The pharmacokinetic profile of intravenous paracetamol in adult patients undergoing major abdominal surgery. Ther Drug Monit. 2012;34:713-721. [PMID: 23149443 DOI: 10.1097/ftd.0b013e31826a70ea]

Influence of general anaesthesia on the intravenous acetaminophen pharmacokinetics in Beagle dogs

OBJECTIVE

To determine if general anaesthesia influences the intravenous (IV) pharmacokinetics (PK) of acetaminophen in dogs.

STUDY DESIGN

Prospective, crossover, randomized experimental study.

ANIMALS

A group of nine healthy Beagle dogs.

METHODS

Acetaminophen PK were determined in conscious and anaesthetized dogs on two separate occasions. Blood samples were collected before, and at 5, 10, 15, 30, 45, 60 and 90 minutes and 2, 3, 4, 6, 8, 12 and 24 hours after 20 mg kg^-1 IV acetaminophen administration. Haematocrit, total proteins, albumin, alanine aminotransferase, aspartate aminotransferase, urea and creatinine were determined at baseline and 24 hours after acetaminophen. The anaesthetized group underwent general anaesthesia (90 minutes) for dental cleaning. After the administration of dexmedetomidine (3 μg kg^-1) intramuscularly, anaesthesia was induced with propofol (2-3 mg kg^-1) IV, followed by acetaminophen administration. Anaesthesia was maintained with isoflurane in 50% oxygen (Fe'Iso 1.3-1.5%). Dogs were mechanically ventilated. Plasma concentrations were analysed with high-performance liquid chromatography. PK analysis was undertaken using compartmental modelling. A Wilcoxon test was used to compare PK data between groups, and clinical laboratory values between groups, and before versus 24 hours after acetaminophen administration. Data are presented as median and range (p < 0.05).

RESULTS

A two-compartmental model best described time-concentration profiles of acetaminophen. No significant differences were found for volume of distribution values 1.41 (0.94-3.65) and 1.72 (0.89-2.60) L kg^-1, clearance values 1.52 (0.71-2.30) and 1.60 (0.91-1.78) L kg^-1 hour^-1 or terminal elimination half-life values 2.45 (1.45-8.71) and 3.57 (1.96-6.35) hours between conscious and anaesthetized dogs, respectively. Clinical laboratory variables were within normal range. No adverse effects were recorded.

CONCLUSIONS AND CLINICAL RELEVANCE

IV acetaminophen PK in healthy Beagle dogs were unaffected by general anaesthesia under the study conditions. Further studies are necessary to evaluate the PK in different clinical contexts.

Extrapolation for a pharmacokinetic model for acetaminophen from adults to neonates: A Latin Hypercube Sampling analysis

Physiological and drug-specific parameters need to be adjusted when extrapolating a pharmacokinetic (PK) model from adults to neonates, so as to reproduce the time profiles of the studied drug(s) consistent with clinical, in vivo data or in vitro cell line measurements. In this paper we present a parameter analysis method, i.e. the Latin Hypercube Sampling (LHS) method for an acetaminophen (APAP) PK model. The original model consists of two compartments (the blood and the urine) with Michaelis-Menten kinetic parameters determined for APAP and its metabolites. The physiological parameters are scaled through allometric laws from adults to neonates, and APAP-specific parameters are adjusted for enzymatic maturational changes. The LHS method is used to statistically investigate the interplay between these parameters. The results for the extrapolated APAP model are consistent with published APAP PK data in neonates. We found the sulphation clearance parameter played a crucial role in the neonatal PK model, but its influence was weakened if the volume of distribution parameters were included. We suggest that this kind of in silico experiment could be valuable as the first step in PK model extrapolation between different ages.

A computational model for hepatotoxicity by coupling drug transport and acetaminophen metabolism equations

The spatial distributions of cytochrome P450 (CYP450) and glutathione (GSH) in liver lobules determine the heterogeneous hepatotoxicity of acetaminophen (APAP). Their interplay in conjunction with blood flow is not well understood. In this paper, we integrate a cellular APAP metabolism model with a sinusoidal blood flow to simulate the temporal-spatial patterns of APAP-induced hepatotoxicity. The heterogeneous distribution of CYP450 and GSH is modeled by linearly varying their reaction rates along the portal triad to the central vein axis of a sinusoid. We found that the spatial distribution of GSH, glutathione S-transferases (GSTs), and CYP450 all contributes to the high acetaminophen protein adduct formation at zone 3 of the lobules. The reversed spatial gradients of CYP450 and GSH cause quick depletion of GSH, which is further accelerated by the distribution of GST. The hepatic flow congestion and hyperperfusion however do not seem to play a significant role in the zonal hepatotoxicity. The simulation results may be useful for understanding the APAP-induced hepatotoxicity and associated pharmaceutical treatment.

Mechanistic and Quantitative Understanding of Pharmacokinetics in Zebrafish Larvae through Nanoscale Blood Sampling and Metabolite Modeling of Paracetamol

Zebrafish larvae are increasingly used for pharmacological research, but internal drug exposure is often not measured. Understanding pharmacokinetics is necessary for reliable translation of pharmacological results to higher vertebrates, including humans. Quantification of drug clearance and distribution requires measurements of blood concentrations. Additionally, measuring drug metabolites is of importance to understand clearance in this model organism mechanistically. We therefore mechanistically studied and quantified pharmacokinetics in zebrafish larvae, and compared this to higher vertebrates, using paracetamol (acetaminophen) as a paradigm compound. A method was developed to sample blood from zebrafish larvae 5 days post fertilization. Blood concentrations of paracetamol and its major metabolites, paracetamol-glucuronide and paracetamol-sulfate, were measured. Blood concentration data were combined with measured amounts in larval homogenates and excreted amounts and simultaneously analyzed through nonlinear mixed-effects modeling, quantifying absolute clearance and distribution volume. Blood sampling from zebrafish larvae was most successful from the posterior cardinal vein, with a median volume (interquartile range) of 1.12 nl (0.676-1.66 nl) per blood sample. Samples were pooled (n = 15-35) to reach measurable levels. Paracetamol blood concentrations at steady state were only 10% of the external paracetamol concentration. Paracetamol-sulfate was the major metabolite, and its formation was quantified using a time-dependent metabolic formation rate. Absolute clearance and distribution volume correlated well with reported values in higher vertebrates, including humans. Based on blood concentrations and advanced data analysis, the mechanistic and quantitative understanding of paracetamol pharmacokinetics in zebrafish larvae has been established. This will improve the translational value of this vertebrate model organism in drug discovery and development. SIGNIFICANCE STATEMENT: In early phases of drug development, new compounds are increasingly screened in zebrafish larvae, but the internal drug exposure is often not taken into consideration. We developed innovative experimental and computational methods, including a blood-sampling technique, to measure the paradigm drug paracetamol (acetaminophen) and its major metabolites and quantify pharmacokinetics (absorption, distribution, elimination) in zebrafish larvae of 5 days post fertilization with a total volume of only 300 nl. These parameter values were scaled to higher vertebrates, including humans.

Hypotensive Response to IV Acetaminophen in Pediatric Cardiac Patients

OBJECTIVES

Acetaminophen is ubiquitously used as antipyretic/analgesic administered IV to patients undergoing surgery and to critically ill patients when enteral routes are not possible. Widely believed to be safe and free of adverse side effects, concerns have developed in adult literature regarding the association of IV acetaminophen and transient hypotension. We hypothesize that there are hemodynamic effects after IV acetaminophen in the PICU and assess the prevalence of such in a large pediatric cardiovascular ICU population using high-fidelity data.

DESIGN

Observational study analyzing an enormous set of continuous physiologic data including millions of beat to beat blood pressures surrounding medication administration.

SETTING

Quaternary pediatric cardiovascular ICU between January 1, 2013, and November 13, 2017.

PATIENTS

All patients less than or equal to 18 years old who received IV acetaminophen. Mechanical support devices excluded.

INTERVENTIONS

None.

MEASUREMENTS AND MAIN RESULTS

Physiologic vital sign data were analyzed in 5-minute intervals starting 60 minutes before through 180 minutes after completion. Hypotension defined as mean arterial pressure -15% from baseline and relative hypotension defined -10%. Only doses where patients received no other medications, including vasopressors, within the previous hour were included. t test and a correlation matrix were used to eliminate correlated factors before a logistic regression analysis was performed. Six-hundred eight patients received 777 IV acetaminophen doses. Median age was 8.8 months (interquartile range, 2-62 mo) with a dose of 12.5 mg/kg (interquartile range, 10-15 mg/kg). Data were normalized for age and reference values. One in 20 doses (5%) were associated with hypotension, and one in five (20%) associated with relative hypotension. Univariate analysis revealed hypotension associated with age, baseline mean arterial pressure, and skin temperature (p = 0.05, 0.01, and 0.09). Logistic regression revealed mean arterial pressure (p = 0.01) and age (p = 0.05) remained predictive for hypotension.

CONCLUSIONS

In isolation of other medication, a hemodynamic response to IV acetaminophen has a higher prevalence in critically ill children with cardiac disease than previously thought and justifies controlled studies in the perioperative and critical care setting. The added impact on individual patient hemodynamics and physiologic instability will require further study.

Population pharmacokinetics of intravenous paracetamol in critically ill patients with traumatic brain injury

PURPOSE

High-dose paracetamol (6 g/day) is a low-cost intervention that may prevent pyrexia. The purpose of this study was to describe the pharmacokinetics of high-dose intravenous paracetamol, in patients with traumatic brain injury (TBI).

MATERIALS AND METHODS

A clinical pharmacokinetic study in adult patients with TBI was performed as a sub-study to a prospective, phase 2B, randomized placebo-controlled study (PARITY). Patients received 1 g of intravenous paracetamol or 0.9% sodium chloride every 4 h for 72 h.

RESULTS

All patients were included in the pharmacokinetic sub-study. The mean age, weight and area under the concentration-time curve for the sampled dosing interval were 34.5 yr, 82.3 kg and 39.9 ± 19.8 mg.h/L, respectively. The concentrations observed in the study patients were well below the threshold of toxicity and there was no evidence of accumulation of paracetamol. Paracetamol clearance was found to be high and variable (25.7 L.h^-1, coefficient of variation (CV) 40.9%), and a wide range of volume of distribution observed (27.6 L, CV 30.6%). A relationship between lower Glasgow coma scores and higher clearance of paracetamol was observed.

CONCLUSION

Due to altered pharmacokinetics, patients experiencing severe TBI may require a higher dose of paracetamol to achieve drug exposure that results in preventing pyrexia.

Neonatal Maturation of Paracetamol (Acetaminophen) Glucuronidation, Sulfation, and Oxidation Based on a Parent-Metabolite Population Pharmacokinetic Model

OBJECTIVES

This study aimed to model the population pharmacokinetics of intravenous paracetamol and its major metabolites in neonates and to identify influential patient characteristics, especially those affecting the formation clearance (CL_formation) of oxidative pathway metabolites.

METHODS

Neonates with a clinical indication for intravenous analgesia received five 15-mg/kg doses of paracetamol at 12-h intervals (<28 weeks' gestation) or seven 15-mg/kg doses at 8-h intervals (≥28 weeks' gestation). Plasma and urine were sampled throughout the 72-h study period. Concentration-time data for paracetamol, paracetamol-glucuronide, paracetamol-sulfate, and the combined oxidative pathway metabolites (paracetamol-cysteine and paracetamol-N-acetylcysteine) were simultaneously modeled in NONMEM 7.2.

RESULTS

The model incorporated 259 plasma and 350 urine samples from 35 neonates with a mean gestational age of 33.6 weeks (standard deviation 6.6). CL_formation for all metabolites increased with weight; CL_formation for glucuronidation and oxidation also increased with postnatal age. At the mean weight (2.3 kg) and postnatal age (7.5 days), CL_formation estimates (bootstrap 95% confidence interval; between-subject variability) were 0.049 L/h (0.038-0.062; 62 %) for glucuronidation, 0.21 L/h (0.17-0.24; 33 %) for sulfation, and 0.058 L/h (0.044-0.078; 72 %) for oxidation. Expression of individual oxidation CL_formation as a fraction of total individual paracetamol clearance showed that, on average, fractional oxidation CL_formation increased <15 % when plotted against weight or postnatal age.

CONCLUSIONS

The parent-metabolite model successfully characterized the pharmacokinetics of intravenous paracetamol and its metabolites in neonates. Maturational changes in the fraction of paracetamol undergoing oxidation were small relative to between-subject variability.

Population pharmacokinetics of intravenous acetaminophen in Japanese patients undergoing elective surgery

INTRODUCTION

Intravenous (i.v.) acetaminophen is administered during surgery for postoperative analgesia. However, little information is available on the pharmacokinetics of i.v. acetaminophen in Japanese patients undergoing surgery under general anesthesia.

METHODS

The study was approved by the Institutional Review Board and registered at UMIN-CTR (UMIN000013418). Patients scheduled to undergo elective surgery under general anesthesia were enrolled after obtaining written informed consent. During surgery, 1 g of i.v. acetaminophen was administered over 15, 60, or 120 min. Acetaminophen concentrations (15 or 16 samples per case) were measured at time points from 0-480 min after the start of administration (liquid chromatography-mass spectrometry/tandem mass spectrometry; limit of quantitation 0.1 μg/mL). The predictive performance of three published pharmacokinetic models was evaluated. Population pharmacokinetics were also analyzed using a nonlinear mixed-effect model based on the NONMEM program.

RESULTS

Data from 12 patients who underwent endoscopic or lower limb procedures were analyzed (male/female = 7/5, median age 55 years, weight 63 kg). Anesthesia was maintained with remifentanil and propofol or sevoflurane. The pharmacokinetic model of i.v. acetaminophen reported by Würthwein et al. worked well. Using 185 datapoints, the pharmacokinetics of i.v. acetaminophen were described by a two-compartment model with weight as a covariate but not age, sex, or creatinine clearance. The median prediction error and median absolute prediction error of the final model were -1 and 10%, respectively.

CONCLUSION

A population pharmacokinetic model of i.v. acetaminophen in Japanese patients was constructed, with performance within acceptable ranges.

Pharmacokinetic Study of Intravenous Acetaminophen Administered to Critically Ill Multiple-Trauma Patients at the Usual Dosage and a New Proposal for Administration

The pharmacokinetic profile of intravenous acetaminophen administered to critically ill multiple-trauma patients was studied after 4 consecutive doses of 1 g every 6 hours. Eleven blood samples were taken (predose and 15, 30, 45, 60, 90, 120, 180, 240, 300, and 360 minutes postdose), and urine was collected (during 6-hour intervals between doses) to determine serum and urine acetaminophen concentrations. These were used to calculate the following pharmacokinetic parameters: maximum and minimum concentrations, terminal half-life, area under serum concentration-time curve from 0 to 6 hours, mean residence time, volume of distribution, and serum and renal clearance of acetaminophen. Daily doses of acetaminophen required to obtain steady-state minimum (bolus dosing) and average plasma concentrations (continuous infusion) of 10 μg/mL were calculated (10 μg/mL is the presumed lower limit of the analgesic range). Data are expressed as median [interquartile range]. Twenty-two patients were studied, mostly young (age 44 [34-64] years) males (68%), not obese (weight 78 [70-84] kg). Acetaminophen concentrations and pharmacokinetic parameters were these: maximum concentration 33.6 [25.7-38.7] μg/mL and minimum concentration 0.5 [0.2-2.3] μg/mL, all values below 10 μg/mL and 8 below the detection limit; half-life 1.2 [1.0-1.9] hours; area under the curve for 6 hours 34.7 [29.7-52.7] μg·h/mL; mean residence time 1.8 [1.3-2.6] hours; steady-state volume of distribution 50.8 [42.5-66.5] L; and serum and renal clearance 28.8 [18.9-33.7] L/h and 15 [11-19] mL/min, respectively. Theoretically, daily doses for a steady-state minimum concentration of 10 μg/mL would be 12.2 [7.8-16.4] g/day (166 [112-202] mg/[kg·day]); for an average steady-state concentration of 10 μg/mL, they would be 6.9 [4.5-8.1] g/day (91 [59-111] mg/[kg·day]). In conclusion, administration of acetaminophen at the recommended dosage of 1 g per 6 hours to critically ill multiple-trauma patients yields serum concentrations below 10 μg/mL due to increased elimination. To reach the 10 μg/mL target, and from a strictly pharmacokinetic point of view, continuous infusion may be more feasible than bolus dosing. Such a change in dosing strategy requires appropriate, pharmacokinetic-pharmacodynamic and specific safety study.

Postmarketing review of intravenous acetaminophen dosing based on Food and Drug Administration prescribing guidelines

STUDY OBJECTIVES

To evaluate the appropriateness of intravenous acetaminophen dosing-prescribed dose, frequency, duration, and indication-based on United States Food and Drug Administration (FDA)-approved prescribing guidelines and to evaluate the adverse effect profile of intravenous acetaminophen.

DESIGN

Retrospective chart review.

SETTING

United States Navy medical center.

PATIENTS

Three hundred patients who received intravenous acetaminophen from August 1, 2011, to August 1, 2012.

MEASUREMENTS AND MAIN RESULTS

The indications, dose, frequency, and duration of intravenous acetaminophen were recorded for each patient. Adverse effects of intravenous acetaminophen were analyzed by thoroughly reviewing any adverse effects documented, including nausea, vomiting, headache, or any symptom specifically attributed to the drug. Baseline liver function tests, including aspartate aminotransferase and alanine aminotransferase levels, and elevations 3 times the upper limit of normal during intravenous acetaminophen therapy were recorded. The average patient weight was 78±21 kg, with 12 patients (4%) weighing less than 50 kg and 288 (96%) patients weighing 50 kg or greater. Two hundred forty-one patients (80%) were appropriately dosed, whereas 59 (20%) patients were not appropriately dosed based on the FDA-approved dosing. No patients exceeded the FDA-approved maximum daily dosing recommendations for intravenous acetaminophen (4 g). Sixty-five patients (22%) received intravenous acetaminophen for longer than 24 hours. Intravenous acetaminophen was well tolerated, without any reported adverse effects, including the commonly reported adverse effects of nausea, vomiting, headache, and insomnia. Ten patients (3%) had a documented history of liver disease and did not experience any adverse effects or increases in liver function tests after the administration of intravenous acetaminophen.

CONCLUSION

Intravenous acetaminophen appeared to be a safe and effective analgesic and antipyretic agent. Dosing for patients weighing less than 50 kg needs to be appropriately weight adjusted. Intravenous acetaminophen can be used alone or in conjunction with opioids and other analgesics. Limitations of this study include its retrospective design, inability to assess outcomes of reducing opioid use, and short-term observation period.

Covariates of intravenous paracetamol pharmacokinetics in adults

BACKGROUND

Pharmacokinetic estimates for intravenous paracetamol in individual adult cohorts are different to a certain extent, and understanding the covariates of these differences may guide dose individualization. In order to assess covariate effects of intravenous paracetamol disposition in adults, pharmacokinetic data on discrete studies were pooled.

METHODS

This pooled analysis was based on 7 studies, resulting in 2755 time-concentration observations in 189 adults (mean age 46 SD 23 years; weight 73 SD 13 kg) given intravenous paracetamol. The effects of size, age, pregnancy and other clinical settings (intensive care, high dependency, orthopaedic or abdominal surgery) on clearance and volume of distribution were explored using non-linear mixed effects models.

RESULTS

Paracetamol disposition was best described using normal fat mass (NFM) with allometric scaling as a size descriptor. A three-compartment linear disposition model revealed that the population parameter estimates (between subject variability,%) were central volume (V1) 24.6 (55.5%) L/70 kg with peripheral volumes of distribution V2 23.1 (49.6%) L/70 kg and V3 30.6 (78.9%) L/70 kg. Clearance (CL) was 16.7 (24.6%) L/h/70 kg and inter-compartment clearances were Q2 67.3 (25.7%) L/h/70 kg and Q3 2.04 (71.3%) L/h/70 kg. Clearance and V2 decreased only slightly with age. Sex differences in clearance were minor and of no significance. Clearance, relative to median values, was increased during pregnancy (F(PREG) = 1.14) and decreased during abdominal surgery (F(ABDCL) = 0.715). Patients undergoing orthopaedic surgery had a reduced V2 (F(ORTHOV) = 0.649), while those in intensive care had increased V2 (F(ICV) = 1.51).

CONCLUSIONS

Size and age are important covariates for paracetamol pharmacokinetics explaining approximately 40% of clearance and V2 variability. Dose individualization in adult subpopulations would achieve little benefit in the scenarios explored.

Clinical pharmacology of intravenous paracetamol in perinatal medicine

Clinical pharmacology aims to predict drug-related effects based on compound and population specific pharmacokinetics (PK, concentration-time), and pharmacodynamics (PD, concentration-effect). Consequently, dosing needs to be based on the physiological characteristics of the individual patient. Pregnancy and early infancy hereby warrant focused assessment. The specific characteristics of both subpopulations will be illustrated based on observations on intravenous (iv) paracetamol PK and PD collected in these specific populations. At delivery, there is a significant higher paracetamol clearance (+ 45%, L/h) when compared to non-pregnant observations. This higher clearance is in part explained by a proportional increase in oxidative metabolite production, but mainly an increase in glucuronidation. When focusing on PD, an association between maternal paracetamol exposure and atopy in infancy and fetal gastroshizis has been reported. In early infancy, paracetamol clearance is significantly lower and mainly depends on size (weight 0.75), while also the distribution volume is higher (L/kg). Reports on hepatic tolerance, haemodynamic stability and impact of body temperature have been published while the concentration effect profile for analgesia seems to be similar between neonates and children. Similar to maternal exposure, there are reports on the association with atopy. Studies on the use of paracetamol to close the patent ductus arteriosus are ongoing. At least, these observations provide evidence on the need to study commonly administered anesthetics in such specific subpopulations with specific focus on both population specific PK and PD to further improve patient tailored pharmacotherapy.