, 2010). In another study after click here 22 weeks of inhalation exposure to the MS of a non-filter reference cigarette at 250 mg TPM/m3, 44 and 33% neutrophils were found for male and female A/J mice, respectively (March et al., 2006). Applying nonlinear regression to the results of the three studies conducted in three different
laboratories, a reasonable inter-laboratory reproducibility for this major inflammatory endpoint with an R2 of 0.90 was obtained. There are some limitations inherent to this A/J mouse model for MS-induced pulmonary tumorigenesis. The most striking difference between this murine model and the human situation is the lack of any reduction of the lung cancer risk in the model upon cessation of MS inhalation, while the relative risk for developing lung cancer in former smokers decreases with the duration since smoking cessation (US Department of Health and Human Services, 1990). Another limitation is the apparent balance of pro-tumorigenic activities with the delaying or inhibiting activities of concomitant MS exposure,
which requires the inclusion of post-inhalation periods at least after shorter-term chronic MS inhalation periods (Stinn et al., 2012). While this is not so much a limitation in long-term comparative inhalation studies, e.g., for a comparison of various types of cigarettes, a situation of smoking cessation or switching to a potential modified risk tobacco product after having smoked conventional cigarettes cannot be modeled by this animal model, and misleading results would be obtained upon such application. Furthermore, there find more is no real relationship between the MS inhalation duration (or accumulated dose) and an increase in lung tumor multiplicity over the sham-exposed control using this A/J mouse model, while smoking duration has been identified as an important parameter Branched chain aminotransferase determining the risk for developing lung cancer
in humans (Flanders et al., 2003 and Hazelton et al., 2005), although with the least impact on adenocarcinoma among the major smoking-associated histologic types of cancer (Kenfield et al., 2008). Lung cancer in humans shows a high malignancy and is often associated with metastasis leading to a fatal outcome. In the current A/J model, MS-induced lung cancers are not the cause of death during the study, which may be due to the lower malignancy compared to the human situation and due to the fact that no metastasis is induced by MS inhalation. In conclusion, data have been accumulated suggesting that the A/J mouse model for long-term MS inhalation-induced pulmonary tumorigenesis is reliable and relevant, two basic requirements towards validation of such models. Reliability was shown for intra- and inter-laboratory reproducibility, the robustness using historic data on spontaneous and ETSS-induced tumorigenesis, and the power to discriminate MS from different cigarette types within limits.