Many pharmaceuticals, agrochemicals and chemicals cause rodent tumours by non-genotoxic mechanisms during long term carcinogenicity studies. Tumours typically occur in liver or thyroid.
Faced with a compound that has caused (or is expected to cause) rodent tumours by a non-genotoxic mechanism, it is necessary to demonstrate lack of human relevance to the satisfaction of regulatory bodies. CXR has worked with global pharmaceutical, chemical and agrochemical companies to demonstrate lack of human relevance of rodent non genotoxic carcinogens, and data generated at CXR are frequently used in regulatory submissions.
Mechanisms of Non-Genotoxic Carcinogenicity
The xenoreceptors Constitutive Androstane Receptor (CAR), Pregnane X Receptor (PXR) and Peroxisome Proliferator Activated Receptor alpha (PPARα) play key roles in rodent non-genotoxic carcinogenicity (NGC). Activation of the human PXR, CAR and PPARα receptors is not believed to result in the carcinogenic response seen in rodents. Therefore, if a compound causes liver tumours in rodents, or if preclinical studies demonstrate the compound is a nuclear receptor ligand, it is critical to:
(i) unambiguously demonstrate the role of specific nuclear receptors in the rodent response, and
(ii) demonstrate lack of response in a model human system.
The CXR approach
CXR Biosciences uses a multidisciplinary approach to evaluate the risk to humans of pharmaceuticals and chemicals that elicit a carcinogenic response in rodents, developed over many years of assisting global pharmaceutical, chemical and agrochemical companies.
1) Mode Of Action (MOA) of liver growth in rodents can be established using short term exposure studies. Animals are treated with test compound, positive control and vehicle control. The extent of compound-induced cell proliferation is measured by BrdU incorporation, and specific receptors involved are established using downstream liver enzyme analysis and/or microarray analysis.
2) Liver carcinogenicity is then initially modelled in primary hepatocytes from rodents, then subsequently assessed in hepatocytes from human donors. Hepatic Cyp450 activity assays are used to compare metabolic activities between rodents and humans, hepatocyte proliferation is compared using S-phase labelling. Non-genotoxic carcinogens typically cause proliferation in rodent hepatocytes, but not in human hepatocytes. Again, comparative gene signatures can be established using microarray analysis. Hepatocytes from suitable knockout animals (see below) can be used to further demonstrate MOA
3) Use of knockout rat or mouse models allow further clarification of the MOA. CXR uses these models to compare the MOA observed in wild-type rodents with transgenic nuclear-receptor knockout (KO) models. The interactions of chemicals with PXR, CAR and PPARα are complex because PXR and CAR regulate common genes, and chemicals can interact with two or more nuclear receptors simultaneously. Use of rats or mice that are KO for these receptors can aid in understanding the relevance of specific modes-of-action for human risk assessment. CXR has extensive experience working with the following models:
- CAR KO rats
- PXR KO rats
- PXR KO – CAR KO rats
- PXR KO mice
- Car KO mice
- PXR KO – Car KO mice
- PPARα KO mice