Quantitative Modeling Uncovers a Potential Limitation in the Putative Mechanism of CCl4 Hepatotoxicity
Drug-induced liver injury (DILI) is one of the leading causes of drug development failures and drug withdrawals. DILIsym® is being developed to identify and mitigate DILI risk through in silico analysis of compounds. The DILIsym® representation of the innate immune response was initially based on acetaminophen (APAP) data. Carbon tetrachloride (CCl4), a compound with hepatotoxic similarities to APAP, was simulated for further evaluation of the innate immune response to liver injury. DILIsym® simulation results for CCl4 pharmacokinetics were consistent with published PK data. CCl4 is generally thought to induce hepatotoxicity via a free radical metabolite which drives lipid peroxidation. However, when CCl4 was simulated via DILIsym®, free radical generation and lipid peroxidation to levels consistent with the public literature were insufficient to drive hepatotoxicity. Analysis demonstrated that saturation of the metabolic pathway generating the free radical limited the extent of lipid peroxidation and thus the extent of cell death. In comparison, the APAP metabolic pathway has a greater dynamic range, resulting in higher levels of lipid peroxidation and cell death. Papers were identified in which lipid peroxidation independent mechanisms of cell death were suggested, including lipid peroxidation independent mitochondrial toxicity. The CCl4 representation was modified to induce mitochondrial electron transport chain (ETC) inhibition. Concurrent induction of lipid peroxidation and ETC inhibition mechanisms permitted reconciliation of simulated hepatotoxicity with published data. For example, >3x ALT elevations were simulated at doses <50 mg/kg, with the highest ALT elevations (>1000 U/L) at doses >100 mg/kg in mice. Simulated CCl4 was then used to evaluate and further refine the innate immune response. In summary, quantitative analysis of CCl4 in DILIsym® not only permitted evaluation of the innate immune response, but also suggested a limitation in the putative primary mechanism of hepatotoxicity.
SOT 53rd Annual Meeting and ToxExpo, March 23-27, 2014, Phoenix, AZ
By Lisl K. M. Shoda; Scott Q. Siler; Paul B. Watkins; Brett A. Howell