Quantitative Systems Toxicology Modeling of Acetaminophen Pharmacokinetics and Hepatic Biomarkers After Overdoses of Extended‐Release and Immediate‐Release Formulations in Adults With Chronic Alcohol Use or Low Glutathione (2025)

1 Introduction

Acetaminophen (APAP) is an over-the-counter analgesic and antipyretic that has been widely used in patients with fever and other conditions causing mild to moderate pain. When ingested in overdose, particularly large intentional overdose, APAP can cause hepatotoxicity, which may result in fulminant hepatic failure and death in severe cases. Hepatic injury from APAP is caused by its reactive metabolite, N-acetyl-ρ-benzoquinone imine (NAPQI), which is formed predominantly by CYP2E1-mediated metabolism of APAP. APAP is mostly metabolized to glucuronide and sulfate conjugates that are excreted in the urine, and only a small percentage (< 10%) of APAP is converted to NAPQI. As such, NAPQI is detoxified by hepatic glutathione without any toxicological consequences at therapeutic dose of APAP. However, if hepatic glutathione stores are depleted after overdose of APAP, the excess NAPQI can cause oxidative stress and mitochondrial dysfunction resulting in liver injury [1]. Patients with acute APAP overdose are treated with N-acetyl cysteine (NAC), which is a precursor of glutathione, if their blood APAP concentration is above the treatment line on the Rumack-Matthew nomogram [2]. In the Rumack-Matthew nomogram, serum concentrations of APAP are plotted in a log scale against the time, starting from 4 h after APAP ingestion at which time absorption of APAP is likely to be complete. The original Rumack-Matthew line started at 200 μg/mL at 4 h and follows a slope representing a half-life of 4 h; a treatment line that is 25% below the original, which starts at 150 μg/mL at 4 h, has been used to guide NAC treatment in APAP overdose patients [3].

APAP has multiple formulations including immediate-release (IR), modified-release (MR), and extended-release (ER) preparations. APAP-IR products are labeled for use on a 4-h basis. APAP-MR and APAP-ER products are designed to achieve prolonged absorption and efficacy for an 8-h period, but MR and ER formulations use different technologies. APAP-MR products, which were previously marketed in Europe and Australia, were associated with markedly delayed absorption, potentially due to formation of a bezoar, and unexpected toxicity [4]. There was also a concern that current treatment guidelines developed for APAP overdose were inappropriate for these MR formulations. As a result, the European Medicines Agency recommended suspending medicines containing APAP-MR from the market in 2017. On the other hand, clinical experiences do not support such concerns for APAP-ER products. APAP-ER products, designed with an IR layer and an erodible ER layer, are marketed in the US and Canada. A recently published consensus statement on the management of APAP poisoning indicated that management of APAP-ER overdose is the same as that for APAP-IR overdose, although an additional blood APAP measurement is recommended for APAP-ER if the APAP concentration from samples drawn 4–12 h after ingestion is below the treatment line but above 10 μg/mL [5].

Consistent with clinical experiences, we have previously reported that quantitative systems toxicology (QST) modeling using DILIsym predicted similar PK and hepatic biomarker profiles for the APAP-ER and APAP-IR formulations in healthy adults after single acute overdoses and repeat supratherapeutic ingestions (RSTI) [6]. DILIsym represents drug exposure predicted by physiologically-based pharmacokinetic (PBPK) models, liver biochemistry, and mechanisms contributing to drug-induced liver injury (DILI) [7]. A mechanistic model of APAP-IR was previously developed within DILIsym and verified with clinical and animal datasets[8-10]. In order to address clinical inter-individual variability, a simulated population (SimPops) that represents variability in model parameters related to APAP exposure and mechanistic hepatotoxicity pathways in healthy subjects was also developed and verified based on existing data[8-10]. In our prior work, the PBPK model of APAP-ER was developed and verified within DILIsym to evaluate potential differences in the PK and metabolism of APAP-ER and APAP-IR by simulation over a range of acute and chronic overdoses [6]. In addition, simulations of PK and hepatic biomarkers used to monitor patients, specifically plasma alanine aminotransferase (ALT), total bilirubin (TB), and international normalized ratio (INR), associated with these overdoses were explored in healthy adults. ALT is the primary biomarker of acute liver injury and an elevation of serum TB in the absence of other causes or cholestasis indicates a decrease in hepatic function [11]. Similar to TB, rising INR indicates functional impairment and represents substantial liver injury. Food and Drug Administration (FDA)'s guidance for DILI suggests that increased incidence of ALT > 3X upper limit of normal (ULN) compared to control indicates that a potential for liver injury is high, and Hy's Law cases, defined as ALT > 3X ULN and TB > 2X ULN, are indicative of severe DILI [12].

While simulations predicted similar PK and hepatic biomarker profiles for the APAP-ER and APAP-IR formulations in healthy adults, it remained unanswered if this similarity would be impacted by patient conditions. Specifically, conditions that can increase NAPQI production, such as induction of CYP2E1 by alcohol [13-15], or conditions that can decrease hepatic glutathione store, such as underling liver disease [16, 17], may impact PK and susceptibility to hepatotoxicity after overdose of APAP-IR and APAP-ER. In the current study, DILIsym was employed to evaluate the PK and hepatic responses of APAP-IR and APAP-ER in adults with chronic alcohol use or low hepatic glutathione (Figure1). APAP-IR and APAP-ER models in moderate chronic alcohol users (MCAU) and excessive chronic alcohol users (ECAU) with no/mild hepatic impairment were developed by updating physiological parameters of previously developed and verified healthy adult APAP models [6] using physiology data from chronic alcohol users. APAP-IR and APAP-ER models in adults with low hepatic glutathione compared to healthy volunteers were developed by updating physiological parameters of previously developed and verified healthy adult APAP models [6] using physiology data from cirrhotic patients. The verified APAP-IR and APAP-ER models in three adult populations (i.e., MCAU, ECAU, low glutathione) were used to simulate PK and hepatic biomarkers after single acute overdose and RSTI of APAP. Simulated PK and hepatic biomarker profiles of APAP-ER were compared with those of APAP-IR by ingested dose for each population.

2 Methods

3 Results

4 Discussion

QST models use a mechanistic, mathematical approach to integrate physiology, drug properties, and biochemical processes to evaluate drug safety. In this study, QST modeling using DILIsym was employed to evaluate the PK exposure and hepatic biomarker responses after overdoses of APAP-IR and APAP-ER in moderate and excessive chronic alcohol users and adults with low hepatic glutathione levels. Simulations predicted that APAP-ER had similar PK and hepatic biomarker profiles compared to APAP-IR in these populations after single acute overdoses and repeat supratherapeutic ingestions.

Chronic alcohol users reportedly have reduced glutathione after excessive consumption and induced CYP2E1 after moderate and excessive consumption [13-15, 17, 29]. However, ethanol has been shown to competitively inhibit CYP2E1 [18], which might offset the effect of CYP2E1 induction. Therefore, both chronic and acute effects of alcohol on the formation and elimination of NAPQI, the toxic reactive metabolite of APAP, have to be considered when evaluating the susceptibility of APAP-mediated hepatotoxicity in chronic alcohol users. Mathematical, mechanistic modeling provides an effective framework to quantitatively predict the net effects of alcohol on these competing and alternating pathways. A previously reported mathematical model suggested that the risk of APAP-induced hepatotoxicity (represented as decrease in fraction viable hepatocytes) after acute overdose was increased if APAP was ingested shortly after alcohol is eliminated from the body in chronic alcohol users with CYP2E1 induction, whereas concurrent consumption of alcohol showed protective effects [30]. While this work provided a quantitative assessment of alcohol effects on hepatotoxicity induced by acute overdose of APAP, the authors did not explore the combined effects of chronic and acute alcohol effects nor represent the alcohol effect on hepatic glutathione. Also, they did not represent specific APAP formulations and clinically relevant outcomes such as hepatic biomarkers. In the current study, APAP-IR and APAP-ER formulations were specifically represented in simulated chronic alcohol users who represent both acute and chronic effects of alcohol on APAP metabolizing enzymes and hepatic glutathione to compare clinically relevant outcomes (i.e., PK and hepatic biomarkers such as plasma ALT and INR) for two formulations.

CYP2E1 enzyme degrades rapidly after alcohol-mediated induction if alcohol consumption is discontinued. It has been reported that CYP2E1 levels were normalized within 3–8 days in newly abstinent heavy drinkers (i.e., ECAU) [14]. As such, newly abstinent ECAU, who still have induced CYP2E1 but do not actively consume alcohol that can directly inhibit CYP2E1, are expected to be the most susceptible to NAPQI-mediated hepatotoxicity among chronic alcohol users. Retrospective studies and case reports suggested that alcohol use in association with APAP overdose may be a risk factor for hepatotoxicity [31, 32]. However, a systematic review of randomized controlled trials demonstrated that repeat therapeutic doses (≤ 4 g/day) of APAP-IR and APAP-ER did not predispose chronic alcohol users, including newly abstinent heavy drinkers who were presumed to have increased risk, to liver injury [28]. Consistent with these data, simulations of repeat therapeutic dosing (4 g/day) of APAP-IR and APAP-ER formulations in newly abstinent ECAU predicted minimal hepatic injury. Simulation outcomes suggested that while newly abstinent ECAU have increased NAPQI synthesis and reduced hepatic glutathione levels, hepatocytes still have sufficient hepatic glutathione to eliminate NAPQI at therapeutic doses of APAP.

Glutathione is a reduced-thiol and tripeptide molecule concentrated in hepatocytes that helps maintain a balanced intracellular redox state by preferentially binding electrophilic molecules such as NAPQI [33]. As such, individuals with reduced hepatic glutathione due to underlying liver disease, malnutrition, and aging may be predisposed to APAP-mediated hepatotoxicity after overdose [34]. The glutathione sub-model within DILIsym represents glutathione synthesis from its precursor, steady-state turnover, depletion, and nuclear factor-like 2 (NRF2)-mediated upregulation of glutathione synthesis [8], which allows for evaluation of APAP-induced hepatotoxicity in individuals with reduced glutathione. In the current work, APAP-ER and APAP-IR representations in adults with low glutathione were developed and verified using data from cirrhotic patients in terms of their hepatic glutathione levels and expression of APAP metabolizing enzymes.

Cirrhotic patients with modest-to-severe hepatic impairment have reduced hepatic glutathione as well as CYP2E1 and other APAP metabolizing enzymes [16, 17, 35-40]. Interestingly, although cirrhotic patients showed increased APAP exposure and reduced systemic clearance compared to healthy adults, the overall biotransformation (e.g., percent urinary recovery of each metabolite) remained unchanged compared to healthy adults [20, 26-28]. In our simulations, the amount of NAPQI formed (i.e., urinary recovery) was similar in healthy adults and individuals with low glutathione, while the formation rate of NAPQI (and other metabolites) was reduced in individuals with low glutathione compared to healthy adults, consistent with clinical data. Simulations of repeat therapeutic dosing (4 g/day) of APAP predicted minimal ALT elevations and no functional impairment in cirrhotic adults with low glutathione, which is consistent with data from prospective clinical studies where stable chronic liver disease does not pose a risk for developing APAP hepatotoxicity at therapeutic doses [25]. Quantitative modeling representing the dynamics of NAPQI formation and elimination, and glutathione dynamics, demonstrated that while cirrhotic patients have a reduced level of hepatic glutathione, the rate of NAPQI formation is also reduced, which led to sufficient NAPQI elimination at a therapeutic dose of APAP-IR and APAP-ER.

The effect of the APAP formulation on its overdose hepatotoxicity in adults with chronic alcohol use and low hepatic glutathione has not been fully characterized. Retrospective studies and case reports are informative but do not provide firm conclusions due to multiple methodological weaknesses and the existence of confounding factors which are not well documented. Also, reliance on the patient's self-reported dose, timing, and formulation introduces great uncertainties. For ethical reasons, it is unlikely that any large randomized controlled trial will be undertaken to understand formulation-specific hepatotoxicity outcomes after APAP overdose in chronic alcohol users (or any patient groups). In the absence of controlled trial data, such information should be extrapolated from a combination of existing PK and hepatic response data by quantitative mechanistic modeling. In our prior work, PK and hepatic response in healthy volunteers after therapeutic dose and overdose (acute and repeat-supratherapeutic ingestions) were developed and verified using clinical data. In the current work, representation of altered physiology in chronic alcohol users and individuals with low hepatic glutathione was verified using APAP PK and hepatic response data after repeated therapeutic doses. Integrating APAP dose-responses and population-specific physiology, modeling was able to evaluate the PK and hepatic biomarker responses to APAP-IR and APAP-ER after acute and repeat supratherapeutic overdoses in special populations.

The modeling and simulation work presented in this study have several assumptions and limitations. First, models and subsequent simulations described in this report did not include concomitant clinical interventions or NAC treatment. Interventions used in the clinic to prevent APAP overdose-associated hepatotoxicity include treatments such as activated charcoal and gastric lavage, each of which has the potential to diminish the fraction of APAP that is absorbed [41-43]. Furthermore, the simulations did not include the administration of the glutathione precursor NAC, an important and standard therapy, after an acute APAP overdose, which decreases APAP hepatotoxicity by restoring glutathione stores and detoxifying the toxic NAPQI metabolite. As a result, APAP hepatotoxicity predictions (e.g., biomarker levels) would very likely have been decreased substantially in the presence of NAC [10].

Second, simulated chronic alcohol users and adults with low glutathione did not represent underlying liver injury. Thus, simulated hepatic effects from individuals with low glutathione do not necessarily represent other causes of underlying liver injury; hepatic biomarker change from the baseline/pretreatment value was assessed to determine APAP-mediated effects. As such, the simulated hepatic biomarker responses represent APAP effects only in stable liver disease and cannot be extrapolated to decompensated liver disease. Lastly, specific alcohol consumption patterns were assumed for the purpose of representing direct competitive inhibition of CYP2E1 by alcohol in chronic alcohol users. The extent of competitive inhibition depends on alcohol exposure, which varies depending on the amount of alcohol consumed and the relative timing of alcohol consumption and APAP ingestion. Despite these limitations, the outcomes for APAP-IR versus APAP-ER can still be compared effectively within each population and condition, as the current modeling and simulations were not meant for cross-population comparisons for the same formulation.

In summary, QST modeling using DILIsym predicted similar PK and hepatic biomarker profiles for the APAP-ER and APAP-IR formulations in moderate and excessive chronic alcohol users and adults with low hepatic glutathione levels after single acute overdoses and repeat supratherapeutic ingestions. These modeling results further support that approaches to manage APAP-IR overdoses can be applied to manage APAP-ER overdoses, as indicated in the recently published consensus statement on the APAP overdose management [5].

Author Contributions

K.Y., J.J.B., B.A.H., J.M., E.A., J.C.K.L., C.K.G., S.S., and E.A. wrote the manuscript; K.Y., J.J.B., B.A.H., J.M., E.A., J.C.K.L., C.K.G., S.S., and E.A. designed the research; K.Y., J.J.B., J.M., and E.A. performed the research; K.Y., J.J.B., J.M., and E.A. analyzed the data.

Disclosure

The authors have nothing to report.

Conflicts of Interest

Kyunghee Yang, James J. Beaudoin, Brett A. Howell, and James Mullin are employees of Simulations Plus Inc. John C.K. Lai, Sury Sista, and Evren Atillasoy are employees of Kenvue. Cathy K. Gelotte is a consultant of Kenvue. Elham Amini has no competing interests to declare.