Contrasting model mechanisms of alanine aminotransferase (ALT) release from damaged and necrotic hepatocytes as an example of general biomarker mechanisms
Authors: Smith Andrew K. , Ropella Glen E.P. , McGill Mitchell R. , Krishnan Preethi , Dutta Lopamudra , Kennedy Ryan C. , Jaeschke Hartmut , Hunt C. Anthony 
Published: December 2019

AbstractInterpretations of elevated blood levels of alanine aminotransferase (ALT) for drug-induced liver injury often assume that the biomarker is released passively from dying cells. However, the mechanisms driving that release have not been explored experimentally. The usefulness of ALT and related biomarkers will improve by developing mechanism-based explanations of elevated levels that can be expanded and elaborated incrementally. We provide the means to challenge the ability of closely related concretized model mechanisms to generate patterns of simulated hepatic injury and ALT release that scale (or not) to be quantitatively similar to the wet-lab validation targets. The validation targets for this work are elevated measures of plasma ALT following acetaminophen (APAP) exposure in mice. We build on a published model mechanism that helps explain the generation of characteristic spatiotemporal features of APAP hepatotoxicity within hepatic lobules. Discrete event and agent-oriented software methods are most prominent. We instantiate and leverage a small constellation of concrete model mechanisms. Their details during execution help bring into focus ways in which particular sources of uncertainty become entangled within and across several levels with cause-effect details. Monte Carlo sampling and simulations comprise a virtual experiment. Falsification of one (or more) of the model mechanisms provides new knowledge and shrinks the model mechanism constellation incrementally. We challenge the sufficiency of four potentially explanatory theories for ALT release. The first model mechanism tested failed to achieve the initial validation target, but each of the three others succeeded. We scale ALT amounts in virtual mice directly to target plasma ALT measures in individual mice. Results for one of the three model mechanisms matched all target ALT measures quantitatively. We assert that the actual mechanisms responsible for ALT measures in individual mice and the virtual causal processes occurring during model execution are strongly analogous within and among real hepatic lobular levels.Author summaryInterpretations of elevated biomarkers for drug-induced liver injury assume passive release during hepatocyte death, yet indirect evidence indicates that plasma levels can increase absent injury. Limitations on measurements make it infeasible to resolve causal linkages between drug disposition and plasma levels of biomarkers. To improve explanatory knowledge, we instantiate within virtual mice, plausible mechanism-based causal linkages between acetaminophen disposition and alanine aminotransferase (ALT) behavior that enables simulation results to meet stringent quantitative validation prerequisites. We challenge the sufficiency of four model mechanisms by scaling ALT measurements in virtual mice to corresponding plasma values. Virtual experiment results in which ALT release is a combined consequence of lobular-location-dependent hepatocyte death and drug-induced cellular damage, matches all validation targets. We assert that the actual mechanisms responsible for plasma ALT measures in individual mice and the virtual causal processes occurring during model execution are strongly analogous within and among real hepatic lobular levels.