Developing new antidotes for poisons with existing effective treatments: a case study of fomepizole in paracetamol poisoning

Introduction: Acetylcysteine is the only effective and licensed therapy for paracetamol poisoning. However, acetylcysteine loses efficacy if treatment is delayed 8-12 hours after paracetamol ingestion, and there is also uncertainty as to whether the dose should be increased in high-risk paracetamol ingestions. Studies have identified potential therapeutic targets, including enzymes that metabolize paracetamol; the pathways causing mitochondrial toxicity via c-Jun N-terminal kinases or superoxide generation; and other specific targets, such as nuclear factor-erythroid factor 2-dependent gene induction and autophagy. With this range of potential additional therapies, how should the speciality of clinical toxicology approach the development of new antidotes for this common poisoning? Historical background: When the first treatments for paracetamol toxicity were developed, the clinical trial and ethical basis of practice were different from today. Acetylcysteine was never subjected to placebo-controlled studies, even by the United States Food and Drug Administration, as it was presumed that the toxicity of high paracetamol concentrations was so evident that placebo-controlled studies were unethical. Thus, the absolute benefit of acetylcysteine remains unknown. In addition, no dose-ranging studies of acetylcysteine in patients were ever done. The weakness of assessing the efficacy of additional antidotes in small groups of patients with moderate poisoning is illustrated by the use of cimetidine in paracetamol poisoning. Current approaches to drug (and antidote) development: The approach required by regulatory authorities today relies on several important steps. First, a clear target for therapeutic effect is sought, normally in a laboratory model. Next, a 'proof of principle' study is required to demonstrate that the target is 'druggable'. Finally, clinical studies to confirm proof of principle applies in humans, followed by a controlled trial with matched patient groups with sufficient power to demonstrate the clinical outcome being sought. Such patient studies can be expensive to conduct, and non-commercial groups suffer the risk of not being funded. Fomepizole: Fomepizole prevents paracetamol-induced hepatic toxicity in mice by inhibiting cytochrome P4502E1, thereby preventing the conversion of paracetamol to its toxic metabolite. Fomepizole also inhibits c-Jun N-terminal kinases, a key pathway in the downstream toxicity on the mitochondria. The present evidence of efficacy in humans is based on small case series with no control groups. The availability of a licensed indication has facilitated off-label use of fomepizole in an unproven indication. Conclusions: Paracetamol poisoning is common, and randomized, controlled clinical trials are possible. The benefit of fomepizole can only be shown by such a study. As clinical trials using fomepizole as an added therapy to acetylcysteine are recruiting in the United States, these should be supported by all clinical toxicologists. In the interim, the publication of small case series using fomepizole should be discouraged by journals.