Proposal to Eliminate Urethane-Treated Positive Control Dose Groups in 26-Week Tg.rasH2 Carcinogenicity Studies

Relevance of Carcinogen-Induced Preclinical Cancer Models

Chemical agents can cause cancer in animals by damaging their DNA, mutating their genes, and modifying their epigenetic signatures. Carcinogen-induced preclinical cancer models are useful for understanding carcinogen-induced human cancers, as they can reproduce the diversity and complexity of tumor types, as well as the interactions with the host environment. However, these models also have some drawbacks that limit their applicability and validity. For instance, some chemicals may be more effective or toxic in animals than in humans, and the tumors may differ in their genetics and phenotypes. Some chemicals may also affect normal cells and tissues, such as by causing oxidative stress, inflammation, and cell death, which may alter the tumor behavior and response to therapy. Furthermore, some chemicals may have variable effects depending on the exposure conditions, such as dose, route, and duration, as well as the animal characteristics, such as genetics and hormones. Therefore, these models should be carefully chosen, validated, and standardized, and the results should be cautiously interpreted and compared with other models. This review covers the main features of chemically induced cancer models, such as genetic and epigenetic changes, tumor environment, angiogenesis, invasion and metastasis, and immune response. We also address the pros and cons of these models and the current and future challenges for their improvement. This review offers a comprehensive overview of the state of the art of carcinogen-induced cancer models and provides new perspectives for cancer research.

Animal use and opportunities for reduction in carcinogenicity studies supporting approved new drug applications in the U.S., 2015-2019

A minimum of 65,341 rats and mice were used in 109 carcinogenicity studies conducted for new drug applications approved by the U.S. Food and Drug Administration from 2015 through 2019. By analyzing how these animals were used, we compared the potential for reducing animal use of implementing existing international guidelines and recommendations. The greatest reduction, 18.7%, would result from evaluating exposure by microsampling blood in main studies to replace toxicokinetics satellites, which used three-fold more mice than rats. A similar reduction, 17.3%, would result from replacing 33 long-term studies in mice with short-term studies in transgenic mice. Based on histopathology findings in chronic studies, 15 long-term studies in rats could have been waived, using 8410 fewer rats. Simply using single, rather than dual, negative control groups would result in a 7.8% reduction, and eliminating positive control groups would use 640 fewer transgenic mice. Combined, an estimated 46% reduction would be achieved, using approximately 29,876 fewer animals. The publication of an addendum to the main carcinogenicity testing guideline promises to decrease the number of long-term studies conducted in rats and mice and presents opportunity to promote full harmonization and implementation of related recommendations that would further dramatically reduce animal use.