Introduction: Paracetamol toxicity is a common cause of acute liver failure in many countries. Most paracetamol is metabolised to the non-toxic sulfate and glucuronide conjugates. Only about 5% is oxidised to the highly reactive toxic metabolite, N-acetyl-p-benzoquinone imine (NAPQI), which can be detoxified by hepatic glutathione(GSH). At higher doses of paracetamol, NAPQI depletes stores of both GSH and cysteine. Hepatic necrosis is observed when GSH consumption is over 70%. As the limiting precursor for GSH synthesis, cysteine can be produced either by the degradation of GSH or the methionine cycle. Therefore, understanding the profile of GSH and methionine is critical to predict paracetamol-induced hepatotoxicity.
Objectives: The aim of this work is to understand GSH depletion and recovery under both therapeutic and overdose conditions.
Methods: We constructed a schema of a quantitative systems pharmacology (QSP) model for paracetamol metabolism. A component of this was available as a previously published mathematical model[1]. The model was modified to accommodate: (1) a non-specific tissue distribution compartment for paracetamol; (2) the irreversibility of paracetamol-protein adduct formation; (3) expansion of the GSH compartment to include the methionine cycle and trans-sulfation; (4) a positive feedback to increase the utilization of methionine(an essential amino acid) when GSH is depleted. The final model consisted of 31 states. The profile of the concentrations of plasma paracetamol and its sulfate and glucuronide conjugates after 20 mg/kg oral administration were compared against published results. The depletion profile of liver GSH was explored under different dose levels, 0 g, 1 g, 5 g, 10 g, 15 g and 20 g. Two mechanisms were proposed for investigating GSH depletion: (i) a reduction in oral intake of methionine and (ii) a positive feedback from GSH to methionine. All simulations were conducted in MATLAB 2017.
Results: The model simulation results for paracetamol and its metabolites in plasma matched the published results. The nadir of depletion of hepatic GSH decreased with higher paracetamol doses. In the absence of mechanism (i) or (ii) GSH recovered while paracetamol concentrations remained high. After including either mechanisms, about 70% of GSH is consumed by NAPQI after 10 g dose. Over 90% of GSH is depleted after 15 g and 20 g doses and did not recover to above 30% of normal GSH concentration after 50 hours. Even for a therapeutic dose, GSH consumption occurs but not to a critical value. When the input of methionine is turned off (mechanism ii), the profile of GSH concentration reaches a new paracetamol-dose-dependent steady state with a recovery based on the resumption of diet.
Conclusion: Our simulation results are consistent with the observation that a single dose of more than 10 g (or 150 mg/kg to 200mg/kg) may be associated with significant depletion of GSH and therefore the potential for liver toxicity. In addition, prolonged depletion of GSH to less than 30% of normal after paracetamol doses of 15 g and 20 g indicates that hepatocyte injury may be ongoing after the initial acute overdose stage. The mechanism of GSH depletion remains unknown. This work is the first step in understanding GSH depletion and potentially cysteine supplementation.
References:
- Ben-Shachar R, Chen Y, Luo S, Hartman C, Reed M, Nijhout HF. The biochemistry of acetaminophen hepatotoxicity and rescue: a mathematical model. Theoretical biology & medical modelling. 2012;9:55.