studies.…”