Conducción del experimento

The NTP studies have not been published in the peer- reviewed literature, although the report was peer-reviewed within the NTP. The purpose of this review was to assess whether there is general agreement in the literature that there is a “cur-

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rent lack of appreciation of the scientific or public health value of negative cancer findings in these models” as stated by Dr. David Schwartz, Director of NIEHS, on September 27, 2005. This has been stated as the rationale for not making public the findings of these studies.

ILSI-ACT

The p53 and Tg.AC transgenic mice models used in the NTP study were also evaluated during a workshop organized by the International Life Sciences Institute Alternative Carcinogenicity Testing (ILSI-ACT) committee. Cohen (2001) summarized the results of assays conducted with 21 chemicals using the standard mouse and rat long-term assay and compared with the results of testing the same chemicals in nine transgenic models and short- term assays. The mechanistic basis for these models, and data ob- tained from studies with a number of chemicals, were reviewed and summarized by workshop participants. These reports were then provided to a group of international cancer experts from government, industry and academic organizations. Following their evaluation, a panel discussion was held with these individ- uals and workshop participants to address the appropriateness of the alternative models to human cancer risk assessment (Pettit, 2001; Popp, 2001). The general consensus in this group was that models had significant value as part of a weight-of-evidence ap- proach to assessing human carcinogenic risk. All of the models

FIG. 2. The role of p19 in the G2-M phase transition of the cell cycle (http://www.biocarta.com/pathfiles/m cellcyclePathway. asp).

were expected to detect genotoxins, with differing responses de- pending on mechanism of action of the carcinogen in question. For example, the carcinogen may act primarily to activate tumor oncogenes or to cause mutation or deletion of tumor suppressor genes. Although all transgenic models have limitations and are not designed to be stand-alone assays, they are considered to be valuable for consideration in an overall risk assessment (Cohen, 2001; Goodman, 2001).

FDA

Jacobson-Kram and colleagues (2004) recently reviewed the use of transgenic mice in carcinogenicity hazard assessment by FDA. Use of transgenic models in carcinogenicity testing proto- cols was formalized in the FDA’s 1997 adoption of the Guidance for Industry S1B Testing for Carcinogenicity of Pharmaceuti- cals (http://www.fda.gov/cder/guidance/1854fnl.pdf), “opening the door” for use of transgenic models in regulatory toxicology assessment. The FDA paradigm is that combining a traditional rat bioassay with a 6-month transgenic study for an overall as- sessment of the weight-of-evidence results in fewer false posi- tives and no increase in false negatives. Analyses to assess the

sensitivity (no false negatives) and specificity (no false positives) to give an overall accuracy score for specific transgenic models and traditional bioassays, as compared to a combined approach, provide support for this paradigm (Table 34) (Pritchard et al., 2003; Jacobson-Kram et al., 2004).

European Regulatory Authorities

In a short opinion paper, van der Lann et al. (2002) stated that similar to long-term rodent bioassays, transgenic models have less-than-perfect accuracy as there are some reports of false negatives and false positives when compared to other supporting data for a specific chemical. With that acknowledgment, the conclusion of a review of the current weight of evidence was that “regulatory authorities cannot neglect the outcome of such studies, but need to be cautious in their interpretation of the data from such models, and the application in risk assessment problems.”

More recently, the ILSI Alternatives to Carcinogenicity Test- ing Committee held a workshop to reassess data from geneti- cally modified mouse model studies that have been evaluated by international regulatory agencies. The perspectives of the U.S.

TABLE 34

Summary of performance of transgenic assays, NTP rodent assays, and combinations (adapted from Pritchard et al., 2003)

+for −for +for −for Overall

Strategy carcinogen noncarcinogen noncarcinogen carcinogen accuracy

p53+/− 21 27 1 10 48/59 (81%)

p53+/−(genotoxic) 16 6 0 4 22/26 (85%)

Tg.AC 17 29 10 6 44/62 (74%)

RasH2 21 18 5 7 39/51 (76%)

p53+/−(genotoxic) and Ras H2 30 14 5 4 44/53 (83%)

p53+/−(genotoxic) and Tg.AC 25 22 10 4 47/61 (77%)

NTP rodent bioassay 23 17 18 0 40/58 (69%)

NTP rodent bioassay and p53+/−(genotoxic) and

Tg.AC (nongenotoxic)

35 13 9 0 48/57 (84%)

NTP rat bioassay and p53+/−(genotoxic) and

RasH2 (nongenotoxic)

33 12 8 0 45/53 (85%)

Note.Definitions: positive for carcinogens, positive assay results for NTP rodent carcinogens; negative for noncarcinogens, negative assay

results for NTP rodent noncarcinogens; positive for noncarcinogens, positive assay results for NTP rodent noncarcinogens; negative for carcinogens, negative assay results for NTP rodent carcinogens.

FDA, the European Committee for Proprietary Medicinal Prod- ucts Safety Working Party, and the Japanese Ministry of Health Labor and Welfare discussed at that workshop were published (MacDonald et al., 2004). Overall agreement was expressed that the alternative assays can and do have an important role in regu- latory carcinogen safety assessment. However, specific concerns were raised.

For the p53+/−model, a study duration of 9 months is rec-

ommended, whereas a 6-month duration is considered adequate for the Tg.AC and Tg.rasH2 models. The number of animals per group should be 25, rather than the originally proposed size of 15 per group, to provide adequate statistical power. This model is considered appropriate for carcinogens that have been shown to be positive in genotoxicity tests, but may not be adequately sensitive to detect nongenotoxic carcinogens.

It is considered advantageous to include positive controls for

all models. For the p53+/−model, the positive control that has

been used is p-cresidine; however, an alternative one is being

sought due to lack of consistent results with p-cresidine. For

the Tg.AC models, TPA administered three times per week is recommended. The Tg.AC model is most appropriate for testing compounds where dermal exposure is relevant. The usefulness of this model for testing compounds where the human exposure will be oral is not well established, regardless of whether the exposure to the mice is dermal or oral.

The genetic background of the parental strain will influence a model’s tumor spectrum and may result in “blind spots” or tumor-resistant organs. This needs to be further investigated in all models. Wild-type parental strains should be included in de- terminations of maximum tolerated dose levels for the genetic models.

Lastly, the generally recognized incidence of tumors at rare sites that should be considered treatment-related was 2 out of 15 animals when the group size is 15. More data will be needed to establish the significant incidence level with larger groups (MacDonald et al., 2004).