Testing Possible Risk Factors for Idiosyncratic Drug-Induced Liver Injury Using an Amodiaquine Mouse Model and Co-treatment with 1-Methyl-d-Tryptophan or Acetaminophen

[Image: see text] Idiosyncratic drug reactions are unpredictable adverse reactions. Although most such adverse reactions appear to be immune mediated, their exact mechanism(s) remain elusive. The idiosyncratic drug reaction most associated with serious consequences is idiosyncratic drug-induced liver injury (IDILI). We have developed a mouse model of amodiaquine (AQ)-induced liver injury that reflects the clinical characteristics of IDILI in humans. This was accomplished by impairing immune tolerance by using PD-1(–/–) mice and an antibody against CTLA-4. PD-1 and CTLA-4 are known negative regulators of lymphocyte activation, which promote immune tolerance. Immune checkpoint inhibitors have become important tools for the treatment of cancer. However, as in our model, immune checkpoint inhibitors increase the risk of IDILI with drugs that have an incidence of causing liver injury. Agents such as 1-methyl-d-tryptophan (D-1-MT), an inhibitor of the immunosuppressive indoleamine 2,3-dioxygenase (IDO) enzyme, have also been proposed as anti-cancer treatments. Another possible risk factor for the induction of an immune response is the release of danger-associated molecular patterns (DAMPs). Acetaminophen (APAP) is known to cause acute liver injury, and it is likely to cause the release of DAMPs. Therefore, either of these agents could increase the risk of IDILI, although through different mechanisms. If true, then this would have clinical implications. We found that co-treatment with D-1-MT paradoxically decreased liver injury in our model, and although APAP appeared to slightly increase AQ-induced liver injury, the difference was not significant. Such results highlight the complexity of the immune response, which makes potential interactions difficult to predict.