Pharmacokinetics, pharmacodynamics and pharmacogenetics associated with nonsteroidal anti-inflammatory drugs and opioids in pediatric cancer patients.

INTRODUCTION: Advancing appropriate and adequate analgesic pharmacotherapy in pediatric patients with cancer is an area of clinical need. Few studies have been performed to evaluate the selection of an analgesic and appropriate dosing corresponding to analgesic effect among pediatric cancer patients. This review describes information related to pharmacokinetic, pharmacodynamic, and pharmacogenomic (when applicable) considerations for analgesics that are commonly used to manage pain experienced by pediatric patients with cancer. Areas covered: Analgesics commonly used to treat pediatric patients with malignancy patterned after the World Health Organization's 'analgesic ladder' for cancer pain management. Expert opinion: Addressing pain management safely and effectively in pediatric patients with cancer will require advances in both drug development, to increase the armament of analgesics available for children, and our pharmacologic understanding of those analgesics in current use. However, performing the necessary types of studies to develop new analgesics, or gain knowledge of existing therapy, within a population that is relatively small, diverse, and who experience pain originating from a variety of sources, is a tremendous challenge.

Population pharmacokinetic-pharmacodynamic modelling to describe the effects of paracetamol and N-acetylcysteine on the international normalized ratio.

Paracetamol is one of the most common pharmaceutical agents taken in self-poisonings, and can increase the prothrombin time (PT) through liver injury, and in overdose without hepatic injury by reducing functional factor VII. PT is a measure of hepatic injury used to predict and monitor hepatotoxicity, reported as the international normalized ratio (INR). The antidote for paracetamol poisoning, N-acetylcysteine (NAC), has been reported to have an effect on the PT. This analysis included patients from a retrospective case series, a prospective inception cohort of paracetamol and psychotropic (control) overdoses, and a cross-over clinical trial. A population pharmacokinetic-pharmacodynamic model describing the pharmacodynamic effects of paracetamol and NAC on the INR was developed in Phoenix NLME. The dataset included 172 patients; the median age was 22 years (range 13-71 years). A one-compartment model with first-order input and linear disposition best described paracetamol pharmacokinetics. The population mean estimate of the concentration that induced a response halfway between the baseline and maximal pharmacological effect of paracetamol was 1302 µmol/L (242), the maximum effect of paracetamol was 0.534 (202; from baseline) and the maximum effect of NAC was 0.325 (9.03; from baseline). Both paracetamol and NAC contributed a pharmacological effect to the elevation of INR. The estimated paracetamol concentration that induced a response halfway between the baseline and maximal pharmacological effect was within the range of plasma paracetamol values studied, fivefold greater than the maximum therapeutic concentration, suggesting that an elevated INR would not be expected within the therapeutic range. Simulated 24 and 48 g paracetamol overdoses with NAC administration produced INR values (50th percentile) that reached the upper limit of, or exceeded, the reference range.

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.

Population pharmacokinetics of intravenous acetaminophen and its metabolites in major surgical patients.

Intravenous acetaminophen is a commonly used analgesic following surgery. The aims of this study were to determine the population pharmacokinetic profile of intravenous acetaminophen and its metabolites in adult surgical patients and to identify patient characteristics associated with acetaminophen metabolism in the postoperative period. 53 patients were included in the dataset; 28 were men, median age (range) 60 years (33-87), median weight (range) 74 kg (54-129). Patients received 1, 1.5 or 2 g of intravenous acetaminophen every 4-6 h. Plasma and urine samples were collected at various intervals for up to 6 days after surgery. Simultaneous modelling of parent acetaminophen and its metabolites was conducted in Phoenix(®) NLME™ to estimate pharmacokinetic parameters. The population mean estimate (CV%) for central (plasma) volume of distribution of parent acetaminophen (VC) was 13.9 (4.41) L, peripheral (tissue) volume of distribution (VT) was 50.9 (2.96) L, and intercompartmental clearance (Q) was 77.5 (9.29) L/h. The population mean (CV%) metabolic clearances for glucuronidation (CLPG) was 8.92 (3.25) L/h, sulfation (CLPS) was 0.903 (3.47) L/h, and oxidation (CLPO) was 0.533 (7.90) L/h. The population mean (CV%) urinary clearances of parent acetaminophen (CLRP) was 0.137 (5.46) L/h, acetaminophen glucuronide (CLRG) was 3.81 (6.71) L/h, acetaminophen sulfate (CLRS) was 3.13 (4.32) L/h, and acetaminophen cysteine + mercapturate (CLRO) was 3.51 (9.98) L/h. Age was found to be a significant covariate on the formation of acetaminophen glucuronide, and renal function (estimated as creatinine clearance) on the urinary excretion of acetaminophen glucuronide.

The pharmacokinetic profile of intravenous paracetamol in adult patients undergoing major abdominal surgery.

BACKGROUND: Intravenous (IV) paracetamol is commonly used in the postoperative period for the treatment of mild to moderate pain. The main pathways for paracetamol metabolism are glucuronidation, sulfation, and oxidation, accounting for approximately 55%, 30%, and 10% of urinary metabolites, respectively. The aim of this study was to describe the pharmacokinetics of IV paracetamol and its metabolites in adult patients after major abdominal surgery. METHODS: Twenty patients were given 1 g of paracetamol by IV infusion at induction of anesthesia (Interval 1) and every 6 hours thereafter, with the final dose given at 48-72 hours (Interval 2). Plasma and urine samples were collected for up to 8 hours after infusion for both intervals. The samples were analyzed by high-performance liquid chromatography to determine the amount of paracetamol and its metabolites. The data were modeled in Phoenix WinNonlin using a user-defined ASCII parent-metabolite model with linear disposition, to obtain the estimates for volume of distribution, metabolic and urinary clearance. RESULTS: Mean (95% confidence interval) metabolic clearance to paracetamol glucuronide increased from 0.06 (0.05-0.08) to 0.14 (0.11-0.18) L · h⁻¹ · kg⁻¹, P value <0.001 and urinary clearance increased from 0.08 (0.07-0.09) to 0.14 (0.10-0.17) L · h⁻¹ · kg⁻¹, P value 0.002. The mean (95% confidence interval) volume of distribution of paracetamol increased from 0.17 (0.12-0.21) to 0.43 (0.27-0.59) L · kg⁻¹, P value 0.032. CONCLUSIONS: After major abdominal surgery, there were apparent increases in the metabolic conversion to paracetamol glucuronide and its urinary clearance suggesting potential induction of paracetamol glucuronidation.