How Reactive Metabolites Induce an Immune Response That Sometimes Leads to an Idiosyncratic Drug Reaction.

Little is known with certainty about the mechanisms of idiosyncratic drug reactions (IDRs); however, there is substantive evidence that reactive metabolites are involved in most, but not all, IDRs. In addition, evidence also suggests that most IDRs are immune mediated. That raises the question of how reactive metabolites induce an immune response that can lead to an IDR. The dominant hypotheses are the hapten and danger hypotheses. These are complementary hypotheses: a reactive metabolite can act as a hapten to produce neoantigens, and it can also cause cell damage leading to the release of danger-associated molecular pattern molecules that activate antigen presenting cells. Both are required for an immune response. In addition, drugs may induce an immune response through inflammasome activation. We have found examples in which the ability to activate inflammasomes differentiated drugs that cause IDRs from similar drugs that do not. There are other hypotheses that do not involve an immune mechanism such as mitochondrial injury and bile salt export pump (BSEP) inhibition. With some possible exceptions, these hypotheses are unlikely to be able to completely explain IDRs. However, some types of mitochondrial injury or BSEP inhibition could produce danger signals. The major mechanism that protects us from IDRs appears to be immune tolerance. Consistent with this hypothesis, we used checkpoint inhibition to develop the first animal model of idiosyncratic drug-induced liver injury that has the same characteristics as the idiosyncratic injury in humans. This was accomplished by treating Pd-1-/- mice with anti-CTLA-4 antibodies and amodiaquine. The combination of the Pd-1-/- mouse and anti-CTLA-4 also unmasks the ability of other drugs such as isoniazid to cause delayed type liver injury. This model should allow rigorous testing of mechanistic hypotheses that was impossible in the past.